JPS6234686B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing alumina that emits less alpha rays. In recent years, as integrated circuits have become ultra-fine, malfunctions of integrated circuits due to alpha rays emitted from the porcelain that houses the integrated circuits, which did not pose a problem in the past, have become a major problem. This α-ray radiation is caused by trace amounts of uranium, thorium, and radium contained in alumina, the main raw material for porcelain, and the α-ray radiation amount of normal alumina is about 0.05 to 0.2 c/cm ² hr (c: cautz). However, it is required to reduce this to 0.05c/cm ² ·hr or less. During the Bayer alumina production process, some of the radioactive substances contained in bauxite are dissolved in sodium aluminate solution, and the majority is discharged as undissolved red mud. The dissolved radioactive substances are incorporated into the aluminum hydroxide crystals precipitated during the precipitation process, so they are not removed even by cleaning or calcination after precipitation and are transferred to the product alumina. Alumina for integrated circuits requires high purity, especially alumina with a low Na ₂ O content, and for this reason, alumina is treated with dilute acid, but even this dilute acid treatment does not remove the above-mentioned radioactive substances. In the case of Bayer process alumina, the aluminum hydroxide itself precipitated in the precipitation process must emit a small amount of alpha rays. The amount of α-ray radiation of aluminum hydroxide precipitated from a sodium aluminate solution varies depending on the Na ₂ O/Al ₂ O ₃ molar ratio (hereinafter simply referred to as molar ratio) of the sodium aluminate solution, precipitation temperature, and the like. The inventors used various Na ₂ O in the range of 70-160 g/
The concentration, molar ratio, amount of seeds, etc. of the sodium aluminate solution were adjusted in various ways, and the amount of α-ray radiation of precipitated aluminum hydroxide was measured. As a result of the experiment, for each temperature of a sodium aluminate solution with a predetermined Na ₂ O concentration, within the range of 40 to 90 °C, the already widely known equilibrium molar ratio (N ₀ ) and the aluminic acid discharged from the precipitation tank were determined. By introducing the difference in the molar ratio (N) of the sodium solution, that is, the degree of supersaturation, the relationship between this and the temperature of the precipitate (T) and the amount of α-ray radiation can be expressed by the following empirical formula with very good compatibility. I succeeded in expressing it as That is, α=0.3×10 ¹³ (N ₀ −N) ³ e ^-11000/T (1) α: Radiation dose as alumina, [c/cm ²・hr] T: Precipitation temperature, [〓] N ₀ : Equilibrium molar ratio, [-] N: Effluent molar ratio, [-] e: Base of natural logarithm However, the applicable range of equation (1) is when the Na ₂ O concentration of the sodium aluminate solution is 70 to 160 g/, α is in the range of 0.2 or less. According to equation (1), it is possible to predict the amount of α-ray radiation of aluminum hydroxide to be precipitated, and by using this relationship, the operation of the precipitation process can be controlled to achieve hydroxylation below the desired amount of α-ray radiation. Aluminum (if the amount of alpha rays emitted by alumina is 1, the amount of alpha rays emitted by aluminum hydroxide is approximately 1/1.5) is precipitated and calcined to produce alumina with a small amount of alpha rays emitted. The above empirical formula (1) applies regardless of the type of industrially used raw material bauxite with an alpha radiation dose of 15 c/cm ² hr or less, and to buyers who extract at 120°C to 250°C for more than 30 minutes. This holds true regardless of the conditions for extraction of bauxite within the legal limits and the conditions for red mud separation within the normal range of the Bayer method. That is, even when bauxite from Southeast Asia with an alpha radiation dose of 10 c/cm ² hr is extracted at 120 to 140°C, a sodium aluminate solution from which red mud is separated is used.
It has been confirmed that the method can be similarly applied using a sodium aluminate solution obtained by extracting 7 c/cm ² ·hr of Australian bauxite at 200 to 230°C and separating the red mud. As mentioned above, alumina for integrated circuits is
It is necessary to keep the amount of α-ray radiation below 0.05c/cm ² ·hr, and by inserting this value into the above equation (1), the following equation can be obtained. e ^11000/T ≧6×(N ₀ −N) ³ ×10 ¹³ (2) In the present invention, in order to obtain alumina with an α-ray radiation dose of 0.05 c/cm ² ·hr or less, in the aluminum hydroxide precipitation step, The gist of this is to operate the precipitation tank that precipitates a dominant portion of the total precipitation amount so as to satisfy the above equation (2). Although the method of the present invention can take various forms, the present invention will be explained below using two embodiments. One of them is a method of precipitating aluminum hydroxide at a constant temperature using one precipitation tank.
Figure 1 shows the flow sheet, in which a sodium aluminate solution F is supplied to a precipitation tank 1 which is kept at a constant temperature, and a classifier 7
of underflow S (containing fine aluminum hydroxide particles used as seeds) is added. An amount corresponding to both amounts is extracted from an appropriate part of the precipitation tank 1 and supplied to the classifier 6, relatively coarse aluminum hydroxide particles are extracted as a product from the underflow H, and the overflow is supplied to the classifier 7. Supplied. In this embodiment, since the precipitation is carried out in one tank, the precipitation conditions must satisfy equation (2). In other words, the liquid temperature in precipitation tank 1 is 75
℃, (N ₀ - N) must be less than 0.96 (see Figure 3. Equation (2) is calculated, and the area below the curve is where Equation (2) holds true) .
Since N ₀ at 75°C is 2.90 at 120 g of Na ₂ O, in the case of a sodium aluminate solution with this Na ₂ O concentration, the N of the liquid extracted from the precipitation tank should be greater than 1.94. Therefore, the residence time or the amount of seeds added may be adjusted. Another embodiment is a method in which aluminum hydroxide is continuously deposited by arranging a plurality of precipitation tanks in series, similar to a normal Bayer process. FIG. 2 is a flow sheet of the process, in which five precipitation tanks are used. A sodium aluminate solution F and an underflow S from a classifier 7 (containing fine particles of aluminum hydroxide as seeds) are added to the precipitation tank 1. The amount of liquid corresponding to both amounts is fed from the precipitation tank 1 to the precipitation tank 2, and in this way, the liquid is sent to the precipitation tank 5 to complete the precipitation, and the liquid after the precipitation is processed in the same manner as in the above embodiment. It is processed. In this embodiment, the liquid temperature in each precipitation tank often decreases sequentially, and as can be seen from Figure 3, the preferred precipitation conditions differ for each precipitation tank, making the operation somewhat complicated. Become. However, since it is sufficient that the amount of α-ray radiation of the product as a whole is below a certain limit, the precipitation conditions of a precipitation tank with a small amount of precipitation do not necessarily have to satisfy the precipitation conditions of equation (2). For example, only the precipitation tank that accounts for most of the precipitation amount needs to satisfy the condition of equation (2), so it can be easily operated by adjusting the residence time, amount of seeds added, liquid temperature, etc. Can be done. Next, the present invention will be further explained by examples based on the embodiments described above and comparative examples. Comparative Example As an aluminum hydroxide precipitation process, five precipitation tanks each having a capacity of 1500 m ³ were arranged in series to carry out continuous precipitation in the same manner as shown in FIG. The operating method was the same as the normal Bayer process, except that the precipitate was allowed to cool naturally instead of being forced to cool. 72℃ in the first tank, Na ₂ O concentration 120g/
, sodium aluminate solution with molar ratio 1.55 250
m ³ /H and seed slurry at 50°C and molar ratio 2.4432
m ³ /H (seed aluminum hydroxide: 17.4t/H)
was added to the precipitation finished liquid 282 discharged from the fifth tank.
m ³ /H was classified using a first classifier, and relatively coarse particles of aluminum hydroxide were taken out at a rate of 18.05 t/H. The overflow of this classifier was further supplied to a second classifier, the overflow was used for bauxite extraction, and the underflow was returned as seeds to the first precipitation tank.

[Table] N ₀ −N in each of the above tanks was outside the applicable range of equation (2). The obtained aluminum hydroxide was washed and calcined to obtain alumina product. The α-ray radiation amount of this alumina was 0.088c/cm ² ·hr. Example 1 A precipitation operation was carried out according to the embodiment shown in FIG. A precipitation tank with a capacity of 1500 m ³ equipped with a stirrer and a heat exchanger was first filled with 1400 m ³ of sodium aluminate solution at 75°C, 120 g of Na ₂ O/molar ratio 2.2, and then 165 t of aluminum hydroxide (moisture
10%), and to this was added 20 m ³ /H of sodium aluminate solution with a molar ratio of 1.5 and 120 g of Na ₂ O at 80°C.
was supplied at the rate of Aluminum hydroxide was precipitated by keeping the liquid temperature at 75° C., and the overflow from the precipitation tank was passed through a first classifier to take out relatively coarse particles of aluminum hydroxide as an underflow at a rate of 1.3 t/H. The overflow is passed through a second classifier, and the clarified liquid is returned to extraction of bauxite.
The entire amount of underflow was used as seeds. In addition,
The molar ratio of the liquid discharged from the precipitation tank was 2.2. As described above, this value satisfies the requirements of equation (2). The α-ray radiation amount of the alumina obtained by washing and calcining the initial seeds and the initial precipitated aluminum hydroxide is as shown in Table 1 below. ^cm2・hr was obtained.

[Table] Example 2 As in the comparative example, five precipitation tanks each having a capacity of 1500 m ³ were arranged in series to carry out continuous precipitation based on the flow sheet shown in FIG. In this case, the precipitation conditions for each tank are (2)
The operation was carried out to satisfy the formula. In the first precipitation tank, a sodium aluminate solution of 184 m ³ /H with a Na ₂ O concentration of 120 g/H and a molar ratio of 1.55 was placed at 86°C.
and 24 m ³ /H of seed slurry at 60°C and a molar ratio of 2.24.
(Seed aluminum hydroxide 12.9t/H) was added,
208m ³ /H of precipitation finished liquid discharged from the 5th precipitation tank
was classified using a first classifier, and coarse particles of aluminum hydroxide were taken out at a rate of 11.18 t/H. The overflow of this classifier was further passed through a second classifier, the clarified liquid was used for the bauxite extraction process, and the entire underflow was returned to the first precipitation tank as seed slurry. The operating conditions of each precipitation tank were as follows.

[Table] The value of N ₀ −N in each tank above satisfies equation (2). The aluminum hydroxide was washed and calcined to obtain alumina product. The α-ray radiation amount of this alumina is
It was 0.025c/ ^cm2・hr. As can be seen from the above comparative examples and examples, according to the method of the present invention, α can be reliably obtained by simply controlling the operation of the aluminum hydroxide precipitation process without using special chemicals or adding equipment. This is an extremely advantageous manufacturing method because it allows the production of alumina with a small amount of radiation, and it greatly contributes to the production of integrated circuits.

[Brief explanation of the drawing]

FIG. 1 shows a flow sheet for one embodiment of the method of the invention, and FIG. 2 similarly shows a flow sheet for another embodiment. FIG. 3 illustrates a region where equation (2) holds true. In Fig. 1 and Fig. 2, 1 to 5 are precipitation tanks;
6 and 7 indicate classifiers.

Claims (1)

[Claims] 1 Bauxite with an α-ray radiation dose of 15 c/cm ² hr or less is heated in a sodium aluminate solution at 120°C to 250°C for 30
In a method of producing alumina by extracting for more than a minute and discharging the undissolved residue as red mud by a conventional method, and precipitating aluminum hydroxide from the solution from which the red mud was separated in the presence of seeds, and calcining this, The aluminum hydroxide precipitation process is operated in a precipitation tank that precipitates the dominant part of the total amount of precipitation, with the temperature of the sodium aluminate solution ranging from 40℃ to 90℃, and the Na ₂ O concentration of the sodium aluminate solution ranging from 70 to 160g/. Then, the temperature T (〓) of the sodium aluminate solution and the equilibrium molar ratio of the same solution at that temperature (Na ₂ /Al ₂ O ₃ ) N ₀
and the molar ratio N of the same solution discharged from the precipitation tank
A method for producing alumina with a small amount of α-ray radiation, characterized in that the method is carried out so that e ^11000/T ≧6×(N ₀ −N) ³ ×10 ¹³ is satisfied.