JP2001089884A - Method for recovering aqueous solution of nitric acid from pickling waste solution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for recovering an aqueous solution of nitric acid of high quality from a pickling waste solution containing nitric acid. SOLUTION: In this method, a pickling waste solution containing nitric acid freed from free acids at need is subjected to neutralizing treatment, the produced precipitates are removed to form into an aqueous solution essentially consisting of calcium nitrate, sulfuric acid is added to the aqueous solution, the produced precipitates of calcium sulfate are removed, and treatment for reducing the concentration of sulfuric ions and/or the concentration of calcium ions contained in the obtained aqueous solution of nitric acid is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for recovering a high-quality aqueous nitric acid solution from a pickling waste liquid containing nitric acid.

[0002]

2. Description of the Related Art In a stainless steel sheet manufacturing process, a descaling method by pickling is generally employed as a method for removing oxide scale formed on a surface after annealing.

A pickling solution generally used for descaling austenitic stainless steel is an aqueous solution called nitric hydrofluoric acid in which nitric acid and hydrofluoric acid are mixed at an appropriate ratio.

For descaling ferritic stainless steel, a method of electrolytic treatment in an aqueous solution of nitric acid is employed. In any case, the pickling consumes the free acid in the solution and, at the same time, the ions of metals such as Fe and Cr dissolve from the stainless steel into the solution. Ability decreases.

[0005] The pickling ability can be recovered to some extent by adding a new acid, but the recovery becomes smaller as the amount of dissolved metal ions increases, so that the pickling liquid exceeding a certain limit is discarded as waste acid. Is done.

However, since such pickling waste acid contains many harmful metal ions, it cannot be discharged into general sewage or the like as it is. Therefore, the acid is usually neutralized with an alkali such as calcium hydroxide. After separating contained metal ions and the like as precipitates such as hydroxides, they are discarded as wastewater.

However, in the case of a pickling waste liquid containing nitric acid, it is difficult to separate the contained nitrate ion (NO ³⁻ ) as a precipitate, so that almost all of the nitrate ion remains.
In recent years, attention has been paid to pollution problems caused by nitrogen contained in industrial wastewater, and as regulations by the law become stricter, development of technology for collecting nitrate ions in wastewater has become increasingly important.

[0008] As a technique for recovering nitrate ions in wastewater, for example, “Resources and Materials '94 (Autumn Games)” 29-3
On page 2 a method for electrolysis using a bipolar membrane is disclosed. This is a method in which a positive electrode and a negative electrode are arranged on both sides of a bipolar membrane having a structure in which an anion exchange membrane and a cation exchange membrane are laminated, and an acid and an alkali are generated by electrolysis in a neutral salt aqueous solution. It is. According to this method, when the neutral salt is calcium nitrate, nitric acid and calcium hydroxide are generated separately. Although this method does not generate by-products such as sludge, it seems to be an excellent method in principle, but it seems that there are still problems in recovery efficiency, membrane durability, etc. It is.

[0009] Also, as long ago, Japanese Patent Publication No. 45-7647 discloses that a waste acid containing nitric acid is neutralized with calcium hydroxide, and the resulting precipitate is removed. A method for obtaining calcium sulfate (gypsum) is disclosed. According to this method, nitric acid can be recovered by a relatively simple treatment. However, according to tests conducted by the present inventor, there was a problem that when used directly for pickling, the pickling efficiency was poor.

[0010]

An object of the present invention is to provide a method for recovering a high-quality aqueous nitric acid solution from a pickling waste liquid containing nitric acid.

[0011]

Means for Solving the Problems The present invention has been made based on the following findings, and the gist is as follows.

(1) The pickling waste liquid containing nitric acid is neutralized with a Ca compound, and the resulting precipitate is removed to form an aqueous solution containing calcium nitrate. Sulfuric acid is added to the aqueous solution to precipitate the generated calcium sulfate. Remove the substance to make nitric acid aqueous solution,
Next, a method for recovering the aqueous nitric acid solution, which performs a treatment for reducing one or both of the sulfate ion concentration and the calcium ion concentration contained in the aqueous nitric acid solution.

(2) The treatment for reducing the sulfate ion concentration is as follows:
The method for recovering an aqueous nitric acid solution according to the above (1), wherein barium nitrate is added to the liquid to be treated, and the generated barium sulfate precipitate is removed.

(3) The method for recovering an aqueous nitric acid solution according to the above (1), wherein the treatment for reducing calcium ions is a treatment using a cation exchange resin.

(4) The pickling waste liquid containing nitric acid is neutralized with barium hydroxide to remove the resulting metal hydroxide precipitate, and sulfuric acid is added to the obtained aqueous solution containing barium nitrate. And a method for recovering a nitric acid aqueous solution that removes the resulting precipitate of barium sulfate.

(5) The method for recovering an aqueous nitric acid solution according to any one of the above (1) to (4), wherein after removing the free acid, the pickling waste liquid containing nitric acid is neutralized.

The present inventor has conducted various experiments and studies on a method of recovering a high-quality aqueous nitric acid solution from pickling waste acid containing nitric acid. As a result, the following findings have been obtained.

A) A pickling waste liquid containing nitric acid is neutralized to remove a precipitate formed to form an aqueous solution containing calcium nitrate as a main component. Sulfuric acid is added to the aqueous solution, and the resulting calcium sulfate (gypsum) When the nitric acid aqueous solution obtained by removing the precipitate of the above) is used as the pickling solution, the pickling efficiency is poor because the recovered nitric acid aqueous solution contains a considerable amount of sulfate ions and calcium ions. That's why. A high quality nitric acid aqueous solution can be obtained as a pickling solution by removing sulfate ions or calcium ions among these ions, and the quality can be further improved if both sulfate ions and calcium ions can be removed.

B) By adding sulfuric acid to an aqueous solution containing calcium nitrate as a main component after the neutralization treatment and adding barium nitrate to an aqueous nitric acid solution obtained by removing calcium as a precipitate of calcium sulfate, sulfate ions can be removed. It can be removed as a precipitate of barium sulfate.

C) A sulfuric acid is added to an aqueous solution containing calcium nitrate as a main component after the neutralization treatment, and an aqueous nitric acid solution in which calcium is obtained as a precipitate of calcium sulfate is treated with a cation exchange resin to obtain calcium ion. Can be removed.

D) Calcium ions can be removed by treating the aqueous nitric acid solution from which sulfate ions have been removed by the above method b) with a cation exchange resin.

E) If barium nitrate is added to the aqueous nitric acid solution from which calcium ions have been removed by the method c), sulfate ions can be removed as barium sulfate precipitates.

F) The pickling waste liquid containing nitric acid after the neutralization treatment is neutralized with barium hydroxide to remove generated precipitates, and sulfuric acid is added to the obtained aqueous solution containing barium nitrate as a main component. By adding and removing barium as a precipitate of barium sulfate, a good quality nitric acid aqueous solution can be obtained.

Incidentally, the pickling waste liquid is neutralized with calcium hydroxide, sulfuric acid is added to the aqueous solution from which the precipitate has been removed, and a considerable amount of sulfate ions and calcium ions are contained in the aqueous nitric acid solution which has been recovered by removing the precipitate. The reason for the inclusion is unknown at present, but it is presumed that calcium sulfate generated by the addition of sulfuric acid dissociates into calcium ions and sulfate ions in the presence of nitric acid.

The reason why the pickling efficiency is reduced when nitric acid containing sulfate ions and calcium ions is used for pickling is unknown at present. This is presumed to be because dissolution of is prevented.

When nitric hydrofluoric acid is prepared using a nitric acid aqueous solution containing calcium ions, a part of the added hydrofluoric acid reacts with calcium ions to form calcium fluoride, which is effective for pickling. It is also considered that the concentration of naphthalic acid decreases.

[0027]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIGS. 1 to 5 are flow charts for explaining the nitric acid aqueous solution recovery method of the present invention.

FIG. 1 shows that the pickling waste liquid containing nitric acid is neutralized with calcium hydroxide to remove metal ions and the like as precipitates to form an aqueous solution mainly containing calcium nitrate. FIG. 4 is a diagram showing a method of using barium nitrate to remove sulfate ions in a nitric acid aqueous solution collected by removing a precipitate of calcium sulfate (gypsum) generated by adding barium nitrate.

To neutralize the pickling waste liquid containing nitric acid, Ca
(OH) ₂ , CaCO _3, CaO, etc.
a) can be used. Ca (O
H) The chemical reaction that occurs when neutralized with ₂ is shown in FIG.
As shown in FIG. By the neutralization treatment, hydroxides of metals such as iron and chromium are generated and separated from an aqueous solution mainly containing calcium nitrate and precipitated. By removing this precipitate and adding sulfuric acid, calcium sulfate (gypsum) and nitric acid are produced by the chemical reaction shown in FIG. However, the nitric acid thus obtained contains a small amount of sulfate ion and calcium ion. By adding barium nitrate to the nitric acid, the sulfate ion is separated from the aqueous nitric acid solution as a precipitate. By such a process, a high-quality aqueous nitric acid solution with less sulfate ions can be recovered.

The barium nitrate used for precipitating sulfate ions can be added as an aqueous solution or as a crystal. In the former case, however, there is a problem that the concentration of nitric acid is reduced by the added water. Therefore, the latter method of adding as crystals is recommended, but in this case, it is desirable to crush the crystals in powder form in advance in order to accelerate the dissolution of the crystals. It can also be recommended to heat the aqueous solution to 40-80 ° C. and stir the solution during and after addition.

FIG. 2 shows a process for removing the precipitate of calcium sulfate and reducing the concentration of calcium ions contained in the obtained aqueous solution of nitric acid, and recovering the aqueous solution of nitric acid when using a cation exchange resin. It is explanatory drawing of a method.

As the cation exchange resin, a commercially available strongly acidic cation exchange resin is preferable. The resin is packed in an appropriate column, and a nitric acid aqueous solution containing calcium ions is poured into the column from above to convert the calcium ions into the resin. The adsorbed nitric acid aqueous solution from which calcium ions have been removed flows out from the lower part. The cation exchange resin does not necessarily need to be packed in a column, and calcium ions can be removed only by mixing with a nitric acid aqueous solution in an appropriate container. Further, the cation exchange resin to be used needs to be washed with hydrochloric acid or sulfuric acid in advance to make the substituent into an acid type.

FIG. 3 is a view showing a method of obtaining a nitric acid aqueous solution from which sulfate ions have been removed by the method shown in FIG. 1 and further removing calcium ions using a cation exchange resin.

FIG. 4 is a view showing a method of removing nitric acid as a precipitate of barium sulfate by adding barium nitrate after obtaining a nitric acid solution from which calcium ions have been removed by the method of FIG.

By the methods shown in FIGS. 3 and 4, a high-quality nitric acid aqueous solution containing almost no sulfate ion and no calcium ion can be obtained.

FIG. 5 is a diagram for explaining a method of using barium hydroxide to neutralize the pickling waste liquid. As shown in the figure, when neutralized with barium hydroxide, hydrates of metals such as iron and chromium are generated, and separated from an aqueous solution containing barium nitrate as a main component and precipitated.

By adding sulfuric acid to the aqueous solution containing barium nitrate as a main component, a high-quality aqueous nitric acid solution can be recovered by the chemical reaction shown in FIG.

According to the method shown in FIGS. 3 to 5, the sulfate ions and calcium ions in the aqueous nitric acid solution can be reduced to such an extent that the pickling ability of the nitric acid hydrofluoric acid pickling solution is not reduced at all. .

When neutralizing the pickling waste liquid containing nitric acid, it is preferable to remove free acids as necessary. In other words, if necessary, the pickling waste liquid is not so used, and a relatively large amount of free acid effective for pickling remains in the pickling waste liquid. If the free acid is removed in advance, the neutralization treatment becomes easy and the recovery cost can be reduced. In addition, a commercially available acid recovery device can be conveniently used for removing the free acid. For example, in the case of APU (Acid Purification System) developed by Ecotech of Canada, by sending an acid solution to a column of ion exchange resin, the free acid in the solution is adsorbed on the resin and metal ions are passed, Next, water is fed in to release the acid adsorbed on the resin. According to this, a solution of a cation of a metal (Fe, Cr, Ni, or the like) containing almost no free acid and a solution of a nitrate ion or a fluoride ion paired with the cation are obtained. Requires less calcium. The recovered free acid aqueous solution can be reused as a pickling solution.

The concentration of nitrate ions in the pickling waste liquid is generally several to several.
Since the concentration is about ten and several percent, the concentration of the aqueous nitric acid solution recovered as a raw material is also about the same or less. The recovered nitric acid aqueous solution having a low concentration of about several percent can be effectively used when a new pickling solution is prepared, but cannot be used for replenishment when the pickling solution has deteriorated. This is because the replenishment reduces the concentration of the entire pickling solution and reduces the pickling ability.

As a method for increasing the concentration of the recovered nitric acid, a method in which calcium hydroxide or barium hydroxide used for neutralizing the pickling waste liquid is used as a powder instead of an aqueous solution, or a method in which calcium nitrate is used as a main component There is a method of using concentrated sulfuric acid as sulfuric acid to be added to an aqueous solution or an aqueous solution containing barium nitrate as a main component.

Increasing the concentration of nitrate ions in the pickling waste acid and the aqueous solution of calcium nitrate and increasing the concentration of nitric acid in the aqueous solution of nitric acid by means such as evaporation concentration and electrodialysis promote the use of the recovered nitric acid. Recommended for

The amount of calcium hydroxide or barium hydroxide for neutralizing the pickling waste liquid depends on the pH of the solution.
And the precipitate such as hydroxide is settled at the bottom of the container, and then the supernatant liquid at the top is filtered as necessary. Further, the amount of sulfuric acid to be added to the aqueous solution containing calcium nitrate or barium nitrate as a main component is determined by analyzing the concentration of calcium or barium in the aqueous solution before addition and determining the amount of CaSO ₄ or BaSO ₄ to be produced without excess or deficiency. Good.

The temperature of the solution at the time of adding sulfuric acid is not particularly limited. However, in the case of an aqueous solution of calcium nitrate, it is preferable to set the temperature to 45 ° C. or higher in order to reduce impurities in the aqueous solution of nitric acid. In the case of barium nitrate aqueous solution, the temperature may be about 10 to 80 ° C.

Calcium sulfate (gypsum) or barium sulfate generated by adding sulfuric acid precipitates as crystals at the bottom of the vessel. Therefore, if only a nitric acid aqueous solution is used, the upper aqueous solution may be extracted. When using gypsum or barium sulfate as a by-product, remove the precipitate at the bottom of the container, filter the solution, and wash with water as necessary.
It may be used after drying.

According to the study of the present inventor, when the concentration of sulfate ions contained in the nitric-hydrofluoric acid pickling solution exceeds 0.2 mass%, the pickling ability obviously decreases. If the treatment for reducing the sulfate ion is not performed, the sulfate ion concentration in the obtained nitric acid aqueous solution is about 0.5 to 1 mass%.
0.2 mass% of sulfate ion concentration in nitric acid hydrofluoric acid pickling solution
It is necessary to limit the amount of the aqueous nitric acid solution to make the amount less than the above. Note that the above-mentioned sulfate ion concentration of 0.5 to 1 mass% differs depending on the concentration of the raw material calcium nitrate aqueous solution, and is a numerical value when the concentration is about 5 to 10 mass%.

On the other hand, as for the concentration of calcium ions in an aqueous nitric acid solution, the pickling ability of nitric hydrofluoric acid prepared using the same tends to decrease. Therefore, the lower the concentration of calcium ions in the aqueous nitric acid solution, the better.

[0049]

EXAMPLE A nitric hydrofluoric acid waste liquid and a nitric acid waste liquid shown in Table 1 were prepared, and a 6 dm ³ amount of each waste liquid was mixed with a powder of calcium hydroxide or barium hydroxide with a concentration of 200 nm while stirring.
g / dm ³ of the suspension was added little by little to the amount shown in Tables 2 and 3, and the pH was adjusted to 7.0 for neutralization.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

A precipitate such as a hydroxide formed by the neutralization was filtered using a filter paper, and the concentration of calcium or barium in the filtrate was analyzed.

[0054] especially a 4 dm ³ from the filtrate, calcium or equimolar number of sulfuric acid and barium included in this using sulfuric acid concentration 18 mol / dm ³ or 6 mol / dm ³ was added in an amount shown in Table 2 and Table 3 And mixed. After the reaction product was filtered using a filter paper, the filtrate was analyzed for nitrate concentration, sulfate ion concentration and calcium ion concentration.
When the ions of the pickling waste liquid were neutralized using barium hydroxide, the filtrate was directly subjected to a pickling test described later.

On the other hand, in the case of neutralization using an aqueous solution of calcium hydroxide, 0.6 dm ³ of the filtrate is separated, and sulfate ions contained therein and an equimolar number of barium nitrate crystals are added with stirring. After dissolution, the reaction product was allowed to settle by standing, and the supernatant was collected and subjected to an acid washing test.

[0056] Further, Separately, the filtrate 0.6Dm ³ pouring column packed with a cation exchange resin 0.2Dm ^3, discard the first 0.1Dm ³ effluent was collected subsequent effluent It was subjected to a pickling test. In addition, Amberlite 20 manufactured by Rohm and Haas Co. was used as a cation exchange resin.
Using 0CT, it was swollen in advance and converted to the acid form before use. Further, another 0.6 dm ³ of the filtrate was collected, and treated with barium nitrate or treated with a cation exchange resin in the same manner as described above. In the former case, a treatment with a cation exchange resin was added thereafter. In the latter case, a process of adding barium nitrate was added thereafter.

The pickling solution used in the pickling test had a concentration of 2 mol.
/ Dm ³ of a new nitric acid nitric acid and the concentration 1 mol / dm recovered nitric acid and the concentration as shown in Table 4 the nitric hydrofluoric acid 0.5 dm ³ containing hydrofluoric acid ³ 15 mol / dm ³ (reagent unused)
And a solution prepared using concentrated hydrofluoric acid (unused reagent) having a concentration of 26 mol / dm ³ .

[0058]

[Table 4]

This pickling solution is placed in a polyethylene container and placed in a container.
C., and a test piece of a stainless steel plate (steel type: SUS304) wet-ground with abrasive paper having a grain size of 600 was immersed.

Tables 2, 3 and 4 also show, for comparison, the results of recovery and pickling tests when no treatment for removing sulfate ions was performed.

As is clear from Tables 2 and 3, by adding barium nitrate, the sulfate ion concentration in the recovered nitric acid could be reduced to less than 0.1 g / dm ³ , but the calcium ion concentration was reduced. 3.1 to 4 g / dm ³ (collection Nos. 2, 7, 12, 17, 24, 2).
9, 34, 39). Further, by treating with a cation exchange resin, the calcium ion concentration in the recovered nitric acid is reduced to 0.1.
Although it could be reduced to less than 1 g / dm ³ , the sulfate ion concentration was in the range of 7.4 to 9.6 g / dm ³ (recovery Nos. ³ , 8, 13, 18, 25, 30, 35) , 4
0).

Further, by performing both the treatment of adding barium nitrate and the treatment with a cation exchange resin,
Both the sulfate ion and calcium ion concentrations in the recovered nitric acid could be reduced to less than 0.1 g / dm ³ (recovery Nos. 4, 5, 9, 10, 14, 15, 19, 2).
0, 26, 27, 31, 32, 36, 37, 41, 4
2). In the column of the ion removal treatment in Tables 2 and 3,
Of the barium nitrate addition treatment and the ion exchange resin treatment, those performed earlier are indicated by those performed later.

On the other hand, when the ion removal treatment was not performed, the sulfate ion concentration in the recovered nitric acid was 3.1 to 4 g.
/ Dm ³ .

As is clear from Table 4, the pickling rate of nitric acid hydrofluoric acid in the case of using the recovered nitric acid in which only the treatment of adding barium nitrate was used (the pickling tests Nos. 1 to 4) of the present invention was as follows. The range was 136 to 143 g / dm ³ / hr. The recovered nitric acid used in this case contained calcium ions in an amount of 3.1 to 4 g / dm ^{3. In} the prepared nitric hydrofluoric acid, these calcium ions contained hydrofluoric acid and Ca ²⁺ + 2HF. → React like CaF ₂ + 2H ⁺ to form a precipitate of calcium fluoride (CaF ₂ ), and the concentration of hydrogen fluoride in nitric hydrofluoric acid is increased from the target value of 1 mol / dm ³ to 0.85 to 0.88 mol /. calcium ion concentration and at the same time reduced to dm ³ it is presumed that dropped below 0.1 g / dm ^3. In the examples of the present invention, when the recovered nitric acid treated only with the cation exchange resin was used (the pickling tests Nos. 5 to 8), the pickling rate was 138 to 1
In the range of 45 g / dm ³ / hr, the pickling rate in the case of using the recovered nitric acid which has been subjected to both the treatment of adding barium nitrate and the treatment with the cationic resin (the pickling tests No. 9 to 16) is as follows: The range was 179 to 183 g / dm ³ / hr. On the other hand, in the case of using a recovered nitric acid which was not subjected to any ion removal treatment as in the comparative example (an acid cleaning test).
Nos. 17 to 24) The pickling rate was much smaller than that of the inventive examples, and was in the range of 71 to 82 g / dm ³ / hr.

[0065]

According to the present invention, a pickling waste solution containing nitric acid, which was conventionally difficult to recover by waste liquid treatment, can be recovered as a high-quality nitric acid aqueous solution, and the recovered nitric acid is reused for pickling stainless steel or the like. It can be used and has a great industrial effect.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a method of recovering a nitric acid aqueous solution using barium nitrate.

FIG. 2 is a diagram for explaining a method for recovering a nitric acid aqueous solution using an ion exchange resin.

FIG. 3 is a diagram for explaining a method for recovering a nitric acid aqueous solution using barium nitrate and an ion exchange resin.

FIG. 4 is a diagram for explaining a method for recovering an aqueous nitric acid solution using an ion exchange resin and barium nitrate.

FIG. 5 is a diagram for explaining a method for recovering an aqueous nitric acid solution using barium hydroxide.

Claims (5)

[Claims] 1. A pickling waste liquid containing nitric acid is neutralized with a Ca compound to remove a generated precipitate to form an aqueous solution containing calcium nitrate. Sulfuric acid is added to the aqueous solution to form a precipitate of calcium sulfate. A nitric acid aqueous solution, and then performing a treatment for reducing one or both of a sulfate ion concentration and a calcium ion concentration contained in the nitric acid aqueous solution. 2. The nitric acid aqueous solution according to claim 1, wherein the treatment for reducing the sulfate ion concentration is a treatment for adding barium nitrate to the liquid to be treated and removing the generated barium sulfate precipitate. Collection method. 3. The method for recovering an aqueous nitric acid solution according to claim 1, wherein the treatment for reducing calcium ions is a treatment using a cation exchange resin. 4. A pickling waste liquid containing nitric acid is neutralized with barium hydroxide to remove a precipitate of metal hydroxide produced, and sulfuric acid is added to the obtained aqueous solution containing barium nitrate. A method for recovering an aqueous nitric acid solution, comprising removing a precipitate of barium sulfate generated. 5. The method for recovering an aqueous nitric acid solution according to claim 1, wherein after removing the free acid, the pickling waste liquid containing nitric acid is neutralized.