COPPER EXTRACTION

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COPPER: PROPERTIES AND APPLICATIONS
Most important non-ferrous metal
Unique set of properties
High electrical conductivity
High thermal conductivity
Adequate mechanical properties
High corrosion resistance
High scrap value
Amenable to fabrication by various
techniques

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MINERALS
Copper mostly exists as sulphides
Chalcopyrite
Oxides and oxidized ores are limited
Common minerals of Cu
Chalcopyrite (CuFeS2), Chalcocite (Cu2S), Bornite (Cu5FeS4), Covellite (CuS), Enargite
(CuAsS4), Cuprite (Cu2O)
Although, these minerals contain large quantity of Cu, actual ore contains 0.5-2% Cu due
to contamination by other sulphides and gangue
0.5-2% Cu considered satisfactory for Cu extraction by pyrometallurgy
For poorer grades of ores hydrometallurgy is used

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CONCENTRATION
Subjected to crushing and grinding in order to liberate sulphide grains from the
gangue
Average particle size of ore after grinding - 40µm
Ground ore is subjected to froth floatation
Differential flotation for ores containing copper sulphide, zinc sulphide and lead
sulphide
Copper concentrate produced contains 15-35% Cu, 15-35% Fe, 25-35% S and 3-
15% gangue

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FROTH FLOATATION
The finely crushed ore is concentrated by Froth-
Floatation process.
The finely crushed ore is suspended in water
containing a little amount of pine oil.
A blast of air is passed through the suspension.
The particles get wetted by the oil and float as a
froth which is skimmed.
The gangue sinks to the bottom

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ROASTING
Purpose: Partially oxidise iron sulphide in copper sulphide concentrate to facilitate
its removal in the form of slag in the next stage i.e. smelting
Extent of roasting determines copper grade of matte produced in smelting stage
which in turn determines amount of copper lost to slag
For concentrates with high iron sulphide content and low copper sulphide content,
(<25% Cu) roasting is required
For high grade copper concentrates (>30% Cu), roasting is often not necessary

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ROASTING
Roasting is carried out in multiple hearth roasters and in newer plants in fluidized
bed roasters with operating temperatures at about 550oC
During roasting several reactions take place simultaneously
Roasted calcine consist of sulphides of copper and iron, oxides of iron and mixed
sulphides of copper and iron
It is transferred to smelting furnace in hot condition to facilitate the separation and
subsequent recovery of copper

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ROASTING REACTIONS

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SMELTING
Smelting seeks separation of metal sulphides in an ore, concentrate or calcine from
gangue
Performed at temperature 1250oC with suitable flux
During smelting two layers of liquid form
 Upper slag layer containing gangue and flux (Specific gravity = 2.8-3.8 g/cc)
 Lower matte layer containing metal sulphides (Specific gravity = 5-5.5 g/cc)
Difference in specific gravities permits clear cut separation
Gangue and iron oxide present in concentrate/calcine to be smelted to form iron
silicate slag
Siliceous flux is needed to provide low melting slag

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SMELTING
Melting point of slag is 1150oC, to obtain fluid slag with low copper content,
smelting is carried out at 1250oC
Exchange reactions take place between oxides and sulphates of copper and iron
sulphide in the charge
6CuO + 4FeS = 3Cu2S + 4FeO + SO2
2CuSO4 + 2FeS = Cu2S + 2FeO + 3SO2
Cu2O + FeS = Cu2S + FeO

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SMELTING
These reactions happen because oxygen has more affinity for iron than copper
Unoxidized iron sulphide reduces higher oxides of iron to ferrous oxide
Resultant FeO and Fe3O4 react with flux to form a slag

10 Fe2O3 + FeS = Fe3O4 + SO2

3Fe3O4 + FeS = 10FeO + SO2
2FeO + SiO2 = Fe2SiO4

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SMELTING
Objective is to produce a matte that contains 35-45% Cu, 20-22% S and 25-35%
Fe
This not only minimizes loss of copper in slag but also provides matte with a
sufficient quantity of iron sulphide for the use in next stage i.e. converting
Iron sulphide oxidation provides all the heat required to ensure autogenous
converting process

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CU LOSS IN SLAG VS CU IN MATTE

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CONVERTING
Purpose is to remove iron, sulphur and other impurities from matte
Molten matte is charged into side blown convertor which is a cylindrical vessel with
capacity of 100-200 tons of matte
Atmosphere is highly oxidizing as compared to the mildly oxidizing or neutral during
smelting
Air or oxygen enriched air (~32 vol% O2)
- injected into molten matte through tuyeres
40 tuyers – each 5cm in diameter
Volume of gas through tuyeres - 600 m3/min
Products - slag and blister copper
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STAGES IN CONVERTING : SLAGGING STAGE
Two distinct stages in converting namely slagging stage and blister formation stage
Iron sulphide present in matte is oxidized and oxide is slagged out by addition of a
siliceous flux

2FeS + 3O2 = 2FeO + 2SO2

2FeO + SiO2 = 2FeO.SiO2

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SLAGGING
Slagging is carried out in stages – adding freshly prepared matte to convertor and
then blowing air
Supernatant slag is skimmed off by tilting cylindrical convertor
Molten slag and matte phases are immiscible

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SLAGGING
Oxidation of iron sulphide generates sufficient heat to overcome heat lost to
surrounding
It also maintains matte and slag in molten state in convertor
Slag obtained typically analyses 2-9% Cu, 40-50% Fe, 20-30% SiO2 and 1-5%
(CaO+MgO)
Cu is recovered from slag produced in the convertor by transferring slag to smelting
furnace
Any matte particles present in the slag settle down at the bottom and slag can be
discarded

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BLISTER FORMATION STAGE
After slagging, convertor contains Cu2S which is called White Metal because of the
appearance
Cuprous sulphide is reduced without any reducing agent
In blister copper formation stage, Cu2S is converted to form copper by a
combination of reactions
Blister formation
2Cu2S (l) + 3O2 (g)  2Cu2O + 2SO2 (g)
Cu2S (l) + 2Cu2O (l)  6Cu(l) + SO2 (g)
Overall: 3Cu2S + 3O2  6Cu + 3 SO2

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BLISTER FORMATION STAGE
When white metal (Cu2S) is oxidized and when
quantity of S reaches 19.5%, bath splits into two
layers:
 top sulphide layer and
 bottom copper layer containing about 1.2%
sulphur
As oxidation continues, volume of sulphide
layer decreases and that of copper layer
increases
When sulphur level eventually reaches 1.2%,
only metallic copper phase remains
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BLISTER FORMATION STAGE Blister copper

At this stage, care should be taken to ensure

that, metal is not overoxidized to Cu2O
Completion of blow can be determined by
casting a small sample of copper and examining
the fractured surface
Blistery appearance of the sample lends the
name blister copper
Blister copper produced contains 0.02-0.05% S
along with 0.2-0.5% dissolved oxygen

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REFINING
Final step in copper extraction – two fold process
1. Obtain metal in a purer form
2. Recover valuable precious metals contained in blister copper
Fire refining – to remove S from liquid blister copper as SO2 by oxidation with air
and to eliminate oxygen by introducing hydrocarbons
Conducted in reverberatory furnace
In refining, surface of the blister copper is oxidized at frequent intervals
Impurities such as S, Fe, Se, Zn are oxidized and solid oxides rise to the top where
they are skimmed off

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FIRE REFINING
Blister Cu contains 97-99% Cu with impurities: Fe, Co, Ni, Pb, Sn, As, Sb, Au, Ag
Cu is fire refined in reverberatory furnace at temperatures ranging from 1100oC -
1200oC
In oxidation of molten Cu, most of the impurities are removed from Cu as they float
on its surface as oxides which eventually form slag
At the end of refining and sometimes in the midst, slags are skimmed off the surface
of the bath
After this, melt is deoxidized to remove remaining cuprous oxide and finished copper
being tapped though taphole and cast into moulds
MECHANISM
Oxygen is expended mainly for interaction with copper as it is in much grater amount
as compared to other impurities
Cu2O is formed, as CuO cannot exist at temperatures above 1030oC in the air
4Cu + O2 = 2Cu2O
Cuprous oxide formed on the surface gets dissolved in copper and thus carried all
over the bath
The rate of this process is governed by intensity of stirring of bath as air is blown
into it
FIRE REFINING
Generally, content of oxygen in fire refining of blister copper is raised to 1.3-1.5%
(11.5-13.5% Cu2O) which maintains copper melt saturated with Cu2O
Impurities dissolved in copper are then oxidized with cuprous oxide which saturates
copper
Cu2O + Me = MeO +2 Cu
Oxides of impurities are formed and float on the surface of the bath giving rise to
slag composed of oxides of impurities, cuprous oxide and furnace lining
FIRE REFINING
Once the oxidation is complete, oxidized copper is reduced by Poling with green
branches
These branches produce hydrocarbons which stir up entire molten bath and create
reducing atmosphere to reduce Cu2O
Recently some new techniques have been developed to bring about reduction of
Cu2O with hydrogen and other reducing agents
Purity of fire refined Cu is usually 99.7%

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ELECTROLYTIC REFINING
Fire refined copper can be further refined by electrolysis
Major portion of the refined copper is utilized in manufacturing of electric wire
Copper anodes in tank of CuSO4 and H2SO4
Cathodes – Parallel sheets of pure Cu
Potential applied ~ 0.34V
Zn, Fe, Ni, As, Sb – pass into electrolyte
Metals more noble than Cu are Ag, Au and compounds like Cu2S, Cu2Se, Cu2Te settle
as anode slime

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