Crushing and pulverizing
Crushing and pulverizing
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; and (5) ultrasonic forces. The last two methods are not in common use.
Crushing and pulverizing are processes in ore dressing needed to reduce valuable ores to the fine size at which the valueless gangue can be separated from the ore. These processes are also used to reduce cement rock to the fine powder required for burning, to reduce cement clinker to the very fine size of portland cement, to reduce coal to the size suitable for burning in pulverized form, and to prepare bulk materials for handling in many processes. See Materials-handling equipment
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.
There are four principal types of primary crushers. The Blake jaw crusher uses a double toggle to move the swinging jaw and is built in a variety of sizes from laboratory units to large sizes having a feed inlet 84 by 120 in. (213 by 305 cm). The Dodge jaw crusher uses a single toggle or eccentric and is generally built in smaller sizes. The Gates gyratory crusher has a cone or mantle that does not rotate but is moved eccentrically by the lower bearing sleeve. The Symons cone crusher also has a gyratory motion, but has a much flatter mantle or cone than does the gyratory crusher. The top bowl is spring-mounted. It is used as a primary or secondary crusher.
Secondary crushers include the single-roll crusher and the double-roll crusher which have teeth on the roll surface and are used mainly for coal. Smooth rolls without teeth are sometimes used for crushing ores and rocks. The hammer crusher is the type of secondary crusher most generally used for ore, rock, and coal. The reversible hammer mill can run alternately in either direction, thus wearing both sides of the hammers.
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.
Ball and tube mills, rod mills, hammer mills, and attrition mills are pulverizers operating by impact and attrition. In ball race and roller pulverizers, crushing and attrition are used. See Pebble mill, Tumbling mill