Introduction

Gravity methods of separation are used to treat a great variety of materials, ranging from heavy metal sulphides such as galena (sp. gr. 7.5) to coal (sp. gr. 1.3), at particle sizes in some cases below 50 ?m.

These methods declined in importance in the first half of the twentieth century due to the development of the froth-flotation process, which allows the selective treatment of low-grade complex ores. They remained, however, the main concentrating methods for iron and tungsten ores and are used extensively for treating tin ores, coal and many industrial minerals.

In recent years, many companies have re-evaluated gravity systems due to increasing costs of flotation reagents, the relative simplicity of gravity processes, and the fact that they produce comparatively little environmental pollution. Modern gravity techniques have proved efficient for concentration of minerals having particle sizes in the 50 ?m range and, when coupled with improved pumping technology and instrumentation, have been incorporated in high-capacity plants (Holland-Batt, 1998). In many cases a high proportion of the mineral in an orebody can at least be pre-concentrated effectively by cheap and ecologically acceptable gravity systems; the amount of reagents and fuel used can be cut significantly when the more expensive methods are restricted to the processing of gravity concentrate. Gravity separation of minerals at coarser sizes as soon as liberation is achieved can also have significant advantages for later treatment stages due to decreased surface area, more efficient dewatering, and the absence of adhering chemicals which could...