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This post was last edited by mkp369 on 2012-11-26 11:21 1 The main reason for high iron alkali caused by shutdown and startup. Soda ash production is a multi-process continuous production process, but for any soda ash system, shutdown and overhaul are necessary. Most soda ash plants shut down for major overhaul once a year, or once every two years. Such a major overhaul requires the entire production unit to be shut down and then restarted after systematic overhaul. During the entire shutdown and start-up process, it has a great impact on product quality and consumption. In particular, product quality is often caused by the shutdown after the start-up, resulting in high-quality iron alkali. It takes several days or even a week to recover high-grade products, which has a great impact on production consumption and market supply. High iron alkali caused by shutdown and start-up mainly occurs in the carbonization process. It is because the corrosion of the tower by the alkali-making liquid causes the iron content of the carbonization liquid to increase significantly, and ultimately the iron content of the soda ash exceeds the standard. Therefore, the quality technology to control the shutdown and start-up of high-alkali iron production is mainly the technical measure of the carbonization process. 2. Changes in Pressure Tower Technology In the history of soda production, the pressure tower technology adopted during shutdowns has undergone several technical changes. Therefore, tower pressing technology has the concepts of old French pressing tower and new French pressing tower. The tower pressure technology introduced in this article is an improvement on the new French pressure tower. Years of practice have proven that this tower pressure technology is crucial to ensuring the quality of soda ash during shutdown and start-up. 2.1 Old French pressure tower The old French pressure tower means that when the tower is shut down, in order to prevent the sodium bicarbonate crystal particles in the tower from blocking the tower, all the crystals in the tower are drained out through the alkali outlet, and the tower is filled with ammonia salt water. When the tower is started, the tower is held to produce alkali. The biggest disadvantage of this pressure tower technology is that the shutdown results in a large amount of waste of in-process products. Moreover, due to the corrosion of the carbonization tower by ammonia salt water after the shutdown, the sodium bicarbonate crystals after the tower is opened are all red and contain extremely high iron oxide. * red base ； Another disadvantage of this pressure tower technology is that it causes great harm to production recovery. It is troubled by red alkali for days or even weeks, and production cannot be normal. For this reason, alkali production engineers developed a new French press tower to replace the old French press tower. 2.2 New French Press Tower The new French press tower also first considers the blocking factor of preventing sodium bicarbonate crystallization. Therefore, the crystallized particles are removed as much as possible before stopping the tower. Generally, it is appropriate to consider the crystallized solid-liquid ratio to be no more than 15%. Then use neutralized water to press back from the bottom of the tower back to the tower. The amount of tower pressure is controlled at the tower pressure before the tower, so that the bottom of the carbonization tower is filled with neutralized water that has been pre-carbonized to a certain extent and the pH value has been reduced to about 9 to reduce corrosion to the carbonization tower. This tower pressing technology not only solves the problem of tower blockage, but also * * Reducing the corrosion intensity of the carbonization tower can reduce the amount of red alkali produced, but it cannot fundamentally solve the problem of red alkali produced, and the problem of waste of in-process products has not been solved. After many soda ash plants shut down, even if they adopt new pressure towers, they still have to produce a certain amount of iron-grade soda ash. 2.3 Cold Mother Hydraulic Tower The compression tower technology (cold mother hydraulic tower) introduced in this article is an improvement based on the new French pressure tower technology. This method includes two parts. First, the medium and process flow of the pressure tower are improved. ； The second is the technical treatment before tower pressing. The corrosiveness of carbonized liquid to the tower mainly depends on the pH of the liquid. Literature shows that the pH value of the ammonia brine gradually decreases as the degree of carbonation increases as the degree of carbonization increases. The pH value of the ammonia brine is 10.5, while the pH value of deeply pre-carbonized neutralized water is 9, and the pH value of the carbonized alkali solution is 8.3. That is to say, the intensity of the corrosion of the carbonization tower by the ammonia brine, neutralized water, and alkali solution is decreasing. From this principle, if we want to reduce the corrosion of the carbonization tower by the alkaline liquid produced after the tower is shut down, the liquid in the tower after the tower is shut down should be fully carbonized mother liquor. This is not only due to the lowering of its pH value, which weakens the electrochemical corrosion of iron. In addition, it is closer to the saturated solution of sodium bicarbonate, which reduces the dissolution of the alkali scar. The alkali scar in the carbonization tower also plays a role in protecting the inner wall of the tower and the bacterial cap. Therefore, it is more scientific to use the mother hydraulic tower when pressing the tower. The use of a mother hydraulic tower requires improvement in the process flow. The main pipe of the carbonization tower contact pipe must be connected to the outlet of the cold mother liquor pump. When pressing the tower, first put the effluent alkali liquid to a solid content of close to 15% of solid crystals, and then use the cold mother liquor pump to backpress the mother liquor back to the carbonization tower. After the tower is stopped, the middle and lower parts of the carbonization tower are all mother liquor with a lower pH value. This method has been proven to be very effective in the early 1990s. The main purpose of using the mother hydraulic tower is to solve the corrosion of the middle and lower parts of the carbonization tower after it is shut down. Necessary technical treatment is also required to reduce corrosion in the middle and upper parts of the carbonization tower. The principle is the same. The pH value of ammonia salt water is the highest, while the pH value of neutralized water is relatively low. Therefore, the upper liquid of the carbonization tower should be pre-carbonized as deeply as possible before stopping the tower. A more effective measure is to circulate the neutralized water through itself in the cleaning tower so that it reaches a saturated state as much as possible, or even precipitates and crystallizes. What needs to be reminded is that: Do not add ammonia salt water to the carbonized alkali production tower before shutting down the tower. 3. Shutdown maintenance, tower shutdown and tower opening technology implementation methods 3.1 Tower shutdown measures 1) Two days before shutdown, the CO2 in the neutralization water and the temperature of the neutralization water should be strictly controlled, and the CO2 in the neutralization water should be maintained at a high limit, preferably greater than 750mL/20mL. Keep the temperature of the neutralizing water at a low limit, preferably around 36°C ； The operation of the cleaning tower is a key link in controlling the height of iron carbide. Several issues to pay attention to are:: ①The cleaning tower pressure must be operated at a high limit to increase the filling coefficient of the tower and minimize the gas phase explosion area in the upper part of the tower. ②Avoid using mid-section air intake, and do not use the method of replenishing a large amount of mid-section air in order to increase the CO2 content of the neutralized water. This will cause overwashing of the middle and upper parts of the cleaning tower. ③The cleaning tower should avoid the operation method of not adding air at the beginning of cleaning and increasing the air volume at the end to increase the degree of carbonization. Instead, from the beginning of cleaning, add enough cleaning gas to maintain the normal cleaning gas volume. The air volume at this time can fully play a stirring role to accelerate the dissolution of the alkali accumulated at the bottom of the tower, and at the same time avoid uneven liquid flow and local overwash caused by liquid deviation. Similarly, because the bottom of the tower absorbs less CO2, the middle and upper parts of the tower increase the absorption of CO2, ensuring the carbonization degree of the middle and upper parts of the cleaning tower, weakening the cleaning effect, and improving the pre-carbonization effect. This operation continues throughout the cleaning cycle, making the cleaning process even. ④Try not to disrupt the operating cycle of the cleaning tower, or in other words, do not disrupt the operating cycle of the alkali-making tower. Make sure that sufficient scarring occurs in the tower during the alkali-making process. 2) The cleaning tower is changed during the mid-shift before the shutdown. If there are multiple groups, one group can be changed every 2 hours, but it must be ensured that the last cleaning tower can produce alkali for 7-8 hours before the shutdown. That is, the new alkali-making tower ensures that the alkali production can reach 7-8 hours, forming a certain amount of scarring in the tower. ； The cleaning of the cleaning tower does not exceed 7-8 hours, that is, part of the alkali scar is retained and is not washed away, and there is also a protective layer of alkali scar. 3) Before the shutdown, according to the tower shutdown time schedule, the cleaning tower begins to return neutralized water to the tower through the neutralizing water pipe and circulates itself to increase the CO2 content of the neutralized water in the upper part of the cleaning tower. Generally, the CO2 content of neutralized water is required to be greater than 750mL/20mL, preferably 850mL/20mL. 4) Before shutdown, the carbonization contact ammonia-reducing brine pump to return ammonia brine, reduce the pressure of the cleaning tower, and completely circulate the neutralized water, making the carbonized liquid close to the precipitated crystallization state. 5) The pressure of the cleaning tower drops to 0.15MPa, and the neutralization water pump and the ammonia brine pump are stopped. 6) Start carbonizing the new French pressure tower. The pressure tower uses a cold mother liquid hydraulic tower. (Add a connecting pipe header at the bottom of the decarbonization tower at the outlet of the cold mother liquid pump, and usually cut it off with a valve. When the mother hydraulic tower is in operation, open this valve and deliver cold mother liquid to the connection pipe of the carbonization tower. ) The specific requirements for the pressure tower are as follows:: ①Close the connection pipe to the main valve of the neutralization water pump, and the alkali making tower of each group presses alkali to the cleaning tower. Before the pressure tower, the pressure of each alkali production tower must be kept at a high limit. By pressing the tower, the pressure of the alkali production tower is reduced and the pressure of the cleaning tower is increased to 0.23-0.24MPa. ②Close the connecting pipe valves of each tower, open the cold mother liquor valve to start the mother hydraulic tower, first press the tower to the cleaning tower to 0.29MPa and stop the tower. After the residual alkali liquid in the alkali outlet pipeline is discharged during the pressure tower, the alkali outlet valve should be closed immediately, and the air should be released to stop the tower to ensure that the alkali outlet pipeline is not blocked during startup. The outlet pressure of the cold mother liquor pump must be strictly controlled at around 0.35MPa to ensure the speed and quality of the pressure tower. ③According to the pressure of each alkali production tower, press the tower one by one. Before pressing the tower, the air intake volume in the middle and lower sections should be adjusted. The air volume is not allowed to be adjusted during the tower pressing process to ensure the accuracy of the pressure during the tower pressing process. Press the tower to 0.29MPa and stop pressing the tower. After the residual alkali liquid in the alkali discharge pipeline is drained, immediately close the alkali discharge valve to ensure that the alkali discharge pipeline is smooth when starting work. The air can be removed after pressing. ④When pressing the tower, the pressure should be at least from 0.24MPa to 0.29MPa. If the capacity of the cold mother liquor pump is sufficient, one group of towers can be pressed at the same time, but a sufficient amount of tower pressure must be ensured, generally based on the tower pressure. It should be noted that the air inlet volume should be reduced as much as possible before the pressure tower to maintain a small air volume to pressure the tower. The air inlet is mainly to ensure sufficient agitation and pressure indication of the liquid in the tower. ； At the same time, ensure that the gas-liquid ratio in the tower is as small as possible, so that the gas phase burst area in the upper part of the tower will be reduced after the tower is stopped. ⑤Before compressing the tower, contact the dispatcher about the compressor usage to ensure the stability of the main pipe pressure during the tower compression process. It is necessary to avoid fluctuations in the main pipe pressure caused by stopping the compressor due to overpressure during the tower pressure process. Once the pressure fluctuates, the actual pressure amount of the carbonization tower cannot be controlled, and even the tower pressure time is affected. Generally, the carbonization tower will promptly contact the dispatcher to stop a compressor based on the condition of the pressure tower and the amount of gas return, rather than scheduling. After stopping the compressor, the tower should promptly adjust the lower section pressure to ensure stable main pipe pressure and ensure the quality of the pressure tower. ⑥During the tower pressure process, it is necessary to pay close attention to the discharge volume of the alkali outlet to prevent the tower pressure from dropping too low, causing the upper part of the tower to explode, or even causing the tower pressure to be too large and the time to be too long, which will delay the tower pressure process and even affect the quality of the products started. ⑦During shutdown, attention should be paid to the stock of ammonia salt water to prevent excessive ammonia salt water from being poured into the stopped carbonization tower after the shutdown, which will have a great impact on the start-up and product quality. 3.2 Start-up measures 1) When starting work, first ensure that the oxygen content of the kiln gas and furnace gas cannot be high, generally no more than 1%. 2) Don’t hold the tower for too long. Generally, alkali will be produced after holding the tower for 15-20 minutes. Don’t check whether the temperature in the middle part reaches the requirement. Because when the tower is stopped, the liquid in the tower is mother liquor, so even if the tower is stopped, the temperature will not rise significantly. Instead, as the alkali solution is released and the tower pressure is reduced, neutralized water is replenished in time to maintain the tower pressure and increase the tower temperature. Note that neutralized water must be added, not ammonia salt water. 3) When holding the tower, you can hold several groups at the same time according to the amount of gas, and open as many towers as possible. The amount of alkali released should not be too small, generally exceeding the amount released during normal operation. The purpose is to release the mother liquor pressed in the net pressure tower as soon as possible. Once crystals are found in the alkali produced, the alkali discharge flow rate must be controlled according to the solid-liquid ratio to prevent the alkali discharge liquid solid-liquid ratio from being too large and the alkali discharge flow rate being too small to block the alkali discharge pipeline. If the dosage is too large, it will affect the titanium temperature and prolong the recovery time. 4) When opening the tower, all the towers in the first group of towers should be closed and the alkali solution should be released. When the oldest alkali-making tower is shut down, the alkali solution should be released. After the mother liquor is drained, it should be pulled out directly and changed to a cleaning tower. Do not pull out all the towers until there is no crystallization. Generally, no more than 15% of the crystallization is sufficient. At the same time, the ammonia brine pump is turned on to supply ammonia brine to the cleaning tower. Ammonia brine must not be supplied directly to each alkali production tower; it must be neutralized water. 5) The cleaning tower before shutdown must no longer be opened as a cleaning tower, but directly changed to an alkali production tower. 6) When starting the operation, the capacity of the carbonization tower should be raised as early as possible to ensure the capacity of the calciner. Try to open more furnaces and recover the furnace gas early. After the furnace gas is recovered, it can promote the recovery of carbonization. Therefore, try to ensure as much carbonization as possible when starting work. 7) After the carbonization tower is put into operation, some towers, such as the cleaning tower before shutdown, will have high iron levels. The iron level of the entire system can be avoided by controlling the volume. If the system allows it, try to release the liquid in the tower as early as possible and replenish fresh neutralizing water as early as possible. The earlier it is released, the faster the recovery will be. 8) For newly opened cleaning towers, try to increase the CO2 content as much as possible, preferably to the highest limit. The middle section of the newly opened cleaning tower must not be used to prevent overwashing of the middle and upper parts of the tower. The cleaning time of the first cleaning tower in each group can be shortened to half the normal cleaning time, or it can be determined based on the analysis of the neutralized water and iron content. The cleaning time can be further shortened to avoid overwashing. 9) To ensure a fast recovery from carbonization start-up, the compressor must be fully prepared when starting the operation, and the start-up must be in place quickly to avoid fluctuations in the carbonization tower caused by delays in starting the compressor and affecting the quality of soda ash at start-up.