Graduate Thesis Or Dissertation
 

A study on electrowinning copper and zinc from low grade ores

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  • Large quantities of copper and zinc bearing ores exist in Oregon. The metals are present as sulfides in small concentrations so that it is difficult and uneconomical to mine them by the conventional sulfuric acid electrowinning process. A new process was developed to make mining of the copper and zinc feasible. In ores of this type an ammonium hydroxide electrowinning process has proved to be more economical and more efficient than the sulfuric acid method. The ore is first ground to approximately 100 mesh and then roasted in air for a few hours at 1200°F. A solution of 15% ammonia in water is then used to leach the copper and zinc from the ore. The ammoniacal solution is selective for copper and zinc since these metals form ammonia complex ions. A countercurrent leaching procedure is recommended since a 91.5% recovery of total copper in 1.5 hours was accomplished in bench scale experiments by this method of leaching. The ammonium hydroxide leach liquor can be reused three times before losing any of its ability to complex the metals. After that time small amounts of concentrated ammonium hydroxide should then be added to the leaching and plating cycle. Experiments showed 0.213 pounds ammonia used per pound of copper refined. The copper is plated from the leach liquor at a current density less than 5.5 amps/ft². By plating below this limit pure copper may be deposited from the solution without plating zinc. After the copper is removed pure zinc is plated at 20-30 amps/ft². A current efficiency of 89.2% was obtained in plating the copper with a power requirement of 1.285 kwhr/lb. Cu. The developed process has many advantages over the sulfuric acid electrowinning process now used in the majority of copper and zinc refineries since both copper and zinc can be recovered in one simple process with little extra cost. Ores containing large amounts of iron are very detrimental to the acid process and elaborate purification techniques are needed before the acid process can be employed at all. Iron and lead cause no difficulty in ammonia leaching since they have no tendency to complex. The ammonia leach is much faster which in turn produces a savings in wages and capital investment. An equal cost batch leach of both ammonium hydroxide and sulfuric acid was conducted and the ammonium hydroxide leach showed a 77.6% recovery of total copper in a six hour period while the acid leach produced only a 55.1% recovery. The power cost for the ammonia process was 0.321 cents per pound of copper which is slightly less than the 0.337 cents per pound for the acid process. This process should prove to be beneficial as a means of extracting copper and zinc from predominately sulfide ores. The advantages of the process over the sulfuric acid method are probably enough to make the products competitive with the metals obtained by acid refineries.
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