Processes of producing potassium fluoroaluminates

Abstract: Potassium tetrafluoroaluminate and potassium hexafluoroaluminate have been produced by three new processes: (a) by reacting alumina trihydrate with freshly produced, hot, solvated potassium fluoride solution in presence of stoichiometric amounts and/or an excess of hydrogen fluoride, or (b) by reacting aluminum trifluoride trihydrate with hot aqueous solution of potassium fluoride or potassium bifluoride, or (c) by generating in situ potassium aluminate by reacting alumina trihydrate with hot aqueous potassium hydroxide and treating the same with aqueous or anhydrous hydrogen fluoride.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION: The present invention relates to several simple, energy efficient and advantageous processes of producing potassium tetrafluoroaluminate, potassium hexafluoroaluminate and their mixtures. DESCRIPTION OF THE RELATED ART: The conventional technical methods of producing potassium tetrafluoroaluminate like (a) reacting alumina trihydrate, hydrofluoric acid and potassium hydroxide, or (b) evaporation of a mixture obtained by combining stoichiometric amounts of the reactive components, or (c) melting together comminuted potassium fluoride and aluminum fluoride at temperatures above 600°C and grinding the resulting solidified melt, suffer from one or several of the following disadvantages such as: product melting at temperatures above 620°C, or low yields, or high energy requirements leading to loss of cost effectiveness of the process. The process described by Willenberg et. al. (U.S. Patent 4,428,920) claims a method for the production of potassium tetrafluoroaluminate melting below 575°C. The process claims the formation and existence of tetrafluoroaluminic acid from a reaction of alumina trihydrate and hydrofluoric acid and forming the tetrafluoroaluminate by subsequent neutralization with potassium hydroxide. The process is claimed to yield a product of melting point not exceeding 575°C. The problems associated with this process are that hydrofluoric acid is difficult to handle and that both reactions are highly exothermic. In addition, the strong base, strong acid neutralization produces a species highly reactive to alumina which ties up potassium quickly. The resultant rapid rate of reaction produces impurities in the product. US-A 3,506,395 pertains to the preparation of artificial cryolite, i.e. Na 3 AlF 6 , useful as a flux in the electrolytic reduction of aluminium metal. Crude sodium bifluoride is dissolved in diluted mineral acid, impurities are precipitated, and the solution is mixed with a solution of aluminium hydroxide in mineral acid. The precipitate of Na 3 AlF 6 is separated, washed and dried. EP-A-0 399 050 discloses a brazing method for bonding of Al or Al alloys. A gas phase containing vapors of potassium fluoroaluminate complexes as flux is provided wherein Al or Al alloys members are brazed. The vapor of potassium fluoroaluminate complex may be produced by vaporizing KAlF 4 , by reaction of KF with metallic Al, it may be generated from a melt of KF and AlF 3 or from metallic complexes comprising K and F such as KZnF 3 , and metallic aluminium. US-A 4,670,067 discloses a flux for aluminium brazing comprising fluoroaluminium complexes of potassium and cesium. The flux may be produced by mixing the fluoroaluminates of potassium and cesium with AlF 3 , by melting AlF 3 , CsF and KF, by mixing AlF 3 , CsF and KF and adding water, by dissolving Al(OH) 3 or Al in HF or a solution of CsOH and KOH and neutralising the product with CsOH/KOH or HF, respectively. Further, the flux may be produced by reacting a mixture of Al(OH) 3 , CsOH and KOH with aqueous HF. SUMMARY OF THE PRESENT INVENTION Potassium tetrafluoroaluminate and potassium hexafluoroaluminate have been produced by the following processes: (a) according to the invention, by reacting alumina trihydrate or aluminum oxide with freshly produced, hot, solvated potassium fluoride solution in presence of stoichiometric amounts and/or an excess of hydrogen fluoride; (b) for comparison by reacting aluminum trifluoride trihydrate with hot aqueous solution in potassium fluoride or potassium bifluoride, or; (c) for comparison, by generating in situ potassium aluminate by reacting alumina trihydrate with hot aqueous potassium hydroxide and treating the same with aqueous or anhydrous hydrogen fluoride. The unique advantages of the present process aid in the production of the said compounds both in pure form as well as mixtures (which exhibit superior performance as fluxes) by only a minor variation in the ratio of the reactants. Further, the process is comprised of a single pot reaction with the internal energy of the system being fully and efficiently utilized to trigger and sustain the reaction to completion. In addition, the present invention leads to the production of pure materials of proven superior performance as established by various physico chemical methods of analysis and performance evaluation tests. Four main objectives of the present invention are to provide: (a) a process of universal applicability for the production of potassium tetrafluoroaluminates; (b) a process making use of the internal energy of the system for the production of the said compounds thus making it energy efficient and superior to all those in prior known art; (c) a process which, with minor variations, can yield different mixtures of the fluoroaluminates which cater to the varied melting behaviors demanded by their use as fluxes and other industrial applications, and; (d) a process that can be carried out in a single pot in convenient batches. Said process thus overcomes the disadvantages of the prior art. Other objects of the present invention and advantageous features thereof will become apparent during the course of the description. DESCRIPTION OF THE PREFERRED EMBODIMENT The first process, hereafter referred to as "Process A", according to the present invention of producing potassium tetrafluoroaluminate or potassium hexafluoroaluminate comprises the following steps: Solvating a potassium fluoride solution with a compound selected from the group hydrofluoric acid, anhydrous hydrogen fluoride, aqueous hydrofluoric acid or hydrogen fluoride; reacting stoichiometric amounts of an alumina compound selected from the group alumina trihydrate or aluminium oxide with the solvated solution to form a resulting suspension; separating potassium tetrafluoroaluminate or potassium hexafluoroaluminate from the resulting suspension. The reacting step preferably includes adding alumina trihydrate within an initial stirring period. Less than stoichiometric amounts of alumina trihydrate are used to produce a eutectic as well as other compositions of the binary system KF/AlF 3 and mixtures of KAlF 4 and K 3 AlF 6 . The invention further comprises a process for producing potassium tetrafluoroaluminate or potassium hexafluoroaluminate comprising the steps of: obtaining a reaction vessel composed of material selected from the group polyethylene, polypropylene, teflon, kynar, rubber, or graphite-lined metal; adding potassium hydroxide of a weight percent between 20% and 46% to water in the reaction vessel; supplying a compound selected from the group hydrofluoric acid, anhydrous hydrogen fluoride, aqueous hydrofluoric acid or hydrogen fluoride; of a weight per cent between 20% and 100% to the reaction vessel to form a solution of a solvated species having the formula KF·nHF, where n=1-6 and a heat of reaction is released; maintaining the temperature of the solution below 110°C by regulating the addition of hydrofluoric acid; with the temperature of the solution 50°C or above, mixing the solution while adding stoichiometric amounts of alumina trihydrate to form a mixture; allowing the mixture time to react utilizing the released heat of reaction to trigger and sustain the reaction to form a resulting suspension; separating the reaction products, potassium tetrafluoroaluminate or potassium hexafluoroaluminate, from the resulting suspension. The solvated potassium fluoride solution is preferably generated fresh by the neutralization of potassium hydroxide with hydrofluoric acid. In order to produce potassium tetrafluoroaluminate, n in the solution with the solvated species having the formula KF·nHF can be 3-4. The said solvated potassium fluoride solution is produced by the neutralization of commercially available potassium hydroxide solution with hydrofluoric acid. The filtrate is recycled. Investigation on the phase equilibria of the aluminum trifluoride and potassium fluoride (B. Philips, C.M. Warshaw, and J. Mocrin in "Journal of the American Ceramics Society" vol. 49 p. 631 (1966)) have revealed the presence of a solid phase with a composition of KAlF 4 for an equimolar mixture of aluminum trifluoride and potassium fluoride with a melting point of 574°C and in addition, an eutectic phase melting at a lower temperature of 560°C which is closer in composition to KAlF 4 but contains also the other potassium fluoroaluminate i.e. the potassium cryolite, K 3 AlF 6 . Thus, as mentioned earlier, with only minor variations of the amounts of reactants, we have produced both the pure species i.e. KAlF 4 or K 3 AlF 6 or the eutectic mixture of the former with the latter to produce a low melting material. It may be mentioned here that the latter mixture is of great applicability as a brazing flux. According to the present invention, (a) for KAlF 4 a preferred molar ratio with respect to the total charge is a molar ratio of potassium to aluminum within the range of 1.0 and 1.14, and (b) for K 3 AlF 6 , a molar ratio of potassium to aluminum 2.95 to 3.01. It is advantageous to freshly prepare the solvated potassium fluoride solution to provide the solvated species KF.nHF, where "n" can be 2-6, preferably 3-4 for KAlF 4 , and 0-3, preferably 0-1 for K 3 AlF 6 . The hydrofluoric acid for generating the solvated potassium fluoride species can be in the concentration range of 20% by weight to 100% by weight, preferably in the range of 50% by weight to 70% by weight. It has been found by experimentation that preparing fresh solvated potassium fluoride solution by the neutralization of potassium hydroxide with hydrofluoric acid produces a more reactive species and ultimately a purer product. An aqueous potassium hydroxide solution of a concentration between 10% by weight to 46% by weight, and more conveniently the commercially available solutions with concentration in the range of 36% by weight to 45% by weight are used for the generation of solvated potassium fluoride species. The reaction of the alumina trihydrate or aluminum oxide with the solvated potassium fluoride solution can be carried out at a temperature within the range between room temperature and the boiling point of the reaction mixture. The preferred temperatures are those between 60°C and 100°C. The precipitation of the reaction product can be effected in any desired manner, but a preferred mode for a batch process would be to generate the solvated potassium fluoride species and then add the alumina trihydrate to the same container. It has been found that a polyethylene reaction vessel is well suited for this process due to the fact that polyethylene does not interfere with the reaction process. When the solvated potassium fluoride solution is prepared through a neutralization of the potassium hydroxide with hydrofluoric acid, a significant amount of heat is released. This heat of reaction is necessary to trigger and sustain the rest of the process, however, polyethylene begins to melt at approximately 130°C. For this reason, the hydrofluoric acid must be added at a rate such that the temperature of the solution does not exceed approximately 110°C. It is possible to use metal reaction vessels, such as stainless steel, monel or nickle, however, it has been determined that the metal reacts with the process causing slight impurities in the final product. A glass reaction vessel would also sustain the heat, however, glass is highly reactive with hydrofluoric acid, thereby rendering it unsuitable for this application. For this reason, plastic containers are preferred such as polyethylene, polypropylene, teflon, or kynar, rubber, or graphite lined metal. The addition of the alumina trihydrate can be completed in 5-30 minutes depending on the size of the batch. It is preferable to stir the reaction mixture during the addition of the alumina trihydrate, the latter being added in a continuous stream. The stirring can then be discontinued. The reaction is allowed to complete over a period of 2 to 6 hours, with no heating at any stage. The exothermicity of the reaction in both the steps eliminates altogether any external energy requirements for the process thus making it a highly energy efficient one. The product can be isolated by adopting conventional separation procedures such as filtration, centrifugation, pressure cartridge or the press mill or drying the whole mass when stoichiometry is used. The process may include pulverizing the reaction product. The yields are stoichiometric, for example, 97% to 99% based on alumina trihydrate or aluminum oxide. Example 1. Small Batch of KAlF 4 according to Process A. In order to produce KAlF 4 in a small batch, 3.35 kg of 45% potassium hydroxide (KOH) were added in a polyethylene vessel and placed on a scale. Carefully, 3.36 kg of 70% hydrofluoric acid was pumped into the mixer and the reaction allowed to take place. HF was added at a rate such that the temperature of the solution reached but did not exceed 110°C due to the released heat of reaction. This is due to the fact that polyethylene begins to melt at approximately 130°C. After all of the HF was added and the temperature of the solution between 60°C and 110°C, 1.844 kg of alumina trihydrate was added while the reaction mixture was stirred. The alumina trihydrate was added quite rapidly, in about 10 minutes during continuous mixing. After the addition of alumina stirring was ceased and there was an induction period where no activity took place. Following the induction period, activity in the mixture became vigorous and the temperature again rose, leveling of at approximately 106°C. This reaction period took about 1 to 3 hours. The mixture was continuously stirred so as to ensure complete reaction and to decrease the time of reaction. The resulting suspension was thick due to the loss of water as a result of the heat of reaction, so H 2 O was added to make a slurry. the slurry was then pumped out and centrifuged. At the conclusion of the centrifuge cycle, the product remaining in the centrifuge basket was collected, dried in a tray oven at 150°C and then pulverized. Upon examination, the process revealed 3.323 kg of KAlF 4 which was analyzed to have a purity of 99.0 percent. Example 2. Large Batch of KAlF 4 according to Process A. In order to produce KAlF 4 in a large batch, a polyethylene vessel was placed on a scale and 166.0 kg of 45% potassium hydroxide (KOH) was added. Carefully, 166.0 kg of 70% hydrofluoric acid (HF) was pumped into the mixer and the reaction allowed to continue. HF was added at a rate such that the temperature of the solution reached but did not exceed 110°C due to the released heat of reaction. HF was added slowly so that the temperature of the solution did not exceed 110°C due to the fact that polyethylene begins to melt at approximately 130°C. After all of the HF was added and the temperature of the solution between 60°C and 100°C, 91.0 kg of alumina trihydrate was added while the reaction mixture was stirred. In contrast, the alumina trihydrate was added quite rapidly, between 15 and 30 minutes during continuous mixing. After the addition of alumina, stirring was ceased and an induction period commenced where no activity took place. Following the induction period, activity in the mixture became vigorous and the temperature again rose, leveling off at approximately 106°C. This reaction period took between one and three hours. The mixture was continuously stirred so as to ensure complete reaction and to decrease the time of reaction. The resulting suspension was very thick due to the loss of water as a result of the heat of reaction, so H 2 O was added to make a slurry. The slurry was then pumped out and centrifuged. At the conclusion of the centrifuge step, the product remaining in the centrifuge basket was collected, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 166 kg of KAlF 4 which was analyzed to have a purity of 99.9%. Example 3. Small Batch of K 3 AlF 6 according to Process A. In order to produce K 3 AlF 6 in a small batch, a polyethylene vessel was placed on a scale and 8.999 kg of 45% potassium hydroxide (KOH) was added. Carefully, 4.686 kg of 70% hydrofluoric acid (HF) was pumped into the mixture and the reaction allowed to take place. HF was added at a rate such that the temperature of the solution reached but did not exceed 110°C due to the released heat of reaction. The temperature of the solution was not allowed to exceed 110°C due to the fact that polyethylene begins to melt at approximately 130°C. After all of the HF was added and the temperature of the solution between 60°C and 100°C, 1.844 kg of alumina trihydrate was added while the reaction mixture was stirred. The alumina trihydrate was added quite rapidly, in about 30 minutes during continuous mixing. After the addition of alumina, stirring was ceased and there was an induction period where no activity took place. Following the induction period, activity in the mixture became vigorous and the temperature again rose, leveling off at approximately 106°C. This reaction period took about one to three hours. The mixture was continuously stirred so as to ensure complete reaction and to decrease the time of reaction. The resulting suspension was very thick due to the loss of water as a result of the heat of reaction, so H 2 O was added to make a slurry. The slurry was then pumped out and centrifuged. At the conclusion of the centrifuge cycle, the product remaining in the centrifuge basket was collected, dried in an oven at 110°C and then pulverized. Upon examination, the process revealed 6.135 kg of K 3 AlF 6 which was analyzed to have a purity of 98.9%. Example 4. Large Batch of K 3 AlF 6 according to Process A. In order to produce K 3 AlF 6 in a large batch, a polyethylene vessel was obtained and placed on a scale and 218.2 kg of 45% potassium hydroxide (KOH) was added. Carefully, 105 kg of 70% hydrofluoric acid (HF) was pumped into the mixture and the reaction allowed to take place. HF was added at a rate such that the temperature of the solution reached but did not exceed 110°C due to the released heat of reaction. The temperature of the solution was not allowed to exceed 110°C due to the fact that polyethylene begins to melt at approximately 130°C. After all of the HF was added and the temperature of the solution was between 60°C and 100°C, 45.45 kg of alumina trihydrate was added while the reaction mixture was stirred. The alumina trihydrate was added quite rapidly in about 30 minutes to an hour during continuous mixing. After the addition of alumina, stirring was ceased and there was an induction period where no activity took place. Following the induction period, activity in the mixture became vigorous and the temperature again rose, leveling off at approximately 106°C. This reaction activity period took about one to three hours. The mixture was continuously stirred so as to ensure complete reaction and to decrease the time of reaction. The resulting suspension was very thick due to the loss of water as a result of the heat of reaction, so H 2 O was added to make a slurry. The slurry was then pumped out and centrifuged. At the conclusion of the centrifuge cycle, the product remaining in the centrifuge basket was collected, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 318 kg of K 3 AlF 6 which was analyzed to have a purity of 96%. Table I shows the weight of reactants used, the product yield and percent yield based on alumina for Examples 1 through 4. Production of KAlF 4 and K 3 AlF 6 from KOH/HF/Al 2 O 3 .3H 2 O KOH (45%) kg HF (70%) kg Alumina kg Yield kg % Yield based on alumina KAlF 4 Example 1 3.353 3.357 1.844 3.323 99.0 Example 2 166.0 166.0 91.0 166 99.9 K 3 AlF 6 Example 3 8.999 4.686 1.874 6.135 98.9 Example 4 218.2 105 45.45 318 96.0 Examples 5-10 show the manner in which additional experiments using Process "A" were carried out. Examples 5-10 disclose all molar ratios where the mole quantities mentioned are all gram moles. Table II lists the reaction conditions. Examples 5-10. KAlF 4 and K 3 AlF 6 according to Process A. In a typical experiment a known amount of potassium hydroxide was taken in a plastic bucket, placed on a scale and the required amount of hydrofluoric acid was carefully pumped into it. After the addition of hydrogen fluoride, alumina trihydrate was added while stirring the reaction mixture. The stirring was discontinued and the stirring assembly removed from the reaction vessel. A vigorous reaction soon followed. The reaction was complete in 30 minutes. The slurry was stirred for another 30 minutes and then filtered. The solid residue was dried in a tray oven. Table II lists the reaction conditions for each Example. KOH Conc. Moles % w/w HF Conc. Moles % w/w A1 2 O 3 .3H 2 O Yield in % based on A1 2 O 3 .3H 2 O Analysis K/A1 F/A1 KA1F 4 Example 5 45 26.95 70 117.5 11.82 99 1.05 4.10 Example 6 45 26.91 70 118.0 11.82 99.5 1.03 4.12 Example 7 45 423.0 70 1843.0 185.1 99.5 1.06 4.08 KA1F 6 Example 8 45 72.32 70 164 12.01 99 3.3 6.3 Example 9 45 1751 70 3670 291 98.5 3.1 6.2 Example 10 45 1751 70 3670 291 98.8 3.0 6.25 PROCESS B (not according to the invention): Also an alternative process, hereafter referred to as "Process B," of producing potassium fluoroaluminates comprises the following steps: reacting a solution of potassium aluminate with hydrofluoric acid, the said potassium aluminate solution can be generated by reacting alumina trihydrate with hot aqueous potassium hydroxide solution. The production of potassium aluminate can be carried out within a range of reaction temperature between 75°C and 116°C, more advantageously between 85°C and 95°C. The heat required can be supplied by traditional methods or generated by dissolution of the potassium hydroxide beads, pellets or powder in water. The addition of alumina trihydrate to the hot potassium hydroxide solution can be done all at once or in increments with no net effect on the reactivity of the potassium aluminate solution obtained. It is also preferable to stir the mixture to facilitate the dissolution. It may be mentioned that the concentration ranges for the potassium hydroxide solution as well as the hydrofluoric acid are the same as described under process A. Example 11. KAlF 4 according to Process B (comparative example). In order to produce KAlF 4 , 1.844 kg of alumina trihydrate was added to 3.353 kg of hot potassium hydroxide (90°C) in a polyethylene vessel during vigorous stirring. The mixture was digested further to produce a clear solution. To the solution, 4.100 kg of hydrofluoric acid was then carefully added. potassium tetrafluoroaluminate precipitated out. The potassium tetrafluoroaluminate product was centrifuged, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 3.323 kg of potassium tetrafluoroaluminate which was analyzed to have a purity of 99%. Example 12. KAlF 4 according to Process B (comparative example). In order to produce KAlF 4 , 7.382 kg of alumina trihydrate was added to 13.415 kg of hot potassium hydroxide (90°C) in a polyethylene vessel during vigorous stirring. The mixture was digested further to produce a clear solution. To the solution, 18.816 kg of hydrofluoric acid was carefully added. Potassium tetrafluoroaluminate precipitated out. The potassium tetrafluoroaluminate product was centrifuged, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 3.323 kg of potassium tetrafluoroaluminate which was analyzed to have a purity of 98.5%. Example 13. K 3 AlF 6 according to Process B (comparative example). In order to produce K 3 AlF 6 , 0.156 kg of alumina trihydrate was added to 7.466 kg of hot potassium hydroxide (90°C) in a polyethylene vessel during vigorous stirring. The mixture was digested further to produce a clear solution. To the solution, 0.504 kg of hydrofluoric acid was carefully added. Potassium tetrafluoroaluminate precipitated out. The product was centrifuged dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 0.511 kg of potassium tetrafluoroaluminate which was analyzed to have a purity of 99%. Example 14. K 3 AlF 6 according to Process B (comparative example). In order to produce K 3 AlF 6 , 0.516 kg of alumina trihydrate was added to 0.759 kg of hot potassium hydroxide (90°C) in a polyethylene vessel during vigorous stirring. The mixture was digested further to produce a clear solution. To this solution, 0.496 kg of hydrofluoric acid was carefully added. Potassium tetrafluoroaluminate precipitated out. The potassium tetrafluoroaluminate product was centrifuged, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 0.512 kg of potassium tetrafluoroaluminate which was analyzed to have a purity of 99.2%. Table III shows the weight of reactants used, the product yield and the percent yield based on alumina, as illustrated in Examples 11-14. Production of KAlF 4 and K 3 AlF 6 from KOH/Al 2 O 3 .3H 2 O/HF KOH (45%) (kg) Alumina (kg) HF (70%) (kg) Yield (kg) % Yield based on alumina KAlF 4 Example 11 3.353 1.844 4.100 3.323 99 Example 12 13.415 7.382 18.816 13.238 98.5 K 3 AlF 6 Example 13 0.7466 0.156 0.504 0.511 99.0 Example 14 0.759 0.156 0.496 0.512 99.2 Comparative examples 15-20 show the manner in which additional experiments using Process "B" were carried out. Examples 15-20 disclose all molar rations where the mole quantities mentioned are all gram moles. Table IV lists the reaction conditions. Example 15-20. KAlF 4 and K 3 AlF 6 according to Process B (comparative examples) In a typical experiment, a known amount of alumina trihydrate was added to a hot potassium hydroxide solution in a plastic bucket with vigorous stirring and the mixture digested further to produce a clear solution. The required amount of hydrofluoric acid was then added carefully to the resulting potassium aluminate solution to precipitate the potassium tetrafluoroaluminate which was then subjected to separation procedures to recover the products. Table IV illustrates the reaction conditions. KOH Conc. Moles % w/w HF Conc. Moles % w/w Al 2 O 3 .3H 2 O Moles Yield in % based on A1 2 O 3 .3H 2 O Analysis K/A1 F/A1 KAlF 4 Example 15 45 26.95 50 102.5 11.82 99 1.2 4.27 Example 16 45 107.8 50 470.4 47.32 98.5 1.08 4.20 Example 17 45 9.88 50 43.12 4.33 99.5 1.09 4.24 K 3 AlF 6 Example 18 45 6.0 50 12.6 1.0 99 3.2 6.4 Example 19 45 6.0 50 12.8 1.0 98.5 3.10 6.1 Example 20 45 6.1 50 12.4 1.0 99.2 3.12 6.3 PROCESS C (not according to the invention): Further, another alternative process, hereafter referred to as "Process C" for producing potassium fluoroaluminates comprises of the following steps: reacting aluminum fluoride trihydrate with hot aqueous potassium fluoride solution, and the said potassium fluoride solution can be generated fresh as in Process A (thereby making use of the heat released to provide the energy input for the reaction) or an already prepared solution can be employed (in which case the process requires an energy input). The concentration of potassium fluoride used, can be within the range of 25% by weight to 42% by weight and more advantageously between 36% by weight to 40% by weight. The temperature of the reaction can be within the range of 70°C and boiling point of the mixture and more advantageously between 80°C and 95°C. Example 21. KAlF 4 according to Process C (comparative example): In order to produce KAlF 4 according to Process C, 0.950 kg of aluminum trifluoride trihydrate was added in a continuous stream with vigorous stirring to 1.099 kg of hot (90°C) aqueous solution of potassium fluoride. The resulting slurry was digested further for one hour, centrifuged, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 0.977 kg of KAlF 4 which was analyzed to have a purity of 99.9%. Example 22. KAlF 4 according to Process C (comparative example): In order to produce KAlF 4 according to Process C, 0.949 kg of aluminum trifluoride trihydrate was added in a continuous stream with vigorous stirring to 1.084 kg of hot (90°C) aqueous solution of potassium fluoride. The resulting slurry was digested further for one hour, centrifuged, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 0.975 kg of KAlF 4 which was analyzed to have a purity of 99.8%. Example 23. K 3 AlF 6 according to Process C (comparative example): In order to produce K 3 AlF 6 according to Process C, 0.156 kg of aluminum trifluoride trihydrate was added in a continuous stream with vigorous stirring to 0.473 kg of a hot (90°C) aqueous solution of potassium fluoride. The resulting slurry was digested further for one hour, centrifuged, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 0.257 kg of K 3 AlF 6 which was analyzed to have a purity of 99.9%. Example 24. K 3 AlF 6 according to Process C (comparative example): In order to produce K 3 AlF 6 according to Process C, 0.156 kg of aluminum trifluoride trihydrate was added in a continuous stream with vigorous stirring to 0.458 kg of a hot (90°C) aqueous solution of potassium fluoride. The resulting slurry was digested further for one hour, centrifuged, dried in an oven at 150°C and then pulverized. Upon examination, the process revealed 0.257 kg of K 3 AlF 6 which was analyzed to have a purity of 99.5%. Table V shows the weight of reactants used, the product yield and percent yield based on alumina for Examples 21 through 24. Production of KAlF 4 and K 3 AlF 6 from KF(aq)/AlF 3 .3H 2 O KF (38%) (kg) AlF 3 .3H 2 O (kg) Yield (kg) % Yield based on AlF 3 .3H 2 O KAlF 4 Example 21 1.099 0.950 0.977 99.9 Example 22 1.084 0.949 0.975 99.8 K 3 AlF 6 Example 23 0.473 0.156 0.257 99.9 Example 24 0.458 0.156 0.257 99.5 Examples 25-30 show the manner in which additional experiments using Process "C" were carried out. Examples 25-30 disclose all molar ratios where the mole quantities mentioned are all gram moles. Table VI lists the reaction conditions. Examples 25-30. KAlF 4 and K 3 AlF 6 according to Process C (comparative examples): In a typical experiment, a calculated amount of aluminum trifluoride trihydrate was added, in a continuous stream, with vigorous stirring to hot aqueous solution of potassium fluoride. The slurry was digested further for 1 hour and then subjected to separation procedures as described earlier to isolate the product. Table VI lists the reaction conditions for this process. KF (aq) Conc. Moles % w/w AlF3.3H2O Moles Yield based on AlF3.3H2O Analysis K/A1 F/A1 KAlF 4 38 7.2 6.89 99.9 1.04 4.10 38 7.1 6.89 99.5 1.02 4.11 38 7.1 6.88 99.8 1.08 4.08 K 3 AlF 6 38 3.1 1.00 99.9 3.06 6.4 38 3.2 1.00 99.0 3.02 6.1 38 3.0 1.00 99.5 3.08 6.02 The preceding Examples serve to illustrate more in detail the processes according to the present invention without, however, being limited thereto. They refer to the production of one or two moles of the fluoroaluminates depending on the type of process. The products have been characterized by the melting point determinations using calibrated muffle furnace and also by differential thermal analysis. Further the products have been characterized by comparing their x-ray powder diffraction patterns with those reported in the literature. The exact composition of the products has been determined by chemical analysis. It may be mentioned that the ratios in Tables I through III are all molar ratios and the mole quantities mentioned are all gram moles. Also as discussed earlier, several variations in the reaction conditions can be effected so that the final composition of the product can be tailored to specific requirement as demanded by the end use. Further it may also be mentioned that a scale up of the processes is easily done without any net loss of efficiency of the process. When maintaining the procedural conditions according to the present invention as described above there is obtained the desired potassium fluoroaluminate in very good and almost theoretical yields, for instance 99% based on the alumina trihydrate for process A, 99% based on alumina trihydrate for process B and 99% based on alumina trifluoride trihydrate for process C. Also effecting the minor variations in terms of the reactant ratios there is obtained a product with the desired melting characteristics. The product thus obtained is suitable as a brazing flux, as a filler in abrasive agents and several other industrial applications.

Claims: A process for producing potassium tetrafluoroaluminate or potassium hexafluoroaluminate comprising the steps of: solvating a potassium fluoride solution with a compound selected from the group hydrofluoric acid, anhydrous hydrogen fluoride, aqueous hydrofluoric acid or hydrogen fluoride; reacting stoichiometric amounts of an alumina compound selected from the group alumina trihydrate or aluminum oxide with the solvated solution to form a resulting suspension; separating potassium tetrafluoroaluminate or potassium hexafluoroaluminate from the resulting suspension. A process according to claim 1, wherein the reacting step includes adding alumina trihydrate within an initial stirring period. A process according to claim 1 wherein a solution with the formula KF·nHF is formed wherein n in the solution with the formula KF·nHF can be 3-6 in order to produce potassium tetrafluoroaluminate. A process according to claim 3 wherein a solution with the formula KF·nHF is formed wherein n in the solution with the formula KF·nHF can be 0-3 in order to produce potassium hexafluoroaluminate. A process according to claim 1, wherein solvating potassium fluoride with aqueous hydrofluoric acid releases a heat of reaction which is utilized in triggering and sustaining the reacting step. A process according to claim 1, wherein less than stoichiometric amounts of alumina trihydrate are used to produce a eutectic as well as other compositions of the binary system KF/AlF 3 and mixtures of KAlF 4 and K 3 AlF 6 . A process for producing potassium tetrafluoroaluminate or potassium hexafluoroaluminate comprising the steps of: obtaining a reaction vessel composed of material selected from the group polyethylene, polypropylene, teflon, kynar, rubber, or graphite-lined metal; adding potassium hydroxide of a weight percent between 20% and 46% to water in the reaction vessel; supplying a compound selected from the group hydrofluoric acid, anhydrous hydrogen fluoride, aqueous hydrofluoric acid or hydrogen fluoride; of a weight percent between 20% and 100% to the reaction vessel to form a solution of a solvated species having the formula KF·nHF, where n=1-6 and a heat of reaction is released; maintaining the temperature of the solution below 110 °C by regulating the addition of hydrofluoric acid; with the temperature of the solution 50 °C or above, mixing the solution while adding stoichiometric amounts of alumina trihydrate to form a mixture; allowing the mixture time to react utilizing the released heat of reaction to trigger and sustain the reaction to form a resulting suspension; separating the reaction products, potassium tetrafluoroaluminate or potassium hexafluoroaluminate, from the resulting suspension. A process according to claim 7, wherein the solvated potassium fluoride solution is generated fresh by the neutralization of potassium hydroxide with hydrofluoric acid. A process according to claim 7, wherein n in the solution with a solvated species having the formula KF·nHF can be 3-4 in order to produce potassium tetrafluoroaluminate.

Inventors: BHAGAT SUDHIR S, CONTRACTOR DINSHAW B, KUMAR SAMPATH H, MESHRI DAYAL T, MESHRI SANJAY, BHAGAT, SUDHIR S., CONTRACTOR, DINSHAW B., KUMAR, SAMPATH H., MESHRI, DAYAL T., MESHRI, SANJAY
Applicants: SOLVAY FLUORIDES INC, SOLVAY FLUORIDES, INC.
Application year: 1993
Main classification: C01F7
IPC codes: C01F7, B23K35