The mixed-control kinetics of ferric chloride leaching of galena
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reasing NaC1 concentration, the reaction rate also increased. These facts are consistent with the observation by Dutrizac I31 that the rate of galena dissolution in ferric chloride media is controlled by the diffusion of PbC12 through the sulfur layer. Thus, if we combine the effects of the surface reaction and the diffusion of PbC12 through the product layer, the kinetics of galena leaching may be unified to include all operating conditions. The possibility that the diffusion of reactant FeE13 might be ratelimiting instead of the product PbC12 was excluded based on the following information: (1) The reaction rate is independent of ferric concentration at levels higher than 0.1 to 0.2 M.t2"3~ (2) The reaction rate increases even when the diffusivity of ferric ion decreases as NaC1 concentration increases. ~21 For an approximate but simple expression of the rate of leaching under the effect of both chemical reaction and pore diffusion, the law of additive reaction times for fluid-solid reactions previously developed taJ was applied in this study in the following form: [1]
t = tr + td
where t = time required to attain a certain conversion; tr = time required to attain the same conversion under infinitely fast intraparticle diffusion; td = time required to attain the same conversion under the control of the pore diffusion through the solid product layer. Previously, Morin e t a l . tS] reported that the mixed kinetic model gave a good representation of the kinetics of galena dissolution. However, they considered different controlling s t e p s i d i f f u s i o n and chemical reaction with respect to ferric i o n I w i t h o u t considering the effect of PbCI2. Furthermore, their experimental conditions were different in that ferric concentration was as low as 1.0 x
10 -2 M .
When the diffusion of the fluid product species (lead chlorocomplexes) is rate-controlling, the reaction time required to attain a certain conversion in a spherical particle is expressed as follows: pro td = 2De(C* - Cb)
1 - 2 X 3
(1 - X) 2/3
[2]
where p (mol/cm3), ro (cm), and X represent the molar density of PbS, the half thickness of a PbS cube, and fractional conversion, respectively. C* and Cb are the saturation and the bulk concentrations (mol/cm 3) of PbCI2, respectively. The effective diffusivity of PbC12, D e, c a n be determined from experimental data obtained under diffusion-controlled conditions by applying Eq. [2]. Computation based on the data in References 2 and 3 assuming rate control by PbC12 diffusion gives the value of effective diffusivity as 9.80 x 10- 5 c m2/ m i n at 50 oC and NaC1 3 M solution and 4.79 x 10 -5 cm2/min at 80 ~ without NaC1 addition, respectively. The latter value, after correcting for the effect of temperature and NaC1 concentration, was much too low to be applicable to the analysis of the rate data in Reference 1. It gave reaction times slower than the experimental values obtained under largely chemically controlled conditions, t~l Thus, the former value was used as the reference value VOLUME 20B, FE
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