New opportunities for metals extraction and waste treatment by electrochemical processing in molten salts
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Molten salt electrolysis is a proven technology for the extraction of metals—all the world's primary aluminum is produced in this manner. The unique properties of molten salts also make them excellent media in which to treat a variety of forms of waste. Of special note in this regard is electrolysis in molten oxides, a concept put forward by the author, initially as a "clean technology" for producing primary metal. However, in the context of waste treatment, electrolysis in molten oxides is a process offering the prospect of changing the valence of dissolved heavy metals while making pure oxygen gas as the main by-product. Laboratory tests conducted at a temperature of 1550 °C on chromate sludge dissolved in melt composed of A12O3, SiO 2 , CaO, and MgO have confirmed electrochemical production of oxygen on a carbon-free anode.
I. INTRODUCTION Molten salt electrolysis is the electrolytic decomposition of a compound dissolved in an ionic melt. A prime example of this process can be found in the production of aluminum. The compound, alumina (A12O3) derived from the mineral bauxite, is dissolved in an ionic melt composed of a multicomponent solution of cryolite (Na 3 AlF 6 ), aluminum fluoride (A1F3), and calcium fluoride (CaF2). The products of electrolysis are molten aluminum and carbon dioxide, the latter due to the attendant consumption of the carbon anode. Primary aluminum is produced in a reactor known as the Hall cell. (The extraction of aluminum by the electrolysis of alumina was invented in 1886 independently by Charles Martin Hall in the United States and by Paul Heroult in France. The reactor for this process is known as the Hall cell in North America and as the Hall-H6roult cell in Europe.) Alternatively, the compound undergoing electrolytic decomposition can be derived from waste. Processing in molten salts, with their capacity to dissolve materials to very high concentrations compared to those attainable in aqueous solutions, can be rather advantageous. High solubilities lead to high limiting current densities, and this, in turn, results in high productivities. For a primer on molten salt electrolysis and a review of the basic literature in the field of molten salts, the reader is directed to an earlier publication of the author.1 II. THE NEED FOR NEW METALLURGICAL TECHNOLOGY The conversion of iron ore to finished steel is both energy intensive and capital intensive, and as a result efforts at improving the process have up to now| been J. Mater. Res., Vol. 10, No. 3, Mar 1995
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focused primarily on reducing operating costs through increases in efficiency. More recently, attention has begun to shift toward the environmental aspects of iron and steelmaking largely in order to attempt to comply with stricter regulations, e.g., the Clear Air Act amendment of November 15, 1990 which targets coke ovens for closure. Today's steelmaking technology, comprising the three main unit operations of coke making, reduction in the blast furnace, and oxidation of exce
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