CA2092238C

CA2092238C - Decreased sodium sulfate production in chlorine dioxide generation

Info

Publication number: CA2092238C
Application number: CA 2092238
Authority: CA
Inventors: Edward J. Bechberger; Gerald Cowley; Maurice C.J. Fredette
Original assignee: Sterling Canada Inc
Current assignee: Superior Plus LP
Priority date: 1992-06-09
Filing date: 1993-05-12
Publication date: 1999-03-30
Anticipated expiration: 2013-05-12
Also published as: CA2092238A1

Abstract

Chlorine dioxide is produced from sodium chlorate, sulfuric acid and methanol in a boiling reaction medium while a subatmospheric pressure is applied, with the concomitant production of a decreased quantity of sodium sulfate by-product. The sodium sesquisulfate produced in the chlorine dioxide generator is metathesized to neutral form and feed solution for the chlorine dioxide generator is electrolyzed to acidify the same and remove sodium ions as useful sodium hydroxide by-product. A decrease of up to 70% in by product sodium sulfate can be attained without increasing the evaporative load on the chlorine dioxide generator.

Description

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The present invention relates to the generation of 5 chlorine dioxide and decreasing the quantity o~ by-product salt-cake which must be handled.
Chlorine d.ioxide, useful as a pulp mill bleaching agent, is produced chemically by reduction of an acid aqueous chlorate solution in accordance with the equation:
Cl03-+ 2H+ + e~ ~ ClO2 + H2O ~ (1) where the electron e~is supplied by various reducing agents, for example, sulfur dioxide, methanol, chloride ion or hydrogen peroxide. In many commercial processes for effecting this reaction, the acidity for the process is provided by sulfuric acid while the chlorate ions are provided by sodium chlorate. The presence of these species leads to the formation of some fo~m of sodium sulfate as a by-product.
One particular embodiment of a commercial process is the so-called "R~" process of the assignee of this application, as described in U.S. Patent No. 4,081,520, assigned to the applicant herein. Improvements in and modifications to that process also are described in the 25 applicant's U.S. Patent~ Nos. 4,465,658, 4,473,540 and 4,6~7,969.
In that chlorine dioxide generating process, the reaction medium is at a high total acid normality of sulfuric acid and is maintained at its boiling point under a subatmospheric pressure applied thereto.
Methanol is used as a reducing agent for chlorate ions, resulting in the formation of chlorine dioxide in a substantially pure form. The boiling nature of the reaction medium produces steam which acts as a diluent for the gaseou~ chlorine dioxide, so as to prevent decomposition of the chlorine dioxide.
The sodium sul~ate by-product builds up in the reaction medium after start-up until the solution is :: : : .
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This process is highly efficient and rapidly produces chlorine dioxide in commercial quantities. As ~0 may be concluded from the above equation (1), for each mole of chlorine dioxide produced a mole of chlorate ion and hence of sodium ion is introduced to the reaction medium. The sodium ions combine with the sulfate ions introduced with the sulfuric acid, to produce a sodium sulfate, which may be sodium bi~ulfate or, more normally under the conditions of an R-8 process, the double salt sodium sesquisulfate, i.e., Na3H(S04)2 (or NaHS04.Na2SO4), depending on the acidity of the solution.
Such by-product sodium sulfate and sodium sesqui~ulfate (sometimes termed "saltcake"), generally have been employed to make up sulfur losses in the pulp mill. However, the adoption of high substitution of chlorine by chlorine dioxide in the chlorination stage of the bleach plant has led to saltcake by-product production ~rom the chlorine dioxide generating process ~xceeding the mill make-up requirements.
In U.S. Patents Nos. 5,122,240 and 5,198,080 [E437-R9], assigned to the applicant hereof, there is described means for decreasing the quantity of salt-cake by-product produced by the R8 chlorine dioxide generating process for the same level of chlorine dioxide production.
As described therein, the portion of the by-product sodium sesquisulfate, which cannot be used by the mill, after making up into an aqueous solution thereof, is acidified in an electrolytic cell while sodium ions are removed from the solution. The acidified solution is ~, .
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recycled to the generator to provide a portion of the acid therein. Commercially, this procedure is known by the applicant's assignee as the "R9" process.
For existing chlorine dioxide generators, while this procedure is able to decrease the quantity of salt-cake that is required to be proces~ed by the mill or otherwise disposed of, this result is achieved at the expense of an increased evaporative load on the chlorine dioxide generator. Increasing the evaporative load increases energy requirements and operating costs are increased.
Furthermore, a substantial capital outlay may be required to adapt existing equipment to this process.
Another problem which exists with re~pect to the sodium susquisulfate by-product is its acidic nature, which necessitates neutralization of the acid values prior to disposal o~ unwanted quantities of the material.
With the trend towards higher chlorine dioxide substitution for chlorine in many mills, the necessity to neutralize increasing quantities of sodium sesquisulfate may result in an imbalance o~ aaustic and chlorine within the mill. The caustic demancl increases while chlorine usage declines, resulting in increased costs to the mills. In addition, the 1QSt acid values of the sodium ses~uisulfate require that make-up sulfuric acid must be fed continuously ts the chlorine dioxide generator to -~ maintain the required acidity.
In U.S. Patent No. 5,116,595, [E439-RlOJ, assigned to the applicant hereof, there are described procedures whereby the acid sodium sesquisulfate is converted by metathesis to neutral sodium sulfate and the acid values are recovered for reuse in the chlorine dioxide generator.
While this procedure is able to provide a neutral by-product from the R8 chlorine dioxide generating process while recovering acid values ~rom the salt-cake, when the aqueous acid product o~ the metathesis process - , :: , . :, : ~ . :: ' :.

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2~!~22~8 is used in the chlorine dioxide generator, the evaporative load on the chlorine dioxide generator again is increased. Commercially, this procedure is known by the applicant's assignee as the "R10" process.
In accordance with the present invention, the inventors have devised a scheme whereby the R8, R9 and R10 processes are combined to achieve the individual benefits thereof without significantly increasing the evaporative load on the chlorine dioxide generator.
This combination process may produce the following significant benefits:
(a) Chlorine dioxide is produced at high efficiency from sodium chlorate, methanol and sulfuric acid by the R~ process, (b) The quantity of by-product sodium sulfate which is produced may be controlled at a level desired by a mill, (c) The by-product sodium sulfate is provided in neutral form, which is more easily handled and requires no further processing, i~ required to be disposed of, (d) Acid values present in the by-product sodium sulfate from the generator are reused in the generation of chlorine dioxide, (e) Sodium hydroxide is produced as a by-product, which may be used elsewhere in the pulp mill without incurring the usual penalty of coproduction of chlorine, and (f) The evaporative load on the chlorine dioxide generator is not increas d to any degree beyond the capacity of the existing equipment while obtain;ng as much as a 70~ decrease in salt-cake production.
In one embodiment of the present invention, by combining the metathesis operation and the electrolysis operation on the by-product sodium ses~uisulfate from the chlorine dioxide generator, an acid recycle stream is - . . .
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Accordingly, in one aspect of the present invention, there is provided a process for the production of chlorine dioxide which comprises a plurality of steps.
Chlorate ions are reduced in an aqueous acid reaction medium having a total acid normality of at least about 4 normal and containing sulfuric acid to form chlorine dioxide in a reaction zone from the aqueous acid reaction medium.
The aqueous acid reaction medium is maintained at its boiling point under a subatmospheric pressure applied to the reaction zsne and a by-~product acid sulfate is -15 precipitated in the raaction zone from the aqueous acid ;reaction medium. The chlorate ions present in the aqueous acid reaction medium usually are provided by sodium chlorate, so that the by-product acid sulfate usually is a sodium acid sulfate. The precipitated by-product acid sulfate is removed from the reaction zone.
The removed by-product acid sulfate is contacted in solid phase with an aqueous medium to effect conversion by metathesis of the solid phase by-product acid sulfate into solid phase neutral anhydrous sulfake and to form an aqueous acid medium.
An aqueous solution of at least one alkali metal salt selected from the group consisting of alkali metal chlorate, alkali metal sulfate and mixtures of alkali metal chlorate and alkali metal sulfate is electrochemically acidified, while at the same time, alkali metal ions are electrochemically removed from the aqueous solution, so as to form an aqueous acidified alkali metal salt solution.
The aqueous acid medium formed by the metathesis step and the aqueous acidified solution of alkali metal : ~. - . . . ..
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salt are forwarded to the aqueous acid reaction medium in the reaction zone.
The chlorine dioxide generating process employed herein involves reduction of chlorate ions in an aqueous acid reaction medium having a total acid normality of at least about 4 normal and containing sulfuric acid, which results in the production of an acid sulfate by-product.
The chlorate ions usually are provided by sodium chlorate, which results in a sodium acid sulfate being produced.
The total acid normality may range upwardly from the at least 4 normal, generally up to about 12 normal.
Chlorine dioxide may be produced at total acid normalities above about 9 normal, or below about 9 normal, preferably about 5 to about 9, such as about 5 to about 7 normal or about 7 to about 9 normal~
Reduction of the chlorate ions present in the aqueous acid reaction medium to form chlorine dioxide generally is effected using methanol as the reducing agent, in view of the advantages which are achieved usiny the R8 process. Other reducing agents which produce chlorine dioxide from chlorate ions in aqueous acid reaction medium, such as, hydrogen peroxide or sulfur dioxide, also may be employed.
The chlorine dioxide is generated from the aqueous acid reaction medium maintained at its boiling point under a subatmospheric pressure. By-product acid sulfate, usually sodium acid sulfate when sodium chlorate is used, precipitable ~rom the aqueous acid reaction medium, after reaching saturation following start up, and is removed from the reaction zone.
The removed by-product acid sulfate is contacted in the solid phase with an aqueous medium to effect conversion by metathesis of the solid phase by-product acid sulPate into solid phase neutral anhydrous sulfate and form an aqueous acid medium. The metathesis , " '' '' " ~" ', , ,, ' .
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operation may be effected in any convenient manner, as described in more detail in the aforementioned UOS.
Patent No. 5,116,595.
As described therein, the metathesis of the acid 5 sulfate, usually sodium acid sulfate, may be ef~ected using water alone or an aqueous solution of sodium chlorate, sodium chloride, methanol, or condensate containing methanol and formic acid. Of these materials, water and aqueous sodium chlorate are preferred. The aqueous acid medium which results from the metathesis contains acid values which are employed in the generation of chlorine dioxide.
In addition, an aqueous solution of an alkali metal sulfake, alkali metal chlorate or mixture thereof, usually in the form of the sodium salt, is electrochemically acidified while alkali metal ions are removed therefrom to form an aqueous acidified alkali metal salt solution, which also is forwarded to the chlorine dioxid~ generator as an acid source thexein.
Such electrochemical acidification may be effected, utilizing the processes described in the aforementioned U.S~ Patents Nos. 5,122,240 and 5,198,080. The aqueous solution which is subjected to electrochemical acidification herein, thus, may comprises an aqueous alkali metal chlorate solution/ usually an aqueous sodium chlorate solution, whereby the electrochemical acidification, which produces an acidic medium containing chloric acid, which is forwarded to the chlorine dioxide generator.
In one embodiment of the invention, the aqueous acid medium formed in the metathesis step which contains some sodium sulfate, forms at least part of the aqueous alkali metal salt solution which is electrochemically acidified and the resultinq acidified aqueous alkali metal salt solution then is forwarded to the aqueous acid reaction medium in the chlorine dioxide generator, so that the .

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acid values released in the metathesis step and formed in the electrochemical acidification step both are recycled to the chlorine dioxide generator in the same stream.
In this embodiment of the invention, sodium chlorate may be added to the aqueous acid medium passed to the electrochemical acidification step by the use of sodium chlorate during the metathesis operation and/or by the addition of sodium chlorate to the ePfluent stream from the metathesis reactor as it passes to the acidification cell. As mentioned in U.S. Patents Nos. 5,1~2,240 and 5,198,0~0, it is pre~erred to acidify an aqueous solution of sodium chlorate and sodium sulfate.
In another embodiment~ the metathesis of the solid ; phase by-product alkali metal acid sulfate is effected using the aqueous acidified alkali metal salt solution produced in the electrochemical acidification step, to form an agueous acid medium in the metathesis step which is forwarded to the chlorine dioxide generator. Part of the solid phase product neutral anhydrous alkali metal sulfate, usually neutral anhydrous sodium sulfate, is formed into an aqueous solution for use as feed to the electrochemical acidification step. Sodium chlorate, or other alkali metal chlorate, may be added to metathesis reactor and/or to the aqueous sodium sulfate solution forwarded to the electrochemical acidification.
The invention is described further, by way of illustration, with reference to the accompanying drawings, in which:
Figure 1 is a schematic flow sheet of a chlorine dioxide generating operation provided in accordance with one embodiment of the invention;
Figure 2 is a schematic flow sheet of a chlorine dioxide generating operation provided in accordance with another embodiment of the invention;

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Figure 3 is a schemakic flow sheet of one embodiment of metathesis operation which may be employed in the metathesis stage in the process of Figure 1 or 2; and Figure 4 is a schematic flow sheet of another embodiment of metathesis operation which may be employed in the metathesis stage in the process of Figure 1 or 2.
Figure 5 is a graphical representation of the lowered energy requirement which is obtained using the process of the invention at significant levels of decrease o* sodium sulfate by-product formation, for the process of the invention, namely the R8/R9/R10 combinations, in comparison to the combination of R8 and R9 processes.
Referring to the drawings, Figure 1 illustrates an embodiment of the application of the present invention to chlorine dioxide production. As seen therein, a chlorine dioxide generating operation 10 comprises a chlorine dioxide generator 12 wherein sodium chlorate fed by line 14, sulfuric acid fed by line'16 and methanol fed by line 18 react together in an aqueous acid reaction medium to produce chlorine dioxide. The reaction medium is ; maintained at its boiling point under a subatmospheric pressure. Chlorine dioxide is removed from the generator 12 by line 20 in gaseous admixture with steam.
Crystalline sodium sesquisulfate is precipitated from the reaction medium and is removed from the generator 12 by line 22 and ~orwarded to a metathesis reactor 24. In the metathesis reactor 24, the solid phase crystalline sodium sesquisulfate is converted into solid phase neutral sodium sulfate, which is removed from the metathesis operation by line 26. The metathesis may be carried out as described in the aforementioned U.S.
Patent No. 5,116,595 [E439]. The procedures which may be used are described below with respect to Figures 3 and 4.
The metathesis may be effected using water fed by line 28 or by an aqueous solution of sodium chlorate fed , ,, ,, ~ - , , . ', ., , , ., ;
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The acid aqueous sodium sulfate solution in line 36 is forwarded to an electrolysis cell 38, which is operated as described in detail in U.S. Patent No.
5,198,080 [E437-R9~ referred to above. The electrolysis cell comprises an anode chamber 40 separated from a cathode chamber 42 by a cation-exchange membrane 44 The cell anode 46 and cell cathode 48 are located in the anode compartment 40 and cathode compartment 42 respectively. Any other convenient form of cell, including an electrolysis cell equipped with a plurality of bipolar membranes and ion exchange membranes, may be employed to effect the acidification.
The aqueous acid solution in line 36, which may be in the form of a slurry, if desired, is fed to the anode compartment 40 while an electrolyte is fed to the cathode compartment by line 50. As a current is applied between the anode and cathode, several reactions occur simultaneously. At the anode 46, water is electrolyzed to oxygen and hydrogen ion, as follows:
H2~ ~ ~~2 + 2H~ + 2e-while at the cathode 48 water is electrolyzed to hydrogen and hydroxyl ion, as follows:
e~+ H2O ~ ~H2 + OH-At the s~me time, sodium ions in the aqueous solution orslurry of a mixture of sodium sulfate and sodium chlorate ,: :
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migrate under the influence of the applied current from the anode compartment 40 across the cation-exchange membrane 44 to the cathode compartment 42. In effect, therefore, the electrolytically-produced hydrogen ions replace the sodium ions in the anode compartment 40 and the transferred sodium ions are available to combine with the electrolytically-produced hydroxyl ions in the cathode compartment 42.
As noted earlier, the sodium sulfate contained in the solution ~eed in line 36 to the cell 38 can be considered to be a deadload circulating via the generator 12 in a closed loop, so that the overall reaction in cell 38 may b~ represented, as follows:
xNaCl03 + 3H20 ~ (x-2) NaCl03 + 2HCl03 + 2NaOH
+ ~o2 + H2 where x is the molar amount of sodium chlorate which is processed. Alternatively, the process may be represented as~
Na3H ( S04) 2 + H20 ~ 2NaHSO4 + NaOH
The acid produced is in addit.ion to the acid already in the stream 42 as a result of metathesis.
The further acidified stream which results from the anode compartment is forwarded by line 52 to the chlorine dioxide generator 12, thereby providing acid and chlorate ions for the chlorine dioxide generating process.
The sodium sulfate removed from the system by lin~
26 corresponds to the proportion of the sulfuric acid and sodium chlorate reactants fed to the chlorlne dioxide generator from external sources, namely by line 16 for sulfuric acid and by unconverted sodium chlorate in line 52 as well as in line 14.
Oxygen is vented from the anode compartment 40 by line 54 while hydrogen is vented from the cathode compartment by line 56. Alternatively, any other anodic reaction producing hydrogen ions can be employed, for example, hydrogen gas oxidation, as described in one . . .. . :: , , ~ ..... .
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embodiment of the procedures described in afor~mentioned U.S. Patent No. 5,198,080 [E437-R9]. The sodium hydroxide solution formed in the cathode compartment i5 removed therefrom by line 57. If desired the sodium hydroxide solution may be cycled through the cathode compartment until a desired concentration is achieved.
The useful sodium hydroxide solution contains sodium which would otherwise have formed sodium sulfate. This sodium hydroxide is employed in the purifying and bleaching operations effected in the bleach plant o~ the pulp mill. The gaseous by-products, namely hydrogen and oxygen also can be utilized in the pulp mill.
Referring now to Figure 2, this Figure illustrates a further embodiment of the application of the present invention to chlorine dioxide production from a chlorine dioxide generator 12'. Elements of the arrangement which are in common with Figure 1 have been designated by the same numerals primed and will only be described as necessary.
In this embodiment, a portion of the solid phase neutral sodium sulfate rasulting from the metathesis reactor 24' is ~orwarded by line 58 to a dissolving tank 60, wherein the sodium sulfat~ is dissolved in water fed by line 62 to ~orm an aqueous solution thereof. Sodium chlorate also may be fed to the dissolving tank 60 by line 64, as well as a portion of the recycled acid medium in line 65. The resulting aqueous solution of sodium sulfate, optionally containing sodium chlorake, is forwarded by line 66 to the anode compartment 40' of the cell 38', for acidification as described above with respect to Figure 1.
In this case, the acidified solution of sodium sulfate, optionally containing sodium chlorate, is forwarded by line 68 ko the metathesis reactor 24' to be employed as a source of water therein. Optionally, a portion or all o~ the acidified solution in line 68 may . .

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2~2238 be forwarded by line 72 to the chlorine dioxide generator 12'. The acid stream resulting from the metathesis reaction, cont~;n;ng acid from the sodium sesquisulfate and formed in the cell 38', is forwarded to the chlorine dioxide generator 12' by line 70.
In both the embodiment of Figure 1 and that of Figure 2, the combined effects of the metathesis and the electrolytic acidification produces an acid stream for feed to the chlorine dioxide generation operation which does not increase the evaporative load on the generator, at least up to a significant decrease in the proportion of sodium sulfate produced per mole of chlorine dioxide.
The metathesis operation effected in the embo~;r-nt of Figures 1 and 2 may comprise one of the procedures illustrated in Figures 3 and 4. In the case of Figure 3, the metathesis is carried out using aqueous sodium chlorate solution, while, in the case of Figure 4, metathesis is carried out using water. As seen in Figure 3, crystalline sodium sesquisulfate, removed in slurry form with spent reaction medium from the chlorine dioxide generator, is forwarded by line 110 to a first filter 112 wherein the crystalline material is separated from spent reaction medium, which is recycled by line 114 to the chlorine dioxide generator. If wash water is desired to assist in separating the crystalline material from entrained reaction medium, it may be fed to the filter 112 by line 116.
The crystalline sodium sesquisulfate is forwarded from the filter 112 by line 118 to a metathesis reactor 120, which preferably takes the form of one or more stirred tanks. To the metathesis reactor 120 is fed an aqueous sodium chlorate solution by line 122 of sufficient concentration and temperature to effect metathesis conversion of the crystalline sodium sesquisulfate to crystalline anhydrous neutral sodium sulfate, with release of acid into the sodium chlorate /

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solution. The resulting slurry is forwarded by line 124 to a second filter 126 for separation of the solid phase sodium sulfate, which is recovered as a product by line 128.
If desired, wash water may be fed by line 129 to the filter 126 to assist in freeing the solid phase from entrained sodium chlorate-containing liquid. The filtrate contains sodium chlorate, sulfuric acid and dissolved sodium sulfate and is recovered by line 130.
If desired, a portion of stream 130 may be recycled and used as at least part of the wash water feed in line 116 to the filter 112, to decrease further volume of water fed to the chlorine dioxide generator.
Turning now to Figure 4, the same elements as in Figure 3 are designated by the same reference numbers in Figure 4 primed and referred to only as necessary. In this embodiment, the solid phase crystalline ses~uisulfate is contacted with water in line 132 in the metathesis reactor 120' to effect metathesis of the sodium sesquisulfate to form neutral anhydrous sodium sulfate. Part of the filtrate from the filter 126' may be recycled by line 134 to the first filter 112', so as to provide wash water employed therein to free the sodium sesquisulfate from spent reaction medium, or may be fed ;: 25 to the metathesis reactor 120' via line 136.
The invention is illustrated further by the following Example:
Example The steam-to-chlorine dioxide ratio (lb/lb) was calculated for various levels of reduction of overall salt cake (sodium sesqui-sulfate) for a chlorine dioxide generator operating at a total acid normality of about 8 N with feeds of 620 g/l sodium chlorate, 20% methanol feed and 93~ H2S04 feed sufficient to produce chlorine dioxide at an efficiency of about 98% to compare an : operation using the combination of R8 and R9 processes, .
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as described in the aforementioned U.S. Patent No.
5,1g8,080 and an operation as described above with respect to Figure 1. The results of these calculations were plotted graphically and appear in Figure 5.
As may he seen from this Figure, above a level of decrease in salt cake production of about 25%, there is a significant saving in steam requirement.
In summary of this disclosure, the present invention provides a novel chlorine dioxide generating procedure which enables a significant decrease in by-product sodium sulfate to be achieved without significantly increasing the evaporative load on the generator by integrating the R8, R9 and R10 processes. Modi~ications are possible within the scope of this invention.

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Claims

1. A process for the production of chlorine dioxide, which comprises:
reducing chlorate ions in an aqueous acid reaction medium having a total acid normality of at least about 5 normal and containing sulfuric acid to form chlorine dioxide in a reaction zone from said aqueous acid reaction medium, maintaining said aqueous acid reaction medium at its boiling point under a subatmospheric pressure applied to said reaction zone and precipitating a by-product acid sulfate in said reaction zone from said aqueous acid reaction medium, removing said precipitated by-product acid sulfate from said reaction zone, contacting said removed by-product acid sulfate in solid phase with an aqueous medium to effect conversion by metathesis of said solid phase by-product acid sulfate into solid phase neutral anhydrous sulfate and to form an aqueous acid medium, electrochemically acidifying an aqueous solution of at least one alkali metal salt selected from the group consisting of alkali metal chlorate, alkali metal sulfate and mixtures of alkali metal chlorate and alkali metal sulfate while, at the same time, electrochemically removing alkali metal ions from said aqueous solution to provide an aqueous acidified alkali metal salt solution, and forwarding said aqueous acid medium and said aqueous acidified solution of alkali metal salt to said aqueous acid reaction medium in said reaction zone.

2. The process claimed in claim 1 wherein said chlorate ions are provided by sodium chlorate and said reducing agent is methanol, whereby said precipitated by-product acid sulfate is a sodium acid sulfate.

3. The process claimed in claim 2 wherein said aqueous acid reaction medium has a total acid normality of about 5 to about 11.

4. The process claimed in claim 3 wherein said aqueous acid reaction medium has a total acid normality of about

5 to about 7 and said sodium acid sulfate is sodium sesqui-sulfate.
5. The process claimed in claim 3 wherein said aqueous acid reaction medium has a total acid normality of about 7 to about 9 and said sodium acid sulfate is sodium sesqui-sulfate.

6. The process claimed in claim 1 wherein said metathesis is effected using water.

7. The process claimed in claim 1 wherein said metathesis is effected using an aqueous solution of sodium chlorate.

8. The process claimed in claim 1 wherein said aqueous solution of an alkali metal salt is an aqueous solution of a sodium salt.

9. The process claimed in claim 8 wherein said aqueous solution of a sodium salt is an aqueous solution comprising sodium sesquisulfate.

10. The process claimed in claim 9 wherein said aqueous solution of a sodium salt is an aqueous solution of sodium sesquisulfate and sodium chlorate.

11. The process claimed in claim 1 wherein said aqueous acid medium forms at least part of said aqueous solution of at least one alkali metal salt which is electrochemically acidified and the resulting acidified aqueous solution of at least one alkali metal salt is forwarded to said aqueous acid reaction medium.

12. The process claimed in claim 11 wherein said chlorate ions are provided by sodium chlorate, whereby said by-product acid sulfate comprises a sodium acid salt and said aqueous solution of at least one alkali metal salt comprises a sodium salt.

13. The process claimed in claim 12 wherein sodium chlorate is present in said aqueous solution of at least one sodium salt.

14. The process claimed in claim 13 wherein said sodium chlorate is present in said aqueous solution as a result of said metathesis reaction being effected using aqueous sodium chlorate solution.

15. The process claimed in claim 13 wherein said sodium chlorate is present in said aqueous solution as a result of said metathesis reaction being effected using water and of sodium chlorate being added to the aqueous sodium salt solution resulting from said metathesis reaction.

16. The process claimed in claim 1 wherein at least part of said aqueous solution of alkali metal salt which is electrochemically acidified is provided by an aqueous solution of part of said solid neutral anhydrous sulfate from said metathesis step, said acidified aqueous solution comprising alkali metal sulfate produced thereby is employed as at least part of said aqueous medium contacting said by-product acid sulfate in said metathesis step, and said aqueous medium, thereby containing said aqueous acidified solution, is forwarded to said aqueous acid reaction medium.

17. The process claimed in claim 16 wherein said chlorate ions are provided by sodium chlorate, whereby said by-product acid sulfate comprises a sodium acid salt, and said aqueous solution of at least one alkali metal salt comprises a sodium salt.

18. The process claimed in claim 17 wherein sodium chlorate is present in said aqueous solution of at least one sodium salt.

19. The process claimed in claim 18 wherein said sodium chlorate is present in said aqueous solution as a result of said metathesis reaction being effected using aqueous sodium chlorate solution.

20. The process claimed in claim 18 wherein said sodium chlorate is present in said aqueous solution as a result of said metathesis reaction being effected using water and of sodium chlorate being added to the aqueous sodium salt solution resulting from said metathesis reaction.