- •1. Gold ores and their metallurgical treatment
- •1.1 Gravity concentration
- •1.2 Amalgamation
- •1.3 Flotation and subsequent processes (refractory ore processing)
- •1.4 Leaching processes
- •1.5 Gold extraction
- •1.6 Cyanide leaching vs. Thio-compound leaching
- •2.2 Microscopy of selected ores and products
- •2. Microscopy of gold ores and treatment products
- •2.1 Tasks and problems of microscopical investigations
- •3. Base metal ores and their metallurgical treatment
- •3.1 Flotation
- •3.2 Development of the flotation process
- •3.3 Principles and mechanisms of flotation
- •3.4 Column flotation
- •3.5 Hydrometallurgy
- •4. Microscopy of base metal ores and milling products
- •4.1 Specific tasks of microscopical investigations
- •4.2 Microscopy of selected base metal ores and milling products (practical exercises)
- •5. Selected bibliography
1.5 Gold extraction
After dissolving the gold by a cyanide solution the metal can be extracted from the liquor by cementation or adsorption.
Cementation of dissolved gold from the leaching solution is performed by pulverised zinc (Merrill-Crowe process).
2[Au(CN)2]- + Zn → 2Au + [Zn(CN)4] 2-
As soon as the zinc dust is added almost the complete gold content of the liquor is immediately precipitated and the cementate can be filtrated and melted to a gold bullion. Before smelting the cementate is washed with acid in order to remove the remainder of zinc.
The adsorption of gold from cyanide solution makes use of the natural affinity of gold for carbon. During adsorption the entire gold cyanide complex becomes attached to the carbon. The gold attached to the carbon is then desorbed by a cyanide solution from which it can be recovered by electrolysis. The desorbed or “stripped” carbon can be regenerated by heating it at approx. 600 °C for 30 minutes in absence of air. From a head value of approx. 2g of gold per tonne of solution a carbon loading of up to 15 kg gold per tonne of carbon is achieved leaving as little as 0,01g of gold per tonne of solution in the tailings.
There are essentially three process varieties that use carbon adsorption:
- The carbon in pulp (CIP) process,
- The carbon in leach (CIL) process and
- The carbon in column (CIC) process.
During the CIP process activated carbon particles are added to the leached pulp in a counter current circuit. The counter current principle is important because the amount of gold adsorbed on the carbon is in equilibrium with the residual gold concentration in the solution, and therefore the low gold concentrations in the final stage can be adsorbed only if the pulp in this stage is contacted with unloaded carbon. The considerable difference in grain size between carbon and pulp particles permits an easy separation of the two by sieving. Whereas pulp particles generally do not exceed 100 microns, the carbon granules appear in fractions between 1 and 3 mm.
The CIL process and the CIC process are essentially variations of the carbon in pulp technique that are used under special conditions.
In principle adsorption can also be accomplished by ion exchange technology with artificial resins. The disadvantages of the existing resins, however, prevented their wider spread use (they are expensive, most of them are non selective for gold and difficult to strip, and if they are selective and easy to strip their loading capacity is very low).
1.6 Cyanide leaching vs. Thio-compound leaching
In recent years thiourea and thiosulphate leaching gained a certain attractivity because it was believed that less toxic or non-toxic reagents are involved. An additional argument for thiourea or thiosulphate are their high leaching rates that can reach 250mg/cm2 per hour exceeding cyanides almost by a factor of 100. The technical implementation of a thio-compound leaching process, however, implies a series of problems and therefore we see very few operations that apply thiourea as an alternative lixiviant. Another adverse factor are the high costs for the reagents because of high reagent consumptions as compared to sodium cyanide. The gold leaching reaction by thiourea and thiosulphate is very sensitive to pH and redox potential and both chemicals are intrinsically unstable decomposing readily to substances that are unable to leach gold. This means that process control is difficult and reagent recycling is very limited if not impossible. The seemingly low toxicity of the two compounds as compared to cyanide is only valid regarding their lethal toxicity. Decomposition of thio-compounds generates a series of partly highly toxic products that must not be released into the ecosystem. In addition to this thiourea is suspected to be carcinogenic.