83. How is zinc slag formed? What are its components?

Zinc slag is primarily the product of zinc-iron reactions, consisting mainly of an iron-zinc alloy layer formed through the combination of zinc and iron. 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, resulting in zinc slag containing 1.5-2% lead (not the lead mixed with zinc). During hot-dip galvanizing of steel pipes, aluminum is intentionally added to achieve a bright galvanized coating, which is why zinc slag may also contain a certain amount of aluminum.
Steel pipes must undergo pretreatment before being immersed in molten zinc. Inadequate cleaning during pretreatment causes iron salts to adhere to the pipe surfaces. At galvanizing temperatures, the zinc bath dissolves the iron-based substrates (pipes, galvanizing pots, and galvanizing machines), enabling mutual diffusion between iron and zinc atoms to form an iron-zinc alloy layer. The resulting phase crystallization detaches from the substrate, settling at the bottom of the galvanizing pot to form zinc sludge. In dry-process hot-dip galvanizing production, zinc sludge typically accounts for 10-20% of total zinc consumption.
The zinc slag formation process involves the following reactions: (1) Iron salt-induced reaction: FeCl₂ + 8Zn → ZnCl₂ + FeZn; FeCl₂ + 14Zn → ZnCl₂ + FeZn; 12 (2) Steel-induced reaction: Fe8C + 21Zn → 3FeZn7 + C; Fe8C + 39Zn → 3Fe8Zn18 + C.