The optimisation of pebble grinding.

Abstract

Secondary pebble milling is an attractive alternative to using ball-mills. It is of particular interest in older mines where tonnage has been reduced, spare mills are available and savings in operating cost are essential to ensure the financial success of the mine. Ball-mills in a conventional two-stage milling circuit can be converted to pebble mills and pebbles can be obtained by allowing a fraction of the feed to bypass the tertiary crusher. The aim of this work was to investigate the effect of pebbles size and mill speed on secondary grinding efficiency at a gold mine. Pebble mills are used at the mine for secondary grinding, using a pebble feed size of 65mm/35mm.

Semi-batch tests were performed on a bulk sample of the ore, using a 1.2 m diameter mill. Various mill speeds and two pebble feed size ranges were tested (i.e. 65mm/35mm and 44mm/28mm). Various size fractions of pebbles were marked using spray paint and weighed before and after each experiment to determine a mass loss function. The steady state size distribution for a given feed size was predicted using an empirical model which incorporates this function.

It is well known that smaller grinding media are more efficient for fine grinding, as a result of a larger surface area to volume ratio. The experiments confirmed this. It was expected that the use of smaller pebbles would result in an increase in pebble consumption, but the pebble consumption actually decreased, when the smaller pebbles were used. This phenomenon is thought to be due to the removal of the larger rocks, resulting in a decrease in impact forces on the pebbles, and hence a lower surface wear rate. A substantial improvement in secondary grinding efficiency (defined as the kWh spent to produce a ton of material finer than 75 μm or Work Index) was observed by reducing the pebble feed size. Grinding efficiency was improved further by reducing the mill speed from 83.5% of critical speed to a typical ball-milling speed of 69%. The results of these batch grinding tests show that simple changes to equipment may lead to a more efficient use of power, a finer grind and better extraction of gold.

Tests in a laboratory-scale mill were used to compare pebble milling to ball milling, using the same size range for the pebbles and balls. The pebbles were more efficient, resulting in a reduction of 26% in the power required per ton of product, for the same product size distribution. However, the reduced power draw requires a 42% increase in mill volume. However if spare mills are available then this poses no problem.