| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | August 15, 1989 |
| PATENT TITLE |
Process for producing silica of high purity |
| PATENT ABSTRACT | This invention provides a process for producing silica from silica sol which comprises preparing an acidic silica sol applied with a hydrogen peroxide treatment from an aqueous solution of alkali silicate, purifying the acidic silica sol by using a cationic exchange resin after pH adjustment and preparing silica from the silica sol obtained by neutralizing the thus purified acidic silica sol, that is, a production process for highly pure silica capable of removing, Ti as much as possible in the same manner as impurities such as Fe and Al. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | August 8, 1988 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A process for producing silica having a Ti impurity concentration of 0.3 to 1.2 ppm. comprising: (a) preparing an acidic silica sol from an aqueous solution of an alkali silicate by reacting an aqueous solution of an alkali silicate with an inorganic acid, an organic acid or by contacting an aqueous solution of the alkali silicate with a strongly acidic cation exchange resin to form an acidic silicate sol composition; (b) treating said acidic silicate sol composition with hydrogen peroxide; (c) adjusting the pH of the aqueous sol composition to 0 to 5 by addition of a base selected from the group consisting of sodium hydroxide and potassium hydroxide; (d) purifying the silica sol obtained by pH adjustment by contacting the acidic silica sol with a cationic ion exchange resin to obtain a silica sol having an Ti impurity level of 0.3 to 1.2 ppm.; (e) neutralizing the silica sol with an aqueous alkali solution; (f) adding the neutralized silica sol to an aqueous solution of ammonium salt; and (g) separating the precipitated silica. 2. A process as defined in claim 1, wherein the hydrogen peroxide is introduced by adding aqueous hydrogen peroxide into the acidic silica sol. 3. A process as defined in claim 2, wherein the concentration of aqueous hydrogen peroxide is from 0.05% by weight to 1.0% by weight based on the acidic silica sol. 4. A process as defined in claim 1, wherein the acidic silica sol is prepared by dropping an aqueous solution of the alkali silicate into an acidic solution of an inorganic acid or an organic acid while stirring said acidic solution. 5. A process as defined in claim 4, wherein the hydrogen peroxide is introduced by adding the aqueous hydrogen peroxide to the aqueous solution of alkali silicate. 6. A process as defined in claim 4, wherein the hydrogen peroxide is introduced by adding aqueous hydrogen peroxide to the acidic solution of the inorganic acid or organic acid. 7. A process as defined in claim 5 or 6, wherein aqueous concentration of the hydrogen peroxide is from 0.05% by weight to 1.0% by weight based on the total weight of the aqueous solution of alkali silicate or the acidic solution. 8. A process as defined in claim 1, wherein the acidic silica sol is prepared by adding the acidic solution of the inorganic acid or organic acid into the aqueous solution of alkali silicate while stirring the solution. 9. A process as defined in claim 8, wherein the hydrogen peroxide is introduced by adding aqueous hydrogen peroxide to the aqueous solution of alkali silicate. 10. A process as defined in claim 8, wherein the hydrogen peroxide is introduced by adding aqueous hydrogen peroxide to the acidic solution of the inorganic acid or organic acid. 11. A method as defined in claim 9 or 10, wherein the concentration of aqueous hydrogen peroxide is from 0.05% by weight to 1.0% by weight based on the total weight of the aqueous solution of alkali silicate or the acidic solution. 12. A process as defined in claim 1, wherein the acidic silica sol is purified to the extend that there exists a Ti impurity concentration range of 0.1 to 0.2 ppm Ti. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention concerns a process for producing synthetic silica of high purity and it particularly relates to an improvement in the process for producing highly pure silica not containing Ti. 2. Description of the Prior Art Heretofore, silica has been utilized in a wide range of application uses as fillers in resin compositions for sealing electronic components and raw materials for multi-ingredient optical fibers, fine ceramics, optical glasses, quartz for use in electronic industry, etc. Particularly, in the case of sealing silica used as filler in resin compositions for sealing electronic components, there is a problem of misoperation referred to as soft errors caused by elements and compounds such as uranium (U) and like other .alpha.-emitters as the density of integration is increased in LSI, SLSI, etc., accordingly, it is required that the content of the .alpha.-emitter material is kept as low as possible. Further, there exists a demand for highly pure silica not containing impurities such as Fe, Ni, Cr, Al and Ti in addition to uranium for use as starting materials for multi-ingredient optical fibers, fine ceramics, optical glasses, quartzs for use in electronic industry, as well as to silica for sealing use. As a process for producing synthetic silica of high purity serving to such application uses, there has been proposed a method of adding an inorganic acid or an organic acid to an aqueous alkali silicate solution or bringing an aqueous alkali silicate solution into contact with an ion exchange resin thereby preparing an acidic silica sol (Japanese Patent Laid Open No. 42217/85), purifying the acidic silica sol by using a cationic exchange resin, adding aqueous ammonia thereto to prepare a neutral or weakly alkaline silica sol, then mixing the sol and an aqueous ammonium salt solution under an alkaline condition to crystallize precipitated silica, recoverying the same by separation and synthesizing silica of high purity. However, the synthetic silica obtained by the conventional method involves a drawback in that it still contains residual Ti at high ratio although Fe, Al, Th, U, Na, etc. are removed favorably and, accordingly, results in a problem that the residual Ti absorbs light and reduces the light transmittance, for example, where it is used in multi-ingredient optical fibers. The present inventor has made an earnest study for such a cause and found that although those ions carrying positive charges such as Fe, Al, Th, U, Na are properly removed through ion exchange upon purifying the acidic silica sol by using a cationic exchange resin in the conventional method, Ti in the form of tetravalent metal ions like that of Si exhibits natures similar with those of Si and remains in a great amount in the acidic silica sol, and not being removed by the cationic exchange resin. SUMMARY OF THE INVENTION This invention has been devised in view of the foregoing and the subject thereof is to provide a process for producing silica of high purity not containing Ti. That is, this invention concerns a process for producing silica of high purity, which comprises preparing an acidic silica sol from an aqueous solution of alkali silicate through a hydrogen peroxide treatment, adjusting the acidic silica sol to pH 0-5, purifying the acidic silica sol after the pH adjustment by using a cationic exchange resin, neutralizing the thus purified acidic silica sol to prepare a silica sol and preparing silica from the silica sol. In such technical means, it is possible to use an aqueous solution of commercially available sodium silicate, potassium silicate etc. usually containing from 20 to 35% by weight of SiO.sub.2 as the alkali silicate for the starting material of the acidic silica sol. However, Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, etc. are present as impurities in the aqueous solution, most of which are present in the form of ions in solution and, in addition, very fine insoluble particles of hydroxides or silicates are sometimes present in the colloidal form, etc. Accordingly, it is desirable to apply purifying procedures prior to the synthesis of the acidic silica sol such as by usual filtration, microfiltration, ultrafiltration, coprecipitation removal with a desired adsorbent or the combination thereof. Further, upon preparing an acidic silica sol from the aqueous solution of the alkali silicate, the aqueous solution as described above is usually used after dilution to a concentration of 2-7% by weight of SiO.sub.2. Then, upon preparing the acidic silica sol from the aqueous solution of the alkali silicate, the alkali silicate is brought into reaction with an inorganic acid or an organic acid silica. The alkali silicate may be dropped into the inorganic acid or the organic acid for reaction or, alternatively, the inorganic acid or the organic acid may be added to the alkali silicate for reaction. In the case of adding the inorganic acid or the organic acid to the alkali silicate for reaction, if the adding speed is slow, the acidic silica sol formed sometimes becomes unstable and gels under the alkaline atmosphere. Thus, instantaneous addition is desirable. Further, it is desirable to uniformly agitate the reaction solution upon addition so that pH does not vary in the reaction solution. Furthermore, hydrochloric acid, sulfuric acid, nitric acid, etc. are usable as the inorganic acid, while formic acid, acetic acid, oxalic acid, etc. are usable as organic acid in this invention. In addition, a method of bringing the aqueous solution of alkali silicate in contact with a strongly acidic cationic exchange resin may be employed as a method of preparing the acidic silica sol. Aqueous hydrogen peroxide is then added to the thus obtained acidic solution to conduct a hydrogen peroxide treatment. In this case, aqueous hydrogen peroxide for electronic industrial use not containing impurities is desirably used. Further, while the hydrogen peroxide treatment is usually applied after the preparation of the acidic silica sol, in the case of adopting an alternative method of reacting the aqueous solution of alkali silicate with the acidic solution of the inorganic acid or organic acid to prepare the acidic silica sol, the aqueous hydrogen peroxide may be added to the aqueous solution of alkali silicate or acidic solution and to conduct the hydrogen peroxide treatment before the preparation of the acidic silica sol. By the hydrogen peroxide treatment, Ti in the acidic silica sol forms a compound with the hydrogen peroxide as shown by the following reaction scheme, by which the nature thereof is made different from that of Si. As a result, Ti changes its nature such that it can be exchanged easily with the cationic exchange resin and removed in the same manner as Fe, Al, Th, U, Na, etc. Ti.sup.4+ +H.sub.2 O.sub.2 .fwdarw.Ti(H.sub.2 O.sub.2).sup.4+ The concentration of hydrogen peroxide during the hydrogen peroxide treatment is desirably higher than 0.05% by weight and, preferably, higher than 0.1% by weight based on the acidic silica sol after preparation considering the reaction rate between Ti and hydrogen peroxide. Further, in the case of applying the hydrogen peroxide treatment before preparation of the acidic silica sol, it is higher than 0.05% by weight and, preferably, higher than 0.1% by weight based on the total weight of the aqueous solution of alkali silicate and acidic solution due to the same reason. While on the other hand, if the concentration of hydrogen peroxide in the hydrogen peroxide treatment exceeds 1.0% by weight, gas bubbles are evolved in the column in the purifying step described next, which hinder the contact between the cationic exchange resin and the acidic silica sol in the column, to reduce the exchanging capability of the cationic exchange resin thereby taking much time for the purification step of the acidic silica sol and, accordingly, the concentration of hydrogen peroxide is desirably lower than 1.0% by weight. Then, for converting the acidic silica sol into a non-gelling stable sol, the acidic silica sol, after pH adjustment 0-5 by the addition of sodium hydroxide, potassium hydroxide, etc., is poured into a column filled with cationic exchange resins for purification to remove impurities contained in the silica sol such as Fe, Al, Th, U, Na, Ti, etc, resulting in an acidic silica sol of high purity. Then, silica powder of high purity is obtained from the thus purified acidic silica sol by the usual method discussed below. That is, an aqueous alkali solution such as of sodium hydroxide is added to the acidic silica sol for neutralization to prepare a neutral or weakly alkaline silica sol, and the neutral or weakly alkaline silica sol is added to an aqueous solution of ammonium salt adjusted to pH 8-9 with a pH controller to obtain silica precipitates. In this case, as the ammonium salt mentioned above, there can be used an inorganic acid salt of ammonium such as ammonium chloride, ammonium nitrate, ammonium carbonate and ammonium sulfate, or an organic acid salt of ammonium such as ammonium formate, ammonium acetate and ammonium oxalate. Then, the precipitates are separated by filtration, washed with water and then with acid, separated by filtration again and washed with water to obtain filter cakes of silica. Then, after drying the cakes, heat treatment is applied at a temperature of about 1000.degree. C. in an electric furnace to obtain silica powder of high purity. In this way, according to this invention, since the acidic silica sol treated with hydrogen peroxide is purified with the cationic exchange resin and the adsorption performance through ion exchange of Ti contained in the acidic silica sol to the exchange resin is increased, Ti can be removed from the acidic silica sol to the same extent as those impurities such as Fe, Al. Accordingly, since the purity can thus be improved, the silica can be used as the starting material for fillers in the resin composition for sealing electronic components, and as starting material of multi-ingredient optical fibers, fine ceramics, optical glasses and quartz for use in the electronic industry. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT |
| PATENT EXAMPLES | Available on request |
| PATENT PHOTOCOPY | Available on request |
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