19/06/2015· The critical speed of a rotating mill is the RPM at which a grinding medium will begin to centrifuge, namely will start rotating with the mill and therefore cease to carry out useful work. Ball mills have been successfully run at speeds between 60 and 90 percent of critical speed, but most mills operate at speeds between 65 and 79 percent of critical speed.
When the ball mill rotates at the critical speed the theoretical trajectory of the charge is such that it would fall outside the shell as centrifuging occur at this stage and the medium is carried around in a fixed position against the shell. The formula to calculate critical speed is given below. N c = 42.305 /sqt N c = critical speed of the mill.
In order for the ball mill to perform normal grinding operations, its working speed must be less than the critical speed. Ball mills generally work in a dropped state. There are many working speeds to achieve this working state, but there must be the most favorable working speed. 1.
13/02/2017· Rod mills generally operate within the range of 6075 % of critical speed in order to avoid excessive wear and tangled rods. Ball and pebble mills are usually operated at 7085 % of critical speed. For dry grinding the speed is usually somewhat lower. Critical speed graph. Example : A mill with 4.5 m dia N obtained = 20 rpm.
June 10, 2020
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