The basic parameters used in ball mill design (power calculations), rod mill or anytumbling millsizing are; material to be ground, characteristics, Bond Work Index, bulk density, specific density, desired mill tonnage capacity DTPH, operating % solids or pulp density, feed size as F80 and maximum chunk size, productsize as P80 and maximum and finally the type of circuit open/closed you are designing for.
In extracting fromNordberg Process Machinery Reference ManualI will also provide 2 Ball Mill Sizing (Design) example done by-hand from tables and charts. Today, much of this mill designing is done by computers, power models and others. These are a good back-to-basics exercises for those wanting to understand what is behind or inside the machines.
W = power consumption expressed in kWh/short to (HPhr/short ton = 1.34 kWh/short ton) Wi = work index, which is a factor relative to the kwh/short ton required to reduce a given material from theoretically infinite size to 80% passing 100 microns P = size in microns of the screen opening which 80% of the product will pass F = size in microns of the screen opening which 80% of the feed will pass
Open circuit grinding to a given surface area requires no more power than closed circuit grinding to the same surface area provided there is no objection to the natural top-size. If top-size must be limited in open circuit, power requirements rise drastically as allowable top-size is reduced and particle size distribution tends toward the finer sizes.
A wet grinding ball mill in closed circuit is to be fed 100 TPH of a material with a work index of 15 and a size distribution of 80% passing inch (6350 microns). The required product size distribution is to be 80% passing 100 mesh (149 microns). In order to determine the power requirement, the steps are as follows:
The ball mill motorpower requirement calculated above as 1400 HP is the power that must be applied at the mill drive in order to grind the tonnage of feed from one size distribution. The following shows how the size or select thematching mill required to draw this power is calculated from known tables the old fashion way.
The value of the angle a varies with the type of discharge, percent of critical speed, and grinding condition. In order to use the preceding equation, it is necessary to have considerable data on existing installations. Therefore, this approach has been simplified as follows:
A = factor for diameter inside shell lining B = factor which includes effect of % loading and mill type C = factor for speed of mill L = length in feet of grinding chamber measured between head liners at shell- to-head junction
Many grinding mill manufacturers specify diameter inside the liners whereas othersare specified per inside shell diameter. (Subtract 6 to obtain diameter inside liners.) Likewise, a similar confusion surrounds the length of a mill. Therefore, when comparing the size of a mill between competitive manufacturers, one should be aware that mill manufacturers do not observe a size convention.
In Example No.1 it was determined that a 1400 HP wet grinding ball mill was required to grind 100 TPH of material with a Bond Work Index of 15 (guess what mineral type it is) from 80% passing inch to 80% passing 100 mesh in closed circuit. What is the size of an overflow discharge ball mill for this application?
Ball mills are used in chemistry and in industry to grind hard solids to a very fine powder. They are very similar to rock tumblers. Basically, the idea is to rotate a container filled with heavy metal balls that crush the substance that you want to grind. Ball mills can be used to grind ceramic material, crystalline compounds, and even some metals.
Note: If you are clever, you can substitute most of these materials with other household items or things you may find in dumpsters. Also, the sizes used are just what I used in my mill and are here to give you a general idea.
Your final goal is to get a container rotating at about 50-100 rpm. If it goes too slowly, the balls will just roll in place- too fast and the centripetal force will be too strong and the balls will just roll along the sides of the container. What you want is the balls to reach the top of the container and then come crashing down.
First figure out what the speed of your motor runs. Most motors should have it labeled, but if it isn't you'll have to time it yourself. This can be done by hooking up the motor to two pulleys with known diameters and then by counting how many times the larger pulley rotates in one minute.
My motor runs at 1725 rpm and I attached a 1-3/4" (diameter) pulley to it. This pulley turns a belt which in turn rotates a 7.5" pulley. This corresponds to a reduction ratio of 1:7.5/1.75 or 1:4.25. This means that my larger pulley runs 4.25 times slower than the motor or at about 400 rpm. The axle that the larger pulley rotates has a diameter of 3/4" (including the rubber hose) and it rotates the 3.5" container. This additional reduction of 1:3.5/0.75 or 1:4.67 means that the container should theoretically rotate at 400/4.67 rpm or about 85 rpm. I measured a speed of 80 rpm- the slightly lower speed due to frictional losses.
Ball milling is a grinding technique that uses media to effectively break down pigment agglomerates and aggregates to their primary particles. Using a rotor or disc impeller to create collisions of the grinding media, the impact and force created by the bead mills collisions break down the pigment agglomerates. The media can consist of either stainless steel, glass, or ceramic materials. The higher the bead hardness or density, the greater the collision force. The ball-milling process uses a higher concentration of grinding media to mill base in which the chambers are designed to maximize the energy transfer.
When a particle size has to be reduced below 10 microns, bead milling is the technique to use. However, if the material has a very low viscosity, ball milling is a better dispersing process than using a high shear mixing (vertical) system.
Currently, the VMA-Getzmann company offers three product lines for bead milling. They can be dedicated stand-alone systems or accessories that can be added to the high-speed vertical disperser models. Depending upon the model, sample quantities can be as low as 20 ml or up to 20,000 ml.
Our Dispermat SL model line is the current horizontal bead mill system. Milling chamber sizes can start at 50 ml to save on raw material costs. The beads are separated from the mill base by a dynamic gap system. The standard gap uses 1.0 mm diameter grinding media; an optional gap is available to use beads down to 0.3 mm diameter. The Dispermat SL can be selected to run as a single pass or as a recirculation configuration.
One of the unique features is an independent pumping system to feed the mill base into the milling chamber. Instead of the speed of the milling rotor controlling the sample volume the operator can control the volume, through the mixing system pump that fits on top of the milling chamber. Separating the rotor speed from the sample feed system provides more control over the milling process.
Basket bead milling is a relatively new design for ball milling applications. The grinding media is contained in a cylinder (basket), and the mill base is circulated through the basket. The VMA-Getzmann basket mill consists of a stainless-steel cylinder with an opening at the top and a sieve filter on the bottom. The standard diameter size of the grinding media is 1.0 mm. however, it can be ordered to use 0.3 mm bead size.
Since the Getzmann basket mill is attached to aHigh-Speed Dispersermodel, those with an adapter allow the user to switch between the basket mill system and a motor shaft for high-shear dispersing easily.
Attached to the bottom of the basket is a cowles blade that rotates at high speed. The purpose of the cowles blade is to circulate the mill base to ensure all materials enter the basket mill. When you have created the desired particle size, the basket mill is then raised out of the sample container, while the grinding media stays in the basket.
The third system for ball milling applications is the APS (air pressure system). The APS is attached to a high shear disperser. It consists of a sample containerwith a sieve filter at the bottom, a stand to elevate the sample container, along with a sealing system around the motor shaft, and a container lid. The mill base and grinding media is mixed 50/50 in the container. Adisk impeller or pearl mill impelleris immersed into the mixture and rotated from 500 to 5000 RPMs depending on the desired particle size. After the dispersion is completed the stop cock that covers the sieve filter is removed, the lid is clamped tight over the vessel. The lid has an air connection; the air is applied to force the sample through the sieve filter separating the mill base from the grinding media. Aside from the ability to produce small quantities of less than 25 milliliters, another advantage of the APS system is their ease of cleaning.
Investigation of a dry fine grinding circuit has shown significant influence of the mill load (powder filling) on the production capacity. To improve the circuit performance at industrial scale, alternative ways of mill load measurement were investigated. Detection of strain changes in the mill shell during mill rotation, by using a piezoelectric strain transducer, provided very interesting results, allowing evaluation of the weight of the mill charge and control of the powder filling to obtain an optimal level. Power draw has thus been increased by about 5% compared to the old configuration where mill motor power input was used to control the mill charge. By measuring mechanical vibration with the transducer, additional useful information has been obtained about the behavior of the cataracting and cascading balls inside the mill shell. Finally an important factor was simplicity and low investment cost of the total installation, as many fine grinding mills operate in relatively small circuits that do not warrant large investment for alternative measurement methods.Get in Touch with Mechanic