JPH04130012A - Production of mullite - Google Patents

Abstract

PURPOSE:To obtain a mullite sintered body having superior strength, thermal stability and creep resistance by mixing an Al(OH)3 slurry with an aq. Na2SiO3 soln., adjusting the mixture to a specified pH, producing a precipitate and carrying out calcination. CONSTITUTION:An Al(OH)3 slurry having <=5mum average particle size and 1-5mol/l concn. is mixed with an aq. Na2SiO3 soln. having 1-5mol/l concn. so as to regulate the weight ratio of Al2O3:SiO2 to (70:30) to (75:25) and the mixture is adjusted to pH4-8, stirred for 1-6hr and allowed to stand to form a precipitate. This precipitate is washed with water and dried to <=30wt.% water content to obtain a precursor of mullite. This precursor is calcined at 1,000-1,600 deg.C in the air and the resulting mullite powder is molded and sintered at 1,500-1,800 deg.C to obtain a mullite sintered body.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing a mullite precursor, and a method for producing a mullite powder and a mullite sintered body using the same.

In this specification, the term "mullite precursor" refers to something that becomes mullite powder by firing. "Mullite powder" is a material that becomes a mullite sintered body by molding and firing, that is, 3AI203/2Si0
Al2O3/5i02 weight ratio expressed as 2 71.8/2
8.2 as well as the weight ratio 65/35 to 80/
20 things.

[Prior Art] A method for producing mullite using inexpensive raw materials, such as an acidic aluminum compound and sodium silicate, is as follows:
It is disclosed in Japanese Patent Application Laid-Open No. 63-1031116.

The method disclosed herein is a method in which a homogeneous mixed aqueous solution of both of the above raw materials is neutralized with an alkali to produce a mullite precursor.

[Problems to be Solved by the Invention] However, in such a method of neutralizing these mixed solutions, since AI and 81 have different precipitation pH and precipitate formation time, it is difficult to simultaneously produce both precipitates from the mixed solution. However, local variations in the At/Si ratio are likely to occur in the mullite precursor. This is particularly noticeable when high concentration raw materials are used. When the mullite precursor is fired, it is difficult to obtain a single mullite phase due to local variations in the Al/Si ratio, and cristobalite (SiO2) and corundum (AI.03) phases precipitate simultaneously as impurity phases. Cheap. When such a mullite precursor is fired to obtain mullite powder, and the powder is molded and fired to produce a mullite sintered body, the product sintering is more difficult than in the case of a single mullite phase due to the presence of impurity phases and glass phases. There are problems such as the body's strength tends to decrease, and thermal stability and creep resistance decrease.

The purpose of the present invention is to use inexpensive raw materials as starting materials,
Moreover, mullite powder containing less cristobalite phase and corundum phase as impurity phases than those obtained by the conventional method as described above, a mullite precursor that can be fired into such mullite powder, and the mullite The object of the present invention is to provide a method for producing a mullite sintered body having excellent strength and the like by using powder as a raw material.

[Means for Solving the Problems] As a result of intensive studies to achieve the above object, the present inventors have found that aluminum hydroxide slurry and sodium silicate aqueous solution are mixed and the mixed solution is adjusted to pl+4 to 8. By forming a precipitate, a mullite precursor is obtained, and by firing this, a mullite powder with less cristobalite phase and corundum phase, which are impurity phases, is obtained.Then, when the mullite powder is molded and fired, it becomes a mullite powder with excellent strength, etc. It was discovered that a mullite sintered body could be obtained, and the present invention was completed.

The details will be explained below.

Examples of the aluminum hydroxide slurry in the present invention include slurries of boehmite, gibbsite, payerite, and the like; neutralized aqueous solutions of acid salts such as aluminum sulfate, aluminum chloride, and aluminum nitrate.

The concentration of the aluminum hydroxide slurry is 5m01/Ω
It is preferable to use the following amount, more preferably 1 mol/Ω or less, but in consideration of productivity, it is preferable to use 1 to 5 mol#l. This is because when the concentration is high, the dispersibility of aluminum hydroxide deteriorates, and the Al/Si ratio becomes locally uneven after mixing with the sodium silicate aqueous solution, resulting in impurity phase when the obtained mullite precursor is fired. This is because it becomes easier to generate.

For the same reason, the average particle diameter of the aluminum hydroxide slurry is preferably 5 μm or less, more preferably 1 μm or less.

Furthermore, the concentration of metal salts other than aluminum salts in the aluminum hydroxide slurry may be 5 wt% or less, preferably 1 wt% or less.

This means that these mullite precursors with high metal salt content are
This is because, compared to a mullite precursor having a low mullite content, the powder produced by the acidification after firing is less likely to turn into mullite, and moreover, it is more likely to generate a glass phase after sintering.

The concentration of the sodium silicate aqueous solution is 5IllO1/
It is preferably less than Ω, more preferably less than i n + ol/Ω, but in consideration of productivity, it is 1 to 5 mol/,
It is best to do this at 11'. This is because if the concentration is high, the pH will locally increase when added to the aluminum hydroxide slurry, and aluminum hydroxide will dissolve, making the Al/Si ratio uneven and potentially creating an impurity phase. It is.

Since the mixed slurry of aluminum hydroxide slurry and sodium silicate aqueous solution becomes alkaline, the pH must be adjusted to 4 to 8 by adding mineral acid or the like.

The aluminum hydroxide slurry, the sodium silicate aqueous solution, and the mineral acid may be mixed by a method in which these three are simultaneously and continuously supplied to the system.

On the other hand, it is also possible to prepare aluminum hydroxide slurry, supply a predetermined amount of sodium silicate aqueous solution thereto, and finally supply mineral acid until the pH of the system reaches a predetermined value. The latter method will be explained below.

First, a sodium silicate aqueous solution is added while stirring the aluminum hydroxide slurry. The amount of sodium silicate aqueous solution added to aluminum hydroxide slurry is
A] 203/5i02 weight ratio 65/35 to 80/20
, preferably from 70/30 to 75/25.

This mixing is preferably completed within one hour in order to prevent the aluminum hydroxide from dissolving as much as possible.

The p)I of the slurry is then adjusted. pH adjustment is p
Adjust the H to 4-8, preferably the pH to 4.5-6.

The pH adjustment is preferably completed within one hour after the mixing is completed so that the aluminum hydroxide slurry is not dissolved as much as possible.

Since silicon compounds require time to precipitate, pt (a certain period of time after completion of adjustment, preferably 1 hour or more, more preferably 6
By allowing the solution to stand for more than an hour while stirring, a uniform precipitate-containing aqueous solution of an aluminum compound and a silicon compound can be obtained.

After stirring, the homogeneous precipitate-containing aqueous solution is filtered and washed to obtain a mullite precursor. For filtration, a conventional method such as vacuum filtration can be used.

It is preferable to wash with water, and it is more preferable to wash with warm water.

The cleaning process reduces impurities in the mullite precursor to A
O, l W of the sum of the weight of l2O3 and the weight of 5in2
It is preferable to do this until it becomes 196 or less, and 0.01ν
It is more preferable to carry out the process until it becomes t% or less.

If drying is carried out so that the water content is about 30 νt% or less, there will be no problem in producing mullite powder.

50~2 by oven drying, spray dryer, etc.
It is preferable to dry in the atmosphere at 00°C, especially 150 to 200°C.

By calcining the above-mentioned dried mullite precursor or its pulverized product at 1000° C. to 1600° C. in the air, a mullite powder containing less cristobalite phase and corundum phase, which are impurity phases, can be obtained.

By molding the mullite powder, preferably pulverizing it, molding it, and firing it at 1500°C to 1800°C, it is possible to obtain a mullite sintered body with a small amount of impurity phases and therefore excellent in strength and the like.

[Function] It is not necessarily clear why the method for producing a mullite precursor of the present invention yields mullite powder with less cristobalite phase and corundum phase, which are impurity phases, but it is presumed as follows. In other words, the fine aluminum hydroxide particles are surrounded by a sodium silicate aqueous solution, and silicon is precipitated as hydroxide on the aluminum hydroxide particles, so that the aluminum and silicon compounds are not precipitated unevenly.
, the mullite precursor is believed to form as a uniform precipitate with a uniform Al/Si ratio.

Due to the uniformity of the Al/Si ratio, it is thought that when the mullite precursor is fired, cristobalite and corundum phases, which are impurity phases, are difficult to form.

[Effects of the Invention] As is clear from the above description, according to the present invention, it is possible to produce a mullite precursor with less cristobalite phase and corundum phase, which are impurity phases, after firing, using inexpensive raw materials. Furthermore, by molding and firing the mullite powder obtained by firing this, a mullite sintered body with excellent strength etc. can be manufactured. The mullite precursor, mullite powder, and mullite sintered body obtained in the present invention are suitably used as engineering ceramics for high temperatures and ceramic substrates.

Example 1 Temperature was kept at 30°C, concentration 2+iol#, 1.5
While stirring the aluminum hydroxide (boehmite) slurry of 1), a sodium silicate aqueous solution with a concentration of 1 got/1) was added to A I 20372 vt%, S i O2
So that the mullite composition is 211vt%, that is,
11 was added over 80 minutes. The p)I of the aqueous solution after mixing was approximately 12. p with a sulfuric acid aqueous solution with a concentration of 1 mol#l
Adjusted to 115. This pH adjustment was completed 30 minutes after the mixing was completed. The generated precipitate was filtered, and the impurities in the mullite precursor were
, 1 wt% or less. The obtained precipitate was left to dry in the air at 180° C. for 24 hours to obtain a mullite precursor.

The obtained mullite precursor was fired at 1450°C to obtain mullite powder. X-rays used for the mullite powder: Cu,
Ka and X-ray generation conditions: The X-ray diffraction pattern under the conditions of 35 Kv and 25 mA is shown in FIG.

Next, the mullite powder was wet-pulverized in ethanol, crushed, and passed through a JIS standard 100 mesh sieve. The mullite powder is 57mm x 84m
A mullite sintered body was obtained by preforming to a size of m x about 5 mm at a molding pressure of 3DOkg/Ctn2, molding with a hydrostatic press at a molding pressure of 2000kg/cn+2, and firing at 1650°C.

A test piece measuring 3 mm + x 4 x about 40 + am was cut out from the obtained sintered body, and the three-point bending strength at room temperature was measured by the method specified in JIS R1601. The average values of the 30 samples are shown in Table 1.

Example 2 The temperature was kept at 30°C and the concentration was 1. While stirring the mol/ρ aluminum sulfate aqueous solution, neutralize it to pH 5 with a 4 mol/ρ sodium hydroxide aqueous solution, filter the obtained precipitate, and convert Na to Na2O with distilled water to calculate the weight of Al2O3 and 5jO2. It was washed with distilled water until the total amount was 0.1vt% or less.

The washed precipitate was dispersed in distilled water to form an aluminum hydroxide slurry having a concentration of 2 mol/Ω.

While stirring 1.5 g of the slurry, the concentration of 1 mol/
ρ sodium silicate aqueous solution with Al20372% by weight,
5i0228 weight 26 mullite composition, ie 1ρ was added in 30 minutes. p of aqueous solution after mixing
H was about 12. The concentration of the mixed slurry is 1 mol.
The pH was adjusted to 5 with an aqueous sodium hydroxide solution of /Ω. p
The H adjustment was completed 30 minutes after the completion of the mixing. The generated precipitate was filtered and washed with distilled water until the impurities in the mullite precursor became 0.1 vt% or less of the total weight of Al2O3 and 8102. The obtained precipitate was left to dry in the air at 180° C. for 24 hours to obtain a mullite precursor.

Mullite powder and then mullite sintered bodies were produced from the obtained mullite precursor under the same conditions as in Example 1, test pieces were cut out, and the three-point bending strength at room temperature was measured. The average values of the 30 samples are shown in Table 1. The X-ray diffraction pattern of the mullite powder was almost the same as that of the example.

Comparative Example While keeping the temperature at 30°C and stirring 1.5 g of an aluminum sulfate aqueous solution with a concentration of 1i + ol, a solution with a concentration of 1mol/j
? Sodium silicate of Al20372 sit%.

1 g of sandpaper was added over 30 minutes to obtain a mullite composition of 810,228% by weight. Immediately thereafter 4 mo
Neutralized with 1# aqueous sodium hydroxide solution. After filtering the generated precipitate, the impurities in the mullite precursor were ^1□0. It was washed with distilled water until the amount was 0.1 νt% or less of the total weight of SiO□ and SiO□.

The obtained precipitate was left to dry in the air at 180° C. for 24 hours to obtain a mullite precursor.

Mullite powder and then mullite sintered bodies were produced from the obtained mullite precursor under the same conditions as in Example 1, test pieces were cut out, and the three-point bending strength at room temperature was measured. The average values of the 30 samples are shown in Table 1. The X-ray diffraction pattern of the above mullite powder is shown in FIG.

[Brief explanation of drawings]

Brief Description of the Figures Figure 1 shows powder X-ray diffraction patterns of mullite powders obtained in Example 1 and Comparative Example. The meanings of the symbols in the figure are as follows. M: Mullite A, corundum (α

Claims (3)

[Claims] (1) A method for producing a mullite precursor, which comprises forming a precipitate by mixing an aluminum hydroxide slurry and an aqueous sodium silicate solution and adjusting the pH of the mixture to 4 to 8. (2) A method for producing mullite powder, which comprises firing the mullite precursor obtained by the method according to claim (1) at 1000°C to 1600°C. (3) A method for producing a mullite sintered body, which comprises molding the mullite powder obtained by the method according to claim (2) and firing it at 1500°C to 1800°C.