Electrowinning copper from solvent extraction acid strip solution using Pb-Sb anodes
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HE native copper deposits in the Upper Peninsula of Michigan have contributed substantially t o the copper needs of the United States; however, the m i n e s are presently closed because of high operating costs for underground mining. The old underground mine workings contain many l a r g e stopes filled with lowg r a d e broken ore or "poor r o c k . " Inplace leaching with ammoniacal ammonium carbonate solutions may be a practical method for extracting copper from this low-grade ore. The Bureau of Mines is currently investigating the feasibility of u s i n g a hydrometallurgical process for treating this material. A previous report described the findings from a preliminary study that indicated the technical feasibility of a process consisting of 1) leaching with dilute ammoniacal ammonium carbonate solution, 2) recovering part of the copper by solvent extraction techniques, 3) oxidizing the remaining copper t o the cupric state, and 4) returning the solution to leach additional copper 1). Dilute solutions were used to minimize losses due t o absorption by the ore particles and copper was recovered from the organic extractant by acid stripping and electrowinning. The primary concern in this investigation was the electrowinning step of the proc e s s , and the remainder of the system was operated to provide the electrolyte necessary for the s t u d y . MATERIALS AND E Q U I P M E N T A 5.4 metric ton s a m p l e of native c o p p e r ore w a s leached by d o w n w a r d percolation in a 1.2 m diameter by 3.7 m high rubber-lined mild steel c o l u m n with 0.031 M (NH4)2CO3 plus 0.063 M N H 4 O H solution.
T. H.JEFFERSand R. D. GROVES are Chemical Engineer and Metallurgist, respectively, Salt Lake City Metallurgy Research Center, Bureau ofMines, Department oftheInterior,Salt Lake City, UT 84112. Manuscript submitted January 12, 1976. METALLURGICAL TRANSACTIONS B
This solution was circulated t o the top of the ore at a rate of 815 liters per sq m per day, and the system was closed to prevent evaporation of reagents. Oxygen was added periodically t o sustain native copper leaching by oxidizing inactive cuprous ions t o active cupric ions. With continued recirculation, a pregnant solution containing 1.3 to 1.4 g per l i t e r copper was produced. A photograph of the solvent extraction-electrowinning apparatus is shown in Fig. 1 and a schematic d i a g r a m of the system is presented in Fig. 2. The organic extractant used in the system was a mixture of 5 pct of General Mills reagent LIX-64N* and 95 pct *Reference to specific company orbrand name is madefor identification only anddoes not imply endorsement by the Bureau &Mines. kerosene. Copper extraction was accomplished in a 7.6 cm diameter by 122 cm high packed tower containing 1.27 cm c e r a m i c saddles. Both pregnant solution and organic extractant were introduced t o the top of the tower and allowed t o percolate downward through the packing. The liquid-liquid contact thus attained was sufficient for extraction of copper from the aqueous phase. In prelimin
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