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Site: Anshan Iron and steel plant

Office address: Xi min, Tiexi District, Anshan, Liaoning

No. 111 Sheng Lu


Safe operation of fifth section zinc pot

Current position: Home >> technical information >> Safe operation of fifth section zinc pot

The so-called normal or safe operation of the zinc pot means that the stable corrosion rate of the inner wall of the zinc pot is less than 4mm/a during use. If we do a good job in the design and manufacture of hot-dip galvanizing furnace and zinc pot, and pay special attention to the temperature measurement and control during heating and operation, then the operation process generally does not need special inspection of the zinc pot, according to the use of time and output to determine the replacement period.

1. heating and control of zinc pot

In addition to the structural design of zinc plating furnace and pot, heating and temperature-rising zinc melting specifications and other factors affecting the life of zinc pot, the heating and control methods of zinc pot in operation also have an important impact on the life of zinc pot. In order to discuss this problem conveniently, the concept of heating intensity is introduced here. The so-called heating intensity refers to the amount of heat supplied by the heat source to the wall of the unit area per unit time. If the total amount of heat provided is fixed, the longer the heating time or the larger the heating area, the smaller the heating intensity; otherwise, the greater the heating intensity.

The heat source is in a dynamic equilibrium state between the heat input to the zinc solution through the wall of the zinc pot and the heat output from the zinc solution heating the plating parts and the zinc surface heat dissipation, so the zinc liquid phase should be in a certain temperature. Only when the dynamic balance is in a suitable state, can the zinc liquid keep the required temperature. When heating a zinc pot with a heat source, a temperature gradient will occur between the inner and outer walls of the pot, which is necessary for heat transfer. There is a functional relationship between the temperature gradient and the heat intensity and the heat transfer coefficient of the steel. The coefficient of heat transfer of steel varies little in a certain temperature range. If it is regarded as a constant, the relationship between the temperature gradient of the wall of the zinc pot and the heating intensity is a positive linear function, as shown in Fig. 1-5. When the heating intensity increases, the wall temperature gradient will increase and the inner wall temperature of the pot will be increased.


Liquid zinc contacts and reacts with the inner wall of the zinc pot. The effective thickness of the wall of the zinc pot will be thinned because of the reaction erosion. The most severe reaction temperature range is 490 ~ 530 C. Therefore, when heating the zinc pot, the temperature of any part of the inner wall of the zinc pot should be lower than the temperature range. If the heating intensity is reduced, the temperature of zinc liquid in and around the inner wall of the zinc pot will also be reduced, and the temperature difference between the inner wall and the working area of the zinc pot will be reduced.

The more uniform the inner wall of the pot, the better, and any local point must be below 490 T., which is what people want. Although the inner wall temperature of a zinc pot is usually not directly measured, it can be seen from the increase in the amount of zinc slag generated if the temperature is often at or above the critical temperature. In addition to the_grain from the workpiece and the wall of the pot, the iron in the inner wall of the pot diffuses into the zinc bath and reacts with zinc through the Fe-Zn alloy layer. The saturation of iron in zinc solution is 0.035%(450 C). Excess iron will cause_grain to grow. The amount of iron diffused into zinc bath depends on the inner wall temperature of the pot. When the formation of zinc slag suddenly increased and no other reasons were found, it is almost certain that the interfacial temperature between the pan wall and the Fe-Zn alloy layer is too high. It is suggested that the output of zinc slag per week should be plotted with a graph (although the amount of zinc slag per week will vary, the average error will not be great every 4-6 weeks). When the amount of zinc slag has an increasing trend and no other reasons are found, the temperature of the inner wall of the pot is higher than the critical value, and the extent of zinc corrosion is greater than the normal situation.

In practice, in order to improve the productivity, it is often hoped that the heating intensity of the zinc pot will be increased, and even the zinc pot will run under the critical heating intensity. In this way, we must first understand what the critical heating intensity of the zinc pot is, that is, the maximum allowable heat transfer per square meter per unit time of the zinc pot wall without serious erosion. The heat transfer coefficient is 698W /(. K) to the liquid zinc through the temperature difference between the critical inner wall temperature of the pot and the liquid zinc. In order to keep the bath temperature constant, the amount of heat needed to be compensated includes: the effective heat needed to raise the temperature of the plating piece to reach the zinc temperature, the heat dissipation of the zinc bath surface, the heat needed to melt the zinc ingot. At constant temperature of zinc bath, the heat input and the heat absorbed by zinc bath reach a balance condition, and then according to the temperature of zinc bath and the thermal parameters of liquid zinc and steel, the output of steel plating parts immersed in different zinc bath temperatures is calculated. The following conclusions can be drawn by calculating and comparing the temperature of zinc bath, the output of plating parts (or the heat supply of zinc pot):

1) The high yield of zinc solution at high temperature will inevitably lead to the operation of the zinc pot above the critical heating strength, resulting in the quick damage of the zinc pot.

2) Under the limit of normal galvanizing pot erosion rate, the yield at higher galvanizing temperature is lower than that at lower galvanizing temperature.

3) the increase of output makes the ratio of effective heat to heat supply increase obviously.

It can be seen that controlling the maximum output per hour and proper bath temperature is the only way to avoid the corrosion of the zinc pot. During the whole production period, the weight of each hour and batch of galvanized parts should be balanced as far as possible. The total output of galvanized parts per shift or every day should not be checked and controlled only to avoid the phenomenon that the output concentrated in a certain period of time and the galvanized pot overloaded.

In practice, the temperature of zinc liquid should be lowered as far as possible, and the heating intensity of the pot wall should be limited to reduce the erosion of zinc liquid to the inner wall of the pot. However, the lowest zinc temperature is determined by the maximum amount of material per immersion. Too low temperature will make the solvent boiling time too long, thus reducing the output and the coating will be too thick. Under normal circumstances, zinc bath temperature maintained at 445~460 C is more suitable.

In a heated zinc pot, the liquid zinc is constantly moving. It flows upward along the surface of the heated pot wall. As it approaches the surface of the zinc bath, the temperature decreases and then turns to flow into the middle of the pot, then downstream. The zeta grains are driven by zinc solution and deposited on the wall of the zinc bath below 100mm. If the hard layer, which is mainly composed of zeta, gradually becomes too thick, it must be carefully washed away. When zinc ingots are added to the zinc bath, the zinc ingots will sink to the bottom of the pot because of the different densities of the solid zinc ingots and the liquid zinc (7.2g/cm3 and 6.6g/cm3, respectively). Dry zinc ingots sink faster than wet zinc ingots because moist water vapor on the ingots is heated and vaporized into vapor, causing the ingots to move back and forth in the zinc bath. When the zinc ingot touches the pot wall, it will destroy the protective iron zinc alloy layer.

2. regularly remove zinc slag.

After the zinc pot is put into operation, zinc slag will continue to produce. As mentioned earlier, zinc slag mainly comes from_particles formed by the reaction of zinc solution with plating parts and the reaction of zinc solution with steel pan wall. Therefore, the amount of zinc slag is related to the yield of hot-dip galvanizing and the technological conditions of hot-dip galvanizing, and the temperature of the inner wall of the pot. The density of the zeta grain is only slightly larger than that of the zinc solution. _grains are carried by the flowing zinc liquid, and most of the grains will eventually fall to the bottom of the zinc bath and form zinc slag, except that some of them adhere to the wall of the zinc pot. The thickness of zinc slag should not exceed 100 mm to avoid being agitated by the galvanized parts. A large number of zinc slag floats in the galvanized bath and then is adsorbed by the galvanized coatings, resulting in roughening of the surface of the galvanized parts. The long-term and excessive accumulation of zinc slag must be prevented. Investigation and Study on some zinc leakage accidents in zinc pots show that the perforation is related to excessive accumulation of zinc slag in the inner wall of the pot. When a large amount of zinc slag is accumulated in the inner wall of the zinc pot, there is no convection of zinc liquid in the pot. The heat is difficult to dissipate and the temperature of the pot wall rises, which accelerates the corrosion of the inner wall of the pot. Long term accumulation of zinc slag will also make zinc slag firmly adhered to the wall of zinc pot and increase the difficulty of cleaning. Therefore, when the zinc slag layer is thicker, slag must be removed by slag remover. The interval between slag removal depends on production, usually once a week. The zinc plating time is short, which can reduce the formation of zinc slag

3. disposal of zinc pots in short term production.

The heat loss on the surface of a clean zinc bath is about 54000kJ/ (H). Short term stop production, for example. On weekends, holidays, equipment maintenance, etc., the entire zinc pot should be covered with a good insulation cover to reduce heat loss; at the same time, to reduce the heating intensity of the zinc pot, keep the zinc bath temperature can be. Some people think that reducing zinc temperature after stopping production can save energy, which is one-sided. The difference between the amount of heat consumed in order to keep the bath temperature from falling and the amount of heat needed to re-heat the bath to normal galvanizing temperature after self-cooling is not very large when the production is stopped for a short time. However, the solubility of iron in zinc bath decreases when the temperature of zinc bath decreases, and the supersaturated iron reacts with zinc to precipitate fine_grains. When heating up, _grain can not be dissolved immediately and suspended in zinc solution, adsorbed on the plating layer and made its surface rough.

In addition, if the heating system is cut off to let the zinc bath cool down naturally, and the corresponding control and alarm system is stopped, the zinc bath will be solidified due to excessive cooling.

For the zinc-plating furnace with the refractory brick structure with large fuel heat storage, the zinc pot must be carefully inspected after adding the insulation cover to ensure that the temperature of zinc bath can not be raised too much.

4. zinc ingots and aluminium and lead addition

According to the consumption of zinc, every shift or two must be added to the zinc pot after the same amount of zinc ingots, not a few days after the concentration of a large number of zinc ingots, resulting in greater fluctuations in zinc temperature. A small amount of zinc ingots can be artificially added. Large quantities of zinc ingots must be cranes or other suitable machinery. If more zinc ingots are to be added at the same time, it should be evenly distributed along the entire zinc pot length.

The aluminum bath in zinc bath can reduce the formation of zinc ash and make the appearance of the coating bright. The maximum mass fraction of aluminum in zinc solution should not exceed 0.02%. Too little aluminum will make the surface of the plated parts yellow, too much aluminum will cause leakage plating and accelerate the corrosion of the zinc pot. Zinc and aluminum alloy must be added to zinc pot regularly.

The maximum solubility of lead in zinc solution is 1.2% (mass fraction) at 450 C. The density of lead is higher than that of zinc. Adding excess lead will sink to the bottom of zinc bath. In pure zinc bath, the addition of lead and maintaining its mass fraction above 0.6% is beneficial for zinc slag to sink into the bottom of the pot. When the lead ingot is put into the zinc bath, it will sink directly to the bottom of the zinc bath and gradually melt. Most of the faults do not dissolve into the zinc bath, but infiltrate into the zinc slag layer and accumulate under the zinc slag layer. Therefore, it is better to regularly disperse small lead particles into lead in zinc bath.