A pilot-scale trial of an improved galvanic deoxidation process for refining molten copper
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INTRODUCTION
CONTROL of undesirable elements, such as dissolved hydrogen, sulfur, and oxygen, in liquid metals and alloys is critical to obtain superior physical and mechanical properties of the final product. Over the past few decades, this has resulted in a search for new metal-refining techniques capable of reducing the concentration of impurities to extremely low levels. Currently, the two most popular refining techniques are vacuum degassing and the addition of reagents to form stable compounds with the undesirable elements.[1] Vacuum degassing is capital intensive, limited by the vacuum pressure that can be created over the melt, and can lead to the loss of desirable elements. In the case of chemical refining, some of the reagent metals can be expensive, and the compounds of the undesirable elements may remain as inclusions in the metal. In addition, the reagent may go into solution in the melt, which may deteriorate the properties. There exists a need for a commercially viable, energy efficient, and environmentally sound moltenmetal refining technique which overcomes the aforementioned disadvantages. A large number of workers have demonstrated the feasibility of using oxygen-ion conduction in stabilized zirconia to extract oxygen from molten metals.[1–8] These processes P. SORAL, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, is with I2 Technologies, Irving, TX 75039. U. PAL, Associate Professor, is with the Department of Manufacturing Engineering, Boston University, Boston, MA 02215. H.R. LARSON, Research Associate, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. B. SCHROEDER, Director, is with the Reading Tube Corporation, Reading, PA 19605. Manuscript submitted June 22, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
involve the diffusion of oxygen through the solid-oxide electrolyte (stabilized zirconia) out of the melt and can be divided into two categories: galvanic and electrolytic. In the electrolytic process, an external potential is used to pump oxygen through the electrolyte. In the galvanic process, the electrolyte is used to separate the oxygen-containing melt from a reducing atmosphere, and the oxygen-chemical-potential gradient acts as the driving force for the migration of oxygen ions across the zirconia membrane.[9] Due to application of an external potential, the electrolytic process would rely on the combustion of fossil fuels to produce electrical energy and, therefore, would have an intrinsic efficiency limitation imposed by the Carnot cycle.[10] In addition, hydrocarbons, NOx/SOx gases, and carbon monoxide are released into the atmosphere during incomplete combustion of fossil fuels. On the other hand, the design of the setup for the galvanic technique is simpler, since no external power supply is used, and it operates on the same principle as a fuel cell. Because a fuel cell converts the chemical energy of the fuel direct
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