White carbon black wastewater treatment by electrodialysis: Salt separation, silicon sol transporting and wastewater recycling

White carbon black (WCB) as an important silica product has been widely used in many industrial processes. At present, precipitation method is commonly adopted for WCB production, in which a large amount of wastewater containing sodium sulfate and silica is produced. Therefore, it is important and urgent to develop an efficient, low-cost wastewater treatment process, which could extract sodium sulfate from wastewater as value-added byproducts and then recycle this wastewater back to WCB process. In this study, electrodialysis (ED) method, as a proven salt-separation technology, was used to treat WCB wastewater. The existence of silica in the wastewater brings more difficulties for its ED treatment. Firstly, to evaluate ED performance on treating WCB wastewater, the effects of applied voltage, flow rate and initial volume ratio on the overall current efficiency (eta), recovery ratio (R), residual rate (gamma) and concentration ratio (omega) were investigated. The results show that under the optimized conditions, the highest recovery ratio of Na2SO4 could reach 86.45%, the lowest overall residual rate of SiO2 could go down to 2.69%, and the dilute solution could achieve the reuse standard of waste water. Secondly, the influence of silica on ED performance was investigated. It is found that only about 5.91% of silica in wastewater was transporting from dilute solution to concentrated solution under the optimized conditions. The SEM images of ED membrane indicate that after ED experiments some SiO2 particles adhered to the surface of ion exchange membrane, which would not affect ED normal performance in a short term. These results illustrate that ED is an effective, low energy-cost and reliable process to treat WCB wastewater.