Recovery of zinc granules from synthetic electroplating wastewater using fluidized-bed homogeneous crystallization process

In the present work, the recovery of zinc granules from synthetic electroplating wastewater was evaluated using fluidized-bed homogeneous crystallization. The effect of carbonate-to-zinc ([CO32-]/[Zn2+]) molar ratio (1.0-2.0), precipitant pH (10.30-11.20), initial zinc concentration (100-500 mg/L), and anions (Cl-, F-, and NO3-) on the removal and granulation efficiencies of zinc was investigated. Results show that the highest granulation efficiency of 96.70% was attained at an influent zinc concentration of 300 mg/L, precipitant pH of 10.60 and [CO32-]/[Zn2+] of 1.2. Meanwhile, the highest removal efficiency of 99.90% was obtained at a precipitant pH of 10.60, [CO32-]/[Zn2+] of 1.2, and influent zinc concentration of 100 mg/L. Moreover, the residual zinc concentration of 0.15 mg/L was attained in the treated effluent, which is within the maximum contaminant level of 5.0 mg/L set by the US Environmental Protection Agency and World Health Organization. The presence of anions had little but insignificant effect on the removal where the treated effluent has a residual zinc concentration of 0.44 mg/L. Based on the X-ray diffraction analysis, zinc granules were recovered in the form of smithsonite and hydrozincite with rhombohedral-hexagonal and monoclinic structures, respectively. A broad size distribution was displayed by zinc granules where majority of the pellet diameters fall within the range of 0.149-2.000 mm. Overall, fluidized-bed homogeneous crystallization produced higher-purity pellets and proved to be an effective alternative to seeded crystallization technology.