M - FROM MINERALS TO ELEMENTS

Introduction

Oxidation and Reduction

Group 7 Elements: The Halogens

Theory Of Solutions

Extracting Copper from its Ore

Acid-Base Theory

Extracting Copper from its Ore

Many metals are found in their ores as oxides or sulphides.  The principal copper-containing ore is chalcopyrite, CuFeS₂.  There are a number of stages required in order to separate the copper from the ore.  After mining the ore is crushed and ground into small pieces in order to liberate the copper crystals.

1. Froth Flotation is a technique used to separate the grains of copper mineral from the rest of the mixture.  Only about 0.5% of the ground rock is chalcopyrite; 99.5% is material of no value.  In froth flotation, a chemical called a collector is added to the water/mineral mixture and binds to the chalcopyrite grains.  Detergent is added and air blown through the mixture.  The chalcopyrite grains become concentrated in the froth and can be removed.  This process concentrates the copper up to about 30%.
   
   
2. Smelting is the process used to separate the copper from the chalcopyrite:
   
   
   4CuFeS₂ + 10.5 O₂ 4Cu + 2FeO + Fe2O₃ + 8SO₂
   
   Iron oxide combines with silica to form a slag which floats to the surface and is removed.  The copper is then cast into anodes for refining.
   
   
3. Electrolytic Refining.  The copper is now 99.6% pure but this is not pure enough for conducting electricity.  The impure copper is made into the anode of an electrolysis cell with a thin pure copper cathode.  The electrolyte is copper(II) sulphate in sulphuric acid.  The impure anode goes into solution as Cu²⁺ ions which are then deposited on the cathode.
   
   

   Anode:	Cu(s) Cu2+(aq) + 2e-
   Cathode:	Cu2+(aq) + 2e- Cu(s)

Extraction of a metal from an ore causes a number of environmental problems:

1. The land where the open mine is becomes devastated.
2. The waste from crushing has to be removed.
3. The waste from froth flotation, called tailings, has to be removed.
4. Sulphur dioxide from smelting can cause acid rain.

In America 10% of all copper produced comes from bacteria which feed off chalcopyrite.  The overall process involves bacteria converting insoluble chalcopyrite into a solution of iron(III) sulphate and copper(II) sulphate.  This process is cheaper, quieter and less polluting than the smelting process, but it is much slower.  The solution produced still needs concentrating, reduction of the copper prior to final purification.

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