Gold Extraction by Chemical Oxidation: Test Procedure VS Cyanide Leaching
There are chemical oxidants such as sodium hypochlorite, and nitric acid that are employed in many chemical processes. The latter is an interesting reagent to be considered in the treatment of refractory auriferous ores.
The reactions involved are:
A. Leaching.
FeS2 + 5HNO3 = 1/2Fe2 (SO4)3 + 1/2H2SO4 + 5NO + 2H2O
B. Gaseous phase.
6NO + 3O2 = 6NO2
C. NO2 absorption.
3NO2 + H2O = 2HNO3 + NO
D. Global reaction.
2FeS2 + 15/2O2 + H2O = Fe2 (SO4)3 + H2SO4
Nitric acid is regenerated and recycled to the process. Chemical oxidation with acid nitric works at atmospherical pressure, 90 oC, and residence time of 1 to 2 hours into the reactor.
The process makes a high oxidation of iron, sulphur, arsenic, antimony, and any type of sulphide material present.
The main advantages are: to work under atmospherical pressure, to employ air instead oxygen, good recoveries of silver, and to promote formation of iron arsenate into the solution.
Also, there are disadvantages such as reuse calcium nitrate, formation of elemental sulphur and its interference during cyanidation. With the years have been implemented a step called Hot-Lime which destroys or dissolves sulphur. This step must be carried out before cyanidation. Finally, it is necessary a good recovery of condensation water and a careful water balance in order to get optimum results.
Tests were done changing the dosage of nitric acid without modifying other parameters in order to find out the influence of nitric acid in the oxidation process. Conditions are listed below.
- Solids: 33%
- Particle size: 85% < 200 mesh
- Temperature: 90 oC
- HNO3: 0.5 to 24 g/l
- Pressure: 1 atmosphere
- Time: 2 hours
- Volume: 15 liters
The Hot-Lime stage was done to 90 oC, and lime added was 58 kg/t.
The next tables show results obtained.
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