Title: Extraction Metallurgy
1Extraction Metallurgy
Part 2 Case studies
Dr. C.B. Perry (C306)
http//www.gh.wits.ac.za/chemnotes
Chem 3033
2Extraction 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.
3Pyrometallurgy of copper
Reminder Pyrometallurgy is the use of heat to
reduce the mineral to the free metal, and
usually involves 4 main steps
- Calcination thermal decomposition of the ore
with associated elimination of a volatile
product. - Roasting a metallurgical treatment involving
gas- solids reactions at elevated temperatures. - Smelting a melting process which separates the
chemical reaction products into 2 or more
layers. - Refining treatment of a crude metal product to
improve its purity.
4Pyrometallurgy 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).
5Pyrometallurgy of copper
The following steps are involved in Cu extraction
- Concentration
- Roasting
- Smelting
- Conversion
- Refining
6Pyrometallurgy 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.
7Pyrometallurgy 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.
8Pyrometallurgy of copper
1. Concentration (cont.)
Froth-flotation
9Pyrometallurgy 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.
10Pyrometallurgy 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.
11Pyrometallurgy 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.
12Pyrometallurgy 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)
13Pyrometallurgy 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.
14Pyrometallurgy 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 - 2Cu2SFeS(l) 3O2(g) SiO2(l) ? 2FeOSiO2(l)
2SO2(g) Cu2S(l) - The oxidized Fe and Si form a slag (insoluble in
matte) that is skimmed off disposed off.
15Pyrometallurgy 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.
16Pyrometallurgy 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.
17Pyrometallurgy of copper
5. Refining (cont.)
- Metallic impurities more active then Cu are
oxidized at the anode, but dont 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) 2e-
- Cu2(aq) 2e- ? Cu(s) E?red -0.83V
18Pyrometallurgy of copper
5. Refining (cont.)
19Pyrometallurgy 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.
20Hydrometallurgy 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
non-existent.
Disadvantages
- Large amount of water used, ? greater potential
for contamination. - Waste waters contain soluble metal compounds,
chelating compounds organic solvents.
21Hydrometallurgy of copper
The following steps are involved
- Ore preparation
- Leaching
- Solution purification
- Metal recovery
22Hydrometallurgy of copper
1. Ore preparation
- Ore undergoes some degree of comminution
(crushing pulverisation) to expose the Cu
oxides sulphides to leaching solution.
23Hydrometallurgy 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.
24Hydrometallurgy 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
- Dump leaching
- (b) Heap leaching
- (c) Bacterial leaching
25Hydrometallurgy 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.
26Hydrometallurgy 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.
27Hydrometallurgy 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)2CO3(s)
2H2SO4(aq) ? 2CuSO4(aq) CO2(g) 3H2O(l)
28Hydrometallurgy 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 ? S6
8e- (oxidation) 2O2 8e- ? 4O2-
(reduction)
29Hydrometallurgy 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.
30Hydrometallurgy of copper
2. Leaching (cont.)
(c) Bacterial leaching
Thiobacilli
- Are acidotolerant some grow at pHs 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).
31Hydrometallurgy 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.
32Hydrometallurgy 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)
33Hydrometallurgy 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
34Hydrometallurgy 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.
35Hydrometallurgy 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
36Hydrometallurgy 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.
37Hydrometallurgy 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.
38Hydrometallurgy 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.).
39Hydrometallurgy 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
40Hydrometallurgy of copper
3. Solution Purification
Solvent extraction
41Hydrometallurgy 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.
42Hydrometallurgy of copper
3. Solution Purification
Solvent extraction
Extractants
- The most successful extractants for copper are of
the ortho-hydroxyoxime type
R alkyl ,phenyl, or H
R1 alkyl
- Function by means of a pH-dependent
cation-exchange mechanism
Cu2 2HA ? CuA2 2H
(where H in HA denotes the replaceable, phenolic
proton)
43Hydrometallurgy 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.
44Hydrometallurgy of copper
3. Solution Purification
Solvent extraction
Extractants
- Cu2 forms square-planar complexes with
hydroxyoxime
- H-bonding between the oximic H and the phenolic O
affords this 21 complex unusual stability. - The formation constant (K2) for the 21 complex
is much greater than for the 11 complex.
45Hydrometallurgy 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.
46Hydrometallurgy 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.
47Hydrometallurgy 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) E?red 0.34 V Fe2(aq)
2e ? Fe(s) 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.
48Hydrometallurgy 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.
49Hydrometallurgy of copper
4. Metal Recovery
Electrochemical recovery
Electrowinning
- An electrochemical process for precipitating
metals from solution.
50Hydrometallurgy 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.
51Hydrometallurgy 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.
52Hydrometallurgy of copper
4. Metal Recovery
Electrochemical recovery
Electrorefining
53Hydrometallurgy of copper
Summary
54Silicon production
- More difficult to extract Si than either Cu or Fe.
?rG? /kJ mol-1
Temperature /?C
55Silicon 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)
56Silicon 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.
57Silicon production
The electric arc furnace
- Electric arc furnaces require huge amounts of
electricity. A mid-sized 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.
58Silicon production
Applications
- Si is the 2nd most abundant element in the
earths 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.
59Silicon 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.
60Silicon production
Purification
- Ultra-pure silicon is required for the production
of semiconductors.
61Silicon 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!
62Silicon 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.
63Silicon production
Purification
The Czochralski process
64Silicon 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!
65Silicon 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 doesnt 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.
66Silicon production
Electrochemical preparation
- The anode is usually graphite, and the reduction
is carried out in a solution of molten CaCl2 at
around 850? C.
- SEM of W-SiO2 electrode before reduction.
- After reduction.
- After washing.
- Side view.
67Silicon 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.
68Silicon 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.
- SEM of SiO2 powder
- reduced Si powder.
69Silicon production
Electrochemical preparation
70Aluminium 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! - Al is a highly reactive metal that forms strong
bonds with O. - Requires a large amount of energy to extract from
Al2O3.
71Aluminium 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 - ?AlOOH (less extractable than
Gibbsite) - Diaspore - aAlOOH (difficult to extract)
72Aluminium 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) AlOOH 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.
73Aluminium 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.
74Aluminium 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 doesnt 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.
75Aluminium 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
76Aluminium 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.
77Aluminium 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.
78Aluminium 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.
79Aluminium production
The Hall-Heroult process
- Cryolite is electrically conductive, and
dissolves Al2O3.
80Aluminium 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.
81Aluminium production
The Hall-Heroult process
- The electrode reactions are as follows
Anode C(s) 2O2-(l) ? CO2(g) 4e- Cathode
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.
82Aluminium 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
F (Faraday constant) magnitude of electric
charge per mole of electrons.
83Aluminium 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
84Aluminium 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.
85Pyrometallurgy 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.
86Pyrometallurgy of iron
Blast furnace
- Hot gas blast
- Melting zone
- Reduction of FeO
- Reduction of Fe2O3
- Pre-heating zone
- Feed of ore, limestone coke
- Exhaust gases
- Column of ore, coke limestone
- Removal of slag
10) Tapping of molten pig iron 11) Waste gas
collection
87Pyrometallurgy 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.
88Pyrometallurgy 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.
89Pyrometallurgy 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.
90Pyrometallurgy of iron
91Pyrometallurgy 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.
92Pyrometallurgy 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, 0.6-1.2 Si, 0.4-2.0 Mn S and P
and needs to be further processed.
93Pyrometallurgy 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)
94Pyrometallurgy 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.
95Pyrometallurgy 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 cant be used as N2 reacts with iron to form
iron nitride which is brittle.
96Pyrometallurgy 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. - Once oxidation complete, contents poured out
various alloying elements added to produce steels.
Iron converter
97Pyrometallurgy 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.
98Pyrometallurgy 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.
99Pyrometallurgy 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)
100Pyrometallurgy of iron
The thermite reaction
101Pyrometallurgy 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.
?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!
102Electrowinning 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.
103Electrowinning 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.
104Electrowinning 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 2e- Fe2 ? Fe3
e-
105Electrowinning of iron
The Pyror process
106Electrowinning 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.
107Electrowinning of iron
The Pyror process
108Gold 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.
109Gold extraction
Gold mining
Historical
- Sluicing water is channelled to flow through a
sluice-box. Sluce-box 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.
110Gold extraction
Gold mining
Modern methods
- Hard rock mining used to extract gold encased
in rock. Either open pit mining or underground
mining.
111Gold 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)
112Gold 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.
113Gold 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.
114Gold 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.
115Gold 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
high-grade ore heap leaching for marginal grade
ores that would otherwise be considered waste
rock.
116Gold 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.
117Gold 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.
118Gold 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 lt
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.
119Gold 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.
120Gold 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.
121Gold extraction
Carbon in pulp
122Gold 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.
123Gold 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.