85. What is the effect of molten zinc temperature on zinc slag? At what iron content in molten zinc can it no longer be used?

When molten zinc reaches high temperatures, significant amounts of iron dissolve into it. For instance, at 510℃, 0.10% of iron dissolves, forming zinc slag that constitutes 1.6% of the total zinc content in the galvanizing pot. Even after cooling to 435℃, residual iron remains at 0.02%. During cooling, this iron precipitates as minute iron-zinc compound crystals that slowly settle at the pot's bottom. To minimize these crystalline zinc residues (iron-zinc alloys), the molten zinc must be maintained at 435℃ for approximately one day post-high temperature treatment. However, such prolonged holding is strictly prohibited in practical operations, necessitating temperature reduction during galvanizing processes.
When the temperature of molten zinc rises, intensified heat convection carries zinc slag upward into the galvanizing pot, contaminating the zinc bath at immersion depth and degrading galvanized layer quality. The presence of zinc slag further disrupts molten zinc flow, eroding the iron-zinc alloy coating on pot walls. This exposes the walls to accelerated corrosion and leads to increased zinc slag accumulation.
Prolonged retention of zinc slag in galvanizing pots leads to its sintering into lumps, with the phenomenon worsening as temperature increases. This not only complicates removal but also impedes heating of the galvanizing pot, resulting in overheating of the pot walls (steel plates) and subsequent zinc leakage through perforation.
In a normally operating hot-dip galvanizing production process, the iron content near the surface of molten zinc should be minimal, typically not exceeding 0.05%. If the iron content reaches or exceeds 0.2%, hot-dip galvanizing should be discontinued. Since the zinc immersion depth is generally around 400 millimeters, the iron content in this area may be relatively higher, necessitating strict control measures.