Primary Crushing

Almost all raw materials require grinding after primary crushing. For coarse grinding, a dry pan or occasionally a wet pan is used. The dry pan is similar to a grist mill but has a perforated bottom through which the crushed material is continuously removed. The wet pan is similar, but has a solid bottom. For very fine grinding, a ring-roll, ball, or impact mill is employed. 

Crushing and Grinding. Some raw materials, such as hard clay and quartzite, must first be crushed to grains small enough for the grinding equipment. Almost all raw materials require grinding after primary crushing. 

Beneficiation. Iron ore coming from the mine must be properly sized. A gyratory crusher is normally used for primary crushing down to approximately 300 mm (see Size reduction). Secondary crushing down to 25 mm can be done in a cone crusher. Fine grinding can be done by rod mills followed by either ball or pebble mills. In some cases, autogenous grinding can be used to replace the cone cmsher and rod mills. 

Smooth jaws produce more numerous flaky pieces than corrugated jaws. Curved jaws produce less fines but more numerous flaky particles. Crusher speed has little effect. The presence of material too small to be crushed has a deleterious effect on the shape of products. Secondary crushers with a small reduction ratio can improve the shape of primary crushed material, but secondary crushers are not inherently different from primary crushers. Slotted screens can remove flaky particles from the product. Impact crushers produce fewer flaky particles than any other type. 

Performance Jaw crushers are applied to the primary crushing of hard materials and are usually followed by other types of crushers. In smaller sizes they are used as single-stage machines. Typical capabilities and specifications are shown in Table 21-9a. 

If the fragmentation of the rock is good on the first blast, then secondary blasting of oversized pieces should not be necessary. Rock size produced during the initial blasting is important since all the ore has to go through some sort of primary crushing system. 

Once the magnesite rock has been blasted, it is generally loaded onto trucks and transported to the primary crushing operation. Primary crushers are heavy-duty machines and handle dry run of mine feed material as large as 1 m. There are two principle types of primary crushers in operation— gyratory crushers and jaw crushers, with the gyratory type being the most common. 

RW. McQuiston, R.S. Shoemaker, Primary crushing plant design . Society of Mining... 

Primary crushing may reduce as mined ore to less than 200 mm and then, in the conventional approach, it is further reduced in secondary and tertiary crushers followed by rod and ball mill grinding to the required particle size. Final sizing classification is usually achieved by operating the ball mill in closed circuit with hydrocyclones. 

The coarsely fragmented material produced by rock blasting has to undergo suitable primary crushing in a mobile or portable plant and has to be fed carefully onto the belt conveyor by means of a special device so as to prevent damage to the... 

Fig.20 Static primary crushing plant with preliminary screening on stepped (multi-stage) grizzlies (Babbitless)...

The pre-drying of raw materials prior to primary size reduction is employed only in exceptional circumstances. Elaborate arrangements to prevent "false" air inleakage into the heated crushers are required in such cases. As a rule, no utilizable waste heat is available at primary crushing plants which have to deal with raw materials with a high natural moisture content, especially as such plants are often... 

The actual crusher may be any of the usual types of primary crushing machines. The machines manufactured in the Federal Republic of Germany for the international cement industry comprise the following types ... 

Crushing the rocks is often achieved in two or three stages. The secondary and tertiary crushing machines resemble the machines used during primary crushing. They consist of vertical cone crushers or horizontal cylinder crushers. The former type is the most widespread. 

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