A process for the production of alkali metal hypophosphites

Abstract: High purity alkali metal hypophosphites are produced in high efficiency by a simple step of using no separating agents other than white phosphorus, alkali metal hydroxides and water as reaction raw materials, resulting in no need of a complicated procedure for removing impurities. The process comprises reacting white phosphorus and an alkali metal hydroxide in an aqueous medium to prepare a crude solution of an alkali metal hypophosphite containing the phosphite, feeding the crude solution to a desalting compartment of an electrodialytic cell in which cation exchange membranes and univalent anion permeable membranes are alternately arranged between a cathode and an anode to form at least one pair of a desalting compartment and a concentrating compartment, so as to separate the hypophosphite from the crude solution, during which a dilute solution of an alkali hypophosphite is fed to the concentrating compartment, subjecting them to electrodialysis, withdrawing the purified solution of the alkali metal hypophosphite from the concentrating compartment and concentrating and crystallizing the thus purified solution.

Description: This invention relates to a process for the production of an alkali metal hypophosphite by an ion-exchange membrane electrodialysis method and a process for the separate production of an alkali metal hypophosphite and an alkali metal phosphite by an ion-exchange membrane electrodialysis method, Alkali metal hypophosphites have been used as synthetic resin stabilizers, catalysts for organic synthesis, reducing agents for industrial use, etc. and in particular, sodium hypophosphite has widely been used as a chemical for electroless plating. Alkali metal hypophosphites are ordinarily prepared by the reaction of white phosphorus with an aqueous solution of alkali metal hydroxide, during which a side reaction of forming the phosphite unavoidably takes place with formation of the hypophosphite. Thus, there is always a considerable amount of the phosphite in the reaction mixture. The amount of the phosphite, depending on the reaction conditions, reaches 50 % when it is formed in most amount. In the general process for the production of an alkali metal hypophosphite, therefore, it is indispensable to provide a step of separating and removing the phosphite from the reaction mixture. Up to the present time, separation of the phosphite has been carried out by precipitating and removing it as calcium phosphite utilizing the fact that calcium hypophosphite is water-soluble, while calcium phosphite is scarcely water-soluble. This nay be accomplished by adding a water-soluble calcium compound as a precipitant to the reaction mixture. When using calcium hydroxide as a precipitant, however, the pH is raised by the precipitating reaction in the initial period, the concentration of calcium ion is lowered and a part of the phosphite remains in the solution so that complete separation of the phosphite is impossible. When using a calcium salt such as calcium chloride, a new impurity i.e. chloride ion is brought into the reaction system, which must subsequently be removed. In order to avoid the disadvantage cause by such a calcium precipitant, it has been Proposed in US Patent No. 2,938,770 to precipitate and separate calcium phosphite with a solution of calcium hypophosphite. In this case, however, it is necessary to use a separation treatment of sodium hypophosphite with an ion exchange resin and the steps including a regenerating treatment are very complicated. Another method for separating the phosphite by-product as a less soluble calcium salt comprises reacting white phosphorus and an alkali metal hydroxide in the presence of calcium hydroxide, for example, by adding white phosphorus to a slurry of calcium hydroxide suspended in an aqueous solution of the alkali metal hydroxide. In this method, the soluble calcium content is contained in the reaction system, so the phosphite by-product is precipitated as less soluble calcium phosphite, as soon as it is formed, without changing the pH of the solution. Consequently, the phosphite is surely separated with certainty and no other impurities are introduced. In the filtrate from which the calcium phosphite has been precipitated and separated, however, there are dissolved excess calcium hydroxide and soluble calcium hypophosphite in addition to the alkali metal hypophosphite and, therefore, it is necessary to remove them. A method of removing the dissolved calcium content has been proposed in German Patent No. 2,006,632, which comprises first neutralizing the filtrate with carbon dioxide to precipitate the excess calcium hydroxide in the form of calcium carbonate, then adding sodium carbonate to precipitate calcium carbonate from the calcium hypophosphite and thereafter neutralizing the remaining sodium carbonate with hypophosphorous acid. US Patent No. 4,379,132 describes a method comprising precipitating and separating a part of the dissolved calcium content with phosphoric acid or an acid phosphate with the co-production of an insoluble calcium phosphate and converting the calcium hypophosphite dissolved in the filtrate into the sodium salt by the use of an ion exchange resin of the sodium type. Furthermore, US Patent Ho. 4,521,391 describes a method comprising adding sodium hydrogen carbonate to a filtrate after separating calcium phosphite to precipitate dissolved calcium hydroxide and further adding sodium carbonate to the filtrate to precipitate and separate the dissolved calcium hypophosphite as calcium carbonate. As described above, the method of the prior art in which the phosphite is subjected to solid-liquid separation as the less soluble calcium salt must necessarily be carried out in a batchwise manner and thus requires the steps of separating calcium phosphite and removing soluble impurities resulting from the precipitants, so that the process is complicated and has a low efficiency. Sodium hypophosphite is very useful as a reducing agent for electroless plating, but of late, requirements for the quality of sodium hypophosphite have become more stringent since the application of the electroless plating process has been introduced to the high tech field and high purity sodium hypophosphite is now required with a decreased content of impurities which have a bad effect on the adhesiveness of plating coatings and the reactivity of plating solutions. However, the prior art has been unable fully to deal with these problems. The present invention provides a process for producing high purity alkali metal hypophosphites in high efficiency by a simple step of using no separating agents other than white phosphorus, alkali metal hydroxides and water as reaction raw materials, which avoids the need for a complicated procedure for removing impurities. The present invention also provides a process for the continuous production of alkali metal hypophosphites using only liquid processing. Furthermore the present invention provides a new process for the separation of alkali metal hypophosphites and alkali metal phosphites and a process for separately producing alkali metal hypophosphites and alkali metal phosphites by the reaction of white phosphorus and an aqueous solution of alkali metal hydroxides. According to the present invention a process for the production of an alkali metal hypophosphite comprises reacting white phosphorus and an alkali metal hydroxide in an aqueous medium to prepare a crude solution of an alkali metal hypophosphite containing the phosphite, feeding the crude solution to a desalting compartment of an electrodialytic cell, in which cation exchange membranes and univalent anion permeable membranes are alternately arranged between a cathode compartment and an anode compartment, which will hereinafter be referred to simply as "cathode and anode", to form at least one pair of a desalting compartment and a concentrating compartment, allowing the hypophosphite in the crude solution of the alkali metal hypophosphite to selectively permeate into the concentrating compartment and thereby separating the hypophosphite from the phosphite remaining in the desalting compartment, then withdrawing the purified solution of the alkali metal hypophosphite from the concentrating compartment, concentrating the purified solution and crystallizing the alkali metal hypophosphite, while withdrawing the solution of the alkali metal phosphite as a byproduct from the desalting compartment. The accompanying drawing illustrates in detail the principle and merits of the present invention. Fig. 1 is a block diagram showing one embodiment of the process for the continuous production of alkali metal hypophosphites according to the present invention. The Applicants have sought to develop a process for producing an alkali metal hypophosphite without effecting a plurality of solid-liquid separating operations as in the prior art, and have found that a high purity alkali metal hypophosphite can be obtained by reacting white phosphorus and an alkali metal hydroxide to obtain a crude solution of alkali metal hypophosphite containing the phosphite and then subjecting the crude solution to electrodialysis using ion-exchange membranes to remove the phosphite. It has been found that a mixed solution containing an alkali metal hypophosphite and an alkali metal phosphite can effectively be separated by an electrodialytic apparatus using specified ion-exchange membranes Including univalent anion permselective membranes, in particular, when the composition and pH thereof is in a specified range. That is, the selective permeability of the hypophosphite ion in the presence of the phosphite ion largely depends on the pH and the composition of the solution. The present invention is based on this finding. According to the present invention, alkali metal hypophosphites can effectively be prepared by a very simple operation without effecting complicated solid-liquid separating operations in many stages, as in the prior art. The present invention provides a process for the production of an alkali metal hypophosphite which comprises reacting white phosphorus and an alkali metal hydroxide in an aqueous medium to prepare a crude solution of an alkali metal hypophosphite containing the phosphite as a byproduct, feeding and electrodialysing the crude solution of the alkali metal hypophosphite in a desalting compartment of an electrodialytic cell, in which cation exchange membranes and univalent anion perm-selective membranes are alternately arranged between a cathode compartment and an anode compartment to form at least one pair of desalting compartment and a concentrating compartment, allowing the hypophosphite in the crude solution of the alkali metal hypophosphite to selectively permeate into the concentrating compartment and thereby separating the hypophosphite from the phosphite remaining in the desalting compartment, then withdrawing the purified solution of the alkali metal hypophosphite from the concentrating compartment, concentrating the purified solution and crystallizing the alkali metal hypophosphite, while withdrawing the solution of the alkali metal phosphite as a byproduct from the desalting compartment. Furthermore, the present invention is concerned with a process for the continuous production of alkali metal hypophosphites, for the separation of alkali metal hypophosphites and alkali metal phosphites and for the separate production of the same, based on the above described fundamental process. In this specification, by alkali metals are meant sodium, potassium and lithium. In Table 1 are shown the compositions of a mixed solution of an alkali metal hypophosphite and alkali metal phosphite with a pH of 10 (hereinafter referred to as "D solution") and a solution obtained by subjecting the mixed solution to electrodialysis in an Ion-exchange membrane electrodialytic apparatus composed of cation-exchange membranes and univalent ion permselective membranes (hereinafter referred to as "C solution"): NaH₂PO₂/(NaH₂PO₂ + Na₂HPO₃) (g/g) D 0.1 0.15 0.2 0.3 0.5 0.6 0.8 0.9 0.97 C 0.45 0.84 0.9 0.94 0.97 0.98 0.99 0.992 0.998 It will clearly be understood from the results of Table 1 that C rapidly increases at about D = 0.1 and the selective permeability of hypophosphite is markedly increased, while C is 0.9 or more at D = 0.2 or more and the selectivity is further increased. When C solution having a C value of 0.9 or more obtained from D solution having a D value of 0.2 or more is again subjected to electrodialysis, the C value is 0.99 or more. That is, the permeability of phosphite is 1/10 or less of that of hypophosphite at D = 0.2 or more and 1/100 or less of that of hypophosphite at D = 0.9 or more. Such a high selective permeability of hypophosphite is particularly remarkable when the solution has a pH of 8 to 13 and when pH is less than 8 the permeability of phosphite is increased, resulting in lowering of the C value and deterioration of the separative capacity. The influences of the concentration of the mixed solution upon the selective permeability are relatively small and the relationship between the D and C values is stable in the range of several percents to several ten percents. In the reaction of the white phosphorus and alkali metal hydroxide, it is confirmed by the inventors that a crude solution of alkali metal hypophosphite with a pH of 8 to 13 and a D value of at least 0.2 can be obtained by the use of about 1 mole or less of an alkali metal hydroxide per 1 gram atom of white phosphorus. Therefore, when this crude solution of alkali metal hypophosphite is introduced into an ion-exchange membrane electrodialytic apparatus without any processing and then subjected to electrodialysis, the hypophosphite can selectively be obtained with high efficiency. This means that alkali metal hypophosphites can be produced by only a simple processing of the solution without necessity of using a separating agent or precipitant and a solid-liquid separating apparatus. Preparation of the crude solution of an alkali metal hypophosphite is ordinarily carried out by dropwise addition of a solution of alkali metal hydroxide to an aqueous dispersion of white phosphorus and it is preferable to adjust the initial concentration of the aqueous dispersion of white phosphorus to at least 5 wt % and the concentration of the solution of alkali hydroxide to at least 10 wt% so as to advance the reaction smoothly and conduct the electrodialysis effectively. The proportion of the alkali metal hydroxide to the white phosphorus is so adjusted that a reaction solution with a pH of 8 to 13 is obtained. White phosphorus and alkali metal hydroxide can be reacted at room temperature, but it is preferable to heat to 50 to 100 °C so as to effect the reaction in an effective manner. Since the reaction rate is high in the initial period and is lower with the progress of the reaction, ageing is carried out if necessary after the alkali metal hydroxide is added. When the crude solution of alkali metal hypophosphite is subjected to electrodialysis, the concentration of the hypophosphite is lowered with the passage of time, the C value of the resulting electrodialyzed solution is gradually lowered and the quantity of the phosphite permeated is increased. Thus, the first electrodialysis is ordinarily carried out up to a C value of about 0.9 and the resulting solution, as D solution, is again subjected to electrodialysis to obtain a solution of alkali metal hypophosphite with a C value of at least 0.99. If the number of electrodialyse is increased, an alkali metal hypophosphite of very high purity can be obtained. As an electrodialytic apparatus for carrying out the process of the present invention there can be used an ordinary electrodialytic cell in which electrodes are provided at both the ends and one or more cation exchange membranes and one or more anion exchange membranes are alternately arranged between the electrodes to form at least one pair of desalting and concentrating compartments. In the present invention, it is essential to use a univalent anion selectively permeable membrane as the anion exchange membrane, but ion-exchange membranes including the anion exchange membranes can be used independently of the quality of the membrane, the process for the production thereof, the principle of permeating, etc. Of compartments partitioned by two kinds of ion-exchange membranes, a compartment provided with a univalent anion selectively permeable membrane at the anode side and a cation-exchange membrane at the cathode side is a desalting compartment, and there is another compartment adjacent to the desalting compartment, which is called a concentrating compartment. A crude solution of an alkali metal hypophosphite is circulated in the desalting compartment and a dilute solution of the alkali metal hypophosphite, as an electrolyte, is circulated in the concentrating compartment. Through both electrode compartments is passed and cycled a solution of sodium sulfate as an electrolyte. Electric current is supplied in a current density of 1 to 3 A/dm², and the area of the ion-exchange membranes, the concentrations of the electrolyte and the electrodialysis time can be determined depending on the concentration of the crude alkali metal hypophosphite solution and the amount thereof. The cations in the crude hypophosphite solution fed to the desalting compartment are permeated through the cation exchange membrane at the cathode side into the adjacent concentrating compartment and the hypophosphite ions as univalent anions are passed through the univalent anion permselective membrane at the anode side into the adjacent concentrating compartment. Since each of the compartments is maintained electrically neutral during the electrodialysis, the permeated amounts of cations and anions are substantially the same, so that the alkali metal hypophosphite is permeated in the concentraing compartment and the alkali metal phosphite is retained in the desalting compartment. Multistage electrodialysis can be carried out by the use of a same cell after removing the solution in the desalting compartment, but can more effectively be carried out when using additionally another electrodialytic cell arranged in series. If necessary, the number of electrodialytic cells is increased to shorten the processing time, whereby the efficiency can further be increased. The solution of alkali metal hypophosphite obtained from the concentrating compartment is concentrated and crystallized in conventional manner to yield hypophosphite crystals. In the present invention, reagents other than the reaction raw materials are not substantially used and the process is simplified to reduce the possibility of contamination with impurities. Thus, the alkali metal hypophosphite obtained by concentrating and crystallizing the solution of the alkali metal hypophosphite in the concentrating compartment contains few impurities except a very small amount of the alkali metal phosphite and is of very high purity. In the prior art method wherein the phosphite is precipitated and separated as a less soluble calcium salt, separating operations in many stages such as filtering, washing, etc. are required, resulting in the necessity of large and complicated apparatus, while in the present invention, the process is so simple that even a small apparatus can be operated in a sufficiently efficient manner and both the installation and operation costs can markedly be lowered. This is very economical. The process for the production of an alkali metal hypophosphite from white phophorus and an alkali metal hydroxide has the advantage of a higher reaction speed in comparison with the other process also using calcium hydroxide, but on the other hand, the former process has so many problems in the purification step that it cannot be put to practical use as described above. Since the problems of removing the phosphite can be solved by the present invention, however, efficient production of alkali metal hypophosphites by only white phosphorus and alkali metal hydroxides, which are highly reactive, can be rendered possible. The present invention provides a process for producing alkali metal hypophosphites in a simple and continuous manner because it is free from the solid-liquid separating operation. In the process for continuous production it is necessary to use white phosphorus always in a quantity in excess over the alkali metal hydroxide. Presence of excess white phosphorus can be achieved by ensuring that unreacted white phosphorus is always present in a reaction vessel. When the amount of white phosphorus used is not in excess, the reaction rate is lowered and the benefit of continuous production is lost. Thus, the reaction is always carried cut in the presence of the unreacted white phosphorus. A process for the continuous production of alkali metal hypophosphite according to the present invention will now be illustrated with reference to the accompanying drawing without limiting the scope of the invention claimed. Referring to Fig. 1, the reaction system as one embodiment suitable for carrying out the process for producing alkali metal hypophosphites consists of a reaction vessel 1, an ageing vessel 2 and a separating vessel 3 connected with each other, the reaction vessel 1 and ageing vessel 2 being provided with stirrers and all the vessels 1 to 3 being provided with exhaust pipes for exhausting waste gases W. Depending on the reaction conditions, the ageing vessel 2 can be omitted so that the reaction vessel 1 and separation vessel 3 can directly be connected. The reaction system is heated in advance to a temperature higher than the melting point of white phosphorus by a heater and the atmosphere is replaced with an inert gas such as nitrogen gas. Molten white phosphorus and a solution of an alkali metal hydroxide are continuously fed to the reaction vessel 1 respectively from reservoirs and reacted with strong agitation. The reaction takes place immediately and rapidly. The reaction mixture containing an excess quantity of white phosphorus is continuously fed to the ageing vessel 2 and is successively reacted adequately by stirring. The reaction mixture is then fed continuously from the ageing vessel 2 to the separating vessel 3 functioning as a decanter, where the unreacted white phosphorus with a density of 1.7 is rapidly precipitated and separated from the crude solution of the alkali metal hypophosphite containing the phosphite. The precipitated white phosphorus is withdrawn from the bottom of the separating vessel 3, recycled to the white phosphorus reservoir and reused. On the other hand, the crude solution of the alkali metal hypophosphite is subjected to electrodialysis in a first electrodialytic cell 6 having a structure as described above. The resulting solution from the concentrating compartment of the first electrodialytic cell 6 is further fed to a second similar electrodialytic cell 7 and subjected to electrodialysis again to further increase the removal ratio of the phosphite. The solution in the concentrating compartment is then concentrated and crystallized through a concentrating vessel 8 and a crystallizing vessel 9 to yield a high purity crystal of alkali metal hypophosphite. On the other hand, the alkali metal phosphite which is obtained from the solution in the desalting compartment has a high phosphite content and is stored in a phosphite recovery vessel 10. In Fig. 1, 4 and 5 respectively designate refluxing condensers. A feature of the process of the present invention lies in obtaining the phosphite by-product as a useful alkali metal phosphite. That is, the crude solution of an alkali metal hypophosphite is subjected to electrodialysis to permeate the alkali metal hypophosphite in the concentrating compartment, while the alkali metal phosphite is not permeated but is retained in the desalting compartment. Accordingly, when the solution in the desalting compartment is concentrated after electrodialysis, there is obtained am alkali metal phosphite substantially free from hypophosphite. Up to the present time alkali metal phosphite by-products from the production of the alkali metal hypophosphite had been subjected to solid-liquid separation through formation of an insoluble alkaline earth metal salt. However, the alkaline earth metal salt of phosphorous acid has no utility value in industry as it is, but as a feed. On the other hand, an alkali metal salt of phosphorous acid is more useful in industry as an additive to polymers, catalysts for the production of polymers, etc. and thus, the alkaline earth metal phosphite is ordinarily converted into the correspoding alkali metal phosphite. The process of the present invention is more advantageous because the phosphite is directly obtained in the form of the useful alkali metal phosphite. The present invention essentially relates to a process for the production of an alkali metal hypophosphite based on the reaction of white phosphorus and an alkali metal hydroxide in an aqueous medium and furthermore provides also a very useful process for the separation of each component from the mixed solution of the phosphite and hypophosphite. The Applicants have found that when the reaction of white phosphorus and an alkali metal hydroxide is performed in the presence of a specified small amount of an alkaline earth metal hydroxide, the amount of the phosphite by-product is decreased and the yield of the hypophosphite is increased. In this case, the amount of the alkaline earth metal hydroxide to be added is preferably 0.01 to 0.2 mol per 1 gram atom of white phosphorus. A part of the thus formed phosphite is precipitated as the alkaline earth metal salt and the rest remains dissolved in the solution. Therefore, after filtering and separating the precipitate of the alkaline earth metal salt from the reaction mixture the filtrate is subjected to electrodialysis in an ion exchange membrane electrodialytic cell to obtain the hypophosphite. This process requires a filtering operation on the alkaline earth metal phosphite, but has an advantage in increasing the yield of the hypophosphite. If the amount of the alkaline earth metal hydroxide added is less than the lower limit, there is no effect of increasing the yield, while if more than the upper limit the amounts of the alkali metal hydroxide and alkaline earth metal hydroxide-dissolved in the solution are increased and free alkali and the less soluble alkaline earth metal phosphite formed during electrodialysis deteriorate the capacity of the ion exchange membranes. Thus, am operation to separate them is also required. This is not desirable. When the amount of the alkaline earth metal hydroxide used is in the range of the present invention, the added alkaline earth metal is mainly precipitated as a less soluble compound so that only a very small amount of the alkaline earth metal is present in the solution and the electrodialytic process is hardly affected. Since the phosphite by product is partly removed as the alkaline earth metal salt, the content of the phosphite in the crude solution of the alkali metal hypophosphite is relatively low and the electrodialysis of the hypophosphite can readily be carried out. In this case, therefore, the time required for the electrodialysis can be shortened and the hypophosphite can be produced in a more effective manner. The ion-exchange membranes used in the electrodialytic process of the present invention are preferably strongly acidic cation-exchange membranes and univalent ion permselective, strongly basic anion-exchange membranes. The commercially available strongly acidic ion-exchange membranes for electrodialysis are based on cation-exchange resins each consisting of a styrene-divinylbenzene copolymer in which sulfonic groups are incorporated. In the past, resins based on styrene-butadiene copolymers were produced. Formation of the membrane is carried out before or after the polymerization reaction, during which polyvinyl chloride is added for the purpose of imparting flexibility to the membrane and reinforcing materials such as fabrics of glass fibers or synthetic fibers are added to increase the membrane strength. As the univalent ion permselective, strongly basic anion-exchange membrane, there can be used commercially available strongly basic anion-exchange resins prepared by incorporating chloromethyl groups or iodomethyl groups into a styrene-divinylbenzene copolymer, styrene-vinylpyridine copolymer or styrene-vinylpyrldine-divinylbenzene copolymer, treating with a tertiary amine and then converting into the quaternary ammonium salt, followed by subjecting to membrane-making and imparting the univalent ion permselective property thereto. The treatment for imparting the univalent ion permselective property is generally carried out by subjecting the surface layer of the membrane to oxidative decomposition to lower the ion-exchange capacity and to suppress swelling by water, or by coating the surface of the membrane with a thin layer consisting of a different resin having a compact network structure. At the present time, the latter method is preferable. The present invention will be further illustrated by reference to the following specific examples, the details of which should not be construed as limitations on the scope of the invention. Example 1 800 ml of water and 150 g of white phosphorus were charged in a reactor of 2000 ml, equipped with a stirrer, nitrogen feed pipe, refluxing condenser (gas discharge), thermometer and dropping funnel, to which 775 g of a 25 % aqueous solution of sodium hydroxide was added dropwise for 2 hours at 60 to 70 °C with agitation while feeding nitrogen gas thereto. After the dropwise addition, the reaction mixture was aged at 60 to 70 °C for 4 hours and a trace amount of the unreacted phosphorus was then separated by filtration, thus obtaining 1800 ml of a crude solution with a pH of 12.3, containing 265 g of sodium hypophosphite and 113 g of sodium phosphite. This crude solution of sodium hypophosphite was subjected to electrodialysis by means of an electrodialytic apparatus for experiment, TS-2-10 type [commercial name, manufactured by Tokuyama Soda Co., Ltd., composed of 10 sets of cation- exchange membranes, NEOSEPTA CM-1 (commercial name -trade mark-manufactured by Tokuyama Soda Co., Ltd.) and univalent anion permselective exchange membranes, NEOSEPTA ACS (commercial name -trade mark- manufactured by Tokuyama Soda Co., Ltd.) and using 800 ml of a 5 % solution of sodium sulfate as an electrolyte]. 800 ml of the above described crude solution of sodium hypophosphite was charged to the desalting compartment and 800 ml of a solution containing 16 g of sodium hypophosphite (chemical reagent first grade) was charged is the concentrating compartment, which were then subjected to electrodialysis at a current density of 2 A/dm² while recycling the solution. The sodium hypophosphite was selectively permeated depending on the dialysis time and there were obtained a solution containing 0.8 g of sodium phosphite and 84.9 g of sodium hypophosphite from the concentrating compartment after an electrodialysis time of 60 minutes and a solution containing 3.2 g of sodium phosphite and 123.7 g of sodium hypophosphite after 100 minutes. Furthermore, 800 ml of the crude solution of sodium hypophosphite was subjected to electrodialysis for 100 minutes under the same conditions to obtain 1005 ml of a solution containing 3.2 g of sodium phosphite and 124.1 g of sodium hypophosphite, which was then withdrawn from the concentrating compartment and again subjected to electrodialysis for 80 minutes under the same conditions to obtain 965 ml of a solution containing 0.1 g of sodium phosphite and 101.8 g of sodium hypophosphite. The resulting solution was concentrated and crystallized to obtain sodium hypophosphite crystals (NaH₂PO₂ · H₂O) whose analytical data are tabulated in Table 1. For comparison, the analytical data of the commercially available sodium hypophosphite are also shown in Table 1. As is evident from this data, the sodium hypophosphite of the present invention has much higher purity as compared with the commercially available material. Analytical Item Commercially Available Article Sodium Hypophosphite of Example 1 Cl ppm 40 20 SO₄ ppm 140 10 Pb ppm 0.5 < 0.2 Fe ppm 0.5 < 0.2 Ca ppm 360 5 As ppm < 1 < 0.5 Na₂HPO₃ % 0.8 0.1 NaH₂PO₂ % 82.9 84.2 Example 2 600 ml of water and 150 g of white phosphorus were charged in the same apparatus as used in Example 1, heated at 60 to 70 °C while feeding nitrogen gas thereto and 1080 g of a 25 % aqueous solution of potassium hydroxide was added dropwise for 2 hours with agitation. After the dropwise addition, the reaction mixture was aged for 4 hours at 60 to 70 °C and a very small amount of the unreacted white phosphorus remaining was separated by filtration to obtain 1900 ml of a crude solution with a pH of 12.2 containing 302 g of potassium hypophosphite and 153 g of potassium phosphite. The whole of the crude solution of potassium hypophosphite was charged to the desalting compartment of the same electrodialytic cell as in Example 1 and 1000 ml of a solution containing 25 g of potassium hypophosphite (prepared by the use of hypophosphorous acid and potassium hydroxide with first grade chemical reagent) was charged to the concentrating compartment thereof, and then subjected to electrodialysis for 180 minutes in an analogous manner to Example 1 while recycling the solution. 1300 ml of a solution containing 240 g of potassium hypophosphite and 5.4 g of potassium phosphite was obtained from the concentrating compartment, again charged to the desalting compartment and again subjected to electrodialysis for 150 minutes under the same conditions to obtain 1100 ml of a solution containing 205 g of potassium hypophosphite and 0.3 g of potsassium phosphite, which was then concentrated and crystallized, thus obtaining crystalline potassium hypophosphite (KH₂PO₂) with a phosphite content of 0.1 %. Example 3 600 ml of water and 200 g of white phosphorus were charged to the same apparatus as used in Example 1 and 520 g of a 25 % aqueous solution of sodium hydroxide was added dropwise for 30 minutes at 90 to 95 °C with agitation. After the dropwise addition, the reaction mixture was aged for 2 hours at 90 to 95°C and the unreacted white phosphorus was separated to obtain 1600 ml of a crude solution with a pH of 8.1 containing 185 g of sodium hypophosphite and 79 g of sodium phosphite. 800 ml of the crude solution of sodium hypophosphite was subjected to electrodialysis for 20 minutes using the same apparatus as used in Example 1. 18.3 g of sodium hypophosphite and 1.3 g of sodium phosphite were permeated in the concentrating compartment thereof at a molar ratio (Na₂HPO₃/NaH₂PO₂) of 0.05. Then, the above described crude solution of sodium hypophosphite was adjusted to a pH of 12 and subjected to electrodialysis in a manner similar to that described above. Consequently, 20.5 g of sodium hypophosphite and 0.3 g of sodium phosphite were permeated in the concentrating compartment with a molar ratio of 0.01. Using sodium hypophosphite of special grade reagent and phosphorous acid and sodium hydroxide of first grade reagent, a solution having the same composition as the above described reaction solution and a pH of 4 was prepared and subjected to electrodialysis in a similar manner. Consequently, 16.8 g of sodium hypophosphite and 5.5 g of sodium phosphite were permeated in the concentrating compartment with a molar ratio of 0.23. As is apparent from the above results, sodium hypophosphite was more effectively selected and permeated from the basic solution. Example 4 Using the apparatus as shown in the accompanying drawing, sodium hypophosphite was continuously prepared. The reaction vessel 1 had a volume of 2000 ml and was provided with a stirrer, nitrogen feed pipe, refluxing condenser, thermometer and two sets of dropping means and the ageing vessel 2 had the same volume as the reaction vessel and was provided with a stirrer, refluxing condenser and thermometer. As a preparatory operation, the crude solution of sodium hypophosphite with a pH of 8.1 was prepared in an analogous manner to Example 3, fed 1000 ml by 1000 ml to the reaction vessel 1 and the ageing vessel 2, heated at 90 to 95 °C and replaced by nitrogen gas. In the reaction vessel 1 and ageing vessel 2, the stirrers were moved and a 12 % aqueous solution of sodium hydroxide and molten white phosphorus were added dropwise thereto. The dropwise addition rate was adjusted to 800 ml/hr of the aqueous solution of sodium hydroxide and 174 g/hr of the molten white phosphorus so that excess white phosphorus was always present in the reaction vessel. The reaction mixture was continuously fed to the ageing vessel 2, heated, stirred and then fed to the separation vessel 3 where it was settled at 50 to 60 °C to separate the white phosphorus from the crude solution of sodium hypophosphite. The unreacted white phosphorus was recovered at a rate of 87 g/hr, stored in a white phosphorus reservoir and recycled in the raw material white phosphorus vessel. The crude solution of sodium hypophosphite was obtained at a rate of 830 ml/hr from the separation vessel 3 and its composition was varied with the passage of time which was, however, approximately 148 g/hr of sodium hypophosphite and 60 g/hr of sodium phosphite. The crude solution of sodium hypophosphite was introduced into the desalting compartment of the same electrodialytic cell as used in Example 1, but using 20 sets of the ion-exchange membranes, and subjected to electrodialysis under the same conditions as those of Example 1. The electrodialysis was carried out using two electrodialytic cells of the same type, connected in series, in which the solution obtained from the concentrating compartment of the first electrodialytic cell was introduced into the desalting compartment of the second electrodialytic cell and again subjected to electrodialysis. The feed rate to the first electrodialytic cell was 830 ml/hr corresponding to the quantity of the crude solution of sodium hypophosphite obtained and the feed rate to the second electrodialytic cell was 800 ml/hr. Consequently, from the concentrating compartment of the second electrodialytic cell was obtained 800 ml of a solution containing 108 g of sodium hypophosphite and 0.2 g of sodium phosphite per hour which was then concentrated and crystallized to obtain a high purity crystalline sodium hypophosphite having a phosphite content of 0.1 %. The solution obtained from the desalting compartment of each of the electrodialytic cells was subjected to electrodialysis in an analogous manner to Example 5 using another electrodialytic cell to crystallize sodium phosphite from the solution of the desalting compartment and the solution of the concentrating compartment was added to the crude solution of sodium hypophosphite obtained from the reaction system, followed by recycling to the electrolytic process. Example 5 570 ml of a solution containing 10 g of sodium hypophosphite and 45 g of sodium phosphite, obtained from the desalting compartment after electrodialysis for 100 minutes in Example 1, was charged in the desalting compartment, while 780 ml of a solution containing 23 g of sodium hypophosphite and 2.8 g of sodium phosphite, obtained from the desalting compartment in the second electrodialysis, was charged to the concentrating compartment, followed by subjecting to electrodialysis for 20 minutes in the same electrodialytic apparatus. Then, 510 ml of a solution containing 0.2 g of sodium hypophosphite and 30 g of sodium phosphite was obtained from the desalting compartment and subjected to concentration and crystallization, thus obtaining a crystalline sodium phosphite containing 0.6 % by weight of sodium hypophosphite. On the other hand, 850 ml of a solution containing 33 g of sodium hypophosphite and 18 g of sodium phosphite in the concentrating compartment was mixed with the crude solution of sodium hypophosphite obtained from the reaction system, followed by recycling to the electrodialytic process. Example 6 800 ml of water, 100 g of white phosphorus and varied amounts of calcium hydroxide were charged in the same apparatus as used in Example 1, to which 440 g of a 25 wt % aqueous solution of sodium hydroxide was dropwise added at 70 to 80 °C for 1 hour with agitation while bubbling nitrogen gas, followed by ageing at 80 to 90 °C, filtering the solid product and washing. The resulting crude solution of sodium hypophosphite was controlled to a liquid quantity of 1500 ml and subjected to composition analysis to obtain results as shown in Table 2. As is evident from Table 2, in the range of calcium hydroxide to white phosphorus molar ratio [Ca(OH)₂/P] of 0.01 to 0.2, the amount of sodium hypophosphite formed is increased and the electrodialysis is hardly affected. If the molar ratio is larger than this range, the amounts of dissolved calcium and free alkali are increased, so that practice of the electrodialysis is difficult. Ca/P Molar Ratio Contents in Crude Solution of Sodium Hypophosphite NaH₂PO₂ (g) Dissolved Ca (g) Free NaOH (g) 0 168 0.00 0.0 0.001 170 0.01 0.0 0.01 180 0.01 0.0 0.1 181 0.05 1.0 0.2 181 0.04 4.0 0.3 180 0.41 14.7 As illustrated above in detail, the present invention is concerned with a process for the production of alkali metal hypophosphites using white phosphorus and alkali metal hydroxides as raw materials and using the ion-exchange membrane electrodialysis method, whereby the following advantages or benefits can be obtained: The process of the present invention does not need complicated apparatus such as multi-stage solid-liquid separation means which are essential for the prior art process and can be accomplished in effective manner by a simple apparatus or simple operation because of a simplified process comprising no solid-liquid operations. Furthermore, the process of the present invention is very economical, since the costs required for installation and operation can be decreased. Since separating agents or precipitants other than the reaction raw materials are not used in the process of the present invention, there can be obtained high purity alkali metal hypophosphites free from impurities. Since the process of the present invention comprises only solution processing, the process can readily be rendered continuous and if the reaction conditions are suitably chosen, the processing time can be shortened to raise efficiency. Furthermore, the phosphite by-product can be obtained as a useful alkali metal salt.

Claims: A process for the production of an alkali metal hypophosphite which comprises reacting white phosphorus and an alkali metal hydroxide in an aqueous medium to prepare a crude solution of an alkali metal hypophosphite containing the phosphite, feeding the crude solution to a desalting compartment of an electrodialytic cell, in which cation exchange membranes and univalent anion perm-selective membranes are alternately arranged between a cathode compartment and an anode compartment to form at least one pair of a desalting compartment and a concentrating compartment, allowing the hypophosphite in the crude solution of the alkali metal hypophosphite to selectively permeate into the concentrating compartment and thereby separating the hypophosphite from the phosphite remaining in the desalting compartment, then withdrawing the purified solution of the alkali metal hypophosphite from the concentrating compartment, concentrating the purified solution and crystallizing the alkali metal hypophosphite, while withdrawing the solution of the alkali metal phosphite as a byproduct from the desalting compartment. A process as claimed in Claim 1 wherein the crude solution of the alkali metal hypophosphite has a pH of 8 to 13. A process as claimed in Claim 1 or Claim 2, wherein the crude solution of alkali metal hypophosphite is prepared by reacting an aqueous dispersion of white phosphorus having an initial concentration of at least 5 % by weight and an aqueous solution of an alkali metal hydroxide having a concentration of at least 10 % by weight. A process as claimed in any one of Claims 1, 2 and 3 wherein the reaction of white phosphorus and the alkali metal hydroxide in an aqueous medium is carried out in the presence of an alkaline earth metal hydroxide in a proportion of 0.01 to 0.2 mol to 1 gram atom of the white phosphorus.

Inventors: KANAYAMA ATSUSHI, ODA MASAOKI, SHIBATA KATSUTOSHI, TAKEUTI TAKASHI, TAKIKAWA YOSHIHIRO, KANAYAMA, ATSUSHI, ODA, MASAOKI, SHIBATA, KATSUTOSHI, TAKEUTI, TAKASHI, TAKIKAWA, YOSHIHIRO
Applicants: PHOSPHORUS CHEM IND, RINKAGAKU KOGYO COMPANY LIMITED
Application year: 1991
Main classification: C01B25
IPC codes: C01B25