Process for production of a highly pure silicic acid aqueous solution

Abstract: A process is disclosed for removing non-alkali metals from an acidic silicate solution or suspension, the process comprising subjecting the solution or suspension to an electrochemical process in the presence of a strong acid and an oxidising agent such that non-alkali metals present form water-soluble salts, and ion-exchanging the solution. The acidic silicate solution or suspension may be generated by ion-exchanging an aqueous alkali silicate solution to remove alkali metals. The invention therefore enables a silicic acid aqueous solution having a low content of metal impurities, for example 1 ppm or less, to be prepared from an alkali metal silicate of low purity.

Description: The present invention relates to a process for the production of a highly pure silicic acid aqueous solution from a low purity alkali silicate aqueous solution. More particularly, it relates to a process for producing a highly pure silicic acid aqueous solution. Considering the fact that low purity alkali silicates have been thought to be unsuitable as starting materials for silica sources of an artificial quartz or an optical fiber, it is surprising that the present invention can use a low purity silicate for such purposes. Silica sources for an artificial quartz or an optical fiber have been produced by pyrolysis of purified silicon tetrachloride or by hydrolysis of purified tetraethyl silicate. Though the silicon tetrachloride and tetraethyl silicate have high purities, they are very expensive and difficult to handle because of their corrosiveness and flammability. Japanese Patent Laid Open No. Sho. 63-21212 discloses a process for producing silica comprising, ion exchanging aqueous solution with cation-exchange resin to get a silicic acid aqueous solution, adding an acid to the solution, ion exhanging the solution with a cation-exchange resin, and pouring the thus treated solution into an ammonium-containing alkaline solution to precipitate silica. Japanese Patent Laid Open No. Sho. 60-191016 also discloses a process for producing precipitated silica by adding a strong acid to an alkali silicate to obtain a free acid concentration of 1 N or more, and precipitating silica at a high temperature from 70°C to 90°C. Japanese Patents Laid Open No. Sho. 60-2041613 and Japanese Patents Laid Open No. Sho. 60-2041614 also disclose processes which comprise producing silica gel from a concentrated alkali silicate aqueous solution by simply rinsing it, and heating the gel in a strong acid repeatedly. These prior processes, however, hardly provide a highly pure silica having metal impurity content of 1 ppm or less because they use low purity alkali silicates as starting materials. Consequently, a highly pure (low impurity level) alkali silicate aqueous solution must be used as a starting material to produce a highly pure silica. Furthermore electrolytic methods of purifying agueous silicic acid solutions are known using a mobile mercury electrode (see CA, 74, no. 10, 1971, 259 no. 45983s) or a membrane cell equipped with iron electrodes (see CA, 106, no. 8, 1987, 144, no 52686e). Moreover US-A-4 857 290 discloses a process of purifying silica sol comprising the steps of a treatment with hydrogen peroxide and a step of ion exchanging. Non-alkali metal impurities exist as silicate complexes in an alkali silicate solution. The complexes are very stable at ordinary temperature; they hardly dissolve in water except when a large amounts of hydrochloric acid is added. When a hydrochloric acid is added, non-alkali metals form water soluble metal chlorides. The previously referred Japanese Patent Laid Open No. Sho. 60-191016 uses this reaction. According to this invention, large amounts of hydrochloric acid are added to an alkali silicate solution ( silica content, 10% by weight or less ) until the solution turns acidic, then the solution is contacted with the ion-exchange resin to remove metal impurities. The metal impurities are removed very efficiently. In the referred processes, silicic acid in a silicate solution under acidic condition forms oligomer or gel in a short time. Once non-alkali metals are captured in silicon oxide oligomer or gel, they hardly dissolve into the aqueous solution. This is the reason why the prior patents failed to obtain non-alkali metal-content of 1 ppm or less. The inventors of the present application have studied the removal of non-alkali metals which exist as complexes with silicic acid oligomers, and have discovered an effective method for removing the metals. The method comprises an electrochemical process which serves to change the metal complexes to water-soluble metallic salts. Once the metallic salts are formed, they are easily removed by ion-exchanging. It is an objection of the present invention to provide a silica source for highly pure silica material. The invention therefore provides a method for removing metal impurities from an alkali silicate solution used as a cheap starting material, and enables a silicic acid aqueous solution having a low content of metal impurities to be prepared from an alkali metal silicate containing metal impurities. The gist of the present invention is to remove alkali metals in an alkali silicate aqueous solution by ion-exchanging, to apply an electrochemical process to the solution in the presence of strong acids and oxidizing agents to form water-soluble salts of non-alkali metals, and to remove the non-alkali metals in solution by ion-exchanging. The application of an electrochemical process to the silicic acid aqueous solution serves to shorten reaction time for metal elements to form water soluble salts, thus metal atoms can be easily removed by ion-exchanging before the silicic acid aqueous solution forms a gel. According to the present invention, commercially available JIS(Japanese Industrial Standard)water glass is preferably used as a cheap starting material, however, any alkali silicate aqueous solution may be used as a starting material. According to the present invention, diluted alkali silicate aqueous solutions having silica content of 1 to 10% by weight are preferably used. In the process of the invention, a silicic acid aqueous solution is first produced by ion-exchanging an alkali silicate aqueous solution to remove alkali metals. Then the electrochemical process is applied to the solution. The term "electrochemical process " in the present invention means a process comprising inserting a pair of electrodes into the silicic acid aqueous solution, and applying an electrical voltage to the electrodes. On the surfaces of electrodes, reduction and oxidation of the metals, existing as silicate compounds in the aqueous solution, occur, thus water-soluble salts are formed. Preferable electrodes are platinum, gold, and gold-platinum alloy electrode, however, any other electrode may be used in the present invention. Especially preferable electrode is a platinum electrode having high surface area, produced by treating the platinum electrode in an aqueous solution of PtCl₄-complex salt such as Na₂(PtCl₄) or K₂(PtCl₄), until the surface of platinum electrode becomes rough. The gold electrode treated similarly may be preferably used. Both direct and alternating currents are used, however, the alternating current is preferable because hydrogen generates when the direct current is used. Voltage of from 0.1 V to 100 V is preferable. The alternating current of from 1 Hz to 1MHz frequency, preferably from 0.1 KHz to 10 KHz frequency is preferable. Further, the alternating current having rectangular wave form is preferable because it has a higher energy density. The silicic acid aqueous solution, to which the electrochemical process is applied, must be acidic, and strong acids having high electronegativity must exist in order to form water-soluble metalic salts. Accordingly, the strong acid should be added previous to the electrolysis reaction. Examples of strong acid are hydrochloric acid, nitric acid and hydrobromic acid. The acid content of 0.01 to 1 % by weight is preferable. The acid is added so as to get a pH of below 7, preferably a pH of 1 to 2 where longer gel time of silicic acid aqueous solution is obtained. According to the present invention, 0.01 to 1 % by weight of oxidizing agents must be added to silicic acid aqueous solution to accelerate oxidation reaction of metals. Preferred oxidizing agents are hydrogen peroxide aqueous solution, hypochlorous acid or nitrous acid, or a combination thereof. If the oxidizing agent is not added, reduction and oxidation reaction of metals is delayed, and gelation of silicic acid proceeds faster than oxidation reaction. Metal atoms forming water soluble salts during the electorochemical process thereafter can be easily removed from the silicic acid aqueous solution by ion-exchanging. However, if gelation of silicic acid occurs, ion exchanging becomes impossible. Consequently, it is preferable to ion exchange the sol while the electrochemical reaction proceeds. More particularly, after the silicic acid aqueous solution is electrochemically treated it is passed through an ion-exchange column, and again the solution is electrochemically treated and thus treated solution is passed through the column. In order to reduce impurity content, it is preferable to repeat the electrochemical process and the ion-exchange process. A small ion-exchange column suffices for the above ion-exchange steps because the amount of metal atoms to be exchanged is very small. The present invention provides a process for producing a silicic acid aqueous solution having a low metal content comprising the steps of: removing alkali metals in the alkali silicate aqueous solution by ion exchanging to obtain the silicic acid aqueous solution, adding the strong acid and the oxidizing agent to the solution. applying the electrochemical process to the solution to form water soluble salts of non-alkali metals existing in water as silicate compound, and ion-exchanging thus treated solution to remove metal atoms existing as water soluble salts. A metal content of 1 ppm or less may be achieved by this process. The results in Table 1 of the Comparative Example which follows clearly show that a process of the invention comprising ion exchanging an alkali silicate aqueous solution to obtain a silicic acid aqueous solution, adding a strong acid and an oxidizing agent to the silicic acid aqueous solution, applying an electrochemical process to the solution to form water soluble salts of non-alkali metals which dissolve in water as silicate compounds, and ion exchanging the solution to remove non-alkali metals, is an excellent process for eliminating metals from sodium silicate. Removing metal elements from an alkali silicate aqueous solution to a metal content of 1 ppm or less has been thought to be impossible. However, according to the present invention, metal elements in an alkali silicate aqueous solution can be removed very easily and effectively to a metal content of 1 ppm or less. Highly pure silica can be produced from the low metal-content-silicic acid aqueous solution, produced thus by adding a large amount of ammonium salt and organic alkali to the silicic acid aqueous solution to precipitate silica, and rinsing precipitates thus formed with pure water. Further, highly pure colloidal silica can be produced from the silicic acid aqueous solution of the present invention. Highly pure silica produced according to the present invention may be used, for example, in various kinds of fillers, fillers for semiconductors, and for quartz glass. Fig. 1 is a schematic view illustrating an apparatus used for removing metals in an alkali silicate aqueous solution in accordance with the present invention. In Fig. 1, 1 is an electrolytic cell, 2 is an ion-exchange tower filled with 2000 ml of Diaion SK1B, and 3 is a storage container, and each of 1,2, and 3 is connected with pipes and a pump. The ion-exchange tower 2 is provided with an inlet 5 for distilled water and hydrochloric acid. An outlet 6 for sample picking-up is provided between the ion-exchange tower 2 and the storage container 3. The following examples will further illustrate the present invention. In the examples, Fe and Al contents per silica were measured by a Inductively Coupled Plasma Atomic Emission Spectroscopy. Example 1 A commercially available concentrated sodium silicate solution (NIPPON Chemical Industries Co. Ltd. JIS No. 3 sodium silicate) was diluted to silica content of 5% by weight. The solution thus diluted was passed through an activated cation-ion-exchange resin (Diaion SK1B) bed to remove sodium in sodium silicate. To this solution, hydrochloric acid and hydrogen peroxide aqueous solution were added to obtain a solution having silica content by 4% by weight, hydrochloric acid content of 0.1% by weight, and hydrogen peroxide aqueous solution content of 0.1% by weight. The pH value of solution was 0.5, and the gelation time was about 72 hours at room temperature. Then the solution of 2000 cm³ was taken into a polypropylene container, and was electrochemically treated in the apparatus shown in Fig. 1. Referring to Figure 1, a silicic acid aqueous solution was pumped to circulate the solution through the electrolytic cell 1, the ion-exchange tower 2, and the storage container 3. A pair of electrodes (golden plates, 20 cm²) were placed at distance of 2 cm to let the solution flow between the electrodes. An alternating current having a rectangular wave form and frequency of 1 KHz, voltage of 2 V was applied between the electrodes. The feed rate adjusted to obtain a linear speed in the tower of 2 cm/min. The circulation was continued for totally 24 hours. Table 1 shows circulation time and Fe and Al contents per silica in the silicic acid aqueous solution. Example 2 A purification process as described in Example 1 was repeated except that platinum electrodes were used instead of gold electorodes. The circulation was continued for 24 hours. Fe and Al contents per silica in the silicic acid aqueous solution, after the circulation, are shown in Table 1. Example 3 The purification process as described in Example 1 was repeated except that the alternative current frequency of 100 Hz was used. The circulation was continued for 24 hours. Fe and Al contents per silica in the silicic acid aqueous solution, after the circulation, are shown in Table 1. Comparative Example The commercially available concentrated sodium silicate solution (Nippon Chemical Industries Co., Ltd. JIS No.3 sodium silicate) was diluted to a silica content of 5% by weight. The aqueous solution thus obtained was passed through the activated cation ion-exchange resin (Daiion SK1B) to remove sodium in sodium silicate. The silicic acid aqueous solution thus obtained was treated with the ion-exchange resin two times. Fe and Al contents per silica in the silicic acid 25 aqueous solution, after ion exchanging, are shown in Table 1. Example Circulation time(hour) Fe(ppm) Al(ppm) 1 0 124 406 1 20 40 4 2.5 4 12 1 or less 1 24 1 or less 1 or less 2 0 124 406 1 30 60 4 10 15 12 1 or less 2 24 1 or less 1 or less 3 0 124 406 1 24 48 4 8 13 12 1 or less 2 24 1 or less 1 or less Comparative Example number of ion exchange times Fe(ppm) Al(ppm) 0 124 406 1 54 198 2 48 198 3 48 198

Claims: A process for preparing a silicic acid aqueous solution, the process comprising: removing alkali metals from a 1% to 10% by weight alkali silicate aqueous solution by ion exchange, applying an electrochemical process to the resulting solution in the presence of 0.01 to 1% by weight of a strong acid selected from hydrochloric acid, nitric acid, hydrobromic acid and mixtures thereof, and from 0.01 to 1% by weight of an oxidising agent selected from a hydrogen peroxide aqueous solution, hypochlorous acid, nitrous acid and mixtures thereof, and removing non-alkali metals which are present as water soluble salts in the solution by ion-exchange. A process according to claim 1 where in the resulting silicic acid aqueous solution has a metal content of 2 ppm or less. A process according to claim 1 or 2 wherein the resulting silicic acid aqueous solution has a metal content of 1 ppm or less. A process according to any one of the preceding claims wherein the electrochemical process comprises applying a voltage of from 0.1 V to 100 V, the voltage being either direct current or an alternating current having a frequency of from 10Hz to 1MHz. A process according to any one of the preceding claims wherein the ion-exchange resin used in ion-exchanging is of the strong acid type, the weak acid type, or of the mixed-acids type. A process according to anyone of the preceding claims which includes, after removal of the non-alkali metals, the additional step of precipitating silica from the solution and, optionally, rinsing the silica with water.

Inventors: KOJIMA MIHO MITSUBISHI MONSANT, SASAKI SHIGEO MITSUBISHI MONSA, KOJIMA, MIHO, MITSUBISHI MONSANTO CHEMICAL CO., SASAKI, SHIGEO, MITSUBISHI MONSANTO CHEMICAL CO.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1990
Main classification: C01B33
IPC codes: C01B33