Equipment suitable for crushing large lumps as they come from the quarry or mine cannot be used to pulverize to fine powder, so the operation is carried on in three or more stages called primary crushing, secondary crushing, and pulverizing. The three stages are characterized by the size of the feed material, the size of the output product, and the resulting reduction ratio of the material. The crushing-stage output may be screened for greater uniformity of product size.
The reduction of materials such as stone, coal, or slag to a suitable size for their intended uses such as road building, concrete aggregate, or furnace firing.
Reduction in size is accomplished by five principal methods:
(1) crushing, a slow application of a large force;
(2) impact, a rapid hard blow as by a hammer;
(3) attrition, a rubbing or abrasion;
(4) sudden release of internal pressure;
(5) ultrasonic forces. The last two methods are not in common use.
In open-circuit pulverizing, the material passes through the pulverizer once with no removal of fines or recirculation. In closed-circuit pulverizing, the material discharged from the pulverizer is passed through an external classifier where the finished product is removed and the oversize is returned to the pulverizer for further grinding.