High Pressure Grinding Rolls
The HPGR is the most energy-efficient comminution device currently available to the minerals processing plant designer, and the focus must be on the development of both machine and flowsheet to maximise the proportion of total comminution performed by the HPGR.
The initial objective must be to minimise the top size of ball mill feed by reducing HPGR product screen aperture and recirculating progressively more material to the HPGR. This will entail changing to wet screening as the separation size falls below about 6 mm, and this in turn will impact circuit design philosophies, as there will be no opportunity to stockpile mill feed.
As the mill feed top size falls, there might be some merit in operating tertiary HPGRs in open circuit and introducing quaternary HPGR crushing to handle screen oversize. The associated moisture would, however, be detrimental to this process, and some form of blending with dry material might be necessary.
Ultimately, with very fine mill feeds, the number and size of conventional wet screens will become unmanageable, while the ball mills will trend ever smaller. The next step is to abandon screens and ball mills entirely and operate HPGRs in closed circuit with air separators, with the final product repulped and fed directly to flotation or other downstream processes.
The technology for this type of circuit already exists and is in operation. An example is the use in Europe of KHD HPGRs and air separators for the production of dry-ground limestone for use in a flue-gas desulphurisation process. Typical air-separator performance in this type of application is 90 m P90, while the finest separation is claimed to be around 20 m P90. Product size is adjustable, and the P80 grind sizes of 75 to 150 m common in minerals processing should be readily achievable.
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