The common processes for recovery of the solubilized gold from solution are (certain processes may be precluded from use by technical factors) [5,6]:
- Carbon Adsorption
- Merrill-Crowe process
- Electrowinning
- Ion-exchange/Solvent Extraction
(i) CarbonAdsorption
The process known as carbon in pulp, or charcoal in pulp or CIP controls the gold precipitation from the cyanide solution by use of activated charcoal (carbon). Activated carbon can be manufactured from wood, nuts shells, coal, petroleum coke and a variety of organic products. Coconut shell carbon is preferred because of its commendable durability and high adsorption capability for gold and silver cyanide [5]. Other modifications include Carbon-in-Leach (CIL) and Carbon-in-Column (CIC).
The technique involves contacting the leached pulp with granular carbon (about -8 to +20 mesh) in a series of gently agitating tanks with a sufficient retention time [2]. The carbon is recycled through the circuit to build up the loading to 8-10 per cent by weight. The loaded charcoal is then separated from the pulp on a suitable vibrating screen, coarse enough to retain the carbon, but fine enough to allow the pulp to pass through. The carbon is next sent to the stripping column for desorption and regeneration. The technique is used on low grade gold and silver ores.
Leached pulp and carbon are transferred in a counter current flow arrangement between a series of tanks, usually numbering 4 to 6 [1] as shown in Figure 2. In the final tank, fresh or barren carbon is put in contact with low grade or tailings solution. At this tank the fresh carbon has a high activity and can remove trace amounts of gold (to levels below 0.01 mg/L Au in solution).
As it moves up the train, the carbon loads to higher and higher concentrations of gold, as it comes in contact with higher grade solutions. Typically concentrations as high as 4000 to 8000 grams of gold per tonne of carbon (g/t Au) can be achieved on the final loaded carbon, as it comes in contact with freshly leached ore and pregnant leach solution (PLS). This can be measured by comparing the amount of gold extracted from the carbon to the amount of carbon used. The final loaded carbon then is removed and washed before undergoing "elution" or desorption of gold cyanide at high temperature and pH. Hot caustic-cyanide (1%NaOH, 0.1 to 0.2%NaCN) solution at nearly boiling temperature in 36 to 72 hours [5]. The elution rate may be accelerated by adding 10% alcohol at temperatures around 60 to 80°C with a stripping time of 4 to 6 hours. The elution of loaded carbon may also be speeded by using pressure elution (120 to 130°C at 70 psig) in 6 to 8 hours - We recommend a CTC 55 US Mesh 6 x 12 grade activated carbon for the absorption of gold.