JPH0124728B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for efficiently producing high-purity silica, which contains significantly less α-emitters such as uranium, thorium, and other impurities than alkali silicates. Traditionally, silica has been widely used as a reinforcing filler for rubber, synthetic resins, etc., and as a diluting and dispersing agent for agricultural chemicals, but recently, with the development of the electronics industry, there has been a demand for particularly high-purity silica as a material for electronic parts. is increasing. For example, high-purity silica is used as a filler in resin compounds for protective containers (IC packages) for LSIs and VLSIs, but α-emitters, especially uranium, thorium, etc. in this IC package material are contained in ppm. If even a trace amount of the unit is present, α-
It has been shown that particles are emitted, which penetrate into IC chips and generate large numbers of electron-hole pairs near the storage nodes of dynamic RAMs and CCDs, thereby inducing so-called soft errors. Moreover, the problem of soft errors caused by α-emitters will become more severe as IC integration increases, so in the future ultra-high purity silica containing α-emitters such as uranium and thorium on the ppb level will be used. It becomes necessary. Such ultra-high purity silica contains sodium, potassium, calcium, aluminum, iron,
It goes without saying that it is also necessary to contain impurity components such as chlorine and sulfate as much as possible. Conventionally, methods for producing ultra-high purity silica have been known, such as decomposing high-purity silicon tetrachloride or silicate ester purified by distillation in the gas phase or hydrolyzing it by wet reaction.
Both methods use expensive silica sources and are extremely uneconomical. On the other hand, as a method for obtaining relatively pure silica using alkali silicate as a silica source, the alkali component in the aqueous alkali silicate solution is removed using an ion exchange resin to obtain acidic silica sol, and silica is precipitated from this acidic silica sol. A method is known in which the powder is separated from the mother liquor and then fired. In this case, the means to precipitate silica from acidic silica sol is to add ammonia water to it.
A method in which the pH is adjusted and then cooled to freeze part or all of it, and then the frozen body is heated and thawed to precipitate silica in the form of fine particles (Special Publication No. 36-9415);
Ammonia per mole of silicic acid in acidic silica sol
A method in which 0.8 mol or more is added and then treated with an inorganic acid, acid anhydride, or acid salt (Japanese Patent Publication No. 18315/1983);
Various methods are known, such as a method of adding a quaternary ammonium salt surfactant to acidic silica sol (Japanese Patent Publication No. 4304/1983). However, in these conventional methods, which the present inventors used commercially available alkali silicate as a silica source, impurities such as sodium and calcium are well removed;
It was found that the removal of important impurities such as uranium and thorium was insufficient, and that this method was completely inappropriate as a method for producing ultra-high purity silica, which is the object of the present invention. According to the study results of the present inventors, in addition to sodium and potassium, normal aqueous alkali silicate solutions contain
aluminum, iron, uranium, thorium, sulfate,
It contains various impurities such as chlorine, and when this is treated with an acid type cation exchange resin and a hydroxyl type anion exchange resin according to the conventional method, sodium and potassium are almost completely adsorbed by the resin and become acidic. However, it was found that many trace components such as aluminum, iron, and thorium remained in the silica sol, making it impossible to achieve the purpose of purification. The present invention is based on the recognition and discovery of these important facts and their application. By going through a series of steps, we can produce high-purity silica using alkali silicate as a silica source with extremely low levels of uranium, thorium, and other impurities. This method has been completed to produce silica efficiently and economically. That is, the gist of the present invention is to add a fine insoluble substance or its precursor to an aqueous alkali silicate solution, mix it uniformly, and then perform solid-liquid separation, and to ionize the aqueous alkali silicate solution obtained from the previous step. The present invention relates to a method for producing high-purity silica, which comprises a step of obtaining a silica sol by treatment with an exchange resin, and a step of heat-treating the silica gel obtained from the sol. As the raw material alkali silicate used in the production method of the present invention, commercially available ordinary sodium silicate, potassium silicate, etc. with an SiO ₂ concentration of 20 to 35% can be used, and the molar ratio (SiO ₂ /M ₂ O)
(However, M represents an alkali metal) has a value of 1 to
4 can be used, although relatively high molar ratios (3-4) are often preferred. The first step of the present invention is to add a fine insoluble substance or its precursor to the alkali silicate aqueous solution, mix uniformly, and then perform solid-liquid separation to purify the alkali silicate aqueous solution. Usually, industrial alkaline silicate aqueous solution contains aluminum,
It contains various impurities such as iron, uranium, thorium, sulfate radicals, and chlorine.Until now, the existence and behavior of trace components in aqueous alkaline silicate solutions have not been clarified in detail, but some of them are in the ionic state. Although these particles can be removed by the following steps, there are colloidal particles that cannot be removed not only by general filtration used in normal solid-liquid separation, but also by pre-coat filters and microfilters.
In particular, trace amounts of α-emitters such as thorium tend to enter the silica of the product.
It is difficult to obtain the desired high purity. Therefore, this step is to remove trace amounts of impurities present in the aqueous alkali silicate solution, by adding and mixing fine insoluble substances or their precursors to the aqueous solution and adsorbing them onto active insoluble fine particles. This is separated and removed for purification. An insoluble substance that can be added to an aqueous alkali silicate solution must be substantially insoluble in the aqueous alkali silicate solution, but even if it has some solubility, it must not be easily separated and removed in a subsequent step. It won't happen. Furthermore, such insoluble substances must have a large surface area and must be active fine particles capable of adsorbing ions or fine colloidal particles. Any such inert substance may be used, including, but not limited to, metal hydroxides (hydrous oxides), metal silicates, or metal silicates such as calcium, magnesium, aluminum, iron, manganese, titanium, zircoconium, and chromium. Examples include aluminosilicates, and other insoluble substances such as silica and activated carbon can also be used. Among these, insoluble materials such as aluminum or wire are particularly desirable from the viewpoint of economy, operability, impurity removal ability, and safety. Further, the precursor of the insoluble substance refers to a water-soluble salt of the above-mentioned metal which can form a virtually insoluble metal hydroxide, metal silicate, or aluminosilicate in an aqueous alkali silicate solution. Such insoluble substances may be either crystalline or amorphous, and one or more of these may be used in combination. Among these, iron, aluminum, titanium, or zirconium hydroxides or their precursors are particularly preferred. The amount of such insoluble substances added to the aqueous alkali silicate solution varies depending on the type of additive, processing conditions, purpose of use of the product, etc., and is not particularly limited, but it is at most the amount of alkali silicate in terms of metal oxide. Up to 10% by weight is sufficient. In particular, if it exceeds 10% by weight, there is no problem, but it can be limited from the standpoint of separation and removal workability and addition efficiency, and on the other hand, the lower limit is naturally set at the effective amount that can be adsorbed and removed. The raw aqueous alkali silicate solution may be either a commercially available concentration or its diluted solution, and in many cases
A suitable range of SiO ₂ is from 2 to 35% by weight. The method of adding and mixing the insoluble substance or its precursor to the aqueous alkali silicate solution does not need to be particularly limited, and any desired means that allows for uniform mixing may be employed. Therefore, in the present invention, adding an insoluble substance or its precursor to an aqueous alkali silicate solution may not only be in the mode of addition, but also simultaneous addition, or if necessary, an aqueous alkali silicate solution may be added to the insoluble substance or its precursor. This shall also include methods for doing so. The insoluble substance or its precursor to be added may be in the form of a fine powder solid, an aqueous slurry, or a solution. Mixing is usually carried out by stirring, but other means such as ultrasonic dispersion mixing, shear dispersion mixing, etc. can be used as necessary, and the temperature during this mixing can range from room temperature to about 200℃. However, in many cases, it is preferable for equipment reasons to reach the boiling point of the mixed system. Mixing time of about 4 hours is sufficient and usually
The duration may range from 10 minutes to 1 hour. After the insoluble substance is added to and mixed with the aqueous alkali silicate solution in this manner, solid-liquid separation is performed using a desired separation device, such as a filter. Through this first step, α-emitters such as thorium and other trace impurities are adsorbed and removed by the insoluble material particles, resulting in a purified aqueous alkali silicate solution. However, depending on the separation operation in this step, there may be the presence of added insoluble substances and other fine colloidal particles that cannot be observed with the naked eye. In such a case, more complete purification can be expected by further performing an ultrafiltration operation, if necessary. The limit device is a normal one and there is no problem.
Further, as the ultrafiltration membrane, various commercially available materials can be used as long as they are alkali-resistant, and the value of the molecular weight cut-off can be appropriately selected depending on the type of impurity to be removed. In addition, if the viscosity of the alkaline silicate aqueous solution used for this filtration is high, the overspeed will be extremely slow, so it is necessary to dilute _it to an appropriate concentration. It is preferable to use an aqueous solution of. Next, in the second two types of silica sol production step, the alkali silicate aqueous solution purified in the previous step is
The concentration of SiO ₂ is adjusted to about 5% by weight or less, or if the solution from the previous step has the above concentration, the ion exchange resin and a dilute aqueous alkali silicate solution are brought into contact with each other in a conventional manner to make the solution acidic. The objective is to generate a silica sol. In this step, the contact treatment can be performed once or multiple times using the same or different ion exchange resins as desired, depending on the liquid properties of the aqueous alkali silicate solution or the intended use of the silica. . Normally, when high-purity silica is desired, a sol can be generated by sequentially processing with a cation exchange resin in acid form, then an anion exchange resin in hydroxyl form, and then a cation exchange resin. preferable. In ion exchange operations, it is common to pass the solution to be treated through a column filled with the resin, but other methods, such as a batch method in which the ion exchange resin and aqueous alkali silicate solution are directly mixed, are also possible. . In this way, the obtained silica sol is high purity silica that does not substantially contain α-emitters,
Depending on the intended use of the product, it can be recovered and made into a product by heat treatment. In this case, it is also possible to perform a concentration treatment to increase the silica sol concentration and then dehydrate and dry it to produce a product. There are several methods for converting silica sol into silica gel, and there is no need to limit the method as long as it does not introduce impurities. It is also preferable from the point of view. That is, this is a method in which silica is precipitated by causing a coagulant to act on purified silica sol, and the precipitate is separated and recovered. In this case, if necessary, the purified silica sol may be appropriately concentrated in advance and then treated with a coagulant. The coagulant must be one that vaporizes during the subsequent heat treatment process, and ammonium salts are suitable in most cases, but other compounds may also be used as long as they have coagulant ability and vaporize when heated. do not have. Among the ammonium salts, one or more of ammonium nitrate, ammonium chloride, ammonium carbonate, etc. are particularly preferred. however,
It goes without saying that these salts themselves require high purity. The particle state of the silica precipitated by this operation generally differs significantly depending on the conditions at the time of precipitation, i.e. the type, concentration, addition order, etc. of the coagulant, so it is an easily dispersible, fine-grained fine powder suitable for uses such as fillers. In order to obtain silica of the following properties, it is necessary to pay sufficient attention to the precipitation conditions. For example, an aqueous solution of the above ammonium salts may be placed in a reactor equipped with a stirrer, and a solution made neutral or alkaline by adding ammonia water to the purified acidic silica sol may be slowly added thereto while stirring. The method is to always precipitate hydrated silica under neutral to alkaline conditions. Under these conditions, high-quality silica with good transient properties can be produced. On this occasion,
The preferred concentration of the aqueous ammonium salt solution charged into the reactor varies depending on the type of salt, for example, 5 to 15% by weight for ammonium chloride, 5 to 20% by weight for ammonium nitrate, and preferably 7 to 13% by weight. . On the other hand, if the silica is precipitated under acidic conditions or if the concentration of ammonium salts is outside this range, part or most of the precipitated silica will be in the form of a transparent agar-like gel. They become silica or become bulky precipitates, significantly deteriorating their sedimentation properties and permeability, and their moisture content is extremely high, making subsequent heating and firing difficult. The product may become flavored, resulting in loss of control over product particle size and deterioration of quality. The amount of ammonium salt aqueous solution charged in advance into the reactor depends on the addition rate of the silica sol to be added,
Although it varies depending on the concentration of the ammonium salt aqueous solution and the desired particle size of the product silica, it is necessary to use at least 1/5 or more of the amount of silica sol to be added. It is better to perform the stirring operation sufficiently; if the stirring is extremely insufficient, the silica will gel. The addition rate of the silica sol can be adjusted as appropriate to obtain a desired particle state, since if it is too fast, it will gel and form a bulky precipitate, and if it is slow, a precipitate with better sedimentation properties will be obtained. The operating temperature may be room temperature, and there is no particular problem even if it is heated. The precipitated silica obtained in this silica precipitation step may be directly transferred to the precipitation heat treatment step, but if higher purity is required depending on the use of the silica, this silica may be subjected to pickling treatment. After that, the next step of heat treatment can be carried out.
For the acid treatment, an acid that can be volatile-decomposed such as nitric acid is preferred, and silica of higher purity can be obtained by washing the silica with an acid at room temperature or at elevated temperature, and then washing with water if necessary. Next, the final step consists in dehydrating the produced hydrated silica by heating it. During this heat treatment, it is necessary to be very careful not to cause contamination from the heating atmosphere or heating device. Heating is carried out under conditions necessary to decompose and vaporize volatile salts, acids, etc. attached to the silica, and to remove as much moisture as possible, but in most cases, the temperature may be 500°C or higher. After heating and firing, the silica can be pulverized if necessary to obtain extremely high purity silica. Thus, according to the method of the present invention, the concentration of impurity components that cannot be obtained by conventional methods using ordinary alkali silicate as a silica source can be reduced to 10 ppm or less of Na ₂ O,
High purity silica with Fe ₂ O ₃ of 10 ppm or less, Th of 10 ppb or less, and U of 1 ppb or less can be obtained industrially advantageously and efficiently. In addition, the product of the present invention is a finely powdered silica with a good particle state and good dispersibility, so the IC
It is expected that it will not only be ideal as a filler for resin compounds for packaging, but also as a practical raw material for a variety of other applications that require a high level of quality. The present invention will be explained below with reference to Examples and Comparative Examples. Example 1 Commercially available No. 3 sodium silicate aqueous solution (SiO ₂ : 28.6% by weight, Na ₂ O: 9.2% by weight, U: 36ppb, Th:
70 ppb) was diluted with water to obtain a 4% by weight sodium silicate aqueous solution as SiO ₂ . To this aqueous sodium silicate solution, aluminum hydroxide produced by a reaction between an aqueous sodium aluminate solution and nitric acid was added to a concentration of 2000 ppm, and after stirring at room temperature for 3 hours, solid-liquid separation was performed to obtain a purified aqueous sodium silicate solution. Next, this solution was passed through a column packed with a cation exchange resin (cation exchange resin IR120B manufactured by Organo Co., Ltd.) that had been converted into H-type with sulfuric acid to prepare an acidic silica sol. Next, 6 ml of 28 wt% ammonia water was added to 500 ml of this sol to make it alkaline, and on the other hand, alkaline silica sol was added dropwise to 12.5 wt% ammonium nitrate aqueous solution at a rate of 3 ml/min with stirring to coagulate and precipitate the silica sol. silica.
After filtering and washing this silica with water, it was heated to 900°C.
After being heat-treated for a period of time, it was pulverized by a conventional method to obtain high-purity silica. When this silica was analyzed,
Impurity content is Na: 10.2ppm, Fe: 40.3ppm,
U: 0.4 ppb or less Th: 12 ppb. In addition, Na and Fe were determined by atomic absorption analysis, and U and Th were determined by activation analysis. Example 2 The precipitated silica obtained in Example 1 was added to 1 mol/
After stirring and mixing with nitric acid at 85℃,
After filtering and washing with water, the mixture was heat-treated in the same manner as in Example 1 to obtain high-purity silica. When this silica was analyzed, the impurity content was Na: 1.5ppm, Fe: 4.1ppm,
U: 0.4 ppb or less, Th: 4 ppb. Comparative Example 1 Silica was obtained under exactly the same conditions and operations as in Example 1 except that aluminum hydroxide was not added. When the impurity content of this product was measured, Na: 11.2ppm, Fe: 45.8ppm, U:
It was 1.0 ppb or less, Th: 18.0 ppb, and Th was hardly removed. Example 3 Ammonia water was added to a ferric chloride aqueous solution in which ferric chloride hexahydrate was dissolved in water to neutralize it (PH8.0).
The produced iron hydroxide was thoroughly washed with water, and added to the same No. 3 sodium silicate dilute aqueous solution containing 4% SiO ₂ as in Example 1 so that the Fe(OH) ₃ concentration in the solution was 2000 ppm. The mixture was stirred at room temperature for 3 hours. Silica that had been subjected to ion exchange treatment and coagulation precipitation treatment and washed with water in the same manner as in Example 1 was calcined at 900° C. for 1 hour to obtain high purity silica. The impurity contents of this silica were Na21.1ppm, Fe50.5ppm, U0.5ppb or less, and Th15ppb. In addition, the impurity content of high-purity silica obtained by washing the silica precipitate with nitric acid in the same manner as in Example 2 is Na: 1.8 ppm Fe:
7.2ppm, U: 0.4ppb or less Th8ppb. Examples 4 to 9, Comparative Example 2 Commercially available No. 3 sodium silicate aqueous solution ( _SiO2 : 28.6% by weight, _Na2O : 9.2% by weight, U: 58ppb, Th:
89ppb) was diluted with water to obtain an 8% by weight sodium silicate aqueous solution as SiO ₂ . After adding and mixing the insoluble substances shown in Table 1 or their precursors to this sodium silicate aqueous solution, solid-liquid separation was performed to obtain a purified sodium silicate aqueous solution. Next, this aqueous sodium silicate solution was treated with an ion exchange resin to obtain a silica sol, and then precipitated silica was recovered from the silica sol and heated to obtain high purity silica. Table 1 shows the processing conditions at this time and the analytical values of the obtained silica. For comparison, silica obtained without adding an insoluble substance was also determined as a blank. 【table】

Claims (1)

[Claims] 1. A step of adding a fine insoluble substance or its precursor to an aqueous alkali silicate solution, mixing uniformly, and then separating the solid-liquid, and treating the aqueous alkali silicate solution obtained from the previous step with an ion exchange resin. 1. A method for producing high-purity silica, comprising the steps of: obtaining a silica sol; and heat-treating silica gel obtained from the sol. 2. The method for producing high-purity silica according to claim 1, wherein the fine insoluble substances are at least one or more selected from metal hydroxides, metal silicates, metal aluminosilicate, silica, and activated carbon. . 3. The method for producing high-purity silica according to claim 1, wherein the precursor of fine insoluble matter is a metal salt capable of reacting with caustic alkali or alkali silicate to produce an insoluble metal hydroxide or metal silicate. . 4. The method for producing high-purity silica according to claim 1, wherein the solid-liquid separation includes ultrafiltration. 5. The method for producing high-purity silica according to claim 1, wherein the silica gel is a precipitated silica gel that is precipitated by causing silica sol to act on a coagulant that is vaporized by heat treatment. 6. The method for producing high-purity silica according to claim 1 or 5, wherein the silica gel is acid-washed.