Extraction of Copper

Extraction of Copper

INTRODUCTION

 Copper is a metallic element of great technological and historical importance. It is valued for its strength, malleability, ductility, corrosion resistance and its ability to conduct electricity and heat.
 Copper is fairly abundant and has been used by man since prehistoric times for a wide variety of purposes. It was discovered and smelted from the oxidised ores to metallic copper by the primitive man.

ORES
Ores of copper are classified as:
Sulphide ores Oxidized ores
(i) Chalcocite, Cu2S (i)Malachite, CuCO3Cu(OH)2
(ii) Chalcopyrite, CuFeS2 (ii)Cuprite, Cu2O
(iii) Bornite, Cu2S.CuS.FeS, (iii)Brochantite, CuSO4.3CuOH2
etc.

OCCURRENCE
The largest and the test known deposits are
(i) the porphyry copper of North and South America.
(ii) the sulphide ores of Butte, Montana, and
(iii) the oxide and sulphide ores of Northern Rhodesia and the Congo.
Chile, Canada and U.S.S.R. are also importance producer of copper.

MINING
The two major methods of obtaining copper ores from the ground are
(i) Open pit mining
(ii) Underground mining
Of these two methods, open pit mining is more extensively used today, since its costs are substantially less than those of underground mining.

ORE CRUSHING
The size of the pieces of ore is reduced, first of all to about that of fine sand. Primary crushing and secondary reduction processes may be used for the purpose.

EXTRACTION
Various steps involved in getting metallic copper from copper ore are:
1 Concentration. 2 Roasting.
3 Smelting. 4 Converting.
5 Refining.

1. Concentration
Since the ore obtained from most mines is quite low in copper content (about 1% by weight), the ore is treated to remove a substantial part of the nonessential material.
Concentration implies elimination of the waste material or gangue present in the ore in order to increase the metal content of the product.

Concentration operation may include
(a) Crushing of ore (b) Grinding
(c) Classification (sizing) (d) Flotation
(e) Drying.

Froth Flotation Process
(i) Froth flotation process is used to separate the useful material from the waste matter.
(ii) A flotation cell operated by compressed air.
(iii) The cell contains water. From the left hand side the conditioned feed (i.e. ground ore) is fed to the cell. The ore before being fed into the cell is treated with a substance called a collecter or promoter, which makes the sulphide ore nonwettable by water without affecting the wettability of the waste material (gangue).
(iv) When air is blown into water, bubbles form and rise to the surface. The mineral (ore) particles adhere to these bubbles and carried to the top of the cell where the useful material is collected and removed. (Since the ore surface has already been made nonwettable, it has an increased tendency to adhere to a bubble).
In order that the bubbles retain their form after reaching the surface of water, a frothing agent is added. It is usually an organic compound of high molecular weight and of relatively low solubility of water; Pine oil and cresylic acid are two substances frequently used for the purpose.
(v) When the bubbles reach the surface, there is no longer water between them, and the bubbles, with the particles of mineral adhering to them, coalesce into a froth.
(vi) The froth is scraped from the top of each cell and subsequently is thickened, filtered, and dried, before roasting.
(vii) The waste matter called gangue gets wetted by water and is removed from the right hand side of the cell as fine and coarse non-floating particles.
(viii) Flotation process results in the elimination of so much unwanted matter that the copper content rises as high as 30-40%, as compared with about 1 % or even less in the original ore.

2. Roasting
Roasting is the operation of heating sulphide ores in air to convert to oxide.
The dried concentrated ore as received from the flotation process is roasted in a multiple-hearth furnace.
(a) Excess sulphur and such elements as arsenic and antimony which form volatile oxides burn off.
(b) Much of the iron sulphide is converted to iron oxide.
(c) Only the desired % of sulphur remains in the concentrated ore.
(d) Ore particles do not fuse.
Roaster reactions:
2Sb2S3 + 9O2 = 2Sb2O3 + 6SO2
2As2S3 + 9O2 = 2As2O3 + 6SO2
FeS2 = FeS + S
S + O2 = SO2
2FeS + 3O2 = 2FeO + 2SO3

3. Smelting
Smelting is a metallurgical operation in which the metal sought is separated in a state of fusion from the impurities with which it may be chemically combined or physically mixed.
Smelting is carried out in a reverberatory furnace heated by coal, oil or gas to a temperature between 1370C and 1480C.
The calcined ore melts and much of the unwanted matter is changed into slag. All copper compounds are converted to cuprous sulphide, and a considerable portion of inn changes to ferrous sulphide.
Cu2O + FeS = Cu2S + FeO
Cuprous sulphide and ferrous sulphide dissolve in each other in the molten state to form matte, which contains about 38 to 45% of copper.
That portion of iron which does not form sulphide remains as iron oxide, and enters the slag with alumina. lime and silica.
Slag being less dense floats over the matte and is periodically removed from the smelting operation. Matte can also be tapped off when required.

4. Converting
Converting operation produces raw (blister) copper from matte which is the product (of the reverberatory furnace) from smelting operation.
For converting operation; two types of converters are used.
(i) The Peirce-Smith converter, and
(ii) The Great Falls converter, both of side-blown construction.
The Peirce-Smith Converter is more favoured. It produces about 60-80 tons of copper per operation.
The conversion of copper matte is carried out in two stages.
(i) During the first stage iron sulphide is decomposed.
2FeS + 3O2 2FeO + 2SO2
The iron is oxidized by the air forced through the converter, and sulphur burns to sulphur dioxide, which escapes from the mouth of the converter.
Raw gold and silver ores, added at intervals, supply the alumina and silica required to combine with the ferrous oxide to form a slag.
FeO + SiO2 2FeSiO3
As the colour of the flame coming out of the mouth of the converter changes to violet, it is the indication that iron has been oxidized and entered the slag leaving cuprous sulphide in the furnace. At this, the air blast is shut off, converter is tilted and slag removed.
(ii) The second stage starts after the slag is removed and the converter is tilted up. The air blast is turned on. The cuprous sulphide starts decomposing and the sulphur burns to sulphur dioxide, leaving metallic copper in the converter. Nearly colourless flame from the mouth of the converter indicates the completion of this reaction.
Cu2S + O2 2Cu + SO2
At this stage if an iron bar is dipped into the melt in the converter, a solidifying copper shell on the iron bar can be noticed.
The molten copper thus obtained may either directly be sent for refining or may be cast for shipment to a refinery.
As the molten copper solidifies, ‘a considerable quantity of gas is given off. The gas bubbles break at the surface of the copper and leave marks resembling those of broken blisters. That is why, the copper is known as Blister Copper.

5. Refining
Blister copper is approximately 98.8% pure. It contains sulphur, iron and arsenic as the main impurities. Sulphur prevents making a usable casting; other impurities reduce electric conductivity and affect malleability.

Blister copper is refined by two processes
(a) Fire refining
(b) Electrolytic refining.
(a)Fire Refining

The fire refining is similar to the process involved in ,smelting
Fire refining involves subjecting a liquid melt to slow, limited oxidation so that the impurities are oxidized but only a minimum of the metal oxidizes.
A reverberatory furnace is used for fire refining purposes.
The blister copper is melted down and then oxidized by blowing compressed air through steel pipes inserted in the molten copper bath. A certain amount of oxidation also occurs during melting.
Some of the impurities are preferentially oxidized and rise out of the copper to form a slag with the flux, generally silica or lime. The slag is skimmed off at intervals.
Oxidation is continued until the copper is considerably oxidized, up to about 0.9% oxygen. When the oxidation is finished, the bath is skimmed and good-grade coke is thrown on the surface. The metal is then vigorously stirred with large poles of green wood. The wooden poles reduce remaining oxides by the reaction of the carbon in the wooden poles, leasing a relatively pure copper product (about 93.3% pure).

(b)Electrolytic Refining
Fire refined copper is suitable for many applications. For others, especially in electrical work, it must be subjected to electrolytic refining, which leaves it 99.95% pure. Fire refining neither effectively removes impurities such as bismuth, tellurium, selenium and nickel nor it recovers the precious metals.
Electrolytic refining is used to make good these deficiencies.

Electrolytic refining is carried out in a tank made of concrete and lined with antimonial lead. The fee refined copper, cast into a suitable shape, (anode) is suspended in a solution (electrolyte) composed of sulphuric acid and copper sulphate. On the passage of current, pure copper from anode passes into solution and is deposited on the cathode which is usually a specially prepared copper sheet.
The impurities associated with the fire refined copper do not deposit on the cathode, but either go into solution or else are deposited on the bottom of the tank.
During electrolytic refining, since the electrolyte becomes contaminated by the impurities, it is necessary to replace a portion of the same by fresh solution to maintain its chemical composition within proper limits.