JP3849010B2 - Sludge removal method for molten salt - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
In an annealing pickling line for stainless steel, a molten salt bath containing molten salt mainly composed of molten alkali such as NaOH and KOH is often arranged on the rear surface of the annealing furnace. The oxide film on the surface of the stainless steel formed in the annealing furnace is difficult to remove by pickling as it is, but when this molten salt is passed through, the oxide film is modified and easily removed by pickling etc. I can do things. The present invention relates to a method and apparatus for handling the molten salt in an annealing pickling line for stainless steel.
[0002]
[Prior art]
The molten salt in the molten salt tank is maintained at 450 to 500 ° C., for example. The stainless steel from the annealing furnace is immersed in the molten salt and travels through. When a stainless steel annealing pickling line is operated for a long time, a large amount of stainless steel passes through the molten salt. When a large amount of stainless steel passes through the molten salt, according to the knowledge of the present inventors, insoluble matter is generated in the molten salt, and this insoluble matter becomes sludge and deposits.
[0003]
This sludge is also deposited on the surface of the stainless steel, and the sludge deposited on the surface of the stainless steel is easily pushed into the surface of the stainless steel by a sink roll or the like provided in the molten salt tank, so that it becomes a pushing rod. For this reason, sludge is mechanically removed in the molten salt tank, but when sludge is removed by a normal method, sludge is generated again immediately after the removal. Therefore, it is not easy to carry out an annealing pickling of stainless steel using a molten salt with little or no sludge.
[0004]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION An object of the present invention is to provide a molten salt sludge removal method and a removal device capable of making molten salt during operation extremely small or without sludge.
[0005]
[Means for Solving the Problems]
In the present invention, (1) in an annealing pickling line for stainless steel, the temperature of the molten salt is lowered by 20 ° C. to 100 ° C. from its operating temperature, the sludge is settled, and the settled sludge is removed. A method for removing sludge from molten salt, characterized in that the temperature is returned to the operating temperature.
[0006]
(2) In the annealing pickling line for stainless steel, a part of the molten salt in the molten salt tank that is arranged adjacent to the molten salt tank is intermittently or continuously received from the molten salt tank. And a sludge pan that can be removed from the settling tank in order to contain sludge that settles from the molten salt in the settling tank and discard the settled sludge at the bottom of the settling tank. The temperature of the molten salt received in the settling tank is lowered by 20 ° C. to 100 ° C. from the temperature in the molten salt tank by using the sludge removing device arranged, and the sludge is settled in the sludge pan. Then, the molten salt treated in the sedimentation tank is returned to the molten salt tank as it is, and the temperature is raised or raised, and then returned to the molten salt tank. It is sludge removal process of the molten salt, wherein.
[0007]
(3) The method for removing sludge from molten salt according to (1) or (2) above, wherein 20 ° C to 100 ° C is 50 ° C to 80 ° C.
[0008]
(4) A sedimentation tank which is arranged adjacent to the molten salt tank and which can receive a part of the molten salt in the molten salt tank intermittently or continuously from the molten salt tank and return it to the molten salt tank after processing. The settling tank contains sludge that settles from the molten salt in the settling tank, and a sludge pan that can be taken out from the settling tank in order to discard the settled sludge is arranged at the bottom of the settling tank, In addition, it is possible to lower the temperature of the molten salt received in the settling tank below the temperature in the molten salt tank, or to further raise the temperature of the molten salt in the settling tank after the temperature is lowered. An apparatus for removing sludge from molten salt, characterized in that a temperature control device is provided.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The temperature of the molten salt in the stainless steel annealing pickling line is 450-500 ° C. As is known, the melting point of NaOH is 370 ° C., and the melting point of KOH is 360 ° C. The melting point of the eutectic component of NaOH and KOH is even lower. Accordingly, NaOH, KOH and mixtures thereof, which are the main components of the molten salt, are liquid at the temperature of the molten salt.
[0010]
When a large amount of stainless steel is passed through the molten salt, sludge is deposited on the bottom of the molten salt tank. As a result of analyzing the molten salt after passing a large amount of stainless steel, the present inventors have found that it contains carbonate impurities. Moreover, as a result of analyzing the sludge in the molten salt tank, it was found that the main component of the sludge was carbonate impurities.
[0011]
In the stainless steel annealing pickling line, a molten salt bath is provided next to the annealing furnace in which CO ₂ is generated. Therefore, the stainless steel immediately after coming out of the annealing furnace in this CO ₂ atmosphere enters the molten salt bath. For these reasons, the molten salt is likely to come into contact with CO ₂ gas, and it is considered that a part of NaOH and KOH becomes carbonate impurities mainly composed of Na ₂ CO ₃ and K ₂ CO ₃ due to this contact. It is done.
[0012]
The inventors pumped a sample of molten salt having a temperature of 500 ° C. through which a large amount of stainless steel was passed, from the molten salt bath, held at 500 ° C. for 10 minutes, and then cooled to 460 ° C. to 460 ° C. Hold for 10 minutes, then cooled to 420 ° C. and held at 420 ° C. for 10 minutes. While the temperature was maintained at 500 ° C., no sludge was generated from the molten salt, and the molten salt was a uniform melt. The carbonate salt impurities contained in the molten salt maintained at 500 ° C. were analyzed, and the content was 36% by weight as shown in FIG. That is, although the molten salt at 500 ° C. contains a large amount of 36% by weight of carbonate-based impurities, the carbonate-based impurities are not sufficiently dissolved and precipitated in the molten salt.
[0013]
When the molten salt at 500 ° C. was cooled to 460 ° C., a large amount of sludge was generated from the molten salt. Incidentally, the carbonate salt impurities of the molten salt held at 460 ° C. were analyzed, and the content thereof was 27% by weight as shown in FIG. That is, when cooling from 500 ° C. to 460 ° C., the content of carbonate-based impurities in the molten salt is reduced and sludge of carbonate-based impurities is generated. Therefore, it is considered that the solubility limit of carbonate impurities is lowered by cooling the molten salt, and the carbonate impurities are precipitated as sludge.
[0014]
Sludge was generated from the molten salt when the 460 ° C molten salt was further cooled to 420 ° C. The amount of carbonate impurities contained in the molten salt at 420 ° C. was about 23% by weight as shown in FIG. 1 (A), which was further reduced from the case of 460 ° C. That is, when the molten salt was further cooled from 460 ° C., the solubility limit of the carbonate-based impurities in the molten salt was further lowered, and thus the carbonate-based impurities contained in the molten salt were further precipitated as sludge.
[0015]
The present inventors collected another molten salt through which a large amount of stainless steel was passed from the molten salt bath, and performed the same test as in FIG. FIG. 1B shows the result. As in the case of FIG. 1A, in the case of FIG. 1B as well, when the temperature of the molten salt is lowered, the solubility limit of the carbonate-based impurities is lowered and sludge of the carbonate-based impurities is generated. In addition, the concentration of carbonate impurities dissolved in the molten salt decreased.
[0016]
When stainless steel is further passed through the molten salt, the molten salt further comes into contact with CO _{2 in the} annealing furnace, and the generation amount of carbonate-based impurities further increases. At this time, the solubility limit of the molten salt at 460 ° C. is 27 wt%. Therefore, the content of carbonate impurities in the molten salt is 27 wt%. Carbonate impurities exceeding 27 wt% become sludge.
[0017]
If no special measures are taken, the sludge is removed by keeping the molten salt at its use temperature, for example, 460 ° C. However, even though this method can remove the deposited sludge, new sludge is immediately generated and deposited. That is, carbonate impurities are generated due to CO ₂ in the annealing furnace, and the annealing furnace continues to generate CO ₂ . Therefore, carbonate impurities are continuously generated after the sludge is removed. On the other hand, the molten salt already contains 27 wt% of carbonate-based impurities, and therefore cannot continuously contain newly generated carbonate-based impurities. Therefore, after the sludge is removed, newly generated carbonate impurities become new sludge in the molten salt tank. Therefore, even if the sludge is removed, a new sludge is generated immediately, and the sludge always exists in the molten salt tank.
[0018]
In the present invention, sludge is removed by lowering the temperature of the molten salt by 20 to 100 ° C. below the operating temperature. That is, the temperature of the molten salt is lowered by 20 to 100 ° C. from 460 ° C., for example, and the sludge is removed at 440 ° C. to 360 ° C. A molten salt of 440 to 360 ° C., for example 420 ° C., contains 23 wt% of carbonate-based impurities as shown in FIG.
[0019]
When the molten salt at 460 ° C. is set to 420 ° C., the carbonate-based impurities that can be contained in the molten salt are changed from 27 wt% to 23 wt%, and 4 wt% of this difference becomes sludge. In the present invention, the 4 wt% sludge is also removed at 420 ° C. Thereafter, the molten salt at 420 ° C. is returned to 460 ° C. At this time, the molten salt at 460 ° C. can contain carbonate impurities up to 27 wt%, but since the sludge was removed at 420 ° C., the molten salt contained only 23 wt%. For this reason, after removing the sludge, it is contained in the molten salt and no sludge is generated until the carbonate impurities reach 27 wt%. In other words, it is possible to operate with a molten salt without sludge until it reaches 27 wt%.
[0020]
When the temperature drop of the molten salt is less than 20 ° C., the amount of sludge generated due to the temperature drop is small, and the effect of the present invention is small. If the temperature drop is over 100 ° C., the energy consumption is large for the rise of the molten salt temperature over 100 ° C. after removing the sludge, and it is difficult to carry out in a short time. For this reason, it is not preferable in terms of energy saving and efficiency. In terms of operation or cost, the temperature of the molten salt is preferably lowered from 50 ° C to 80 ° C.
[0021]
FIG. 2 is an example of a sludge removing apparatus for carrying out the present invention. In the figure, 1 is a molten salt bath and 2 is a molten salt. The removal apparatus shown in FIG. 2 is disposed adjacent to the molten salt tank 1, and a deposition tank for receiving a part of the molten salt 2 in the molten salt tank 1 from the molten salt tank 1 intermittently or continuously and returning it to the molten salt tank 1. 3. In the figure, 6 is an example of a molten salt receiving pump, and 7 is an example of a molten salt returning pump. This receiving and returning may be performed by another method, for example, by forming a circulation channel or by forming an overflow channel. It can also be done by doing.
[0022]
The settling tank 3 accommodates a sludge 4 that settles from the molten salt 2 in the settling tank 3, and a sludge pan 5 that can be removed from the settling tank in order to discard the sludge 4 is disposed at the bottom of the settling tank. In addition, the temperature of the molten salt 2 received in the settling tank 3 can be made lower than the temperature of the molten salt 2 in the molten salt tank 1, or after the temperature is lowered, the molten salt 2 in the settling tank 3 is further reduced. It has a temperature control device 8 that can also raise the temperature. In addition, 9 is a stirrer in the figure and is used as needed.
[0023]
When removing sludge using the removing device of FIG. 2, for example, the molten salt receiving pump 6 is operated to receive the molten salt into the settling tank 3, and the temperature adjusting device 8 is used to melt the molten salt 2 in the settling tank 3. Is lower by 20 ° C. to 100 ° C. than the molten salt 2 in the molten salt bath 1. As a result, the melting salt 2 in the settling tank 3 has a lower limit for dissolving carbonate-based impurities, and the carbonate-based impurities that can no longer be dissolved in the molten salt 2 in the settling tank 3 become sludge 4 in the sludge pan 5. Settling. The sludge 4 is lifted together with the sludge pan 5, for example, and taken out from the settling tank 3, and the sludge 4 is discarded in a sludge disposal site (not shown). The molten salt 2 in the settling tank 3 is returned to the molten salt tank 1 by operating the return pump 7 as it is or after raising the temperature. Since the molten salt returned from the settling tank 3 to the molten salt tank 1 is unsaturated in the content of carbonate impurities, the carbonate impurities increased thereafter in the salt tank 1 are dissolved, and the molten salt tank 1 Prevents the formation of sludge.
[0024]
In FIG. 2, the temperature adjusting device 8 and the stirring device 9 are shown immediately above the sludge pan 5 for the sake of simplicity. For this reason, it seems that it is difficult to lift the sludge pan 5 upward, but as is well known, these can be arranged on the wall surface of the settling tank 3, or can be a movable setting type that can be easily installed and removed. . Therefore, these do not make it difficult to lift the sludge pan 5.
[0025]
In FIG. 2, for example, the temperature of the molten salt in the molten salt bath 1 is set to 500 ° C. The temperature of the molten salt in the settling tank 3 is set to 420 ° C., for example. At this time, no sludge is generated until the molten salt in the molten salt tank 1 contains 36 wt% of the carbonate impurities, which is the solubility limit of 500 ° C. from FIG. The content of carbonate impurities immediately after being treated in the settling tank 3 and returned to the molten salt tank 1 is 23 wt%, which is a solubility limit of 420 ° C. When sludge is removed quickly and frequently by the settling tank 3, the ratio of the molten salt immediately after the treatment in the settling tank 3 increases in the molten salt tank 1. Therefore, the content of carbonate-based impurities in the molten salt in the molten salt tank 1 approaches 23 wt%. This molten salt does not generate sludge until the carbonate-based impurities are 36 wt%. That is, if sludge is removed quickly and frequently using the sedimentation tank 3 so that the carbonate-based impurities of the molten salt at 500 ° C. in the molten salt tank 1 does not exceed 36 wt%, a large amount of stainless steel passes. Even after the operation, the molten salt tank can be operated without generating any sludge.
[0026]
【The invention's effect】
By practicing the present invention, the molten salt during operation can be brought into a state where there is very little or no sludge. As a result, it is possible to significantly reduce the occurrence of stainless steel indentations.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph showing allowable limits of carbonate impurities at temperatures of 500 ° C., 460 ° C., and 420 ° C. of a molten salt.
FIG. 2 is a diagram showing an example of a sludge removing apparatus for carrying out the present invention.
[Explanation of symbols]
1: molten salt tank, 2: molten salt, 3: sedimentation tank, 4: sludge, 5: sludge pan, 6: molten salt receiving pump, 7: molten salt return pump, 8: temperature adjusting device, 9: stirrer .

Claims (2)

Oite of NaOH and KOH in molten salt bath provided in annealing pickling line of the molten salt was contained stainless steel 450 to 500 ° C. mainly, of the molten salt bath is arranged adjacent to the molten salt bath A settling tank capable of receiving a part of the molten salt intermittently or continuously from the molten salt tank, and returning the molten salt tank to the molten salt tank after processing, and containing sludge that settles from the molten salt in the settling tank; A sludge pan that can be taken out from the settling tank to discard the settled sludge is circulated between the molten salt tank and the settling tank by using a sludge removing device arranged at the bottom of the settling tank. decreased 20 ° C. ~ 80 ° C. than the temperature of the temperature of the molten salt melt salt tank that received in, allowed to settle sludge to Surajipan, precipitated sludge is taken from the sedimentation tank with Surajipan Remove the molten salt from the sludge disposal site, return the molten salt treated in the sedimentation tank to the molten salt tank and raise the temperature, or return to the molten salt tank after raising the temperature. Method. The method for removing sludge from molten salt according to claim 1, wherein 20 ° C. to 80 ° C. is 50 ° C. to 80 ° C.

Images