The design and operation of small gold concentrators with gravity equipment is a great deal more subtle and complex than normally considered. It is well known that small scale gold operations are characterized based on small reserves with variable gold grades. In this way, quality and flexibility and flexibility are factors that have impact on the operation and they are as important as the processing capacity of the plant. Basically high grade zones need more careful treatment than low grade because of their relevance to project profitability. Then, the project needs an operating and metallurgical analysis and probably with more details from that normally required on large gold operation with cyanidation and/or flotation circuits where success is based on high operating availabilities and maximizing processing rate.
Pre-concentration is an interesting alternative for minimizing the effects of excessive dilution and giving a consistent feed tonnage to the concentrator. Hand picking is the oldest pre-concentration technique, but it is usually influenced by labor costs and security aspects. The size of range processed is normally 0.2-75 mm and if the material is coarser than 19 or 25 mm, processing is most the time done in rotating drums. If the gold ore is finer, centrifugal separators with shaking tables are a good option. The pre-concentration is then fine crushed before rejoining the finer fractions of material for processing. The value of gold justifies the design of complex circuits when is required. A designer who knows and understands the trade-off necessary in small gold concentrators and the specialized equipment to recover gold is the right person to design and develop small gold concentrators. It is well known that cyanidation and flotation have dominated and simplified gold ore processing in the last years that the subtleties of physical gold processing have been partially reduced. Then, it is important to review and understand the principles needed to design gold concentrators.
A small gold concentrator must avoid overgrinding at all costs. Make provision for regrinding substantial tonnages of middlings in at least one subsidiary grinding circuit. Efficient sizing and classification is important to provide appropriate material to the recovery process. The concentrator must include rougher, scavenger and cleaner stages. It is important to remember that stage separation efficiency are lower that flotation. Probably, at least one of each unit circuit middling must be reprocessed by regrinding or in a different section of the plant. Control the plant by monitoring gold content in tails. Gravity concentrators are used to take care themselves and some streams must be reprocessed when is necessary.
Good results can be obtained on gold ores using a spiral concentrator that is able to treat crushed gravel grading 1-2 g/t of gold and produce a concentrate grading 15-25 g/t of gold with recoveries as high as 85-90%. The fine material from tailings can be reprocessed in centrifugal concentrators such as Knelson or Falcon. An important advantage of pre-concentration and gravity concentration is that final tailings are coarser and consequently easier and cheaper to dispose. Also, a special disposal allows recovering gold for cyanidation and/or flotation.