JPH0892800A - Treatment of neutral salt electrolyte, and treating device therefor, descaling of stainless steel and device therefor - Google Patents

Abstract

PURPOSE: To efficiently recover heavy metal ions from an electrolyte by using one of sulfoxylic acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, peroxosulfuric acid, peroxodisulfuric acid, polythionic acid and hydrosulfurous acid as a reducing agent. CONSTITUTION: Stainless steel 1 is dipped into an electrolytic bath using a neutral salt aq. solution of sodium sulfate as the electrolyte 21 and is descaled by electrolysis. The electrolyte 21 in the electrolytic bath 2 is received in a reducing vessel 4 through a reserve tank 3. The electrolyte 21 in the reducing vessel 4 is adjusted in pH to 1-2 by adding sulfuric acid and sodium hydroxide and is reduced by adding an acid or a metallic salt of the acid. Sodium hydroxide is added into the reduced electrolyte 21 transported to a neutralizing vessel 5 to make pH >=5 and to convert the heavy metal into the hydroxide. The electrolyte 21 is passed through a precipitator 6 and a filter 7 to remove the heavy metal and is fed to the reserve tank 3 as the clean electrolyte 21.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for treating an electrolytic solution used for electrolytic descaling of stainless steel, and
TECHNICAL FIELD The present invention relates to a neutral salt electrolytic solution treatment method, a treatment apparatus, and a stainless steel descaling method and apparatus that can effectively remove hexavalent chromium ions from an electrolytic solution and reuse them.

[0002]

2. Description of the Related Art Generally, an oxide scale is formed on the surface of a cold rolled steel strip of stainless steel by an annealing process in an oxidizing atmosphere. Therefore, it is necessary to remove (descale) oxide scale. As the descaling method, a pickling method using an acid such as sulfuric acid, hydrochloric acid, or nitric hydrofluoric acid is used, but the cold-rolled steel strip of stainless steel contains a large amount of chromium oxides, and its removal is simply Only pickling is very difficult. Therefore, conventionally, a descaling method as shown in JP-B-53-13173, JP-A-63-286600, JP-A-1-96398, etc. has been used.
This descaling method improves the descaling performance and surface quality of the stainless steel strip by electrolyzing in a neutral salt solution.

In order to continue stable descaling performance by electrolysis, it is necessary to control the concentration of hexavalent chromium ions in the electrolytic solution. The electrolytic solution contains a large amount of eluted hexavalent chromium ions. Electrolysis is generally performed by indirect energization in which an electric current is indirectly passed through a steel strip by passing an electric current between a plurality of electrodes installed on the steel strip. In this method, both anode and cathode reactions occur on the steel strip. Therefore, when the concentration of hexavalent chromium ions in the electrolytic solution becomes high, the reduction reaction of hexavalent chromium ions competitively proceeds in addition to the hydrogen generation reaction which is a cathode reaction. When the reduction reaction of hexavalent chromium ions occurs, chromium oxides and hydroxides are generated and adhere to the steel strip, so that the descaling performance deteriorates. When the descaling performance deteriorates, not only the electric energy required for descaling becomes large, but also the electrolysis time becomes long (in the actual pickling line, the length of the electrolytic cell becomes long, that is, the treatment electrolysis is performed. The problem is that the amount of liquid increases, and the electrolysis efficiency also decreases. Therefore, in order to perform efficient descaling at low cost, it is important to control the concentration of the electrolytic solution, especially the concentration of hexavalent chromium ions.

Therefore, in order to prevent the precipitation of chromium oxide, a method has been proposed in which the electrode is arranged so that the steel strip serves as an anode at the end of electrolysis. In addition, Japanese Patent Publication 5-344
Japanese Patent Publication No. 39 and Japanese Patent Publication No. 5-34440 disclose that the chromium ion concentration of the electrolytic solution is limited. However, in these publications, specific removal of hexavalent chromium ion concentration,
There is no description about the management method.

The conventional method for controlling the hexavalent chromium ion concentration is to discharge a part of the electrolytic solution by a fixed amount,
It is a secondary management by replenishing a new solution. However, this method cannot effectively reduce the concentration of hexavalent chromium ions. Further, this waste liquid contains harmful hexavalent chromium ions, and a waste liquid treatment step for removing this is further required. Further, since the solution discharged by the liquid exchange is disposable, not only the cost becomes high, but also the system for managing the liquid concentration is inefficient and the solution is inefficient because it is performed in a batch system.

In order to overcome these problems, Japanese Patent Laid-Open No.
In the technique described in Japanese Patent No. 39600, after reducing hexavalent chromium ions contained in a used electrolytic solution using a sodium metabisulfite solution, chromium is precipitated as chromium hydroxide and filtered to electrolyze. Remove it from the liquid,
The filtrate is recovered as a reusable electrolyte.

[0007]

However, in this method, if the reaction time is sufficiently long, chromium ions can be removed to 5 mg / l or less, but since the reaction rate is slow, the actual chromium ion concentration can be shortened if treated in a short time. Is higher than 5 mg / l, which causes a problem of reduced efficiency. In order to improve efficiency, it is necessary to increase the amount of processing, but for that purpose the size of the reducing tank must be increased, the amount of reducing agent also increases, and a large amount of equipment cost and electric cost are required. There is a problem that it will occur.

Japanese Unexamined Patent Publication (Kokai) No. 63-286600 does not show a step of converting Cr ₂ (SO ₄ ) ₃ produced by neutralization into Cr (OH) ₃ and Na ₂ SO ₄ .

An object of the present invention is to remove hexavalent chromium ions efficiently in a short time and to provide an electrolyte solution after the removal in order to solve the above problems and achieve low cost and efficient descaling. It is an object of the present invention to provide a method for treating a neutral salt electrolyte solution that can be reused, and an apparatus for descaling stainless steel.

[0010]

The present invention is a method for treating or descaling an aqueous solution of a neutral salt, which is an electrolytic solution used for electrolysis for descaling of stainless steel, in which a reducing agent is added to the electrolytic solution. Then, a reduction step of reducing the dissolved heavy metal ions, a neutralization step of adding a base to the electrolytic solution that has undergone the reduction step to convert the reduced heavy metal ions into hydroxides, and a precipitation step, and a neutralization step were obtained. A neutral salt electrolytic solution treatment method is provided, which comprises a filtration step of removing the precipitated heavy metal hydroxide from the electrolytic solution. However, the neutral salt that is the solute of the electrolytic solution is a sulfate, and the reducing agent is a sulfoxylic acid, a dithionous acid, a sulfurous acid, a pyrosulfurous acid, a pyrosulfuric acid, a thiosulfuric acid, a peroxodisulfuric acid, a peroxodisulfuric acid, a polythione. Any one of an acid and hydrosulfite, or a metal salt thereof. The reducing agent can be changed according to the composition of the stainless steel, and sodium metabisulfite can be used as the reducing agent.

In the neutralization step, the pH of the electrolytic solution is adjusted to 5.0.
It is desirable to have a step of adjusting as described above. In the reducing step, when an acid is used as the reducing agent, p of the electrolytic solution is used.
It preferably has a step of adjusting H to 1.0 to 2.0, and when a salt is used as a reducing agent, it has a step of adjusting the pH of the electrolytic solution to 1.5 to 3.0.

Further, the neutral salt which is a solute of the electrolytic solution is sodium sulfate, the reducing step has a step of adding at least one of sulfuric acid and sodium hydroxide, and the neutralizing step has a hydroxylation step. It is desirable to have a step of adding sodium.

Further, according to the present invention, there is provided an apparatus for treating an aqueous solution of a neutral salt which is an electrolytic solution used for electrolysis of stainless steel, wherein at least one of the reducing agents is added to the electrolytic solution. Reduction tank, an acid and base supply mechanism that supplies at least one of an acid and a base to the reduction tank, a reducing agent supply mechanism that supplies a reducing agent to the reduction tank, and a base is added to the electrolytic solution after adding the reducing agent. Provided is a neutral salt electrolytic solution treatment device including a neutralization tank for performing a neutralization tank, a base supply mechanism for a neutralization tank for supplying a base to the neutralization tank, and a filtration mechanism for removing precipitates from the electrolytic solution after the base is added To be done. The neutral salt is
Sodium sulfate, the acid supplied by the acid and base supply mechanism is sulfuric acid, the base supplied by the acid and base supply mechanism is sodium hydroxide, and the base supplied by the neutralization base supply mechanism is sodium hydroxide. Is preferred.

This neutral salt electrolytic solution treating apparatus comprises an electrolyzer pH meter for detecting the pH of a solution held in the electrolytic cell, an oxidation-reduction potentiometer for detecting the oxidation-reduction potential of the solution held in the electrolytic cell, Chromium ion concentration meter for detecting the chromium ion concentration of the solution held in the electrolytic cell, and p of the solution held in the neutralization vessel
It is desirable to further include a neutralization tank pH meter that detects H, and a control device that includes a reducing condition adjusting unit and a neutralizing unit.

Here, the reducing condition adjusting means is p for the electrolytic cell.
The acid and base supply mechanisms are controlled according to the pH value detected by the H meter, and the pH of the solution holding at least one of the acid and the base in the electrolytic cell is 1.0 if the reducing agent is an acid.
2.0, if the reducing agent is a salt, the means for supplying it so as to be 1.5 to 3.0, the redox potential detected by the redox potentiometer, and the chromium ion concentration detected by the chromium ion concentration meter The reducing agent supply mechanism is controlled so that the reducing agent is supplied so that the redox potential of the solution held in the electrolytic cell becomes higher than a predetermined potential and the chromium ion concentration becomes equal to or lower than the predetermined concentration. And means. The predetermined potential is preferably 550 mV, and the predetermined chromium ion concentration is preferably 2 mg / l.

The neutralizing means controls the neutralization tank base supply mechanism according to the pH value detected by the neutralization tank pH meter,
It has a means for supplying the base so that the pH of the solution held in the electrolytic cell becomes 5.0 or more.

Further, according to the present invention, there is provided a stainless steel descaling device for electrolytically descaling stainless steel in an electrolytic solution which is an aqueous solution of a neutral salt, which is an electrolytic bath for immersing the stainless steel in the electrolytic solution. Also provided is a stainless steel descaling device including the neutral salt electrolytic solution treatment device, and a mechanism for supplying the filtrate supplied from the filtration mechanism to the electrolytic cell. The electrolytic cell may be equipped with a mechanism for changing the composition of the reducing agent according to the composition of the stainless steel.

The present invention comprises the step (a) of anodic electrolysis of scale-formed stainless steel in a neutral salt aqueous solution, and the stainless steel treated in the step is further subjected to cathodic electrolysis in a nitric acid aqueous solution or nitric hydrofluoric acid. A method including a step of performing immersion treatment in a mixed aqueous solution, or (b) steps of anodic electrolysis or immersion treatment in an alkaline aqueous solution in the order of (a) step (b) step or (b) step (a) step And (b) a neutral salt aqueous solution electrolytic cell having a plurality of positive and negative electrodes, and (b) an alkaline aqueous solution electrolytic cell having a plurality of positive and negative electrodes. Both the (a) and (b) tanks of the alkaline aqueous solution immersion tank should be in the order of (a) tank (b) tank or (b)
Achieved from a continuous descaling device for stainless steel, which is equipped with a tank (a) in this order, and a nitric acid aqueous solution electrolytic tank or a nitric hydrofluoric acid mixed aqueous solution immersion treatment tank with a plurality of positive and negative electrodes behind both tanks. It is characterized by including the above-mentioned neutral salt electrolytic treatment method or apparatus. As stainless steel, austenitic or ferritic stainless steel is used, such as AISI 304, 316, 410, 430 series.

Further, each electrode provided in each electrolytic cell of the continuous descaling device is achieved by an insoluble electrode arranged so as to face a continuously moving high speed stainless steel strip.

In particular, the descaling in the present invention is to remove at a high speed oxide scale which is annealed in a non-oxidizing atmosphere and slightly formed on the surface. It is suitable for a surface scale amount of 100 μg / cm ² or less.

By carrying out this method with this apparatus, a stainless steel having substantially scale removed and having excellent gloss and smoothness on the surface can be obtained easily at a high speed.

According to the present invention, in the descaling method for stainless steel, the step of removing the chromium oxide tank formed on the outermost surface of the stainless steel, the step of removing the chromium oxide tank, and the Mn
And the step of removing the chromium oxide bath containing Fe and the step of removing iron oxide are sequentially performed chemically by an optimum solution.

Further, according to the present invention, in the descaling method of stainless steel, the step of dissolving chromium oxide in the scale formed on the surface of the stainless steel as Cr ₂ O ₇ ^2- , the chromium oxide in the scale is dissolved. It is characterized in that the step of dissolving as CrO ₄ ²⁻ and the step of dissolving iron oxide in the scale as Fe ² + are chemically removed by optimum solutions.

According to the present invention, in the high-speed consistent continuous production method of stainless steel, a step of hot rolling, followed by cold rolling of the descaled stainless steel strip, a step of cold rolling, and a step of electrically heating and annealing in a non-acidic atmosphere, After annealing, it is cooled and then the stainless steel strip is subjected to anodic cathodic electrolysis in an aqueous solution of a neutral salt, immersed in an alkaline aqueous solution or anodic cathodic electrolyzed, and anodic cathodic electrolyzed in an aqueous nitric acid solution or nitrofluoride. The method is characterized by including the step of immersing in an acid aqueous solution, and electrolyzing the neutral salt aqueous solution described above.

The present invention relates to a cold rolling machine for cold rolling a descaled stainless steel strip after hot rolling, an annealing furnace for conducting electric current annealing in a non-oxidizing atmosphere after this cold rolling, and a cooling after this annealing. In the continuous continuous production apparatus for stainless steel strips provided with the cooling device for cooling and the descaling device for descaling after cooling, the descaling device is as described above, and the neutral salt electrolytic treatment device is as described above.

[0026]

In order to save the consumption of the electrolytic solution and reduce the burden of wastewater treatment, it is effective to reuse the neutral salt electrolytic solution after the heavy metal removal treatment. Therefore, in the present invention, at least a part of the electrolytic solution, which is a neutral salt aqueous solution, is taken out from the electrolytic cell, and hexavalent chromium ions eluted in the solution by electrolysis are reduced to trivalent chromium using a reducing agent. After that, it is neutralized and deposited as chromium hydroxide to be removed out of the system, and a solute (neutral salt) is generated by oxidizing and neutralizing the reducing agent to regenerate the neutral salt electrolytic solution. The present invention was realized by the present inventors finding a particularly effective reducing agent for the reduction of heavy metal ions, and as the reducing agent, sulfoxylic acid, dithionous acid, sulfurous acid, or pyrosulfite is used. , Sulfurous acid, thiosulfuric acid, peroxomonosulfuric acid, peroxodisulfuric acid, polythionic acid, and sulfuric acid of any of hydrosulfurous acids, or metal salts of these acids. When sodium sulfate is used as the solute of the electrolytic solution, the metal salt is preferably a sodium salt. That is, when a salt is used as the reducing agent, sodium sulfoxylate, sodium dithionite, sodium sulfite, sodium pyrosulfite, sodium pyrosulfate, sodium thiosulfate,
It is preferable to use any one of sodium peroxomonosulfate, sodium peroxodisulfate, sodium polythionate, and sodium hydrosulfite. The chemical formula of polythioic acid is H ₂ S _x O ₆ (x = 2 to 6), and the chemical formula of polythionate is M _x H _y S _z O _v (x = 0 to 2, y = 0 to 2, z = 2
˜6, v = 2 to 6, M is a metal).

When all of these reducing agents were actually used for regenerating the electrolytic solution, it was found that the reaction efficiency was higher than that of sodium metabisulfite. These reducing agents not only effectively reduce hexavalent chromium ions, but also effectively reduce other heavy metal ions. Therefore, according to the present invention, chromium, iron, It is possible to efficiently recover nickel and the like. In addition, these reducing agents are converted to sulfate ions by reduction of metal ions (that is, oxidation of the reducing agent), so by neutralizing them,
A sulfate that is a solute of the electrolytic solution is obtained. Therefore, according to the present invention, effective reduction can be performed, and solute can be generated by the reduction to restore the solute concentration of the electrolytic solution.

As an example of the reduction reaction of a hexavalent chromium compound to a trivalent chromium compound by the above reducing agent, dichromic acid when sulfurous acid, sodium thiosulfate, or sodium dithionite is used as a reducing agent. The reaction formulas of the reduction reaction are shown as reaction formulas 1, 2, and 3, respectively.

H ₂ Cr ₂ O ₇ + 3H ₂ SO ₃ → Cr ₂ (SO ₄ ) ₃ + 4H ₂ O (Reaction Formula 1) 4H ₂ Cr ₂ O ₇ + 3Na ₂ S ₂ O ₃ + 9H ₂ SO ₄ → 4Cr ₂ ( SO ₄ ) ₃ + 3Na ₂ SO ₄ + 13H ₂ O (Reaction formula 2) H ₂ Cr ₂ O ₇ + Na ₂ S ₂ O ₄ + 2H ₂ SO ₄ → Cr ₂ (SO ₄ ) ₃ + Na ₂ SO ₄ + 3H ₂ O (Reaction formula 3) As can be seen from the reaction formulas 2 and 3, when a salt is used as the reducing agent, the liquidity in the system changes to the alkaline side due to the reduction. , It is necessary to adjust the pH of the system to a range suitable for reduction, but as can be seen from the reaction formula 1, when the oxygen acid of sulfur is used as the reducing agent, sulfuric acid is generated by the oxidation of the reducing agent itself, and therefore, especially when external It is not necessary to add sulfuric acid from the. When a sodium salt is used as a reducing agent, sodium sulfate is produced by the reaction, as can be seen from the reaction formulas 2 and 3. Therefore,
When sodium sulfate is used as the neutral salt of the electrolytic solution, the concentration of the electrolytic solution can be recovered by the generated sodium sulfate.

As can be seen from the reaction formulas 1 to 3, the reduced chromium ions are once dissolved in the solvent as sulfates (that is, trivalent chromium ions and sulfate ions). As shown in reaction formula 4 of 1., by neutralizing this with sodium hydroxide, trivalent chromium hydroxide can be recovered and further sodium sulfate can be obtained.

Cr ₂ (SO ₄ ) ₃ + 6NaOH → 2Cr (OH) ₃ + 3Na ₂ SO ₄ (Reaction Formula 4) The toxicity of trivalent chromium is much weaker than that of hexavalent chromium. Further, the resulting chromium hydroxide (III) is insoluble in water, its solubility product is at ^{17 ℃ [Cr 3 +] [} OH -] 3
= 5.4 × 10 ⁻³¹ (physical and chemical dictionary, 3rd edition, expanded edition (1981, published by Iwanami Shoten), page 672). Therefore, this chromium (III) hydroxide can be easily taken out of the system in a safe form by the filtration step.

The scale formed on the surface of the stainless steel strip by the annealing treatment is a spinel type oxide. Normal (80
FeCr ₂ containing Fe ₃ O ₄ in the annealing heat treatment (0 ° C or more)
Iron / chromium spinel oxide composed of O ₃ is produced. The electrolysis of the stainless steel strip having scale in each of the neutral salt aqueous solution, the alkaline aqueous solution and the nitric acid aqueous solution used in the method including the treatment step for removing the scale has the following effects.

In the neutral salt electrolysis, it has a function of mainly dissolving Kushintaka in iron / chromium spinel oxide. That Cr-H ₂ O system potential -pH view of FIG. 2 (M.Pourbaix: At
lasof Electrochemical Equilibria in Aque-ous Solut
ions (1966) PergamonPress), chrome is neutral ~
It is shown that it is dissolved as Cr ₂ O ₇ ²⁻ by anodic polarization above +0.2 V based on the saturated calomel electrode in the acidic pH range. In ordinary neutral salt electrolysis, Na ₂ SO ₄ is used as an electrolytic solution. Na ₂ SO ₄ has the function of increasing the conductivity of the electrolytic solution. Since the pH is usually electrolyzed in the neutral to weakly acidic region, the scale is dissolved as Cr ₂ O ₇ ²⁻ .

The electrolytic treatment in an alkaline aqueous solution such as an aqueous NaOH solution or an aqueous KOH solution has the following effects. That is, chromium in the scale is dissolved as CrO ₄ ²⁻ .
In this case, it can be seen that the electrolytic potential at pH 13 to 14 is obtained by anodic polarization at a noble potential of about -0.35 V or higher based on the saturated calomel electrode. That is, as compared with the above-mentioned neutral salt electrolysis, the chromium oxide CrO ₄ ²⁻ can be dissolved and removed efficiently by dissolving the chromium oxide as CrO ₄ ²⁻ at a considerably lower potential.

In the nitric acid aqueous solution electrolysis, it has a function of dissolving iron in the scale. In this case, the electrode is a stainless steel strip as the cathode. That is, iron in the spinel oxide scale is mixed with divalent and trivalent iron, but divalent iron dissolves in an ordinary acid aqueous solution, but trivalent iron has a very low dissolution rate. However, a practical dissolution rate can be obtained by reducing trivalent iron to divalent iron. In cathodic electrolysis in a nitric acid aqueous solution, electrons are supplied to a stainless steel strip to reduce trivalent iron to divalent as described below, and at the same time, nitric acid is dissolved and removed as Fe ² +.

Fe ³ + (oxide) + e ⁻ → Fe ² + (ion) The spinel-type oxide scale produced on the stainless steel strip by the above three types of electrolytic treatments has high efficiency, high activity,
Can be removed at high speed.

In the combination of the three kinds of electrolytic treatments of the present invention, the effect is the same regardless of which of neutral salt aqueous solution electrolysis and alkaline aqueous solution electrolysis is first. It is effective to use nitric acid aqueous solution electrolysis in the final step after removing chromium oxide, which is difficult to remove.

In the present invention, workability is remarkably improved because it does not involve the high temperature treatment of the conventional alkali molten salt. Further, the problem of scale dissolution rate due to slightly lower efficiency of neutral salt aqueous solution electrolysis in neutral salt aqueous solution electrolysis → nitric acid aqueous solution electrolysis is solved by performing highly efficient alkaline electrolysis, and the scale removal rate is improved.

It is more effective if the neutral salt aqueous solution electrolysis and the alkaline aqueous solution electrolysis are mainly anodic electrolysis, and the nitric acid electrolysis is mainly cathodic electrolysis.

[0040]

【Example】

Examples 1 to 20 (1) System Configuration This example is an example in which the present invention is applied to a pickling step in a stainless steel continuous annealing line. FIG. 1 shows the configuration of a stainless steel descaling device for the neutral salt treatment device of this embodiment. In FIG. 1, arrows indicate the flow of the solution, and dotted lines indicate the flow of the signal.

The descaling apparatus of this embodiment is equipped with an electrolytic bath 2 for electrolytically pickling the annealed stainless steel strip 1 in an electrolytic solution 21 which is a solution of a neutral salt (sodium sulfate). The stainless steel strip 1 which is the object of pickling is sent from the right to the left in the drawing at a constant speed by a carrying mechanism (not shown). In this embodiment, the electrolyte 21 is 180 g /
A sodium sulphate solution with a concentration of 1 is used.

Further, the descaling apparatus of this embodiment has a reserve tank 3 for supplying the electrolytic solution 21 to the electrolytic cell 2.
And a pump 31. The electrolytic cell 2 and the reserve tank 3, the reserve tank 3 and the pump 31, and the pump 31 and the electrolytic cell 2 are communicated with each other by a flow path, respectively, and the electrolytic solution 21 contains the electrolytic bath 2, the reserve tank 3, and the pump 31.
It is designed to circulate. Also, reserve tank 3
A new liquid tank 32 is connected to the via a flow path via a pump 34. The fresh solution tank 32 holds a sodium sulfate solution having a concentration higher than that of the electrolytic solution 21, and a solution having a high concentration (200 g / l in this embodiment) of sodium sulfate solution is transferred from the fresh solution tank 32 to the reserve tank 3. By supplying, the sodium sulfate concentration of the electrolytic solution is adjusted. Furthermore, the descaling device of this embodiment is provided with the electrolytic solution 21.
As an electrolytic solution treatment device for regenerating
A neutralization tank 5 and a filtration mechanism (precipitation tank 6 and filter 7) are provided, and between the reserve tank 3 and the reduction tank 4, between the reduction tank 4 and the neutralization tank 5, between the neutralization tank 5 and the precipitation tank 6, and,
Pump 3 is used between the settling tank 6 and the reserve tank.
The channels are connected via 3, 41, 51 and 71. A filter 7 is provided in the flow path between the settling tank 6 and the pump 71 to remove solids from the solution flowing through the flow path.

The electrolytic solution 21 in the reserve tank 3 is continuously withdrawn and sent to the reducing tank 4 via the pump 33. The electrolytic solution 21 reduced in the reduction tank 4 is sent to the neutralization tank 5 via the pump 41. The electrolytic solution 21 neutralized in the neutralization tank 5 is sent to the settling tank via the pump 51 to precipitate the precipitated salts, and then returned to the reserve tank 3 via the filter 7 and the pump 71. The filter 7 is provided to filter out the precipitate that has not completely settled and remove it from the electrolytic solution 21.

Further, the descaling apparatus of this embodiment is equipped with a sodium hydroxide tank 8 and pumps 81 and 82 as a mechanism for supplying a base for neutralization. The sodium hydroxide tank 8 is connected to the neutralization tank 5 via the pump 81 and to the reducing tank 4 via the pump 82 by the flow paths, respectively, and the sodium hydroxide solution held in the sodium hydroxide tank 8 is held therein. Is supplied to the neutralization tank 5 and the reduction tank 4, respectively. Furthermore, the descaling apparatus of this embodiment includes a reducing agent tank 9 and a pump 91 as a mechanism for supplying a reducing agent for reducing the electrolytic solution 21. The reducing agent tank 9 is communicated with the reducing tank 4 via a pump 91,
The reducing agent held by is supplied to the reducing tank 4. Also,
The descaling device of the present embodiment has p
A sulfuric acid tank 10 and a pump 101 are provided as a mechanism for supplying an acid for preparing H. The sulfuric acid tank 10 is connected to the reducing tank 4 via a pump 101, and the sulfuric acid tank 1
The sulfuric acid held at 0 is supplied to the reduction tank 4. In addition,
In this embodiment, the sodium hydroxide tank 8 is shared between the reduction tank 4 and the neutralization tank 5, but a base supply mechanism may be provided separately.

Further, the descaling device of this embodiment is a means for measuring the sodium sulfate concentration of the solution, a sodium sulfate concentration meter 14, and a means for measuring the hydrogen ion concentration (pH) and the redox potential of the solution. pH meter 11,
A chromium ion concentration meter 12, which is a means for measuring the chromium ion concentration of the solution, and a control device 13 for controlling the supply of the new liquid to the reserve tank 3 and the reduction by the reducing tank 4 and the neutralization by the neutralizing tank 5. ing. In this embodiment, one pH meter 11 capable of measuring both pH and redox potential is provided, but it may be measured by two detection devices that detect the respective values separately.

The sodium sulfate concentration meter 14 includes a specific gravity sensor 141 installed in the reserve tank 3. The sodium sulfate concentration meter 14 detects the concentration of sodium sulfate based on the specific gravity of the solution detected by this sensor 141, and notifies the controller 13 of the concentration. . p
The H meter 11 includes an electrode 111, and the electrode 111 is installed inside the reduction tank 4 and the neutralization tank 5. pH meter 11
Detects the pH or the oxidation-reduction potential from the potential detected by the electrode 111, and notifies the control device 13. The pH meter 11 of this embodiment uses a calomel electrode as a reference electrode. The chromium ion concentration meter 12 includes a hexavalent chromium ion ion-selective electrode 121 installed in the reduction tank 4, and detects the concentration of hexavalent chromium ion from the potential detected by the ion-selective electrode 121. Then, the control device 13 is notified.

In the present embodiment, the control unit 13 has at least a central processing unit (CPU) 13 as shown in FIG.
1 is an information processing device including a main storage device 132,
As shown in FIG. 3, neutral salt concentration adjusting means 301, reducing condition adjusting means 302, and neutralizing means 303 are provided. Each of these means 301 to 303 is realized by the CPU 131 executing an instruction previously held in the main storage device 132.

The neutral salt concentration adjusting means 301 is a means for controlling the operation of the pump 34 for supplying a new liquid based on the information detected by the sodium sulfate concentration meter 14. The reducing condition adjusting means 302 includes a pump 91 for supplying a reducing agent, a pump 101 for supplying sulfuric acid, and a sodium hydroxide based on the information detected by the pH meter 11 and the chromium ion concentration meter 12. Is a means for controlling the operation with the pump 82 for supplying. The neutralizing means 303, based on the information detected by the pH meter 11,
It is a means for controlling the operation of the pump 81 for supplying sodium hydroxide.

(2) Flow of Electrolyte Treatment When the stainless steel strip 1 after annealing is electrolytically pickled in a neutral salt solution, the flow of treatment of the neutral salt solution 21 used in electrolysis will be described below. To do.

The neutral salt solution 21 used for electrolysis in the electrolytic cell 2 is circulated between the electrolytic cell 2 and the reserve tank 3. The solution held in the reserve tank 3 is
It is continuously taken out and introduced into the reduction tank 4 via the pump 33. In the reducing tank 4, the p
A sulfuric acid tank 10 and a sodium hydroxide tank 8 for adjusting H,
A reducing agent tank 9 for reducing hexavalent chromium ions contained therein to trivalent is added.

For the reducing agent tank 9, sulfuric acid or a sodium salt of sulfuric acid is used. Examples of the sulfur acid include sulfoxylic acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, peroxomonosulfuric acid, peroxodisulfuric acid and polythionic acid. As the sodium salt of sulfuric acid, sodium sulfoxylate, sodium dithionite, sodium sulfite, sodium pyrosulfite, sodium pyrosulfate, sodium thiosulfate, sodium peroxomonosulfate, sodium peroxodisulfate,
Examples include sodium polythionate and sodium hydrosulfite.

When an acid other than sulfoxylic acid and dithionous acid is used as a reducing agent, it is not necessary to add sulfuric acid, so a mechanism for supplying sulfuric acid (sulfuric acid tank 1
0, the pump 101, and the flow path for connecting the sulfuric acid tank and the reducing tank) may not be provided. When sulfoxylic acid is used as a reducing agent, sulfoxylic acid is generated by adding sulfuric acid to sulfoxylate, and this is used. When dithionous acid is used as a reducing agent, sulfuric acid is added to dithionite to generate dithionous acid, which is used.

In the reducing tank 4, the sodium hydroxide solution held in the sodium hydroxide tank 8 or the sulfuric acid tank 10 is held depending on the internal hydrogen ion concentration (detected by the pH meter 11). Sulfuric acid is injected and the solution is adjusted to a predetermined pH. Further, a reducing agent is injected from the reducing agent tank 9 into the reducing tank 4 in accordance with the hexavalent chromium ion concentration (detected by the chromium ion concentration meter 12) to maintain a predetermined reducing agent concentration. Be adjusted as required.

The solution reduced in the reduction tank 4 is then introduced into the neutralization tank 5. The sodium hydroxide solution held in the sodium hydroxide tank 8 is introduced into the neutralization tank 5 until the pH of the internal solution reaches a predetermined value (5.0 in this embodiment). Trivalent chromium ions are precipitated as chromium hydroxide when the pH reaches 5.0. Also,
Iron ions and nickel ions mixed in the solution also precipitate as hydroxides.

The precipitate is collected in the settling tank 6 and the filter 7.
When solid-liquid separation is carried out with, a regenerated electrolytic solution from which excess metal has been removed is obtained as a filtrate. This regenerated electrolytic solution is returned to the reserve tank 3.

The neutral salt (sodium sulfate in this embodiment) concentration in the reserve tank is adjusted by injecting the new liquid held in the new liquid tank 32 so as to maintain a predetermined concentration. ing. The fresh solution held in the fresh solution tank 32 has a neutral salt concentration higher than a predetermined concentration of the electrolytic solution.

In this embodiment, the injection rates of the new liquid, the acid, the base and the reducing agent are controlled by the controller 13 according to a predetermined procedure.

(3) Control Flow of Control Device 13 First, the processing of the neutral salt concentration adjusting means 301 will be described. The control flow of the neutral salt concentration adjusting means 301 of this embodiment is shown in FIG.
Shown in.

The neutral salt concentration adjusting means 301 of this embodiment is
If the sodium sulfate concentration in the reserve tank 3 input from the sodium sulfate concentration meter 14 is 180 g / l or less (step 401), the pump 34 for supplying the new liquid is operated to discharge the new liquid from the new liquid tank 32. It is supplied to the reserve tank 3 (step 402), and the process is returned to step 401. When the sodium sulfate concentration is higher than 180 g / l (step 401), the neutral salt concentration adjusting means 301 returns the process to step 401 without supplying the new solution. In the present embodiment, the neutral salt concentration adjusting means 301 always performs this process (steps 401 to 40) while the descaling device is operating.
2) is repeated.

Next, the processing of the reduction condition adjusting means 302 will be described. FIG. 5 shows a control flow of the reducing condition adjusting means 302 of this embodiment. The pH at the time of reduction varies depending on the reducing agent, but here, the case where the pH is adjusted to 1.5 to 3.0 will be described as an example.

In the reducing condition adjusting means 302 of this embodiment, first, according to the pH in the reducing tank 4 inputted from the pH meter 11, if the pH is larger than 3.0 (step 501),
The pump 101 for acid injection is operated to inject sulfuric acid from the sulfuric acid tank 10 into the reducing tank 4 to lower the pH (step 50
2) The process is returned to step 501. If the pH is less than 1.5 (step 503), the base injection pump 82 is operated to inject the sodium hydroxide solution from the sodium hydroxide tank 8 into the reducing tank 4 to raise the pH (step 504), and then step 501. Return processing to. These steps 501-50
According to 4, the pH in the reduction tank is adjusted to 1.5 to 3.0.

Next, the reducing condition adjusting means 302 sets the redox potential in the reducing tank 4 inputted from the pH meter 11 to 55.
If it is higher than 0 mV (step 505) or the hexavalent chromium ion concentration in the reducing tank 4 input from the chromium ion concentration meter 12 is 2 mg / l or more (step 50).
6) The pump 91 for injecting the reducing agent is operated to inject the reducing agent from the reducing agent tank 9 into the reducing tank 4 (step 507),
The process is returned to step 501. By the reducing agent injection processing (step 507), hexavalent chromium ions existing in the solution are reduced to trivalent. Also, when the hexavalent chromium ion concentration is less than 2 mg / l (step 506), the reducing condition adjusting means 302 returns the process to step 501.
The above steps 501 to 507 are always repeated during the operation of the present descaling device.

Finally, the processing of the neutralizing means 303 will be described. FIG. 6 shows a control flow of the neutralizing means 303 of this embodiment.

The neutralization means 303 of this embodiment is a pH meter 11
If the pH in the neutralization tank 5 input from the above is less than 5.0 (step 601), the pump 81 for supplying the base is operated to transfer the sodium hydroxide solution from the sodium hydroxide tank 8 to the neutralization tank 4. Inject to raise the pH (step 602) and return the process to step 601. When the pH is 5.0 or higher (step 601), the neutralizing means 303 returns the process to step 601 without supplying the base. In this embodiment, the neutralizing means 301 always repeats this processing (steps 601 to 602) while the descaling device is operating.

(4) Evaluation of Each Reducing Agent First, the pickling efficiency of the electrolytic solution reduced by each reducing agent and regenerated will be described. Table 1 shows the descaled state of the stainless steel strip 1 performed using the above descaling device for each reducing agent.

[0066]

[Table 1]

In Table 1, under the experimental condition A, anodic electrolysis (4 A / dm ² ) was carried out for 35 seconds in a neutral salt electrolyte solution (pH 5.0 to 8.0) at 80 ° C., and then at 50 ° C. The descaled state of the stainless steel strip 1 immersed in hydrofluoric acid for 28 seconds was observed. Further, in the experimental condition B, in the neutral salt electrolytic solution (pH 5.0 to 8.0) at 80 ° C., the anodic electrolysis (4 A /
dm ² ) for 18 seconds, followed by cathodic electrolysis (4 A / d
m ² ) was carried out for 18 seconds, and then the descaled state of the stainless steel strip 1 immersed in nitric hydrofluoric acid at 50 ° C. for 28 seconds was observed.

In Comparative Example 1, when a sodium sulfate solution having a concentration of 180 g / l (referred to as undiluted solution a) which had not been used for electrolysis was used as the electrolytic solution, both the experimental conditions A and B were excellent in descaling. The state was obtained.

In Comparative Example 2, when an electrolytic solution (referred to as the undiluted solution b) after being used for electrolysis was used, it was possible to perform descaling in a good condition under the experimental condition A, but it was slightly inferior in the experimental condition B. It was a result.

In Comparative Example 3, about 10% of the stock solution b was used as the stock solution a.
Using the solution exchanged with (partially called exchange solution), good descaling could be performed under the experimental condition A.
Under the experimental condition B, the result was rather good.

This is because the stock solution b and the partially exchanged solution have a high hexavalent chromium ion concentration in the solution, and the descaling performance deteriorates in the step including the cathodic electrolysis (experimental condition B). In the process of anodic electrolysis alone (experimental condition A), the descaling performance is not deteriorated, but in the usual case, indirect electrolysis is performed in electrolysis, so that the cathodic process is inevitably included. Therefore, even under the experimental condition B, an electrolytic solution having a good descaling performance is desirable.

Therefore, in Examples 1 to 5, sulfur dioxide (sulfoxylic acid, dithionous acid, sulfurous acid, pyrosulfurous polythionoic acid) was used as a reducing agent to regenerate the stock solution b by the descaling apparatus described above. (Filtrate of filter 7)
Was obtained, and descaling was performed using this, and good results were obtained for both experimental conditions A and B regardless of which reducing agent was used. The pH during reduction is 1.0-
It was set to 2.0.

Further, in Examples 6 to 10, the sodium salt of sulfuric acid (sodium sulfite, sodium pyrosulfite, sodium hydrosulfite, sodium dithionite, sodium polythionate) was used as the reducing agent, and the above-mentioned descaling apparatus was used. The undiluted solution b was regenerated by using the above to obtain a regenerated solution (filtrate of the filter 7), and descaling was performed using this, and good results were obtained for both experimental conditions A and B when reduced with either reducing agent. Was obtained. The pH during the reduction was set to 1.5 to 3.0.

Next, the regeneration efficiency of each reducing agent will be described. Table 2 shows the amounts of heavy metals contained in the stock solutions a and b, the partial exchange solution, and the regeneration solution.

[0075]

[Table 2]

In Table 2, the Cr ⁶ + concentration is Cr
It was determined by summing the concentrations of O ₄ ²⁻ and Cr ₂ O ₇ ²⁻ .

In Comparative Examples 4 to 6, the heavy metal contained in each of the stock solution a, the stock solution b and the partially exchanged solution was measured. The stock solution a before being used for the electrolytic treatment does not contain heavy metals. However, the stock solution b, which is a solution after being used for electrolysis, contains many heavy metals eluted from the stainless steel to be treated, and in particular, the hexavalent chromium ion concentration that reduces the descaling performance under the experimental condition B is high. It can be seen from the results of Comparative Example 5 that the value is high. Traditionally,
Although the electrolytic solution is regenerated by exchanging a part of the stock solution b with a new solution, it can be seen from Comparative Example 6 that the partially-exchanged solution has a very high hexavalent chromium ion concentration. .

On the other hand, in Examples 11 to 20, the stock solution b was
Sulfuric acid (sulfoxylic acid, dithionous acid, sulfurous acid, pyrosulfite polythionic acid) or sodium salt of sulfur acid (sodium sulfite, sodium pyrosulfite, sodium hydrosulfite, sodium dithionite, sodium polythionate) as reducing agent It was regenerated and the heavy metal content of the obtained regenerated liquid (filtrate of filter 7) was measured. It was found that the regenerated liquid thus obtained had a low content of heavy metals, and particularly the hexavalent chromium ion concentration was 2 mg / l or less, so that it was effectively removed.

When conventionally known sodium metabisulfite was used as a reducing agent (Comparative Example 7),
The chromium ion concentration of the regenerant was 5 mg / l or less. The chromium ion concentration of the electrolytic solution has a great influence on the brightness of the treated stainless steel. When the concentration of Cr ₆ + is 5 mg / l, good brightness cannot be obtained, but when it is 2 mg / l, good brightness is obtained as a product.

(5) Effects of Examples As described above, since the electrolytic solutions regenerated in Examples 1 to 20 have low hexavalent chromium ion concentration, electrolysis is performed by a step including cathode electrolysis. However, the descaling performance does not deteriorate, and the same good descaling as in the case of using an unused electrolytic solution can be performed. Moreover, Examples 1-10
According to the above, it was possible to obtain a product having the same brightness as when an unused electrolytic solution was used.

Further, according to each of Examples 1 to 20 above, not only can hexavalent chromium ions, which are harmful to the environment, be taken out of the system as trivalent chromium hydroxide from the electrolytic solution, but also unused. It was possible to recover an electrolytic solution with descaling performance comparable to that of the electrolytic solution. As the reducing agent, pyrosulfuric acid, thiosulfuric acid, peroxomonosulfuric acid, peroxodisulfuric acid, sodium sulfoxylate, sodium sulfite, sodium pyrosulfite, sodium pyrosulfate, sodium thiosulfate, sodium peroxomonosulfate, sodium peroxodisulfate are used. Even in the case where it was present, the same results as in Examples 1 to 20 could be obtained. The pH at the time of reduction was set to 1.0 to 2.0 when sulfur acid was used as the reducing agent, and set to 1.5 to 3.0 when salt was used.

As can be seen from Comparative Example 7, the chromium ion concentration remained about 5 mg / l even when sodium metabisulfite was used as a reducing agent, but according to Examples 11 to 20, up to 2 mg / l. Chromium ion concentration can be reduced. Furthermore, according to Examples 11 to 20, according to the reducing agent of the present invention, iron ions and nickel ions can also be removed with higher efficiency than other methods.

Example 21 FIG. 7 is a perspective view of a stainless steel descaling integrated device for descaling while continuously moving a stainless steel strip. The stainless steel strip cold-rolled by the cold rolling mill is wound on a coil and supplied from the pay-off reel 701. The stainless steel strip is cut to an appropriate length by an up-cut shear 702 and welded by a welding machine 703. The integrally connected stainless steel strips are degreased by the alkali degreasing device 705 after being adjusted in speed by the entrance looper 704, fed into the annealing furnace 706 and fed, and then forcibly cooled by the cooling device 707. The cooled stainless steel strip is a neutral salt aqueous solution electrolytic treatment tank 708, an alkaline aqueous solution electrolytic treatment tank 7
09, nitric acid aqueous solution electrolytic treatment tank 710, nitric hydrofluoric acid mixed aqueous solution treatment tank, and after being descaled, exit looper 7
13 and is wound up by the tension reel 714.
The alkaline aqueous solution electrolytic treatment tank 709 and the nitric acid aqueous solution electrolytic treatment tank 710 may be simply immersed. Either nitric acid or mixed acid can be used, or both can be used. Before entering each tank, each tank is passed through the washing tanks 715 to 718.

FIG. 8 is a concrete sectional view of each electrolytic cell.
The neutral salt aqueous solution electrolytic treatment tank 708 in this embodiment is the same as that in the first embodiment.
The electrolytic solution treatment devices shown in FIGS.

In the neutral salt aqueous solution electrolytic treatment tank 708, Na
₂ SO ₄ 20% concentration, pH 6 aqueous solution is filled, a positive voltage is applied to the stainless steel strip 1 by a pair of upper and lower positive electrodes 803, and a pair of upper and lower counter electrodes 803 ′ serves as negative electrodes on both sides. An electric current flows from the zone 1 to the counter electrode 803 'through the Na ₂ SO ₄ aqueous solution. With this current, chromium in the scale becomes Cr ₂ O ₇ ²⁻ and dissolves. The electrolytic solution treatment apparatus of the neutral salt aqueous solution electrolytic treatment tank 708 is the first embodiment.
Is the same as. Next, the stainless steel strip 1 is washed with water 71
Step 8 is to wash Na ₂ SO ₄ remaining on the surface. Next, the washing water is squeezed with lingualol and then introduced into the alkaline aqueous solution electrolytic treatment tank 709. The alkaline aqueous solution electrolytic treatment tank 709 is filled with an aqueous solution having a 40% concentration of NaOH, a positive voltage is applied to the stainless steel strip 1 by a pair of upper and lower positive electrodes 807, and a current is passed through the aqueous NaOH solution to form a pair of upper and lower counter electrodes 807 '. Flow to. Due to the current flowing at this time, the chromium oxide in the scale becomes CrO ₄ ²⁻ and is dissolved and removed. Chromium oxide is removed on the surface of the stainless steel strip 1, leaving iron oxide. Next, the stainless steel strip 1 enters the water washing tank 716, and Na remaining on the surface
The OH is removed by washing with water, and the washing water is squeezed out with Ringalol. Next, the stainless steel strip 1 is introduced into the nitric acid aqueous solution electrolytic treatment tank 710. Nitric acid aqueous solution electrolytic treatment tank 7
10 is filled with a 10% concentrated nitric acid aqueous solution, in which a current flows through the stainless steel strip through a pair of upper and lower positive electrodes 811 provided on the left and right, and a pair of upper and lower counter electrodes 811 'at the center are negative electrodes. Becomes Positive and negative electrodes 811 and 81
For 1 ', an insoluble electrode such as a titanium-palladium-coated plate or a titanium-platinum-coated plate is used in order to prevent dissolution and consumption in an aqueous nitric acid solution. These electrodes may be provided partially over the entire length of the width of the steel strip or they may be provided over its entire length. In the present embodiment, the electrode is not in contact with the steel strip, but it can be contacted with the steel strip, but the former is preferable. Here, since stainless steel is used as a cathode electrode, Fe in the scale is used as described above.
(III) becomes Fe (II) and is eluted as Fe ² + in the solution. By the above three kinds of electrolytic treatments, the scale made of iron-chromium spinel oxide on stainless steel is removed with high efficiency and at high speed. Further, the stainless steel strip 1 was washed with water to remove residual HNO ₃ in a washing tank 717.
As is clear from the above, in the examples according to the present invention, the scale was completely removed, and the stainless steel surface after the scale removal had a smooth, glossy and beautiful mirror surface.

On the other hand, in the conventional method shown in Table 3, the scale removal was incomplete or the stainless steel surface was clouded after the removal, resulting in surface roughness. In this embodiment, the stainless steel strip 1 that has left the aqueous nitric acid solution electrolytic treatment tank 710 is washed with water.
In step 16, the HNO ₃ remaining on the surface is washed, drained with ringer rolls 13, dried with a dryer 14, and sent to the next step.

In the electrolytic treatment of this embodiment, it is natural that the scale can be easily removed by raising the temperature of the electrolytic solution.

Table 3 shows the conventional method (electrolysis of neutral salt aqueous solution + electrolysis of nitric acid aqueous solution, electrolysis of neutral salt aqueous solution + immersion of mixed nitric hydrofluoric acid solution) for comparison with the descaling condition of the stainless steel treated in Example 1. The cases are also shown as Comparative Examples 8 and 9.
The stainless steel used is 0.5 mm of ferritic SUS430.
It is a thick plate. The electrolysis conditions are: neutral salt solution electrolysis: anode electrolysis, current density 6 A / dm
² Alkaline aqueous solution electrolysis: Anode electrolysis, current density 3A / d
m ² nitric acid aqueous solution electrolysis: anode electrolysis, current density 2 A / dm ² .

In this example, as a result of performing the above-mentioned electrolytic treatment while moving the above-mentioned AISI430 steel strip as stainless steel at 100 m / min, the same results as in Table 3 were obtained.

[0090]

[Table 3]

Also, AISI 304 as stainless steel was descaled by immersion in a nitric hydrofluoric acid mixed aqueous solution instead of the last electrolysis in a nitric acid aqueous solution, but it was confirmed that efficient descaling can be performed. .

It is to be noted that the anodic electrolysis and the cathodic electrolysis can be alternately switched over the predetermined length of the steel strip by the electrolysis of the aqueous solution of the neutral salt and the nitric acid.

The descaled steel strip is rinsed and, if necessary, is coiled through a bridal roll. The annealing furnace 206 may use a method of directly heating the stainless steel strip in a non-oxidizing atmosphere such as N ₂ by Joule heat by energization. The heating by direct energization is performed by passing a large current through the steel strip between the turn rollers 20 and 21 for a predetermined length. The annealing temperature is 850 to 1150 ° C., and the annealing is performed within about 3 minutes. Cooling after annealing is forcibly carried out by flowing a non-oxidizing gas flow at a high speed along the steel strip, and is cooled to room temperature.

By the above descaling method, a continuous integrated manufacturing process of cold rolling → annealing → descaling becomes possible, and the processing can be performed at a speed of 100 m / min or more.

In this embodiment, sodium metabisulfite can be further used as a reducing agent for electrolytic treatment. In addition, although descaling including immersion in an alkaline aqueous solution and its electrolytic treatment was described in this example, descaling that does not include these can be performed while treating the electrolytic solution in the same manner as in Example 1.

The heating by direct energization is carried out by applying a high current to the steel strip over a predetermined length with the turn roller. Annealing temperature is 850 ~ 1150 ℃, about 3
Done within minutes. Cooling after annealing is forcibly performed by flowing a non-oxidizing gas along the steel strip at a high speed, and is cooled to room temperature.

Embodiments 22 to 27 In this embodiment, the electrolytic solution treating apparatus is used as in the case of the first embodiment, and the neutral salt aqueous solution electrolytic treating tank 708 and the alkaline aqueous solution electrolytic treating tank 709 are exchanged in the order of steps. Example 22 uses a scale method. That is, the stainless steel strip was first electrolyzed in an alkaline aqueous solution by applying a positive voltage to the stainless steel strip in an alkaline aqueous solution electrolysis tank.
Then, a positive voltage was applied to the stainless steel strip in a neutral salt aqueous solution electrolyzer to electrolyze the neutral salt aqueous solution. After that, a negative voltage was applied to the stainless steel strip in the nitric acid aqueous solution electrolytic bath to perform electrolytic treatment. The cleaning treatment and draining during the electrolysis treatment and after the electrolysis of the nitric acid aqueous solution are the same as in Example 21. By this method, a stainless steel strip having a smooth and glossy surface with the scale completely removed was obtained. Table 4 shows the processing conditions and the processing results.

Table 4 shows other embodiments of the present invention and the processing results thereof as Embodiments 22 to 27.
Similar to the first embodiment, these embodiments also have an electrolytic solution treating apparatus.

[0099]

[Table 4]

[0100]

EFFECTS OF THE INVENTION According to the present invention, by using an excellent reducing agent, it is possible to efficiently reduce, neutralize and filter heavy metal ions, particularly chromium ions, in the electrolytic solution and recover them. This not only prevents harmful hexavalent chromium ions from being left in the waste liquid, but also allows the electrolytic solution to be reused efficiently, so that effective descaling can be performed at low cost.

Further, according to the present invention, since the descaling is performed according to the scale component while adjusting the neutral salt aqueous solution electrolyte, the descaling process can be sped up to enable continuous continuous production of stainless steel sheets.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a descaling device according to an embodiment.

FIG. 2 is a hardware configuration diagram of a control device according to the embodiment.

FIG. 3 is a functional block diagram of a control device according to the embodiment.

FIG. 4 is a flow chart showing a control flow of the neutral salt concentration adjusting means in the embodiment.

FIG. 5 is a flowchart showing a control flow of the reducing condition adjusting means of the embodiment.

FIG. 6 is a flowchart showing a control flow of neutralizing means processing of the embodiment.

FIG. 7 is a perspective view of a stainless steel descaling consistent apparatus.

FIG. 8: Neutral salt aqueous solution electrolysis tank, alkaline aqueous solution treatment tank,
Sectional drawing which shows a nitric acid aqueous solution processing tank and a mixed aqueous solution processing tank.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Stainless steel strip, 2 ... Electrolyte tank, 3 ... Reserve tank, 4 ... Reduction tank, 5 ... Neutralization tank, 6 ... Precipitation tank, 7 ... Filter, 8 ... Sodium hydroxide tank, 9 ... Reducing agent tank, 10 … Sulfuric acid tank, 11… pH meter, 12… Chromium ion concentration meter, 13
... Control device, 31, 34, 41, 51, 71, 81, 8
2, 91, 101 ... Pump, 32 ... New liquid tank, 111
... electrodes, 121 ... ion selective electrodes, 131 ... central processing unit, 132 ... main memory, 141 ... specific gravity sensor,
301 ... Neutral salt concentration adjusting means, 302 ... Reduction condition adjusting means, 303 ... Neutralizing means, 705 ... Alkaline degreasing device, 7
06 ... Annealing apparatus, 708 ... Neutral salt aqueous solution electrolytic treatment tank, 7
09 ... Alkaline aqueous solution electrolytic treatment tank, 710 ... Nitric acid aqueous solution electrolytic treatment tank, 711 ... Nitric-hydrofluoric acid mixed aqueous solution treatment tank.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiko Ito 3-10-2 Bentencho, Hitachi-shi, Ibaraki Hitachi Kyowa Industry Co., Ltd.

Claims (23)

[Claims] 1. A method for treating a neutral salt electrolyte, which comprises electrolytically treating stainless steel with an aqueous solution of a neutral salt and descaling,
A reducing step of adding a reducing agent to the electrolytic solution to reduce dissolved heavy metal ions, and a base to the electrolytic solution that has undergone the reducing step to neutralize to precipitate the reduced heavy metal ions as hydroxides. The reducing agent is a sulfoxylic acid, a dithionous acid, a sulfurous acid, a pyrosulfurous acid, a pyrosulfuric acid, a thiosulfuric acid, and a peroxomonomer. A method for treating a neutral salt electrolytic solution, which is one of sulfuric acid, peroxodisulfuric acid, polythionoic acid, and hydrosulfurous acid, or one of metal salts of these acids. 2. The method according to claim 1, wherein the reducing step includes a step of adjusting the pH of the electrolytic solution to 1.0 to 2.0, and the neutralizing step includes adjusting the pH of the electrolytic solution to 5.0. A neutral salt electrolytic solution treatment method comprising the steps of adjusting as described above. 3. The neutral salt according to claim 1, wherein the neutral salt is sodium sulfate, and the reducing step includes a step of adding at least one of sulfuric acid and sodium hydroxide,
The neutralizing electrolytic solution treatment method, wherein the neutralizing step includes a step of adding sodium hydroxide. 4. The reducing agent according to claim 1, wherein the reducing agent is sulfoxylic acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, peroxomonosulfuric acid, peroxodisulfuric acid,
A neutral salt electrolytic solution treatment method, which is a metal salt of any one of polythionic acid and hydrosulfite. 5. The neutral salt according to claim 4, wherein the neutral salt is sodium sulfate, and the metal salt is a sodium salt.
The neutralizing electrolytic solution treatment method, wherein the neutralizing step includes a step of adding sodium hydroxide. 6. The method according to claim 4, wherein the reducing step has a step of adjusting the pH of the electrolytic solution to 1.5 to 3.0, and the neutralizing step has a pH of the electrolytic solution of 5.0 or more. A method for treating a neutral salt electrolytic solution, comprising the step of: 7. A neutral salt electrolytic solution treating apparatus for electrolytically descaling stainless steel using an aqueous neutral salt solution, comprising a reducing tank for adding a reducing agent to the electrolytic solution, and an acid in the reducing tank. And an acid and base supply mechanism for supplying at least one of a base, a reducing agent supply mechanism for supplying a reducing agent to the reducing tank, a neutralizing tank for adding a base to the electrolytic solution after the reducing agent is added, and A neutralizing tank base supply mechanism for supplying a base to the neutralization tank and a filtration mechanism for removing precipitates from the electrolyte solution after the base addition, and the reducing agent supplied by the reducing agent supply mechanism is sulfoxyl. Acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, peroxomonosulfuric acid, peroxodisulfuric acid, polythionic acid, and / or hydrosulfurous acid, or any of these metal salts. Neutral salt electrolyte treatment apparatus according to symptoms. 8. The pH of the electrolytic cell solution according to claim 7.
PH meter for electrolytic bath, redox potential meter for detecting redox potential of the electrolytic bath solution, chromium ion concentration meter for detecting chromium ion concentration of the electrolytic bath solution, and neutralization bath solution It has a pH meter for a neutralization tank for detecting pH, and a control device, and the control device is provided with a reducing condition adjusting means and a neutralizing means, and the reducing condition adjusting means is a pH meter for the electrolytic cell. A means for controlling the acid and base supply mechanism according to the detected pH value so as to supply at least one of the acid and the base so that the pH of the solution in the electrolytic cell becomes 1.0 to 2.0; The redox potential detected by the reduction potentiometer, and
The reducing agent supply mechanism is controlled according to the chromium ion concentration detected by the chromium ion densitometer, and the redox potential of the solution held in the electrolytic cell of the reducing agent becomes larger than a predetermined potential, and the chromium And a means for supplying the ions so that the ion concentration becomes equal to or lower than a predetermined concentration, and the neutralization means is a base supply mechanism for the neutralization tank according to the pH value detected by the pH meter for the neutralization tank. And a means for supplying a base so that the pH of the solution held in the electrolytic cell becomes 5.0 or more. 9. The neutral salt electrolytic solution treating apparatus according to claim 8, wherein the predetermined potential is 550 mV and the predetermined chromium ion concentration is 2 mg / l. 10. The neutral salt according to claim 7, wherein the neutral salt is sodium sulfate, the acid supplied by the acid and base supply mechanism is sulfuric acid, and the base supplied by the acid and base supply mechanism is sodium hydroxide. And the base supplied by the neutralization base supply mechanism is sodium hydroxide. 11. A neutral salt electrolytic solution treatment method of electrolytically treating stainless steel with an aqueous neutral salt solution and descaling, wherein a reducing agent is added to the electrolytic solution to reduce dissolved heavy metal ions. And a neutralizing step of adding a base to the electrolytic solution that has undergone the reducing step to precipitate the reduced heavy metal ions as hydroxides, and a filtration for removing hydroxides from the electrolytic solution that has obtained the neutralizing step. And a step of changing the composition of the reducing agent according to the composition of the stainless steel. 12. A neutral salt electrolytic solution treating apparatus for electrolytically treating stainless steel using an aqueous solution of a neutral salt for descaling, and a reducing tank for adding a reducing agent to the electrolytic solution, and an acid in the reducing tank. And an acid and base supply mechanism for supplying at least one of a base, a reducing agent supply mechanism for supplying a reducing agent to the reducing tank, a neutralizing tank for adding a base to the electrolytic solution after the reducing agent is added, and A neutralization tank base supply mechanism for supplying a base to the neutralization tank, and a filtration mechanism for removing precipitates from the electrolytic solution after addition of the base are provided, and the composition of the reducing agent according to the composition of the stainless steel. An apparatus for treating a neutral salt electrolyte, which has a mechanism for changing the temperature. 13. A descaling method for stainless steel, comprising descaling by electrolytic treatment using a neutral salt electrolytic solution, wherein a reducing agent is added to the electrolytic solution to reduce dissolved heavy metal ions. A neutralizing step of adding a base to the electrolytic solution that has undergone the reducing step to precipitate the reduced heavy metal ions as hydroxide, and a filtering step of removing the hydroxide from the electrolytic solution that has undergone the neutralizing step. And the reducing agent is sulfoxylic acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, peroxomonosulfuric acid, peroxodisulfuric acid, polythionic acid, and hydrosulfurous acid. Any of the above or any of these metal salts, The descaling method of the stainless steel characterized by the above-mentioned. 14. A stainless steel descaling device for descaling by electrolytic treatment using a neutral salt electrolytic solution, comprising a reducing tank for adding a reducing agent to the electrolytic solution, and an acid and a base in the reducing tank. An acid and base supply mechanism for supplying at least one of them, a reducing agent supply mechanism for supplying a reducing agent to the reducing tank, a neutralization tank for adding a base to the electrolytic solution after the addition of the reducing agent, and the neutralization tank A base supply mechanism for supplying a base to the base and a filtration mechanism for removing precipitates from the electrolytic solution after the base addition, and the reducing agent is sulfoxylic acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiol. Descaling of stainless steel, characterized by being any one of sulfuric acid, peroxomonosulfuric acid, peroxodisulfuric acid, polythionic acid, and hydrosulfurous acid, or any of these metal salts. Apparatus. 15. A descaling method for stainless steel, comprising descaling by electrolytic treatment using a neutral salt electrolytic solution, wherein a reducing step is added to the electrolytic solution to reduce dissolved heavy metal ions, A neutralizing step of adding a base to the electrolytic solution that has undergone the reducing step to precipitate the reduced heavy metal ions as hydroxide, and a filtering step of removing the hydroxide from the electrolytic solution that has undergone the neutralizing step. A descaling method for stainless steel, comprising: changing the composition of the reducing agent according to the composition of the stainless steel. 16. The reducing agent is one of sulfoxylic acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, peroxomonosulfuric acid, peroxodisulfuric acid, polythionic acid, sodium methanebisulfite and hydrosulfurous acid. The method for descaling stainless steel according to claim 1. 17. A descaling apparatus for descaling stainless steel by electrolytic treatment using a neutral salt electrolytic solution, comprising: a reducing tank for adding a reducing agent to the electrolytic solution; and at least an acid and a base in the reducing tank. An acid and base supply mechanism for supplying either, a reducing agent supply mechanism for supplying a reducing agent to the reducing tank, a neutralizing tank for adding a base to the electrolytic solution after the reducing agent is added, and a neutralizing tank for the neutralizing tank. A neutralization tank base supply mechanism for supplying a base and a filtration mechanism for removing precipitates from the electrolyte solution after the base addition, and a mechanism for changing the composition of the reducing agent according to the composition of the stainless steel. A stainless steel descaling device characterized in that 18. A method of descaling an annealed and continuously moving stainless steel strip, which comprises a step of annealing the stainless steel strip while continuously moving the stainless steel strip, wherein the stainless steel strip is electrolyzed in a neutral salt aqueous solution. Step, including a step of electrolyzing the stainless steel strip after the step in a nitric acid aqueous solution or a step of dipping in a nitric hydrofluoric acid mixed aqueous solution, by adding a reducing agent to the electrolytic solution consisting of the neutral salt aqueous solution,
A reduction step of reducing dissolved heavy metal ions, a neutralization step of adding a base to the electrolytic solution that has undergone the reduction step to precipitate the reduced heavy metal ions as hydroxides, and the electrolysis that has undergone the neutralization step A step of removing the hydroxide from the liquid, and changing the composition of the reducing agent according to the composition of the stainless steel. 19. A method of descaling the annealed and continuously moving stainless steel strip, which comprises a step of annealing the stainless steel strip while continuously moving the stainless steel strip, wherein the stainless steel strip is electrolyzed in a neutral salt aqueous solution. And a step of electrolyzing in an alkaline aqueous solution or a step of dipping, and a step of electrolyzing the stainless steel strip that has undergone both steps in a nitric acid aqueous solution or a step of dipping in a nitric hydrofluoric acid mixed aqueous solution, Add a reducing agent to the electrolytic solution consisting of a neutral salt solution,
A reduction step of reducing dissolved heavy metal ions, a neutralization step of adding a base to the electrolytic solution that has undergone the reduction step to precipitate the reduced heavy metal ions as hydroxides, and the electrolysis that has undergone the neutralization step A method of descaling stainless steel, comprising a filtration step for removing the hydroxide from the liquid, and changing the composition of the reducing agent according to the composition of the stainless steel. 20. An apparatus for descaling the annealed stainless steel strip, comprising an annealing furnace for annealing while continuously moving the stainless steel strip, wherein the descaling device comprises a plurality of positive and negative electrodes. A neutral salt aqueous solution electrolysis cell, a nitric acid aqueous solution electrolysis cell having a plurality of positive and negative electrodes or a nitric hydrofluoric acid mixed aqueous solution cell behind the electrolysis cell. A reducing tank for adding a reducing agent to the liquid, an acid and base supplying mechanism for supplying at least one of an acid and a base to the reducing tank, a reducing agent supplying mechanism for supplying a reducing agent to the reducing tank, A neutralization tank for adding a base to the electrolytic solution after the addition of the reducing agent, a base supply mechanism for the neutralization tank for supplying a base to the neutralization tank, and a filtration mechanism for removing deposits from the electrolytic solution after the addition of the base. And a set of the above stainless steels Descaling device stainless steel and having a mechanism for changing the composition of the reducing agent in accordance with the. 21. An apparatus for descaling the annealed stainless steel strip, comprising an annealing furnace for annealing while continuously moving the stainless steel strip, wherein the descaling device comprises a plurality of positive and negative electrodes. Neutral salt aqueous solution electrolysis tank having a plurality of positive and negative electrodes, or an alkaline aqueous solution dipping tank having a plurality of positive and negative electrodes, and nitric acid aqueous solution electrolysis having a plurality of positive and negative electrodes behind the electrolytic cells. A tank or a nitric hydrofluoric acid mixed aqueous solution immersion tank is provided, and the neutral salt aqueous solution electrolytic tank supplies a reducing tank for adding a reducing agent to the electrolytic solution and at least one of an acid and a base to the reducing tank. An acid and base supply mechanism, a reducing agent supply mechanism that supplies a reducing agent to the reducing tank, a neutralization tank that adds a base to the electrolytic solution after the reducing agent has been added, and a base that supplies a base to the neutralization tank. A Japanese tank base supply mechanism, A descaling apparatus for stainless steel, comprising: a filtration mechanism for removing precipitates from the electrolytic solution after the addition of the base, and a mechanism for changing the composition of the reducing agent according to the composition of the stainless steel. 22. A cold rolling mill for cold rolling a descaled stainless steel strip after hot rolling, an annealing furnace for annealing while continuously moving the stainless steel strip after the cold rolling,
An integrated manufacturing apparatus for a stainless steel strip, comprising a cooling device for cooling after annealing, a descaling device for descaling while continuously moving the stainless steel strip after cooling, the descaling device comprising a plurality of descaling devices. Neutral salt aqueous solution electrolyzer with positive and negative electrodes, a plurality of positive,
A reducing bath for adding a reducing agent to the electrolytic solution comprising the neutral salt aqueous solution, comprising a nitric acid aqueous solution electrolytic bath having a negative electrode or a nitric hydrofluoric acid mixed aqueous solution bath, and at least one of an acid and a base in the reducing bath. An acid and base supplying mechanism for supplying the reducing agent, a reducing agent supplying mechanism for supplying a reducing agent to the reducing tank, a neutralizing tank for adding a base to the electrolytic solution after the reducing agent is added, and a base for the neutralizing tank. And a mechanism for changing the composition of the reducing agent according to the composition of the stainless steel. An integrated manufacturing equipment for stainless steel. 23. A cold rolling mill for cold rolling a descaled stainless steel strip after hot rolling, an annealing furnace for annealing while continuously moving the stainless steel strip after the cold rolling,
An integrated manufacturing apparatus for a stainless steel strip, comprising a cooling device for cooling after annealing, a descaling device for descaling while continuously moving the stainless steel strip after cooling, the descaling device comprising a plurality of descaling devices. A neutral salt aqueous solution electrolytic cell having positive and negative electrodes, an alkaline aqueous solution electrolytic cell having a plurality of positive and negative electrodes, or an alkaline aqueous salt dipping tank is provided, and a plurality of positive and negative electrodes are provided behind the both electrolytic cells. A nitric acid aqueous solution electrolytic bath or a nitric hydrofluoric acid mixed aqueous solution dipping bath, and a reducing tank for adding a reducing agent to the electrolytic solution composed of the neutral salt aqueous solution, and at least one of an acid and a base in the reducing tank. Acid and base supply mechanism for supplying the reducing agent, a reducing agent supply mechanism for supplying a reducing agent to the reducing tank, a neutralizing tank for adding a base to the electrolytic solution after the reducing agent is added, and a base for the neutralizing tank. During supply A stainless steel characterized by having a tank base supply mechanism and a filtration mechanism for removing precipitates from the electrolytic solution after addition of the base, and having a mechanism for changing the composition of the reducing agent according to the composition of the stainless steel. Integrated steel manufacturing equipment.