PRODUCTION OF FLUORINE-CONTAINING COMPOUNDS WITH THE AID OF AN ELECTROCHEMICAL MEASURING CHAIN

Abstract

The present invention relates to processes for producing fluorine-containing compounds by means of precipitation reactions from solutions containing fluoride ions wherein the pH value is determined using an electrochemical measuring chain and to electrochemical measuring chains for determining pH values.

Claims

1. A process for producing a fluorine-containing compound by a precipitation reaction, wherein the fluorine-containing compound is precipitated from a reaction medium and wherein during the course of the precipitation reaction the pH value of the reaction medium is determined using an electrochemical measuring chain.

2. The process as claimed in claim 1 wherein the fluorine-containing compound is selected from the group consisting of at least one alkali metal fluoride, an alkali metal hydrogenfluoride, an alkaline earth metal fluoride, an alkaline earth metal hydrogenfluoride, a transition metal fluoride, a fluoride of an element of the third main group, an ammonium fluoride, an alkali metal fluoroaluminate, an alkaline earth metal fluoroaluminate, hydrates thereof, HF adducts thereof and mixtures thereof.

3. The process as claimed in claim 1 wherein the fluorine-containing compound is selected from the group consisting of KAlF.sub.4, K.sub.2AlF.sub.5, K.sub.3AlF.sub.6, CsAlF.sub.4, Cs.sub.2AlF.sub.5, Cs.sub.3AlF.sub.6, LiAlF.sub.4, Li.sub.2AlF.sub.5, Li.sub.3AlF.sub.6, NaAlF.sub.4, Na.sub.2AlF.sub.5, Na.sub.3AlF.sub.6, KBF.sub.4, BaF.sub.2, KHF.sub.2, NH.sub.4F, NH.sub.4F.sub.2, hydrates thereof, HF adducts thereof and mixtures thereof.

4. The process as claimed in claim 1 wherein the reaction medium contains fluoride ions at commencement of the precipitation reaction and hydroxide ions are added to the reaction medium during the precipitation reaction.

5. The process as claimed in claim 1 wherein the reaction medium contains hydroxide ions at commencement of the precipitation reaction and fluoride ions are added to the reaction medium during the precipitation reaction.

6. The process as claimed in claim 1 wherein the fluoride-containing compound is selected from the group consisting of KAlF.sub.4, K.sub.2AlF.sub.5, K.sub.3AlF.sub.6, hydrates thereof and mixtures containing KAlF.sub.4, K.sub.2AlF.sub.5, K.sub.3AlF.sub.6 and/or hydrates thereof, the reaction medium contains fluoroaluminic acid at commencement of the precipitation reaction and the precipitation reaction is effected by addition of an aqueous potassium hydroxide solution to the reaction medium.

7. The process as claimed in claim 1 wherein the addition of the reaction partner is terminated as soon as the pH value in the reaction medium determined by the electrochemical measuring chain achieves a value of 2.5 to 7.

8. The process as claimed in claim 1 wherein the addition of the reaction partner during the course of the precipitation reaction is controlled by the pH value determined using the electrochemical measuring chain.

9. A fluorine-containing compound produced by the process according to claim 1.

10. An electrochemical measuring chain containing a hydrogen gas diffusion electrode 7 as an indicator electrode and at least one reference electrode, wherein the electrodes form a constructional unit with a housing 1 having an outlet 14 and wherein the housing 1 comprises a reference electrolyte 11 in contact with the reference electrode and a sealed-off gas space 19 above the reference electrolyte 11.

11. The electrochemical measuring chain as claimed in claim 10 wherein the at least one reference electrode comprises a hydrogen gas diffusion electrode 8 or an Ag/AgCl electrode 17, preferably a hydrogen gas diffusion electrode 8.

12. The electrochemical measuring chain as claimed in claim 10 wherein the housing 1 comprises a temperature sensor 15 in contact with the reference electrolyte 11 and a temperature sensor 15 in contact with the measurement medium 9.

13. The electrochemical measuring chain as claimed in claim 10 comprising a hydrogen gas diffusion reference electrode 8 and an Ag/AgCl reference electrode 8.

14. The electrochemical measuring chain as claimed in claim 10 wherein the temperature of the reference electrolyte may be controlled by means of a heat transferor.

15. A measuring instrument containing the electrochemical measuring chain according to claim 10.

16. The process as claimed in claim 3 wherein the fluorine-containing compound is selected from the group consisting of KAlF.sub.4, K.sub.2AlF.sub.5, K.sub.3AlF.sub.6, hydrates thereof and mixtures containing KAlF.sub.4, K.sub.2AlF.sub.5, K.sub.3AlF.sub.6 and/or hydrates thereof.

17. The process as claimed in claim 7 wherein the addition of the reaction partner is terminated as soon as the pH value in the reaction medium determined by the electrochemical measuring chain achieves a value of 3.0 to 5.0.

18. The process as claimed in claim 17 wherein the addition of the reaction partner is terminated as soon as the pH value in the reaction medium determined by the electrochemical measuring chain achieves a value of 3.5 to 4.5.

19. The electrochemical measuring chain as claimed in claim 10, wherein the reference electrolyte is an aqueous solution of at least one metal salt.

20. The electrochemical measuring chain as claimed in claim 19, wherein the at least one metal salt is a halogen metal salt of the first or second main group of the periodic table.

Description

WORKING EXAMPLES

Example 1Production of a Soldering Flux

[0049] 614 kg of hydrogen fluoride in the form of a hydrofluoric acid containing 20% by weight of hydrogen fluoride and 550 kg of aluminum hydroxide (99% purity) are reacted to form a fluoroaluminic acid in a stirred reactor fitted with the electrochemical measuring chain according to FIG. 1. At a temperature of 80 C. in the reactor and a temperature of 50 C. in the reference electrolyte 11, the reference electrolyte 11 consisting of an aqueous KCl solution, the obtained reaction mixture is admixed with a potassium hydroxide solution containing 15% by weight of KOH which was obtained by dissolution of 550 kg of KOH. A suspension of precipitating potassium salts of complex aluminum fluorides is formed. During the addition of potassium hydroxide solution the pH value is continuously monitored using the electrochemical measuring chain and the addition of potassium hydroxide solution is terminated upon achieving a pH value of 4.0. The product formed is filtered and the filter-damp residue is dried in a flow dryer at 570 C. The described process results in an advantageous filterability of the suspension and higher yield and purity of the resulting product. The process further results in a saving of hydrofluoric acid and potassium hydroxide solution since the additions of hydrofluoric acid and potassium hydroxide solution can be terminated with pinpoint accuracy resulting in neither underproduction nor overproduction.

Example 2Production of a Soldering Flux

[0050] 614 kg of hydrogen fluoride in the form of a hydrofluoric acid containing 50% by weight of hydrogen fluoride and 550 kg of aluminum hydroxide (99% purity) are reacted to form a fluoroaluminic acid in a stirred reactor. The stirred reactor is fitted with a bypass which is equipped with an electrochemical measuring chain according to FIG. 1 and through which by means of a pump the reaction mixture is withdrawn from the stirred reactor and after measurement by means of said electrochemical measuring chain according to FIG. 1 released back into the stirred reactor. At a temperature of 80 C. in the bypass and in the stirred reactor and a temperature of 50 C. in the reference electrolyte 11, the reference electrolyte consisting of an aqueous KCl solution, the obtained reaction mixture is admixed with a potassium hydroxide solution containing 15% by weight of KOH which was obtained by dissolution of 550 kg of KOH. A suspension of precipitating potassium salts of complex aluminum fluorides is formed. During the addition of potassium hydroxide solution the pH value is continuously monitored using the electrochemical measuring chain and the addition of potassium hydroxide solution is terminated upon achieving a pH value of 4.5. The product formed is filtered and the filter-damp residue is dried in a flow dryer at 570 C. The described process results in an advantageous filterability of the suspension and higher yield and purity of the resulting product. The process further results in a saving of hydrofluoric acid and potassium hydroxide solution since the additions of hydrofluoric acid and potassium hydroxide solution can be terminated with pinpoint accuracy resulting in neither underproduction nor overproduction. The use of a bypass furthermore results in a markedly smaller construction of the electrochemical measuring chain according to FIG. 1.