Intrinsic kinetics of the hydrogen reduction of copper sulfate: Determination by a nonisothermal technique
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I.
INTRODUCTION
RISING costs of energy and environmental control are subjecting traditional pyrometallurgical processes to economic stress. The prospects for the utilization of hydrogen as a reductant for the extraction of some metals appear to be promising. Solutions containing copper are obtained by leaching various copper ores. Copper is selectively extracted from the leach solution by solvent extraction. The organic phase is stripped by aqueous sulfuric acid to form CuSO4 solution from which CuSO4 9 5HeO may be crystallized. After the dehydration of CuSO4 9 5HeO, metallic copper can be obtained by the hydrogen reduction of CuSO4. This process has potential technical importance as an alternative to electrowinning, because CuSO4 is reduced to high-purity metal at relatively low temperatures. In spite of the considerable potential of this process, very little work has been done in this area. HegediJs and Fukker ~ showed that metallic copper was formed by the reduction of CuSO4 with H2. Vo Van and Habashi 2 reported that Cu2SO~ was formed as an intermediate solid product and SO: and H:O as the gaseous products during the reduction. They also presented the results of a study on the reaction kinetics. However, their work does not indicate the particle size of the sample, the determination of the preexponentlal factor, and the dependence of rate on the gaseous reactant concentration and the amount of solid unreacted. It is the objective of this paper to report the results of an investigation to determine the intrinsic kinetics of the hydrogen reduction of copper sulfate. A nonisothermal technique was used which yields the temperature dependence of the rate constant from a single run.
II.
T H E O R E T I C A L CONSIDERATIONS
A. Thermodynamics
CuSO4(s) + 2H2(g) = CuIs) + SO qg) + 2H_,O(g)
the value of the equilibrium constant, KI, is about 4 x 10 ~5 in the temperature range of interest. Table I shows the standard free energy for this reaction, obtained from JANAF Thermochemical data) Also shown in Table I is the heat of reacuon at different temperatures. As can be seen, the reaction is somewhat exothermic. A more comprehensive thermodynamic analysis of the hydrogen reduction of copper sulfate under various partial pressures of the gaseous species revolved and involving intermediate solid species is quite complex. Jacinto et al. z reported the results of such an analysis. In this paper we are concerned with the situation in which diffusion and mass transfer are very rapid so that the solid ~s always in direct contact with the reactant hydrogen. The reaction rate thus obtained will then represent the intrinsic kinetics. B. Kinetics
The intnnsic kinetics for the reaction of a solid with a gas may follow a number of different rate expressions, which can in general he written as dX dt
k .f,~p~).
Table I.
METALLURGICALTRANSACTIONS B
[2]
- k "J,(p,)"f:(X)
where X is the fractional conversion of the solid reactant, k the reaction-rate constant, fj(pa) the dependence of rate on the gaseous reactant concentration, a
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