Determination of the optimum conditions for the leaching of Cd–Ni residues from electrolytic zinc plant using statistical design of experiments

Abstract

This research is part of a continuing effort to reduce environmental conflicts and occupational hazards of cadmium bearing zinc plant residues (ZPR's) and to break through this problem and recover valuable constituents of the wastes. In this paper, effects of influential factors on extraction efficiencies of Cd, Zn, Ni, Pb and Cu from Cd–Ni filtercake as a major ZPR were investigated. In the view of above, the systematical and analytical evaluation method of Taguchi quality engineering has been applied for the leaching of the Cd–Ni filtercake to evaluate the optimal experimental conditions and hence to achieve the highest leaching performance and the best robustness of quantitation from the least number of trials in a batch laboratory scale. An L 25 orthogonal array (OA, five factors in five levels) was employed to evaluate the effects of temperature ( T = 25, 35, 45, 55 and 65 °C), acid concentration ( C = 0.31, 0.51, 0.82, 1.22 and 1.84 M), time ( t = 15, 30, 45, 60 and 90 min), pulp density (S/L = 125, 142.86, 166.67, 200 and 250 g/l) and stirring speed ( R = 200, 300, 400, 500 and 800 rpm) on extraction percent of the individual metals. Statistical analysis, ANOVA, was also employed to determine the relationship between experimental conditions and yield levels. The results showed that increasing temperature reduced performance characteristics. Two approaches were taken into consideration for the experiments, i.e., selective and collective leaching, and then the optimum conditions were sought for each considered approach. The experimental results for selective leaching showed that under optimal leaching conditions ( T: 45 °C, C: 0.82 M, t: 60 min, (S/L): 250 g/l, R: 400 rpm), the extraction of cadmium, zinc, nickel, lead and copper were 90.29, 96.86, 8.66, 0.11 and 0.01, respectively. However, the same results for optimal collective leaching conditions ( T: 25 °C, C: 1.22 M, t: 60 min, (S/L): 142.86 g/l, R: 200 rpm) showed that the extraction of Cd, Zn and Ni could be over 95%, with negligible dissolution of lead and copper.