Copper Extraction: A Comprehensive Process Analysis
Copper Extraction Process
Copper is mainly extracted from its sulphide ores. The different steps involved in the extraction of copper from copper pyrites ore (CuFeS2) are:
1) Crushing and Pulverization
The sulphide ore is crushed in a big jaw crusher and pulverized in a ball mill.
2) Concentration
The powdered ore is concentrated by the froth flotation process. In this process, the powdered ore is mixed with water containing a little pine oil in a big tank and agitated by blowing air. The sulphide ore comes to the surface along with the froth, and impurities settle to the bottom. The impurities are also known as gangue. The concentrated ore is removed as froth for roasting.
3) Roasting
The concentrated ore is heated strongly in a current of air on the hearth of a reverberatory furnace. During roasting, the following changes take place:
- Moisture and volatile impurities are driven out.
- The impurities like S, P, As, etc., present in the ore are oxidized into their gaseous oxides and removed.
- The pyrites are converted to cuprous and ferrous sulphides.
- These sulphides are partially oxidized to cuprous oxides and ferrous oxides.
In this way, the roasted ore mainly contains a mixture of cuprous sulphide, cuprous oxide, ferrous sulphide, and ferrous oxide.
4) Smelting
The roasted ore is mixed with coke and sand and heated strongly in a water-jacketed blast furnace. The blast furnace is about 15-20 feet in height and 3-5 feet in diameter, made of steel plates lined inside with fire clay bricks. Hot air at 800°C is introduced from tuyeres near the base of the furnace. The coke burns, and a temperature of 1000°C is produced. At this temperature, ferrous sulphide is oxidized to ferrous oxide.
Alumina and some of the iron oxide combine with silica to form slag.
The slag, being lighter, floats on the surface, forming a layer removed through the slag hole from time to time.
During roasting, if any oxide of copper is formed, it combines with iron sulphide (FeS) and is converted back into Copper Sulphide (Cu2S).
In this process, FeS is not completely removed. Copper sulphide containing traces of FeS is called copper matte. The molten matte forms the lower layer, which is taken out from the tapping hole at the bottom.
5) Bessemerization
The molten matte is mixed with a little silica and charged into an egg-shaped Bessemer converter. The converter is made of steel and is lined inside with lime and magnesium oxide. A hot blast of air mixed with sand is blown into the molten mass through the side tuyeres. If any sulphur, arsenic, etc., are still present, they escape as their respective oxides. Ferrous sulphide (FeS) present in matte is oxidized to ferrous oxide (FeO), which combines with silica to form slag.
When all the iron has been removed as slag, some of the cuprous sulphide undergoes oxidation to form cuprous oxide. At this stage, Cu2S and Cu2O react to produce metallic copper and SO2.
When the reaction completes, the converter is tilted, and molten copper is poured into molds. As the metal solidifies, the dissolved sulphur dioxide escapes, producing blisters on the metal surface. The product so obtained is known as blister copper. It is about 98% pure.
6) Refining
Blister copper contains about 2% of impurities and can be further purified by:
1) Poling
In this process, blister copper is melted and stirred with green poles of wood. The hydrocarbons present in these poles reduce the cuprous oxide to metallic copper. Copper thus obtained is 99.5% pure.
2) Electrolytic Refining
Impurities like Ag, Au, Ni, Zn, Fe, etc., can be removed by the electrolytic method. In this process, a thin sheet of pure copper is made as the cathode, and a block of crude copper is made as the anode. Both electrodes are placed in an acidified copper sulphate solution (CuSO4, 15% + H2SO4, 5%). When electric current is passed through the solution, impure copper from the anode goes into the solution, and pure copper from the solution gets deposited at the cathode. The impurities like Zn, Ni, Fe, Ag, etc., are collected below the anode as anode mud. Thus, the Cu obtained is 99.99% pure.