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Extraction of leachable metals and recovery of alumina from utility coal ash.pdf

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Extraction of leachable metals and recovery of alumina from utility coal ash

Resources and Conservation, 9 (1982) 131-142 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands 131 EXTRACTION OF LEACHABLE METALS AND RECOVERY OF ALUMINA FROM UTILITY COAL ASH R. C. GABLER, JR., and R. L. STOLL U.S. Department of the Interior, Bureau of Mines, Avondale Research Center, Avondale, Maryland 20782 (U.S.A.) ABSTRACT The classification of coal ash as a special waste by the Environmental Protection Agency (EPA) stimulated questions concerning the environmental im- pact of the disposal of this volumineus byproduct of coal-fired power plants. Both industry and Government face possible expensive disposal problems because of the large volume of coal ash produced annually. The extraction of metals from coal ash was investigated as part of a Bureau of Mines program to convert waste materials from a disposal problem to poten- tial secondary resources. The objectives of this study were to produce a more environmentally acceptable waste and offset waste disposal costs with recovered metal values. The research was conducted in two steps: determination of parameters affecting extraction of aluminum from coal ash, and development of extraction-recovery procedures. The major parameter affecting the extraction of metals from coal ash was the calcium content of the fly or bottom ash. Other parameters, such as furnace type and operating temperature, had lesser effects on metal extraction. Two procedures were studied for metal extraction: an acid leach for high-calcium ashes and a lime sinter-acid leach for low-calcium ashes. The first procedure removed about 75% of the metals in high-calcium ash, and the second removed about 90% of the metals in low-calcium ash. An alumina product was recovered that, with sufficient washing, meets Hall Cell specifications. The leach residues pass the EPA EP toxicity test and are classified as nonhazardous was tes. INTRODUCTION In late 1978, the U.S. Environmental Protection Agen