Comprehensive Study Guide to Metallurgy and Extractive Metallurgy

Principles of Metallurgy and Ore Mineralogy

• Fundamental Definitions:

  • Metallurgy: The entire scientific and technological process used for the isolation and extraction of metals from their ores.
  • Natural Occurrence of Metals:
  • Native/Free State: Unreactive metals (e.g., $Au$, $Pt$, and partly $Ag$, $Cu$) occurring in elemental form.
  • Combined State: Reactive metals existing in nature as oxides, sulphides, carbonates, halides, sulphates, and silicates.
  • Minerals: Naturally occurring, inorganic chemical substances with definite chemical compositions and crystalline structures found in the Earth's crust, obtained through mining.
  • Ores: Minerals from which a metal can be extracted economically, conveniently, and viably. All ores are minerals, but all minerals are not ores.
  • Gangue (Matrix): Earthy, rocky, or undesired waste impurities (such as sand, limestone, clay, rocks) associated with the ore deposits.
  • Flux: A chemical substance added to the charge in a furnace to lower the fusion/melting point of the ore and to convert infusible gangue impurities into a fusible, low-melting waste substance called slag.
  • Slag: The fusible mass formed by the chemical combination of flux with gangue during pyrometallurgical operations: $\text{Gangue} + \text{Flux} \rightarrow \text{Slag}$.

• Classification of Fluxes:

  • Acidic Flux: Used to remove basic impurities (such as $FeO$, $CaO$, $MgO$). Common acidic fluxes include silica ($SiO_2$), boron trioxide ($B_2O_3$), and phosphorus pentoxide ($P_4O_{10}$).
  • Reaction with basic impurity: $FeO + SiO_2 \rightarrow FeSiO_3$
  • Basic Flux: Used to remove acidic impurities (such as $SiO_2$, $P_4O_{10}$). Common basic fluxes include calcium oxide ($CaO$), limestone ($CaCO_3$), magnesite ($MgCO_3$), and magnesium oxide ($MgO$).
  • Reaction with acidic impurity: $SiO_2 + CaO \rightarrow CaSiO_3$

• Comprehensive Table of Important Ores and Minerals:

  • Aluminium ($Al$):
  • Bauxite: $AlO_x(OH)_{3-2x}$ (where $0 < x < 1$), typically represented as $Al_2O_3 \cdot 2H_2O$
  • Kaolinite (clay): $[Al_2(OH)_4Si_2O_5]$ or $Al_2Si_2O_5(OH)_4$
  • Corundum: $Al_2O_3$
  • Cryolite: $Na_3AlF_6$
  • Feldspar: $KAlSi_3O_8$
  • Mica: $K_2O \cdot 3Al_2O_3 \cdot 6SiO_2 \cdot 2H_2O$
  • Iron ($Fe$):
  • Haematite (chief ore): $Fe_2O_3$
  • Magnetite: $Fe_3O_4$
  • Siderite: $FeCO_3$
  • Limonite: $Fe_2O_3 \cdot 3H_2O$ or $FeO(OH) \cdot H_2O$
  • Iron pyrites ("Fool's Gold"): $FeS_2$
  • Copper ($Cu$):
  • Copper pyrites (Chalcopyrite, chief ore): $CuFeS_2$
  • Malachite: $CuCO_3 \cdot Cu(OH)_2$
  • Azurite: $2CuCO_3 \cdot Cu(OH)_2$
  • Cuprite (Ruby copper): $Cu_2O$
  • Copper glance (Chalcocite): $Cu_2S$
  • Zinc ($Zn$):
  • Zinc blende (Sphalerite): $ZnS$
  • Calamine: $ZnCO_3$
  • Zincite: $ZnO$
  • Tin ($Sn$):
  • Cassiterite (Tin stone): $SnO_2$
  • Lead ($Pb$):
  • Galena: $PbS$
  • Magnesium ($Mg$):
  • Magnesite: $MgCO_3$
  • Dolomite: $MgCO_3 \cdot CaCO_3$
  • Epsom salt: $MgSO_4 \cdot 7H_2O$
  • Carnallite: $KCl \cdot MgCl_2 \cdot 6H_2O$
  • Kieserite: $MgSO_4 \cdot H_2O$
  • Barium ($Ba$):
  • Baryte: $BaSO_4$
  • Silver ($Ag$):
  • Argentite (Silver glance): $Ag_2S$
  • Horn silver: $AgCl$

Preliminary Ore Dressing and Concentration Techniques

• Crushing and Pulverisation:

  • Large mineral rocks extracted from mines are crushed into small lumps using mechanical jaw crushers.
  • The crushed lumps are subsequently pulverized into a fine powder using stamp mills or ball mills.

• Concentration of Ore (Dressing or Benefaction):

  • Removal of non-metallic, rocky gangue materials from pulverized ore.
  • Selection of concentration method depends on physical property differentials (density, magnetic susceptibility, surface wetting) between ore minerals and gangue, regional availability of resources, and environmental considerations.

• Hydraulic Washing (Gravity Separation):

  • Principle: Difference in specific gravities (densities) of the ore and gangue particles.
  • Operation: Powdered ore is treated with an upward stream of running water over a vibrating, sloping table fitted with corrugated boards (riffles). The lighter gangue particles are washed away down the slope, while the denser ore particles settle into the grooves.
  • Application: Heavily used for dense oxide and native ores, particularly tin stone ($SnO_2$), haematite ($Fe_2O_3$), chromite ($FeCr_2O_4$), and galena ($PbS$) to remove lighter siliceous gangue.

• Magnetic Separation:

  • Principle: Exploitation of differences in magnetic susceptibility between ore minerals and gangue.
  • Operation: Finely powdered ore is fed onto a moving conveyor belt stretched across two rollers, one of which houses a strong electromagnet. The magnetic component is attracted toward the magnetic roller and falls into an inner collection hopper, whereas the non-magnetic fraction drops off tangentially into an outer hopper.
  • Applications:
  • Separation of magnetic Wolframite ($FeWO_4$) from non-magnetic Cassiterite ($SnO_2$).
  • Concentration of Chromite ore ($FeO \cdot Cr_2O_3$) from non-magnetic siliceous impurities ($SiO_2$).

• Froth Floatation Process:

  • Principle: Differences in the wetting characteristics of ore minerals and gangue by oil and water (preferential wetting). Mineral particles are preferentially wetted by oil (hydrophobic behavior), while gangue particles are wetted by water (hydrophilic behavior).
  • Applicability: Exclusively utilized for sulphide ores (e.g., $ZnS$, $PbS$, $CuFeS_2$).
  • Reagents and Chemical Additives:
  • Collectors: Reagents that adsorb onto mineral surfaces to enhance non-wettability by water (e.g., pine oil, fatty acids, potassium/sodium ethyl xanthates). The non-polar organic tail points outwards, imparting hydrophobicity.
  • Froth Stabilisers: Substances that lower surface tension and stabilize bubbles to sustain froth longevity (e.g., aniline, cresols).
  • Depressants: Reagents added to selectively inhibit the floatation of one sulphide mineral over another in complex multi-sulphide ores.
    • Separation of Galena ($PbS$) from Sphalerite ($ZnS$) and Pyrite ($FeS_2$): Addition of sodium cyanide ($NaCN$) or potassium cyanide ($KCN$) in alkaline medium ($Na_2CO_3$).
    • $PbS$ does not react and floats with the froth.
    • $ZnS$ reacts to form a water-soluble coordination complex that stays in the aqueous pulp: $ZnS + 4NaCN \rightarrow Na_2[Zn(CN)_4] + Na_2S$
  • Activators: Reagents added to restore the floating ability of previously depressed minerals.
    • Activation of $ZnS$: Addition of copper sulphate ($CuSO_4$) replaces zinc surface atoms with copper sulphide ($CuS$), restoring collector adsorption and enabling $ZnS$ to float in a secondary stage.
  • Operational Mechanism: Ore pulp, water, collector, and frother are agitated vigorously in a tank by a rotating paddle that draws in compressed air. The oil-coated sulphide particles adhere to rising air bubbles, accumulating on the surface as a mineral-rich froth, which is skimmed off, filtered, and dried.

• Leaching (Hydrometallurgical Extraction):

  • Principle: Chemical dissolution where the valuable metal or mineral selectively reacts with a suitable aqueous solvent into a soluble complex or salt, leaving insoluble gangue impurities behind as residue.
  • Applications:
  • Leaching of Red Bauxite (Baeyer's Process): Powdered bauxite containing impurities of $Fe_2O_3$, $SiO_2$, and $TiO_2$ is digested with $40\%$ aqueous $NaOH$ at elevated temperatures ($473 - 523\,K$) under pressure: $Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]$
    • The insoluble red mud ($Fe_2O_3$, $TiO_2$) is removed by filtration.
  • Leaching of Silver and Gold (MacArthur-Forrest Cyanide Process): Powdered argentite ($Ag_2S$) or native gold/silver is treated with dilute ($0.1 - 0.2\%$) $NaCN$ or $KCN$ solution aerated with oxygen: $Ag_2S + 4NaCN \rightleftharpoons 2Na[Ag(CN)_2] + Na_2S$
    • Air oxidizes the sodium sulphide, preventing backward equilibrium shifts.

Thermal Conversion and Pyrometallurgical Reduction

• Conversion of Concentrated Ores to Oxides:

  • Metal oxides undergo thermodynamic reduction much more readily than sulphides, carbonates, or hydrated matrices. Hence, dressed ores are converted to oxides via calcination or roasting.

• Calcination vs. Roasting:

  • Calcination:
  • Thermal process carried out in the absence or strictly limited supply of air at temperatures below the fusion/melting point of the metal.
  • Endothermic thermodynamic process.
  • Applied primarily to carbonate and hydrated oxide ores.
  • Function: Drives off moisture and crystal water, decomposes carbonates with evolution of $CO_2$, expels volatile impurities ($S$, $As$, $Sb$) as elemental vapours, and makes the ore porous.
  • Chemical reactions:
    • Hydrated ferric oxide: $Fe_2O_3 \cdot xH_2O \xrightarrow{\Delta} Fe_2O_3 + xH_2O$
    • Calamine: $ZnCO_3 \xrightarrow{\Delta} ZnO + CO_2$
    • Dolomite: $CaCO_3 \cdot MgCO_3 \xrightarrow{\Delta} CaO + MgO + 2CO_2$
  • Roasting:
  • Thermal process carried out in an excess of air or oxygen at temperatures below the melting point of the charge.
  • Exothermic thermodynamic process.
  • Applied primarily to sulphide ores.
  • Function: Converts sulphides into metal oxides, burns out organic matter, and expels volatile non-metallic impurities ($S$, $As$, $Sb$, $P$) as volatile oxides ($SO_2$, $As_2O_3$, $Sb_2O_3$, $P_4O_{10}$).
  • Can be used to oxidise lower metal oxides to higher oxidation state oxides: $2FeO + \frac{1}{2}O_2 \rightarrow Fe_2O_3$
  • Chemical reactions:
    • Sphalerite: $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$
    • Galena: $2PbS + 3O_2 \rightarrow 2PbO + 2SO_2$
    • Copper glance: $2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$

• Thermodynamic Principles of Reduction (Pyrometallurgy):

  • Pyrometallurgy refers to the reduction of metal oxides into crude metals at high temperatures utilizing suitable reducing agents (such as carbon, carbon monoxide, or electropositive metals).
  • The thermodynamic spontaneity of a reduction reaction is dictated by the standard Gibbs free energy change: $\Delta_r G^\circ = \Delta_r H^\circ - T\Delta_r S^\circ$
  • For a reaction to proceed spontaneously, $\Delta_r G^\circ$ must be negative. The more negative the value of $\Delta_r G^\circ$, the higher the thermodynamic feasibility and spontaneity of the system.
  • Coupling of non-spontaneous oxide reduction with an oxidation reaction yielding an overall negative $\Delta_r G^\circ$ drives the extraction.

Ellingham Diagrams and Thermodynamic Reduction

• Fundamental Features of Ellingham Diagrams:

  • An Ellingham diagram represents the temperature dependence of the standard Gibbs free energy of formation ($\Delta_r G^\circ$) of oxides per mole of dioxygen ($O_2$) consumed: $\frac{2x}{y}M + O_2 \rightarrow \frac{2}{y}M_xO_y$
  • Positive Slopes: Most metal oxidation lines display a positive slope because gaseous $O_2$ is converted into solid metal oxide, resulting in a decrease in entropy ($\Delta S^\circ < 0$). Since $\frac{d(\Delta G^\circ)}{dT} = -\Delta S^\circ$, the slope $-\Delta S^\circ$ is positive.
  • Phase Transformations: Abrupt increases (inflection breaks) in the positive slope of a line indicate a phase transition (melting or vaporization) of the reactant metal or the product oxide. When a metal vaporizes, the entropy of the reactants increases sharply, making $\Delta S^\circ$ for the oxidation reaction much more negative, leading to a steeper upward slope.
  • Carbon Combustion Lines:
  • $C(s) + O_2(g) \rightarrow CO_2(g)$: 1 mole of gas forms 1 mole of gas, yielding $\Delta S^\circ \approx 0$. The curve is an almost horizontal line with a slope close to zero.
  • $2C(s) + O_2(g) \rightarrow 2CO(g)$: 1 mole of gas forms 2 moles of gas, yielding $\Delta S^\circ > 0$. The curve exhibits a negative slope that trends downwards as temperature rises.
  • $2CO(g) + O_2(g) \rightarrow 2CO_2(g)$: 3 moles of gas form 2 moles of gas, yielding $\Delta S^\circ < 0$. The curve exhibits a positive slope.
  • Relative Reactivity and Reduction Hierarchy:
  • Elements situated lower on the Ellingham diagram have more negative $\Delta_r G^\circ$ values and exhibit higher affinities for oxygen than elements above them.
  • Any metal whose oxidation line lies lower on the diagram can thermodynamically reduce the oxide of any metal whose line lies above it at a given temperature.
  • The intersection point ($X$) between two oxidation lines marks the equilibrium threshold where $\Delta_r G^\circ = 0$. At temperatures exceeding this intersection, the line that drops below the other serves as the reducing agent for the higher oxide.

• Mechanisms of Pyrometallurgical Reduction:

  • Carbon Reduction (Smelting):
  • Zinc oxide reduction: $ZnO + C \xrightarrow{\sim 1400^\circ\text{C}} Zn + CO$
  • Iron oxide reduction: $Fe_2O_3$ is reduced by gaseous $CO$ below approximately $1073\,K$ ($800^\circ\text{C}$), and by solid carbon (coke) above $1073\,K$.
  • Solid-Gas vs. Solid-Solid Kinetics: Gaseous $CO$ acts as the chief reducing agent in the blast furnace stack because the interfacial contact and collision frequency between a gaseous reductant ($CO$) and porous solid ore ($Fe_2O_3$) is vastly superior to the solid-solid contact between lumpy coke ($C$) and ore particles.
  • Reduction by Hydrogen ($H_2$):
  • Applied when metals form refractory carbides with carbon at high temperatures, rendering carbon reduction ineffective.
  • Reactions:
    • Tungsten oxide: $WO_3 + 3H_2 \rightarrow W + 3H_2O$
    • Nickel oxide: $NiO + H_2 \rightarrow Ni + H_2O$
  • Constraints: Highly flammable and explosive handling conditions; expensive operating costs; cannot reduce electropositive metal oxides such as $Al_2O_3$.
  • Reduction by Electropositive Metals (Metallothermic Reduction):
  • Kroll's Process: Used for refractory titanium tetrachloride ($TiCl_4$):
    • $TiCl_4 + 2Mg \rightarrow Ti + 2MgCl_2$
  • Goldschmidt Thermite Process (Aluminothermic Reduction):
    • Oxides of metals with extremely high melting points (e.g., $Cr$, $Mn$, $Ti$, $Mo$, $Fe$) are exceptionally difficult to reduce with carbon due to carbide formation and intense energy requirements.
    • Powered aluminium ($Al$) is employed as a powerful reducing agent due to its high oxygen affinity and the massive negative heat of formation of $Al_2O_3$.
    • Reaction: $3Mn_3O_4 + 8Al \rightarrow 4Al_2O_3 + 9Mn$
    • Initiation: Heat is supplied by an ignition mixture containing barium peroxide ($BaO_2$) and magnesium powder ($Mg$).
    • The reactions are fiercely exothermic; the massive thermal output maintains superheated temperatures such that reduced metals are tapped in the molten state.
  • Auto-Reduction (Self-Reduction):
  • Occurs without adding an external chemical reducing agent in sulphide roasting furnaces.
  • A fraction of the sulphide ore is roasted by oxygen to its oxide or sulphate, which subsequently reacts at higher temperatures with the remaining unchanged metal sulphide.
  • Reactions:
    • Copper: $2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$, followed by $Cu_2S + 2Cu_2O \rightarrow 6Cu + SO_2$
    • Lead: $2PbS + 3O_2 \rightarrow 2PbO + 2SO_2$, followed by $PbS + 2PbO \rightarrow 3Pb + SO_2$ and $PbS + PbSO_4 \rightarrow 2Pb + 2SO_2$
    • Mercury: $2HgS + 3O_2 \rightarrow 2HgO + 2SO_2$, followed by $HgS + 2HgO \rightarrow 3Hg + SO_2$
  • Electrolytic Reduction (Electrometallurgy):
  • Applied to highly electropositive metals ($K$, $Na$, $Ca$, $Mg$, $Al$) whose oxides possess extreme thermodynamic stability.
  • Carbon reduction cannot be used because:
    1. The thermodynamic temperatures required are impractically high, causing astronomical fuel costs.
    2. Highly reactive metals form stable interstitial metal carbides at extreme temperatures.
    3. Liberated metals re-oxidize rapidly unless maintained in an inert environment.
  • Conducted via the electrolysis of fused halides, oxides, or hydroxides containing fluxing salts (e.g., extraction of sodium in Down's cell via fused $NaCl$: Cathode: $Na^+ + e^- \rightarrow Na$; Anode: $2Cl^- \rightarrow Cl_2 + 2e^-$).

Metal Refining and Purification Processes

• Distillation:

  • Applicable to volatile metals with low boiling points that are contaminated with higher-boiling impurities.
  • The impure metal is boiled in a retort; the pure metal vaporizes, distills over, and condenses in a separate cooled receiver.
  • Examples: Zinc ($Zn$, b.p. $1180\,K$), Cadmium ($Cd$), Mercury ($Hg$, b.p. $630\,K$).

• Liquation:

• Applicable to metals having significantly lower melting points than their associated impurities.

• The block of crude metal is placed at the upper end of a sloping hearth in a reverberatory furnace heated just above the metal's melting point.

• The low-melting metal liquefies and flows down the inclined hearth into a collection receiver at the bottom, while higher-melting impurities remain behind on the hearth as a solid residue (dross).

• The impurity fraction must be sufficiently high; otherwise, fine molten metal droplets can carry impurities along mechanically.

• Examples: Refining of Tin ($Sn$, m.p. $505\,K$), Lead ($Pb$), and Bismuth ($Bi$).

  • Electrolytic Refining:

• Cell Configuration:

  • Anode: Thick slab of crude, impure metal.
  • Cathode: Thin strip or sheet of highly pure metal.
  • Electrolyte: Aqueous bath containing a soluble salt of the metal acidified with mineral acid.

• Mechanism: Upon passing direct electrical current, metal atoms from the impure anode oxidize into solution as cations: $M \rightarrow M^{n+} + ne^-$. At the cathode, identical cations deposit as pure metal: $M^{n+} + ne^- \rightarrow M$.

• Impurity Fate:

  • More basic/electropositive metals dissolve and remain in the electrolyte bath as cations.
  • Less basic/noble metals (lower in electrochemical series) do not dissolve at the working cell voltage and settle below the anode as insoluble anode mud (anode sludge).

• Electrolyte Baths for Specific Metals:

  • Copper ($Cu$): Acidified copper sulphate ($CuSO_4$ solution + $H_2SO_4$).
  • Zinc ($Zn$): Acidified zinc sulphate ($ZnSO_4$ solution).
  • Tin ($Sn$): Tin sulphate ($SnSO_4$ solution).
  • Nickel ($Ni$): Nickel nitrate ($Ni(NO_3)_2$ solution).
  • Silver ($Ag$): Silver nitrate ($AgNO_3$ solution containing $10\%$ $HNO_3$).
  • Lead ($Pb$): Lead fluorosilicate ($PbSiF_6$ solution with hydrofluorosilicic acid $H_2SiF_6$, Bett's process).
  • Gold ($Au$): Gold chloride ($AuCl_3$ solution with $HCl$).
  • Aluminium ($Al$): Molten mixture of $(Al_2O_3 + Na_3AlF_6) + BaF_2$.

• Anode Mud Components:

  • In copper electrolytic refining, anode mud contains valuable unreactive elements: $Ag$, $Au$, $Pt$, $Sb$, $Se$, $Te$.

  • Zone Refining (Fractional Crystallization):

• Principle: Segregation coefficient principle—impurities are significantly more soluble in the molten (melt) state of a metal than in its solid crystalline phase.

• Process: A circular mobile induction heating coil (ring furnace) is fitted around a rod of impure metal placed inside an inert noble gas atmosphere (e.g., argon). The heater slowly travels along the length of the rod. As the molten zone sweeps forward, pure metal crystallizes out behind it at the cooling interface. The impurities concentrate in the liquid molten zone and are pushed to one terminal end of the rod. The process is repeated multiple times in the same direction, after which the impurity-laden end is sawn off.

• Applications: Production of ultra-high-purity semiconductor-grade elements (purity up to $99.9999\%$): Germanium ($Ge$), Silicon ($Si$), Boron ($B$), Gallium ($Ga$), and Indium ($In$).

  • Vapour Phase Refining:

• Underlying Criteria:

  1. The target metal must readily form a volatile chemical compound with an available reagent at moderate temperatures.
  2. The resulting volatile compound must decompose back into the pure metal and reagent at elevated temperatures.

• Mond's Process (for Nickel):

  • Impure nickel is heated in a stream of carbon monoxide at $330 - 350\,K$ to form volatile, toxic nickel tetracarbonyl gas, leaving solid impurities behind: $Ni(\text{impure}) + 4CO \xrightarrow{330 - 350\,K} [Ni(CO)_4]$
  • The volatile complex is piped to a decomposition tower maintained at $450 - 470\,K$, where it decomposes to yield pure nickel pellets and recyclable $CO$: $[Ni(CO)_4] \xrightarrow{450 - 470\,K} Ni(\text{pure}) + 4CO$

• Van Arkel-de Boer Process (for Zirconium and Titanium):

  • Used to eliminate oxygen, nitrogen, and carbon impurities from metals like $Zr$, $Ti$, $Hf$, $V$, $W$, $U$, and $B$.

  • The crude metal is heated in an evacuated vessel with iodine ($I_2$) to form a covalent, volatile metal iodide: $Zr + 2I_2 \xrightarrow{\Delta} ZrI_4$

  • The gaseous iodide is circulated across an electrically heated tungsten filament maintained at approximately $1800\,K$. The compound decomposes upon contact, depositing high-purity crystalline metal on the filament while liberating iodine vapor for reuse: $ZrI_4 \xrightarrow{1800\,K} Zr(\text{pure}) + 2I_2$

  • Poling:

• Applicable when the crude molten metal contains significant amounts of its own oxide as an impurity (e.g., blister copper containing $Cu_2O$; tin containing $SnO_2$).

• Molten metal in a reverberatory furnace is stirred vigorously with long poles of green, unseasoned wood. The green wood releases hydrocarbons and reducing gases (such as methane, $CH_4$), which reduce the dissolved metal oxide back to metallic form: $3Cu_2O + CH_4 \rightarrow 6Cu + CO + 2H_2O$.

• The surface of the melt is covered with powdered coke to prevent atmospheric re-oxidation.

  • Cupellation and Bessemerisation (Pyrometallurgical Oxidation):

• Employed when associated impurities have a much higher affinity for oxygen than the parent noble or semi-noble metal.

• In cupellation, argentiferous lead is heated in an oval hearth (cupel) made of bone ash or porous cement; lead oxidizes to volatile litharge ($PbO$) that is blown away or absorbed into the porous cupel, leaving pure silver.

  • Chromatographic Methods:

• Column chromatography operates on the differential adsorption capacities of diverse chemical species on a stationary adsorbent matrix ($Al_2O_3$, silica gel) eluted by a mobile liquid solvent.

• Invaluable for the isolation and purification of elements present in trace/minute quantities where impurities possess chemical properties nearly identical to the extracted element (e.g., separation of lanthanoids and actinoids).

Industrial Furnaces in Metallurgy

• Blast Furnace:

  • A vertical, continuous counter-current chimney-like reactor built of steel plate lined with refractory firebrick.
  • Operates via downward descending solid charge (ore, flux, coke) moving against an upward blast of hot preheated air.
  • Extensively utilized for the smelting of iron and copper matte.

• Reverberatory Furnace:

  • A furnace where the fuel is burned in a separate firebox, and the long luminous flames and burning gases are deflected (reverberated) by a low curved arched roof down onto the shallow hearth containing the charge.
  • The ore does not come into physical contact with the solid fuel, only with heated oxidizing/reducing flue gases.
  • Chiefly deployed for calcination, roasting, and smelting of copper and tin ores.

• Muffle Furnace:

  • A closed refractory chamber (muffle) that holds the metallurgical charge, heated externally by fuel gases or electrical resistance coils.
  • Completely isolates the heated material from fuel combustion products, preventing contamination.
  • Used for small-scale operations, precious metal assaying, and analytical heat-treatments.

• Electric Arc Furnace:

  • Achieves extremely high temperatures (up to $3300\,K$) through an electric arc struck between large graphite electrodes.
  • Employed where immense thermal energy is required without introducing flue gas impurities, particularly in the manufacture of alloy steels, ferro-alloys, and calcium carbide.

Extractive Metallurgy of Iron ($Fe$)

• Principal Ores:

  • Haematite ($Fe_2O_3$), Magnetite ($Fe_3O_4$), Limonite ($FeO(OH) \cdot H_2O$).
  • Iron pyrites ($FeS_2$) is not commercially used because the high residual sulphur content renders the extracted iron hot-short and extremely brittle.

• Preparation and Dressing:

  • Pulverisation followed by gravity concentration (hydraulic washing) and subsequent magnetic separation to eliminate siliceous gangue.
  • Calcination/roasting in a reverberatory furnace removes moisture, evolves $CO_2$ from carbonates, oxidizes $FeO$ to $Fe_2O_3$, and expels $S$, $As$, and $P$ as volatile oxides.

• Blast Furnace Operations and Chemistry:

  • Charge Ratio: Sintered Ore : Limestone ($CaCO_3$) : Coke ($C$) = $8 : 1 : 4$.
  • Introduced at the furnace top via a double cup-and-cone hopper mechanism.
  • Hot air blast injected at the lower section through water-cooled copper nozzles (tuyeres).
  • Thermal and Chemical Zones:
  1. Zone of Combustion (Tuyere Zone, $1500 - 1600^\circ\text{C}$, $\sim 2170\,K$):
     • Coke burns intensely in the air blast: $C + O_2 \rightarrow CO_2$ ($\Delta H = -393.5\,kJ\,mol^{-1}$, highly exothermic).
     • As $CO_2$ ascends through red-hot coke, it is reduced: $CO_2 + C \rightarrow 2CO$
  2. Zone of Fusion (Lower Bosh, $1200 - 1500^\circ\text{C}$):
     • Spongy iron melts at around $1300^\circ\text{C}$ and trickles down to collect in the furnace hearth.
     • Low-density molten slag floats over the molten iron, creating a protective blanket against re-oxidation.
  3. Zone of Slag Formation (Middle Zone, $800 - 1000^\circ\text{C}$):
     • Limestone decomposes thermally: $CaCO_3 \rightarrow CaO + CO_2$
     • Basic flux combines with silica gangue: $CaO + SiO_2 \rightarrow CaSiO_3$ (calcium silicate slag).
     • Reduction of trace impurities into the iron: $P_4O_{10} + 10C \rightarrow 4P + 10CO$; $SiO_2 + 2C \rightarrow Si + 2CO$; $MnO_2 + 2C \rightarrow Mn + 2CO$.
  4. Zone of Reduction (Upper Stack, $300 - 800^\circ\text{C}$, $500 - 1073\,K$):
     • Spongy iron formation via sequential reduction by ascending carbon monoxide:
       • $3Fe_2O_3 + CO \xrightarrow{300 - 400^\circ\text{C}} 2Fe_3O_4 + CO_2$
       • $Fe_3O_4 + CO \xrightarrow{500 - 600^\circ\text{C}} 3FeO + CO_2$
       • $FeO + CO \xrightarrow{700^\circ\text{C}} Fe + CO_2$
     • Carbon monoxide partially disproportionates to deposit soot: $2CO \rightleftharpoons CO_2 + C$

• Commercial Forms of Iron:

  • Pig Iron: Direct raw product tapped from the blast furnace hearth. Contains approximately $93\%$ $Fe$, $4\%$ carbon, and residual quantities of $Si$, $P$, $S$, and $Mn$. Highly impure and brittle.
  • Cast Iron: Produced by remelting pig iron with scrap iron and coke in a cupola furnace under a hot air blast. Contains slightly lower carbon content ($\sim 2.5 - 5\%$). Very hard, brittle, non-malleable, cannot be welded, but casts cleanly into complex molds.
  • Wrought Iron: The purest commercial variety of iron ($0.25 - 0.3\%$ carbon). Manufactured by refining cast iron on the hearth of a reverberatory furnace lined with haematite ($Fe_2O_3$). The haematite oxidizes the carbon and impurities: $Fe_2O_3 + 3C \rightarrow 2Fe + 3CO$. Extremely tough, ductile, malleable, and weldable.
  • Steel: Alloy of iron containing intermediate carbon levels ($0.25 - 2.0\%$), imparting tensile strength and elasticity.

Extractive Metallurgy of Copper ($Cu$)

• Extraction from Copper Pyrites ($CuFeS_2$):

  • Concentration: Froth floatation utilizing pine oil collector and potassium ethyl xanthate.
  • Roasting: Conducted in a reverberatory furnace with controlled excess air to expel volatile oxides ($SO_2$, $As_2O_3$, $Sb_2O_3$) and partially oxidize the sulphides:
  • $2CuFeS_2 + O_2 \rightarrow Cu_2S + 2FeS + SO_2$
  • $2FeS + 3O_2 \rightarrow 2FeO + 2SO_2$
  • $2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$
  • Smelting in Blast Furnace: Roasted ore is mixed with crushed coke and silica ($SiO_2$ flux):
  • Any $Cu_2O$ formed reacts back with remaining $FeS$: $Cu_2O + FeS \rightarrow Cu_2S + FeO$
  • $FeO$ combines with silica to form slag: $FeO + SiO_2 \rightarrow FeSiO_3$
  • The molten mass settles into two layers: an upper light slag layer ($FeSiO_3$) and a dense lower layer called copper matte ($Cu_2S$ with a trace of $FeS$).
  • Bessemerisation: Molten copper matte is transferred into a silica-lined, pear-shaped Bessemer converter, and a hot air blast is blown through tuyeres:
  • Residual $FeS$ oxidizes first and forms slag: $2FeS + 3O_2 \rightarrow 2FeO + 2SO_2$, followed by $FeO + SiO_2 \rightarrow FeSiO_3$.
  • Partial oxidation of $Cu_2S$: $2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$.
  • Auto-reduction: The remaining cuprous sulphide reacts with cuprous oxide: $Cu_2S + 2Cu_2O \rightarrow 6Cu + SO_2$.
  • Blister Copper: Solidified copper ($\sim 98\%$ purity) displays a blistered appearance due to the effervescence of escaping dissolved sulphur dioxide ($SO_2$) bubbles.
  • Refining: Blister copper is treated by poling in a reverberatory furnace followed by final electrolytic refining.

• Hydrometallurgical Extraction of Low-Grade Copper:

  • Employed for low-grade ores and mine tailings where smelting is economically inviable.
  • The ore is leached with dilute sulphuric acid ($H_2SO_4$) or through bacterial bio-leaching to extract aqueous $Cu^{2+}$ ions.
  • Copper is precipitated using scrap iron ($Fe$) or hydrogen gas ($H_2$):
  • $Cu^{2+}(aq) + Fe(s) \rightarrow Cu(s) + Fe^{2+}(aq)$
  • $Cu^{2+}(aq) + H_2(g) \rightarrow Cu(s) + 2H^+(aq)$
  • While zinc is electrochemically more reactive than iron, scrap iron is preferred industrially due to its much lower cost.

Extractive Metallurgy of Tin, Lead, and Zinc

• Extraction of Tin ($Sn$) from Cassiterite ($SnO_2$):

  • Dressing: Crushing, pulverisation, and hydraulic washing to wash away lighter gangue.
  • Roasting: Expels sulphur and arsenic as volatile oxides; copper and iron pyrites convert to oxides and soluble sulphates.
  • Leaching/Washing: Washed with hot water to dissolve out $CuSO_4$ and iron sulphates. The dense residual oxide is termed black tin ($60 - 70\%$ $SnO_2$).
  • Smelting: Black tin is mixed with anthracite/coke and limestone/silica flux and heated to $1200^\circ\text{C}$ in a reverberatory furnace: $SnO_2 + 2C \xrightarrow{1200^\circ\text{C}} Sn + 2CO$.
  • The resulting product is cast into blocks as block tin ($\sim 99.5\%$ purity), which is further purified via liquation and electrolytic refining.

• Extraction of Lead ($Pb$) from Galena ($PbS$):

  • Concentration: Froth floatation.
  • Roasting: Heated in a reverberatory furnace with controlled airflow for partial oxidation:
  • $2PbS + 3O_2 \rightarrow 2PbO + 2SO_2$
  • $PbS + 2O_2 \rightarrow PbSO_4$
  • Smelting via Self-Reduction: Air supply is throttled, furnace temperature raised, and additional fresh concentrated galena is charged:
  • $2PbO + PbS \rightarrow 3Pb + SO_2$
  • $PbSO_4 + PbS \rightarrow 2Pb + 2SO_2$
  • Purification of Crude Lead:
  • Softening: Melted in an oxidizing reverberatory furnace; impurities ($Cu$, $Sn$, $Bi$, $As$, $Sb$) oxidize to form a surface dross/scum that is skimmed off.
  • Desilverisation: Removal of trace silver via Parke's Process (addition of zinc to molten lead; zinc-silver alloy forms a solid crust) or Pattinson's Process (fractional crystallization of argentiferous lead).
  • Electrolytic Refining (Bett's Process): Anode of impure lead, cathode of pure lead sheet, and electrolyte bath of lead fluorosilicate ($PbSiF_6$) and hydrofluorosilicic acid ($H_2SiF_6$). Anode mud yields $Ag$, $Au$, and $Sb$.

• Extraction of Zinc ($Zn$) from Sphalerite ($ZnS$):

  • Concentration: Froth floatation.
  • Roasting: Roasted in excess air at $900^\circ\text{C}$: $2ZnS + 3O_2 \xrightarrow{900^\circ\text{C}} 2ZnO + 2SO_2$.
  • Carbon Reduction: Roasted $ZnO$ is mixed with crushed coke and heated to $1400^\circ\text{C}$ ($\sim 1673\,K$) in vertical retorts: $ZnO + C \rightarrow Zn(g) + CO$.
  • Zinc vaporizes (b.p. $907^\circ\text{C}$) and is condensed into liquid form, solidifying as spelter (impure zinc containing $Pb$, $Fe$, $Cd$, $As$).
  • Refining: Volatile cadmium ($Cd$) is removed by fractional distillation at $800^\circ\text{C}$. High-purity zinc is attained by electrolytic refining using an impure zinc anode, pure aluminium sheet cathode, and an acidified zinc sulphate ($ZnSO_4$) electrolyte.

Extractive Metallurgy of Magnesium, Silver, and Gold

• Extraction of Magnesium ($Mg$):

  • Highly electropositive metal extracted via electrolysis of anhydrous fused halides mixed with alkali/alkaline earth chlorides ($NaCl$, $CaCl_2$) to lower operating melting points and enhance electrical conductivity.
  • From Sea Water (Dow's Process):
  • Sea water containing $0.13\%$ $Mg^{2+}$ is treated with slaked lime: $MgCl_2 + Ca(OH)_2 \rightarrow Mg(OH)_2\downarrow + CaCl_2$
  • The precipitate is neutralized with hydrochloric acid: $Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$
  • Crystallized as hydrated hexahydrate: $MgCl_2 \cdot 6H_2O$.
  • Dehydration: Cannot be directly flame-heated (hydrolysis occurs: $MgCl_2 \cdot 6H_2O \rightarrow MgO + 2HCl + 5H_2O$). It must be dehydrated by heating gradually in a stream of dry $HCl$ gas.
  • Electrolysis: Fused electrolyte of anhydrous $MgCl_2$ ($35\%$), $NaCl$ ($50\%$), and $CaCl_2$ ($15\%$) electrolysed at $700^\circ\text{C}$ in an inert gas atmosphere:
    • Cathode: $Mg^{2+} + 2e^- \rightarrow Mg$
    • Anode: $2Cl^- \rightarrow Cl_2 + 2e^-$
  • From Carnallite ($KCl \cdot MgCl_2 \cdot 6H_2O$): Dehydrated in a dry $HCl$ stream and electrolysed.
  • From Dolomite (Pidgeon Process): Thermal reduction with ferrosilicon ($FeSi$):
  • Calcination: $CaCO_3 \cdot MgCO_3 \xrightarrow{\Delta} CaO \cdot MgO + 2CO_2$
  • Reduction: $2MgO + 2CaO + FeSi \xrightarrow{1273\,K} 2Mg + Fe + Ca_2SiO_4$

• Extraction of Silver ($Ag$) and Gold ($Au$):

  • MacArthur-Forrest Cyanide Process:
  • Ground argentite ($Ag_2S$) is aerated with dilute $NaCN$ solution:
    • $Ag_2S + 4NaCN \rightleftharpoons 2Na[Ag(CN)_2] + Na_2S$
    • Aeration oxidizes the sodium sulphide, preventing the reverse reaction: $4Na_2S + 5O_2 + 2H_2O \rightarrow 2Na_2SO_4 + 4NaOH + 2S$
  • For Gold: $4Au + 8NaCN + 2H_2O + O_2 \rightarrow 4Na[Au(CN)_2] + 4NaOH$
  • Precipitation (Displacement): Zinc dust is added to the filtered aurocyanide/argentocyanide solution:
    • $2Na[Ag(CN)_2] + Zn \rightarrow Na_2[Zn(CN)_4] + 2Ag\downarrow$
    • $2Na[Au(CN)_2] + Zn \rightarrow Na_2[Zn(CN)_4] + 2Au\downarrow$
  • Refining: Silver is purified by electrolytic refining using an electrolyte of $AgNO_3$ with $10\%$ $HNO_3$ (Cathode: pure $Ag$; Anode: impure $Ag$).

Extractive Metallurgy of Aluminium ($Al$)

• Purification of Bauxite Ore:

  • Baeyer's Process (for red bauxite containing $Fe_2O_3$ as chief impurity):
  • Digestion with concentrated $NaOH$ at $150^\circ\text{C}$: $Al_2O_3 \cdot 2H_2O + 2NaOH \rightarrow 2NaAlO_2 + 3H_2O$
  • Insoluble $Fe_2O_3$ (red mud) is filtered off.
  • The sodium meta-aluminate solution is diluted and seeded with freshly precipitated $Al(OH)_3$: $NaAlO_2 + 2H_2O \rightarrow Al(OH)_3\downarrow + NaOH$.
  • Precipitate is filtered, washed, dried, and ignited: $2Al(OH)_3 \xrightarrow{\Delta} Al_2O_3 + 3H_2O$.
  • Hall's Process (for red bauxite):
  • Fused with sodium carbonate: $Al_2O_3 + Na_2CO_3 \xrightarrow{1500^\circ\text{C}} 2NaAlO_2 + CO_2$.
  • Extracted with water, warmed, and $CO_2$ is bubbled through to precipitate aluminium hydroxide: $2NaAlO_2 + 3H_2O + CO_2 \rightarrow 2Al(OH)_3\downarrow + Na_2CO_3$.
  • Serpeck's Process (for white bauxite containing silica, $SiO_2$, as chief impurity):
  • Bauxite is heated with coke in an atmosphere of nitrogen at $1800^\circ\text{C}$:
    • $Al_2O_3 + 3C + N_2 \rightarrow 2AlN + 3CO$
    • $SiO_2 + 2C \rightarrow Si\uparrow + 2CO$ (silica reduced and volatilized away)
  • Aluminium nitride is hydrolyzed with water: $AlN + 3H_2O \rightarrow Al(OH)_3\downarrow + NH_3\uparrow$.
  • Ignition of $Al(OH)_3$ yields pure alumina ($Al_2O_3$).

• Electrolytic Reduction of Alumina (Hall-Héroult Process):

  • Technical Hurdles:
  1. Pure alumina is an electrical insulator (very poor conductor of electricity).
  2. Pure alumina has an extraordinarily high fusion temperature ($\sim 2050^\circ\text{C}$, $2323\,K$).
  • Electrolyte Bath Solution: Alumina ($Al_2O_3$, $20\%$) is dissolved in molten cryolite ($Na_3AlF_6$, $60\%$) and fluorspar ($CaF_2$, $20\%$). This mixture decreases the melting point to approximately $950^\circ\text{C}$ ($\sim 1223\,K$) and imparts high electrical conductivity.
  • Cell Setup:
  • Cathode: Carbon/graphite lining on the inside of the steel bath.
  • Anode: Series of graphite rods suspended into the molten bath.
  • Electrode Reactions:
  • Ionisation: $Al_2O_3 \rightarrow 2Al^{3+} + 3O^{2-}$
  • Cathode: $Al^{3+} + 3e^- \rightarrow Al(l)$ (molten aluminium sinks to the bottom)
  • Anode: Carbon anodes are consumed by oxygen:
    • $C + O^{2-} \rightarrow CO + 2e^-$
    • $C + 2O^{2-} \rightarrow CO_2 + 4e^-$
  • Overall Cell Reaction: $2Al_2O_3 + 3C \rightarrow 4Al + 3CO_2$
  • Due to oxidation by liberated oxygen, approximately $0.5\,kg$ of carbon anode is consumed for every $1\,kg$ of aluminium metal produced.

• Refining of Aluminium (Hoope's Three-Layer Electrolytic Process):

  • Operates using three immiscible liquid layers stratified by density inside a carbon-lined cell:
  1. Bottom Layer (Anode): Impure crude aluminium alloyed with molten copper to artificially elevate its density.
  2. Middle Layer (Electrolyte): Molten mixture of cryolite ($Na_3AlF_6$), aluminium fluoride ($AlF_3$), and barium fluoride ($BaF_2$) saturated with alumina. The addition of $BaF_2$ adjusts the density to maintain its position as an intermediate layer.
  3. Top Layer (Cathode): Pure molten aluminium floating on the electrolyte with suspended graphite cathode rods.
  • Operational Mechanism:
  • Anode (Bottom): $Al(\text{impure}) \rightarrow Al^{3+} + 3e^-$
  • Cathode (Top): $Al^{3+} + 3e^- \rightarrow Al(\text{pure})$
  • Generates aluminium with purity exceeding $99.99\%$.

Commercial Alloys and Industrial Applications

• Compositions of Prominent Alloys:

  • Brass: Copper ($Cu$, $60\%$), Zinc ($Zn$, $40\%$).
  • Bronze: Copper ($Cu$, $80 - 90\%$), Tin ($Sn$, $10 - 20\%$).
  • German Silver: Copper ($Cu$, $25 - 30\%$), Zinc ($Zn$, $25 - 30\%$), Nickel ($Ni$, $40 - 50\%$).
  • Coinage Alloy: Copper ($Cu$) alloyed with Nickel ($Ni$).
  • Gun Metal: Copper ($Cu$), Tin ($Sn$), Zinc ($Zn$), and Lead ($Pb$).
  • Solder: Lead ($Pb$, $\sim 50\%$), Tin ($Sn$, $\sim 50\%$).
  • Mischmetal: Lanthanoids ($Ln$, $\sim 95\%$), Iron ($Fe$, $\sim 5\%$), and trace amounts of $S$, $C$, $Ca$, $Al$. Used in cigarette lighter flints.
  • Alloy Steels:
  • Nickel Steel: Contains nickel; exhibits extreme toughness and thermal resistance. Used in structural cables, automobile and aircraft components, pendulum bobs, and precision measuring tapes.
  • Chrome Steel: Contains chromium; high hardness and impact resistance. Used in cutting tools, rock crushers, and ball bearings.
  • Stainless Steel: Iron with chromium ($18\%$) and nickel ($8\%$); resists chemical corrosion. Used in cutlery, domestic utensils, surgical instruments, and automotive parts.

• Industrial Applications of Refined Metals:

  • Aluminium ($Al$):
  • Thin foils used for packaging food, pharmaceuticals, and confectionery.
  • Fine aluminium powder/dust used in anti-corrosive silver paints and lacquers.
  • Powerful metallothermic reducing agent for the isolation of chromium and manganese from their oxides.
  • Overhead electrical transmission cables due to high electrical conductivity per unit weight.
  • Structural lightweight alloys (e.g., Duralumin) used in aerospace and automotive frameworks.
  • Copper ($Cu$):
  • Electrical wiring and power circuitry due to exceptional electrical conductivity.
  • Industrial water piping, steam condensers, and heat exchangers due to corrosion resistance and high thermal conductivity.
  • Component metal in heavy-duty engineering alloys (brass, bronze, coinage alloys).
  • Zinc ($Zn$):
  • Galvanisation of structural iron and steel to prevent atmospheric rusting.
  • Large-scale anode component in primary dry batteries and accumulator cells.
  • Component in brass and German silver.
  • Zinc dust used as a specialized laboratory and industrial reducing agent in manufacturing dyes, drugs, and paints.
  • Iron ($Fe$):
  • Cast iron used for heavy castings: stoves, railway sleepers, gutter pipes, sash weights, and heavy machine housings.
  • Fundamental feed material in the manufacture of wrought iron and industrial steels.
  • Wrought iron used where high ductility and magnetic permeability are needed: ship anchors, safety chains, structural bolts, wrought fencing, and agricultural tools.