Metallurgy – Chemistry Study Notes

Occurrence of Ores and Mineralogy

In the Earth’s crust, metals exist in varied states depending on their chemical reactivity. Unreactive metals such as gold, platinum, and sometimes copper occur in the native or free state.

Conversely, reactive metals like sodium, potassium, aluminium, and iron are found combined with non-metals in the form of compounds called minerals. Minerals are naturally occurring substances in the earth’s crust containing metals.

Among all minerals, only a select few are economically viable for extracting metals on an industrial scale; these specific minerals are termed ores. Gangue or matrix refers to the earthly, unwanted impurities—such as silica, rocky matter, and sand—that are invariably associated with the mined ore. Understanding the specific chemical composition of an ore dictates the extraction pathway we choose.

• Oxide Ores: Haematite ($\text{Fe}_2\text{O}_3$), Bauxite ($\text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O}$), Cuprite ($\text{Cu}_2\text{O}$).
• Sulphide Ores: Zinc Blende ($\text{ZnS}$), Copper Pyrites ($\text{CuFeS}_2$), Galena ($\text{PbS}$).
• Carbonate Ores: Limestone/Calcite ($\text{CaCO}_3$), Siderite ($\text{FeCO}_3$), Malachite ($\text{CuCO}_3 \cdot \text{Cu(OH)}_2$).
• Halide Ores: Rock Salt ($\text{NaCl}$), Cryolite ($\text{Na}_3\text{AlF}_6$), Horn Silver ($\text{AgCl}$).

Concentration (Beneficiation) of Ores

Before any chemical reduction takes place, the concentration step removes bulk quantities of the unwanted gangue. This physical or physicochemical process dramatically increases the percentage of the metal compound in the raw ore. The choice of concentration method depends entirely on the physical and chemical differences between the ore particles and the gangue impurities.

Gravity separation (hydraulic washing) relies on differences in specific gravities. Heavy ore particles settle down in a current of water while lighter earthy impurities are washed away, commonly used for dense oxide ores like haematite and tin stone ($\text{SnO}_2$). Conversely, magnetic separation is applied when either the ore or the gangue is magnetic; for example, separating non-magnetic tin stone from magnetic wolframite ($\text{FeWO}_4$).

Froth Flotation Method: This technique is specifically designed for concentrating sulphide ores. It depends on the preferential wetting of ore and gangue particles by oil and water respectively. Pine oil acts as the frother, collectors (like xanthates) enhance non-wettability of the ore, and depressants (like $\text{NaCN}$ for $\text{ZnS}$ in $\text{PbS}$ separation) selectively prevent specific sulphides from floating.

Leaching is a chemical concentration method where the crushed ore is treated with a suitable solvent that dissolves the ore while leaving the impurities insoluble. A classic example is Bayer’s process for purifying bauxite, where bauxite is digested with hot concentrated $\text{NaOH}$ to form soluble sodium meta-aluminate, leaving behind iron oxides and silica as insoluble red mud.

Conversion to Oxides: Calcination and Roasting

Once concentrated, it is generally much easier to reduce a metal oxide than a sulphide, carbonate, or halide. Therefore, thermal pre-treatment processes convert concentrated ores into their corresponding metal oxides. These two foundational high-temperature processes are calcination and roasting.

Calcination involves heating the ore strongly below its melting point in the absence of air or in a limited supply of air. This process is primarily used for carbonate and hydrated oxide ores to expel volatile matter and moisture, and to drive off carbon dioxide. For instance, heating limestone ($\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$) or bauxite ($\text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O} \rightarrow \text{Al}_2\text{O}_3 + x\text{H}_2\text{O}$) are standard calcination reactions.

Roasting involves heating the ore strongly below its melting point in the regular presence of excess air. It is typically applied to sulphide ores. Roasting converts volatile sulphides into oxides, drives off volatile impurities like arsenic, sulphur, and phosphorus as gaseous oxides, and makes the ore porous. For example, roasting zinc blende proceeds as: $2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$.

Thermodynamic Principles and Reduction Techniques

Extracting a metal from its oxide requires chemical reduction. Thermodynamics provides the guiding principles via Gibbs Free Energy ($\Delta G$). For a reduction process to be spontaneous, the overall $\Delta G$ must be negative ($\Delta G < 0$). Ellingham diagrams plot the standard Gibbs free energy of formation of metal oxides ($\Delta_f G^\circ$ vs $T$) as a straight line sloping upwards, because entropy decreases when oxygen gas is consumed to form a solid oxide.

A metal can reduce the oxide of another metal if the curve of the reducing metal lies lower (more negative $\Delta G^\circ$) on the Ellingham diagram at that specific temperature. Carbon, carbon monoxide, and reactive metals like aluminium or magnesium act as common reducing agents depending on where their oxidation lines sit relative to the metal oxide line.

• Smelting: Reduction of metal oxides using carbon (coke) at high temperatures, typical for iron in a blast furnace. Flux is added to combine with remaining gangue to form a fusible slag (e.g., $\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3$).
• Auto-reduction (Self-reduction): Used for less active metals like copper, mercury, and lead. When roasting their sulphide ores partially, the unreacted sulphide reacts with the newly formed oxide without needing an external reducing agent: $2\text{Cu}_2\text{O} + \text{Cu}_2\text{S} \rightarrow 6\text{Cu} + \text{SO}_2$.
• Aluminothermy (Goldschmidt Process): Highly reactive metals like aluminium reduce refractory metal oxides such as $\text{Cr}_2\text{O}_3$ or $\text{MnO}_2$ due to the immense exothermicity of forming $\text{Al}_2\text{O}_3$.
• Electrolytic Reduction: Highly electropositive metals (alkali, alkaline earth metals, and aluminium) cannot be reduced by carbon because their oxides are extremely stable. They are extracted by the electrolytic reduction of their fused anhydrous chlorides or oxides (e.g., Hall-Héroult process for aluminium).

Refining of Metals

The metal obtained from reduction processes usually contains impurities and requires purification, known as refining. The method chosen depends entirely on the nature of the metal and the specific impurities present.

1. Distillation: Used for low-boiling volatile metals like zinc, mercury, and cadmium. Impure metal is vaporized and condensed, leaving high-boiling impurities behind.
2. Liquation: Applied to metals with low melting points (like tin or bismuth) compared to their impurities. The impure metal is placed on a sloping hearth and heated; the pure metal melts and flows down, leaving solid impurities behind.
3. Electrolytic Refining: Widely used for copper, zinc, aluminium, and silver. The impure metal serves as the anode, a pure strip of the metal acts as the cathode, and a soluble salt solution of the metal serves as the electrolyte. Metal dissolves from the anode and deposits pure on the cathode.
4. Zone Refining: Based on the principle that impurities are more soluble in the molten state than in the solid state of a metal. Used for ultra-pure semiconductor elements like silicon, germanium, boron, gallium, and indium.
5. Vapour Phase Refining:
   • Mond Process for Nickel: Nickel is heated with carbon monoxide to form a volatile $\text{Ni(CO)}_4$ complex, which is subsequently decomposed at higher temperatures to yield pure nickel.
   • Van Arkel Method for Zirconium/Titanium: Crude metal is heated with iodine gas to form volatile tetraiodide, which decomposes on a hot tungsten filament at $1800\text{ K}$ to yield ultra-pure metal.
6. Chromatographic Methods: Based on selective adsorption of components of a mixture on an adsorbent column.

Important Facts / Formulas

Previous Year Question Hints

• Question Type 1 (Matching / Identification): Aspirants are frequently asked to identify the correct depressant used in the froth flotation separation of galena ($\text{PbS}$) and zinc blende ($\text{ZnS}$). Hint: Sodium cyanide ($\text{NaCN}$) selectively depresses $\text{ZnS}$ by forming a soluble complex $\text{Na}_2[\text{Zn(CN)}_4]$, allowing $\text{PbS}$ to float.
• Question Type 2 (Ellingham Diagram interpretation): Questions test why carbon can reduce zinc oxide at higher temperatures, or why aluminium can reduce chromium oxide. Hint: Look at the intersection points of standard Gibbs free energy lines ($\Delta G^\circ = 0$).
• Question Type 3 (Refining Technique match): Expect direct conceptual match-ups between high-purity element production (e.g., Silicon or Germanium) and Zone Refining.

Quick Revision Summary

• Ores vs Minerals: All ores are minerals, but not all minerals are economically viable ores.
• Gravity Separation: Suited for dense oxide ores; relies on specific gravity differences.
• Froth Flotation: Specifically concentrates sulphide ores utilizing surface wetting properties and collectors.
• Calcination: Thermal decomposition of carbonates and hydrated oxides in the absence of air.
• Roasting: Strong heating of sulphide ores in the presence of excess air to convert them into oxides.
• Ellingham Diagram: Graphical representation of $\Delta_f G^\circ$ vs $T$, determining the feasibility of thermal reduction.
• Blast Furnace: Employs carbon reduction where slag formation ($\text{CaSiO}_3$) eliminates silica gangue.
• Auto-reduction: Spontaneous reduction of less reactive metal sulphides and oxides without external reducing agents.
• Zone Refining: Produces ultra-high purity semiconductors based on differences in impurity solubility between solid and molten states.
• Vapour Phase Refining: Purifies metals via volatile intermediate compounds (Mond process for $\text{Ni}$, Van Arkel for $\text{Zr/Ti}$).