Implementation of High Pressure Grinding Rolls and Grinding Circuits
In greenfield installations, HPGRs will have their place as the tertiary crushing stage in front of ball mills. Even this commitment still leads to many different flowsheet configurations.
The secondary crusher and the HPGR, which replaces conventional tertiary crushers, are operated in closed circuit with dry screens. The product of the crushing circuit is stockpiled. In this configuration, the crushing circuit and the ball mill circuit are decoupled, allowing both circuits to be operated at a different utilisation. However, this decoupling has two implications. First, an additional stockpile is required. Stockpiling of the HPGR product, which contains a lot of fines, remains a challenge and requires extensive dust suppression. Second, screening of the HPGR product has to be done dry because a wetscreen undersize cannot be stockpiled. This entails a coarser product.
HPGR in closed circuit with dry screens
Wet screening of the HPGR discharge may provide significant improvements. It is advantageous from the point of energy efficiency to shift as much grinding work as possible to the HPGR and feed the ball mills with a finer product. This approach requires a finer mesh size for the screen, 4 to 6 mm. Fine screening usually has a lower efficiency, especially if the discharge from the HPGR is in the form of highly compacted flakes. Wet screening will address the disagglomeration of the HPGR discharge and will definitely improve screening efficiency. It also will facilitate wetting of the material. A flowsheet illustrating a wet-screen arrangement for an HPGR is shown in Figure 4. The HPGR and ball mill circuits are combined, whereas the secondary crusher is decoupled. Alternatively, if the circuit consists of multiple crushing and grinding units, all three can be combined, eliminating the stockpile by oversizing the equipment. This arrangement allows for the lower availability of the crushers, whereas the HPGR availability is expected to be high enough for in-line operation with the ball mills.
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