Method to reduce the potential salt cake content of chlorine dioxide generator spent acids

A method of generating chlorine dioxide that comprises forming chloric acid by the action of sulphuric acid on a chlorate in a generator in the liquid phase. The chloric acid is reduced to produce chlorine dioxide. The chlorate is fed to the generator as a solid.

FIELD OF THE INVENTION 
This invention relates to a process for the generation of chlorine dioxide 
gas. 
DESCRIPTION OF THE PRIOR ART 
Chlorine dioxide is used in pulp and paper mills for the production of 
bleaching solution. The waste acid from the generating process is 
generally directed to the recovery process to provide sulphur and sodium 
makeup within the pulp cooking circuit. 
Chlorine dioxide is produced by four principle processes; the Mathieson, 
Solvay, R-2 and R-3 processes. 
All these processes reduce a chlorate, usually sodium chlorate, in a strong 
sulphuric acid medium. Generally speaking, the processes differ in the 
reducing agent used. In the Mathieson process the reducing agent is 
sulphur dioxide, in the Solvay process the reducing agent is methanol and 
in the R-2 and R-3 processes, the reducing agent is sodium chloride. 
An essential part of all these processes is the reaction to produce chloric 
acid, which is directly reduced by the reducing agents to produce chlorine 
dioxide gas. The chlorine dioxide gas is removed from the reaction 
solution as a 10-15% gaseous mixture in air. 
The four processes mentioned may be described by the following simplified 
equations: 
##STR1## 
All the above processes require a continuous supply of sulphuric acid to 
maintain the requisite level of acidity for efficient chlorine dioxide 
production. Equation 2 shows that the Mathieson process does not consume 
any acid, since sulphuric acid sufficient to combine with sodium added as 
chlorate is generated from the sulphur dioxide used as the reducing agent. 
Thus, the Mathieson process offers an excellent possibility of generating 
chlorine dioxide without the consumption of fresh acid. In the Solvay 
process, acid is consumed in the initial generation of chloric acid, and 
additional acid is consumed for the generation of hydrochloric acid, as in 
the case with the R-2 and R-3 processes. 
The desirability of not using sulphuric acid in the generation system, is 
two-fold. First there is the economic advantage of dispensing with an 
expensive compound. Secondly it avoids the need to reduce sulphur as 
sulphate in the recovery cycle. The current practice necessitates the 
incineration of spent acids from Mathieson, Solvay and R-2 processes which 
creates the problem of sulphidity control in the digester cooking liquor. 
It was from this that the R-3 system, which is described and claimed in 
U.S. Pat. No. 3,446,584, was invented. In the R-3 system a chlorine 
dioxide generator could operate at low normalities in order to separate 
the neutral salts in sulphuric acid. However the process requires the 
complete scrapping of existing generation equipment. 
Effluents from chlorine dioxide generators have weight compositions in the 
range 20-30% sodium sulphate; 25-35% sulphuric acid; balance water and 
dissolved chlorine dioxide, chlorine, SO.sub.2, ferric, calcium, chloride 
and chlorate ions. For example, a typical effluent from a Mathieson or 
Solvay process would have the following composition: 
______________________________________ 
Sodium Sulfate (Na.sub.2 SO.sub.4) 
24.5% W/W 
Sulphuric Acid (H.sub.2 SO.sub.4) 
28.2% W/W 
Water 46.3% W/W 
Sodium Chlorate (NaClO.sub.3) 
0.6% W/W 
Sodium Chloride (NaCl) 
0.1% W/W 
Gases, Etc. 0.1% W/W 
______________________________________ 
A typical effluent from the R-2 process has the following approximate 
composition: 
______________________________________ 
Sodium Sulfate (Na.sub.2 SO.sub.4) 
22.3% W/W 
Sulphuric Acid (H.sub.2 SO.sub.4) 
33.6% W/W 
Water 44.0% W/W 
Sodium Chlorate (NaClO.sub.3) 
0.33% W/W 
Sodium Chloride (NaCl) 
0.20% W/W 
Gases, Etc. 0.10% W/W 
______________________________________ 
In the case of the R-3 process where neutral sodium sulfate is formed in 
the generator, the waste product would have the approximate following 
composition: 
______________________________________ 
Sodium Sulfate (Na.sub.2 SO.sub.4) 
90% W/W 
Sulphuric Acid (H.sub.2 SO.sub.4) 
Trace 
Sodium Chlorate (NaClO.sub.3) 
" 
Sodium Chloride (NaCl) 
" 
Water 10% W/W 
______________________________________ 
The analysis of salt cake emanating from the filter in the R-3 process is 
variable and dependent on the amount of wash water supplied on the filter 
cake. If one examines a phase diagram of the system Na.sub.2 SO.sub.4 
--H.sub.2 O--H.sub.2 SO.sub.4, it becomes clear that it is difficult to 
isolate sodium sulfate from the mixture. It will of course be realized 
that reference to the phase diagram is an over simplification, as the 
effluent from the chlorine dioxide generator also contains chlorate, 
chloride, traces of chlorine dioxide, chlorine and various other 
chemicals. However, generally the effluents all lie in that area of the 
phase diagram where trisodium hydrogen disulphate Na.sub.3 
H(SO.sub.4).sub.2 or sesqui salt crystallize if the effluent is cooled. 
The acid recovery process (ARP), is described in U.S. Pat. No. 4,104,365. 
In the process sulphuric acid is separated from neutral sodium sulphate in 
spent chlorine dioxide generator liquor with an 85% precipitation 
efficiency under normal conditions with total recycle of the sulphuric 
acid after concentration from the distilled product. There are therefore 
two available systems for recovery of sodium salts from the generator 
waste acid; ARP and the R-3 ClO.sub.2 process. 
The recovery of sodium salts from the ARP system relies upon the 
polarization of water within the waste acid, thus moving the chemical 
equilibrium over to the neutral salt zone. The R-3 process relies upon the 
salting out of salt cake in the generator by the continuous addition and 
recycle of sodium chlorate solutions to provide a common ion effect, thus 
precipitating the neutral salt. The acid recovery process which comprises 
an added chemical plant to the existing chlorine dioxide plant, represents 
the only true method known wherein all the sulphuric acid is recovered in 
the Mathieson process for eventual recycle back into the primary 
generator. However, the extra equipment and space requirement is somewhat 
expensive. Moreover, the distillation and recovery of methanol, and 
reconcentration of sulphuric acid, can be onerous and energy consuming. It 
should be noted that all chlorine dioxide processes use sulphuric acid to 
convert NaClO.sub.3 to HClO.sub.3. Furthermore, sulphuric acid is required 
to maintain a 9N acidity in the ClO.sub.2 generator, since NaClO.sub.3 is 
fed in aqueous solution at 40-50% concentration by weight. These 
conditions are well recognized in the industry and are adequately shown in 
equations (1)-(4). 
SUMMARY OF THE INVENTION 
The present invention seeks to provide a process that, except for startup, 
does not use sulphuric acid within the generator. In instances where 
chlorine dioxide is being generated using salt or methanol as the reducing 
agent, the amount of sulphuric acid entering the generator will be 
significantly reduced. 
Accordingly the present invention is in a method of generating chlorine 
dioxide that comprises forming chloric acid by the action of sulphuric 
acid on a chlorate in a generator in the liquid phase, and reducing the 
chloric acid to produce chlorine dioxide and is the improvement that 
comprises feeding the chlorate to the generator as a solid. 
In accordance with the usual practice the chlorate will usually be sodium 
chlorate. In a preferred aspect the reducing agent will be sulphur 
dioxide. The feeding of dry crystalline NaClO.sub.3 into the ClO.sub.2 
reactor offers a unique opportunity to reduce, and in the case of the 
Mathieson process to eliminate completely, the use of H.sub.2 SO.sub.4 as 
a continuous feed stock to the generator. The H.sub.2 SO.sub.4 consumed in 
equation (1) above is returned in equation (2) above, and since chemical 
reactions are far from perfect, more `by product` acid will be returned as 
a result of ClO.sub.2 generation inefficiency. 
In the case where SO.sub.2 reacts with produced ClO.sub.2, then even more 
H.sub.2 SO.sub.4 is produced as follows: 
EQU 6H.sub.2 O+5SO.sub.2 +2ClO.sub.2 .fwdarw.5H.sub.2 SO.sub.4 +2HCl (6) 
A further side reaction within the Mathieson process will also produce 
extra H.sub.2 SO.sub.4 as follows: 
EQU HClO.sub.3 +3SO.sub.2 +3H.sub.2 O.fwdarw.3H.sub.2 SO.sub.4 +HCl (7) 
In practical terms, a Mathieson type ClO.sub.2 generator using 46% W/W 
NaClO.sub.3 as feed stock will demand and produce the following amounts of 
H.sub.2 SO.sub.4 /ton ClO.sub.2, 
______________________________________ 
Gen. NaClO.sub.3 
H.sub.2 SO.sub.4 
SO.sub.2 
Na.sub.2 SO.sub.4 
H.sub.2 SO.sub.4 
Eff. % 
In In In Out Out 
______________________________________ 
100 1.58 1.32 0.48 1.05 1.32 
96 1.65 1.26 0.6 1.1 1.38 
94 1.68 1.22 0.66 1.12 1.40 
92 1.72 1.17 0.73 1.14 1.43 
90 1.76 1.14 0.80 1.17 1.47 
______________________________________ 
There is more H.sub.2 SO.sub.4 in the generator waste acid than in the 
feed, at the lower efficiencies. This acid is produced through either 
equation (6) or (7) as a result of system inefficiency. It therefore 
follows, that once a Mathieson process generator has been primed with 
acid, the reaction inefficiencies will sustain the required acidity at 
between 7N and 12N providing dilution water is added to control the rise 
in acidity. 
With a dry NaClO.sub.3 crystal feed an acid salt will be produced as a 
precipitate in the generator after cooling in an external vessel. This 
acid salt is a form that lies in the Na.sub.3 H(SO.sub.4).sub.2 phase of 
the equilibrium diagram. It is also known from the work of Pascal and Ero 
in Bulletin de la Societe Chimique (4) 25, 1919--page 44, that sulphuric 
acid of 30% by weight in a saturated solution of sodium sulphate up to 
90.degree. C. will produce a precipitate of sesqui salt. This point is 
demonstrated in FIG. 3 which plots the temperatures between 15.degree. C. 
and 97.degree. C. and shows only 1 anomaly at about 25.degree. C.

FIG. 1 illustrates an apparatus able to carry out the present process. The 
apparatus comprises a primary chlorine dioxide generator 2, a secondary 
chlorine dioxide generator 4 and an absorption column 6. Solid chlorate is 
fed from a hopper 8 into the primary generator 2 through a volumetric 
feeder 10. An air bleed through a line 12 allows a free flow of chlorate, 
that is it prevents crystal sticking or hanging up. Sulphuric acid at 
93-96% concentration is fed through line 14 and is used solely for startup 
in a Mathieson process generator 2 and at much reduced levels of 
concentration in Solvay and chloride reduction processes. The produced 
chlorine dioxide gas leaves the primary and secondary generators through 
line 16 to pass to the absorption column 6. Foam or liquid from generator 
2 passes along line 18 to the secondary chlorine dioxide generator 4 and 
liquid overflow from the secondary generator 4 passes through line 20 to 
discharge line 24 of a salt cake filter 26 and then into a standard 
recovery cycle. Vacuum for the salt cake filter 26 is provided by a steam 
eductor 28 through which steam passes to reduce pressure in a salt cake 
receiver 30 that communicates with filter 26 through a line 32. In the 
case of the Mathieson process dilution water is fed through line 34 to 
control acid normality. Filtrate from the filter 26 passes to the primary 
generator 2 through lines 36 and 38 by the action of pump 40. 
Dilution air is cycled from the absorption tower 6 exhaust gas through line 
42 under the influence of an exhaust fan 44. The amount of air is 
controlled by a chlorine dioxide gas analyzer 46 to ensure a chlorine 
dioxide partial pressure in the absorption column 6 between 0.05 and 0.2 
atmospheres. Dilution air is also fed through line 48 in generator 2 and 
the feed of this air is controlled by a valve 50. There is a take-off pipe 
52 so that dilution air may be mixed with sulphur dioxide gas passing 
along line 54. A mixture of sulphur dioxide and dilution air is fed to the 
primary and secondary chlorine dioxide generators at their bases through 
line 54. The quantity of dilution air is proportional to the reducing 
agent added, sulphur dioxide, sodium chloride or methanol and compliments 
recycled gas fed into the primary generator 2 through line 42 from 
absorption column 6. Control of the recycled gas in line 42 is controlled 
by a valve 56. Primary generator solution at approximately 70.degree. C. 
leaves the generator 2 through line 58. Generally the temperature will be 
in the range of 45.degree.-90.degree. C. and the generator solution is 
saturated with sulphate ion at between 7N and 12N sulphuric acid and 
contains dissolved chlorine dioxide and chlorine gases. The solution 
enters cooling vessel 60 and is chilled to approximetaly 40.degree. C. 
generally in the range 20.degree.-60.degree. C., dependent upon the 
primary generator liquid phase temperature. It should be noted that both 
the primary and the secondary chlorine dioxide generators are provided 
with temperature control coils 62 and 64. 
Cooling vessel 60 comprises a jacketted titanium tank with a conical bottom 
and a vented top to permit exhaust of chlorine dioxide and chlorine gases 
through line 66 into the main gas line 16. A paddle type agitator is 
mounted in cooling vessel 60 and has PTFE scrapers to prevent buildup of 
sulphate crystal on the interior walls of the vessel. Supernatant liquor 
from the cooling tank 60 flows through line 68 to the suction side of the 
filtrate recycle pump 40. 
The flow from the cooling tank 60 passes to salt cake filter 26 and the 
solids are discharged to the recovery section of the mill. The filtrate is 
recycled back to the chlorine dioxide generator through lines 32, 36 and 
38. 
The desired product is fed from the absorption column through line 70. 
FIG. 2 is a phase diagram indicating the above statement that it is 
difficult to isolate sodium sulphate from a mixture of sodium sulphate, 
water and sulphuric acid. As indicated generally the effluents all lie in 
that area of the phase diagram where trisodium hydrogen disulphate or the 
sesqui salt crystallize if the effluent is cooled. 
The following examples illustrate the invention. Experiments were conducted 
to check equilibrium diagram data as published by Seidell, "Solubilities 
of Inorganic and Metal Organic Compounds". 
EXAMPLE 1 
A batch of 9N H.sub.2 SO.sub.4 solution was prepared then saturated with 
Na.sub.2 SO.sub.4 at 70.degree. C. Sample 1A consisted of 100 mls of the 
above solution with 4 grams of NaClO.sub.3 added. The chlorate was stirred 
in and dissolved. The mass was cooled to 40.degree. C. and the sulphate 
salts filtered off and vacuum dried at 50.degree. C. 
The dried salt was then titrated for acidity, and found to contain 24.1% 
H.sub.2 SO.sub.4. 
Sample 1B--procedure as above, but with 6 gms NaClO.sub.3 in 100 mls of 
solution. Result 25.1% acid in salt. 
Sample 1C--as above, but with 8 gms of NaClO.sub.3 in 100 mls of solution. 
Result 24.6% acid in salt. 
Sample 1D--as above, but with 10 gms of NaClO.sub.3 in 100 mls of solution. 
Result 23.6% acid in salt. 
EXAMPLE 2 
Example 1 was repeated, utilizing a 70.degree. C. saturated salt cake 
solution in a 9N H.sub.2 SO.sub.4 medium. 
Sample 2A--was identical to Sample 1A in Example 1, except 1 ml. of 
CH.sub.3 OH was added at 70.degree. C. and ClO.sub.2 gas was evolved after 
consumption of the NaClO.sub.3 by the methanol. The sample was cooled to 
40.degree. C. and the sulphate salts filtered off and vacuum dried at 
50.degree. C. The dried salt was then titrated for acidity, and found to 
contain 21.3% H.sub.2 SO.sub.4. 
Sample 2B--procedure as above but with 6 gms NaClO.sub.3 and 1.5 mls 
CH.sub.3 OH. Result 21.3% acid in salt. 
Sample 2C--as above but with 8 gms NaClO.sub.3 and 2 mls CH.sub.3 OH Result 
22.0% H.sub.2 SO.sub.4 
Sample 2D--as above but with 10 gms NaClO.sub.3 and 2.5 mls CH.sub.3 OH. 
Result 22.4% H.sub.2 SO.sub.4 
The above experiments show beyond doubt that cooling of saturated sulphate 
solutions in 9N sulphuric acid down to 40.degree. C. produces an acid salt 
Na.sub.3 H(SO.sub.4).sub.2 containing 18.7% H.sub.2 SO.sub.4 with trace 
bisulphate (NaHSO.sub.4) impurities. 
Since this new process deals with evolution of chlorine dioxide at an 8-11 
acid normality and saturated sulphate conditions at 40.degree. C., a 
series of experiments was conducted to determine differences of ClO.sub.2 
evolution between existing processes with the same acid normalities. 
EXAMPLE 3 
Samples were prepared using a standard Mathieson ClO.sub.2 generator 
solution which is 24% W/W Na.sub.2 SO.sub.4 in a 9N H.sub.2 SO.sub.4 
solution. 
Four 100 ml. samples were measured out as follows: 
______________________________________ 
NaClO.sub.3 
CH.sub.3 OH 
Exotherm .degree.C. 
______________________________________ 
Sample 3A 2 g. 0.5 mls -0.5 
Sample 3B 4 g. 1.0 mls +5.5 
Sample 3C 6 g. 1.5 mls +13.5 
Sample 3D 8 g. 2.0 mls +20.0 
______________________________________ 
The starting temperature for each sample was 70.degree. C. and the exotherm 
represents the maximum temperature rise (or drop) when methanol was added 
to the sample after sodium chlorate was dissolved. 
The results obtained are plotted in FIG. 4, together with the results from 
the following Examples. 
EXAMPLE 4 
A half liter batch of 9N H.sub.2 SO.sub.4 solution was prepared, with 
Na.sub.2 SO.sub.4 saturated at 70.degree. C. 
Four 100 mil samples were measured out as follows: 
______________________________________ 
NaClO.sub.3 
CH.sub.3 OH 
Exotherm .degree.C. 
______________________________________ 
Sample 4A 2 g. 0.5 mls -2 
Sample 4B 4 g. 1.0 mls -1 
Sample 4C 6 gt 1.5 mls +3.5 
Sample 4D 8 g. 2.0 mls +11.0 
______________________________________ 
The starting temperature and conditions were the same as Example 3. 
EXAMPLE 5 
A sample was prepared exactly as Example 4 except for the following: 
The solution was cooled to 40.degree. C. and the salt filtered off. 
Five 100 mil samples were measured out as follows: 
______________________________________ 
NaClO.sub.3 
Ch.sub.3 OH 
Exotherm .degree.C. 
______________________________________ 
Sample 5A 2 g. 0.5 mls -1 
Sample 5B 4 g. 1.0 mls +0.5 
Sample 5C 6 g. 1.5 mls +7.5 
Sample 5D 8 g. 2.0 mls +14.5 
Sample 5E 10 g. 2.5 mls +19.0 
______________________________________ 
The starting temperature for each sample was 70.degree. C., the same as 
Example 4. 
EXAMPLE 6 
A sample was prepared exactly as Example 4 with a saturated sulphate 
solution in 9N sulphuric acid. 
______________________________________ 
NaClO.sub.3 
CH.sub.3 OH 
Exotherm .degree.C. 
______________________________________ 
Sample 6A 2 0.5 mls -2 
Sample 6B 4 1.0 mls -1.5 
Sample 6C 6 1.5 mls -1.0 
Sample 6D 8 2.0 mls +2.5 
Sample 6E 10 2.5 mls +12.5 
______________________________________ 
The starting temperature for each sample was 59.degree. C. 
SUMMARY 
Conclusion from Test Work 
Graphical results are tabulated in FIG. 4. It is apparent that higher 
sulphate concentrations in the generator acid inhibit the evolution of 
chlorine dioxide gas. To achieve comparable or improved reaction rates, 
the ClO.sub.2 generator temperature must be increased over the norm; the 
norm being typically 45.degree. C. for Mathieson, 60.degree. C. for 
Solvay, and 50.degree. C. for the chloride reduction process. 
Furthermore, sodium chlorate residuals must be increased to create a 
greater driving force with the appropriate reducing agent. From the graph 
it can be determined that an approximate increase of 20 gpl as NaClO.sub.3 
is necessary in the generator liquor to achieve the same exothermic 
temperature when considering Examples 1 and 5. 
Sodium Chlorate Losses in Salt Cake 
Since a greater driving force is required in the generator (when the salt 
content is increased) between the oxidizing and reducing agent, it might 
be considered important for the sesqui-salt cake to be as dry as possible 
in order to reduce entrained sodium chlorate losses. 
Experiments showed that sesqui-salt may be filtered readily to 75% W/W 
solids. 
With a sodium chlorate residual of 60 gpl in the generator, the losses 
would amount to 22 lbs. of NaClO.sub.3 per ton of ClO.sub.2. This equates 
to only a 0.6% overall loss of sodium chlorate. 
Absorption Tower Gas Recycle 
It was shown that the reaction rate to produce ClO.sub.2 is depressed as 
the temperature is lowered. To produce a comparable or increased 
production of ClO.sub.2, the driving force and reaction temperature must 
be increased. 
To overcome the potential problem of an increased ClO.sub.2 gas temperature 
and therefore an increased energy release whenever the gas decomposes or 
`puffs`, it is desirable to recycle the cold absorption tower off-gases 
back to the vapour phase of the generator. 
There is a four-fold advantage in adopting this technique, which takes 
advantage of the chilled water already entering the absorption system. 
1. The trace amounts of chlorine and chlorine dioxide gas emanating from 
the absorption column, have a further chance to be absorbed by recycling. 
2. By eliminating at least 50% of diluent air through the liquid mass of 
the ClO.sub.2 generator, a greater volume of reaction liquor is available 
for use; and in the case of the SO.sub.2 reduction process (Mathieson), a 
greater driving force is created between the SO.sub.2 gas and chloric acid 
in solution. 
3. Where side or secondary reactions occur to produce entrained acids in 
the gaseous phase, such as is the case in equations (6) and (7), these 
acids will be condensed by introducing cold recycled gas from the 
absorption column. 
4. Less energy will be used in compressing decreased diluent air through 
the generator sparger plates and liquid mass. 
Solid Sodium Chlorate Unloading and ClO.sub.2 Generator Feed 
The present practice of unloading crystal sodium chlorate from a tank car 
by using an equal weight of hot water at approximately 70.degree. C. will 
be eliminated. Furthermore, storage of the solution in holding tanks 
equipped with steam coils will not be necessary. 
The process of the invention calls for the unloading of sodium chlorate 
and/or chloride crystal from a tank car or truck using an air compressor, 
gas separation cyclone and storage bin. 
Feed to the ClO.sub.2 generator will be from the chlorate storage bin or 
hopper, into a volumetric feeder with accurate volume control, then into 
the ClO.sub.2 generator, as shown in FIG. 1.