Selection and Sizing of Autogenous and Semi-Autogenous Mills. Pilot Plant Testwork Basis, страница 3

•         Those for which a pilot plant program was not possible or practical.

The basic parameters which are used to calculate the applied power to the primary mills are:

•         The size analysis of the mill feed, whether by primary crushing alone or primary crushing supplemented by some form of pre-crushing

•         The specific power consumption, kWh/t, or range thereof for different ore composites, lithologies, mineralizations, alteration types, mineable ore zones, or operating conditions: ball charge volume/total mill charge volume combinations, top ball size, mill speeds, pebble crushing, and pulp densities

•         The size analysis of the transfer product from primary grinding as new feed to secondary grinding.

In some methodologies without pilot plant testwork it is quite common to calculate the total specific power consumption for primary and secondary grinding first, then select a transfer product size (80% passing T₈₀, microns) and lastly, calculate the split in specific power consumption between primary and secondary grinding. At least one of these methodologies can be used to incorporate the results of pilot plant testwork on primary grinding and predict the specific power consumption for secondary grinding. Contingencies can be applied either to primary grinding, secondary grinding, or both; for instance, a contingency could be applied to primary grinding kWh/t to account for production of a finer transfer size than sampled in a pilot plant, as well as a contingency applied to secondary grinding kWh/t to account for a coarser feed (transfer size). This approach is a common one for porphyry copper ores in which there is usually a contrast between softer (weathered or secondary mineralized zones) and harder (silicified or primary mineralized zones) ore types, as well as differential grinding phenomena in ball milling (e.g., accumulation of harder porphyritic feldspar grains and variable circulating loads), as distinct from normal variations in new feed rate.

Pilot Plant Testwork Basis

A pilot plant test program will have been run either in an accredited facility or in a custom-built plant that is located in a remote exploration camp. A composite bulk sample, or a number of samples, will have been selected for testwork. For the principal bulk sample, tests will have been run to stable operating conditions, recorded, and sampled for selected circuit types on the recommendations of the supervising engineer to the project development team. These basic circuit types, from which to choose, are:

•         Single-stage or two-stage circuit to final product sizing

•         Autogenous, with and without pebble crushing

•         Semi-autogenous, with and without pebble crushing

•         Pre-crushing of AC or SAO test feed, this is particularly important for application to harder ores in low aspect primary mills with limitations of feed sizing, or in high aspect mills if perceived limitations on mill size and construction have been reached, e.g., 40 ft dia. currently

•         100% recycle of crushed pebbles to the primary mill feed, or whole/partial advancement to secondary grinding, use of high pressure grinding rolls (HPGR) in this context

•         Secondary grinding as pebble milling, preferably in closed-circuit with a cyclone

•         Secondary grinding as ball milling, although this is optional in a pilot plant test program if it is decided to size the ball mills by power-based methods using Bond work indices. If crushed pebbles are advanced to secondary grinding, then it should be piloted. A pilot-scale ball mill should be closed with a cyclone to establish circulating loads, size analyses of streams, pulp densities for optimum operation, and power efficiency.