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Title: Extraction Metallurgy


1
Extraction Metallurgy
Part 2 Case studies
Dr. C.B. Perry (C306)
http//www.gh.wits.ac.za/chemnotes
Chem 3033
2
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.

3
Pyrometallurgy 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.

4
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).
5
Pyrometallurgy of copper
The following steps are involved in Cu extraction
  • Concentration
  • Roasting
  • Smelting
  • Conversion
  • Refining

6
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.

7
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.

8
Pyrometallurgy of copper
1. Concentration (cont.)
Froth-flotation
9
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.

10
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.
11
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.

12
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)

13
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.

14
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
  • 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.

15
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.

16
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.

17
Pyrometallurgy 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

18
Pyrometallurgy of copper
5. Refining (cont.)
19
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.

20
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
    non-existent.

Disadvantages
  • Large amount of water used, ? greater potential
    for contamination.
  • Waste waters contain soluble metal compounds,
    chelating compounds organic solvents.

21
Hydrometallurgy of copper
The following steps are involved
  • Ore preparation
  • Leaching
  • Solution purification
  • Metal recovery

22
Hydrometallurgy of copper
1. Ore preparation
  • Ore undergoes some degree of comminution
    (crushing pulverisation) to expose the Cu
    oxides sulphides to leaching solution.

23
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.

24
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
  • Dump leaching
  • (b) Heap leaching
  • (c) Bacterial leaching

25
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.

26
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.

27
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)2CO3(s)
2H2SO4(aq) ? 2CuSO4(aq) CO2(g) 3H2O(l)
28
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 ? S6
8e- (oxidation) 2O2 8e- ? 4O2-
(reduction)
29
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.

30
Hydrometallurgy 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).

31
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.

32
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)
33
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
34
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.

35
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
36
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.

37
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.

38
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.).

39
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

40
Hydrometallurgy of copper
3. Solution Purification
Solvent extraction
41
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.

42
Hydrometallurgy 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)
43
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.

44
Hydrometallurgy 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.

45
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.

46
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.

47
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) 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.

48
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.

49
Hydrometallurgy of copper
4. Metal Recovery
Electrochemical recovery
Electrowinning
  • An electrochemical process for precipitating
    metals from solution.

50
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.

51
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.

52
Hydrometallurgy of copper
4. Metal Recovery
Electrochemical recovery
Electrorefining
53
Hydrometallurgy of copper
Summary
54
Silicon production
  • More difficult to extract Si than either Cu or Fe.

?rG? /kJ mol-1
Temperature /?C
55
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)
56
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.

57
Silicon 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.

58
Silicon 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.

59
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.

60
Silicon production
Purification
  • Ultra-pure silicon is required for the production
    of semiconductors.

61
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!

62
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.

63
Silicon production
Purification
The Czochralski process
64
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!

65
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 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.

66
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.
  • SEM of W-SiO2 electrode before reduction.
  • After reduction.
  • After washing.
  • Side view.

67
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.

68
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.
  • SEM of SiO2 powder
  • reduced Si powder.

69
Silicon production
Electrochemical preparation
70
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!
  • Al is a highly reactive metal that forms strong
    bonds with O.
  • Requires a large amount of energy to extract from
    Al2O3.

71
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 - ?AlOOH (less extractable than
    Gibbsite)
  • Diaspore - aAlOOH (difficult to extract)

72
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) 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.

73
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.

74
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 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.

75
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
76
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.

77
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.

78
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.

79
Aluminium production
The Hall-Heroult process
  • Cryolite is electrically conductive, and
    dissolves Al2O3.

80
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.

81
Aluminium 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.
82
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

F (Faraday constant) magnitude of electric
charge per mole of electrons.
83
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

84
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.

85
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.

86
Pyrometallurgy 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
87
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.

88
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.

89
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.

90
Pyrometallurgy of iron
91
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.

92
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, 0.6-1.2 Si, 0.4-2.0 Mn S and P
    and needs to be further processed.

93
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)
94
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.

95
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 cant be used as N2 reacts with iron to form
    iron nitride which is brittle.

96
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.
  • Once oxidation complete, contents poured out
    various alloying elements added to produce steels.

Iron converter
97
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.

98
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.

99
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)
100
Pyrometallurgy of iron
The thermite reaction
101
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.
?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!
102
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.

103
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.

104
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 2e- Fe2 ? Fe3
e-
105
Electrowinning of iron
The Pyror process
106
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.

107
Electrowinning of iron
The Pyror process
108
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.

109
Gold 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.

110
Gold extraction
Gold mining
Modern methods
  • Hard rock mining used to extract gold encased
    in rock. Either open pit mining or underground
    mining.

111
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)
112
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.

113
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.

114
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.

115
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
    high-grade ore heap leaching for marginal grade
    ores that would otherwise be considered waste
    rock.

116
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.

117
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.

118
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 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.

119
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.

120
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.

121
Gold extraction
Carbon in pulp
122
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.

123
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.
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