At this stage of the process, the chemical reactions begin. They convert the copper minerals into copper metal.
We can illustrate the types of process using the example of chalcopyrite - CuFeS2. From the formula, it is clear that iron and sulphur have to be removed in order to produce copper.
In roasting, we:
change some of the CuFeS2 to copper oxide
remove some of the sulphur as sulphur dioxide.
We do this by heating the concentrated ore from froth floatation. It is heated to betweem 500°C and 700°C in air. The product from the roaster is called calcine. It is a solid mixture of oxides, sulphides and sulphates. One reaction that takes place is:
2CuFeS2(s) + 3O2(g) → 2FeO(s) + 2CuS(s) + 2SO2(g)
As SO2 is hazardous, it has to be removed from the gases released into the environment. One way of solving the problem is to convert it to sulphuric acid. This is a valuable by-product and helps to offset the costs of copper extraction.
Smelting with fluxes
The calcine is heated to over 1200°C with fluxes such as silica and limestone. The calcine melts and reacts with the fluxes. Some impurities form a slag which floats on the surface of the liquid (like oil on water) and is easily removed.
This is very similar to the removal of impurities in the blast furnace. The liquid left is a mixture of copper sulphides and iron sulphides. It is called a matte.
Picture 7. Blister copper being taken from the converter.
Conversion of matte to copper blister
The liquid matte is oxidised with air to form blister copper in a converter.
The reactions are:
a) Elimination of iron sulphide by oxidation to iron oxide which forms a slag:
b) Formation of blister copper by reduction of copper sulphide:
Picture 8. This huge wheel is used to cast the anodes. Molten copper is poured into a mould and the wheel is turned for the next mold. Meanwhile, the molten copper cools and solidifies into anodes over a metre tall.
The blister copper produced by this process is 99% pure copper. The name 'blister' copper comes from the fact that this final process produces bubbles of sulphur dioxide on the surface of the copper. The blister copper is cast into anodes ready for electrolytic refining.