The main purpose of heating the phosphating pool is to provide protection to the base metal. Simply put, the metal provides protection to the base metal, preventing it from being corroded and making it more susceptible to corrosion. Temperature control is one of the important factors in phosphating reaction, which directly affects the rate of phosphating reaction, the quality of film formation, the decomposition of accelerators, and the amount of precipitation produced by phosphating solution.
Different solutions have different working temperature ranges. When the temperature exceeds the specified range, it will accelerate the decomposition of the dihydrogen phosphate salt contained in the phosphating solution, rapidly increase the free acid, and significantly increase the sediment. If sodium nitrite or sodium chlorate is used as a promoter in the solution, it will accelerate their decomposition, lose the original equilibrium of the phosphating solution, and affect the quality of the film formation. Therefore, controlling the working temperature during phosphating is very important.
The heating method of the phosphating tank has a significant impact on the control effect of the solution temperature. Some of the solution heating methods for immersion or spray phosphating tanks used in domestic phosphating production adopt steam heating by installing coils along the tank wall. Usually, the solution is not stirred, so the convection of the solution in the tank is not good during heating, and the temperature of the solution is not easily uniform.
Due to the high temperature of the steam medium, the surface temperature of the heating tube is much higher than the working temperature of the solution, causing the solution near the heating tube to decompose due to local overheating, producing a large amount of phosphate precipitation and free acid. This not only wastes chemical materials but also destroys the stability of the solution. It also causes sediment to deposit on the surface of the steam tube, forming a hard shell, which not only reduces thermal efficiency but also wastes energy.
It is best to use low-temperature thermal oil to heat the phosphating solution, reducing the temperature difference between the heat source and the solution and slowing down the decomposition rate of the solution. Generally, manufacturers will make coils in the phosphating tank and use an electric heating heat transfer oil furnace to indirectly heat the heat transfer oil inside, heating the phosphating tank to the desired temperature.
What are the benefits of temperature control, distribution, heating, and thermal oil furnace in phosphating tank? Easy to install, connect the equipment and coil, and then add heat transfer oil to heat it up. Heat transfer oil also has the advantage of slow heat dissipation, which can better maintain insulation. The phosphating temperature of the phosphating pool is generally around 50-90 degrees.
Taking a 10 cubic meter phosphating tank as an example: equipped with a 96KW electric heating heat transfer oil furnace, the heating time is about 5 hours. If you need to accelerate the heating time, the heating power can be increased. However, according to national regulations, equipment exceeding 100KW (including 100KW) is considered special equipment and requires annual inspection. If you do not want annual inspection, it is best to set the heating power below 100KW. So currently, this heating method is highly favored by phosphating manufacturers, which can solve practical temperature control problems without the need for professional personnel to supervise, saving time, effort, and space.
At present, the automation level of electric heating and heat transfer oil furnaces is high, that is, the automatic adjustment of heat load is achieved by feeding back the set temperature to the control system. The control technology combines fuzzy control and self-tuning PID control, and the temperature control accuracy can reach ± 1 ℃ to ± 0.1 ℃, even more accurately and can be networked with computers. The control system can provide signals to the DCS system, such as the heater being in operation, overheating, stopping, temperature signals, interlock status, etc., and can accept automatic and stop operation commands issued by the DCS.