Process for the production of chlorine dioxide

A process for production of chlorine dioxide by reacting in a reaction vessel an alkali metal chlorate, a mineral acid and methanol as a reducing agent in proportions to generate chlorine dioxide in a reaction medium maintained at a temperature from about 50.degree. C. to about 100.degree. C. and at an acidity within the interval from about 2 to about 11N and subjected to an subatmospheric pressure. Thereby water is evaporated and a mixture of chorine dioxide, water vapor and gaseous by-products is withdrawn from an evaporation region in the reaction vessel. The alkali metal sulphate is precipitated in a crystallization region in the reaction vessel. The content of formic acid in the reaction vessel is increased by addition of formic acid to a content of formic acid exceeding about 0.3M. The gaseous by-products are condensed to obtain formic acid and the content of formic acid in the reaction vessel is increased by recirculation of the condensate.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for the production of chlorine 
dioxide from an alkali metal chlorate, a mineral acid and methanol as a 
reducing agent. The process is carried out in a vessel operated under 
subatmospheric pressure, whereby water is evaporated and withdrawn 
together with chlorine dioxide and the alkali metal salt of the mineral 
acid is crystallized within the reaction vessel and withdrawn therefrom. 
According to the invention the efficiency of the process is improved by 
increasing the content of formic acid in the reaction vessel by addition 
of formic acid. 
Chlorine dioxide used as an aqueous solution is of considerable commercial 
interest, mainly in pulp bleaching but also in water purification, fat 
bleaching, removal of phenols from industrial wastes, etc. It is therefore 
desirable to provide processes by which chlorine dioxide can be 
efficiently produced. 
The predominant chemical reaction involved in such processes is summarized 
by the formula 
EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 
O [1] 
The chlorate ions are provided by alkali metal chlorate, preferably sodium 
chlorate, the chloride ions by alkali metal chloride, preferably sodium 
chloride, or by hydrogen chloride, and the hydrogen ions by mineral acids, 
normally sulfuric acid and/or hydrochloric acid. Processes for producing 
chlorine dioxide are described in e.g. U.S. Pat. Nos. 3,563,702 and 
3,864,456. 
In existing processes for production of ClO.sub.2 there is often also a 
by-product Cl.sub.2 formation, due to the use of chloride ions as reducing 
agent according to formula [1]. This chlorine by-product has formerly been 
used as such in the paper mills as a bleaching agent in aqueous solution. 
Today there is a tendency towards a more extensive chlorine dioxide 
bleaching for environmental reasons and thus there is a decreasing need 
for chlorine as a bleaching agent. 
It is also known to use other reducing agents, which do not produce 
chlorine as a by-product. In U.S. Pat. No. 3,933,988 sulfur dioxide is 
used as a reducing agent and in U.S. Pat. Nos. 4,081,520, 4,145,401, 
4,465,658 and 4,473,540 methanol is used as reducing agent. The methanol 
is very poorly utilized in a process according to e.g. U.S. Pat. No. 
4,465,658. The consumption of methanol is 190-200 kg/ton produced chlorine 
dioxide whereas the theoretical consumption is only 79 kg/ton according to 
the formula 
EQU 6NaClO.sub.3 +CH.sub.3 OH+4H.sub.2 SO.sub.4 .fwdarw.6ClO.sub.2 +CO.sub.2 
+5H.sub.2 O+2(Na.sub.3 H(SO.sub.4).sub.2 [ 2( 
Thus only about 40% of the methanol charged are used efficiently in 
existing processes. A thorough study of the reaction products from earlier 
known processes shows that parts of the added methanol leave the reactor 
without having reacted. This loss can be as high as 30 to 40 percent. 
However, the direct reaction between chlorate ions and methanol is very 
slow and the true reducing agent in this case is chloride ions reacting 
according to [1]. The produced chlorine then reacts with methanol to 
regenerate chloride ions according to the formula 
EQU CH.sub.3 OH+3Cl.sub.2 +H.sub.2 O.fwdarw.6Cl.sup.- +CO.sub.2 +6H.sup.+[ 1] 
It is therefore often necessary to continuously add a small amount of 
chloride ions in order to obtain a steady production. 
A more efficient process with methanol as a reducing agent is described in 
U.S. Pat. No. 4,770,868. According to this patent it appears that the 
methanol losses are strongly dependent on the mode of addition of the 
methanol to the reactor. According to the U.S. patent an improved yield is 
obtained by introducing the reducing agent in the crystalization zone of 
the reactor. 
The U.S. Pat No. 4,770,868 shows a considerably improved process, but 
losses of methanol are still obtained as a result of by-reactions. The 
main by-reaction takes place according to the following net formula: 
EQU 12NaClO.sub.3 +3CH.sub.3 OH+8H.sub.2 SO.sub.4 .fwdarw.12ClO.sub.2 
+4Na.sub.3 H(SO.sub.4).sub.2 +3HCOOH+9H.sub.2 O [4] 
The formed formic acid and the methanol which has not been consumed are 
by-products which only constitute losses in the system. According to known 
methods they are condensed together with formed water vapour and are added 
to the absorption tower for the chlorine dioxide absorption. The formic 
acid and the methanol which has not reacted are thus incorporated in the 
obtained chlorine dioxide water, which will give as a result that they 
after the chlorine on the waste water from the bleach plant. Another draw 
back with formic acid in the chlorine dioxide water is a reduced stability 
of the water. U.S. Pat. No. 4,770,868 suggests addition of small amounts 
of catalysts to influence the oxidation of methanol to carbon dioxide in a 
favourable way.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention, as it appears from the claims, provides an improved 
process with a considerable reduction of the losses in the form of 
by-products, without the use of catalysts. According to the process of the 
invention formic acid is added to the reactor. As a result thereof a 
higher steady state concentration of formic acid is obtained in the 
reactor. Surprisingly this leads to an increased conversion of methanol to 
carbon dioxide according to the formulae 2 (and 3). Thereby the 
consumption of methanol is reduced as the conversion of methanol to formic 
acid according to the by-reaction 4 is reduced. Besides, a more pure 
chlorine dioxide is obtained. 
A study of the relation between the content of formic acid in the reactor 
and the amount of formic acid leaving the reactor shows that the amount of 
formic acid leaving the reactor increases with increasing content of 
formic acid. However, from FIG. 1 it is evident that when the content in 
the reactor is increased above a certain concentration, the amount of 
produced formic acid will be strongly reduced. This fact indicates that 
the reaction producing formic acid is favoured at low concentrations, 
whereas at high concentrations the reaction consuming formic acid is 
favoured. By addition of formic acid to the reactor the concentration can 
be increased to the concentration range where the formic acid consuming 
reaction is dominating, whereby the conversion of methanol to carbon 
dioxide instead of to formic acid is increased. 
The concentration of formic acid in the reactor should be above about 0.3 
M, preferably above about 0.6 M. The upper limit for the concentration of 
formic acid depends on solubility and vapour pressure for the formic acid, 
i.e. it depends on the temperature at which the reactor is run. This 
temperature can easily be tried out. It has been found practical not to 
exceed 3.5 M. 
The addition of formic acid to the reactor can be made in the same way as 
the other chemicals. The formic acid can also be added by recirculation of 
the condensed by-products to the chlorine dioxide reactor. The unconsumed 
methanol is then also brought back to chlorine dioxide production 
resulting in a reduced methanol consumption. By recirculation of the 
condensed by-products these are reused in a useful way instead of loading 
the waste water. Recirculation of the condensed by-products is a preferred 
way of adding formic acid to the reactor. 
The process according to the invention is applied to those chlorine dioxide 
processes which are performed in one single reaction vessel, 
generator--evaporator--crystallizer, at a reduced pressure. At the process 
water is vaporized and withdrawn together with chlorine dioxide. The 
alkali metal salt of the mineral acid is crystallized and withdrawn from 
the reaction vessel. A suitable reactor is a SVP.RTM. (single vessel 
process) reactor. 
The invention is illustrated by means of the flow chart in FIG. 2 and the 
diagram of the condensation in FIG. 3. Dissolved alkali metal chlorate 2, 
methanol 3 and mineral acid 4 are added to the reactor R. Reactor gas 
leaves the reactor via the conduit 1 and is brought to a condenser 5. The 
reactor gas consists mainly of formed chlorine dioxide, water vapour, 
carbon dioxide and the by-products and formic acid and unreacted methanol. 
The conditions in the condenser are regulated so that the water vapour, 
the formic acid and the methanol are condensed and withdrawn as a flow 6 
to be brought back to the reactor. The chlorine dioxide gas is brought to 
an absorption tower A for the absorption of the chlorine dioxide in water. 
The temperature in the condenser should be between 8.degree. and 
40.degree. C. at an absolute pressure of between 90 and 400 mm Hg, a 
preferred temperature interval is between 12.degree. and 25.degree. C. and 
a preferred pressure is between 110 and 250 mm Hg. From the diagram in 
FIG. 3 it is evident that at these conditions the main part of the formic 
acid will condense together with water and methanol. However, it is only a 
very small part of the chlorine dioxide that will condense, e.g. at 
10.degree. C. only about 0.8%. Further, to reduce the amount of condensed 
chlorine dioxide in the condensate air can be blown in at 7 in FIG. 2, 
whereby any condensed chlorine dioxide is desorbed. 
The condensate of by-products can be brought back completely or partly to 
chlorine dioxide production. The condensate can be brought to the tank for 
chlorate dissolving and replace fresh water. The need for fresh water is 
then reduced, which can be an advantage when there is a problem with the 
purity of the fresh water. However, the recirculation of the formic acid 
results in an increased risk of corrosion as the chlorate solution becomes 
more acid. Therefore the condensate of by-products can suitably be 
concentrated and brought directly to the chlorine dioxide reactor or via 
the tank for methanol storage. Several different processes can be used for 
concentration, e.g. azeotropic distillation, adsorption or membrane 
separation as e.g. reverse osmosis and ultra filtration. 
The amount of condensate being recirculated can be varied within wide 
limits, from a small amount of the condensate to 100% of it. The degree of 
recirculation for the condensate is decided from such parameters as the 
amount of water vapour that is desirable to bring back to the reactor, and 
if a step for concentration of the condensate has been used. If the 
condensate is concentrated a larger amount can be recirculated without 
bringing back too much water to the system. 
The production of chlorine dioxide is carried out by continuous addition of 
the reactants to the reactor. The reaction is run at a temperature of 
50.degree.-100.degree. C., preferably 50.degree.-75.degree. C. and at a 
subatmospheric pressure, suitably at 60-400 mm Hg. The acid strength can 
be kept within a wide interval, between 2 and 11 N. When the acid strength 
is kept between about 2 and 4.8 the reaction can be carried out in the 
presence of a small amount of catalyst, which can be one or a combination 
of two or more metals chosen from the group antimony, molybdenum, 
technetium, ruthenium, rhodium, palladium, rhenium, osmium, iridium, 
platinum, or a combination of one or several of these with manganese or 
vanadium. Sulphuric acid or hydrochloric acid or mixtures of these are 
mineral acids suitable to use, but other mineral acids can also be used. 
If hydrochloric acid is not used as a mineral acid in the process it can 
be suitable to add a smaller amount of chloride ions, preferably in the 
form of alkali metal chloride, so that the concentration of the chloride 
ions in the reactor is within the interval from 0.001 and up to 0.8 moles 
per liter. The process is not limited to one of the alkali metals, but 
sodium is the one most preferred. 
The invention is illustrated by the following examples in which by parts 
and per cent are meant parts and per cent by weight if nothing else is 
said: 
EXAMPLE 1 
A chlorine dioxide reactor of the SVP type was run at boiling and 100 mm Hg 
and with additions of 320 g of NaClO.sub.3 /h, 5 g of NaCl/h and 196 g of 
H.sub.2 SO.sub.4 /h. A flow of 30 g/h of methanol was added. No 
recirculation of the condensate to the reactor took place. 202 g/h of 
ClO.sub.2, 7.5 g/h of CO.sub.2, 8.5 g/h of CH.sub.3 OH and 23 g/h of HCOOH 
left the reactor together with water vapour. 
EXAMPLE 2 
The same reaction conditions as in example 1 were used, but the leaving 
gases were condensed at 15.degree. C. A flow of condensate containing 19.3 
g/h of HCOOH and 4.1 g/h of CH.sub.3 OH was taken out. The concentration 
of formic acid was 41 g/1. 71 percent of the condensate were brought back 
to the tank for dissolving chlorate from which the condensate was added to 
the chlorine dioxide reactor. The total degree of recirculation for the 
formic acid was 60 percent. By recirculation of the condensate the 
addition of methanol could be reduced to 25 g/h or, with 17 percent, at 
the same time as the production of formic acid decreased with 60 percent 
and the departing gas flow contained 202 g/h of ClO.sub.2, 13 g/h of 
CO.sub.2, 6.1 g/h of CH.sub.3 OH and 9 g/h of HCOOH. The chlorine dioxide 
water from this example showed a higher stability than the one in example 
1, depending on higher purity.