Mill speed controls the angle of break

Mill speed controls the angle of break

Experimental work conducted in our laboratory and supplemented by our pictures in slow motion definitely indicates that the action inside the Mill drum is not a haphazard stirring and throwing of the charge.
There is a specific operating speed for most efficient 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.
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 break:
1.Speed of Mill
2.Amount of grinding media
3.Amount of material
4.In wet grinding, the consistency or viscosity
As this section deals entirely with speeds, we will confine our discussion to this item and cover the other factors in their respective categories.
While, 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.
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.
A Pebble Mill the same size as a Ball Mill is expected to run at a slightly faster speed. This is due to the smaller inside diameter of the Pebble Mill with its lining, which is lacking in the Ball Mill.
In the production of bronze and aluminum powders, the Mills are run almost a critical speed so that the balls are drooped to give the same effect as a hammer blow. Without this action the product grinds finely but no flaking of any consequence can be obtained and aluminum or bronze powders are only effective as coatings when they are used in flake form.With its high quality products such as , , , , , Henan Hongxing mining machinery Co.Ltd has ascended in the front rank of the world in the exporting of mining equipments.