what's the difference between sag mill and ball mill - jxsc machine

what's the difference between sag mill and ball mill - jxsc machine

A mill is a grinder used to grind and blend solid or hard materials into smaller pieces by means of shear, impact and compression methods. Grinding mill machine is an essential part of many industrial processes, there are mainly five types of mills to cover more than 90% materials size-reduction applications.

Do you the difference between the ball mill, rod mills, SAG mill, tube mill, pebble mill? In the previous article, I made a comparison of ball mill and rod mill. Today, we will learn about the difference between SAG mill vs ball mill.

AG/SAG is short for autogenous mill and semi-autogenous mill, it combines with two functions of crushing and grinding, uses the ground material itself as the grinding media, through the mutual impact and grinding action to gradually reduce the material size. SAG mill is usually used to grind large pieces into small pieces, especially for the pre-processing of grinding circuits, thus also known as primary stage grinding machine. Based on the high throughput and coarse grind, AG mills produce coarse grinds often classify mill discharge with screens and trommel. SAG mills grinding media includes some large and hard rocks, filled rate of 9% 20%. SAG mill grind ores through impact, attrition, abrasion forces. In practice, for a given ore and equal processing conditions, the AG milling has a finer grind than SAG mills.

The working principle of the self-grinding machine is basically the same as the ball mill, the biggest difference is that the sag grinding machine uses the crushed material inside the cylinder as the grinding medium, the material constantly impacts and grinding to gradually pulverize. Sometimes, in order to improve the processing capacity of the mill, a small amount of steel balls be added appropriately, usually occupying 2-3% of the volume of the mill (that is semi-autogenous grinding).

High capacity Ability to grind multiple types of ore in various circuit configurations, reduces the complexity of maintenance and coordination. Compared with the traditional tumbling mill, the autogenous mill reduces the consumption of lining plates and grinding media, thus have a lower operation cost. The self-grinding machine can grind the material to 0.074mm in one time, and its content accounts for 20% ~ 50% of the total amount of the product. Grinding ratio can reach 4000 ~ 5000, more than ten times higher than ball, rod mill.

Ball mills are fine grinders, have horizontal ball mill and vertical ball mill, their cylinders are partially filled with steel balls, manganese balls, or ceramic balls. The material is ground to the required fineness by rotating the cylinder causing friction and impact. The internal machinery of the ball mill grinds the material into powder and continues to rotate if extremely high precision and precision is required.

The ball mill can be applied in the cement production plants, mineral processing plants and where the fine grinding of raw material is required. From the volume, the ball mill divide into industrial ball mill and laboratory use the small ball mill, sample grinding test. In addition, these mills also play an important role in cold welding, alloy production, and thermal power plant power production.

The biggest characteristic of the sag mill is that the crushing ratio is large. The particle size of the materials to be ground is 300 ~ 400mm, sometimes even larger, and the minimum particle size of the materials to be discharged can reach 0.1 mm. The calculation shows that the crushing ratio can reach 3000 ~ 4000, while the ball mills crushing ratio is smaller. The feed size is usually between 20-30mm and the product size is 0-3mm.

Both the autogenous grinding mill and the ball mill feed parts are welded with groove and embedded inner wear-resistant lining plate. As the sag mill does not contain grinding medium, the abrasion and impact on the equipment are relatively small.

The feed of the ball mill contains grinding balls. In order to effectively reduce the direct impact of materials on the ball mill feed bushing and improve the service life of the ball mill feed bushing, the feeding point of the groove in the feeding part of the ball mill must be as close to the side of the mill barrel as possible. And because the ball mill feed grain size is larger, ball mill feeding groove must have a larger slope and height, so that feed smooth.

Since the power of the autogenous tumbling mill is relatively small, it is appropriate to choose dynamic and static pressure bearing. The ball bearing liner is made of lead-based bearing alloy, and the back of the bearing is formed with a waist drum to form a contact centering structure, with the advantages of flexible movement. The bearing housing is lubricated by high pressure during start-up and stop-up, and the oil film is formed by static pressure. The journal is lifted up to prevent dry friction on the sliding surface, and the starting energy moment is reduced. The bearing lining is provided with a snake-shaped cooling water pipe, which can supply cooling water when necessary to reduce the temperature of the bearing bush. The cooling water pipe is made of red copper which has certain corrosion resistance.

Ball mill power is relatively large, the appropriate choice of hydrostatic sliding bearing. The main bearing bush is lined with babbitt alloy bush, each bush has two high-pressure oil chambers, high-pressure oil has been supplied to the oil chamber before and during the operation of the mill, the high-pressure oil enters the oil chamber through the shunting motor, and the static pressure oil film is compensated automatically to ensure the same oil film thickness To provide a continuous static pressure oil film for mill operation, to ensure that the journal and the bearing Bush are completely out of contact, thus greatly reducing the mill start-up load, and can reduce the impact on the mill transmission part, but also can avoid the abrasion of the bearing Bush, the service life of the bearing Bush is prolonged. The pressure indication of the high pressure oil circuit can be used to reflect the load of the mill indirectly. When the mill stops running, the high pressure oil will float the Journal, and the Journal will stop gradually in the bush, so that the Bush will not be abraded. Each main bearing is equipped with two temperature probe, dynamic monitoring of the bearing Bush temperature, when the temperature is greater than the specified temperature value, it can automatically alarm and stop grinding. In order to compensate for the change of the mill length due to temperature, there is a gap between the hollow journal at the feeding end and the bearing Bush width, which allows the journal to move axially on the bearing Bush. The two ends of the main bearing are sealed in an annular way and filled with grease through the lubricating oil pipe to prevent the leakage of the lubricating oil and the entry of dust.

The end cover of the autogenous mill is made of steel plate and welded into one body; the structure is simple, but the rigidity and strength are low; the liner of the autogenous mill is made of high manganese steel.

The end cover and the hollow shaft can be made into an integral or split type according to the actual situation of the project. No matter the integral or split type structure, the end cover and the hollow shaft are all made of Casting After rough machining, the key parts are detected by ultrasonic, and after finishing, the surface is detected by magnetic particle. The surface of the hollow shaft journal is Polished after machining. The end cover and the cylinder body are all connected by high-strength bolts. Strict process measures to control the machining accuracy of the joint surface stop, to ensure reliable connection and the concentricity of the two end journal after final assembly. According to the actual situation of the project, the cylinder can be made as a whole or divided, with a flanged connection and stop positioning. All welds are penetration welds, and all welds are inspected by ultrasonic nondestructive testing After welding, the whole Shell is returned to the furnace for tempering stress relief treatment, and after heat treatment, the shell surface is shot-peened. The lining plate of the ball mill is usually made of alloy material.

The transmission part comprises a gear and a gear, a gear housing, a gear housing and an accessory thereof. The big gear of the transmission part of the self-grinding machine fits on the hollow shaft of the discharge material, which is smaller in size, but the seal of the gear cover is not good, and the ore slurry easily enters the hollow shaft of the discharge material, causing the hollow shaft to wear.

The big gear of the ball mill fits on the mill shell, the size is bigger, the big gear is divided into half structure, the radial and axial run-out of the big gear are controlled within the national standard, the aging treatment is up to the standard, and the stress and deformation after processing are prevented. The big gear seal adopts the radial seal and the reinforced big gear shield. It is welded and manufactured in the workshop. The geometric size is controlled, the deformation is prevented and the sealing effect is ensured. The small gear transmission device adopts the cast iron base, the bearing base and the bearing cap are processed at the same time to reduce the vibration in operation. Large and small gear lubrication: The use of spray lubrication device timing quantitative forced spray lubrication, automatic control, no manual operation. The gear cover is welded by profile steel and high-quality steel plate. In order to enhance the stiffness of the gear cover, the finite element analysis is carried out, and the supporting structure is added in the weak part according to the analysis results.

The self-mill adopts the self-return device to realize the discharge of the mill. The self-returning device is located in the revolving part of the mill, and the material forms a self-circulation in the revolving part of the mill through the self-returning device, discharging the qualified material from the mill, leading the unqualified material back into the revolving part to participate in the grinding operation.

The ball mill adopts a discharge screen similar to the ball mill, and the function of blocking the internal medium of the overflow ball mill is accomplished inside the rotary part of the ball mill. The discharge screen is only responsible for forcing out a small amount of the medium that overflows into the discharge screen through the internal welding reverse spiral, to achieve forced discharge mill.

The slow drive consists of a brake motor, a coupling, a planetary reducer and a claw-type clutch. The device is connected to a pinion shaft and is used for mill maintenance and replacement of liners. In addition, after the mill is shut down for a long time, the slow-speed transmission device before starting the main motor can eliminate the eccentric load of the steel ball, loosen the consolidation of the steel ball and materials, ensure safe start, avoid overloading of the air clutch, and play a protective role. The slow-speed transmission device can realize the point-to-point reverse in the electronic control design. When connecting the main motor drive, the claw-type Clutch automatically disengages, the maintenance personnel should pay attention to the safety.

The slow drive device of the ball mill is provided with a rack and pinion structure, and the operating handle is moved to the side away from the cylinder body The utility model not only reduces the labor intensity but also ensures the safety of the operators.

ball mill refiners vs roller refiners - community | thechocolatelife

ball mill refiners vs roller refiners - community | thechocolatelife

There are differences in flavor and texture with every different combination of equipment used in the production chain. Most people never have the luxury to experiment with different production methods until they outgrow one and have to step up to a new one.

It's not always true that roll refiners and ball mills are mutually exclusive: I know of one Italian equipment manufacturer that offers a "turnkey" system that includes a roaster, cracker/winnower, roll refiner (used a liquor grinder), AND a ball mill. One of their customers has purchased an old-style melangeur and goes from the ball mill (particle size reduction) to the melangeur (using it like a conche for flavor development).

There are some well-known and respected companies that use ball mills: Domori is one; of course, Netzsch's ChocoEasy machines incorporate their ball mils - ball mills are ideally suited to continuous production lines. One of the knocks against most conventional ball mills is uneven particle size distribution. The peak tends to be wider than other methods and their is often a bump in the tail where there are large sizes.

I would have to disagree that most startups use ball mills, though. Apart from the Netzsch machines, which are very expensive - a 50kg machine costs over $90,000 - the only other small ball mill unit I know of is from BLT. At close to $100k their "turnkey" systems are still too expensive for most startups.

My experience is that most startups start out with a small (5kg) kitchen appliance wet grinder and then graduate to one or more of the larger ones as it is comparatively economical to grow the business this way and, because the technique is essentially the same the finished product is not too different. The challenge is the support equipment. Finding comparably scaled (and priced) roasters, crackers, and winnowers is not so easy. I just solved the winnower problem (~50 lbs/hr for about $5k) and am working on an alternative to the most-used small cracker that uses an entirely different principle and should create much smaller quantities of "fines" (which increases yield). I will have a prototype sometime this summer. If it works, we plan to open-source the plans as well as offer it in kit form for those who don't want to build one themselves.

We've corresponded privately about your soon-to-open school in Irvine and I think it's something that's absolutely necessary for the industry. I applaud Qzina for taking the initiative on this. I think that starting small (with respect to batch sizes) and offering the widest possible variety of equipment to work with is a sound approach. Making good chocolate is as much (or more) ART as Science. No one way is better than another; they're all different and can all produce good chocolate. Which way (if any) is better depends on what the desired outcome is.

Any chance you'd give us more info about the bean cracker you're working on? After all, the biggest strength of open source is in the development process--"given enough eyeballs, all bugs are shallow" and all that.

I'm particularly interested as this is something I've been thinking about lately, too. I've read of bean crackers that fling the beans against a metal plate and been thinking of a way to build something like this myself. I'd probably start with metal plates attached to a drill and dropping beans through them. Hopefully, they'd get hit a couple times on the way down.

I would be more than happy to share what we're doing. At the moment, if this were a software project, I would characterize it as "we're still in the early design phase, have a direction, and want to do one proof of concept before we open things up to the community." Very definitely when we get to the point that we have our first physical prototype - we'll open it up for community review.

I can tell you that we are working on the impact principle. There is a long way to go from your description of a drill with plates to something that works ("hoping" the beans crack is not an option unless you have an automatic return for uncracked beans on your winnower - otherwise huge amounts of extra manual labor are involved). We actually took a look at four different approaches that I had come up with after talking to a lot of people with a lot of experience cracking cocoa and building machines, and narrowed it down to one approach that we can easily prototype.

There are a bunch of fun challenges to solve, including finding an inexpensive way to control the rate at which the beans enter the cracker. The brute force method the Crankandstein relies on won't work for this method.

Another thing we're set on doing is incorporating a small digital controller - based on an Arduino - so that users can program the speed of the central cracking mechanism as well as the feed mechanism. We'll open source that, too.

IMO, there aren't a lot of people looking to set up production facilities in par with the scale of TCHO. Their Universal is a 3MT machine and is fronted by a ball mill. Theoretically, they could be producing 15-20MT/week in their plant - from liquor as they have no roaster and, on the pier, they are not likely to.

The real growth in the market (as I see it) is how to help companies that are in their real startup phase (i.e., producing in 5Kg batches) or in small production (e.g., producing 40-50kg batches) move up to the next level of production and to do so cost effectively. Going the Mast Brothers route (without the Selmi) a used convection oven, Crankandstein, CPS (or similar) winnower, and a 65L CocoaTown costs about $10k.

What's the next step? Spending over $70K for the basic (15Kg) BLT setup is too big a jump for most startups. It's not enough increase in production to justify the price differential. The cost structure goes up but the throughput to pay for the increased costs does not go up at the same rate.

If I was looking to spend roughly 110K Euros on new equipment there are solutions which offer up to 400Kg/day throughput (not including tempering/molding) for that amount of money. The throughput increase is great enough to justify the cost difference.

It's also really, really, really important to note that once you get above a certain production size, what really matters is materials handling. Everything can be moved around by hand when you're doing up to a couple of hundred kilos a day, but above that you really do need to consider where you're going to be storing beans (receiving/cleaning, storage, staging before and after roasting), where you're going to be storing chaff and nib, how you're moving/pumping (and storing) liquor and finished chocolate. The issues associated with materials handling are critical when considering growth above a certain size.

Where's the tipping point? I don't know - it depends on how much experience you have. I visited Pralus's factory in 2009 and they still move everything around by hand. They have 3, 250Kg Universals (two dark, one dedicated to milk). There's a 35kg ball roaster and a cracker/winnower. I did not see where they store/age their chocolate after it comes out of the Universal and before it gets molded or what they store it in. But Pralus did not start out at this production level, he's grown into it over many years. If he started from scratch today, with little experience is that how he'd set it up? Probably not.

With respect to your question about conching. It's really about three things: final particle size reduction, breaking up agglomerates and covering all the powder particles with fat, and flavor development. The "beauty" of the Netzsch approach is that it decouples the physical processes from the flavor development processes. You run it through the ball mill until you get the particle size you want and then stop pumping the chocolate through the ball mill and only beat it and aerate it to evaporate out aromatics you don't want.

You could do the physical processes another way (e.g., a grinder into a roll refiner then into a universal for a short while) and then use a device like the Duyvis-Wiener taste changer for final flavor development. (You could also blow a lot of air through the universal using both push and pull fans.)

What's important to know - and this is where having a variety of equipment on hand helps - is that the optimum time required for the the physical processes is not the same as the optimum time required to develop flavor. If it takes 48 hours of continuous grinding to get the texture where you want it you run the risk of driving off a lot of interesting flavors.

I think, in the realm of information that is likely to be publicly shared with you or freely available, what you find is going to be heavily influenced by the type of mfr you speak to (ie the refiner guys will tell you that their product is the best, ball milling guys the same). It will be heavily influenced by tradition and unproven beliefs (you will find folks in every camp that believe their way is the best simply because that's what they've always done, or that's what their predecessor or admired teacher told them - chefs are notorious for this). You yourself are already predisposed towards roll refiners; however in the above text you're only comparing it to one other production method (i can think of at least 8 particle size reduction technologies that are used commonly), and chances are very high that at least some of the chocolates you've consumed, you have no idea what process was used to convert it. Additionally, you will find that those who have spent the time and resources to do a scientific study to determine and quantify the differences in the various production methods will guard their results closely, as there are distinct competitive advantages that can be leveraged if one understands the options at a detailed level.

I will tell you that it is quite possible to make very similar (read: indistinguishable via the consumer) chocolate via multiple production methods (for example, i can make a dark chocolate on a refiner as well as another type (or types) of production kit that you will not be able to distinguish from one another. There are also chocolates that require a very specific type of kit to make, and that you simply can not make via another method. It is very dependent upon the type of chocolate you are making, and the specifics of that chocolate's physical and sensory components become very important. There is no single answer to the question you seek.

Generally speaking, each approach will have thematic pros and cons associated with it. Some are more heavily weighted towards product (ie how it handles raw materials and converts to finished product) while others are more heavily weighted towards throughput and energy utilization. Chocolate processing has far more science behind it than most admit to, as quite frankly most don't understand it, and the industry abounds with myth and misperception. Many companies continue to do things 'because that's the way it's always worked' and have, over time, created hypothesis as to why something does or doesn't work - but by and large, those theories haven't really been tested or challenged in a valid fashion. There are exceptions, of course, and those exceptions will, for the most part, be treated as trade secrets due to the advantage the understanding offers.

vertimill vs ball mill - grinding & classification circuits - metallurgist & mineral processing engineer

vertimill vs ball mill - grinding & classification circuits - metallurgist & mineral processing engineer

Vertical roller mills. Such mills normally have integrated classifier. This means plant design is more compact / simple in comparison to ball mill / classifier circuit. On the other hand fineness is limited.Agitated vertical mills. They have much higher specific grinding energy and from there are for production of finer products than conventional ball mills.

I think that the vertical mills are better than conventional ball mills because they have a good pneumatic classification. And other better characteristic is its flexibility for change between different clays.

A VRM is more energy efficient than a ball mill. It consumes less electricity.The VRM takes up less space in the milling complex than a ball mill.VRM is a sure choice of the modern day milling in an energy depleted world.

The metallurgical dictionary can be confusing! So what type of milling are we talking about?We manufacture and market high intensity vertical wet bead mills called a Commett Millwhich principle parts can be installed in a Deswik mill and dry autogenous micropulva mills ( no balls ) http://is.gd/ybiAKy 

Large rocks are effectively broken by crushing/compression.Fine material by impact grinding/cataractingUltra fine grinding / attrition are effective on for production of -75 micron down to 2 micron product.

As I see, nobody consider the capacities for vertical mills during operation. Basically, vertical mills in different models have less capacity than ball mills.As I know, just in cement raw material grinding, vertical roller mills have been developed.

IIT has patented new vertical milling technology that combines low energy input with maximum grinding force to accurately control the specification of fine powder output. Throughput capacities can also be controlled by both the size of the grinding chamber and a modular approach that enables multiple grinding systems to be incorporated in special twin mill configurations. The system is now being taken up in a range of material and mineral processing operations.

I see that several have answered based on dry grinding, i.e. Vertical Roller Mill vs. conventional Ball Mill.If we instead focus on wet grinding, we should differentiate between stirred media grinding, which can be both vertical and horizontal, and tumbling mills with manufactured grinding bodies. A continuous grate-discharge tumbling mill with grinding balls does seldom have ball size below 20 mm since you then will have unacceptable media losses or problem with grate throughput. To get energy efficient grinding below approx. 40 m you need to have finer grinding balls than are suitable for the tumbling mill. So, the case for the stirred media mill that often has media in the range 5-10 mm if used as a tertiary grinding step or for re-grinding.

Generally, the VMs are successfully used for very fine grinding. This equipment is very advantageous in comparison with BMs. During the last 3 years, I replaced the BMs with VMs for two magnetite projects. FYI, the power consumption!

I think it is horses for courses - it may not be realistic to replace a wet horizontal ball mill with a wet vertical mill (stirred), but rather have both in the same plant to achieve your ultimate particle size (i.e. wet grind, separate / classify then onto the next stage of fine or ultrafine grinding). Similarly, in dry grinding, the vertical roller mill with integral classifier has a specific task to do. You can also have a horizontal ball mill with air classifying capability. Each machine has a specific role and each plant / process has specific requirements. Yes there are overlaps, but this is where the analysis and evaluation comes in - can we replace one piece of equipment with another, what are the tradeoff'sand benefits?

Maximal feed size accepted by VMs is 6 mm. You can judge if their use is adequate to the comminution circuit in function of the process requirements (product final size, production capacity etc.). Certainly, the VMs are characterized by lower power and grinding media consumption than BMs. Even the CAPEX can be lower, depending of the production capacity. The maintenance costs are lower too in comparison with BMs.

It would be difficult to provide an accurate answer without a reference point as a base to compare the two types of comminution equipment you mentioned. That reference point would have to be throughput rate and target grind size.

It seems from the above that each application will be specific and will require a specific analysis. Vertical roller mills can have lower power than ball mills in most applications for dry grinding. One comment that I did not see here is the potential for differential grinding, especially in vertical mills, but also in ball mills. That is when inter-grinding different materials, the softer materials tend to be ground finer. In air swept vertical mills, this can leave a higher quantity of the harder to grind material in the mill. If this material is also abrasive, it may accelerate wear inside the mill. Which leads to an additional comment vertical mill tend to be more complex and have a higher level of maintenance expertise required? Wear rates (measured in metal loss) tend to be less, but replacement parts are more expensive. This is not to endorse ball mills, which appear to becoming a dying breed. Just to note some more of the differences.

Just add small comments to the previous, you will not need classification equipment such as vibrating screen or hydrocyclone for Isa Mill or Verti-Mill circuit. A signature plot test work to determine the energy requirement per ton of feed, the media consumption of the specific feed is recommended.

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ball mills

ball mills

In all ore dressing and milling Operations, including flotation, cyanidation, gravity concentration, and amalgamation, the Working Principle is to crush and grind, often with rob mill & ball mills, the ore in order to liberate the minerals. In the chemical and process industries, grinding is an important step in preparing raw materials for subsequent treatment.In present day practice, ore is reduced to a size many times finer than can be obtained with crushers. Over a period of many years various fine grinding machines have been developed and used, but the ball mill has become standard due to its simplicity and low operating cost.

A ball millefficiently operated performs a wide variety of services. In small milling plants, where simplicity is most essential, it is not economical to use more than single stage crushing, because the Steel-Head Ball or Rod Mill will take up to 2 feed and grind it to the desired fineness. In larger plants where several stages of coarse and fine crushing are used, it is customary to crush from 1/2 to as fine as 8 mesh.

Many grinding circuits necessitate regrinding of concentrates or middling products to extremely fine sizes to liberate the closely associated minerals from each other. In these cases, the feed to the ball mill may be from 10 to 100 mesh or even finer.

Where the finished product does not have to be uniform, a ball mill may be operated in open circuit, but where the finished product must be uniform it is essential that the grinding mill be used in closed circuit with a screen, if a coarse product is desired, and with a classifier if a fine product is required. In most cases it is desirable to operate the grinding mill in closed circuit with a screen or classifier as higher efficiency and capacity are obtained. Often a mill using steel rods as the grinding medium is recommended, where the product must have the minimum amount of fines (rods give a more nearly uniform product).

Often a problem requires some study to determine the economic fineness to which a product can or should be ground. In this case the 911Equipment Company offers its complete testing service so that accurate grinding mill size may be determined.

Until recently many operators have believed that one particular type of grinding mill had greater efficiency and resulting capacity than some other type. However, it is now commonly agreed and accepted that the work done by any ballmill depends directly upon the power input; the maximum power input into any ball or rod mill depends upon weight of grinding charge, mill speed, and liner design.

The apparent difference in capacities between grinding mills (listed as being the same size) is due to the fact that there is no uniform method of designating the size of a mill, for example: a 5 x 5 Ball Mill has a working diameter of 5 inside the liners and has 20 per cent more capacity than all other ball mills designated as 5 x 5 where the shell is 5 inside diameter and the working diameter is only 48 with the liners in place.

Ball-Rod Mills, based on 4 liners and capacity varying as 2.6 power of mill diameter, on the 5 size give 20 per cent increased capacity; on the 4 size, 25 per cent; and on the 3 size, 28 per cent. This fact should be carefully kept in mind when determining the capacity of a Steel- Head Ball-Rod Mill, as this unit can carry a greater ball or rod charge and has potentially higher capacity in a given size when the full ball or rod charge is carried.

A mill shorter in length may be used if the grinding problem indicates a definite power input. This allows the alternative of greater capacity at a later date or a considerable saving in first cost with a shorter mill, if reserve capacity is not desired. The capacities of Ball-Rod Mills are considerably higher than many other types because the diameters are measured inside the liners.

The correct grinding mill depends so much upon the particular ore being treated and the product desired, that a mill must have maximum flexibility in length, type of grinding medium, type of discharge, and speed.With the Ball-Rod Mill it is possible to build this unit in exact accordance with your requirements, as illustrated.

To best serve your needs, the Trunnion can be furnished with small (standard), medium, or large diameter opening for each type of discharge. The sketch shows diagrammatic arrangements of the four different types of discharge for each size of trunnion opening, and peripheral discharge is described later.

Ball-Rod Mills of the grate discharge type are made by adding the improved type of grates to a standard Ball-Rod Mill. These grates are bolted to the discharge head in much the same manner as the standard headliners.

The grates are of alloy steel and are cast integral with the lifter bars which are essential to the efficient operation of this type of ball or rod mill. These lifter bars have a similar action to a pump:i. e., in lifting the product so as to discharge quickly through the mill trunnion.

These Discharge Grates also incorporate as an integral part, a liner between the lifters and steel head of the ball mill to prevent wear of the mill head. By combining these parts into a single casting, repairs and maintenance are greatly simplified. The center of the grate discharge end of this mill is open to permit adding of balls or for adding water to the mill through the discharge end.

Instead of being constructed of bars cast into a frame, Grates are cast entire and have cored holes which widen toward the outside of the mill similar to the taper in grizzly bars. The grate type discharge is illustrated.

The peripheral discharge type of Ball-Rod Mill is a modification of the grate type, and is recommended where a free gravity discharge is desired. It is particularly applicable when production of too many fine particles is detrimental and a quick pass through the mill is desired, and for dry grinding.

The drawings show the arrangement of the peripheral discharge. The discharge consists of openings in the shell into which bushings with holes of the desired size are inserted. On the outside of the mill, flanges are used to attach a stationary discharge hopper to prevent pulp splash or too much dust.

The mill may be operated either as a peripheral discharge or a combination or peripheral and trunnion discharge unit, depending on the desired operating conditions. If at any time the peripheral discharge is undesirable, plugs inserted into the bushings will convert the mill to a trunnion discharge type mill.

Unless otherwise specified, a hard iron liner is furnished. This liner is made of the best grade white iron and is most serviceable for the smaller size mills where large balls are not used. Hard iron liners have a much lower first cost.

Electric steel, although more expensive than hard iron, has advantage of minimum breakage and allows final wear to thinner section. Steel liners are recommended when the mills are for export or where the source of liner replacement is at a considerable distance.

Molychrome steel has longer wearing qualities and greater strength than hard iron. Breakage is not so apt to occur during shipment, and any size ball can be charged into a mill equipped with molychrome liners.

Manganese liners for Ball-Rod Mills are the world famous AMSCO Brand, and are the best obtainable. The first cost is the highest, but in most cases the cost per ton of ore ground is the lowest. These liners contain 12 to 14% manganese.

The feed and discharge trunnions are provided with cast iron or white iron throat liners. As these parts are not subjected to impact and must only withstand abrasion, alloys are not commonly used but can be supplied.

Gears for Ball-Rod Mills drives are furnished as standard on the discharge end of the mill where they are out of the way of the classifier return, scoop feeder, or original feed. Due to convertible type construction the mills can be furnished with gears on the feed end. Gear drives are available in two alternative combinations, which are:

All pinions are properly bored, key-seated, and pressed onto the steel countershaft, which is oversize and properly keyseated for the pinion and drive pulleys or sheaves. The countershaft operates on high grade, heavy duty, nickel babbitt bearings.

Any type of drive can be furnished for Ball-Rod Mills in accordance with your requirements. Belt drives are available with pulleys either plain or equipped with friction clutch. Various V- Rope combinations can also be supplied.

The most economical drive to use up to 50 H. P., is a high starting torque motor connected to the pinion shaft by means of a flat or V-Rope drive. For larger size motors the wound rotor (slip ring) is recommended due to its low current requirement in starting up the ball mill.

Should you be operating your own power plant or have D. C. current, please specify so that there will be no confusion as to motor characteristics. If switches are to be supplied, exact voltage to be used should be given.

Even though many ores require fine grinding for maximum recovery, most ores liberate a large percentage of the minerals during the first pass through the grinding unit. Thus, if the free minerals can be immediately removed from the ball mill classifier circuit, there is little chance for overgrinding.

This is actually what has happened wherever Mineral Jigs or Unit Flotation Cells have been installed in the ball mill classifier circuit. With the installation of one or both of these machines between the ball mill and classifier, as high as 70 per cent of the free gold and sulphide minerals can be immediately removed, thus reducing grinding costs and improving over-all recovery. The advantage of this method lies in the fact that heavy and usually valuable minerals, which otherwise would be ground finer because of their faster settling in the classifier and consequent return to the grinding mill, are removed from the circuit as soon as freed. This applies particularly to gold and lead ores.

Ball-Rod Mills have heavy rolled steel plate shells which are arc welded inside and outside to the steel heads or to rolled steel flanges, depending upon the type of mill. The double welding not only gives increased structural strength, but eliminates any possibility of leakage.

Where a single or double flanged shell is used, the faces are accurately machined and drilled to template to insure perfect fit and alignment with the holes in the head. These flanges are machined with male and female joints which take the shearing stresses off the bolts.

The Ball-Rod Mill Heads are oversize in section, heavily ribbed and are cast from electric furnace steel which has a strength of approximately four times that of cast iron. The head and trunnion bearings are designed to support a mill with length double its diameter. This extra strength, besides eliminating the possibility of head breakage or other structural failure (either while in transit or while in service), imparts to Ball-Rod Mills a flexibility heretofore lacking in grinding mills. Also, for instance, if you have a 5 x 5 mill, you can add another 5 shell length and thus get double the original capacity; or any length required up to a maximum of 12 total length.

On Type A mills the steel heads are double welded to the rolled steel shell. On type B and other flanged type mills the heads are machined with male and female joints to match the shell flanges, thus taking the shearing stresses from the heavy machine bolts which connect the shell flanges to the heads.

The manhole cover is protected from wear by heavy liners. An extended lip is provided for loosening the door with a crow-bar, and lifting handles are also provided. The manhole door is furnished with suitable gaskets to prevent leakage.

The mill trunnions are carried on heavy babbitt bearings which provide ample surface to insure low bearing pressure. If at any time the normal length is doubled to obtain increased capacity, these large trunnion bearings will easily support the additional load. Trunnion bearings are of the rigid type, as the perfect alignment of the trunnion surface on Ball-Rod Mills eliminates any need for the more expensive self-aligning type of bearing.

The cap on the upper half of the trunnion bearing is provided with a shroud which extends over the drip flange of the trunnion and effectively prevents the entrance of dirt or grit. The bearing has a large space for wool waste and lubricant and this is easily accessible through a large opening which is covered to prevent dirt from getting into the bearing.Ball and socket bearings can be furnished.

Scoop Feeders for Ball-Rod Mills are made in various radius sizes. Standard scoops are made of cast iron and for the 3 size a 13 or 19 feeder is supplied, for the 4 size a 30 or 36, for the 5 a 36 or 42, and for the 6 a 42 or 48 feeder. Welded steel scoop feeders can, however, be supplied in any radius.

The correct size of feeder depends upon the size of the classifier, and the smallest feeder should be used which will permit gravity flow for closed circuit grinding between classifier and the ball or rod mill. All feeders are built with a removable wearing lip which can be easily replaced and are designed to give minimum scoop wear.

A combination drum and scoop feeder can be supplied if necessary. This feeder is made of heavy steel plate and strongly welded. These drum-scoop feeders are available in the same sizes as the cast iron feeders but can be built in any radius. Scoop liners can be furnished.

The trunnions on Ball-Rod Mills are flanged and carefully machined so that scoops are held in place by large machine bolts and not cap screws or stud bolts. The feed trunnion flange is machined with a shoulder for insuring a proper fit for the feed scoop, and the weight of the scoop is carried on this shoulder so that all strain is removed from the bolts which hold the scoop.

High carbon steel rods are recommended, hot rolled, hot sawed or sheared, to a length of 2 less than actual length of mill taken inside the liners. The initial rod charge is generally a mixture ranging from 1.5 to 3 in diameter. During operation, rod make-up is generally the maximum size. The weights per lineal foot of rods of various diameters are approximately: 1.5 to 6 lbs.; 2-10.7 lbs.; 2.5-16.7 lbs.; and 3-24 lbs.

Forged from the best high carbon manganese steel, they are of the finest quality which can be produced and give long, satisfactory service. Data on ball charges for Ball-Rod Mills are listed in Table 5. Further information regarding grinding balls is included in Table 6.

Rod Mills has a very define and narrow discharge product size range. Feeding a Rod Mill finer rocks will greatly impact its tonnage while not significantly affect its discharge product sizes. The 3.5 diameter rod of a mill, can only grind so fine.

Crushers are well understood by most. Rod and Ball Mills not so much however as their size reduction actions are hidden in the tube (mill). As for Rod Mills, the image above best expresses what is going on inside. As rocks is feed into the mill, they are crushed (pinched) by the weight of its 3.5 x 16 rods at one end while the smaller particles migrate towards the discharge end and get slightly abraded (as in a Ball Mill) on the way there.

We haveSmall Ball Mills for sale coming in at very good prices. These ball mills are relatively small, bearing mounted on a steel frame. All ball mills are sold with motor, gears, steel liners and optional grinding media charge/load.

Ball Mills or Rod Mills in a complete range of sizes up to 10 diameter x20 long, offer features of operation and convertibility to meet your exactneeds. They may be used for pulverizing and either wet or dry grindingsystems. Mills are available in both light-duty and heavy-duty constructionto meet your specific requirements.

All Mills feature electric cast steel heads and heavy rolled steelplate shells. Self-aligning main trunnion bearings on large mills are sealedand internally flood-lubricated. Replaceable mill trunnions. Pinion shaftbearings are self-aligning, roller bearing type, enclosed in dust-tightcarrier. Adjustable, single-unit soleplate under trunnion and drive pinionsfor perfect, permanent gear alignment.

Ball Mills can be supplied with either ceramic or rubber linings for wet or dry grinding, for continuous or batch type operation, in sizes from 15 x 21 to 8 x 12. High density ceramic linings of uniform hardness male possible thinner linings and greater and more effective grinding volume. Mills are shipped with liners installed.

Complete laboratory testing service, mill and air classifier engineering and proven equipment make possible a single source for your complete dry-grinding mill installation. Units available with air swept design and centrifugal classifiers or with elevators and mechanical type air classifiers. All sizes and capacities of units. Laboratory-size air classifier also available.

A special purpose batch mill designed especially for grinding and mixing involving acids and corrosive materials. No corners mean easy cleaning and choice of rubber or ceramic linings make it corrosion resistant. Shape of mill and ball segregation gives preferential grinding action for grinding and mixing of pigments and catalysts. Made in 2, 3 and 4 diameter grinding drums.

Nowadays grinding mills are almost extensively used for comminution of materials ranging from 5 mm to 40 mm (3/161 5/8) down to varying product sizes. They have vast applications within different branches of industry such as for example the ore dressing, cement, lime, porcelain and chemical industries and can be designed for continuous as well as batch grinding.

Ball mills can be used for coarse grinding as described for the rod mill. They will, however, in that application produce more fines and tramp oversize and will in any case necessitate installation of effective classification.If finer grinding is wanted two or three stage grinding is advisable as for instant primary rod mill with 75100 mm (34) rods, secondary ball mill with 2540 mm(11) balls and possibly tertiary ball mill with 20 mm () balls or cylpebs.To obtain a close size distribution in the fine range the specific surface of the grinding media should be as high as possible. Thus as small balls as possible should be used in each stage.

The principal field of rod mill usage is the preparation of products in the 5 mm0.4 mm (4 mesh to 35 mesh) range. It may sometimes be recommended also for finer grinding. Within these limits a rod mill is usually superior to and more efficient than a ball mill. The basic principle for rod grinding is reduction by line contact between rods extending the full length of the mill, resulting in selective grinding carried out on the largest particle sizes. This results in a minimum production of extreme fines or slimes and more effective grinding work as compared with a ball mill. One stage rod mill grinding is therefore suitable for preparation of feed to gravimetric ore dressing methods, certain flotation processes with slime problems and magnetic cobbing. Rod mills are frequently used as primary mills to produce suitable feed to the second grinding stage. Rod mills have usually a length/diameter ratio of at least 1.4.

Tube mills are in principle to be considered as ball mills, the basic difference being that the length/diameter ratio is greater (35). They are commonly used for surface cleaning or scrubbing action and fine grinding in open circuit.

In some cases it is suitable to use screened fractions of the material as grinding media. Such mills are usually called pebble mills, but the working principle is the same as for ball mills. As the power input is approximately directly proportional to the volume weight of the grinding media, the power input for pebble mills is correspondingly smaller than for a ball mill.

A dry process requires usually dry grinding. If the feed is wet and sticky, it is often necessary to lower the moisture content below 1 %. Grinding in front of wet processes can be done wet or dry. In dry grinding the energy consumption is higher, but the wear of linings and charge is less than for wet grinding, especially when treating highly abrasive and corrosive material. When comparing the economy of wet and dry grinding, the different costs for the entire process must be considered.

An increase in the mill speed will give a directly proportional increase in mill power but there seems to be a square proportional increase in the wear. Rod mills generally operate within the range of 6075 % of critical speed in order to avoid excessive wear and tangled rods. Ball and pebble mills are usually operated at 7085 % of critical speed. For dry grinding the speed is usually somewhat lower.

The mill lining can be made of rubber or different types of steel (manganese or Ni-hard) with liner types according to the customers requirements. For special applications we can also supply porcelain, basalt and other linings.

The mill power is approximately directly proportional to the charge volume within the normal range. When calculating a mill 40 % charge volume is generally used. In pebble and ball mills quite often charge volumes close to 50 % are used. In a pebble mill the pebble consumption ranges from 315 % and the charge has to be controlled automatically to maintain uniform power consumption.

In all cases the net energy consumption per ton (kWh/ton) must be known either from previous experience or laboratory tests before mill size can be determined. The required mill net power P kW ( = ton/hX kWh/ton) is obtained from

Trunnions of S.G. iron or steel castings with machined flange and bearing seat incl. device for dismantling the bearings. For smaller mills the heads and trunnions are sometimes made in grey cast iron.

The mills can be used either for dry or wet, rod or ball grinding. By using a separate attachment the discharge end can be changed so that the mills can be used for peripheral instead of overflow discharge.

vrm vs ball mill for cement grinding - page 1 of 1

vrm vs ball mill for cement grinding - page 1 of 1

Good Days! Gentlemen, Anybody in this forum who have had some practical/extensive experience on Vertical Roller Mill being used for cement grinding? We are planning to put up a new cement grinding plant using Vertical Roller Mill(300-350 TPH) and before implementing the project, we would like to gather as much information regarding the actual operation of this mill during cement grinding and the quality of the product in terms of the fineness/particle size distribution and how does it compare with the product of a ball mill relative to these parameters. Other parameters(VRM vs. Ball Mill) to be considered are the following: Specific power consumption Maintenance costs Process control/process technology Plant/system auxiliaries Reliability & Product quality It will be highly appreciated if we will receive immediate feedbacks from the knowledgeable members of this forum regarding this topic as this will help us to move forward with the implementation of this project. my e-mail: [email protected] Best regards, Abdullah Bazarah

Anybody in this forum who have had some practical/extensive experience on Vertical Roller Mill being used for cement grinding? We are planning to put up a new cement grinding plant using Vertical Roller Mill(300-350 TPH) and before implementing the project, we would like to gather as much information regarding the actual operation of this mill during cement grinding and the quality of the product in terms of the fineness/particle size distribution and how does it compare with the product of a ball mill relative to these parameters.

It will be highly appreciated if we will receive immediate feedbacks from the knowledgeable members of this forum regarding this topic as this will help us to move forward with the implementation of this project.

Specific power consumption- high for ball mill and Low for VRM Maintenance costs- high for VRM and low for Ball mill Process control/process technology- ball is operation friendlt and simple in operation but VRM requires some PID or control technology. Plant/system auxiliaries- less auxillaries in ball mill and hydraulic circuit to be maintained for VRM . Reliability & Product quality- good partcicle size distribution in ball mill as we can play wtth Grinding media filling. VRM has less flexibility in controlling particle size distribution. Overall ball miil is good option for Cement mill but only has limitations for higher capacity, as two ball mill to be installed for One VRM, but in case of breakdown one ball mill can be in operation. Trust this clear your clarifications on equipment selection for cment production. Regards SRP...

Overall ball miil is good option for Cement mill but only has limitations for higher capacity, as two ball mill to be installed for One VRM, but in case of breakdown one ball mill can be in operation.

construction of ball mill/ ball mill structure | henan deya machinery co., ltd

construction of ball mill/ ball mill structure | henan deya machinery co., ltd

Structurally, each ball mill consists of a horizontal cylindrical shell, provided with renewable wearing liners and a charge of grinding medium. The drum is supported so as to rotate on its axis on hollow trunnions attached to the end walls (attached figure 1 ball mill). The diameter of the mill determines the pressure that can be exerted by the medium on the ore particles and, in general, the larger the feed size the larger needs to be the mill diameter. The length of the mill, in conjunction with the diameter, determines the volume, and hence the capacity of the mill.

The feed material is usually fed to the mill continuously through one end trunnion, the ground product leaving via the other trunnion, although in certain applications the product may leave the mill through a number of ports spaced around the periphery of the shell. All types of mill can be used for wet or dry grinding by modification of feed and discharge equipment.

Mill shells are designed to sustain impact and heavy loading, and are constructed from rolled mild steel plates, buttwelded together. Holes are drilled to take the bolts for holding the liners. Normally one or two access manholes are provided. For attachment of the trunnion heads, heavy flanges of fabricated or cast steel are usually welded or bolted to the ends of the plate shells, planed with parallel faces which are grooved to receive a corresponding spigot on the head, and drilled for bolting to the head.

The mill ends, or trunnion heads, may be of nodular or grey cast iron for diameters less than about 1 m. Larger heads are constructed from cast steel, which is relatively light, and can be welded. The heads are fibbed for reinforcement and may be flat, slightly conical, or dished. They are machined and drilled to fit shell flanges(attached figure 2 tube mill end and trunnion). figure 2 Tube mill end and trunnion Trunnions and bearings The trunnions are made from cast iron or steel and are spigoted and bolted to the end plates, although in small mills they may be integral with the end plates. They are highly polished to reduce bearing friction. Most trunnion bearings are rigid highgrade iron castings with 120-180 degree lining of white metal in the bearing area, surrounded by a fabricated mild steel housing, which is bolted into the concrete foundations (attached figure 3 oil-lubricated trunnion bearing). figure 3 oil-lubricated trunnion bearing The bearings in smaller mills may be grease lubricated, but oil lubrication is favoured in large mills, via motor-driven oil pumps. The effectiveness of normal lubrication protection is reduced when the mill is shut down for any length of time, and many mills are fitted with manually operated hydraulic starting lubricators, which force oil between the trunnion and trunnion bearing, preventing friction damage to the beating surface, on starting, by re-establishing the protecting film of oil (attached figure 4 Hydraulic starting lubricator). figure 4 Hydraulic starting lubricator Some manufacturers install large roller bearings, which can withstand higher forces than plain metal bearings (attached figure 5 Trunnion with roller-type bearings ). Trunnion with roller-type bearings Drive Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated. Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing. The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry. Liners The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

The trunnions are made from cast iron or steel and are spigoted and bolted to the end plates, although in small mills they may be integral with the end plates. They are highly polished to reduce bearing friction. Most trunnion bearings are rigid highgrade iron castings with 120-180 degree lining of white metal in the bearing area, surrounded by a fabricated mild steel housing, which is bolted into the concrete foundations (attached figure 3 oil-lubricated trunnion bearing). figure 3 oil-lubricated trunnion bearing The bearings in smaller mills may be grease lubricated, but oil lubrication is favoured in large mills, via motor-driven oil pumps. The effectiveness of normal lubrication protection is reduced when the mill is shut down for any length of time, and many mills are fitted with manually operated hydraulic starting lubricators, which force oil between the trunnion and trunnion bearing, preventing friction damage to the beating surface, on starting, by re-establishing the protecting film of oil (attached figure 4 Hydraulic starting lubricator). figure 4 Hydraulic starting lubricator Some manufacturers install large roller bearings, which can withstand higher forces than plain metal bearings (attached figure 5 Trunnion with roller-type bearings ). Trunnion with roller-type bearings Drive Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated. Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing. The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry. Liners The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

The bearings in smaller mills may be grease lubricated, but oil lubrication is favoured in large mills, via motor-driven oil pumps. The effectiveness of normal lubrication protection is reduced when the mill is shut down for any length of time, and many mills are fitted with manually operated hydraulic starting lubricators, which force oil between the trunnion and trunnion bearing, preventing friction damage to the beating surface, on starting, by re-establishing the protecting film of oil (attached figure 4 Hydraulic starting lubricator). figure 4 Hydraulic starting lubricator Some manufacturers install large roller bearings, which can withstand higher forces than plain metal bearings (attached figure 5 Trunnion with roller-type bearings ). Trunnion with roller-type bearings Drive Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated. Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing. The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry. Liners The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

Some manufacturers install large roller bearings, which can withstand higher forces than plain metal bearings (attached figure 5 Trunnion with roller-type bearings ). Trunnion with roller-type bearings Drive Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated. Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing. The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry. Liners The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated.

Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing.

The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry.

The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used.

Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost.

Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings.

The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts.

A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported.

To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines.

Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner.

The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

hammermills versus roller mills | world-grain.com | november 03, 2010 10:10

hammermills versus roller mills | world-grain.com | november 03, 2010 10:10

This wasnt always the case. In the early days of compound feed milling, when raw materials were homegrown and power sources were either wind or water, the effort needed to grind or flake cereals into a form where animal uptake was optimized dictated that roller milling was a more economic and popular means of size reduction.

Thus, roller milling was the traditional method of preparing cereals and fodder for on-farm consumption by livestock. Today, millers have the option of using either method, or both, and there are many factors that impact their choice.

First, and in some peoples eyes the most important consideration, is power consumption per tonne of grinded product. In this case, I am referring to general processing of cereals and proteins, and my comments do not relate to the specialist grinding of micro-ingredients and high-fat raw materials, which both need careful and specific attention when being ground.

In recent years, hammermill diameters have gradually increased, which obviously allows for greater peripheral beater tip speed at lower revolutions. This has meant the impact effect on cereals at the outer extremities of the grinding chamber is increasingly severe. Consequently, power consumption levels in such hammermills have been reduced to a minimum. This is partly due to a combination of increased screen-hole diameter that complements the increased peripheral beater tip speed by accelerating the impact of individual particles between beater and screen.

This increased diameter of hammermill grinding chambers has led to the adoption of machines with greater throughput capacity, and there has been a progressive shift toward the adoption of post-grinding techniques in most mills built today. In this case, post grinding refers to the positioning of the hammermill after the blending stage as opposed to pre-grinding positioning, when hammermills are placed at the early stage in the mill flow, before ingredients are combined together.

There are distinct advantages to adopting the post-grind position for hammermills. Building layout is simplified, overall bulk ingredient storage capacity is reduced, and capital costs are thus minimized in new installations. There are some disadvantages, however, as millers are aware, particularly those who have been called out in the early hours of the morning when a hammermill has broken down and there has been no reserve of ground product for manufacture through the pellet mills while the hammermill is being repaired.

Essentially, hammermills rely on the impact of screens and beaters on the product being ground to reduce it to the desired granularity for incorporation into a balanced ration. Roller mills simply roll or crush product between two revolving cylinders. This latter process has the distinct advantage of requiring considerably less power, although it is not possible to achieve the fineness of final grind through a roller mill that can be achieved through a hammermill. In a hammermill, the screen-hole diameter controls the maximum finished particle size of any ground product. When using roller mills, there is no screen being used, and unless the product is sifted and the coarse fraction reprocessed, the resultant particle size is purely reliant on the millers skill in setting the roller mill effectively.

Roller mills, particularly single pass installations, require more care and attention than hammermills in order to achieve a consistent and accurate grind. Ensuring the feed is spread thinly across the face of the roller mills can present some problems as mechanical feed gates can easily become obstructed, impairing the smooth and regular flow of product into the nip of the roller mill. Variability of raw material also needs regular adjustments as opposed to the all-encompassing grinding nature of the hammermill.

The available capacity is also a major consideration when using roller mills as there is a need for machines of considerable size or number to achieve the similar capacity as that of hammermills in the same circumstances. There are other general considerations that may affect capacity such as the cleanliness of the grain and the presence of foreign objects that may restrict flow through the roller mill feed mechanisms.

However, there are some circumstances when roller mills have the edge and it is not completely desirable to reduce ingredients down to a very fine particle size. Ruminant animals prefer to consume flaked cereals, as do horses and outdoor pigs. In such instances, the roller mill comes very much to the fore, particularly where coarse or open rations are being produced and fed. In the case of beef lots, where the finished feed is not required to be pelleted for purposes of cost-effective transportation, the roller mill can be used quite effectively and can be a key part of reducing power consumption at the mill.

One advantage of using flaked cereals is that the ability to incorporate liquid ingredients into a ration is enhanced. The greater surface area presented by a flake allows for greater absorption of liquids. At the very least, it allows for coating of a greater surface area if absorption is not fully achievable with such ingredients as molasses and some fats and oils. In the brewing industry, a standard grist is required that has been proven to allow optimum application and absorption of enzymes into the mash stage of the process. This can only be achieved by the use of roller mills, often triple roller mills where product is ground twice to achieve the desired grist spectrum.

It should be stressed that the roller mill, when equipped with fluted or corrugated roll chills, can achieve a relatively fine grind, particularly when moisture content of cereals is optimized. The use of differential roll drive arrangements, which create a sheer effect between the chills, not only allows for a finer particle size output, but the sheering effect the roller mill has upon starch granules in cereals is advantageous to the nutritionist when compiling rations. This is especially true for young stock, such as baby piglets and veal calves, where the digestive tract is undeveloped and its sensitivity needs to be respected and treated kindly in early stage diets. The use of HTD belt drives to achieve differential roll speeds of up to 2.5:1 is now well proven, and as a result of such engineering technology there is little need for lubrication of the modern roller mill.

One of the biggest disadvantages of using roller mills is that when the roll chills become worn, replacing them with new chills and subsequently recorrugating the old chills is a major endeavor in terms of time and expense. The good news, however, is there are no screens that can burst or become damaged.

Another positive aspect of using roller mills is that they require little or no air flow to operate effectively due to the fact that, with the rollers being mounted horizontally, product passes through by gravity. Hammermills require a steady and balanced airflow in order to operate efficiently and to keep screens clear and unimpeded. The cost of moving that air, the capital cost of filters and fans, and the space requirement must all be borne in mind when drawing comparisons between grinding techniques.

Recent hammermill designs have been quite innovative, and we have seen the combination of roller mill and hammermill technologies begin to emerge. By using a roller mill, or adopting roller grinding principles as part of the feed mechanism on entry to the hammermill, the raw material is partially ground at that point, which then allows the hammers and screens in the grinding chamber of the hammermill to be fully effective, with often excellent grinding efficiency results.

Not only is a finer grind achievable with far less power consumption, but the control the miller has on the resultant particle size of the grinded ingredient is enhanced tremendously. By partial preparation of the product between the rollers in transit to the hammermill grinding chamber in such an arrangement, the best of both worlds is achieved.

As power consumption becomes increasingly important, you will likely see greater use of roller milling technology as part of overall grinding techniques. Rolls of up to eight inches in diameter are being adopted as feed mechanisms with differential drives and variable gap settings. Compared to conventional, straight forward hammermilling, these new hybrid arrangements can reduce power consumption by around 15%, which cannot be ignored in these stringent times.

The key to successful size reduction, however, is diligence and, as with all aspects of mill management, attention to detail is paramount. The daily walk around the mill, keenly observing minor daily changes in operations, will always prove to be the best defense against rising costs.

Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J.B. Bradshaw Ltd. He has extensive experience in flour and feed milling in Africa, the Americas, Europe and the Caribbean. He may be contacted at: [email protected]?.

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