JPH0260607B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing alumina, and more particularly, to a method for producing alumina from an acidic aluminum solution via an oxalic acid complex of aluminum.

[Conventional technology]

There are many methods for producing alumina, including the Bayer method. In a method other than the Bayer method that involves some intermediate compound, distilled water is added to aluminum alkoxide (for example, aluminum isopropoxide [Al(OC ₃ H ₇ ) ₃ ]) and hydrolyzed with heating and stirring to produce alumina hydrate. There is an alkoxide method in which alumina powder is obtained by obtaining a precipitate of AlOOH and calcining this precipitate.

Also, aluminum sulfate and ammonium sulfate are dissolved in water under heating. Add sulfuric acid to aluminum hydroxide. Add ammonium water to the acidic aluminum sulfate solution. There is an ammonium alum method in which alum is crystallized, heated, and thermally decomposed to obtain alumina.

[Problem that the invention seeks to solve]

However, conventional alkoxide methods use aluminum alkoxide as a starting material, resulting in high costs. In addition, the ammonium alum method requires a long recrystallization process to achieve high purity, resulting in higher decomposition temperatures and the generation of corrosive decomposition gases.

This invention was made in view of the above-mentioned circumstances, and its purpose is to provide a method for producing highly pure and ultrafine alumina at low cost and at a low decomposition temperature.

[Means for solving problems]

In order to achieve the above-mentioned object, the present inventor conducted various tests and researches and found that it is effective to produce alumina from an acidic aluminum solution through an oxalic acid complex of aluminum,
We have now completed this invention.

That is, the method for producing alumina of this invention is as follows.
It includes the steps (a) to (c).

(a) A process of neutralizing an acidic aluminum aqueous solution with a basic substance to generate aluminum hydroxide.

(b) A step of dissolving the aluminum hydroxide in a mixed solution of oxalic acid and ammonium oxalate, concentrating this, and crystallizing aluminum ions and an oxalic acid complex.

(c) A step of heating the oxalic acid complex to thermally decompose it into alumina.

In a preferred embodiment of the present invention, the acidic aluminum aqueous solution can be used as an acidic aluminum-containing waste liquid.

As an embodiment of this invention, the oxalate complex of aluminum can be ammonium tris(oxalite)aluminum()ate.

In a desirable embodiment of the present invention, in the step (c), the oxalic acid complex may be thermally decomposed into alumina by firing.

As another embodiment, in the step (c), the oxalic acid complex can be thermally decomposed into alumina by spraying it into plasma.

This invention will be explained in more detail below.

In step (a) of the alumina production method according to the present invention, aluminum hydroxide is produced by neutralizing with an acidic aluminum aqueous solution and a basic substance.

The acidic aluminum aqueous solution that can be used in this invention is an acidic aluminum aqueous solution containing chloride ions, sulfate ions, phosphate ions, nitrate ions, etc., or mixed ions thereof.
Although the aluminum ion concentration is arbitrary in this invention, the concentration can be adjusted by diluting or concentrating as desired. As the acidic aluminum aqueous solution used in this invention, for example, etching waste liquid discharged during the manufacture of aluminum electrolytic capacitors can be used. This waste liquid is a strongly acidic aluminum waste liquid containing chloride ions, nitrate ions, phosphate ions, nitrate ions, etc., and has conventionally been used as a neutralizing agent for alkaline waste liquid in pulp production. If this waste liquid is used as a starting material for this invention, aluminum can be recovered, which is effective from the viewpoint of resource recovery.

The basic substances used in this step (a) are:
Examples include sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and ammonia, with ammonia being preferred. It is desirable that the amount of the basic substance added be changed as appropriate depending on the concentration, acidity, etc. of the acidic aluminum solution.

The aluminum hydroxide precipitate obtained by neutralization is preferably filtered off and washed with water. (b) of this invention
In the process, the aluminum hydroxide is dissolved in a mixture of oxalic acid and ammonium oxalate.
If there is undissolved aluminum hydroxide, it is desirable to filter it out. The dissolution is performed by heating, for example, on a water bath at 80°C. The above mixed solution in which aluminum hydroxide is dissolved is concentrated. This concentration is
For example, it is performed by heating to evaporate and remove water as a solvent. The degree of concentration varies depending on the concentration of the original mixed liquid, and is, for example, about 1/10 by volume. In addition to the above-mentioned concentrations, the crystallization of the oxalate complex of aluminum can be promoted by cooling. The oxalate complex of aluminum to be crystallized is, for example, the trihydrate of tris(oxalato)aluminum()ammonium complex represented by (NH ₄ ) ₃ [Al(C ₂ O ₄ ) ₃ ]·3H ₂ O. be. The obtained oxalic acid complex is preferably purified by operations such as recrystallization.

In step (c) of the production method of this invention, the aluminum oxalate complex is heated and thermally decomposed into alumina. Thermal decomposition of this complex can be carried out, for example, by calcining the obtained crystals of the complex at a temperature higher than the thermal decomposition temperature of the complex, for example, about 430°C,
It can also be carried out by spraying the obtained complex dispersion or solution into an inductively coupled plasma device. This thermal decomposition is preferably performed in an oxidizing atmosphere.

The morphology of alumina obtained by pyrolysis of the complex is determined by the method of pyrolysis. For example, the above-mentioned calcination method yields amorphous alumina, and the plasma method yields γ-alumina. Additionally, these aluminas may be converted into other crystalline forms using known methods. The obtained alumina can be used for general purposes.

[Effect]

The method for producing alumina of the present invention is thought to proceed based on the following reaction and mechanism of action. Note that this explanation is for understanding the present invention, and is not intended to limit the scope of the present invention.

When an acidic aluminum aqueous solution is neutralized with a basic substance, aluminum hydroxide is produced as shown in the following reaction formula (). Note that this formula uses ammonium as the basic substance.

Al ³⁺ +3NH ₄ OH →Al(OH) ₃ ↓+3NH ⁴⁺ …() When aluminum hydroxide is dissolved in a mixture of oxalic acid and ammonium oxalate and concentrated, its complex, tris(oxalato)aluminum() acid Ammonium crystallizes as shown in the reaction formula () below. Note that the crystals to be crystallized are trihydrate, dihydrate, or 1.5 hydrate depending on the temperature, and this formula () is for the trihydrate.

2Al(OH) ₃ +3(NH ₄ ) ₂ C ₂ O ₄ +3H ₂ C ₂ O → 3
(NH ₄ ) ₃ [Al(C ₂ O ₄ ) ₃ ]・3H ₂ O↓ …() Next, when the aluminum oxalate complex is heated in an oxidizing atmosphere and thermally decomposed, alumina is formed as shown in the following formula (). generate.

2(NH ₄ ) ₃ [Al(C ₂ O ₄ ) ₃ ]・3H ₂ O＋3O ₂ →Al ₂ O ₃
+6NH ₃ +12CO ₂ +9H ₂ O...() Almost 100% of aluminum in the solution is recovered in the above reaction formula (), and in the same way, almost 100% proceeds in the above reaction formula (). Therefore, the recovery rate of aluminum can be increased.

〔Example〕

Hereinafter, the present invention will be specifically explained with reference to Examples.

Example 1 Hydrochloric acid waste liquid with aluminum ion concentration 15g/
400ml was diluted 5 times with water to 2. 380 g of 28% ammonia water was added to this diluted solution with an aluminum ion concentration of 3 g/min to form a precipitate of aluminum hydroxide, which was filtered and washed with water. Note that this aluminum hydroxide is obtained in the form of a hydrate or hydrous aluminum hydroxide.

Next, 213 g (1.5 mol) of ammonium oxalate and 189 g (1.5 mol) of oxalic acid were dissolved in water 4, and 35 g of the above aluminum hydroxide was added to this mixed solution.
The mixture was dissolved in hot water at 80°C, and after about 1 hour, the precipitated aluminum hydroxide was filtered off. The obtained filtrate (3800 ml) was concentrated to about 400 ml to precipitate crystals.
Recrystallization was performed for purification, and after reprecipitation, the product was washed with alcohol.

The purified crystals were thermally decomposed at about 500°C in an oxygen atmosphere for about 1 hour. The obtained amorphous alumina is
10.8g (95% recovery rate), and its primary particle size is:
It was 0.2 μm.

Example 2 In the same manner as in Example 1, an ammonium tris(oxalato)aluminum()ate complex solution was prepared, and after concentrating, recrystallizing, and washing, crystals of the complex were obtained. 50 g of this crystal was crushed and dissolved during distillation to prepare a solution (aluminum concentration 1M/2).
This liquid was sprayed into an inductively coupled plasma device at 6000-6500K to thermally decompose it into spherical γ-alumina.
The obtained alumina was 6.33 g (recovery rate 99%), and its particle size was 100 to 200 Å.

〔Effect of the invention〕

By the manufacturing method of the present invention, the following effects can be obtained.

(a) The reaction proceeds quantitatively from the aluminum solution, and the recovery rate of aluminum is determined by the purification process by recrystallization. If there are few cationic impurities in the stock solution, they can be almost completely removed at the aluminum hydroxide filtration stage, so almost 100% recovery is possible.
Approximately 90
% recovery is possible. In addition, since the oxalate complex has a large negative charge against anionic impurities, even if large amounts of chlorine ions, phosphate ions, oxalate ions, sulfate ions, etc. are present, the recrystallization process Purity can be easily increased by melting.

(b) separating aluminum from the solution as a complex;
Since the complex crystals are easy to recrystallize, there are few impurities mixed into the final alumina, and high purity alumina can be produced.

(c) Since the oxalate complex of aluminum is thermally decomposed, the thermal decomposition temperature can be lowered, e.g.
Ultrafine alumina particles of 0.01 to 0.2 μm can be obtained.

(d) If acidic aluminum waste liquid is used as an acidic aluminum solution (starting material), alumina can be produced at low cost, and it can also contribute to its recovery and reuse from the viewpoint of resource conservation.

(e) When spraying inductively coupled plasma, conventionally nitrates and chlorides were used, which caused the problem of nozzle clogging, but using Tris salt eliminates nozzle clogging and enables efficient production. be able to.

Claims (1)

[Scope of Claims] 1. A method for producing alumina including the following steps (a) to (c). (a) A process of neutralizing an acidic aluminum aqueous solution with a basic substance to generate aluminum hydroxide. (b) A step of dissolving the aluminum hydroxide in a mixed solution of oxalic acid and ammonium oxalate, concentrating this, and crystallizing aluminum ions as an oxalate complex. (c) A step of heating the oxalic acid complex to thermally decompose it into alumina. 2. The manufacturing method according to claim 1, wherein the acidic aluminum aqueous solution is an acidic aluminum-containing waste liquid. 3 The oxalic acid complex is tris(oxalato)
The manufacturing method according to claim 1 or 2, wherein ammonium aluminium()ate is used. 4. In the step (c), the oxalic acid complex is fired and thermally decomposed into alumina.
2. The manufacturing method according to item 2, item 3, or item 3. 5. The manufacturing method according to claim 1, 2, or 3, wherein in step (c), the oxalic acid complex is sprayed into plasma and thermally decomposed into alumina.