Current production of aluminum in the United States is approximately 3.8 x 106 metric tons per year. The presently used system for the production of aluminum is the Hall-Herault cell. It is operated at 950oC to 980oC and consists of a carbon cavity (lining) which conducts current from a molten aluminum cathode on the bottom of the cathode cavity. Consumable carbon anodes are suspended in a cryolite electrolyte (bath) which floats on the alumina cathode (metal pad). Alumina powder is fed periodically or continuously, carbon anodes are consumed continuously (with periodic replacement in prebaked anode plants), and molten metal product is siphoned approximately daily. In this work the PI will examine new electrode materials and new concepts for electrochemical cell design and operation which could save energy and lower operating costs on retrofit older plants, as well as decrease both capital and operating costs for new aluminum reduction plants. The aluminum industry has for decades been pursuing a non-consumable anode that can be substituted for the consumable carbon anodes in the Hall-Herault cells. The PI plans to test a new alumina/carbon composite anode made of a Ni-Fe-Cu cermet material. This composite anode technology utilizes the same reaction as the Hall-Herault technology but the alumina in the anode is dissolved at the anode-bath interface at exactly the location where oxide ions are discharged on the carbon matrix. The results is that high alumina solubility in the bath is not required, and low-temperature baths could be used. The bath to be used in this research is a mixture of sodium and lithium cryolites having a freezing point less than 670oC and operating at about 700oC. Titanium diboride sleeves inside the crucible will serve as an alternate cathode to graphite. Phase I results have shown that the electrolyte compositions have the appropriate properties for cell operation and process feasibility has been proven. The overall objectives of the Phase II research are to develop the basic engineering and materials science required for the Phase III and the following commercialization of the new technology. The specific objectives are: a. Materials Science o Determine optimum particle size of alumina and treatments to avoid or control agglomeration to minimize the formation of muck in the cell. o Determine optimum corrosion-resistant alloy anode composition. o Develop quantitative understanding of oxide growth rates and corrosion rates of alloy and cermet anodes. o Develop understanding of the role of lithium ions in electrical conduction phenomena for alloy and cermet anodes. b. Engineering Science o Develop quantitative fluid dynamics and mass transport correlations for bath/alumina slurry/gas bubble system. o Develop scaling laws for mass and heat transport and current distribution in the cells. o Develop quantitative understanding of conditions of convection and diffusion that avoid or result in solid salt cathode deposits. o Determine current efficiency and identify mechanisms of inefficiency in the low-temperature slurry electrolyte system. o Determine potential balance in cell: reversible potential, activation and concentration overpotentials and ohmic drops in electrodes and bath. The results of Phase II will thus provide information for scaleup to commercial size cells.
