Zinc dross is primarily the product of the reaction between zinc and iron. Its main component is the iron-zinc alloy layer phase formed by 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 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 molten zinc after pretreatment. During pretreatment, if cleaning is not thorough, iron salts may adhere to the surface of the steel pipes to be galvanized. At galvanizing temperatures, iron-based materials such as steel pipes, steel galvanizing pots, and pipe galvanizing machines are dissolved by molten zinc. The mutual diffusion of iron and zinc atoms results in the formation of an iron-zinc alloy layer. Among them, certain phases (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 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) Reaction caused by iron salts
FeCl₂ + 8Zn → ZnCl₂ + FeZn
FeCl₂ + 14Zn → ZnCl₂ + FeZn₁₂
(2) Reaction caused by steel
Fe₈C + 21Zn → 3FeZn₇ + CFe₈
C + 39Zn → 3Fe₈Zn₁₈ + C (Note: The chemical formula for carbon in the second reaction should be noted as it doesn't change in this context, but it's important to recognize that carbon is a reactant.)
