Reduction of phosphate ores by carbon: Part I. Process variables for design of rotary kiln system
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I.
INTRODUCTION
PHOSPHORIC acid from the wet acid process contains nearly all the impurities found in the original ore with the exception of calcium, much of which is removed by precipitation as gypsum or hemihydrate. Although the impure acid is acceptable for many agricultural uses, industrial applications require a purer product which has been produced by the electric furnace method for nearly a century. In this method phosphorus is recovered from the ore as it is reduced with carbon. The reaction is highly endothermic and is driven by the in s i t u heating of the charge with a submerged electric arc. Significant increases in the price of electricity have raised the cost of operating the furnace to the point where the development of alternative processes is desirable. A very promising alternative process is called "Kiln Phosphoric Acid process" (KPA), which was developed by Occidental Research Corporation (first described by Buckholtz et al.). ~ Several recent patents have been issued. 24 In this process, partially beneficiated ore is reduced by carbon in a rotary kiln, the heat for the endothermic reaction being supplied directly by oxidation of various gaseous components, including phosphorus and carbon monoxide. KPA makes efficient use of this heat and is expected to be economically attractive. In a recent paper Mu et al. 6 have already reported on some aspects of the reduction of phosphate ore by carbon. The reactants were formed into pellets and reduced to varying levels in a thermogravimetric analyzer (TGA). The residues were evaluated by optical microscopy, X-ray diffraction (XRD), and with an electron microprobe. Coexisting crystalline phases such as mono- , di- , and tri-calcium silicates were identified and their concentration FREDERIC LEDER, Manager. Research, and WON C. PARK, Research Scientist, are wUh Dowell Schlumberger, Tulsa, OK 74137. HOWARD REISS is Professor, Chemistry Department, UCLA, Los Angeles, CA 90024. JACOB MU is Senior Research Engineer, U.S. Borax Research, Anaheim, CA 92810 JOSEPH MEGY is Technical Director, Albany Titanium, Inc., Albany, OR 97321. ROBERT A. HARD is Manager. Research, Cabot Corporation, Bdlerica, MA 01821. Manuscript submitted November 15, 1985.
METALLURGICALTRANSACTIONS B
levels were measured as functions of the degree of reduction. The authors found that multiple reaction stages appeared to be involved, and that melt formation was an important factor. Melt viscosities were measured with the aid of 29Si nuclear magnetic resonance (NMR). The observed degree of melting was in fact compared with predictions based on existing ternary phase diagrams, and reasonable agreement was found. Finally, empirical data on the rates of reduction were available from the TGA measurements. On the basis of these findings, it was possible to suggest a mechanism for the reduction process; however, the paper concerned itself primarily with the melt characteristics of the reactant agglomerate. The present set of papers presents a more detailed investigation of the mechanism with spec
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