Copper wastewater treatment with high concentration in a two-stage crystallization-based combined process

Crystallization process in a fluidized bed reactor (FBR) to treat metal wastewaters has been considered as a suitable alternative to the chemical precipitation. However, the process efficiency was high under low initial concentrations, but decreased with the increase of metal concentration due to the unwanted homogeneous precipitation. Aiming at the treatment of heavy metal wastewater with high concentration, a modified FBR (mFBR) was investigated and the two-stage crystallization-based combined process was proposed. mFBR was more suitable for copper removal with high concentration than FBR. The copper concentration was reduced to 9.92 mg/L after the two-stage crystallization, corresponding to the removal rate of 96.6%. After filtration and ion exchange, the copper concentration was further reduced to 0.739 and 0.175 mg/L. During crystallization precipitation, the median size of the silica sand was gradually increased from 0.182 to 0.260 mm. The glossy surface of silica was gradually densely covered with short crystal precipitate, and obvious angularity gradually disappeared due to the deposition of copper precipitate. In addition, the water content of the crystallization precipitate was about 13.7% and much lower than the traditional chemical sludge. Copper precipitate was mainly composed of CuCO₃ and Cu(OH)₂.