This article will describe units, circuits and strategies used to recover gold that is either liberated or present in very high-grade gold particles, referred to as gravity-recoverable gold, (GRG), as well as gold present in much lower grades in sulfidic particles, typically pyrite and arsenopyrite, that can in their own right be recovered by gravity. These particles will be referred to as gold carriers.
Recovery strategies for GRG and gold carriers vary, as only GRG can be substantially recovered at very low-weight recovery into the concentrate, or yield (less than 0.1%), typical of the type of semi-continuous units used today, whereas gold carriers such as pyrite and arsenopyrite are recovered by continuous units capable of much higher yields, which typically match or slightly exceed the sulfide content of the stream treated. Gravity separation has been utilized in gold plants as the primary recovery mechanism or alternatively ahead of other downstream processes such as flotation and cyanidation since the inception of mineral processing.
The recovery of free and sulfide (pyrite, arsenopyrite and telluride)-associated gold from the primary grinding circuit featured in all these installations. Virtually, almost every gold mine incorporated gravity recovery in the primary grinding circuit.
The treatment of the concentrates ranged from full gravity via tables through amalgamation and in many cases cyanidation was used to recover gold from the concentrates. The use of amalgamation was featured in many mines but has since been phased out due to health and environmental issues. Until recently, the only common option was the shaking table, despite its lower efficiency. Rotating devices are also used in a very limited number of plants. Intensive cyanidation, notwithstanding its higher recoveries, never achieved a high degree of acceptance, possibly because of the lack of a commercial unit.
Issues such as the poor operability, security and maintenance of these circuits combined with rapid advances and the elegance of the carbon-in-pulp (CIP) and carbon-in-leach (CIL) process, capable of achieving very high recoveries, saw a reduction in reliance on gravity as a primary means of concentration. This was amplified by the move towards simplified, low capital plants with low manning levels and automated processes. This drove down operating costs, which in turn made possible the treatment of lower grade ores.
However, some orebodies have been found to have attributes that do not lend themselves to high recovery through the direct cyanidation route. Coarse free gold and gold associated with complex sulfide minerals tend to complicate the cyanidation process. Coarse gold increases the residence time required to achieve high recoveries by cyanidation. Complex metallurgy can cause coatings on gold that render it impervious to cyanidation, while other forms of gold such as gold locked in a sulfide lattice as solid-solution gold or attached to a sulfide particle can report to the tails stream of a conventional cyanidation plant. These problems are generally amplified for the coarse grinds normally associated with low-grade ores. A better understanding of these problems and the development of larger, more reliable gravity units, as well as intensive cyanidation, have heralded a return to gravity recovery.