JP3129561B2 - Neutral salt electrolytic treatment method and apparatus for descaling of chromium iron alloy strip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention makes it possible to carry out a highly stable descaling treatment of a chromium iron alloy band such as a stainless steel band economically and avoiding the adverse effects on quality caused by accumulation of hexavalent chromium ions. The present invention relates to a neutral salt electrolytic treatment method for descaling a chromium iron alloy strip and an apparatus suitable for carrying out the method.

[0002]

2. Description of the Related Art A chromium iron alloy strip represented by a stainless steel strip is subjected to an annealing treatment in order to remove the effect of work hardening by cold rolling in a manufacturing process. Generates scale (metal oxide). If the scale remains, there are various inconveniences in the subsequent steps. Therefore, it is necessary to perform a descaling treatment. As this descaling method,
After performing pretreatment such as electrolytic treatment in aqueous sodium sulfate solution (hereinafter sometimes simply referred to as neutral salt electrolytic treatment) or immersion treatment in a mixed molten salt of caustic soda and sodium nitrate,
A process of immersing in an aqueous solution of nitric hydrofluoric acid, performing an electrolytic treatment in an aqueous solution of nitric acid, and a combination thereof are widely performed. Of these, neutral salt electrolysis is
This is a descaling treatment method that has been widely adopted in recent years since it was disclosed in JP-A-162.

[0003] This neutral salt electrolytic treatment is easy to obtain beautiful surface properties as compared with the immersion treatment in the mixed molten salt,
Since the pH of the aqueous solution is close to neutral, it has advantages such as easy handling and low pickling cost. The action of this neutral salt electrolytic treatment is described in JP-B-38-12.
As disclosed in Japanese Patent No. 162, it has been conventionally considered that a chromium iron alloy band having a scale is subjected to a dissolving action of the scale during anodic electrolysis, and is not particularly affected during the cathodic electrolysis treatment, and only a hydrogen generating reaction occurs. . However, Japanese Patent Application Laid-Open No. 63-161194 discloses that during cathodic electrolysis treatment of a chromium iron alloy band, metal ions such as Cr and Fe dissolved in a neutral salt electrolytic bath are reduced, and as a result, These precipitate out to form scale-like substances, and the subsequent descaling of nitric acid is not easy even if the subsequent nitric hydrofluoric acid immersion treatment is performed. It is disclosed to limit the current density. Japanese Patent Application Laid-Open No. 1-96398 discloses a 6 salt solution in a neutral salt electrolytic bath.
To limit the chromium ion concentration and the pH, JP-A-1-96399 and JP-A-1-96400 disclose that the anodic electrolytic section and the cathodic electrolytic section are separated and the neutral salt electrolytic bath conditions are changed. Japanese Unexamined Patent Publication (Kokai) No. 2-173300 discloses that a reducing agent is added to a neutral salt electrolytic bath to reduce and change hexavalent chromium ions. It is disclosed that descaling can be performed by preventing the formation of scale-like substances that are not easy to perform.

[0004] However, these methods are not practical due to the necessity of remodeling equipment and an increase in work load such as adjustment of current density during electrolytic treatment. Further, the method disclosed in Japanese Patent Application Laid-Open No. 63-286600 discloses a method of monitoring the oxidation-reduction potential of a neutral salt electrolytic bath under acidic conditions of pH ₂ to 3 while using Na ₂ as a reducing agent. Since the method is characterized by reducing chromate ions in the neutral salt electrolytic bath by adding a sulfite such as SO ₃ or Na ₂ S ₂ O ₃ , the oxidation-reduction potential under acidic conditions is reduced. There are problems such as excessive introduction of the reducing agent due to low reliability of the measurement accuracy, and generation of sulfurous acid gas due to the use of sulfite.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a neutral salt electrolysis treatment for descaling a chromium iron alloy band represented by a stainless steel strip, and to a hexavalent chromium ion formed in a neutral salt electrolytic bath. Method of easily reducing in a neutral salt electrolytic bath to avoid adverse effects on the quality of the chromium iron alloy strip due to the accumulation of hexavalent chromium ions and to maintain a highly stable neutral salt electrolytic processing ability And an apparatus suitable for carrying out this method.

[0006]

The present inventor first investigated the mechanism of hexavalent chromium ion formation in a neutral salt electrolytic bath in order to solve the above-mentioned problems. Was confirmed to be oxidized and dissolved in the neutral salt electrolytic bath and accumulated as hexavalent chromium ions in the neutral salt electrolytic bath. As the hexavalent chromium ions accumulate in the neutral salt electrolytic bath, the neutral salt electrolytic treatment capacity, that is, the scale altering action as a pretreatment for descaling decreases, and as a result, the subsequent nitric hydrofluoric acid bath To increase the amount of acid used due to increased descaling load, sometimes causing poor descaling, and sulfuric acid to maintain the sodium sulfate concentration in the neutral salt electrolytic bath at an appropriate value. Even when soda was replenished, it was also confirmed that, if the hexavalent chromium ion concentration in the neutral salt electrolytic bath was high, problems such as poor dissolution of sodium sulfate occurred due to solubility.

[0007] Accordingly, further studies have been made on a method for removing hexavalent chromium ions from the neutral salt electrolytic bath. Conventionally,
As a method for removing chromate ions from the neutral salt electrolytic bath, the entire amount or a part of the bath has been regularly updated.
The liquid taken out at the time of this renewal is rendered harmless by the facility for detoxifying waste liquid containing hexavalent chromium ions and then discarded into public waters. Has been implemented. In this chemical injection method, a reducing agent is generally added to hexavalent chromium ions to add
This is a method in which a chromium ion is converted to a valent chromium ion, and an alkali is further added to convert it to a hydroxide.Then, the solid-liquid separation is performed by a solid-liquid separation device. Often reused as raw material. As described above, the conventional method for removing hexavalent chromium ions from the neutral salt electrolytic bath is passive and is certainly wasteful, but the hexavalent valent ion is directly added to the neutral salt electrolytic bath by adding a reducing agent. The reduction of chromium ions is a method that has been practiced unavoidably because it has been difficult to carry out due to various problems. On the other hand, when attention is paid to the detoxification of general hexavalent chromium ion-containing waste liquid, the above-mentioned chemical injection reduction method, electrolytic reduction method, and ion exchange membrane method are widely known. Since the method other than the reduction method is not practical because of the high operating cost, the chemical injection reduction method is currently used widely. However, this chemical injection reduction method also has various problems, and is not necessarily an established technology. In this chemical injection reduction method, sulfites such as Na ₂ SO ₃ゃ Na ₂ S ₂ O ₃ and irons such as FeSO ₄ and FeCl ₂ are generally used as reducing agents. However, when a sulfite-based reducing agent is used, there is a problem that sulfur dioxide gas is generated or a complex salt is formed with hexavalent chromium ion when excessively added. On the other hand, when an iron-based reducing agent is used, iron hydroxide sludge is used. Occurrence or OR
There is a problem that it is difficult to control chemical injection using a P-meter (redox potential measurement meter) [Industrial Pollution Prevention Association, published June 30, 1990, No. 247 of "Pollution Prevention Technology and Regulations"
See description on page].

Therefore, the present inventor has tried to apply the method for removing hexavalent chromium ions in a neutral salt electrolytic bath by improving the chemical injection reduction treatment method itself. As described above, there are many problems with the sulfite-based reducing agent as described above. On the other hand, ferrous sulfate and ferrous chloride can be easily obtained as pickling waste liquor by the present applicant. It was assumed that a reducing agent was used.

A major problem when using an iron-based reducing agent is that it is difficult to use an ORP meter as a sensor for controlling chemical injection as described above. Thus, as a result of investigating the relationship between the pH of the hexavalent chromium ion-containing aqueous solution and the output of the ORP meter, the output change of the ORP meter before and after the hexavalent chromium ion reduction treatment was sharpened by fixing the pH in the range of 6 to 12. And it turned out to be bigger. That is, although it is known that there is a close relationship between the pH of the hexavalent chromium ion-containing aqueous solution and the output of the ORP meter, the pH of the aqueous solution of the iron-based reducing agent itself is as low as about 3; It was overlooked that the pH of the hexavalent chromium ion-containing aqueous solution changes depending on the amount of the reducing agent added. Therefore, by simultaneously adding an alkali according to the amount of the reducing agent added, 6
As a result of suppressing the fluctuation of the pH of the aqueous solution containing valent chromium ions,
It was found that the output change of the ORP meter before and after the hexavalent chromium ion reduction treatment was sharp and large.
Although hexavalent chromium ions the pH of the aqueous solution containing a higher iron reducing agent is a concern that hydroxides of, in the present inventor's investigation to exhibit sufficient reduction effect even in the form of Fe (OH) ₂ confirmed. The above results were confirmed for both iron-based reducing agents, ferrous sulfate and ferrous chloride.

On the other hand, looking at the operating conditions of the neutral salt electrolytic bath, the pH of the aqueous sodium sulfate solution immediately after the bathing is about 6,
As a result of the generation of hexavalent chromium ions during use, the pH of the neutral salt electrolytic bath gradually decreases. For this reason, an aqueous solution of caustic soda is added as an alkali to prevent the pH of the neutral salt electrolytic bath from lowering. This is because roughening of the chromium iron alloy band occurs when the pH of the neutral salt electrolytic bath decreases.
Therefore, it was thought that if the pH of the neutral salt electrolytic bath was maintained at pH 6 immediately after the bath was added by adding an alkali, the chemical injection control of the iron-based reducing agent using the ORP meter as a sensor could be performed. However, when ferrous chloride is added to the neutral salt electrolytic bath as a reducing agent, there is a risk of causing accumulation of chloride ions that cause defects such as pitting in the chromium iron alloy band in the neutral salt electrolytic bath,
Avoided its use. Furthermore, the pH of the neutral salt electrolytic bath was increased to the upper limit value of 12, which confirmed that the chemical injection control by the ORP meter was possible. Was not found. However, it is meaningless to operate the neutral salt electrolytic bath at an unnecessarily high pH, and the electrolysis efficiency is reduced by the generation of oxygen.
6 is the pH range in which it is determined that chemical injection control by the meter is possible.
In consideration of と 12, an operation in a pH range of 6 to 8 was tried. As a result, it was confirmed that there was no problem in quality and that the chemical injection control by the ORP meter became possible, and that the hexavalent chromium ion concentration in the neutral salt electrolytic bath was maintained at a low level. Further, since sodium sulfate is generated by the reaction between Na ions of the aqueous sodium hydroxide solution for pH adjustment and SO ₄ ions of the aqueous ferrous sulfate solution as a reducing agent, there is an advantage that the sodium sulfate concentration can be replenished. I understood.

The preferred conditions of the neutral salt electrolytic bath for practicing the present invention are that the sodium sulfate concentration is 100 to 200 g /
1, a pH of 6 to 8, a liquid temperature of 70 to 90 ° C., a chromate ion of 1 g / l or less, and a sludge concentration of 5 g / l or less are recommended. This is because if the sodium sulfate concentration is less than 100 g / l, the electric conductivity of the neutral salt electrolytic bath decreases, so that the electrolysis power increases. If it exceeds 200 g / l, the removal of sodium sulfate by the chromium iron alloy strip may occur. This is because the possibility that sodium sulfate is not dissolved but accumulates in the tank increases due to the solubility in water. The reason for setting the pH to 6 to 8 is as described above. The reason for setting the liquid temperature to 70 to 90 ° C. is that the higher the liquid temperature, the higher the solubility of sodium sulfate in water and the higher the electric power of the neutral salt electrolytic bath. There are advantages such as an increase in conductivity.
C. is preferred. The lower the chromate ion, the better the effect of neutral salt electrolysis and the effect of chromate ion on the quality of the chromium iron alloy band, but it is easy to add too much reducing agent when trying to completely remove it. As a result of a factory experiment, the optimum value of was confirmed to be economical if it was 1 g / l or less, and that the chromate ions had little effect on the quality of the chromium iron alloy strip. Although the sludge concentration depends on the capacity of the sludge removing device, in a factory test, if the sludge is 5 g / l or less, there is no occurrence of scratches on the chromium iron alloy strip due to the sludge being caught in the immersion roll in the electrolytic cell. Has been confirmed.

[0012] Based on the above findings, the reduction of the chromate ion concentration in the neutral salt electrolytic bath by adding a reducing agent to descale the chromium iron alloy band by the neutral salt electrolytic treatment is carried out. Of the pH of the neutral salt electrolytic bath due to the addition of the agent is suppressed by the addition of an alkali to reduce the pH of the neutral salt electrolytic bath.
A chromium iron alloy strip descaling medium according to the present invention, characterized in that a reducing agent is added in accordance with a change in the oxidation-reduction potential of the neutral salt electrolytic bath while maintaining H in the range of 6 to 8. This led to the completion of the method for electrolytically treating a salt.

The details of the apparatus of the present invention suitable for carrying out the neutral salt electrolytic treatment method for descaling a chromium iron alloy strip according to the present invention will be described below with reference to FIG. FIG. 1 is a schematic explanatory view showing a relevant portion of a neutral salt electrolytic treatment apparatus for descaling a chromium iron alloy strip according to the present invention, which relates to the removal of hexavalent chromium ions. The description is omitted because it is not directly related to the method. In FIG. 1, the chromium iron alloy strip S that has been annealed is continuously passed into the neutral salt electrolytic treatment tank 1. Neutral salt electrolytic treatment tank 1
Neutral salt electrolytic bath 1a consisting mainly of aqueous sodium sulfate solution
And a dipping roll 1b for supporting the chromium iron alloy strip S, and an electrode 1c for electrolysis. This neutral salt electrolytic bath 1a
Is continuously sent from the neutral salt electrolysis tank 1 to the electrolyte circulation tank 2 at a certain flow rate. Further, the neutral salt electrolysis bath 1a in the electrolyte circulation tank 2 removes sludge by the sludge removing device 3. After that, the solution is sent to the neutral salt electrolytic treatment tank 1 via the pump 4.

An ORP meter 5a is provided in the neutral salt electrolytic treatment tank 1.
Is installed, and the oxidation-reduction potential of the neutral salt electrolytic bath 1a with respect to a reference electrode such as an Ag / AgCl electrode is continuously measured.
On the other hand, the electrolyte circulation tank 2 contains FeSO 3 as a reducing agent.
₄ Reducing agent storage tank 6a storing aqueous solution, pump 6b and valve 6
c is connected by a pipe, and based on the signal of the ORP meter 5a, the addition amount control device 8
, The pump 6b and the valve 6c are operated to add a required amount of reducing agent. In this description, the reducing agent is an aqueous solution in consideration of workability. However, when a large amount is required, crystal powder may be used in order not to increase the liquid amount. Similarly, an alkaline aqueous solution storage tank 7a storing an alkaline aqueous solution of caustic soda, an alkaline aqueous solution addition device 7 including a pump 7b and a valve 7c are connected to the electrolytic solution circulation tank 2 by piping.
The pump 7b is controlled via the addition amount control device 8 so as to adjust the pH of the neutral salt electrolytic bath 1a within the range of 6 to 8 based on the signal of the pH meter 5b installed in the neutral salt electrolytic treatment tank 1. And valve 7c
Operates to add a required amount of an alkaline aqueous solution.

In the above configuration, the neutral salt electrolytic bath 1a
A preferable management method for preventing accumulation of hexavalent chromium ions in the composition will be described. With the continuous implementation of neutral salt electrolysis,
As the scale mainly composed of chromium oxide of the chromium iron alloy band S accumulates in the neutral salt electrolytic bath 1a, the pH of the neutral salt electrolytic bath 1a decreases. So neutral salt electrolytic bath
When the pH of 1a falls below 6, the addition amount control device 8 is activated based on a signal from the pH meter 5b, and the electrolyte solution is supplied from the alkaline aqueous solution storage tank 7a of the alkaline aqueous solution addition device 7 via the pump 7b and the valve 7c. An alkaline aqueous solution is added to the circulation tank 2 to maintain the pH of the neutral salt electrolytic bath 1a in the range of 6 to 8. By continuing the above operation, the neutral salt electrolytic bath 1a
Although H is maintained in the range of 6 to 8, chromate ions are only electrically neutralized with sodium ions in the neutral salt electrolytic bath 1a by this operation alone, and are accumulated as hexavalent chromium ions. Go. The accumulation state of this hexavalent chromium ion
Periodic investigation by chemical analysis and ORP immediately after bathing
Preliminary survey of the relationship between the transition of the output of the total 5a and the hexavalent chromium ion concentration, and the preliminary survey of the relationship between the cumulative surface area of the chromium iron alloy strip passed through the neutral salt electrolyzer and the hexavalent chromium ion concentration It is possible. In the above means, when the hexavalent chromium ion concentration in the neutral salt electrolytic bath 1a exceeds 1 g / l, the addition amount control device 8 operates based on the signal from the ORP meter 5a, and the reducing agent addition device 6 The aqueous solution of the reducing agent is added from the reducing agent storage tank 6a to the electrolyte circulation tank 2 via the pump 6b and the valve 6c, and the amount of the reducing agent added is preferably controlled by the following control method. That is, the addition amount controller 8 is provided with a circuit for sequentially differentiating the change in the output of the ORP meter 5a simultaneously with the addition of the reducing agent, and when the differential value of the output of the ORP meter 5a suddenly changes, the reducing agent is added. Is a method of stopping the addition of This is because addition of an excessive reducing agent is not only wasteful, but also the pH of the neutral salt electrolytic bath 1a is 6 to 8
As a result, the iron content of the reducing agent forms a hydroxide and exceeds the processing capacity of the sludge removing device 3, so that the neutral salt electrolytic bath
This is because a problem of increasing sludge during 1a occurs. On the other hand, if the reducing agent is added too small, hexavalent chromium ions will remain. Since the pH of the aqueous ferrous sulfate solution as a reducing agent is as low as about 3, the pH of the neutral salt electrolytic bath 1a is adjusted to the lower limit pH 6 at which it is determined that the chemical injection control by the ORP meter 5a is possible. It may be lower. For this reason, the pH of the aqueous ferrous sulfate solution at the working concentration is determined in advance.
The required amount of alkali per unit volume of the aqueous solution of FeSO ₄ required to reduce the volume to about 7 is calculated or measured, and the required amount of alkali that satisfies the above relationship can be added simultaneously with the addition of the reducing agent. preferable. This operation
It is preferable that the addition amount control device 8 is incorporated as a logic circuit.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of carrying out the method of the present invention on an actual line with an apparatus having the structure shown in FIG. 1 are shown below. Neutral salt electrolytic bath 1a
After the bath was prepared, a 10 ml sample of the neutral salt electrolytic bath 1a which had been subjected to the neutral salt electrolytic treatment for a certain period of time was sampled, and a tabletop type O was added while adding an aqueous solution of caustic soda with the aim of maintaining the pH at 7.
FIG. 2 shows the results of investigating the output of the ORP meter 5a by adding a 200 g / l aqueous solution of FeSO ₄ as a reducing agent using the RP meter 5a. Based on this result, the pH meter 5a, the addition amount control device 8 and the alkaline aqueous solution addition device 7 are operated on the neutral salt electrolytic bath 1a of 20 m ³ to adjust the pH of the neutral salt electrolytic bath 1a to 7. Then, the ORP meter 5a, the reducing agent addition device 6 and the addition amount control device 8 were operated to add the reducing agent. The output of the ORP meter 5a and its differential value at this time are the same as those in FIG. 2 which is the result of the beaker test, and the differential value of the output of the ORP meter 5a shows a large change. At the same time, the addition of the reducing agent was stopped. 4.3 m ^{3 of} reducing agent added). Table 1 shows the results of an examination of the state of the neutral salt electrolytic bath 1a before and after this series of treatments. This indicates that the removal of hexavalent chromium ions in the neutral salt electrolytic bath 1a can be easily and reliably performed by implementing the method of the present invention. Further, the pH of the neutral salt electrolytic bath 1a was adjusted to 6, 8, 9, 10,
It was also confirmed that hexavalent chromium ions could be easily and reliably removed in the cases of Nos. 11 and 12. However, an increase in the metal hydroxide sludge in the neutral salt electrolytic bath 1a was observed with the increase in the pH of the neutral salt electrolytic bath 1a,
It did not exceed the capacity of the sludge removing device 3.
In carrying out the method of the present invention, the ORP meter 5a is an ORP electrode 6491 / converter HD38D manufactured by Electrochemical Instruments.
PH meter 5b is a pH electrode 6462 /
A converter HD36D was used.

[0017]

[Table 1]

[0018]

[Comparative Example] A table-top ORP meter 5a was used to maintain a pH of 2 by adding an aqueous solution of caustic soda to a 10 ml sample of the used neutral salt electrolytic bath 1a collected in the above-mentioned embodiment. FIG. 3 shows the result of investigating the output of the ORP meter 5a by adding 200 g / l of FeSO ₄ . This indicates that it is difficult to control the amount of the reducing agent added by the ORP control when the pH is 2.

[0019]

As described in detail above, the method of the present invention and the apparatus of the present invention suitable for carrying out the method of the present invention have various advantages as listed below, and their industrial value. Is big. As a result, hexavalent chromium ions generated in the neutral salt electrolytic bath can be removed or maintained at a low concentration, and the electrolytic treatment capacity of the neutral salt electrolytic bath can be easily and reliably maintained. Since the electrolytic treatment capacity can be stabilized at a high level, descaling by subsequent nitric acid treatment can be facilitated, the amount of these acids used can be reduced, and the production cost and waste acid treatment cost can be reduced. The removal of environmentally unfavorable hexavalent chromium ions in the neutral salt electrolytic treatment apparatus makes it possible to reduce the operating cost and labor required for the waste liquid treatment for the aging waste liquid of the neutral salt electrolytic bath. Since the removal of environmentally unfavorable hexavalent chromium ions in the neutral salt electrolyzer has become possible, it is not necessary to install a waste liquid processor dedicated to the aging of the neutral salt electrolyzer, or the scale can be reduced. Therefore, it is possible to reduce the facility cost when newly installing the neutral salt electrolytic treatment apparatus and to effectively use the site. Since the work of updating or partially updating the neutral salt electrolytic bath due to the aging of the neutral salt electrolytic bath is not required, the labor required for the adjustment is reduced.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a relevant portion of a neutral salt electrolytic treatment apparatus for descaling a chromium iron alloy strip according to the present invention, which relates to hexavalent chromium ion removal.

FIG. 2 shows the output of the ORP meter when a reducing agent is added while adding caustic soda with the aim of maintaining the pH at 7 in a neutral salt electrolytic bath sample that has been subjected to neutral salt electrolytic treatment for a certain period. It is a figure which shows the differential value.

FIG. 3 shows a reducing agent while adding caustic soda with the aim of maintaining a pH of 2 in a sample of a neutral salt electrolytic bath which has been subjected to a neutral salt electrolytic treatment for a certain period of time as used in the test of FIG. FIG. 10 is a diagram showing an output of the ORP meter when adding O.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Neutral salt electrolysis tank 1a Neutral salt electrolysis bath 1b Immersion roll 1c Electrode for electrolysis 2 Electrolyte circulation tank 3 Sludge removal device 4 Pump (for electrolytic solution sending) 5a ORP meter 5b pH meter 6 Reducing agent addition device 6a Reducing agent storage tank 6b Pump (for sending reducing agent) 6c Valve (for adjusting reducing agent flow rate) 7 Alkaline aqueous solution addition device 7a Alkaline aqueous solution storage tank 7b Pump (for sending alkali) 7c Valve (for adjusting alkali flow rate) 8 Addition Quantity control device S Chrome iron alloy strip

Claims (5)

(57) [Claims] 1. A method for removing a chromium iron alloy zone by neutral salt electrolysis while maintaining a low chromate ion concentration in a neutral salt electrolytic bath by adding a reducing agent, the addition of a reducing agent. Change in the oxidation-reduction potential of the neutral salt electrolytic bath while maintaining the pH of the neutral salt electrolytic bath in the range of 6 to 8 by suppressing the decrease in the pH of the neutral salt electrolytic bath due to the addition of an alkali. A neutral salt electrolytic treatment method for descaling a chromium iron alloy strip, characterized by adding a reducing agent according to the following conditions. 2. A neutral salt electrolysis for descaling a chromium iron alloy strip according to claim 1, wherein an aqueous solution of caustic soda is used as an alkali for adjusting pH, and an aqueous solution of FeSO ₄ is used as a reducing agent for reducing chromate ions. Processing method. 3. The chromium according to claim 1, wherein the change in the oxidation-reduction potential is sequentially differentiated simultaneously with the addition of the reducing agent, and the addition of the reducing agent is stopped when the differentiated value changes abruptly. Neutral salt electrolytic treatment method for descaling of iron alloy strip. 4. The neutral salt electrolytic bath conditions include a Na ₂ SO ₄ concentration of 100-200 g / l, a pH of 6-8, and a liquid temperature of 70-9.
The neutral salt electrolytic treatment method for descaling a chromium iron alloy strip according to any one of claims 1 to 3, wherein the chromate ion is 1 g / l or less and the sludge concentration is 5 g / l or less at 0 ° C. 5. The oxidation-reduction potential of a neutral salt electrolysis bath (1a) in a neutral salt electrolysis tank (1) in which a chromium iron alloy strip (S) is continuously passed and neutral salt electrolysis is performed. ORP meter (5a) and pH meter (5b) for continuously measuring pH and pH, and the neutral salt electrolytic treatment tank
(1) An electrolytic solution circulating tank (2) to which a neutral salt electrolytic bath (1a) is continuously fed at a certain flow rate, and the electrolytic solution circulating tank (2)
Sludge removal device for removing sludge from neutral salt electrolytic bath (1a) and then sending it to neutral salt electrolytic treatment tank (1)
(3) and electrolysis for adjusting the pH of the neutral salt electrolytic bath (1a) to be in the range of 6 to 8 by a signal from the addition amount control device (8) based on the value measured by the pH meter (5b). Liquid circulation tank (2)
Aqueous solution addition device that adds an alkaline solution to the inside
(7) and a signal from the addition amount control device (8) based on the differential value of the oxidation-reduction potential measured by the ORP meter (5a) to the neutral salt electrolytic bath (1a) in the electrolyte circulation tank (2). A neutral salt electrolytic treatment apparatus for descaling a chromium iron alloy strip, comprising a reducing agent adding device (6) for adding a reducing agent.