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Transcript
Extraction Metallurgy
Part 2: Case studies
Dr. C.B. Perry (C306)
http://www.gh.wits.ac.za/chemnotes
Chem 3033
Extraction Metallurgy
Part 2: Case studies
• Copper – Pyrometallurgy route and environmental concerns. The
hydrometallurgical alternative.
• Hydrometallurgical processes – ion exchange processes,
solvent extraction, and bacterial leaching.
• Iron – Pyrometallurgy and the blast furnace.
• Silicon – The electric arc furnace. Purification by the Czochralski
process.
• Aluminium – Electrolytic reduction.
• The siderophiles – The extraction of Au and the Pt group metals
and their purification.
Pyrometallurgy of copper
Reminder: Pyrometallurgy is the use of heat to reduce
the mineral to the free metal, and usually
involves 4 main steps:
1. Calcination: thermal decomposition of the ore with
associated elimination of a volatile product.
2. Roasting: a metallurgical treatment involving gassolids reactions at elevated temperatures.
3. Smelting: a melting process which separates the
chemical reaction products into 2 or more layers.
4. Refining: treatment of a crude metal product to improve its
purity.
Pyrometallurgy of copper
Cu ore usually associated with sulphide minerals.
Most common source of Cu ore is the mineral
chalcopyrite (CuFeS2), which accounts for ± 50% of Cu
production.
Other important ores include:
chalcocite [Cu2S],
malachite [CuCO3 • Cu(OH)2],
azurite [2CuCO3 • Cu(OH)2],
bornite (3Cu2S • Fe2S3),
covellite (CuS).
Pyrometallurgy of copper
The following steps are involved in Cu extraction:
1. Concentration
2. Roasting
3. Smelting
4. Conversion
5. Refining
Pyrometallurgy of copper
1. Concentration
Finely crushed ore concentrated by the froth-flotation
process:
• Ground ore mixed with xanthates (salts & esters of
xanthic acid), dithiophosphates, or thionocarbamates.
These make the ore surface hydrophobic.
• Ore then introduced into a water bath where air is
bubbled through the suspension.
• Finely divided hydrophobic ore particles latch on to the
air bubbles and travel to the surface where a froth is
formed.
Pyrometallurgy of copper
1. Concentration (cont.)
• The froth containing the Cu ore is skimmed off and
reprocessed.
• The remaining material (sand particles & other
impurities) sink to the bottom & is discarded or
reprocessed to extract other elements.
Pyrometallurgy of copper
1. Concentration (cont.)
Froth-flotation
Pyrometallurgy of copper
2. Roasting
• Involves partial oxidation of the sulphide mineral with
air at between 500C and 700C.
• For chalcopyrite, the main reactions are:
CuFeS2(s) + 4O2(g) → CuSO4(s) + FeSO4(s)
4CuFeS2(s) + 13O2(g) → 4CuO(s) + 2Fe2O3(s) + 8SO2(g)
• Reactions are exothermic,  roasting is an autogenous
process requiring little or no additional fuel.
• NB, not all the sulphides are oxidised, only around 1/3.
Rest remain as sulphide minerals.
• The gases produced contain around 5 – 15% SO2, which
is used for sulphuric acid production.
Pyrometallurgy of copper
2. Roasting (cont.)
Objectives of roasting:
1)
Remove part of the sulphur.
2)
Convert iron sulphides into iron oxide and iron
sulphate to facilitate removal during smelting.
3)
To pre-heat the concentrate to reduce amount of
energy needed by the smelter.
Pyrometallurgy of copper
3. Smelting
• Smelting consists of melting the roasted concentrate
to form 2 molten phases:
1) a sulphide “matte”, which contains the iron-copper
sulphide mixture.
2) an oxide slag, which is insoluble in the matte, and
contains iron oxides, silicates, and other impurities.
• Smelting is carried out at around 1200C, usually with a
silica flux to make the slag more fluid.
• The matte layer sinks to the bottom, and the slag layer
floats on top of the matte & is tapped off & disposed of.
Pyrometallurgy of copper
3. Smelting (cont.)
• The main reaction is the reduction of copper oxides
(formed during roasting) back into copper sulphide to
ensure that they migrate into the matte phase:
FeS(l) + 6CuO(l) → 3Cu2O(l) + FeO(l) + SO2(g)
FeS(l) + Cu2O(l) → FeO(l) + Cu2S(l)
Cu2S(l) + FeS(l) → Cu2S•FeS(l) (matte)
Pyrometallurgy of copper
4. Conversion
• After smelting, matte contains from between 30 to
80% Cu in the form of copper sulphide.
• The sulphur is removed by selective oxidation of the
matte with O2 to produce SO2 from S, but leave Cu
metal.
• Converting is carried out in two stages: 1) an iron
removal stage, and 2) a copper-making stage.
Pyrometallurgy of copper
4. Conversion (cont.)
Iron removal
• A silica flux is added to keep the slag (see below)
molten.
• Air is blown into the converter to oxidize the iron
sulphide according to the following reaction:
2Cu2S•FeS(l) + 3O2(g) + SiO2(l) → 2FeO•SiO2(l) + 2SO2(g) + Cu2S(l)
• The oxidized Fe and Si form a slag (insoluble in
matte) that is skimmed off & disposed off.
Pyrometallurgy of copper
4. Conversion (cont.)
Copper making
• The sulphur in the Cu2S can now be oxidized to
leave behind metallic copper according to the
following reaction:
Cu2S(l) + O2(g) → 2Cu(l) + SO2(g)
• The end product is around 98.5% pure & is known as
blister copper because of the broken surface created
by the escape of SO2 gas.
Pyrometallurgy of copper
5. Refining
• The copper is refined by electrolysis.
• The anodes (cast from blister copper) are placed into
an aqueous CuSO4/H2SO4 solution.
• Thin sheets of highly pure Cu serve as the cathodes.
• Application of a suitable voltage causes oxidation of
Cu metal at the anode.
• Cu2+ ions migrate through the electrolyte to the
cathode, where Cu metal plates out.
Pyrometallurgy of copper
5. Refining (cont.)
• Metallic impurities more active then Cu are oxidized at
the anode, but don’t plate out at the cathode.
• Less active metals are not oxidized at the anode, but
collect at the bottom of the cell as a sludge.
• The redox reactions are:
Cu(s)  Cu2+(aq) 2eCu2+(aq) + 2e-  Cu(s) Ered = -0.83V
Pyrometallurgy of copper
5. Refining (cont.)
Pyrometallurgy of copper
Environmental impact
• Large amount of gases produced present air pollution
problems, in particular SO2 gas  acid rain.
• Dust produced contains heavy metals such as
mercury, lead, cadmium, zinc  health problems.
• Waste water contaminated with:
Insoluble substances, mostly waste sludge (finely ground rock).
Soluble substances (heavy metals, sulphates).
Chemicals from flotation process.
Hydrometallurgy of copper
Advantages
• Much more environmentally friendly than
pyrometallurgy.
• Compared to pyrometallurgy, only a fraction of the
gases liberated into the atmosphere.
• Emissions of solid particles comparatively nonexistent.
Disadvantages
• Large amount of water used,  greater potential for
contamination.
• Waste waters contain soluble metal compounds,
chelating compounds & organic solvents.
Hydrometallurgy of copper
The following steps are involved:
1. Ore preparation
2. Leaching
3. Solution purification
4. Metal recovery
Hydrometallurgy of copper
1. Ore preparation
• Ore undergoes some degree of comminution
(crushing & pulverisation) to expose the Cu oxides &
sulphides to leaching solution.
Hydrometallurgy of copper
1. Ore preparation (cont.)
• Amount of comminution depends on quality of ore:
Higher grade ore – more comminution.
Lower grade ore – less comminution.
(Why??)
• If possible, ore is pre-concentrated; reject ore that
contains very little Cu.
Hydrometallurgy of copper
2. Leaching
Definition : The dissolution of a mineral in a solvent, while
leaving the gangue (rock or mineral matter of no value)
behind as undissolved solids.
• Cu is normally leached by one of three methods:
(a) Dump leaching
(b) Heap leaching
(c) Bacterial leaching
Hydrometallurgy of copper
2. Leaching (cont.) (a) Dump leaching
•
•
•
•
Leaching solution trickled over a dump.
Runoff solution collected & the Cu recovered from it.
A slow process that takes months or years to complete.
Typically only around 60% of the Cu in the dump is
recovered.
Hydrometallurgy of copper
2. Leaching (cont.) (b) Heap leaching
• Similar to dump leaching except ore not simply dumped on a
hillside, but is crushed to gravel size & piled onto an artificial
pad.
• After leaching (6 months to 1 year) gangue is removed from
pad, disposed of & replaced with fresh ore.
Hydrometallurgy of copper
2. Leaching (cont.)
Leaching reactions
Nature of ore determines if leaching is non-oxidative or
oxidative.
Non-oxidative leaching: No change in oxidation state.
e.g. (1) dissolution of copper sulphate by water:
CuSO4(s) + H2O(l)  Cu2+(aq) + SO42-(aq)
(2) dissolution of alkaline materials by acid:
Cu2(OH)2•CO3(s) + 2H2SO4(aq)  2CuSO4(aq) + CO2(g) + 3H2O(l)
Hydrometallurgy of copper
2. Leaching (cont.)
Oxidative leaching: Many ores only soluble once oxidised.
e.g. covellite (CuS) much more soluble if oxidised to CuSO4
CuS(s) + O2(g)  CuSO4(aq)
CLASS EXERCISE : work out which species is oxidised, and
which is reduced, and write out the balanced half reactions for
each.
SOLUTION: CuS  Cu = +2, S = -2
O2
O=0
CuSO4  Cu = +2, O4 = -8, S = +6
S-2  S+6 + 8e(oxidation)
2O2 + 8e-  4O2- (reduction)
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
• Several bacteria, especially Thiobacilli, are able to
solubilise metal minerals by oxidising ferrous to ferric
iron, as well as elemental sulphur, sulphide, and other
sulphur compounds to sulphate or sulphuric acid.
• 20 to 25% of copper produced in the USA, and 5% of
the worlds copper is obtained by bacterial leaching.
• Very slow process; takes years for good recovery
• But low investment and operating costs.
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Thiobacilli
• Are acidotolerant; some grow at pH’s as low as 0.5
• Are tolerant against heavy metal toxicity.
• Are chemolithoautotrophs (C source is CO2 & energy
derived from chemical transformation of inorganic
matter).
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms
Generalised reaction : M(II)S + 2O2  M2+ + SO42-
• Two mechanisms: (a) indirect mechanism involving
the ferric-ferrous cycle, and (b) direct mechanism
involving physical contact of the organism with the
sulphide mineral.
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms: Indirect
First step: ferrous sulphate is converted into ferric
sulphate by the action of Acidithiobacillus ferrooxidans:
4FeSO4 + O2 + 2H2SO4  2Fe2(SO4)3 + 2H2O
CLASS EXERCISE : work out which is ferric- and which is
ferrous sulphate, and write out the balanced half reactions for
each.
FeSO4 – SO42-  Fe2+ (ferrous)
2Fe2(SO4)3 – 3 × SO42- = -6, but 2 × Fe Fe3+ (ferric)
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms: Indirect
2Fe2+  2Fe3+ + 2e- (oxidation)
O2 + 2e-  2O2(reduction)
• Ferric sulphate is a strong oxidising agent capable of
dissolving a range of sulphide minerals.
• In the case of chalcopyrite:
CuFeS2 + 2Fe2(SO4)3  CuSO4 + 5FeSO4 + 2S
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms: Indirect
• The elemental S produced by the indirect method can
be converted to H2SO4 by Acidithiobacillus
ferrooxidans:
• The H2SO4 helps maintain the pH at levels favourable
for bacterial growth.
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms: Direct
• Bacteria actually adheres to the mineral surface prior
to enzymatic attack.
• The mineral is oxidised with oxygen to sulphate and
metal cations without any detectable intermediate
occurring.
• In the case of covellite:
CuS + 2O2  CuSO4
Hydrometallurgy of copper
3. Solution Purification
• Leaching reactions not perfectly selective  other
elements in solution as well, not just Cu. These need
to be removed.
• After leaching, Cu in solution can be very dilute. 
need a way to concentrate it.
• Both of these are generally done using ion exchange
processes, the two most common being ion exchange
chromatography, and solvent extraction.
Hydrometallurgy of copper
3. Solution Purification
Ion exchange chromatography
• DEFINITION: a solution containing a mixture of metal
ions is contacted with a resin that is insoluble in the
metal-ion solution.
• Ion-exchange resin consists of an inert solid phase to
which labile functional groups are chemically bonded.
• Functional groups can either be acidic (H+) or basic
(OH–) groups that exchange with cations (M+) or
anions (M–), respectively.
• The ion-exchange process is reversible.
Hydrometallurgy of copper
3. Solution Purification
Ion exchange chromatography
• Carboxyl groups exchanges the ion it currently holds
(H+) for a Cu2+ ion.
• The Cu2+ is later released by contacting it with a
stripping solution (very high H+ conc.).
Hydrometallurgy of copper
3. Solution Purification: Solvent extraction
• DEFINITION: a method to separate compounds
based on their relative solubilities in 2 different
immiscible liquids.
• In industry, this is usually set up as a continuous
process
Hydrometallurgy of copper
3. Solution Purification: Solvent extraction
Hydrometallurgy of copper
3. Solution Purification: Solvent extraction
• Organic + aqueous stream pumped into a mixer.
• Organic (containing an extractant) and aqueous
components mix, and ion transfer occurs between them.
• Once ion transfer is complete (equilibrium), mixture is
allowed to separate.
• Aqueous solution is removed & the organic phase
(containing the Cu2+) is mixed with an aqueous stripping
solution.
• Cu2+ moves back into the aqueous phase, and the two
phases are again allowed to separate.
• The aqueous phase (containing the Cu2+) is removed &
the organic phase is recycled back into the first mixer.
Hydrometallurgy of copper
3. Solution Purification: Solvent extraction
Extractants
• The most successful extractants for copper are of the orthohydroxyoxime type:
OH
OH
N
R = alkyl ,phenyl, or H
R
R
R1 = alkyl
1
• Function by means of a pH-dependent cation-exchange
mechanism:
Cu2+ + 2HA  CuA2 + 2H+
(where H in HA denotes the replaceable, phenolic proton)
Hydrometallurgy of copper
3. Solution Purification: Solvent extraction
Extractants
• At low pH (1.5 – 2.0) the ortho-hydroxyoxime extractant
complexes the Cu.
• During back-extraction (stripping stage) the pH is lowered
further, releasing the Cu, and regenerating the hydroxyoxime
for recycle to the extraction stage.
• Aqueous feeds (leach solution) typically contain more iron
per litre than copper. For commercial success, the extractant
must  have a greater selectivity for Cu than Fe.
Hydrometallurgy of copper
3. Solution Purification: Solvent extraction
Extractants
• Cu2+ forms square-planar complexes with hydroxyoxime:
R
1
N
O
R
Cu
1
N
O
R
R
O H
O H
• H-bonding between the oximic H and the phenolic O affords
this 2:1 complex unusual stability.
• The formation constant (K2) for the 2:1 complex is much
greater than for the 1:1 complex.
Hydrometallurgy of copper
3. Solution Purification: Solvent extraction
Extractants
• The tris(salicylaldoximato)iron(III) complex is octahedral, and
no extended planar ring structure is possible between the 3
oxime ligands.
•  stability of Fe(III) complex is less than Cu(II) complex,
which allows the extraction of Cu to be carried out at lower
pH than what is required for efficient Fe extraction.
Hydrometallurgy of copper
4. Metal Recovery:
• At this point, the metal needs to be recovered from
solution in the solid form.
• This is either achieved chemically, or
electrochemically.
Hydrometallurgy of copper
4. Metal Recovery:
Chemical recovery
• Dissolved copper will plate out on an iron surface
according to the following reaction:
Cu2+(aq) + Fe(s)  Fe2+(aq) + Cu(s)
Why??
Reduction half-reactions:
Cu2+(aq) + 2e–  Cu(s)
Fe2+(aq) + 2e–  Fe(s)
Ered = +0.34 V
Ered = -0.44 V
• Ered for the Cu2+ half-reaction is more positive than for
the Fe2+ half reaction which leads to Cu being
reduced and Fe oxidised.
Hydrometallurgy of copper
4. Metal Recovery:
Chemical recovery
• Solutions containing dissolved copper are thus run
through a bed of shredded scrap iron, resulting in the
copper ions being plated out as solid Cu on the iron
surface.
• For the process to be efficient, the surface of the
scrap iron must be large.
Hydrometallurgy of copper
4. Metal Recovery:
Electrochemical recovery
Electrowinning
• An electrochemical process for precipitating metals
from solution.
Hydrometallurgy of copper
4. Metal Recovery:
Electrochemical recovery
Electrowinning
• A current is passed from an inert anode through a
liquid leach solution containing the metal so that the
metal is extracted as it is deposited onto the cathode.
• The anode is made out of a material that will not
easily oxidise or dissolve, such as lead or titanium.
Hydrometallurgy of copper
4. Metal Recovery:
Electrochemical recovery
Electrorefining
• The anodes consist of unrefined impure metal.
• Current passes through the acidic electrolyte
corroding the anode into the solution.
• Refined pure metal deposited onto the cathodes.
• Metals with a greater Ered than Cu (such as Zn and
Fe) remain in solution.
• Metals with a lower Ered than Cu (Au, Ag) accumulate
as an “anode sludge”  collected & sold for further
refining.
Hydrometallurgy of copper
4. Metal Recovery:
Electrorefining
Electrochemical recovery
Hydrometallurgy of copper
Summary:
Silicon production
rG /kJ mol-1
• More difficult to extract Si than either Cu or Fe.
Temperature /C
Silicon production
• Silicon of between 96 to 99% purity is achieved by
reduction of quartzite or sand (SiO2, also called silica)
• High temperatures required achieved in an electric arc
furnace.
• Reduction carried out in the presence of excess silica
to prevent accumulation of silicon carbide (SiC) :
SiO2(l) + 2C(s)  Si(l) + 2CO2(g)
2SiC(s) + SiO2(l)  3Si(l) + 2CO(g)
Silicon production
The electric arc furnace
• Silica and carbon fed in
through the top, liquid Si
collected at the bottom.
• Temps of ± 2000K
achieved by an electric
arc burning between
graphite electrodes.
• An arc forms between the charge and the electrodes.
• The charge is heated both by current passing through the charge and
by the radiant energy evolved by the arc.
Silicon production
The electric arc furnace
• Electric arc furnaces require huge amounts of electricity. A midsized furnace would have a transformer rated about 60,000,000
volt-amperes with a secondary voltage between 400 and 900
volts and a secondary current in excess of 44,000 amperes.
Silicon production
Applications
• Si is the 2’nd most abundant element in the earth’s
crust (~28%).
• Principal constituent of natural stone, glass, concrete
& cement.
• Largest application of pure Si (metallurgical grade) is
in the manufacture of Al-Si alloys to produce cast
parts (for automotive industry).
• Important constituent of electrical steel (modifies the
resistivity & ferromagnetic properties).
• Added to molten cast iron to improve its performance
in casting thin sections.
Silicon production
Applications
• 2’nd largest application is in the production of
silicones. These are polymers containing Si-O and SiC bonds. Typically heat-resistant, nonstick, and
rubberlike, they are frequently used in cookware,
medical applications, sealants, lubricants, and
insulation.
• Electronics industry – ultra-pure silicon wafers used in
electronic components such as transistors, solar cells,
integrated circuits, microprocessors & various
semiconductor devices.
Silicon production
Purification
• Ultra-pure silicon is required for the production of
semiconductors.
Silicon production
Purification
• Semiconductor-grade Si produced by converting
crude Si to more volatile compounds like SiCl4.
• These are then purified by exhaustive fractional
distillation.
• Reduced back to Si with pure H2.
• Finally, the high-purity Si is melted and large single
crystals are grown by the Czochralski process.
• Electronic grade Si is required to be 99.999999999%
pure!
Silicon production
Purification: The Czochralski process
• Ultra-pure Si (only a few ppm of impurities) is melted in a crucible.
• Dopant impurities (B or
P) can be added to
make n-type or p-type
silicon (influences the
electrical conductivity).
• A seed crystal mounted
on a rod is dipped into
the molten Si.
• Seed crystal rod pulled up & rotated at the same time.
• By carefully controlling the temp gradients, rate of pulling, and
rotation speed, a large single-crystal (called a boule) can be
extracted from the melt.
Silicon production
Purification: The Czochralski process
Silicon production
Purification: The Czochralski process
• The boule is then ground down
to a standard diameter and
sliced into wafers, much like a
salami.
• The wafers are etched and
polished, and move on to the
process line.
• A point to note however, is that due to "kerf" losses (the width
of the saw blade) as well as polishing losses, more than half
of the carefully grown, very pure, single crystal silicon is
thrown away before the circuit fabrication process even
begins!
Silicon production
Electrochemical preparation:
• A new method that uses electrolysis to reduce SiO2 to
elemental Si.
• Advantageous because it avoids the high energy costs
associated with the older carbothermic route, and also
reduces the CO2 emissions considerably.
• SiO2 is usually an insulator, and doesn’t conduct electricity,
but it has been shown that a tungsten wire sealed within a
quartz tube with the tungsten end exposed, can act as a
cathode.
Silicon production
Electrochemical preparation:
• The anode is usually graphite, and the reduction is carried
out in a solution of molten CaCl2 at around 850 C.
a) SEM of W-SiO2 electrode
before reduction.
b) After reduction.
c) After washing.
d) Side view.
Silicon production
Electrochemical preparation:
• Conversion of quartz to Si occurs at the three-phase
boundary between the SiO2, the electrolyte, and the flattened
end of the tungsten wire.
• This provides enough impetus for the electrochemistry to
kick in properly as the silica is gradually converted to
conducting silicon.
• This reaction should theoretically propagate through the
silica electrode, but in reality it grinds to a halt very quickly.
• Reason for this is that the molten electrolyte cannot
penetrate through the newly formed Si layer on the surface.
 three-phase boundary formation halted.
Silicon production
Electrochemical preparation:
• Solution: replace solid quartz electrode with SiO2 powder
pressed into pellets & sintered.
• Resulting electrode porous enough to allow electrolyte to
penetrate deeply into the material.
a) SEM of SiO2 powder
b) reduced Si powder.
Silicon production
Electrochemical preparation:
Aluminium production
• Most abundant metallic element in the earth’s crust.
• But, extremely rare in its free form.
• Once considered as a precious metal more valuable
than gold!
• Al is a highly reactive metal that forms strong bonds
with O.
• Requires a large amount of energy to extract from
Al2O3.
Aluminium production
• Cannot be reduced directly by carbon since Al is a
stronger reducing agent than C.
• Must therefore be extracted by electrolysis.
• Aluminium production involves two steps: 1) purifying
Al2O3 from bauxite (the Bayer process) and 2)
converting Al2O3 to metallic Al (The Hall-Heroult
process).
• Primary Al ore is bauxite, which consists of:
Gibbsite - Al(OH)3 (most extractable form)
Boehmite - AlO•OH (less extractable than Gibbsite)
Diaspore - αAlO•OH (difficult to extract)
Aluminium production
The Bayer process: Step 1: Dissolution
• The hydrated aluminium oxides are first selectively
dissolved from bauxite:
Al(OH)3 + NaOH  NaAlO2 + 2H2O (Gibbsite dissolution)
AlO•OH + NaOH  NaAlO2 + H2O (Boehmite dissolution)
• An undesirable side reaction is the formation of “red
mud”, which occurs when Al(OH)3 reacts with
dissolved Kaolinite clay:
5Al2Si2O5(OH)4 + 2Al(OH)3 + 12NaOH  2Na6Al6Si5O17(OH)10 + 10H2O
• Red mud formation consumes dissolved Al and 
represents a Al loss.
Aluminium production
The Bayer process: Step 2: Solid-Liquid Separation
• The digested bauxite now consists of 1 liquid and 2
solid components:
Caustic liquid soln. with dissolved Al.
Undissolved coarse material (sand).
Precipitated fines (red mud).
• Sand (mainly undissolved silicates) easily removed
since they settle very rapidly.
• The red mud is removed by adding a flocculent to
increase the settling rate.
• The Al content of the red mud is recovered & forms
part of the liquid layer.
Aluminium production
The Bayer process: Step 3: Precipitation
• The remaining solution is supersaturated, containing
around 100-175 grams of dissolved Al2O3 per litre.
• Al(OH)3 is precipitated out by adding seed crystals
since Al(OH)3 doesn’t crystallise out easily on its own.
• Once the crystals have reached the desired size, they
are removed, washed, and filtered.
• The spent liquor is reheated, recausticised and
recycled.
Aluminium production
The Bayer process: Step 4: Calcination
• Wet crystals of Al(OH)3, obtained from the
precipitation step are dried by heating to around 1300
– 1500 C.
• This process also converts the Al(OH)3 to Al2O3:
2Al(OH)3  Al2O3 + 3H2O
Aluminium production
The Bayer process: Problems
• Problems result from the coordination chemistry of Al
in basic solutions. Generally accepted structures:
• Leads to extensive H-bonding between aluminate ion
& solvent, which in turn leads to high viscosity of these
solutions.
• In turn leads to problems with materials handling &
heat exchange.
Aluminium production
The Bayer process: Problems
• In addition, the inertness of Al(III) leads to slow rates
of crystallisation, requiring large vessels & large
volumes of circulating solution & seed material.
Aluminium production
The Hall-Heroult process:
• Reactive metals (e.g. Mg and Na) can be produced by
electrolysing a molten chloride salt of the metal.
• Not the case for AlCl3 since it sublimes rather than
melts.
• Even under sufficient pressure, molten AlCl3 is an
electrical insulator & cannot be used as an electrolyte.
Would have to be dissolved in a conductive salt (NaCl
or KCl).
• Commercially viable production of Al only commenced
once the use of cryolite (Na3AlF6) was discovered.
Aluminium production
The Hall-Heroult process:
• Cryolite is electrically conductive, and dissolves Al2O3.
Aluminium production
The Hall-Heroult process:
• Anhydrous Al2O3 melts at over 2000C which is too
high to be used as a molten medium for electrolytic
reduction of Al.
• Al2O3 dissolved in cryolite has a m.p. of 1012C & is a
good electrical conductor.
• Graphite rods are used as anodes & are consumed in
the electrolytic process.
• The cathode is a steel vessel, lined with graphite.
Aluminium production
The Hall-Heroult process:
• The electrode reactions are as follows:
Anode: C(s) + 2O2-(l)  CO2(g) + 4eCathode: 3e- + Al3+(l)  Al(l)
CLASS EXERCISE : Write out the balanced overall reaction
4Al3+(l) + 6O2-(l) + 3C(s)  4Al(l) + 3CO2(g)
CLASS EXERCISE : Calculate the mass of Al that will be
produced in 1.00 hr by the electrolysis of molten AlCl3, using a
current of 10.0 A.
Aluminium production
The Hall-Heroult process:
• Step 1: calculate the number of coulombs (C) from the
current (I) and the time (t):
Q=I×t
= 10.0 A × 3600 s
= 3.60 × 104 C
• Step 2: find the number of moles of electrons:
Q
3.60 104 C
n 
 0.373 mol electrons
-1
F 96500 C mol
F (Faraday constant) = magnitude of electric charge per mole
of electrons.
Aluminium production
The Hall-Heroult process:
• Step 3: find the mass of Al produced:
3 mole of electrons needed to produce 1 mol of Al:
3e- + Al3+(l)  Al(l)
 Number of mol Al = 0.124
Mr Al = 26.98 g mol-1
 mass Al = 26.98 g mol-1 × 0.124 mol
= 3.34 g Al
Aluminium production
The Hall-Heroult process:
• Electrolytic reduction of Al is costly (3 e- required for
every atom of metallic Al reduced).
• The electrical voltage used is only around 5.25 V, but
the current required is very high, typically 100,000 to
150,000 A or more!
• Electrical power is the single largest cost in Al
production,  Al smelters are typically located in areas
with inexpensive electric power, like S.A.
Pyrometallurgy of iron
• Still the most important pyrometallurgical process
economically.
• The most important sources of iron are hematite
(Fe2O3) and magnetite (Fe3O4).
• Prehistorically, iron was prepared by simply heating it
with charcoal in a fired clay pot.
• Today, the reduction of iron
oxides to the metal is
accomplished in a blast
furnace.
Pyrometallurgy of iron
Blast furnace:
1) Hot gas blast
2) Melting zone
3) Reduction of FeO
4) Reduction of Fe2O3
5) Pre-heating zone
6) Feed of ore,
limestone + coke
7) Exhaust gases
8) Column of ore, coke
+ limestone
9) Removal of slag
10) Tapping of molten pig iron
11) Waste gas collection
Pyrometallurgy of iron
• The iron ore, limestone, and coke are added to the
top of the furnace.
• Coke is coal that has been heated in an inert
atmosphere to drive off volatile components (~ 80 –
90% C).
• Coke is the “fuel”, producing heat in the lower part of
the furnace. Is also the source of the reducing gases
CO & H2.
• Limestone (CaCO3) serves as the source of CaO
which reacts with silicates & other impurities in the ore
to form slag.
Pyrometallurgy of iron
Slag:
• Most rocks are composed of silica (SiO2) and silicates
(SiO32-) & are almost always present in the ore.
• These compounds don’t melt at the furnace
temperature & would eventually clog it up.
• An important chemical method to remove these is by
use of a flux which combines with the silica & silicates
to produce a slag.
• Slag collects at bottom of furnace & doesn’t dissolve
in the molten metal.
Pyrometallurgy of iron
Slag:
• The heat of the furnace decomposes the limestone to
give calcium oxide.
• CaO (a basic oxide) reacts with silicon dioxide to give
calcium silicate.
CaCO3(s)  CaO(s) + CO2(g)
CaO(s) + SiO2(s)  CaSiO3(l)
• Slag helps protect the molten iron from re-oxidation.
• Slag is used in road making, and can also be
combined with cement.
Pyrometallurgy of iron
Pyrometallurgy of iron
• Air is blown into the bottom of the furnace, and
combusts with the coke to raise the furnace temp up
to 2000C :
2C(s) + O2(g)  2CO(g)
H = -221 kJ
• H2O in the air also reacts with the coke:
C(s) + H2O(g)  CO(g) + H2(g)
H = +131 kJ
• Since this reaction is endothermic, if the blast furnace
gets too hot, water vapor is added to cool it down
without interrupting the chemistry.
Pyrometallurgy of iron
• Molten iron is produced lower
down the furnace & removed.
• Slag is less dense than iron &
can be drained away.
• The iron formed (called pig iron)
still contains around 4-5% C, 0.61.2% Si, 0.4-2.0% Mn + S and P
and needs to be further
processed.
Pyrometallurgy of iron
• At around 250C (top of the furnace), limestone is
calcinated:
CaCO3(s)  CaO(s) + CO2(g)
• Also at the top of the furnace, hematite is reduced:
3Fe2O3(s) + CO(g)  2Fe3O4(s) + CO2(g)
• Reduction of Fe3O4 occurs further down the furnace
(~700C):
Fe3O4(s) + CO(g)  3FeO(s) + CO2(g)
• Near the middle of the furnace (1000C) Fe is
produced:
FeO(s) + CO(g)  Fe(s) + CO2(g)
Pyrometallurgy of iron
Cast iron
• Cast iron is made by remelting pig iron & removing
impurities such as phosphorous and sulphur.
• The viscosity of cast iron is very low, & it doesn’t
shrink much when it solidifies.
•  ideal for making castings.
• BUT, it is very impure, containing up to 4% carbon.
This makes it very hard, but also very brittle.
• Shatters rather than deforms when struck hard.
• These days cast iron is quite rare, often being
replaced by other materials.
Pyrometallurgy of iron
Steelmaking
• Pig iron is brittle, and not directly very useful as a
material.
• Typically, pig iron is drained directly from the blast
furnace (referred to as hot metal), and transported to
a steelmaking plant while still hot.
• The impurities are removed by oxidation in a vessel
called a converter.
• The oxidising agent is pure O2 or O2 mixed with Ar.
• Air can’t be used as N2 reacts with iron to form iron
nitride which is brittle.
Pyrometallurgy of iron
Steelmaking
• O2 blown directly into molten
metal.
• Reacts exothermically with
C, Si + other impurities.
• C & S expelled as CO and
SO2 gas.
• Si oxidised to SiO2 &
incorporates into the slag
layer.
Iron converter
• Once oxidation complete,
contents poured out &
various alloying elements
added to produce steels.
Pyrometallurgy of iron
Types of iron & steel
• Wrought iron – iron with all the C removed. Soft &
easily worked with little structural strength. No longer
produced commercially.
• Mild steel – iron containing around 0.25% C. Stronger
& harder than pure iron. Has many uses including
nails, wire, car bodies, girders & bridges, etc.
• High carbon steel – contains around 1.5% C. Very
hard, but brittle. Used for things like cutting tools, and
masonry nails.
Pyrometallurgy of iron
Types of iron & steel
• Stainless steel – iron mixed with chromium and nickel.
Resistant to corrosion. Uses include cutlery, cooking
utensils, kitchen sinks, etc.
• Titanium steel – iron mixed with titanium. Withstands
high temperatures. Uses include gas turbines,
spacecraft parts, etc.
• Manganese steel – iron mixed with manganese. Very
hard. Uses include rock-breaking machinery, military
helmets, etc.
Pyrometallurgy of iron
The thermite reaction
• Aluminium metal can reduce Iron(III) oxide (Fe2O3) in
a highly exothermic reaction.
• Molten iron is produced at around 3000C.
• Reaction used for thermite welding, often used to join
railway tracks.
Fe2O3(s) + 2Al(s)  2Fe(s) + Al2O3(s)
Pyrometallurgy of iron
The thermite reaction
Pyrometallurgy of iron
The thermite reaction
Fe2O3(s) + 2Al(s)  2Fe(l) + Al2O3(s)
CLASS EXERCISE : calculate the thermal energy that is
released in the reaction.
Component
Hfo (kJ/mol)
Fe2O3(s)
-822.2
Al(s)
Al2O3(s)
0
-1,669.8
Fe(s)
0
Horxn = (1 mol)(HfoAl2O3) + (2 mol)(HfoFe) - (1 mol)(HfoFe2O3) - (2 mol)(HfoAl)
Horxn = (1 mol)(-1,669.8 kJ/mol) + (2 mol)(0) - (1 mol)(-822.2 kJ/mol) - (2mol)(0 kJ/mol)
Horxn = -847.6 kJ mol-1
Exothermic!
Electrowinning of iron
The Pyror process:
• Studies into iron extraction by electrowinning from
sulphate solutions were first carried out around 50
years ago, then subsequently forgotten.
• May become important again in the future as new,
more environmentally friendly methods are sought for
steelmaking.
Electrowinning of iron
The Pyror process:
• First step is to convert iron pyrite (FeS2) into an acid
soluble form (FeS). Achieved by either calcining at
800 to 900 C to expel a loosely-bound S, or by
smelting in an electric furnace.
• Step 2 is a leaching step using H2SO4 to extract iron
from FeS:
FeS(s) + H2SO4(l)  FeSO4(l) + H2S(g)
• Step 3: before entering the electrowinning cells, the
solution is purged with air to remove any remaining
H2S.
Electrowinning of iron
The Pyror process:
• Step 4: Electrolysis. Iron is reduced and deposited on
the cathode, while O2 is evolved, and H2SO4 is
regenerated at the anode. More specifically:
At the cathode:
Fe2+ + 2e-  Fe(s)
2H+ + 2e-  H2(g)
Fe3+ + e-  Fe2+
At the anode:
SO42- + H2O  H2SO4 + 1/2O2 + 2eFe2+  Fe3+ + e-
Electrowinning of iron
The Pyror process:
Electrowinning of iron
The Pyror process:
• The process was shown to
be quite efficient. During a 2
year pilot-plant project, a
quantity of iron close to 150
tonnes was produced.
• Electrolysis was run for several weeks before
stripping was performed, resulting in deposits of
13mm or more in thickness.
Electrowinning of iron
The Pyror process:
Gold extraction
Gold mining
Historical:
• Panning – sand and gravel
containing gold is shaken around
with water in a pan. Gold is much
denser than rock, so quickly
settles to the bottom of the pan.
Gold extraction
Gold mining
Historical:
• Sluicing – water is channelled to
flow through a sluice-box. Slucebox is essentially a man-made
channel with riffles (barriers) at the
bottom. Riffles create dead-zones
in the water current which allows
gold to drop out of suspension.
• Sluicing and panning results in the direct recovery of
small gold nuggets and flakes.
Gold extraction
Gold mining
Modern methods:
• Hard rock mining – used
to extract gold encased in
rock. Either open pit
mining or underground
mining.
Gold extraction
Gold ore processing
Gold cyanidation:
• The most commonly used process for gold extraction.
• Used to extract gold from low-grade ore.
• Gold is oxidised to a water-soluble aurocyanide
metallic complex.
• In this dissolution process, the milled ore is agitated
with dilute alkaline cyanide solution, and air is
introduced:
4Au(s) + 8NaCN(l) + O2(g) + 2H2O(l)  4NaAu(CN)2(l) + 4NaOH(l)
Gold extraction
Gold ore processing
Gold cyanidation:
• At a slurry concentration of around 50% solids, the
slurry passes through a series of agitated mixing tanks
with a residence time of 24 hrs.
• The gold-bearing liquid is then separated from the
leached solids in thickener tanks or vacuum filters &
the tailings are washed to remove Au and CN- prior to
disposal.
Gold extraction
Gold ore processing
Gold cyanidation:
• The aurocyanide complex has an exceptionally high
stability constant, 2 [Au(CN)2]- = 2 × 1038.
• This high stability constant means that dissolution can
be achieved even in the presence of considerable
amounts of other metals (Cu, Zn, and Ni).
• At this point, the dissolved Au needs to be recovered
from the cyanide solution. Two methods commonly
used to achieve this are 1) the Carbon in pulp
process, and 2) the Merrill-Crowe process.
Gold extraction
Gold ore processing
Heap Leaching
• Is an alternative to the agitated leaching process.
• Drastically reduced gold recovery costs of low grade
ore.
• Ore grades as low as 0.3 g per ton can be
economically processed by heap leaching.
Gold extraction
Gold ore processing
Heap Leaching
• Generally requires 60 to 90 days for processing ore
that could be leached in 24 hrs in a conventional
agitated leach process.
• Au recovery is around 70% as compared with 90% in
an agitated leach plant.
• BUT, has gained wide favour due to vastly reduced
processing costs.
• Frequently, mines will use agitated leaching for highgrade ore & heap leaching for marginal grade ores
that would otherwise be considered waste rock.
Gold extraction
Gold ore processing
Gold cyanidation:
1) Carbon in pulp – overview:
• Dissolved aurocyanide is mixed with free activated
carbon particles in solution and agitated in leach tanks.
• The carbon particles are much larger than the ground
ore particles.
• Gold has a natural affinity for C, and the aurocyanide
complex is adsorbed onto the C.
• The coarse C particles with bound [Au(CN)2]- are then
removed by screening using a wire mesh. Finely ground
ore passes through the mesh.
Gold extraction
Gold ore processing
Gold cyanidation:
1) Carbon in pulp – details:
• On completion of cyanidation, pregnant pulp is
transferred to Carbon In Pulp (CIP) process.
• Pregnant pulp passed through a number of tanks (5 or
6) in series. Tanks are mechanically stirred.
• Granulated carbon is pumped counter-current to the
pulp through the tanks.
• In the final tank, fresh, or barren carbon comes into
contact with low-grade or tailings solution.
Gold extraction
Gold ore processing
Gold cyanidation:
1) Carbon in pulp – details:
• In this tank, the barren carbon has a high activity, and
can remove trace amounts of Au < 0.01 mg / L.
• As the carbon passes through the tanks, it collects
increasing quantities of Au from the solution. This is
termed loading.
• Typically, concentrations as high as 4000 to 8000 g Au /
ton of C can be achieved on the final loaded C.
Gold extraction
Gold ore processing
Gold cyanidation:
1) Carbon in pulp
• The loaded carbon is separated from the pulp in the final
tank & transferred to the elution circuit.
• Barren pulp is dewatered (to recycle water & remove
cyanide for reuse in the process).
• In the elution circuit, the loaded carbon is treated with a
hot cyanide & caustic solution to remove the Au.
• The barren carbon is reactivated & recycled for use in
the process.
Gold extraction
Gold ore processing
Gold cyanidation:
1) Carbon in pulp
• The cyanide & caustic solution is transferred to an
electrowinning circuit where the Au is plated out onto
steel wool.
• The Au-plated steel wool is transferred to the smelting
circuit to produce gold bullion.
Gold extraction
Carbon in pulp
Gold extraction
Gold ore processing
Gold cyanidation:
2) Merrill-Crowe process
• Traditional method for Au recovery from pregnant
cyanide solutions.
• Once dissolution of Au is complete, the remaining rock
pulp if filtered off through various filters &
diatomaceous earth to produce a sparkling clear
solution.
• O2 is removed from the clarified solution by passing
the solution through a vacuum deaeration column.
Gold extraction
Gold ore processing
Gold cyanidation:
2) Merrill-Crowe process
• Zinc dust is then added to the cyanide solution to
chemically reduce the gold to the metal.
• The metallic gold is then filtered out & refined.