Method for re-use of aqueous co-product from manufacture of sodium dithionite

Aqueous co-product, produced by distillation of reaction filtrate from a batch reaction for manufacturing sodium dithionite by reacting sodium formate, sodium hydroxide, and sulfur dioxide in aqueous methanol solution, is rapidly evaporated to remove about 80% of its water content, so that only the determined quantity of water remains with its determined contents of alkali metal compounds, and is utilized as an admixed component of a succeeding batch.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of anhydrous alkali dithionites 
by reacting an alkaline formate, an alkali metal agent, and sulfur dioxide 
in an alcohol/water solvent. It particularly relates to improving this 
process by reusing aqueous column bottoms from the solvent recovery 
distillation to produce additional dithionite. 
In the process for the manufacture of alkali metal dithionites from an 
alkali metal salt of formic acid, an alkali metal hydroxide, carbonate or 
bicarbonate, and sulfur dioxide, the product precipitates in an 
alcohol/water solution. Upon completion of the dithionite reaction, the 
product is separated from the reaction filtrate, also termed the mother 
liquor, by filtration. The filter cake is washed with alcohol to remove 
the adhering filtrate, and the product is dried. The alcohol in both the 
filtrate and the wash alcohol is purified for re-use by distillation. The 
water phase from the distillation is disposed of as a co-product of the 
manufacturing process. As such, this material has little or no commercial 
value. It does contain a mixture of metal salts which include formate, 
metabisulfite, sulfite, thiosulfate, and sulfate. Formate, metabisulfite, 
and sulfite are either fed directly to the synthesis as raw materials, or 
are produced as intermediates in the production of dithionites. 
It is well known in the manufacture of dithionites that a portion of the 
dithionite product decomposes during the course of the reaction to form 
thiosulfate. Furthermore, this decomposition is autocatalytic with respect 
to thiosulfate; as the concentration of thiosulfate increases, so does its 
rate of formation. For this reason, the aqueous co-product containing 
thiosulfate cannot be re-used in the process. 
It is also known that certain organic compounds are capable of reacting 
with or complexing thiosulfates. For example, U.S. Pat. No. 4,622,216 
describes a method in which certain organic compounds are added during the 
course of producing dithionites to react with thiosulfate and thus 
minimize the decomposition reaction. These thiosulfate-reactive compounds 
include epoxy compounds such as ethylene oxide, propylene oxide, butyl and 
isobutyl oxide, epichlorohydrin, and epibromohydrin as well as halogenated 
hydrocarbons of the general formula RX or XRX, where R is an alkyl group 
of carbon number 1 to 8, or an allyl, methallyl, or ethylallyl group, and 
X is a halogen. 
When such thiosulfate-reactive compounds are added to a batch reactor, they 
destroy thiosulfate ions as they are being formed within the reaction 
vessel and minimize destruction of the sodium dithionite product. The 
yield of anhydrous sodium dithionite is thereby increased. 
Japanese Patent Publication No. 28,397/75 teaches a process for 
manufacturing anhydrous sodium dithionite in an alcohol/water solvent from 
sodium formate, an alkali compound, and sulfur dioxide, followed by 
filtering the sodium dithionite crystals from the mother liquor. The 
publication discloses a method for recycling a portion of the reaction 
filtrate with reduced distillation of the filtrate by treating the 
filtrate with 1-to-4-fold excess on a molar basis of ethylene oxide, 
propylene oxide, or a mixture thereof over the amount of sodium 
thiosulfate contained in the reaction filtrate and by allowing the 
reaction mixture to stand for several hours at room temperature. The 
reaction filtrate is combined with the methanol used in washing the 
separated crystals of sodium dithionite to form a mixture of which a part 
is distilled to recover the methanol and isolate the additional product, 
which is discarded, of sodium thiosulfate and ethylene oxide or propylene 
oxide. 
Japanese Patent Disclosure No. 110,407/83 teaches a method for producing 
dithionites by reacting a formic acid compound, an alkali compound, and 
sulfur dioxide in a water-organic solvent mixture and by adding an epoxy 
compound, a halogenated hydrocarbon of the general formula R--X, or a 
mixture of two or more compounds of these types to the reaction mixture in 
the final stage of the reaction. Suitable epoxy compounds include ethylene 
oxide, propylene oxide, butylene oxide, isobutylene oxide, styrene oxide, 
cyclohexene oxide, epichlorohydrin, and epibromohydrin. In the halogenated 
hydrocarbon, R is a primary or secondary alkyl group having 1-8 carbons, 
an allyl group, a 2-methylallyl group, or a 2-ethallyl group, and X is a 
halogen. The filtrate obtained by isolating the dithionite crystals, the 
organic solvent used for washing the crystals, or a mixture thereof is 
recycled and reused in the reaction. Both the filtrate and the washing 
liquid were demonstrated to be equivalent to distilled methanol as the 
organic solvent for producing sodium dithionite. 
In European Patent Publication No. 68,248 and in U.S. Pat. No. 4,388,291, a 
process is disclosed for producing anhydrous dithionite in which the 
washing liquid discharged from the washing step is sequentially divided 
into two portions, a first discharge liquid and a second discharge liquid, 
the former being treated to convert undesirable substances inhibiting the 
production of dithionites into substances which do not exert an adverse 
influence on the production of dithionites by adding an organic compound 
selected from the group consisting of compounds represented by Formulas I 
and II and cyclohexene oxide. Formula I is as follows: 
##STR1## 
wherein R.sub.1 group containing from 1 to 8 carbon atoms, a halogenated 
alkyl group containing 1 or 2 carbon atoms, a phenyl group, or a 
substituted phenyl group. The compound represented by this formula 
includes ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, 
epibromohydrin, and styrene oxide. Formula II is as follows: 
EQU R.sub.2 --X, 
wherein R.sub.2 is a primary or secondary alkyl group containing from 1 to 
8 carbon atoms, an allyl group or a 2-methylallyl or 2-ethallyl group, and 
X is a halogen atom. Suitable compounds include methyl iodide and allyl 
chloride. 
A portion of the treated first discharge liquid was mixed with nearly twice 
as much of the untreated second discharge liquid and used to prepare 
sodium dithionite after adjusting for the amount of water in the discharge 
liquid mixture. The resulting purities and yields for the sodium 
dithionite product were substantially identical to those obtained with 
pure methanol. 
U.S. Pat. No. 4,693,880 describes a dithionite production process in which 
the washings first discharged are distilled by a conventional method for 
recovering the organic solvent, and the subsequent washings are used as 
the reaction solvent without distillation. 
Japanese Patent Publication No. 079,805/83 teaches the addition of an epoxy 
compound (selected from the group comprising epichlorohydrin, 
epibromohydrin, butylene oxide, propylene oxide, ethylene oxide, styrene 
oxide, and cyclohexene oxide) and acid (to bring the pH below 6) to the 
mother liquor and recycling the treated solution to the next dithionite 
production stage. 
Japanese Patent Publication No. 091,004/83 teaches the addition of 
propylene oxide or ethylene oxide to the wash liquid and recycling the 
treated liquid to dithionite production. 
Japanese Patent Publication No. 110,406/83 relates to washing the filtered 
dithionite crystals with an organic solvent containing an epoxy compound 
or a hydrocarbon halide and recycling the used washing solvent to obtain 
dithionite of high purity in high yields. 
The objective in all of these patents is to recycle a portion of the 
filtrate, or the wash methanol, or both. The filtrate contains all of the 
unused raw materials, as much as 25% of the quantity fed to the batch 
reaction, plus most of the decomposition products. The filtrate also 
contains all of the water fed to the batch as aqueous solutions of both 
sodium hydroxide and sodium formate plus the water of reaction produced in 
the batch as shown in the overall chemical reaction: 
EQU NaOH+HCOONa+2SO.sub.2 .fwdarw.Na.sub.2 S.sub.2 O.sub.4 +CO.sub.2 +H.sub.2 
O. 
It also contains all of the alcohol fed to the batch. Since there is a very 
narrow range of water to alcohol ratio appropriate to the manufacture of 
sodium dithionite via the formate process, it is patently impossible to 
recycle all of the filtrate to the next batch. If attempted, no water in 
which to dissolve the sodium hydroxide and sodium formate would be 
available, nor would alcohol be available in which to dissolve the sulfur 
dioxide used as a raw material. Finally, if these obstacles were somehow 
overcome, the water made via reaction in the recycle batch would create an 
excessive water to alcohol ratio and severely damage or destroy the 
product quantity and quality. 
In order to recover for re-use all of the unused raw material present in 
the filtrate, it is necessary to do two things: 
A. remove all of the water except that quantity which would normally be 
present with the raw materials in the filtrate if they were virgin raw 
materials, and 
B. prevent to the greatest extent possible, the formation of sodium 
thiosulfate both during the batch reaction and during subsequent 
processing of the filtrate to produce the partially dehydrated material 
appropriate for re-use to make additional sodium dithionite. 
Whether a thiosulfate-reactive compound is added during the 
dithionite-producing reaction as taught in U.S. Pat. No. 4,622,216, to the 
reaction filtrate before re-use thereof as taught in Japanese N. 
110,407/83, or to the first wash discharge liquid before re-use thereof as 
taught in Japanese 091,004/83, decomposition of dithionite and formation 
of thiosulfate continues to occur after the compound has been consumed. If 
the reaction filtrate is distilled to produce co-product, such thiosulfate 
formation also continues to occur so that the co-product is not usable as 
a raw material for the reaction. 
Nevertheless, the necessary raw materials to make sodium dithionite are 
present in the filtrate and in fact some sodium dithionite is produced 
during distillation. The sodium dithionite that is produced quickly 
decomposes to form, among other compounds, sodium thiosulfate, because 
sulfur dioxide that is present as sodium bisulfite in the filtrate is 
released during distillation. Under typical conditions, the amount of 
sodium thiosulfate increases about 20-25% during distillation. 
It accordingly seemed reasonable that the same compounds could be used to 
react with thiosulfate present in the largely aqueous co-product, thereby 
making it possible to re-use the valuable compounds present in the 
material for manufacture of additional dithionite. However, all of the 
water that is originally present in the reaction mixture and all of the 
water formed by chemical reactions during such manufacture is inherently 
isolated in the co-product. An amount of water must consequently be 
removed from the co-product which represents the water made during the 
dithionite reactions plus that associated with the original feed 
solutions, leaving behind only the water associated with the equivalent 
amount of raw materials contained in the co-product. 
A process for treating the co-product that can enable it to be usable as a 
raw material for the dithionite-producing reaction is accordingly needed. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide a process for re-use 
of the aqueous co-product produced by distillation of the reaction 
filtrate. 
In accordance with these objects and the principles of this invention, it 
has been discovered that the aqueous co-product can be evaporated under 
vacuum until the remaining water is only the water normally associated 
with the equivalent amount of co-product raw materials that are useful in 
dithionite-producing reactions. 
It has also been discovered that the sulfur dioxide remaining in the 
co-product can be neutralized prior to the distillation by adding an 
alkali, preferably sodium hydroxide. 
It has further been discovered that the increase in sodium thiosulfate 
content of the co-product during the evaporation is linearly time 
dependent. 
In addition, the problem of thiosulfate formation during distillation is 
minimized by adding a sufficient quantity of an alkali metal compound, 
such as sodium hydroxide, to the reaction filtrate to prevent sodium 
thiosulfate formation by synthesis and subsequent decomposition of 
Na.sub.2 S.sub.2 O.sub.4 during distillation. The alkali metal compound 
converts a portion of the sodium bisulfite in the filtrate to sodium 
sulfite, preventing the release of free sulfur dioxide during distillation 
of the filtrate, thereby minimizing or eliminating sodium thiosulfate 
formation during distillation. This alkali addition is necessary in order 
to minimize the amount of ethylene oxide or other sodium thiosulfate 
complexing agent which must be used in the co-product re-use synthesis to 
produce sodium dithionite, as disclosed in U.S. Pat. No. 4,622,216. 
It was noted that the co-product evaporation required between 1 and 2 hours 
to complete. During the evaporation, the sodium thiosulfate content of the 
co-product increased by 0.38 pounds/100 pounds co-product, notwithstanding 
addition of NaOH. This increase was perceived to be linearly time 
dependent. 
Evaporating the water very rapidly using a wiped film evaporator or similar 
apparatus, involves a retention time of one minute or less and results in 
an increase of sodium thiosulfate of only 0.01 pounds/100 pounds 
co-product, compared to the increase of 0.38 pounds/100 pounds co-product 
when using a retention time of one to two hours. 
Consequently, the use of evaporative techniques involving retention times 
of one minute or less is the most preferred embodiment of this invention. 
After concentration to remove approximately 80% of the water present, the 
co-product is then used to make additional sodium dithionite in a separate 
reactor identical to that in the typical dithionite manufacturing process. 
In accordance with the principles of this invention, an aqueous co-product, 
produced by distillation of a caustic treated reaction filtrate from an 
earlier batch reaction for making a dithionite by reacting an alkali metal 
formate, sulfur dioxide, and an alkali metal hydroxide, carbonate, or 
bicarbonate in aqueous methanol solution, is utilized as feed for a 
subsequent dithionite batch reaction by means of the following process: 
A. determining the contents of alkali metal formate and equivalent alkali 
metal hydroxide in the co-product; 
B. determining the quantity of water normally associated with these alkali 
metal compounds as used in the batch reactions for making anhydrous alkali 
metal dithionites; 
C. heating the co-product and evaporating water therefrom, (preferrably at 
a retention time of one minute or less) until only the determined quantity 
of water remains with the determined contents, to produce concentrated 
co-product; 
D. selectively admixing the following materials, at standard feed rates, 
under standard heating conditions, and at standard pressures, to produce a 
succeeding batch reaction mixture: 
1. the standard amounts of sulfur dioxide and methanol, minus the 
determined equivalent amount of sulfur dioxide contained in the 
concentrated co-product, 
2. the standard amount of the alkali metal formate and of the alkali metal 
hydroxide, carbonate, or bicarbonate minus the determined amounts of these 
alkali metal compounds, 
3. the standard amount of water minus the determined associated amount of 
water, 
4. the concentrated co-product, and 
5. at least one compound selected from the group consisting of ethylene 
oxide, propylene oxide, butylene oxide, isobutylene oxide, 
epichlorohydrin, styrene oxide, methyl iodide, allyl chloride, and 
cyclohexene oxide; 
E. heating the mixture to the standard reaction temperature and maintaining 
this temperature throughout the course of the reaction to produce a 
completed reaction mixture containing fully reacted alkali metal 
dithionite; 
F. cooling the reaction mixture; 
G. filtering the cooled reaction mixture to produce crude alkali metal 
dithionite and reaction filtrate; 
H. separately distilling from the reaction filtrate to produced methanol 
and a aqueous co-product which may be treated again as previously 
described or purged as a waste from the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the FIGURE, the dithionite manufacturing process according to 
this invention utilizes a pressurized reactor 1 having a reflux condenser, 
a filter/dryer 2, a filtrate tank 3, a distillation unit 4, and a wiped 
film evaporator 5, a reuse reactor 6, a re-use filter/dryer 7, a separate 
filtrate tank 8, and a separate distillation unit 9. Materials that are 
charged to reactor 1 include an SO.sub.2 stream 10, an HCOONa stream 11, a 
sodium hydroxide stream 12, an ethylene oxide stream 13, and a recycle 
CH.sub.3 OH stream 14. CO.sub.2 off-gas stream 15 leaves reactor 1 through 
a condenser during the reaction. Additional CH.sub.3 OH is fed to a 
scrubber to capture the volatile methyl formate, but it is not shown in 
the FIGURE. After the reaction has been completed and at the end of a 
succeeding cooling period, reaction mixture 16 is produced, as a slurry of 
dithionite crystals in aqueous methanol which is fed to filter/dryer 2. 
The slurry is filtered in filter/dryer 2 to produce filtrate stream 17 and 
crude dithionite crystals which are washed with methanol stream 18 and 
then dried to produce anhydrous Na.sub.2 S.sub.2 O.sub.4 product stream 
19. Filtrate stream 17 is fed to filtrate tank 3 with NaOH stream 20, 
producing treated filtrate. Wash liquid leaves filter/dryer 2 as part of 
stream 17. 
Treated filtrate stream 21 is fed to distillation unit 4, producing recycle 
CH.sub.3 OH stream 22 and aqueous co-product stream 23 which is fed to 
evaporator 5 to produce concentrated co-product stream 24, for feeding to 
re-use reactor 6, and water stream 25 which is discarded. 
Also fed to re-use reactor 6 are SO.sub.2 stream 26, a sodium hydroxide or 
sodium formate stream 27, an ethylene oxide stream 28, and a recycle 
methanol stream 29. CO.sub.2 off-gas stream 30 leaves reactor 6 through 
condensers and a scrubber. Additional methanol is fed to the scrubber but 
is not shown in the FIGURE. 
The cooled reaction slurry 31, is fed to filter/dryer 7; the cake is washed 
with methanol stream 32, producing filtrate stream 33 and, after vacuum 
drying, product stream 34. Filtrate stream 33 including wash methanol 
stream 32 is collected in tank 8, from which it is fed as stream 35 to 
distillation unit 9. The overhead from distillation unit 9 is recycle 
methanol stream 36, while the aqueous bottoms stream 37 is sent to waste. 
EXAMPLE 1 
Co-product Generation and Standard Yield (Weight Basis) 
To a 100-gallon reactor, 150 pounds of distilled recovered methanol 
containing 2.89% methyl formate and 1.32% sulfur dioxide were added as a 
first feed. Next, as a second feed, 5 pounds of 96% sodium formate 
dissolved in 4 pounds of water were added to the reactor. The reactor 
contents were heated to 50.degree. C. with agitation. At this temperature, 
a third feed, consisting of 310 pounds of distilled recovered methanol, of 
the same composition as the first feed and containing sulfur dioxide of a 
quantity such that in the first and third feeds there would be a total of 
201 pounds of sulfur dioxide, began to be fed to the reactor. The feed 
rate was controlled so that 80% of its total amount was fed to the reactor 
in 67.6 minutes. The fourth feed consisted of 127 pounds of 96% sodium 
formate, 104 pounds of water, and 67 pounds of 99% sodium hydroxide. The 
fourth feed was started 2.6 minutes after the third feed, and its feed 
rate was controlled so that it was fed in its entirety in 65 minutes. A 
fifth feed of 3.3 pounds of pure ethylene oxide was started simultaneously 
with the fourth feed. Its feed rate was controlled so that it was fed in 
its entirety in 195 minutes. 
Owing to the exothermic nature of the reaction, the mixture self-heated to 
84.degree. C. over a 15-minute period. Temperature control was then 
initiated to maintain 84.degree. C. throughout the course of the reaction. 
Owing to the evolution of carbon dioxide, the reactor pressure increased 
to 40 psig during this 15-minute period, and pressure control was then 
initiated to maintain 40 psig throughout the course of the reaction. 
The vented carbon dioxide left the reactor through condensers and a 
scrubber which was fed at a rate of 0.34 pounds/minute with essentially 
pure recovered methanol. When the fourth feed terminated, the rate of feed 
of the third feed was reduced so that the remaining 20% was fed over an 
additional 65 minutes. At the conclusion of this feed, an additional 
65-minute period was allowed for the reaction to go to completion, at 
which time the ethylene oxide feed ended. The reactor contents were cooled 
to 73.degree. C. and were discharged to a filtering apparatus wherein the 
mother liquor was separated from the crude product which was then washed 
with 190 pounds of essentially pure recovered methanol. The filter cake 
was vacuum dried to yield the anhydrous product. The yield was 236 pounds 
of 91.37 weight percent sodium dithionite. 
The filtrate was treated with 0.76 pound of sodium hydroxide/100 pounds of 
filtrate, prior to recovery of the methanol by atmospheric distillation. 
The amount of sodium thiosulfate produced during the act of distillation 
was reduced from 0.41 pound of thiosulfate/100 pounds of filtrate without 
sodium hydroxide addition to 0.0065 pound/100 pounds with sodium hydroxide 
addition. The co-product from the distillation was collected, analyzed, 
and shown to contain substances equivalent, on a weight basis, to 1.32% 
sodium thiosulfate, 7.35% sodium hydroxide, 9.48% sulfur dioxide, 11.13% 
sodium formate, and 68.20% water. Using aliquots of this co-product, the 
following small scale experiment was conducted. 
EXAMPLE 2 
By conventional batch evaporation of 5,068 parts of the co-product from 
Example 1 under vacuum in the synthesis reactor, 2,880 gm of water were 
removed over a two hour period. During the course of this evaporation, the 
sodium thiosulfate concentration increased from 1.32% on a co-product 
basis to 1.70%, again on a co-product basis. This is a 29% increase during 
the evaporation. To the concentrated co-product was added 1,773 gm of 
recovered methanol containing 2.72% methyl formate and 0.31% sulfur 
dioxide. The mixture was heated, with agitation, to approximately 
70.degree. C., at which time 54 gm of propylene oxide was added over a 
10-minute period. The mixture was maintained at 70.degree. C. for one 
hour, at which time 210 gm of 96% sodium formate dissolved in 140 gm water 
was introduced to the reactor. A first feed mixture was prepared which 
consisted of 687 gm SO.sub.2 and 840 gm recovered methanol of the same 
composition as that added to the concentrated co-product. A second feed 
was prepared which consisted of 410 gm of pure recovered methanol and 30 
gm of propylene oxide. Propylene oxide was used in these small scale 
experiments rather than ethylene oxide because of safety and health 
considerations. 
Initially, 17.5% of the first feed was introduced to the reactor 
immediately. This is the quantity of sulfur dioxide calculated to convert 
the sodium sulfite in the concentrated co-product to sodium bisulfite. The 
rate of the first feed was then adjusted to allow 49.2% of the feed to be 
added at an equal rate over 65 minutes. Simultaneously, the second feed 
was started and the rate adjusted so that it was fed in its entirety in 
195 minutes. 
Upon addition of the second portion of the first feed, an exothermic 
reaction began which caused the contents of the reactor to self-heat. At 
83.degree. C., temperature control was initiated to maintain 83.degree. C. 
throughout the course of the reaction. The reaction evolved CO.sub.2, and 
the reactor pressure was maintained at 30 psig by venting excess CO.sub.2 
through a condenser. After 65 minutes, the first feed rate was adjusted to 
allow the remaining 33.3% to feed in 65 minutes. The mixture was then 
maintained at 83.degree. C. and 30 psig for 65 minutes, at which time the 
second feed ended. The contents of the reactor were cooled to 73.degree. 
C. and then filtered. The solid was washed with 1,400 gm of pure recovered 
methanol, and the filter cake was vacuum dried. The yield was 1,135 gm of 
78.04 weight percent sodium dithionite. 
EXAMPLE 3 
In order to make a pilot plant scale co-product re-use run, it was 
necessary to make three additional co-product generation batches similar 
to Example 1. The combined co-product from these runs, 635 lb, was 
analyzed and shown to contain substances equivalent, on a weight basis, to 
2.25% sodium thiosulfate, 5.41% sodium hydroxide, 6.51% sulfur dioxide, 
14.52% sodium formate, and 69.50 water. 
Using a Kontro Co., Inc., one square foot, horizontal, tapered, agitated 
film evaporator, 626 lb of the combined co-product was evaporated under 
vacuum to remove 341 lb of water. During the course of this evaporation, 
the sodium thiosulfate concentration increased from 2.28% on a co-product 
basis to 2.29%, again on a co-product basis. This is only a 0.4% increase 
as compared to the 29% increase using conventional evaporation as in 
Example 2. 
The 244 lb of concentrated co-product was placed in the reactor and to it 
was added 221 lb of recovered methanol containing 1.8% methyl formate and 
0.32% sulfur dioxide. The mixture was agitated, and to it was added 6 lb 
of ethylene oxide. A first feed mixture was prepared consisting of 94 lb 
of sulfur dioxide dissolved in 239 lb of recovered methanol of the same 
composition as that added to the concentrated co-product in the reactor. A 
second feed was prepared consisting of 9 lb of 99% sodium hydroxide 
dissolved in 41 lb of water. This additional water gave the proper water 
to alcohol ratio for optimum results. A third feed consisted of 3.3 lb of 
ethylene oxide. 
Initially, 13.2% of the first feed was added to the reactor. This is the 
quantity of sulfur dioxide calculated to convert the sodium sulfite in the 
concentrated co-product to sodium bisulfite. The rate of the first feed 
was then adjusted so that 67.6% of the total feed would be added uniformly 
over 65 minutes. Simultaneously, the second and third feeds were started, 
adjusted so that the second was fed in its entirety in 50 minutes, and the 
third in its entirety in 195 minutes. 
During addition of the second portion of the first feed, an exothermic 
reaction occurred which caused the reactor contents to self heat. At 
84.degree. C. temperature control was initiated to maintain 84.degree. C. 
throughout the remaining course of the reaction. The reaction evolved 
carbon dioxide, and the reactor pressure was maintained at 40 psig by 
venting carbon dioxide from the reactor through two condensers and a 
scrubber. After 65 minutes, the feed rate of the first feed was adjusted 
so that the remaining 19.2% was fed during the next 65 minutes. The 
mixture was then maintained at 84.degree. C. and 40 psig for an additional 
65 minutes. At the conclusion of this third 65 minute period the third 
feed terminated, and the reactor contents were cooled to 73.degree. C. and 
filtered. After washing the filter cake with 190 lb of methanol, the cake 
was vacuum dried to yield 135 lb of 85.20% sodium dithionite. The 
collected filtrate and wash methanol combined were distilled to recover 
the methanol. The aqueous still bottoms from this re-use batch were not 
saved for additional re-use, but went to waste. 
In Example 2 the equivalent sodium hydroxide content of the concentrated 
co-product was adequate to supply the alkali requirement of the re-use 
run, but added sodium formate was needed. In Example 3 the opposite was 
true; the sodium formate was adequate for the re-use run, but sodium 
hydroxide had to be added. 
The examples illustrates the flexibility of the novel process of this 
invention. As has been previously pointed out, it is necessary to balance 
the reactants contained in the co-product with the supplemental raw 
materials introduced into the reactor. Therefore, the exact composition of 
the co-product is not critical since the appropriate amount of raw 
reactant can be added depending upon the co-product composition as 
illustrated in the above examples. Optimal re-use of the co-product is 
very much dependent on accurate analytical determination of the various 
chemicals dissolved in the co-product. 
The economic importance of re-use of the co-product is shown in Table 1 
which compares the pounds of purchased raw materials required to make one 
pound of 100% sodium dithionite via the conventional process (Example 1), 
and via the re-use process (Example 3). 
TABLE 1 
______________________________________ 
Purchased Raw Material Usage/Pound of 
100% Sodium Dithionite 
Standard 
Re-Use 
Example 1 
Example 3 
______________________________________ 
Sulfur Dioxide 0.930 0.817 
Sodium Hydroxide 0.287 0.078 
Sodium Formate 0.655 0 
Ethylene Oxide 0.015 0.081 
Total 1.798 0.976 
______________________________________ 
In Example 2, the co-product evaporation required between 1 and 2 hours to 
complete. During the evaporation, the sodium thiosulfate content of the 
co-product increased by 0.38 lb/100 lb co-product. This increase was found 
to be linearly time dependent, so that the preferred embodiment is to 
evaporate the water very rapidly, using a wiped film evaporator or similar 
apparatus which involves a retention time of one minute or less. When 
using an apparatus of this kind, as in Example 3, the increase in sodium 
thiosulfate content was only 0.01 lb/100 lb of co-product. 
After concentration to remove approximately 80% of the water present, the 
co-product is then used to make additional sodium dithionite in a separate 
reactor identical to that used in the typical manufacturing process.