United States Patent (19) (11) 4,061,723 Feser Et Al
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United States Patent (19) (11) 4,061,723 Feser et al. 45) Dec. 6, 1977 (54 PROCESS FOR PREPARING 58) Field of Search ........................ 423/276, 240, 293 CONCENTRATED TETRAFLUOROBORIC (56) References Cited ACD U.S. PATENT DOCUMENTS Inventors: Manfred Feser, Frankfurt am Main; (75) 2,526,777 10/1950 Smith et al. ..................... 423/240 X Egon Joerchel, Hochheim, Main; 3,353,911 1 1/1967 Moller et al. .................... 423/240 X Jirgen Korinth, Hofheim, Taunus, all 3,920,825 1 1/1975 Becher et al. ........................ 423/276 of Germany 3,995,005 1 1/i976 Teller ............................... 423/240 X (73) Assignee: Hoechst Aktiengesellschaft, Primary Examiner-G. O. Peters Frankfurt am Main, Germany Attorney, Agent, or Firm-Curtis, Morris & Safford (21) Appl. No.: 741,399 57 ABSTRACT 22 Filed: Nov. 12, 1976 Process for the preparation of a concentrated tetra Foreign Application Priority Data fluoroboric acid from boron/oxygen compounds and (30) hydrogen fluoride by reacting a solid boron/oxygen Nov. 15, 1975 Germany ............................. 255433 compound with a hydrogen fluoride-containing gas. (51) Int. Cl’.............................................. CO1B 35/06 (52) U.S.C. .................................................... 423/276 10 Claims, 2 Drawing Figures U.S. Patent Dec. 6, 1977 Sheet 1 of 2 4,061,723 U.S. Patent Dec. 6, 1977 Sheet 2 of 2 4,061,723 u1 4,061,723 2 of hydrogen fluoride of the gases used is not critical for PROCESS FOR PREPARING CONCENTRATED the reaction. It may be in the range of 0.03 to 100% by TETRAFLUOROBORCACD volume. Surprisingly, the quantities of heat set free may readily be dissipated. Up to a content of 20% by volume The present invention relates to the preparation of 5 of HF the heat set free is absorbed by the residual gas concentrated tetrafluoroboric acid. and dissipated. The process may, consequently, be car Tetrafluoroboric acid is generally prepared by react ried out in especially good manner with contents of ing hydrofluoric acid of 40 to 50% strength with a hydrogen fluoride of this order of magnitude, especially slight excess over the calculated quantity of boric acid of about 0.05 to 10% by volume. External cooling with while intensively cooling according to the reaction 10 water is indicated for contents of hydrogen fluoride of equation: more than 20% by volume. w The form of the reaction vessel is not critical. Tubu HBO + 4 HF - HBF + 4 HO lar reactors are especially suitable. Horizontally ar The crude tetrafluoroboric acid containing consider ranged tubular reactors, for example, may be used, pro able quantities of water in said process is purified in 15 vided that they permit the withdrawal of the HBF4 complicated manner in most cases. In order to obtain produced through openings in the reactor shell at the tetrafluoroboric acid of high strength, there must be lowest portion of the bottom thereof. The process ac used as starting compounds reactants containing as little cording to the invention may be carried out especially water as possible, especially anhydrous hydrogen fluo advantageously when using vertical tubular reactors. ride, crystalline boric acid, metaboric acid, or boron 20 Thereby the hydrogen fluoride-containing gas is suit trioxide. As the reaction takes a highly exothermal ably introduced at the bottom. The gases liberated from course (27.7 Kcal are set free during the reaction of 1 HF leave the reactor at the upper part. The tetra mol of boric acid), difficulties arise for dissipating the fluoroboric acid is obtained as an aqueous solution, quantities of heat set free when adding solid boric acid which consequently, collects at the bottom. By intro to hydrofluoric acid. For this reason only absorption 25 ducing the hydrogen fluoride-containing gas at the top processes for HF using aqueous solutions of boric acid there is obtained a tetrafluoroboric acid containing con have been known hitherto. siderable quantities of hydroxyfluoroboric acids, as may In U.S. Pat. No. 3,353,911 there is disclosed a process be seen from the F/B ratio. in which a gaseous mixture consisting of hydrogen The dimensions of the reaction vessel are not critical chloride, hydrogen fluoride and hydrogen halide con 30 for the process of the invention either. They should pounds is reacted with a diluted aqueous boric acid however be such that they permit a control of the tem solution.U.S. Pat. No. 3,016,285 describes the selective ab perature increase caused by the quantity of hydrogen sorption of hydrogen fluoride from gas mixtures con fluoride introduced per unit of time. Aqueous concen 35 trated tetrafluoroboric acid begins to decompose at a taining SO, and HF by means of boric acid solutions in temperature of about 140 C. The reaction is therefore a spray tower. advantageously performed at a temperature below 140 According to French Pat. No. 1,427,940 potassium fluoride is added to the aqueous absorption solution of C, preferably below 110' C, especially below 80 C. A boric acid in order to improve the absorption of hydro temperature greater than 10 C, especially of 20' to 30 gen fluoride. 40 C is especially suitable. , Therefore, potassium tetrafluoroborate is formed. In The particle size of the corresponding boron/oxygen all cited processes there are formed only diluted solu compounds may vary within wide limits. Products hav tions of tetrafluoroboric acid containing moreover con ing a particle size of 0.5 to 5 mm may be used, for exam siderable quantities of an excess of free boric acid, ple. Greater particle sizes make a lengthening of the owing to the fact that the end of the reaction can only 45 reactor necessary, owing to the fact that the character be difficultly determined. istic reactivity of the compounds would be reduced in According to German Pat. No. 2,239,131 gases con this case. When the particle sizes are too small there is taining hydrogen fluoride are reacted with a solution of the danger of a blowing out of the reactive boron com solid boric acid in tetrafluoroboric acid of high pound by the gas current introduced. The reaction may strength. This process certainly permits preparation of 50 be carried out continuously or discontinuously. highly concentrated solutions of tetrafluoroboric acid, The invention will now be described, by way of ex but the forced circulation of hydroxyfluoroboric acid ample, with reference to the accompanying drawings makes necessary the use of specialized apparatus. which show diagrammatically an apparatus suitable for A process for the preparation of highly concentrated the carrying out the process of the invention. tetrafluoroboric acid has now been found that involves 55 FIG. 1 shows a possible arrangement of a vertical reacting hydrogen fluoride with boron/oxygen com tubular reactor for discontinuous operation. The reac pounds, which comprises reacting a solid boron/oxy tor is composed of a cylindrical tube 1 consisting of a gen compound directly with hydrogen fluoride-con material resistant to hydrogen fluoride, for example taining gases. Suitable boron/oxygen compounds are nickel, polyethylene or polypropylene, which contains especially boric acid, metaboric acid and boron oxide. a perforated plate 2, on which a charge 3 of the reacting The gas used may contain further components besides boron/oxygen compound, for example boron acid, is hydrogen fluoride, for example fluorine-containing hy placed. The hydrogen fluoride-containing gas is intro drocarbons, fluorochloro-alkanes, sulfur dioxide, hy duced into the reactor via conduit 4. The gases liberated drogen chloride or inert gases such as nitrogen. from HF leave at the top of the reactor via conduit 5. The process according to the invention permits both 65 Tetrafluoroboric acid formed during the reaction accu the preparation of highly concentrated tetrafluoroboric mulates in the lower part of the reactor as a liquid phase acid and a simultaneous substantially selective removal 6. It is withdrawn through a valve 7 continuously or of hydrogen fluoride from the gases used. The content discontinuously. The heat generated during the reaction 4,061,723 3 4. may be dissipated by a cooling jacket 8 provided with chamber the crude gas free from hydrogen fluoride, an inlet 9 for cooling water and an outlet 10 for heated which has been fed to the reactor via 17 meets with the cooling water. The reaction course may be observed by hydrochloric acid-containing tetrafluoroboric acid in a means of thermometers 11. When the diameter of the counter-current flow, is saturated with HCl and leaves cylinder is less than about 10 cm, the boron/oxygen the blowing chamber 15 at the upper part via conduit compound will not move downwardly. In this case an 20. Conduit 20 discharges into conduit 5. A body of increase of the temperature at the highest measuring HBF, 21 practically free from hydrochloric acid col point 11 indicates that the reactor must be charged lects at the lower bed of the blowing chamber 15. It is anew. The consumption of boric acid, boron trioxide or withdrawn continuously or discontinuously via the methaboric acid may also be previously calculated from 10 valve 12. the hydrogen fluoride content of the gases used. The invention may also be used when the gases con In the case of rather large tube diameters the charg taining hydrogen fluoride are obtained discontinuously, ing with boric acid and the formation of tetrafluorobo for example for the purification of receptacles, in which ric acid may also be carried out continuously. When HF had been stored. The purification may be carried working continuously two reactors may also be oper 15 out in such a way that air is drawn in through the recep ated alternately. tacle. The current of air containing hydrogen fluoride The process according to the invention permits re may then pass through the reactor as described above duction of the hydrogen fluoride content of the gases and be liberated from HF.