Technical Evaluation of Two Alternative Aluminium Production Processes

Abstract

The Hall Héroult process used for industrial production of primary aluminium metal is an energy intensive and an inherently energy inefficient process, which also contributes to a considerable amount of greenhouse gas emissions. The fundamental disadvantage of this process lies in the use of consumable carbon anodes for electrolysis of alumina within the reduction cells. As a result, over the years much effort has gone into research and development of alternative aluminium processes which utilize non consumable anodes to overcome many of the limitations with the Hall Héroult process. The first part of the work undertaken in this thesis includes the technical evaluation of two such promising routes for aluminium production. These are: 1. Alumina reduction utilizing metallic inert anodes and wettable cathodes arranged in vertical/slanted electrode configurations in a low temperature bath. 2. Production of aluminium via electrolysis of AlCl3 derived directly from bauxite (i.e. Bauxite chlorination route) utilizing efficient bipolar cells. The evaluation carried out, took into account seven fundamental aspects of each technology and was developed based on a scoring/ranking system which allowed calculation of a final weighted average score for each process route. The outcome of the evaluation resulted in an overall score of 0.62 (62%) for inert anode technology and an overall score of 0.63 (63%) for the bauxite chlorination route. However, these scores were considered to be very similar due to the likely uncertainty of the scoring system employed. Hence, both routes were considered to have very similar merits for further development. Nevertheless, the evaluation revealed two likely significant technical constraints for inert anode technology. These were: (a) unavailability of electrode materials and (b) uncertainty of the feasibility of low temperature operation. In contrast, only one significant technical constraint was apparent for the bauxite chlorination route which was the unavailability of a proven raw material production route. For further investigation, in the second part of this thesis, the bauxite chlorination route was selected as the most promising route based on the findings of the evaluation. In particular, the removal of FeCl3 from AlCl3 was selected as a focus for an experimental investigation as this is known to be a critical unknown which could be a technical limitation of the whole bauxite chlorination route, for the production of cell grade AlCl3. Hence, an experimental study of FeCl3 removal from AlCl3 was conducted based on a selective reduction technique utilizing iron as the reductant. Feasibility of removing FeCl3 from AlCl3 using this method was confirmed in this investigation by demonstrating that AlCl3 products with impurity levels well below 1% residual FeCl3 content can be achieved (from an original content of 10% FeCl3). The best removal of FeCl3 was achieved by taking advantage of the differences in volatilities of the FeCl3 and AlCl3 compounds coupled with selective reduction using iron. The highest purity AlCl3 condensate sample achieved in this investigation in a single trial consisted of a residual FeCl3 impurity level of 0.23%. Although cell grade purity was not achieved in this study, this work demonstrates the potential of this process to be improved, to achieve product purity levels of those approaching cell grade AlCl3. Furthermore, this process can be utilized as a refining stage in a prospective bauxite chlorination route which incorporates other methods of iron removal.