Improved grindability and gold liberation by microwave pretreatment of a free-milling gold ore

The gravity concentration of gold is very efficient if the metal is fully liberated and the particle size of the gold is relatively coarse. Liberation is usually achieved by comminution, but due to the association of the gold with the other minerals in the ore, overgrinding occurs in conventional comminution circuits and slime generation leads to inefficient recovery. The liberation of minerals can be improved by adding grinding aids, which modify the mechanical properties of the ore and allow breakage at lower stress levels. In this research, microwave pretreatment was used to augment the grinding of a free-milling gold ore containing quartz, silicates and iron oxides. Under microwave irradiation, selective heating of the different mineral components resulted in thermal stresses that caused cracking. These inter granular and transgranular fractures were confirmed by scanning electron microscopy. After microwave processing, the grindability of the ore was improved and the crushing strength and the Bond Work Index were reduced by 31.2% and 18.5%, respectively. In addition to the enhanced grindability, gold was released from the matrix of the host minerals at a coarser size, resulting in a significant increase in free gold recovery by gravity concentration. For a gold ore with a head grade of 6.4 g the gold recovery improved from about 28% to 40 A after microwave pretreatment. (C) 2005 Institute of Materials, Minerals and Mining and Australasian Institute of Mining and Metallurgy. Published by Maney on behalf of the Institutes.