The economic benefit of gold gravity recovery ahead of cyanidation or flotation circuits is often overlooked or misunderstood. It is seldom more than 1-3% contribution over the overall recovery, even when gravity recovery is high, 45 to 65%. Ahead of flotation, it can be significantly more, especially for low grade gold-copper ores or for lead-zinc ores, the former because of their very high upgrading ratios and the latter because of the increased selectivity of flotation. Net smelter returns is often higher for a gravity concentrate, 99.5-99.7%, compared to a copper concentrate (91-97%), and even more so a zinc concentrate. Other ores that benefit greatly from gravity recovery are severe pre-robbers.
Essentially, increased overall recovery may not be the only economic factor when considering gravity recovery. Additional factors include decreased costs for the downstream circuit, both capital and operating, reduced gold inventory, and increased security. The latter is a significant factor in the renewed interest in gravity recovery in many places in South Africa. Many Australian plants use coarse grinding and very short retention times in cyanidation circuits, both factors contributing significantly to the economic impact of gravity recovery. Many Australian operations also boast very poor water quality, total dissolved solids of 100,000 to 300,000 ppm, and abundant coarse gold.
It is important to indicate that the first step when considering gold gravity recovery is to estimate the potential economic impact of gravity recovery as a function of how much is recovered. The basic approach is to consider a linear relationship between gravity recovery and its economic benefit, and to use reported results of similar gold operations to estimate relationships. For example, an operation is planned for the production of 4 tonnes of gold every year. A similar operation reports an impact for gravity recovery of 1% extra gold recovery at 40% gravity recovery. Based on the proposed production, an additional overall recovery of gold, 40 kg/year or a contribution of 1,000 g per year per 1% gravity recovery, at $ U.S. 30/g, a 40% gravity recovery represents $ U.S. 1,200,000 per year, each extra 1% in gravity recovery adds $U.S. 30,000 per year.
Because the economic befit of gravity recovery can vary significantly from project to project, the extent to which gravity should be used can also vary, even when the amount of gravity recoverable gold is relatively constant. Contrast two Carbon-in-Pulp operations, one in Canada with 48-h cyanidation time, a 80% passing 75 µm and excellent water quality, to the same operation in Australia with a 16-h cyanidation time, a 80% passing 170 µm and very poor water quality. In this case, the benefit of gold gravity recovery is much more significant for the Australian operation. The two examples are typical characteristics of gold operations in both countries. The Canadian operations typically a high grade underground, and the Australian one a low grade open circuit or combination, underground open cut, with most of the tonnage from the open cut. In south Africa, the increased security of a hands-off gravity recovery circuit is considered a very important benefit, as gold accumulating in the grinding circuit is considered a high security risk that can be reduced with a gravity recovery circuit.