Merrill Crowe-Zinc Precipitation precious metals recovery systems

The clarification process is designed to produce a clean pregnant solution (leachate) with a final suspended solids content less than 10 ppm. A clean solution increases precipitation efficiency and reduces excess loading of non-precious metal solids in the precipitation filters. Automatic backwash clarifier filters are used to complete this process stage.

Following clarification, the deaeration tower removes dissolved oxygen to less than 1.0 ppm, and preferably below 0.7 ppm, from the pregnant solution, allowing an efficient zinc precipitation reaction. The pregnant leach solution is sprayed onto high surface-area plastic tower packing, exposing the oxygen to the tower vacuum, where it is removed by the vacuum pump. The deaeration solution collects in the tower reservoir, which acts as a surge tank upstream of the zinc addition and precipitation pump steps.

Powdered zinc addition to the deaeration solution causes the precious metals to precipitate out of solution via a cementation reaction. Zinc is precisely fed via a screw feeder or turntable into a FLS zinc cone, where it is well mixed with a sodium-cyanide solution to form a slurry. A gravity or suction cone feeder with steady head tank feeds the zinc slurry into the clarified and deaerated leachate, precipitating out the precious metals, which are recovered by a FLS filter press.

The precoat and body feed processes assist clarification and precipitate filtering. Precoat involves placing a thin layer of diatomaceous earth onto the filters, facilitating solids removal during filter servicing. Body feed adds volume to the precipitate and allows the extended build-up of caked material.

For non-mercury-containing ores, a standard drying oven is used to dry the precious metal precipitate; mercury-containing ores are dried in an FLS mercury retort. The dried or retorted precipitate is mixed with fluxes in an FLS flux mixer/furnace feeder, or by hand, and smelted at 2000+°C in a melt furnace. During smelting, impurities are removed into the slag, which is poured into slag pots. The remaining molten precious metal is poured into doré moulds, cooled, weighed, and sold.

The Merrill Crowe process is widely used to recover gold and silver from cyanide-based leachates. Known for its efficiency and low operating costs, it uses a zinc cementation reaction to precipitate out the precious metals for recovery by filter press.

The Merrill Crowe process relies on the reaction of powdered zinc with a metal cyanide complex, which causes the metal to precipitate and the zinc to go into solution.