Extraction Metallurgy: Part 2: Case Studies
Extraction Metallurgy: Part 2: Case Studies
Extraction Metallurgy: Part 2: Case Studies
http://www.gh.wits.ac.za/chemnotes
Chem 3033
Extraction Metallurgy
Part 2: Case studies
Copper Pyrometallurgy route and environmental concerns
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
1.
associated elimination of a volatile product.
2. Roasting: a metallurgical treatment involving gassolids reactions at elevated temperatures.
3. Smelting:
g a melting
gp
process which separates
p
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
1.
2 Roasting
2.
3 Smelting
3.
4 Conversion
4.
5 Refining
5.
Pyrometallurgy of copper
1. Concentration
Only ~0
0.7%
7% of the extracted ore contains Cu
Finely crushed ore concentrated by the froth-flotation
process:
Copper ore slurry mixed with:
Lime water to give basic pH
Pine oil to make bubbles
An alcohol to strengthen bubbles
A chemical collector
Pyrometallurgy of copper
S
1. Concentration (cont.)
Na3PS2O 2
S
R1
R2
Hydrophobic
H
d
h bi attracted
tt t d
to hydrocarbon pine oil
Hydrophilic
H
d
hili attracted
tt t d
to sulphide minerals
CH3
H3C
S
O
Potassium
amyl xanthate
Pyrometallurgy of copper
1. Concentration (cont.)
Raising the pH causes the polar ends to ionize more
more,
thereby preferentially sticking to chalcopyrite (CuFeS2)
and leaving pyrite (FeSs) alone.
Air is bubbled through
g the suspension.
p
Finely divided hydrophobic ore particles latch on to the
air
i b
bubbles
bbl and
d ttravell tto th
the surface
f
where
h
a ffroth
th iis
formed.
The froth containing the Cu ore is skimmed off and
reprocessed.
reprocessed
Pyrometallurgy of copper
1. Concentration (cont.)
In this manner,
manner the ore is
concentrated to an eventual
value of over 28% Cu.
The remaining
g material ((sand
particles & other impurities) sink
to the bottom & is discarded or
reprocessed
d tto extract
t t other
th
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 500C and 700C.
For chalcopyrite, the main reactions are:
CuFeS2(s) + 4O2(g) CuSO4(s) + FeSO4(s)
4CuFeS2(s) + 13O2(g) 4CuO(s) + 2Fe2O3(s) + 8SO2(g)
exothermic roasting is an autogenous
Reactions are exothermic,
process requiring little or no additional fuel.
NB
NB, nott allll th
the sulphides
l hid are oxidised,
idi d only
l around
d 1/3
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)
2)
3)
Pyrometallurgy of copper
3. Smelting
Smelting consists of melting the roasted concentrate
to form 2 molten phases:
1) a sulphide matte
matte , which contains the iron
iron-copper
copper
sulphide mixture.
2) an oxide slag,
slag which is insoluble in the matte
matte, and
contains iron oxides, silicates, and other impurities.
Smelting is carried out at around 1200C, 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) Cu2SFeS(l) (matte)
Pyrometallurgy of copper
4. Conversion
After smelting,
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
S, but leave Cu
metal.
Converting is carried out in two stages: 1) an iron
removal stage,
stage and 2) a copper-making
copper making stage.
stage
Pyrometallurgy of copper
4. Conversion (cont.)
Iron removal
A silica flux is added to keep the slag (see below)
molten.
lt
Air is blown into the converter to oxidize the iron
sulphide according to the following reaction:
2Cu2SFeS(l)
S FeS(l) + 3O2(g) + SiO2(l) 2FeOSiO
2FeO SiO2(l) + 2SO2(g) + Cu2S(l)
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
Th
d product
d t iis around
d
98.5% pure & is known as
bli t copper because
blister
b
off
the broken surface
created
t d by
b th
the escape off
SO2 gas.
Pyrometallurgy of copper
5. Refining
Almost all copper is refined by electrolysis
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
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 dont p
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
sludge.
The redox reactions are:
Cu(s) Cu2+(aq) + 2eCu22+(aq) + 2e- Cu(s) Ered = -0.83V
0 83V
Pyrometallurgy of copper
5. Refining (cont.)
Pyrometallurgy of copper
Environmental impact
Large amount of gases produced present air pollution
problems, in p
p
particular SO2 g
gas acid rain.
Dust p
produced contains heavy
y 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.
Pyrometallurgy of copper
Environmental impact
T i l Li
Typical
Liquid
id Effl
Effluents
t
Hydrometallurgy of copper
Advantages
Much more environmentally friendly than
pyrometallurgy.
Compared to pyrometallurgy,
pyrometallurgy only a fraction of the
gases liberated into the atmosphere.
Emissions of solid particles comparatively nonexistent.
Disadvantages
L
Large amountt off water
t used,
d greater
t potential
t ti l for
f
contamination.
Waste waters contain soluble metal compounds,
chelating compounds & organic solvents.
Hydrometallurgy of copper
The following steps are involved:
1 Ore preparation
1.
2 Leaching
2.
3 Solution purification
3.
4 Metal recovery
4.
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??)
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
C iis normally
ll lleached
h db
by one off th
three methods:
th d
(a) Dump leaching
(b) Heap leaching
(c) Bacterial leaching
Hydrometallurgy of copper
2. Leaching (cont.) (a) Dump leaching
Hydrometallurgy of copper
2. Leaching (cont.) (b) Heap leaching
Similar to dump leaching except ore not simply dumped on a
hill id b
hillside,
butt iis crushed
h d tto gravell size
i & piled
il d onto
t an artificial
tifi i l
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
Non
oxidative leaching: No change in oxidation state.
e.g. (1) dissolution of copper sulphate by water:
2 (aq) + SO 22
CuSO4(s) + H2O(l) Cu2+
4 (aq)
Hydrometallurgy of copper
2. Leaching (cont.)
Oxidative leaching: Many ores only soluble once oxidised
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,
reduced and write out the balanced half reactions for
each.
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Several bacteria,
bacteria especially Thiobacilli,
Thiobacilli are able to
solubilise metal minerals by oxidising ferrous to ferric
iron as well as elemental sulphur
iron,
sulphur, sulphide
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
leaching.
Very slow process; takes years for good recovery
But low investment and operating costs
costs.
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Thiobacilli
Are
A acidotolerant;
id t l
t some grow att pHs
H as llow as 0
0.5
5
Are
A ttolerant
l
t against
i t heavy
h
metal
t l ttoxicity.
i it
A
Are chemolithoautotrophs
h
lith
t t h (C source iis CO2 & energy
derived from chemical transformation of inorganic
matter).
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
th fferric-ferrous
the
i f
cycle,
l and
d (b) direct
di t mechanism
h i
involving physical contact of the organism with the
sulphide
l hid mineral.
i
l
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms: Indirect
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms: Indirect
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms: Indirect
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Mechanisms: Direct
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Compared to other extraction techniques:
Hydrometallurgy of copper
2. Leaching (cont.) (c) Bacterial leaching
Compared to other extraction techniques:
ADVANTAGES:
Economical: Simpler, cheaper, less infrastructure.
More environmentally friendly; no SO2 emissions, less
landscape damage.
DISADVANTAGES:
Economical: Very slow compared to smelting; less
profit Delay in cash flow for new plants
profit.
plants.
Environmental; Toxic chemicals sometimes produced.
H2SO4 pollution.
pollution Precipitation of heavy ions (Fe
(Fe, Zn,
Zn
As) pollution.
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
removed.
After leaching,
leaching Cu in solution can be very dilute
dilute.
need a way to concentrate it.
Both of these are generally done using ion exchange
processes the two most common being ion exchange
processes,
chromatography, and solvent extraction.
Hydrometallurgy of copper
3. Solution Purification
Ion exchange chromatography
DEFINITION: a solution containing a mixture of metal
i
ions
iis contacted
t t d with
ith a resin
i th
thatt iis iinsoluble
l bl iin th
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 th
thatt exchange
h
with
ith cations
ti
(M+) or
anions (M), respectively.
The ion-exchange process is reversible.
Hydrometallurgy of copper
3. Solution Purification
Ion exchange chromatography: Theory
Analyte molecules retained on a column (stationary
phase)
h
)b
based
d on coulombic
l bi (i
(ionic)
i ) iinteractions.
t
ti
St
Stationary
ti
phase
h
has
h ionic
i i functional
f
ti
l groups (R-X)
(R X)
that interact with analyte ions of opposite charge.
Two types: cation exchange chromatography:
R X-C+ + M+B- R-X
R-X
R X-M+ + C+ Banion
i exchange
h
chromatography:
h
t
h
R-X+A- + M+B- R-X+M- + M+ A-
Hydrometallurgy of copper
3. Solution Purification
Cu Ion exchange chromatography
C
Carboxyl
b
l groups exchanges
h
the
h iion iit currently
l h
holds
ld
(H+) for a Cu2+ ion.
2 is
The
Th Cu
C 2+
i llater
t released
l
db
by contacting
t ti it with
ith a
stripping solution (very high H+ conc.).
Hydrometallurgy of copper
3. Solution Purification: Solvent extraction
DEFINITION: a method to separate compounds
b
based
d on th
their
i relative
l ti solubilities
l biliti iin 2 diff
differentt
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
t mix,
i and
d iion ttransfer
f occurs between
b t
them.
th
Once ion transfer is complete (equilibrium), mixture is
allowed to separate
separate.
Aqueous solution is removed & the organic phase
(containing the Cu2+) is mixed with an aqueous stripping
solution.
Cu2+ moves back into the aqueous
q
p
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
R = alkyl ,phenyl,
phenyl or H
R
R1 = alkyl
F
Function
i b
by means off a pH-dependent
Hd
d
cation-exchange
i
h
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
Cu.
back extraction (stripping stage) the pH is lowered
During back-extraction
further, releasing the Cu, and regenerating the hydroxyoxime
for recycle
y
to the extraction stage.
g
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
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.
Hydrometallurgy of copper
4. Metal Recovery:
At this point, the metal needs to be recovered from
solution
l ti iin th
the solid
lid fform.
Thi
This is
i either
ith achieved
hi
d chemically,
h i ll or
electrochemically.
Hydrometallurgy of copper
4. Metal Recovery:
Chemical recovery
Ered = +0.34 V
Ered = -0.44
0 44 V
Hydrometallurgy of copper
4. Metal Recovery:
Chemical recovery
Hydrometallurgy of copper
4. Metal Recovery:
Electrochemical recovery
Electrowinning
Hydrometallurgy of copper
4. Metal Recovery:
Electrochemical recovery
Electrowinning
Hydrometallurgy of copper
4. Metal Recovery:
Electrochemical recovery
Electrorefining
d sludge
l d collected
ll t d & sold
ld ffor ffurther
th
refining.
Hydrometallurgy of copper
4. Metal Recovery:
Electrorefining
Electrochemical recovery
Hydrometallurgy of copper
Summary:
Silicon production
rG /kJ
J mol-1
Elli h
Ellingham
di
diagrams
Temperature /C
Ellingham diagrams
Lower the position of a metal in the
Ellingham diagram = greater stability
of its oxide.
A metal found in the Ellingham
diagram can act as a reducing agent
for a metallic oxide found above itit.
Stability of metallic oxides decrease
with increase in temperature.
Intersection of two lines imply the equilibrium of oxidation and
reduction reaction between two substances. Reduction possible
at the intersection p
point and higher
g
temperatures
p
where the G
line of the reductant is lower on diagram than the metallic
oxide.
Silicon production
rG
G /kJ m
mol-1
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) :
2SiO2(l) + 3C(s) Si(l) + 2CO2(g) + SiC(s)
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 byy an electric
arc burning between
graphite electrodes.
An arc forms between the charge and the electrodes.
Th
The charge
h
is
i heated
h t d both
b th b
by currentt passing
i th
through
h th
the charge
h
and
d
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 2nd most abundant element in the earths
crust (~28%).
Principal constituent of natural stone
stone, glass
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
2nd largest application is in the
production of silicones. These are
polymers containing Si-O and Si-C
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.
i
d t
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
Finally, the high-purity
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
P ifi ti
Purification:
Th Czochralski
The
C
h l ki process
Ultra-pure
Ultra pure Si (only a few ppm of impurities) is melted in a crucible
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
gradients, rate of pulling
pulling, and
rotation speed, a large single-crystal (called a boule) can be
extracted from the melt.
Silicon production
P ifi ti
Purification:
Th Czochralski
The
C
h l ki process
Silicon production
P ifi ti
Purification:
Th Czochralski
The
C
h l ki process
The boule is then ground down
to a standard diameter and
sliced into wafers
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"
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
El t h i l preparation:
Electrochemical
ti
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 doesnt conduct electricity,
but it has been shown that a tungsten wire sealed within a
quartz tube with the tungsten end exposed,
exposed can act as a
cathode.
Silicon production
El t h i l preparation:
Electrochemical
ti
The anode is usually graphite,
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
f reduction.
c) After washing.
d) Side view
view.
Silicon production
El t h i l preparation:
Electrochemical
ti
Conversion of quartz to Si occurs at the three-phase
three phase
boundary between the SiO2, the electrolyte, and the flattened
end of the tungsten
g
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,
electrode but in reality it grinds to a halt very quickly
quickly.
y cannot
Reason for this is that the molten electrolyte
penetrate through the newly formed Si layer on the surface.
three-phase boundary formation halted.
Silicon production
El t h i l preparation:
Electrochemical
ti
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.
Silicon production
El t h i l preparation:
Electrochemical
ti
Aluminium production
Most abundant metallic element in the earths crust.
But, extremely rare in its free form.
Once considered as a precious metal more valuable
than gold!
g
Al is a highly
g y reactive metal that forms strong
g 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
g reducing
g agent
g
than C.
Must therefore be extracted by electrolysis.
Aluminium production involves two steps: 1) purifying
Al2O3 from bauxite ((the Bayer
y process)
p
) 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 - AlOOH (less extractable than Gibbsite)
Diaspore - AlOOH (difficult to extract)
Aluminium production
Th Bayer
The
B
process: Step 1: Dissolution
The hydrated
h drated aluminium
al mini m oxides
o ides are first selecti
selectively
el
dissolved from bauxite:
Al(OH)3 + NaOH NaAlO2 + 2H2O (Gibbsite dissolution)
AlOOH + NaOH NaAlO2 + H2O (Boehmite dissolution)
An undesirable side reaction is the formation of red
mud, which occurs when Al(OH)3 reacts with
mud
dissolved Kaolinite clay:
5Al2Si2O5(OH)4 + 2Al(OH)3 + 12NaOH 2Na6Al6Si5O17(OH)10 + 10H2O
Aluminium production
Th Bayer
The
B
process: Step 2: Solid-Liquid Separation
The digested bauxite now consists of 1 liquid and 2
solid components:
Caustic liquid soln
soln. with dissolved Al
Al.
Undissolved coarse material (sand).
Precipitated fines (red mud).
Aluminium production
Th Bayer
The
B
process: Step 3: Precipitation
The remaining solution is supersaturated
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
doesntt crystallise out easily on its own
own.
size, they
Once the crystals have reached the desired size
are removed, washed, and filtered.
The spent liquor is reheated, recausticised and
recycled.
recycled
Aluminium production
Th Bayer
The
B
process: Step 4: Calcination
Wet crystals of Al(OH)3, obtained from the
precipitation step are dried by heating to around 1300
1500 C
C.
This process also converts the Al(OH)3 to Al2O3:
2Al(OH)3 Al2O3 + 3H2O
Aluminium production
Th Bayer
The
B
process: Problems
Problems result from the coordination chemistry of Al
in basic solutions. Generally accepted structures:
Aluminium production
Th Bayer
The
B
process: Problems
In addition,
addition the inertness of Al(III) leads to slow rates
of crystallisation, requiring large vessels & large
volumes of circulating solution & seed material
material.
Aluminium production
Th Hall-Heroult
The
H ll H
lt process:
Reactive metals (e
(e.g.
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
electrolyte.
Would have to be dissolved in a conductive salt (NaCl
or KCl)
KCl).
Commercially viable production of Al only commenced
once the use of cryolite (Na3AlF6) was discovered.
Aluminium production
Th Hall-Heroult
The
H ll H
lt process:
Cryolite is electrically conductive
conductive, and dissolves Al2O3.
Aluminium production
Th Hall-Heroult
The
H ll H
lt process:
Anhydrous Al2O3 melts at over 2000C
2000 C which is too
high to be used as a molten medium for electrolytic
reduction of Al
Al.
Al2O3 dissolved in cryolite has a m
m.p.
p of 1012C
1012 C & is a
good electrical conductor.
Graphite rods are used as anodes & are consumed in
the electrolytic process
process.
vessel lined with graphite
graphite.
The cathode is a steel vessel,
Aluminium production
Th Hall-Heroult
The
H ll H
lt process:
The electrode reactions are as follows:
Anode: C(s) + 2O2-(l) CO2(g) + 4eC th d
Cathode:
3 - + Al3+(l) Al(l)
3e
CLASS EXERCISE : Write out the balanced overall reaction
Aluminium production
Th Hall-Heroult
The
H ll H
lt process:
Step 1: calculate the number of coulombs (C) from the
current (I) and the time (t):
Aluminium production
Th Hall-Heroult
The
H ll H
lt process:
Step 3: find the mass of Al produced:
Aluminium production
Th Hall-Heroult
The
H ll H
lt 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
high, typically 100
100,000
000 to
150,000 A or more!
Electrical power is the single largest cost in Al
production Al smelters are typically located in areas
production,
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.
furnace
Pyrometallurgy of iron
Blast furnace:
1)) Hot g
gas blast
2) Melting zone
3) Reduction of FeO
4) Reduction
R d ti off F
Fe2O3
5) Pre-heating zone
6) Feed of ore
ore,
limestone + coke
7)) Exhaust g
gases
8) Column of ore, coke
+ limestone
9) Removal
R
l off slag
l
10) Tapping of molten pig iron
11) W
Waste
t gas collection
ll ti
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.
slag
Pyrometallurgy of iron
Slag:
Most rocks are composed of silica (SiO2) and silicates
(SiO32-) & are almost always present in the ore.
These compounds dont 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 & doesnt dissolve
in the molten metal.
Pyrometallurgy of iron
Slag:
The heat of the furnace decomposes the limestone to
give calcium oxide (e.g. of a calcination reaction).
CaO (a basic oxide) reacts with silicon dioxide to give
calcium silicate.
CaCO3(s)
CaO(s) + CO2(g)
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 2000C :
2C(s) + O2(g) 2CO(g)
H = -221
H
221 kJ
H = +131 kJ
Pyrometallurgy of iron
Molten iron is produced lower
down the furnace & removed.
Slag
g 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 250C (top of the furnace), limestone is
calcinated:
CaCO3(s) CaO(s) + CO2(g)
Also
Al att the
th top
t off the
th furnace,
f
hematite
h
tit is
i reduced:
d
d
3Fe2O3(s) + CO(g) 2Fe3O4(s) + CO2(g)
Reduction of Fe3O4 occurs further down the furnace
( 700C):
(~700C):
Fe3O4(s) + CO(g) 3FeO(s) + CO2(g)
Near the middle of the furnace (1000C) 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 doesnt
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,
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
cantt be used as N2 reacts with iron to form iron
nitride which is brittle.
Pyrometallurgy of iron
Steelmaking
Iron converter
Pyrometallurgy of iron
Types of iron & steel
Wrought iron iron with all the C removed
removed. Soft &
easily worked with little structural strength. No longer
produced commercially
commercially.
Mild steel iron containing around 0
0.25%
25% C
C. Stronger
& harder than pure iron. Has many uses including
nails wire
nails,
wire, car bodies
bodies, girders & bridges
bridges, etc
etc.
1.5%
5% C
C. Very
High carbon steel contains around 1
hard, but brittle. Used for things like cutting tools, and
masonry nails
nails.
Pyrometallurgy of iron
Types of iron & steel
Stainless steel iron mixed with chromium and nickel
nickel.
Resistant to corrosion. Uses include cutlery, cooking
utensils kitchen sinks
utensils,
sinks, etc
etc.
Titanium steel iron mixed with titanium
titanium. Withstands
high temperatures. Uses include gas turbines,
spacecraft parts
parts, etc
etc.
manganese. Very
Manganese steel iron mixed with manganese
hard. Uses include rock-breaking machinery, military
helmets etc
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 3000C.
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
l
d iin the
h reaction.
i
Component
Fe2O3(s)
Al(s)
Al2O3(s)
Fe(s)
Hfo (kJ/mol)
-822.2
0
-1,669.8
1 669 8
0
Electrowinning of iron
The Pyror process:
Studies into iron extraction by electrowinning from
sulphate solutions were first carried out around 50
years ago
ago, then subsequently forgotten
forgotten.
May become important again in the future as new
new,
more environmentally friendly methods are sought for
steelmaking.
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
loosely-bound
bound S
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.
Electrolysis Iron is reduced and deposited on
the cathode, while O2 is evolved, and H2SO4 is
regenerated at the anode
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:
Electrowinning of iron
The Pyror process:
Gold extraction
G ld mining
Gold
i i
Historical:
Gold extraction
G ld mining
Gold
i i
Historical:
Gold extraction
G ld mining
Gold
i i
Modern methods:
Gold extraction
G ld ore processing
Gold
i
Gold cyanidation:
Gold extraction
G ld ore processing
Gold
i
Gold cyanidation:
Agitated leaching
Gold extraction
G ld ore processing
Gold
i
Gold cyanidation:
Gold extraction
G ld ore processing
Gold
i
Gold cyanidation:
Heap leaching
Gold extraction
G ld ore processing
Gold
i
Gold cyanidation:
Heap leaching
Gold extraction
G ld ore processing
Gold
i
Gold cyanidation:
Heap leaching
Gold extraction
G ld ore processing
Gold
i
Gold recovery:
Gold extraction
G ld ore processing
Gold
i
Gold recovery:
On completion
p
of cyanidation,
y
p
pregnant
g
p
pulp
p is
transferred to Carbon In Pulp (CIP) process.
Pregnant pulp passed through a number of tanks (6 to
8) 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
low grade or tailings solution.
Gold extraction
G ld ore processing
Gold
i
Gold recovery:
Gold extraction
G ld ore processing
Gold
i
Gold recovery:
1) Carbon in pulp
Gold extraction
G ld ore processing
Gold
i
Gold recovery:
1) Carbon in pulp
Gold extraction
Carbon in pulp
Gold extraction
G ld ore processing
Gold
i
Gold recovery:
2) Merrill
Merrill-Crowe
Crowe process
Gold extraction
G ld ore processing
Gold
i
Gold recovery:
2) Merrill
Merrill-Crowe
Crowe process