JP2775550B2 - Treatment of waste nitric acid containing aluminum - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating waste nitric acid containing phosphoric acid and aluminum, and more particularly to a method for recovering nitric acid from the waste nitric acid and effectively using the waste liquid after the recovery of nitric acid. is there. INDUSTRIAL APPLICABILITY The present invention can be applied to, for example, treatment of waste liquid discharged from a production process of an aluminum electrolytic capacitor.

[0002]

2. Description of the Related Art In the electrolytic foil industry, high-quality aluminum electrolytic foil is produced by performing electrical and chemical treatments such as etching and chemical formation. Conventionally, a single acid such as hydrochloric acid or nitric acid has been used for the chemical treatment of aluminum foil, and the waste liquid discharged from this step has a simple composition in which aluminum is dissolved in the single acid. However, improvement of capacitor performance such as miniaturization of products is desired, and processing techniques such as etching and chemical formation are becoming more sophisticated to meet the demand. Along with this, a mixed acid obtained by adding phosphoric acid and / or sulfuric acid to a monobasic acid such as hydrochloric acid or nitric acid is used as the acid to be used, and the composition of the discharged waste liquid is complicated,
The processing has become very difficult. In particular, aluminum-containing waste nitric acid, unlike aluminum-containing waste hydrochloric acid, had no use as a coagulant in paper mills, and was in trouble in treating it. Conventionally, aluminum-containing waste nitric acid discharged from factories related to the electrolytic foil industry that manufactures aluminum electrolytic capacitors has been discarded after neutralization treatment. In the neutralization treatment of the waste liquid, not only a large amount of alkali is consumed, but also it is difficult to treat aluminum hydroxide by-produced by the neutralization. Since nitric acid is more expensive than sulfuric acid or hydrochloric acid, it is needless to say that a means for recovering nitric acid from aluminum-containing waste nitric acid is extremely desirable. Also, in recent years, environmental problems have been highlighted, and from various viewpoints, such as the problem of enormous processing costs, the technology for treating waste nitric acid has become a major issue. As a method for recovering nitric acid from such aluminum-containing waste nitric acid, a method for regenerating waste nitric acid using an ion exchange resin (Japanese Patent Laid-Open No. 4-103378) is known. However, problems such as the treatment of recycled wastewater and the deterioration of resin remain, and no effective solution has been reached. As described above, development of an efficient treatment method for the waste nitric acid has been desired.

[0003]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems and efficiently recovers nitric acid from waste nitric acid containing phosphoric acid and aluminum, which is generated in large quantities from the manufacturing process of aluminum electrolytic capacitors. It is another object of the present invention to provide a method for effectively utilizing residual waste liquid after nitric acid recovery. If the waste nitric acid is simply distilled and concentrated, the recovery rate of nitric acid is poor. However, the present inventor has surprisingly found that adding sulfuric acid to the waste nitric acid and then performing concentration by distillation reduces nitric acid to 9%.
It has been found that it can be recovered at a high rate of 0% or more. The nitric acid recovered at such a high rate can be reused. Furthermore, the present inventor has found a method of treating the residual liquid after the recovery of nitric acid to derive a product that can be effectively used (for example, a flocculant) or a reusable product. The present invention is based on these findings.

[0004]

Accordingly, the present invention comprises phosphoric acid and aluminum, and comprises phosphoric acid and nitric acid.
To waste nitric acid with a content of mechanical acid of about 1% or less , SO ₄ / A
adding sulfuric acid so that the molar ratio becomes 3 or more;
The solution obtained in the above sulfuric acid addition step is prepared by adding
A distillation and concentration step of distilling and recovering nitric acid by distilling and concentrating the mixture to a concentration of 5 to 50% by weight, and a method for treating waste nitric acid containing phosphoric acid and aluminum. Further, the present invention provides a step of adjusting the aluminum sulfate in the residual liquid after distilling and recovering the nitric acid by the distillation concentration step to be in a supersaturated state, and crystallizing aluminum sulfate crystals from the obtained supersaturated solution. And a process for treating waste nitric acid containing phosphoric acid and aluminum. Furthermore, the present invention relates to a residual liquid after distilling and recovering nitric acid by the above-mentioned distillation concentration step.
And the molar ratio of free sulfuric acid to the ferrous compound in the residual liquid (free sulfuric acid / ferrous compound) was adjusted to 0.1. 5 and adding to become less than, characterized in that it comprises a step of oxidizing the divalent iron in the resulting solution, also relates to a method of treating waste nitrate containing phosphate and aluminum.

The waste liquid to be treated in the method of the present invention is not particularly limited as long as it is a waste liquid produced by performing an aluminum metal treatment with a treatment liquid containing nitric acid or nitric acid and phosphoric acid as a main component. Waste liquid generated in large quantities from the manufacturing process of aluminum electrolytic capacitors. In addition to nitric acid and phosphoric acid and their aluminum salts, the waste liquid may contain other various inorganic acids (for example, sulfuric acid, hydrochloric acid, etc.) in an amount of about 1% or less.

According to the method of the present invention, sulfuric acid is added to the waste liquid in the sulfuric acid addition step, and the nitrate in the waste liquid is reacted with sulfuric acid to convert the nitrate into nitric acid. By distilling, nitric acid can be recovered in high yield. In addition, the presence of sulfuric acid can eliminate the azeotropic point between nitric acid and water. In the distillation concentration step, almost no nitric acid is distilled off until the concentration of free sulfuric acid in the waste liquid reaches about 15% by weight, Since only distillate can be removed, a high concentration of nitric acid can be recovered. The amount of sulfuric acid added was SO
The molar ratio of ₄ / Al is 3 or more (preferably 3.8 to 5.
Adjust to 0). SO ₄ / A in the waste nitric acid
If an attempt is made to recover nitric acid having a molar ratio of 1 of less than 3 and 50% or more, aluminum sulfate crystals are crystallized in a distillate residue of nitric acid by evaporation and concentration, and the concentrating can is clogged. Therefore, the recovery rate of nitric acid is practically 50% or less. In the method of the present invention, it is preferable that the amount of sulfuric acid added is as small as possible. For example, when the molar ratio of SO ₄ / Al exceeds 5, acidic crystals are crystallized later when aluminum sulfate crystals are obtained. It is not preferable.

[0007] In the distillation concentration step, the concentration of free sulfuric acid in the waste liquid is adjusted to 25 to 50% by weight. If the concentration of free sulfuric acid exceeds 50% by weight, aluminum nitrate crystals crystallize out of the residual concentrated liquid and clog the concentrator. If the concentration of free sulfuric acid in the waste liquid is less than 25% by weight, the recovery rate of nitric acid is remarkable. The nitric acid recovery rate does not reach 70%.
The concentration operation can be performed, for example, by evaporative concentration under reduced pressure. About 90% or more of the nitric acid in the waste nitric acid is recovered by the sulfuric acid addition step and the distillation concentration step.
In addition, the free sulfuric acid mentioned here is free sulfuric acid generated by the following reaction.

[0008]

[Formula 1] Al (NO ₃ ) ₃ + mH ₂ SO ₄ → 0.5 Al ₂ (SO ₄ ) ₃ + 3HNO ₃ + (m−1.5) H ₂ SO ₄ (where m is 1. The number of phosphoric acids in the waste nitric acid is not present in a free state,
It exists in the form of Al (H ₂ PO ₄ ) ₃ .

Next, after distilling and recovering nitric acid by the above-mentioned distillation concentration step, aluminum sulfate in the residual liquid is adjusted to be a supersaturated solution, and aluminum sulfate crystals are crystallized from the obtained supersaturated solution to obtain a supersaturated solution. The resulting aluminum sulfate crystals are separated from the mother liquor. For example, while adding water at a water temperature of about 30 to 35 ° C. to the residual liquid from which nitric acid has been recovered and adjusting the concentration so that aluminum sulfate crystals do not precipitate, an aluminum sulfate supersaturated solution is prepared. The solution is cooled and seeded with aluminum sulphate at about 55-40 ° C, and the water temperature is raised to about 25 ° C at 2-5 ° C / hr.
Cooling is carried out at a rate of 2 to precipitate aluminum sulfate crystals. Next, the aluminum sulfate is separated by filtration. The obtained aluminum sulfate can be used, for example, as a flocculant for wastewater treatment. Further, since the filtrate sufficiently contains sulfuric acid, the sulfuric acid can be recovered by, for example, a diffusion dialysis treatment and used in the above-mentioned sulfuric acid addition step. Further, calcium can be added to the filtrate or the residual solution of the diffusion dialysis treatment to induce a calcium-aluminum sulfate and a phosphate-rich filtrate, and the calcium-aluminum sulfate can be used, for example, as gypsum or as a filler material. The phosphate-rich filtrate can be used, for example, for nutrients in plant cultivation.

After distilling and recovering nitric acid by the above-mentioned distillation and concentration step, iron is added to the residual liquid to convert free acids (particularly free sulfuric acid) in the residual liquid into iron salts. Here, the amount of iron added is determined by the molar ratio of free sulfuric acid to the ferrous compound (particularly ferrous sulfate) in the residual liquid (free sulfuric acid /
Less than 0.5 (preferably 0.35-
0.45). The molar ratio is 0.5
Above this is not preferable because free acid is present. The iron that can be added is not particularly limited as long as it is a compound containing ferrous iron, and examples thereof include hydrated iron oxide and triiron tetroxide. Iron temperature 7
It can be added at 0-90 ° C and dissolved in 1-4 hours.

Following the iron addition step, an iron oxidation step is performed. In the residual liquid, a small amount of nitric acid that could not be recovered in the distillation concentration step remains, and due to the action of the nitric acid, almost oxidation proceeds by air oxidation,
Very little oxidizing agent such as sodium nitrite is needed. The oxidation step can be carried out, for example, by leaving at about 70 ° C. for about 3 to 5 hours while being in contact with air. Further, it is preferable to adjust the concentration of sulfuric acid (dilution with water) before adding iron to the residual liquid, and then to recover a part of the free sulfuric acid in the residual liquid by diffusion dialysis. The sulfuric acid thus recovered can be reused in the sulfuric acid addition step, and the amount of iron to be added can be reduced since free acid in the residual liquid is reduced by this treatment. Diffusion dialysis treatment can be performed by a conventional method. For example, a diffusion dialysis tank configured in a filter press type in which a large number of anion exchange membranes are arranged is used, and the residual liquid and water are passed through the anion exchange membrane in an alternating current. This is done by: The supply rates of the residual liquid and water are not particularly limited, but may be about 0.5 to 3 liters / hr · m ² . Since the anion exchange membrane has the property of not allowing cations other than hydrogen ions to permeate, the acid diffuses toward water due to the concentration difference between the residual solution and water, and an acid solution (recovered acid) is obtained. The free acid concentration is low. When the insoluble matter is removed by filtration following the iron addition step and the oxidation step, a solution containing aluminum sulfate, ferric sulfate, and phosphoric acid as a main component is obtained. It can be used as a liquid flocculant which makes the components in the waste liquid coagulate and precipitate to form harmless waste liquid.

[0012]

EXAMPLES The present invention will be described below in more detail with reference to examples, but these examples do not limit the scope of the present invention. Example 1 Waste nitric acid 2 generated by etching aluminum foil 2
0 liters (17.2 g / liter as Al, PO ₄
12.8 g / liter, NO ₃ 155 g /
Liter), and 6.1 kg of 98% by weight sulfuric acid was added thereto, and the free sulfuric acid concentration in the waste liquid was reduced to about 43 by a reduced pressure method (40 mmHg).
% By weight to obtain a distillate of about 17 liters (174 g / liter as NO ₃ , Al, PO ₄ , S
0.01 g / liter or less as O ₄ ). The recovery of nitric acid was 95%. Next, the distillate residue (A
58.1 g / liter as 1 and 43.0 as PO ₄
g / liter, 21.5 g / liter as NO ₃ , S
About 4.3 liters of water was added to about 5.9 liters (989 g / l as O ₄ ), and the concentration of free sulfuric acid in the distillate was adjusted to about 30% by weight. This solution is heated from 60 ° C to 3 ° C.
After cooling at a rate of / hr, seed crystals (300 g) were added at 54 ° C., and cooled to 25 ° C. to precipitate aluminum sulfate crystals. Thereafter, 3.9 kg of aluminum sulfate crystals and 7.3 liters of mother liquor were separated and collected using a centrifuge. Analytical values 7.3 wt% aluminum sulfate crystals as Al, NO ₃ as 0.1 wt%, 0.4 wt% as PO _4, was 42.7 wt% as SO _4, the mother liquid Al
16.1g as 10.8 g / l, as NO ₃
/ Liter, 32.4 g / liter as PO ₄ , SO
₄ was 587 g / liter.

Example 2 Waste nitric acid 1 produced by etching an aluminum foil
0 liter (18.0 g / liter as Al, PO ₄
144g as 13.5 g / l, as NO ₃ /
After adding 2.4 kg of 98% by weight sulfuric acid to the liter), the free sulfuric acid concentration in the waste liquid was reduced to about 39 by a reduced pressure method (40 mmHg).
Concentration by distillation to a weight percentage of about 9 liters of distillate (N
151 g / liter as O ₃ , Al, PO ₄ , SO ₄
0.01 g / liter or less). The nitric acid recovery was 94%. Next, about 2.1 parts of the above distillate residue
About 4.3 liters of water was added to 7 liters to adjust the free sulfuric acid concentration in the distillate residue to about 18% by weight, and 6.4 liters of the solution (27.8 g / liter as Al and 20.000 as PO ₄₎ . 8 g / l, 11.3 g / l as NO _{3 and} 358 g / l as SO ₄ ). Diffusion dialysis treatment was performed using this solution. The diffusion dialysis treatment was performed using a filter press type diffusion dialysis tank equipped with 19 anion exchange membranes (effective area: 0.397 m ² ), and the solution and water were supplied in countercurrent. The feed rate of the solution is 23
0 ml / hr and water supply rate set to 280 ml / hr
Time diffusion dialysis was performed. As a result, about 4.9 liters of recovered acid (0.4 g / liter as Al, 2.5 g / liter as PO ₄ , 10.0 g / liter as NO ₃ , 167 g / liter as SO ₄ ) and dialysis waste liquid 5.2 Liter (24.2 g / liter as Al, P
16.0 g / l as O ₄ , 0.5 as NO ₃
7 g / l and 159 g / l as SO ₄ ). 5 liters of the dialysis waste liquid is concentrated to obtain about 3 liters of a solution composed of 40.3 g / liter of Al, 26.6 g / liter of PO ₄ , 0.95 g / liter of NO ₃ , and 265 g / liter of SO _4. Was. Approximately 135 g of hydrated iron oxide (43.4% by weight as Fe ³⁺ , 11.4% by weight as Fe ²⁺ ) was dissolved in the concentrated solution at a water temperature of 80 ° C. for 3 hours, air-oxidized, and insoluble matter was removed. Is removed by filtration, and about 3 liters of Fe-based flocculant (Fe ³⁺
24.6 g / l as Al and 40.2 g /
Liter, 26.5 g / l as PO ₄ , NO ₃
0.1 g / l as SO ₄ and 264 g / l as SO ₄ ).

[0014]

According to the present invention, high-purity and high-concentration nitric acid can be efficiently recovered from waste nitric acid containing phosphoric acid and aluminum, which is extremely effective in preventing pollution.
In addition, by-products that can be effectively used, such as an inorganic flocculant, can be derived from the nitric acid recovery residual liquid.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C01F 7/74 C01B 21/46 C02F 1/04

Claims (3)

(57) [Claims] Claims: 1. A method comprising the steps of:
The waste nitrate content of the acid and an inorganic acid other than nitric acid is less than about 1%, and adding sulfuric acid as SO 4 / _Al molar ratio is 3 or more, the solution obtained by the sulfuric acid addition step A distillation concentration step of distilling and concentrating by distilling and concentrating nitric acid until the concentration of free sulfuric acid reaches a ratio of 25 to 50% by weight, and treating the waste nitric acid containing phosphoric acid and aluminum. 2. A step of adjusting the aluminum sulfate in the residual liquid after distilling and recovering nitric acid by the distillation concentration step according to claim 1 so as to be in a supersaturated state, and crystallizing aluminum sulfate crystals from the obtained supersaturated solution. Separating the waste nitric acid containing phosphoric acid and aluminum. 3. A process according to claim 1 in the residual liquid after distillation recovery of nitric acid by distillation concentration step described, the iron, the molar ratio of free sulfuric acid to the first iron compounds retentate (free sulfuric acid / first
Iron compound) . 5 and adding to become less than,
Oxidizing divalent iron in the resulting solution. A method for treating waste nitric acid containing phosphoric acid and aluminum.