2. Cu
Ores of copper
Copper exists in nature together with Ni, Fe, Co, Zn etc. metal. The ores of
Copper mainly oxides and sulfides such as:
Name of ores Chemical Formula
i. Chalcocite or copper glace Cu2S
ii. Covellite CuS
iii. Chalcopyrite or copper pytites CuFeS2
iv. Bornite Cu5FeS4
v. Digenite Cu9S5
vi. Cuprite Cu2O
vii. Tenorite CuO
viii. Malachite CuCO3.Cu(OH)2
ix. Azurite 2CuCO3.Cu(OH)2
x. Bronchantite CuSO4.3Cu(OH)2
3. Hydrometallurgical extraction of copper
A variety of methos are available for carrying out the hydrometallurgical copper
recovery process but ‘Freeport process’ were choosed because, in this process,
each individual step is suited to the others and to the overall process for
manufacturing a high quality metallic copper product efficiently and
economically. This process also avoids air pollution due to the production of SO2.
Process description
In this process a water slurry of floatation concentrate of CuFeS2 along with some
recycle streams is fed on a continuous basis into an autoclave system operating
at about 425oF and 500 psig pressure.
Oxygen is feed into the first stage autoclave in which the copper and iron
suffides are oxidized to water soluble sulfates. The heat generated during this
step makes the reaction self-sustaining.
To the second stage autoclave a controlled amount of lime or limestone is added
to precipitate the dissolved iron and neutralize excess acid.
From the second stage autoclave an acid slurry containing essentially all the
copper dissolved as copper sulfate, with small amounts of iron and other
impurities in the solution and gangue elements in the solid phase is obtained.
4. The classified liquor from this operation is fed directly to electrolytic cells for
production of copper cathodes. Electrical efficiencies indicate that about 90% of
copper is recovered and the 10% not recovered is precipitated from a bleed
stream from the cells and recycled. The acid which is generated in the
electrolysis step can be used by the refinery for oxide leaching or neutralized
with limestone for disposal as calcium sulfate.
In the process there are essentially three key areas that required to be careful
for economic, engineering and technical evaluation, which are as follows:
i) Autocalve oxidation-neutralization
ii) Separation of copper leach liquor and solids
iii) Electrowining for efficiency and quality of copper.
6. The oxidation-neutralization step:
In this critical operation, it is necessary to dissolve nearly 100% of the copper and
also reject impurities.
For this operation, a chalcopyrite flotation concentrate, containing between
about 20-30% copper along with recycle wash water, containing residual amounts
of copper sulfate and sulfuric acid and recycle acidic slurry of copper sulfide is
fed continuously to the high temperature, high pressure operation which is
carried out in a multi-compartment, horizontal autoclave system equipped with
agitators. The slurry temperature is maintained by the introduction of high
quality oxygen so a to provide an oxygen overpressure of 50 to 200 psi.
The reactions that occur during this step are as follows-
The flotation concentrated (CuFeS2) is oxidized to copper sulfate and ferrous
sulfate and the recycle copper sulfide is oxidized to copper sulfate. The major
portion of heat evolved is generated in this step.
(i) CuFeS2 + 4O2 → CuSO4 + FeSO4 + heat
CuS + 2O2 → CuSO4 + heat
It is necessary to reject the major portion of the iron to prepare the solution for
the electrolytic cells. The initial steps of iron rejection involves, oxidation of iron
from Ferrous to Ferric.
(ii) 2FeSO4 + H2SO4 + 0.5 O2 → Fe2(SO4)3 + H2O
7. Ferric sulfate, when at temperature of about 425oF as exist in the autoclave, is
hydrolyzed to Ferric oxide and sulfuric acid.
(iii) Fe2(SO4)3 + 3H2O ⇌ Fe2O3 ↓ + 3H2SO4
Unfortunately, this reaction stops when the acid concentration gets too high, so
limestone is added to neutralize the acid.
(iv) CaCO3 + H2SO4 → CaSO4 + H2O + CO2
This results all the iron being converted to insoluble iron oxide while copper
sulfate remains unchanged throughout all of these steps. Overall reaction is
(v) CuSO4 + 3CaCO3 + Fe2(SO4)3 → Fe2O3 + 3CaSO4 + CuSO4 + CO2
Before leaving the slurry is kept for an additional 20 min of retention time in the
autoclave system at 425-450oF, under oxygen over pressure, with constant
mechanical agitation. During this time, additional iron is precipitated and further
oxidation & dissolution of copper may occur. For these, a vapor-space bleed-
stream is vented to atmosphere to purge gradual buildup of contaminant
nitrogen.
The stream leaving the slurry discharge end compartment of the autoclave made
up of a liquid phase composed of gangue, anhydrite and hematite. Before the
liquid phase is separated from these solids, the slurry would be subjected to a
flashing operation.
Flashing operation is an indirect cooling operation, so, exiting the autocalve, the
slurry is first cooled with water in an indirect heat exchanger to approximately
300-340oF, while maintaining same pressure.
8. In the process of being flashed to atmospheric pressure, additional water vapor is
lost and copper concentration of the liquor further increased. After flashing, the
temperature of the slurry should be approximately 230oF. Further cooling of the
liquor may be carried out before the separation of the liquid and solids.
Fig. Oxidation – Neutralization step.
9. Separation of Copper leach liquor and solids
The resultant cooled slurry is fed to a solids-liquid separation system, with the separated liquor going to
a cooling operation and the solids going to a washing systems. A barometric condenser may be used to
cool the stream.
In a barometric condenser, additional water is removed which further increases the copper
concentration of the leach liquor. By these steps, a copper sulfate solution with 60 to 70 g/L of copper is
obtained, with very low in iron and is free of impurities such as arsenic, bismuth and antimony.
In the washing system, the solids are washed with water to recover the retained copper sulfate solution
and the wash water is recycled back to the autoclave. The washed solids (tailing) may be sent to a
tailing pond or to further treatment for the recovery of gold and silver etc.
Fig. Solid separation & washing
10. Efficiency of the electrowinning step
In the electrolytic deposition process, carbon is used as anode and copper rod
used as cathod and liquor from leach solution containing 75 g/L Cu used as
electrolyte. When potential is applied upon the cell, the Cu2+ ions are deposited
on cathod surface as 99.9% copper-the electrowinning are carried out about 3
stage. Where the spent liquor from stage-1 us used as the electrolyte of stage-2
and similarly the spent liquor from stage-2 is used as the electrolyte of stage -3.
Heat is liberated during this operation and should be removed inorder to keep
the temperature in the electrolyte cells at about 150oF or less. The spent
electrolyte from third stage containing about 5 to 10 g/L copper and about 100 to
150 g/L H2SO4.
The spent solution is then treated with a sulfiding agent to precipitate the
copper sulfide. Any sulfiding agent such as hydrogen sulfide, ammonium sulfide
and sodium sulfide etc. may be used in this process.
Hydrogen sulfate is preferred because it causes precipitation of virtually all of
the copper present in the spent electrolyte. The resultant slurry is processed in a
thickener and the thickened acidic copper-sulfate slurry is recycled to the
autoclave. The acidic supernatant liquor stream from the thickener may be
treated with a limestone slurry and the resultant gypsum slurry is then pumped
to waste i.e., to holding ponds or land-fill areas.
11. Aluminium (Al)
Ores of Aluminium
Aluminium occupies first place among the metals available in nature. The
amount of Al in aluminium ores max 10-12% Al. Some ores are given below:
Name of ores Chemical Formula
i. Corundum Al2O3
ii. Diaspore Al2O3 or AlOOH
iii. Bauxite Al2O3.2H2O
iv. Gibbsite Al2O3.3H2O or Al(OH)3
v. Cryolite Na3AlF6
vi. Feldspar KAlSi3O8
vii. Kaoline Al2O3.2SiO2.2H2O
Uses of Al: Aluminium is one of the most important and widely used metals in the transport,
construction, packaging and electrical sectors. In transport, aluminium is used in cars (engine
blocks, cylinder heads, transmission housings and body panels), trucks and buses (sheet and plate
for bodies), in railway stock and in aircraft.
12. Extraction of Aluminium from its ores:
Now a days, aluminum is extracted from its bauxite ore. Bauxite designate
various kind of aluminium ores consisting mainly of aluminium hydroxide. There
are three types of Al(OH)3 which occur in bauxite, gibbsite, bohmite and
diaspore. They differ considerable in their physical properties such as crystal
system, hardness, sp. gravity, refractive index, solubility etc.
Bauxite, differ widely in color from the white of pure Al(OH)3 through cream to
dark brown when the iron content is high. The composition of typical bauxite
are-
Al2O3 40-60%
SiO2 1-6%
Fe2O3 2-25%
TiO2 1-5%
CaO+MgO 0.2-0.6%
Loss on ignition 10-30%
Trace amount Ga2O3, K2O, P2O5, V2O5, F,
Zr, Nb etc.
The main occurrences of bauxite are in Jamaica, Surinam, U.S.S.R, Guyana,
France U.S.A and Hungary.
13. Process description:
There are three methods are available, these are as follows:
(i) Bayer process
(ii) Hall process
(iii) Serpeck’s process
(i) Bayer process:
At first, crushed bauxite is usually washed to remove clay, dried in a rotary kiln
then ground to 60-100 mesh. The drying process is essential to facillate grinding,
destroy organic matter and oxidize ferrous minerals to ferric. The drying
temperature should not exceed 150oC, otherwise a part of the combined water is
expelled and the solubility in NaOH will be greatly decreased, if the ore is to be
treated by the Bayer process.
The use of NaOH to leach bauxite was first proposed in 1888 by Karl Josep Bayer
in Russia. Leaching of bauxite is carried out with agitators. As a result the
reaction between bauxite and NaOH form a soluble sodium aluminate.
Al2O3.2H2O + 2NaOH → 2Na[Al(OH)4]+H2O
15. But alkaline impurities, Fe2O3 does not react with NaOH. So, these are separated
as in insoluble impurities.
Fe2O3 + 3H2O → 2 Fe(OH)3 or Fe2O3.3H2O
Then, water is added to the produced solution and insoluble Fe2O3 is separated
by filtration as red mud.
Then, freshly prepared Al(OH)3 is added as seed to the filtrate solution with
continuous agitation at 10-30oC. As a result of hydrolysis Na[Al(OH)4], Al(OH)3 is
precipitated.
Na[Al(OH)4] → NaOH + Al(OH)3 ↓
The produced Al(OH)3 is now filtrated and calcined at 1200oC to produced Al2O3.
2Al(OH)3
Δ
Al2O3 + 3H2O
Electrometallurgy of aluminium:
Purified bauxite in the form of alumina is dissolved in cryolite Na3AlF6 and the
molten solution is electrolyzed in a steel tank lined with carbon inside. The lining
is made the cathode and anode is also made of carbon which dip in the molten
mass. Voltage of the cell adjusted to ensure the deposition of Aluminum at the
bottom but not sodium which remains in solution. Na3AlF6 and CaF2 acts as
catalyst.
The reaction occurs in the electrode are
Al2O3 ⇌ 2 Al3+ + 3O2- (l)
16. At cathode: Al3+ + 3e- → Al (liquid)
At anode : 2O2- - 4e- → O2
C to → CO+O2 → CO2
Fig. Electrolytic cell for Al from Bauxite.
17. Molten Al is drawn off from the tap hole and more Al2O3 is added and some
carbon powder also added to protect the corrosion of anode to the fused
electrolyte. Al produced 99% pure unless Cu, Fe, Si, Al2O3 etc. as impurities.
Electro-refining of aluminum:
Aluminum obtained from electrolysis of bauxite is not pure. So for purifying it, the fused
electrolyte is poured to another electrolysis cell. Fused bath consists of three layers:
(i) Bottom layer is a fused alloy of Cu-Al, which acts as anode
(ii) Top layer is pure molten Al which acts as cathode and
(iii) The middle layer consists of the electrolyte of a fused mixture of BaF2, AlF3 and NaF,
saturated with Al2O3.
The three layers are maintained by the difference in their densities during electrolysis.
On electrolysis, Al from Cu-Al alloy goes into solution in the middle layer as Al3+, leaving
the impurities at the anode. The Al3+ is now reduced at the cathode and the upper layer of
Al grows which is drawn off and impure metal is added to the bottom layer. So, pure layer
is withdrawn from upper layer as 99.99% pure Al. The reaction occurs at the electrode is as
follows:
AlF3 ⇌ Al3+ + 3F-
At Cathode: Al3+ + 3e- → Al
At anode: 3F- → 3/2 F2 + 3e-
The produced F2 is reacts with Alumina which are ready for further used.
Al2O3 + 3F2 → 2AlF3 + 3/2 O2
18.
19. Nickel (Ni)
Hydrometallurgical extraction of Ni:
Name Formula
Pentlandite (Ni, Cu, Fe)S
Smaltite (Ni, Co, Fe) As2
Milirite NiS
Nicolite NiAs
Nickel Glance NiAsS
Garnierite (Ni, Mg) SiO3.xH2O
Nickel ores with chemical formula is given below:
From the above, Pentlandite is the most important ore for the extraction of
Nickel.
20. Principle:
Ni, Co, Cu, Zn, ferrous ions and number of other metals combine with NH3 in aqueous
solution, presence of oxygen to form complex [M(NH3)n]2+. This complex is soluble.
Solubility depends on concentration of metal ion, amount of NH3 and type of anions present
e.g. OH-, CO3
2-, NO3
-, Cl-, SO4
2-.
MS + nNH3 + 2O2 → [M(NH3)n]2+ + SO4
2-
The extraction method has been classified into four steps-
(i) Leaching
(ii) Boiling
(iii) Oxyhydrolysis and
(iv) Hydrogen reduction
(i) Leaching:
A finely ground flotation concentrate of sulfied minerals containing about,
Ni 10-14% or 12-16%
Cu 1-2%
Co 0.3-0.4%
Fe 33-40%
S 28-34%
Insoluble 8-14%
Precious metal 0.02%
21. (i) The following conditions for leaching are maintained:
(a) Sufficient oxygen be supplied in solution
(b) Sufficient ammonia be supplied to neutralize acid forming by oxidation of sulfides.
(c) To form higher-amines of Cu, Ni, Co together with the unbound NH3 to establish
equilibrium with ammonia.
(d) Sufficient anion, SO4
2- be present.
(e) Unbound NH3 be not so high to reduce solubility of amines.
(f) Temperature be such that reaction proceed at an acceptable high rate.
The sulfide concentrate is mixed with water and ammonia and leached in autoclaves under
air pressure of 100 psi and at 70-80oC for 20-24 h. Reaction is exothermic and therefore
extra heating of the autoclaves is not required. Reactions are-
NiS.FeS + 3FeS + 7O2 +10NH3 +4H2O → [Ni(NH3)6]SO4 + 2Fe2O3.H2O + 2(NH4)2S2O3 (1)
2(NH4)2S3O6 + 2O2 →(NH4)2S3O6 (poly or trithionate)+ (NH4)2SO4 (2)
(NH4)2S3O6 + 2O2 + NH3 + H2O → NH4.SO3.NH2 + 2(NH4)2SO4 (3)
Equation (1) heterogeneous reaction involving mineral particles, H2O, NH3 and dissolved O2.
Oxidation of thiosulfate to thionate, sulfamate and sulfate occurs homogeneously. Reaction
rate influenced by –
(i) Temperature
(ii) Oxygen pressure
(iii) Concentration of reactants in solution
(iv) Agitation of pulp and
(v) The size of mineral particles.
22. (ii) Boiling:
The leach solution contains beside nickel and cobalt excess ammonia, copper, thiosulfates
and thionates. Ammonia is removed by distillation and is recovered in scrubbers. During
distillation most of the dissolved copper is precipitated as sulfide.
S3O6
2- + Cu2+ + 2H2O → 2SO4
2- + CuS + 4H+
S2O3
2- + Cu2+ + H2O → SO4
2- + CuS + 2H+
NH3 is boiled off in a series of pot stills condensed and recovered 15% NH3 and recycled
heated about 250oF to complete the precipitation of CuS.
10[Cu(NH3)4]SO4 + 6(NH4)2S2O3 + 3(NH4)2S2O6 → 4CuS + 3Cu2S + 14(NH4)2SO4 + 20 NH3 +
5S
(4)
[Cu(NH3)4]SO4 + H2S → 2NH3 + CuS + (NH4)2SO4
(5)
The filtrate contains 45 g/L Ni, 0.8 g/L Co and 0.001 g/L Cu are recycled to the leaching
stage.
(iii) Oxyhydrolysis:
The precipitated metal is contaminated with the sulfur during precipitation by hydrogen in
the leach solution of nickel or cobalt. For this reason the copper free solution is then
digested at 175-200oC in an autocalve. This process is carried out in the presence of
compressed air at 600 psi. There are two reasons behind this
(i) To oxidize thiosulfates and thionates to sulfates.
(ii) To oxidize traces of ferrous ion to ferric, which is hydrolyzed and precipitated.
Reactions are-
(NH4)2S2O3 + (NH4)2S3O6 + 4O2 + 3H2O + 6NH3 →5(NH4)2SO4 (6)
NH4SO3.NH2 + H2O → (NH4)2SO4 (7)
23. (iv) Hydrogen reduction:
The solution contains about 45 g/L of nickel, 1 g/L of Co, 350 g/L of ammonium sulfate and
the concentration of free ammonia is adjusted to give two moles of this for each mole of
nickel and cobalt present.
A batch of the solution is pumped into the reduction autoclave and seeded with a suitable
solid of very small particle size. Reduction is carried out at 200oC with hydrogen at 500 psi
and when complete the nickel particles are allowed to settle.
Ni(NH3)2SO4 + H2 → Ni + (NH4)2SO4 (8)
[Ni(NH3)2]SO4 + [Co(NH3)2]SO4 + 2H2S → NiS+CuS+2(NH4)2SO4 (9)
The nickel deposited on the walls of the autoclave and stirrer. It is dissolved in ammonia
solution in the presence of air and recycled to the reduction plant.
The nickel powder is filtered off, washed to remove all states and dried in an inert or
reducing atmosphere. It is either sold as powder, compressed and sintered to briquette or
rolled as metal strip.
24. Fig. Flow sheet of Sherritt-Gordon process for
production of Ni & Co metals from sulfide ore
25. Cobalt recovery
The solution after precipitation of Ni, it contains about 1g/L of Ni and 1g/L of Co, which
are precipitated with H2S at 80oC and the filtered solution passed for recovery of the
(NH4)2SO4.
The sulfides are dissolved in H2SO4 in the presence of air at 120oC and 100 psi, the pH
adjusted to 5.1, the solution aerated to oxidize the small amount of Fe present and the
Fe(OH)3 removed by filtration. The cobalt is then oxidized with air in the presence of
excess NH3 at 70oC and 100 psi giving aquopentaammio cobaltic sulfate
[Co(NH3)5.H2O]2(SO4)3. The solution is acidified with H2SO4 and the double salt, nickel
ammonium sulfate, crystallizes out and filtered off.
The cobalt is reduced to the bivalent state with metallic cobalt and cobalt metal
precipitated by H2 in a similar manner to that used for nickel, using an ammoniacal solution
and cobalt sulfide seed. The ammonium sulfate is recovered from the final spent solution
after stripping out residual cobalt.
