CN1313411A - Neutral salt electrolytic liquid for treating device of stainless steel, and method and apparatus for descaling stainless steel - Google Patents

Abstract

The present invention relates to a neutral salt electrolytic liquid for treating device of stainless steel, and method and apparatus for descaling stainless steel.

Description

Treatment device for neutral salt electrolyte of stainless steel, and method and device for descaling stainless steel

The present invention relates to a method for treating an electrolytic solution used for electrolytic descaling of stainless steel, and more particularly to a method for treating a neutral salt electrolytic solution, a treatment apparatus and a stainless steel descaling apparatus, which can effectively remove hexavalent chromium ions from the electrolytic solution and can reuse the hexavalent chromium ions.

Generally, on the surface of a cold-rolled steel strip of stainless steel, oxide scales (oxide films) are formed by an annealing step in an oxidizing atmosphere. Therefore, removal of the oxidized scale (descaling) is necessary. As the descaling method, an acid pickling method using an acid such as sulfuric acid, hydrochloric acid, or hydrofluoric/nitric acid is used, but a cold-rolled steel strip of stainless steel contains many chromium oxides, and it is very difficult to remove the chromium oxides by only acid pickling. In the past, the descaling methods disclosed in Japanese patent application laid-open Nos. 53-13173, 63-286600, and 1-96398 have been used. The descaling method is a method for improving the descaling performance and surface property of the stainless steel band by electrolysis in neutral salt electrolyte.

In order to continue the stable descaling performance of electrolysis, the concentration of hexavalent chromium ions in the electrolyte must be controlled. The electrolyte contains a large amount of dissolved hexavalent chromium ions. Generally, electrolysis is performed by passing a current between a plurality of electrodes provided on a steel strip, thereby indirectly passing a current through the steel strip. In this method, both anodic and cathodic reactions take place on the steel strip. Thus, when the concentration of hexavalent chromium ions in the electrolytic solution becomes high, the reduction reaction of hexavalent chromium ions proceeds in a mixed manner in addition to the hydrogen generation reaction as the cathode reaction. Because of the reduction reaction of hexavalent chromium ions, chromium oxides and chromium hydroxides are generated and attached to the steel strip, and the descaling performance is reduced. When the descaling performance is lowered, not only the electric energy required for descaling but also the electrolysis time becomes long (in the actual pickling line, the length of the electrolytic bathbecomes large, that is, the amount of the electrolyte to be treated increases), and the electrolysis efficiency also decreases. Therefore, in performing the descaling at low cost and high efficiency, the concentration control of the electrolyte, that is, the concentration control of hexavalent chromium ions is very important.

Here, in order to prevent the precipitation of chromium oxide, a method of disposing an electrode using a steel strip as an anode at the end of electrolysis has been proposed. Further, JP-B-5-34499 and JP-B-5-34440 disclose the limitation of the chromium ion concentration of the electrolyte, but these publications do not disclose a specific method for removing hexavalent chromium ions and controlling the concentration thereof.

A method for controlling the concentration of hexavalent chromium ions, which has been used in the past, is a method for sequentially controlling the concentration by discarding a part of an electrolyte by a certain amount and then replenishing a new electrolyte. However, the concentration of hexavalent chromium ions cannot be effectively reduced by this method. In addition, the waste liquid contains harmful hexavalent chromium ions, and a waste liquid treatment step for removing the harmful hexavalent chromium ions is necessary. Further, since the solution discharged by the liquid exchange cannot be recycled, not only the cost is increased, but also the solution can be intermittently discharged, and therefore, there is a problem that the concentration control and efficiency of the electrolyte solution are poor.

In order to overcome the above problems, the technique described in Japanese unexamined patent publication Hei 5-39600 is used to reduce hexavalent chromium ions contained in a spent electrolyte with a sodium metabisulfite solution, precipitate chromium as chromium hydroxide, remove the chromium hydroxide from the electrolyte by filtration, and recover the filtrate as a reusable electrolyte.

Although this method can remove chromium ions at 5mg/l or less for a sufficient reaction time, the reaction rate is low, and therefore, the actual chromium ion concentration becomes higher than 5mg/l in a short treatment time, resulting in a problem of low efficiency. In order to improve the efficiency, the amount of treatment must be increased, and therefore, not only the size of the reduction tank is increased, but also the amount of reducing agent used is increased, which causes a problem of enormous equipment cost and electrical cost.

JP-A63-286600 does not disclose Cr2 (SO) produced by neutralization₄)₃Conversion to Cr (OH)₃And Na₂SO₄The process (2).

The present invention has been made to solve the above problems and an object of the present invention is to provide a method and an apparatus for treating an electrolytic solution of an electric salt, and an apparatus for treating a stainless steel scale, which can remove hexavalent chromium ions efficiently in a production time and reuse the removed electrolytic solution, and to provide a method and an apparatus for treating a stainless steel scale.

The invention relates to a method for treating or descaling neutral saline solution, which is electrolyte used in the descaling electrolysis of stainless steel. The invention provides a treatment method of neutral salt electrolyte, which comprises a reduction process of adding a reducing agent into the electrolyte and reducing dissolved heavy metal ions; a neutralization step of adding an alkali to the electrolyte solution having undergone the reduction step to precipitate the reduced heavy metal ions as hydroxides; and a filtration step of removing the hydroxide of the heavy metal precipitated from the electrolyte solution obtained in the neutralization step. However, the neutral salt as a solute of the electrolyte is a sulfate, and the reducing agent is any one of hyposulfuric acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, caronic acid, peroxodisulfuric acid, polythionic acid, and dithionous acid or a metal salt thereof. The reducing agent may be changed corresponding to the composition of stainless steel, and sodium metabisulfite may be used as the reducing agent.

The neutralization step preferably includes a step of adjusting the PH of the electrolyte to 5.0 or higher. The reduction step preferably includes a step of adjusting the pH of the electrolyte to 1.0 to 2.0 when an acid is used as the reducing agent, and a step of adjusting the pH of the electrolyte to 1.5 to 3.0 when a salt is used as the reducing agent.

Preferably, the neutral salt as a solute of the electrolyte solution is sodium sulfate, the reduction step includes a step of adding at least one of sulfuric acid and sodium hydroxide, and the neutralization step includes a step of adding sodium hydroxide.

The present invention also provides a neutral salt electrolyte treatment apparatus for treating a neutral salt aqueous solution of an electrolyte used in the electrolysis of stainless steel, comprising a reduction tank for adding at least one of the reducing agents to the electrolyte, an alkali supply means for supplying at least one of an acid and an alkali to the reduction tank, a reducing agent supply means for supplying the reducing agent to the reduction tank, a neutralization tank for adding an alkali to the electrolyte to which the reducing agent is added, an alkali supply means for supplying an alkali to the neutralization tank, and a filtration means for removing a precipitate precipitated from the electrolyte to which the alkali is added. Preferably, the neutral salt issodium sulfate, the acid supplied from the acid and alkali supply means is sulfuric acid, the alkali supplied from the acid and alkali supply means is sodium hydroxide, and the alkali supplied from the neutralizing alkali supply means is sodium hydroxide.

Preferably, the neutral salt electrolyte treatment apparatus further comprises a pH meter for the electrolytic cell for detecting a pH of the solution in the electrolytic cell, an oxidation-reduction potentiometer for detecting an oxidation-reduction potential of the solution in the electrolytic cell, a chromium ion concentration meter for detecting a chromium ion concentration of the solution in the electrolytic cell, a pH meter for the neutralization cell for detecting a pH of the solution in the neutralization cell, and a control device provided with a reducing condition adjusting means and a neutralizing means.

Wherein the reducing condition adjusting means comprises means for controlling the supply of an acid and a base based on a pH detected by a pH meter for the electrolytic cell, means for supplying at least one of an acid and a base so that the pH of the solution in the electrolytic cell is 1.0 to 2.0 when the reducing agent is an acid and 1.5 to 3.0 when the reducing agent is a salt, and means for controlling the supply of the reducing agent based on an oxidation-reduction potential detected by the oxidation-reduction potential meter and a chromium ion concentration detected by the chromium ion concentration meter, and supplying the reducing agent so that the oxidation-reduction potential of the solution in the electrolytic cell is higher than a predetermined potential and the chromium ion concentration is not higher than a predetermined concentration. The predetermined potential is preferably 550mV, and the predetermined chromium ion concentration is preferably 2 mg/l.

The neutralization means has a means for controlling the alkali supply means in the neutralization tank so that the pH of the solution in the electrolytic tank becomes 5.0 or more by supplying alkali based on the detected pH by the pH meter for the neutralization tank.

The present invention also provides a stainless steel descaling apparatus for descaling by electrolyzing stainless steel in an electrolyte solution of a neutral salt aqueous solution, comprising an electrolytic bath for immersing the stainless steel in the electrolyte solution, and a neutral salt electrolyte treatment device for supplying a filtrate from a filter means to the electrolytic bath. The electrolytic cell may be provided with a mechanism for changing the composition of the reducing agent in accordance with the composition of the stainless steel.

The present invention is characterized by a method comprising the steps of (a) anodizing a stainless steel in a neutral aqueous salt solution and subjecting the stainless steel treated by the steps to a further cathodic electrolysis in an aqueous nitric acid solution or an immersion treatment in an aqueous hydrofluoric/nitric acid mixture solution, or the method comprising (b) conducting the step of anodizing or immersion treatment in an aqueous alkaline solution in the order of the steps (a) and (b) in the order of the steps (a), and (i) a tank for an aqueous alkaline solution having a plurality of positive and negative electrodes or (ii) a tank for an aqueous alkaline solution immersion tank for carrying out the method, wherein (i) the tank for an aqueous alkaline solution having a plurality of positive and negative electrodes or (i) the tank for an aqueous alkaline solution immersion tank are arranged in the order of (i) the tank (ii) or in the order of (i) the tank (i), and wherein the tank having a plurality of positive and negative electrodes, a plurality of positive electrodes, a negative electrode, and a negative electrode, the continuous descaling device of the stainless steel ofthe nitric acid aqueous solution electrolytic bath or the nitric acid fluoric acid mixed aqueous solution dipping treatment tank of the negative electrode. And is characterized by comprising the neutral salt electrolysis treatment method or device. As the stainless steel, austenitic or ferritic stainless steel, AISI304, 316, 410, 430 series steel, and the like are used.

In addition, each electrode in each electrolytic cell of the continuous descaling device is realized by an insoluble electrode disposed opposite to a stainless steel belt which continuously moves at a high speed.

In particular, the descaling in the present invention is a high-speed removal of oxide scale which is annealed in a non-oxidizing atmosphere and is formed on the surface in a very small amount. It is preferable that the amount of oxide scale on the surface is 100. mu.g/cm²The following. Then, by carrying out the above method by the above apparatus, it is possible to easily obtain stainless steel having substantially removed oxide scale and having excellent gloss and smoothness on the surface at a high speed.

The present invention is characterized in that in the descaling method of stainless steel, chemical removal is performed by using an optimum solution in the order of a step of removing a chromium oxide layer formed on the outermost surface of stainless steel, a step of removing a chromium oxide layer containing Mn and Fe after chromium oxide removal, and a step of removing iron oxide.

The present invention is also characterized in that in the descaling method for stainless steel, chromium oxide in the scale formed on the surface of stainless steel is converted into Cr₂O^2- ₇The step of dissolving chromium oxide in the scale as CrO^2- ₄Dissolving the iron oxide in the scale as Fe²⁺In the step of dissolution of the form (1), chemical removal is carried out using an optimum solution.

The present invention is characterized in that the high-speed continuous manufacturing method of stainless steel comprises a step of cold rolling stainless steel which is descaled after hot rolling, a step of performing electric heating annealing in a non-oxidizing atmosphere after cold rolling, a step of cooling after annealing, and then subjecting the stainless steel strip to the same steps as described above to the step of anodizing in a neutral aqueous salt solution, a step of immersion in an alkaline aqueous solution or anodizing, and a step of anodizing in a nitric acid aqueous solution or an aqueous solution of hydrofluoric acid, and the neutral salt electrolysis is performed.

A continuous plant for producing a stainless steel strip is provided with a cold rolling mill for cold rolling a hot-rolled, descaled strip, an annealing furnace for annealing the cold-rolled strip by electric heating in a non-oxidizing atmosphere, a cooling device for cooling the annealed strip, a descaler for cooling the cooled strip, and a neutral salt electrolysis treatment device. In this plant, the above-mentioned descaling apparatus has the same structure as the above-mentioned descaling apparatus, and the above-mentioned neutral salt electrolysis treatment apparatus is also the above-mentioned neutral salt electrolysis treatment apparatus.

In order to reduce the consumption of the electrolyte and reduce the load of wastewater treatment, it is effective to reuse the neutral salt electrolyte after heavy metal removal treatment. Therefore, in the present invention, at least a part of the neutral salt water-soluble electrolytic solution is taken out from the electrolytic bath, hexavalent chromium ions dissolved in the solution are reduced to trivalent chromium ions by a reducing agent for electrolysis, and then neutralizedto be precipitated and removed as chromium hydroxide, and at the same time, a solute (neutral salt) is generated by oxidation and neutralization of the reducing agent, and the neutral salt electrolytic solution is regenerated. The present invention has been achieved by the inventors' discovery of a particularly effective reducing agent for reducing heavy metal ions. As the reducing agent, at least one of hyposulfuric acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, caronic acid, peroxodisulfuric acid, polythionic acid and dithionous acid, or a metal salt thereof is used. When sodium sulfate is used as a solute of the electrolyte solution, the metal salt is preferably a sodium salt. That is, when a salt is used as the reducing agent, sodium hyposulfite, sodium dithionite, sodium sulfite, and sodium metabisulfite are preferably usedAt least one of sodium pyrosulfate, sodium thiosulfate, sodium caroate, sodium peroxodisulfate, sodium polythionate, and sodium dithionite. In addition, polythionic acid has the formula H₂S_xO₆(X =2-6), M of the formula of polythionates_XH_YS_ZO_V(X =0-2, Y =0-2, Z =2-6, V =2-6, M is a metal).

It has been found that when these reducing agents are actually used for regenerating the electrolyte, the reaction efficiency is higher than that when sodium pyrosulfite is used. Since these reducing agents are effective not only in reducing hexavalent chromium ions but also in reducing other heavy metal ions, according to the present invention, chromium, iron, nickel, and the like can be efficiently recovered from the used electrolytic solution. In addition, since these reducing agents are changed to sulfate radicals by reduction of metal ions (i.e., oxidation of the reducing agents), sulfate salts as solutes of the electrolyte can be obtained by neutralization thereof. Therefore, according to the present invention, effective reduction can be performed, and further, the solute concentration of the electrolyte can be restored by reducing the generated solute.

Further, taking as an example a reduction reaction in which a hexavalent chromium compound is reduced to a trivalent chromium compound by the above-mentioned reducing agent, reaction formulae of a reduction reaction of dichromic acid when sulfurous acid, sodium thiosulfate or sodium dithionite is used as the reducing agent are shown in reaction formulae 1, 2 and 3, respectively. … … (reaction type 1) … … (reaction type 2) … … (reaction type 3)

As can be seen from the reaction formulae 2 and 3, when the reducing agent is a salt, the PH of the reaction system must be adjusted to a range suitable for reduction by adding sulfuric acid because the liquid property in the reaction system is changed to the alkaline side, but as can be seen from the reaction formula 1, when sulfuric acid (oxyden acid contacting sulfuric acid) is used as the reducing agent, the sulfuric acid is generated by oxidation of the reducing agent itself, and therefore, it is not necessary to add sulfuric acid into the reaction system from the outside. When a sodium salt is used as a reducing agent, sodium sulfate is generated by the reaction as shown in reaction formulas 2 and 3. Therefore, when sodium sulfate is used as a neutral salt of the electrolyte, the concentration of the electrolyte can be recovered by the sodium sulfate thus produced.

Further, as shown in the reaction formulas 1 to 3, when the reduced chromium ions are once dissolved in the solvent as sulfate (i.e., trivalent chromium ions and sulfate), the solution is neutralized with sodium hydroxide as shown in the following reaction formula 4,and the hydroxide of trivalent chromium can be recovered, and thenSodium sulfate can be obtained in one step. … … (reaction type 4)

The toxicity of trivalent chromium is far lower than that of hexavalent chromium. In addition, the chromium (III) hydroxide formed is insoluble in water and has a solubility [ C]at 17 ℃³⁺ _r〕〔OH^-1〕³=5.4×10^-31(physiochemical dictionary, 3 rd edition supplement (published by rock Bow bookshop 1981.) therefore, this chromium (III) hydroxide can be easily discharged out of the reaction system in a safe form by the filtration step.

The oxide scale formed on the surface of the stainless steel strip after the annealing treatment is spinel-type oxide. Fe is generated and contained in the annealing treatment (at 800 ℃ C. or higher) in general₃O₄FeCr (a)₂O₃The composite Fe-Cr spinel oxide. The electrolysis of the stainless steel strip containing the oxide scale in each of the neutral aqueous salt solution, the alkaline aqueous solution and the nitric acid aqueous solution used in the method including the treatment step for removing the oxide scale has the following functions.

Neutral salt electrolysis mainly has the function of dissolving chromium in the iron-chromium spinel oxide. Namely, Cr-H from FIG. 9₂It can be seen from the O-system potential-pH diagram (M. Pourbaix: Atlas of electrochemical Reguliba in Aqueous Solutions (1966) Pergamon Press) that by anodic polarization at a pH in the neutral-acidic range with respect to a saturated calomel electrode, chromium is converted to Cr by anodic polarization at a voltage of +0.2V or more₂o^2- ₇The form of (a) is dissolved. Na is used as an electrolytic salt in normal neutral salt electrolysis₂SO₄。Na₂SO₄With the power supplyThe electrolyte has high conductivity. Usually, the electrolysis is carried out in a range of pH neutral to weakly acidic to cause the scale to be oxidized with Cr₂O^2- ₇The form of (a) is dissolved.

The electrolytic treatment in an alkaline aqueous solution such as an aqueous NaOH solution or an aqueous KOH solution has the following effects. Namely, chromium in the oxide scale is changed into CrO^2- ₄The form of (a) is dissolved. The electrolytic potential in this case can be obtained by anodizing at pH 13 to 14 with an anodic polarization potential of about-0.35V or more based on a saturated calomel electrode. That is, chromium oxide can be converted to CrO at a much lower electrolytic potential than the neutral salt^2- ₄Form (1) ofDissolved and removed efficiently.

The electrolysis of aqueous solutions of nitric acid has the effect of dissolving the iron in the oxidized scale. In this case, electrolysis was carried out with a stainless steel strip as a cathode. That is, ferrous iron and ferric iron in the spinel-type oxide scale are present in a mixed state, and ferrous iron is dissolved in a normal acid aqueous solution, but the dissolution rate of ferric iron is extremely low. However, a practical dissolution rate can be obtained by reducing ferric iron to ferrous iron. Cathodic electrolysis in aqueous nitric acid solution can supply electrons to a stainless steel strip to reduce ferric iron to divalent as shown below while using nitric acid to reduce it as F²⁺The form is dissolved and removed.

The spinel-type oxide scale formed on the stainless steel strip can be removed at a high speed with high efficiency and high operability by the above 3 kinds of electrolytic treatments.

In the combination of the three electrolytic treatments of the present invention, which one of the former and latter effects does not change. The electrolysis of aqueous nitric acid is more effective as a final step after removal of chromium oxide, which is difficult to remove.

In the present invention, since high-temperature treatment such as alkaline molten salt is not carried out, the workability is remarkably improved. In addition, in the neutral aqueous salt solution electrolysis → nitric acid aqueous solution electrolysis, the problem of dissolution rate of the oxidized scales due to a somewhat low electrolysis efficiency of the neutral aqueous salt solution is solved by the efficient alkaline aqueous solution electrolysis, and the removal rate of the oxidized scales is increased.

In addition, the effects are better when the electrolysis of the neutral aqueous salt solution and the electrolysis of the alkaline aqueous solution are mainly performed by anodic electrolysis and when the electrolysis of the nitric acid is mainly performed by cathodic electrolysis.

Examples 1 to 20

(1) Constitution of the System

In this example, the present invention was applied to the pickling step in the continuous annealing line for stainless steel. Fig. 1 shows the structure of the stainless steel descaling apparatus with neutral salt treatment apparatus according to the present embodiment. In fig. 1, the solid arrow lines indicate the solution flow system and the dash-dot lines indicate the signal system.

The electrolytic solution 21 in the descaling apparatus of this example is a solution of neutral salt (sodium sulfate), and an electrolytic bath 2 for electrolytically pickling the annealed steel strip 1 is provided. The stainless steel strip 1 to be pickled is fed from right to left at a constant speed by a conveying mechanism (not shown). In the present example, a sodium sulfate solution having a concentration of 180g/l was used as the electrolyte solution 21.

Further, the scale remover of the present embodiment is provided with a tank 3 for supplying the electrolyte 21 to the electrolytic bath 2 and a pump 31. The electrolytic bath 2, the liquid storage tank 3 and the pump 31, and the pump 31 and the electrolytic bath 2 are connected by a pipe, respectively, to circulate the electrolyte 21 among the electrolytic bath 2, the liquid storage tank 3, and the pump 31. Further, the tank 3 is connected to the new tank 32 through a pipe by means of a pump 34. The fresh liquid tank 32 holds a sodium sulfate solution having a concentration higher than that of the electrolyte 21, and the sodium sulfate concentration of the electrolyte is adjusted by supplying a more concentrated (200 g/l in this embodiment) sodium sulfate solution from the fresh liquid tank 32 to the liquid storage tank 3.

In addition, in the descaling device of the present embodiment, the reduction tank 4, the neutralization tank 5 and the filtering mechanism (the precipitation tank 6 and the filter 7) are provided as the electrolyte treatment device for regenerating the electrolyte 21, and the communication between the liquid storage 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 between the precipitation tank and the liquid storage tank 3 is made by the pumps 33, 41, 51, 71 and the piping, respectively. In addition, a filter 7 is provided on the line between the precipitation tank 6 and the pump 71 to remove solids from the solution flowing through the line.

The electrolyte 21 in the tank 3 is continuously pumped out to the reduction tank 4 by the pump 33. The electrolyte 21 reduced in the reduction tank 4 is sent to the neutralization tank 5 by a pump 41. The electrolyte 21 neutralized in the neutralization tank 5 is sent to the precipitation tank 6 by the pump 51, and the precipitated salts are precipitated and then returned to the liquid tank 3 through the filter 7 and the pump 71. The filter 7 is provided to remove non-precipitated precipitates from the electrolyte 21.

In the scale removing apparatus of this embodiment, a sodium hydroxide tank 8 and pumps 81 and 82 are provided as a means for supplying a neutralizing alkali. The sodium hydroxide tank 8 is connected to the neutralization tank 5 and the reduction tank 4 via pumps 81 and 82 and a pipe, respectively, and sodium hydroxide held in the sodium hydroxide tank 8 is supplied to the neutralization tank 5 and the reduction tank 4, respectively. Further, in the descaling device of the present embodiment, a reducing agent tank 9 and a pump 91 are provided as means for supplying a reducing agent for reducing the electrolytic solution 21. The reducing agent tank 9 is communicated with the reducing tank through a pump 91 and a pipe, and the reducing agent in the reducing agent tank 9 is supplied to the reducing tank 4. In the descaling apparatus of the present embodiment, the sulfuric acid tank 10 and the pump 101 are provided as a means for supplying an acid for adjusting the PH at the time of reducing the electrolyte 21. The sulfuric acid tank 10 is communicated with the reduction tank 4 by a pump 101, and sulfuric acid in the sulfuric acid tank 10 is supplied to the reduction tank 4. In the present embodiment, the reducing tank 4 and the neutralizing tank 5 share the single sodium hydroxide tank 8, but alkali supply means may be provided separately.

Further, in the scale removing device of the present embodiment, a sodium sulfate concentration meter 14 for measuring the sodium sulfate concentration of the solution is provided; a PH meter 11 as a means for measuring the hydrogen ion concentration (PH) and the redox status of the solution; a chromium ion concentration meter 12 for measuring the chromium ion concentration of the solution; and a control device 13 for controlling the supply of the fresh liquid to the liquid storage tank 3, the reduction in the reduction tank 4, and the neutralization in the neutralization tank 5. In the present embodiment, one PH meter 11 capable of measuring both PH and oxidation-reduction potential is provided, but two detection devices may be used to detect the respective values of both.

The sodium sulfate concentration meter 14 has a specific gravity sensor 141 provided in the tank 3, and measures the concentration of sodium sulfate by the specific gravity of the solution detected by the specific gravity sensor 141, and notifies the controller 13 of the concentration. The pH meter 11 has an electrode 111, and the electrode 111 is disposed inside the reduction tank 4 and the neutralization tank 5. The PH meter 11 detects PH or an oxidation-reduction potential from the potential detected by the electrode 111, and notifies the control device 13 of the detected PH or oxidation-reduction potential. In addition, in the PH meter 11 of the present embodiment, a calomel electrode was used as a reference electrode. The chromium ion concentration meter 12 has an ion-selective electrode 121 for hexavalent chromium ions provided in the reduction tank 4, and detects the concentration of hexavalent chromium ions from the potential detected by the ion-selective electrode 121, and notifies the controller 13 of the concentration.

In the present embodiment, the control device 13 is an information processing device having at least a Central Processing Unit (CPU)131 and a main memory device 132 as shown in fig. 2, and has a neutral salt concentration adjusting means 301, a reducing condition adjusting means 302, and a neutralizing means 303 as shown in fig. 3. The above-mentioned means 301-303 are realized by the CPU executing the instruction pre-held in the main memory 132.

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

(2) Electrolyte treatment process

The following describes a treatment flow of the neutral salt solution 21 used for electrolysis when the annealed steel strip 1 is electrolytically pickled in the neutral salt solution.

The neutral salt solution 21 used for electrolysis in the electrolytic cell 2 is circulated between the liquid tanks 3 of the electrolytic cell 2. The solution held in the tank 3 is continuously pumped out and introduced into the reduction tank 4 by the pump 33. In the reduction tank 4, sulfuric acid for PH adjustment, sodium hydroxide, and a reducing agent for reducing hexavalent chromium contained therein to trivalent chromium are added to the introduced solution.

As reducing agent, sulfuric acid or a sodium salt of sulfuric acid is used. Examples of sulfuric acid include hyposulfuric acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, caronic acid, peroxodisulfuric acid, polythionic acid, and dithionous acid. Examples ofthe sodium salt of sulfuric acid include sodium hyposulfite, sodium dithionite, sodium sulfite, sodium metabisulfite, and sodium pyrosulfate. Sodium thiosulfate, sodium caroate, sodium peroxodisulfate, sodium polythionate and sodium dithionite.

When an acid other than hyposulfuric acid and dithionous acid is used as the reducing agent, since it is not necessary to add sulfuric acid, a mechanism for supplying sulfuric acid (the sulfuric acid tank 10, the pump 101, and a line between the sulfuric acid tank and the reduction tank) is not necessary. In addition, in the case of using hyposulfuric acid as the reducing agent, hyposulfuric acid produced by adding sulfuric acid to hyposulfuric acid salt may be used. In addition, in the case of using dithionite as a reducing agent, dithionite produced by adding sulfuric acid to a dithionite salt may be used.

The solution is adjusted to a predetermined pH by injecting a sodium hydroxide solution held in a sodium hydroxide tank 8 or sulfuric acid held in a sulfuric acid tank 10 into the reduction tank 4 in accordance with the hydrogen ion concentration in the tank (detected by a pH meter 11). Further, the reducing agent in the reducing agent tank 9 is injected into the reducing tank 4 based on the hexavalent chromium ion concentration (detected by the chromium ion concentration sensor 12) so as to maintain a predetermined reducing agent concentration.

Then, the solution reduced in the reduction tank 4 is 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 solution in the tank reaches a predetermined value (5.0 in this example). At pH5.0, trivalent chromium ions precipitate as chromium hydroxide. In addition, iron ions and nickel ions mixed in the solution are also precipitated as hydroxides.

When the precipitate is subjected to solid-liquid separation by the precipitation tank 6 and the filter 7, a regenerated electrolyte from which excess metals have been removed can be obtained as a filtrate. The regenerated electrolyte is returned to the tank 3.

The concentration of neutral salt (sodium sulfate in this embodiment) in the tank can be adjusted to a predetermined concentration by the fresh liquid injected into the fresh liquid tank 32. The concentration of neutral salt of the fresh liquid in the fresh liquid tank 32 is higher than the concentration of the preset electrolytic solution.

In the present embodiment, the injection rates of the fresh liquid, the acid, the alkali, and the reducing agent are controlled by the control device 13 according to a preset program.

(3) Control flow of the control device 13

First, the treatment of the neutral salt concentration adjusting means 301 will be described. The control flow of the neutral salt concentration adjusting means 301 of the present embodiment is shown in fig. 4.

When the sodium sulfate concentration in the tank 3 fed from the sodium sulfate concentration meter 14 is not more than 180g/l (step 401), the neutral salt concentration adjusting means 301 of the present embodiment starts the new liquid supply pump 34 to supply new liquid from the new liquid tank 32 to the tank 3 (step 402), and returns the process to step 401. When the concentration of sodium sulfate is higher than 180g/l (step 401), the neutral salt concentration adjusting means 301 does not supply a new solution, and the process returns to step 401. In this embodiment, the neutral salt concentration adjusting means 301 repeats the process during the operation of the descaler (step 401-402).

The reduction condition adjusting means 302 is explained below. The control flow of the reducing condition adjusting means 302 of this embodiment is shown in FIG. 5. The pH at the time of reduction differs depending on the subsequent starting agent, and the case of adjusting the pH to a range of 1.5 to 3.0 will be described.

In the reducing condition adjusting means 302 of this embodiment, first, based on the pH in the reducing tank 4 inputted from the pH meter 11, if the pH is higher than 3.0 (step 501), the acid injection pump 101 is started to inject sulfuric acid from the sulfuric acid tank 10 into the reducing tank 4 to lower the pH (step 502), and the process returns to step 501. If the pH is less than 1.5 (step 503), the alkali injection pump 82 is started to inject sodium hydroxide from the sodium hydroxide tank 8 into the reduction tank 4 to raise the pH (step 504), and the process returns to step 501. The pH in the tank is adjusted to 1.5-3.0 by the above steps 501-504.

Then, when the oxidation-reduction potential in the reduction vessel 4 inputted from the PH meter is higher than 550mV (step 505) or the hexavalent chromium ion concentration in the reduction vessel 4 inputted from the chromium ion concentration meter 12 is higher than 2mg/l (step 506), the reducing agent injection pump 91 is started to inject the reducing agent from the reducing agent vessel 9 into the reduction vessel 4 (step 507), and the process returns to step 501. The hexavalent chromium ions present in the solution are reduced to trivalent by a reducing agent injection treatment (step 507). When the concentration of hexavalent chromium ions is less than 2mg/l (step 506), the reduction condition adjustment means 302 returns the treatment to step 501. The above steps 501 and 507 are repeated all the time during the operation of the descaler of the present invention.

Finally, the processing of the neutralization means 303 will be described. The control flow of the neutralization means 303 of the presentembodiment is shown in fig. 6.

In the neutralization means 303 of the present embodiment, when the PH in the neutralization tank 5 fed from the PH meter 11 is 5.0 or less (step 601), the alkali supply pump 81 is started, sodium hydroxide is fed from the sodium hydroxide tank 8 into the neutralization tank 5 to raise the PH (step 602), and the process is returned to step 601. When the pH is 5.0 or more (step 601), the neutralization means 302 returns the treatment to 601 without supplying an alkali. In this embodiment, the neutralizing means 303 repeats the process during the operation of the descaling device of the present invention (step 601-602).

(4) Evaluation of various reducing agents

First, the pickling efficiency of the electrolyte reduced and regenerated by various reducing agents will be described. The state of stainless steel descaling by the above-mentioned descaling apparatus is shown in Table 1 for each reducing agent.

TABLE 1

Experiment number	Electrolyte solution	Reducing agent	pH at reduction	Experimental Condition A	Experimental strip Part B
						Comparative example 1 Comparative example 2 Comparative example 3 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10	Stock solution a Stock solution b A part of the exchange liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid	- - - Hyposulfuric acid Dithionic acid Sulfurous acid Pyrosulfurous acid Polythionic acid Sodium sulfite Sodium metabisulfite Sodium dithionite Sodium dithionite Sodium polythionate	- - - 1·0-2·0 1·0-2·0 1·0-2·0 1·0-2·0 1·0-2·0 1·5-3·0 1·5-3·0 1·5-3·0 1·5-3·0 1·5-3·0	○ △ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○	○ × △ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○

○ good △ good x poorIn Table 1, it is observed that 35 seconds of anodic electrolysis (4A/dm) was carried out under the experimental condition A, i.e., in a neutral salt electrolyte (pH5 · 0-8.0) at 80 deg.C²) Then, the stainless steel 1 was immersed in hydrofluoric/nitric acid at 50 ℃ for 28 seconds to obtain a scale-removed state. In addition, it was observed that under the experimental condition B, i.e., in a neutral salt electrolyte (pH5.0-8.0) at 80 ℃, anodic electrolysis was carried out for 18 seconds (4A/dm)²) Thereafter, cathodic electrolysis (4A/dm) was carried out for 18 seconds²) Then, the stainless steel 1 was immersed in hydrofluoric/nitric acid at 50 ℃ for 28 seconds to obtain a scale-removed state.

In comparative example 1, when a sodium sulfate solution (referred to as stock solution a) having a concentration of 180g/l, which had not been used for electrolysis, was used as the electrolyte solution, a good scale-removed state was obtained under all of experimental conditions A, B.

In comparative example 2, when the electrolytic solution used in electrolysis (referred to as stock solution B) was used, although the scale removal was favorably performed under experimental condition a, the results obtained were inferior under experimental condition B.

In comparative example 3, when 10% of the electrolyte (referred to as a partial exchange liquid) of the stock solution B was exchanged with the stock solution a, good scale removal was possible under the experimental condition a, but only good results were obtained under the experimental condition B.

This is because in a part of the exchange solution of the stock solution B, the concentration of hexavalent chromium ions in the solution is high, and the descaling performance in the step including cathodic electrolysis (experimental condition B) is lowered. In the step including the anodic electrolysis (experimental condition a), although no decrease in the scaling property was observed, the cathodic step was required because the indirect energization was performed in the electrolysis in a normal case. Therefore, an electrolyte having good descaling performance under the experimental condition B is required.

Therefore, in examples 1 to 5, sulfuric acid (hyposulfuric acid, dithionous acid, sulfurous acid, pyrosulfurous acid, polythionic acid) was used as a reducing agent, and the raw solution b from the above-mentioned descaling device was regenerated to obtain a regenerated solution (filtrate of the filter 7), and when descaling was performed using this regenerated solution, good results were obtained under all of experimental conditions A, B in the case of reduction using any reducing agent, and the PH during reduction was 1.0 to 2.0.

In examples 6 to 10, sodium salts of sulfuric acid (sodium sulfite, sodium metabisulfite, sodium dithionite, and sodium polythionate) were used as reducing agents, and the stock solution b from the above-mentioned descaling apparatus was regenerated to obtain a regenerated solution (filtrate of the filter 7), and when descaling was performed using this regenerated solution, good results were obtained under the experimental condition A, B in the case of reduction with any reducing agent. In addition, the pH during reduction is 1.5 to 3.0.

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

TABLE 2

Experiment number	Electrolyte solution	Reducing agent	While reducing pH of (1)	Iron ion Concentration of (mg/l)	Nickel ion Concentration of (mg/l)	Chromium ion Total concentration of (mg/l)	Cr6+ Concentration of (mg/l)
								Comparative example 4 Comparative example 5 Comparative example 6 Practice ofExample 11 Example 12 Example 13 Example 14 Example 15 Example 16 Example 17 Example 18 Example 19 Example 20 Comparative example 7	Stock solution a Stock solution b A part of the exchange liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid Regeneration liquid	- - - Hyposulfuric acid Dithionic acid Sulfurous acid Pyrosulfurous acid Polythionic acid Sodium sulfite Sodium metabisulfite Sodium dithionite Sodium dithionite Sodium polythionate Sodium metabisulfite	-	0	0	0	0
-	400～ 800	200～ 300	6000～ 12000	6000～ 10000
					-	200～ 600	100～ 300				5000～ 8000	5000～ 7000
1.0～ 2.0	100～ 150	1 00～ 150	≤2	≤2
					1.0～ 2.0	100～ 150	100～ 150				≤2	≤2
1.0～ 2.0	100～ 150	1 00～ 150	≤2	≤2
					1.0～ 2.0	100～ 150	100～ 150				≤2	≤2
1.0～ 2.0	100～ 150	100～ 150	≤2	≤2
					1.5～ 3.0	100～ 150	100～ 150				≤2	≤2
1.5～ 3.0	100～ 150	100～ 150	≤2	≤2
					1.5～ 3.0	100～ 150	100～ 150				≤2	≤2
1.5～ 3.0	100～ 150	100～ 150	≤2	≤2
					1.5～ 3.0	100～ 150	100～ 150				≤2	≤2
2.0～ 5.0	150～ 200	150～ 200	≤5	≤5

In Table 2, Cr is⁶⁺At a concentration of CrO^2- ₄And Cr₂O^2- ₇The total concentration of (b) is obtained.

In comparative examples 4 to 6, the heavy metals contained in the stock solution a, the stock solution b, and a part of the exchange liquid were measured. The stock solution a before the electrolytic treatment does not contain heavy metals. However, the stock solution B which was the solution used in the electrolysis contained a large amount of heavy metals eluted from the stainless steel to be treated, and it was found from the results of comparative example 5 that the concentration of hexavalent chromium ions was high, particularly, the descaling performance under the experimental condition B was lowered. Conventionally, the regeneration of the electrolytic solution was carried out by exchanging a part of the stock solution b with a new solution, but it is apparent from comparative example 6 that the concentration of hexavalent chromium ions in this part of the exchanged solution is also extremely high.

On the other hand, in examples 11 to 20, the heavy metal content of the regenerated liquid (filtrate of the filter 7) obtained by regenerating the raw liquid b using sulfuric acid (hyposulfuric acid, dithionous acid, sulfurous acid, pyrosulfurous acid, polythionic acid) or a sodium salt of sulfuric acid (sodium sulfite, sodium metabisulfite, sodium dithionite, sodium polythionate) as a reducing agent was measured. The resulting regenerant had a low metal content, particularly a hexavalent chromium concentration of less than 2mg/l, thereby indicating that it had been effectively removed.

When a known sodium pyrosulfite was used as a reducing agent (comparative example 7), the chromium ion concentration of the regenerated solution was 5mg/l or less. The chromium ion concentration of the electrolyte has a great influence on the brightness of the treated stainless steel. Cr (chromium) component⁶⁺When the concentration of (A) is 5mg/l, good brightness cannot be obtained, but when it is 2mg/l, good brightness can be obtained as a product.

(5) Effects of the embodiments

As described above, since the electrolyte solutions regenerated in examples 1 to 20 have low concentrations of hexavalent chromium ions, the descaling performance does not deteriorate even when electrolysis is performed in a step including cathodic electrolysis, and thus, the descaling performance can be improved as well as when an unused electrolyte solution is used. Further, according to examples 1 to 10, products having the same brightness as when an unused electrolyte was used were obtained.

In addition, by the above examples 1 to 20, not only hexavalent chromium ions harmful to the environment in the electrolyte can be discharged out of the system as hydroxides of trivalent chromium, but also an electrolyte having descaling performance not inferior to that of an unused electrolyte can be recovered. In addition, the same results as in examples 1 to 20 were obtained when pyrosulfuric acid, thiosulfuric acid, caronic acid, peroxosulfuric acid, sodium hyposulfite, sodium sulfite, sodium metabisulfite, sodium pyrosulfate, sodium thiosulfate, sodium caronate, and sodium peroxodisulfate were used as the reducing agent. The pH in the reduction is 1.0-2.0 when sulfuric acid is used as the reducing agent, and 1.5-3.0 when a salt thereof is used.

As can be seen from comparative example 7, the use of sodium peroxodisulfate as the reducing agent also resulted in a residual chromium ion concentration of about 5mg/l, but the chromium ion concentration could be reduced to 2mg/l according to examples 11 to 20. In addition, according to examples 11 to 20, iron ions and nickel ions can be removed more effectively with the reducing agent of the present invention than with other methods.

Example 21

Fig. 7 is a perspective view of a stainless steel descaling kit for descaling while continuously moving a stainless steel strip. The stainless steel strip cold-rolled by the cold rolling mill is wound into a coil and supplied from an uncoiler 701. The stainless steel strip is cut at an appropriate length by an up-cut cutter 702 and welded by a welding machine 703. The speed of the integrated stainless steel strip is adjusted by an entry-side looper 704, and the stainless steel strip is degreased by an alkaline degreasing apparatus 705, and then enters an annealing furnace to be annealed, and then is forcibly cooled by a cooling apparatus 707. The cooled stainless steel strip is descaled in the neutral salt electrolysis treatment tank 708, the alkaline electrolysis tank 709, the nitric acid electrolysis tank 710, and the mixed acid tank of nitric acid and hydrofluoric acid, and then passed through the exit side looper 713, and wound into a coil by a tension winder. The alkaline electrolytic bath 709 and the nitric acid electrolytic bath 710 may be impregnated alone. Either or both of nitric acid and mixed acid may be used. Before entering the tanks, the wastewater passes through a water washing tank 715-718.

FIG. 8 is a sectional view of each electrolytic cell. The neutral saline solution electrolytic processing bath 708 in this embodiment is provided in the electrolytic solution processing apparatus shown in embodiments 1 to 20.

The neutral saline solution electrolytic treatment tank 708 was filled with Na having a pH of 6 and a concentration of 20%₂SO₄The aqueous solution is applied with positive voltage to the stainless steel strip 1 through a pair of upper and lower positive electrodes 803, the pair of upper and lower electrodes 803' at both ends are negative electrodes, and current passes through Na₂SO₄The aqueous solution flows from the stainless steel strip 1 to the electrode pair 803'. The current oxidizes chromium in the scale to Cr₂O^2- ₇And dissolved. The electrolyte treatment apparatus of the neutral saline solution electrolytic bath 708 is the same as in example 1. Then, the stainless steel strip 1 is washed with Na remaining on the surface thereof in a rinsing bath 4₂SO₄. Then, after squeezing out the washing water by a squeezing roller 5Enters an alkaline aqueous solution electrolytic treatment tank 709. The alkaline electrolytic processing bath 709 is filled with a 40% NaOH aqueous solution, a positive voltage is applied to the stainless steel strip 1 through a pair of upper and lower positive electrodes 807, and a current flows to the upper and lower counter electrodes 807' through the NaOH aqueous solution. Then, by passing current, chromium oxide in the scale is changed into CrO^2- ₄Iron oxide remains after the chromium oxide on the surface of the stainless steel strip 1 is removed by dissolution. Then, the stainless steel strip enters a washing tank 716, is washed with water to remove NaOH remaining on the surface, and is further squeezed out of the washing water by a squeeze roller. Subsequently, the stainless steel strip 1 is introduced into an electrolytic treatment tank 710 for an aqueous nitric acid solution. The electrolytic nitric acid solution treatment tank 710 is filled with a 10% nitric acid solution, and current is supplied to the stainless steel strip through upper and lower pairs of positive electrodes 811 disposed on the left and right sides, and the upper and lower pairs of electrodes 811' in the middle are negative electrodes. In order to prevent the positive and negative electrodes 811 and 811' from being dissolved and consumed in the aqueous nitric acid solution, an insoluble electrode such as a titanium palladium-coated plate or a titanium platinum-coated electrode is used. These electrodes may be provided partially or entirely across the width of the strip, and in this embodiment, the electrodes are not in contact with the strip. However, a phase connection is also possible, and the former is more preferable. Here, since stainless steel is used as the anode,as described above, Fe (III) in the oxidized scale is changed into Fe (II) and Fe is used as Fe in the solution²⁺The form of (2) is dissolved out. By the three electrolytic treatments, oxide scale composed of hercynite oxide on stainless steel can be removed efficiently and at high speed. Washing the stainless steel strip 1 with water in a washing tank 717 to remove residual HNO₃As is clear from Table 3, in the examples of the present invention, not only the scale was completely removed, but also the surface of the stainless steel from which the scale was removed was smooth and glossy, and exhibited a beautiful mirror surface.

In contrast, the conventional method shown in Table 3 did not completely remove the oxidized scale, or the surface of the stainless steel after removal was dull and not smooth. In this example, the stainless steel strip 1 having passed through the nitric acid aqueous solution electrolytic bath 710 was introduced into the washing bath 716 to wash the HNO remaining on the surface₃Is squeezed off by a squeezing roller 13The water is dried by the dryer 14 and then sent to the next step.

In the electrolytic treatment of the present example, it is needless to say that the oxide scale can be easily removed by raising the temperature of the electrolytic solution.

The descaling conditions of the stainless steel treated in example 21 and comparative examples 8 and 9 in which the conventional methods (electrolysis of a neutral saline solution + electrolysis of an aqueous nitric acid solution, electrolysis of a neutral saline solution + immersion of an aqueous nitric-fluoric acid mixture) were used as a comparison are shown in table 3. The stainless steel used was a 0.5mm plate of ferrite-type SUS 430. In addition, the electrolysis conditions were:

electrolysis of neutral saline solution: anode electrolysis with current density of 6A/dm²(ii) a Electrolysis of alkaline aqueous solution: anode electrolysis with current density of 3A/dm²(ii) a Electrolysis of aqueous solution of nitric acid: cathode electrolysis with current density of 2A/dm²。

In this example, the results of the above-described electrolytic treatment while using the above-described AISI430 steel strip as a stainless steel and moving it at a speed of 100 m/min were the same as those shown in Table 3.

TABLE 3

	Time of electrolysis			Descaling situation	Surface of stainless steel Condition of the condition
						Neutral base Na2SO4,20％ 80℃	Alkali NaOH 40％70℃	Nitric acid	10％50℃
	Example 21 Comparative example 8 (neutral salt electrolysis + nitric acid electrolysis) Relieving) Comparative example 9 (neutral salt electrolysis + nitrofluoro Acid pickling)	60 60 60	5 - -							15 25 -	◎ △ ◎	Gloss, smoothness Hair blackening and hair blackening face Hair blacking, superficial Coarse
(10％HNO 33% HF40 deg.C, 15 seconds)

Further, it was found that the stainless steel AISI304 was also effectively descaled by immersion in a nitric-fluoric acid mixed solution instead of electrolysis in a final nitric acid aqueous solution.

Alternatively, the anodic and cathodic electrolysis in the electrolysis of neutral salts and aqueous nitric acid can be carried out interchangeably over a defined length of the steel strip.

The descaled steel strip is washed and passed through a brush roll as the case may be, and wound into a coil.

Annealing furnace 706 can be used in N₂Or the like, by directly passing joule heat generated by energization to the stainless steel strip in a non-oxidizing atmosphere. The direct electrical heating is carried out by passing a large current through the steel strip over a predetermined length between the rotating rollers. The annealing temperature is 850-1150 ℃, and the annealing time is within about 3 minutes. The cooling after annealing is forced by passing a stream of non-oxidizing gas at high velocity along the strip until it is cooled to room temperature.

The use of the above-described descaling method enables a continuous manufacturing process of cold rail → annealing → descaling, and can be carried out at the above-described speed of 100 m/min or more.

In this example, sodium metabisulfite was also used as a reducing agent for the electrolytic treatment. In addition, although the description has been given of the alkaline aqueous solution immersion and the descaling including the electrolytic treatment in this example, the descaling not including the alkaline aqueous solution immersion and the descaling including the electrolytic treatment may be performed while the electrolytic solution treatment is performed as in example 1.

Examples 22 to 27

In this example, the same electrolyte treatment apparatus as in example 1 was used, and a case of the descaling method in which the order of the neutral aqueous salt solution electrolytic bath 708 and the alkaline aqueous solution electrolytic bath 709 was changed was defined as example 22. That is, first, electrolysis of an alkaline aqueous solution is performed by applying a positive voltage to a stainless steel strip in an alkaline aqueous solution electrolytic bath. Then, a positive voltage was applied to the stainless steel strip in a neutral aqueous solution electrolytic bath to perform electrolysis of a neutral aqueous salt solution. Then, electrolytic treatment was performed by applying a negative voltage to the stainless steel strip in an electrolytic bath of an aqueous nitric acid solution. The cleaning treatment and the wringing treatment between the respective electrolytic treatments and after the electrolysis of the nitric acid aqueous solution were the same as in example 21. The method can completely remove the oxidation scale to obtain the stainless steel band with smooth and glossy surface. The treatment conditions and the treatment results are shown in Table 4.

Embodiments of the invention and the results of their treatment are shown in Table 4 as examples 22-27. These examples also have the same electrolyte treatment apparatus as in example 1.

TABLE 4

	Time of treatment			Descaling Condition of the condition	Surface of stainless steel Status of state
						Example 22 Example 23 Example 24 Example 25 Example 26 Example 27	Alkali electrolysis NaOH，40％,70℃ 10 seconds Neutral salt electrolysis Na2SO4,20％,80℃ 60 seconds Alkaline impregnation NaOH,60％,20℃ 20 seconds Neutral salt electrolysis Na2SO4,20％,80℃ 60 seconds Alkali electrolysis NaOH,40％,70℃ 10 seconds Neutral salt electrolysis Na2SO4,20％,80℃ 50 seconds	Neutral salt electrolysis Na2SO4,20％,80℃ 50 seconds Alkaline impregnation NaOH,60％,90℃ 20 seconds Neutral salt electrolysis Na2SO4,20％,60℃ 60 seconds Alkali electrolysis NaOH,40％,70℃ 5 seconds Neutral salt electrolysis Na2SO4,20％,80℃ 50 seconds Alkaline impregnation NaOH,60％,90℃ 30 seconds	Electrolysis of nitric acid HNO3,10％,50℃ 15 seconds Electrolysis of nitric acid HNO3,10％,50℃ 15 seconds Electrolysis of nitric acid HNO310％,50℃ 15 seconds Impregnation with hydrofluoric/nitric acid HNO3,7％，HF2％,60℃ 10 seconds Impregnation with hydrofluoric/nitric acid HNO3,7％,HF2％,60℃ 10 seconds Impregnation with hydrofluoric/nitric acid HNO3,7％,HF2％,60℃	◎ ○ ○ ◎ ◎ ◎	Gloss, smoothness Gloss, smoothness Gloss, smoothness The hair is slightly blackened, and the hair is slightly blackened, rough surface The hair is slightly blackened, and the hair is slightly blackened, rough surface The hair is slightly blackened, and the hair is slightly blackened, rough surface

Wherein ◎ was completely removed, ○ was removed, △ was slightly remained, and x was remained in a large amount to indicate the state of scale removal

According to the present invention, by using an excellent reducing agent, heavy metal ions, particularly chromium ions, in an electrolytic solution can be efficiently reduced, neutralized, filtered, and recovered. Therefore, harmful hexavalent chromium ions are not left in the wasteliquid, and the electrolytic solution can be effectively reused, so that the descaling can be efficiently performed at low cost.

According to the present invention, since the descaling is performed according to the components of the oxidized scale while adjusting the neutral aqueous electrolyte, the speed of the descaling process is increased, so that the stainless steel sheet can be continuously manufactured.

FIG. 1 is a schematic view of a scale removing apparatus according to an embodiment.

Fig. 2 is a hardware configuration diagram of the control device of the embodiment.

Fig. 3 is a functional schematic diagram of a control device according to an embodiment.

FIG. 4 is a control flowchart of the neutral salt concentration adjusting means of the embodiment.

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

Fig. 6 is a control flow chart of the neutralization section process in the embodiment.

Fig. 7 is an oblique view of a stainless steel descaling kit.

FIG. 8 is a sectional view showing a neutral aqueous salt solution electrolytic bath, an alkaline aqueous solution treatment bath, a nitric acid aqueous solution treatment bath, and a mixed aqueous solution treatment bath.

FIG. 9 shows Cr-H₂potential-pH diagram of O system.

The description of the symbols in the above figures.

1-stainless steel band; 2-an electrolytic cell; 3-a liquid storage tank; 4-a reduction tank; 5-a neutralization tank; 6-a precipitation tank; 7-a filter; 8-sodium hydroxide tank; 9-a reducing agent tank; 10-sulfuric acid tank; 11-a pH meter; 12-chromium ion concentration meter; 13-a control device; 31. 34, 41, 51, 71, 81, 82, 91, 101-pump; 32-a fresh liquid tank; 111-electrode, 121-ion selective electrode; 141-specific gravity sensor; 131-a central processing unit; 132-a master memory device; 301-neutral salt concentration adjustment means; 302-reduction condition adjustment means; 303-neutralization means; 705-alkaline degreasing equipment; 706-annealing means; 708-a neutral brine solution electrolysis treatment tank; 709-alkaline aqueous solution electrolytic cell; 710-nitric acid aqueous solution electrolytic treatment tank; 711-hydrofluoric nitrate mixed aqueous solution treatment tank.

Claims (20)

1. A neutral salt electrolyte processing apparatus, which is a neutral salt water solution processing apparatus of an electrolyte used in stainless steel electrolysis, is characterized by comprising a reduction tank for adding a reducing agent to the electrolyte; acid and alkali supply means for supplying at least one of an acid and an alkali to the reduction vessel; a reducing agent supply mechanism for supplying a reducing agent to the reduction tank; a neutralization tank for adding an alkali to the electrolyte solution after the addition of the reducing agent; a neutralizing tank for supplying an alkali to said neutralizing tank, an alkali supply means for supplying an alkali to said neutralizing tank, and a filtering means for removing a precipitate precipitated from said electrolyte after addition of an alkali, wherein said reducing agent supplied by said reducing agent supply means is at least one of hyposulfuric acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, caronic acid, peroxodisulfuric acid, polythionic acid, and dithionous acid, or a salt thereof.

2. The apparatus of claim 1, having a pH meter for the electrolytic cell for detecting the pH of the solution in said electrolytic cell; an oxidation-reduction potentiometer for detecting the oxidation-reduction potential of the solution in said electrolytic cell; a chromium ion concentration meter for detecting the chromium ion concentration of the solution in the electrolytic bath; a pH meter for a neutralization tank for detecting the pH of the solution in the neutralization tank, and a control device having a reducing condition adjusting means and a neutralizing means, wherein the reducing condition adjusting means has a means for controlling an acid and/or alkali supply means so as to supply at least one of an acid and an alkali to the solution in the electrolysis tank so that the pH of the solution in the electrolysis tank is 1.0 to 2.0, based on the pH value detected by the pH meter for the electrolysis tank, and a means for controlling a reducing agent supply means so as to supply the reducing agent so that the oxidation-reduction potential of the solution in the electrolysis tank is higher than a predetermined potential and the chromium ion concentration is equal to or lower than the predetermined chromium ion concentration, based on the oxidation-reduction potential detected by the oxidation-reduction potential meter and the chromium ion concentration detected by the chromium ion concentration meter, and the neutralizing section has the pH value detected by the pH meter for the neutralization tank, and detecting an alkali supply mechanism in the neutralization tank, and supplying an alkali to the alkali supply mechanism so that the pH of the solution in the electrolytic tank is 5.0 or more.

3. The device of claim 2, wherein said predetermined potential is 500mV and said predetermined concentration of chromium ions is 2 mg/l.

4. The apparatus of claim 1, wherein the neutral salt is sodium sulfate, the acid supplied by the acid and alkali supply means is sulfuric acid, the alkali supplied by the acid and alkali supply means is sodium hydroxide, and the alkali supplied by the neutralization alkali supply means is sodium hydroxide.

5. The apparatus of claim 1, having a pH meter for the electrolytic cell for detecting the pH of the solution in said electrolytic cell; an oxidation-reduction potentiometer for detecting the oxidation-reduction potential of the solution in said electrolytic cell; a chromium ion concentration meter for detecting the chromium ion concentration of the solution in the electrolytic bath; a pH meter for a neutralization tank for detecting the pH of the solution in the neutralization tank, and a control device having a reducing condition adjusting means and a neutralizing means, wherein the reducing condition adjusting means has a means for controlling the supply of at least one of an acid and a base by controlling the acid and base supply means based on the pH value detected by the pH meter for an electrolysis tank so that the pH of the solution in the electrolysis tank is 1.5 to 3.0, and a means for controlling the reducing agent supply means so that the reducing agent is supplied together so that the oxidation-reduction potential of the solution in the electrolysis tank is higher than a predetermined potential and the chromium ion concentration is equal to or lower than the predetermined chromium ion concentration, based on the oxidation-reduction potential detected by the oxidation-reduction potential meter and the chromium ion concentration detected by the chromium ion concentration meter, and the neutralizing means has the pH value detected by the pH meter for a neutralization tank, and a means for controlling the alkali supply mechanism in the neutralization tank to supply alkali so that the pH of the solution in the electrolysis tank is 5.0 or more.

6. The device of claim 5, wherein said predetermined potential is 550mV and said predetermined concentration of chromium ions is 2 mg/1.

7. The apparatus of claim 1, wherein the neutral salt is sodium sulfate, the metal salt is sodium salt, the acid supplied by the acid and alkali supply means is sulfuric acid, the alkali supplied by the acid and alkali supply means is sodium hydroxide, and the alkali supplied by the neutralization alkali supply means is sodium hydroxide.

8. A neutral salt electrolyte treatment device is characterized in that the neutral salt electrolyte treatment device for descaling stainless steel through electrolysis treatment by using a neutral salt aqueous solution is provided with a reduction tank for adding a reducing agent into the electrolyte; an acid and alkali supply mechanism for supplying at least one of an acid and an alkali to the reduction vessel; a reducing agent supply mechanism for supplying a reducing agent to the reduction tank; a neutralization tank for adding an alkali to the electrolyte solution to which the reducing agent has been added; a neutralization tank alkali supply mechanism for supplying alkali to the neutralization tank; a filtering means for removing precipitates from the electrolyte solution after the addition of the alkali, and having a structure in which the composition of the reducing agent is changed in accordance with the composition of the stainless steel.

9. A method for descaling stainless steel by electrolytic treatment with a neutral salt electrolyte, characterized by comprising a reduction step of adding a reducing agent to the electrolyte to reduce dissolved heavy metal ions; a neutralization step of adding an alkali to the electrolyte solution having undergone the reduction step to precipitate the reduced heavy metal ions as hydroxides; a filtering step of removing the hydroxide from the electrolyte solution having undergone the neutralizing step, wherein the neutral salt is a sulfate, and the reducing agent is any one of hyposulfurous acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, caronic acid, peroxodisulfuric acid, polythionic acid, and dithionous acid, or any one of metal salts thereof.

10. A method for descaling stainless steel by electrolytic treatment with a neutral salt electrolyte, comprising a reduction step of adding a reducing agent to the electrolyte to reduce dissolved heavy metal ions; a neutralization step of adding an alkali to the electrolyte solution having undergone the reduction step to precipitate the reduced heavy metal ions as hydroxides; a filtering step of removing the hydroxide from the electrolyte solution having undergone the neutralizing step, and changing the composition of the reducing agent in accordance with the composition of the stainless steel.

11. The method of claim 10, wherein the reducing agent is any one of hyposulfurous acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, caronic acid, peroxodisulfuric acid, polythionic acid, sodium metabisulfite, and dithionous acid.

12. A method for descaling stainless steel, characterized by comprising a step of annealing while continuously moving a stainless steel strip, the method for descaling the continuously moving stainless steel after annealing comprises a step of electrolyzing the stainless steel strip in a neutral saline solution, a step of electrolyzing the stainless steel strip subjected to the step in a nitric acid aqueous solution or a step of immersing the stainless steel strip in a mixed aqueous solution of nitric acid and hydrofluoric acid, and comprises a reduction step of adding a reducing agent to an electrolyte solution composed of the neutral saline solution to reduce dissolved heavy metal ions, a neutralization step of adding an alkali to the electrolyte solution subjected to the reduction step to precipitate the reduced heavy metal ions as hydroxides, a filtration step of removing the hydroxides from the electrolyte solution subjected to the neutralization step, and a composition of the reducing agent is changed according to the composition of the stainless steel.

13. A method for descaling stainless steel, characterized by comprising a step of continuously moving a stainless steel strip while annealing the stainless steel strip, wherein the method for descaling the continuously moving stainless steel after annealing comprises a step of electrolyzing the stainless steel strip in a neutral saline solution, a step of electrolyzing the stainless steel strip in an alkaline aqueous solution or a step of immersion-treating the stainless steel strip in a nitric acid aqueous solution or a step of immersion-treating the stainless steel strip in a mixed aqueous solution of nitric acid and hydrofluoric acid, and comprises a step of adding a reducing agent to an electrolytic solution composed of the neutral saline solution to reduce dissolved heavy metal ions, a neutralization step of adding an alkali to the electrolytic solution subjected to the reduction step to precipitate the reduced heavy metal ions as hydroxides, and a filtration step of removing the hydroxides from the electrolytic solution subjected to the neutralization step, and the composition of the reducing agent is changed according to the composition of the stainless steel.

14. A stainless steel descaling device is a stainless steel sharp-descaling device for conducting descaling through electrolysis in a neutral saline solution electrolyte of stainless steel, and is characterized by comprising an electrolytic bath for immersing the stainless steel in the electrolyte; a reduction tank for holding the electrolyte from said electrolytic tank; an acid and alkali supply mechanism for supplying at least one of an acid and an alkali to the reduction vessel; a reducing agent supply mechanism for supplying a reducing agent to the reduction tank; a neutralization tank for holding the electrolyte from the reduction tank; a neutralization tank alkali supply mechanism for supplying alkali to the neutralization tank; a filtering means for removing a precipitate precipitated from the electrolyte solution from the neutralizing tank, and a means for supplying the filtrate from the filtering means to the electrolytic tank, wherein the reducing agent supplied by the reducing agent supplying means is any one of hyposulfuric acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, caro acid, peroxodisulfuric acid, polythionic acid, and dithionous acid, or a metal salt thereof.

15. A stainless steel descaling device for descaling by electrolytic treatment with a neutral salt electrolyte, characterized by comprising a reduction tank for adding a reducing agent to the electrolyte; a reducing agent supply mechanism for supplying a reducing agent to the reduction tank; a neutralization tank for adding an alkali to the electrolyte solution after the reducing agent; an alkali supply mechanism for adding alkali to the neutralization tank; and a filtering means for removing a precipitate from the electrolyte solution after the addition of the alkali, wherein the reducing agent is any one of hyposulfurous acid, dithionous acid, sulfurous acid, pyrosulfurous acid, pyrosulfuric acid, thiosulfuric acid, caronic acid, peroxodisulfuric acid, polythionic acid, and dithionous acid, or any one of metal salts thereof.

16. A stainless steel descaling device is characterized in that the stainless steel descaling device for descaling by electrolytic treatment with neutral salt electrolyte is provided with a reduction tank for adding a reducing agent into the electrolyte; an acid and alkali supply mechanism for supplying at least one of an acid and an alkali to the reduction vessel; and a reducing agent supply mechanism for supplying a reducing agent to the reduction tank; a neutralization tank for adding an alkali to the electrolyte solution to which the reducing agent has been added; a neutralization tank alkali supply mechanism for supplying alkali to the neutralization tank; a filtering means for removing precipitates from the electrolyte solution after the addition of the alkali, and a means for changing the composition of the reducing agent in accordance with the composition of the stainless steel.

17. A stainless steel descaling apparatus comprising an annealing furnace for continuously moving a stainless steel strip to perform annealing and a descaling device for descaling the annealed stainless steel strip, characterized in that the descaling apparatus is provided with a neutral aqueous salt solution electrolytic bath having a plurality of positive and negative electrodes, an alkaline aqueous solution electrolytic bath or an alkaline aqueous solution immersion bath having a plurality of positive and negative electrodes, a nitric acid aqueous solution electrolytic bath or a hydrofluoric acid mixed aqueous solution bath having a plurality of positive and negative electrodes provided behind the both electrolytic baths, a reducing bath for adding a reducing agent to an electrolyte composed of the neutral aqueous salt solution, an acid and alkali supply means for supplying at least one of an acid and an alkali to the reducing bath, a reducing agent supply means for supplying a reducing agent to the reducing bath, a neutralization bath for adding an alkali to the electrolyte to which the reducing agent has been added, a neutralization tank alkali supply means for supplying alkali to the neutralization tank, a filtration means for removing precipitates from the electrolyte after addition of the alkali, and a means for changing the composition of the reducing agent in accordance with the composition of the stainless steel.

18. A descaling device for stainless steel, comprising an annealing furnace for continuously moving and annealing a stainless steel strip, and a descaling device for descaling the annealed stainless steel strip, characterized in that the descaling device comprises a neutral aqueous salt solution electrolytic bath having a plurality of positive and negative electrodes, an alkaline aqueous solution electrolytic bath or an alkaline aqueous solution immersion bath having a plurality of positive and negative electrodes, a nitric acid aqueous solution electrolytic bath or a nitric acid-fluoric acid mixed aqueous solution bath having a plurality of positive and negative electrodes provided behind the both electrolytic baths, a reducing bath for adding a reducing agent to an electrolyte composed of the neutral aqueous salt solution, an acid and alkali supply means for supplying at least one of an acid and an alkali to the reducing bath, a reducing agent supply means for supplying a reducing agent to the reducing bath, and a neutralization bath for adding an alkali to the electrolyte to which the reducing agent has been added, a neutralization tank alkali supply means for supplying alkalito the neutralization tank, a filtration means for removing precipitates from the electrolyte after addition of the alkali, and a means for changing the composition of the reducing agent in accordance with the composition of the stainless steel.

19. A complete stainless steel manufacturing apparatus provided with a cold rolling mill for cold rolling a hot-rolled descaled stainless steel strip, an annealing furnace for continuously moving and annealing the cold-rolled stainless steel strip, an apparatus for cooling the annealed stainless steel strip, and a descaler for continuously moving and descaling the cooled stainless steel strip, characterized in that the descaler comprises a neutral aqueous salt solution electrolytic bath having a plurality of positive and negative electrodes, a nitric aqueous nitrate electrolytic bath or a nitric/hydrofluoric acid mixed aqueous solution immersion bath having a plurality of positive and negative electrodes provided behind the electrolytic bath, a reduction bath for adding a reducing agent to an electrolyte composed of the neutral aqueous salt solution, an acid and alkali supply means for supplying at least one of an acid and an alkali to the reduction bath, and a reducing agent supply means for supplying a reducing agent to the reduction bath, the apparatus comprises a neutralization tank for adding an alkali to the electrolytic solution to which the reducing agent has been added, an alkali supply means for supplying an alkali to the neutralization tank, a filtration means for removing precipitates from the electrolytic solution to which the alkali has been added, and a means for changing the composition of the reducing agent in accordance with the composition of the stainless steel.

20. A complete stainless steel manufacturing apparatus provided with a cold rolling mill for coldrolling a hot-rolled descaled stainless steel strip, an annealing furnace for continuously moving and annealing the cold-rolled stainless steel strip, an apparatus for performing the annealing, and a descaler for continuously moving and descaling the cooled stainless steel strip, characterized in that the descaler comprises a neutral aqueous salt solution electrolytic bath having a plurality of positive and negative electrodes, an alkaline aqueous solution electrolytic bath or an alkaline aqueous solution immersion bath having a plurality of positive and negative electrodes, a nitric acid aqueous solution electrolytic bath or a nitric acid-fluoric acid mixed aqueous solution immersion bath having a plurality of positive and negative electrodes provided behind the both electrolytic baths, a reducing bath for adding a reducing agent to an electrolyte composed of the neutral aqueous salt solution, and acid and alkali supply means for supplying at least one of an acid and an alkali to the reducing bath, a reducing agent supply means for supplying a reducing agent to the reduction tank, a neutralization tank for adding an alkali to the electrolyte solution to which the reducing agent has been added, an alkali supply means for supplying an alkali to the neutralization tank, and a filtration means for removing precipitates from the electrolyte solution to which the alkali has been added, and further comprises a means for changing the composition of the reducing agent in accordance with the composition of the stainless steel.

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