The recovery process of the gold plant operation is defined as including all of the unit operations, including handling and gold ore preparation, required to obtain a gold concentrate or pregnant solution with gold values. Probably, gravity concentration and flotation are less complicated and operator`s skill and experience play an important role. However, leaching is not easy all the time and could be necessary to perform a preliminary treatment. For example, roasting may be necessary, either to improve their physical properties during subsequent processing or to increase solubility of gold during cyanidation. The cyanidation is a key operation in the mill process and the type of recovery options such activated carbon or Merrill-Crowe influences the entire flowsheet arrangement. This aspect has an effect on the economics of the operation and must be selected properly.
Cyanidation is universally employed in many operations and is effective with most ores and usually needs fine liberation to obtain high gold recoveries. The process is performed with air addition and cyanide concentration, leaching time, pH, oxidation potential and temperature are among the controlled variables. Pressure leaching may be employed in some occasions and has been considered for use under special conditions (presence of refractory gold ores). The final step in the extraction phase of the milling process is the separation of the pregnant solution from waste solids and depends on the recovery system. Depending upon subsequent operation, this step may either produce a clarified solution or classified, dilute, slurry of solution and fine solids. The clarity of the solution is more important for Merrill-Crowe process and activated carbon can tolerate some degree of turbidity. The cost of clarifiers and selection of the equipment is important due to the different alternatives of the market.
Concentration and purification of the gold from dilute and impure cyanide solutions is pre-requisite to the production of a final, high grade Dore bar. Cyanidation plants may be sufficiently selective to select the purification step. The Merrill-Crowe process needs a special care due to the cementation process is sensitive to the presence of some contaminants in the solution pregnant (e.g. copper, mercury). The other two alternatives are activated carbon and ion-exchange resins. The first one is applicable to the treatment of slurries and clarified solutions in alkaline environment. Resins are more efficient with clarified solutions, but its application on slurries is not impossible. The final selection must be based on metallurgical performance and environmental considerations. Try to consider one aspect is not a good idea and some problems could appear during the full scale industrial operation and the economics of the operation would be affected.