The largest single operating loss in many hydrometallurgical plants is the poor recovery of leached product in the solids sent to tailings. Any reduction in the concentration of the valuable species in this stream will improve the project economics._x000D_
Conventional design, to reduce the soluble loss, is to increase the number of wash stages or the wash/water ratio. This approach is limited in effectiveness as it can only reduce the soluble loss to that dictated by the concentration in the returning solution. For copper plants this concentration is set by the recovery in the solvent extraction (SX) plant. When using a single grade of SX feed a single grade of SX raffinate is produced generally with five to seven per cent of the copper still remaining in solution._x000D_
An alternative approach has been developed by Cognis Corporation and implemented on a number of recent projects using engineering input from Miller Metallurgical Services. The SX plant feed is split' into two different streams: a high-grade stream containing up to 75 per cent of the copper and a low-grade stream with the remaining copper. This Split-Circuit concept has significant benefits in terms of both copper recovery and reduction in acid costs._x000D_
The high-grade stream has the bulk of the copper removed and the raffinate sent directly back to the leach; to fully re-use the acid that has been generated during extraction in the SX process. The recycling of this higher concentrated acid stream considerably reduces the acid in the raffinate sent to washing steps. It represents a significant saving in acid consumed via the tails stream. The low-grade stream has a significantly lower SX feed grade and hence produces a lower raffinate acid and copper concentration. This lowers the economic species in the wash solution and hence reduces the soluble loss accordingly. Other operating improvements are realised with lower neutralisation costs (with the lower acid in the wash stream) and from a decreased need for in-circuit neutralisation to improve SX performance._x000D_
The SX plant itself is also split into a high-grade extraction and a low-grade extraction stream to enable the low-grade raffinate to be produced. A number of circuit alternatives have been utilised that have provided a range of benefits. The circuit concept has been included into both new and existing plant operation. As a result of this experience other methods of incorporating the concept into a circuit have been developed.