Carbothermic Processes to Replace the Hall-Heroult Process

Transcription

1 Carbothermic Processes to Replace the Hall-Heroult Process Abstract Yaghoub Sayad-Yaghoubi CTO at Calsmelt Pty Ltd, 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 m 3 helium gas is required. For the stepwise production of 1 kilogram alumina from bauxite at C, the required argon would be about m 3. From this data, it is estimated that higher than 1200 m 3 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 ( ) were terminated primarily because of the

2 company s economic status together with the concurrent downward trend then being experienced by the aluminium industry 1. 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 Al 2 O 3 -Al 4 C 3 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 ( ). 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 Al 2 O 3 -Al 4 C 3 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 Al 2 O 3 -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 continuing 1. 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) -Al 2 O (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 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 Al 2 O 3 -C mixtures during heating, and to the characteristics of molten slag. The Al 2 O 3 -C feed material produces Al 2 O (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: Al 2 O 3 + 2C =Al 2 O (g) +2CO (g) (1) Al 2 O 3 + 3C = 2Al (g) +3CO (g) (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; nal 2 O 3 + mc q(al 2 O 3 +xal 4 C 3 ) (l) + z(al 4 C 3 ) (s) + {aco+bal+cal 2 O} (g) Furthermore, once the carbide saturated slag (Al 2 O 3 +xal 4 C 3 ) is formed and the temperature of slag is not increased immediately up to the metal producing temperature, the slag can dissociate to produce Al 2 O gas according to the following reactions: Al 2 O 3 + Al 4 C 3 = 3Al 2 O (g) + 3C (3) 2Al 2 O 3 + Al 4 C 3 = 2Al (l) + 3CO + 3Al 2 O (g) (4) 5Al 2 O 3 + 2Al 4 C 3 = 6CO + 9Al 2 O (g) (5) In order to avoid excessive production of Al 2 O 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 Al 2 O gas, or remain as a layer of molten alumina which will require carbide make-up in order to react to produce further metal. Al 2 O 3 + 4Al = 3Al 2 O(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 Al 2 O 3 -Al 4 C 3 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

4 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(al 2 O 3 + xal 4 C 3 ) (l) + z(al 4 C 3 ) (s) jal (l) + {dco + eal + fal 2 O} (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 Al 2 O 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. Al 2 O 3(s) + 3C (s) = 2Al (g) +3CO (g) (7) Theoretically, in these conditions aluminium should be formed as a vapour without the formation of Al 4 C 3 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 mbar pressure range and 1,025 C- 1,800 C temperature range. Balomenos et al. 3 showed that under vacuum (0.1 mbar) and at around 1500 C, theoretically, a full conversion of Al 2 O 3 to Al (g) and CO (g) can occur. Therefore, these workers carried out experiments in the C temperature range and at a total pressure in the range of 3.5 to 12 mbar using the reactants Al 2 O 3 and charcoal. In these experiments, the aluminium yield reached 19% by condensation of the produced fume.

5 Carbide (Al 4 C 3 ) and oxycarbide (Al 4 O 4 C) 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 Al 2 O (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 mbar and in the temperature range C. In a series of experiments in the pressure range of mbar, they observed that Al 4 O 4 C and Al 4 C 3 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 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 C temperature range and at a CO partial pressure in the mbar range using Al 2 O 3-3C 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 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 Al 4 C 3, Al 4 O 4 C 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 ( 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

6 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 K( 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 SiO 2, 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) Al (g) H 2(g) H (g) (8) AlO(OH) +2C = 2.00CO (g) Al (g) H 2(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 SiO 2 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) SiO C = 3.3CO (g) + 1.0Al (g) H 2(g) + 0.1SiC(s) + 0.1SiO (g) Al 2 O (g) Si (g) (10) AlO(OH) + 0.2SiO C = 2.30CO (g) Al (g) H 2(g) +0.1SiO (g) +0.1SiC (s) Al 2 O Si (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) Al (g) H2 (g) + 0.1Fe (g) (12)

7 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) FeO(OH) C = 2.56CO (g) Al (g) H2 (g) Fe (g) H (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 Al 2 O Fe 2 O 3 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. Al 2 O Fe 2 O C = 3.7CO (g) + 2Al (g) Fe (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 (Al 2 O Fe 2 O 3 ), the equilibrium formed is shown as Reaction (15): Al 2 O Fe 2 O C = 3.43CO (g) Al (g) Fe (g) Al 2 O (g) +0.37Fe (S) Al 4 C 3( 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 <3 wt%). A mixture of calcined bauxite and charcoal was heated up to 1,600 C at an initial pressure of 10-7 bar in a corundum reactor tube. A condensation of β-sic, Al 4 O 4 C, Al, Al 4 C 3, and α-alumina appeared on the cold wall of the reactor. The remaining residue in the crucible included α- alumina, TiC, and FeO in the hot zone of the reactor. As the thermodynamic conditions of this experiment were similar to Reaction (12) conditions, it seems that back reactions between CO, Al and Fe were active and converted the products into carbides and oxycarbide. These results indicate that similar to the vacuum carbothermic reduction of alumina, issues concerning a practical process for vacuum carbothermic reduction of bauxite are: 1- very high energy requirement

8 The total theoretical energy (heat and work) requirement for vacuum carbothermic can be as high as 23.6 kwh/kg Al for gibbsite and 16.7 kwh/kg Al for boehmite and diaspore. These values do not include any energy required for feed preparation or the energy which is required for the treatment of condensates for Al purification, 2- low yield due to back reactions The condensation of aluminium vapour from its mixture with CO is not a fast process. Under low pressure and presence of non-condensable gas (CO), a diffusional resistance reduces the heat transfer rate for the condensation process. This delays condensation of gaseous aluminium, therefore, back reactions take place, and 3- lack of an efficient technique to separate Si (g), SiO (g) and Fe (g) from the gas mixture. Si (g), SiO (g) and Fe (g) are produced in the reactor gas phase. According to the results, the content of these impurities in the product is proportional to their oxide content in bauxite and the level of elemental Si and Fe in the condensates can be as high as 20 wt% and 32 wt% respectively. Therefore, pure aluminium production from the condensates does not seem to be viable. 5. Carbothermic Reduction of Alumina Using Inert Gas for Carbide Production Solid state carbothermic reduction of alumina was carried out at reduced CO partial pressure. Carbothermic reduction of alumina into aluminium carbide in argon, helium and hydrogen atmospheres was experimentally studied by Li et. al. 8. The experiments were carried out at 1,500 1,700 C temperature range using high purity alumina and graphite powder mixture with a molar ratio of C/Al 2 O 3 =6. The mixtures were pressed as pellets. In these experiments, pellets of about 1 g weight were used. At 1,600 C, the maximum production rate of CO gas reached moles per minute per mole of the oxygen present in the sample when helium and hydrogen gases were used. In argon and at 1,600 C, the CO production rate reached a maximum value of about mole/min.moleo. During the reaction course, the CO partial pressure did not reach more than 4.5 mbar (3.4 torr) in the reactor atmosphere with a flow of 1 lit/min of the inert gas. Based on the maximum value of the reaction rate, the inert gas (He or H2) consumption was about 30 m 3 /mole of carbide. Based on these results, issues concerning a practical process for alumina reduction into carbide at an inert gas are: 1- low reaction rate of reduction and 2- very high inert gas consumption. In the Li s work, the average reaction rate for oxygen removal (carbide production) was about go/min.go, in helium atmosphere at 1,600 C. A process for aluminium carbide production based on this scenario will need about 550 m 3 helium per kilogram carbide. If using argon, the required inert gas volume will be even higher. The cost of using this volume of inert gas (purchased or recycled including cleaning, pumping and maintenance) will be very high. 6. Stepwise Carbothermic Reduction of Bauxite Using Inert Gas for Alumina Production Carbothermic reduction of bauxite was examined for alumina and ferroalloy production by Yeh and Zhang 9. Bauxite ores were reduced stepwise in argon. The results showed that the

9 ores can be reduced stepwise, in the solid state, at different temperatures. Below 1,100 C, only iron oxides were reduced to metallic iron. A ferroalloy phase was formed at 1,200 C and above. In this work, Western Australia bauxite powder (<212 µm) with a composition of 40% Al 2 O 3, 19% iron oxides, 17% silica and ~1% titania, was mixed with graphite powder (<20 µm) in a C/O molar ratio of 1.2:1 and then the mixture, in the form of a tablet (1 g), was heated in argon from 850 C to 1,600 C at 2 C/min rate and thereafter was kept at 1,600 C for two hours. The residue in the crucible included a ferroalloy phase (82.6 wt%fe, 13.5%C, 2.3%Si, 1.5%Al) embedded in a porous non-metallic phase containing mainly alumina and some iron oxide and carbon. In this sample, carbides SiC and TiC were formed in the matrix of the ferroalloy and precipitated out of the ferroalloy phase. Additionally, in this sample it was observed that the metallic phase was molten but that the non-metallic phase remained solid. During the course of reaction (6 hours) in which the sample was heated continuously to 1,600 C, the CO pressure changed in the range of mbar (0.4-2 torr) in the reactor atmosphere with a flow of 1 lit/min of the argon gas. These results show that the iron oxide and all of silica left the bauxite sample. The ferroalloy phase was very rich in iron indicating that the iron from the bauxite sample was recovered as metal but this was not the case with silica, indicating that Si (g) and/or SiO (g) entered in the gas phase. At the same time, the aluminium value in the ferroalloy phase was very low, indicating limited reduction of alumina into metal. These results show that alumina can be separated from bauxite by direct reduction of bauxite at temperatures lower than 1,600 C and at CO pressures lower than 2.5 mbar. Regarding these results, issues concerning a practical process for stepwise bauxite reduction in an inert gas include: 1- Very high usage of inert gas. Using m 3 Ar/kg Al 2 O 3 seems unlikely to be feasible. Therefore, the process would not be able to compete with Bayer process economically. 2- Once the alumina is generated, then the treatment of alumina to produce carbide or metal will need additional inert gas. The total inert gas requirement for metal production from bauxite can reach higher than 1200 m 3 /kgal. 3- Treatment of Si(g) and SiO(g) in the exit gas and the separation of solid alumina from the ferroalloy produced in the process are other required functions for alumina production 4- Lack of an efficient technique to separate Al from the ferroalloy phase or from condensate above the reaction zone. 7. The Thermical Approach for Carbothermic Production of aluminium In contrast to the conventional process, the Thermical carbothermic smelting process 10,11,12,13 uses a different approach. This process starts with the production of charge material by heating mixtures of aluminium metal, carbon and alumina particles in (4+x)Al+3C+Al 2 O 3 proportion. During heating, carbon reacts exothermically with aluminium metal to produce aluminium carbide. The carbide particles, together with the oxide, form an intimate mixture known as charge. By choosing the appropriate carbon content in the initial mixture of Al 2 O 3 -

10 C, the charge composition can be controlled and fixed in stoichiometric proportions for metal production. For practical purposes, the desired temperature range for carbide production is 1,550-1,650 C. In this temperature range, any gas such as Al 2 O (g), Al (g) and CO is absent and the carbon reaction with metal is complete. Therefore, this reaction alone reduces the troubles found with the first stage of the conventional carbothermic aluminium smelting approach. At 1,550-1,650 C, Al 4 O 4 C and Al 4 C 3 are the thermodynamically stable components of the charge. Therefore, when a charge sample is subsequently heated in an electric arc furnace, the charge temperature is rapidly increased and metal is produced at atmospheric pressure in 1,750-1,850 C temperature range, depending on the quantity of hydrocarbon gas used. Metal is produced initially at the contact point of Al 4 O 4 C and Al 4 C 3. But, as the reaction progresses, the reactants are separated. Therefore, the reaction continues through a gaseous route. These reactions are: 3Al 2 O (g) +Al 4 C 3(s) = 10Al (l) + 3CO (g) (16) 3AlO (g) + Al 4 C 3(s) = 7Al (l) + 3CO (g) (17) In this system, theoretically, AlO (g) and Al 2 O (g) suboxides are generated at above 1650 C and can then react with carbide in the charge body. Reactions (16) and (17) use these gases and reduce their volume before they are released from the metal production zone. Furthermore, the Thermical process introduces more favourable kinetics features over the conventional carbothermic aluminium process. The charge particles undergoing metal production are in stoichiometric proportion and therefore do not need any make-up to maintain the appropriate material balance for the reaction. Aluminium metal in the charge plays an important role. When the charge is produced with some aluminium metal content, the particles of carbide and oxide(s) are physically attached to the metal. Therefore, this pre-existing metal acts as a conductive medium and assists in the coalescence of metal nuclei produced in the metal production zone to enhance the development and flow of the metallic liquid phase. Another role of the pre-existing metal in the charge is its effect on the reaction rate between aluminium oxide and carbide. In experimental work, undertaken by the author, it has been established that the rate of reaction between oxide and carbide in the charge is higher when the charge contains metal 10. In this work, it has been shown that at temperatures above 1,750 C, the reaction rate in the charge containing metal increases rapidly and at 1,830 C, the rate of reaction reaches one order of magnitude higher than the rate of reaction found in charge samples without metallic content. In the Thermical process, the consumption of electrode and refractory is lower than the conventional carbothermic processes. The lower consumption is due to the high content of carbon in the charge-making zone and the high carbide content in the metal production zone. In summary, by using the Thermical process, metal can be produced at atmospheric pressure and at lower temperatures than in previously attempted conventional carbothermic processes. This is due to the following factors: 1- There are much lower reactions kinetics barriers and higher reactions rates, based on solid-solid reactions with gaseous root rather than sluggish ionic reactions in molten slag. 2- There is a catalytic effect of aluminium in the charge

11 3- Lower CO partial pressure in the reactor due to the presence of hydrogen when methane gas (or other hydrocarbons) is used as a carrier gas and a carbon supplier The higher efficiency of the Thermical process is characterized by: 1- Efficient and rapid delivery of high temperature at the metal production zone by applying a submerged or inductive heating. 2- Ability to operate at high heating rates of the charge and at very rich carbide conditions, so ensuring the generation of very low Al 2 O and Al (g). 3- The low operation temperature for carbide and metal production reduces the carbon content dissolved in metal. The low operation temperature also avoids excessive aluminous gas generation 4- A rich content of carbide and the stable production of charge ensures low consumption of electrodes and refractory 8. Conclusions The Thermical process reduces the occurrence of problems that have been associated with heating Al 2 O 3 -C mixtures, liquid slag and product fluid dynamics, treatment of the gas phase and treatment of metal product with a high content of carbide as well as the high consumption of graphite electrodes and refractory lining. These are inherent problems that have long troubled conventional carbothermic aluminium production. These problems are simply reduced by the Thermical process s ability to rapidly heat charge and, thereafter to rapidly cool the metal product, together with a lower temperature requirement for the production of the charge and metal itself in a highly reactive medium with much lower kinetics barriers. The Thermical process does not use vacuum or inert gas in order to lower the process temperature. 9. References 1- White C. V., Mikkelsen O., and Roha D., Status of the Alcoa Carbothermic Aluminium Project, International Smelting Technology (Incorporating the 6th advances in Sulfide Smelting Symposium) TMS (The Minerals, Metals, & Material Society), Bruno M. J., Aluminium Carbothermic Technology, Alcoa Centre Report to USA Department of Energy, December 31, Balomenos E., Panais D., Paspaliaris I., Friedrich B., Jaroni B., Steinfled A., Guglielmini E., Halmann M., Epstein M., Vishnevsky I., Carbothermic Reduction of Alumina: A Review of Developed Processes and Novel Concepts, Proceedings of EMC 2011, European Metallurgical Conference 2011, Dusseldorf, pp Qin-chun Y., Hai-bin Y., Fu-long Z., Han Z., Chen W., Da-chun L. and Bin Y., Carbothermic reduction of Alumina with Carbon in Vacuum, Iournal Central South University Press, Springer-Verlag Berlin Heidelberg, 2012, Vol 19, pp Halmann M., Steinfeld A., Epstein M., and VishnevetskyI., Vacuum Carbothermic Reduction of Alumina, Mineral Processing & Extractive Metallurgy Review, Jully 2013, Vol 35, pp

12 6- Halmann M., Epstein M., and Steinfeld A., Vacuum Carbothermic Reduction of Bauxite Components: A Thermodynamic Study, Mineral Processing & Extractive Metallurgy Review, 2012, Vol 33, pp Goldin B.A., Grass V. E. and Ryabkov, Vacuum Carbothermal Processing of Low-Iron Bauxites, Glass and Ceramics, 1998, Vol 55, pp Li J., Zhang G., Liu D., and Ostrovski O., Low-temperature Synthesis of Aluminium Carbide, ISIJ International, Vol. 51 (2011), No. 6, pp Yeh C., and Zhang G., Stepwise Carbothermal Reduction of Bauxite Ores, International Journal of Mineral Processing, 2013, Vol 124, pp Sayad-Yaghoubi Y., Smith G., Thermical Carbothermic Production of Aluminium Concept and Technology, Calsmelt Pty. Ltd., Australia-Melbourne, Oct Sayad-Yaghoubi Y., Carbothermic Processes, International pct/au2006/001048, Australian patent , US patent 7,824,468 B2, Chinese patent ZL , European Patent , Canadian Patent 2,633,210, Eurasian patent , Indian patent Sayad-Yaghoubi Y., Carbothermic Processes, International pct/au2007/001986, Australian patent , US patent 7,896,945 B2, Chinese patent ZL , Eurasian patent Sayad-Yaghoubi Y., Carbothermic Processes, International pct/au2009/000577, Australian patent , US patent A1.