Zinc dross is primarily the product of the reaction between zinc and iron, and its composition is mainly the ζ-phase of the iron-zinc alloy layer formed after the combination of zinc and iron. Therefore, the iron content in zinc dross is generally around 3-6%, and the zinc content is around 94-97%. In some hot-dip galvanizing processes, lead is added to protect the galvanizing pot. In such cases, zinc dross contains 1.5-2% lead. Sometimes, aluminum is also added to improve the surface finish and structural properties of the galvanized layer, so there will also be some aluminum content in the zinc dross.
The iron in zinc dross mainly originates from iron salts attached to the surface of steel pipes during pretreatment, as well as from the steel pipes themselves, the galvanizing pot, and galvanizing equipment. After dissolution by zinc liquid and mutual diffusion of iron and zinc atoms, an iron-zinc alloy layer is formed, with the crystalline ζ-phase layer (drifting layer) detaching from the steel substrate and accumulating at the bottom of the galvanizing pot. This gradually accumulates to form a zinc dross layer.
The reactions during the formation of zinc dross are:
Reactions caused by iron salts:
FeCl₂+8Zn→ZnCl₂+FeZn₇
FeCl₂+14Zn→ZnCl₂+FeZn13
Reactions caused by steel:
Fe3C+21Zn→3FeZn13+C
Fe3C+39Zn→3FeZn13+C
Properties of zinc dross:
The melting point of zinc dross is higher than that of pure zinc.
The density of zinc dross is greater than that of pure zinc. According to measurements by Schurmann, the densities of zinc dross with iron contents of 6.2%, 7.0%, 9.0%, and 22% are 7.15, 7.24, 7.24, and 7.36 kg/dm³ respectively.
The crystalline shape of zinc dross belongs to the symmetric monoclinic system and hexagon, with unique angular characteristics.
The color of zinc dross is grayish-white with a silvery.
Zinc dross contains impurities such as lead, aluminum, antimony, cadmium, copper, and insoluble compounds of acids.
The zinc content in zinc dross is about 94-97%.
The iron content in zinc dross is about 3-6%.
Zinc dross is brittle compared to pure zinc, thus lacking flexibility.
9) The essence of zinc dross is an alloy formed by the inclusion of the drifting ζ-phase and栅状δ1-phase layers in zinc, along with other impurities.
