Extraction Metallurgy Student Copy

Extraction MetallurgyPart 2: Case studies

Dr. C.B. Perry (C306)y ( )

http://www.gh.wits.ac.za/chemnotes

Chem 3033

Extraction MetallurgyPart 2: Case studies

• Copper – Pyrometallurgy route and environmental concerns The• Copper – Pyrometallurgy route and environmental concerns. The hydrometallurgical alternative.

• Hydrometallurgical processes – ion exchange processes, solvent extraction, and bacterial leaching.

• Iron – Pyrometallurgy and the blast furnace.

• Silicon – The electric arc furnace. Purification by the Czochralski process.

• Aluminium – Electrolytic reduction.

• The siderophiles – The extraction of Au and the Pt group metals and their purification.

Pyrometallurgy of copperReminder: 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 with1. Calcination: thermal decomposition of the ore with associated elimination of a volatile product.

2. Roasting: a metallurgical treatment involving gas-solids reactions at elevated temperatures.solids reactions at elevated temperatures.

3. Smelting: a melting process which separates the g g p pchemical reaction products into 2 or more layers.

4. Refining: treatment of a crude metal product to improve its purity.

Pyrometallurgy of copperCu 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 copperThe following steps are involved in Cu extraction:

1 Concentration1.Concentration

2 Roasting2.Roasting

3 Smelting3.Smelting

4 Conversion4.Conversion

5 Refining5.Refining

Pyrometallurgy of copper1. ConcentrationOnly ~0 7% of the extracted ore contains CuOnly 0.7% of the extracted ore contains Cu

Finely crushed ore concentrated by the froth-flotationFinely crushed ore concentrated by the froth-flotationprocess:

• Copper ore slurry mixed with:Lime water – to give basic pH

Pine oil – to make bubblesPine oil – to make bubbles

An alcohol – to strengthen bubbles

A chemical collector

Pyrometallurgy of copperS1. Concentration (cont.)

ROS

X

S

• Chemical collectors such as xanthates (salts & esters of xanthic acid),

Na3PS2O2

( ),dithiophosphates, or thionocarbamates make the ore surface hydrophobic. NO

R1

S

Hy pR2

NO

H d hili tt t dH d h bi tt t d Hydrophilic – attracted to sulphide minerals

Hydrophobic – attracted to hydrocarbon pine oil

SCH3

O SK

CH3Potassium

amyl xanthate

Pyrometallurgy of copper1. Concentration (cont.)• Raising the pH causes the polar ends to ionize more• Raising the pH causes the polar ends to ionize more,

thereby preferentially sticking to chalcopyrite (CuFeS2) and leaving pyrite (FeSs) alone.and leaving pyrite (FeSs) alone.

• Air is bubbled through the suspension.g p

• Finely divided hydrophobic ore particles latch on to the i b bbl d t l t th f h f th iair bubbles and travel to the surface where a froth is

formed.

• The froth containing the Cu ore is skimmed off and reprocessedreprocessed.

Pyrometallurgy of copper1. Concentration (cont.)• In this manner the ore is• In this manner, the ore is

concentrated to an eventual value of over 28% Cu.value of over 28% Cu.

• The remaining material (sand g (particles & other impurities) sink to the bottom & is discarded or

d t t t threprocessed to extract other elements.

Pyrometallurgy of copper1. Concentration (cont.)

Froth-flotation

Pyrometallurgy of copper2. Roasting• Involves partial oxidation of the sulphide mineral with• 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 autogenousReactions are exothermic, ∴ roasting is an autogenous process requiring little or no additional fuel.

NB t ll th l hid idi d l d 1/3• NB, not all the sulphides are oxidised, only around 1/3. Rest remain as sulphide minerals.

• The gases produced contain around 5 – 15% SO2, which is used for sulphuric acid production.

Pyrometallurgy of copper2. 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 smelterenergy needed by the smelter.

Pyrometallurgy of copper3. Smelting• Smelting consists of melting the roasted concentrate• Smelting consists of melting the roasted concentrate

to form 2 molten phases:1) a sulphide “matte” which contains the iron copper1) a sulphide matte , which contains the iron-copper

sulphide mixture.2) an oxide slag which is insoluble in the matte and2) an oxide slag, which is insoluble in the matte, and

contains iron oxides, silicates, and other impurities.

• Smelting is carried out at around 1200°C, usually with a silica flux to make the slag more fluid.

• The matte layer sinks to the bottom, and the slag layer floats on top of the matte & is tapped off & disposed of.

Pyrometallurgy of copper3. Smelting (cont.)• The main reaction is the reduction of copper oxides• 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:ensure that they migrate into the matte phase:

FeS(l) + 6CuO(l) 3Cu O(l) + FeO(l) + SO (g)FeS(l) + 6CuO(l) → 3Cu2O(l) + FeO(l) + SO2(g)FeS(l) + Cu2O(l) → FeO(l) + Cu2S(l)

Cu S(l) + FeS(l) → Cu S•FeS(l) (matte)Cu2S(l) + FeS(l) → Cu2S•FeS(l) (matte)

Pyrometallurgy of copper4. Conversion• After smelting matte contains from between 30 to• 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 O to produce SO from S but leave Cumatte 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 stageremoval stage, and 2) a copper-making stage.

Pyrometallurgy of copper4. Conversion (cont.)

Iron removalIron removal• A silica flux is added to keep the slag (see below)

ltmolten.

• Air is blown into the converter to oxidize the ironAir is blown into the converter to oxidize the iron sulphide according to the following reaction:

2Cu S FeS(l) + 3O (g) + SiO (l) 2FeO SiO (l) + 2SO (g) + Cu S(l)2Cu2S•FeS(l) + 3O2(g) + SiO2(l) → 2FeO•SiO2(l) + 2SO2(g) + Cu2S(l)

• Si added to help form FeO slag.Si added to help form FeO slag.

• The oxidized Fe and Si form a slag (insoluble inThe oxidized Fe and Si form a slag (insoluble in matte) that is skimmed off & disposed off.

Pyrometallurgy of copper

Copper making

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)Th d d t i dThe end product is around 98.5% pure & is known as bli t b fblister copper because of the broken surface

t d b th fcreated by the escape of SO2 gas.

Pyrometallurgy of copper5. Refining

• Almost all copper is refined by electrolysis• Almost all copper is refined by electrolysis.

• The anodes (cast from blister copper) are placed into• 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 anodeCu metal at the anode.

• Cu2+ ions migrate through the electrolyte to the• Cu2+ ions migrate through the electrolyte to the cathode, where Cu metal plates out.

Pyrometallurgy of copper5. Refining (cont.)

• Metallic impurities more active then Cu are oxidized at the anode, but don’t plate out at the cathode.p

• Less active metals are not oxidized at the anode, but collect at the bottom of the cell as a sludgecollect at the bottom of the cell as a sludge.

• The redox reactions are:

Cu(s) → Cu2+(aq) + 2e-

Cu2+(aq) + 2e- → Cu(s) E° = 0 83VCu2 (aq) + 2e → Cu(s) E red = -0.83V

Pyrometallurgy of copper5. Refining (cont.)

Pyrometallurgy of copperEnvironmental impact

• Large amount of gases produced present air pollution problems, in particular SO2 gas → acid rain.p p 2 g

• Dust produced contains heavy metals such as p ymercury, 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 copperEnvironmental impact

Typical Air Emissions T i l Li id Effl tTypical Air Emissions Typical Liquid Effluents

Hydrometallurgy of copperAdvantages• Much more environmentally friendly thanMuch more environmentally friendly than

pyrometallurgy.• Compared to pyrometallurgy only a fraction of the• Compared to pyrometallurgy, only a fraction of the

gases liberated into the atmosphere.• Emissions of solid particles comparatively non-

existent.

DisadvantagesL t f t d t t ti l f• Large amount of water used, ∴ greater potential for contamination.

• Waste waters contain soluble metal compounds, chelating compounds & organic solvents.

Hydrometallurgy of copperThe following steps are involved:

1 Ore preparation1.Ore preparation

2 Leaching2.Leaching

3 Solution purification3.Solution purification

4 Metal recovery4.Metal recovery

Hydrometallurgy of copper1. Ore preparation• Ore undergoes some degree of comminutionOre undergoes some degree of comminution

(crushing & pulverisation) to expose the Cu oxides & sulphides to leaching solution.sulphides to leaching solution.

Hydrometallurgy of copper1. Ore preparation (cont.)• Amount of comminution depends on quality of ore:Amount of comminution depends on quality of ore:

Higher grade ore – more comminution.Lower grade ore – less comminution.(Why??)

• If possible, ore is pre-concentrated; reject ore that contains very little Cu.

Hydrometallurgy of copper2. LeachingDefinition : The dissolution of a mineral in a solvent, while ,leaving the gangue (rock or mineral matter of no value) behind as undissolved solids.

C i ll l h d b f th th d• Cu is normally leached by one of three methods:(a) Dump leaching

(b) Heap leaching

(c) Bacterial leaching

Hydrometallurgy of copper2. Leaching (cont.) (a) Dump leaching

• Leaching solution trickled over a dump.R ff l ti ll t d & th C d f it• 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• Typically only around 60% of the Cu in the dump is

recovered.

Hydrometallurgy of copper2. Leaching (cont.) (b) Heap leaching

• Similar to dump leaching except ore not simply dumped on a hill id b t i h d t l i & il d t tifi i lhillside, but is crushed to gravel size & piled onto an artificial pad.

• After leaching (6 months to 1 year) gangue is removed from• After leaching (6 months to 1 year) gangue is removed from pad, disposed of & replaced with fresh ore.

Hydrometallurgy of copper2. Leaching (cont.)

Leaching reactionsLeaching reactions

Nature of ore determines if leaching is non-oxidative or oxidative.

Non-oxidative leaching: No change in oxidation state.Non oxidative leaching: No change in oxidation state.

e.g. (1) dissolution of copper sulphate by water:2 2CuSO4(s) + H2O(l) → Cu2+(aq) + SO4

2-(aq)

(2) dissolution of alkaline materials by acid:Cu2(OH)2•CO3(s) + 2H2SO4(aq) → 2CuSO4(aq) + CO2(g) + 3H2O(l)

Hydrometallurgy of copper2. Leaching (cont.)Oxidative leaching: Many ores only soluble once oxidisedOxidative leaching: Many ores only soluble once oxidised.

e.g. covellite (CuS) much more soluble if oxidised to CuSO4CuS(s) + O (g) → CuSO (aq)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 forwhich is reduced, and write out the balanced half reactions for each.

Hydrometallurgy of copper2. Leaching (cont.) (c) Bacterial leaching

• Several bacteria especially Thiobacilli are able to• Several bacteria, especially Thiobacilli, are able to solubilise metal minerals by oxidising ferrous to ferric iron as well as elemental sulphur sulphide and otheriron, 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 leachingthe worlds copper is obtained by bacterial leaching.

• Very slow process; takes years for good recovery• Very slow process; takes years for good recovery

• But low investment and operating costs• But low investment and operating costs.

Hydrometallurgy of copper2. Leaching (cont.) (c) Bacterial leachingThiobacilliThiobacilli

A id t l t t H’ l 0 5• Are acidotolerant; some grow at pH’s as low as 0.5

A t l t i t h t l t i it• Are tolerant against heavy metal toxicity.

A h lith t t h (C i CO &• Are chemolithoautotrophs (C source is CO2 & energy derived from chemical transformation of inorganic matter)matter).

Hydrometallurgy of copper2. Leaching (cont.)Mechanisms

(c) Bacterial leachingMechanisms

Generalised reaction : M(II)S + 2O2 → M2+ + SO42-

• Two mechanisms: (a) indirect mechanism involving th f i f l d (b) di t h ithe ferric-ferrous cycle, and (b) direct mechanisminvolving physical contact of the organism with the

l hid i lsulphide mineral.

Hydrometallurgy of copper2. Leaching (cont.)Mechanisms: Indirect

(c) Bacterial leachingMechanisms: Indirect

First step: ferrous sulphate is converted into ferric sulphate by the action of Acidithiobacillus ferrooxidans:

4FeSO + O + 2H SO → 2Fe (SO ) + 2H O4FeSO4 + O2 + 2H2SO4 → 2Fe2(SO4)3 + 2H2O

CLASS EXERCISE : work out which is ferric- and which isCLASS EXERCISE : work out which is ferric and which is ferrous sulphate, and write out the balanced half reactions for each.



• Ferric sulphate is a strong oxidising agent capable of dissolving a range of sulphide mineralsdissolving a range of sulphide minerals.

• In the case of chalcopyrite:• In the case of chalcopyrite:

CuFeS2 + 2Fe2(SO4)3 → CuSO4 + 5FeSO4 + 2S



• The elemental S produced by the indirect method can b t d t H SO b A idithi b illbe converted to H2SO4 by Acidithiobacillus ferrooxidans:

SO f• The H2SO4 helps maintain the pH at levels favourable for bacterial growth.

Hydrometallurgy of copper2. Leaching (cont.)Mechanisms: Direct

(c) Bacterial leachingMechanisms: Direct

• Bacteria actually adheres to the mineral surface prior t ti tt kto enzymatic attack.

Th i l i idi d ith t l h t d• The mineral is oxidised with oxygen to sulphate and metal cations without any detectable intermediate

ioccurring.

I th f llit• In the case of covellite:

CuS + 2O CuSOCuS + 2O2 → CuSO4

Hydrometallurgy of copper2. Leaching (cont.)Compared to other extraction techniques:

(c) Bacterial leachingCompared to other extraction techniques:

• Traditional methods expensive (i.e. roasting + lti ) & i hi h t ti f C ismelting) & require high concentrations of Cu in ore.

B t i ff ti l d l ith l [C ] th• Bacteria can effectively deal with low [Cu] as they simply ignore surrounding waste materials.

• Up to 90% extraction efficiency.

Hydrometallurgy of copper2. Leaching (cont.)Compared to other extraction techniques:

(c) Bacterial leachingCompared 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

DISADVANTAGES:

profit. Delay in cash flow for new plants.• Environmental; Toxic chemicals sometimes produced.

H SO pollution Precipitation of heavy ions (Fe ZnH2SO4 pollution. Precipitation of heavy ions (Fe, Zn, As) – pollution.

Hydrometallurgy of copper3. Solution Purification• Leaching reactions not perfectly selective ∴ other• Leaching reactions not perfectly selective ∴ other

elements in solution as well, not just Cu. These need to be removedto be removed.

• After leaching Cu in solution can be very dilute ∴• 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 exchangeprocesses, the two most common being ion exchange chromatography, and solvent extraction.

Hydrometallurgy of copper3. Solution Purification

Ion exchange chromatographyIon exchange chromatography• DEFINITION: a solution containing a mixture of metal

i i t t d ith i th t i i l bl i thions 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.which labile functional groups are chemically bonded.

• Functional groups can either be acidic (H+) or basic (OH ) th t h ith ti (M+)(OH–) groups that exchange with cations (M+) or anions (M–), respectively.

• The ion-exchange process is reversible.


Ion exchange chromatography: TheoryIon exchange chromatography: Theory• Analyte molecules retained on a column (stationary

h ) b d l bi (i i ) i t tiphase) based on coulombic (ionic) interactions.

St ti h h i i f ti l (R X)• Stationary phase has ionic functional groups (R-X) that interact with analyte ions of opposite charge.

• Two types: cation exchange chromatography:R X C+ M+B R X M+ C+ BR-X-C+ + M+B- ↔ R-X-M+ + C+ B-

i h h t hanion exchange chromatography:R-X+A- + M+B- ↔ R-X+M- + M+ A-


Cu Ion exchange chromatographyCu Ion exchange chromatography

C b l h h i i l h ld• Carboxyl groups exchanges the ion it currently holds (H+) for a Cu2+ ion.Th C 2 i l t l d b t ti it ith• The Cu2+ is later released by contacting it with a stripping solution (very high H+ conc.).

Hydrometallurgy of copper3. Solution Purification: Solvent extraction

• DEFINITION: a method to separate compounds b d th i l ti l biliti i 2 diff tbased on their relative solubilities in 2 different immiscible liquids.

• In industry, this is usually set up as a continuous process


• Organic + aqueous stream pumped into a mixer.• Organic (containing an extractant) and aqueous

t i d i t f b t thcomponents mix, and ion transfer occurs between them.• Once ion transfer is complete (equilibrium), mixture is

allowed to separateallowed to separate.• Aqueous solution is removed & the organic phase

(containing the Cu2+) is mixed with an aqueous stripping(containing the Cu ) is mixed with an aqueous stripping solution.

• Cu2+ moves back into the aqueous phase, and the two q p ,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 copper3. Solution Purification: Solvent extractionExtractants

• The most successful extractants for copper are of the ortho-hydroxyoxime type:

Extractants

hydroxyoxime type:

NOHOH

R = alkyl phenyl or HR

R = alkyl ,phenyl, or HR1 = alkyl

R1

F i b f H d d i h• Function by means of a pH-dependent cation-exchange mechanism:

Cu2+ + 2HA → CuA + 2H+Cu + 2HA → CuA2 + 2H

(where H in HA denotes the replaceable, phenolic proton)

Hydrometallurgy of copper3. Solution Purification: Solvent extractionExtractants

• At low pH (1.5 – 2.0) the ortho-hydroxyoxime extractant complexes the Cu

Extractants

complexes the Cu.

• During back-extraction (stripping stage) the pH is loweredDuring back extraction (stripping stage) the pH is lowered further, releasing the Cu, and regenerating the hydroxyoxime for recycle to the extraction stage.y 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 copper3. Solution Purification: Solvent extractionExtractantsExtractants

• Cu2+ forms square-planar complexes with hydroxyoxime:R

OH

OCu

RN

OH

R1

Cu

RN

O HR

1O

• H-bonding between the oximic H and the phenolic O affords this 2:1 complex unusual stability.

O H

this 2:1 complex unusual stability.

• The formation constant (K2) for the 2:1 complex is much ( 2)greater than for the 1:1 complex.

Hydrometallurgy of copper3. Solution Purification: Solvent extractionExtractantsExtractants

• The tris(salicylaldoximato)iron(III)tris(salicylaldoximato)iron(III) complex is octahedral, and no extended planar ring structureextended planar ring structure is possible between the 3 oxime ligands.

∴ stability of Fe(III) complex is less than Cu(II) complex, which allows the extraction of Cu to be carried out at lower pH than what is required for efficient Fe extraction.

Hydrometallurgy of copper4. Metal Recovery:

• At this point, the metal needs to be recovered from l ti i th lid fsolution in the solid form.

Thi i ith hi d h i ll• This is either achieved chemically, or electrochemically.

Hydrometallurgy of copper4. Metal Recovery: Chemical recovery

• Dissolved copper will plate out on an iron surface according to the following reaction:g g

Cu2+(aq) + Fe(s) → Fe2+(aq) + Cu(s)Why??Why??

Reduction half-reactions:Cu2+(aq) + 2e– → Cu(s) E°

red = +0.34 VFe2+(aq) + 2e– → Fe(s) E° = 0 44 VFe2 (aq) + 2e → Fe(s) E red = -0.44 V• E°

red for the Cu2+ half-reaction is more positive than for 2the Fe2+ half reaction which leads to Cu being

reduced and Fe oxidised.

Hydrometallurgy of copper4. Metal Recovery: Chemical recovery

• Solutions containing dissolved copper are thus run through a bed of shredded scrap iron, resulting in the g p gcopper ions being plated out as solid Cu on the iron surface.

• For the process to be efficient, the surface of the pscrap iron must be large.

Hydrometallurgy of copper4. Metal Recovery: Electrochemical recoveryElectrowinning

• An electrochemical process for precipitating metals f l ti

Electrowinning

from solution.

Hydrometallurgy of copper4. Metal Recovery: Electrochemical recoveryElectrowinning

• A current is passed from an inert anode through a

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 cathodemetal is extracted as it is deposited onto the cathode.

• The anode is made out of a material that will not• The anode is made out of a material that will not easily oxidise or dissolve, such as lead or titanium.

Hydrometallurgy of copper4. Metal Recovery: Electrochemical recoveryElectrorefining

• The anodes consist of unrefined impure metal.

Electrorefining

• Current passes through the acidic electrolyte corroding the anode into the solution.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.Fe) remain in solution.

• Metals with a lower E°red than Cu (Au, Ag) accumulate “ d l d ” ll t d & ld f f thas an “anode sludge” → collected & sold for further

refining.

Hydrometallurgy of copper4. Metal Recovery: Electrochemical recoveryElectrorefiningElectrorefining

Hydrometallurgy of copperSummary:

Silicon productionElli h diEllingham diagrams

• Follow the 2nd law of th d ithermodynamics:

∆G = ∆H - T∆S

J m

ol-1 • Plot ∆G vs T

(y = m × X + c)

Δ rG°

/kJ (y m × X c)

Δ

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• A metal found in the Ellingham diagram can act as a reducing agent for a metallic oxide found above itfor a metallic oxide found above it.

• Stability of metallic oxides decrease with increase in temperature.

• Intersection of two lines imply the equilibrium of oxidation and• Intersection of two lines imply the equilibrium of oxidation and reduction reaction between two substances. Reduction possible at the intersection point and higher temperatures where the ∆G p g pline of the reductant is lower on diagram than the metallic oxide.

Silicon production

• More difficult to extract Si than either Cu or Fe.

mol

-1G°

/kJ

mΔ r

G

Temperature /°C

Silicon production

• Silicon of between 96 to 99% purity is achieved by reduction of quartzite or sand (SiO2, also called silica)reduction of quartzite or sand (SiO2, also called silica)

• High temperatures required achieved in an electric arcHigh temperatures required achieved in an electric arc furnace.

• Reduction carried out in the presence of excess silica to prevent accumulation of silicon carbide (SiC) :to prevent accumulation of silicon carbide (SiC) :

2SiO (l) + 3C(s) → Si(l) + 2CO (g) + SiC(s)2SiO2(l) + 3C(s) → Si(l) + 2CO2(g) + SiC(s)

2SiC(s) + SiO2(l) → 3Si(l) + 2CO(g)

Silicon productionThe electric arc furnace

• Silica and carbon fed in

The electric arc furnaceSilica and carbon fed in through the top, liquid Si collected at the bottom.

• Temps of ± 2000K achieved by an electric yarc burning between graphite electrodes.

• An arc forms between the charge and the electrodes.

Th h i h t d b th b t i th h th h d• The charge is heated both by current passing through the charge and by the radiant energy evolved by the arc.

Silicon productionThe electric arc furnaceThe 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.

Silicon productionApplicationsApplications• Si is the 2’nd most abundant element in the earth’s

crust (~28%).

• Principal constituent of natural stone glass concretePrincipal 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 theImportant constituent of electrical steel (modifies the resistivity & ferromagnetic properties).

• Added to molten cast iron to improve its performance in casting thin sections.

Silicon productionApplicationsApplications• 2’nd 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 productionPurificationPurification• Ultra-pure silicon is required for the production of

i d tsemiconductors.

Silicon productionPurificationPurification• Semiconductor-grade Si produced by converting

crude Si to more volatile compounds like SiCl4.

• These are then purified by exhaustive fractionalThese are then purified by exhaustive fractional distillation.

• Reduced back to Si with pure H2.

• Finally the high-purity Si is melted and large singleFinally, the high purity Si is melted and large single crystals are grown by the Czochralski process.

• Electronic grade Si is required to be 99.999999999% pure!

Silicon productionP ifi ti Th C h l kiPurification:

• Ultra pure Si (only a few ppm of impurities) is melted in a crucible

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 toP) can be added to make n-type or p-typesilicon (influences the electrical conductivity).

• A seed crystal mounted on a rod is dipped intoon 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• By carefully controlling the temp gradients, rate of pulling, and rotation speed, a large single-crystal (called a boule) can be extracted from the melt.

Silicon productionP ifi ti Th C h l kiPurification: The Czochralski process

Silicon productionP ifi ti Th C h l kiPurification: The Czochralski process

The boule is then ground down• The boule is then ground down to a standard diameter and sliced into wafers much like asliced 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 widthA point to note however, is that due to kerf losses (the width of the saw blade) as well as polishing losses, more than half of the carefully grown, very pure, single crystal silicon is thrown away before the circuit fabrication process even begins!

Silicon productionEl t h i l ti

A new method that uses electrolysis to reduce SiO to

Electrochemical preparation:

• A new method that uses electrolysis to reduce SiO2 to elemental Si.

• Advantageous because it avoids the high energy costs associated with the older carbothermic route, and also ,reduces the CO2 emissions considerably.

• SiO2 is usually an insulator, and doesn’t conduct electricity, but it has been shown that a tungsten wire sealed within a quartz tube with the tungsten end exposed can act as aquartz tube with the tungsten end exposed, can act as a cathode.

Silicon productionEl t h i l ti• The anode is usually graphite and the reduction is carried

Electrochemical preparation:The anode is usually graphite, and the reduction is carried out in a solution of molten CaCl2 at around 850° C.

a) SEM of W-SiO2 electrode before reduction.

) fb) After reduction.c) After washing.d) Side viewd) Side view.

Silicon productionEl t h i l ti• Conversion of quartz to Si occurs at the three-phase

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.g

• 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 quicklysilica electrode, but in reality it grinds to a halt very quickly.

• Reason for this is that the molten electrolyte cannot ypenetrate through the newly formed Si layer on the surface. ∴ three-phase boundary formation halted.

Silicon productionEl t h i l ti• Solution: replace solid quartz electrode with SiO2 powder

Electrochemical preparation:Solution: replace solid quartz electrode with SiO2 powder pressed into pellets & sintered.

• Resulting electrode porous enough to allow electrolyte to penetrate deeply into the material.

a) SEM of SiO powdera) SEM of SiO2 powderb) reduced Si powder.

Silicon productionEl t h i l tiElectrochemical preparation:

Aluminium production• Most abundant metallic element in the earth’s crust.

• But, extremely rare in its free form.

• Once considered as a precious metal more valuable than gold!g

• Al is a highly reactive metal that forms strong bonds g y gwith O.

• Requires a large amount of energy to extract from Al2O3.2 3

Aluminium production• Cannot be reduced directly by carbon since Al is a

stronger reducing agent than C.g g g

• Must therefore be extracted by electrolysis.

• Aluminium production involves two steps: 1) purifying Al2O3 from bauxite (the Bayer process) and 2) 2 3 ( y p ) )converting Al2O3 to metallic Al (The Hall-Heroult process).)

• Primary Al ore is bauxite, which consists of:Gibbsite Al(OH) (most extractable form)Gibbsite - Al(OH)3 (most extractable form)Boehmite - γAlO•OH (less extractable than Gibbsite)Diaspore αAlO•OH (difficult to extract)Diaspore - αAlO•OH (difficult to extract)

Aluminium productionTh B

The h drated al mini m o ides are first selecti el

The Bayer process: Step 1: Dissolution

• The hydrated aluminium oxides are first selectively dissolved from bauxite:

Al(OH)3 + NaOH → NaAlO2 + 2H2O (Gibbsite dissolution)AlO•OH + NaOH → NaAlO2 + H2O (Boehmite dissolution)

• An undesirable side reaction is the formation of “red mud” which occurs when Al(OH) reacts withmud , 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 ∴• Red mud formation consumes dissolved Al and ∴represents a Al loss.

Aluminium productionTh B• The digested bauxite now consists of 1 liquid and 2

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 AlCaustic liquid soln. with dissolved Al.Undissolved coarse material (sand).Precipitated fines (red mud).Precipitated fines (red mud).

• Sand (mainly undissolved silicates) easily removed since they settle very rapidlysince 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• The Al content of the red mud is recovered & forms part of the liquid layer.

Aluminium productionTh B• The remaining solution is supersaturated containing

The Bayer process: Step 3: Precipitation

• The remaining solution is supersaturated, containing around 100-175 grams of dissolved Al2O3 per litre.

• Al(OH)3 is precipitated out by adding seed crystals since Al(OH)3 doesn’t crystallise out easily on its ownsince Al(OH)3 doesn t crystallise out easily on its own.

• Once the crystals have reached the desired size theyOnce the crystals have reached the desired size, they are removed, washed, and filtered.

• The spent liquor is reheated, recausticised and recycledrecycled.

Aluminium productionTh B• Wet crystals of Al(OH)3 obtained from the

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 °C1500 C.

• This process also converts the Al(OH)3 to Al2O3:This process also converts the Al(OH)3 to Al2O3:2Al(OH)3 → Al2O3 + 3H2O

Aluminium productionTh B• Problems result from the coordination chemistry of Al

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& solvent, which in turn leads to high viscosity of these solutions.

• In turn leads to problems with materials handling & heat exchange.

Aluminium productionTh B

• In addition the inertness of Al(III) leads to slow rates

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 materialvolumes of circulating solution & seed material.

Aluminium productionTh H ll H lt• Reactive metals (e g Mg and Na) can be produced by

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 electrolyteelectrical insulator & cannot be used as an electrolyte. Would have to be dissolved in a conductive salt (NaCl or KCl)or KCl).

• Commercially viable production of Al only commenced once the use of cryolite (Na3AlF6) was discovered.

Aluminium productionTh H ll H lt• Cryolite is electrically conductive and dissolves Al2O3

The Hall-Heroult process:• Cryolite is electrically conductive, and dissolves Al2O3.

Aluminium productionTh H ll H lt• Anhydrous Al2O3 melts at over 2000°C which is too

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 Alreduction of Al.

• Al2O3 dissolved in cryolite has a m p of 1012°C & is aAl2O3 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 processthe electrolytic process.

• The cathode is a steel vessel lined with graphiteThe cathode is a steel vessel, lined with graphite.

Aluminium productionTh H ll H lt• The electrode reactions are as follows:

The Hall-Heroult process:• The electrode reactions are as follows:Anode: C(s) + 2O2-(l) → CO2(g) + 4e-

C th d 3 Al3+(l) Al(l)Cathode: 3e- + Al3+(l) → Al(l)

CLASS EXERCISE : Write out the balanced overall reactionCLASS EXERCISE : Write out the balanced overall reaction

CLASS EXERCISE : Calculate the mass of Al that will be produced in 1.00 hr by the electrolysis of molten Al2O3 , using a p y y 2 3 gcurrent of 10.0 A.

F (Faraday constant) = magnitude of electric charge per mole of electrons = 96500 C mol-1.

Aluminium productionTh H ll H ltThe Hall-Heroult process:• Step 1: calculate the number of coulombs (C) from the• Step 1: calculate the number of coulombs (C) from the

current (I) and the time (t):

• Step 2: find the number of moles of electrons:Step 2: find the number of moles of electrons:

Aluminium productionTh H ll H ltThe Hall-Heroult process:• Step 3: find the mass of Al produced:• Step 3: find the mass of Al produced:

Aluminium productionTh H ll H ltThe Hall-Heroult process:• Electrolytic reduction of Al is costly (3 e- required for• 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 tothe 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 areasproduction, ∴ Al smelters are typically located in areas with inexpensive electric power, like S.A.

Pyrometallurgy of iron• Still the most important pyrometallurgical process

economically.

• The most important sources of iron are hematite (Fe2O3) and magnetite (Fe3O4).

• Prehistorically, iron was prepared by simply heating it with charcoal in a fired clay pot.

• Today, the reduction of iron oxides to the metal isoxides to the metal is accomplished in a blast furnacefurnace.

Pyrometallurgy of ironBlast furnace:1) Hot gas blast) g2) Melting zone3) Reduction of FeO4) R d ti f F O4) Reduction of Fe2O35) Pre-heating zone6) Feed of ore6) Feed of ore,

limestone + coke7) Exhaust gases) g8) Column of ore, coke

+ limestone9) R l f l9) Removal of slag10) Tapping of molten pig iron11) W t ll ti11) Waste gas collection

Pyrometallurgy of iron• The iron ore, limestone, and coke are added to the

top of the furnace.

• Coke is coal that has been heated in an inert atmosphere to drive off volatile components (~ 80 –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 (CaCO ) serves as the source of CaO• Limestone (CaCO3) serves as the source of CaO which reacts with silicates & other impurities in the ore to form slagto form slag.

Pyrometallurgy of ironSlag:• Most rocks are composed of silica (SiO2) and silicates Slag:

(SiO32-) & are almost always present in the ore.

• These compounds don’t melt at the furnace temperature & would eventually clog it up.

• An important chemical method to remove these is by use of a flux which combines with the silica & silicatesuse of a flux which combines with the silica & silicates to produce a slag.

• Slag collects at bottom of furnace & doesn’t dissolve in the molten metal.

Pyrometallurgy of ironSlag: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)Δ

CaO(s) + SiO2(s) → CaSiO3(l)

• Slag helps protect the molten iron from re-oxidation.

Sl i d i d ki d l b• Slag is used in road making, and can also be combined with cement.

Pyrometallurgy of iron

Pyrometallurgy of iron• Air is blown into the bottom of the furnace, and

combusts with the coke to raise the furnace temp up to 2000°C :

2C(s) + O2(g) → 2CO(g) ΔH° = -221 kJ2C(s) O2(g) → 2CO(g) ΔH 221 kJ

• H O in the air also reacts with the coke:• H2O in the air also reacts with the coke:C(s) + H2O(g) → CO(g) + H2(g) ΔH° = +131 kJ

• Since this reaction is endothermic, if the blast furnace gets too hot water vapor is added to cool it downgets too hot, water vapor is added to cool it down without interrupting the chemistry.

Pyrometallurgy of iron• Molten iron is produced lower

down the furnace & removed.

• Slag is less dense than iron & gcan 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.

Pyrometallurgy of iron• At around 250°C (top of the furnace), limestone is

calcinated:CaCO3(s) → CaO(s) + CO2(g)

Al t th t f th f h tit i d d• 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):(~700°C):

Fe3O4(s) + CO(g) → 3FeO(s) + CO2(g)

• Near the middle of the furnace (1000°C) Fe is produced:produced:

FeO(s) + CO(g) → Fe(s) + CO2(g)


• Cast iron is made by remelting pig iron & removing

Cast iron• Cast iron is made by remelting pig iron & removing

impurities such as phosphorous and sulphur.

• The viscosity of cast iron is very low, & it doesn’t shrink much when it solidifies.

• ∴ ideal for making castings.

• BUT, it is very impure, containing up to 4% carbon. This makes it very hard, but also very brittle.

• Shatters rather than deforms when struck hard.

• These days cast iron is quite rare, often being replaced by other materials.


• Pig iron is brittle and not directly very useful as a

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 vesselThe impurities are removed by oxidation in a vessel called a converter.

• The oxidising agent is pure O2 or O2 mixed with Ar.

• Air can’t be used as N2 reacts with iron to form ironAir can t be used as N2 reacts with iron to form iron nitride which is brittle.

Pyrometallurgy of ironSteelmakingSteelmaking

• O2 blown directly into molten metal.

• Reacts exothermically with C Si + other impuritiesC, Si + other impurities.

• C & S expelled as CO and SO gasSO2 gas.

• Si oxidised to SiO2 & incorporates into the slag layer.

Iron converter• Once oxidation complete,

contents poured out & various alloying elements added to produce steels.


• Wrought iron – iron with all the C removed Soft &

Types of iron & steel• Wrought iron – iron with all the C removed. Soft &

easily worked with little structural strength. No longer produced commerciallyproduced commercially.

• Mild steel – iron containing around 0 25% C Stronger• Mild steel – iron containing around 0.25% C. Stronger & harder than pure iron. Has many uses including nails wire car bodies girders & bridges etcnails, wire, car bodies, girders & bridges, etc.

• High carbon steel – contains around 1 5% C VeryHigh carbon steel contains around 1.5% C. Very hard, but brittle. Used for things like cutting tools, and masonry nailsmasonry nails.


• Stainless steel – iron mixed with chromium and nickel

Types of iron & steel• Stainless steel – iron mixed with chromium and nickel.

Resistant to corrosion. Uses include cutlery, cooking utensils kitchen sinks etcutensils, kitchen sinks, etc.

• Titanium steel – iron mixed with titanium Withstands• Titanium steel – iron mixed with titanium. Withstands high temperatures. Uses include gas turbines, spacecraft parts etcspacecraft parts, etc.

• Manganese steel – iron mixed with manganese VeryManganese steel iron mixed with manganese. Very hard. Uses include rock-breaking machinery, military helmets etchelmets, etc.

Pyrometallurgy of ironThe thermite reactionThe thermite reaction

• Aluminium metal can reduce Iron(III) oxide (Fe O ) in• 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)


Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s)Fe2O3(s) 2Al(s) → 2Fe(l) Al2O3(s)

CLASS EXERCISE : calculate the thermal energy that is l d i h i

Component ΔHfo (kJ/mol)

released in the reaction.

Fe2O3(s) -822.2Al(s) 0Al O (s) 1 669 8Al2O3(s) -1,669.8Fe(s) 0

Electrowinning of iron

Studies into iron extraction by electrowinning from

The Pyror process:

• Studies into iron extraction by electrowinning from sulphate solutions were first carried out around 50 years ago then subsequently forgottenyears ago, then subsequently forgotten.

• May become important again in the future as new• May become important again in the future as new, more environmentally friendly methods are sought for steelmakingsteelmaking.


First step is to convert iron pyrite (FeS ) into an acid

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 by800 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: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 remainingsolution is purged with air to remove any remaining H2S.


• Step 4: Electrolysis Iron is reduced and deposited on

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:regenerated at the anode. More specifically:At the cathode:

Fe2+ + 2e- → Fe(s)2H+ + 2e- → H2(g)Fe3+ + e- → Fe2+

At the anode:At the anode:SO4

2- + H2O → H2SO4 + 1/2O2 + 2e-

Fe2+ → Fe3+ + e-

Electrowinning of ironThe 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 ofstripping was performed, resulting in deposits of 13mm or more in thickness.

Gold extractionG ld i iGold mining Historical:

• Panning – sand and gravel• Panning – sand and gravel containing gold is shaken around with water in a pan Gold is muchwith water in a pan. Gold is much denser than rock, so quickly settles to the bottom of the pansettles to the bottom of the pan.

Gold extractionG ld i iGold mining Historical:

• Sluicing – water is channelled to• Sluicing – water is channelled to flow through a sluice-box. Sluce-box is essentially a man-madebox is essentially a man-made channel with riffles (barriers) at the bottom Riffles create dead-zonesbottom. Riffles create dead-zones in the water current which allows gold to drop out of suspensiongold to drop out of suspension.

• Sluicing and panning results in the direct recovery of• Sluicing and panning results in the direct recovery of small gold nuggets and flakes.

Gold extractionG ld i iGold mining Modern methods:

• Hard rock mining – used• Hard rock mining – used to extract gold encased in rock Either open pitrock. Either open pit mining or underground miningmining.

• World’s deepest mine near Carletonville – 3778 m b l d & 1949 b l l l!below ground & 1949 m below sea level!

Gold extractionG ld iGold ore processing• The most commonly used process for gold extraction

Gold cyanidation:

• The most commonly used process for gold extraction.

• Used to extract gold from low-grade ore• Used to extract gold from low-grade ore.

• Gold is oxidised to a water-soluble aurocyanide• 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 iswith dilute alkaline cyanide solution, and air is introduced (Elsener equation):

4A ( ) 8N CN(l) O ( ) 2H O(l) 4N A (CN) (l) 4N OH(l)4Au(s) + 8NaCN(l) + O2(g) + 2H2O(l) → 4NaAu(CN)2(l) + 4NaOH(l)

Gold extractionG ld iGold ore processing Gold cyanidation:

Agitated leaching

• At a slurry concentration of around 50% solids the

Agitated leaching

• At a slurry concentration of around 50% solids, the slurry passes through a series of agitated mixing tanks with a residence time of 20 - 40 hrswith a residence time of 20 - 40 hrs.

• The gold-bearing liquid is then separated from the• 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 tothe tailings are washed to remove Au and CN prior to disposal.

Gold extractionG ld iGold ore processing• The aurocyanide complex has an exceptionally high

Gold cyanidation:

• The aurocyanide complex has an exceptionally high stability constant, β2 [Au(CN)2]- = 2 × 1038.

• This high stability constant means that dissolution can be achieved even in the presence of considerablebe 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 commonlyfrom the cyanide solution. Two methods commonly used to achieve this are 1) the Carbon in pulpprocess and 2) the Merrill-Crowe processprocess, and 2) the Merrill Crowe process.


Heap leaching

• Is an alternative to the agitated leaching process.

Heap leaching

g g p

• Drastically reduced gold recovery costs of low grade oreore.

• Ore grades as low as 0.3 g per ton can be g g peconomically processed by heap leaching.


Heap leaching• Crushed ore placed in a pile on an impervious barrier.

CN l ti ll d t l t th h th h d

Heap leaching

• CN- solution allowed to percolate through the crushed ore & leaches out the Au.

• Gold-laden pregnant solution drains out the bottom.• Gold recovered by either Carbon in pulp process or y p p p

Merrill-Crowe process.

Gold extractionG ld iGold ore processingHeap leaching

Gold cyanidation:

• Generally requires 60 to 90 days for processing ore that

Heap leaching

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.g p

• BUT, has gained wide favour due to vastly reduced processing costsprocessing costs.

• Frequently, mines will use agitated leaching for high-grade ore & heap leaching for marginal grade ores that would otherwise be considered waste rock.

Gold extractionG ld iGold ore processing Gold recovery:

1) Carbon in pulp – overview:• Dissolved aurocyanide is mixed with free activated

carbon particles in solution and agitated in leach tanks

1) Carbon in pulp overview:

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 aurocyanideGold has a natural affinity for C, and the aurocyanide complex is adsorbed onto the C.

Th C ti l ith b d [A (CN) ] th• The coarse C particles with bound [Au(CN)2]- are then removed by screening using a wire mesh. Finely ground

th h th hore passes through the mesh.


1) Carbon in pulp – details:

• On completion of cyanidation, pregnant pulp is

1) Carbon in pulp details:

p y p g p ptransferred to Carbon In Pulp (CIP) process.

• Pregnant pulp passed through a number of tanks (6 to• 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 or tailings solution.contact with low grade or tailings solution.


1) Carbon in pulp – details:

• In this tank, the barren carbon has a high activity, and

1) Carbon in pulp details:

can remove trace amounts of Au < 0.01 mg / L.

• As the carbon passes through the tanks, it collects increasing quantities of Au from the solution. This is termed loading.

T i ll t ti hi h 4000 t 8000 A /• Typically, concentrations as high as 4000 to 8000 g Au / ton of C can be achieved on the final loaded C. This represents a gold reco er of 90 99%represents a gold recovery of 90-99%.


1) Carbon in pulp• The loaded carbon is then washed with dil. HCl to

remove CaCO that precipitates on the C

1) Carbon in pulp

remove CaCO3 that precipitates on the C.• Acid washed C then separated from the pulp in the final

tank & transferred to the elution circuit.• Barren pulp is dewatered (to recycle water & remove p p ( y

cyanide for reuse in the process).• In the elution circuit the loaded carbon is treated with a• In the elution circuit, the loaded carbon is treated with a

hot cyanide & caustic solution to remove the Au. Reversal of adsorption process Produces a small vol ofReversal of adsorption process. Produces a small vol. of soln. With a high Au conc.


1) Carbon in pulp• The barren carbon is reactivated & recycled for use in

the process

1) Carbon in pulp

the process.

• The cyanide & caustic solution containing the Au is• The cyanide & caustic solution containing the Au is either transferred to an electrowinning circuit or Au is plated out onto steel woolplated out onto steel wool.

• The Au obtained by either method is then transferred toThe Au obtained by either method is then transferred to the smelting circuit to produce gold bullion.

Gold extractionCarbon in pulpCarbon in pulp


2) Merrill-Crowe process

• Traditional method for Au recovery from pregnant

2) Merrill Crowe process

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.


2) Merrill-Crowe process

• Zinc dust is then added to the cyanide solution to

2) Merrill Crowe process

chemically reduce the gold to the metal.

• The metallic gold is then filtered out & refined.

Extraction Metallurgy Student Copy

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