rp-4 gold shaker table sale

rp-4 gold shaker table sale

The RP-4 shaker table is the most widely used and most successful gold gravity shaking concentrating table worldwide, used by small and large mining operations and the hobbyist. The patented RP-4 is designed for separation of heavy mineral and gemstone concentrate. The RP-4 table can process up to 600 (typically 400) lbs. per hour of black sand magnetite or pulverised rock with little to no losses. The RP-4 uses a unique reverse polarity of rare earth magnets, which will cause the magnetite to rise and be washed off into the tails. This allows the micron gold to be released from the magnetite, letting the gold travelling to the catch. The RP-4 is compact and weighs 60 lbs. With a small generator and water tank, no location is too remote for its use. The RP-4 is a complete, ready to go gold recovery machine. THERE ARE NO SCREEN INCLUDED with the small shaking table. Use was reservoirsgreater than 250 gallon and recycle all your water. Only 400 Watt of power drawn by typical pump. The small RP4 gold shaking has a mini deck of 13wide x 36 long = 3.25 square feet of tabling area. The RP-4 is the best and longest selling small miner shaker table still on the market today. With many 1000s of units sold during the last 10 years! Review the RP-4 Operating Manual and Installation Guide lower on this page.

The RP-4 uses a unique reverse polarity of rare earth magnets which will cause the magnetite to rise and be washed off into the tails and allowing the micron gold to be released from the magnetite leaving the gold travelling to the catch.

When assembling the RP-4, it is very important to set it up correctly to get the best recovery. The unit needs to be bolted preferably to a concrete pad or bedrock when in the field. It can be weighted down with seven or eight large sandbags. Wooden stands will set up harmonics and vibrations in the unit. Vibrations will create a negative effect on the concentrating action of the deck and create a scattering effect on the gold. We would strongly advise getting the optional stand to mount it. See a detailed RP4 Shaker Table review.

Once you have the RP-4 mounted or weighted down, you will want to level it, place a level under the machine on the bar running attached to the two mounting legs. Use washers to get a precise level adjustment. Once mounted and leveled, use the adjustment screw to adjust the horizontal slope of the deck. It took me about 10 minutes of playing with the adjustment till you are satisfied the slope angle was where it needed to be. A general rule for good recovery is less grade for the table deck and as much water as possible without scouring off the fine gold particles.

When the table is set, wet down your black sand concentrates with water and a couple drops of Jet-Dry to help keep any fine gold from floating off the table. You are now ready to start feeding the RP-4.

DO NOT dump material into the feed tray. You want a nice steady feed without overloading the table. Use a scoop and feed it steadily. Watch the back where the small gold should concentrate. If you see fine gold towards the middle, adjust your table angle just a bit at a time till it is where it needs to be.

Run a few buckets of black sand tailings that already panned out just in case there might have been some gold left behind. Its a good thing, too, because I pulled almost three pennyweights of gold out of my waste materials. Thats a pennyweight per bucket!

You could run all of you concentrates over this awesome little RP-4 Gravity Shaker Table. Some ran bottles No. 1 and No. 2 over the table a second time and cleaned it up some more, getting out almost all of the sand in No. 1 and removing more than half the sand from No. 2. It was amazing to see a nice line of fine gold just dancin down the table into the bottle. And, to think you were was about to throw away all of that black sand that still had color in it! This machine is small enough for the prospector and small-scale miner who, like me, wants all of the gold for his or her hard work. The 911MPE-RP-4 Gravity Shaker Table is also big enough to clean up bucket after bucket of concentrates from a big operation! The RP4 people came up with the solution for getting all of the gold!

All RP4 shaker tables operate best when firmly secured to a dense solid mounting base. Wooden stands will set up harmonics and vibrations. Dense concrete or solid bedrock is preferred or a heavy braced steel table sitting on concrete. Mount shaker table to solid bed rock if possible when operating in the field. When that is not an option, six or seven sand bags may also be used if concrete or bedrock is not available for mounting.

Place a level on top of the steel bar that extends between the two bolts down mounting feet.Use flat washers installed under either end of the mounting feet for precise level adjustment in the long axis.

At no time should sand or slime be re-circulated back with mill water. Large, calm, surface areas are required to settle slimes. Buckets, barrels or any deep containers with turbulent water will not allow slimes to settle. Tailings should discharge into a tails pond or into a primary holding vessel before entering slime settling ponds. Surface area is more important than depth. A small 10 x 20 ft. settling pond can be installed in about 30 minutes. Shovel a 6 high retainer wall of earth and remove all gravel. Lay a soft bed of sand in the bottom. A small raised wall area (with the top approximately 2 blow water level) should be placed around the pump area. Roll out plastic liner and fill with water. Desert areas require a plastic cover to retard evaporation. Use a 24 wood across pond and lay plastic.

As with ponds, at no time should sand or slime be re-circulated back with mill water. A calm surface is needed in the final two barrels to settle slimes. (In lieu of the last two barrels, the discharge from barrel two may be directed to a settling pond as outlined above.)Turbulent water will not allow slimes to settle. Tailings are discharged into the first container.

A small compact tailings thickener introduces tailings feed at a controlled velocity in a horizontal feed design that eliminates the conventional free settling zone. The feed particles quickly contact previously formed agglomerates. This action promotes further agglomeration and compacting of the solids. Slowly rotating rakes aid in compacting the solids and moving them along to the discharge pipe, these solids are eventually discharged at the bottom of the unit. Under flow from the thickener 60-65% solids are processed through a vacuum filter and a90-95% solids is sent to the tailings area. Tailings thickeners are compact and will replace ponds. A 23 ft. diameter will process flow rates at 800 gpm or 50 tph.

Pine oils and vegetation oils regularly coat the surface of placer gold. Sometimes up to 50% of the smaller gold will float to the surface and into the tails. The pine oil flotation method for floating gold is still in use today. A good wetting agent will aid in the settling and recovery of oil coated gold.

Separation of concentrate from tails Minerals or substances that differ in specific gravity of2.5 or to an appreciable extent, can be separated on shaker tables with substantially complete recovery. A difference in the shape of particles will aid concentration in some instances and losses in others. Generally speaking, flat particles rise to the surface of the feed material while in the presence of rounded particles of the same specific gravity. Particles of the same specific gravity but varying in particle size, can be separated to a certain extent, varying in particle size, can be separated to a certain extent, removing the larger from the smaller, such as washing slime from granular products.

Mill practice has found it advantageous in having the concentrate particles smaller than the tailing product. Small heavy magnetite particles will crowd out larger particles of flat gold making a good concentrate almost impossible with standard gravity concentrating devices. The RP-4 table, using rare earth reverse polarity magnets, overcame this problem by lifting the magnetite out and above the concentrate material thus allowing the magnetite to be washed into the tails. This leaves the non-magnetics in place to separate normally.

No established mathematical relationship exists for the determination of the smallest size of concentrate particle and the largest size of tailing particle that can be treated together. Other factors, such as character of feed material, shape of particles, difference in specific gravity, slope or grade of table dock and volume of cross flow wash water will alter the final concentrate.

Size of feed material will determine the table settings. Pulverized rod mill pulps for gravity recovery tables should not exceed 65-minus to 100-minus 95% except where specific gravity, size, and shape will allow good recovery. Recovery of precious metals can be made when processing slime size particles down to 500-minus, if the accompanying gangue is not so coarse as to require excessive wash water or excessive grade to remove the gangue, (pronounced gang), to the tails. Wetting agents must be used for settling small micron sized gold particles. Once settled, 400-minus to 500 minus gold particles are readily moved and saved by the RP-4shaker table head motion. Oversized feed material will require excess grade to remove the large sized gangue,thus forcing large pieces of gold further down slope and into the middling. Too much grade and the fine gold will lift off the deck and wash into the tailings. Close screening of the concentrate into several sizes requires less grade to remove the gangue and will produce a cleaner product. A more economical method is to screen the head ore to window screen size (16-minus) or smaller and re-run the middling and cons to recover the larger gold. This concept can be used on the RP-4 shaker tables and will recover all the gold with no extra screens. A general rule for good recovery is less grade for the table deck and as much was water as possible without scouring off the fine gold. Re-processing on two tables will yield a clean concentrate without excess screening. Oversized gold that will not pass through window screen size mounted on RP-4 shaker tables, will be saved in the nugget trap. Bending a small 1/4 screen lip at the discharge end of the screen will trap and save the large gold on the screen for hand removal.

On the first run, at least one inch or more of the black concentrate line should be split out and saved into the #2 concentrate bin. This concentrate will be re-run and the clean gold saved into the #1 concentrate pocket. Argentite silver will be gray to dull black in color and many times this product would be lost in the middling if too close of a split is made.

The riffled portion of the RP-4 shaker table separates coarse non-sized feed material better than the un-riffled cleaning portion. Upon entering the non-riffled cleaning plane, small gangue material will crowd out and force the larger pieces of gold further down slope into the middling. Screen or to classify.

The largest feed particles should not exceed 1/16 in size. It is recommended that a 16-minus or smaller screen be used before concentrating on the RP-4 shaker table, eliminating the need for separate screening devices. Perfect screen sizing of feed material is un-economical, almost impossible, and is not recommended below 65-minus.

A classified feed is recommended for maximum recovery, (dredge concentrates, jig concentrates, etc.) The weight of mill opinion is overwhelmingly in favor of classified feed material for close work. Dredge concentrates are rough classified and limiting the upper size of table feed by means of a submerged deck screen or amechanical classifier is all that is necessary. A separate screen for the sand underflow is used for improved recovery when using tables.

Head feed capacity on the RP-4 tables will differ depending on the feed size, pulp mixture and other conditions. Generally speaking, more head feed material may be processed when feeding unclassified, larger screened sized material and correspondingly, less material may be processed when feeding smaller sized classified rod or ball mill pulps. Smaller classified feed material will yield a cleaner concentrate. Ultimately, the shape of the feed material particles and a quick trial test will determine the maximum upper size.

The width between the riffles of the RP-4 table is small and any particle over 1/8 may cause clogging of the bedding material. A few placer operators will pass 1/8 or larger feed material across the RP-4 table, without a screen, with the intent of making a rough concentrate for final clean up at a later date. This method will work, but excess horizontal slope/grade of the table deck must not be used as some losses of the precious metals will occur. Magnetite black sands feed material, passing a 16-minus screen (window screen size if 16-minus + or -) will separate without losses and make a good concentrate at approximately 500 to 600lbs feed per hour for the RP-4. Head feed material must flow onto the RP-4 screen, at a constant even feed rate. An excess of head feed material placed onthe table and screen at a given time will cause some gold to discharge into the tailings nugget trap. Head feed material should be fed at the end of the water bar into the pre-treatment feed sluice. Do not allow dry head feed material to form thick solids. The wash water will not wash and dilate the head feed material properly, thus allowing fine gold to wash into the tails.

Feed material should disperse quickly and wash down slope at a steady rate, covering all the riffles at the head end,washing and spilling over into the tails trough. A mechanical or wet slurry pump feeder (75% water slurry) is recommended for providing a good steady flow of feed material. This will relieve the mill operator of a tedious chore of a constantly changing concentrate line when hand feeding.

Eight gallons of water per minute is considered minimum for black sands separation/concentration on the RP-4 shaker table. 15 gallons of water per minute is consideredoptimum and will change according to feed material size, feed volume and table grade. A 1 inch hose will pass up to 15 gpm, for good recovery, wash water must completely cover the feed material 1/4 or more on the screen.

The PVC water distribution bar is pre-drilled with individual water volume outlets, supplying a precision water flow. Water volume adjustment can be accomplished by installing a 1 mechanical PVC ball valve for restricting the flow of water to the water distributing holes. Said valve may be attached between the garden hose attachment and water distributing bar.

More water at the head end and less water at the concentrate end is the general rule for precise water flow. More feed material will occupy the head end of the RP-4 shaker table deck in deep troughs and less material will occupy the concentrate end on the cleaning plane. A normal water flow will completely cover the feed material over the entire table and flow with no water turbulence.

A rubber wave cloth is installed to create a water interface and to smooth out all water turbulence. This cloth is installed with holes. Holes allow water to run underneath and over the top of the cloth and upon exiting will create a water interface smoothing out all the water turbulence. Bottom of water cloth must contact the deck.

Avoid excessive slope and shallow turbulent water.For new installations, all horizontal grade/slope adjustments should be calculated measuring from the concentrate end of the steel frame to the mounting base. For fine gold, the deck should be adjusted almost flat.

All head feed must be fed as a 75% water pulp. Clean classified sand size magnetite will feed without too much problem when fed dry. Ground rod or ball mill feed material 65-minus or smaller must be fed wet, (75% water slurry by weight or more) and evenly at a constant rate, spilling over into the tails drain troughat the head end of the table. Feed material without sufficient water will not dilute quickly andwill carry concentrate too far down slope or into the tails. A good wet pulp with a deflocculant and a wetting agent will aid the precious metals to sink and trap within the first riffles, thus moving onto the cleaning plane for film sizing. Round particles of gold will sink instantly and trap within the first riffles. The smaller flat gold particles will be carried further down slope to be trapped in the mid riffles. Potential losses of gold can occur if the table deck is overloaded by force feeding at a faster rate than the smaller flat gold can settle out. Under-feeding will result in the magnetites inability to wash out of the riffles, thus leaving a small amount of magnetiteconcentrated with the gold. A small addition of clean quartz sand added to a black sand concentrate will force the magnetite to the surface and will aid in its removal. Slimes require a separate table operation.

In flotation, surface active substances which have the active constituent in the positive ion. Used to flocculate and to collect minerals that are not flocculated by the reagents, such as oleic acid or soaps, in which the surface active ingredient is the negative ion. Reagents used are chiefly the quaternary ammonium compounds, for example, cetyl trimethyl ammonium bromide.

A substance composed of extremely small particles, ranging from 0.2 micron to 0.005 micron, which when mixed with a liquid will not gravity separate or settle, but remain permanently suspended in solution.

A crusher is a machine designed to reduce large rocks into smaller rocks, gravel, or rock dust. Crushers may be used to reduce the size, or change the form, of waste materials so they can be more easily disposed of or recycled, or to reduce the size of a solid mix of raw materials (as in rock ore), so that pieces of different composition can be differentiated. Crushing is the process of transferring a force amplified by mechanical advantage through a material made of molecules that bond together more strongly, and resist deformation more, than those in the material being crushed do. Crushing devices hold material between two parallel ortangent solid surfaces, and apply sufficient force to bring the surfaces together togenerate enough energy within the material being crushed so that its molecules separate from (fracturing), or change alignment in relation to (deformation), each other. The earliest crushers were hand-held stones, where the weight of the stone provided a boost to muscle power, used against a stone anvil. Querns and mortars are types of these crushing devices.

A basic alkali material, such as sodium carbonate or sodium silicate, used as an electrolyte to disperse and separate non-metallic or metallic particles. Added to Slip to increase fluidity. Used to aid in the beneficiation of ores, to convert into individual very fine particles, creating a state of colloidal suspension in which the individual particles of gold will separate from clay or other particles. This condition being maintained by the attraction of the particles for the dispersing medium, water, purchase at any chemical house.

Manner in which the intensity and direction of an electrical or magnetic field change as a function of time that results from the superposition of two alternating fields, (+/-) that differ in direction and in phase.

The smelting of metallic ores for the recovery of precious metals, requiring a furnace heat. Each milligram of recovered precious metal is gravimetric weighed and reported as one ounce pershort ton. Atomic Absorption (AA finish) is the preferred method for replacing the gravimetric weighing system.

A reagent added to a dispersion of solids in a liquid to bring together the fine particles to form flocs and which thereby promotes settling, especially in clays and soils. For example, lime alters the soil pH and acts as a flocculent in clay soils. Acid reagents and brine are also used as a flocculent.

The method of mineral separation in which a froth created in water with air and by a variety of reagents floats some finely crushed minerals, whereas other minerals sink. Separate concentrates are made possible by the use of suitable depressors and activators.

An igneous oxide of iron, with a specific gravity of 5.2 and having an iron content of 65-70% or more. Limonite crystals, sometimes mistaken for magnetite, occurs with the magnetite and sometimes may contain gold. Vinegar will remove gold locked in limonite coated magnetite.

In materials processing a grinder is a machine for producing fine particle size reduction through attrition and compressive forces at the grain size level. See also CRUSHER for mechanisms producing larger particles. Since the grinding process needs generally a lot of energy, an original experimental way to measure the energy used locally during milling with different machines was proposed recently.

A typical type of fine grinder is the ball mill. A slightly inclined or horizontal rotating cylinder is partially filled with balls, usually stone or metal, which grinds material to the necessary fineness by friction and impact with the tumbling balls. Ball mills normally operate with an approximate ball charge of 30%. Ball mills are characterized by their smaller (comparatively) diameter and longer length, and often have a length 1.5 to 2.5 times the diameter. The feed is at one end of the cylinder and the discharge is at the other. Ball mills are commonly used in the manufacture of Portland cement and finer grinding stages of mineral processing. Industrial ball mills can be as large as 8.5 m (28 ft) in diameter with a 22 MW motor, drawing approximately 0.0011% of the total worlds power. However, small versions of ball mills can be found in laboratories where they are used for grinding sample material for quality assurance.

A rotating drum causes friction and attrition between steel rods and ore particles. But note that the term rod mill is also used as a synonym for a slitting mill, which makes rods of iron or other metal. Rod mills are less common than ball mills for grinding minerals.

Screening is the separation of solid materials of different sizes by causing one component to remain on a surface provided with apertures through which the other component passes. Screen size is determined by the number of openings per running inch. Wire size will affect size of openings. -500=500 openings per inch is maximum for gravity operations due to having a solid disperse phase.

Long established in concentration of sands or finely crushed ores by gravity. Plane, rhombohedra deck is mounted horizontally and can be sloped about its axis by a tilting screw. Deck is molded of ABS plastic, and has longitudinal riffles dying a discharge end to a smooth cleaning area. An eccentric is used to create a gentle forward motion, compounded to full speed and a rapid return motion of table longitudinally. This instant reverse motion moves the sands along, while they are exposed to the sweeping and scouring action of a film of water flowingdown slope into a launder trough and concentrates are moved along to be discharged at the opposite end of the deck.

A material of extremely fine particle size encountered in ore treatment, containing valuable ore in particles so fine, as to be carried in suspension by water. De-slime in hydrocyclones before concentrating for maximum recovery of precious metals.

A mixture of finely divided, micron/colloidal particles in a liquid. The particles are so small that they do not settle, but are kept in suspension by the motion of molecules of the liquid. Not amenable to gravity separation. (Bureau of Mines)

Flotation process practiced on a shaking table. Pulverized ore is de-slimed, conditioned with flotation reagents and fed to table as a slurry. Air is introduced into the water system and floatable particles become glom rules, held together by minute air bubbles and positive charged edge adhesion. Generated froth can be discharged into the tailings launder trough or concentrates.

The parts, or a part of any incoherent or fluid material separated as refuse, or separately treated as inferior in quality or value. The gangue or valueless refuse material resulting from the washing, concentration or treatment of pulverized head ore. Tailings from metalliferous mines will appear as sandy soil and will contain no large rock, not to be confused with dumps.

A substance that lowers the surface tension of water and thus enables it to mix more readily with head ore. Foreign substances, such as natural occurring pine oils, vegetation oils and mill grease prevent surface wetting and cause gold to float. Addition agents, such as detergents, (dawn), wetting out is a preliminary step in deflocculating for retarding gold losses.

RP4 shaker table for sale mini gold shaker table RP4 shaker table instructions RP4 shaker table dimensions RP4 gold shaker table RP 4 gravity shaker table utech RP4 shaker table RP 4 gravity shaker table price used RP4 shaker table for sale

Global mining solutions warrants that all mining equipment manufactured will be as specified and will be free from defects in material and workmanship for a period of one year for the RP-4. Providing that the buyer heeds the cautions listed herein and does not alter, modify or disassemble the product, gms liability under this warranty shall be limited to the repair or replacement upon return to gms if found to be defective at any time during the warranty. In no event shall the warranty extend later than the date specified in the warranty from the date of shipment of product by GMS. Repair or replacement, less freight, shall be made by gms at the factory in Prineville, Oregon, USA.

All bearings are sealed and no grease maintenance is required. Do not use paint thinners, or ketones to clean your deck. A small amount of grease should be applied to the adjustable handle which is used for the changing the slope of the deck.

Do not allow the RP-4 to stand in direct sunlight without water. Always keep covered and out of the sun when not in use. Heat may cause the deck to warp. Do not lift or pull on the abs plastic top, always lift using the steel frame. Do not attach anything to the abs plastic top. Do not attach PVC pipe to concentrate discharge tubes, constant vibration from the excess weight will cause stress failure of the plastic.

ball mill - an overview | sciencedirect topics

ball mill - an overview | sciencedirect topics

The ball mill accepts the SAG or AG mill product. Ball mills give a controlled final grind and produce flotation feed of a uniform size. Ball mills tumble iron or steel balls with the ore. The balls are initially 510 cm diameter but gradually wear away as grinding of the ore proceeds. The feed to ball mills (dry basis) is typically 75 vol.-% ore and 25% steel.

The ball mill is operated in closed circuit with a particle-size measurement device and size-control cyclones. The cyclones send correct-size material on to flotation and direct oversize material back to the ball mill for further grinding.

Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.

By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:

where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.

The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.

The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.

where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.

A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.

The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.

Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.

When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.

In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).

Planetary ball mills. A planetary ball mill consists of at least one grinding jar, which is arranged eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to that of the grinding jars according to a fixed ratio. The grinding balls in the grinding jars are subjected to superimposed rotational movements. The jars are moved around their own axis and, in the opposite direction, around the axis of the sun wheel at uniform speed and uniform rotation ratios. The result is that the superimposition of the centrifugal forces changes constantly (Coriolis motion). The grinding balls describe a semicircular movement, separate from the inside wall, and collide with the opposite surface at high impact energy. The difference in speeds produces an interaction between frictional and impact forces, which releases high dynamic energies. The interplay between these forces produces the high and very effective degree of size reduction of the planetary ball mill. Planetary ball mills are smaller than common ball mills, and are mainly used in laboratories for grinding sample material down to very small sizes.

Vibration mill. Twin- and three-tube vibrating mills are driven by an unbalanced drive. The entire filling of the grinding cylinders, which comprises the grinding media and the feed material, constantly receives impulses from the circular vibrations in the body of the mill. The grinding action itself is produced by the rotation of the grinding media in the opposite direction to the driving rotation and by continuous head-on collisions of the grinding media. The residence time of the material contained in the grinding cylinders is determined by the quantity of the flowing material. The residence time can also be influenced by using damming devices. The sample passes through the grinding cylinders in a helical curve and slides down from the inflow to the outflow. The high degree of fineness achieved is the result of this long grinding procedure. Continuous feeding is carried out by vibrating feeders, rotary valves, or conveyor screws. The product is subsequently conveyed either pneumatically or mechanically. They are basically used to homogenize food and feed.

CryoGrinder. As small samples (100 mg or <20 ml) are difficult to recover from a standard mortar and pestle, the CryoGrinder serves as an alternative. The CryoGrinder is a miniature mortar shaped as a small well and a tightly fitting pestle. The CryoGrinder is prechilled, then samples are added to the well and ground by a handheld cordless screwdriver. The homogenization and collection of the sample is highly efficient. In environmental analysis, this system is used when very small samples are available, such as small organisms or organs (brains, hepatopancreas, etc.).

The vibratory ball mill is another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vials capacities in the vibratory mills are smaller (about 10 ml in volume) compared to the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6) at very high speed, as high as 1200 rpm.

Another type of the vibratory ball mill, which is used at the van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig. 1.7).

The mill is evacuated during milling to a pressure of 106 Torr, in order to avoid reactions with a gas atmosphere.[44] Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.

A ball mill is a relatively simple apparatus in which the motion of the reactor, or of a part of it, induces a series of collisions of balls with each other and with the reactor walls (Suryanarayana, 2001). At each collision, a fraction of the powder inside the reactor is trapped between the colliding surfaces of the milling tools and submitted to a mechanical load at relatively high strain rates (Suryanarayana, 2001). This load generates a local nonhydrostatic mechanical stress at every point of contact between any pair of powder particles. The specific features of the deformation processes induced by these stresses depend on the intensity of the mechanical stresses themselves, on the details of the powder particle arrangement, that is on the topology of the contact network, and on the physical and chemical properties of powders (Martin et al., 2003; Delogu, 2008a). At the end of any given collision event, the powder that has been trapped is remixed with the powder that has not undergone this process. Correspondingly, at any instant in the mechanical processing, the whole powder charge includes fractions of powder that have undergone a different number of collisions.

The individual reactive processes at the perturbed interface between metallic elements are expected to occur on timescales that are, at most, comparable with the collision duration (Hammerberg et al., 1998; Urakaev and Boldyrev, 2000; Lund and Schuh, 2003; Delogu and Cocco, 2005a,b). Therefore, unless the ball mill is characterized by unusually high rates of powder mixing and frequency of collisions, reactive events initiated by local deformation processes at a given collision are not affected by a successive collision. Indeed, the time interval between successive collisions is significantly longer than the time period required by local structural perturbations for full relaxation (Hammerberg et al., 1998; Urakaev and Boldyrev, 2000; Lund and Schuh, 2003; Delogu and Cocco, 2005a,b).

These few considerations suffice to point out the two fundamental features of powder processing by ball milling, which in turn govern the MA processes in ball mills. First, mechanical processing by ball milling is a discrete processing method. Second, it has statistical character. All of this has important consequences for the study of the kinetics of MA processes. The fact that local deformation events are connected to individual collisions suggests that absolute time is not an appropriate reference quantity to describe mechanically induced phase transformations. Such a description should rather be made as a function of the number of collisions (Delogu et al., 2004). A satisfactory description of the MA kinetics must also account for the intrinsic statistical character of powder processing by ball milling. The amount of powder trapped in any given collision, at the end of collision is indeed substantially remixed with the other powder in the reactor. It follows that the same amount, or a fraction of it, could at least in principle be trapped again in the successive collision.

This is undoubtedly a difficult aspect to take into account in a mathematical description of MA kinetics. There are at least two extreme cases to consider. On the one hand, it could be assumed that the powder trapped in a given collision cannot be trapped in the successive one. On the other, it could be assumed that powder mixing is ideal and that the amount of powder trapped at a given collision has the same probability of being processed in the successive collision. Both these cases allow the development of a mathematical model able to describe the relationship between apparent kinetics and individual collision events. However, the latter assumption seems to be more reliable than the former one, at least for commercial mills characterized by relatively complex displacement in the reactor (Manai et al., 2001, 2004).

A further obvious condition for the successful development of a mathematical description of MA processes is the one related to the uniformity of collision regimes. More specifically, it is highly desirable that the powders trapped at impact always experience the same conditions. This requires the control of the ball dynamics inside the reactor, which can be approximately obtained by using a single milling ball and an amount of powder large enough to assure inelastic impact conditions (Manai et al., 2001, 2004; Delogu et al., 2004). In fact, the use of a single milling ball avoids impacts between balls, which have a remarkable disordering effect on the ball dynamics, whereas inelastic impact conditions permit the establishment of regular and periodic ball dynamics (Manai et al., 2001, 2004; Delogu et al., 2004).

All of the above assumptions and observations represent the basis and guidelines for the development of the mathematical model briefly outlined in the following. It has been successfully applied to the case of a Spex Mixer/ Mill mod. 8000, but the same approach can, in principle, be used for other ball mills.

The Planetary ball mills are the most popular mills used in MM, MA, and MD scientific researches for synthesizing almost all of the materials presented in Figure 1.1. In this type of mill, the milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial (milling bowl or vial) and the effective centrifugal force reaches up to 20 times gravitational acceleration.

The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed, as schematically presented in Figure 2.17.

However, there are some companies in the world who manufacture and sell number of planetary-type ball mills; Fritsch GmbH (www.fritsch-milling.com) and Retsch (http://www.retsch.com) are considered to be the oldest and principal companies in this area.

Fritsch produces different types of planetary ball mills with different capacities and rotation speeds. Perhaps, Fritsch Pulverisette P5 (Figure 2.18(a)) and Fritsch Pulverisette P6 (Figure 2.18(b)) are the most popular models of Fritsch planetary ball mills. A variety of vials and balls made of different materials with different capacities, starting from 80ml up to 500ml, are available for the Fritsch Pulverisette planetary ball mills; these include tempered steel, stainless steel, tungsten carbide, agate, sintered corundum, silicon nitride, and zirconium oxide. Figure 2.19 presents 80ml-tempered steel vial (a) and 500ml-agate vials (b) together with their milling media that are made of the same materials.

Figure 2.18. Photographs of Fritsch planetary-type high-energy ball mill of (a) Pulverisette P5 and (b) Pulverisette P6. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).

Figure 2.19. Photographs of the vials used for Fritsch planetary ball mills with capacity of (a) 80ml and (b) 500ml. The vials and the balls shown in (a) and (b) are made of tempered steel agate materials, respectively (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).

More recently and in year 2011, Fritsch GmbH (http://www.fritsch-milling.com) introduced a new high-speed and versatile planetary ball mill called Planetary Micro Mill PULVERISETTE 7 (Figure 2.20). The company claims this new ball mill will be helpful to enable extreme high-energy ball milling at rotational speed reaching to 1,100rpm. This allows the new mill to achieve sensational centrifugal accelerations up to 95 times Earth gravity. They also mentioned that the energy application resulted from this new machine is about 150% greater than the classic planetary mills. Accordingly, it is expected that this new milling machine will enable the researchers to get their milled powders in short ball-milling time with fine powder particle sizes that can reach to be less than 1m in diameter. The vials available for this new type of mill have sizes of 20, 45, and 80ml. Both the vials and balls can be made of the same materials, which are used in the manufacture of large vials used for the classic Fritsch planetary ball mills, as shown in the previous text.

Retsch has also produced a number of capable high-energy planetary ball mills with different capacities (http://www.retsch.com/products/milling/planetary-ball-mills/); namely Planetary Ball Mill PM 100 (Figure 2.21(a)), Planetary Ball Mill PM 100 CM, Planetary Ball Mill PM 200, and Planetary Ball Mill PM 400 (Figure 2.21(b)). Like Fritsch, Retsch offers high-quality ball-milling vials with different capacities (12, 25, 50, 50, 125, 250, and 500ml) and balls of different diameters (540mm), as exemplified in Figure 2.22. These milling tools can be made of hardened steel as well as other different materials such as carbides, nitrides, and oxides.

Figure 2.21. Photographs of Retsch planetary-type high-energy ball mill of (a) PM 100 and (b) PM 400. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).

Figure 2.22. Photographs of the vials used for Retsch planetary ball mills with capacity of (a) 80ml, (b) 250ml, and (c) 500ml. The vials and the balls shown are made of tempered steel (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).

Both Fritsch and Retsch companies have offered special types of vials that allow monitoring and measure the gas pressure and temperature inside the vial during the high-energy planetary ball-milling process. Moreover, these vials allow milling the powders under inert (e.g., argon or helium) or reactive gas (e.g., hydrogen or nitrogen) with a maximum gas pressure of 500kPa (5bar). It is worth mentioning here that such a development made on the vials design allows the users and researchers to monitor the progress tackled during the MA and MD processes by following up the phase transformations and heat realizing upon RBM, where the interaction of the gas used with the freshly created surfaces of the powders during milling (adsorption, absorption, desorption, and decomposition) can be monitored. Furthermore, the data of the temperature and pressure driven upon using this system is very helpful when the ball mills are used for the formation of stable (e.g., intermetallic compounds) and metastable (e.g., amorphous and nanocrystalline materials) phases. In addition, measuring the vial temperature during blank (without samples) high-energy ball mill can be used as an indication to realize the effects of friction, impact, and conversion processes.

More recently, Evico-magnetics (www.evico-magnetics.de) has manufactured an extraordinary high-pressure milling vial with gas-temperature-monitoring (GTM) system. Likewise both system produced by Fritsch and Retsch, the developed system produced by Evico-magnetics, allowing RBM but at very high gas pressure that can reach to 15,000kPa (150bar). In addition, it allows in situ monitoring of temperature and of pressure by incorporating GTM. The vials, which can be used with any planetary mills, are made of hardened steel with capacity up to 220ml. The manufacturer offers also two-channel system for simultaneous use of two milling vials.

Using different ball mills as examples, it has been shown that, on the basis of the theory of glancing collision of rigid bodies, the theoretical calculation of tPT conditions and the kinetics of mechanochemical processes are possible for the reactors that are intended to perform different physicochemical processes during mechanical treatment of solids. According to the calculations, the physicochemical effect of mechanochemical reactors is due to short-time impulses of pressure (P = ~ 10101011 dyn cm2) with shift, and temperature T(x, t). The highest temperature impulse T ~ 103 K are caused by the dry friction phenomenon.

Typical spatial and time parameters of the impactfriction interaction of the particles with a size R ~ 104 cm are as follows: localization region, x ~ 106 cm; time, t ~ 108 s. On the basis of the obtained theoretical results, the effect of short-time contact fusion of particles treated in various comminuting devices can play a key role in the mechanism of activation and chemical reactions for wide range of mechanochemical processes. This role involves several aspects, that is, the very fact of contact fusion transforms the solid phase process onto another qualitative level, judging from the mass transfer coefficients. The spatial and time characteristics of the fused zone are such that quenching of non-equilibrium defects and intermediate products of chemical reactions occurs; solidification of the fused zone near the contact point results in the formation of a nanocrystal or nanoamor- phous state. The calculation models considered above and the kinetic equations obtained using them allow quantitative ab initio estimates of rate constants to be performed for any specific processes of mechanical activation and chemical transformation of the substances in ball mills.

There are two classes of ball mills: planetary and mixer (also called swing) mill. The terms high-speed vibration milling (HSVM), high-speed ball milling (HSBM), and planetary ball mill (PBM) are often used. The commercial apparatus are PBMs Fritsch P-5 and Fritsch Pulverisettes 6 and 7 classic line, the Retsch shaker (or mixer) mills ZM1, MM200, MM400, AS200, the Spex 8000, 6750 freezer/mill SPEX CertiPrep, and the SWH-0.4 vibrational ball mill. In some instances temperature controlled apparatus were used (58MI1); freezer/mills were used in some rare cases (13MOP1824).

The balls are made of stainless steel, agate (SiO2), zirconium oxide (ZrO2), or silicon nitride (Si3N). The use of stainless steel will contaminate the samples with steel particles and this is a problem both for solid-state NMR and for drug purity.

However, there are many types of ball mills (see Chapter 2 for more details), such as drum ball mills, jet ball mills, bead-mills, roller ball mills, vibration ball mills, and planetary ball mills, they can be grouped or classified into two types according to their rotation speed, as follows: (i) high-energy ball mills and (ii) low-energy ball mills. Table 3.1 presents characteristics and comparison between three types of ball mills (attritors, vibratory mills, planetary ball mills and roller mills) that are intensively used on MA, MD, and MM techniques.

In fact, choosing the right ball mill depends on the objectives of the process and the sort of materials (hard, brittle, ductile, etc.) that will be subjecting to the ball-milling process. For example, the characteristics and properties of those ball mills used for reduction in the particle size of the starting materials via top-down approach, or so-called mechanical milling (MM process), or for mechanically induced solid-state mixing for fabrications of composite and nanocomposite powders may differ widely from those mills used for achieving mechanically induced solid-state reaction (MISSR) between the starting reactant materials of elemental powders (MA process), or for tackling dramatic phase transformation changes on the structure of the starting materials (MD). Most of the ball mills in the market can be employed for different purposes and for preparing of wide range of new materials.

Martinez-Sanchez et al. [4] have pointed out that employing of high-energy ball mills not only contaminates the milled amorphous powders with significant volume fractions of impurities that come from milling media that move at high velocity, but it also affects the stability and crystallization properties of the formed amorphous phase. They have proved that the properties of the formed amorphous phase (Mo53Ni47) powder depends on the type of the ball-mill equipment (SPEX 8000D Mixer/Mill and Zoz Simoloter mill) used in their important investigations. This was indicated by the high contamination content of oxygen on the amorphous powders prepared by SPEX 8000D Mixer/Mill, when compared with the corresponding amorphous powders prepared by Zoz Simoloter mill. Accordingly, they have attributed the poor stabilities, indexed by the crystallization temperature of the amorphous phase formed by SPEX 8000D Mixer/Mill to the presence of foreign matter (impurities).

high-speed milling guidelines for hardened tool steels |
 

 moldmaking technology

high-speed milling guidelines for hardened tool steels | moldmaking technology

This photo contrasts a general-purpose end mill geometry with that of an end mill specifically designed for high-speed milling hardened tool steels.

In a survey of die and mold manufacturers in 1995, the question was asked, "What are the most important technologies in die and mold manufacturing in the next two years? Reliable toolpath generation software (CAM) won first place with 61 percent. High-speed milling of hardened die steels was second choice at 49 percent. Since then, it has gained more acceptance in the industry.1The following observations from the Engineering Research Center for Net Shape Manufacturing (ERC/ NSM) at The Ohio State University may be useful to shops contemplating getting into high-speed milling.

Fully engaged situations occur when sharp changes of direction are programmed in the cutter path. It is best to add small arcs where corners are to be machined. A typical 90-degree turn without an arc causes the machine tool to slow down too much - decreasing chip load and causing a momentary rubbing action that heats up the tool. At the same time, the tool engagement angle increases from a steady 10 to 15 degrees to approximately 90 degrees - increasing the load on the tool. ERC/NSM experiments show that even a programmed radius as small as 0.005" makes a big difference in maintaining a steady milling regime. There also are dynamic effects when a cutter engages the corner of a pocket - such as the number of inserts in the cut being drastically changed. This in turn affects the tooth passing frequency, and consequently, the stability of the cutting system.

Helical interpolation helps maintain a steady load on the tool, which improves tool life, as well as surface finish.2This technique also can be used for roughing cavities. The key is steady, uniform load on the tool. High-speed milling is less tolerant of sudden changes in tool loading.Figure 1shows an inserted face mill in helical interpolation Due to the helical nature of the path, tool entry will be smooth. Tool loading will smoothly increase to a maximum and stay there during the cut.

Do not rely on old conventional milling parameters with which you may be familiar. High speeds for milling of tool steels range from about 1,500 to 3,000 sfm. Always consider the actual "sharpness" of your tool. Typical tool edge radius (sharpness) varies between 0.0002" and 0.0004"; therefore, realistic chip loads should be a minimum of 0.0010". Considering tool diameters, heat generated, etc., maximum chip load should probably be less than 0.010".Figure 2shows two cutting edges, one theoretically sharp, the other only "real world" sharp. In order to get under the material, our chip load should be larger than the expected sharpness of the cutting edge.

Inserts with chip breakers tend to be more fragile. In our tool life experiments, inserts with flat rake faces (strongest) performed best. It is important to choose those tools and inserts that were specifically designed for high-speed milling of hardened tool steels.

Tools with minimal flutes are stronger as well as more rigid. Traditional end mills usually have deeper flutes and are thus less rigid. During high-speed milling we will be taking lighter cuts and creating fewer chips per rotation of the tool. Deep flutes won't be necessary or useful; instead, rigidity and dynamic balance are more important.Photo 1contrasts a general-purpose end mill geometry with that of an end mill specifically designed for high-speed milling hardened tool steels.

Toolholders that use set screws are not suitable because they are inherently unbalanced. Even collet toolholders are thought to be unsuitable past 10,000 rpm. Toolholders with hydraulic chucks are the accepted norm between 10,000 and 20,000 rpm. Higher speeds demand shrink fit and balanced tooling for best results.

It is important to relieve the tool when machining thin-walled parts. One should machine the most flexible region of the workpiece first and then work toward the less flexible or fixtured areas. Once the most flexible region is cut, it is important that it does not come in contact with the tool again. This is best accommodated by relieving the tool(see Figure 3).

Milling, by its very nature is an "interrupted" process. The cutting edges of a milling tool go in and out of cut all of the time - heating up within the cut and cooling down outside of the cut. High-speed milling generates higher temperatures than conventional milling. Typically, casing and other workpiece suppliers coat the surface of the workpiece with a rust preventive oil. During cutting, the oil carries away a lot of the heat, drastically reducing the temperature of the cutting edge. This drastic fluctuation in temperature can cause premature cracking and fracture of the cutting edge, commonly referred to as thermal shock. It is imperative to eliminate such residual oils as much as possible. The same theory applies to cutting fluid. Typical flood coolant usually causes thermal shock to the cutting edges reducing tool life. High-pressure air supplied through the spindle is usually a good choice for clearing the chips away from the cutting zone.

Chatter or other vibration-related problems often become the limiting factor in determining how much of the available speed and power can be incorporated into the milling process. Therefore, they are very important when dealing with high-velocity machining. As seen inFigure 4, every system exhibits a tendency that can be represented by stability lobes. A system is stable when the generated chatter frequency is equal to the tooth passing frequency or some harmonic of it. The stability lobes tell the user as to what rpms and depths-of-cut to run the tool so that the material removal rates are maximized.Some aspects of high-speed milling technology have just been reviewed, but complete coverage is beyond the scope of any single article. It is important to remember that in contrast to conventional milling practice, high-speed milling requires a complete engineering of the process (a systems approach) for success.References1"Survey of the U.S. Die and Mold Manufacturing Industry," P.FallBoehmer, Report No. ERC/NSM-D-95-41, July 1995.2"Tool Materials and Cutting Strategies for High-Speed Machining," J.Fernandez, Report No. HPM/ERC/NSM-98-R-017, February 1998.

Both copper and graphite provide approximately the same end result, so it is important for a shop to consider the advantages and disadvantages of each material in order to discover what would work best in their shopfloor environment.

end mills. the essential beginners guide

end mills. the essential beginners guide

End mills, slot drills, routers, milling cutters, drill bits, V-bits and burrs - what does it all mean?And which bit do I need for what job? For instance, which are the best end mills? and which is the best end mill for aluminium, and which are the best end mills for stainless steel.This article gives you the low down on milling cutters and CNC tooling.

The craft market has recently exploded with exciting compact, table top CNC Routers and Mini-milling machines. CNC Routers are now affordable enough to allow DIY enthusiasts access to this high- precision milling tool for carving and engraving.

4. The exception to this rule are diamond drill bits which have a flat end rather than pointed or fluted. (Unless it is a diamond twist drill which is not used for drilling but for expanding already existing holes such as in beads)

There are multiple types of End mills, each designed with a variety of different factors to enable you to choose the right end mill to match the material youre working on, and the type of project youre going to use it for.

Often more expensive, these coatings are added to the bit to reduce wear and friction. However, not all coatings are suitable for all materials and whilst a particular coating may be good for productivity on one material, it may be not be on another.

ball nose milling strategy guide - in the loupe

ball nose milling strategy guide - in the loupe

Ball nose end mills are ideal for machining 3-dimensional contour shapes typically found in the mold and die industry, the manufacturing of turbine blades, and fulfilling general part radius requirements. To properly employ a ball nose end mill (with no tilt angle) and gain the optimal tool life and part finish, follow the 2-step process below (see Figure 1).

A ball nose end mills Effective Cutting Diameter (Deff) differs from its actual cutting diameter when utilizing an Axial Depth of Cut (ADOC) that is less than the full radius of the ball. Calculating the effective cutting diameter can be done using the chart below that represents some common tool diameters and ADOC combinations or by using the traditional calculation (see Figure 2).

Given the new effective cutting diameter a Compensated Speed will need to be calculated. If you are using less than the cutter diameter, then its likely your RPMs will need to be adjusted upward (see Figure 3).

If possible, it is highly recommended to use ball nose end mills on an incline () to avoid a 0 SFM condition at the center of the tool, thus increasing tool life and part finish (Figure 4). For ball nose optimization (and in addition to tilting the tool), it is highly recommended to feed the tool in the direction of the incline and utilize a climb milling technique.

Given the new effective cutting diameter a compensated speed will need to be calculated. If you are using less than the cutter diameter, then its likely your RPMs will need to be adjusted upward (see Figure 6).

Thank you for this milling strategy guide. I especially appreciate your insight on milling with a tilt angle. I was unaware that this could extend the life of the bit. I will keep this in mind while milling in the future.

ball milling

ball milling

These notes are based on observations made while on a recent trip through the West, for the purpose of studying the practical operation of the ball mill. The writer takes this opportunity to express his thanks for courtesies extended at the many plants visited as well as for the valuable data received.

While there are several types of ball mill on the market, particular attention will here be given to the diaphragm type, as the open-trunnion type, especially the conical mill, has been thoroughly discussed in the Transactions.

There is a prevailing impression that the ball mill is a recent development; however, ball mills were used extensively in Montana and other western states for crushing ores for concentration as far back as 1898. Its present prominence is due in part to its recent successful application by one of the large copper companies. Without any reference to dry grinding, the first successful ball mill for wet crushing, which is still in operation, was built 10 years ago. This mill, designed by Erminio Ferraris for crushing Sardinian ores for concentration, is of more than passing interest. It embodies the peripheral discharge with grates, large forged-steel balls, and the principal features of the modern ball mill. The results approach present-day practice, the chief differences being that the mechanical construction has been unproved in the modern types.

The action of the balls and the principles of crushing have been studied by several investigators. Their conclusions are confirmed by results obtained by the writer in experimenting with a small machine built at the Allis-Chalmers factory, and serve to explain the reasons for some of the results obtained in practice. A ball mill may be revolved so fast that the balls will cling to the shell during the entire revolution, while at slow speeds they will be carried up only a short distance and roll back. On the other hand, at the critical speed, they will cascade as shown in Fig. 1. At the critical speed the balls ascending on the layer next to the shell start from rest at a point S and cling to the shell without revolving or rolling, which has often been ascribed to them. These balls are held at rest by centrifugal force until they reach a point G, the location of which is dependent on the speed of rotation. Beyond the point G, gravity overcomes centrifugal force and the balls fall with increasing velocity in a parabolic curve which is the resultant of the above two forces, striking at a point W, the force of the impact being expended in crushing the material.

The several layers of balls lying on top of those next to the shell follow a similar cycle except that, due to relative difference in the two forces, their paths become more nearly vertical. The outer layers, spreading more than the inner layers, increase the area in the zone of the falling balls. Within the circuit thus formed is a neutral axis or a sluggishly rotating kidney-shaped mass in which little actual work is performed.

The material being crushed is thoroughly distributed throughout the mass by filling the interstices between the balls, and follows in the same circuit. It is, therefore, evident that the material is crushed mainly by impact of the striking balls as the whole mass falls. There can be very little grinding by attrition due to the rotation of balls, except at the point S where the shell picks up the mass and accelerates it to the rotative speed of the shell. The argument has often been advanced that fine material cannot be produced by impact alone and that fine grinding is done entirely by attrition or rubbing of adjoining balls. It is only necessary to break up a few small pieces of rock on an anvil with a hammer to prove that fines are unavoidably produced by impact. Screen analyses of the discharges from tube-mills in open and in closed circuits lead to the conclusion that in many instances an ore fragment may pass through the mill six to eight times before it is crushed to the desired fineness. Quoting directly from the article by Hermann Fischer referred to above:

The grinding action, therefore, depends upon the height of the drop of the balls, i.e:, the height of the curve vertex above the point where the ball strikes, the speed of the shell, the weight and number of balls.

The speed of the drum must be so determined that the curves can develop themselves properly. The weight of the balls and the height of drop are interrelated and their product must be sufficient to break the ore according to its size and hardness. Hard materials require heavier balls or greater height of drop than soft ones and steel balls in small diameter cylinders will do the same work as flint pebbles in large diameter cylinders.

The free fall of the balls is dependent upon the volume of ball load. With a charge equal to or greater than half the volume of the mill the free fall of the balls is decreased, the charge is held together, and the size of the inactive kidney-shaped mass is increased. When the charge is about one-third of the volume of the mill the size of the kidney-shaped mass is reduced and the balls fall from their maximum free height. Operating results bear out the above facts in that the greatest number of tons crushed to a certain mesh per kilowatt-hour are obtained with ball charges equal to approximately one-third the volume of the mill.

There is a general impression that the grate acts as a screen or sizer. This is true to a limited extent, but it is not of primary importance. The fineness of product delivered by a ball mill, the size of feed, ball charge, and speed remaining constant, depends upon the tonnage fed, the density of the pulp (water to solids ratio), size of balls, and, when operating in closed circuit, on the efficiency of the external classifying apparatus. The screen analyses plotted in Fig. 2 show the effect of varying tonnages, other factors remaining constant. They are from actual results with a 6 by 4-ft. mill.

The screen analyses plotted in Fig. 3. show the difference in product when the initial charge included only 5-in. and 2-in. balls, and when the same charge contained a large percentage of 4, 3, and 2-in. balls. In some respects, these results do not agree with what would be expected, but I will not attempt to propound a theory to explain the deviations at this writing.

give a fine product and a large amount a coarse product. As the discharge is entirely at the periphery, and does not depend upon any classifying action to overflow the, finished product, the greater the amount of water added the quicker the pulp will pass through the mill and the coarser the product.

In mills provided with means for raising the discharge or pulp level from the periphery to some intermediate height between the periphery and the trunnion, the fineness and the amount of oversize can be controlled within certain limits. No.figures are available showing these differences, but from practical results in the field it appears that a wide variation can be obtained by this means.

The grate should, of course, retain some oversize, but this action can be carried to extremes, especially when a fine product is desired, as the consequent diminished capacity is not compensated by the reduction of oversize. In all cases when a fine product is desired, it is advisable to run the mill in closed circuit with an efficient external classifier. The principal function of the grate is to retain the ball charge in the mill, while permitting a peripheral discharge. The efficiency of the classifier, when a ball mill is run in closed circuit, directly affects both tonnage and fineness. This will be discussed under capacity.

Capacity of ball mills depends upon the following factors: fineness of grinding, weight or volume of ball charge, hardness of material, size of grate openings, and size of balls, other factors remaining constant. Practically speaking, the most important limiting factors for capacity have

As previously shown, tonnage and fineness are interrelated and the capacity of a ball mill should be figured on the following basis when sufficiently reliable figures have been collected. The kw.-hours required to crush a ton of ore from and to a certain mesh should be arrived at from average operating conditions. A ball mill has a certain definite maximum power rating depending upon its ball load. Multiplying the kw.-hours per ton by the tons required to be crushed per hour, the product will represent the power required, and the mill nearest to that power rating should be selected. Fig. 4 is a preliminary power curve based on the recommended maximum ball charge, together with all available data at hand at the present time; however, 60 or more carefully taken power records would be needed for even an approximately correct curve.

Operating a mill at less than its maximum capacity for a given ball charge will result in excessive wear on lining and balls and produce a finer product than necessary. To crush a ton of ore of a certain hardness and size to a given fineness represents a definite amount of work; hence the capacity of a mill depends upon (a) the hardness, and (b) the ratio of reduction, the latter affecting capacity far more than the former.

It is useless to expect a large capacity from a mill operated with balls of a size too small to crush the ore, or when the balls are of a composition that will not withstand the shock of impact and shatter themselves to fragments. Hard ores, when fed direct from a crusher, require a proper percentage of 5-in. steel balls to do effective work. A 4-in. steel ball is often sufficient for some of the softer porphyry ores. Smaller steel balls may be used for regrinding work, but the charge should contain a per-

Where a fine product is desired together with a minimum amount of oversize, the grate opening should not be diminished. Smaller grate openings will reduce the amount of oversize but the decreased tonnage is not compensated. In such cases it is advisable to depend on an external classifier and operate the mill in closed circuit; the grate bars should be set with at least 1/8-in. opening. Where a coarse product is desired, for example for concentrating table work, the grate may be used as a sizer and an open-circuit scheme adopted.

When the mill is operated in closed circuit the efficiency of the classifier directly affects the capacity and it is important that the classifier be of proper size and properly operated. In one case observed, a classifier of the mechanical drag type was set with the wrong slope; correcting the slope approximately doubled the capacity of the mill. Classifiers of the mechanical drag type, in order to make an efficient separation, must be operated with proper consistency of pulp in the classifying zone, the slope and length of the sand plane must be correct, and the speed of the drag must be suited to the material.

Power depends principally upon the weight of ball charge, an approximate figure being 9 to 10 hp. per ton. However, the power per ton of balls will vary according to the percentage of volume the ball charge occupies in the mill. An approximate curve from data at hand is given in Fig. 6, from which it will be seen that the power required per ton of balls is least when the mill is loaded half full and that the curve rises very rapidly as the ball load is reduced. A charge greater than half full causes a balancing effect until, when the mill is full, the power required is practically only that necessary to take care of friction after starting.

When the volume of ball charge is reduced, within certain limits, the power consumption per unit of ball charge is increased, because the center of gravity of the charge is further from the axis of the mill; but as the mass of balls is more active and circulates more freely, the crushing efficiency is increased proportionately to the increase in power consumption per-ton of ball load.

There are a number of ball mill installations for fine crushing in the West. Most of these are arranged in two or more stages where a product finer than 100-mesh is desired, and there seems to be little difference of opinion as to the advantage of such an arrangement. Where coarser products are desired say, through 48-mesh, both single-reduction and stage-crushing installations are found. Stage crushing seems to have higher efficiency, but when first cost and simplicity are considered, the single-reduction installation seems to be more desirable, especially for small plants.

The curves (Fig. 7) plotted from recent tests show the power required per ton of material crushed under-varying capacities. It can be seen that the power rises rapidly at the expense of capacity when a fine product

is desired, and when compared with an average power curve it would make a saving to run a large tonnage through several stages. The phrase single reduction as applied to ordinary ball mill practice is misleading, because in the most common application of the ball mill, running in closed circuit for preparing feed for flotation, a great deal of the material is returned from once to six or seven times before it is finally reduced. The most efficient installations in practice are undoubtedly those which have a large return circuit and the mill is crowded, making a small reduction at each pass through the mill, but handling a large tonnage at the same time.

The ball mill is not to be recommended for all and sundry problems in the milling field. It is not suitable for concentration work where the ore contains a large amount of coarse mineral easily pulverized. Where crushing to 12-mesh and finer is necessary to release the mineral, the ball mill makes a suitable product when properly operated, and is as good as any other regrinding machine. The installation of concentrating

The use of ball mills for reducing crusher product to 85 per cent,: below 200-mesh in two stages, as practised at the United Eastern, Tom Reed, and Montana mines, in Arizona, is a distinct advance in fine crushing. The simplicity, small floor space and large capacity of these installations are especially notable.

While there is not such economy in power nor so small a number of repairs as compared with a stamp- battery and tube-mill plant of the same capacity, the operating troubles and attendance are much reduced.

The most desirable method of feeding coarse material is the arrangement as installed at the Tom Reed mill. The crusher product is fed direct from a bin to an apron feeder, the speed of which is controlled by a Reeves variable-speed transmission device, having a small hand crank, sprocket, and chain conveniently situated for the mill operator. This insures absolute control and allows quick changes.

When a ball mill having a proper crushing load is rotated at the critical speed, the balls strike at a point on the periphery about 45 below horizontal, or S in Fig. 1. An experienced operator is able to judge by the sound whether a mill is crushing at maximum efficiency, or is being over- or under-fed. Excessive rattling denotes under-feeding; a sound of impact at W (Fig. 1) indicates overloading; while under proper conditions, the impact will be heard near S.

When a ball mill fitted with a diaphragm is over-fed, the mill fills up to a certain level, then stops, crushing and discharges any additional feed back through the feed trunnion. Once over-fed, it takes from 30 min. to 2 hr. to free itself. Ball mills, therefore, should be provided with a central opening in the diaphragm connecting with the discharge trunnion, to prevent over-feeding and the delays incidental thereto.

The greatest difficulty in feeding most ball mills, when running on large tonnages and coarse feed, say, to 3 in., is due to the restricted area of the feed trunnion, which limits the quantity of coarse material that can be fed through it. A few simple calculations will show the velocity necessary to pass a given quantity feed through the trunnion. It can also be shown mathematically that the average spiral in the trunnion liner does not advance the feed rapidly enough; therefore, instead of aiding, it retards the feeding. These results are confirmed in practice. A smooth liner, tapering from the feeder into the mill, does not retard the flow of the feed, and is, therefore, more efficient than the spiral. Experiments with small models, as well as experiments in the field, corroborate these conclusions. A short trunnion with large diameter is essential for feeding a large tonnage to a ball mill.

The engineering department of the Allis-Chalmers Manufacturing Co. has recently conducted some experiments with feeders modeled after the various types in use, on a scale of 1 in. per foot. The feeders were operated at constant speed conformable with present practice, the material delivered in a given time being weighed. The following con-

clusions were drawn: The intake of a single-scoop feeder has far greater capacity than the throat or trunnion of the mill, and there is no good reason for using a double- or triple-scoop feeder, the capacity of the feeder not being controlled by the quantity it will pick up, but by the quantity that it can discharge through the throat or trunnion. These experiments further demonstrated that the capacity of a spiral feeder is in direct proportion to the length of the path of the spiral. In other words, a spiral feeder embodies all the principles of the Frenier sand pump, in which the long path of the spiral increases the pressure which forces the feed into the trunnion opening.

Fig. 10 shows a double-scoop feeder without a partition; Fig. 11 shows the same feeder with the two spirals connected across the center of the trunnion opening, making a partition so that the material taken up cannot drop from one scoop into the other. Fig. 12 shows a single-spiral feeder; Fig. 13 shows a triple-spiral feeder; and Fig. 14 shows a standard combination feeder which has a single spiral.

Disregarding the influence of the trunnion liner as determining the relative capacity of feeders, the experiments demonstrated that No. 12, the single-spiral feeder, has the greatest capacity; No. 11, double-spiral feeder with the partition across the trunnion opening, gave the next best capacity, which, however, was less than 50 per cent., that of No. 12. The capacity of No. 10 was only about 25 per cent, that of No. 12. The capacity of the triple-scoop feeder, Fig. 13, was but very little greater than that of No. 11. The results clearly demonstrate that increasing the number of spirals of scoops does not add to the capacity of a feeder.

The ratio of moisture to solids is important in ball mill work. From actual operation it has been observed that fine grinding is best done when water constitutes 33 to 40 per cent, of the pulp, or the water-to- solids ratio is 1 : 2 or 1 : 1. Where a minimum of fine material is desired, 50 per cent, and upward of water is desirable.

Ball consumption varies with the fineness of the product, hardness of the ore, quality of ball, and whether a mill is run in closed or open circuit. The ball, consumption for mills delivering a coarse product, all passing 8-mesh- and containing 10 to 20 per cent. below 200-mesh, the mill being run in open circuit, is about lb. per ton for steel balls and 1 lb. for cast composition balls.

The average ball consumption for mills in closed circuit has been plotted in Fig. 15 for steel balls and for cast composition balls. Enough data are not available to plot curves for hard and soft ores, and individual figures will vary considerably from the average of the curves, which are given merely a guide as to what may be expected and also to show the increased consumption with finer grinding. It should be noted that the curves apply to products practically all of which are finer than the meshes indicated, up to 65-mesh. Points on the curves representing finer products are for mills generally regrinding 10- to 20-mesh feed; hence corresponding amounts must be added to give the total ball consumption for reducing from crusher size to 100-mesh and finer.

Average consumption of shell liners, for both chrome and manganese steel, is 1/3-lb. per ton of ore crushed. The consumption of lining seems to be fairly constant regardless of the hardness of the ore, fineness of product, or other conditions. The greatest wear on the lining is probably caused by the impact of the balls and by their slippage on the shell during the period of acceleration. If the mill is running below capacity the wear will increase.

is the general increase in weight and thickness. The proportion of scrap has been very high, and the consumption stated above may be reasonably expected to be diminished with heavier and thicker liners. Regarding the shape of liner, there is considerable difference of opinion. The smooth liner is probably as efficient as any of the others if run at slightly higher speed.

Hard-iron liners have not been found satisfactory when used with balls of 5 and 4-in. diameter, as, they have invariably failed by cracking and breaking, but with balls of 2-in. diameter and smaller they are sufficiently durable. It is possible that a heavy hard-iron liner backed and set in cement mortar might be successful, but this has not yet been tried as far as we know.

The loosening of liners may be avoided by using deeply countersunk bolts of large diameter with double nuts. When the liners are first put in place, after running the mill for several hours the bolts should be gone over again and the nuts tightened with a short wrench and hammer. Later, after the feed is on, they should be gone over once more. Leakage around bolt holes is caused entirely by loosening of the bolts due to lack of tightening or a worn-out lining. If candlewicking is used as packing around a bolt; between the shell and the washer, and the nut is kept tight, no leakage will occur until the liners are worn out.

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