Determination of the Optimum Conditions for the Leaching of Lead from Zinc Plant Residues in NaCl–H₂SO₄–Ca(OH)₂ Media by the Taguchi Method

This research is part of a continuing effort to reduce environmental conflicts and occupational hazards of lead-bearing zinc plant residues (ZPRs), and to break through this problem and recover lead of the wastes. The residue with an assay of 14.4% Pb was used in chloride leaching for lead recovery, and sulfate was controlled in the leaching stage by the addition of Ca­(OH)₂. In this paper, the effects of influential factors on extraction efficiency of Pb from ZPRs were investigated. Taguchi’s method based on orthogonal array design (OAD) has been used to arrange the experimental runs in order to maximize lead extraction from a ZPR. Orthogonal array (OA) L₈(2⁷) consisting of seven parameters, each with two levels, was employed to evaluate the effects of NaCl concentration (<i>C</i> = 300 and 400 g/L), stirring speed (<i>R</i> = 500 and 700 rpm), reaction temperature (<i>T</i> = 55 and 65 ◦C), reaction time (<i>t</i> = 1 and 8 h), liquid-to-solid ratio (L/S = 6 and 20), acidic pH (pH_a= 2 and 3.5), and neutral pH (pH_b= 4 and 5.5) on lead extraction percent. Statistical analysis, ANOVA, was also employed to determine the relationship between experimental conditions and yield levels. The results showed that the pulp density, and NaCl concentration were significant parameters, and increasing pulp density reduced leaching efficiency of lead. However, increasing NaCl concentration promoted the extraction of lead. The obtained optimum conditions from this study were <i>C</i>₂, 400 g/L; <i>R</i>₂, 700 rpm; <i>T</i>₁, 55 °C; <i>t</i>₁, 1 h; (L/S)₂, 20; (pH_a)₂, 3.5; and (pH_b)₁, 4. But only two significant factors (<i>C</i>₂, 400 g/L; and (L/S)₂, 20) were used to estimate the performance at the optimum conditions. The calculated leaching percent (85.91%) was in reasonable agreement with the experimental results in optimum conditions