Solar production of aluminum by direct reduction: Preliminary results for two processes

The production of aluminum or silicon by reduction of their oxides with car bon is a technical challenge. The temperature required, in the range 2100-2 300 degreesC, is too high for practical process heat addition from a combus tion sour ce alone. Wizen an electrothermal process is used, only about a t hird of the energy contained in the fuel used to generate electricity enter s the process. Thus, for materials produced electrolytically or in an elect ric furnace, the energy cost dominates the cost of the final product. By co ntrast, highly-concentrated solar energy is capable of supplying large amou nts of process heat at very high temperatures, and may have real advantages for metals reduction processes. An are introduces too much energy to the r eaction zone, hz the case of aluminum, the metal floats and it short circui ts the nl c. Ideally, the heat would enter at the bottom or side of a react or, which could be achieved with solar process heat. Among industries, the primary aluminum industry is a major consumer of electricity. It uses about 10 percent of the electricity generated globally for industrial purposes, and about half comes from coal-fired generation stations. This represents a bout 5 percent of the electricity generated for all sectors. A solar-therma l process would drastically reduce the emission of climate-altering gases, reduce the reliance on electricity, and might be a critical factor in makin g a direct thermal route Si om the ore to metal possible. Two industrially- developed processes appear to be attractive candidates for a solar process. Preliminary tests have been performed using a black-body cavity receiver p laced at the focus of the Paul Scherrer Institute's 70kW tracking parabolic concentrator, and though the experiment had to be ended earlier than plann ed, a small amount of 61/37 weight percent Al/Si alloy was formed, and the partially reacted pellets showed conversion to Al4C3 and SiC. Further quali tative tests have been performed using the facilities at Odeillo in a 2 kW solar furnace, where the onset of production of both aluminum by direct car bothermal reduction, and Al-Si alloy via carbothermal reduction of a mixtur e of alumina, silica and carbon could be directly observed.