Extraction Metallurgy Modified from notes by Dr C Perry

Extraction Metallurgy Modified from notes by Dr C Perry
Part 2: Case studies Dr. S Durbach (C205) Modified from notes by Dr C Perry Chem2017 & Chem 3030

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. Siderophile elements, chemical elements such as iridium or gold that tend to bond with metallic iron, as described by the Goldschmidt classification

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. Eg. Of a roasting rxn: CuS + 1.5O2 → CuO + SO2

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).

The following steps are involved in Cu extraction: Concentration Roasting Smelting Conversion Refining

1. Concentration Only ~0.7% of the extracted ore contains Cu Finely crushed ore concentrated by the froth-flotation process: Copper ore slurry mixed with: Lime water – to give basic pH Pine oil – to make bubbles An alcohol – to strengthen bubbles A chemical collector

1. Concentration (cont.) Chemical collectors such as xanthates (salts & esters of xanthic acid), dithiophosphates, or thionocarbamates make the ore surface hydrophobic. Hydrophobic – attracted to hydrocarbon pine oil Hydrophilic – attracted to sulphide minerals Potassium amyl xanthate

1. Concentration (cont.) Raising the pH causes the polar ends to ionize more, thereby preferentially sticking to chalcopyrite (CuFeS2) and leaving pyrite (FeS2) alone. 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. The froth containing the Cu ore is skimmed off and reprocessed.

1. Concentration (cont.) In this manner, the ore is concentrated to an eventual value of over 28% Cu. The remaining material (sand particles & other impurities) sink to the bottom & is discarded or reprocessed to extract other elements.

1. Concentration (cont.) Froth-flotation

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. Autogenous = self-produced or self generating. Fe2O3 is hematite

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. Up until the early 20th century, roasting was started by burning wood on top of ore. This would raise the temperature of the ore to the point where its sulfur content would become its source of fuel, and the roasting process could continue without external fuel sources. Today, roasting is carried out in special roasting furnaces.

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. Matte smelting is carried out in a neutral or slightly reducing atmosphere to prevent overoxidation of the charge. Overoxidation leads to the formation of magnetite which is not molten at Cu smelting temps. This makes the slag more viscous which is not desirable since the more viscous the slag, the more droplets of matte get trapped in it. 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.

3. Smelting (cont.) The main reaction is the reduction of copper oxides (formed during roasting) back into copper sulphide to ensure that they migrate into the matte phase: FeS(l) + 6CuO(l) → 3Cu2O(l) + FeO(l) + SO2(g) FeS(l) + Cu2O(l) → FeO(l) + Cu2S(l) Cu2S(l) + FeS(l) → Cu2S•FeS(l) (matte) The FeS (iron sulphide) was originally in the ore.

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.

4. Conversion (cont.) Iron removal A silica flux is added to keep the slag (see below) molten. Air is blown into the converter to oxidize the iron sulphide according to the following reaction: 2Cu2S•FeS(l) + 3O2(g) + SiO2(l) → 2FeO•SiO2(l) + 2SO2(g) + Cu2S(l) Si added to help form FeO slag. The oxidized Fe and Si form a slag (insoluble in matte) that is skimmed off & disposed off. NB, maybe do something with the redox potentials of Cu and Fe, since Fe is more readily oxidized than Cu.

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.

5. Refining Almost all 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.

5. Refining (cont.) Metallic impurities more active then Cu are oxidized at the anode, but don’t plate out at the cathode. Less active metals are not oxidized at the anode, but collect at the bottom of the cell as a sludge. The redox reactions are: Cu(s)  Cu2+(aq) + 2e- Cu2+(aq) + 2e-  Cu(s) Ered = -0.83V

5. Refining (cont.)

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.

Environmental impact Typical Air Emissions Typical Liquid Effluents

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.

The following steps are involved: Ore preparation Leaching Solution purification Metal recovery Worldwide, around 20% of copper is produced this way

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

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.

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 Gangue pronounced “gang”

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. dump leaching, relatively cheap.

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.

2. Leaching (cont.) Leaching reactions Nature of ore determines if leaching is non-oxidative or oxidative. Non-oxidative leaching: No change in oxidation state. e.g. (1) dissolution of copper sulphate by water: CuSO4(s) + H2O(l)  Cu2+(aq) + SO42-(aq) (2) dissolution of alkaline materials by acid: Cu2(OH)2•CO3(s) + 2H2SO4(aq)  2CuSO4(aq) + CO2(g) + 3H2O(l)

2. Leaching (cont.) Oxidative leaching: Many ores only soluble once oxidised. e.g. covellite (CuS) much more soluble if oxidised to CuSO4 CuS(s) + O2(g)  CuSO4(aq) CLASS EXERCISE : work out which species is oxidised, and which is reduced, and write out the balanced half reactions for each. SOLUTION: CuS  Cu = +2, S = -2 O2  O = 0 CuSO4  Cu = +2, O4 = -8, S = +6 S-2  S+6 + 8e (oxidation) 2O2 + 8e-  4O2- (reduction)

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.

2. Leaching (cont.) (c) Bacterial leaching Thiobacilli Are acidotolerant; some grow at pH’s as low as 0.5 Are tolerant against heavy metal toxicity. Are chemolithoautotrophs (C source is CO2 & energy derived from chemical transformation of inorganic matter). Chemolithoautotroph - An organism that obtains energy from inorganic compounds and carbon from CO2.

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.

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)

2. Leaching (cont.) (c) Bacterial leaching Mechanisms: Indirect 4Fe2+  4Fe3+ + 4e- (oxidation) O2 + 4e-  2O (reduction) Check this balancing!!! 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

2. Leaching (cont.) (c) Bacterial leaching Mechanisms: Indirect The elemental S produced by the indirect method can be converted to H2SO4 by Acidithiobacillus ferrooxidans: Sulphur is oxidized. The H2SO4 helps maintain the pH at levels favourable for bacterial growth.

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

2. Leaching (cont.) (c) Bacterial leaching Compared to other extraction techniques: Traditional methods expensive (i.e. roasting + smelting) & require high concentrations of Cu in ore. Bacteria can effectively deal with low [Cu] as they simply ignore surrounding waste materials. Up to 90% extraction efficiency.

2. Leaching (cont.) (c) Bacterial leaching Compared to other extraction techniques: ADVANTAGES: Economical: Simpler, cheaper, less infrastructure. More environmentally friendly; no SO2 emissions, less landscape damage. Currently it is more economical to smelt copper ore rather than to use bioleaching, since the concentration of copper in its ore is generally quite high. The profit obtained from the speed and yield of smelting justifies its cost. DISADVANTAGES: Economical: Very slow compared to smelting; less profit. Delay in cash flow for new plants. Environmental; Toxic chemicals sometimes produced. H2SO4 pollution. Precipitation of heavy ions (Fe, Zn, As) – pollution.

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.

Ion exchange chromatography
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.

Ion exchange chromatography: Theory
Hydrometallurgy of copper 3. Solution Purification Ion exchange chromatography: Theory Analyte molecules retained on a column (stationary phase) based on coulombic (ionic) interactions. 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+B- anion exchange chromatography: R-X+A- + M+B-  R-X+B- + M+A-

Cu Ion exchange chromatography
Hydrometallurgy of copper 3. Solution Purification Cu 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.).

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.

3. Solution Purification: Solvent extraction

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.

3. Solution Purification: Solvent extraction Extractants The most successful extractants for copper (II) 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)

3. Solution Purification: Solvent extraction Extractants At low pH (1.5 – 2.0) the ortho-hydroxyoxime extractant complexes the Cu(II). During back-extraction (stripping stage) the pH is lowered further, releasing the Cu(II), 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. The acid reverses the reaction shown on the previous slide.

3. Solution Purification: Solvent extraction Extractants Cu2+ forms square-planar complexes with hydroxyoxime: 2+ H-bonding between the oximic H and the phenolic O affords this 2:1 complex unusual stability. The formation constant (K2) for the 2:1 complex is much greater than for the 1:1 complex.

3. Solution Purification: Solvent extraction Extractants 3+ 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.

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.

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 = V Fe2+(aq) + 2e–  Fe(s) Ered = 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.

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.

4. Metal Recovery: Electrochemical recovery Electrowinning An electrochemical process for precipitating metals from solution.

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. In 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 in an electroplating process onto the cathode. In electrorefining, the anodes consist of unrefined impure metal, and as the current passes through the acidic electrolyte the anodes are corroded into the solution so that the electroplating process deposits refined pure metal onto the cathodes.

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.

4. Metal Recovery: Electrochemical recovery Electrorefining

Summary: Extra froth flotation step to remove any remaining organic liquid from solvent extraction step as it will cause problems with the electrochemical cells.

Silicon production Ellingham diagrams
Follow the 2nd law of thermodynamics: ∆G = ∆H - T∆S Plot ∆G vs T (y = m  X + c) Ellingham diagram. A change of slope = a change of state (boiling point) rG /kJ mol-1 Temperature /C

Ellingham diagrams Lower the position of a metal in the Ellingham diagram = greater stability of its oxide. A metal found in the Ellingham diagram can act as a reducing agent for a metallic oxide found above it. Stability of metallic oxides decrease with increase in temperature. Ellingham diagram 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 ΔG line of the reductant is lower on diagram than the metallic oxide.

Silicon production More difficult to extract Si than either Cu or Fe.
Ellingham diagram rG /kJ mol-1 Temperature /C

Silicon production 2SiC(s) + SiO2(l)  3Si(l) + 2CO(g)
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) : SiC has a very high mp. Around 2700 deg C. Is also a ceramic. 2SiO2(l) + 3C(s)  Si(l) + 2CO2(g) + SiC(s) 2SiC(s) + SiO2(l)  3Si(l) + 2CO(g)

Silicon production The electric arc furnace
Silica and carbon fed in through the top, liquid Si collected at the bottom. Temps of ± 2000K achieved by an electric arc burning between graphite electrodes. An arc forms between the charge and the electrodes. The charge is heated both by current passing through the charge and by the radiant energy evolved by the arc.

Silicon production The electric arc furnace
Electric arc furnaces require huge amounts of electricity. A 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 production Applications
Si is the 2’nd most abundant element in the earth’s crust (~28%). Principal constituent of natural stone, glass, concrete & cement. Largest application of pure Si (metallurgical grade) is in the manufacture of Al-Si alloys to produce cast parts (for automotive industry). Important constituent of electrical steel (modifies the resistivity & ferromagnetic properties). Added to molten cast iron to improve its performance in casting thin sections. O around 46%.

Silicon production Applications
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. O around 46%. Electronics industry – ultra-pure silicon wafers used in electronic components such as transistors, solar cells, integrated circuits, microprocessors & various semiconductor devices.

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

Silicon production Purification
Semiconductor-grade Si produced by converting crude Si to more volatile compounds like SiCl4. These are then purified by exhaustive fractional distillation. Reduced back to Si with pure H2. Finally, the high-purity Si is melted and large single crystals are grown by the Czochralski process. Electronic grade Si is required to be % pure! Czochralski(pronounced "cha-krawl-ski")

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). The property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are often referred to as extrinsic. A seed crystal mounted on a rod is dipped into the molten Si. Seed crystal rod pulled up & rotated at the same time. By carefully controlling the temp gradients, rate of pulling, and rotation speed, a large single-crystal (called a boule) can be extracted from the melt.

Silicon production Purification: The Czochralski process

Silicon production Purification: The Czochralski process
The boule is then ground down to a standard diameter and sliced into wafers, much like a salami. The wafers are etched and polished, and move on to the process line. A point to note however, is that due to "kerf" losses (the width of the saw blade) as well as polishing losses, more than half of the carefully grown, very pure, single crystal silicon is thrown away before the circuit fabrication process even begins!

Silicon production Electrochemical preparation:
A new method that uses electrolysis to reduce SiO2 to elemental Si. Advantageous because it avoids the high energy costs associated with the older carbothermic route, and also reduces the CO2 emissions considerably. SiO2 is usually an insulator, and doesn’t conduct electricity, but it has been shown that a tungsten wire sealed within a quartz tube with the tungsten end exposed, can act as a cathode.

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.

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.

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. Sintering is a method for making objects from powder, by heating the material in a sintering furnace[1] below its melting point (solid state sintering) until its particles adhere to each other. SEM of SiO2 powder reduced Si powder.

Silicon production Electrochemical preparation:

Aluminium production Most abundant metallic element in the earth’s crust. But, extremely rare in its free form. Once considered as a precious metal more valuable than gold! Al is a highly reactive metal that forms strong bonds with O. Requires a large amount of energy to extract from Al2O3. Napoleon III, emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the other guests had to make do with gold

Aluminium production Cannot be reduced directly by carbon since Al is a stronger reducing agent than C. Must therefore be extracted by electrolysis. Aluminium production involves two steps: 1) purifying Al2O3 from bauxite (the Bayer process) and 2) converting Al2O3 to metallic Al (The Hall-Heroult process). Primary Al ore is bauxite, which consists of: Gibbsite - Al(OH)3 (most extractable form) Boehmite - AlO•OH (less extractable than Gibbsite) Diaspore - αAlO•OH (difficult to extract)

Aluminium production The Bayer process:
Step 1: Dissolution The hydrated aluminium oxides are first selectively dissolved from bauxite: Al(OH)3 + NaOH  NaAlO2 + 2H2O (Gibbsite dissolution) AlO•OH + NaOH  NaAlO2 + H2O (Boehmite dissolution) An undesirable side reaction is the formation of “red mud”, which occurs when Al(OH)3 reacts with dissolved Kaolinite clay: 5Al2Si2O5(OH)4 + 2Al(OH)3 + 12NaOH  2Na6Al6Si5O17(OH) H2O Red mud formation consumes dissolved Al and  represents a Al loss.

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). Flocculants, or flocculating agents, are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc. action differs from precipitation in that the solute coming out of solution does so at a concentration generally below its solubility limit in the liquid. 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.

Step 3: Precipitation The remaining solution is supersaturated, containing around grams of dissolved Al(OH)3 per litre. Al(OH)3 is precipitated out by adding seed crystals since Al(OH)3 doesn’t crystallise out easily on its own. Once the crystals have reached the desired size, they are removed, washed, and filtered. The spent liquor is reheated, recausticised and recycled.

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

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.

Problems In addition, the inertness of Al(III) leads to slow rates of crystallisation, requiring large vessels & large volumes of circulating solution & seed material.

Aluminium production The Hall-Heroult process:
Reactive metals (e.g. Mg and Na) can be produced by electrolysing a molten chloride salt of the metal. Not the case for AlCl3 since it sublimes rather than melts. Even under sufficient pressure, molten AlCl3 is an electrical insulator & cannot be used as an electrolyte. Would have to be dissolved in a conductive salt (NaCl or KCl). Commercially viable production of Al only commenced once the use of cryolite (Na3AlF6) was discovered.

Cryolite is electrically conductive, and dissolves Al2O3.

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.

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 Al2O3 , using a current of 10.0 A. F (Faraday constant) = magnitude of electric charge per mole of electrons = C mol-1.

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:

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 = g mol-1 mass Al = g mol-1 × mol = 3.34 g Al

Electrolytic reduction of Al is costly (3 e- required for every atom of metallic Al reduced). The electrical voltage used is only around 5.25 V, but the current required is very high, typically 100,000 to 150,000 A or more! Electrical power is the single largest cost in Al production,  Al smelters are typically located in areas with inexpensive electric power, like S.A.

Pyrometallurgy of iron
Still the most important pyrometallurgical process economically. The most important sources of iron are hematite (Fe2O3) and magnetite (Fe3O4). Prehistorically, iron was prepared by simply heating it with charcoal in a fired clay pot. Today, the reduction of iron oxides to the metal is accomplished in a blast furnace.

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

The iron ore, limestone, and coke are added to the top of the furnace. Coke is coal that has been heated in an inert atmosphere to drive off volatile components (~ 80 – 90% C). Coke is the “fuel”, producing heat in the lower part of the furnace. Is also the source of the reducing gases CO & H2. Limestone (CaCO3) serves as the source of CaO which reacts with silicates & other impurities in the ore to form slag.

Slag: Most rocks are composed of silica (SiO2) and silicates (SiO32-) & are almost always present in the ore. These compounds don’t melt at the furnace temperature & would eventually clog it up. An important chemical method to remove these is by use of a flux which combines with the silica & silicates to produce a slag. Slag collects at bottom of furnace & doesn’t dissolve in the molten metal. Limestone decomposes to CaO & then combines with the Si. Limestone is the flux.

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. Limestone decomposes to CaO & then combines with the Si. Limestone is the flux. 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.

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.

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, % Si, % Mn + S and P and needs to be further processed. Magnetite is Fe3O4. Magnetite can also be written as FeO.Fe2O3. FeO is Wüstite. Hematite is Fe2O3.

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)

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.

Steelmaking Pig iron is brittle, and not directly very useful as a material. Typically, pig iron is drained directly from the blast furnace (referred to as hot metal), and transported to a steelmaking plant while still hot. The impurities are removed by oxidation in a vessel called a converter. The oxidising agent is pure O2 or O2 mixed with Ar. Air can’t be used as N2 reacts with iron to form iron nitride which is brittle.

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

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.

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.

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)

The thermite reaction

The thermite reaction Fe2O3(s) + 2Al(s)  2Fe(l) + Al2O3(s) CLASS EXERCISE : calculate the thermal energy that is released in the reaction. Component  Hfo (kJ/mol) Fe2O3(s) -822.2 Al(s) Al2O3(s) -1,669.8 Fe(s) Enthalpy of formation. By definition, the standard enthalpy of formation of the most stable form of any element is zero, because there is no formation reaction needed when the element is already in its standard state. 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)( kJ/mol) - (2mol)(0 kJ/mol) Horxn = kJ mol-1 Exothermic!

Electrowinning of iron
The Pyror process: Studies into iron extraction by electrowinning from sulphate solutions were first carried out around 50 years ago, then subsequently forgotten. May become important again in the future as new, more environmentally friendly methods are sought for steelmaking. Was actually a copper producing mine in Norway, but the ore they extracted contained a large amount of iron pyrite which initially was just going to waste. So they decided to set up a pilot electrochemical plant for the reduction of FeS in sulphate solutions.

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.

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-

The Pyror process:

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.

The Pyror process:

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

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.

Modern methods: Hard rock mining – used to extract gold encased in rock. Either open pit mining or underground mining. World’s deepest mine near Carletonville – 3778 m below ground & 1949 m below sea level!

4Au(s) + 8NaCN(l) + O2(g) + 2H2O(l)  4NaAu(CN)2(l) + 4NaOH(l)
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 (Elsener equation): NaCN also ensures that the solution remains at a pH > 10.5 to prevent the formation of highly toxic HCN gas. 4Au(s) + 8NaCN(l) + O2(g) + 2H2O(l)  4NaAu(CN)2(l) + 4NaOH(l)

Gold extraction Gold ore processing
Gold cyanidation: 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 hrs. The gold-bearing liquid is then separated from the leached solids in thickener tanks or vacuum filters & the tailings are washed to remove Au and CN- prior to disposal.

Gold cyanidation: The aurocyanide complex has an exceptionally high stability constant, 2 [Au(CN)2]- = 2 × 1038. This high stability constant means that dissolution can be achieved even in the presence of considerable amounts of other metals (Cu, Zn, and Ni). At this point, the dissolved Au needs to be recovered from the cyanide solution. Two methods commonly used to achieve this are 1) the Carbon in pulp process, and 2) the Merrill-Crowe process.

Gold cyanidation: Heap leaching Is an alternative to the agitated leaching process. Drastically reduced gold recovery costs of low grade ore. Ore grades as low as 0.3 g per ton can be economically processed by heap leaching.

Gold cyanidation: Heap leaching Crushed ore placed in a pile on an impervious barrier. 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 Merrill-Crowe process.

Gold extraction Gold ore processing Gold cyanidation:
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.

Gold recovery: 1) Carbon in pulp – overview: Dissolved aurocyanide is mixed with free activated carbon particles in solution and agitated in leach tanks. The carbon particles are much larger than the ground ore particles. Gold has a natural affinity for C, and the aurocyanide complex is adsorbed onto the C. The coarse C particles with bound [Au(CN)2]- are then removed by screening using a wire mesh. Finely ground ore passes through the mesh.

Gold recovery: 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 (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.

Gold recovery: 1) Carbon in pulp – details: In this tank, the barren carbon has a high activity, and can remove trace amounts of Au < 0.01 mg / L. As the carbon passes through the tanks, it collects increasing quantities of Au from the solution. This is termed loading. Typically, concentrations as high as 4000 to 8000 g Au / ton of C can be achieved on the final loaded C. This represents a gold recovery of 90-99%.

Gold recovery: 1) Carbon in pulp The loaded carbon is then washed with dil. HCl to 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 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. Reversal of adsorption process. Produces a small vol. of soln. With a high Au conc.

Gold recovery: 1) Carbon in pulp The barren carbon is reactivated & recycled for use in the process. The cyanide & caustic solution containing the Au is either transferred to an electrowinning circuit or Au is plated out onto steel wool. The Au obtained by either method is then transferred to the smelting circuit to produce gold bullion.

Gold extraction Carbon in pulp

Gold recovery: 2) Merrill-Crowe process Traditional method for Au recovery from pregnant cyanide solutions. Once dissolution of Au is complete, the remaining rock pulp if filtered off through various filters & diatomaceous earth to produce a sparkling clear solution. O2 is removed from the clarified solution by passing the solution through a vacuum deaeration column.

Gold recovery: 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.

Extraction Metallurgy Modified from notes by Dr C Perry

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Extraction Metallurgy Modified from notes by Dr C Perry

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