A ball mill and a semi-autogenous grinding (SAG) mill are similar in that both are industrial machines for grinding up a substance unto uniform pieces. Both use a rotating drum with steel balls to break up substances as varied as metallic ores, concrete, oats, wheat flour and rock candy. SAG mills have a few extra features such as lifting plates and large rocks as well as the steel balls. SAG mills are also smaller than ball mill and usually used in the first step of the grinding process. Ball mills may do very fine grinding, using small steel balls.
Remove the lifting plates. SAG mills have lifting plates to move the steel balls and rocks higher up the drum before they fall, so they will fall with more impact. Removing these plates is important because it makes the grinding action more uniform. It also reduces the energy need to rotate the drum because the mill is no longer lifting anything up the side of the drum--product, steel balls and rocks are only lifted up as far as centrifugal force takes them before falling back down
Remove the large rocks. Bouncing ball do a more uniform job of grinding because all the steel balls are the same size and do not fall from a great height. Ball mills control the fineness of the grinding process by changing out the gate valve size. Another granularity controlling device is the screen on the exit size of the mill. If the converted SAG mill is to be the only ball mill, the ball size and screen size might have to be changed several times --- unless the grinding is simple enough to do in one step.
Install a mill lining system. There are several commercially available systems to line the inside of a metal mill with hard synthetic plates. This makes the grinding smoother and has the very desirable side effect of reducing the noise that the ball mill makes while running. Most of the grinding is accomplished between product and balls --- not by collisions with the side of the drum --- so the lining does not slow down the grinding process.
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