There are four principal typesof primary crushers. The Blake jaw crusher uses a double toggle tomove the swinging jaw and is built in a variety of sizes fromlaboratory units to large sizes having a feed inlet 84 by 120 in.(213 by 305 cm). The Dodge jaw crusher uses a single toggle oreccentric and is generally built in smaller sizes. The Gates gyratorycrusher has a cone or mantle that does not rotate but is movedeccentrically by the lower bearing sleeve. The Symons cone crusheralso has a gyratory motion, but has a much flatter mantle or conethan does the gyratory crusher. The top bowl is spring-mounted. It isused as a primary or secondary crusher.

Secondary crushers include thesingle-roll crusher and the double-roll crusher which have teeth onthe roll surface and are used mainly for coal. Smooth rolls withoutteeth are sometimes used for crushing ores and rocks. The hammercrusher is the type of secondary crusher most generally used for ore,rock, and coal. The reversible hammer mill can run alternately ineither direction, thus wearing both sides of the hammers.

Crushing and pulverizing areprocesses in ore dressing needed to reduce valuable ores to the finesize at which the valueless gangue can be separated from the ore.These processes are also used to reduce cement rock to the finepowder required for burning, to reduce cement clinker to the veryfine size of portland cement, to reduce coal to the size suitable forburning in pulverized form, and to prepare bulk materials forhandling in many processes. See also Materials-handling equipment.

Equipment suitable forcrushing large lumps as they come from the quarry or mine cannot beused to pulverize to fine powder, so the operation is carried on inthree or more stages called primary crushing, secondary crushing, andpulverizing. The three stages are characterized by the size of thefeed material, the size of the output product, and the resultingreduction ratio of the material. The crushing-stage output may bescreened for greater uniformity of product size.

The reduction of materialssuch as stone, coal, or slag to a suitable size for their intendeduses 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 rapidhard blow as by a hammer; (3) attrition, a rubbing or abrasion; (4)sudden release of internal pressure; and (5) ultrasonic forces. Thelast two methods are not in common use.

In open-circuit pulverizing,the material passes through the pulverizer once with no removal offines or recirculation. In closed-circuit pulverizing, the materialdischarged from the pulverizer is passed through an externalclassifier where the finished product is removed and the oversize isreturned to the pulverizer for further grinding.