Carbothermic Processes to Replace the Hall-Heroult Process Yaghoub Sayad-Yaghoubi CTO at Calsmelt Pty Ltd,
[email protected] [email protected]
Abstract For aluminium production, the conventional carbothermic reduction of alumina has inherent problems such as generation of aluminous fumes, floatation of the produced aluminium on the top of the slag, a high content of carbon in the product and excessive consumption of the graphite electrode and the refractory furnace walls. These problems are all rooted in the behaviour of the feed material during heating and of the alumina-rich molten slag intermediate that the process is based on. Other proposals to replace the Hall-Haroult process include the carbothermic reduction of alumina or bauxite under vacuum or inert gas conditions. Analyses of the results obtained from these recent studies indicate that an application for aluminium production based on these approaches could not be practical. The problems with these approaches include very low reaction rates, low yield, very high energy requirements (above 24 kWh/kg Al) and very high level of inert gas usage. Based on the reaction rate data obtained in these studies, for production of 1 kg carbide from alumina at 1600°C temperature, about 550 m3 helium gas is required. For the stepwise production of 1 kilogram alumina from bauxite at 850-1600°C, the required argon would be about 500-800 m3. From this data, it is estimated that higher than 1200 m3 inert gas is required for the production of 1 kg aluminium from bauxite. At the end of these processes, the separation of aluminium from the carbide and ferroalloy phases produced in the reactor, or from deposits produced from fume condensation, are additional issues for consideration. In contrast, the Thermical™ process is not based on liquid slag and doesn’t use vacuum or inert gas to operate. The feasibility of the Thermical™ process is based on its ability to rapidly heat the “charge”, which is an aluminium carbide, alumina and aluminium mixture and thereafter, to rapidly cool the metal produced, together with a lower temperature requirement. Keywords: Carbothermic, Aluminium, Smelting, Reduction, Energy, Slag, Vacuum, Inert Gas, Replace. 1. Introduction Over number of decades now, the carbothermic production of aluminium has been the subject of many projects for replacing the Hall-Heroult process. However, these attempts have not resulted in a commercial technology. Some of the projects were discontinued because of technical hurdles which proved to be impossible to overcome. Other projects were stopped as the results obtained indicated that a practical application for aluminium production based on these approaches could face difficult economical challenges. Projects such as those executed by the Reynolds Metals Company (1971-1984) were terminated primarily because of the
company’s economic status together with the concurrent downward trend then being experienced by the aluminium industry1. These carbothermic processes attempted for replacement of the Hall-Heroult process included: 1- conventional carbothermic reduction of alumina, 2- vacuum carbothermic reduction of alumina and bauxite and 3- inert gas carbothermic reduction of alumina and bauxite. This paper summaries the results of recent efforts and considers the viability of these processes. 2. Conventional Carbothermic Reduction of Alumina Since the earliest attempts at carbothermic reduction of alumina, in the first stage of this multistage process, mixtures of alumina and carbon have been heated to produce an aluminarich molten slag of an Al2O3-Al4C3 mixture. Thereafter, the molten slag is further heated (during a second stage of the process) to produce metal under atmospheric pressure. Several variations of this conventional carbothermic reduction concept have been attempted by different major aluminium producers. Among these attempts, significant results were obtained by the Reynolds Metals Company (1971-1984). Alcoa reviewed the situation of the technology in 1998 and then commissioned its own aluminium carbothermic technology (ACT-ARP)2. However, in a similar manner to that of the previous attempts, the ACT-ARP process was based on separate compartments (i.e., different reactors) for the two stages of the process. This process also used molten Al2O3-Al4C3 slag and was demonstrated to function at temperatures in the region of 2,000°C to 2,250°C. Evidently, the process had many of the same challenges that were evident in earlier attempts to develop an effective carbothermic process for aluminium. Later, Alcoa used a single reactor compartment, but still based on molten slag. In the reactor, first, an Al2O3-C mixture is melted, at which point, a molten slag and some solid carbide are produced. Further heating of the molten slag then occurs and metal is formed. The metal produced inside the slag floats to the surface and a metallic layer is generated on the top of the molten slag. Thereafter, the metal is tapped and treated for refining. Calsmelt understands that this project is still continuing1. Generally, in this “conventional” type of carbothermic aluminium production process, inherent problems occur due to high temperature required to melt the feed material and prepare the molten slag. Thereafter, an even higher temperature is required during metal production. As a result, the production of aluminous fume Al(g)-Al2O(g) is unavoidable. In addition, the molten slag is alumina rich and ionic medium which is aggressive to both the refractory walls of the furnace and the electrodes used. The key problems in this approach are: 1- reactions which produce aluminous fumes, 2- aluminium floating on the molten slag,
3- a very high carbon content in the product and 4- excessive electrode consumption and severe refractory attack. Solving these problems is a very challenging proposition as these problems are rooted in the behaviour of Al2O3-C mixtures during heating, and to the characteristics of molten slag. The Al2O3-C feed material produces Al2O(g) and aluminium vapour during heating and in the slag-making reaction. These emissions may either be lost or can be treated and recycled back into the reactor. The reactions producing these gases are: Al2O3 + 2C =Al2O(g) +2CO(g) Al2O3 + 3C = 2Al(g) +3CO(g)
(1) (2)
These reactions become more significant when the heating rate of the feed is slow, and/or when the reactants particle size is not fine enough which may lead to slow slag formation. Under such conditions, Stage 1 of the process can be summarized as: Stage 1;
nAl2O3 + mC → q(Al2O3+xAl4C3)(l) + z(Al4C3)(s) + {aCO+bAl+cAl2O}(g)
Furthermore, once the carbide saturated slag (Al2O3+xAl4C3) is formed and the temperature of slag is not increased immediately up to the metal producing temperature, the slag can dissociate to produce Al2O gas according to the following reactions: Al2O3 + Al4C3 = 3Al2O(g) + 3C 2Al2O3+ Al4C3 = 2Al(l) + 3CO + 3Al2O(g) 5Al2O3+ 2Al4C3 = 6CO + 9Al2O(g)
(3) (4) (5)
In order to avoid excessive production of Al2O gas, the slag should quickly be transferred into the metal production reactor and then rapidly be heated to the metal production temperature. But, slag transfer into the second reactor cannot be undertaken quickly, especially when slag viscosity is high due to higher content of carbide particles. Therefore, the process can become restricted by the achievable flow rate of the slag and/or by the content of solid carbide. The liquid slag entering the second stage of the process is rich in oxide (in a carbide saturated slag, the alumina mole fraction is about 4 times higher than carbide). Therefore, in the second stage, as metal production proceeds, carbide depletion will gradually occur to the point that metal production may stop. This scenario can arise because the slag composition is not stoichiometric for metal production. Therefore, the oxide that is in excess of the stoichiometric value should either be consumed by a reaction such as Reaction (5) and/or Reaction (6) and be converted to Al2O gas, or remain as a layer of molten alumina which will require carbide make-up in order to react to produce further metal. Al2O3 + 4Al = 3Al2O(g)
(6)
As a result, in order to sustain metal production, the carbide value in the melt has to be maintained by make-up from the first stage, or by a direct addition of carbide or Al2O3-Al4C3 agents into the second stage reactor. However, it seems that due to fluidity/mass transfer restrictions, the addition of carbide will always be less than effective and the process will lose
some of its oxide into the gas phase. Under such conditions, the chemical reaction in Stage 2 of the process can be summarized as: Stage 2;
q(Al2O3+ xAl4C3)(l) + z(Al4C3)(s)→ jAl(l) + {dCO + eAl + fAl2O}(g)
In order to reduce mass transfer restrictions, the transport properties of the slag must be enhanced. A higher temperature is one option, but, as the reaction temperature is increased, emissions of Al vapour and Al2O gas have been previously found to make the process difficult and it becomes inefficient. This process also needs installations to recover the emissions and fume that may be generated. Furthermore, high temperatures required promote a high solubility of carbon in the produced aluminium. As a result the metal product needs additional further treatment. Furthermore, in Stage 2, CO gas passes through the metal layer that is produced and which accumulates on the top of the slag. If the temperature of this layer is lower than the metalmaking temperature in the reactor, the CO gas can react with the metal and oxidize it. In aluminium carbothermic processes, graphite electrodes are used to generate heat. The electrodes may be installed vertically (top entering) and/or horizontally (introduced through the reactor side walls). In either case, the electrodes are partially submerged in the molten slag. In the conventional process the molten slag is rich in alumina and it aggressively reacts with graphite electrodes. The same reaction occurs between alumina rich slag and graphite refractory in the reactor. Therefore, the graphite electrode and refractory consumption can be very high. As pointed out above, all of these challenges are rooted in the behaviour of the feed material during heating and of alumina-rich molten slag that the processes are based on. As a result, no truly practical process based on this approach has been forthcoming to date. 3. Vacuum Carbothermic Reduction of Alumina for Metal Production Research has been conducted by carrying out the carbothermic reduction of alumina under vacuum conditions. It is believed that under vacuum, the equilibrium of Equation (7) should shift to the right and so the onset temperature for metal production should be lower. Al2O3(s) + 3C(s) = 2Al(g) +3CO(g)
(7)
Theoretically, in these conditions aluminium should be formed as a vapour without the formation of Al4C3 and oxycarbides. Thus, to obtain liquid aluminium, the generated fume is condensed and the aluminium so formed is separated from CO gas. Experiments were carried out by researchers in 0.03-12 mbar pressure range and 1,025°C1,800°C temperature range. Balomenos et al.3 showed that under vacuum (0.1 mbar) and at around 1500°C, theoretically, a full conversion of Al2O3 to Al(g) and CO(g) can occur. Therefore, these workers carried out experiments in the 1027-1727°C temperature range and at a total pressure in the range of 3.5 to 12 mbar using the reactants Al2O3 and charcoal. In these experiments, the aluminium yield reached 19% by condensation of the produced fume.
Carbide (Al4C3) and oxycarbide (Al4O4C) were also produced within the crucible. Based on the measured CO generated in the experiments, the reaction extent reached 55% at 1727°C. However, the conclusions derived from these experiments indicated that even if the formation of aluminium carbides is avoided or controlled, there exists a mechanism that prevents the full reduction of alumina to aluminium through the formation of sub-oxide Al2O(g). This mechanism exists due to the comproportionation reaction of aluminium species.3 Qing-chun et. al.4 carried experiments across a lower pressure range 0.2-1.5 mbar and in the temperature range 1420-1580°C. In a series of experiments in the pressure range of 0.4-1.5 mbar, they observed that Al4O4C and Al4C3 appeared at above 1430°C in the crucible residue. At these conditions, aluminium metal was absent in the crucible and in the condensation that collected on the colder side of the reaction tube. To further investigate the effect of pressure, they reduced the pressure to 0.2 mbar over a temperature range between 1500-1580°C. However, due to this lower temperature, the reaction rates were so low that after two hours duration at temperature (and perhaps also due to back reactions) aluminium was absent in the fumes and crucible residues. Halmann, et. al.5 conducted experiments isothermally in the 1400-1800°C temperature range and at a CO partial pressure in the 0.03-3.5 mbar range using Al2O3-3C mixtures. The reactants were prepared as pellets using charcoal and pure alumina powder. The duration at reaction temperature was terminated when the released CO level dropped to zero. In tests at temperatures in the range of 1,500-1,600°C, with CO pressure at 0.2-0.3 mbar, and at 1800°C with CO pressure at 3.5 mbar, the reactants were almost completely consumed with only minor residual amounts left in the crucible. In these tests, elementary aluminium was observed as aluminium drops and with a content of 60-80% in a gray powder that deposited on the reaction tube walls. A layer of yellowish colour was also deposited on the wall. This layer contained Al4C3, Al4O4C and some corundum. It is believed that aluminium vapours left the crucible and condensed on a colder surface of reaction tube, but the vapour was still hot enough for the reverse reactions with CO to occur and so produce carbide and oxycarbides. These workers have argued that the major issues in carbothermic reduction of alumina under vacuum are: 1. high actual energy consumption The total theoretical energy required for Reaction (7) comprises sensible heat of reactants and products, the reaction heat, and pumping work for isothermal expansion of gases in the reactor (product gas and carrier gas if any). These authors reported that the energy consumption at reactor pressure of about 1 mbar (10-3 bar) and without using argon is equal to the Hall-Heroult process (14-15 kWh/kgAl). At lower pressures, such as 0.2 mbar (10-3.7 bar) and a gas composition of Ar/CO = 2 mole ratio, the energy consumption reaches a formidable value close to 24 kWh/kg Al. 2. low yield Due to the difficulties in preventing the re-oxidation, or carburization of the gaseous aluminium that condenses on the walls of the reactor, the yield is low. Under
vacuum, the metal is produced at lower pressures, therefore, the heat transfer in the gas phase is controlled by diffusion barriers. 4. Vacuum Carbothermic Reduction of Bauxite for Metal Production For metal production, the possibility of carbothermic reduction of bauxite minerals Al(OH)3 (Gibbsite) and AlO(OH) (Boehmite and diaspore) in vacuum (10-7 bar pressure) and 1400– 1800°K(1127-1527°C) temperature range have been examined by Halmann et al.6. In this work, using thermochemical equilibrium calculations, the equilibrium compositions as a function of temperature of the minerals with the stoichiometric values of carbon (C/O= 1 mole ratio) were determined. Furthermore, the effects of SiO2, and FeO(OH) on reaction equilibria were also studied. Based on these results, it was shown that for the carbothermic reduction of these minerals at 1400°K (1127°C) temperature and at a low 10-7 bar pressure, the equilibrium becomes: Al(OH)3 + 3C = 3.00CO(g) + 1.00Al(g) + 1.48H2(g) + 0.04H(g)
(8)
AlO(OH) +2C = 2.00CO(g) + 1.00Al(g) + 0.50H2(g)
(9)
In these conditions, theoretically, Al(g) is produced at 1,127°C and all aluminium enters into the gas phase. All of the carbon would be consumed and converted into CO. The effect of SiO2 with a composition of 20 mole% in iron free bauxite is shown in Reactions (10) and (11) at 1,400°K(1,127°C) temperature and at 10-7 bar pressure. Al(OH)3 + 0.2SiO2 + 3.4C = 3.3CO(g) + 1.0Al(g) + 1.48H2(g) + 0.1SiC(s) + 0.1SiO(g) + 0.003Al2O(g)+ 0.001Si(g) (10) AlO(OH) + 0.2SiO2 + 2.4C = 2.30CO(g) + 0.99Al(g) + 0.49H2(g)+0.1SiO(g)+0.1SiC(s) +0.006Al2O3 +0.0009Si(g)+0.02H(g) (11) In Reaction (10), at 1,400°K(1,127°C) there is no significant interference of Si(g) on the production of Al(g). The content of Si(g) in the product becomes significant at higher temperatures. For example at 1,600°K(1,327°C), its content is about 10 mole%. In Reaction (11), Si(g) appears at 1,400°K(1,127°C, and at 1,800°K(1,527°C) all of the silica enters the gaseous phase and the content of gaseous silicon in the product reaches 20 mole%. As the content of silicon increases in the fume and deposit, production of pure Al becomes an issue in a commercial process. The second issue with this process is the high energy requirement. The theoretical heat and work (vacuum generation) required for Reaction (10) is about 23.6kWh/kgAl and for Reaction (11) is about 16.7 kWh/kgAl. The effect of iron was tested by using AlO(OH) + 0.1FeO(OH) system as a relatively low iron content bauxite. At 1,400°K(1,127°C) temperature and 10-7 bar pressure, the reaction equilibrium becomes: AlO(OH) + 0.1FeO(OH) + 2.2C = 2.2CO(g) + 1.00Al(g) + 0.54H2(g) + 0.1Fe(g) (12)
The elemental molar ratio Fe/Al=0.1 in the gaseous phase remains fixed at temperatures above 1,400°K. The content of Fe in the product according to Reaction (12) is about 10 mole%. System AlO(OH)+0.28FeO(OH) was tested as an example of bauxite with a high iron content. At 1,400°K(1,127°C), the equilibrium was represented by Reaction (13): AlO(OH) + 0.28FeO(OH) + 2.56C = 2.56CO(g) + 1.00Al(g) + 0.63H2(g)+ 0.28Fe(g) + 0.026H(g) (13) The elemental molar ratio of Fe/Al=0.28 (36.7wt% Fe) in the gas phase remains fixed at temperatures above 1,400°K. At these temperatures separation of Al(g) from Fe(g) would not be possible and pure aluminium could not be obtained. Carbothermic reduction of calcined bauxite was explored at different levels of vacuum. The equilibrium compositions versus temperature for Al2O3+0.23Fe2O3 system at 10-4, 10-5 and 10-6 bar pressures were calculated. A complete reduction of alumina, as it is shown by Reaction (14) occurred at 1,800°K(1,527°C), 1,600°K(1,327°C) and 1,500°K(1,227°C) respectively. Al2O3 + 0.23Fe2O3 + 3.7C = 3.7CO(g) + 2Al(g) + 0.46Fe(g)
(14)
According to Reaction (14), the content of iron in the product would be about 32 wt%. At 1,400°K(,1123°C) and 10-6 bar pressure for the same system (Al2O3+0.23Fe2O3), the equilibrium formed is shown as Reaction (15): Al2O3 + 0.23Fe2O3 + 3.7C = 3.43CO(g) + 1.14Al(g) + 0.087Fe(g) + 0.26Al2O(g) +0.37Fe(S) + 0.088Al4C3(s) (15) According to Reaction 15, solid iron is 80% of the total iron produced. The iron content in the gaseous metallic product Al(g)+Fe(g)is about 13.7 wt%. Comparing Reaction (14) at 1,500°K(1,227°C) and 10-6 bar pressure with Reaction (15) at 1,400°K(1,123°C) and 10-6 bar pressure shows that better separation of iron from aluminium is expected at lower vacuum pressures and lower temperatures. However, at lower temperatures the kinetics is not favourable and the lower pressure needs higher work done and causes lower yield as suboxides are generated. Goldin et al.7 tried vacuum carbothermic reduction of low-iron bauxite (iron oxide
