The treatment methods for zinc slag can be divided into two categories: wet process and pyrometallurgical process. For the pyrometallurgical process, distillation is the core method. Based on the type of distillation equipment, it can be further categorized 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 treating hot-dip galvanizing zinc slag shares the same advantages and disadvantages as horizontal tank distillation for treating hot-dip galvanizing zinc ash. Line-frequency coreless induction furnaces and arc furnaces for distilling hot-dip galvanizing waste slag are rarely used by manufacturers due to high equipment investment, low production capacity, complex condenser selection, and unsatisfactory condensation efficiency. Additionally, zinc slag is produced in dispersed locations and is not easily collected. On the other hand, the continuous distillation furnace is a new type of furnace specifically designed for treating hot-dip galvanizing waste slag. It completely overcomes the disadvantage of discontinuous processes in other pyrometallurgical methods for treating hot-dip galvanizing waste slag, enabling continuous production. Moreover, it boasts high zinc recovery rates, flexible equipment processing capacity, lower equipment investment, and reduced labor intensity compared to horizontal tank distillation, making it popular among enterprises 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. One 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 ethylenediaminetetraacetic acid (EDTA) as the electrolyte. Under the action of direct current, the anode continuously dissolves, and zinc precipitates out at 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 makes it difficult to remove iron from the electrolyte, limiting the industrial application of this method. 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.
