CHEMISTRY A LEVEL(FORM SIX) NOTES – INORGANIC CHEMISTRY 1.1-EXTRACTION OF METALS

INORGANIC CHEMISTRY 1.1-EXTRACTION OF METALS

Definition: Extraction of metals refers to the process of removing or obtaining metallic elements from their respective ores.

What is an ore?

An ore is the mineral from which a particular metal can be extracted conveniently and economically. Examples include Haematite (Fe₂O₃), Iron pyrite (FeS₂), galena (PbS), Zinc blende (ZnS), tin-stone or cassiterite (SnO₂).

What are minerals?

• Are naturally occurring metallic compounds found in the earth’s crust which can be obtained by mining. Examples include Magnetite (Fe₃O₄), Gypsum (CaSO₄.2H₂O), Bauxite (Al₂O₃.2H₂O), Malachite (CuCO₃.Cu(OH)₂), Limestone (CaCO₃), etc.

NB: All minerals are not ores but all ores are minerals.

NATURAL OCCURRENCE OF METAL ORES

Metals occur naturally in two states:

1. NATIVE OR UNCOMBINED STATE
2. COMBINED STATE

A. NATIVE STATE

The elements are said to occur in the native state when they are found in their elementary form (free metals).

Examples: Copper (Cu), Platinum (Pt), Gold (Au), Silver (Ag), and Mercury (Hg).

B. COMBINED STATE

The elements are said to occur in combined state when they are found in the form of compounds. Generally, the reactive metals occur as oxides, sulphides, sulphates, silicates, carbonates, chlorides, nitrates, etc. Elements that occur in combined state include Fe, Cu, Al, Pb, Sn, Ca, Mg, Na, Mn, Cr, Co, etc.

QUESTION: What are the main ores from which tin, copper, and aluminum are extracted?

NATURAL OCCURRENCE OF METAL ORES OF TIN, COPPER, AND ALUMINIUM

a) ALUMINIUM

Aluminium is the third most abundant element after silicon and oxygen. It is the most abundant metal in the earth’s crust. Aluminium is a reactive metal and hence does not occur as a free metal.

The following are different forms in which aluminium occurs in nature:

1. Corundum (Al₂O₃) – Free oxide
2. Bauxite (Al₂O₃.2H₂O) – Hydrated oxide
3. Silicates (Al₂Si₂O₇.2H₂O), KAlSi₃O₈, KH₂Al₃(SiO₄)₃ – Kaolin (China clay), Feldspar, Mica
4. Cryolite (Na₃AlF₆)

Aluminium ores have different colours due to the presence of other metals which are impurities. Examples: Ruby (red) containing Iron and Titanium, Sapphire (blue) containing Cobalt and Titanium.

Bauxite is the most economic ore from which Aluminium is extracted. Deposits of bauxite are exploited in South–East Europe, India, Australia, Brazil, USA, and West Indies.

b) TIN (Sn)

Tin does not occur as a free element because it is a moderately reactive metal. Tin occurs in the form of oxide (tin-stone SnO₂) and sulphide (tin-pyrite CuS.FeS.SnS₂). The important ore of tin is cassiterite (tin-stone) which contains 0.5-10% of the metal as SnO₂ and the rest are impurities, i.e., sulphides of Fe, Cu, and other substances such as sand, silicious matter, earth matter, etc.

Miners call tin-stone “black tin” to distinguish it from “white tin,” which is the name given to the metal. Tin deposits occur in Malaysia, Indonesia, Bolivia, China, Burma, Thailand, and Nigeria.

c) COPPER (Cu)

Copper occurs as a free element as well as in combined state. In combined state, copper occurs as copper pyrite (CuFeS₂), azurite (2CuCO₃.Cu(OH)₂), and malachite (CuCO₃.Cu(OH)₂). Malachite and azurite are basic carbonates. Deposits of copper occur in Zambia and the Democratic Republic of Congo. CuFeS₂ is the chief ore of copper.

NB:

1. Copper occurs as free metal in Late Superior in Canada.
2. Copper also occurs naturally in the form of oxide (ruby ore – Cu₂O).
3. Minerals and ores differ because minerals contain a low percentage of the metal, while ores contain a large percentage of the metal.
4. The metal cannot be extracted from the mineral; it can be extracted from an ore.

STAGES OF METAL EXTRACTION

There are three major stages of metal extraction:

1. Concentration of ore
2. Reduction
3. Refining or purification

I. CONCENTRATION OF ORE

Concentration of ore means increasing the metallic content of the ore by removing the impurities (gangue). The ore which is mined usually contains large amounts of rocky impurities such as sand, clay, limestone, which are called gangue.

Definition: Gangue is the worthless or useless earthy impurities which are found in an ore.

Before concentration, proper separation of ore is carried out. The ore is broken up into lumps which are then ground down to a fine powder suitable for the next operation. Concentration of the ore is carried out by any of the following methods depending upon the nature of the ore and gangue.

1. GRAVITY SEPARATION (WASHING WITH WATER)

This process is applied where there is a well-marked difference in the densities of the gangue and the ore. The crushed ore is washed by a current of water on a sloping table filled with a series of corrugated boards. The table is kept vibrating all the time. The lighter particles of gangue are washed off leaving behind heavier particles of the ore.

2. MAGNETIC SEPARATION

The magnetic impurities present in an ore are separated by a magnet. The powdered ore is allowed to fall on a rubber belt moving around two rollers. One of the rollers is a strong magnet. The ore moves with the belt towards the magnetic roller and then the belt takes a turn near the magnet. The magnetic components are attracted by the magnet and form a separate heap. The non-magnetic components (e.g., tin-stone) are separated from magnetic components (e.g., Wolframite FeWO₄).

3. FROTH FLOTATION

This method is commonly used for concentration of low sulphide ores like galena (PbS), copper iron pyrite (CuFeS₂), or Cu₂FeS₂, and Zinc blende (ZnS). This method of concentration is based on the different wetting characteristics of the sulphide ore and the gangue particles.

The finely powdered ore is mixed with water in a tank (flotation cell). Some amount of pine oil (froth) and sodium ethyl xanthate (collectors) are added to it. Air is then blown through the mixture vigorously. The sulphide ore particles, which are wetted by the oil, rise up to the surface of the liquid. The sulphide ore forms a stable froth which is skimmed off. The gangue particles, which are wetted by water, sink to the bottom of the tank. The froth with sulphide particles is collected in a separate tank.

NB: The role of sodium ethyl xanthate is to collect sulphide ore particles. This compound attaches to sulphide ore particles and makes them water-repellent. As a result, the sulphide ore particles pass into the froth easily.

4. LEACHING

In this method, the ore containing impurities is leached with an aqueous solution of a suitable dissolving reagent so that the metal in the ore is converted into a simple salt or complex compound while the impurities remain insoluble in the reagent and are removed by filtration. The metal is then extracted from the simple salt or complex compound formed. For example, impurities in bauxite (Al₂O₃.2H₂O), i.e., FeO, Fe₂O₃, and SiO₂, can be removed by heating the crushed bauxite with sodium hydroxide solution (Bayer’s process). Aluminium present in the ore is converted into a soluble complex compound (aluminate complex – NaAlO₂) while the impurities (FeO, Fe₂O₃, and SiO₂) remain insoluble and hence are filtered out.

5. CALCINATION OF THE ORE

Calcination of ore refers to the process of heating the concentrated ore strongly in a limited supply of air or in the absence of air. Calcination brings about the following changes:

1. The carbonate ores are decomposed to their representative metal oxides.
   e.g.,
2. Water of crystallization in the hydrated oxide gets lost in the form of water vapour.
   e.g.,
3. Organic matter, if present in the ore, gets lost or expelled and the ore becomes porous.

6. ROASTING

Roasting is the process of heating the concentrated ore strongly in excess air or oxygen below its melting point. Roasting can be done at moderate or high temperature. The roasting is generally for sulphide ores. When roasting takes place at moderate temperature, some portion of the sulphide ore like galena (PbS), Zinc blende (ZnS) is converted into metallic oxide and the remaining portion is converted into metallic sulphates.

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

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

Examples:

THE DIFFERENCE BETWEEN CALCINATION AND ROASTING

1. Calcination is used for hydrated oxides or carbonates while roasting is for sulphide ores.
2. In calcination, the ore is heated strongly in the absence of air or oxygen, while in roasting, the ore is heated in excess air or oxygen.
3. In calcination, the hydrated oxide or carbonate is converted to metallic oxide, while in roasting, the sulphide ore is converted into metallic oxide, sulphates, or metallic element.

I. REDUCTION

During the extraction of a metal from its ore, the metal ions are reduced to metal atoms by accepting electrons. Thus, extraction of a metal from its ore is a reduction process. Reduction of metal ores can be done by thermal or electrolytic reduction methods.

In the thermal reduction method, heat is used to extract the metal from its ore. A suitable reducing agent may be used, for example, coke, coal, Al, Co, Carbon, Mg, Ca, etc.

The elements extracted by this method are those which are moderately reactive, for example, Zn, Pb, Fe, Sn, Cu, etc. The reaction takes place either in an electric furnace, blast furnace, or suitable container.

In electrolytic reduction, electricity is used to extract metals from their ores. This method is used for extracting metals which are strongly electropositive (more reactive) like Na, K, Ca, Mg, and Al. These metals cannot be reduced by normal reducing agents like Co or Carbon because their compounds (ores) are stable. Even if the metal is formed, carbon reacts with the metal produced to form carbide at high temperature. These reactive metals are extracted by passing through their molten or fused chlorides, oxides, or hydroxides.

II. PURIFICATION (REFINING) OF IMPURE METALS

Purification is the process of removing impurities from the extracted metal. The metal obtained contains many impurities and hence needs to be purified. The impurities include other metals, non-metals, unreduced oxides, sulphides of metal, and slag. The method used for purification depends on the nature of the metal, the impurities to be removed, and the purposes for which the metal is to be used.

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

1. PHYSICAL METHODS

1. LIQUEFACTION PROCESS

This process is used for the purification of metals which melt at lower temperatures than each of the impurities associated with them. This method is used for purification of Sn, Zn, and Pb.

2. FRACTIONAL DISTILLATION

This process is used to purify metals which are volatile and whose impurities are non-volatile, and vice versa. The vapor of the metal is condensed in a separate vessel while the non-volatile impurities remain behind in the vessel in solid state. Example: Purification of Zn where As, Cd, Pb, and Fe are impurities.

2. CHEMICAL METHOD

1. OXIDATIVE PROCESS

This process is used to remove metallic and non-metallic elements present as impurities, for example, Mn, Cu, Pb, Sn, Fe, Ag, C, P, S, Si, etc. When oxygen or air is passed through the impure molten metal, the impurities are easily oxidized into their oxides.

These oxides:

1. May form scum on the surface of the metal and hence can easily be removed by skimming.
2. May be volatile and hence can escape through the mouth of the furnace.
3. May form a slag with the lining on the side surface of the furnace and may thus be removed.

2. THERMAL DECOMPOSITION METHOD

In this method, the impure metal is converted into a suitable volatile compound which on being heated decomposes to give pure metal. Example: Purification of nickel by Mond’s process (carbonyl process).

3. AMALGAMATION PROCESS

This process is used for extraction of silver and gold. Both silver and gold form amalgams with mercury (example Au/Hg and Ag/Hg). When Au/Hg or Ag/Hg is distilled in an iron retort, mercury being more volatile, distills off and Au or Ag metal is left behind in the retort.

4. ELECTROLYTIC PROCESS

Purification by electrolytic method gives metal of high purity. In this process, the impure metal is made the anode of the electrolytic cell while the pure metal is made the cathode. An aqueous solution of a suitable sample or complex salt of the metal having some corresponding acid (if necessary) is used as an electrolyte. When electric current of appropriate strength is passed through the electrolyte, the metal from the impure plate (anode) migrates to the pure plate (cathode) where it is deposited. The soluble impurities go into the solution while the insoluble settle down at the bottom below the anode. The matter settled below the anode is called anode mud or anode sludge. Sometimes the anode mud contains valuable metals which can be extracted from it. Metals that can be purified by this method include Sn, Pb, Cu, Ag, Ni, Zn, Cr, etc.

THERMAL REDUCTION OF TIN STONES (SnO₂)

Tin is generally extracted from tin stone (SnO₂). This contains SiO₂ as an impurity (acidic impurity). To remove this impurity and reduce tin stone to tin metal, the oxide is mixed with anthracite coal and a basic flux like lime (CaO). The mixture is heated in a reverberatory furnace at 1200°C–1300°C. Then tin stone (SnO₂) is reduced to tin metal (molten tin) and the impurity SiO₂ combines with CaO to form a slag of CaSiO₃ (Calcium Silicate). The molten tin (heavier) forms the lower layer and slag forms the upper layer.

The molten tin is tapped and cast into blocks or ingots. The block contains about 70% metallic tin.

Some SnO is also formed during the reduction of SnO₂ to Sn.

The SnO and CaO react with silica (SiO₂) to form slag of SnSiO₃ and CaSiO₃.

The slag of SnSiO₃ and CaSiO₃, being lighter, floats on the molten tin metal as an upper layer which is easily removed. Thus, SiO₂ (impurity) is removed in the form of slag. The tin metal can also be recovered from SnSiO₃ by melting it with limestone (CaCO₃) in a blast furnace or reverberatory furnace.

NB:

1. Flux is a substance which reacts with the gangue (impurities) during smelting to form a fusible compound which can easily be removed from molten metal, example CaO and SiO₂. The CaO is a basic flux while SiO₂ is an acidic flux. The acidic flux removes the basic impurities (e.g., CaO, FeO, MgO), and the basic flux removes acidic impurities like SiO₂, P₂O₅, etc.
2. Slag is a fusible compound obtained by the combination of the flux and gangue present in the ore.
   i.e., Flux + Gangue → Slag
   CaO + SiO₂ → CaSiO₃
3. The gangue may be acidic (example SiO₂) or basic (example CaO, MgO, FeO, etc.).
4. Smelting is the process of conversion of roasted or calcined ore into molten metal using a suitable reducing agent at high temperature in a current of air.

ELECTROLYTIC REDUCTION (ELECTROMETALLURGY)

This method is used for extraction of metals which are very reactive and form very stable compounds. For example, the oxides, chlorides, hydroxides, etc., of more reactive metals like Na, K, Mg, Ca, Al are very stable and hence cannot be reduced to free metals either by carbon or aluminium. However, the oxides of the above metals can be reduced by carbon at very high temperature, but as soon as the metal is formed, it reacts with carbon to form metal carbide. Therefore, the metals are obtained by reducing their suitable molten (fused) salts like chlorides, hydroxides, etc., by electrolytic reduction. The metal is liberated at the cathode. An aqueous electrolyte is not used in electrolytic reduction because the metal obtained at the cathode reacts with water present in the solution.

In order to make an electrolytic reduction successful, the following conditions (criteria) must be satisfied:

1. The liberated metal should not be miscible with the melt (soluble in the melt). For example, potassium cannot be obtained by electrolysis of (KCl and CaCl₂) melt, since the liquid potassium is soluble in CaCl₂ melt.
2. The products obtained as a result of electrolysis may react and hence must be collected separately.
3. Some other salts may be added to lower the melting point of compounds that melt at very high temperature. The decrease in melting point makes the process of electrolysis take place at a lower temperature.
4. The cell and electrodes should be made of materials which are not affected by the electrolyte or the products obtained after electrolysis.

ELECTROLYTIC REDUCTION OF ALUMINIUM FROM ALUMINIUM OXIDES (Al₂O₃)

In the electrolytic reduction of aluminium, pure Al₂O₃ is mixed with Cryolite (Na₃AlF₆) and then the mixture is melted in an electric furnace. The fused mixture of Al₂O₃ and Na₃AlF₆ is taken in an iron tank lined with carbon from inside. This carbon lining acts as the cathode and carbon rods which are dipped into the molten Al₂O₃ and Na₃AlF₆ act as the anode. On passing electric current, aluminium metal in the molten state is liberated at the cathode and sinks to the bottom of the cell from which it is removed periodically.

Oxygen from the electrolyte is liberated at the anode. Some of the oxygen combines with the carbon anodes to form carbon monoxide and carbon dioxide. The anodes therefore are gradually worn out.

NOTE: The cryolite serves as a solvent and is not decomposed.

Reaction at the cathode (reduction):

Reaction at the anodes (oxidation):

Thus, O₂, CO, and CO₂ are liberated at the anodes.

PURIFICATION OF BLISTER COPPER

Copper obtained from copper-Iron pyrite (CuFeS₂) is called BLISTER COPPER. This copper contains many impurities like Fe, Ni, Zn, Ag, Au, etc.

In refining blister copper, a thick plate of impure copper is made the anode and a thin plate of pure copper is made the cathode. The cathode is coated with graphite and oil which helps in scraping the copper easily that gets deposited on the cathode. A 15% CuSO₄ solution containing 5% H₂SO₄ is used as an electrolyte.

On passing electric current through the electrolyte, ionization of CuSO₄ tends to take place.

The copper ions (Cu²⁺) obtained move towards the cathode where they accept electrons and become neutral Cu atoms. Thus, the cathode (pure copper plate) goes on becoming thicker and thicker in size as the electrolysis goes on.

The ions move towards the anode where they lose electrons and become sulphate radicals. These radicals have a strong tendency to combine with Cu atoms of the copper anode (impure copper plate) to form CuSO₄ which moves into the solution where they are again ionized to give Cu²⁺ ions. These Cu²⁺ ions migrate to the cathode where they are deposited as free Cu atoms.

On adding the equations, we obtain:

Or Cu (impure copper) → Cu²⁺ + 2e^– (oxidation)

The anode goes on becoming thinner and thinner in size as the reaction proceeds. From the reaction at the anode, the pure copper is transferred from the impure copper plate to the pure plate (cathode).

The soluble impurities like Fe²⁺, Ni²⁺, and Zn²⁺ pass into solution as sulphates while the insoluble impurities (example Ag and Au) which are not affected by H₂SO₄-CuSO₄ solution settle down below the anode as ANODE MUD or ANODE SLUDGE.