Title of Invention

A PROCESS FOR THE PREPARATION OF PURE POTASSIUM IODATE SOLUTION FROM IODINE AND POTASSIUM HYDROXIDE FOR SALT IODIZATION

Abstract

A process for the preparation of pure potassium iodate solution from iodine and potassium hydroxide for salt iodization This invention relates to a process for the preparation of pure potassium iodate solution from iodine and potassium hydroxide for salt iodization. This process is carried out in a two-compartment anion exchange membrane flow cell by allowing a solution of 1.75% (w/v) iodine in 1.25% (w/v) potassium hydroxide through anode and 2-6%(w/v) of potassium hydroxide or sodium hydroxide through cathode compartments at 25-50 ml/min under gravity. An expanded precious metal oxide coated titanium as anode and an expanded thin stainless steel sheet as cathode are used. The current is varied across the electrodes between 30-60 A against 3 - 5 V at 30-40 °C.

Full Text

The present invention relates to a process for the preparation of pure potassium iodate solution from iodine and potassium hydroxide for salt iodization using an anion exchange membrane electrochemical flow cell. Alkali iodates and calcium iodate are known to be useful in the iodization of salt for edible purposes to prevent thyroid gland diseases, in the preparation of certain medicines, as conditioning agents in bakeries and as a reagent in chemical laboratories. The solution of potassium iodate in dilute sulfuric acid is used for the preliminary surface cleaning of plastics before the metal coatings (T. A. Arzhanova, Russ. RU 2,077,605,20 Apr. 1977). Potassium iodate is preferred to potassium iodide as an iodizing agent in the manufacture of iodized salts for human consumption in countries like India. Iodine is required in trace quantities for the various metabolic activities of the living beings. Its deficiency for long time, leads to biological disorders like gout, mental retardation, deaf mutism, short stature, cretinism and risk of death during the childhood (Scientific Encyclopedia, Van Nostrand's, 8th ed. D. M. Considine, Ed. Van Nostrand Reinhold, New York, p!751-1755, 1995). It is supplemented by iodization of edible salt as most of people consume salt independent of religion, caste creed and sex. Potassium'iodate is preferred to iodine and its other salts for the iodization of salt in all Asian countries including India because of weather conditions and its stability. Reference is made to S. Balagopalan et. al. (CSIR, Indian IN 173, 008, 22nd Jan. 1994) have disclosed that a flow reactor with an externally insulated stainless steel tank as cathode and a cylindrical lead oxide coated graphite anode placed at a distance of 1.5 - 3 mm gap can be used for the preparation of potassium iodate from elemental iodine in a potassium hydroxide solution. The product is cooled and isolated. The product obtained by this method contains impurities like lead, which from the viewpoint of health, is hazardous and hence not useful as a micronutrient in food ingredients. Moreover, the product potassium iodate and hypo iodite are not stable at the cathode. Sodium and potassium iodates can be prepared by a multi-stage method as reported independently by J. von Liebig (Ogg. Ann. 1832, 24,362) and G. S. Serullas (Ann. Chim. Phys. 1830, 43, 125). It comprises saturating chlorine gas in water containing a fine suspension of iodine, neutralizing the liquid with the respective carbonate; and again passing chlorine until the iodine is all dissolved, and further neutralizing with the carbonate. The solution is to be evaporated to one-tenth of its volume, mixed with half its volume of alcohol, and the sodium/potassium chloride was washed from the precipitate by means of aqueous alcoholic solutions. The sodium carbonate is to be removed by treatment with acetic acid, and the sodium acetate is then washed with alcohol. This method is not advantageous as it is a multi stage process and involves the use of hazardous chlorine gas. Moreover, it requires heating to concentrate the solutions and adding alcohol to precipitate the iodate salts. J. L. Gay Lussac (Ann. Chim. Phys. 1814, P7(l), 5) has prepared the potassium iodate by the action of elemental iodine on hot potassium hydroxide. The salt is separated by the addition of alcohol after the concentration. In this method, one-sixth of the total iodine only gets converted to potassium iodate and the rest to potassium iodide, which needs to be oxidized again to potassium iodate by an additional oxidizing agent. O. Henry (Journ. Pharm. 1862, 18, 345) has obtained potassium iodate by melting together an intimate mixture of potassium iodide and potassium chlorate up to the temperature at which chlorate begins to decompose. The product is separated from the un-reacted potassium chlorate by the fractional crystallization. This method requires special equipment and intensive care, as the potassium chlorate is a strong oxidizing agent, corrosive and explosive. J. S. Stas (Mem. Acad. Belgque, 1865, 35, 3) has made potassium iodate by heating elemental iodine or potassium iodide with potassium chlorate. The drawbacks of this method that it requires special equipment and intensive care as the potassium chlorate is a very strong oxidizing agent, corrosive and explosive. N. H. Hugo et. al. (Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1,121,540; 1 May 1996, Ch.) have disclosed a method of preparing potassium iodate by electrolysis using elemental iodine and potassium hydroxide. In this method iodine is reacted with 30-40 lit of potassium hydroxide in a cooling reactor and subjected to electrolysis in multistage series-connected electrolytic cells. The voltage of each cell is maintained at 2.0 - 3.5 V and the flow rate at 5 - 10 lit/min. The current density was between 0.1 and 0.2 A/m2 while the temperature of the last cell was reported to be 80-100 °C. The potassium iodate is crystallized and separated. 1 - 3 g/lit potassium dichromate is added to the electrolytes to prevent the reduction of iodates and hypo iodite on the cathode. The iodate obtained by this method is not useful for edible purposes as they contain trace quantities of dichromate and/or chromate, which are known to be carcinogens. According to L. Henry (Ber. 1870, 3, 893), potassium iodate can be prepared by passing chlorine gas into finely divided suspension of elemental iodine in water until all was dissolved. Latter, a mol of potassium chlorate per gram atom of iodine is added to the resulting solution. The mixture is warmed to get potassium iodate with the evolution of chlorine gas. Chlorine is an air pollutant and corrosive. It requires specially made vessels to store and transport. Hence, the use of this method is limited. Moreover, this method needs to purify the product from potassium chloride. L. P. de St. Giles (Compt. Rend. 1858, 46, 624) has reported that the potassium iodate can alternately be prepared by the oxidation of potassium iodide with a solution of costly potassium permanganate.. The product obtained by this method again needs to purify from manganese dioxide in order to make it useful for edible purposes. Recently P. K. Ghosh et. al. have disclosed (1057/DEL/2000) the preparation of solid alkali iodates from the corresponding alkali iodides using an ion exchange membrane flow reactor. However, this method requires pure alkali iodides as the starting materials, which enhance the cost of their production. The main object of the present invention is to provide a method for in-situ preparation of pure 2.8-3.0% potassium iodate solution for direct salt iodization from elemental iodine and potassium hydroxide using an ion-exchange membrane electrochemical flow cell which obv idles the drawbacks as detailed above. Another object of the present invention is to use an ion-exchange membrane as solid polymer electrolyte for supporting and enhancing the electrolysis and to keep the products formed at the cathode and anode compartments separately. Still another object of the present invention is to use metal oxide coated titanium as stable catalytic anode by replacing the corrosive chlorine and costly oxo-compounds that are used earlier. Still another object of the present invention is to use membrane cell for obtaining potassium iodate solution from iodine without the problem of separation of side products. Accordingly the present invention provides a process for the preparation of pure potassium iodate solution from iodine and potassium hydroxide for salt iodization which comprises characterized in that the electrochemical oxidation of 1.75% (w/v) iodine in 1.25% (w/v) potassium hydroxide being carried out in a two compartmental anion exchange membrane cell flowing through anode compartment and 3-5%(w/v) potassium hydroxide or sodium hydroxide through cathode compartment at 25-50 ml/min under gravity using an expanded precious metal oxide coated titanium anode and an expanded stainless steel thin sheet as cathode and by controlling the current between 30-60 A against 3-5 V at a temperature of 30-40 °C to obtain pure potassium ioadte. In an embodiment of the present invention, an anion exchange membrane placed between the two working electrodes may work as the solid polymer electrolyte wherein the mobility of the counter ion in the membrane drives the cell current at reduced cell resistance. In another embodiment of the present invention, a solution of 1.75% (w/v) iodine in 1.25% (w/v) potassium hydroxide may be used as anolyte. In yet another embodiment of the present invention, a solution in the range of 2-6% (w/v) potassium hydroxide or sodium hydroxide may be used as the catholyte. In yet another embodiment of the present invention, the anolyte and catholyte solutions may be passed through the respective compartments in the range of 25-50 ml/min under gravity. In yet another embodiment of the present invention, an expanded precious triple metal oxide coated titanium anode having surface area in the range of 500-1200 cm2 and an expanded stainless steel having surface area in the range of 800-1300 cm2 may be used. In yet another embodiment of the present invention, the current at the electrodes may be varied in the range of 30-60 A against a cell voltage in the range of 3-5 V. In yet another embodiment of the present invention, the cell temperature may be in the range of 3 0-40 °C. According to the present invention, elemental iodine rapidly dissolves in alkaline solution according to the equation (eq. 1). The iodide in turn gets oxidized to iodine by the loss of one electron per atom at the anode producing elemental iodine and potassium ion (eq. 2). 3I2 + 6KOH » 5KI + KIO3 (1) 2KI -» I2 + 2K+ + 2e~ (2) The counter reaction at the cathode is the reduction of water liberating hydrogen gas with the release of hydroxyl (OFT) ions (eq. 3). 2H2O + 2e~ -> H2 + 2 OH~ (3) The hydroxyl (anion) ions liberated in the cathode compartment migrate towards anode compartment for charge balancing, by passing over the ion-exchange membrane producing an equal amount of potassium hydroxide as the co-product. The overall cell reaction is then given as eq. 4. 2KI + 2H2O ->• I2 + H2 + 2KOH(4) The iodine and the potassium hydroxide that are produced in the anode compartment readily react together to produce potassium iodate and potassium iodide as shown in eq.5. 3I2 + 6KOH -» SKI + KIO3 (5) This was conducted in a pilot scale with a rectangular PVC cell (72.5 cm x 34.5 cm x 5.5 cm) embodied with an expanded sheet of special triple metal oxide coated titanium anode (1000 cm2) and an expanded sheet of stainless steel (1010 cm2) cathode on either side of the membrane. The capacity of each of the electrode compartments was about 1.6 liter. A solution of 1.75% (w/v) iodine dissolved in 1.25% (w/v) potassium hydroxide was used as the anolyte for this purpose. A solution of 2-6% (w/v) potassium hydroxide or sodium hydroxide was used as catholyte. An anion exchange membrane of surface area of about 1930 cm2 'vas used. The cell temperature remained at 30 to 40 °C through out the electrolysis. The anolyte and catholyte solutions were allowed to flow at 25-50 ml/min through the respective electrode compartments under gravity while the electrolysis was on. A constant current between 30-60 A was applied at anode. The cell potential was found to be constant between 3-5 V. It is preferable to oxidize the solution moderately at high current between 30-40 A to achieve pure potassium iodate solution at the reasonably high flow rates with high coulombic efficiency. It is generally preferable to use 2-6% potassium hydroxide or sodium hydroxide as catholyte to get pure potassium iodate solution. The process according to the present invention was started at 28 °C at the initial stages and increased to 30-40 °C depending on the current applied at the anode. The temperature did not change during the operation. No appreciable loss was found either in potassium iodide or potassium iodate content due to transportation to the cathode compartment. The membrane and the cell body were also found to be intact even after carrying the experiments for several hours. In all the cases, the anolyte solution at the end of electrolysis was basic having pH between 12.1 and 12.3. During the electrolysis, one ml of the anolyte solution was collected during the electrolysis time. It was analyzed for the iodate composition by volumetric procedure using standard sodium thiosulfate, Na2S2O3 using 10 ml of 4% potassium iodide solution and 10 ml of 2 N sulfuric acid and starch as an indicator (A. I. Vogel A Text-Book of Quantitative Inorganic Analysis, 3rd ed., The English Language Book Society, Longman, London 1969, 349). The important inventive steps of the present invention are i) a mixture of iodine (1.75%, w/v) and potassium hydroxide (1.25%, w/v) in 1:2 mole ratio is used, ii) no additional supporting electrolyte is required, iii) the cell works at 30-60 A against 3 - 5 V at 30 to 40 °C, iv) the potassium iodate solution is obtained at 25-50 ml/min under single pass conditions with a maximum cell efficiency, v) an indigenous anion exchange membrane is used and vi) the anion exchange membrane keeps the potassium iodate away from the cathode. The following examples are given by way of illustrations of the present invention and should not be construed to limit the scope of the present invention. EXAMPLE 1: The preparation of potassium iodate solution was carried out by the electrolysis of 1.75% (w/v) iodine in 1.25% potassium hydroxide solution at 30 A against 3.4 V at a flow rate of 27 ml/min anolyte. A solution of 5% sodium hydroxide was circulated through the cathode compartment at speed of 30 ml/min. The temperature of the solution was measured as 30 °C. The potassium iodate in the anolyte solution was 3% (w/v) with about 100% coulombic efficiency. The pH of the solution was 12.23. EXAMPLE 2: Using the same cell and the solutions having the composition as in example 1, i. e. 1.75% (w/v) iodine in 1.25% potassium hydroxide solution as anolyte and of 5% sodium hydroxide as catholyte solution, the electrolysis was carried out at a current of 40 A. The anolyte and catholyte were set to flow at 32 ml/min. The cell potential was 4.2 V. The solution temperature during the operation was remained constant at 35 °C. The potassium iodate content in the solution as estimated to be 2.996% (w/v) corresponding to nearly 100% coulombic efficiency. The pH of the solution was 12.25. EXAMPLE 3: A solution having 1.75% (w/v) of iodine in 1.25% potassium hydroxide was electrolyzed in the above cell at a current of 50 A while a solution of 5% sodium hydroxide was served as catholyte. The anolyte and catholyte were set to flow at 40 ml/min. The cell potential was steady 4.6 V and the anolyte temperature was found to be at 39 °C. 2.996% (w/v) potassium iodate was obtained. The coulombic efficiency was around 100%. The pH of the solution was 12.21. EXAMPLE 4: A solution having 1.75% (w/v) of iodine in 1.25% potassium hydroxide was electrolyzed in the above cell at a current of 60 A while a solution of 5% sodium hydroxide was served as catholyte. The anolyte and catholyte were set to flow at 50 ml/min. The cell potential was steady 5.1 V and the anolyte temperature was found to be at 40 °C. 3% (w/v) potassium iodate was obtained. The coulombic efficiency was around 100%. The pH of the solution was 12.26. EXAMPLES: A solution having 1.75% (w/v) of iodine in 1.25% potassium hydroxide was electrolyzed in the above cell at a current of 40 A while a solution of 5% potassium hydroxide was served as catholyte. The anolyte and catholyte were set to flow at 32 ml/min. The cell potential was steady 4.7 V and the anolyte temperature was found to be at 35 °C. 3% (w/v) potassium iodate was obtained. The coulombic efficiency was around 100%. The pH of the solution was 12.22. EXAMPLE 6: A solution having 1.75% (w/v) of iodine in 1.25% potassium hydroxide was electrolyzed in the above cell at a current of 40 A while a solution of 3% potassium hydroxide was served as catholyte. The anolyte and catholyte were set to flow at 32 ml/min. The cell potential was steady at 4.5 V and the anolyte temperature was found to be at 35 °C. Potassium iodate greater than 2.95% (w/v) was obtained. The coulombic efficiency was above 98%. The pH of the solution was 12.21. The main advantages of the present invention are: 1. it is an easy and alternate method to the existing ones for preparing a solution of potassium iodate from iodine to use directly in salt iodization, 2. it occurs at low energy inputs and low current densities with high coulombic efficiency, 3. there is no need for the addition of any other compound, electrolyte, or special catalyst which adds to the production cost and complicates the purification, 4. it does not involve any corrosive oxidants like chlorine and other oxo-compounds which attacks the materials of the reaction vessels, 5. it operates under ambient conditions of temperature and pressure which is energy saving, 6. the procedure does not involve any toxic gases or release any unwanted by products, 7. hydrogen is liberated as a by-product which can be recycled in a fuel cell, if desired, 8. the technique involves a compact electrolysis cell which has the advantage of conserving energy by avoiding certain steps like heating/cooling, addition of neutralizers or separation of unwanted materials, 9. the method involves an inexpensive and easily moldable plastic cell with an inexpensive cathode and a non-polarizable anode for effecting the electrolysis and 10. the membrane is stable towards iodide and iodate, hence the life of the cell is unlimited. We Claim: 1. A process for the preparation of pure potassium iodate solution from iodine and potassium hydroxide for salt iodization which comprises characterized in that the electrochemical oxidation of 1.75% (w/v) iodine in 1.25% (w/v) potassium hydroxide being carried out in a two compartmental anion exchange membrane cell flowing through anode compartment and 3-5%(w/v) potassium hydroxide or sodium hydroxide through cathode compartment at 25-50 ml/min under gravity using an expanded precious metal oxide coated titanium anode and an expanded stainless steel thin sheet as cathode and by controlling the current between 30-60 A against 3-5 V at a temperature of 30-40 °C to obtain pure potassium ioadte. 2. A process as claimed in claim 1, wherein an anion exchange membrane placed between the anode and the cathode at a distance of 4-6 mm, was used. 3. A process as claimed in claims 1-3, wherein an expanded precious triple metal oxide coated titanium anode having 500-1200 cm2 surface area and an expanded stainless steel sheet cathode having 800-1300 cm were used. 4. A process for the preparation of pure potassium iodate solution from iodine and potassium hydroxide for salt iodization substantially as herein described with reference to the examples accompanying this specification.

Documents:

479-DEL-2003-Abstract-(02-01-2009).pdf

479-del-2003-abstract.pdf

479-DEL-2003-Claims-(02-01-2009).pdf

479-DEL-2003-Claims-(28-01-2009).pdf

479-del-2003-claims.pdf

479-DEL-2003-Correspondence-Others-(02-01-2009).pdf

479-DEL-2003-Correspondence-Others-(03-02-2009).pdf

479-DEL-2003-Correspondence-Others-(28-01-2009).pdf

479-del-2003-correspondence-others.pdf

479-del-2003-correspondence-po.pdf

479-DEL-2003-Description (Complete)-(28-01-2009).pdf

479-del-2003-description (complete).pdf

479-DEL-2003-Form-1-(03-02-2009).pdf

479-DEL-2003-Form-1-(28-01-2009).pdf

479-del-2003-form-1.pdf

479-del-2003-form-18.pdf

479-del-2003-form-2.pdf

479-DEL-2003-Form-3-(02-01-2009).pdf

479-del-2003-form-3.pdf

479-DEL-2003-Petition-137-(03-02-2009).pdf