Galvanizing Ash Recycling New Technology Research

Hot dip galvanizing ash with the zinc content up to 65%～80% is generated during galvanizing of steel and iron, and is considered as an appreciable secondary zinc resource. In this paper, a new process of recovering zinc from hot dip galvanizing ash was studied.Hot dip galvanizing ash was separated to undersize material and oversize material by dry grinding and sieving. Chlorine in undersize material was removed by alkali washing. The results show that the better conditions for the chlorine removing efficiency by alkali washing are as follows:reaction temperature,363 K; Na2CO3 concentration,8 g·L-1; reaction time,90 min; ratio of liquid to solid,12.5:1(ml·g-1). In this experiment conditions, chlorine content in undersize material was decreased to 0.4 wt% and chlorine removing efficiency is 88.17%. Zinc chloride in oversize material was removed by evaporating in vacuum. Chlorine content in oversize material was decreased to 0.12 wt% and chlorine removing efficiency is 57.1% under the following experimental conditions:a temperature of 723 K, a vacuum evaporation time of 30 min and a residual gas pressure of 35 Pa.Zinc was recovered from oversize material by vacuum distillation after removing chlorine. Pure metal zinc(99.84 wt%) was obtained under the following experimental conditions:briquetting at a pressure of 20 MPa,distilling at a temperature of 898 K, a distilling time of 20 min and a residual gas pressure of 20 Pa.After removing chlorine, ZnO in undersize material was reduced to metal zinc by silicon in vacuum. Based on the thermodynamical calculations and analysis, reducing ZnO by silicon is thermodynamically possible at 1100-1500 K. However, about 50% of the ZnO was not reduced because the SiO2 generated by the reduction of ZnO with silicon can react with ZnO and produce 2ZnO·SiO2. Upon the addition of CaO, it can react with SiO2 before ZnO to inhibit the production of 2ZnO·SiO2, and ZnO can be reduced to Zn completely. The results showed that the reduction efficiency of ZnO was 92.81% and the yield of metal zinc was 89.82%, the metal zinc obtained was well crystallized under the following experimental conditions:a temperature of 1448 K, a vacuum reduction time of 120 min and a residual gas pressure of 20 Pa.