WO1990012637A2

WO1990012637A2 - Electrodialytic water splitting process for the treatment of aqueous electrolytes

Info

Publication number: WO1990012637A2
Application number: PCT/CA1990/000116
Authority: WO
Inventors: Michael Paleologou; Richard M. Berry
Original assignee: Pulp And Paper Research Institute Of Canada
Priority date: 1989-03-15
Filing date: 1990-04-09
Publication date: 1990-11-01
Also published as: WO1990012637A3

Abstract

A process for dealkalization or acidification of aqueous salt solutions or for the splitting of the salt of such solutions employs a water splitting system of bipolar membranes (3) in conjunction with ion selective membranes (2); a two-compartment cell employs cation permselective membranes (2) to define acid and base compartments with the bipolar membranes (3), and a three compartment cell employs anion permselective and cation permselective membranes to define with the bipolar membrane salt, acid and base compartments; the process has particular applicability to the dealkalization of a monosodium peroxide solution containing sodium hydroxide produced in a hydrogen peroxide generator.

Description

ELECTRODIALYTIC WATER SPLITTING PROCESS FOR THE TREATMENT OF AQUEOUS ELECTROLYTES

TECHNICAL FIELD

This invention is concerned with processes for dealkalization or acidification of aqueous salt solutions, as well as for the splitting of the salt of such solutions employ¬ ing a water splitting system.

More especially the invention relates to the de¬ alkalization and/or acidification of alkaline or non-alkaline aqueous solutions of various compositions; and to the dealka¬ lization and/or splitting of the salt of alkaline or non- alkaline aqueous solutions of various compositions.

BACKGROUND ART

The simplest case involves the partial dealka- lization of an alkaline but otherwise pure solution of water to a minimum conductivity dictated by the efficient operation of a 2-compartment or 3-ccmpartment water splitter. Assuming that sodium hydroxide is the base to be removed the minimum operational conductivity is about 20 mS/cm and the correspond- ing concentration of sodium hydroxide is about 0.1M. For a better efficiency in terms of power consuπption a conductivity of 30 mS/cm corresponding to a concentration of NaOH of 0.15M is recommended. The cation of the base could be the type that does not hydrolyze (e.g. Na+) or the type that does (e.g. NH₄+) .

In the case of a 3-con_partment water splitter the co-products are soclium hydroxide (base compartment) and water (acid coπpartment) . In order to maintain conductivity in the acid cαtpartment an electrolyte (acid, base or salt) should be added of a concentration sufficient to maintain a minimum con¬ ductivity of 20 mS/cm. This solution can be continuously recirculated through the aσid loop. A second case in a first errfoodiment involves the partial or complete dealkalization and/or acidification of an alkaline or non-alkaline solution of a salt of the type whose cation and anion do not hydrolyze in water (e.g. NaCl) using a 2-cxxιpartment water splitter. In this case the removal of alkali cations can continue beyond the cαnplete dealkalization point thus acidifying the aqueous salt solution by replacing the displaced alkali cations with hydrogen ions; this process is covered in U.S. Patent No. 4,391,680 by Mani and Chlanda. The acidification process in this case can continue until cαηpletion since the conductive salt is being replaced by an acid which is even more conductive than the salt itself.

A second en±>odiment of the second case involves the partial or complete dealkalization and/or splitting of the salt of an alkaline or non-alkaline solution of a salt of the type whose cation and anion do not hydrolyze in water (e.g. NaCl)using a 3-cc^*πρart_^*ment water splitter. In this case since the hydroxyl anions are more mobile and in addition are better bases than any other ions they would be expected to pre- ferentially migrate to the acid ccropartment over the competing aniαns of the salt. Depending on the relative migration rates of hydroxyl and the anion of the salt, the salt may have to be partially depleted before being completely dealkalized. As in case 1 an electrolyte may have to be added to the acid compart- ent in order to maintain conductivity in the system. Electro- hydrolysis can continue beyond the complete dealkalization point thus splitting the salt into its corresponding acid (e.g. HCl) and base (e.g. NaOH) . This process which has been covered in U.S. Patent No. 2,829,095 to Oda et al, can efficiently continue until the conductivity of the salt compartment reaches abojif 20ms/cm.

A third case in a first embodiment involves the partial or ccxrplete dealkalization and/or acidification of an alkaline or non-alkaline solution of a salt of the type whose cation does not hydrolyze but whose anion does to produce

T hy roxyl ions, (e.g. NaHO„)using a 2-ccmpartment water splitter. In this case the alkali cation removal process can continue beyond the complete dealkalization point thus modi¬ fying the aqueous salt solution by replacing the displaced alkali cations with hydrogen cations. In this case, however, the extent of alkali cation removal is limited by the degree of ionization of the acid produced, which is usually very weak. Since, during the alkali cation removal process the original salt is being replaced by a very weak acid, this process can only efficiently continue as long as the decreasing con¬ ductivity of the depleted salt solution remains above 20 mS/cm. A second embodiment of the third case involves the partial or complete dealkalization and/or splitting of the salt of an alkaline or non-alkaline solution of a salt of the type whose cation does not hydrolyze but whose anion does to produce hydroxyl ions, (e.g. NaHO^,) using a 3-ccmpartment water + splitter. In this case the alkali cations (e.g. Na ) migrate to the base compartment in which alkali metal and/or non-metal hydroxide (e.g. NaOH) is formed whereas the hydroxyl anions as well as the anions of the salt (e.g. HO--) migrate to the acid ccπpartment in which water and the acid of the anion of the salt (e.g. H D-) form. Since, however, both the hydroxyl anions and the anions of the salt migrate simultaneously to the acid coπpartment it is not possible to remove all caustic from the salt cαtpartment without deleting the salt itself. Another problem is that none of the compounds forming in the acid coiφartment (e.g. H-,0 and H_0₂) are conductive. . The solution to both of these problems is the addition of alkali metal and/or non-metal hydroxide (e.g. NaOH) electrolyte into the acid cαrpartment. In this way the formation of the acid of the anion of the salt (e.g. H.-0.-) is avoided and instead the salt of the anion is formed (e.g. NaHO^). Thus, the desired ratio of alkali metal and/or non-metal hydroxide to salt of the

SUBSTITUTE SHEET migrating anion can be achieved in the acid loop by adjusting the original concentration of alkali metal and/or non-metal hydroxide in this loop and/or the extent of electrohydrolysis. A^" fourth case involves the partial or complete dealkalization and/or acidification and/or splitting of the salt of an alkaline or non-alkaline solution of a salt of the type whose anion does not hydrolyze but whose cation does to produce hydrogen ions (e.g. NH.C1) . In the presence of a hydroxide of an alkali metal and/or non-metal (e.g. NaOH) the hydrolysis equilibrium of the cation shifts completely to the right, e.g.

NH_jCl + NaOH ^ ,NH. + NaCl + H_0

thus forining a solution of alkali metal chloride; its dealkalization and/or acidification and/or splitting of the salt is, therefore expected to be as in case 2. Since the anion is of the type that does not hydrolyze the acidification is expected to proceed as in case 2. The process can continue until the conductivity in the salt c_aτpartment is about 20 mS/cm. A fifth case involves the partial or complete dealkalization and/or acidification and/or splitting of the salt of an alkaline or non-alkaline solution of a salt of the type whose cation as well as anion hydrolyze to produce hydroxyl and hydrogen ions respectively, (e.g. NH.HO„). The initial pH of the solution of such a salt will depend on the relative hydrolysis constants of the cation and anion. In an alkaline solution of a salt of this type the hydrolysis of the anion is suppressed and the hydrolysis of the cation is carried to completion. This suggests that the dealkalization of such solutions will proceed as in case 2. As the dealkalization proceeds, however, the hydrolysis of the cation will increasingly be suppressed and the hydrolysis of the anion would be carried closer to completion; therefore, the latter -5-

part of the dealkalization process and the splitting of such salts is likely to proceed as in case 3. In an acidic solution of the salt in question the hydrolysis of the cation would be completely suppressed and the hydrolysis of the anion would be

5 carried to completion; therefore the acidification of such solutions will proceed as in case 3.

The sixth case involves alkaline or non-alkaline solutions of the types discussed in cases 1 to 5 above, but also containing water soluble but non-conductive ccnpounds

10 (e.g. ethanol) and/or insoluble but suspended compounds (e.g. Mg(OH)- colloidal particles).

In all of the cases described above employing a 3-compartment water splitter the co-products are the hydroxide of the alkali metal and/or non-metal cation that was removed

15 from the original solution and the acid of the anion of the salt.

A problem exists in processes for the complete dealkalization of an alkaline nronosodium peroxide (NaH0_) solution. Solutions such as these are produced by the reduction

20 of oxygen in electrolytic cells employing sodium hydroxide as the electrolyte in the anode ccmpart ent, (e.g. the Dow on-site peroxide generator, U.S. Patent Nos. 4,224,129 and 4,317,704). Since, however, completely dealkalized solutions of monosodium peroxide are needed for the efficient bleaching of mechanical

25 pulps the need exists for the dealkalization of these solutions. Alternate approaches, such as acidification of the solution from an external source, consume the acid added, waste caustic soda and furthermore change the nature of the solution, since a new salt is formed as a result of the neutralization

30 reaction.

Approaches other than the addition of acid from an external source, for the dealkalization and/or acidification of aqueous solutions, include: electrolytic systems (e.g. U.S.

Patent No. 4,671,863 by Tejeda) , ion-exchange systems employing

35 strong-acid cation resins, weak-acid cation resins, and anion

SUBSTITUTE SHEET resins (e.g. McGarvey, F. X., Power, 128(8), 59-60, 1984), systems employing magnetic ion-exchange resins in fluidized bed adsorbers (e.g. Bolto et al, J. Chem. Technol. Biotechnol., 29(6), 325-31, 1979) and electrodialytic systems (e.g. U.S. Patent No. 3,893,901 by Tejeda) . The ion-exchange systems referred to above are primarily intended for the removal of trace amounts of alkali from water for purification purposes; the scale-up of such systems, for the removal of large quantities of alkali frcm industrial streams, would be uneconomical because of the high cost associated with the regeneration of ion-exchange columns. In addition, in such systems, alkali is obtained in the form of a salt and not caustic. Electrodialytic systems, on the other hand, recover the alkali in the form of caustic, the energy costs associated with dealkalization, however, are significantly higher than those associated with water splitting techniques.

Membrane systems involving stacked pairs of mem¬ branes have been recoπmended for various applications. These include desalination (U.S. Patent No. 3,654,125 to Leitz), springing of sulfur dioxide from aqueous sulphite and bisulfite solutions (U.S. Patent No. 4,082,835 to Chlanda et al), the removal of alkali metal cations from aqueous alkali metal chloride solutions so as to produce an acidified salt solution and sodium hydroxide (U.S. Patent No. 4,391,680 to Mani and Chlanda) and the recovery of valuable metal or ammonium values frcm materials comprising a salt of a first acid while avoiding the formation of gas bubbles (U.S. Patent No. 4,592,817 to Chlanda and Mani). In none of the aforementioned systems, however, suggestion is made for their application to the partial or cαtplete dealkalization of alkaline alkali metal and/or non-metal salt solutions. ϊurthermore, for only one type of salt (case 2) reference is made to the acidification of solutions of salts (U.S. Patent No. 4,592,817 to Chlanda and Mani). Membrane systems involving water splitters in the three-ccmpartment configuration have been recommended for various applications. These include the recovery of fluorine values from fluorosilic acid aqueous streams by electrodialytic water splitting of fluoride salt to hydrofluoric acid and hydroxide base (U.S. Patent 3,787,304 to Chlanda et al), the recovery of TiO_ from ilmenite-type ores by digestion with hydrofluoric acid, in which hydrofluoric acid and aπrπoni^*um hydroxide are recovered by an electrodialytic water-splitting process from by-product aqueous ammonium fluoride (U.S. Patent No. 4,107,264 by Nagasubramanian and Liu), the conversion of alkali metal sulfate values, such as sodium or potassium values in spent rayon spin bath liquors, into alkali metal hydroxide and alkali metal sulfate/sulfuric acid (U.S Patent No. 4,504,373 by Mani and Chlanda) and the recovery of metal or aπmonium values from materials comprising a salt of a first acid while avoiding formation of gas bubbles in the electro- hydrolysis cells. In none of the aforementioned systems, how¬ ever, suggestion is made for their application to the partial or complete dealkalization of alkaline alkali metal and/or non-metal salt solutions; furthermore, no attempt is made to cover the splitting of the dealkalized or neutral salts of various types as described above.

DISCLOSURE OF THE INVENTION This invention seeks to provide a process for dealkalization or acidification of aqueous salt solutions.

This invention also seeks to provide a process for the splitting of the salt of an alkaline or non-alkaline aqueous salt solution. Still further this invention seeks to provide such processes employing a two conpartment membrane cell based on a cation permselective membrane and bipolar membranes.

SUBSTITUTE SHEET Still further this invention seeks to provide such processes employing a three cc-mpartment membrane cell based on anion permselective and cation permselective membranes and bipolar membranes. In accordance with the invention there is provided a process which comprises the steps of: a) providing a cell coπprising an anode, a cathode and at least two coπpartments ^•therebetween defined at least in part by a cation permselective membrane and at least a pair of bipolar membranes; the bipolar eanbranes have a cation side facing the cathode and an anion side facing the anode, b) feeding an aqueous solution of a salt MX into a first of the ccπpartments to contact a first side of the cation permselective membrane, wherein M is a cation selected frcm alkali metal and non-metal cations and X is an anion of an acid, c) feeding a liquid comprising water into a second of the ccmpartments to contact an anion side of a the bipolar πiemhrane, d) passing a direct current through the cell between the anode and cathode to effect: i) migration of the cation M from the first cσmp__rtment through the cation perm- selective membrane into the second compartment, and ii) split¬ ting of water by the bipolar membrane of the second ccnpart-nent with accumulation of hydroxide ions of the cation in the second cαnpartment and removal of hydrogen ions of the water frcm the second cαπpartment, e) removing accumulated MOH frcm the second cαipart ent, and f) removing MX from the first cαπpartment.

The hydrogen ions in d)ii) effectively accumulate in the adjacent cαrpartment incorporating the cationic side of the bipolar membrane of the second cαipartment. In one aspect the aqueous salt solution is an alkaline solution and the process serves to dealkalize the solution. In another aspect the invention can be applied to the generation of useful products by splitting of the salts and can, for example, be applied to the treatment of waste effluents containing salts, to generate useful chemicals.

DESCRIPTION OF PREFERRED EMBODIMENTS i) Two Coπpartment Cell

In accordance with the invention it has been found that a dealkalized solution of an alkali metal salt can be generated frcm an alkaline solution of the salt without the addition of acid from an external source by using a two com¬ partment water splitter employing a cation membrane and a bipolar membrane. The process can, in particular, be used to generate a dealkalized monosodium peroxide solution from an alkaline monosodium peroxide solution. The co-product is aqueous alkali metal hydroxide (e.g. NaOH) which may have a concentration which is considerably higher than the concent¬ ration of the caustic in the alkaline salt solution. Depend¬ ing on the current used and the desired concentration of the co-product very high current efficiencies can be achieved during the dealkalization process.

Thus the invention contemplates a process which includes the following steps: a) feeding an aqueous alkaline monosodium peroxide solution into a two cαrpartment water splitter composed of alternating cation and bipolar membranes; the solution is introduced into each acid ccπpartment between a cation membrane and the cation side of a bipolar membrane; b) feeding a liquid comprising water into each base cαipart¬ ment, between a cation membrane and the anion side of a bipolar membrane;

SUBSTITUTE SHEET c) passing a direct current through the water splitter, theireBy causing the transfer of alkali metal cations from the alkaline monosodium peroxide solution to the base compartment with dealkalization of the alkaline monosodium peroxide solution and basification of the liquid comprising water; d) bleeding from the acid cαtpartments a dealkalized aqueous monosodium peroxide solution; e) bleeding from the base cαipartments a liquid comprising aqueous alkali metal hydroxide. In step d) the dealkalized aqueous monosodium pero ide ^' solution bled frcm the acid compartments may, in particular, comprise between 0 and x moles of sodium hydroxide per x moles of monosodium peroxide. This solution of mono¬ sodium peroxide can be used directly for the bleaching of mechanical pulps, especially if the dealkalization is complete (i.e. x = 0).

In step e) the alkali metal hydroxide bled from the base ccπpartments may suitably be recycled to a hydrogen peroxide generator or may be used directly for bleaching of chemical or mechanical pulps.

The two ccr artment water splitter employed in the process of the invention may be any of the systems described in U.S. Patent No. 4,082,835 to Chlanda et al (1979). The two ccπpartment water splitter is composed of a large number of cationic and bipolar membranes alternately stacked between two elθbtrode_5.

Bipolar membranes are composite membranes consisting of three parts, a cation selective region, an anion selective region and the interface between the two regions. When a direct current is passed across a bipolar membrane with the cation selective side toward the cathode, electrical conduction is achieved by the transport of H and OH ions which are obtained from the dissociation of water. The water splitter employs suitable bipolar membranes, that can be of the type described, for example, in U.S. Patent No. 2,829,095 to Oda et al, in U.S. Patent 4,024,043 (single film bipolar membranes), in U.S. Patent No. 4,116,889 (cast bipolar membranes) or any other type which effectively converts water into hydrogen and hydroxy ions. The cation membranes useful in the process of the invention can be weakly acidic or strongly acidic cation pe mselective membranes. Examples of suitable cation membranes are Nafion (Trade Mark) R 110,901 and 324 of E. I. Du Pont de Nemours & Co.; but other commercially available cationic membranes can be used.

In general, stacks that are suitable for electro- dialysis can be used for the water splitter. Such stacks are available commercially from Asahi Glass Co., 1-2, Marunouchi 2-chcme, Chiyoda-Ku, Tokyo, Japan; Ionics Inc., Watertown, Massachussets and other commercial sources.

The operating temperature of the two cαtpartment water splitter may be any temperature compatible with the membranes and above the freezing point of the solutions, preferably in the 20 to 60 C. range. The alkaline salt could be any soluble salt consist¬ ing of a onovalent cation, for example, the Group la alkali metals or other monovalent cations, for example, ammoniijm cations; and any anion, for example, the anions of the Group Vila elements or other anions, for example, sulphate, acetate, oxalate, perhydroxyl, etc.

The operation of the water splitter is further described below:

The aqueous alkaline monosodium peroxide fed into the acid compartments of the electrohydrolysis stack is typically a 1:1 caustic to monosodium peroxide mixture but may also be composed of different ratios. Magnesium sulphate (120ppm) is added to the said feed solution in order to prevent the decomposition of hydrogen peroxide into oxygen and water which is otherwise quite pronounced. In the absence of the magnesium sulphate stabilizer bubbling due to oxygen evolution

SUBSTITUTE hinders considerably the normal operation of the system. Any concentration between 100 and 400 ppm is sufficient to prevent decomposition of hydrogen peroxide in stationary aqueous solutions or such solutions in an electrσhydrodynamic environ- menc (e.g. water splitter). In the indicated concentration range magnesium sulphate is in the form of colloidal magnesium hydroxide particles suspended in solution.

Typically, the concentration of the alkaline mono¬ sodium peroxide solution is 0.5M in caustic and 0.5M in mono- sodium peroxide. These concentrations, however, can be higher or lower without adversely affecting the normal operation of the system. Preferably, the feed solution is free of divalent and trivalent cations in large quantities (e.g. Ca , Mg ,

Fe or Fe ), that will either migrate across the cation membrane and precipitate in the by-product alkali metal hydroxide solution or will precipitate and foul the cation -membrane, however, the process can tolerate trace amounts of such divalent and trivalent metal ion impurities and their corresponding anions. In this application, however, the problem arising from such cations are minimal, if not non-existent, since in the presence of excess sodium hydroxide, colloidal hydroxides are formed riαich due to their slightly negative charge are not expected to migrate through the cationic membrane towards the cathode.

The liquid fed to the base compartments may be water alone, or may be water with any of a variety of electrolytes in it. Preferably, this liquid is neutral or basic (pH 7-14).

The current passed through the water splitter in conventional fashion is direct current of a voltage dictated by the resistance of the membranes and the various solution streams between the two electrodes. Current densities between about 50 and about 150 mAs per square centimeter are preferred. Higher or lower current densities are contemplated, however, for certain specific applications. The result of the current flow is electrodialysis to produce a dealkalized monosodium peroxide solution in the acid compartments and a liquid comprising sodium hydroxide in the base compartments. It is contemplated that the concentration of the product sodium hydroxide solution may be quite different frcm the sodium hydroxide concentration in the feed alkaline monosodium peroxide solution. This can be accomplished by adjusting the feed rates into the two coπpartments or the con¬ centrations of the feed solutions. Representative monosodium peroxide concentrations in the feed solution are between 0.25 and 2M, while sodium hydroxide concentrations in the same solutions are 0.25 to 3M. The relative ratio of caustic to monosodium peroxide is dictated by the mode of operation of the hydrogen peroxide generator. If one begins with a feed solution which is 0.5M in monosodium peroxide and sodium hydroxide, which is a typical Dow Hydrogen Peroxide Generator solution, then the output of the acid compartments of the water splitter will be adjusted to be 0.0M in sodium hydroxide and 0.5M in monosodium peroxide. The output of the base compartments is usually set to be 1M.

The residence time of the aqueous alkaline alkali metal salt solution in the acid compartments is sufficient to cause this solution to have a molar ratio of NaOH to NaH0_ between 0 and 1. In particular this residence time is sufficiently long to dealkalize completely the solution and to acidify it to a molar ratio of acid (H„O ) to salt (NaHO_^) of between 0 and 1.13.

The liquid comprising aqueous alkali metal hydroxide withdrawn from the base compartments suitably has a concent- ration between about 4 and about 10 weight percent alkali metal hydroxide, ii) Three Coπpartments

In accordance with the invention it has been found that a dealkalized solution of an alkali metal salt can be generated frcm an alkaline solution of the salt, without the

ITUTE SHEET addition of acid frcm the external source, by using a three cαnpartment water splitter employing alternate cationic, bipolar and anionic membranes. The process can, in particular, be used to generate a dealkalized monosodium peroxide solution from an alkaline πonosodium peroxide solution. The co-products are aqueous alkali metal hydroxide (e.g. NaOH), which may have a concentration which is considerably higher than the concent¬ ration of the caustic in the alkaline salt solution, and a depleted alkaline alkali metal salt solution. Depending on the current used and the desired concentration of the co-product high current efficiencies can be achieved during the dealkalization process.

The present invention contemplates a process which includes the following steps: a) feeding an aqueous alkaline monosodium peroxide solution into a three ccmpartinent water splitter composed of alternating cation, bipolar, and anionic membranes; the solution is intro¬ duced into each salt compartment between a cation membrane and an anion membrane; b) feeding a liquid comprising water into each base com¬ partment, between a cation membrane and the anion side of a bipolar membrane; c) feeding into each acid compartment, between an anion membrane and the cation side of a bipolar membrane, a solution of sodium hydroxide of a concentration equivalent to the desired concentration of monosodium peroxide to be produced; d) passing a direct current through the water splitter thereby causing the transfer of alkali metal cations from the alkaline nonosodium peroxide solution to the base compartment thereby causing basification of the liquid comprising water, and in addition causing the transfer of hydroxide and per- hydroxide anions to the acid cαipartment thereby causing the formation of water and hydrogen peroxide; the latter reacts with the added sodium hydroxide in this compartment to produce monosodium peroxide.

HEET e) bleeding from the salt compartments a partially dealkalized and depleted aqueous monosodium peroxide solution; f) bleeding frcm the base compartments a liquid comprising aqueous alkali metal hydroxide; g) bleeding from the acid compartments a dealkalized mono¬ sodium peroxide solution.

The solution bled from the salt compartment in d) may be fed to another water splitter for further dealkalization, or recycled to a hydrogen peroxide generator. Part of the liquid bled frcm the base compartments b) can- be recycled to the acid compartment of the water splitter and part can be recycled to a hydrogen peroxide generator or used directly for the bleaching of chemical or mechanical pulps. In step g) the bled solution may comprise between 0 and x moles of monosodium peroxide per x moles of sodium hydroxide initially added to this coπpartment. This solution can be used directly for the bleaching of mechanical pulps if the ratio of monosodium peroxide to sodium hydroxide is adjusted to be 1:0.

The three compartment water splitter employed in the process of the invention may be any of the systems described in U.S. Patent No. 4,592,817 to Chlanda et al. The three compart¬ ment water splitter is composed of a large number of cationic, bipolar and anionic membranes alternatively stacked between two electrodes.

Bipolar membranes used in the three compartment water splitter may be those described above for the two com¬ partment water splitter. The cation membranes useful in the three cαipartment water splitter may be those described above for the two com¬ partment water splitter.

The anion membranes employed in the process of the invention can be weakly basic or strong basic membranes such as those available from Ionics Inc., Watertown, Massachusetts (sold as Ionics 204-UZL-386 - Trade Mark) , from Asahi Chemical Industry Co., frcm Asahi Glass Co. (AMV anion membranes), from Tokyama Soda or from R. A. I. Research Corporation. The latter membranes are preferred since they are resistant to oxidation. In general, stacks that are suitable for electro- dialysis can be used for the three compartment water splitter. Such stacks are available commercially frcm Asahi Glass Co., 1-2, Marunochi 2-chome, Chiyoda-ku, Tokyo, Japan; Ionics Inc., Watertown, Massachussetts and other commercial sources. The operating temperature of the three coπpartment water splitter may be any temperature compatible with the membranes and above the freezing point of the solutions, perferably in the 20-60 C. temperature range.

The alkaline salt could be any soluble salt consist- ing of a monovalent cation, for example, the Group la alkali metals or non-metal monovalent cations, for example, ammonium cations, and any anion, for example, the anions of the Group Vila elements or other anions, for exaπple, sulphate, acetate, oxalate, perhydrαxyl, etc. The operation of the water splitter is further described below:

The aqueous alkaline monosodium fed into the salt cctrpartments of the electrαhydrolysis stack is typically a 1:1 caustic to monosodium peroxide mixture but may also be composed of different ratios. Typically, the concentration of the alkaline πonosodium peroxide solution is 0.5M in caustic and 0.5M in monosodium peroxide. These concentrations, however, can be higher or lower without adversely affecting the normal operation of the system. Preferably, the feed solution is free of large amounts of divalent and/or trivalent cations (e.g. Ca , Mg , Fe or Fe ) that will either migrate across the cation membrane and precipitate in the by-product alkali metal hydroxide solution or will precipitate and foul the cation membrane. In this application, however, the problems arising from such cations, in trace amounts, are minimal, if not non- existent, since in the presence of excess sodium hydroxide colloidal hydroxides are formed which due to their slightly negative charge are not expected to migrate through the cationic membrane towards the cathode. Magnesium sulphate is beneficial as a stabilizer in the feed solution in the amounts described for the two cαipart¬ ment cell and for the same reasons.

The current passed through the water splitter in conventional fashion is direct current of a voltage dictated by the resistance of the membranes and the various solution streams between the two electrodes. Current densities between about 50 to about 150 mAs per square centimeter are preferred. Higher or lower current densities are contemplated, however, for certain specific applications.

The result of the current flow is electro ialysis to produce a partially dealkalized and depleted monosodium per- oxide solution in the salt compartments, a liquid comprising sodium hydroxide in the base compartments and a liquid com¬ prising monosodium peroxide of the desired concentration and alkalinity or acidity in the acid compartments. It is con¬ templated that the concentration of the product sodium hydroxide solution from the base compartment may be quite different from the sodium hydroxide concentration in the feed alkaline monosodium peroxide solution. This can be accomplished by adjusting the feed rates into the salt and/or acid compartments or the concentrations of the feed solutions. Representative monosodium peroxide concentrations in the feed solution are between 0.25 and 2M, while sodium hydroxide concentrations in these same solutions are 0.5 to 3M. The relative ratio of caustic to monosodium peroxide is dictated by the mode of ^' operation of the hydrogen peroxide generator. If one begins with a feed solution which is 0.5M in

SUBSTITUTE SHEET raαnosodium peroxide and sodium hydroxide, which is a typical Dow Hydrogen Peroxide Generator solution, then the output of the acid csaπpartments of the water splitter will be adjusted to be 0.0M in sodium hydroxide and 0.5M in monosodium peroxide. The output of the base ccmpartments is usually set to be 1M.

The residence time of the aqueous alkaline alkali metal salt solution in the salt ccmpartments is suitably sufficient to cause the output of the acid ccmpartments to have molar ratios of NaH0~ to NaOH between O and 1. In particular, the residence time is sufficient to permit transfer of sufficient perhydroxyl anions to the acid compartment to prOduce a solution rich in the acid of the salt, for example, hydrogen peroxide.

- Suitably, the liquid comprising aqueous alkali metal hydroxide withdrawn from the base cαipartment has a concent¬ ration between about 2 and about 10 weight percent alkali metal hydroxide, iii) Applications of the Process

The process of the invention has been particularly described with reference to an especially important embodiment in which an alkaline solution of an alkali metal salt is dealkalized; and in particular an alkaline solution of mono¬ sodium peroxide is dealkalized with sodium hydroxide solution being generated as a by-product. The afore-mentioned process has particular utility in the pulp and paper industry since it provides a means of efficiently dealkalizing an alkaline solution of monosodium peroxide for bleaching of mechanical pulps.

The process also has wide application in the pro- cessing of aqueous salt solutions of various types. Thus it can be used to dealkalize alkaline solutions of a wide range of alkali metal salts.

The process can also be applied to generate useful products from non-alkaline salt solutions, a first class of salts comprises salts whose anions hydrolyze but cations do not, for example, alkali metal peroxides; a second class of salts comprises salts whose cations hydrolyze but whose anions do not, for exaπple, ammonium chloride; a third class of salts comprise salts whose cations and anions hydrolyze, for exaπple, ammonium peroxide.

Thus considering the first class of salts, a salt such as sodium peroxide may be treated by the process of the invention to generate sodium hydroxide and hydrogen peroxide. Since hydrogen peroxide is not electrically conductive it is necessary to introduce an electrolyte into the compartment in which the hydrogen peroxide is generated; the selection of the electrolyte will depend on the intended use of the hydrogen peroxide and more particularly an electrolyte, for exaπple, sodium chloride, is selected whose presence in the hydrogen peroxide is not detrimental or otherwise unacceptable in the product hydrogen peroxide. Other salts in this first class are the alkali metal salts of organic acids, for example, the salts of carboxylic acids and phenolates.

Considering the second class of salts, a salt such as aπroonium chloride may be treated by the process of the invention to generate ammonium hydroxide and hydrochloric acid from an aqueous solution of the salt. Other salts in this class are the aπmonium salts and amine salts, for example, trimethylamine and triethylamine salts of mineral acids. Considering the third class of salts, a salt such as cuniϋnium peroxide may be treated, in aqueous solution, to generate cumionium hydroxide and hydrogen peroxide. As in the case of the first class of salts an electrolyte is required in the ccsrpartment in which the hydrogen peroxide is generated. Other salts in this class are the ammonium salts and amine salts, for example, trimethylamine and triethylamine salts of organic acids, for example, the ammonium and amine salts of carboxylic acids and phenol.

SUBSTITUTE SHEET These applications may have particular utility in the recovery of useful chemical reagents frcm salts in waste effluent streams. Such effluent streams have in many cases been discharged into rivers and lakes, but environmental concerns increasingly make such practice unacceptable if not illegal.

BRIEF DESCRIPTION OF DRAWINGS

The invention is illustrated in preferred embodi¬ ments by reference to the accompanying drawings in which: FIG. 1 illustrates schematically a two cαπpartment cell;

FIG. 2 illustrates schematically a three compartment cell electrohydrolysis stack of the invention.

MODES FOR CARRYING OUT THE INVENTION Fig. 1 illustrates the process of the invention employing a two compartment cell. An electrohydrolysis stack 1 is shown with cationic membranes 2 and bipolar membranes 3 alternately stacked together between an anode 4 and a cathode 5. In Fig. 1 three cationic membranes 2a, 2b and 2c and two bipolar membranes 3a and 3b are shown, however, a much greater number of pairs can be incorporated between the two electrodes. A minimum of two cation membranes and one bipolar membrane are needed for a complete two cαipartment unit. The bipolar mem¬ branes are oriented with the cation permeable face towards the cathode 5.

An alkaline alkali metal salt containing MX, MOH and H-O is fed in stream 6 into the electrohydrolysis stack 1. A first portion 6a of aqueous salt stream 6 is fed between the cation permeable side of bipolar membrane 3a and the base cation membrane 2b. Similarly, aqueous stream 6b is fed between the positive side of bipolar membrane 3b and cationic membrane 2c. Simultaneously, a water stream 7, which may contain an electrolyte, and especially low concentrations of sodium hydroxide, is fed into electrohydrolysis stack 1 through stream 7a between cation membrane 2a and the anion permeable side of bipolar membrane 3a and stream 7b, between cation membrane 2b and the anion permeable side of bipolar membrane 3b.

An alkaline solution such as MOH hydroxide is fed to the compartments adjacent the anode and cathode in streams 8a and 8b from a reservoir, and return from these ccmpartments to the reservoir, after degassing to remove hydrogen and oxygen. Current is passed between anode 4 and cathode 5 through the electrohydrolysis stack 1, causing alkali metal cations to migrate toward the cathode across the cationic membranes 2a, 2b and 2c. In addition water is induced to be split in bipolar membranes 3a and 3b with the hydrogen ions migrating to the ccmpartments from which the alkali metal cations M have migrated and the hydroxide anions migrating into the same cαipartments that the alkali metal ions M have migrated into. Accordingly, alkali metal hydroxide MOH is formed in each of the ccmpartments between a cation membrane and the minus side of a bipolar membrane, and this alkali metal hydroxide is MDH bled frcm the electrohydrolysis stack 1 through streams 9a and 9b and collected in stream 9 as aqueous alkali metal hydroxide. The ccmpartments between the positive side of each bipolar and the adjacent side cation membrane will contain fully or partially dealkalized alkali metal salt which is bled frcm these ccmpartments through streams 10a and 10b which are combined as a base product in stream 10. In a parti¬ cular eanbodiment MX is monosodium peroxide and MOH is sodium hydroxide. Fig. 2 illustrates the process of the invention employing a three compartment cell. An electrohydrolysis stack 1 is shown with bipolar membranes 2, anionic membranes 3, and cationic membranes 4, alternately stacked together between an anode 5 and a cathode 6. ^'in Fig. 2 two bipolar membranes 2a and 2b, one anionic membrane 3, and one cationic membrane 4,

SUBSTITUTE SHEET are shown, however, a much greater number of such units can be incorporated between the two electrodes as suggested by the three dots near each electrode in Fig. 2. A minimum of two bipolar, one anionic and one cationic membranes are needed for a complete three compartment unit. The bipolar membranes are oriented with the cation permeable face towards the cathode 6.

An alkaline alkali metal salt MOH/MX is fed in stream 8 into the electrohydrolysis stack 1 between the cationic and anionic membrane (salt compartment S). Simultaneously, a water stream 9, which may contain an electrolyte, and especially low concentrations of MOH, is fed into electrohydrolysis stack 1 between cation membrane 4 and the anion permeable side of bipolar membrane 2b (base ccmpartment B) . Moreover, a MOH solution is fed in stream 7 into the electrohydrolysis stack 1 between anion membrane 3 and the positive side of bipolar membrane 2a (acid cαipartment A) . An alkaline solution such as MOH is fed to the ccmpcurtments adjacent the anode and cathodefrom a reservoir, and returned frcm these ccmpartments to the reservoir, after degassing to remove hydrogen and oxygen.

Current is passed between anode 5 and cathode 6 through the electrohydrolysis stack 1, causing alkali metal

+ cations M to migrate toward the cathode across the cationic membrane 4 and hydroxyl as well as X anions to migrate towards the anode across anionic membrane 3. In addition water is induced to be split in bipolar membranes 2a and 2b with the hydrogen ions migrating into the acid ccmpartments and the hydroxide anions migrating into the base ccmpartments. Accord¬ ingly, alkali metal hydroxide MDH is formed in each of the ccmpartments between a cation membrane and the minus side of a bipolar membrane, and this alkali metal hydroxide MDH is bled from the electrohydrolysis stack 1 through stream 12. The stream can be further split into two other streams, 7 and 13, with stream 7 going to ^' the acid compartment of the water splitter. The ccmpartments between the positive side of each bipolar and the adjacent side anion membrane will contain fully or partially dealkalized alkali metal salt which is bled frcm these compartments through stream 10. The ccmpartments between the cationic and the anionic membranes will contain a partially dealkalized and depleted MDH/MX which is bled frcm stream 11.

In a particular embodiment MDH is sodium hydroxide and MX is monosodium peroxide. In this case stream 8 parti¬ cularly originates from a hydrogen peroxide generator and stream 10 containing dealkalized monosodium peroxide is passed to a bleaching stage in pulp treatment. Stream 13 may be split frαn stream 12 to deliver by-product sodium hydroxide solution to the hydrogen peroxide generator and stream 11 containing partially dealkalized and depleted alkaline monosodium peroxide solutions may similarly be returned to the hydrogen peroxide generator.

EXAMPLES Exaπple 1

The cell shown in Fig. 1 was used to demonstrate how an alkaline NaHO- solution (i.e. Dow Hydrogen Peroxide Generator Output) can be dealkalized. A pilot cell stack consisting of eight, 2-coπpartment cells arranged between two electrodes was used. The arrangement of the cell is illustrated in Fig. 1. For simplicity this figure shows two cells with electrodes on either side. The cationic membranes were Nafion (Trade Mark) R 110 fluorocarbon membranes and the bipolar membranes were of the type having an amine-crosslinked polystyrene-vinylbenzyl chloride anion layer prepared in accordance with U.S. Patent 4,116,889 to Chlanda et al. Each

2 membrane has an exposed area of 125cm . A 10% NaOH hydroxide solution was fed to the coπpartments adjacent to the anode and the cathode from a reservoir and returned to the reservoir, after degassing to remove hydrogen and oxygen. Each cell compartment was connected to its appropriate reservoir tank and the acid coπpartments operated in the batch mode while the base

SUBSTITUTE SHEET compartments were in the feed and bleed mode. A batch mode refers to the case in which the same solution is being recirculated continuously through the system while the feed and bleed mode refers to a case in which fresh solution is continuously fed into and bled from the system.

Initially, the salt loop was filled with a solution that was 0.68M in NaHO- and 0.32M in NaOH while the base compartment was filled with a 1M solution of NaOH in order to maintain conductivity in the system in the early stages of the run. The feed soltuion into the base compartments was water at a feed rate of 36 mL/min. The initial electrical input was 13 amps at 23 V. At time intervals during the run small samples were taken from the reservoir of the acid compartment and analyzed for alkalinity by titration with HCl. These same samples were analyzed for H-0- through an iodine titration.

As shown in Table 1, once the power was turned on, the concentration of sodium hydroxide in the acid compartments decreased over twenty two minutes from 0.32M to 0.05M, indicating that sodium cations are crossing the cationic membranes thus moving from the acid into the base compartments. It is also shown in Table 1 that the concentration of NaHO- in the acid compartments remains approximately the same throughout the run thus indicating that very small amounts, if any, of hydrogen peroxide is crossing over into the base compartments.

Table 1 •

Time Cone. NaOH Cone. NaHO- mins in Acid Compt. ,M in Acid Con

0 0.32 0.68

11 0.20 0.67

22 0.05 0.66 Example 2

The cell of Fig 1, referred to in example 1, was also used in example 2. In this case, however, both the acid and base compartments were operated in the feed and bleed mode. Table I shows the conditions used for the operation of the cell stack.

Table 2 :

Base Salt Initial Concentration, M

NaOH 1.2 0.0

NaHO- 0.0 0.5

MgS0₄ 120 ppm

Circulation Rate , gpm 1.0 0.83 P Prreessssuurree ,, ppssii 2 2..77 2.7

Feed Soln. Concentration, M

NaOH 0.0 0.5

NaHO- 0.0 0.5

M MggSS0O₄, 120 ppm

Feed Rate, mL/min 44 126

Actual Feed Rate, meq/min

NaOH 0.0 63.0

NNaaHHOO-- 00..00 63.0

Initially, the acid compartments were filled with a solution 0.5M in NaHO-, containing also 120ppm of MgS0₄ whereas the base compartments were filled with a 1.2M sodium hydroxide solution. Magnesium sulphate was added to the acid compartments in order to prevent the decomposition of hydrogen peroxide. The decomposition of this species is otherwise quite pronounced for equimolar 0.5M NaOH, H-O- solutions as shown in Table 3. Table 3 presents the residual concentration (M) of hydrogen peroxide in such solutions at various time intervals and concentrations of added magnesium sulphate stabilizer (ppm) . As shown in this table, at very small concentrations (e.g. 1 ppm ) MgS0₄ acts as a destabilizer, whereas at concentrations over 10 ppm it begins to act as a stabilizer. At concentrations over about 100 ppm there is almost no decomposition of hydrogen peroxide. From mass balance studies we found that at these levels MgS0₄ can act as an effective stabilizer in the electrohydrodyna ic environment of the water splitter as well. Concentrations of MgS0₄, however, higher than 400 ppm should be avoided since precipitation of Mg(0H)₂ begins to occur within the water splitter and in addition the stabilizing effect is greatly diminished.

Table 3 :

Time/MgS04 ,r PPm 0 1 10 60 120

Hours

0 0.49 0.49 0.49 0.49 0.49

24 0.40 0.31 0.47 0.47 0.49

90 0.32 0.14 0.37 0.41 0.48

The reason for filling the various compartments with the solutions indicated in Table 2 was to maintain high conductivity throughout the stack during the initial stages of the run. The feed solution for the acid compartments was a simulated Dow Generator solution (0.5M

NaOH ,0.5M NaHO.) 120ppm in MgS0₄. The feed solution for this base compartments was water. The circulation rates, that is the rates at which solutions are circulated through the membrane compartments and the corresponding recirculation tanks, were adjusted at 1.0 and 0.83 gpm so that the pressure in the two compartments would be about the same at 2.7 psi. The feed rates into the acid and base compartments were 126 and 44mL/min respectively. The former feed rate was chosen in order to achieve a 0.52M concentration of Na⁺ in the bleed solution of the acid compartments. At higher or lower feed rates the Na⁺ concentration of the bleed solution from the acid compartments is higher or lower respectively than 0.52M and hence a 1:1 ratio of NaOH to H-0- can not be achieved (see table 4) .

Table 4 :

Acid Compt. Na' cone. HO- ^"conc. Na⁺/^"OOH

Feed Rate M M Ratio mL/min

88 0.47 0.52 0.90

113 0.46 0.52 0.88

123 0.50 0.52 0.97

126 0.52 0.52 1.04

131 0.50 0.52 0.97

172 0.70 0.52 1.35

At 126 mL/min, conductivity in the acid compartments is 30 mS/cm, which is not optimum in terms of power efficiency; it is, however, within the practically useful range. The electrical input was 13 amps at 24.5V.

Small samples from the reservoirs of the acid compartments were taken at intervals and analyzed for alkalinity by titration with HCl. These same samples were also analyzed for H-0₂ through an iodine titration. Samples were also taken from the base compartments and analyzed for alkalinity by titration with HCl. On the basis of the latter analysis the current efficiency of the system (see Table 5) was calculated. A constant current efficiency of over 95% was observed over the duration of the run which was 6.5 hrs. Table 5 :

Base Acid

Bleed Soln. Concentration, M

NaOH 1.29 0.00

NaH0₂ 0.00 0.51

Bleed Rate , mL/min 48.3 122

Actual Bleed Rate , meq/min

NaOH 62.3 0.0

NaH0₂ 0.0 62.2

Net Production Rate , meq/min

NaOH 62.3 -63.0

NaH0₂ 0.0 -0.8

Current Efficiency for Na⁺,% 96

Under the indicated conditions, the bleed solution from the acid compartments was 0.52M in NaHO- at 114mL/min (1:1 NaOH, H-O- solution) while from the base compartments 1.2M NaOH at 53 mL/min. These figures suggest that Na⁺ cations are crossing the cationic membrane and entering the base compartments. In terms of net production rates 63meq/min of NaOH are removed from the acid compartments and 63 meq/min of NaOH are produced in the base compartments. These figures suggest that Na⁺ cations are crossing the cationic membrane and entering the base compartments. Moreover, these figures show that the loss of hydrogen peroxide crossing the cationic membrane or being decomposed is minimal in the hydrodynamic environment of the two-compartment water splitter.

Example 3

TUTE SHEET The cell employed in examples 1 and 2 was also used to examine the effect of co-product concentration on the current efficiency and on product bleed rate. The experiment was designed so that a 10%(2.8M) caustic solution would be produced from the base compartments instead of the 5% caustic solution produced in example 1. All experimental variables were held the same as in example 2 except for the feed rates into the acid and base compartments which were adjusted so that the desired product concentrations can be achieved (see Table 6) .

Table 6 :

Base Salt

Initial Concentration, M

NaOH 1.2 1 0.0

NaHO- 0.0 0.5

MgS0₄ 120 ppm

Circulation Rate , gpm 1.0 1 0.83

Pressure , psi 2.7 2.7

Feed Soln. Concentration, M

NaOH 0.0 0.5

NaH0₂ 0.0 0.5

MgS0₄ 120 ppm

FFeeeedd RRaattee,, mmLL//mmiinn 1155..8 111.3

Actual Feed Rate, meq/min

NaOH 0.0 55.7

NaHO, 0.0 55.7

The feed rate into the acid compartments was 111.3 mL/min and into the base compartments was 15.8mL/min. As shown in Table 7, at steady state, a solution of 0.51M NaHO- is produced from the acid compartments whereas a 2.82M solution of NaOH is produced from the base compartments. Table 7 :

Base Acid

Bleed Solution Concentration, M

NaOH 2.82 0.0 NaHO, 0.0 0.51

Bleed Rate , mL/min 20.8 110.2 Actual Bleed Rate ,meq/min

NaOH 58.7 0. 0 NaHO- 0. 0 56.2

Net Production Rate, meq/min

NaOH 58 .7 -55. 65 NaHO- 0. 0 -0. 55

The current efficiency in this process is quite high at 90.6 + 0.7% even though it is lower than that obtained in Example 2 by about 5%. A reduced bleed rate of about 20.7 mL/min is obtained as a result of the reduced current efficiency. The complete dealkalization, however, of the alkaline NaHO- feed solution continues to be feasible under the new experimental conditions.

Example 4

The cell shown in Fig 2 was used to demonstrate how an alkaline NaH0₂ solution (i.e. Dow Hydrogen Peroxide Generator Output) can be completely dealkalized. A pilot cell stack consisting of eight, 3-compartment cells arranged between two electrodes was used. The arrangement of the cell is illustrated in Fig 2. For simplicity this figure shows one cell with electrodes on either side. The cationic membranes were Dupont's Nafion® 110 flourocarbon membranes, the bipolar membranes were of the type having an amine-crosslinked polystyrene- vinylbenzyl chloride anion layer prepared in accordance with U.S. Pat. 4,116,889 to Chlanda et al and the anionic membranes were of a type resistant to oxidants supplied by the RAI Research Corporation. Each membrane had an exposed area of 125cm². A 10% NaOH hydroxide solution is fed to the compartments adjacent to the anode and the cathode from a reservoir and returned to the reservoir, after degassing to remove hydrogen and oxygen. Each cell compartment was connected to its appropriate reservoir tank and all compartments operated in the batch mode. A batch mode refers to the case in which the same solution is being recirculated continuously through the system while the feed and bleed mode refers to a case in which fresh solution is continuously fed into and bled from the system. Table 8 shows the conditions used for the operation of the cell stack.

Table 8:

Acid Base Salt Initial Concentration, M

NaOH 0.49 1.78 0.88

NaH0₂ 0.0 0.0 0.86

MgS0₄ 120 ppm

Circulation Rate , ggppmm 0.8 0.8 0.8 Initial Loop Volumeι ,, LL 3.5 3.4 4.6

Initially, the Acid compartments were filled with a solution that was 0.49M in NaOH, the Base compartments with a solution 1.78 M in NaOH and the salt compartment with a solution 0.88M in NaOH and 0.86M in NaHO-. The circulation rates, that is the rates at which solutions are circulated through the membrane compartments and the corresponding recirculation tanks were adjusted at 0.8 gpm in all compartments. The initial loop volumes in the three compartments were 3.5, 3.4 and 4.6 liters for the acid, base and salt compartments respectively. The initial electrical input was 13 amps at 25 V. At time intervals during the run small samples were taken from the reservoirs of the three compartments and analyzed for alkalinity by titration with HCl. These same samples were analyzed for H-O- through an iodine titration.

Table 9 demonstrates the changes in the concentration of sodium hydroxide and monosodium peroxide in the three compartments once the power was turned on and the system allowed to run for 120 minutes.

Table 9 :

Time NaOH Cone. ,M NaH0₂ Cone. , M

Min Acid Base Salt cid Base Salt

0 0.49 1.78 0.88 0 0 0.86

20 0.39 2.00 0.70 0.10 0 0.81

50 0.30 2.35 0.50 0.19 0.01 0.72

70 0.20 2.59 0.33 0.28 0.01 0.66

120 0.00 3.00 0.12 0.46 0.02 0.36

As shown in Table 9 the concentration of sodium hydroxide in the salt compartments decreased over the duration of the experiment from 0.88M to 0.12M while the concentration of sodium hydroxide in the base compartment increased from 1.78 M to 3.00 M; this indicates that sodium cations are crossing the cationic membranes thus moving from the salt into the base compartments. It is also shown in Table 9 that the concentration of monosodium peroxide in the salt compartments decreased over the duration of the experiment from 0.86M to 0.36M while the concentration of monosodium peroxide in the acid compartment increased from 0 to 0.46M; this indicates, that perhydroxyl anions are crossing the anionic membranes thus moving from the salt into the acid compartments. The concentration of monosodium peroxide in the base compartments remained at about o M during the run thus indicating that very small amounts, if any, of perhydroxyl anions are crossing the cationic membrane into the base compartments.

Table 10 presents the current efficiencies obtained for the transport of Na⁺ to the base compartment and the transport of H0₂ ^" and OH^" to the acid compartment in the duration of the experiment.

Table 10

Time Current Efficiency % mins Na⁺ H0₂ ^" OH-

0-20 74.2 24.7 49.5

20-50 77.3 21.0 56.3

50-70 92.4 24.7 67.7

70-120 74.5 27.4 47.1

0-120 78.3 25.0 53.3

As shown in Table 10 the sodium cation current efficiencies are on the average about 78% and typically about 75%. For the perhydroxyl anion the current efficiencies are on the average 25% and about 25% throughout the duration of the experiment. The reason for the relatively low current efficiency for the perhydroxyl ions is that they compete with the hydroxyl ions in crossing over to the acid compartment.

Example 5

The cell of Fig 2 referred to in example 4 was also used in example 5. In this case, however, all three compartments were operated in the feed and bleed mode. Table 11 shows the conditions used for the operation of the cell stack . Table 11

Acid Base Salt

Initial Concentration, M

NaOH 0.00 1.10 0.50

NaHO- 0.80 0.00 0.50

MgS0₄ 120 ppm

Circulation Rate , gpm 0.80 1.0 1.0

Feed Solution Cone. ,M NaOH 1.09 0.0 0.5

NaHO- 0.00 0.0 0.5

Feed Rate, mL/min 16.8 53.5 112.6

Actual Feed Rate, meq/min

NaOH 18.3 0.0 56.3

NaH0₂ 0.0 0.0 56.3

Initially, the acid compartments were filled with a solution 0.8M in NaH02, containing also 120 ppm of MgS0₄ whereas the base compartments were filled with a 1.10M sodium hydroxide solution. The salt compartments were filled with a solution 0.50 M in NaOH and 0.50 M in NaHO-. Magnesium sulphate was added to the acid compartments in order to prevent the decomposition of hydrogen peroxide migrating into these compartments. The decomposition of this species is otherwise quite pronounced for equimolar 0.5M NaOH, H-O- solutions as shown in Table 12. Table 12 presents the residual concentration (M) of hydrogen peroxide in such solutions at various time intervals and concentrations of added magnesium sulphate stabilizer (ppm) . As shown in this table, at very small concentrations (e.g. 1 ppm ) MgS0₄ acts as a destabilizer, whereas at concentrations over 10 ppm it begins to act as a stabilizer. At concentrations over about 100 ppm there is almost no decomposition of hydrogen peroxide. From mass balance studies we found that at these levels MgS0₄ can act as an effective stabilizer in the electrohydrodynamic environment of the water splitter as well. Concentrations of MgS0₄, however, higher than 400 ppm should be avoided since precipitation of Mg(0H)₂ begins to occur within the water splitter and in addition the stabilizing effect is greatly diminished.

Table 12 :

Time/MgS04 , ppm 0 1 10 60 120 240

Hours

0 0.49 0.49 0.49 0.49 0.49 0.49 24 0.40 0.31 0.47 0.47 0.49 0.49

90 0.32 0.14 0.37 0.41 0.48 0.49

The reason for filling the various compartments with the solutions indicated in Table 12 was to maintain high conductivity throughout the stack during the initial stages of the run. The feed solution for the salt compartments was a simulated Dow Generator solution (0.5M NaOH ,0.5M NaH0₂) . The feed solution for the base compartments was water and for the acid compartments was 1.09 M NaOH, 120 ppm in MgS0₄. The feed rates into the acid, base and salt were 16.8, 53.8 and 112.6 mL/min respectively. These particular rates were chosen in order to achieve the desired ratio of 0:1 NaOH/NaH0₂ in the output of the acid compartment and a concentration of NaHO- of about O.δM.The circulation rates, that is the rates at which solutions are circulated through the membrane compartments and the corresponding recirculation tanks, were adjusted at 0.8, 1.0 and 1.0 gpm. The electrical input was 13 amps at 27V.

Small samples from the reservoirs of the acid, base. and salt compartments were taken at intervals and analyzed for alkalinity by titration with HCl. These same samples were also analyzed for H₂0₂ through an iodine titration. On the basis of the latter analysis the current efficiency of the system (see Table 13) was calculated. A constant current efficiency of about 90% for Na⁺ and 30% for H0₂ ^' was observed over the duration of the run which was 6.0 hrs. The rather low current efficiency for HO^2" is due to the fact that OH^" anions are also crossing the anionic membrane, thus competing with the the perhydroxyl ions.

Table 13

Acid Base Salt

Bleed Solution Concentration ,M

NaOH 0.00 1.10 0.160

NaHO, 0.79 0.03 0.35

Bleed Rate , mL/min 24.7 52.3 97.0 Actual Bleed Rate , meq/min

NaOH 0.0 57.5 15.5

NaH0₂ 19.5 1.6 34.0

Net Production Rate , meq/m 1 Li1nTli-

NaOH -18.3 57.5 -40.8

NaH0₂ 19.5 1.6 -22.3

Current Efficiency for Na⁺ ,% 90

H0₂''% 30

As shown in Table 13, the bleed solution from the acid compartments was 0.79M in NaH0₂ at 24.7 mL/min (1:1 NaOH, H₂0₂ solution) whil e from the base compartments 1.10M NaOH at 52.3 mL/min. The concentration of the bleed solution from the base compartment was specifically adjusted to be 1.1M in order to demonstrate that it can be of a concentration that can be fed directly into the acid compartment as shown in Fig 2 ; since, however, sodium hydroxide at this concentration is required only at the rate of 18.3 mL/min in the acid compartment the excess 34 mL/min of sodium hydroxide can be recycled to the hydrogen peroxide generator or be used for other purposes. The bleed solution from the salt was 0.16 M in NaOH and 0.35 M in NaH0₂ at 97 mL/min. In terms of net production rates for NaOH, 40.8 meq/min of NaOH are being removed from the salt compartment and 18.3 meq/min are being consumed in the acid compartment while 57.5 meq/min are being produced from the base compartment. In terms of net production rates for NaH0₂ , 22.3 meq/min are being removed from the salt compartment, while 19.5 meq/min are produced from the acid compartment. These figures suggest that Na+ cations are crossing the cationic membrane and entering the base compartments and HO^2" are crossing the anionic membranes and entering the acid compartments. Moreover, these figures show that the loss of hydrogen peroxide to decomposition is minimal and hence magnesium hydroxide at 120 ppm can act as an effective stabilizer in the electrohydrodynamic environ¬ ment of the three-compartment water splitter.

SUBSTITUTE SHEET

Claims

1. A process which comprises the steps of: a) providing a cell comprising an anode, a cathode and at least two coπpartments ^•therebetween defined at least in part by a cation permselective membrane and at least a pair of bipolar membranes, said bipolar membranes having a cation side facing said cathode and an anion side facing said anode, b) feeding an aqueous solution of a salt MX into a first of said coπpartments to contact a first side of said cation perm¬ selective membrane, wherein M is a cation selected frcm alkali metal and non-metal cations and X is an anion of an acid, c) feeding a liquid comprising water into a second of said ccmpartments to contact an anion side of a said bipolar membrane, d) passing a direct current through said cell between said anode and cathode to effect: i) migration of said cation M from said first compartment through said cation permselective membrane into said second compartment, and ii) splitting of water by said bipolar membrane of said second compartment with accum ulation of hydroxide ions of the water in said second compart¬ ment and removal of hydrogen ions of the water from said second com¬ partment, e) removing accummulated MOH from said second crcxipartment, and f) removing MX from said first compartment, wherein said solution of salt MX is selected frcm the group consisting of:

A) aqueous alkaline solutions of a salt MX,

B) aqueous non-alkaline solutions in which MX is a salt whose anions hydrolyze but cations do not,

C) aqueous non-alkaline solutions in which MX is a salt whose cations hydrolyze but whose anions do not,

D) aqueous non-alkaline solutions in which MX is a salt whose cations and anions hydrolyze.

2. A process according to claim 1, wherein said cell is a two compartment cell, said first coπpartment being defined between said first side of said cation permselective membrane and a cation side of a first bipolar membrane, and said second compartment being defined between a second side of said cation permselective membrane and an anion side of a second bipolar itiembrane.

3. A process according to claim 1 or 2, wherein said solution in b) is an aqueous alkaline solution comprising monosodium peroxide and sodium hydroxide.

4. A process according to claim 1, wherein acid and salt chambers are defined in said first compartment separated by an anion permselective membrane, said salt chamber being defined between said anion permselective membrane and said cation permselective membrane, said acid chamber being defined between said anion permselective membrane and a cation side of a said bipolar membrane, and said second compartment defining a

S BSTIT TE SHEET base chamber; step b) comprises feeding said solution of a salt MX into said salt chamber; and step c) comprises feeding a liquid comprising water into said base chamber.

5. A process according to claim 4, further including feeding a liquid comprising water into said acid chamber.

6. A process according to claim 5, vherein step f) comprises removing HX frαn said acid chamber.

7. - A process according to claim 6, further including removing depleted MX solution from said salt chamber.

8. A process according to claim 7, wherein said solution in b) is an aqueous alkaline solution comprising monosodium peroxide and an alkali.

9. A process for the dealkalization of an aqueous alkaline alkali metal peroxide solution comprising: a) establishing an electrolytic cell comprising an anode, a cathode and a plurality of membranes therebetween, said plurality of membranes including spaced apart bipolar membranes and a cation permselective membrane disposed between each pair of bipolar membranes, said bipolar membranes each having an anion side facing said anode and a cation side facing said cathode, and a plurality of side-by-side coπpartments defined between adjacent membranes of said plurality, b) feeding an aqueous alkaline solution of a salt MX and a hydroxide MDH, in which M is an alkali metal cation and X is an anion of an acid into a first coπpartm:.nt of said plurality to contact a first side of a said cation perm¬ selective membrane, c) feeding a liquid comprising water into a second compartment of said plurality to contact an anion side of a said bipolar membrane, d) passing a direct current through said cell between said anode and cathode to effect: i) migration of said cation M from said first compartment through said cation permselective membrane into said second compartment, ii) splitting of water by said bipolar membrane of said second compartment with accummulation of hydroxide ion of the water in said second cαipart¬ ment and removal of hydrogen ions of the water from said second compart¬ ment e) removing accumulated MDH from said second compartment, and f) removing dealkalized MX from said first compartment.

10. A process which comprises the steps: a) feeding an aqueous alkaline or non-alkaline solution of a salt MX in which M is a cation selected from alkali metal and non-metal cations and X is an anion of an acid, into each acid compartment between a cation membrane and a cation side of a bipolar membrane of an electrodialytic water splitter coπprised of alternating cation and bipolar membranes positioned between an anode and a cathode; b) feeding into each base coπpartment between a cation membrane and an anion side of a bipolar membrane, a liquid comprising water; c) passing a direct current through said water splitter thereby causing dealkalization or acidification of said aqueous alkaline or non-alkaline salt solution and basification of said liquid comprising water through transfer of alkali metal cations M from said aqueous salt solution said liquid comprising water, d) bleeding frcm the base ccmpartments a liquid comprising aqueous MDH, e) bleeding frcm the acid ccmpartments a dealkalized or acidified aqueous solution of said salt MX, wherein said solution of salt MX is selected from the group consist¬ ing of:

A) aqueous alkaline solutions of a salt MX,

11. The process of claim 10, wherein said salt in a) is NaHO- in a solution comprising NaOH.

12. The process of claim 11, wherein the initial molar ratio of NaOH to NaHO- is 1:1.

13. The process of claim 12, wherein the initial molar concentration of NaOH and NaHO- in said aqueous solution in step a) is 0.5M NaOH and 0.5M NaHO-.

14. The process of claim 13, wherein magnesium sulphate at a concentration of 120 ppm is added to said salt solution for preventing the decomposition of hydrogen peroxide.

15. The process of claim 11, wherein the residence time of said aqueous salt solution in the acid coπpartments is sufficient to cause this solution to have a molar ratio of NaOH to NaHO- between 0 and 1.

16. The process of claim 11, wherein the residence time of the aqueous salt solution in the acid ccmpartments is sufficiently long to dealkalize completely this solution and furthermore acidify it to have molar ratios of acid (H-0-.) to said (NaH0₂) between 0 and 1.13.

17. The process of claim 15, wherein the liquid compris¬ ing aqueous MDH withdrawn from the base ccmpartments has a concentration between about 4 and about 10 weight percent alkali metal hydroxide.

18. The process of claim 15, wherein a dealkalized alkali metal salt solution bled from the acid ccmpartments in e) is passed directly to a unit for bleaching of mechanical pulps.

ET

19. The process of claim 16, wherein the aqueous MDH bled frαn the base ccmp__rtments in d) is passed to a unit for bleaching of chemical or mechanical pulps or is recycled to a hydrogen peroxide generator.

20. The process of claim 10, vherein said salt MX is of the type whose ions do not hydrolyze in water.

21. The process of claim 10, wherein said salt MX is of the type whose anions hydrolyze but cations do not.

22. The process of claim 10, wherein said salt MX is of the type whose cations hydrolyze but whose anions do not.

23. The process of claim 10, wherein said salt is of the type whose cations and anions hydrolyze.

24. The process of claim 10, wherein said salt solution contains soluble non-conductive compounds and/or insoluble but suspended compounds.

25. A process which comprises the steps: a) feeding an aqueous alkaline or non- alkaline solution of an alkali metal and/or non-metal salt into a three cαrpartment water splitter composed of alternating cation, bipolar and anionic membranes, said solution being intro¬ duced into each salt compartment between a cation membrane and an anion membrane; b) feeding a liquid comprising water into each base compartment, between a cation membrane and an anion side of a bipolar membrane, c) feeding into each acid cαipartment,

SUBSTITUTE SHEE between an anion membrane and a cation side of a bipolar membrane, a solution of alkali metal and/or non-metal hydroxide of a concentration equivalent to a desired concentration of a salt to be produced; d) passing a direct current through said water splitter thereby causing transfer of alkali metal cations frαn said solution of a salt to the base compartment thereby causing basification of said liquid com¬ prising water and in addition causing the transfer of hydroxide anions and anions of the salt to the acid compartment thereby causing the formation of water and the acid of the anion of the salt, said acid reacting with the added alkali metal hydroxide in said acid compartment to produce the salt of the anion and water; e) bleeding from the salt compartments a partially dealkalized and depleted aqueous salt solution, f) bleeding frcm the base ccmpartments a liquid comprising aqueous alkali metal hydroxide; and g) bleeding from the acid ccmpartments a dealkalized salt solution.

26. The process of claim 25, wherein said salt in a) is NaH0„ in a solution comprising NaOH.

27. The process of claim 26, wherein the initial molar ratio of NaOH to NaH0₂ is 1:1.

28. The process of claim 27, wherein the initial con¬ centration is 0.5M in NaoH and NaHO-.

29. The process of claim 28, wherein magnesium sulphate at a concentration of 120 ppm is added to said acid compart¬ ments for preventing the decomposition of hydrogen peroxide.

30. The process of claim 26, wherein the residence time of said aqueous alkaline alkali metal salt solution in the salt ccxipartments is sufficient to cause the output of the acid <_cmpartments to have molar ratios of NaHO- to NaOH between 0 to 1.

31. The process of claim 25, wherein the residence time of said aqueous alkaline alkali metal salt in the salt coπpart¬ ments is sufficiently long to permit the transfer of sufficient perhydroxyl anions to the acid compartment to produce a solution rich in the acid of the salt.

32. The process of claim 30, wherein the liquid com¬ prising aqueous alkali metal hydroxide withdrawn from the base ccmpartments has a concentration between about 2 and about 10 weight percent alkali metal hydroxide.

33. The process of claim 30, wherein the output of the acid ccmpartments is passed directly to a unit for bleaching of mechanical pulps.

34. The process of claim 30, wherein part of said alkali metal hydroxide is recycled to the acid compartment of the water splitter and part is re-cycled to a hydrogen peroxide generator or to a unit for the bleaching of chemical or mechanical pulps.

35. The process of claim 30, wherein said output of the salt ccnpartment is fed into another water splitter or recycled into a hydrogen peroxide generator.

36. The process of claim 25, wherein said salt is of the type whose ions do not hydrolyze in water.

37. The process of claim 25, wherein said salt is of the type whose anions hydrolyze but cations do not.

38. - The process of claim 25, wherein said salt is of the type whose cations hydrolyze but whose anions do not.

39. The process of claim 25, wherein said salt is of the type whose cations and anions hydrolyze.

40. The process of claim 25, wherein said salt solution contains soluble non-conductive compounds and/or insoluble but suspended compounds.

41. The process of claim 14, wherein trace amounts of divalent and trivalent metal ion impurities and corresponding anions are present in the salt solution.

42. The process of claim 29, wherein trace amounts of divalent or trivalent metal ion iπpurities and corresponding anions are present in one or more of said solution of an alkaline metal introduced to said salt coπpartment, said liquid comprising water fed to said base cαipartment and said solution of hydroxide fed to said acid compartment.