An Introduction to Pyrometallurgical Technologies

Metals in ores exist in the form of oxides, sulphides, carbonates and other compounds. Metal ores also contain impurities or gangue materials. In the processing of metal ores to metals which are in common industrial use – steel, aluminium, nickel, copper, titanium, and many others, metal oxides or other compounds are converted to the metallic state by a reduction process and separated from gangue materials before or after reduction. Two major metallurgical routes to produce industrial metals are pyrometallurgy and hydrometallurgy.

Pyrometallurgical technologies include the thermal treatment of metal ores to extract valuable metals, while hydrometallurgy is based on the use of aqueous chemicals at much lower temperatures.

Pyrometallurgical routes are used in the commercial production of steel, aluminium, metallurgical silicon, manganese, chromium, titanium, and many other metals and alloys. The significant energy consuming processes in metals extraction are the reduction and smelting stages with formation of two immiscible phases – molten metal and the predominantly metal oxide phase, slag.

The temperature and energy required for the conversion of a metal oxide (or other compound) to the pure metal are defined by reaction thermodynamics and kinetics. A major reductant for metal oxides is carbon in the form of coke or char. Carbothermal reduction of a metal oxide MOx can be presented by the reaction:

MO_x + xC = M + xCO (1)

Metal M contains dissolved carbon and impurities which are also partially reduced. In the reduction of manganese, chromium and some other oxides, carbides MC_y are formed:

MO_x + (y+x)C = MC_y + xCO (2)

Industrial metallurgical processes based on carbothermal reduction of metal oxides are energy intensive. Energy input to the smelting/reduction furnace in some industrial processes is presented in Table 1. Table 1 also includes the enthalpy of formation of oxides, standard Gibbs free energy change for the reduction reaction leading to the formation of CO and the equilibrium reduction temperature.

Carbothermal reduction of stable metal oxides such as MnO, Cr₂O₃, SiO₂, TiO₂ and Al₂O₃ requires high temperatures. To have high productivity, industrial processes are run at much higher temperatures than those listed in Table 1. Thus, ferromanganese is produced at 1450- 1500^oC; reduction of iron oxides in a blast furnace takes place at about 1000^oC; temperature in the ferrochromium production is above 1700^oC, in silicon production – above 1800^oC.

Carbothermal reduction of titania to titanium carbide is conducted at 1700-2100^oC.

The high temperature required for the carbothermal reduction of alumina is a major obstacle in the development of carbothermal technology for aluminium production. The major technology for aluminium production is electrolysis using the Hall-Herault process.

In general, energy and exergy consumption increases with the further away the reaction takes place from the equilibrium temperature [6]. However, near-equilibrium processes are too slow to be commercially viable.

However, the efficiency of carbothermal reduction processes can be improved by decreasing the reduction temperature and improving the reaction kinetics. This can be achieved by decreasing the CO partial pressure and/or increasing mass transfer in the gas phase by running reduction processes in an inert atmosphere or in hydrogen, to aid the reduction reaction.

Table 1: Enthalpy of oxides formation, reduction reactions, standard Gibbs free energy change, equilibrium temperature and energy input to the smelting/reduction furnace

At low temperatures, reduction of oxides takes place in the solid state. It is well recognised that carbothermal reduction of metal oxides in the solid state occurs through the gas phase.

Overall reactions for reduction of oxide MO_x to metal M (reaction (1)) can be presented by reactions (3) and (4):

MO_x + xCO = M + xCO₂ (3)

CO₂ + C = 2CO (4)

Reduction of metal oxide to carbide MC_y (reaction 2) can be presented by reactions (5) and (4):

MO_x + (x + 2y)CO = MC_y + (x+y)CO₂ (5)

When carbothermal reduction takes place in the hydrogen-containing gas atmosphere, methane is formed by the reaction of carbon with hydrogen (reaction (6)). It changes the mechanism of reduction which then proceeds via reaction (7).

C + 2H₂ = CH₄ (6)

MO_x + (x+y)CH₄ = MC_y + xCO + 2(x+y)H₂ (7)

Reduction of manganese, chromium and titanium oxides by methane-containing gas was studied in [7-14], while carbothermal reduction of stable metal oxides in different gas atmospheres was examined in [15-25].

In reactions (3) and (4) or (5) and (4), carbon and oxygen are transferred between solid phases by CO and CO₂, respectively. In the reduction of stable metal oxides like manganese or titanium oxides, CO₂ partial pressure is very low, below 10^-4 atm (subject to the applicable temperature); the mass transfer can then be a limiting stage for the reaction rate. In reactions (6) and (7), carbon is transferred between solid carbonaceous material and oxide by CH₄ with the formation of CO and H₂ which are transferred to the gas phase. The partial pressure of CH₄ is much higher than the partial pressure of CO₂, this allows the carbothermal reduction of manganese and titanium oxides in the H₂-containing gas to occur at a much faster rate.

The gas phase also plays an important role when reduction proceeds with the formation of metal or metal oxide vapour, as in the case of the reduction of alumina [25]. Gas species are directly involved in the carbothermal reactions; the composition of the gas phase therby affects the reaction rate.

Our research in carbothermal reduction of manganese, titanium and aluminium oxides showed [15-25] that reduction temperature can be decreased by 300-400^oC.

Figure 1. Reduction of manganese oxide MnO in different gas atmospheres at 1275^oC

Further, it has been shown that the control of the gas atmosphere in reduction reactions is an important factor in the development of pyrometallurgical technologies.

References

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