Leaching kinetics of malachite in ammonium carbonate solutions
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I.
INTRODUCTION
AMONGthe
oxidized copper minerals, malachite is the most common. With the depletion of world supplies of the relatively higher grade copper sulfides, more and more attention will be focused on the extraction of the oxidized copper ores. Generally leaching is done with sulfuric acid as lixiviant, but often basic ores consume excessive acid. A more basic lixiviant such as ammonium carbonate then becomes attractive. For this reason it was used as a leachant for the mineral in this study. Malachite occurs as a secondary copper mineral in the upper oxidized zone of copper deposits. Chemically it is a basic carbonate CuCO3Cu(OH)2 , and it crystallizes with monoclinic symmetry. 1,2,3,~ Kopylov and Orlovs studied the kinetics of dissolution of malachite in sulfuric acid in different concentrations (5 to 30 g/l) for specified periods (1 to 120 minutes). The concentration of H2SO4 had a pronounced effect on the rate of dissolution. The reaction was first order with respect to H § and was diffusion-controlled. Roman and Benner 6 showed that the rate of leaching of malachite from a sandstone ore followed a shrinking core model and was controlled by the diffusion of H § ions through the host rock. Semenishin, Aksel'rud, and Limarenko7 reported that leaching of malachite with H2SO4 from the pores of an inert material (polystyrene) was initiated with a rapid reaction on the surface of malachite accompanied by evolution of CO2 and precipitation of the solid phase CuSO4- 5HzO into the bulk solution. Bryden 8 derived a Langmuir adsorption type PAUL D. OUDENNE is Research Engineer, Laboratory of Mineral Ore Dressing, Catholic University of Louvain, Place St. Barbe, 2, B 1349, Louvain la Neuve, Belgium. FERRON A. OLSON is with the Department of Metallurgy and Metallurgical Engineering, 412 Browning Building, University of Utah, Salt Lake City, UT 84112. Manuscript submitted August 28, 1981.
METALLURGICALTRANSACTIONS B
model to describe the kinetics of leaching of a copperbearing ore containing primarily malachite and chrysocolla with ammonium sulfate solution as the leaching agent. He reported 48.5 kJ/mole (11.6 kcal/mole) as the activation energy for the rate-controlling step, indicating that the slowest step was probably surface reaction. Bryden9 also studied the kinetics of leaching of the mineral malachite in the presence of ammonium sulfate solutions at three pH levels. At the lower pH, (pH < 2), mixed kinetics rate control was observed and the kinetics were described by the coupled diffusion chemical equation: 1 - 2 / 3 a - (1 - ~)2/3 + /311 - (1 - a) 1/3] = 7Ct
[1]
where a = fraction of copper dissolved from the mineral,/3 and 3' are constants, 9 and C is concentration of hydrogen ion. The reaction was characterized by two apparent activation energies: a pore diffusion energy of 41.8 kJ/mole (10 kcal/mole) and a surface reaction activation energy of 16.7 kJ/mole (4 kcal/mole). At the intermediate pH of 5.4, the reaction was controlled by surface reaction and was characterized by the apparent activation en
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