Why Carbothermic Smelting of Aluminium?
The Aluminium industry is facing increasing issues as its ability to achieve continuous improvement through production process efficiencies has gradually approached theoretical limits. Currently, the industry requires high levels of capital and energy intensive technology, especially for its smelting processes.
An investment in the aluminium industry needs a stable political and economic environment, given the level of capital and supporting infrastructure required for a current technology-based smelter. Meanwhile, steel and plastics are lower cost alternatives, which can effectively replace aluminium in various applications. In addition, the industry faces considerable environmental issues. Hence, Calsmelt argues that the aluminium industry needs to consider a radical smelting process change and therefore the commercial introduction of Thermical™ technology should be very timely.
In order to sustain long-term competitiveness across the aluminium industry, essential areas of research and industry-wide goals have been identfied and published in the “Alumina Technology Roadmap” (produced by AMIRA International in November 2001) and in the Aluminium Industry’s “Technology Roadmap” published by the Aluminum Association in February 2003. Both of these roadmaps emphasize the high priority gaps requiring further technology development for the industry. The two roadmaps report that continuous improvements through incremental changes, as well as significant advances, through innovative step change, should be considered.
Therefore, recent ongoing R&D around the world has included both activities for continuous improvement in electrolytic cell technology, as well as by more discontinuous approaches that attempt to replace that presently ubiquitous cell technology.
The focus of the work for cell technology improvement is primarily targeted towards the reduction of capital and the lowering of operational costs incrementally, although as pointed out above, technology improvements in the current electrolytic smelting process, especially, are trending towards a theoretical limit and therefore the value of further improvements is unlikely to be as significant as was achieved in the past.
To reduce capital cost, the industry’s focus has been on increasing the cell efficiency and its lifetime. During the last 20 years, breakthroughs have been achieved by changing cell and electrode designs, through increases in the cell current, and via the application of new materials. However, each modification has also introduced extra costs and has provided some new barriers to the process.
Theoretically, the possible further energy reduction for electrochemical cell technology is between 4% and 7% of total operating cost (i.e., a reduction is possible from the typical consumption value 14.5 kWh/kg to the ideal value of 9.5 kWh/kg).
Meanwhile, the target for carbon and fluoride reduction over the coming 20 years is about 2%. Thus, the total targeted cost reduction is around 9% over the next two decades. In Calsmelt’s view, these figures indicate that incremental improvement will not offer a sufficiently dramatic economic gain to protect the future of the industry.
Based on these data, a significant enough change in the efficiency and cost of current cell technology does not seem at all probable. The industry has recognized this situation and has expressed an interest in embracing a dramatic change and, if possible, a replacement for the present technology.
In the above mentioned technology roadmaps, pyrometallurgical options were suggested as an approach to offer a radical step change. In pyrometallurgy, there are technologies based on the carbothermic production route that have been shown to be promising. It has been estimated elsewhere that these technologies have the potential to reduce capital investment by around 50% and operational costs by between 25% and 30% for aluminium smelting.
However, the informed reader will be also be aware that Aluminium carbothermic smelting technologies have been the subject of considerable R&D that has continued intermittently over the last 70 years, involving significant investment by many of the major aluminum producers, together with government support in some circumstances (Motzfeldt, et al, 1989). The most recent major activity was conducted jointly by Alcoa and Elkem. In that work, a new approach known as “Aluminium Carbothermic Technology - Advanced Reactor Processing” (ACT-ARP) was considered (2004). The ACT-ARP process was demonstrated to function at temperatures in the region of 2,000°C to 2,250ºC, but was shown to have many of the same challenges that were also evident in earlier attempts to develop an effective carbothermic process for aluminium.
In those previous carbothermic research programs for aluminium smelting, the challenges and critical factors were demonstrated to include the recovery of molten and gas phase products, the accurate control of reactants, and the availability of effective refractory materials to sustain effective operations at temperatures above 2,000ºC. In fact, the optimization of a preferred set of process conditions probably could not be achieved within the required temperature range, Calsmelt contends. Therefore, as the industry generally recognizes, past attempts to commercialize carbothermic technology for aluminium smelting have been less than totally successful and the industry’s demand for a radical process change remains unsatisfied.