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STAFF, 1978 ENGINEERING AND MINING JOURNAL INTERNATIONALDIRECTORY OF MINING, 1978, P. 276.A BAN ON SCHEELITE ORE EXPORTS, ENGINEERING AND MININGJOURNAL, SEPT. 1977, P. 44.LEFOND, S. J., BRAZILIAN MINING: RELAXED GOV'T. ATTITUDESPAVE THE WAY FOR EXPLOITING CRITICAL RESERVES, MININGENGINEERING, NOV. 1973, P. 44.STAFF, MINING MAGAZINE, JAN. 1979, P. 55.YIH, STEPHEN W.H. AND WANG, CHUN T.; TUNGSTEN: SOURCES,METALLURGY, PROPERTIES, AND APPLICATIONS., PLENUM PRESSNEW YORK, 1979. P 47.ROSKILL INFORMATION SERVICES LTD., THE ECONOMICS OF TUNGSTEN6'TH EDITION 1990. P 55.