Copper is mostly extracted from the sulphide ore, copper pyrites, by means of the following two processes:
Modern copper metallurgy is based on the smelting process. Following steps are involved:
i. CONCENTRATION OF THE ORE
The finely powdered ore is concentrated by froth flotation process. The powdered ore is mixed with water and a small amount of pine oil (a foaming agent). This mixture is taken in a flotation tank. A blast of air is passed through the suspension. The oil forms foam (or froth) with air. The ore particles which are wetted by oil, rise to the surface along with the foam. The gangue (rocky material) is wetted by water and settles down slowly to the bottom. The ore particles in the form of froth are skimmed off from the surface as shown in the diagram.
The concentrated ore is heated strongly in the presence of hot air in a furnace called reverberatory furnace. During roasting the copper pyrite decomposes to Cu2S and FeS and the sulphur is oxidized to SO2 gas.
2CuFeS2+O2 → Cu2S + FeS + SO2
Arsenic and antimony present as impurities are removed in the form of their volatile oxides.
S +O 2 → SO2
4As + 3O2 → 2As2O3
4Sb+3O2 → 2Sb2O3
The roasted ore is mixed with coke and sand. It is then transferred to a water-jacketed blast furnace, where it is heated in the presence of excess of air. The combustion of the ore also provides a lot of heat; hence less amount of coke is usually needed. During the process of smelting, the sulphides ore converted to the oxides.
FeS + 3O2 → 2FeO + 6SO2
This ferrous oxide then combines with the sand and forms iron silicate in the form of a slag.
FeO + SiO2 → FeSiO3 (slag)
Some of the cuprous sulphide is also converted to cuprous oxide.
2Cu2S + 3O2→ 2Cu2O + 2SO2
This cuprous oxide then reacts with iron sulphide’ changing to cuprous sulphide and ferrous oxide.
Cu2O+ FeS → Cu2S + FeO
The ferrous oxide also combines with sand and forms iron silicate as slag. The slag is removed from the hole. The molten mass containing mostly cuprous sulphide with a little ferrous sulphide is called “matte” and is taken out front the hole at the bottom of furnace.
The molten “mate” is now transferred to a Bessemer converter which is a pear shaped furnace. It can be tilted in any position. It is fitted with pipes known as twyers through which sand and hot air is blown.
The molten metal is poured off into sand moulds by tilting the converter and allowed to stand. On cooling, the dissolved SO2 gas escapes out causing blisters on the surface of copper. Thus the copper metal obtained is called blister copper and is about 98% pure.
v. REFINING OF BLISTER COPPER
The refining of blister copper is done by an electrolytic method. The impure copper is cast into blocks which arc used as anode. The cathode is made up of thin sheets of pure copper. The cathode and anode are dipped in a solution of copper sulphate and dilute H2SO4. On passing an electric current, copper from the impure anode is oxidized and goes into solution as Cu+2 ions. These Cu+2 ions from the solution are reduced and pure copper is deposited on the cathode.
Impure Cu → Cu+2 + 2e (Anode)
Cu+2 + → Cu (Pure) (Cathode)
Impurities like nickel, zinc and iron pass into solution as sulphates. While impurities like silver and gold settle down undissolved as anode mud. The copper thus obtained is almost 100% pure.
HYDROMETALLURGY OF COPPER
This process is used for the extraction of copper from low grade sulfide ore. In this process, the crushed and ground ore in bulk (about 100,000 tons) is exposed to air and water in big tanks. The air in the presence of water gradually oxidizes copper and iron sulphides to their surphides. The process is completed in about one year.
The pale green liquid draining from the bottom contains copper sulphate which on treatment with iron, forms copper. CuSO4 + Fe → Cu + FeSO4
The precipitated copper is dried, melted and refined.