65. Under what conditions is zinc slag produced? What are its components and properties?

Zinc slag primarily consists of the reaction products between zinc and iron, with its main component being the ζ-phase iron-zinc alloy layer formed through their combination. Consequently, zinc slag typically contains 3-6% iron and 94-97% zinc. In some hot-dip galvanizing processes, lead is added to protect the galvanizing pot bottom, resulting in zinc slag containing 1.5-2% lead. Additionally, aluminum is sometimes incorporated to enhance the surface smoothness and microstructure of the galvanized layer, leading to trace amounts of aluminum in the zinc slag.
The iron in zinc slag primarily originates from iron salts adhering to steel pipe surfaces during pretreatment, along with iron-zinc alloy layers formed through the dissolution of zinc liquid and mutual diffusion between iron and zinc atoms within the pipes, galvanizing pots, and plating equipment. The crystallized ζ-phase layer (known as the drift layer) detaches from the steel substrate and accumulates at the bottom of the galvanizing pot, gradually forming the zinc slag layer.
The formation of zinc slag involves two main reactions: 1) Iron salt-induced reaction: FeCl₂ + 8Zn → ZnCl₂ + FeZn₇; FeCl₂ + 14Zn → ZnCl₂ + FeZn₁₃. 2) Steel-induced reaction: Fe₃C + 21Zn → 3FeZn₁₃ + C; Fe₃C + 39Zn → 3FeZn₁₃ + C. Key properties of zinc slag include: 1) A higher melting point than pure zinc; 2) Greater density than pure zinc. According to Schullman's method, the zinc slag densities at iron contents of 6.2%,7.0%,9.0%, and 22% are 7.15,7.24,7.24, and 7.36 kg/dm⁻⁸, respectively.
3) The crystalline morphology of zinc slag exhibits symmetrical monoclinic and hexagonal structures with distinctive angular features; 4) It displays a grayish-white color with a silvery luster; 5) The slag contains impurities such as insoluble compounds of lead, aluminum, antimony, cadmium, copper, and acids; 6) The zinc content ranges from 94% to 97%; 7) Iron content is approximately 3% to 6%; 8) Due to its brittleness, zinc slag lacks the flexibility of pure zinc; 9) Essentially, zinc slag is an alloy formed by zinc mixed with the ζ phase of the floating layer, δ1 phase of the lattice layer, and other impurities.