Methods of leaching sulphide ores can be:
- in absence of oxidizing agents
- in presence of oxidizing agents
- Bacterial leaching
Absence of oxidizing agents
Dilute acids dissolve some metal sulphides with liberation of H2S, e.g.
ZnS + 2H+ ^ Zn2+ + H2S
Three types of sulphides can be encountered.
- Completely soluble sulphides: Zns, CoS, NiS, FeS
- Slightly soluble, e.g. FeS2.
- Insoluble: PbS, CuS, AS2S3, Sb2S3, CaS
- Alkali cyanides
Sodium cyanide has a solvent action on sulphides.
- Alkali sulphides:
Sodium sulphide solution reacts with sulphides of arsenic, antimony, tin and mercury to form soluble thiosalts, additions of NaOH helping prevent hydrolysis of Na2S.
- Alkali hydroxides:
NaOH may be used to leach PbS and ZnS forming soluble plumbite (Na2Pb02) and zincate solutions respectively (Na2Pb02) respectively.
- Double decomposition
Copper sulphate can be used to separate Copper from Nickel in a sulphide ore or Copper from Fe in Cu -Fe sulphide ore. CuSO4 solution can also be used for removing Pb, Zn, and Fe from copper sulphide ores.
In presence of oxidizing agents:
- Ferric ion:
Ferric sulphate or chloride react with metal sulphides liberating elemental sulphur, e.g.
Zns + 2Fe3+ ^ Zn2 + 2Fe2+ + S
And in the case of cuprous sulphide
Cu2S + 2Fe3+ ^ CuS + Cu2+ + 2Fe2+
CuS + 2Fe3+ ^ Cu2+ + 2Fe2+ + S
During leaching with Ferric ion a critical balance between Fe2+ and Fe3+ ions in the leach solution is essential to maintain efficiency of leaching and subsequent electrolysis. The ferric ion is also very sensitive to pH changes, hydrolysis and precipitation of basic ferric salts starting at pH 3.5.
2. Chlorine water and hypochlorite:
Mixtures of hypochlorous and hydrochloric acid (the former a very powerful oxidizing agent) help leach sulphide ores, the overall reaction being:
MS + Cl(aq) ^ MCl2 (aq) + S
The technique can be used for Lead (PbS), copper (Cu2S and CuS), Iron (FeS2 and FeS) and Molybdenum (MoS2).
3. Nitric acid and Nitrates:
The oxidizing action of nitric acid and nitrates can be utilized to leach sulphides, but this is seldom adopted because of costs.
4. Concentrated sulphuric acid:
Most sulphides are oxidized to sulphate. From the sulphated ore, all of the zinc, copper and iron can be leached with water while lead, silver, gold and antimony will remain in the insoluble residue.