Extraction of Crude Metals from Concentrated Ore
The concentrated ore is usually converted to oxide before reduction, as oxides are easier to reduce.
Thus, isolation of crude metal from concentrated ore involves two major steps:
- Conversion to oxide.
- Reduction of the oxides to metal.
Conversion to oxide
- Calcination: - It is the process of converting an ore into its oxides by heating it strongly, below its melting point in a limited supply of air or in absence of air.
During calcination, volatile impurities as well as organic matter and moisture are removed.
- Fe2 O3 x H2O (∆)à Fe2 O3 (s) + x H2 O (g)
- ZnCO3 (s) (∆)à ZnO(s) + CO2 (g)
- Roasting: - In roasting, the ore is heated in a regular supply of air in a furnace at a temperature below the melting point of the metal. This process is commonly used for sulphide ores and is carried out in blast furnace or reverberatory furnace. Roasting helps to remove the non-metallic impurities and moisture.
Some of the reactions involving sulphide ores are:
- 2ZnS + 3O2 → 2ZnO + 2SO2
- 2PbS + 3O2 → 2PbO + 2SO2
- 2Cu2S + 3O2 → 2Cu2O + 2SO2
If the ore contains iron, it is mixed with silica before heating.
Iron oxide ‘slags of’ as iron silicate and copper is produced in the form of copper matte which contains Cu2S and FeS.
- FeO + SiO2 → FeSiO3
- Meaning of slag: - During metallurgy, ‘flux’ is added which combines with ‘gangue’ to form ‘slag’. Slag separates more easily from the ore than the gangue. This way, removal of gangue becomes easier.
Reduction of oxide to the metal
- The roasted or the calcined ore is then converted to the free metal by reduction. Reduction method depends upon the activity of metal.
- Metals which are low in the activity series (like Cu, Hg, and Au) are obtained by heating their compounds in air: metals which are in the middle of the activity “cries (like Fe. Zn, Ni, Sn) are obtained by heating their oxides with carbon while metals which are very high in the activity series (e.g., Na, K, Ca, Mg, Al) are obtained by electrolytic reduction method.
- Using the concepts of thermodynamics will help us to know the metallurgical transformations.
- Gibb’s Energy:- The change in Gibbs energy i.e. ∆G = ∆H - T∆S equation(A)
Where, ΔH is the enthalpy change and ΔS is the entropy change for the process.
- This equation can also be written as:- ΔG(-) = – RTlnK equation(1)
Where, K is the equilibrium constant of the ‘reactant – product’ system at the temperature, T. A negative ΔG implies a +ve K in equation (1).
- Following conclusions can be made:-
- When the value of ΔG is negative in equation (A), only then the reaction will proceed.
- If reactants and products of two reactions are put together in a system and the net ΔG of the two possible reactions is –ve, the overall reaction will occur.
- During reduction, the oxide of metal decomposes:
MxO(s) à xM (solid or liquid) + (1/2) O2 (g) Equation (2)
The reducing agent takes away the oxygen. Equation (2) is the reverse of the oxidation of the metal. And then, the Δf G (-) value is written in the usual way: xM(s or l) + (½) O2 (g) → MxO(s) [ΔG (-) (M, MxO)] equation (B)
- If reduction is being carried according to equation(2), the oxidation of the reducing agent(e.g. C or CO) will be:-
- C(s) + (1/2) O2(g) à CO (g) [ΔG(-) (C,CO)] equation (3)
- CO(s) + (1/2) O2(g) à CO2 (g) [ΔG(-) (C,CO)] equation(4)
- If carbon is taken, there may also be complete oxidation of the element to CO2:
- (1/2) C(s) + (1/2) O2(g) à (1/2) CO2(g) [(1/2) ΔG(C,CO2)] equation(5)
- On subtracting equation (B) from one of the three equations (3, 4 or 5).
- MxO(s) + C(s) → xM(s or l) + CO(g)
- MxO(s) + CO(g) → xM(s or l) + CO2(g)
- MxO(s) + (1/2) C(s) → xM(s or l) +( ½) CO2(g) equation(8)
- These reactions describe the actual reduction of the metal oxide, Mx
Extraction of Iron from oxide
- Oxide ores of iron, after concentration through calcination/roasting (to remove water, to decompose carbonates and to oxidise sulphides) are mixed with limestone and coke and fed into a Blast furnace from its top. In this case, the oxide is reduced to the metal.
- One of the main reduction steps in this process is:
- FeO(s) + C(s) → Fe(s/l) + CO (g)
- Consider the above reaction as 2 simpler reactions, in One reduction of FeO takes place and in another C is being oxidised to CO:
- FeO(s) → Fe(s) +( ½) O2(g) [ΔG(FeO, Fe)] equation (C)
- C(s) + (1/2) O2 (g) → CO (g) [ΔG (C, CO)]
- When both the reactions take place to yield the equation (8),
The net Gibbs energy change becomes:
ΔG (C, CO) + ΔG (FeO, Fe) = ΔrG equation (9)
- The resultant reaction will take place if RHS of the equation (9) is negative.
- In ΔG (-) vs T plot representing reaction by equation (C), the plot goes upward and that representing the change C→CO (C, CO) goes downward.
- At temperatures above 1073K (approx.),
- The (C, CO) line comes below the Fe, FeO line [ΔG (C, CO) < ΔG (Fe, FeO)].
- So in this range, coke will be reducing the FeO and will itself be oxidised to CO.
- In a similar way the reduction of Fe3O4 and Fe2O3 at relatively lower temperatures by CO can be explained on the basis of lower lying points of intersection of their curves with the CO, CO2
(Graph 1) Gibbs energy (ΔGV) vs T plots (schematic) for formation of some oxides (Ellingham diagram)
- In the Blast furnace, reduction of iron oxides takes place in different temperature ranges.
- Hot air is blown from the bottom of the furnace and coke is burnt to give temperature upto about 2200K in the lower portion itself.
- The burning of coke therefore supplies most of the heat required in the process. The CO and heat moves to upper part of the furnace.
- In upper part, the temperature is lower and the iron oxides (Fe2O3 and Fe3O4) coming from the top are reduced in steps to FeO.
- Thus, the reduction reactions taking place in the lower temperature range and in the higher temperature range, depend on the points of corresponding intersections in the ΔrG(-) vs T plots.
- Following are the reactions which are taking place:-
- At 500 – 800 K (lower temperature range in the blast furnace)–
- 3Fe2O3 + CO à 2 Fe3O4
- Fe3O4 + 4 CO à 3Fe + 4CO2
- Fe2O3 + CO à 2 FeO + CO2
- At 900 – 1500 K (higher temperature range in the blast furnace):
- C + CO2 à 2 CO
- FeO + CO à Fe + CO2
Products formed in Blast Furnace
- Limestone is also decomposed to CaO which removes silicate impurity of the ore as slag. The slag is in molten state and separates out from iron.
- The iron obtained from Blast furnace contains about 4% carbon and some other impurities. This iron is known as pig iron.
- Cast iron is different from pig iron and is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (about 3%) and is extremely hard and brittle.
- Wrought iron or malleable iron is the purest form of commercial iron and is prepared from cast iron by oxidising impurities in a reverberatory furnace lined with haematite.
- This haematite oxidises carbon to carbon monoxide:
- Fe2O3 + 3 C → 2 Fe + 3 CO
- Limestone is added as a flux and sulphur, silicon and phosphorus are oxidised and passed into the slag. The metal is removed and freed from the slag by passing through rollers.