The kinetics of the dissolution of chalcocite in alkaline cyanide solution
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DROMETALLURGICAL treatment of copper concentrates has received considerable attention in r e c e n t y e a r s partly as a result of emission standards imposed upon smelters and partly due to an inc r e a s e d awareness of resource conservation. Emphasis is b e i n g placed on processes that may have h i g h e r recoveries, t r e a t lower g r a d e materials, provide by-product recovery or minimize air pollution. In addition, processes that are optimal for a partic u l a r ore type r a t h e r than all copper ores are becoming increasingly important. Sufficient quantities of chalcocite a r e available t o justify a process that is not economical for concentrates containing chalcopyrite. In Arizona and New Mexico, about 5000 tons per day of such concentrates are produced. Although final concentrates are typically f i n e r than 50 pct minus 325 mesh, the u s u a l intimate association of chalcocite and p y r i t e m a k e s separation by flotation difficult. Consequently, most concentrators treating such ores have copper extractions as low as 80 pct and p y r i t e content in the concentrate as high as 70 pct. The resultant effects on s m e l t e r economics f o r c e the production of concentrates that are suitable for smelting at the expense of copper recovery. One approach t o achieving higher overall copper recovery that has been proposed is to extract additional copper from flotation tails by leaching. Chalcocite remaining in the tails can be leached with cyanide solutions without oxidation t o produce soluble cuprous cyanide ions and sulfide ions.1-3 The copper contained in the pregnant l e a c h solution can be precipitated as cuprous sulfide by acidification of the solution and the cuprous sulfide product sent t o a s m e l t e r for conventional smelting and refining. In addition, direct cyanide leaching of low g r a d e ores has been proposed.4 Another approach to this problem is t o develop a hydrometallurgical extraction technique that recovers copper from chalcocite concentrates without attacking other minerals such as pyrite. Since p y r i t e and other minerals would not be effected during the leaching process, such a process would allow a concentrator to achieve higher copper reROBERT SHANTZ is Metallurgist, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM and WALTER W. FISHER is Metallurgist, Arizona Bureau of Mines, Tucson, AZ. Manuscript submitted September 2 7 , 1976. METALLURGICAL
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covery at the expense of concentrate g r a d e . T h e r e a r e s e v e r a l advantages t o leaching chalcocite in a cyanide solution: the reaction is rapid, p y r i t e is not attacked, and SO2 is not produced. Cyanide leaching will also r e c o v e r the precious m e t a l values. Because copper enters the solution as cuprous ion, only half as much reductant is required for copper recovery compared to acid or ammonia l e a c h solutions. In addition, since the copper extraction rate is high, it may be feasible to t r e a t the c o a r s e rougher concentrate directly. Furthermore, a p
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