An ore is the mineral from which a particular metal can be extracted conveniently and economically. Examples include:

1. Haematite (Fe₂O₃)
2. Iron pyrite (FeS₂)
3. Galena (PbS)
4. Zinc blende (ZnS)
5. Tin-stone or cassiterite (SnO₂)

Minerals are naturally occurring metallic compounds found in the Earth's crust, which can be obtained by mining. Examples include:

1. Magnetite (Fe₃O₄)
2. Gypsum (CaSO₄·2H₂O)
3. Bauxite (Al₂O₃·2H₂O)
4. Malachite (CuCO₃·Cu(OH)₂)
5. Limestone (CaCO₃)

Note: All ores are minerals, but not all minerals are ores.

Metals occur naturally in two states:

1. Native or uncombined state: Elements found in their elemental form (free metals), e.g., copper (Cu), platinum (Pt), gold (Au), silver (Ag), and mercury (Hg).
2. Combined state: Elements found in the form of compounds, e.g., Fe, Cu, Al, Pb, Sn, Ca, Mg, Na, Mn, Cr, Co, etc.

Question: What are the main ores from which tin, copper, and aluminium are extracted?

Aluminium

Aluminium is the third most abundant element after silicon and oxygen and the most abundant metal in the Earth's crust. Being reactive, aluminium does not occur as a free metal. It occurs in nature in the following forms:

1. Corundum (Al₂O₃) – free oxide
2. Bauxite (Al₂O₃·2H₂O) – hydrated oxide
3. Silicates (e.g., Al₂Si₂O₇·2H₂O, KAlSi₃O₈)
4. Cryolite (Na₃AlF₆)

Bauxite is the most economical ore for extracting aluminium, with major deposits in South-East Europe, India, Australia, Brazil, the USA, and the West Indies.

Tin (Sn)

Tin does not occur as a free element due to its moderate reactivity. The primary ores of tin include:

1. Cassiterite (SnO₂)
2. Tin-pyrite (CuS·FeS·SnS₂)

Deposits occur in Malaysia, Indonesia, Bolivia, China, Burma, Thailand, and Nigeria.

Copper (Cu)

Copper occurs in both free and combined states. In the combined state, it is found as:

1. Copper pyrite (CuFeS₂)
2. Azurite (2CuCO₃·Cu(OH)₂)
3. Malachite (CuCO₃·Cu(OH)₂)

Deposits are found in Zambia, the Democratic Republic of Congo, and Canada.

The extraction of metals involves three main stages:

1. Concentration of ore: Increasing the metallic content of the ore by removing impurities.
2. Reduction: Converting metallic ions to metallic atoms by accepting electrons.
3. Refining or purification: Removing impurities to obtain a pure metal.

Concentration of ore

This involves increasing the metal content by removing gangue (earthy impurities). Common methods include:

1. Gravity separation: Using differences in density to separate ore from gangue.

   

2. Magnetic separation: Separating magnetic impurities from non-magnetic components.

   

3. Froth flotation: Using pine oil and air bubbles to separate sulphide ores from gangue.

   

4. Leaching: Dissolving ore in a suitable reagent to remove impurities.

   • Calcination: Heating ore in limited air to remove volatile impurities.

     Calcination brings about the following changes:

     a) The carbonates ores are decomposed to their representative metal oxide, e.g.:

$\begin{aligned} FeCO_3 &\xrightarrow{\Delta} FeO + CO_2 \\ PbCO_3 &\xrightarrow{\Delta} PbO + CO_2 \\ CuCO_3 \cdot Cu(OH)_2 &\xrightarrow{\Delta} 2CuO + H_2O + 2CO_2 \end{aligned}$

 b) Water of crystallization in the hydrated oxide gets lost in form of water vapour, e.g.:

$\begin{aligned} Fe_2O_3 \cdot 3H_2O_{(s)} &\xrightarrow{\Delta} Fe_2O_{3(s)} + 3H_2O_{(g)} \quad \text{(Limonite)} \\ Al_2O_3 \cdot 2H_2O &\xrightarrow{\Delta} Al_2O_{3(s)} + 2H_2O_{(g)} \quad \text{(Bauxite)} \end{aligned}$

 c) Organic matter, if present in the ore gets lost or expelled and the ore becomes porous.

5. Roasting: Heating ore in excess air to convert sulphides into oxides or metals.

$$\begin{aligned} &2PbS_{(s)} + O_{2(g)} \xrightarrow{\Delta} PbO_{(s)} + 2SO_{2(g)} \\ &PbS_{(s)} + 2O_{2(g)} \xrightarrow{\Delta} PbSO_{4(s)} \\ &\text{Also}\quad ZnS_{(s)} + O_{2(g)} \xrightarrow{\Delta} ZnO_{(s)} + SO_{2(g)} \\ &ZnS_{(s)} + 2O_{2(g)} \xrightarrow{\Delta} ZnSO_{4(s)} \end{aligned}$$

When roasting takes place at high temperature in the presence of oxygen, some sulphide ore give metallic oxide:

$$2Cu_2S_{(s)} + 3O_{2(g)} \xrightarrow{\Delta} 2Cu_2O_{(s)} + 2SO_{2(g)}$$

The sulphide ores of some metals like Cu, Pb, Hg, Sb, etc. when heated strongly in excess of air or oxygen are reduced directly to the metallic elements, e.g.:

$$Cu_2S + O_2 \xrightarrow{\Delta} 2Cu_{(s)} + 2SO_{2(g)} \quad \text{(Copper glance)}$$ $$PbS_{(s)} + O_{2(g)} \xrightarrow{\Delta} Pb_{(s)} + SO_{2(g)} \quad \text{(Galena)}$$

Reduction

Reduction is achieved either thermally or electrolytically:

1. Thermal reduction: Using heat and reducing agents (e.g., carbon, coke, or metals like Mg).
2. Electrolytic reduction: Using electricity to extract reactive metals (e.g., Na, K, Al).

Purification is the process of removing impurities from extracted metals. The metal obtained often contains various impurities, such as other metals, non-metals, unreduced oxides, and sulfides of metal slag. The purification method depends on the nature of the metal, the impurities to be removed, and the intended use of the purified metal.

The methods used for refining impure metals can be categorized into four main categories:

1. Physical methods

a) Liquefaction process

This process is used for purifying metals that melt at a lower temperature than their impurities. It is applicable for metals such as tin (Sn), zinc (Zn), and lead (Pb).

b) Fractional distillation

This process purifies volatile metals from non-volatile impurities. The metal vapour is condensed in a separate vessel, while the non-volatile impurities remain in solid form. An example is the purification of zinc (Zn) from impurities such as arsenic (As), cadmium (Cd), lead (Pb), and iron (Fe).

2. Chemical methods

a) Oxidative process

This method removes metallic and non-metallic impurities by oxidation. Examples of impurities include manganese (Mn), copper (Cu), lead (Pb), tin (Sn), silver (Ag), carbon (C), phosphorus (P), sulfur (S), and silicon (Si).

During the process, oxygen or air is passed through the molten metal, oxidizing the impurities into oxides. These oxides:

1. Form scum on the metal's surface and can be skimmed off.
2. May escape as volatile oxides through the furnace mouth.
3. Combine with furnace linings to form a slag, which can be removed.

b) Thermal decomposition method

In this method, the impure metal is converted into a volatile compound that decomposes upon heating, leaving pure metal behind. An example is the purification of nickel using Mond's process (carbonyl process).

3. Amalgamation process

This process is used to extract and purify silver (Ag) and gold (Au). Both metals form amalgams with mercury (Hg), such as Au/Hg and Ag/Hg. The amalgam is distilled in an iron retort, where mercury distils off (being more volatile), leaving behind purified silver or gold.

4. Electrolytic process

Electrolytic purification produces high-purity metals. In this method, the impure metal is made the anode, and the pure metal serves as the cathode in an electrolytic cell. A suitable aqueous solution or complex salt of the metal is used as the electrolyte.

When an electric current is passed:

1. The metal migrates from the anode to the cathode, depositing as pure metal.
2. Soluble impurities dissolve in the electrolyte.
3. Insoluble impurities settle as anode mud or sludge, which may contain valuable metals for further extraction.

Metals purified by this method include tin (Sn), lead (Pb), copper (Cu), silver (Ag), nickel (Ni), zinc (Zn), and chromium (Cr).

Tin is extracted from tin stone (SnO₂), which contains silicon dioxide (SiO₂) as an acidic impurity. To remove this impurity and reduce tin stone to molten tin:

1. The tin stone is mixed with anthracite coal and a basic flux such as lime (CaO).
2. The mixture is heated in a reverberatory furnace at 1200°C–1300°C.
3. SiO₂ reacts with CaO to form a slag of calcium silicate (CaSiO₃), which is removed.

The molten tin is tapped and cast into blocks containing approximately 70% metallic tin. Any SnSiO₃ formed can be recovered by melting it with limestone in a furnace.

Key terms

1. Flux: A substance that reacts with impurities during smelting to form fusible compounds (e.g., CaO for basic flux and SiO₂ for acidic flux).
2. Slag: A fusible compound formed by the reaction of flux with impurities (e.g., CaO + SiO₂ → CaSiO₃).
3. Gangue: Impurities in the ore, which can be acidic (e.g., SiO₂) or basic (e.g., CaO, MgO, FeO).
4. Smelting: The process of converting roasted or calcined ore into molten metal using a reducing agent at high temperatures.