WO2009113088A2

WO2009113088A2 - A process for manufacturing of potassium fluoride

Info

Publication number: WO2009113088A2
Application number: PCT/IN2009/000030
Authority: WO
Inventors: Sanjaya Ranjana Mohapatra; Rahul Jadhav.; Mandal Sisir; Narendrasingh Patil; Ravi Kelkar; Prashant Mickey Puri
Original assignee: Aditya Birla Science and Technology Co Ltd
Current assignee: Aditya Birla Science and Technology Co Ltd
Priority date: 2008-01-14
Filing date: 2009-01-09
Publication date: 2009-09-17
Anticipated expiration: 2010-07-14
Also published as: WO2009113088A3

Abstract

In the present application is provided a process for manufacturing high potassium fluoride wherein a by product of the fertilizer industry predominantly containing hydrofluorosilicic acid is used as the staring material. Solid potassium fluoride as obtained by the process of the present application has purity ranging from 99% to 99.99%.

Description

A PROCESS OF MANUFACTURING OF POTASSIUM FLUORIDE

Field of the Invention

This invention relates to a process of manufacturing of Potassium Fluoride.

Background of the Invention

Potassium fluoride [7789-23-3] is a chemical compound with the formula

KF. It is an alkali metal halide and occurs naturally as the rare mineral carobbiite. In Organic chemistry, after hydrogen fluoride, Potassium fluoride is the preferred source of fluorine in the conversion of chlorocarbons to fluorocarbons.

There are two known methods for manufacturing Potassium Fluoride.

In the first method, potassium hydroxide is reacted with hydrofluoric acid and the resulting salt can then be purified by recrystallization.

In the second method, potassium metal is reacted vigorously with a halogen to form potassium halide.

These conventional processes suffer from disadvantages like low productivity and high cost.

Existing knowledge

GB1419393 and US3842161 disclose a method for preparation of metal fluoride which comprises two stages. In the first stage, KOH solution is treated with solid potassium silico fluoride at molar ratio less than 4 to form potassium fluoride solution and then separating off the silica. In the second stage, the remaining KOH is reacted with potassium silica fluoride. The silica is separated by filtration and potassium fluoride solution is recovered as a product.

The method suffers from many disadvantages like high silica content, contamination with heavy metals because of alkaline pH, involves many stages of reaction and separation.

There is thus felt a need for a process of manufacturing potassium fluoride having low silica content and heavy metals in a cost-effective manner.

Object of the Invention

It is an object of the present invention is to provide a process for preparation of potassium fluoride having low silica content and heavy metals.

Another object of the present invention is to provide a cost-effective and economically feasible process for preparation of potassium fluoride.

Yet another object of the invention is to provide a process for preparation of potassium fluoride by cheaply available raw materials.

Yet another object of the invention is to provide a process for preparation of potassium fluoride in high yield and high purity.

Still another object of this invention is to provide a process for preparation of potassium fluoride which reduces the wastage of toxic byproducts. Summary of the Invention

In accordance with the present invention there is provided a process for manufacturing high purity potassium fluoride comprising the following steps: a. reacting a by product of the fertilizer industry predominantly containing hydrofluorosilicic acid with a suspension containing potassium chloride, potassium fluoride and silica cake to obtain a first resultant mass containing potassium silicofluoride and hydrochloric acid; b. cooling the first resultant mass to room temperature; c. filtering the first resultant mass to obtain a first residue containing potassium silicofluoride; d. washing the first residue with water to obtain a washed mass predominantly containing potassium silicofluoride; e. forming an aqueous slurry containing the washed mass of potassium silicofluoride; f. reacting the slurry with potassium hydroxide to obtain a second resultant mass predominantly containing potassium fluoride in solution and silica; g. filtering the second resultant mass to obtain a first filtrate predominantly containing potassium fluoride, unreacted potassium hydroxide and soluble silica and a second residue containing insoluble silica; h. reacting the first filtrate with hydrogen fluoride or hydrofluorosilicic acid to adjust the pH between 6.5 to 8.5 and to obtain a third resultant mass; i. filtering the third resultant mass to obtain a second filtrate containing potassium fluoride and a third residue predominantly containing potassium silicofluoride; and j. heating the second filtrate to obtain solid potassium fluoride having purity ranging from 99% to 99.99%.

In accordance with the process of the present invention, the reactants have the following relationship a. for 2 moles of KCl at least 1 mole of hydrofluorosilicic acid is added; and b. for each 6 moles of KF present along with KCl at least 2 moles of hydrofluorosilicic acid are added along with about 1 mole of silica.

Typically, the concentration of hydrofluorosilicic acid is in the range of 5% to 25 % in the process step (a).

Typically, the process step (a) is carried out at 40-100 C for 40-45 minutes.

Preferably, the potassium chloride in the process step (a) is a by-product of organic fluorination reaction.

Typically, the potassium chloride is present in the range of 0% to 100% along with potassium fluoride and with or without organic impurities.

Preferably, the suspension of potassium chloride is added as a slurry or solid form to the hydrofluorosilicic acid in the process step (a). Typically, the first resultant mass is filtered to remove hydrochloric acid in the process step (c).

Typically, the potassium silicofluoride is dried in an oven to remove moisture in the process step (d).

Preferably, the potassium silicofluoride is obtained as a white solid powder having purity more than 99% in the process step (d).

Preferably, the molar ratio of potassium hydroxide to potassium silicofluoride is in the range of 3.5: 1 to 4.1 : 1 in the process step (f).

Typically, the potassium hydroxide is used as a solid or as an aqueous solution in the process step (f).

Preferably, the potassium hydroxide is added slowly over a period of 5 min. to 300 min. in the process step (f).

Preferably, the concentration of potassium silicofluoride in aqueous slurry is in the range of 5 to 50 % in the process step (f).

Typically, the process step (g) is carried out at 5 to 105⁰C for 40-45 minutes.

Preferably, the insoluble silica is washed with water to remove potassium fluoride or unreacted potassium hydroxide present with silica in the process step (g). Typically, the silica is obtained as a silica cake having 40% moisture content in the process step (g).

Typically, the silica cake is dried at 110⁰C for 4 hrs. to obtain dry silica in the process step (g).

Preferably, the dry silica is used to prepare potassium silicofluoride.

Preferably, the washing solution is used to prepare slurry of potassium fluorosilicate or potassium hydroxide in the process step (g).

Typically, the process step (h) is carried out at 20-100⁰C for 10 to 120 minutes.

Typically, the third resultant mass is filtered out and the third residue obtained in the process step (i) can be reused.

Preferably, the solid potassium fluoride is obtained by heating the second filtrate to remove water in the process step (j).

Typically, the solid potassium fluoride is obtained by air drying the filtrate in the process step (J). Brief Description of the accompanying Drawings:

The invention will now be described with reference to accompanying drawings, in which

Figure 1 illustrates the X-Ray powder diffractogram of potassium silicofluoride (K₂SiF₆);

Figure 2 illustrates the X-ray Powder diffractogram of organic fluorination by-product KCl-KF mixture;

Figure 3 illustrates the X-ray Powder diffractogram of Potassium fluoride (KF)

Figure 4 shows a flowchart of the process for preparation of Potassium fluoride in accordance with the invention.

Detailed Description of the Invention

Potassium fluoride is an alkali metal halide occurs naturally as the rare mineral carobbiite. Potassium fluoride is the preferred source of fluorine in the conversion of chlorocarbons to fluorocarbons. The conventional methods for manufacturing potassium fluoride suffers from many disadvantages like use of expensive raw materials, low productivity and involves many reaction steps. Accordingly, the present invention provides an effective process for manufacturing of potassium fluoride, a source of fluorine for organic fluorinations in high yield and purity. A process in accordance with the present invention is also a cost-effective process over the other conventional methods used for manufacturing of potassium fluoride.

Hydrofluorosilicic acid, which is a raw material useful for producing potassium fluoride, is a by-product produced in large amount in the manufacture of well known phosphate fertilizers. Rock phosphate is a raw material used in the manufacture of most commercial phosphate fertilizers. Rock phosphate containing fluorides such as calcium fluoride and silica as impurity and mineral phosphates having 5% to 10% calcium fluoride is treated with previously diluted sulphuric acid. This reaction converts the tertiary calcium phosphate into primary phosphate rendering it soluble in water and liberates the fluorine of the calcium fluoride, forming hydrofluoric acid. This acid in turn combines with the silica forming silicon tetrafluoride, a gas, escapes into the air and is detrimental to surroundings. In order to prevent the escape of this gas, the gas is then sprayed with water in condensing towers into a series of scrubbers and dissolved in water and this decomposes the silicon tetrafluoride into Hydrofluorosilicic acid and silicic acid . This is the crude form of fluorosilicic acid. The purified form is obtained by distillation of the crude acid. The hydrofluorosilicic acid can also be prepared by the reaction of hexafluorosilicate, apatite and/or fluorite (fluorspar) with sulphuric acid.

During organic fluorination reaction, the source of fluoride ion which is an alkali metal fluoride (normally potassium fluoride) and solvent are charged in the reactor. The heat is applied to dry the system by distilling out any water with some of the solvent. After drying, the substrate which is usually aryl chloride and phase transfer catalyst are then fed into the reactor. The reaction is carried out at the predetermined temperature for the required time to yield potassium salts like potassium chloride and potassium fluoride as a residue with any involatile organic byproducts. This halogen exchange is usually referred as a Halex fluorination. The by-product of the organic fluorination reactions can also be utilized for the preparation of potassium fluoride in a cost-effective manner.

Until now, Hydrofluorosilicic acid has been barely utilized, and mostly discarded as a toxic waste. However, in recent times increasing attention has been paid to the utilization of hydrofluorosilicic acid in view of saving natural resources, and reducing environmental pollution. The present invention envisages the use of Hydrofluorosilicic acid, a byproduct of the fertilizer industry and the aforesaid by-product of organic fluorination reaction for the preparation of potassium fluoride. Thus, in accordance with this invention, a process for manufacturing high purity potassium fluoride comprises the following steps. A by product of the fertilizer industry predominantly containing hydrofluorosilicic acid having concentration in the range of 5% to 25% by mass is heated to a temperature of 95-98 C in a vessel and is reacted with a suspension containing potassium chloride (a by-product of organic fluorination reaction containing (potassium fluoride 10%, potassium chloride (in slurry form or in solid form) 89.5% and other impurities 0.5%) and silica cake (having 40% moisture). The reaction is continued for 40-45 min. at 40-100⁰C to obtain a first resultant mass containing insoluble potassium silicofluoride and hydrochloric acid.

In accordance with the process of the present invention, the reactants have the following relationship for 2 moles of KCl at least 1 mole of hydrofluorosilicic acid is added and for each 6 moles of KF present along with KCl at least 2 moles of hydrofluorosilicic acid are added along with about 1 mole of silica.

The first resultant mass is cooled to room temperature and then filtered to obtain a first residue containing potassium silicofluoride. The first residue is washed with water to obtain a washed mass containing potassium silicofluoride and to remove hydrochloric acid.

The product is then dried inside oven at 130 C for 3 hrs. to obtain a white solid powder of Potassium fluorosilicate free from moisture having more than 99% purity and isolated product yield.

The resultant Potassium fluorosilicate and water are added to the pp conical flask and the aqueous slurry is heated to 88 to 92⁰C. Then the 45% KOH solution is added slowly over a period ranging from 5 min. to 300 min. More preferably, the KOH solution is added within 20 min. The reaction is continued for 40-45 min. at 88 to 92⁰C to obtain a second resultant mass predominantly containing potassium fluoride in solution and silica as residue.

In accordance with the present invention the potassium silicofluoride is employed as aqueous slurry having concentration in the range of 5% to 50%.

In accordance with the present invention, the potassium hydroxide is used as a solid or as an aqueous solution.

In accordance with the present invention, the molar ratio of potassium hydroxide to potassium silicofluoride is in the range of 3.5:2 to 4.1 : 1. The second resultant mass is then cooled to room temperature and then it is filtered to obtain a first filtrate predominantly containing potassium fluoride, unreacted potassium hydroxide, soluble silica and a second residue containing insoluble silica. The insoluble silica is removed. This silica cake residue having 40% moisture is washed with water to remove any potassium fluoride and un-reacted potassium hydroxide. The washing solution is reused to prepare slurry of potassium silicofluoride or potassium hydroxide in the next reaction.

The pH of the first filtrate is more than 8 (generally 10) is adjusted to 6.5 - 7.5 using 48% aqueous solution of hydrogen fluoride or hydrofluorosilicic acid. The reaction mixture is heated for 10-120 min. at a temperature of 10 to 120 min. and then cooled at room temperature to obtain a third resultant mass. The third resultant mass is then filtered to obtain a second filtrate containing potassium fluoride and a third residue containing potassium silicofluoride and fluorosilicates of heavy metals. The residue containing potassium fluorosilicate, fluorosilicates and other heavy metals is filtered out. The residue is reused for next batch. The second filtrate (potassium fluoride solution) is obtained having about 29% (by wt.) of potassium fluoride. Solid potassium fluoride having purity in the range of 99% to 99.9% is obtained by removing water by heating.

In accordance with the present invention, the solid potassium fluoride is obtained by air drying the second filtrate.

Following reaction represents the preparation of potassium fluoride in accordance with the present invention. 1. H₂SiF₆ + 2KCl = K₂SiF₆ + 2HCl 2H₂SiF₆ + SiO₂ + 6KF = 3K₂SiF₆ + 2H₂O

SiO₂ + 3H₂SiF₆ + 2KCl + 6KF = 4K₂SiF₆ + 2HCl + 2H₂O

2. 4KOH + K₂SiF₆ = 6KF + SiO₂ + 2H₂O

The invention will now be described with respect to the following examples which do not limit the invention in any way and only exemplify the invention

Example-1 Preparation of Potassium Fluorosilicatc (K2SiF6)

1.697 Kg hydrofluorosilicic acid solution (18 % w/w solution in water) was heated to 98⁰C in a 2 liter four necked round bottom flask equipped with overhead stirrer. 9.26 gm silica cake (having 40 % moisture) was added to it and then 322.261 gm of by product of organic fluorination (mixture of Potassium chloride and Potassium fluoride) reaction was added with continuous stirring to round bottom flask. The reaction was further continued for 45 minutes at 100⁰C.

The reaction mixture was cooled to room temperature and then filtered. The product was isolated and washed with 1.5 ltr water to remove any solubles including hydrochloric acid generated in the reaction. The resultant product was dried inside oven at 13O⁰C for 3 hrs to obtain 482.7 gm of white solid powder of Potassium fluorosilicate. The product has more than 99 % purity and more than 99 % isolated product yield.

Example 2: Preparation of Potassium Fluoride 66.67 gm of wet cake of Potassium Fluorosilicate (having moisture content 40%) and 58.33 gm of water were added to 500 ml poly propylene conical flask equipped with overhead stirrer. Then, the mixture was heated to 92⁰C and 85.2 gm of potassium hydroxide solution (45%) was added slowly within 20 minutes. Mole ratio of potassium hydroxide to Potassium silico fluoride was 3.76: 1. The reaction was continued for 40 minute at 92⁰C. The reaction mixture was cooled to room temperature (25 deg C) and stirred for another 30 minutes and then filtered. 184 gm of filtrate having 29 % potassium fluoride (by wt) and 0.4% unreacted potassium hydroxide was obtained and it was used for making neutral potassium fluoride solution. 25.8 gm silica wet cake residue having 40% moisture was obtained and it was washed with 140gm of water. Then it was filtered through filter paper to remove soluble potassium fluoride & un-reacted potassium hydroxide from silica.

20.8 gm of wet residue (mostly silica) was obtained and it was dried at 110 deg C for 4 hr to get 12.5 gm of dry silica. This silica was used in preparation of potassium fluorosilicate in next batch of reaction. Out of total potassium hydroxide used in reaction, 97% converted to potassium fluoride and 3% remained as unreacted. Recovery of used potassium fluoride and unreacted potassium hydroxide in reaction was close to 100%. 91.8% potassium silicofluoride has been converted to potassium fluoride. The pH of filtrate of was 10.5.

Example-3 Effect of mole ratio of KOH: K₂SiF₆

The reaction was carried out for different reaction mole ratio of potassium hydroxide: Potassium silico fluoride following the same process as described in Example 2, wherein 66.67 gm of wet cake of Potassium Fluorosilicate (moisture: 40%) and 58.33 gm of water added to 500 ml poly propylene conical flask equipped with overhead stirrer. Reaction was carried out with different amount of 45% potassium hydroxide solution.

The results showed that if the mole ratio of potassium hydroxide to potassium silicofluoride was kept less than 3.7, the conversion of potassium silicofluoride was found to be low making the process uneconomical. On the other hand, if the mole ratio of potassium hydroxide to potassium silicofluoride was kept more that 4, the amount of unreacted potassium hydroxide and silica present in solution was found to be high. This also makes process uneconomical as its neutralization cost in next step was found to be increased. Example-4 Effect of concentration of potassium silicofluoridc in aqueous slurry

30 gm of potassium silicofluoride (99.3%) having different slurry concentration was heated to 95⁰C in PP conical flask and 35.2gm of potassium hydroxide pellets (87%) were added. The reaction was carried out at 95-100 C. Addition time for solid potassium hydroxide was 20 minutes. Agitator speed 450 rpm. Mole ratio of (potassium hydroxide: potassium silicofluoride) = 4: 1.

The observations obtained from the above table showed that the potassium hydroxide was preferably added in its aqueous form. On the other hand, it was found to be desirable that if potassium hydroxide was added in solid form then the concentration of potassium silicofluoride in aqueous slurry was kept below 25%. Example-5 Effect of addition of KOH

In example- 1, 45% solution of potassium hydroxide was reacted with potassium silicofluoride slurry and in Example-4, potassium hydroxide pellets were added to the potassium silicofluoride slurry within 10 minutes. It was found that upon fast addition of potassium hydroxide into the reaction mass, the lumps were formed making the stirring difficult. Thus, slow addition of potassium hydroxide either as a solution or as a solid form was found to be desirable.

Example 6: Purification of Potassium fluoride

184 gm of filtrate having 29 % potassium fluoride (by wt) and 0.4% unreacted potassium hydroxide was obtained from example- 1 was used for making neutral potassium fluoride solution.

Example-6A

The pH of filtrate (184 gm) was adjusted to 7.0 using 1.5 gm of 48% aqueous solution of hydrofluoric acid. It was heated for 30minutes at 105⁰C. Then it was cooled to room temperature (25 deg C) and filtered. 180 gm of filtrate (potassium fluoride solution) was obtained having about 29.5 % (by weight) of potassium fluoride. Water was removed by heating to obtain solid potassium fluoride. Yield: 53.1 gm. Percentage yield: 99.5% Percentage Purity: 99.3% Example-6B

The pH of filtrate (184 gm) was adjusted to 7.0 using 16 gm of 18% aqueous solution of flurosilicilic acid. It was heated for 30 minutes at 105⁰C. Then it was cooled to room temperature (25 deg C) and filtered. 191 gm of filtrate (potassium fluoride solution) was obtained having about 27.36% (by weight) of potassium fluoride. Water was removed by heating to get solid potassium fluoride. Yield: 52.26 gm. Percentage Yield: 98% Percentage purity: 99.3%.

Technical advancement:

The process as described herein above offers several advancement over processes disclosed in the prior art in terms of yield, purity, cost- effectiveness and the recoverability of almost all the reagents. Furthermore, the process uses toxic waste materials from fertilizer industry and organic fluorination reaction for effective manufacture of potassium fluoride in high purity (more than 99%) and high yield which could have wasted otherwise which in turn saves the natural resources and reduces the environmental pollution. The process in accordance with the present invention results into potassium fluoride having low silica content and heavy metals.

While considerable emphasis has been placed herein on the specific steps of the preferred process, it will be appreciated that many steps can be made and that many changes can be made in the preferred steps without departing from the principles of the invention. These and other changes in the preferred steps of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

Claims

Claims:

1. A process for manufacturing high purity potassium fluoride comprising the following steps: a. reacting a by product of the fertilizer industry predominantly containing hydrofluorosilicic acid with a suspension containing potassium chloride, potassium fluoride and silica cake to obtain a first resultant mass containing potassium silicofluoride and hydrochloric acid; b. cooling the first resultant mass to room temperature; c. filtering the first resultant mass to obtain a first residue containing potassium silicofluoride; d. washing the first residue with water to obtain a washed mass predominantly containing potassium silicofluoride; e. forming an aqueous slurry containing the washed mass of potassium silicofluoride; f. reacting the slurry with potassium hydroxide to obtain a second resultant mass predominantly containing potassium fluoride in solution and silica; g. filtering the second resultant mass to obtain a first filtrate predominantly containing potassium fluoride, unreacted potassium hydroxide and soluble silica and a second residue containing insoluble silica; h. reacting the first filtrate with hydrogen fluoride or hydrofluorosilicic acid to adjust the pH between 6.5 to 8.5 and to obtain a third resultant mass; i. filtering the third resultant mass to obtain a second filtrate containing potassium fluoride and a third residue predominantly containing potassium silicofluoride; and j. heating the second filtrate to obtain solid potassium fluoride having purity ranging from 99% to 99.99%.

2. A process as claimed in claim 1, wherein the reactants have the following relationship a. for 2 moles of KCl at least 1 mole of hydrofluorosilicic acid is added; and b. for each 6 moles of KF present along with KCl at least 2 moles of hydrofluorosilicic acid are added along with about 1 mole of silica

3. A process as claimed in claim 1, wherein the concentration of hydrofluorosilicic acid is in the range of 5 to 25 in the process step (a).

4. A process as claimed in claim 1, wherein the process step (a) is carried out at 40-100⁰C for 40-45 minutes.

5. A process as claimed in claim 1, wherein the potassium chloride in the process step (a) is a by-product of organic fluorination reaction.

6. A process as claimed in claim 4, wherein the potassium chloride is present in the range of 0% to 100% along with potassium fluoride and with or without organic impurities.

7. A process as claimed in claim 1, wherein the suspension of potassium chloride is added as a slurry or solid form to the hydrofluorosilicic acid in the process step (a).

8. A process as claimed in claim 1, wherein the first resultant mass is filtered to remove hydrochloric acid in the process step (c).

9. A process as claimed in claim 1, wherein the potassium silicofluoride is dried in an oven to remove moisture in the process step (d).

10. A process as claimed in claim 1 , wherein the potassium silicofluoride is obtained as a white solid powder having purity more than 99% in the process step (d).

1 1. A process as claimed in claim 1, wherein the molar ratio of potassium hydroxide to potassium silicofluoride is in the range of 3.5: 1 to 4.1 : 1 in the process step (f).

12. A process as claimed in claim 1, wherein the potassium hydroxide is used as a solid or as an aqueous solution in the process step (f).

13. A process as claimed in claim 1, wherein the potassium hydroxide is added slowly over a period of 5 min to 300 min in the process step

14.A process as claimed in claim 1, wherein the concentration of potassium silico fluoride in aqueous slurry is in the range of 5 to 50 % in the process step (f).

15.A process as claimed in claim 1, wherein the process step (g) is carried out at 5 to 105⁰C for 40-45 minutes.

16.A process as claimed in claim 15, wherein the insoluble silica is washed with water to remove potassium fluoride or unreacted potassium hydroxide present with silica in the process step (g).

17. A process as claimed in claim 15, wherein the silica is obtained as a silica cake having 40% moisture content in the process step (g).

18. A process as claimed in claim 17, wherein the silica cake is dried at 110⁰C for 4 hrs. to obtain dry silica in the process step (g).

19.A process as claimed in claim 18, wherein the dry silica is used to prepare potassium silicofluoride.

20. A process as claimed in claim 17, wherein the washing solution is used to prepare slurry of potassium fluorosilicatc or potassium hydroxide in the process step (g).

2 LA process as claimed in claim 1, wherein the process step (h) is carried out at 20-100 ⁰C for 10 to 120 minutes.

22.A process as claimed in claim 1, wherein the third resultant mass is filtered out and the third residue obtained in the process step (i) can be reused.

23.A process as claimed in claim 1, wherein the solid potassium fluoride is obtained by heating the second filtrate to remove water in the process step Q).

24.A process as claimed in claim 1, wherein the solid potassium fluoride is obtained by air drying the filtrate in the process step Q).