Sulphide Leaching

Methods of leaching sulphide ores can be:

1. in absence of oxidizing agents
2. in presence of oxidizing agents
3. Bacterial leaching

   

Absence of oxidizing agents

Acid leaching:

Dilute acids dissolve some metal sulphides with liberation of H₂S, e.g.

ZnS + 2H+ ^ Zn²⁺ + H2S

Three types of sulphides can be encountered.

1. Completely soluble sulphides: Zns, CoS, NiS, FeS
2. Slightly soluble, e.g. FeS₂.
3. 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:

1. 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.

Leaching of an Arsenical Copper Sulfide Concentrate