The profile of gold mining operations is very similar in many locations around the world. First, alluvial deposits are mined, later small, high grade, coarse grained and hard rocks. Also tailings from old operations are reworked when the gold process rises strongly or when a new process or technology is available. The successful opening of a small hardrock deposit is often a trade-off between capital exposure and gold production. Cyanidation maximizes plant investment costs and gold gravity concentration tends to reduce investment, but does not achieve the recovery obtained by cyanidation. Basically, where the amount of available capital is low and gold ore characteristics permit, the arguments and preferences for gravity concentration are strong.
Small scale gold operations are notorious for extreme variability in gold grade and ore hardness. This variation can be difficult to for small operations to handle with efficiency. Other problem that they have to face are small budgets, limited ore reserves and the nature of high grade occurrences, as few constraints as possible should be placed on the mine engineers in their attempts to recover all the high grade ore without consider technical aspects such as dilution, which is not acceptable in big operations. Pre-concentration is a useful technique for eliminating the effects of excessive dilution and ensuring a smaller and more consistent feed tonnage to the mill. The objective is to reject part of low grade ore in order to reduce operation costs produced by crushing and grinding circuits.
South Africa has mined hard rock deposits since late 19th century and its development on gold recovery processes is important. Gold recovery by gravity is still used, but basically simply as a unit cell operation in the primary grinding circuit to recover small portions of free gold particles. Sometimes, trying to get simplicity, this option is overlooked and bad interpreted by some gold plant designers. The trend is South America is more or less similar to South Africa. Countries such as Brazil or Peru processes hard rock deposits and the inclusion of any gravity equipment such as a Knelson concentrator, Falcon Concentrator or Jig is a common patron in many operations. The start of a new operation with gravimetric equipment is not simple and operators and metallurgists have to apply their experience and knowledge.
When there are gold projects the investment capital plays an important role in the development of the project. In this way, the criterion that really must be studied and analyzed is the gold output per unit of invested capital. With care in tailings disposal, the possibility of later tailings retreatment by cyanidation is also kept open. In this way, the capital investment of a common and simple gravity circuit must be approximately half that of a comparable tonnage cyanidation operation. Assuming a total recovery of 90%, and gold recovery by gravity concentration of 50%, the investment could be recovered in the short term from gravity circuit. The other recovery can be obtained by cyanidation in the future or at the same time if the design includes gravity and cyanidation.