Question

Discuss the most important factors that influence the oxidation processes of sulphide minerals. Use the relevant equations/information whenever it is appropriate. (maximum 250 words)

Answer 1

Sulphide minerals are the minerals containing sulphide(S2-) or persulphide(S22-) as anions. Eg - Pyrite (Iron Ore), Chalcopyrite (Copper Ore), Molybdenite( Molybdenum ore) etc. Among various sulphide minerals, the most common is Pyrite (FeS2).

Oxidation of these minerals can occur by direct contact with atmospheric oxygen and water during mining activity or also by the use of micro-organisms. The micro-organisms gain energy from oxidation of the compounds and take carbon from the atmosphere. Thus, the major factors that affect oxidation of sulphide minerals are-

1. Availability of oxygen

2. Availabilitty of carbon dioxide

3. Bacteria: Compound Ratio

4. Inhibition of bacteria

3. and 4. affect the process of oxidation only if bio-oxidation is carried out.

The overall reaction of metal sulphides like Pyrite can be shown by the equation-

FeS2 + O2 (excess) + H2O Fe2+ + 2SO42- + 2H+

The ferrous ion further gets oxidised to ferric ion.

Fe2+ + O2 + 2H+ Fe3+ + H2O

The ferric ion hydrolyses into ferric hydroxide as ppt.

Fe3+ + 3H2O Fe(OH)3 (ppt) + 3H+

However, the second process i.e. oxidation of ferrous ion to ferric ion is a very slow process for which bacteria are used for oxidation of ferrous ions.

Same process is followed for oxidation of other sulphide minerals.