Chemistry · Chapter 27
Study notes aligned to the official NEB syllabus.
Copper is one of the earliest metals used by humans and remains industrially important because of its excellent electrical and thermal conductivity, malleability and ductility. It occurs both in the native (free) state and, more abundantly, in the combined state as sulphide, oxide and carbonate ores. This unit covers the occurrence of copper, its extraction from copper pyrites, its physical and chemical properties, its alloys, and its important compounds.
Copper occurs in nature both as the free metal and combined with other elements. The principal ores of copper are:
Since copper pyrites is by far the most widely worked ore, the extraction process described below is based on it.
Crushing and pulverisation. The lumps of copper pyrites mined from the earth are first broken down in a jaw crusher and then ground to a fine powder in ball mills.
Concentration. As the ore is a sulphide, it is concentrated by the froth flotation process. The powdered ore is mixed with water and a small amount of pine oil in a large tank, and a stream of air is blown through the mixture. The sulphide ore particles are wetted by the oil and rise to the surface as a froth, while the heavier earthy and siliceous impurities (gangue) settle down and are removed.
Roasting. The concentrated ore is heated strongly in a reverberatory furnace in the presence of excess air. Roasting serves two purposes: it drives off volatile impurities and it converts part of the ore into sulphides that are easier to smelt.
Sulphur, arsenic and phosphorus present as impurities escape as their volatile oxides: $$ \begin{aligned} S + O_2 \rightarrow SO_2 \ 4As + 3O_2 \rightarrow 2As_2O_3 \ 4P + 5O_2 \rightarrow 2P_2O_5 \end{aligned} $$
Copper pyrites itself is partly converted into a mixture of copper(I) sulphide and iron(II) sulphide: $$2CuFeS_2 + O_2 \rightarrow Cu_2S + 2FeS + SO_2$$
A part of the iron(II) sulphide and copper(I) sulphide so formed is further oxidised: $$ \begin{aligned} 2FeS + 3O_2 \rightarrow 2FeO + 2SO_2 \ 2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2 \end{aligned} $$
Smelting. The roasted ore is mixed with coke and sand (silica) and charged into a blast furnace, where a hot blast of air is blown in from the lower part of the furnace. Inside the furnace, the ferrous oxide formed during roasting combines with the added silica to form fusible iron(II) silicate slag, which floats above the heavier molten sulphides and is run off separately: $$ \begin{aligned} 2FeS + 3O_2 \rightarrow 2FeO + 2SO_2 \ FeO + SiO_2 \rightarrow FeSiO_3 \ (\text{slag}) \end{aligned} $$
Some of the copper(I) oxide formed reacts back with any remaining iron(II) sulphide, regenerating copper(I) sulphide and more iron(II) oxide, which again slags off with silica: $$ \begin{aligned} Cu_2O + FeS \rightarrow Cu_2S + FeO \ FeO + SiO_2 \rightarrow FeSiO_3 \ (\text{slag}) \end{aligned} $$
A small amount of copper(I) oxide is also reduced directly by the coke: $$Cu_2O + C \rightarrow 2Cu + CO$$
The molten slag, being lighter, collects as the top layer and is tapped off as waste, while the lower layer, a molten mixture of copper(I) sulphide with a little iron(II) sulphide, is tapped out separately. This mixture is called matte and contains about 50% copper.
Bessemerisation. The hot molten matte is transferred into a Bessemer converter, a pear-shaped furnace made of steel and lined inside with a basic refractory material such as lime or magnesia, and mounted on trunnions so that it can be tilted. A blast of hot air mixed with sand is blown in through tuyeres at the base.
The remaining iron(II) sulphide is oxidised to iron(II) oxide, which combines with the added silica to form slag, and this slag floats on top of the molten mass and is removed once all the iron has been eliminated: $$ \begin{aligned} 2FeS + 3O_2 \rightarrow 2FeO + 2SO_2 \ FeO + SiO_2 \rightarrow FeSiO_3 \ (\text{slag}) \end{aligned} $$
Once the iron has been removed, part of the remaining copper(I) sulphide is oxidised to copper(I) oxide, which then undergoes self-reduction with the rest of the copper(I) sulphide to give metallic copper: $$ \begin{aligned} 2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2 \ 2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2 \end{aligned} $$
The blast of air is then stopped, the converter is tilted, and the molten copper is poured out and allowed to cool. Dissolved $SO_2$ escapes as the metal solidifies, leaving blister-like marks on the surface, so the copper obtained at this stage (about 98% pure) is called blister copper.
Electrolytic refining. Blister copper is impure, containing traces of metals such as silver, gold, iron and zinc, and must be purified before use. A large block of impure blister copper is made the anode and a thin sheet of pure copper is made the cathode; both electrodes are dipped into a bath of copper sulphate solution acidified with a little dilute sulphuric acid.
When current is passed through the cell, copper dissolves from the impure anode and an equivalent amount of pure copper is deposited on the cathode: $$ \begin{aligned} \text{At anode: } Cu \rightarrow Cu^{2+} + 2e^- \ \text{At cathode: } Cu^{2+} + 2e^- \rightarrow Cu \end{aligned} $$
More reactive impurities such as iron and zinc pass into the solution as ions, while the more noble impurities, silver and gold, do not dissolve and fall below the anode as a valuable residue called anode mud. The copper deposited at the cathode by this method is about 99.9% pure.
Copper is one of the earliest metals used by humans and remains industrially important because of its excellent electrical and thermal conductivity, malleability and ductility. It occurs both in the native (free) state and, more abundantly, in the combined state as sulphide, oxide and carbonate ores. This unit covers the occurrence of copper, its extraction from copper pyrites, its physical and chemical properties, its alloys, and its important compounds.
Copper occurs in nature both as the free metal and combined with other elements. The principal ores of copper are:
Since copper pyrites is by far the most widely worked ore, the extraction process described below is based on it.
Crushing and pulverisation. The lumps of copper pyrites mined from the earth are first broken down in a jaw crusher and then ground to a fine powder in ball mills.
Concentration. As the ore is a sulphide, it is concentrated by the froth flotation process. The powdered ore is mixed with water and a small amount of pine oil in a large tank, and a stream of air is blown through the mixture. The sulphide ore particles are wetted by the oil and rise to the surface as a froth, while the heavier earthy and siliceous impurities (gangue) settle down and are removed.
Roasting. The concentrated ore is heated strongly in a reverberatory furnace in the presence of excess air. Roasting serves two purposes: it drives off volatile impurities and it converts part of the ore into sulphides that are easier to smelt.
Sulphur, arsenic and phosphorus present as impurities escape as their volatile oxides:
Copper pyrites itself is partly converted into a mixture of copper(I) sulphide and iron(II) sulphide:
A part of the iron(II) sulphide and copper(I) sulphide so formed is further oxidised:
Smelting. The roasted ore is mixed with coke and sand (silica) and charged into a blast furnace, where a hot blast of air is blown in from the lower part of the furnace. Inside the furnace, the ferrous oxide formed during roasting combines with the added silica to form fusible iron(II) silicate slag, which floats above the heavier molten sulphides and is run off separately:
Some of the copper(I) oxide formed reacts back with any remaining iron(II) sulphide, regenerating copper(I) sulphide and more iron(II) oxide, which again slags off with silica:
A small amount of copper(I) oxide is also reduced directly by the coke:
The molten slag, being lighter, collects as the top layer and is tapped off as waste, while the lower layer, a molten mixture of copper(I) sulphide with a little iron(II) sulphide, is tapped out separately. This mixture is called matte and contains about 50% copper.
Bessemerisation. The hot molten matte is transferred into a Bessemer converter, a pear-shaped furnace made of steel and lined inside with a basic refractory material such as lime or magnesia, and mounted on trunnions so that it can be tilted. A blast of hot air mixed with sand is blown in through tuyeres at the base.
The remaining iron(II) sulphide is oxidised to iron(II) oxide, which combines with the added silica to form slag, and this slag floats on top of the molten mass and is removed once all the iron has been eliminated:
Once the iron has been removed, part of the remaining copper(I) sulphide is oxidised to copper(I) oxide, which then undergoes self-reduction with the rest of the copper(I) sulphide to give metallic copper:
The blast of air is then stopped, the converter is tilted, and the molten copper is poured out and allowed to cool. Dissolved escapes as the metal solidifies, leaving blister-like marks on the surface, so the copper obtained at this stage (about 98% pure) is called blister copper.
Electrolytic refining. Blister copper is impure, containing traces of metals such as silver, gold, iron and zinc, and must be purified before use. A large block of impure blister copper is made the anode and a thin sheet of pure copper is made the cathode; both electrodes are dipped into a bath of copper sulphate solution acidified with a little dilute sulphuric acid.
When current is passed through the cell, copper dissolves from the impure anode and an equivalent amount of pure copper is deposited on the cathode:
More reactive impurities such as iron and zinc pass into the solution as ions, while the more noble impurities, silver and gold, do not dissolve and fall below the anode as a valuable residue called anode mud. The copper deposited at the cathode by this method is about 99.9% pure.