Acid bake-leach-flotation treatment of offgrade molybdenite
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MOLYBDENITE recovered as a by-product from copper porphyry ores represents one of the w o r l d ' s principal sources of molybdenite. In o r d e r t o meet m a r k e t specifications of low lead and copper in a h i g h g r a d e molybdenite concentrate, recovery frequently must be sacrificed. This problem is encountered at Kennecott Copper Corporation's Nevada Mines Division (NMD) w h e r e a marketable product frequently cannot be obtained, and thus an offgrade concentrate is stockpiled for subsequent reworking with excessive l o s s . A new acid bake-leach-flotation p r o c e s s(ABLF) *Patent pending. has been developed as a m e a n s of solving the problem. Fig. l(a) shows typical crystallites of NMD molybdenite characterized by rough surfaces and i r r e g u l a r shape. Contrast this texture with the thin plates and smooth surfaces shown in Fig. l ( b ) . T h e s e l a t t e r c r y s tals were sampled from a molybdenite ore well known for its excellent amenability t o flotation concentration. The NMD molybdenite contains 0.2 pct rhenium in solid solution; the second sample contains only 0.02 pct rhenium. Sutulov has noted that molybdenite with high rhenium content is usually more difficult t o concentrate by flotation than is molybdenite which is r e l a tively free of this impurity.1 This poor flotability of the high rhenium molybdenite is probably r e l a t e d as much t o its physical mode of occurrence in the deposit as it is t o its high rhenium content.~ Easily floated molybdenite appears to o c c u r as disseminated p a r t i c l e s , whereas m a t e r i a l which is difficult t o float is found as fracture coatings and in veins. The f r a c t u r e coatings and v e i n s happen to be w h e r e n a t u r e concent r a t e s rhenium. PROCESS SELECTION Kennecott recovers rhenium by scrubbing the volatile rhenium oxides from the g a s e s evolved during the H. R. SPEDDEN, J. D. PRATER, P. B. QUENEAU, G. G. FOSTER, and W. S. PICKLES are ResearchDirector,Section Head, Hydrometallurgy, Scientist,Scientific Aide, and Senior Design Engineer, respectively, Kennecott Copper, Corp., Kennecott ResearchCenter, Salt Lake City, Utah. Manuscript submittedFebruary 19, 1971. METALLURGICAL TRANSACTIONS
molybdenite roasting. Thus, possible processing routes t o upgrading the r o a s t e r feed were l i m i t e d to methods which did not substantially a l t e r the molybdenite mineralization and its associated r h e n ium content. Our literature s e a r c h turned up two promising c h e m i c a l methods which s e e m e d applicable; oxidation of impurities w i t h hot concentrated sulfuric acid 3 and oxidation of impurities w i t h chlorine gas at 300°C.4 T h e s e methods have s i n c e been investigated b o t h in the laboratory and on pilot plant s c a l e . This paper is limited t o the development of the concent r a t e d sulfuric acid method. In 1933, H a r r y Morgan demonstrated that copperiron sulfides could be r e n d e r e d water-soluble by baking with hot concentrated sulfuric a c i d . Contained molybdenite
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