High pressure oxidation of refractory gold minerals
Many researches consider this process as the best alternative to the treatment of refractory material containing gold and silver, mainly when the precious metals are encapsuled into pyrite and arsenopyrite. The process works to high temperature (160 to 220 oC), high pressures (12 to 30 atmospheres), and residence time of 1 to 4 hours. At the beginning, the process was used successfully in the treatment of sulphide ores and concentrates.
Oxidation can be done under acid or alkaline conditions without air contamination with SO2 or arsenic compounds. However, during the process must be avoided the formation of sulphur and jarosites because gold particles could be occluded with some sulphide particles creating a high consumption of cyanide during the leaching stage. Usually, It is included a Hot-Lime step in order to destroy jarosites and improve dissolution of precious metals.
Pyrite and arsenopyrite oxidation involves several reactions with formation of ferrous sulphate, ferric sulphate, sulphate ions, sulphur, and arsenates. The main reactions are listed below:
Pyrite.
- FeS2 + 7/2O2 + H2O = FeSO4 + H2SO4
- 2FeSO4 + 1/2O2 + H2SO4 = Fe2(SO4)3 + H2O
Under specific conditions of temperature and concentration of Fe+3, and H2SO4, some precipitates can be formed.
- Fe2(SO4)3 + 3H2O = Fe2O3 + 3 H2SO4
- Fe2(SO4)3 + 2H2O = 2FeOHSO4 + H2SO4
- 3Fe2(SO4)3 + 14H2O = 2H3OFe3(SO4)2 (OH)6 + 5H2SO4
Arsenopyrite.
- FeAsS+ 13/2O2 + 3H2O = 2H3AsO4 + 2FeSO4
- 4FeSO4 + O2 + 2H2SO4 = 2Fe2(SO4)3 + 2H2O
Similarly, under specific conditions of temperature and concentration of Fe+3, H3AsO4, and H2SO4, some precipitates can be formed.
- 4FeAsS+ 11O2 + 2H2O = 4HAsO2 + 4FeSO4
- 4FeSO4 + O2 + 2H2SO4 = 2Fe2(SO4)3 + 2H2O
- 4HAsO2 + O2 + 2H2O = 2H3AsO4
- 2Fe2(SO4)3 + 2H3AsO4 = 2FeAsO4 + 3H2SO4
Tests were done to different conditions of oxygen pressure and temperature. Other variables were no changed. Conditions are listed below.
- Solids: 33%
- Temperature: 160 – 200 oC
- Oxygen pressure: 10 – 20 atmospheres
- Particle size: 85% < 200 mesh
- Time: 2 hours
The Hot-Lime stage was done to 90 oC, and lime added was 55 kg/t. It lasted two hours.
The next tables show results obtained.
Oxidation.
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