Hydrometallurgical valorization of chromium, iron, and zinc from an electroplating effluent

Abstract

Hydrometallurgical removal of heavy metals (like chromium, iron, and zinc) from an industrial effluent, and their valorization into less-toxic species has been studied. At first, the solvent extraction of Cr(VI) was investigated with tri-butyl phosphate (TBP) to separate this carcinogenic metal from an electroplating effluent containing 3.43 g·L−1 Cr, 1.3 g·L−1 Zn, 0.41 g·L−1 Fe, and 1.23 mol·L−1 free acid of chloride medium. The study based on parametric variation revealed the spontaneity in adduct formation (HCrO3Cl·2TBP) into organic phase through the exothermic extraction process (ΔH°, −18.8 kJ·mol−1). The extracted species of Cr(VI) was efficiently stripped (∼99%) as less-toxin Cr(III) by 30 min of contact with 2.0 mol·L−1 ascorbic acid solution. Thereafter, the hydrolytic precipitation of Cr(III) from stripped solution at pH ∼9.0, and Fe(III) from raffinate at pH 3.5 under continuous air-flow could yield the precipitates of Cr(OH)3 and FeOOH·2H2O, respectively. From the Fe-removed solution, a 5% stoichiometric excess of oxalic acid could effectively precipitate >99.8% zinc in 30 min at 50 °C.