Heap leach is a method developed by Henin and Lindstrom to process low-grade ore on a large scale with small production cost so that the levels below the cut-of grade can still be economical. Heap leach is the process of leaching of low grade gold rocks thrown, then the solution containing the metal particles are screened and separated by electrolysis. Heap leach done repeatedly and on a large scale.
There are several steps that are used in processing gold ores using heap leaching system
- Preparation of the foundation of leaching with an angle of 1 degree to 6 degrees. : The foundation of this leaching can be made by permanently using a hardening of the concrete floor or foundation and geo-coated membrane (High Density Polyethylene)
- Then the rock minerals that have been taken from the mine area was destroyed. Destruction of rock done to get smaller sized rocks. Size obtained usually ± ½ or up to 1 inch. machines used in the process of destruction is the engine or Stone Mill Hammer Crusher and Crusher Jaws.
- After the material to be crushed and refined, then placed into the foundation of leaching that has been provided
- Then through the spray in the spray solution of sodium sianide. And in general the use of sodium sianide NaCN. This process aims to take the gold extraction is formed in the chlorination process and transformed into compounds Dicyano Aurrat (Au (CN) 2). Because the compound that would be too absorption within the pores of activated carbon
In the cyanidation process consists of two important stages, namely the process of dissolution and separation of gold from the solution process. Solvents commonly used in the process cyanidasi is NaCN, KCN, Ca (CN) 2, or a mixture of all three. The solvent most often used is NaCN, being able to dissolve gold better than any other solvent.
From this it will spray cyanide in the extraction process produces a chemical reaction in the form:
Au + NaCN + O2 + H2O = NaAu (CN) 2 + NaOH (from equation Elsener and Adamson)
Au + NaCN + O2 + H2O = NaAu (CN) 2 + H2O2 + NaOH (from equation Adamson)
Au + NaCN + O2 + H2O = NaAu (CN) 2 + H2O2 + NaOH (from equation Adamson)
Gold extraction is generally effective at pH 9.5 to 11. Optimum near 10.5. The more acidic, hydrolysis to hydrogen cyanide had cyanide (HCN), the more alkaline reaction the more it will slow down. To speed up the process of gold dissolution in cyanide solution will be added to the catalyst. Namely lead Nitrate (PbNO3), because the catalyst can also increase the recovery of gold to be 95%. Generally cyanide complex solution containing between 1 ppm to 3 ppm.
- Once the cycle is completed the leaching process of leaching in the wash with water to remove residual cyanide is used peroxide. Decision-stop solution if the gold content down to 0.005 ounces of gold per ton of solution. Taking gold in the solution can be done by using activated carbon or by direct receipts using zinc cementation process (Merrill - Crowe Process). In addition to using the carbon and cementation, other techniques can be used is elektrorefining and ion exchange resins. Elektrorefining is the process of attachment of the metal contained in the solution on the cathode rod, in this process is stailess steel rod anode and cathode in the form of steel brush.
In heap leaching using carbon adsorption techniques in coulumn Carbon (CIC) in which the solution containing the gold complex flows in tanks containing activated carbon in series and terraced. Activated carbon can absorb gold from 100 up to 400 ounces per ton of carbon, the efficiency will drop if content of the solution decreases. Precious metals in the capture of carbon by passing a hot solution of caustic soda and cyanide mixture (1% NaOH and 1% NaCN) Activated carbon can be reused by reactivation in advance using the rotary kiln at a temperature of 730 degrees Celsius for 20 minutes. In traditional mining usually carbon that has been filled with gold complexes directly in the fuel, the dust from the burning of carbon was then taken to be melted down. Contained metal can be purified directly. This is done because the aged and technological considerations.
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