PROCESS FOR THE PREPARATION OF PURE SILICA

Abstract: Pure silica is prepared by adding to a hexafluorosilicic acid solution a minor amount of an aminopolycarboxylic acid or a salt thereof, contacting the mixture with an ammonium hydroxide solution and separating off the silica precipitate formed. The silica produced is suitable as catalyst support or as substitute for iota quartz or mountain crystal.

Description: PROCESS FOR THE PREPARATION OF PURE SILICA The invention relates to a process for the preparation of pure silica by conversion of a solution of hexafluorosilicic acid with an ammonium hydroxide solution and separation of the silica precipitate thus formed. Such a conversion is known from, for instance, United States Patents 3,271,107 and 4.026.997. A disadvantage of these known processes is that the silica obtained is severely contaminated. Thus, in par ticular the metal content is rather high. The product thus obtained therefore is applicable to only few technical purposes, for instance as additive for paint and rubber products. The invention now provides a process allowing the preparation of a very pure silica on the basis of solutions of hexafluorosilicic acid that may even be strongly contaminated. According to the invention this is achieved by adding a small amount of an aminopolycarboxylic acid or a salt thereof to the hexafluorosilicic acid before contacting the latter with the ammonium hydroxide solution. The aminopolycarboxylic acids or salts thereof that can be applied include inter alia ethylene diaminotetra-acetic acid, nitrilotriacetic acid, diethylene triaminopenta-acetic acid or hydroxyethylethylenediaminotriacetic acid or the potassium, sodium or lithium salts of these. Particularly suitable were found to be ethylenediaminetetra-acetic acid or alkali metal salts thereof, especially the 1,2-disodium salt. In the subject process the aminopolycarboxylic acid, respectively the salts thereof, may be added in the form of an aqueous solution, for instance as a 1-10 wt.-% solution. It is, however, preferred to add a solid aminopolycarboxylic acid or salt thereof, so that dilution of the reaction medium is avoided. The amount of aminopolycarboxylic acid or salt thereof that is to be added may vary within wide limits but is, of course, determined by the degree.of which the hexafluorosilicic acid used is contaminated. In general an amount is applied that stoichiometrically is at least equal to the amount of multivalent metals present in the hexafluorosilicic acid. By preference the aminopolycarboxylic acid or the salt thereof is added in an amount of 1.5-15 gram molecules per gram atom of multivalent metals present in the hexafluorosilicic acid. The application of aminopolycarboxylic acids and alkali metal salts for sequestration of multivalent metals in solution is known as such. It is also known, however, that the resultant complexes with these metals decompose in acid environments, so that these compounds, which are called sequestering, chelating or complexing agents, are applied practically only in neutral and basic environments. It has now been found that when the aminopolycarboxylic acids are added to the highly acid hexafluorosilicic acid, after ammoniation thereof a silica precipitate is obtained which has a substantially lower heavy metals content than a silica obtained in a similar conversion in which the aminopolycar- boxylic acid is added to the wholly or partly neutralized hexafluorosilicic acid solution. In the process according to the invention the concentrations of the hexafluorosilicic acid solution used and the ammonium hydroxide solution, as well as the amounts used of these reactants and the temperature applied, may be varied within wide limits. For instance, use is made of a hexafluorosilicic acid solution having a concentration of at most 15 wt.-%, an NH40H solution having a concentration of 15-35 wt.-Z, and a temperature of between 25 and 45 OC, as is described in the European patent application 94.138. From the reaction mixture formed in the conversion, which consists mainly of an ammonium fluoride solution with solid silica, the silica is separated, for instance by filtration, decantation or centrifugation. The separated product can, if desired, be further processed in a known way, for instance by washing and/or calcining. The product consists practically quantitatively of silica and is suitable, for instance, for use as catalyst support or as substitute of, for instance, iota quartz or mountain crystal. The reaction liquid remaining after separation of silica, which consists mainly of an ammonium fluoride solution, may be upgraded, for instance by concentrating it, into technical-grade ammonium fluoride, or it may be converted in, for instance, alkali metal fluorides, aluminium fluorides or hydrofluoric acid. It has been found that the metals present are fully complexed and may be regarded as inert when, or instance, the solution is discharged or processed. The invention will be elucidated in the following examples, without being restricted thereto. Example I An amount of 0.5 1 of an H2SiF6 solution with a concentration of about 10 wt.-X was introduced into a 3-litre reaction vessel equipped with an agitator and a cooler. This solution had been prepared by diluting a raw, approx. 29.5 wt.-X H2SiF6 solution, obtained by washing Si and F containing off-gases from a phosphoric acid plant, with water. While the solution was being stirred at a stirring speed of 200 revolu tions per minute, 0.84 g solid 1,2-disodiumethylene diaminotetra-acetic acid was added to it, the temperature meanwhile being maintained at about 32 OC. While stirring was continued and the temperature was kept constant, a 25 wt.-Z ammonium hydroxide solution was injected just subcutaneous of the liquid surface, the metering rate being 16.25 cm3 NH40E solution per minute. Metering was continued until the pH in the reaction mixture was approximately 9.0. The reaction mixture was subsequently filtered through a polypropylene filter; the filter cake thus obtained (2 g/minute) was washed with distilled water and subsequently air-dried at 120 "C. The product was analysed using atom spectrometry. The results are summarized in Table I. Example II The process of Example I was repeated, but now 0.17 g solid 1,2-disodiumethylene diaminotetra-acetic acid was added. The analytical results are also summarized in Table I. Comparative example A The process of Example I was repeated, but now no 1,2-disodiumethylene diaminotetra-acetic acid was added. The analytical results are also summarized in Table I. Table I Comparative Example II Example I example A Sodium 40 ppm 23 ppm 14 ppm Potassium 30 ppm 8 ppm 4 ppm Lithium 0.1 ppm < 0.1 ppm < 0.1 ppm Copper 3 ppm 2 ppm < 2 ppm Iron 30 ppm 22 ppm 2 ppm Nickel 4 ppm < 1 ppm < 1 ppm Calcium 600 ppm 14 ppm 3 ppm Magnesium 30 ppm 2 ppm 2 ppm Aluminium 15 ppm 14 ppm 4 ppm Phosphorus 50 ppm < 10 ppm < 10 ppm Comparative example B The process of Example I was repeated, but now the 1,2-disodiumethylene diaminotetra-acetic acid was added to the neutralized reaction mixture at pR 9.0. On analysis it was found that the metal content of the silica obtained was substantially higher than in Example I. In particular the concentrations of iron, aluminium, calcium and magnesium were only slightly lower than in the blank test of com- parative example A. Example III The process of Example I was repeated, but now the filtered product, after having been washed with water, was four times washed with a 16 wt.-Z HN03 solution at about 80 OC and was subsequently afterwashed with distilled water and dried at 120 OC. On analysis it was found that the Na content had dropped to 2 ppm, the Ca content to C 1 ppm, the Mg content to about 1 ppm and the Al content to C 1 ppm. The other results were virtually the same as those of Example I.

Claims: CLAIMS 1. Process for the preparation of pure silica by converting a solution of hexafluorosilicic acid with an ammonium hydroxide solution and separating the silica precipitate formed, characterized in that a small amount of an aminopolycarboxylic acid or a salt thereof is added to the hexafluorosilicic acid solution prior to contacting the latter with the ammonium hydroxide solution. 2. Process according to claim 1, characterized in that ethylene diaminotetra-acetic acid or an alkali metal salt thereof is added to the hexafluorosilicic acid solution. 3. Process according to claim 1 or 2, characterized in that the 1,2-disodium salt of ethylene diaminotetra-acetic acid is added. 4. Process according to any one of claims 1-3, characterized in that the aminopolycarboxylic acid or salt thereof is added in an amount of 1.5-15 gram molecule per gram atom of multivalent metals present in the hexafluorosilicic acid. 5. Process according to any one of claims 1-4, characterized in that the aminopolycarboxylic acid or salt thereof is added in solid form.

Inventors: VAN MONTFOORT, ABRAHAM, WETERINGS, CORNELIS ANTONIUS MARIA
Applicants: STAMICARBON B.V.
Application year: 1983
Main classification: C01B33
IPC codes: C01C1, C01B33