Selection and Sizing of Autogenous and Semi-Autogenous Mills. Pilot Plant Testwork Basis, страница 6

For semi-autogenous grinding, the Bond work indices for low energy impact crushing (W_ic), rod milling (W_irм), and ball milling (W_iвм) can be used in power-based models to predict the specific power consumption for primary grinding without direct reference to other specific operations. Testwork results would be obtained in the normal manner as input to the model in use. Rock strength data and JK Tech drop weight test data would be obtained specifically for the sample of interest.

Confidence in specific power consumptions which are predicted by power-based models depends upon their precision which, in turn, depends upon the basis of the underlying equations and the database that supports them. Their precision carries more importance in respect of the capital cost of large-scale projects and the size of die applied contingency than it does for smaller projects.

Power-based models are offered by Orway Mineral Consultants, Minproc, and Fluor ("GRINDPOWER"). "GRTNDPOWER" is considered by many to be the most versatile in view of its mill sizing capabilities for both autogenous and semi-autogenous grinding, pebble milling, and secondary ball nulling, the ability to input pilot plant results, and to expeditiously test sensitivities of mill operating parameters for both primary and secondary grinding, changes in ore type, maximum power capability, and the power split between primary and secondary grinding (Matthews and Barratt 1991). Its "CHARGEPOWER" sub-program can produce a series of spread sheets, each of which details net power draw across a range of ball charge volumes and total mill charge volumes for a particular mill speed, and which in aggregate covers a range of mill speeds. This capability becomes useful when specific power consumptions for different ore types (lithologies, mineralizations, alteration types, and mineable ore zones) are converted to applied net power draws for a particular mill size (see "tent" diagrams in Figure Nos. 7 to 11).

Figure 3   Relationship between Bond work indices for design of the Batu Hijau Project (Barratt et alia 1996)

The effects of ore variability (grindability) on the specific power consumptions for primary and secondary grinding can also be assessed using power-based models. "GRINDPOWER" was used to design the 120,000 tpd plant for Ваtu Hijau (Barrett et alia 19%, MacLaren et alia 2001) on the basis of testing 105 composites for the three Bond work indices (Wic, Wi_RM, and Wi_BM) with a contingency for secondary ball milling to account for the effects of pebble crushing. These Bond work indices, composited by mine production year from weighted bench composites, are illustrated in Figure 3 in which the relationship between the higher values of Wi_RM vis a vis Wi_c and Wi_BM is fairly consistent Pebble crushing was included in grinding circuit design as a direct result of Wi_c < Wi_RM > Wi_BM. Subsequent testwork on a new set of infill composites has shown a correlation of 0.985 between new feed rate t/h and Wi_RM at constant power draw in the primary and secondary mills with a variable grind to flotation. Such a good correlation is specific to Ваtu Hijau and the opportunity to test its efficacy on other projects has not yet arisen. JK SimMet simulations were run on a few macro lithological/alteration composites to support the circuit design. Pilot plant testing for AQ/SAG grinding on this project was not practical due to environmental and logistical reasons.

The Minnovex SPI test (SAG power index) is now widely used to test for ore variability in SAG circuits and to predict mill throughput based on a simple test procedure (Kosick and Bennett 1999). The SPI represents the specific power consumption (kWh/t) required to produce an 80% passing 10 mesh (1,680 microns) product Values of SPI can be obtained from drill core and fitted into the geological model. Mill throughput can be predicted from mill power draws, either for the circuit as a whole, or if the SAG mill is limiting, or if the ball mill is limiting. The program has been extended to include economic criteria, СЕЕТ (Kosick et alia 2001). The SPI test and its associated software is a powerful tool that, like JK SimMet simulations, can complement pilot plant testwork and the more accurate power-based models for mill sizing.