Ball mill efficiency and Speed of ball mill

Ball mill efficiency

Milling operations are known to be notoriously energy inefficient. Ball mill grinding is based on a work index value developed to directly reflect industrial milling practices. Mill selection is based on this value and related empirical equations. Improved grinding efficiency requires making a change from an energy per ton basis to an efficient particle breakage basis.

Recently, investigators concluded that grinding efficiency is limited to less than 20%, and grinding efficiency can be increased beyond this limit by making three changes in operating practice as follows:

Use correct make-up media size.
Operate the mill in a cataracting mode.
Control milling circuit in a modified mode.

Speed of ball mill

There is a specific operating speed for most efficient ball mill grinding. At a certain point, controlled by the Mill speed, the load nearest the wall of the cylinder breaks free and it is so quickly followed by other sections in the top curves as to form a cascading, sliding stream containing several layers of balls separated by material of varying thickness. The top layers in the stream travel at a faster speed than the lower layers thus causing a grinding action between them. There is also some action caused by the gyration of individual balls or pebbles and secondary movements having the nature of rubbing or rolling contacts occur inside the main contact line.

Ball mill break point

It is important to fix the point where the charge, as it is carried upward, breaks away from the periphery of the Mill. We call this the “break point”, or “angle of break” because we measure it in degrees. It is measured up the periphery of the Mill from the horizontal.

There are four factors affecting the angle of ball milling break:

Speed of Mill
Amount of grinding media
Amount of material
In wet grinding, the consistency or viscosity

In the old days, operating speeds were determined by trial and error, we have been able to establish practical operating speeds through correlation with the critical speed, which is the speed at which the grinding media, without material, begin to centrifuge. Therefore, to determine the critical speed for any given size Mill, we use the following formula: 54.19 divided by the square root of the radius in feet.

The smaller the Mill the faster in RPM it must run to attain critical speed. Our 4.5” diameter Specimen Jar has a critical speed of 125 RPM, and our size #00 90” diameter Ball Mill 28 RPM.

Ball mill grinding and dispersing problems

For most grinding and dispersing problems, we strive to attain the cascading, sliding action described earlier, and to accomplish this we have found that the most desirable angle of break ranges from 50 to 60 degrees from the horizontal.

The lower range is recommended for most wet grinding operations like paints and soft dry materials, and the higher break point (which provides a more severe grinding action) for most dry materials and wet grinding such hard products as enamel frit and glaze.

It is also known that the grinding action in a larger Mill is more severe than in the smaller sizes and, consequently, we are of the opinion that the angle of break should be lower for the larger Mills than for the smaller.

The rule of speed applies regardless of the type of grinding media.

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