The leaching of chalcopyrite with ferric sulfate
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I. I N T R O D U C T I O N
THE kinetics of leaching of chalcopyrite with ferric chloride and cupric chloride have been studied previously.~'2'3 Those studies have been extended to the leaching of chalcopyrite with ferric sulfate. Ferric sulfate is one of the most important oxidative leaching reagents for sulfides. In sulfuric acid solutions containing ferric sulfate, chalcopyrite dissolves according to the following reaction: CuFeS2 + 4Fe 3+ = Cu 2+ + 5Fe 2+ + 2S
[1]
The stoichiometry of this reaction has been proved experimentally by many investigators. 4-7 A number of investigations have been conducted by Dutrizac et al. ,4.8-~0 Wadsworth and his c o w o r k e r s y Jones and Peters, H and other investigators 5'~2'~3 to elucidate the reaction kinetics and to delineate the important leaching variables with chalcopyrite, but some problems still remain to be solved. Most of these researchers concluded that the leaching of chalcopyrite with ferric sulfate follows a parabolic rate law. 4-7 In contrast, Jones and Peters H found that the leaching curve is linear over an extended leaching period. Previously, it was reported that observations of the leached surface provide useful information on the leaching mechanism. 1.2,3This study was conducted to obtain a fundamental understanding of the reaction kinetics of chalcopyrite leaching with ferric sulfate. For this purpose, the kinetics of the chemical and electrochemical leaching of chalcopyrite crystals of museum grade were investigated.
II.
EXPERIMENTAL PROCEDURES
A. Sample and Chemicals Chalcopyrite crystals of museum grade from Shakanai mine, Akita prefecture, Japan, were used in the leaching
experiments. Their copper, iron, and sulfur contents were 34.8 pct, 30.4 pct, and 34.8 pct, respectively. The purity of chalcopyrite crystals was observed microscopically and confirmed by electron probe microanalysis. It was found that the crystals used did not contain any other copper minerals and any gangue minerals. The disk specimen was prepared by cementing a chalcopyrite crystal in epoxy resin such that one surface (0.3 to 0.5 cm 2) was exposed to the leaching solution. Moreover, electrical contact was made by soldering copper wire with silver paste on the rear face of each specimen for electrochemical measurements. After fine wet grinding with emery paper, the exposed surface area was measured prior to each experiment by using a planimeter on an enlarged photograph. Deionized water whose specific resistivity was 5 x 106 ~~cm and reagent grade chemicals were used to prepare all electrolyte solutions.
B. Leaching Experiments A 0.25 dm 3 glass vessel with a silicone rubber plug having three holes was used as a leaching reactor. The center hole of the plug was used to accommodate the rod holding the chalcopyrite disk. The other two holes were used for a thermometer for temperature control and a sampling tube. The reaction vessel had a Nichrome heater on its outer wall, and the temperature of the leaching solutions was kept constant during the experiment. The rate curve
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