The leaching kinetics of chalcopyrite (CuFeS 2 ) in ammonium lodide solutions with iodine
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I.
INTRODUCTION
CHALCOPYRITE is the most abundant copper sulfide mineral and is the major source of copper. Numerous leaching processes, such as ammoniacal systems,[1,2,3] acid sulfate systems,[4–7] chloride systems,[4,7–12] and nitrate systems,[13] have been introduced and investigated. In oxygenated ammoniacal solutions, chalcopyrite dissolves according to the following reaction:[1,2,3] 17 CuFeS2 1 4NH3 1 O 1 2OH2 4 2 1 5 Cu(NH3)21 1 Fe2O3 1 2SO422 1 H2O 4 2
[1]
Commercial-scale ammonia leaching of copper sulfide concentrates has been accomplished. The Arbiter process[1] is carried out at 60 7C to 90 7C with an oxygen pressure of 5 to 40 psig. Intensive mixing is employed to improve the rate of oxygen transfer from the gas phase to the liquid phase and to prevent the formation of a protective hematite surface layer. Warren and Wadsworth[14] studied the anodic dissolution of chalcopyrite in ammoniacal solutions. They found that at low overpotentials a deficient layer of Cu12xFeS2 was formed. At higher overpotentials, the anodic dissolution of chalcopyrite can be described as CuFeS2 1 4NH3 1 9OH2 5 Cu(NH3)421 1 Fe(OH)3 1 S2O
22 3
1 Fe(OH)3 1 S2O22 3
9 2 I 5 Cu(NH3)21 4 2 3 27 2 1 3H2O 1 I 2
[4]
The standard free energy change for Reaction [4] was calculated and found to be 21086.25 kJ/mole (thermodynamic data were taken from Helgeson et al.[17] and Lide[18]). The large, negative standard free energy change indicates that chalcopyrite should readily be attacked by ammonia in the presence of triiodide, and the reaction can be assumed to be irreversible. The purpose of this study was to search for alternative oxidants for the ammonia leaching of chalcopyrite, so that leaching can take place at a lower temperature. Iodine was selected for this study because it has been demonstrated to be an effective oxidant for leaching of gold and other precious metals. It has a number of advantages, such as its moderate oxidation potential, higher solubility in the presence of iodide, and easy regeneration for recycling use in the system. The parameters investigated were: total ammonia concentration, iodine concentration, pH of the solution, disc rotating speed, reaction temperature, and the reaction product layer. II.
Iodine has already been established as an effective oxidant for the dissolution of gold.[15,16] In an excess of I2, iodine will react with I2 and form triiodide (I23 ). The triiodide ion can serve as an oxidant for dissolution of chalcopyrite according to the following electrochemical reaction: [3]
Y. CHARLES GUAN, Process Engineer, is with Bateman Engineering Inc., Denver, CO 80235, and KENNETH N. HAN, Distinguished Professor, is with the Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701-3995. Manuscript submitted November 11, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
CuFeS2 1 4NH3 1 9OH2 1
[2]
1 3H2O 1 9 e
9 2 27 2 I3 1 9 e 5 I 2 2
The overall reaction for leaching of chalcopyrite in ammoniacal solutions with iodine as an oxidant can be
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