Achieve Maximum Efficiency in Precious Metal Recovery
Merrill Crowe Process for Gold and Silver
The Merrill-Crowe process is used to separate gold from the solution formed when cyanide leaching is used on gold ores. Leaching removes precious metals from ore by converting the valuable metals into soluble salts. Once the salts have been washed out and processed, pure metal remains.
WesTech’s process expertise helps recover the valuable liquids and/or solids via inter-stage mixing and state-of-the-art sedimentation. Our solutions include countercurrent decantation (CCD) and screening.
Precious Metal Applications
Countercurrent Decantation (CCD)
Rock passes into a dump hopper and is then transferred to a vibrating grizzly screen. The oversized material is sent to a jaw crusher. The crushed product is combined with the grizzly undersize and the ore is conveyed to the coarse ore bin.
Coarse ore is ground, sized, slurried with pregnant liquor, and pumped to the hydrocyclone cluster. Oversized material from the hydrocyclone cluster underflow is returned to the ball mill for further grinding. The hydrocyclone overflow is sent to a trash screen that rejects oversized material and debris.
The trash screen underflow slurry is fed to the high rate pre-leach thickener. Diluted flocculant and barren solution are added to the feed of the thickener to assist in solids settling and thickening. Thickener underflow is pumped to the leaching circuit. Overflow pregnant solution containing precious metals dissolved into solution is pumped to the clarifier.
Cyanide solution is added to leach the gold from the slurry using a series of mixed leach tanks. Oxygen is injected into first leach tank using a recirculation pump to maintain sufficient dissolved oxygen in the slurry for leaching. Slaked lime slurry is used to increase pulp pH levels.
The countercurrent decantation (CCD) circuit recovers precious metals leached into solution using a multistage countercurrent thickener. Slurry from the leaching process reports to the first CCD thickener. The thickened underflow is pumped to the next CCD thickener, where it is washed with recovered solution from the previous CCD thickener. Thickened underflow slurry from the final CCD thickener is pumped to the tailings paste thickener. The wash solution will flow countercurrent to the solids flow, increasing in precious metal concentration as it proceeds to the first CCD thickener. The pregnant solution from the first CCD thickener is pumped to the buoyant media clarifier.
A buoyant media clarifier provides initial clarification of the solution. Clarifier overflow is pumped to the polishing filter circuit. The remaining suspended solids in the clarified solution are removed by pressure leaf filters. Diatomaceous earth is used to precoat the filters and as a body feed. Filtered pregnant solution is discharged to the vacuum deaeration column, which removes oxygen from the solution in a packed tower.
A zinc feeder with an auger is used for the addition of zinc. The zinc powder displaces the gold from solution. A rich pregnant solution allows for better utilization of the zinc.
Final recovery of the precious metals is accomplished by filtering the solution using filter presses. The recessed plate-type filter press collects the filter cake between the filter plates in the chambers formed by the recessed plates. At the end of the filtration cycle, the free liquid is displaced from the filter cake by an air blow step. The filter press is opened and the cake falls from between the plates. It is then collected and sent to the smelter.
Carbon-in-Leach Circuit (CIL)
For over a hundred years, miners have used dilute alkaline cyanide solutions (e.g., sodium cyanide [NaCN] around pH 10-11) to leach (dissolve) gold and other precious metals, from their ores. The gold is usually in the form of small flakes mixed with other minerals. It is difficult to separate mechanically, so it is dissolved and then recovered by other means.
Activated carbon removes gold out of dilute cyanide solution by adsorption (sticking). Carbon adsorption (with other extraction steps) is often the best method to follow gold cyanidation.*†
The carbon-in-leach process adds the leaching agent (cyanide solution) and activated carbon together into the slurry of ore and water. This prevents other carbonaceous materials (wood, clay, etc.) in the ores from adsorbing the gold first (“preg-robbing”).
In this step:
- Mills grind the ore, exposing gold particles.
- Water joins the ore to form slurry.
- A trash linear screen rejects wood and debris so that it does not disrupt later operations.
- A high-rate thickener removes excess water from the grinding stage.
This process includes several vessels where:
- Cyanide solution leaches the gold from the slurry so that it can be adsorbed by carbon. ‡
- Slurry flows downstream (pumped or by gravity). Carbon retention screens keep the larger-sized carbon from going downstream at each stage.
- Pumps force the carbon-rich slurry upstream.
- Countercurrent net transfer: slurry flows downstream – with less gold at each stage; carbon flows upstream – loaded with more gold at each stage.
- A high-rate thickener or paste thickener dewaters the tails (waste), before disposal in a tailings pond, or as a paste deposition.
This process includes:
- Elution – A hot, concentrated cyanide solution pulls the gold from the carbon.
- Regeneration – A kiln reactivates carbon before the circuit reuses it.
- Electrowinning – Electricity passes through the gold-loaded (pregnant) solution, causing gold to form at a cathode and cyanide at an anode. A smelter refines the gold.
Carbon Fines Recovery
Slurry leaves the final adsorption stage through a linear screen, which catches any residual carbon fragments. These are recycled.
A carbon sizing, linear screen ejects the carbon fines from the adsorption circuit.
A settling tank (e.g., AltaFlo™ Thickener) or filter collects carbon fines, and reclaims water.
*But if the silver content in the ore is high, download our “CCD – Merrill-Crowe Gold Silver” flow sheet.
†There are three subsets of the carbon adsorption approach:
- Carbon-in-Pulp – Most efficient for slurries. Process leaches the gold first, adds carbon separately.
- Carbon-in-Leach – Effective for carbonaceous ore slurries. Process adds leaching agent and carbon together, keeping “preg-robbing” material (like wood) from adsorbing the gold.
- Carbon-in-Column – For non-slurries, solution-only. Typical for heap leach applications.
‡Leaching detention time is dependent on:
- Particle Size – Finer particles dissolve quicker, less time needed.
- Dissolved Oxygen – Rate of dissolution is directly proportional to amount of oxygen present.