Alternative Al production methods

Abstract

The production of Al from its ores at present relies on the Bayer (alumina production) and the Hall–Heroult (Al production) process. The cost associated with alumina production and apparent disadvantages of the Hall–Heroult process have led to intensive research to find alternative routes for Al production. The direct carbothermal reduction process has been thoroughly investigated as an alternative technique. Another alternative includes the indirect carbothermal reduction route where alumina (or aluminous ores) is first reduced to intermediate Al compounds before reduced further to Al. The present study reviews and provides systematic thermodynamic analyses of alternative Al production routes. In this paper (Part 1), a comprehensive review of alternative Al production techniques focusing on the indirect carbothermal reduction routes is presented. These include carbochlorination, carbonitridation and carbosulphidation routes for the formation of intermediate Al compounds, followed by various Al extraction processes.

Keywords

Carbothermal reduction of alumina Carbochlorination of alumina Carbonitridation of alumina Carbosulphidation of alumina Thermal dissociation Electrolysis Disproportionation

Introduction

Aluminium is the most abundant metallic element in the Earth's crust (∼8%) and the second most widely used metal next to steel. The production of Al has increased by nearly four times in the last three decades (Mahadevan et al., 1996). Pure Al cannot be found in nature because of its high affinity to oxygen. In general, it is found in a variety of minerals combined with various elements such as silicon, oxygen, phosphates, fluorine and hydroxides. In the last century, a number of methods have been developed to extract Al from its ores. The current commercial production method relies on two energy intensive processes developed in the late 1800s: the Bayer process which produces pure alumina from bauxite ore, and the Hall–Heroult electrolysis process which produces Al metal from the alumina. The cost of the Bayer process represents ∼27% of the cost of Al production (IAI, 2010) and the Hall–Heroult process uses about three quarters of the total energy requirement, though these numbers vary depending on the specifics of the ores and sources of energy used.

The Bayer process is the principal industrial technique to refine bauxite to pure alumina. There have been little changes to the basic process since the first plant opened in 1893. In this process, bauxite (containing typically 10–30 wt-%Fe₂O₃, 4–8 wt-%SiO₂ and 2–5 wt-%TiO₂ as major impurities) is dried, ground, digested in sodium hydroxide under pressure (Rao and Soleiman, 1986). Impurities are separated by filtration, before alumina hydroxide is precipitated and calcined to produce commercial alumina. The relatively high purity alumina produced from the Bayer process is then transferred to the Hall–Heroult electrochemical cell for electrolysis.

The Hall–Heroult process was patented in 1886 and the basic process layout remains unchanged except for the form of the carbon anode (Brooks et al., 2007). There has been significant improvement in the process over last hundred years, through increasing cell size, greater automation and control, and improved control of emissions. Tarcy et al. (2011) provided an outline of the improvements in a recent review, showing that the overall energy usage per kg of metal has decreased from typically over 30 kWh kg⁻¹ Al in 1914 to below 14 kWh kg⁻¹ Al by 2000.

In the Hall–Heroult process, Al metal is extracted through electrolysis of Al₂O₃ dissolved in NaF–AlF₃ (cryolite) solution according to the following overall reactions (1) (2) In this process, liquid Al is produced at the bottom of the cell and CO and CO₂ gases are evolved from the bath. The Hall–Heroult process also generates greenhouse gasses such as CF₄ and C₂F₆ from the carbon electrode and reactions with liquid cryolite (Na₃AlF₆). Per fluorocarbons (PFCs) may also forms during the so called anode effect when the electrolyte becomes depleted in alumina (Namboothiri et al., 2007). In general, the process has high capital costs, low productivity compared to other metallurgical processes and consumes significant energy (from 12·9 to15 kWh kg⁻¹ Al) (Motzfeldt et al., 1989; Huglen and Kvande, 1994; Steinfeld and Thompson, 1994). The production of each kg of Al consumes between 0·4 and 0·5 kg carbon in the form of anodes and in total, Al production contributes to 2·5% of the world anthropogenic CO₂ equivalent emissions (Halmann et al., 2007). This value will vary from plant to plant as it depends on energy source, the details of the cell and quality of the feed materials (Tabareeux, 1994).

The apparent disadvantages of the Hall–Heroult process have led to numerous researches to develop alternative routes throughout the twentieth and early twenty-first centuries. The most important alternatives that have been envisaged include modified Hall–Heroult process utilising inert anodes, direct carbothermal reduction of alumina (Russell et al., 1973), and indirect carbothermal reduction of alumina (e.g. formation of AlCl₃ intermediate followed by Al extraction in the Alcoa process).

In this paper, a comprehensive review of various Al production methods, focusing on the indirect carbothermal reduction routes, is presented. There have been a number of major review papers on Al production routes. In 1964, Stroup (1964) provided a detailed review on the carbothermal production processes of Al. Russell (1981) provided another major review on various new technologies in 1981, including review and progress on the development of Alcoa process. This paper briefly reviews the recent development in the direct carbothermal routes followed by systematic reviews of different indirect carbothermal reduction routes. In Part 2, systematic thermodynamic analyses of selected indirect carbothermal reduction routes are presented. These include evaluation of Gibbs free energy formation and equilibrium calculations of various intermediate Al compounds and extraction of Al from these compounds.

Direct carbothermal reduction process

The direct carbothermal reduction of alumina to Al has potential for greater productivity, lower capital investment, less consumption of electric power, and lower overall greenhouse gases emission, compared to the Hall–Heroult process. It is well established that direct carbothermal reduction of iron oxide, through blast furnace technology, offers far greater productivity and energy efficiency than any comparable electrolytic process. It is this desire to duplicate the merits of pyrometallurgy processing to Al that has driven interest in the carbothermal reduction of alumina. The overall reactions for carbothermal reduction of alumina can be presented as (3) (4) These two reactions are thermodynamically favoured above 2057 and 1497°C, respectively (Stroup, 1964). Although these reactions look simple, in practice they are difficult to achieve with high Al yield. The reaction steps are not straight forward as intermediate and volatile subcompounds are formed during the process. During cooling down of the products, mixtures of carbide and oxycarbide can form due to reactions between the products. The chemistry and thermodynamics of the Al–C–O system have been studied extensively (Moissan, 1894; Askenasy et al., 1908; Askenasy and Lebedeff, 1910; Fraenkel, 1913; Ruff et al., 1918; Kohlmeyer and Lundquist, 1949; Hoch and Johnston, 1954; Filonenko et al.; 1959; Motzfeldt, 1962; Cox and Pidgeon, 1963; Stroup, 1964; Huglen and Kvande, 1994; Steinfeld and Thompson, 1994; Lacamera, 2002; Halmann et al., 2007; Walker, 2010).

Recent developments in carbothermic production of Al evolved in the direction of a two-stage process. Cochran and Fitzgerald (1981) invented a stack type reactor in which Al₂O₃ and C is reacted in a high temperature upper reaction zone and produces liquid mixture of Al₂O₃ and Al₄C₃ which is then transferred to a lower reaction zone for Al extraction. Alcoa and Elkem companies thoroughly investigated carbothermic production of Al using the stack type reactor (advanced reactor process) (Johansen and Aune, 2002; Bruno, 2003; Johansen, 2003; Aune and Johansen, 2004; Fruehan et al., 2004a, b; Garcia-Osorio and Ydstie, 2004). In this process, alumina is carbothermally reduced at 2000°C in the first reaction compartment of a vessel producing liquid slag (50Al₂O₃–50Al₄C₃). Further reaction between alumina and Al carbide took place in the second compartment at 2200°C to produce an aluminium–carbon alloy. The reactions associated with each stage are given below (5) (6) The advanced reactor process included a method for vapour recovery for recycling the Al and Al suboxide vapours generated. Fruehan et al. (2004a, b) studied the mechanism and reactions of Al₂O_(g), Al_(g) with carbon source. They found that the rate of formation of Al₄C₃ from Al₂O and Al gases in CO is controlled by diffusion of the reactant gases through the Al₄C₃ product layer. In 2011, Alcoa continued the development of the carbothermal process on its own with the establishment of the Alcoa Norway Carbothermic group, in which the test reactor with auxillary systems located at Alcoa Lista, Southern Norway. They continue to improve the off-gas cooling system for stable operation of the reactor. White et al. (2012) reported a successful test campaigns that run for several weeks, with each tap generates several hundred kilograms of metal with total yield in tonnage scale.

Similar two-zone concept was also researched by Persson (1983), and Dewing et al. (1981). Dmitriev and Karasev (2000) proposed an induction shaft furnace for Al₄C₃ production from alumina, where Al is then extracted by electrolysis in the lower zone of the furnace.

Other recent developments include the vacuum carbothermic reduction currently being developed under the ENEXAL project (Balomenos et al., 2011). This process uses an electric arc furnace with integrated shaft attachment. The electric arc furnace is the main reactor where liquid Al is produced and the shaft acts as condenser for the Al and Al₂O vapours. The formation of Al rich vapour is favoured over liquid Al as the process is carried out under vacuum. The process can also be carried out at lower temperatures, e.g. 1500°C at 10 Pa pressure. Preliminary experimental study has been carried out using a solar furnace to demonstrate the process. At temperatures 1027–1727°C and pressures 350–1200 Pa, Al (up to 19%) along with Al₄C₃ and Al₄O₄C can be produced upon condensation (Murray et al., 1995; Kruesi et al., 2011). It is unclear whether yield can be improved and whether the proposed process has potential for commercialisation.

Sayad-Yaghoubi (2007) proposed a carbothermic process for the production of Al carbide by injection of carbon and Al₂O₃ into superheated Al (>1400°C). The reaction between the injected carbon with molten Al follows (7) The formed Al₄C₃ attaches to Al₂O₃ and forming a mixture of Al₄C₃–Al₂O₃. Aluminium is extracted from the mixture in a separate zone through reaction given in equation (6) at 1700–2000°C. This is carried out by maintaining a sufficiently low gas pressure in the second zone, e.g. by extracting the CO gas. Sayad-Yaghoubi (2007, 2008, 2009) also proposed various type of carbonaceous materials (such as CH₄ and other hydrocarbons) and additional reactants (such Al scrap and dross) to be used in the process.

Despite the significant scientific and technological developments to date, no processes based on direct carbothermal reduction have been successfully commercialised to full plant scale. Direct carbothermal processes have to date suffered generally from problems with yield, associated with the formation of other compounds other than Al.

Indirect carbothermal reduction processes

Researchers and industry have also directed their attention to a multistage process, in particular indirect carbothermal reduction of alumina. This involves two or more steps where in the first step (stage 1) alumina or Al ores are converted to intermediate Al compounds. The intermediate Al compounds are then further reduced to Al metal in subsequent steps (stage 2). This is schematically shown in Fig. 1.

Figure 1

Schematic diagram showing indirect carbothermic aluminium production method from alumina

The following subsections review the various indirect carbothermal productions of Al, focusing on the carbochlorination, carbosulphidation and carbonitridation routes in stage 1 followed by various Al extraction techniques in stage 2.

Carbochlorination route

This route involves the formation of Al–chloride intermediate compounds from reactions of alumina (or aluminous ores) with carbon and chlorine sources. The production of volatile metal halides (including Al trichloride) from alumina has been known for many years, but most of the processes have difficulties in terms of operation and control. One of the earliest patented processes for the production of Al trichloride was from Ferguson and Cranford (1948), where they developed a continuous process using fluidised mixture reactor. In the case of carbochlorination of alumina, the process can be represented by the following reactions (8) (9) The chlorination temperature is in the range 400–1000°C depending upon the reacting agents. There have been a number of studies on the chlorination of alumina investigating the effect of various kinetic parameters (chlorination media, partial pressure of gaseous components, particle shape and size and reactor design) on the reactions in the temperature range between 427 and 1027°C (Milne, 1976; Milne and Wibberley, 1978; Bertoti et al.1981; Toth et al., 1982). Unless a high purity alumina source is used, other elements that are generally present such as iron, silicon and titanium are also chlorinated and must be separated from the Al trichloride. Yuan et al. (2010) carried out study of carbochlorination of alumina in vacuum and reported that AlCl_(g) is formed at temperatures 1430–1580°C at pressures 40–150 Pa.

Grob and Richarz (1984) attempted to chlorinate alumina in kaolinitic clay in Cl₂ and CO gas mixtures. They found that the reactivity of alumina in the raw material increases when the material is first pretreated using ammonium sulphate (NH₄)₂SO_4(g) or sulphur trioxide SO_3(g). At temperatures below 727°C, the chlorination rates were found to be comparable to those reported for using pure gamma alumina as starting material. However, above 727°C, SiCl₄ formed and interacted with alumina, leading to a decrease in the rate of Al chlorination.

Huapin et al. (1977) produced Al trichloride from alumina through reaction with carbon and chlorine in alkali metal halides and alkaline earth metal halides molten salt baths (AlCl₃–NaCl–LiCl) at temperatures 400–950°C. Becker and Das (1978) used a mixture of high purity activated carbon and alumina heated in bubbling chlorine at temperatures 500–775°C and was able to obtain 100% conversion to AlCl₃ vapour.

Toth (1971) patented a two-stage process for Al production where in stage 1, alumina is reacted with manganese chloride in the presence of carbon at temperatures 900–1300°C producing Al trichloride and manganese, through the reaction (10) Wyndham et al. (1978) and Lippman et al. (1978) (of Toth Al Corporation) used alumina based ores as the source for Al. In the case of alumina silica ores, the reaction was carried out in the presence of sulphur to improve the halogenation. Sulphur acted as a catalyst and the process could be carried out a temperature as low as 400°C through this reaction (11) There are other processes for Al trichloride production, including various Al extraction techniques from the chloride. Table 1 summarises selected processes and technological development associated with the chloride route (both stages 1 and 2).

Table 1

Summary of previous major works/patents in chloride route

Year	Author	Reactions and Parameters	Process Information
2002	Tomaswick^p (Alcoa)	AlCl₃ molten bath with NaCl, LiCl, KCl, MgCl₂, CaCl₂, BeCl₂, BaCl₂. T = 300°C preferably (150–200°C)	Electrolysis of Al₂O₃ in a low temperature bath containing AlCl₃ and an alkali metal chloride. The bath is at 300°C and solid Al is produced at the cathode in the frozen layer of the alkali metal chloride (2nd stage)
2002	Tomaswick^p (Alcoa)	Conc. of alkali metal chloride = 10–50 mol.-%; voltage, <2·0 V; current density, >0·2 A inch⁻²
2000	Sharma^p		Electrolytic production of Al metal from Al trichloride using chloride fluoride salts bath. Al fluoride is separated from the bath and electrolytically reduced to metallic Al (2nd stage)


		Electrolytes: Na₃AlF₆–NaCl–NaF; Li₃AlF₆–LiCl–LiF; K₃AlF₆–KCl–KF; temperature: 750°C
1990	Wilkening^p (VAW Vereinigte Aluminium-Werke AG)	Electrolyte melt: >50% consist of NaCl–KCl, 10–40% at least of cryolite, alkali fluorides and alkaline earth fluorides; AlCl₃, 3–5%.	Production of Al from alumina through electrolysis where the bath consists of alkali halides and/or alkali earth halides. Alumina is charged as briquette with carbon which also acts as anode. Bipolar electrodes designs are also used (2nd stage)
1990	Wilkening^p (VAW Vereinigte Aluminium-Werke AG)	T = 680–850°C
1986	Cohen et al.^p (Pechiney)	Al₂O_3(s)+3Cl_2(g)+3C_(s) = 2AlCl_3(g)+3CO_(g)	Al production method through carbochlorination of alumina, followed by electrolysis of Al trichloride in a molten salt bath containing at least one alkali/alkali earth metal halides. Flowsheet of a continuous process (1st and 2nd stages)
		T = 660–800°C
		Chlorinating gas: Cl₂, CCl₄, C₂Cl₆, phosgene
		Electrolysis: Conc. of AlCl₃ in bath = 10–40 mol.-%
		T = 450–900°C
1986	Rao et al.^p (Washington Res. Foundation)	Example for MgF₂ catalyst: MgF_2(s) = MgF_2(g)	The use of catalysts for carbochlorination of Al₂O₃ at 750–950°C. These include: alkali fluorides, alkaline earth fluorides, alkaline earth carbonates, alkaline earth chlorides, alkaline earth bromides, alkaline earth oxides, and their mixtures (1st stage)
		3Al₂O_3(s)+2MgF_2(g)+9CO_(g)+9Cl_2(g) = 2MgAl₃F₂Cl_9(g)+9CO_2(g)
		MgAl₃F₂Cl_9(g) = MgF_2(g)+3AlCl_3(g)
1981	Sinha^p (CSIRO Australia)	Dehydration T = 300–400°C	Development of a process to produce anhydrous Al chloride from Al chloride hexahydrate. The process involved dehydration and chlorination in a fluidised bed reactor (1st stage)
		Chlorination T = 400–500°C
		2AlCl₃.6H₂O_(s)+12CO_(g)+12Cl_2(g) = 2AlCl_3(g)+2CO_2(g)+24HCl_(g)
		Gas mixture: 40–50%Cl₂, 30–50%CO, 5–15%CO₂, 5–15%H₂
1981	Mueller et al.^p (Swiss Al Ltd.)	Dilution agents: quartz, corundum, MgO	Improved carbochlorination process of aluminous ores in a fluidised bed reactor by the addition of an inert solid dilution agent to the bed (1st stage)
1981	Mueller et al.^p (Swiss Al Ltd.)	Size: comparable to starting materials, amount 10–90 wt-%; gas flowrates: 2–30 cm s⁻¹; conversion: 58–65%
1981	Loutfy et al.^p (Dept of Energy of USA)	Al₂O₃.Fe₂O_3(s)+6Cl_2(g)+6C_(s) = 2AlCl_3(g)+2FeCl_3(g)+6CO_(g); T = 900–1200 K	Production of Al trichloride from aluminous material contains iron, titanium and silicon compound reacting with carbon and a chlorine containing gas. Produced chloride gases are cooled down to 400 K or lower to condense the Al chloride and iron chloride gases then heated to 800 K and passed into intimate contact with Al sulphide to precipitate solid iron sulphide (1st stage)
		Al₂S_3(s)+2FeCl_3(g) = Fe₂S_3(s)+2AlCl_3(g); T = 800 K
		Yield: 99·8%
1979	Ishikawa et al.^p (Nippon Light Metal Co Ltd)	Electrolyte bath: CaCl₂/MgCl₂(15–70 wt-%)–(15–83 wt-%)NaCl; with conc. AlCl₃ = 2–15 wt-%; T = 680–780°C; current density: 0·5–2·0 A cm⁻²; current efficiency: 90–100%	Production of Al by electrolysis of fused Al chloride together with an alkali metal halide (2nd stage)
1978	Harvey et al.^p (Westinghouse Electric Corp.)	Al₂O₃.SiO_2(s)+5C+5Cl₂ = 2AlCl_3(g)+SiCl_4(g)+5CO_(g)	Production of Al through chlorination of chemically bonded Al₂O₃and SiO₂ to produce AlCl₃ and SiCl₄. AlCl₃ is then reacted with Mn to produce elemental Al and MnCl₂. Oxidising MnCl₂ produces MnO. Reducing SiCl₄ by hydrogen to yield silicon which can be used to reduce MnO producing Mn which is recycled to the previous step to reduce AlCl₃ (1st and 2nd stage)
		2AlCl_3(g)+3Mn_(l) = 3MnCl_2(g)+2Al_(l)
		SiCl_4(g)+2H_2(g) = Si_(l)+4HCl_(g)
		MnCl_2(g)+O_2(g) = Mn₂O_3(s)+Cl_2(g)
		Si_(l)+Mn₂O_3(s) = SiO_2(s)+Mn_(l)
		T = 1227–1727°C
1978	Becker et al.^p (Alcoa)	Al₂O_3(s)+3Cl_2(g)+3C_(s) = 2AlCl_3(g)+3CO_(g)	Production of Al chloride by using high purity activated carbon and alumina and bubbling chlorine gas. Al chloride was remove in vapour from and condensed (1st stage)
		Cl₂ flowrate: 0·95–3 L min⁻¹
		Chlorine to AlCl₃: 100%; T = 500–775°C
1978	Lippman et al.^p (Toth Al Corp.)	Bauxite/clay_(s)+Cl_2(g)+C_(s) = AlCl_3(g)+FeCl_3(g)+SiCl_4(g)+TiCl_4(g)+CO_(g)+CO_2(g); T = 750–850°C	Process for the production of pure Al chloride and alumina from aluminous ores through carbochlorination followed by chlorides separation and purification (1st stage)
1978	Lippman et al.^p (Toth Al Corp.)	Vaporisation of AlCl₃–FeCl₃ mixture, followed by separation of AlCl₃ by distillation
1978	Wyndham et al.^p (Toth Al Corp.)	Al₂O₃.2SiO_2(s)+7C_(s)+7Cl_2(g)+sulphur = 2AlCl_3(g)+2SiCl_4(g)+7CO_(g)	Improved halogenation of aluminous ores by sulphur treatment; allowing low temperature processing. Sulphur acts as catalyst but no detailed mechanisms were presented (1st stage)
1978	Wyndham et al.^p (Toth Al Corp.)	T = 400–1000°C (∼700°C)
1978	Pope^p (Alcoa)	N/A	Development of a fluidised bed reactor for chlorination of alumina with multilayer lining (1st stage)
1977	Holliday et al.^p (Comalco Ltd)	Fe₂O_3(s) _{(in bauxite)}+3SO_2(g)+7CO_(g) = Fe₂S_3(s)+CO_2(g)	Removal of iron from bauxite by reaction with SO₂/CO gas mixtures followed by chlorination (1st stage)
		Fe₂S_3(s) _{(in bauxite)}+Cl_2(g) = FeCl_3(g)+purified bauxite
		T = 430–750°C
		Purified bauxite+Cl_2(g)→AlCl_3(g) (with low Fe content)
		T = 650–750°C
1977	Haupin et al.^p (Alcoa)	Al₂O_3(s)+3Cl_2(g)+3C_(s) = 2AlCl_3(g)+3CO_(g)	Production of Al chloride by reaction of Al oxide, carbon and chlorine in an alkali metal halides and alkaline earth metal halides molten salt bath. Al chloride was separated by vaporisation (1st stage)
		Molten bath: AlCl₃–NaCl–LiCl
		Cl₂ conversion to AlCl₃: 99%; T = 400–950°C
1975	Terry et al.^p (Toth Al Corp.)	3Mn_(s)+2AlCl_{3(g, l)} = 3MnCl_{2(s, soln)}+2Al_(s)	A method of producing Al wherein Al chloride in liquid phase reacted with manganese in solid phase. The manganese reduces Al chloride and forms essentially elemental Al and manganese chloride (2nd stage)
1975	Terry et al.^p (Toth Al Corp.)	Pressure range: 15–450 psi; temperature: 180–600°C; time: ∼2 h; AlCl₃ flowrate: 0·75–3·20 g h⁻¹
1975	Dell et al.^p (Alcoa)	AlCl_3(l) = Al_(l)+1·5Cl_2(g)	Design and development of electrolytic cell with intermediate bipolar electrodes. This allows the flow of bath melt and settling of Al metal. Anode–cathode distance/spacing of less than 3/4 inch (2nd stage)
1975	Dell et al.^p (Alcoa)	Electrolyte: 50–75%NaCl and 25–50%LiCl₂; Conc. AlCl₃: 1·5–10 wt-%; T = 660–730°C
1975	Othmer^p	3AlCl_(g) = 2Al_(l)+AlCl_3(g)	Extraction of Al from Al trihalide by halogenation and/or disproportionation in a flash condenser (2nd stage)
		2AlCl_3(g)+6S_(l) = 2Al_(l)+3S₂Cl₂(g)
		T = 600–1000°C
		Other reactions above 1500°C:
		Al₂O_3(s)+S₂Cl_2(g)+3C_(s) = 2AlCl_(g)+2S_(l)+3CO_(g)
		2Al₂O_3(s)+2S₂Cl_2(g)+O_2(s) = 4AlCl_(g)+4SO_2(g)
1974	Othmer^p	Cl_2(g)+Al₂O_3(s)+3C_(s) = 2AlCl_(g)+3CO_(g)	Separation of Al from aluminous ores (bauxites, clays, feldspar, etc) through the formation of volatile tri-halide, followed by disproportionation of tri-halide to metallic Al. (1st and 2nd stages)
		SiCl_4(g)+2Al₂O_3(s)+4C_(s) = 4AlCl_(g)+4CO_(g)+SiO_2(s)
		AlCl_3(g)+Al₂O_3(s)+3C_(s) = 3AlCl_(g)+3CO_(g)
		T = 1000–1200°C
		Removal of Fe impurities:
		2AlCl_2(g)+Fe₂O_3(s) = 2FeCl_3(g)+Al₂O_3(s)
		Disproportionation T = 700°C
		3AlCl_(g) = 2Al_(l)+AlCl_3(g)
1973	Russell et al.^p (Alcoa)	AlCl_3(l) = Al_(l)+1·5Cl_2(g); T = 660–730°C	Production of Al by continuous electrolysis of Al trichloride. Increase of current efficiency of the process (avoiding the formation of sludge) using proposed operating conditions (2nd stage)
		Electrolyte: NaCl–LiCl₂ (50∶50, w/w)
		Conc. AlCl₃ in bath: 1–15 wt-%; oxide impurities: <0·25 wt-%
		ACD: 1 in; current density: 10 A; voltage: <5 V; current efficiency: 80%
1971	Toth^p (Applied Al Research Corp.)	Al₂O_3(s)+3MnCl_2(l)+3C_(s) = 3Mn_(l)+2AlCl_3(g)+3CO_(g)	Production of Al by a two-steps process comprising the reaction of alumina with manganese chloride in the presence of carbon to form Al trichloride and manganese and the subsequent reaction of Al trichloride with manganese at a temperature sufficient to reduce Al trichloride to Al (1st and 2nd stages)
		3Mn_(l)+2AlCl_3(g) = 2Al_(l)+3MnCl_2(l)
		T = 900–1300°C
1948	Ferguson et al.^p (Standard Oil Development Co)	Al₂O_3(s)+3C_(s)+3Cl_2(g) = 2AlCl_3(g)+3CO_(g)	Production of Al chloride and Al bromides. Development of a continuous process using a ‘fluidised mixture’ type reactor (1st stage)
1948	Ferguson et al.^p (Standard Oil Development Co)	T = 649–816°C
1939	Willmore^p (Alcoa)	3AlF_(l) =2Al_(s)+AlF_3(s)	Disproportionation of Al halides, including Al chlorides (2nd stage)
1939	Willmore^p (Alcoa)	T = 900–1000°C

^*^p refers to patent; 1st stage refers to production of intermediate Al chlorides; 2nd stage refers to extraction of Al from the chlorides.

Extraction of Al from Al trichloride

The common routes for extracting Al from its chlorides are through disproportionation and electrolysis reactions. Other routes include distillation and direct reduction with other metals.

Disproportionation of Al halides (including chlorides)

Willmore (1939) discovered that AlF₃ and several other fluorine compounds selectively distil Al at 900–1300°C. Klemm and Voss (1943) indicated that this occurs because of a formation of a monovalent compound (in this case AlF) at a temperature 1200°C. This compound was then disproportionated at a lower temperature of 800°C according to the following reaction (12) In this process the Al condensed as a fine powder, mixed with solid AlF₃. Therefore, further process using molten flux was necessary to separate and agglomerate the Al. It was then found that the above reaction also occurs with other Al halides. The halide formation has been the basis of Al production and refining process using various Al halides and other subcompounds. Belyayev and Firsanova (1961) conducted a survey on various Al subcompounds and concluded that subfluoride and subchloride provide the most potential for practical refining processes.

Gross (1944, 1949) proposed a process in which AlCl₃ vapour is passed through a bed of impure crushed Al alloy above 1000°C and Al is extracted and condensed by disproportionation at 700°C according to the reaction (13) The reformed AlCl₃ was not condensed and was recirculated through the reaction furnace. As only Al was condensed, further separation process from the AlCl₃ was not necessary.

In the 1960s, Alcan developed an alternative process for production of Al which involved carbothermal reduction of aluminous ores followed by monochloride purification (Russell, 1981). This work was stopped due to problems associated handling of chlorides which included stress–corrosion cracking and inefficiency in separating manganese impurities. In the 1970s, Othmer (1974, 1975) proposed a separation of Al from aluminous ores (bauxite, clays, feldspar) through carbochlorination (to form AlCl₃) followed by disproportionation or halogenation in a flash condenser. One of the recent works associated with carbochlorination of alumina followed by disproportionation was from Yuan et al. (2010). They carried out the carbochlorination in vacuum and observed AlCl_(g) at temperatures 1430–1580°C and pressures from 40 to 150 Pa. The AlCl_(g) was then disproportionate into Al and AlCl_3(g) below 660°C. They were able to obtain Al metal with average purity of 95·32 wt-%.

Electrolysis of Al trichloride

It has been claimed that Al chloride electrolysis method is attractive both from an economic and technical point of view (Grjotheim et al., 1977). Ishikawa and Ichikawa (1979) argued that the potential advantages of Al chloride salt electrolysis process include:

chloride salts are much less corrosive than fluoride salts which results a longer cell life

the electrolysis process requires a closed system which limit the emission of gasses

iii

chloride salts have higher conductivities compared to the fluoride salts resulting lower energy consumption, higher power and current efficiencies

the electrolysis process has a very broad operational range of Al concentration which results in no ‘anode effect’

it is possible to design an electrolytic process cell with bipolar electrodes which results in a much more compact cell with increased production potential per unit volume.

In the 1970s, Alcoa developed alternative processes to produce Al which focused in three major directions:

direct carbothermal reduction of alumina

indirect carbothermal reduction of aluminous ores that involved carbochlorination, monchloride purification

iii

electrolysis of Al chloride (Russell, 1981).

The general schematic of the process routes is shown in Fig. 2.

Figure 2

Schematic diagram showing process routes considered by Alcoa (Russel, 1981)

Russell et al. (1973) from Alcoa developed an Al chloride electrolysis process using NaCl–LiCl (50∶50, w/w) electrolyte at temperatures 660–730°C. They suggested operation with 1–15 wt-% concentration of AlCl₃ in the bath. The anode–cathode distance in the process was suggested to be ∼25 mm. They were able to improve the current efficiency (up to 80%) and avoid the formation of sludge from the impurities using these proposed operating conditions. Dell et al. (1975) improved the process by developing a new design with bipolar electrodes. This allowed the process to operate with a lower anode–cathode distance (<19 mm). Alcoa developed the Al trichloride electrolysis route in the 1970s to a commercial level but halted the operations in the 1980s, because of difficulties associated with production and handling of pure Al trichloride (Thonstad et al., 2001).

Ishikawa and Ichikawa (1979) from Nippon Light Metal electrolysed AlCl₃ in a molten AlCl₃–LiCl–NaCl salt bath at 700°C. Additions of LiCl, CaCl₂ or MgCl₂ salts to the bath improved the electrical conductivity and the current efficiency. They were able to obtain 90–100% current efficiency. Cohen et al. (1986) from Pechiney proposed a continuous process through carbochlorination of alumina to form AlCl₃ followed by electrolysis in AlCl₃–alkali/alkali–earth metal halides electrolyte. One of the recent works on the electrolysis of AlCl₃ was from Sharma (2000) where Al is produced through electrolysis in a chloride–fluoride salt bath at 750°C. The electrolytes used include Na₃AlF₆–NaCl–NaF, Li₃AlF₆–LiCl–LiF and K₃AlF₆–KCl–KF.

Wilkening (1990) produced Al through electrolysis where the bath consists of alkali halides and/or alkali earth halides. A mixture of Al oxide and carbon was used as a feed material. Different arrangements of the electrolytic cell were also proposed in the work. Tomaswick (2002) from Alcoa electrolysed Al₂O₃ directly in a bath containing AlCl₃ and an alkali metal chloride (NaCl, LiCl, KCl, MgCl₂, CaCl₂, BeCl₂, BaCl₂). The process was carried at 300°C and Al was extracted as a solid metal at the cathode in a frozen layer of the alkali metal chloride.

Other Al extraction methods from Al trichloride

There are other methods used to extract Al from AlCl₃. One example is through direct reduction using another metal (e.g. manganese). Toth (1971) patented a two-stage process where in stage 1, alumina is reacted with manganese chloride in the presence of carbon producing Al trichloride and manganese, as given in reaction in equation (10). The Al was extracted by reacting AlCl₃ with manganese in stage 2 at a temperature sufficient to reduce AlCl₃ to Al, according to the following reaction (14) The flowsheet of the process is shown in Fig. 3. Terry et al. (1975) modified the process and carried out the reaction at lower temperatures (180–600°C) and higher pressures (0·1–3·04 MPa) using solid manganese to form solid Al directly according to the reaction (15)

Figure 3

Flowsheet of Toth process (Toth, 1971)

Carbosulphidation route

Another alternative Al production method that has received a less attention is through the formation of Al sulphides (Al₂S₃ and AlS) as intermediate compounds. One of the first patented processes on the carbothermic production of Al₂S₃ was by Haglund (1931). Haglund produced a mixture of Al₂O₃ and Al₂S₃ from the reaction between Al₂O₃ containing ores, carbon as reduction agent, and sulphur sources in the form of FeS and ZnS. FeS was added in the form of a lump (mixed with uncalcined bauxite) where it was sink through the formed slag into the liquid iron alloy underlying the slag. Al sulphide was formed as part of the reaction products, while Al was contained in the iron alloy.

Weiss (1958) proposed a method for producing Al from a mixture of aluminous ores and Al₂S₃ reacted with carbon at temperatures above 1000°C at pressures below atmospheric pressure. The reaction resulted in vapours of Al subsulphide (such as Al₂S) which upon cooling disproportionated into Al sulphide (Al₂S₃) and Al precipitate. The reactions are given below (16) (17) Weiss proposed the process to be carried out at 1000–1200°C at pressure below 5 mmHg. The process could be carried out at higher pressures if the temperature of the process is increased, e.g. at a temperature above 2000°C, the process can be carried out at atmospheric pressure.

Loutfy et al. (1981c) studied the thermodynamics of the carbosulphidation of alumina and patented an Al production process in 1981 (Loutfy et al., 1981b). Aluminous ores were reacted with carbon and sulphur containing gas at temperatures 1027–1227°C to obtain molten Al sulphide (Al₂S₃) and CO gas according to the following reaction (18) The Al₂S₃ was further heated at temperatures 1327–1627°C to produce Al monosulphide (AlS) and sulphur. Then AlS was cooled to a temperature 927–1097°C where it disproportionated to molten Al sulphide (Al₂S₃) and Al metal following the reactions (19) (20) The Al₂S_3(l) produced from the reaction in equation (20) was then electrolysed to extract the Al. The flowsheet of the process is shown in Fig. 4.

Figure 4

Flowsheet of aluminium production through carbosulphidation followed by combination of disproportionation and electrolysis proposed by Loutfy et al. (1981b)

Sportel and Verstraten (2003) (of Corus Al GmbH) produced Al₂S₃ from γ-Al₂O₃ using CS₂ gas at 850°C according to the following reactions (21) (22) The carbon disulphide is produced by reacting methane with sulphur gas through the following reactions (23) (24) The process is preferably performed at temperatures of 750–1100°C and pressures of 5×10⁵–35×10⁵ Pa. Typically temperature of 850°C and pressure of 30×10⁵ Pa are applied if solid Al₂S₃ is required (Sportel and Verstraten, 2003). This temperature was proposed to avoid transformation of γ-Al₂O₃ to α-Al₂O₃. It was shown that the reaction rate between γ-Al₂O₃ and CS_2(g) is higher compared to that of reaction between α-Al₂O₃ and CS_2(g). It was proposed that the resulting Al₂S₃ is to be electrolysed to extract the aluminium. This route (carbosulphidation followed by electrolysis) is the basis of the Corus – Compact Aluminium Production Process (CAPP). The proposed flowsheet for this process is shown in Fig. 5.

Figure 5

Flowsheet of compact aluminium production process (CAPP) (Sportel and Verstraten, 2003)

Xiao et al. (2004) studied the kinetics of the sulphidation of alumina with CS₂ gas at ambient pressure. They suggested that the optimum temperature of 850°C for sulphidation of γ-Al₂O₃. In the experimental conditions studied, they reported that the particle size of γ-Al₂O₃ has no effect on the reaction rate. A maximum conversion ratio of ∼40% after 5 h reaction at the conditions studied was reported.

Li et al. (2006) investigated the possibility of direct formation of Al monosulphide (AlS) by reacting alumina–carbon mixture with FeS according to the following reaction (25) They suggested the above reaction can be carried out at temperatures as low as 407–597°C at pressure range of 15–300 Pa.

Previous major works on the production of Al from alumina (or aluminous ores) through carbosulphidation reactions followed by various Al extraction process (stages 1 and 2) are summarised in Table 2.

Table 2

Summary of previous major works/patents in sulphide route

Year	Author	Reactions and process parameters	Process details
2007	Xiao et al.	Al₂S₃ = 2Al+1·5S_2(g)	The electrochemical behaviour of Al₂S₃ in molten salt on a laboratory scale. Temperature and cryolite addition have a positive effect on the current density (2nd stage)
		Electrolytes: MgCl₂–NaCl–KCl
		Temperature: 700°C
		Yield: 42%, current efficiency: 62%
		Energy required: 13·4 kWh kg⁻¹
2006	van der Plas et al.^p (Corus Aluminium)	Temp: 800–900°C	Continuous production of Al from alumina including a first step of converting alumina into Al sulphide (Al₂S₃) and a second step of separation of Al from Al sulphide in a separating reactor. In the first step, alumina is dissolved in a molten salt. Sulfur containing gas is fed through the melt. It is proposed that Al extraction is carried through electrolysis in multipole cell (1st and 2nd stages)
		Pressure: ≥3×10⁵ Pa
		Salt mixture: KCl–NaCl–10 wt-% cryolite
		Sulphidation gas: mainly CS₂
2004	Xiao et al.	CH_4(g)+2S_2(g) = CS_2(g)+2H₂S_(g)	Investigation of sulphidation kinetics of Al₂O₃ with CS₂ gas. The alumina with a higher specific surface area results in a higher sulphidation ratio. The particle size of γ-Al₂O₃ has no effect on the reaction ratio (1st and 2nd stages)
		Al₂O_3(γ)+3CS_2(g) = Al₂S₃+3CO_(g)+1·5S_2(g)
		Al₂S₃ = 2Al+1·5S_2(g)
		Sulphidation T = 850°C; yield: 40%
2004	Van Der Plas^p (Corus Technology BV)	Cathode: Al³⁺+3e⁻ = Al	Production of Al by electrolysis of Al₂S₃ in multipolar cell, using a molten chloride salt bath. The sulphur gas was collected and recycled to produce CS₂ which was used in the sulphidation step. MgCl₂ is added to the bath to increase solubility of Al₂O₃. Cryolite may also be added (2nd stage)
		Anode: 2S₂₋ = S_2(g)+4e⁻
		Overall: Al₂S₃ = 2Al+1·5S_2(g)
		Electrolytes: MgCl₂–NaCl–KCl
		T = 700–800°C; current efficiency: 80%
2003	Sportel et al.^p (Corus Aluminium)	3H₂S+1·5O₂ = 3S+3H₂O	Production of primary Al from alumina comprising the step of converting alumina into Al sulphide (Al₂S₃) and subsequently Al is separated from Al sulphide. γ-alumina is preferred for the reaction due to better kinetics. Three reactors were proposed: for manufacturing CS₂, for manufacturing Al₂S₃ from Al₂O₃ and CS₂ and electrolysis cell for Al extraction (1st stage)
		CH₄+2S₂ = CS₂+2H₂S (T = 550–650°C)
		2Al₂O₃+6CS₂ = 2Al₂S₃+6CO+3S₂
		Al₂O₃+3CS₂ = Al₂S₃+3COS
		T = 750–1000°C; P = 1×10⁵⁻40×10⁵ Pa
1984	Minh et al.^p (Dept of Energy of USA)	MgCl_2(soln)+Al₂S_3(s) = 2AlSCl_(soln)+MgS_(s)	Production of Al through electrolysis of Al₂S₃ utilising an electrolytic bath containing alkali metal chloride and/or alkaline earth metal chloride to provide improved operating characteristics of the process (2nd stage)
		Temperature: 700–800°C; voltage: <2–3 V
		Conc. Al₂S₃ in bath 2–10 mol.-%
		Electrolytes: (20–70 mol.-%)MgCl₂–NaCl–KCl
		MgCl₂–NaCl–KCl–(1–10 mol.-%)AlCl₃
		Current efficiency: 75–85%
1981	Loutfy et al.^p (Dept of Energy of USA)	Al₂O₃+3C+3S = Al₂S₃+3CO; T = 1027–1227°C	Al ore is reacted with carbon and sulphur containing gas at elevated temperatures forming molten Al sulphide which is decomposed to Al subsulphide (AlS). Then Al monosulphide is cooled to below its disproportionation temperature to produce Al metal. Blended aluminous ores and coke were used (1st and 2nd stages)
		Al₂S₃ = 2AlS+S; T = 1327–1627°C
		3AlS = Al₂S₃+Al; T = 927–1097°C	Note: current thermodynamic analyses indicate that the proposed reactions of the 2nd stage of the process do not occur at the conditions specified
1958	Weiss^p (Vereinigte Aluminium-Werke AG)	2Al₂O_3(s)+Al₂S₃(s)+6C_(s) = 3Al₂S_(g)+3CO_(g)	Production of Al from aluminous ores by reactions of a mixture of aluminous ores, Al₂S₃ and carbon. The reaction produce vapours of Al subsulphide which upon cooling produce Al sulphide and Al precipitate (1st and 2nd stages)
		3Al₂S_(g) = Al₂S_3(g)+4Al_(s)
		T = 1000–1200°C at P<5 mmHg
		T>2000°C at P = 0·1 MPa
1931	Haglund^p	N/A	Production of mixture of Al₂O₃ and Al₂S₃ by reaction between Al₂O₃ containing materials, reduction agents (carbon) and sulphur sources (FeS, ZnS) (1st stage)

^*^p refers to patent; 1st stage refers to production of intermediate Al sulphides; 2nd stage refers to extraction of Al from the sulphides.

Extraction of Al from Al sulphides

The literature review on the production of Al through carbosulphidation route revealed that the methods proposed to extract Al from its sulphides are disproportionation and electrolysis.

Disproportionation of Al subsulphides

The basis of Al extraction by dispropotionation is through dissociation of Al subsulphides to Al sulphide (Al₃S₂) and Al upon cooling (e.g. at temperature below the disproportionation temperature of associated subsulphides). Weiss (1958) suggested the disproportionation of Al₂S_(g) subsulphide as per reaction given in equation (17) at pressure 5 mmHg and temperature of 1200°C. Loutfy et al. (1981c) proposed the extraction of liquid Al through disproportionation of AlS_(l) subsulphide at temperatures between 927 and 1097°C following reaction in equation (20). Loutfy et al. (1981c) did not provide information on the pressure for the process. Thermodynamic analysis carried out by Dewan et al. (2012) showed that the reaction in equation (20) does not occur at ambient pressure. Although mentioned in a number of patents (Weiss, 1958; Loutfy et al., 1981c), there is a limited published work on the details of thermodynamics and kinetics of the disproportionation reactions of Al subsulphides. Further studies on the thermodynamics and kinetics of Al subsulphide disproportionation are needed for optimisation of the process.

Electrolysis of Al sulphide

The extraction of Al metal by electrolysis of Al sulphide in molten salts is attractive from the viewpoint of energy utilisation. By improving the cell design and electrolyte composition, the theoretical energy consumption can be reduced to 8·41 kWh kg⁻¹ Al which is considerably lower than the value of 14 kWh kg⁻¹ Al in the current Hall–Heroult process (Xiao et al. 2007). Al sulphide (Al₂S_3(s)) has a lower theoretical decomposition potential, compared to Al₂O_3(s) and AlCl_3(l) (Minh et al. 1982), and can be electrolytically decomposed in a molten cryolite at 727–927°C (Loutfy et al. 1981b) to produce molten Al and sulphur gas that can be recycled for CS₂ production.

The early works on the electrolysis of Al₂S₃ in a mixture of cryolite with NaCl were from German and Russian scientists (in 1930s and early 1940s). In these studies, a maximum current efficiency of 55% was reported (Rontgen and Borchers, 1933; Khazanov and Belyaev, 1935; Khazanov and Komarov, 1940). The overall reaction for electrolysis of Al₂S₃ in molten electrolyte follows (26) In the early 1980s, Minh et al. (1982a, b) electrolysed Al₂S₃ to extract Al in an electrolytic bath containing alkali metal chloride and alkaline earth metal chlorides (MgCl₂–NaCl–KCl and MgCl₂–NaCl–KCl–AlCl₃). They carried out the process at 750°C and found out that the limiting current density for the process was at the graphite anode. Within the current density range investigated (0·2–1·2 A cm⁻²), current efficiencies of 75–85% were claimed. Minh et al. (1984) further patented the work in 1984. In this work, it was suggested that the process be carried out at 700–800°C, with MgCl₂ concentrations of 20–70 mol.-% with a balance of NaCl, KCl and AlCl₃. The solubility of the Al₂S₃ is ∼3 wt-% at 750°C, but could be enhanced by the presence of MgCl₂ and AlCl₃ according to the following reactions (27) (28) In the 2000s, Corus developed a process for Al extraction through electrolysis of Al₂S₃, as part of development of the CAPP process. Lans (Lans et al., 2003; Xiao et al., 2003) electrolysed Al₂S₃ and obtained Al melt and sulphur gases at the anode according to reaction given in equation (26). A molten electrolyte of MgCl₂–NaCl–KCl (50∶30∶20, mol/mol/mol) was used and cryolite (∼10 wt-%) was added to enhance the solubility of Al₂S₃ and increase the activities of Al and S in the melt. This resulted in an increase in the current density up to three times. This work was the basis of the Corus patent (van der Plas, 2004). Further studies were carried out by Xiao et al. (2007) to investigate in detail the mechanism of the process. It was concluded that the electrolytic process was governed by its ohmic drop. The production of sulphur bubbles at the anode was suggested to significantly affect the ohmic drop. Addition of cryolite changed the characteristic of the sulphur bubbles layer thus decreasing the ohmic drop. Increased Al₂S₃ solubility due to the cryolite addition resulted in a higher limiting current density. It was reported that a current efficiency of 62% can be achieved.

In 2006, Corus (van der Plas and Xiao, 2006) developed a continuous production process of Al from alumina. In the first stage, alumina was dissolved in a molten salt and a sulphur containing gas (in particular CS₂) was fed through the melt; this resulted in a partial conversion of alumina to Al sulphide. Al was then separated from Al sulphide in a multipole electrolysis cell. The process was carried out at temperatures 800–900°C, at pressures above 3×10⁵ Pa in a KCl–10 wt-%NaCl cryolite mixture.

Carbonitridation route

Selvaduray and Sheet (1993) and Haussonne (1995) provided a review on various methods for the synthesis of Al nitride. In general the methods can be classified into: carbonitridation reactions, direct nitridation, floating nitridation, chemical vapour deposition, vapour phase reactions and reactions utilising organometallic precursors (Selvaduray and Sheet, 1993). Only the carbonitridation route will be described in this paper, as it is the route most relevant to industrial scale production of metal.

Carbonitridation of alumina and aluminous ores is a well known process. It follows an overall reaction of (29) Rather than for Al production, the original aim of the process was to produce Al nitride or to purify alumina from its ores. In the later case, for example in the Serpek (1907) process, a mixture of bauxite and coke is heated in an electric furnace in the presence of nitrogen to produce Al nitride. Al nitride can then be hydrolysed through reaction with water to form pure alumina and ammonia. The reaction can be carried out even at ambient temperature (Li et al., 2006; Highfield and Bowen, 1989) but at slow reaction rates (Galvez et al., 2009).

Considerable disagreement exists in the literatures concerning the detailed reaction steps for nitride formation. Hirai et al. (1989) synthesised AlN_(s) from Al₂O_3(s) and graphite at temperatures between 1500 and 1700°C. They observed a small quantity of Al oxynitride (AlON), when alumina graphite mixture is heated at 1700°C. Their results indicated that the reaction rate is not affected by the grain size of graphite, pellet diameter and flow rate of N₂; and they suggested that the diffusion of the reactant gas through the AlN_(s) layer (formed around Al₂O_3(s)) is the rate determining step. Lefort and Billy (1993) argued that the rate determining step suggested by Hirai et al. is unlikely as the apparent activation energy (∼530 kJ mol⁻¹) is beyond what would be expected from a limiting gaseous diffusion. They suggested that the rate limiting process is the combustion of carbon and suggested the following reaction steps (30) (31) (32) The dissociation of alumina was suggested to be controlled by the very low partial pressure of oxygen in contact with carbon, and followed by fast nitridation of the Al vapour formed. Lefort and Billy (1993) recommended temperatures 1550–1600°C for the process. Chen and Lin (1994a), studied a carbonitridation of alumina at temperatures 1375–1552°C and developed a kinetic model for the process. A mixed mechanism was proposed comprising reaction between Al₂O_3(s) and C_(s) to form Al₂O_(g) and CO_(g), followed by surface reaction between Al₂O_(g) and N_2(g) to form AlN_(s) (Chen and Lin, 1994a, b). They suggested that the formation of Al₂O_(g) is the rate limiting step according to (33) Al monocyanide, AlCN_(s), can also form as described by Perieres and Bollack (1961) according to (34) A preferred temperature of 1700°C was recommended for nitride preparation as the formation of AlCN_(g) increases above 1700°C, which explains the apparent decrease in nitride formation and the presence of Al carbide in the condensates (Perieres and Bollack, 1961). Al monocyanide vapour can react with Al vapour forming solid encrustations of carbide and nitride.

Ide et al. (1999) produced AlN_(s) through carbonitridation of alumina at temperatures 1350–1450°C using CaF_2(s) as catalyst. They reported that nitridation rate tended to increase with decreasing particle size of alumina and was affected by the history of the alumina used in the reaction. They observed the formation of intermediate compounds CaO.6Al₂O₃ (CA₆) and CaO.2Al₂O₃ (CA₂) and suggested that the process proceeded through the nitridation of the intermediate compounds from liquid phase system CaF₂–CA₆–CA₂. Molisani and Yoshimura (2010) investigated the effect of various additives (0·5–3 wt-%CaF₂, Y₂O₃, Li₂CO₃ and SrCO₃) on the carbonitridation of alumina at 1300–1400°C. The addition of these additives (and their mixture combinations) was reported to reduce the synthesis temperature by a maximum of 200°C. They attributed this to the formation of aluminate phases that easy to vapourise at lower temperatures.

Bartnitskaya et al. (2002) studied the carbonitridation reaction at temperatures 1800–1900°C in a static condition (closed system) under nitrogen atmosphere of 0·2–0·3 MPa. They obtained isometric AlN particles of 2–6 μm as opposed to fibrous type particles usually obtained in reaction carried out in a stream of nitrogen gas. They suggested that in the case of reaction at a high nitrogen pressure, alumina dissociation is suppressed which excludes the possibility of reaction in the gas phase, hence the formation of isometric particles. Chowdhury et al. (2006) synthesised nanosize AlN_(s) by nitridation of C–Al₂O₃ composite particles at 1500–1600°C in an over pressure (0·4 MPa) flowing nitrogen gas. A mixture of fibrous and spherical particles was obtained.

Joo and Jung (2008) investigated the effect of CO content in the N₂–CO gas mixture for carbonitridation reaction from 1000 to 1600°C. They used Al hydroxosuccinate as precursor for the reaction. The carbonitridation reaction was found to be retarded with increasing content of CO in the mixed gas. They observed conversion sequence from ρ-Al₂O₃ to γ-Al₂O₃ to AlN_(s) (or to δ-Al₂O₃) depending on the CO content in the gas mixture. Qin et al. (2008) prepared C–Al₂O₃ composite particles from Al(NO₃)₃.9H₂O, CO(NH₂)₂ and C₆H₁₂O₆.H₂O and carried out the nitridation at 1000–1600°C in a flowing nitrogen gas. They reported a conversion to γ-Al₂O₃ followed by direct conversion to AlN_(s) upon nitridation. Complete conversion was observed at 1400°C.

Galvez et al. (2009) produced AlN_(s) using concentrated thermal radiation through reduction of Al₂O₃ using activated C (and CH₄) in flowing nitrogen. The radiative fluxes used were equivalent to solar concentration exceeding 4500 kW m⁻² and the reaction was carried out at 1827–2027°C. In the case of carbothermal–nitridation reaction (reaction given in equation (28)), the experimental data were fitted into a solid–solid reactions model with an apparent activation energy of 360 kJ mol⁻¹. In the case of reduction using CH₄ (methanothermal reduction), the overall reaction follows (35) In this case, a low conversion (∼10%) was observed. It was suggested that the diffusion is hindered by a carbon layer produced from the decomposition of CH₄. The kinetics, however, may be enhanced by doping the alumina with metallic particles (such as Ni and Fe). Best results were obtained when Ni is used, where it acts as nucleation site for carbon filament growth.

Galvez et al. (2007) also investigated the carbothermal reduction of alumina in nitrogen atmosphere at temperatures between 1500 and 1700°C. The kinetics were described as solid–solid reactions and fitted to Jander and Ginstling-Brounshtein models with activation energies of 815 and 757 kJ mol⁻¹ respectively. They also evaluated various types of carbon sources and reported that the reaction rates vary for different carbons (listed in decreasing order of reaction rates: petcoke, activated carbon, wood charcoal and carbon black) (Galvez et al., 2008).

Baik et al. (1994) and Kuang et al. (2003) produced Al nitride through carbonitridation using sucrose precursors. Zhang and Gao (2006) produced nanocrystalline Al nitride from δ-Al₂O₃ nanoparticles in flowing ammonia. They reported that the nanocrystalline δ-Al₂O₃ was converted into AlN_(s) completely at temperatures 1350–1400°C within 5 h in a single step synthesis process. Xi et al. (2008) synthesised AlN_(s) by carbothermal reduction using a mechanically activated Al₂O₃. They showed that carbonitridation can be carried out at temperature as low as 1100°C and completed thoroughly at 1250°C when milled Al₂O₃ (for 20 h) was used.

There are two primary methods to prepare AlN powders at an industrial scale (Selvaduray and Sheet, 1993): direct nitridation of Al with N₂ (2Al+N_2 =2AlN), and carbothermal reduction of Al₂O₃ with carbon black (or other carbonaceous sources) in the presence of N_2(g) at 1500–2000°C (reaction in equation (29)). The former is beyond the scope of this paper. In terms of technological developments, a number of Al nitride production processes by carbonitridation in nitrogen or mixed gas atmosphere have been patented. A summary of these studies is presented in Table 3.

Table 3

Summary of previous major works/patents in nitride route

Year	Author	Reactions and parameters	Process details
2008	Joo and Jung	Al₂O₃+3C+N₂ = 2AlN+CO	The carbothermal reduction and nitridation (CRN) of alumina to Al nitride was investigated in a gas environment consisting of N₂ and CO. The CRN reaction of alumina is slowing with increasing content of CO in the mixed gas
2008	Joo and Jung	Temperature: 800–1600°C
1998	Kotaka et al.^p (Toshiba Ceramics Co.)	Calcination of γ-Al₂O₃: 300–1100°C	A method of manufacturing Al nitride by calcining γ-Al₂O₃ in ammonia and hydrocarbon gas to obtain Al nitride having both carbon content and an oxygen content of 1 wt-% of less
		Nitridation temperature: 1200–1700°C
		Gas: mixtures of NH₃ and hydrocarbons
		Purity: 99%
1997	Ravenel et al.^p (ELF Atochem SA)	Al₂O₃+3C+N₂ = 2AlN+CO	A continuous process for production of Al nitride by the carbonitriding process of alumina
		Temperature: 1450–1500°C
		Time: 12 h
		Residual carbon: <700 ppm
		Oxygen content: <1·1%
		Flowrate of granule: 4·4 kg h⁻¹
		Flowrate of N₂: 12 kg h⁻¹
1996	Ravenel et al.^p (ELF Atochem SA)	Al₂O₃+3C+N₂ = 2AlN+CO	Continuous production of Al nitride by carbonitriding of alumina in a moving bed reactor reaction zone comprising at least one conduit
		Exchange surface area/volume: 5–50 m⁻¹
		C/Al₂O_3 =3
		Granules feedrate: 5 kg h⁻¹
		N₂ feedrate: 20 kg h⁻¹
		Yield: 100%
		Time: 12 h
		Temperature: 1350–2000°C
1993	Dorn et al.^p (Hoechst)	Calcination of mixture at 400–1000°C	Production of fine Al nitride by reaction between Al hydroxide with carbon in the presence of flowing nitrogen
1993	Dorn et al.^p (Hoechst)	Nitridation at 1400–1700°C (1–100 h)
1992	Dunn et al.^p (Dow Chemical Co)	Al₂O₃+3C+N₂ = 2AlN+3CO	Purification of Al nitride produced by carbothermal reaction of Al oxide, carbon and nitrogen. Significant amount of the surface portions are removed by mechanical agitation or mechanical abrasion to leave behind Al nitride pellets having significantly increased purity
		Temperature: 1650–1750°C
		Time: 90 min
		Frequency: 1100 vibration min⁻¹
		Yield: 97 wt-%
1991	Nakano et al.^p (Sumitomo Chemical Co Ltd)	Al₂O₃+3C+N₂ = 2AlN+3CO	Production of Al nitride by firing and reacting alumina and carbon mixture in a nitrogen containing atmosphere
		Temperature: 1500–1700°C
		Time: 2–10 h, starting materials <5 μm
		O, <2 wt-%
		Fe, <20 ppm
1989	Bolt^p (Du Pont)	Al₂O₃+3C+N₂ = 2AlN+3CO	Production of Al nitride fibers/films through carbontiridation of precursor fibers/films containing source of alumina and carbon
1989	Bolt^p (Du Pont)	T nitridation = 1550–1800°C
1986	Kuramoto et al.^p (Tokuyama)	Al₂O₃+3C+N₂ (NH₃) = 2AlN+3CO	Production of Al nitride fine powder (not more than 2 μm) by firing alumina carbon mixture after optionally drying it
		Temperature: 1400–1700°C
		Yield: 94 wt-%
1963	Paris and Perieres^p (Pechiney)	Al₂O₃+3C+N₂ = 2AlN+3CO	Manufacturing of Al nitride from the Al ores reacting with nitrogen and a reducing agent (hydrocarbon), which is relatively free of impurities and unreacted components
		nAl₂O₃+3C_nH_2n+2+nN₂ = 2nAlN+3nCO+3(n+1) H₂
		CH₄+½O₂₊2N_2 =CO+2N₂+2H₂
		Al₂O₃+3CH₄+N₂ = 2AlN+3CO+6H₂
		Temperature: 1200–1750°C
1962	Clair^p (Pechiney)	T = 1750°C	Development of reactors for continuous production of Al nitride. Counter current reactor, vertical shaft. A mixture of alumina, carbon and calcium aluminate binder
1961	Perieres and Bollack^p (Pechiney)	Al₂O₃+3C+N₂ = 2AlN+3CO	A process for making Al nitride having a degree of purity above 97%, particularly free from any substantially amounts of alumina or carbon
		AlN+C = AlCN
		Yield: 99%
		Temperature: 1700°C (nitridation)
		700–800°C (roasting)
		Time: 2 h
		Nitrogen flowrate: 100 L min⁻¹
1918	Shoeld^p (Armour Fert. Works)	Al₂O₃+3C+N₂ = 2AlN+3CO	Production of Al nitride from comminuted carbon, alumina, and a binder mixture, and a nitrogen containing gas travelling in opposite directions by passing a heating current of electricity
		Temperature: 1800–2000°C
		Time: 3–4 h

^*^p refers to patent.

One of the first patents in the carbonitridation of alumina is from Shoeld in 1918 where he carried out the reaction at 1800–2000°C. In the 1960s, Pechiney (Perieres and Bollack, 1961; Clair, 1962; Paris et al., 1986) patented various carbonitridation processes. Perieres and Bollack (1961) produced AlN powder to 99% by roasting the produced AlN_(s) from carbonitridation reaction in an oxygen free atmosphere at 700–800°C to burn off the excess carbon. Clair (1962) developed a vertical shaft counter current reactor for continuous production of Al nitride, while Paris and Perieres (1986) used a mixed environment of nitrogen and hydrocarbon gas for carbonitridation of alumina at 1200–1700°C. The hydrocarbon gas was cracked to elemental carbon in a highly reactive state in uniform distribution throughout the Al oxide particle.

Kuramoto et al. (1986) produced AlN powder from alumina and carbon by firing the mixture at 1400–1700°C in nitrogen (or ammonia) atmosphere followed by heating at 600–900°C to improve the purity to 94%. Bolt (1989) patented a method to produce Al nitride films (and fibres) using precursors containing alumina and carbon.

In the 1990s, a number of AlN_(s) production processes were patented by various companies. Nakano et al. (1991) patented a process using very fine source materials (<5 μm). Dunn et al. (1992) increased the purity of AlN_(s) produced by carbonitridation by applying mechanical agitation. Most of the impurities remain in the outer zone of the pellet and a significant amount of the surface portion is removed to improve the purity of the final Al nitride product. Dorn et al. (1996) produced AlN_(s) through calcinations of Al hydroxide–carbon mixtures at temperatures from 400 to 1000°C followed by nitridation from 1400 to 1700°C. Ravenel et al. (1996, 1997) patented a process for continuous production of Al nitride from 1350 to 2000°C using a moving bed reactor. Kotaka et al. (1998) patented a process to produce AlN_(s) from γ-Al₂O₃ using mixtures of ammonia and hydrocarbons from 1200 to 1700°C and claimed to obtain purity of 99%.

Extraction of Al from Al nitride

One of the ways to extract aluminum from aluminum nitride is through thermal dissociation. The thermal dissociation of Al nitride occurs at a temperature above 2400°C at 0·1 MPa according to reaction (36) The temperature of the dissociation can be reduced by operating the process at lower pressures or in a vacuum condition. Thermodynamic calculations (Dewan et al., 2012) showed that the dissociation temperature can be reduced to ∼1700°C if the pressure is reduced to 0·1 kPa.

The theoretical decomposition voltage of Al nitride to Al and nitrogen is 0·75 V at 700°C. This is a lower decomposition voltage than that of AlCl_3(l), Al₂S_3(s) and Al₂O_3(s) at the same temperature. From the thermodynamic point of view, electrolysis has a good potential for extracting aluminum from Al nitride. One of the major challenges is to find the appropriate electrolytes that can dissolve the very stable AlN_(s). Bonomi et al. (1982) carried out a limited study on the solubility and the electrolysis of AlN in molten salt (Li₃N–LiCl) bath at 660–700°C. The current efficiency for this system was 83% with a cathodic current density of ∼1·5 A cm². No further results indicating the successful of the process have been presented. Goto et al. (2005) used the salt system of LiCl–KCl–Li₃N to deposit AlN film onto Al substrate. Figure 6 shows the E-pN³⁻ diagram for the Al–N system in a LiCl–KCl eutectic melt at 450°C showing the region where Al solid is stable. Yan (2008) conducted a direct electrochemical reduction of AlN cathode in a CaCl₂–NaCl melt at 1133 K and observed a pure Al droplet. The yield, however, was low, i.e. 3–5%.

Figure 6

Potential-pN³⁻ diagram for Al–N system of LiCl–KCl eutectic melt in presence of 10⁻³ cation fraction of Al³⁺ at 450°C (Goto et al., 2005).

Conclusion

Many researchers have pursued an alternative commercial method for Al production. Although carbothermal reduction of alumina or aluminous ores (direct or indirect) offers the potential for lower energy consumption and improved productivity, compared to existing process, to date, none of the proposed processes have been successfully commercialised in a plant scale. In the case of direct carbothermal reduction of alumina/aluminous ores, a mixture of Al carbide and metallic Al is formed. Equilibrium studies show that the driving forces for both Al carbide and Al metal formation by carbothermic reduction of alumina are similar, therefore it is difficult to obtain high yield of pure Al. This route has other difficulties, including a high operating temperature and yield problems associated with Al vapour loss and back reactions with carbon monoxide forming Al oxides and carbides. The recent technology in this area has shifted to a multi stage process, the Alcoa-Elkem process, where a slag of Al₂O₃ –Al₄C₃ is first produced followed by extraction of Al. Alcoa continued the development of the carbothermal process on its own, particularly on the improvement of the off-gas cooling system. They reported a successful test campaigns that run for several weeks, with each tap generates several hundred kilograms of metal with total yield in tonnage scale.

In the case of multistage production of aluminum through the formation of intermediate compounds; there had been intense developments of the chloride route in the 1960s and throughout the 1970s with major players including Alcoa and Toth Al Corporation. At one stage, the Alcoa chloride process (carbohclorination followed by electrolysis) was used to produce Al commercially. This, however, was halted in the mid-1980s due to problems associated with production and handling of pure Al chloride and chlorine at high temperatures.

In the sulphide route, Al₂S_3(l) can be produced by carbosulphidation of alumina or aluminous ores. Al metal can be extracted from Al₂S_3(l) by disproportionation and electrolysis processes. In the 2000s, Corus developed a process for making Al through carbosulphidation followed by electrolysis. This, however, is not yet commercialised.

There is also the nitride route where AlN_(s) is produced from alumina–graphite mixture reacted at 1700°C in nitrogen containing gas. One of the major challenges is to find the economical route to extract Al from AlN_(s). The nitride is a very stable phase but can be dissociated to Al metal at >2400°C at 0·1 MPa pressure. The theoretical decomposition voltage of Al nitride to Al and nitrogen is 0·75 V at 700°C. It is the lowest compared to the decomposition voltage of AlCl_3(l), Al₂S_3(s) and Al₂O_3(s) at the same temperature. Therefore, electrolysis has potential to be used for extracting aluminum from Al nitride. However, to date no appropriate electrolytes are available to dissolve the very stable AlN.

In summary, direct carbothermic reduction has yet to be commercialised because of problems with extreme operating conditions and yield, while, two stage indirect carbothermic reduction using Cl, S and N sources to form intermediates have been investigated but still require significant developments. No systematic thermodynamic examination of all the options has been published in the literature.

Footnotes

Acknowledgements

The current study is funded by the ‘CSIRO – Breakthrough Technologies for Primary Aluminium’ Research Cluster.

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