PROCESS FOR THE PREPARATION OF BORIC ACID AS WELL AS PROCESS FOR THE PREPARATION OF SILICA USING BORIC ACID THUS OBTAINED

Abstract: Boric acid is prepared by reaction of a fluoroboric acid solution at elevated temperature with calcium carbonate and separation of the calcium fluoride precipitate formed from the remaining solution of boric acid. The fluoroboric acid solution used is preferably obtained by conversion of hexafluorosilicic acid with boric acid (formed from fluorobor­ ic acid) and separation of the preciptated silica. Highly pure silica and valuable calcium fluoride can be prepared in a cyciic process using by-product hexafluorosili­ cic acid and cheap calcium carbonate.

Description: PROCESS FOR THE PREPARATION OF BORIC ACID AS WELL AS PROCESS FOR THE PREPARATION OF SILICA USING BORIC ACID THUS OBTAINED The invention relates to a process for the preparation of boric acid by reaction of a boron and fluorine containing compound with a calcium compound, as well as a process for the preparation of pure silica starting from hexafluorosilicic acid by converting this using a boric acid thus obtained. It is known (see Zeitschrift fr Anorganische und Allgemeine Chemie, Band 300 (1959) pp. 152-158)that when boron trifluoride is mixed with calcium chloride dihydrate in an aqueous environment a reaction takes place in which boric acid is formed besides calcium fluoride and hydrochloric acid. This known conversion is applied as analytical method for boron trifluoride. For technical preparation of boric acid, however, this known method is not suitable, particularly because the separation of boric acid from such a reaction mixture is a highly laborious operation. The reason is that, as has beenfound, the calcium fluoride formed is present in the form of an extremely fine precipitate, which can practically not be separated off (for instance by filtration). The separation of boric acid from a mixture containing hydrochloric acid also is a technically difficult method. Furthermore, such a reaction mixture is highly corrosive, leading to high costs of the separation equipment to be used. Finally, this known process has the disadvantage that the cost prices of the reactants used are relatively high. The present invention provides a process for the preparation of boric acid in which the above-mentioned disadvantages are wholly or to a large extent eliminated. According to the invention this is achieved by reacting an aqueous solution of fluoroboric acid with calcium carbonate at elevated temperature and separating the resultant precipitate of calcium fluoride from the remaining aqueous boric acid solution. The reaction is preferably carried out at a temperature of between 50 and 90 OC, for when lower temperatures are used the reaction proceeds extremely slowly, whereas at higher temperatures fluoroboric acid decomr poses into gaseous borontrifluoride and hydrofluoric acid. In the present process an aqueous solution of fluoroboric acid (= RBF4) is used as B and F containing reactant. The concentration of the HBF4 solution used does not appear to be critical. In general a 5-45 wt.-%, in particular a 10-20 wt.-% RBF4 solution is used. As calcium compound, according to the invention use is made of calcium carbonate. In principle this calcium carbonate can be added in solid form. By preference a 10-30 wt.-Z suspension of calcium carbonate in water is added, because it has been found that this yields a virtually quantitative conversion at a relatively short reaction time. Though it is preferred to use calcium carbonate in the present process, in principle use may also be made of calcium hydroxide. The amounts of fluoroboric acid and calcium carbonate may vary. By preference approximately stoichiometric amounts are used, for when using-a-large-excess o± fluoroboric acid a-proportion of non-converted fluoroboric acid remains, and it is rather difficult to separate this from the reaction mixture, and when a large excess of calcium carbonate is used, insoluble calcium borates ar formed. The calcium fluoride precipitate formed in the process can be separated from the liquid reaction mixture in various ways, for instance by centrifuging or decanting. By preference the calcium fluoride is separated by filtering, in particular by vacuum filtration. It has been found that the filtration of the precipitate, which has a relatively fine grain size, can be improved substantially if a small amount of a nonionogenic flocculant is added to the reaction mixture. Suitable flocculants include polyacrylamides, polyacrylonitriles, copo lymers of acrylamides or acrylonitrile with acrylates or vinyl esters. As is common practice, the flocculant is added to the reaction mixture in the form of an aqueous solution, for instance a 0.1-5 wt.-Z solution. The amount of flocculant to be added may vary within broad limits, for instance from 0.001 to 0.04 wt.-Z, relative to the total weight of the reaction mixture. The calcium fluoride separated off can be processed further in a way known per se, for instance by washing and drying. The product can be sold as such or be applied, for instance, as raw material for the preparation of hydrofluoric acid or other fluorine compounds. The liquid reaction mixture remaining after separation of the calcium fluoride consists practically quantitatively of an aqueous solution of boric acid. From this, solid boric acid can be recovered in a way known per se by evaporation of the liquid. Of course it is also possible to recover the boric acid as a dilute or concentrated solution. Though the present process can be generally applied for converting fluoroboric acid solutions into boric acid, it is preferred to use a fluoroboric acid solution that is obtained as by-product in the preparation of silica by converting a solution of hexafluorosilicic acid with boric acid and separating the precipitated silica. Such a silica preparation, which is described, for instance, in US patent 2,799,559, is very attractive from a technical and economic point of view, because starting from the cheap, impure hexafluorosilicic acid, which is formed as by-product in the phosphoric acid and phosphate industry, a very pure, high-value silica product is obtained. The fluoroboric acid solution obtained as by-product in such a synthesis can according to the invention be converted into boric acid and calcium fluoride using calcium carbonate, and the boric acid thus formed can advantageously be utilized for the conversion of hexafluorosilicic acid. In this way it is possible, starting from the by-product hexafluorosilicic acid, to prepare a very pure silica in a cyclic process, the required boric acid being regenerated in situ and, in addition, a cheap calcium carbonate being converted into the relatively high-value calcium fluoride. The reactions occurring in such a cyclic process can be represented by the following reaction equations: a) 2H2SiF6 + 3 H3B03 + 3 HBF4 + 2 Si02 + 5 R20 b) 3 BBF4 + 6 CaC03 + 3 R20 + 3 H3B03 + 6 CaF2 + 6 C02 This results in the following gross reaction equation: c) 2 H2SiF6 + 6 CaC03 + 2Si02 + 6 CaF2 + 6 CO2 + 2 R2 The silica obtained from raw, contaminated hexafluorosilicic acid in such a process appears to be very pure. Moreover, it has been found that a product is obtained with a very low content of metals if the silica is washed with a dilute fluoroboric acid solution. The wash liquor thus obtained can without any problems be added to the fluoroboric acid solution obtained in the conversion of R2SiF6, upon which the mixture is processed into boric acid and calcium fluoride in accordance with the invention. When the process is carried out continuously it will of course be necessary to drain part of the metal-containing liquid at regular inter vals so as to avoid metal accumulation. The invention will be elucidated in the following examples, without being restricted thereto. Example I 500 g of a 15 wt.-% RBF4 solution having a temperature of about 80 "C was fed into a stirred reactor. Subsequently, 850 g of a 20 wt.-Z suspension of CaC03 in water was added to the EBF4 solution in approximately 2 hours' time, use being made of a peristaltic pump, after which 10.5 ml of a 0.2 wt.-Z solution of a nonionogenic flocculant was supplied to the reaction mixture, which was meanwhile being stirred. As -Elocculant use was made of a product marketed under the name Rercofloc 527 N by Rercules. The reaction mixture formed was subsequently vacuum-filtered through a glass filter and the filter cake was dried at 150 OC. Via Xray diffraction analysis it was demonstrated that the precipitate thus separated consisted practically quantitatively of CaF2. The remaining filtrate was evaporated to dryness with the aid of a rotating film evaporator, the yield being 51.7 g of solid residue. X-ray diffraction analysis showed that the residue consisted of pure boric acid. The process efficiency was 98.5 Z. Example II 480 gthour of an R2SiF6 solution with a concentration of 18 wt.-Z and 50 g/hour of solid H3B03 were continuously and simultaneously fed to a stirred reactor. The H2SiF6 solution had been obtained by diluting a raw 35.8 wt.-Z H2SiF6 solution from the evaporation section of a phosphate processing plant with demineralized water. The dilute solution had a fluorine and P205 content of 14.4 wt.-% and 0.23 wt.-Z respectively and further contained, besides 330 ppm chlorine, substantial amounts of metals, such as 30 ppm Na, 223 ppm Ca, 64 ppm Fe, 16 ppm As. As boric acid use was made of the evaporated product obtained in example I. The residence time of the reactants in the reactor was about 1 hour, while the temperature was kept between about 80 and 85 "C. The reaction mixture formed was subsequently filtered through a glass filter. As filtrate an approximately 8.1 wt.-% HBF4 solution was obtained, which could be concentrated with a rotating evaporator to a 34.5 wt.-% solution (temperature 40 OC) and a 44.5 wt.-Z solution (temperature 80 "C). Part of the filter cake obtained upon filtration, which contained 53.1 wt.-% moisture, was dried at 150 OC. Via X-ray diffraction analysis the dried cake was demonstrated to consist of pure, amorphous SiO2. The product was subsequently analysed. The results are summarized in Table I. The remaining part of the filter cake was subjected to eleven lashings on the filter, use beingemade of demineralized water, and was subsequently dried at 150 OC. The washed product was also analysed. The results are summarized in Table I. Example III The process of Example II was repeated, but this time the filter cake on the filter was first washed four times with a 1 wt.-% HBF4 solution and subsequently eight times with demineralized water. The wash liquors were added to the filtrate, resulting in a decrease of the latter's concentration to about 7.2 wt.-% HBF4. The washed product was analysed. The results are also summarized in Table I. Table I Unwashed product Washed product Washed product Example II Example III Copper 1.0 ppm 0.310 ppm < 0.05 ppm Nickel 2.9 ppm 0.020 ppm 40.02 ppm Chromium 6.0 ppm 0.014 ppm 0.003 ppm Cobalt 0.02 ppm sic 0.005 ppm a: 0.001 ppm Manganese 0.7 ppm 0.019 ppm < 0.019 ppm Vanadium 0.1 ppm < 0.1 ppm < 0.01 ppm Iron 41 ppm 2 ppm 0.53 ppm Example IV The process of Example II was repeated, but this time a 37 wt.-t H2SiF6 solution was used. The SiO2 obtained was practically equal to that of Example II. As filtrate a 14.7 wt.-Z HBF4 solution was obtained.

Claims: CLAIMS 1. Process for the preparation of boric acid by reaction of a boron and fluorine containing compound with a calcium compound, characterized in that an aqueous solution of fluoroboric acid is reacted with calcium carbonate at elevated temperature and the resultant calcium fluoride precipitate is separated from the remaining aqueous solution of boric acid. 2. Process according to claim 1, characterized in that the reaction is carried out at a temperature of between 50 and 90 "C. 3. Process according to claim 1 or 2, characterized in that the calcium carbonate-is added as a 10-30 wt.-% suspension. 4. Process according to any of claims 1-3, characterized in that the precipitated calcium fluoride is separated by vacuum filtration. 5. Process according to claim 4, characterized in that a nonionogenic flocculant is added to the reaction mixture prior to filtration. 6. Process according to any one of claims 1-5, characterized in that solid boric acid is recovered by evaporation of the aqueous reaction mixture that- remains after separation of calcium fluoride. 7. Process according to any of claims 1-6, characterized in that an aqueous solution of fluoroboric acid is used, which is obtained by conversion of a solution of hexafluorosilicic acid with boric acid formed from the fluoroboric acid and separation of the precipitated silica. 8. Process according to claim 7, characterized in that the separated silica precipitate is washed with a solution of fluoroboric acid and the resultant acid wash liquor Is added to the aqueous fluoroboric acid solution to be converted.

Inventors: SPIJKER, RUUD
Applicants: STAMICARBON B.V.
Application year: 1983
Main classification: C01B35
IPC codes: C01B35, C01F11, C01B33