MerrillCrowe

The Merrill-Crowe process is a zinc-based precipitation method used to recover gold and silver from cyanide solutions produced during the hydrometallurgical processing of gold ores. It is one of the principal cyanide precipitation techniques and is typically used after leaching and solution clarification to recover precious metals before final solid-liquid separation. The method competes with alternative recovery methods such as carbon-in-pulp and resin-in-leach in certain flowsheets.

Process details: After leaching, the pregnant solution is clarified to remove solids. The clarified solution is rendered alkaline (pH around 10-11) to suppress zinc dissolution and to favor precipitation. Zinc dust is added under vigorous agitation and aeration; the dissolved gold and silver are displaced from cyanide complexes and coprecipitate onto the zinc or as a piece of sponge-like sludge. The precipitate is then collected by filtration or thickening.

The precipitate is smelted to recover gold and silver; zinc is removed during melting. The process is simple and relatively low in capital cost, and is effective for solutions with low concentrations of copper and other base metals. Disadvantages include zinc consumption and handling hazards, the need for alkaline conditions, and sensitivity to copper doping which reduces recovery. It is often used for pregnant solutions from tailings dumps, heap leaches, or when carbon-based methods are not feasible.

Environment and safety: The process uses cyanide solutions and zinc dust; proper handling, storage, and waste treatment are required, with attention to preventing releases and protecting workers.