Production of chlorine dioxide with conversion of by-product solid phase sodium acid sulphate to its neutral form

A highly efficient chlorine dioxide generating process which produces chlorine-free chlorine dioxide and neutral solid phase sodium sulphate from sodium chlorate, sulphuric acid and methanol is described. Solid phase sodium acid sulphate recovered from the high acidity reaction medium is metathesized using controlled quantities of water and controlled quantities of methanol to form the solid phase neutral sodium sulphate. Sulphuric acid recovered from the acid sulphate by the metathesis is recycled to the generator along with a part of the methanol used in the metathesis.

FIELD OF INVENTION 
The present invention relates to the production of chlorine dioxide at high 
efficiency. 
BACKGROUND TO THE INVENTION 
In U.S. Pat. No. 4,081,520, assigned to the assignee of this application, 
the disclosure of which is incorporated herein by reference, there is 
described a process for the production of chlorine dioxide at high 
efficiency using sodium chlorate, sulphuric acid and methanol. The 
mechanism whereby chlorine dioxide is formed is that chlorine which is 
coproduced with the chlorine dioxide is reacted with the methanol to form 
chloride ions which then reduce the chlorate ions to form chlorine dioxide 
and chlorine. The overall reaction may be represented by the following 
equation: 
EQU 2NaClO.sub.3 +2H.sub.2 SO.sub.4 +CH.sub.3 OH.fwdarw.2ClO.sub.2 
+2NaHSO.sub.4 +HCHO+2H.sub.2 O 
The reaction medium from which the chlorine dioxide is formed and which 
contains sodium chlorate, methanol and sulphuric acid is maintained at its 
boiling point, generally in the range of about 50.degree. to about 
85.degree. C., under a subatmospheric pressure. The evaporated water 
serves to dilute the chlorine dioxide for removal from the reaction zone. 
The reaction medium has a high total acid normality in excess of about 9 
normal and the by-product deposited from the reaction medium once 
saturation is reached after start up is a sodium acid sulphate, which may 
be sodium bisulphate (NaHSO.sub.4) or sodium sesquisulphate (Na.sub.3 
H(SO.sub.4).sub.2). 
The process of the prior patent is highly efficient in terms of the 
conversion of chlorate ions to chlorine dioxide. Close to 100% efficiency 
is attained and the chlorine dioxide is removed from the reaction zone 
virtually uncontaminated by chlorine, which may be beneficial in many 
instances of end use of the chlorine dioxide. 
The drawback to this prior art process which heretofore has inhibited 
commercial implementation thereof is the form and nature of the solid 
by-product. Since the sodium sulphate precipitates in an acid form, 
removal of this material from the reaction zone results in a loss of the 
acid values therein. In addition, sodium acid sulphates are difficult to 
handle physically and exhibit deliquescence. 
SUMMARY OF INVENTION 
The present invention enables this prior art problem to be overcome by 
metathesizing the solid phase sodium acid sulphate in a unique manner to 
form solid phase neutral sodium sulphate and recovering the acid values 
for recycle to the reaction zone. 
Although the present invention is particularly described herein with 
respect to the conversion of sodium acid sulphate formed in the procedure 
of the aforementioned U.S. Pat. No. 4,081,520 to a neutral sodium 
sulphate, the present invention is broadly applicable to the conversion to 
a neutral sulphate of a solid acid sulphate recovered from any high 
acidity sulphuric acid based chlorine dioxide generating process in which 
the reaction medium is maintained at its boiling point under a 
subatmospheric pressure and in which the acid sulphate precipitates from 
the reaction medium in the reaction zone. 
For example, the process of the invention is applicable to the conversion 
of sodium acid sulphate by-product recovered from a chlorine dioxide 
generator wherein added chloride ions are used as the reducing agent for 
the chlorate in the presence of high acidity sulphuric acid, as described 
in Canadian Pat. No. 825,084, to the assignee of this application. 
Further, the chlorine dioxide generating process producing the sodium acid 
sulphate may be one in which sulphur dioxide is used to reduce the 
coproduced chlorine to chloride ions, in analogous manner to the methanol 
in the process of U.S. Pat. No. 4,081,520 as discussed above. The use of 
sulphur dioxide is described in U.S. Pat. No. 3,933,988, assigned to 
Hooker Chemicals & Plastics Corporation. 
In the process of the present invention, the solid phase acid sulphate 
removed from the reaction zone is contacted with water in the presence of 
a water-soluble alcohol or ketone, preferably methanol, to form solid 
phase neutral sulphate and to recover the sulphuric acid. 
GENERAL DESCRIPTION OF INVENTION 
There has been a prior suggestion to utilize water and methanol or other 
water-soluble alcohol or ketone to convert sodium acid sulphate to neutral 
sodium sulphate. Such procedure is described in U.S. Pat. No. 4,104,365 in 
the names of Howard and Lobley. The process of the latter patent is 
directed to the recovery of neutral sodium sulphate from the liquid phase 
effluent from high total acid normality sulphuric acid based chlorine 
dioxide producing processes. Such processes do not operate at the boiling 
point of the reaction medium and do not crystallize the by-product sodium 
sulphate from the reaction medium in the reaction vessel. The starting 
material in this prior art process is an aqueous solution of the sodium 
acid sulphate, in contrast to the solid phase starting material used in 
this invention. 
The prior art procedure involves an initial stripping step which is said to 
be required to remove dissolved gases and residual sodium chlorate which 
otherwise inhibit the reaction. Such an operation is not required for the 
process of this invention since the starting material is solid phase 
sodium acid sulphate produced in the chorine dioxide generating reaction 
zone. 
The process of the present invention is distinguished from the prior art 
procedure not only on the ground that the starting materials are in 
different physical forms but also since the volumes of water and methanol 
required to be added to the aqueous effluent to form solid phase neutral 
sodium sulphate in the prior art process are very much greater than the 
volumes used in the process of this invention. As a consequence of these 
large volumes of water and methanol, considerable evaporation is required 
in the prior art process, first of all to recover the methanol used and 
secondly to concentrate the aqueous sulphuric acid to a concentration 
which is suitable for reuse in the generator. 
In the process of the present invention, the weight ratio of water to 
sodium acid sulphate (calculated as Na.sub.3 H(SO.sub.4).sub.2) is about 
0.4:1 to about 1.4:1, preferably about 0.6:1 to about 0.8:1. Ratios of 
water to sodium acid sulphate within the recited range are critical to the 
process of the present invention, in that less than an 0.4:1 weight ratio 
leads to only poor conversions of acid sulphate to neutral sulphate while 
greater than a 1.4:1 weight ratio leads to large quantities of sodium 
sulphate being dissolved in the aqueous phase. By using weight ratios 
within this critical range, the aqueous phase which results contains 
sulphuric acid of a sufficient acid normality to permit recycle thereof to 
the chlorine dioxide generating process without concentration. 
In contrast, in a typical operation according to the process of U.S. Pat. 
No. 4,104,365 referred to above, the weight ratio of water to sodium acid 
sulphate (considered as Na.sub.3 H(SO.sub.4).sub.2) is about 1.83:1 and 
the weight of water required to be removed to enable the sulphuric acid 
solution to be reused is about 3.6 lb. per lb. of neutral sodium sulphate 
recovered (or about 4.14 lb. per lb. of ClO.sub.2 formed). 
The weight ratio of methanol to sodium acid sulphate (calculated as 
Na.sub.3 H(SO.sub.4).sub.2) is less critical than the water weight ratio 
and may vary up to about 2:1. The weight ratio of methanol to sodium acid 
sulphate is preferably about 0.3:1 to about 0.8:1 for the preferred weight 
ratio of water to sodium and sulphate referred to above. In view of its 
miscibility in water, the presence of the methanol decreases the volume of 
water in which the sodium sulphate is able to dissolve and hence inhibits 
dissolution of the neutral sodium sulphate in the aqueous phase. 
As the weight ratio of methanol increases, the proportion of neutral sodium 
sulphate dissolved in the aqueous phase decreases until a weight ratio of 
methanol is reached beyond which further quantities of methanol do not 
increase the yield of solid phase neutral sodium sulphate, the yield limit 
being about 80 to 85 wt.%. No benefit, therefore, is gained by increasing 
the weight ratio of methanol to sodium acid sulphate beyond about 2:1. 
When the preferred process of the invention is adopted, namely, when the 
chlorine dioxide is produced by the process of the above-mentioned U.S. 
Pat. No. 4,081,520, a proportion of the aqueous phase resulting from the 
metathesis may be recycled directly to the chlorine dioxide producing 
reaction medium to provide at least part, preferably all, of the methanol 
requirement thereof. This recycle stream also provides part of the 
sulphuric acid requirement of the chlorine dioxide-forming process. 
Methanol which is recycled to the reaction medium in this way does not 
require to be recovered from the recycled proportion of the aqueous phase. 
Methanol is removed from the remainder of the aqueous phase to provide a 
sulphuric acid solution suitable for recycle to the generator. 
The quantities of methanol used in this invention contrast markedly with 
those used in the prior art procedure of U.S. Pat. No. 4,104,365 wherein 
typically a weight ratio of methanol to sodium acid sulphate (considered 
as Na.sub.3 H(SO.sub.4).sub.2) of about 9.33:1 is used and must be 
recovered for reuse and so that the sulphuric acid solution can be 
concentrated to an acid normality suitable for recycle to the chlorine 
dioxide producing process. 
The steam requirement of the process of the present invention for 
evaporation of the aqueous phase is limited to that required to strip 
methanol and in a typical preferred embodiment of the invention gives rise 
to a cost of about $2.50 per ton of chlorine dioxide produced (calculated 
at a cost of $3 per 1000 lb. of steam). This steam requirement is 
substantially less than for the process of U.S. Pat. No. 4,104,365 wherein 
heat is required to strip substantial quantities of methanol and to 
concentrate the sulphuric acid solution and in a typical embodiment 
thereof gives rise to a cost of about $35.00 per ton of chlorine dioxide 
produced, i.e. nearly fifteen times the cost of steam required for the 
process of this invention. 
Usually the water and methanol are added to the solid phase sodium acid 
sulphate as a solution containing the required proportions of water and 
methanol. Since, however, the role of the methanol is exclusively to 
effect the decrease in solubility of the neutral sodium sulphate in the 
aqueous phase, the methanol may be added after initial addition of the 
water. 
The above description of the process of the invention has been made with 
respect to the use of methanol, in view of the ready availability of the 
solvent and the effectiveness of the solvent in the process of the 
invention. Other water-soluble alcohols and ketones, however, may be used, 
if desired, for example, methanol, n-propanol, isopropanol and acetone. 
The metathesis reaction used in this invention may be effected over a wide 
range of temperatures, usually from about 10.degree. to about 70.degree. 
C. The reaction proceeds effectively at room temperature (about 20.degree. 
to 25.degree. C.) although elevated temperatures usually are preferred as 
the rate of reaction increases with increasing temperature. Preferably, 
the temperature is in the range of about 20.degree. to about 50.degree. C. 
The metathesis reaction of the present invention may be effected in any 
convenient manner. Although a batch operation may be effected, continuous 
operation is preferred since the process of the invention is associated 
with a continuous chlorine dioxide producing process. 
The metathesis may be effected in a simple reaction vessel or in a 
decantation-washing column, such as is described in detail in U.S. patent 
application Ser. No. 971,790 filed Dec. 21, 1978 (E140), assigned to the 
assignee of this application, the disclosure of which is incorporated 
herein by reference. 
Intermixing of the water-methanol solution with the sodium acid sulphate to 
effect the metathesis reaction may be assisted by stirring in a reaction 
vessel. Although stirring speeds up the mass transfers involved in the 
metathesis, high shear is unnecessary and gentle stirring only need be 
used, although consuming a longer period of time. 
The reaction time required for completion of the metathesis may vary 
widely, and usually is from about 10 minutes at high stirring to about 60 
minutes in a decantation washer.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to the drawing, a chlorine dioxide generator 10 produces a 
gaseous mixture of chlorine dioxide and steam in line 12 from which 
chlorine dioxide is absorbed into water to provide an aqueous solution 
thereof for utilization in bleaching wood pulp or any other desired end 
use. 
The generator 10 produces the chlorine dioxide in accordance with the 
procedure of the aforementioned U.S. Pat. No. 4,081,520 from sodium 
chlorate solution fed to the generator 10 by line 14, sulphuric acid fed 
to the generator 10 by line 16 and methanol fed to the generator 10 by 
line 18. 
The aqueous reaction medium, which has a total acid normality of greater 
than about 9, is maintained at its boiling point below a temperature above 
which substantial decomposition of chlorine dioxide occurs, usually in the 
range of about 30.degree. to about 85.degree. C., under a subatmospheric 
pressure corresponding to the boiling point, usually in the range of about 
20 to about 400 mmHg, and sodium acid sulphate continuously precipitates 
from the reaction medium once the reaction medium reaches saturation after 
start up. 
The volume of the reaction medium in the generator 10 is maintained 
substantially constant by balancing the volume of aqueous phase entering 
the generator 10 with the volume of water evaporated from the reaction 
medium to form the gaseous product stream 12 and the volume of water 
removed as slurry medium for the solid phase sodium acid sulphate. 
The sodium acid sulphate precipitated from the reaction medium in the 
generator 10 is forwarded by line 20 to a reactor 22. The sodium acid 
sulphate, usually sodium sesquisulphate, may be removed from the generator 
10 in the form of a slurry with reaction medium and separated therefrom by 
filtration prior to passage to the reactor 22. 
In the reactor 22, the sodium acid sulphate is contacted with water fed by 
line 24 and methanol fed by line 26. The weight ratio of water to sodium 
acid sulphate (calculated as Na.sub.3 H(SO.sub.4).sub.2) in the reactor is 
about 0.4:1 to about 1.4:1, preferably about 0.6:1 to about 0.8:1. The 
weight ratio of methanol to sodium acid sulphate (calculated as Na.sub.3 
H(SO.sub.4).sub.2) is up to about 2:1, preferably about 0.3:1 to about 
0.8:1. 
The temperature of the media contacting the solid phase sodium acid 
sulphate in the reactor 22 is preferably about 20.degree. to about 
50.degree. C. and the metathesis reaction produces solid anhydrous neutral 
sodium sulphate. The metathesis of the sodium acid sulphate produces 
sulphuric acid in addition to the neutral acid sulphate. The neutral 
sodium sulphate and the aqueous phase are separated in any convenient 
manner, such as on a filter, and the neutral sodium sulphate is removed by 
line 28, for utilization as desired, typically to make up sodium and 
sulphur values in a pulp mill with which the chlorine dioxide generator 10 
is associated. 
The aqueous phase, containing sulphuric acid, methanol and some dissolved 
sodium sulphate, is removed by line 30 and is split into two streams, with 
typically approximately one-third of the volume of the aqueous phase being 
recycled by line 32 to the methanol feed line 18 to the chlorine dioxide 
generator so as to provide at least part, preferably all, of the methanol 
requirement of the generator 10, any remainder of such requirement being 
fed by line 33 to the methanol feed line 18. The sulphuric acid content of 
the aqueous phase in line 32 provides part of the sulphuric acid 
requirement of the reaction medium in the generator 10. 
The remaining typically approximately two-thirds of the volume of the 
aqueous phase is forwarded by line 34 to a methanol stripper 36 wherein 
the methanol is removed from the aqueous phase. The methanol vapor is 
passed by line 38 to a condenser 40 to result in liquid methanol, which is 
forwarded by line 42 to the methanol feed line 26 to the metathesis 
reactor 22, the balance of the methanol requirement of the reactor 22 
being fed by line 44. 
The methanol-depleted sulphuric acid solution is recycled by line 46 to the 
sulphuric acid feed stream for the chlorine dioxide generator 10 in line 
16. Additional sulphuric acid requirement is fed by line 48 to the 
sulphuric acid feed line 16. 
The process described above with respect to the drawing, therefore, 
produces chlorine dioxide which is essentially chlorine-free at high 
efficiency without the necessity for the addition of any catalytic species 
to the reaction medium. At the same time, the by-product sodium sulphate 
is obtained in a neutral, preferably anhydrous neutral form, so that no 
sulphuric acid is lost from the system with the by-product. The system 
uses one of the reactants, namely, methanol, in the conversion of the acid 
sulphate to the neutral sulphate. 
EXAMPLES 
EXAMPLE 1 
This Example illustrates the preparation of sodium acid sulphate in 
accordance with the procedure of U.S. Pat. No. 4,081,520. 
A chlorine dioxide generator was run to form chlorine dioxide from sodium 
chlorate, sulphuric acid and methanol. The reaction medium was held at its 
boiling point under a subatmospheric pressure and sodium sesquisulphate 
deposited from the reaction medium. The operating parameters are set forth 
in the following Table I: 
TABLE I 
______________________________________ 
Operating conditions: 
temperature 74.degree. C. 
pressure 135 mm Hg 
Reactants concentration and feed 
rate: 
MeOH 33%, 3.4 ml/min. 
H.sub.2 SO.sub.4 9M, 3.6 ml/min. 
NaClO.sub.3 6.74M, 10.5 ml/min. 
Generator liquor concentrations: 
H.sub.2 SO.sub.4 9.3N 
NaClO.sub.3 1.1M 
Crystal Na.sub.3 H(SO.sub.4).sub.2 
Chlorine dioxide production: 
rate 0.48 g/l/min. 
efficiency based on 
chlorate &gt; 99% 
gas analysis ClO.sub.2 &gt; 99%, Cl.sub.2 &lt; 1% 
______________________________________ 
EXAMPLE II 
This Example illustrates metathesis of sodium sesquisulphate. 
A series of experiments was conducted in which 100 g samples of solid 
sodium sesquisulphate were contacted with water and/or methanol under a 
variety of conditions. In each case, the weight of sodium sulphate 
recovered, the proportion of sulphuric acid remaining in the sodium 
sulphate and the normality of the aqueous phase at the end of the 
experiment were determined. The results are reproduced in the following 
Table II: 
TABLE II 
__________________________________________________________________________ 
Stirring Na.sub.2 SO.sub.4 
% H.sub.2 SO.sub.4 
Normality 
Expt. 
Wt. Ratio 
Wt. Ratio Temp. 
Time Type of 
Recovered 
in of Aq. 
No. H.sub.2 O:Na.sub.3 H(SO.sub.4).sub.2 
MeOH:Na.sub.3 H(SO.sub.4).sub.2 
.degree.C. 
Mins. 
Stirring 
g.sup.(1) 
Na.sub.2 SO.sub.4 
Phase.sup.(2) 
__________________________________________________________________________ 
1 0.6:1 0 40 7 SM.sup.(3) 
--.sup.(4) 
9.8 6.4 
2 0.6:1 0.8:1 40 8 SM -- 6.5 6.4 
3 0 1.1:1 20 10 FM.sup.(5) 
99 15.2 very high 
4 0.8:1 2.5:1 20 10 SM -- 2.9 4.8 
5 0.8:1 2.5:1 20 11 SM -- 19.0 --.sup.(6) 
6 0.8:1 2.5:1 20 11 none -- 12.7 4.8 
7 0.8:1 2.5:1 20 10 none -- 14.7 4.8 
8 0.8:1 2.5:1 20 10 none -- 17.6 4.8 
9 0.4:1 0 62 10 SM -- 6.3 9.6.sup.(7) 
10 0.6:1 0 40 10 SM -- 2.7 6.4 
11 0.8:1 0 43 10 SM -- 0.1 4.8 
12 1.0:1 0 45 10 SM -- 1.2 3.8 
13 1.2:1 0 36 10 SM -- 0.96 3.2 
14 1.4:1 0 40 10 SM None -- 2.7 
15 0.6:1 0 40 11 SM 30.1 1.4 6.3 
16 0.8:1 0 47 10 SM 22.3 0.4 4.8 
17 1.0:1 0 47 10 SM 12 0.4 3.8 
18 0.8:1 0 45 10 HP.sup.(8) 
-- 22.9 4.8 
19 1.0:1 0 45 10 HP -- 1.4 3.8 
20 1.2:1 0 45 10 HP -- 9.7 3.2 
21 1.4:1 0 45 10 HP -- 9.8 2.7 
22 0.857:1 0.1:1 20 15 SM 37.5 0.73 4.4 
23 0.75:1 0.2:1 20 17 SM 52.3 0.94 5.1 
24 0.5:1 0.4:1 20 18 SM 74.5 5.0 7.6.sup.(9) 
25 0.25:1 0.6:1 20 15 SM 88 11.6 15.3.sup.(10) 
26 0.125:1 0.7:1 20 15 SM 93.3 18.4 30.6 
27 0.75:1 0.2:1 48 14 SM 46.3 0.72 5.1 
28 0.5:1 0.4:1 48 14 SM 67.3 0.84 7.6 
29 0.25:1 0.6:1 48 15 SM 84.5 10.4 15.3.sup.(11) 
30 0.75:1 0.4:1 19 15 SM 64.5 0.56 5.1 
31 0.75:1 0.6:1 19 15 SM 67.3 0.34 5.1 
32 0.75:1 0.8:1 19 15 SM 70.7 0.13 5.1 
33 0.75:1 1.0:1 19 15 SM 64.5 0.43 5.1 
34 0.645:1 0.34:1 19 15 SM 63.3 0.53 5.9 
35 0.555:1 0.29:1 19 15 SM 71.3 2.0 6.9.sup.(12) 
36 0.555:1 0.29:1 19 15 SM 71.3 3.3 6.9.sup.(13) 
37 0.555:1 0.29:1 19 15 HM 65.8 4.9 6.9.sup.(14) 
38 0.555:1 0.29:1 50 15 SM 38.8 2.4 6.9.sup.(15) 
39 0.555:1 0.29:1 50 15 HM 44.5 0.35 6.9 
40 0.6:1 0.4:1 19 15 SM 66 1.8 6.4 
41 0.75:1 0.4:1 19 15 SM -- 0.34 5.1.sup.(16) 
42 0.75:1 0.4:1 19 15 SM -- 0.26 5.1.sup.(17) 
43 0.75:1 0.4:1 19 15 SM -- 0.17 5.1.sup.(18) 
44 0.77:1 0.41:1 19 10 SM 60.5 0.46 5.1.sup.(19) 
45 0.75:1 0.4:1 19 ON.sup.(20) 
VS.sup.(21) 
55 0.31 5.1 
__________________________________________________________________________ 
Notes: 
.sup.(1) A yield of 81.3 g of Na.sub.2 SO.sub.4 is attainable at 100% 
conversion. 
.sup.(2) Calculated for liberated H.sub.2 SO.sub.4 in aqueous phase. 
.sup.(3) SM means slow magnetic stirring just sufficient to form a vortex 
.sup.(4) -- means that the quantity of sodium sulphate was not determined 
.sup.(5) FM means fast magnetic stirring sufficient to cause splashing. 
.sup.(6) In this experiment NaHSO.sub.4 was converted to Na.sub.3 
H(SO.sub.4).sub.2. 
.sup.(7) 6.4 N actually reached due to incomplete metathesis. 
.sup.(8) HP means hand pouring from one beaker to another steadily for th 
"stirring time". 
.sup.(9) 5.6 N actually reached due to incomplete metathesis. 
.sup.(10) 5.8 N actually reached due to incomplete metathesis 
.sup.(11) 6.8 N actually reached due to incomplete metathesis. 
.sup.(12) 6.5 N actually reached due to incomplete metathesis. 
.sup.(13) 5.7 N actually reached due to incomplete metathesis. 
.sup.(14) 5.1 N actually reached due to incomplete metathesis. 
.sup.(15) 6.0 N actually reached due to incomplete metathesis. 
.sup.(16) 16.7 ppm Cr (VI) added to liquor. 
.sup.(17) 167 ppm Cr (VI) as K.sub.2 CrO.sub.4 added to liquor. 
.sup.(18) 1670 ppm Cr (VI) added to liquor. 
.sup.(19) Cr (III) present in Na.sub.3 H(SO.sub.4).sub.2 49.8 ppm and in 
Na.sub.2 SO.sub.4 76.2 ppm. 
.sup.(20) ON means overnight. 
.sup.(21) VS means very slow at approx. 75 rpm. 
The results of the above Table II illustrate a number of points. A 
comparison of experiments 1 and 2 illustrates that the addition of 
methanol for additional volume is not very helpful in the metathesis. 
Experiment 3 shows that methanol alone is unable to achieve metathesis. A 
comparison of Experiment 4 with Experiment 2 shows that increasing the 
volume of water also increases the metathesis. Experiments 6, 7 and 8 show 
that little or no metathesis is attained in the absence of stirring. 
Experiments 9 to 14 show the effect of increasing quantities of water and 
were used to determine the minimum weight ratio of water to sodium 
sesquisulphate required for complete metathesis. Experiments 15 to 17 show 
the actual quantities of sodium sulphate (Na.sub.2 SO.sub.4) recovered 
using water only. Experiments 18 to 21 were used to determine the effect 
of minimal stirring and some or all the acid sulphate was converted to 
Na.sub.2 SO.sub.4. 
Experiments 22 to 26 illustrate the effect of water and methanol mixtures 
on metathesis at room temperature while Experiments 27 to 29 illustrate 
the effect of such mixtures at 48.degree. C. Experiments 30 to 33 
illustrate the effect of increasing quantities of methanol on yield of 
Na.sub.2 SO.sub.4. 
Experiments 34 and 35 attempt to determine the minimum water requirement 
while Experiments 36 to 39 attempt to determine the effect of temperature 
and stirring on marginal cases. 
Experiments 41 to 44 illustrate the effect of chromium on the process while 
Experiment 45 shows the effect of prolonged slow stirring. 
EXAMPLE III 
This Example illustrates the use of solvents other than methanol. 
The procedure of Example II was repeated except that acetone and ethanol 
were substituted for methanol in two runs effected at 19.degree. C. for 10 
minutes with slow stirring. The results are reproduced in the following 
Table III: 
TABLE III 
__________________________________________________________________________ 
Wt. of 
Weight Ratio 
Na.sub.2 SO.sub.4 
% H.sub.2 SO.sub.4 
Weight Ratio 
Solvent: 
Recovered 
in Normality of 
Solvent 
H.sub.2 O:Na.sub.3 H(SO.sub.4).sub.2 
Na.sub.3 H(SO.sub.4).sub.2 
(g) Na.sub.2 SO.sub.4 
Aqueous Phase 
__________________________________________________________________________ 
Acetone 
0.75:1 0.4:1 62.4 1.6 5.1 
Ethanol 
0.75:1 0.4:1 75.9 0.13 5.1 
__________________________________________________________________________ 
The results of the above Table III illustrate the utility of acetone and 
ethanol in the metathesis reaction. 
SUMMARY OF DISCLOSURE 
In summary of this disclosure, the present invention provides a highly 
efficient chlorine dioxide generating process in which chlorine dioxide 
uncontaminated by chlorine is formed and neutral sodium sulphate 
by-product is produced. Modifications are possible within the scope of 
this invention.