JP2001048531A - Production of ammonium hexafluoroaluminate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing ammonium hexafluoroaluminate, by which the ammonium hexafluoroaluminate crystals having a large particle diameter and a large bulk density can stably and easily be obtained, by simultaneously adding hexafluoroaluminic acid and ammonia to water to react the hexafluoroaluminic acid with the ammonia in a neutral or near pH region and further adding a specific amount of seed crystals. SOLUTION: This method for producing ammonium hexafluoroaluminate comprises simultaneously adding hexafluoroaluminic acid and ammonia to water and further adding seed crystals. The hexafluoroaluminic acid is obtained by dissolving aluminum hydroxide to hydrofluoric acid. The seed crystals preferably comprise (NH4)3AlF6 from the aspect of purity, have an average particle diameter of about >=100 μm, and is added in an amount of preferably about >=2 wt.%, especially about 2 to 40 wt.%, based on the ammonium hexafluoroaluminate to be crystallized. The reaction is desirably carried out at pH near to neutrality at a temperature of about >=40 deg.C. The slurry produced by the reaction is subjected to a solid-liquid separation treatment, washed and then dried at a temperature of about >=150 deg.C to give the (NH4)3AlF6 which is suitable for producing NF3 and has an average particle diameter of about >=300 μm and a bulk density of about >=1.0 g/cm3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the hexafluoro aluminum ammonium [(NH _{4) 3} AlF used as a raw material for producing useful nitrogen trifluoride as a gas or the like for gas cleaning in a semiconductor manufacturing apparatus (NF _3)
6 ].

[0002]

2. Description of the Related Art Nitrogen trifluoride (NF ₃ ) is usually a colorless gas having a boiling point of about −129 ° C. and a melting point of about −208 ° C., and is used for gas cleaning in semiconductor manufacturing equipment. Various methods for producing NF ₃ have been proposed. For example, US Pat. No. 3,304,248 discloses that gaseous nitrogen can be passed through a plasma arc at a temperature exceeding 1000 ° C. and gaseous fluorine can be used as an anode. A method for obtaining NF ₃ by introducing it into the post arc region as close as possible is disclosed.

[0003] In addition, gas phase reactions such as a reaction between hydrazoic acid gas and oxygen difluoride and a direct fluorination of ammonia are known. Also, molten salt electrolysis of ammonium / oxyfluoride is known. However, in any of these methods, since the reaction is in a gas phase, it is necessary to prevent the reaction from being difficult to control or to prevent the generation of an atmosphere containing flammable or explosive hydrogen.

Further, Japanese Patent Publication No. 55-8926 discloses a method of reacting ammonium oxyfluoride with gaseous fluorine in a molten state. However, since this method is a gas-liquid reaction, it is not always easy to control the reaction, the apparatus is significantly corroded, and the yield of NF ₃ is low, which is not an industrially sufficient method.

As a solution to such a disadvantage, Japanese Patent Application Laid-Open No. Sho 60-71503 discloses a method of reacting a solid metal fluoride, for example, (NH ₄ ) ₃ AlF ₆ with elemental fluorine at room temperature or higher. It has been disclosed.

[0006] Although industrial production of (NH ₄ ) ₃ AlF ₆ is not common, as a method for producing a small amount, fresh aluminum hydroxide is put into an aqueous ammonium fluoride solution and boiled. There is known a method of reacting an aqueous solution of ammonium fluoride with a solution of aluminum hydroxide dissolved in hydrofluoric acid.

However, the thus obtained (N
Since H ₄ ) ₃ AlF ₆ has a small particle size and a small bulk density, it is not always sufficient as a raw material for producing NF ₃ . Specifically, when the particle size is small, the reaction with elemental fluorine proceeds rapidly, and it is not easy to control the temperature of the reaction system, and the reaction temperature rises significantly, so that (NH ₄ ) ₃ AlF ₆ Causes self-decomposition, and the yield of NF ₃ decreases. On the other hand, if the bulk density is small, the volumetric efficiency of the device will be poor. In addition, there is a problem in handling such as generation of dust.

To solve the above-mentioned disadvantage, Japanese Patent Application Laid-Open No. 6-64917 discloses a method in which hexafluoroaluminic acid and ammonia are added simultaneously to achieve a reaction pH of 6 to 6.
This object is achieved by crystallizing in the range of 8.5. The point of this reaction is to maintain the pH in the range of 6 to 8.5 throughout the reaction.

However, in order to maintain the reaction pH range, it is important to control the flow rate of both raw materials, but it is very difficult to control this narrow pH range.
In particular, in the early stage of the reaction, the pH of the mother liquor in a dilute state is greatly fluctuated, and control is difficult. In addition, the fluctuation in pH at the beginning of the reaction is the largest cause of generation of fine particles, and the particle diameter and bulk density of (NH ₄ ) ₃ AlF ₆ crystals obtained are reduced. Therefore, it is difficult to constantly obtain a large crystal of (NH ₄ ) ₃ AlF ₆ .

[0010]

[Specific means for solving the problem]
As a result of intensive studies to solve the above problems, in the step of simultaneously adding and reacting hexafluoroaluminic acid and ammonia, at the time of the reaction, it is possible to achieve such an object by adding a seed crystal and reacting. And reached the present invention.

That is, according to the present invention, (NH ₄ ) ₃ A is crystallized.
By lF ₆ by adding 2 wt% or more seed against reacted by adding and ammonia hexafluoro aluminate simultaneously, pH control is very easy, even and variation in pH occurs particles An object of the present invention is to provide a method for constantly obtaining (NH ₄ ) ₃ AlF ₆ having a large diameter and a high bulk density.

The raw material hexafluoroaluminic acid used in the reaction can be easily obtained by dissolving aluminum hydroxide in hydrofluoric acid. Ammonia may be a gas or a liquid, and is not particularly limited.

The pH during the reaction is preferably around neutral,
When the pH fluctuates temporarily, the desired (NH ₄ ) ₃ AlF ₆ can be obtained if the pH is in the range of ₃ to 10, but it is necessary to adjust the pH at the end of the reaction to near neutrality.

In the reaction, it is preferable to simultaneously add raw materials of hexafluoroaluminic acid and ammonia.

The reaction temperature is not particularly limited. However, since the reaction is an exothermic reaction, in order to carry out the reaction at a low temperature, the amount of raw materials to be added is restricted and productivity is deteriorated. Considering these conditions, the reaction temperature is preferably 40 ° C. or higher.

Under the above conditions, the seed crystal to be added at the time of the reaction is preferably at least 2% by weight, more preferably 2 to 40% by weight, based on (NH ₄ ) ₃ AlF ₆ to be crystallized. If the amount is less than this range, the effect is small, and if it exceeds this range, no effect is observed on the powder properties such as the particle diameter and the bulk density. The kind of the seed crystal is not particularly limited, but (NH ₄ ) ₃ AlF ₆ is preferable in terms of purity, and the average particle size is preferably 100 μm or more.

The reaction slurry thus obtained is solid-liquid separated, washed and dried at about 150 ° C. to obtain NF.
₃ manufacturing the suitable average particle size 300μm or more, a bulk density of 1.0 g / cm ³ or more can be obtained (NH _{4) 3} AlF _6.

[0018]

EXAMPLES The present invention will be described below in more detail with reference to Examples, but it should not be construed that the invention is limited thereto.

EXAMPLES 1-6, COMPARATIVE EXAMPLE 1 In a 100 l polytetrafluoroethylene tank, water 24
(NH ₄ ) ₃ Al with an average particle size of 100 μm
At a reaction temperature of 60 ° C. while stirring a predetermined amount of F ₆ , 3
The reaction was carried out for 5 hours while adjusting the pH to a range of 6 to 8.5 with 39 kg of 0% hexafluoroaluminic acid and 17 kg of 25% aqueous ammonia. The reaction slurry was dried at 150 ° C. after solid-liquid separation and washing with water. The results are shown in Table 1.
The bulk density was measured according to JIS K5101.

[0020]

[Table 1]

Examples 7-12, Comparative Example 2 Water 100 was placed in a 100 l polytetrafluoroethylene tank.
(NH ₄ ) ₃ Al with an average particle size of 100 μm
At a reaction temperature of 60 ° C. while stirring a predetermined amount of F ₆ , 3
While adjusting 39 kg of 0% hexafluoroaluminic acid and 17 kg of 25% aqueous ammonia to a pH range of 3 to 10,
The reaction took place over time. Finally, the pH was adjusted to be around neutral. After the reaction slurry is solid-liquid separated and washed with water,
Dried at 50 ° C. Table 2 shows the results. still,
The bulk density was measured according to JIS K5101.

[0022]

[Table 2]

[0023]

According to the method of the present invention, (NH ₄ ) ₃ AlF ₆ having a large average particle size and a large bulk density can be easily obtained as a raw material advantageous for producing nitrogen trifluoride.

Claims (2)

[Claims] 1. A process for producing hexafluoroaluminum ammonium, comprising the step of simultaneously adding and reacting hexafluoroaluminic acid and ammonia with a seed crystal during the reaction. 2. The method for producing hexafluoroaluminum ammonium according to claim 1, wherein the amount of the seed crystal added is 2% by weight or more based on the hexafluoroaluminum ammonium crystallized.