Lesson 1/Learning Event 3
FIGURE 10. MATERIAL FLOW THROUGH THE SECONDARY UNIT
OF A 35-tph CRUSHING AND SCREENING PLANT
The secondary unit is not capable of being used as a primary unit in gravel pit operations unless some field
expedient is employed to prescreen the material to control the top size being fed into the machine, and to control
the rate of flow.
Crushed rock product from the jaw crusher is discharged into the feed hopper of the secondary unit. Material that
was crushed to product size in the jaw chamber will pass the top screen. The material that is oversize to the top
screen is retained and directed through the dual roll crusher. It is further reduced in size and discharged onto the
under crusher conveyor. It is then carried to and deposited into the revolving elevator wheel which picks it up and
drops it back on to the over crusher conveyor which routes it back to the top screen. If any of this material is still
too large, it will be sent through the roller crusher again. This is known as a "closed circuit" crushing system.
The product setting of the roller crusher must be equal to or somewhat less than the top screen opening size. The
material retained on the bottom screen represents the product size range. It is directed into a chute to a side
delivery conveyor which carries it to the product stockpile. The product will contain the various particle sizes
ranging between the top screen opening size and the bottom screen opening size. The material that is small
enough to pass the bottom screen will be carried to the by-product stockpile.
Production Considerations. The jaw crusher can be expected to produce anywhere between 20 and 90 tons per
hour. The actual rate depends primarily upon the toughness of the raw material and the size of the finished
product. The minimum product setting is 1-1/2 inches and the maximum product setting is 4-1/2 inches.
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