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Gold Carbon In Leach Plants Updated January 2017




Gold Processing Plant, Carbon In Leach (CIL) Plant






A Carbon In Leach Gold Recovery Plant. This plant processes 4 million tons of ore per year with an average gold content of 0.043 ounces of gold per ton of ore. Approximately 85% of the gold is recovered in the plant. From the ball and/or autogenous mill, slurry is pumped to the CIL leach tanks where lime adjusts the pH, if necessary, and it is agitated for the desired period of time. Air is added to increase the reaction rate and shorten the leach time. Cyanide is added, typically in the first tank. Each carbon leach tank has a interstage screen in it for carbon retention. Slurry can gravity flow between tanks or be pumped. Typically there are 4 to 6 tanks in series.

From the first tank, the loaded carbon is pumped using a recessed impeller pump (to not degrade the carbon), or another suitable type of pump, to a loaded carbon screen, where the slurry passes through the screen and retains the carbon.

From there the loaded carbon may be acid washed to remove material, such that might interfere (take longer) for the carbon stripping process. After the acid wash, the acid is neutralized with NaOH.

Then the carbon is sent to the carbon stripping process. This can be either at atmospheric pressure of a higher pressure. The higher pressure Zadra method takes less time for stripping and is popular, for that reason. The stripping solution is typically a heated solution of cyanide and sodium hydroxide. Stripping can take a couple of hours. Then, the pregnant concentrated solution is pumped to the electrowinning cells, where the precious metals plate onto the cathodes, which are steel or stainless steel wool.



 
A flowsheet, showing the Carbon In Leach (CIL) process  This circuit utilizes a thickener, to maintain a constant percent solids in the ore feed, necessitating less water used in the process, smaller tanks and a smaller pond. It also gives much more control over the process that a conventional CIL plant without a thickener. The CIL process is viable if there is sufficient gold content, carbonates/clays in the ore which will "rob" some gold in solution, and it leaches in 8-13 hours.
 
A Stainless Steel Cathode from the electrowinning cell in the gold room. Notice the gold that has plated onto the surface of the stainless steel mesh. After plating is completed, the steel wool cathodes are washed with high pressure water to remove the gold. The gold powder is then melted into a dory bar, which is typically 80%-90% or more pure. The dory bars are usually sent to refineries for processing to 99.9% AU or Ag, which are the precious metal ingots that are sold worldwide for the spot market, or contract price per ounce.
 


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