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Investigation of Rare-Earth Alumino-Silicate Refractories for Molten Aluminum Alloy Containment Michael Hughes O'brien Iowa State University

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Investigation of Rare-Earth Alumino-Silicate Refractories for Molten Aluminum Alloy Containment Michael Hughes O'brien Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1988 Investigation of rare-earth alumino-silicate refractories for molten aluminum alloy containment Michael Hughes O'Brien Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Materials Science and Engineering Commons Recommended Citation O'Brien, Michael Hughes, "Investigation of rare-earth alumino-silicate refractories for molten aluminum alloy containment " (1988). Retrospective Theses and Dissertations. 9707. https://lib.dr.iastate.edu/rtd/9707 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. INFORMATION TO USERS The most advanced technology has been used to photo­ graph and reproduce this manuscript from the microfilm master. UMI films the original text directly from the copy submitted. Thus, some dissertation copies are in typewriter face, while others may be from a computer printer. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyrighted material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are re­ produced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each oversize page is available as one exposure on a standard 35 mm slide or as a 17" x 23" black and white photographic print for an additional charge. Photographs included in the original manuscript have been reproduced xerographically in this copy. 35 mm slides or 6" X 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. lilUMIAccessing the World's Information since 1938 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA Order Number 8826428 Investigation of rare-earth alumino-silicate refractories for molten aluminum alloy containment O'Brien, Michael Hughes, Ph.D. Iowa State University, 1988 UMI 300N.ZcebRd. Ann Aibor, MI 48106 PLEASE NOTE: In all cases this material has been filmed in the best possible way from the available copy. Problems encountered with this document have been identified here with a check mark V 1. Glossy photographs or pages 2. Colored illustrations, paper or print 3. Photographs with dark background ^ 4. Illustrations are poor copy 5. Pages with black marks, not original copy 6. Print shows through as there is text on both sides of page 7. Indistinct, broken or small print on several pages 8. Print exceeds margin requirements 9. Tightly bound copy with print lost in spine 10. Computer printout pages with indistinct print 11. Page(s) lacking when material received, and not available from school or author. 12. Page(s) seem to be missing in numbering only as text follows. 13. Two pages numbered . Text follows. 14. Curling and wrinkled pages 15. Dissertation contains pages with print at a slant, filmed as received 16. Other UMI [ Investigation of rare-earth alumino-silicate refractories for molten aluminum alloy containment by Michael Hughes O'Brien A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Department: Materials Science and Engineering Major: Ceramic Engineering Approved: Signature was redacted for privacy. Signature was redacted for privacy. For the Major Department Signature was redacted for privacy. Fo ïte College Iowa State University Ames, Iowa 1988 il TABLE OF CONTENTS Page GENERAL INTRODUCTION 1 Explanation of Dissertation Format 4 SECTION I. MOLTEN ALUMINUM ALLOY ATTACK ON RARE-EARTH DOPED ALUMINO-SILICATES 5 ABSTRACT 6 INTRODUCTION 7 EXPERIMENTAL 9 Materials 9 Procedures 11 RESULTS AND DISCUSSION 17 Alumino-silicate Refractories 17 Rare-earth Doped Bauxite 28 CONCLUSIONS 48 REFERENCES 49 SECTION II. THE ROLE OF CERIA DOPING ON THE MOLTEN ALUMINUM ALLOY RESISTANCE OF ALUMINO-SILICATE REFRACTORIES 51 ABSTRACT 52 INTRODUCTION 53 EXPERIMENTAL APPROACH 56 MATERIALS AND PROCEDURES 59 CeOg/SiOg and CeOg/AlgO^/SiOg Systems 59 BigOg/Bauxite System 63 Molten Alloy Attack Tests 54 iii Page Analysis of Refractories 65 RESULTS 66 Alloy Attack 66 Microstructure of Doped and Undoped Bauxite Compositions 68 Microstructure of Ceria/Alumina/Silica Specimens 73 Microstructure of Attacked Samples 81 DISCUSSION 98 CONCLUSIONS 103 REFERENCES 104 SECTION III. CALCULATION OF LATTICE ENERGIES AND ENTHALPIES OF FORMATION OF RARE-EARTH PYROSILICATES 106 ABSTRACT 107 INTRODUCTION 108 APPROACH 113 PROCEDURE 114 RESULTS AND DISCUSSION 120 Crystals with Known Thermodynamic Values 120 Rare-earth Pyrosilicates 128 CONCLUSIONS 137 REFERENCES 138 GENERAL SUMMARY 139 ACKNOWLEDGMENTS 140 1 GENERAL INTRODUCTION In the aluminum industry, there is a long-standing problem of aluminum alloy attack on refractory materials. Industrial production of aluminum is an expensive process, involving several steps. In review, the Bayer process is used to separate the free alumina (AlgO^) from an alumino-silicate ore (commonly bauxite). In the Bayer process, a caustic soda solution is used at an elevated temperature to form a soluble compound, sodium aluminate. After decanting, the solution is supersaturated with respect to alumina and crystallized as gibbsite. To claim the alumina found combined with silica, a lime-soda sintering procedure is performed subsequent to the Bayer process. Aluminum is made from alumina by electrolytic smelting, often called the Hall process. During smelting and later remelting, molten aluminum metal attacks the refractory brick baths in which the charge is contained. This attack results in the destruction of the refractory bath, the loss of aluminum and the contamination of the aluminum product. Primary reduction of alumina to produce aluminum is done in an electrolytic reduction cell charged with a mixture of alumina and cryolite (Na^AlFg). Cryolite and other fluorite salts are used primarily as electrolytes and powerful fluxes. Reduction cells are commonly constructed with tri-layer walls. The outer structural wall is steel. The middle layer is constructed from insulating brick, often an alumino-silicate firebrick. The inner wall must be very 2 resistant to both molten aluminum attack and dissolution in mixed fluorides. Often carbon-carbon composite materials or graphite are used for the inner wall because of their high electrical conductivity. The bottom of the cell is the cathode. Carbon anode rods are hung into the molten salts, roughly three inches above the level of the molten aluminum. In this semicontinuous process, alumina is periodically added on top of the frozen crust, where it is preheated prior to mixing. A low-voltage direct current decomposes the alumina, releasing oxygen to the anode and aluminum to the cathode bottom. Several ceramic oxide materials have been tested as inner wall surfaces, but an economical solution has not yet been found. Alkali and alkaline-earth oxides have substantially higher thermodynamic stability than alumina, but also have high solubilities in molten cryolite. Alumina walls also dissolve readily, but do not contaminate the charge. With carbides, the formation of aluminum carbide is a limitation. For borides and nitrides, thermal shock is often a problem, but high cost is the greatest drawback. Secondary holding, refining and remelting furnaces present refractory problems closely related to those found in the primary production process. Molten aluminum is tapped from the reduction cell such that no cryolite passes into these furnaces. Here, a chloride mixture is often used as a flux. These furnaces are generally gas-fired reverberatory designs, characteristically having reducing atmospheric conditions. Currently, several high-cost refractories are being used in secondary processing, including magnesite-chrome. 3 Mg-Al spinel and high purity alumina. These materials are more vulnerable to thermal shock than less expensive alumino-silicates and hence, are not perfect answers to the problem. The economic problem of refractory performance is a function of many variables, not the least of which are energy cost, down-time and metal loss. There is a considerable economic investment in aluminum processing prior to secondary handling. The metal lost by penetration into and reaction with the refractories is not recoverable. Production down-time results when a bath requires rebuilding due to refractory failure. Rebuilding can be dictated by either loss of product purity or simply the mechanical failure of the walls. The labor costs for demolishing and relaying these brick walls can also be substantial. Relative to all of the other costs described above, many producers have apparently determined the cost of the refractory brick to be small, as indicated by the trend of employing more resistant, higher cost compositions. In spite of many years of work by numerous researchers, the problem of aluminum attack of refractories is possible more critical than ever before. Evolution in the aluminum industry has created the demand for higher product purity, shorter production time and higher energy efficiency.
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