The treatment of zinc slag can be categorized into two main methods: wet process and pyrometallurgical process. The core of the pyrometallurgical process is distillation. Depending on the distillation equipment used, it can be further divided into horizontal tank distillation, line-frequency coreless induction furnace distillation, arc furnace distillation, and continuous distillation furnace distillation. The distillation products can be metallic zinc, zinc powder, or higher-quality zinc oxide, depending on the needs. Horizontal tank distillation for the treatment of hot-dip galvanizing zinc slag shares the same advantages and disadvantages as horizontal tank distillation for the treatment of hot-dip galvanizing zinc ash. The use of line-frequency coreless induction furnaces and arc furnaces for the treatment of hot-dip galvanizing waste slag is currently limited among manufacturers due to high equipment investment, low production capacity, complex condenser selection, and unsatisfactory condensation efficiency. Additionally, the dispersed origin of zinc slag makes it difficult to collect. On the other hand, the continuous distillation furnace is a new type of furnace specifically designed for the treatment of hot-dip galvanizing waste slag. It fully overcomes the discontinuity of other pyrometallurgical processes for treating hot-dip galvanizing waste slag, enabling continuous production. Moreover, it boasts a high zinc recovery rate, flexible equipment processing capacity, low equipment investment, and reduced labor intensity compared to horizontal tank distillation, making it popular among companies specializing in zinc slag treatment and thus widely used.
The wet process for treating hot-dip galvanizing waste slag can be divided into two completely different methods based on the products obtained. The first is the soluble anode electrolysis method, where the waste slag is cast or die-cast into an anode, with an aluminum plate as the cathode and an aqueous solution of sulfuric acid or sulfuric acid esters as the electrolyte. Under the action of direct current, the anode continuously dissolves, and zinc precipitates out on the cathode, ultimately producing electrolytic zinc. The advantages of this method include high zinc recovery rates. However, the major drawback is the rapid accumulation of iron in the electrolyte, which is difficult to remove, thus limiting its industrial application. The other method is the production of zinc sulfate heptahydrate. This method involves dissolving the zinc slag in an aqueous sulfuric acid solution, removing impurities such as iron, and then concentrating and crystallizing the aqueous zinc sulfate solution to obtain zinc sulfate heptahydrate.
