WO1990010733A1

WO1990010733A1 - Process for generating chloric acid and chlorine dioxide

Info

Publication number: WO1990010733A1
Application number: PCT/CA1990/000084
Authority: WO
Inventors: Richard M. Berry; Richard D. Mortimer
Original assignee: Pulp And Paper Research Institute Of Canada
Priority date: 1989-03-15
Filing date: 1990-03-14
Publication date: 1990-09-20
Also published as: AU5336990A; CA2020278A1

Abstract

A three compartment water splitting system comprised of repeating cationic, anionic and bipolar membranes is used to remove alkali metal cations from an alkali metal chlorate solution thereby producing an alkali metal hydroxide and chloric acid. When the chloric acid reaches a certain concentration, chlorine dioxide is formed. Alternatively the chloric acid can be used for the generation of chlorine dioxide.

Description

PROCESS FOR GENERATING CHLORIC ACID AND CHLORINE DIOXIDE

TECHNICAL FIELD

This invention relates to a process for the production of chloric acid and chlorine dioxide.

BACKGROUND ART

Chloric acid is a strong oxidant which is an intermediate in the production of chlorine dioxide which in turn is used as a bleaching agent in the pulp and paper industry. Chloric acid is formed through the action of concentrated sulphuric acid solutions on sodium chlorate. The process leaves sodium sulphate as a by-product which may or may not be usable in other parts of the pulp and paper process. Another method of generating chloric acid is by the action of a soluble chloric acid salt, (e.g. barium chlorate) which forms a precipitable salt (barium sul¬ phate) with a suitable acid (sulphuric acid). The method is of laboratory interest only but a variation of the method could be used industrially. An ion exchange membrane could be used as the "precipitable" salt and then regenerated with a suitable acid, (e.g. sulphuric acid) after the passage of the chloric acid salt. The salt (e.g. sodium sulphate) formed through the regeneration process and present in the waste stream again may or may not be usable in other parts of the pulp and paper process.

Membrane systems involving stacked pairs of membranes have been recommended for various applications. All these applications appear to stem from a disclosure in C.A., Vol. 53, 11070b (1959) (Oda et al) of two compartment electrodialytic water splitting of aqueous neutral salts. Examples- of individual applications in two and three compartment . cell configurations are Mani and Chlanda (U.S. Patent Nos. 4,504,373 and 4,391,680), Gancy et al (U.S. Patent Nos. 4,238,305 and 4,219,396), Chlanda et al (U.S. Patent Nos. 4,116,889, 4,107,015 and 4,028,835), and Dege et al (U.S. Patent No. 4,024,043). Neither the separation of chloric acid from sodium chlorate nor the generation of chlorine dioxide from chloric acid either inside or outside a cell are envisaged in any of this prior art.

Other literature describes the generation of chlorine dioxide from sodium chlorate. All this literature, however, describes the same initial step of acidifying sodium chlorate with a strong acid before various reducing agents are applied to the resulting solution. The step of initially generating chloric acid separately in an electrochemical cell is not specified in the prior art. By using the process in this invention chlorine dioxide can be formed with sodium hydroxide as the coproduct. Sodium hydroxide cannot be generated as a coproduct in any of the other methods described and sodium hydroxide is a chemical, the need for which is presently high and will grow in the future.

DISCLOSURE OF THE INVENTION

This invention seeks to provide a method for the production of chloric acid.

This invention also seeks to provide a method for the production of chlorine dioxide. In accordance with the invention it has been found that chloric acid and an alkali metal hydroxide can be generated from an alkali metal chlorate by using an electrochemical cell comprising a three compartment water splitter employing a cation membrane, an anion. membrane and a pair of bipolar membranes. It has also been found that the chloric acid above a certain concentration forms chlorine dioxide in this type of cell stack and depending on the current use and the desired concentration of the coproduct, very high current efficiencies can be achieved with such cell. Thus in accordance with one aspect of the invention there is provided a process for producing chloric acid from which chlorine dioxide can be generated which comprises establishing an electrochemical cell having an anode and a cathode and at least one unit disposed between the anode and cathode. The unit com¬ prises an acid compartment, a salt compartment and a base compartment. An aqueous alkali metal chlorate solution is fed to the salt compartment and water is fed to the acid and base compartments and a direct electric current is applied across the unit between the anode and cathode. The salt compartment is defined by an anion permselective membrane and a cation permselective membrane. The acid compartment is defined by the anion permselective membrane and a first bipolar membrane which has a cation portion facing the acid compartment and an anion portion facing the anode. The base compartment is defined by the cation permselective membrane and a second bipolar membrane which has an anion portion facing the base compartment and a cation portion facing the cathode. The establishment of the flow of direct current effects several phenomena in the cell: alkali metal cations move or migrate from the salt compartment, in the direction of the cathode, through the cation perm- selective membrane to the base compartment; chlorate anions move or migrate from the salt compartment, in the direction of the anode, through the anion permselective membrane to the acid compartment; water dissociates within the first and second bipolar membranes and hydro¬ gen cations move or migrate through the cation portion of the bipolar membranes, in the direction of the cathode, and accumulate in the acid compartment, and hydroxide anions move or migrate through the anion portion of the bipolar membranes, in the direction of the anode and accumulate in the base compartment.

In this way chloric acid derived from the hydrogen ions and chlorate ions accumulates in the acid compartment; and alkali metal hydroxide derived from the alkali metal ions and hydroxide ions accumulates in the base compartment.

In operation of the cell spent chlorate solution is removed from the salt compartment and chloric acid and alkali metal hydroxide solutions may be with- drawn from the acid and base compartments, respectively. As an alternative to withdrawing chloric acid from the acid compartment, the concentration of the acid may be allowed to increase in the acid compartment until it dissociates to liberate chlorine dioxide which can then be recovered from the acid compartment.

DESCRIPTION OF PREFERRED EMBODIMENTS

The cell suitably includes a multiplicity of the units between the anode and cathode, in such case the invention involves a process which includes the following steps: a) feeding an aqueous alkali metal chlorate solution into a three compartment water splitter composed of repeating anion, cation and bipolar membranes; this solution is introduced between the cation and anion membranes (compartment 1); b) feeding a water solution into each compartment between a cation membrane and the anion side of a bipolar membrane (compartment 2); c) feeding a water solution into each compartment between an anion membrane and the cation side of a bipolar membrane (compartment 3); d) passing a direct current through the water splitter thereby causing the transfer of alkali metal cations and chlorate ions from the salt solution in all compartments numbered 1), e) bleeding from compart¬ ments numbered 2) an alkali metal hydroxide solution, f) bleeding from compartments numbered 3) a chloric acid solution which can then be fed to a conventional chlorine dioxide generator. The alkali metal chlorate solution, and the alkali metal chlorate itself in step a) is, in parti¬ cular, free or substantially free of alkali metal chloride and more especially the solution consists essentially of alkali metal chlorate in water. The addition of a reducing agent to the chloric acid externally of the cell, for example, NaCl, HC1, CH-OH or SO_ suffices for the formation of chlorine dioxide from the chloric acid. Alternatively the chloric acid solution in each compartment 3) can be raised to a high enough concentration in the stack for chlorine dioxide to be formed directly, for this purpose it is appropriate to raise the chloric acid content in each compartment 3) to a concentration of above about 1.0 molar. The three compartment water splitter referred to in steps a) to e) incorporates a large number of cation, anion and bipolar membranes arranged in an alternating fashion between two electrodes to provide alternating base, acid and- salt compartments that form an electrodialytic stack.

Bipolar membranes are composite membranes consisting of three parts, a cation selective region, an anion selective region and an interface region between the ion selective regions.

The bipolar membranes are permeable or porous to neutral species, for example, water and consequently water migrates from the acid and base compartments through the ion selective regions to the interface region.

When a direct current is passed across a bipolar membrane with the cation selective side towards the cathode, electrical conduction is achieved by the transport of H and OH ions which are obtained from the dissociation of water within the interface region.

Hydrogen cations migrate from the interface region through the cation selective region in the direction of the cathode, and hydroxide anions migrate from the interface region through the permselective region in the direction of the anode.

Hydrogen ions migrating to the cathode produce hydrogen which is drawn off from the cell at the cathode, and hydroxide ions migrating to the anode produce oxygen which is drawn off from the cell at the anode.

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 general, stacks that are suitable for electrodialysis can be used for the water splitter. Such stacks are avail- able commercially from Asahi Glass Co., Chiyoda Ku, Tokyo, Japan; Ionics, Inc., Watertown, Massachusetts and other commercial sources.

In general, for efficient operation, it is preferred to establish an acid content in the acid compartment and an alkali content in the base compartment prior to applying the direct current. This is achieved by introducing an acid and an alkali, respectively, to the acid and base compartments. Suitably the start-up acid for the acid com¬ partment is chloric acid and the start-up alkali for the base compartment is the same as the alkali to be generated, however, this is not essential and may depend on the intended use and purity required, in the products of the cell.

In general it is preferred that the start-up acid be solely chloric acid and that the start-up base be solely the base which is to be generated in the base compartment, for example, sodium hydroxide. As the alkali metal chlorate there is prefer¬ ably used sodium or potassium chlorate.

BRIEF DESCRIPTION OF DRAWINGS

The invention is illustrated in particular and preferred embodiments by reference to the accompanying drawings in which:

FIG. 1 illustrates schematically an electro¬ chemical cell for use in the process of the invention;

FIG. 2 illustrates schematically a process system in accordance with a preferred embodiment of the invention; and -8-

FIG. 3 illustrates schematically a process system in accordance with another embodiment of the invention.

MODES FOR CARRYING OUT THE INVENTION The operation of the water splitter and process of the invention is further described below by reference to Fig. 1. The concentration of the solution of aqueous alkali metal chlorate fed into the salt compartment of the three-compartment cell may be as low as 0.3 molar and as high as the saturation concentration for the parti¬ cular sait. However a 2 to 5 molar solution is pre¬ ferred. Solutions of low concentration should be avoided because of diminished conductivity in such solutions.

The solution fed to the acid compartment preferably contains more than 0.3 molar chloric acid and is free of other acids such as hydrogen chloride. Solutions of concentrations above 1.3 molar should be used with care because of the reactions which generate chloride dioxide from the chloric acid. This solution may be a stream exiting from a chlorine dioxide generator which will be depleted in chloric acid.

The solution fed to the base compartment preferably contains alkali metal hydroxide, for example, sodium hydroxide, preferably at a concentration between 1 and 5 molar. This concentration may be achieved by recycling the stream until the desired concentration is reached. The movement or migration of the various ions is illustrated schematically in Fig. 1.

Fig. 2 schematically illustrates the preferred embodiment of the process of the present invention which uses a three compartment electrodialytic water splitter. A portion of the spent chlorine dioxide generator solution is taken from a generator 100 via line 101 to the acid compartment A of- a three compartment electro- dialytic water splitter. .Alternatively if the process described here is used to generate chlorine dioxide directly the line from the generator 100 is excluded. The three compartment electrodialytic water splitter has unit cells defined by four membranes, including two bipolar membranes 200, an anion permselective membrane 201 and a cation permselective membrane 202 which form acid A, salt S and base B compartments. Chlorate ions migrate from the salt compartment S into the acid com¬ partment A and therein combine with hydrogen ions generated at the cation face of the bipolar membrane 200. An aqueous chloric acid solution enriched in chloric acid is removed from the acid compartment A via line 121 to a reservoir 120 which has two lines coming from it. The first line 122 leads back to line 101 while the second line 123 leads to the generator 100. A third line 124 delivers water to the reservoir 120. The rates of flow through the lines 101, 121, 122, 123 and 124 determines the concentration of the solution in line 123 which is either feeding the chlorine dioxide generator 110 or is being taken to a stripper where chlorine dioxide generated in the stack is removed. Sodium chlorate is added via line 131 to the salt compartment recycle tank 130 in the form of a solid, slurry or aqueous solution. Make-up water, if necessary, is added to recycle tank 130 via line 132. An aqueous solution of sodium chlorate is removed from the recycle tank 130 and forwarded via line 133 to salt compartment S. Sodium cations migrate through the cation perm¬ selective membrane 202 from salt compartment S into base compartment B and chlorate ions migrate from salt com- partment S through anion permselective membrane 201 to acid compartment A. An aqueous sodium chlorate solution containing a decreased amount of sodium chlorate is removed from salt compartment S via line 135. Water is added via line 141 to the base com¬ partment B recycle tank 140. Two lines lead from the tank. Line 142 leads to a storage tank (not shown) from where the sodium hydroxide can be taken for use in the mill processes. Line 143 leads to the base compartment B. Sodium ions migrate from the salt compartment S through the cation permselective membrane 202 where they combine with hydroxide ions introduced at the anion face of bipolar membrane 200 to form aqueous sodium hydroxide. An aqueous sodium hydroxide solution containing an increased amount of sodium hydroxide is removed from the base compartment B via line 144.

General Experimental Chloric Acid Generation

The electrodialytic water splitter used in the experiments (Fig. 3) was a cell equipped at each end with platinum electrodes 6 and 7 connected to a DC power source. Several types of compartment were set up: anolyte 1, base 2, acid 3, salt 4 and catholyte 5. The compartments 2, 3 and 4 form a unit which was repeated 8 times. Each compartment was separated by ion exchange

2 membranes with an exposed area of 1000 cm . Membranes 8 and 11 were Nafion (Trade Mark) 110 membranes manu¬ factured by DuPont; 9 was a bipolar membrane manufactured by Aquatech; and anion exchange membrane 10 was com- mercially available from Ionics Inc., under the code 204-UZL-386. Pumps 15, 16 and 17 were used to circulate solutions through the cell. The anolyte/catholyte reservoir 12 was charged with 0.5M Na₂SO.. This solution was circulated to the anolyte compartment as stream 26 and returned to the reservoir 12 via line 29 and to the catholyte compartment as stream 27 and returned to the reservoir via line 30. The base compartment 2 was fed from reservoir 13 by stream 35 and was returned to the reservoir 13 via line 31. The salt compartment 4 was fed from reservoir 114 by stream 36 and was returned to the reservoir 14 via line 32. The acid compartment 3 was fed from reservoir 19 by stream 37 and was returned to the reservoir 19 via line 33.

Chlorine Dioxide Generation

The cell stack was allowed to run until the chloric acid concentration increased to the point where chlorine dioxide formed. Alternatively, chloric acid was slowly heated with a number of reducing agents in a glass beaker.

Example 1

The salt tank was charged with 1 molar NaClO.,. The acid tank contained 0.3 molar HC10- and the base tank contained 0.3 molar NaOH. The circulation rates in the base, acid and salt loops were 3 L per minute. The voltage

was maintained below 30 volts by varying the current. Table I shove that over a period of 80 minutes, the concentration of NaC10₃ in the salt loop was diminished while the concentration of HC10₃ in the acid loop increased. Likewise the NaOH concentration in the base loop Increased.

TABLE I

Time, Current_/ Voltage, Concentration, min A V moles L

NaClOa HClOs NaOH

0 7.3 28.0 1.0 0.3 0.3

30 12.2 29.5 0.7 0.6 0.6

60 11.6 29.5 0.5 0.8 0.8

80 9.3 29.0 0.3 1.0 1.0

The chloric acid solution was then treated with a number of reducing agents. The results are shown in Table II. To model the Hoist process, 60 m of 1M Chloric acid was slowly heated while stirring. During this period SOa was bubbled through the solution. Between 40 and 50°C the solution turned yellow indicating the production of chlorine dioxide. The colour increased with time and was confirmed to be due to chlorine dioxide by titration.

To model the R3 process, the same conditions were used as in the Hoist process except that SOa was replaced with the stoichiometric quantity of NaCl. The solution turned yellow between 50 and 60°C, a slightly higher temperature than in the Hoist process model. The presence of chlorine dioxide was again confirmed by titration.

The R8 process was modelled .as above except methanol was the reductant. NaCl was added to the solution to give a concentration of 0.02 M/L as required by the R8 process. The solution turned yellow at 65°C.The presence of chlorine dioxide was again confirmed by titration. TABLE II

Process Reducing Agent Temperature at which C10_ was observed,

^•C

Hoist SO: 45

R3 NaCl 55

R8 CH_OH 65

Example 2. The salt tank was charged with 1 molar NaC . The acid tank contained 0.64 molar HClOa and the base tank contained 0.3 molar NaOH. The circulation rates in the three loops were 3 L per minute . The voltage was maintained below 30 volts by varying the current. T»able III shows that over a period of 120 minutes, the concentration of HClOa in the acid loop increased until at a concentration of about 1.3 moles L chlorine dioxide was generated in the cell stack.

TABLE III

Time, Current, Voltage, Concentration, min A V moles L

HClOa

0 13.0 29.0 0.65

60 13.0 29.0 0.94

110 13.0 29.0 1.24

120 13.0 29.0 CLOa formed

Claims

1. A process for the production of chloric acid comprising: i) establishing an electrochemical cell having an anode and a cathode and at least one unit comprising an acid compartment, a salt compartment and a base compartment disposed between said anode and cathode, said salt compartment being defined by an anion permselective membrane and a cation permselective membrane, said acid compartment being defined by said anion permselective membrane and a first bipolar membrane, said first bipolar membrane having a cation portion facing said acid compartment and an anion portion facing said anode, said base compartment being defined by said cation permselective membrane and a second bipolar membrane, said second bipolar membrane having an anion portion facing said base compartment and a cation portion facing said cathode; ii) feeding aqueous alkali metal chlorate solution to said salt compartment and water to said acid and base compartments, iii) applying a direct electric current across said unit between said anode and cathode to effect: a) movement of alkali metal cations from said salt compartment through said cation permselective membrane to said base compartment, b) movement of chlorate, anions from said salt compartment through said anion permselective membrane to said acid compartment. c) dissociation of water within said first and second bipolar membranes to provide hydrogen cations at said cation portion of said first bipolar membrane, with accumulation of chloric acid in said acid compartment and hydroxide anions at said anion portion of said bipolar membrane with accumulation of alkali metal hydroxide in said base compartment.

2. A process according to claim 1, further including establishing an acid content in said acid compartment and an alkali content in said base com¬ partment prior to applying said direct current to iii).

3. A process according to claim 1, including recovering accumulated chloric acid from said acid compartment.

4. A process according to claim 3, including a step of generating chlorine dioxide from the recovered chloric acid externally of the cell.

5. A process according to claim 2, including forming chlorine dioxide in said acid compartment from accumulated chloric acid and recovering chlorine dioxide from said acid compartment.

6. A process according to claim 5, wherein said accumulation of chloric acid in said acid compartment is to a concentration above about 1.0 molar.

7. A process according to claim 2, wherein said alkali metal chlorate is sodium chlorate and said alkali metal hydroxide is sodium hydroxide.

8. A process according to claim 1, including a step of feeding a low concentration aqueous chloric acid solution to said acid compartment to establish an acid condition in said acid compartment, said chloric acid being the sole acid fed to said acid compartment.

9. A process according to claim 8, including a step of feeding an aqueous alkali metal hydroxide solution to said base compartment to establish a base condition in said base compartment.

10. A process according to claim 2, wherein said at least one unit comprises a multiplicity of units in which an anion portion of each said first bipolar membrane defines a base compartment with an adjacent cation permselective membrane; and a cation portion of each said second bipolar membrane defines an acid compartment with an adjacent anion permselective membrane.

11. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, wherein said aqueous alkali metal chlorate solution fed to said salt compartment in ii) is free of alkali metal chloride.

12. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, wherein said aqueous alkali metal chlorate solution fed to said salt compartment in ii) consists essentially of alkali metal chlorate and water.

13. A process according to claim 11, in which said hydroxide anions developed by dissociation of water within said first bipolar migrate to said anode to produce oxygen and including recovering oxygen from said anode.

14. A process according to claim 12, in which said hydroxide anions developed by dissociation of water within said first bipolar migrate to said anode to produce oxygen and including recovering oxygen from said anode.