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Extraction of Copper

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Copper manufacturing

Theoretical copper 

Chalcopyrite CuFeS­4 (34.5),

chalcocite Cu­2S (79.8),

bornite Cu5FeS­4 (55.5),

covellite CuS(66.5)

enargite CuAsS­4(48.4),

malachite, CuCO3. Cu (OH)2 ( 57.3),

azurite, 2CuCO­3.Cu (OH)­2  (55.1)

cuprite Cu­2O (88.8) andScienceCopper manufacturing

chrysocolla CuSiO­3.2H­2O (36.2),

 of the aforestated minerals, chalcopyrite CuFeS2 is the commercially the most important ore.

 Large quantities of copper, the actual ores containing only a small amount of copper because of contamination by other sulphides and the gangue. In fact, a sulphide ore that is 0.5-2.0 per cent copper is considered satisfactory for copper extraction by concentration followed by pyrometallurgy.  In the case of poorer grades,copper is extracted by hydrometallurgical processes.

EXTRACTION OF COPPER FROM SULPHIDE ORES

steps involved in the extraction of the copper by the cooventional route

namely,

1.Concentration, 

2. Roasting,

3.Smelting

4.Converting

5.Refining

1.Concentration:

Sulphide ore usually contains 0.5-2 per cent copper. 

Initially, this ore is crushing and grinding in order to liberate the sulphide grains from the gangue.  The particle size of the ore after grinding should be 40 um.

 Next, the ground ore is subjected to froth flotation.  During froth flotation, the pH is controlled by the addition of CaO and a xanthate reagent as used as a collector. lead sulphide, and zinc sulphide, a differential flotation procedure is employed to selectively recover the copper sulphide. 

The copper sulphide concentrate is thus produced contains

Cu- 15-35 per cent

Fe- 15-35 per cent,

S- 25-35 per cent , and

Gangue- 3-15 per cent.

 

2.Rosting:

The purpose of the roasting is to partially oxidize the iron sulphide present in the copper sulphide concentration in order to facilitate its removal in the form of slag in the smelting stage.

Roasting determines the copper grade of the matte (i.e., the percentage copper i produced in the smelting stage, which, in turn, determines the amount of copper grade of the matte, therefore concentrates for a high iron sulphide content and a low copper content ( <25 per cent Cu), roasting is resorted to, but for high-grade copper concentrates(>30 per cent Cu), roasting is often not necessary

 

In conventional plants, roasting is carried out in multiple hearth roasters with the feed travelling from one hearth to the other .  The temperature of each hearth is gradually  increased by fuel firing so that the charge gets heated up from the room temperature 550 ° C. 

In the newer plants, fluidized bed roasters , operating at about 550 ° C are employed.  Not only does this provide a more uniform feed for smelting purposes  but also produces a concentrated SO2  gas stream which can be diverted for the manufacture of sulphuric acid.

  During roasting.  Several reactions take place simultaneously.  The roasted calcine consists of copper and iron, oxides of iron and mixedminhatesaf concerts and iron.  Someon suphides or Maypical reactions are

2CuFeS + 6.5O2  ---à2Cuo + Fe2O3+ 4SO2. 

CuFeS + 4O2  ---à2CuSO4 + FeSO4

3CuFeS2+ 9.5O2 ---à 3Cuo + Fe3O4+6SO2.

 6CuFeS2+ 13O­­2--------à 3CuS + 2Fe3O4 + 9SO2. 

 The roasted calcine is transferred to the smelting furnace in the hot condition itself.

 

3.Smelting:

Smelting seeks to separate the metal sulphides in an ore, Concentrate or calcine from the gangue. Such a separation can be achieved by smelting the chare with a suitable flux at a temperature about 1250 ° C.  During smelting, two layers of liquid are formed, namely, an upper layer that consists of the gngue and the flux and a lower matte layer that contains metallic sulphides. 

The specific gravity of the slag layer ranges from 2.8gm /cc   to3.8gm / cc, whereas that of the Matte layer ranges from5gm /cc  to 5.5gm / cc.  This difference in specific gravities permits a clear-cut separation of one phase from the other.The gangue and the iron orides present in the concentrate / calcine to be smelted together with each other during smelting to form an iron silicate slag.  If the amount of iron oxides is insufficient, lime is added as a flux to form an iron - calcium silicate slag.  In most instances, a siliceous flux is needed to provide a low melting slag.  The melting points of the slags  thus formed are approximately 1150 ° C, and to obtain afluid slag with a low copper content, the smelting is carried out at a temperature of about 1250 ° C. 

During smelting, both the copper sulphide minerals and the slag met.  In addition, exchange motions take place between the oxides and sulphates of copper and iron sulphide present in the furnace charge:

6CuO + 4FeS → 3Cu2S + 4FeO + SO2,

2CuSO4+ 2FeS --à Cu2S + 2FeO+ 3SO2,

Cu2o + FeS → Cu2S + FeO. 

These reactions occur because oxygen has a higher affinity for iron than for copper. The unoxidized iron sulphide reduces the oxidation of iron to ferrous oxide as shown by the reactions.

 

10Fe2O3 + Fes   -à 7Fe3O4  + 5O2

 

the resuliant ferrous oxide (FeO) and magnetite (Fe3O4) react with the flux (silica or lime) to form a slag. 

Generally, be object to produce a matte that contains 35 - 45 per cent,20-22 per cent S and 25 - 35 per cent Fe.This not only minimizes the loss of copper tote also provides a matte.  W.

Conventionally, a smelting operation is carried out in reverberatory furnaces with either coal or oil.  Smelting has also been carried out in electric furnaces.  An electric.  Furnace is more advantageous than a reverberatory furnace if hydroelectric power is available freely and inexpensively because a large volume of combustion gases is avoided.  |  This facilitates both the recovery of so, and the cleaning of the furnace gases, which is usually carried out by our electrostatic precipitator in order to recover the copper-bearing dust.  However, an electric furnace consumes a large amount of energy, when fossil fuel is burnt especially to Renerate electricity. 

 

4.Converting:

The purpose of converting is to remove iron,sulphur and other impurities from matte.For this, the molten matte is produced as a result of smelting is charged into a side - blown converter which is a cylindrical vessel with a  capacity of 100 - 200 tons of matte.. A typical vessel is 4m in Diameter and 9m in length and is lined with a layer of chrome - magnesite refractory (about 40m thick).

 

In the converter, the atmosphere is highly oxidizing compared with the neutral or mildly oxidizing atmosphere during smelting. Air or oxygen-enriched air (up to a maximum of 32 vol% oxygen in the blast) is injected into the molten matte through the tuyeres.  tuyeres is about 600 m3/ min. The products of the converter are sag and blister copper.

 

4.a.Slaging stage:

In the  slagging stage,the iron sulphide present in the matte is oxidized and the oxide is slagged out by the addition of a siliceous flux.The reactions are

2FeS + 3O2 -à2FeO+ 2SO2. 

2FeO+ SiO2 à2FeO.SO2, (Fayalite).

 Part of FeO IS also oxidized to magnetite.

  Slagging is carried  out in stages, i.  e, by first adding freshly obtained furnace matte to the converter and then blowing air.  Next, the supernatant slagf formed is skimmed off by tilting the cylindrical converter .  The molten slag and matte phases are an immiscible. 

During converting,  the oxidation of the iron sulphide generates sufficient heat to overcome the heat lost to the surroundings and to maintain the matte and slag .  It may be noted that in a large converter, where the blowing rate is high consequently iron sulphide oxidation rates are also high, extra heat is generated, which is utilized for melting scrap copper or cement copper is commonly produced in the plant.

The slag achieved typically analyzes 2 – 9 per cent Cu, 40 - 50 per cent Fe, 20 – 30 per cent SiO2 and  1 - 5 per cent (Cao + Mgo).  The iron oxide is produced as a result of FeS oxidation is present in the slag mostly as fayalite saturated with magnetite.  The magnatite forms a  coating on the chrome - magnesite refractory, providing some protection for the refraractory against attack by the slag.

 4.b.Blister formation stage:

After slagging has been completed, the converter is essentially contains Cu2S which is called white metal because of its appearance.  In the blister copper formation stage, Cu2S is oxidized to form copper by a combination of reactions

2Cu2S (l) + 3O2 (g) à2Cu2O(l) + 2SO2(g),

Cu2S(l) + 2Cu2O(l)à 6Cu (l) + SO2(g) 

The overall reaction is

 3Cu2S+ 3O2à6Cu + 3SO2 

We see that the cuprous sulphide is reduced without any reducing agent. When white metal (essentially Cu2S)  is oxidized and when the quantity of sulfur in the bath decreases to about  19.5 per cent, the bath splits into two layers, i.c, a top sulphide layer (less dense) and a bottom copper layer (more dense) containing about 1.2 per cent sulfur . As the oxidation continues, the volume of the sulphide layer  (top layer ) decreases and the volume of the copper layer(bottom layer) increases.The relative volumes of the top two layers can be determined by the Iever rule.  When the sulfur level eventually reaches 1.2 per cent, only  metallc copper remains. At this stage, care is exercised to ensure that the metal is not overoxidized to Cu2O.  The completion of the blow can be determined by casting a small sample of the copper and examining the fracture of this sample.  The blistery appearance of the sample lends the name blister copper to this product.  In the industrial practice, the blister copper contains .02 - 0.05 per cent S along with 0.2 – 0.5 per cent dissolved oxygen.

 

Freside - Blowne The presence of Buperiments has fineness for the Lorprocious mer J / the byproduct moontent or mplayed for ojustify electro pitallic phase EXTRACTION OF METALS FROM SULPHIDE ORIS 351 lawn converter blows air into the sulphide phase and not into the blister copper.  During the two phases of the ions the copper of the conversion can be taken advantage nts showing that the precious metals and impurities such as As, Bi, and Sb have an affinity  for the metallic copper.  Consequently, in the subsequent electrolytic refining step  the precious metals and impurity elements can be recovered as valuable byproducts.  The value byproducts compensates for is  the high cost incurred during the refining operation.If  the content of precious metals in the blister copper is low, it can be directly fire refined and employed for alloying purposes. 

5.Refining:

The purpose of refining- the final step in copper extraction - is twofold.  First, to obtain the metal in a purer form (for a higher degree of electrical conductivity) and second, to recover the valuable precious metals contained in the blister copper produced as a result of converting. 

5.a.Fire refining:

  The main purpose of fire refining  is to remove sulphure from  liquid blister copper as SO2 by oxidation with air and to eliminate oxygen by introducing hydrocarbons.

 Fire refining is the first step in refining and is conducted in a reverberatory furnace with a capacity of up to 400 tons of copper.The fuel used is fuel oil or natural gas or pulverized coal, the oil consumption being 10-11 per cent of the weight of the charge (  blister copper). The time taken for the charge to be refined is 12 - 16 hours.  The reverts from the fire - refining fornace contain an appreciable amount of copper and is re-treated with the matte in a converter for recovering the copper. 

In the refining furnace, the surface of the blister copper is oxidized at frequent intervals.The  furnace doors are kept open in order to allow a constant mild blast of air.  As a result of this slow oxidation process (2-4 hour duration), impurities such as S, Fe, Se, and Zn are oxidized and the solid oxides rise to the top where they are skimmed off.Some copper is also oxidized during the process;  There is, however, a limit to which such oxidation may be permitted.Normally, the process is discontinued when the level of Cu2O in the bulk of the Cu2O in the bulk of the metal reaches 6 per cent.At this stage, almost all the sulfur is eliminated from the metal.Once the oxidation is complete, the oxidized copper is reduced by poling with green branches.  These branches, on ignition, give off hydrocarbons which not only stir up the entire molten bath but also  create a reducing atmosphere which helps reduce the Cu2O.   The purity of the fire-refined copper is 99.7 per cent.

5.b.Electrolytic refining:

Fire-refined copper can be further refined by electrolysis. An electrolytic refining tank(cell) is made of either concrete or wood and is well insulated.  Each tank is 3 - 5m long,  1 – 1.1m wide and 1 – 1.3m deep.  The design of the cell is based on the utilization of the minimum cathode and anode areas.  During electrolysis, a number of such tanks arranged in a cascade formation. Fire-refined copper is in the form of an anode that weighs in 250 – 320 kg.  The cathode is   made of a pure copper sheet that is greased in order to facilitate the subscquent stripping of the metal deposited on it.  The clectrolyte consisted of copper sulphate (Cu 35 gm / litre,H2SO4 (200 gm /litre)), and some addition agents such as glue and alcohol.Electrolysis is carried out at a temperature of 50-60oC using a current density of about 200 A/m2.

  During electrolysis,the copper is transferred from crude anode to the pure cathode. Impurities in the blister copper such as Fe, Co, Ni, Se, and Te go into solution and the precious metals collect below the anod in what is known as the anode slime.  As the electrolys is proceeds,some difficulties are likely to be encountered.  For example, in the vicinity of the cathode, the concentration of Cu2+ ions decreases and the water in the electrolyte  is electrolyzed, releasing  hydrogen which lowers the current efficiency.On the other hand,vicinity of the anode,the concentration of Cu2+  ions increases and CuSO4 may tend to crystallize out.These setbacks can be overcome by the proper stirring and circulation of the electrolyte.

It should be noted that the CuSO4  content of the electrolyte increases beyond a certain limit as a result of the dissolution of the residual Cu2O in blister copper, it may become necessary to  clectrolyze the electrolyte using an insoluble inert anode made of lead.  This procedure would bring down the CuSO4  content of the electrolyte.  In addition to removal of the copper from the electrolyte, impurities such as As, Sb, Bi, and Ni have to be removed.  As, Sb, and Bi are removed during the last phase of copper refining with an insoluble inert lead anode.  Nickel is removed with iron and cobalt as a sulphate from the copper - free bleed - off.  The volume of the bleed solution, which has to be purified, varies depending on the impurity content of the anode and lies in the range 0.2 - 0.6m3 / ton of copper produced.

 The approximate composition of the anode slime is as follows:

Cu = 10 - 40 per cent. 

Ag = 8500 - 14, 000 gm / ton,

Au = 250 - 1700 gm / ton,

Pb = 2 - 20 per cent,

As = 0.  5 - 5 per cent,

Te = 0 - 4 per cent,

Ni - 0 - 25 per cent,

S - 2 - 10 per cer

Fe = 0.  5 - 2 per cent,

and Se = 0 - 25. 

 

 

 

 

 

 

 

 

 

 

 

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