Leaching kinetics of natural cobalt triarsenide in chlorine solutions
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I.
INTRODUCTION
CHLORIDE hydrometallurgy has developed extensively in recent years 1because of a number of undoubted advantages, among which is the high solubility of many metal chlorocomplexes as well as the high leaching rates observed in chloride media. The use of chlorine as leaching agent, however, is limited both by difficulties in handling and availability of resistant materials as well as by the unfavorable stoichiometry (sulfate formation) in its reactions with many common sulfides. 2-6 Recently, however, as the materials problem can be solved, chlorine leaching has shown itself to be attractive for treating a variety of products such as oxidic base metal residues, v'8 materials containing refractory gold, 9 or for the treatment of products containing both oxidic materials and noble metals. 1~ Moreover, other potential applications of chlorine leaching will be the treatment of valuable minerals such as molybdenite n and particularly the ores, mattes, or speiss of cobalt and nickel. Along this line, the development in recent years of the Falconbridge Chlorine Leach Process 12 on a commercial scale in Norway has demonstrated its competitiveness with a conventional pyrometallurgical process, the electro-refining (Hybinette) process for treating nickel matte. This paper presents the results obtained from a kinetic study of chlorine leaching of natural cobalt triarsenide (skutterudite). Experimental work includes stoichiometry and particle size, chlorine and hydrochloric acid concentrations, stirring speed, and temperature effects. The results have been interpreted in terms of mixed transport/ chemical control. II. T H E AQUEOUS SOLUTIONS OF C H L O R I N E Gaseous chlorine is moderately soluble in water. In addition to C12/aq) these solutions contain other species13 since a J. VII~/ALS, Professor of Extracnve Metallurgy, C. NUIqEZ, Professor of Extractive Metallurgy and Department Head, and J. OLIVERAS, Graduate Student, are with the Department of Metallurgy, University of Barcelona, Avda. Diagonal 647, 08028-Barcelona, Spain. Manuscript submitted February 28, 1986.
METALLURGICALTRANSACTIONSB
hydrolysis reaction takes place quickly. Similarly, there is an association reaction in chloride medium. 14 The nature of these solutions can therefore be described in the following terms: C12(g/ ~ C12(aq)
Kt~298)= 6.2 • 10-2
[1] Clzfaq) + H 2 0 ~
H + + C1- + HC10 K2(298) = 3.94 z 10 -4 [2]
C12(aq I + C1- ~- C13
K3(298) =
1.95 • 10 -1
[3] Calculation of the distribution of chlorine species (Table I) in hydrochloric acid solutions of varying concentration and temperature, i.e., under conditions of interest for the present kinetic study, was based on the equilibrium constants involved and the corresponding balances of mass and charge. For the hydrolysis reaction, assuming YClz = YHCIOand with y• as the mean ionic activity coefficient of HC1, we have:
K2 =
aH+acl aHCIO IH+IIEVIIHC10[ ac,~,. Ic12(~q)l y~.c,
[4]
The.K2 values given by Connick and Chia 15 were used at different temperatures. For temperatures abov
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