Solar Production of Aluminum by Direct Reduction: Preliminary Results for Two Processes

The production of aluminum or silicon by reduction of their oxides with carbon is a technical challenge. The temperature required, in the range 2100–2300°C, is too high for practical process heat addition from a combustion source alone. When an electrothermal process is used, only about a third of the energy contained in the fuel used to generate electricity enters the process. Thus, for materials produced electrolytically or in an electric furnace, the energy cost dominates the cost of the final product. By contrast, highly-concentrated solar energy is capable of supplying large amounts of process heat at very high temperatures, and may have real advantages for metals reduction processes. An arc introduces too much energy to the reaction zone. In the case of aluminum, the metal floats and it short circuits the arc. Ideally, the heat would enter at the bottom or side of a reactor, 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 about 5 percent of the electricity generated for all sectors. A solar-thermal process would drastically reduce the emission of climate-altering gases, reduce the reliance on electricity, and might be a critical factor in making a direct thermal route from 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 placed at the focus of the Paul Scherrer Institute’s 70 kW tracking parabolic concentrator, and though the experiment had to be ended earlier than planned, 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 qualitative 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 carbothermal reduction, and Al-Si alloy via carbothermal reduction of a mixture of alumina, silica and carbon could be directly observed.