JPH04304393A - Treatment of salt bath solution - Google Patents

Abstract

PURPOSE:To recover again free acid from a salt of a silver nitrate contained solution base and to recover also alkali in surface treatment of a steel material with a salt bath containing nitrate compound or caustic soda. CONSTITUTION:The chemicals contained in a mixed solution of the over flow solution 10 from each vessel 7, 9 with a cleaning water 13 for removing the impurities in the component of the salt bath is recovered. The mixed solution is mixed with the nitric acid contained solution for pickling the steel material and after that allowed to make alkali by supplying caustic soda 24. In this way, the solved metal material in the treating solution is separated into the insoluble metal hydroxide solid 15 and the soluble salt solution 16 without mixing insoluble alkali.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention relates to a method for treating salt bath liquid, and in particular, the present invention relates to a method for treating salt bath liquid, and in particular, to treatment of waste liquid generated during the rapid cooling treatment of salt-treated stainless steel and precipitate accumulated in the salt bath of steel materials. A method for treating salt bath liquid that can use wastewater from a familiar treatment system as the necessary chemicals when treating wastewater generated during cleaning salt adhering to objects, and that can also remove harmful substances. Regarding. Furthermore, the present invention relates to a method for treating a salt bath liquid that can be applied particularly to fields that require the treatment of stainless steel materials and where chemicals contained in the waste liquid are associated with environmental problems.

0002

[Prior Art] Bath composition chemicals taken out of the salt bath for salt treatment of stainless steel are expensive and, at the same time, the salts cannot be made insoluble even by neutralization treatment, so they may be contained in wastewater. Since nitrate roots serve as a nitrogen source for eutrophication, their removal has become a problem.

Sodium nitrate, which is the main component of these bath compositions, is a soluble salt, and the only way to remove it from the system for drainage is to perform a concentration operation. Even if it is more concentrated, it cannot be reused unless it is converted into anhydrous salt for practical applications.

Furthermore, the chemicals brought out of the salt bath contain chromium molecules, which are one of the constituents of the stainless steel being treated, but due to the high treatment temperature, these chromium molecules are more concentrated than Cr3+. Changes to Cr6+. This Cr6+ becomes soluble in water and is highly toxic. For this reason, reliable treatment is required as a pollution control measure, but since it is difficult to manage, problems arise in waste treatment.

[0005] Conventionally, therefore, Cr6+ is reduced to Cr3+ by coexisting with a reducing agent, and then removed by coagulation treatment. However, it has not been possible to remove nitrate radicals from water systems.

Alternatively, as a reducing agent for hexavalent chromium,
It has also been proposed that trivalent iron metal salts be purchased from outside and used for the desired treatment. Special application 1986-1998
No. 80 describes that the free acid in the pickling bath is recovered using a diffusion dialysis membrane, and the resulting wastewater, which is mainly composed of metal components, is used as a reducing neutralizing agent. However, efforts are being made to reduce the amount of chemicals contained in wastewater by concentrating the nitrate radicals contained in the liquid that has undergone rapid cooling treatment, separating and recovering free nitric acid and caustic soda, and promoting reuse. There is no mention of effort.

[0007]

Problems to be Solved by the Invention The chemicals used in salt baths are expensive and at the same time difficult to separate from the dissolved solution and concentrate.

On the other hand, the aged acid bath solution in the nitric acid-based acid treatment solution is generally neutralized and discharged alone, but part of the iron contained in the aged acid bath solution in the nitric acid-based acid processing solution is 6
Effectively works on reducing chromium. Therefore, by mixing these liquids, it is possible to direct the reduction of the amount of drugs to be used.

[0009] Therefore, the present invention selects a chemical with a reducing function from other locations in a factory where the same operation is being carried out, and uses this agent effectively against toxic and soluble chromium compounds contained in the processing liquid. to act on. In order to make effective use of resources, the salt contained in the nitrate radical-containing liquid system is recovered as a free acid, and at the same time, this is recovered as an alkali agent, which can be used at various locations during the process, thereby achieving the above-mentioned results. The purpose is to solve problems.

0010

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a method for surface treating steel materials using a salt bath containing a nitrate compound or caustic soda. overflowing liquid generated by immersing the steel material in water and cooling it; overflowing liquid generated by neutralizing the alkali agent adhering to the steel material after this cooling treatment; and the bottom of the salt bath. The chromium metal compounds in the treated steel composition are removed from the salt bath in order to recover the cleaning solution generated during cleaning to remove the salt bath composition deposited on the steel slag and the removed steel slag. After reduction, these compounds in the mixture of the above-mentioned solutions, which have changed into hexavalent water-soluble salts and are dissolved, are treated with insoluble hydroxide, and the steel material is immersed in acid as a chemical for this treatment. Injecting a part of the acid solution containing metal ions that are dissolved in a bath containing at least nitric acid for washing treatment and leaving free acid as a reducing agent for the mixed solution into the solution after the neutralization operation, By maintaining the environment for this reduction reaction at high temperature and acidity, the hexavalent salts contained in the liquid to be treated are reduced to trivalent salts, and then made alkaline with caustic soda, thereby removing the dissolved metals. Separates into insoluble metal hydroxide solids and soluble salt solutions such as soluble sodium nitrate and sodium chloride without mixing in insoluble alkali agents.

[0011]

[Action] The characteristics of the composition that makes up the salt bath are:
It is as follows. (A) The composition of the salt solution is mainly composed of sodium hydroxide and sodium nitrate. These agents dissolve the dissolved metal in the form of a salt, and the concentration of metal ions contained is high. (B) Since the temperature inside the salt bath is as high as 600°C, the dissolved chromium compound changes into a stable compound that is oxidized to hexavalence and dissolves. (C) Other metals are also dissolved in the form of salts, and when these compounds come into contact with water for quenching, they dissolve in the liquid system and migrate to the waste water side. However, chromium that is alkaline and oxidized to hexavalent is soluble, but the others are insoluble, and when the dispersion is separated, a) soluble neutral salt - sodium nitrate and b) alkaline agent - caustic soda , c)
Separated into dispersed fine particles and metal oxides and hydroxides. (D) On the other hand, it is common for factories whose purpose is to treat the surface of stainless steel to have a treatment bath that uses nitric acid at the same time. , must be disposed of. By the way, the bath liquid contains trivalent iron, and iron that has been oxidized with nitric acid has a particularly strong reducing power.It efficiently reduces hexavalent chromium, first reduces toxicity, and then oxidizes chromium. It has the function of separating things.

[0012] In addition, the acid bath used for the electrolytic stripping operation using other nitric acid baths and the waste liquid contained in the cleaning water also work effectively. In addition, in the treatment solution after the reduction treatment, metal ions are still dissolved because free acids remain, and by adding an alkaline agent, it is possible to make them insoluble and separate the metal hydroxides. can.

[0013] The selection of the alkaline agent used at this time is important. The present invention takes advantage of the fact that dissolved salts can be separated into free acid and alkali through electrolytic separation. When viewed from the separated metal hydroxide side, it becomes a form with increased impurities,
This problem can be avoided by recycling caustic soda, which does not significantly impede the recycling operation of the separated metal hydroxide.

[0014] This makes it possible to reuse valuable metal compounds contained in large amounts, and also generates expensive chemicals necessary for reduction and separation operations during the treatment process.
Cost reduction is achieved.

That is, first, the hexavalent chromium of the above-mentioned soluble metal salts to chromic acid sodas is treated to detoxify and insolubilize the chromium using the acid bath waste liquid shown in item (D) above, and then Separate and recover salts dissolved in the solution.

[0016] As an apparatus for separating free acid radicals and alkaline radicals of dissolved salts, a cylindrical anode and a cathode are installed facing each other, and a cylindrical anode and a cathode are installed between the electrodes. It is preferable to install a plurality of separation membranes made of ion exchange membranes, which have high chemical resistance and low electrical resistance, as required. The target alkaline solution is supplied to a membrane chamber close to the anode chamber isolated by this separation membrane, and caustic soda, which has been electrophoretically separated from the anode chamber, accumulates in the opposing cathode chamber.

[0017] As the caustic soda accumulates on the cathode, free nitric acid is generated on the anode chamber side. If the concentration of nitric acid produced is low and there is a problem with reuse, the sample can be supplied to the isolation membrane chamber placed in front of the diaphragm in front of the anode chamber, so that the nitric acid can be diffused into the anode chamber. You can avoid this problem by placing it.

[0018] In such a case, the current efficiency of the electrolytic separation work is high, and the process can be carried out at the same cost as purchasing each chemical, so it can be said that significant resource saving is possible from an economical point of view.

[0019]

[Example] (1) Nitric acid bath solution (2.3N nitric acid, 0.57N iron (of which 70% was trivalent) used for pickling stainless steel in the pickling treatment tank 1 shown in the figure.
)2 was stored in the waste acid bath liquid tank 3. (2) On the other hand, the composition of the salt 5 in the salt treatment tank 4 was 7:3 by weight of sodium nitrate and sodium hydroxide, and the solid powder was heated to a temperature of 400 to 600°C to form a uniform solution. After the SUS304 stainless steel material 6 is immersed in the salt 5 in the salt treatment tank 4, the steel material 6 is immersed in the cooling liquid 8 in the quenching tank 7 to be rapidly cooled, and the scale is made into a porous state for descaling in the post-pickling treatment. can be done effectively.

[0020] The composition of this quenching liquid 8 changes with time, but the nitrate radical is 50 gr/l and the Na salt is 34 gr/l.
/l, 2000 ppm of Cr6+ was dissolved, and 5000 ppm of iron hydrolyum dispersion coexisted, and the pH of the liquid was about 13, a value indicating strong alkalinity. (3) Next, the stainless steel 6 after being rapidly cooled is transferred to a neutralization treatment tank 9.
The neutralization treatment tank 9 contains the above-mentioned waste acid bath liquid tank 3.
Neutralization treatment was performed using acid bath solution 2 stored in . (4) The neutralization treatment at this time can be carried out as well or better than the conventional neutralization treatment using water, and it is necessary to ensure that no alkali exists in the pores of the scale. It has become possible.

[0021] Also, the liquid 10 overflowing from this neutralization treatment tank 9
was used as the water supplied to the quench tank 7. This liquid 10 contained a large amount of soluble sodium nitrate and sodium chloride, and also contained a large amount of metal hydroxide. (5) Next, these liquids 10 were stored in the storage tank 11 in order to perform an operation to recover the medicine contained in the overflowing liquid.

Furthermore, foreign matter 12 such as insoluble metal oxides and silica sand deposited at the bottom of the salt bath is removed and washed to prevent scattering of chromium compounds coexisting with the attached salt, and the cleaning water is 13 was also flowed into the storage tank 11 and stored.

The stored liquid in the storage tank 11 was first neutralized with the above-described waste acid bath solution, and at the same time, chromium was reduced by adding active trivalent iron remaining in the acid bath under acidic conditions. This situation was monitored with an oxidation-reduction potential measuring device immersed in the reaction tank. In addition, by maintaining the temperature of the reaction tank liquid at 40 to 50°C or higher, complete treatment could be ensured.

After the reduction treatment has been completed, the dissolved metal ions are insolubilized by adding an alkaline agent again, and the dispersion 15 is separated in the neutralization/reduction dispersion separator 14 which removes these dispersions. , a clean filtrate 16 is obtained, and this is stored in a storage tank 17.
It was stored in (6) This stored clean filtrate 16 is supplied to an electrolytic separation tank 18 using an ion exchange membrane as a diaphragm, and the circulating salt solution 20 in the circulation tank 19 on the anode chamber side is electrolytically operated to recover free acid solution. 21 and the circulation tank 2 on the cathode chamber side.
It was possible to separate and recover the alkaline caustic soda 24 from the circulating salt solution 23 of No. 2. (7) The recovered free acid was 2.5N nitric acid, which could be reused as an acid bath.

Furthermore, the alkaline solution was 2.7N, and was also reused to neutralize the reduced solution.

[0026]

[Effect of the invention] As explained above, according to this invention,
Concentrated sodium nitrate generated during salt treatment of stainless steel can be recovered without being discharged outside the system as waste liquid and used again as an acid bath composition, and furthermore, its generation is difficult to suppress with high-temperature salt treatment6. A means for converting valent chromium into trivalent chromium and a drug for separating insoluble chromium from a liquid system.
By using the same wastewater, the drugs contained in the wastewater can be recovered at the same time, making it possible to reduce drug resources, recover valuable drugs, and purify the environment by reducing soluble nitrogen sources. , the effect is enormous.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of an embodiment of the method of the present invention.

[Explanation of symbols]

1 Pickling treatment tank 4 Salt treatment tank 7 Quenching tank 9 Neutralization treatment tank 10 Liquid 11 Storage tank 13　　　　Washing water 14 Neutralization/reduction dispersion separator 18 Electrolysis tank 21　　　　Recovered free acid solution 24 Alkaline caustic soda

Claims (2)

[Claims] [Claim 1] When surface-treating a steel material using a salt bath containing a nitrate compound or caustic soda, the steel material is cooled by immersing it in water after being immersed in a high-temperature melting furnace and taken out. The overflowing liquid generated by neutralizing the alkali agent adhering to the steel material after this cooling treatment, and the overflowing liquid generated by neutralizing the alkaline agent adhering to the steel material that has finished this cooling treatment, and the salt bathing liquid deposited at the bottom of the salt bath and adhering to the steel slag that has been taken out. In order to recover the cleaning solution generated during cleaning to remove the composition and the composition agent contained in it,
The chromium metal compounds in the composition of the treated steel material are converted into hexavalent water-soluble salts in a salt bath, and these compounds in the mixture of the respective solutions are reduced and then treated with insoluble hydroxide,
As a chemical for this treatment, a part of the acid solution containing metal ions dissolved in a bath containing at least nitric acid for immersion pickling of the steel material and leaving a free acid is used as a reducing agent in the mixed solution. The hexavalent salt contained in the solution to be treated is reduced to trivalent by injecting it into the solution after the neutralization operation, and the environment for this reduction reaction is maintained at high temperature and acidity. By making it alkaline with Soda, dissolved metals can be separated into insoluble metal hydroxide solids and soluble salt solutions such as soluble Sodium nitrate and Sodium nitric acid without mixing with insoluble alkaline agents. A method for treating salt bath liquid. 2. When recovering the chloride contained in the treated solution as a free acid and an alkaline agent as valuable substances, a cation exchange membrane is installed between the cylindrical positive and negative electrodes as an ion selective separation diaphragm. The free nitric acid is supplied to an electrolytic cell installed concentrically, and an electric charge is supplied between both electrodes, and free nitric acid is recovered from an isolated chamber near the anode electrode chamber, while a solution of caustic soda is collected from the cathode electrode chamber side. 2. The method for treating a salt bath liquid according to claim 1, wherein the salt bath liquid is recovered and used again in a steel treatment process.