Zinc dross is primarily the product of the reaction between zinc and iron. Its composition mainly consists of the iron-zinc alloy phase formed after the combination of zinc and iron. Therefore, zinc dross generally contains about 3 to 6% iron and about 94 to 97% zinc. In some hot-dip galvanizing processes, lead is added to protect the galvanizing pot. In such cases, zinc dross may contain 1.5 to 2% lead (not referring to lead mixed together). Generally, in the hot-dip galvanizing of steel pipes, aluminum is intentionally added to obtain a bright galvanized layer. Therefore, zinc dross may also contain a certain amount of aluminum.
Steel pipes can only be immersed in the zinc bath after pretreatment. During pretreatment, incomplete cleaning may cause iron salts to adhere to the surface of the steel pipe to be galvanized. At the galvanizing temperature, iron-based materials such as steel pipes, steel galvanizing pots, and pipe galvanizing machines undergo dissolution in the zinc bath. The mutual diffusion of iron and zinc atoms results in the formation of an iron-zinc alloy layer. Among them, the phase crystals can detach from the substrate and sink to the bottom of the galvanizing pot, accumulating to form zinc dross. Generally, in "dry method" hot-dip galvanizing production of steel pipes, the amount of zinc dross accounts for about 10 to 20% of the total zinc consumption.
During the formation of zinc dross, the reactions are as follows:
(1) Reactions caused by iron salts
FeCl₂ + 8Zn → ZnCl₂ + FeZn
FeCl₂ + 14Zn → ZnCl₂ + FeZn₁₂
(2) Reactions caused by steel
Fe₈C + 21Zn → 3FeZn₇ + CFe₈
C + 39Zn → 3Fe₈Zn₁₈ + C
