Optimization of Lead and Silver Extraction from Zinc Plant Residues in the Presence of Calcium Hypochlorite Using Statistical Design of Experiments

Abstract

In this work, a chloride/hypochlorite leaching process was performed for zinc plant residues. Sodium chloride and calcium hypochlorite were used as leaching and oxidizing agents, respectively. Fractional factorial method has been used to test main effects, and interactions among factors were investigated. The statistical software named Design-Expert 7 has been utilized to design experiments and subsequent analysis. Parameters and their levels were reaction time (t = 16 and 120 minutes), reaction temperature [T = 303 K and 343 K (30 °C and 70 °C)], solid-to-liquid ratio (S/L = 1/6 and 1/38), pH (pH = 0.5 and 2), and Ca(OCl)2 concentration (C = 0.6 and 3 g/L). Analysis of variance was also employed to determine the relationship between experimental conditions and yield levels. Results showed that reaction temperature and pH were significant parameters for both lead and silver extractions but solid-to-liquid ratio had significant effect only on lead extraction. Increasing pH reduced leaching efficiency of lead and silver. However, increasing reaction temperature promoted the extraction of lead and silver. Ultimate optimum conditions from this study were t 1: 16 min, T 2: 343 K (70 °C), (S/L)2: 1/38, pH1: 0.5, and C 1: 0.6 g/L. Under these conditions, extractions of lead and silver were 93.60 and 49.21 pct, respectively.