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HaiGang Yunyi
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Tianjin Haigang Steel Coil
Service first, quality assured. More dedicated and focused on crafting even better boards.
14
2020
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09
The Generation and Control of Zinc Dust and Zinc Slag
Zinc dross and zinc slag not only seriously compromise the quality of the zinc-plated layer, resulting in an uneven coating and the formation of zinc nodules, but also significantly increase the cost of hot-dip galvanizing. Typically, each ton of workpiece requires 80 to 125 kg of zinc for plating; however, if the zinc dross and slag are particularly severe, the zinc consumption can rise to between 140 and 200 kg. To effectively control zinc dross and slag, it is crucial to maintain proper temperature control, thereby reducing surface oxidation of the zinc bath that leads to slag formation. Some Chinese manufacturers use fire-resistant sand or carbon ash as a covering material. Abroad, it is common practice to employ ceramic or glass beads—materials with low thermal conductivity, high melting points, low specific gravity, and no chemical reaction with the zinc bath. These beads not only help minimize heat loss but also prevent oxidation. Moreover, these spherical particles are easily carried away by the workpieces and do not adhere to them at all.
The formation of zinc dross in the zinc bath is primarily due to the mobility deviation of non-ferrous metals—specifically, when the iron content dissolved in the zinc bath exceeds its solubility at a given temperature. The zinc content in zinc dross can reach as high as 95%, making it the root cause of increased costs in hot-dip galvanizing. As shown by the solubility curve of iron in the zinc bath, the amount of iron dissolved—that is, the iron loss—varies depending on both the temperature and the duration of thermal insulation. At 500°C, the iron loss increases significantly with rising temperature and longer insulation time, exhibiting an almost linear relationship. Below or above the range of 480–510°C, the increase in iron loss with time slows down considerably. Therefore, the temperature range of 480–510°C is referred to as the "worsening dissolution zone." Within this temperature range, the zinc bath exhibits more severe corrosion of both the workpiece and the zinc pot. Above 560°C, the iron loss continues to rise sharply, and once the temperature exceeds 660°C, the zinc bath causes devastating corrosion of the iron substrate, leading to a dramatic increase in zinc dross and rendering galvanizing impossible. Consequently, hot-dip galvanizing is currently carried out predominantly within two temperature ranges: 450–480°C and 520–560°C.
Control of Zinc Slag Quantity
To reduce zinc slag, it is necessary to lower the composition of the zinc liquid—specifically, by addressing the many factors that contribute to iron dissolution.
⑴ During plating and thermal insulation, avoid the iron’s melting peak range—specifically, do not carry out galvanizing operations when the temperature is between 480°C and 510°C.
⑵ The raw materials for zinc pots should, whenever possible, be thick steel plates welded electrically with low carbon and silicon contents. High carbon content accelerates the corrosion of the zinc pot by the molten zinc, while high silicon content promotes the erosion of iron by the molten zinc. Currently, 08F high-quality alloy steel plates are widely used. These plates have a carbon content of 0.087% (ranging from 0.05% to 0.11%), a silicon content of no more than 0.03%, and contain elements such as nickel and chromium that can inhibit iron corrosion. Carbon structural steel must not be used, as it would lead to excessive zinc consumption and a shorter service life for the zinc pot. Some people have also suggested using carbon-composite materials to fabricate molten-zinc tanks. Although this approach could address the issue of significant iron loss, challenges remain in terms of design and manufacturing processes.
⑶ Regularly skim off the dross. First, raise the temperature to the upper limit of the processing temperature range to facilitate separation of the zinc dross from the molten zinc. Then, lower the temperature below the processing temperature range so that the zinc dross settles to the bottom of the tank and can be scooped out using a ladle. Parts immersed in the molten zinc should also be promptly removed.
⑷ Avoid allowing the fluxing agent to be carried into the zinc bath along with the workpieces. After a certain period of use, the fluxing agent will gradually convert into light-brown iron-containing compounds; therefore, it should be filtered out regularly. It is best to maintain the pH value of the fluxing agent around 5.
⑸ When the aluminum content in the plating bath falls below 0.01%, the conversion of residues will be accelerated. Proper amounts of aluminum not only enhance the fluidity of the zinc bath and improve the color tone of the coating, but also help reduce zinc dross and zinc ash. A small amount of aluminum floating on the liquid surface is beneficial for minimizing air oxidation; however, excessive aluminum can adversely affect coating quality and lead to pinholes.
⑹ Heating and temperature raising must be uniform to avoid overheating and localized overtemperature.
Strip steel, galvanized strip steel, galvanized steel strip, galvanized strip steel manufacturer, cold-rolled hard strip steel
We primarily produce various products including galvanized strip steel, pickled strip steel, and cold-rolled strip steel. These products are widely used in industries such as construction, mechanical manufacturing, automotive, and household appliances.
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