Secondary crushing

The initial quarrying, primary, and secondary crushing and screening of raw materials are nol shown in the flowsheet. See also Gypsum. 

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. 

Primary crushers used are jaw, gyratory, impact, and toothed roll crushers. Impact mills are limited to limestone and softer stone. With rocks containing more than 5 percent quartz, maintenance of hammers may become prohibitive. Gyratory and cone crushers dominate the field for secondary crushing of hard and tough stone. Rod mills have been employed to manufacture stone sand when natural sands are not available. 

Secondary Crushing.— After the work of the primary breakers has been done, the rock is in pieces varying according to the size of the original breaker, from 2 or 2 K up to 6 in. in diameter. In industrial use, the rock, unless it be limestone flux or ore for furnace use, requires further reduction therefore a second stage in reduction is required— this stage we classify as crushing. It may be performed in machines of a number of different kinds. 

Secondary crushing, as has already been pointed out, may be performed in machines of various types, but for such work, the gyratory principle is being generally preferred. 

Granulating.—In many industries, it is necessary to produce material in granular form—a form in which the particles are considerably smaller than is ordinarily produced by secondary crushing, and not so small as when a pulverulent product is required. The requirements may be such that the product wanted is anything from 40 mesh to 10 mesh, the mesh meaning the number of apertures per lineal inch. Reduction of this character can be performed in any one of many different ways, but according to usual practice, is accomphshed by swing-hammer machines, rolls or ball tube mills. 

As the material is conveyed to the secondary crusher, steel is typically removed by an electromagnetic separator- Secondary crushing further breaks down the RCM, which is then screened to the desired gradation... 

The production shaft is serviced by a groimd-moimted friction winder with a nominal production capacity of 3.5Mt/y and a hoisting depth of 1,100m. A surface conveyor system transports ore from the tipping bin at the shaft to a surface ROM stockpile adjacent to a new secondary crushing cuid screening plant. 

The location would also suit the proposed secondary crushing station and road haulage route to Mount Isa. 

Jaw crushers are sensitive to moist and plastic feed material and tend to choke if there is a substantial proportion of fine particles in the feed. The attainable reduction ratio is between about 6 1 and 8 1. For obtaining a product of favourable size for feeding to grinding mills it is generally necessary to apply secondary crushing in another type of crusher. 

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