Occurrence Of Metals

Metals have played a significant role in human history and civilization, with different periods even named after metals. The ability to extract metals from their natural sources transformed society, providing materials for tools, weapons, ornaments, and utensils.

While some elements like carbon, sulfur, gold, and noble gases exist in their free state, most metals are found in the Earth's crust in combined forms as various chemical compounds.

Metals vary in their abundance. Aluminum is the most abundant metal in the Earth's crust, ranking as the third most abundant element overall (around 8.3% by weight). It is found in minerals like mica and clays and is the main component of bauxite ore. Iron is the second most abundant metal, forming a variety of compounds and being essential in biological systems.

A mineral is a naturally occurring chemical substance in the Earth's crust obtained by mining. An ore is a mineral from which a metal can be extracted economically and efficiently.

The selection of an ore for metal extraction depends on factors like its abundance, ease of extraction, and environmental considerations (e.g., avoiding ores that produce polluting gases).

Principal ores of some important metals:

Metal	Ores	Composition
Aluminium	Bauxite Kaolinite (a form of clay)	AlO$_x$(OH)$_{3-2x}$ [where 0 < x < 1] Al$_2$(OH)$_4$Si$_2$O$_5$
Iron	Haematite Magnetite Siderite Iron pyrites	Fe$_2$O$_3$ Fe$_3$O$_4$ FeCO$_3$ FeS$_2$
Copper	Copper pyrites Malachite Cuprite Copper glance	CuFeS$_2$ CuCO$_3$.Cu(OH)$_2$ Cu$_2$O Cu$_2$S
Zinc	Zinc blende or Sphalerite Calamine Zincite	ZnS ZnCO$_3$ ZnO

The scientific and technological process of isolating a metal from its ore is called metallurgy. Metallurgical processes are based on chemical principles and aim to be chemically feasible and commercially viable.

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Concentration Of Ores

Ores are typically contaminated with unwanted earthly or rocky materials called gangue. Removing gangue from the ore is known as concentration, dressing, or benefaction. Ores are usually crushed before concentration.

The choice of concentration method depends on the physical properties of the ore and gangue particles. Some important methods:

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Hydraulic Washing

Based on the difference in specific gravities of the ore and gangue particles (gravity separation). Powdered ore is washed with an upward stream of running water. Lighter gangue particles are washed away, leaving heavier ore particles behind.

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Magnetic Separation

Used when either the ore or the gangue has magnetic properties. Powdered ore is dropped onto a conveyor belt passing over a magnetic roller. Magnetic particles are attracted to the roller and fall closer to it, separating them from non-magnetic particles.

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Froth Floatation Method

This method is mainly used for concentrating sulphide ores. A suspension of powdered ore is made in water with collectors (e.g., pine oils, fatty acids) and froth stabilisers (e.g., cresols, aniline).

Collectors make the mineral particles water-repellent (non-wettable by water) and oil-wettable. Froth stabilisers make the froth stable. A rotating paddle agitates the mixture, drawing in air, which forms froth. The oil-wet mineral particles are carried by the froth to the surface and skimmed off. Gangue particles, being water-wet, settle down. Depressants (e.g., NaCN for ZnS/PbS ore) can be used to selectively prevent one sulphide ore from forming froth.

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Leaching

Leaching is used when the ore is soluble in a suitable solvent. It involves treating the powdered ore with a reagent that dissolves the metal compound, leaving impurities undissolved. The solution is then processed to recover the metal.

• Leaching of alumina from bauxite: Bauxite ($\textsf{Al}_2\text{O}_3$.xH$_2\text{O}$) contains impurities like $\textsf{SiO}_2$, iron oxides, $\textsf{TiO}_2$. Powdered bauxite is heated with concentrated $\textsf{NaOH}$ solution at high temperature and pressure (digestion). Alumina dissolves as sodium aluminate ($\textsf{Na[Al(OH)}_4]$), and $\textsf{SiO}_2$ dissolves as sodium silicate ($\textsf{Na}_2\text{SiO}_3$). Other impurities don't dissolve.
  • $\textsf{Al}_2\text{O}_3\text{(s)} + 2\textsf{NaOH(aq)} + 3\textsf{H}_2\text{O(l)} \to 2\textsf{Na[Al(OH)}_4]\text{(aq)}$
  The sodium aluminate solution is then neutralised by passing $\textsf{CO}_2$ gas, precipitating hydrated alumina ($\textsf{Al}_2\text{O}_3$.xH$_2\text{O}$). Seeding (adding fresh hydrated alumina) aids precipitation.
  • $2\textsf{Na[Al(OH)}_4]\text{(aq)} + 2\textsf{CO}_2\text{(g)} \to \textsf{Al}_2\text{O}_3.\textsf{xH}_2\text{O(s)} + 2\textsf{NaHCO}_3\text{(aq)}$
  Hydrated alumina is filtered, dried, and heated strongly (calcined) to get pure alumina.
  • $\textsf{Al}_2\text{O}_3.\textsf{xH}_2\text{O(s)} \xrightarrow{\text{1470 K}} \textsf{Al}_2\text{O}_3\text{(s)} + \textsf{xH}_2\text{O(g)}$
• Leaching of silver and gold: These metals are leached with dilute solutions of $\textsf{NaCN}$ or $\textsf{KCN}$ in the presence of air ($\textsf{O}_2$). Metals form soluble complexes.
  • $4\textsf{M(s)} + 8\textsf{CN}^{-}\text{(aq)}+ 2\textsf{H}_2\text{O(aq)} + \textsf{O}_2\text{(g)} \to 4[\textsf{M(CN)}_2]^{-} \text{(aq)} + 4\textsf{OH}^{-}\text{(aq)}$ (M= Ag or Au)
  The metal is then recovered by displacement using a more electropositive metal, like zinc powder.
  • $2[\textsf{M(CN)}_2]^{-}\text{(aq)} + \textsf{Zn(s)} \to [\textsf{Zn(CN)}_4]^{2-}\text{(aq)} + 2\textsf{M(s)}$

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Extraction Of Crude Metal From Concentrated Ore

After concentration, the ore must be converted into a form suitable for reduction to the metal. Usually, sulfide ores are converted to oxides because oxides are easier to reduce than sulfides.

This stage involves two main steps:

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Conversion To Oxide

• Calcination: Heating the ore strongly in the absence or limited supply of air. Used for carbonate and hydrated ores. It removes volatile matter like moisture and carbon dioxide.
  • $\textsf{Fe}_2\text{O}_3$.xH$_2\text{O(s)} \xrightarrow{\Delta} \textsf{Fe}_2\text{O}_3\text{(s)} + \textsf{xH}_2\text{O(g)}$ (Hydrated iron oxide)
  • $\textsf{ZnCO}_3\text{(s)} \xrightarrow{\Delta} \textsf{ZnO(s)} + \textsf{CO}_2\text{(g)}$ (Zinc carbonate)
  • $\textsf{CaCO}_3.\textsf{MgCO}_3\text{(s)} \xrightarrow{\Delta} \textsf{CaO(s)} + \textsf{MgO(s)} + 2\textsf{CO}_2\text{(g)}$ (Dolomite)
• Roasting: Heating the ore strongly in a regular supply of air in a furnace at a temperature below the metal's melting point. Primarily used for sulfide ores to convert them into oxides. $\textsf{SO}_2$ gas is produced.
  • $2\textsf{ZnS(s)} + 3\textsf{O}_2\text{(g)} \xrightarrow{\text{Roasting}} 2\textsf{ZnO(s)} + 2\textsf{SO}_2\text{(g)}$ (Zinc sulfide)
  • $2\textsf{PbS(s)} + 3\textsf{O}_2\text{(g)} \xrightarrow{\text{Roasting}} 2\textsf{PbO(s)} + 2\textsf{SO}_2\text{(g)}$ (Lead sulfide)
  • $2\textsf{Cu}_2\text{S(s)} + 3\textsf{O}_2\text{(g)} \xrightarrow{\text{Roasting}} 2\textsf{Cu}_2\text{O(s)} + 2\textsf{SO}_2\text{(g)}$ (Copper(I) sulfide)
  Sulphide ores of copper often contain iron. During roasting in a reverberatory furnace, silica ($\textsf{SiO}_2$) is added, which reacts with iron oxide formed to produce easily removable slag (iron silicate, $\textsf{FeSiO}_3$). Copper is obtained as copper matte ($\textsf{Cu}_2\text{S} + \textsf{FeS}$).
  • $\textsf{FeO} + \textsf{SiO}_2 \to \textsf{FeSiO}_3$ (Slag)
  The produced $\textsf{SO}_2$ can be used for $\textsf{H}_2\textsf{SO}_4$ manufacture.

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Reduction Of Oxide To Metal

Metal oxides are reduced to crude metal, typically by heating with a reducing agent like carbon (coke), carbon monoxide ($\textsf{CO}$), or another metal. The reducing agent removes oxygen from the metal oxide.

$\textsf{M}_\text{x}\textsf{O}_\text{y} + \textsf{yC} \to \textsf{xM} + \textsf{yCO}$

The ease of reduction varies for different metal oxides.

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Thermodynamic Principles Of Metallurgy

Thermodynamics provides a framework to understand the feasibility of metallurgical reduction processes and choose suitable reducing agents. The criterion for a reaction to be spontaneous (feasible) at a given temperature (T) is that the change in Gibbs energy ($\Delta \textsf{G}$) must be negative:

$\Delta \textsf{G} = \Delta \textsf{H} - \textsf{T}\Delta \textsf{S}$

Where $\Delta \textsf{H}$ is the enthalpy change and $\Delta \textsf{S}$ is the entropy change. For thermal reduction of a metal oxide (decomposition), the reaction is typically $\textsf{M}_\text{x}\textsf{O} \to \textsf{xM} + \frac{1}{2}\textsf{O}_2\text{(g)}$. This reaction involves an increase in entropy (formation of gas), so $\Delta \textsf{S}$ is positive. At high temperatures, the $-\textsf{T}\Delta \textsf{S}$ term becomes large and negative, making $\Delta \textsf{G}$ more negative and favoring decomposition.

Reduction of a metal oxide $\textsf{M}_\text{x}\textsf{O}$ by a reducing agent ($\textsf{A}_{\text{red}}$) involves two coupled reactions:

1. Reduction of the metal oxide: $\textsf{M}_\text{x}\textsf{O} \to \textsf{xM} + \frac{1}{2}\textsf{O}_2\text{(g)}$ ($\Delta \textsf{G}_1$)
2. Oxidation of the reducing agent: $\textsf{A}_{\text{red}} + \frac{1}{2}\textsf{O}_2\text{(g)} \to \textsf{A}_{\text{red}}\textsf{O}$ ($\Delta \textsf{G}_2$)

Overall reaction: $\textsf{M}_\text{x}\textsf{O} + \textsf{A}_{\text{red}} \to \textsf{xM} + \textsf{A}_{\text{red}}\textsf{O}$ ($\Delta \textsf{G}_{\text{overall}} = \Delta \textsf{G}_1 + \Delta \textsf{G}_2$)

The reduction is feasible if $\Delta \textsf{G}_{\text{overall}}$ is negative. This occurs when the $\Delta \textsf{G}$ for the oxidation of the reducing agent ($\Delta \textsf{G}_2$) is sufficiently negative to make the sum ($\Delta \textsf{G}_1 + \Delta \textsf{G}_2$) negative. A reducing agent is chosen such that its oxidation reaction has a more negative $\Delta \textsf{G}$ than the reduction reaction of the metal oxide at the chosen temperature.

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Applications

Ellingham Diagram: These are plots of $\Delta_\text{f}\textsf{G}^0$ (standard Gibbs energy of formation of metal oxides per mole of oxygen) vs temperature (T). They are used to predict the feasibility of reducing a metal oxide with a particular reducing agent at a given temperature.

• Each plot shows the $\Delta \textsf{G}^0$ for reactions like $2\textsf{M(s)} + \textsf{O}_2\text{(g)} \to 2\textsf{MO(s)}$. The slope is usually positive because $\Delta \textsf{S}$ for oxide formation from gas is negative ($\Delta \textsf{G} = \Delta \textsf{H} - \textsf{T}\Delta \textsf{S}$). Phase changes (melting, boiling) cause abrupt changes in slope.
• The intersection point of two curves (e.g., $\textsf{M}_1\text{O}$ formation and $\textsf{M}_2\text{O}$ formation) represents the temperature where $\Delta \textsf{G}^0$ for the reaction $\textsf{M}_1\text{O} + \textsf{M}_2 \to \textsf{M}_1 + \textsf{M}_2\text{O}$ is zero.
• Above the intersection point, the element on the lower curve can reduce the oxide of the element on the upper curve (because the overall $\Delta \textsf{G}^0$ becomes negative). Below the intersection, the reverse reaction is feasible.
• The greater the vertical distance between the reducing agent's oxidation curve (e.g., C $\to$ CO or CO $\to$ $\textsf{CO}_2$) and the metal oxide formation curve, the easier the reduction.

Limitations of Ellingham Diagram:

• Based on thermodynamics, it only predicts feasibility, not reaction kinetics (how fast).
• Assumes reactants/products are in equilibrium, which isn't always true in commercial processes.

(a) Extraction of iron from its oxides (e.g., Haematite, $\textsf{Fe}_2\text{O}_3$) in a Blast furnace:

• Concentrated ore, coke (reducing agent), and limestone (flux) are fed into a Blast furnace. Hot air is blown from the bottom.
• Temperature gradient exists: very high at the bottom ($\sim 2200$ K), lower at the top (500-800 K).
• Coke burns to provide heat and $\textsf{CO}$: $\textsf{C} + \textsf{O}_2 \to \textsf{CO}_2$, then $\textsf{CO}_2 + \textsf{C} \to 2\textsf{CO}$. $\textsf{CO}$ is the main reducing agent.
• Reduction reactions occur at different temperatures:
  • 500-800 K: $\textsf{Fe}_2\text{O}_3 \to \textsf{Fe}_3\text{O}_4 \to \textsf{FeO}$ by $\textsf{CO}$.
  • 900-1500 K: $\textsf{FeO}$ is reduced to $\textsf{Fe}$ by $\textsf{CO}$ or $\textsf{C}$. Looking at the Ellingham diagram, the C,CO line is below the Fe,FeO line above $\sim 1073$ K, making C a better reducing agent than CO at higher temperatures in the furnace.
• Limestone (flux) decomposes ($\textsf{CaCO}_3 \to \textsf{CaO} + \textsf{CO}_2$) and $\textsf{CaO}$ removes silicate impurities as slag: $\textsf{CaO} + \textsf{SiO}_2 \to \textsf{CaSiO}_3$.
• Molten iron and slag are collected separately at the bottom.

The iron obtained (pig iron) contains $\sim 4\%$ C and impurities. Cast iron has $\sim 3\%$ C. Wrought iron is the purest form of commercial iron ($\sim 0.15\%$ C), produced by oxidising impurities from cast iron.

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(b) Extraction of copper from cuprous oxide ($\textsf{Cu}_2\text{O}$):

• $\textsf{Cu}_2\text{O}$ is relatively high in the Ellingham diagram, indicating it's easier to reduce.
• Oxide ores can be reduced directly with coke: $\textsf{Cu}_2\text{O(s)} + \textsf{C(s)} \to 2\textsf{Cu(s/l)} + \textsf{CO(g)}$.
• Sulphide ores ($\textsf{Cu}_2\text{S}$) are roasted to oxides first. If iron is present, silica is added during smelting to remove FeO as slag ($\textsf{FeO} + \textsf{SiO}_2 \to \textsf{FeSiO}_3$). Copper matte ($\textsf{Cu}_2\text{S} + \textsf{FeS}$) is formed.
• Copper matte is put in a silica-lined converter, and hot air is blown. $\textsf{FeS}$ oxidises to $\textsf{FeO}$ (forms slag with $\textsf{SiO}_2$). $\textsf{Cu}_2\text{S}$ is partially oxidised to $\textsf{Cu}_2\text{O}$. $\textsf{Cu}_2\text{O}$ then reacts with remaining $\textsf{Cu}_2\text{S}$ (self-reduction) to produce copper: $2\textsf{Cu}_2\text{O} + \textsf{Cu}_2\text{S} \to 6\textsf{Cu} + \textsf{SO}_2$.
• Blister copper is obtained, containing trapped $\textsf{SO}_2$ bubbles.

(c) Extraction of zinc from zinc oxide ($\textsf{ZnO}$):

• $\textsf{ZnO}$ is reduced by coke at a higher temperature than for copper extraction ($\sim 1673$ K).
• $\textsf{ZnO(s)} + \textsf{C(s)} \xrightarrow{\text{1673 K}} \textsf{Zn(s/l)} + \textsf{CO(g)}$.
• Zinc distils off due to its low boiling point and is collected by rapid chilling.

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Electrochemical Principles Of Metallurgy

Electrochemical methods (electrolysis) are used for reducing metal ions in solution or molten state to obtain metals, especially for highly reactive metals where thermal reduction with common reducing agents is difficult or impossible. These methods are based on electrochemical principles, related to electrode potentials and Gibbs energy.

The feasibility of reducing a metal ion ($\textsf{M}^{\text{n}+}$) by electrolysis is related to the standard electrode potential ($\textsf{E}^0$) of the $\textsf{M}^{\text{n}+}$/M redox couple and the Gibbs energy change ($\Delta \textsf{G}^0$) of the reduction process:

$\Delta \textsf{G}^0 = - \textsf{nF} \textsf{E}^0$

Where n is the number of electrons transferred, and F is Faraday's constant. A more negative $\textsf{E}^0$ (meaning the element is more reactive or a stronger reducing agent) corresponds to a more positive $\Delta \textsf{G}^0$ for reduction, indicating that the reduction is thermodynamically less favorable. To drive the reduction of a highly electropositive metal ion, a large negative external potential must be applied at the cathode.

In electrolysis, $\textsf{M}^{\text{n}+}$ ions are discharged (reduced) at the cathode (negative electrode) and deposited as metal. Precautions are needed based on the reactivity of the metal produced. Fluxes may be added to make the molten mass more conducting.

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Oxidation Reduction

Some extraction processes involve oxidation, particularly for non-metals or when recovering metals from complexes formed after leaching. For example, chlorine is extracted from brine (aqueous $\textsf{NaCl}$) by electrolysis, which involves the oxidation of $\textsf{Cl}^{-}$ ions:

$2\textsf{Cl}^{-}\text{(aq)} + 2\textsf{H}_2\text{O(l)} \to 2\textsf{OH}^{-}\text{(aq)} + \textsf{H}_2\text{(g)} + \textsf{Cl}_2\text{(g)}$ ($\Delta \textsf{G}^0 = + 422$ kJ, $\textsf{E}^0 = -2.2$ V)

This non-spontaneous reaction requires an external voltage greater than 2.2 V for electrolysis to occur, producing $\textsf{H}_2$ and $\textsf{NaOH}$ as by-products.

Extraction of gold and silver often involves leaching with $\textsf{CN}^{-}$ in the presence of $\textsf{O}_2$, which is an oxidation of the metal:

$4\textsf{Au(s)} + 8\textsf{CN}^{-}\text{(aq)} + 2\textsf{H}_2\text{O(aq)} + \textsf{O}_2\text{(g)} \to 4[\textsf{Au(CN)}_2]^{-}\text{(aq)} + 4\textsf{OH}^{-}\text{(aq)}$

The metal is then recovered by reducing the complex using a more electropositive metal, like zinc (displacement reaction):

$2[\textsf{Au(CN)}_2]^{-}\text{(aq)} + \textsf{Zn(s)} \to 2\textsf{Au(s)} + [\textsf{Zn(CN)}_4]^{2-}\text{(aq)}$ (Zinc acts as a reducing agent).

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Refining

The crude metal obtained after extraction usually contains impurities. Refining is the process used to obtain metals of high purity. Various techniques are used, chosen based on the differences in properties between the metal and its impurities.

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Distillation

Useful for low boiling metals like zinc, mercury, and cadmium. The impure metal is heated to evaporate the pure metal, which is then condensed to obtain the distillate.

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Liquation

Used for metals that melt at a lower temperature than their impurities, such as tin, lead, and bismuth. The impure metal is heated on a sloping hearth. The low-melting metal flows down, separating from the higher-melting impurities.

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Electrolytic Refining

This is a widely used method for obtaining high-purity metals like copper, zinc, nickel, silver, gold, and aluminum. The impure metal is made the anode, and a thin strip of the pure metal is the cathode. Both electrodes are immersed in an electrolytic bath containing a soluble salt of the same metal.

During electrolysis:

• At the Anode (impure metal): The metal dissolves ($\textsf{M} \to \textsf{M}^{\text{n}+} + \textsf{ne}^{-}$). More reactive (more basic) impurities remain in solution as ions. Less reactive (less basic) impurities fall to the bottom as anode mud (e.g., Ag, Au, Pt).
• At the Cathode (pure metal): The metal ions from the electrolyte solution are reduced and deposited as pure metal ($\textsf{M}^{\text{n}+} + \textsf{ne}^{-} \to \textsf{M}$).

For copper refining: Impure copper anode, pure copper cathode, acidified $\textsf{CuSO}_4$ electrolyte. $\textsf{Cu}$ dissolves from anode ($\textsf{Cu} \to \textsf{Cu}^{2+} + 2\textsf{e}^{-}$), $\textsf{Cu}^{2+}$ ions deposit on cathode ($\textsf{Cu}^{2+} + 2\textsf{e}^{-} \to \textsf{Cu}$). Impurities like Sb, Se, Te, Ag, Au, Pt collect as anode mud.

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Zone Refining

Based on the principle that impurities are generally more soluble in the molten state (melt) of the metal than in the solid state. Used for producing very high purity semiconductors (Ge, Si, B, Ga, In).

A mobile circular heater is slowly moved along a rod of the impure metal. A narrow molten zone is created and moves with the heater. As the heater moves, the pure metal solidifies behind it, and the impurities, being more soluble in the melt, preferentially stay in the molten zone and move forward with the heater. Repeating this process concentrates impurities at one end of the rod, which is then cut off.

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Vapour Phase Refining

The impure metal is converted into a volatile compound, separated, and then decomposed to obtain the pure metal. Requirements: Metal forms a volatile compound with a reagent, and the volatile compound can be easily decomposed to pure metal.

• Mond Process (for Ni refining): Impure nickel is heated with $\textsf{CO}$ at 330-350 K to form volatile nickel tetracarbonyl ($\textsf{Ni(CO)}_4$). The complex is heated to 450-470 K to decompose it, yielding pure $\textsf{Ni}$ and $\textsf{CO}$.
• $\textsf{Ni(s)} + 4\textsf{CO(g)} \xrightarrow{\text{330-350 K}} \textsf{Ni(CO)}_4\text{(g)}$
• $\textsf{Ni(CO)}_4\text{(g)} \xrightarrow{\text{450-470 K}} \textsf{Ni(s)} + 4\textsf{CO(g)}$
• van Arkel Method (for Zr or Ti refining): Used for removing O$_2$ and N$_2$ impurities. Crude metal is heated with iodine in vacuum to form volatile metal iodide. Iodide is decomposed on a hot tungsten filament ($\sim 1800$ K) to deposit pure metal.
• $\textsf{Zr(s)} + 2\textsf{I}_2\text{(g)} \xrightarrow{\Delta} \textsf{ZrI}_4\text{(g)}$
• $\textsf{ZrI}_4\text{(g)} \xrightarrow{\text{1800 K, W filament}} \textsf{Zr(s)} + 2\textsf{I}_2\text{(g)}$

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Chromatographic Methods

Chromatography is useful for purifying elements available in minute quantities or when impurities have chemical properties very similar to the element to be purified (where other methods are less effective). Different chromatographic techniques (column chromatography, gas chromatography, etc.) can be used based on differences in adsorption, partition, or exchange properties of the metal and impurities.

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Uses Of Aluminium, Copper, Zinc And Iron

Metals are widely used in various industries and applications:

• Aluminium: Used in foils for food wrapping, fine dust in paints/lacquers, as a reducing agent (e.g., in aluminothermite process to extract Cr, Mn), electricity conductors (wires), and in light alloys (e.g., duralumin) used in aircraft, automobiles.
• Copper: Used in electrical wires, water and steam pipes, and alloys (brass with zinc, bronze with tin, coinage alloy with nickel) which are often stronger than pure copper.
• Zinc: Used for galvanising iron (coating with zinc to prevent rust), in batteries (dry cells), and in alloys like brass, german silver (with Cu and Ni). Zinc dust is a reducing agent in organic chemistry and dye manufacture.
• Iron:
  • Cast iron: Most important commercial form, used for casting (stoves, pipes, toys). Contains $\sim 4\%$ C, hard and brittle. Used to make wrought iron and steel.
  • Wrought iron (malleable iron): Purest commercial iron ($\sim 0.15\%$ C). Tough and malleable. Used for making anchors, wires, bolts, chains, agricultural implements.
  • Steel: Iron with controlled carbon content (0.1-1.5%) and other elements. Many uses. Alloy steels (with added metals like Ni, Cr, V, Mn) have specific properties for various applications (cables, automobile parts, cutting tools, stainless steel utensils).

Summary table for selected metals:

Metal	Occurrence	Common method of extraction	Remarks
Aluminium	Bauxite, Cryolite	Electrolysis of Al$_2$O$_3$ dissolved in molten Na$_3$AlF$_6$ (Hall-Heroult process)	Requires large amount of electricity. Conductive cathode lining (carbon), sacrificial anode (graphite).
Iron	Haematite, Magnetite, Siderite, Iron pyrites	Reduction of oxide with CO and coke in Blast furnace	High temperature involved. CO is main reducing agent at lower T, C at higher T. Produces pig iron.
Copper	Copper pyrites, Copper glance, Malachite, Cuprite	Roasting of sulphide partially and reduction (Pyrometallurgy) Sulphuric acid leaching (Hydrometallurgy for low grade ores)	Self-reduction occurs in a converter. Easily reduced oxide.
Zinc	Zinc blende (Sphalerite), Calamine, Zincite	Roasting followed by reduction of oxide with coke	Reduction at higher T than copper. Zinc vapor is collected. Purified by fractional distillation.

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