Selection and Sizing of Autogenous and Semi-Autogenous Mills. Pilot Plant Testwork Basis, страница 14

SUMMARY AND CONCLUSIONS

Pilot testing is normally required to develop design criteria for AG/SAG circuits, but is not essential. In the context of engineering work preparatory to equipment purchase, power-based models that rely on bench-scale testwork can also be used, either in the absence of pilot plant testwork or to supplement it. This is particularly true for semi-autogenous grinding for which projects ranging in size from 500 tpd to 1 20,000 tpd have been designed without pilot plant testwork and operated successfully. For autogenous grinding and pebble milling, pilot plant testwork is preferred, but it can be sacrificed if project logistics dictate and a comprehensive database of geotechnical parameters is available with a reference to existing autogenous operations and bench-scale ore competency testwork.

It is now common place to assess the variability of grinding characteristics by testing a large number of ore samples that are related to geology, the geological model, and the mineable resource, as well as the results of pilot plant testwork or more comprehensive bench-scale testwork. The Minnovex SPI test is widely used in this regard.

The more precise power-based models (e.g., "GRINDPOWER") are used to scale-up pilot-scale or bench-scale test results to the appropriate range of applied power draw, mill operating conditions, and to develop mill sizes.

Power numbers have been developed from several mill surveys and are are related to a range of mill speeds and total mill charge volumetric loadings. They are considered to be accurate to within 2%-3% and take into account motion within the mill charge and losses due to heat, sound, windage and no-load.

This mill sizing methodology is used as a basic engineering tool and as a check on technical information that is provided by mill suppliers at the mill bidding stage.

Selection of a basic mill size is analyzed prior to equipment purchase for its integrity in terms of feed chute design, grate open area, shell liner/lifter design, diameter: length ratio, rated motor power and rated speed, motor torque capability, operating speed range, and the range of mill charge operating conditions. All of these parameters are mounted into a series of "tent" diagrams that are designed to assist in optimizing the capability of the motor in satisfying the power demands of the mill and its charge, as well as the production objectives.

In conclusion, criteria for selecting the diameter of the primary mill, its diameter: length ratio, and its applied power are influenced by the hardness, or competency of the ore with respect to impact, abrasion, or both, as well as the desired rated speed and the torque capability of the motor as challenged by power demands of the mill, especially with high ball charge volumes of 15% v/v or more.

For the more competent ores, there should be a return toward a 3:1 ratio for D:L from the present 2:1 for high aspect primary mills and, for higher mill throughputs, mill diameters in excess of 12.192 m (40 ft), both being necessary (together with optimized shell liner/lifter design) for more efficient delivery of impact energy.

For low aspect mills at higher mill speeds with grid shell liners, the technology is proven with D:L as 1:2. Use of this mill profile at lower mill speeds and D:L ranging from 1:1.5 to 1:2 requires confirmation of the optimum feed size, possibly with some form of pre-crushing, to achieve production objectives. The D:L ratio for lower speed primary mills is generally in the range 1:1 to 1:1.1.

In the area of pulp and pebble discharge, more industrial-scale surveys and engineering studies are necessary to establish design criteria for grate open area, "dry" mills, and efficient pulp transfer with curved pulp lifters.

Whereas pebble mills are sized using the same principles and formula as is applied to the sizing of autogenous mills, secondary ball mills are sized using established Bond formulae with contingencies for maximum transfer size, maxJTnuni ball charge volume, and the highest operating mill speed in setting the rated motor power.