One of the two main processes for recovering gold from pregnant cyanide solution. The other is carbon adsorption. The Merrill-Crowe process involves precipitation with zinc dust and is applicable particularly to gold ores with high silver content due to better silver recovery than by carbon adsorption.
Before gold can be precipitated with zinc dust, all solids must be removed from the pregnant solution. The Merrill-Crowe process then comprises three stages:
(i) de-aeration to prevent gold from re-dissolving;
(ii) zinc dust precipitation;
(iii) precipitate filtration: the zinc dust and gold are mixed with sulphuric acid to dissolve the zinc, filtered, and the remaining solids smelted to a gold bullion bar.