Selective leaching of zinc from hazardous As-bearing zinc plant purification filter cake

- Leaching behavior of ZPHPFC was investigated in acidic, neutralized and alkaline medias.
- Leaching in pH 5 resulted suitable efficiency for Zn and also low amounts of impurities.
- Taguchi method was applied to pursue the possibility of reducing filtrate impurities.
- Ultimate optimal leaching conditions were chosen.
- Obtained leaching filtrate could be used to produce cathodic zinc.

This article is part of a continuing endeavor to decrease environmental conflicts and industrial hazards of As-bearing zinc plant hot purification filter cakes (ZPHPFC's) and to break through this problem and extract Zn selectively from filter cakes to obtain a filtrate containing Zn capable of feeding to zinc electrowinning stage. In this part of research, a process was investigated with a view to selective Zn leaching from ZPHPFC, as well as diminishing the quantity of the ZPHPFC to be dumped. Firstly, pH-dependent leaching tests were done by H2SO4 or NaOH solution with the pH ranging from 1 to 13 to monitor Zn and impurities leaching behaviors and finding suitable pH zone for selective Zn leaching. Then, orthogonal array (OA) of Taguchi's method was employed to arrange the experimental runs in order to distinguish the most notably affecting factors and the optimum leaching conditions for maximizing Zn extraction with minimum impurities dissolution. OA L16 (45) consisting of five factors, each with four levels, was utilized to scrutinize the effects of reaction temperature [T = 303, 313, 323, 333 (30, 40, 50, 60) K (°C)], reaction time (t = 30, 60, 90, 120 min), pH (pH = 4.5, 5, 5.5, 6), pulp density (S/L = 50, 100, 150, 200 g/L), and stirring speed (R = 375, 500, 625, 750 rpm), on leaching efficiency of the individual metals. Statistical analysis of variance (ANOVA) was done to conclude the relationship between experimental conditions and metals leaching efficiencies. The results showed that increasing pH reduced Zn, Fe and Cd leaching efficiencies. Likewise, increasing of temperature declined extraction of impurities such as Ni, Cd, and Co. Also, time increasing, especially in high temperature, decreased Ni and Co leaching efficiencies. This is due to that conditions in leaching media are suitable for cementation of Ni and Co by metallic zinc that exists in the ZPHPFC in the presence of As. The experimental results for selective leaching showed that under ultimate optimal leaching conditions (T4: 333 K (60 °C), t4: 120 min, pH2: 5, (S/L)3: 150 g/L, R3: 625 rpm), the leaching efficiency of Zn could be almost 95%, with low dissolution of impurities.

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