Bulletin of Eledrochemistry 1 (5) Sep.-Od. 1985, pp. 483488 A CRITICAL APPRAISAL AND REVIEW OF ALUMINIUM CHLORIDE ELECTROLYSIS FOR THE PRODUCTION OF ALUMINIUM C N KANNAN and PS DESIKAN Central Electrochemical Research Institute, Karaikudi - 623 006, ABSTRACT This paper is an attempt to examine the current state of art of aluminium chloride electrolysis through a review of all the available published literature and patents so that this could help the formulation of the plans of work for any serious R&D effort to develop the chloride te&nolcgy in this country. Even though the development of technology for aluminium chloride electrolysis is being carried out in a big way by ALCOA and a few other multinational companies for the past several years, many of the data and information are lacking in published literature and the answers to various critical questions have to be found only through inferences from the meagre information available in patents. It was therefore thought fit to undertake a thorough review of all the basic applied and R&Dwork that are reported in this field and critically assess the various problems to be tackled to evolve a viable technology. This review confines itself to the electrolytic aspect and those relating to the material preparation will be taken up separately later. Key words : Production of aluminium. Molten salt electrolysis, chlorination of alumina INTRODUCTION The most obvious advantages of the aluminium chloride smelting are 131 : he need to develop a new technology for aluminium metal production 1. Substantially lower working temperature (700°C) compared to Hall- T has been felt very much in the major aluminium producing countries Heroult cell (980°C). and considerable amount of time and money were spent for the research and development efforts in this line. 2. Relatively higher current densities could be applied since the critical current density for the anode effect is fairly high. By this the throughputs per The Bayer-Hall-Hemult process established about 100 years ago, did cell can be considerably increased thereby reducing the capital costs. not face much competition in the past because of its simplicity and easier technical and practical operations involved even at large industrial scale. Of 3. It does not require a consumable carbonaceous anode which for Hall- late the Hall-Heroult process is also being subjected to various Heroult's process accounts for more than 7 % of the total cost impmvements so that the energy efficiency and other operating parameters are improved. Use of refractory metal cathodes like titanium diboride or 4. Added freedom one can get in the choice of raw materials. hwcost non- zirconium diboride, inert oxide anodes, modified bath employing different bauxite ores can also be beneficially used. cryolite ratios, improvement in cell linings etc. are some onhe fields where 5. No environmental pollution is involved. much attention is given and considerable research is going on. 6. Chloride process would be the most energy efficient because it is operated Despite these improvements, the other factors such as the low energy at the lowest temperature. efficiency, high capital and operating costs involved, depletion in bauxite and petroleum coke reserves in certain countries and serious environment 7. Near theoretical power efficiencies are possible in the advanced chloride- problems like fluorine emission from the cell pots etc. have led the producers bipolar cells thereby providing direct saving in energy costs. to think towards the development of an alternate technology for aluminium metal production. 8. The chloride cells can survive power interruptions more easily than the Hall-Heroult cells. This is possible because the chloride cell with its high Prominent alternate proceuer efficiency causes little excess heat generation and hence the cell is well insulated. In addition the chloride cell has a much lower temperature liquid Of the various processes thought of, the following are considered to be the range for the electrolyte than the fluoride cells. most significant and leading producers of aluminium metal like the ALCOA, ALCAN, Nippon Light Metal etc. have shown considerable interest. 9. Chloride electrolysis provides metal of superior purity. Undenirable contamination of sodium as found in Hall-Heroult metal are greatly reduced 1. Pechiney-Alcoa process (2) Alcan process or sub-halide process (3) Toth in this system. process and (4) chloride electrolysis. Details of these processes have been dealt with elsewhere in literature by various authors [l-41. There are certain disadvantages also in the chloride electrolysis. The chlorination process is an extra step in the process, while Bayer alumina is Out of the four processes mentioned above, the last mentioned one is the starting material in the Hall's cell. The aluminium chloride and its considered to be the best and attracted the eyes of aluminium producers very compounds are highly corrosive to many construction materials. The high much. An attempt is made to describe the salient features of this process in volatility of the electiolyte also pones problems for the recovery of the following pages. aluminium chloride from the fumes. Possibilities are there for the formation of phosgene and other poisonous gases during the chlorination of alumina Kannan and Desikan - Aluminium by aluminium chloride electrolysis The decomposition potential of aluminium chloride with an inert anode the cathodes. With carbon electrode, the reduction of chloro aluminate is 1.8 Vat 7W°C compared to 1.2 Vat 970°C for the aluminium oxide with occurred simultaneously with the reduction of alkali metal ion and also of a consumable carbon anode. However, the operating cell voltage for the the formation of aluminium carbide (A14C3).At the other electrodes like chloride electrolysis is much lower (around 3 V) compared to Hall-Heroult silver, platinum, tantalum, titanium boride, etc. it was evident that there was cell (about 4.5 V). This is possible because of the lower polarisation voltage, the formation of alloy or compound. Chronopotentiometric study revealed iR and electronic voltage drops in the case of chloride electrolysis. The that the chloroaluminate reduction corresponds to the following equation chloride electrolysis claims less energy consumption to the extent of at least 30% especially when the bipolar cells are operated. A brief comparison of the chloride process with Hall-Heroult process is Under certain conditions, the reaction is found to be reversible. given in Table I 151. Studies repding anodic overvoltages of chlorine liberation at carbon Table I : Comparison of chloride process-with best present Bayer-Hall- electrodes in melts containing aluminium chloride had been found to be Heroult processes helpful, to calculate energy and heat balances 171. The study on reaction mechanism at carbon electrode helps to understand the nature of Best present Future improved ALCOA interactions between the electrode products and anode material and to Bayer-Hall- Bayer-Hall cell (*) (AICI, foresee and prevent the oxidation and destruction of electrodes and so also to Heroul t Heroult electrolysis) reduce the anodic overvoltage. The anodic overvoltage is found to be mainly dependent on aluminium chloride content in the melt. 1. Fixed capital invest- References are cited in literature regarding electrochemical studies mentlannual short on certain low temperature molten salt systems containing aluminium ton Al $ 1750 1750 1450 chloride which undergoes a series of Lewis acid base reactions with other 2. Electrical energy chlorides [8]. They form low melting chloro-aluminates with alkali kWh/kgofAl 14.3t015.4 11-12.1 9.9 chlorides. The most important equilibria in the melts can be represented 3. Carbon kg/kgofAl 0.44-0.5 0.44-0.5 0.36-0.4 by the equation : 4. Direct operating costs per kg of Al 35-44 33-40 30-40 (Raw materials and The sodium chloride containing aluminium chloride melts at 175OC, the energy only) equilibrium constant being 1.06~ Some metal chloride6 dissolvein the melt enabling codeposition to form alloys of aluminium. (*) Bipolar cell -includes Bayer process for chlorination step (Alcoa) -- Electrolysis of aluminium chloride has been carried out by several Electrochemirtry of aluminium chloride electrolyrir research workers employing different salt systems and operating parameters [4,9,10,1 I, 121 .Operating temperatures rangin from 1.50" C to 760°C and The electrolysis of aluminium chloride is not a new idea In fact it can be current densities varying from 0.9 to 2.8A/cm 4 are reported. traced back to as early as 1854 much earlier to the discovery of Hall-Heroult process, when an electrolysis of fused NaAICI, was carried out to separate Factom influencing the ideal operation of electrolytic cell alu'minium metal. Several works in this line were continued even after the If an electrolytic cell has to be operated under ideal conditions many factors establishment of the Hall-Heroult process. Molten binary and ternary such as cell design, operating temperature, electrode materials, refractory mixtures containing LiCI, NaCI, KCI, MgC12 and BaClz in addition to AICI3 lining, bath composition and control etc. are to be carefully studied and the have also been studied and established as potential electrolytes. The cell best conditions are to be incorporated. reaction in aluminium chloride electrolysis is Cell demign Based on energy and production considerations on ideal electrolytic cell For this reaction at 7W°C A G = + 123.6 kilo calories and AH = 155.0 kilo should satisfy the following requirements 1131 : calories. Using these values the reversible decomposition voltage works out to be about 1.8 volts and the minimum enerm consumption of 6.7 kwWka. a) The cell should be operated at a voltage very near to the theoretical At an actual energy consumption of 10 kwhzg the energy efficiency wouG voltage. be 6716. b) The electrodes should be dimensionally stable and the designs of the Several researchers have studied the electrochemical reduction of same should facilitate minimum losses in current efficiency.