Selection and Sizing of Autogenous and Semi-Autogenous Mills. Pilot Plant Testwork Basis, страница 2

- Mine blasting practice

- Comminution testwork on predominant ore types in the mine production plan, whether by bench-scale tests, pilot plant tests, or a combination of both, coupled with an assessment of the variability of ore grindability

- Assessment of autogenous grinding vs semi-autogenous grinding

- Methodologies for scaling up testwork results.

•         Definition of the primary mill size and aspect ratio which will ensure that the required throughput range can be processed and the required power can be drawn (kWh/t x t/h) over a defined range of ore hardnesses, mill speeds, and other operating conditions:

- Primary crusher setting

- The necessity for pre-crushing part or all of the primary mill feed

- The necessity for pebble crushing

- Ball charge volume and top ball size

- Total mill charge volume    

- Assessment of voids between ore/balls and pulp

- Pulp density

- Discharge classification type(s)

- Mill charge size distribution

- Circulating load variation

- Shell liner and lifter geometry and design

- Grate open area requirement

- Pulp discharger design.

•         Application of known engineering and manufacturing procedures and practices with respect to ensuring the structural integrity and operability of both the mill and motor at the most economical cost without compromising standards, quality assurance, quality control and inspection, and engineering co-ordination between the mill supplier, motor supplier, and engineer-of-record with third party review.

This paper deals particularly with a summary of the types of testwork required for assessing AG/SAG grinding and ore variability, methodologies for scaling up testwork results, definition of the applied net power, and criteria required for definition of the primary mill size and aspect ratio.

In assessing the applied net power, the question of a contingency often arises and, while its impact is not as noticeable for smaller AG/SAG mills, the principles governing its application are the same and have to be quantified for the larger mills, namely:

•         Whether an AG mill is to be converted to a SAG mill

•         Definition of the maximum operating ball charge volume in combination with the total mill charge volume to give the maximum operating mill charge density

•         Definition of the maximum operating mill speed for power-efficient grinding of harder ores

•         Definition of the optimum shell liner/lifter design for power-efficient grinding of harder ores using modem design and simulation concepts

•         Definition of the required grate open area and pulp discharger capacity and their design to efficiently discharge the anticipated maximum flow of pulp and pebbles.  These parameters, particularly grate open area, can determine the optimum mill diameter.

Therefore, the applied contingency is made up of a number of intrinsic components and it does not necessarily have a predetermined value.

Whereas a lot of dry autogenous grinding mills have been converted to wet semi-autogenous grinding, this paper concentrates on wet grinding. It comments briefly on the sizing of secondary mills, whether they are ball mills or pebble mills.

TESTWORK AND SCALE-UP

Testwork methodology, for both bench-scale and pilot plant, is described in the relevant chapters elsewhere in this book. Requirements for the determination of the applied net power to the primary AG or SAG mills for the purposes of sizing these mills are dictated by the results from such testwork and the quality of that testwork. This paper discusses these requirements in the context of the basic engineering phase and issue of technical specifications preparatory to equipment purchase and detailed engineering. These requirements can be sub-divided into:

•         Those benefiting from a pilot plant program