INTEGRATED CHLORINE DIOXIDE PRODUCTION SYSTEM

SYSTEME INTEGRE POUR LA PRODUCTION DU DIOXYDE DE CHLORE

English Abstract

INTEGRATED CHLORINE DIOXIDE PRODUCING SYSTEM
ABSTRACT OF THE DISCLOSURE
Chlorine dioxide and chlorine are produced by
reduction of sodium chlorate with chloride ions in an
aqueous acid medium having a total acid normality of 2 to
4.8 normal. The reaction medium is maintained at its
boiling point under a subatmospheric pressure to provide
steam dilution of the chlorine dioxide and chlorine and
anhydrous neutral sodium sulphate is deposited therefrom.
Chlorine dioxide is recovered as an aqueous solution containing
some dissolved chlorine while the bulk of the chlorine is
recovered in gaseous form. Part of the recovered chlorine
is reacted with sulphur dioxide and water, preferably in
heat exchange relationship with the reaction medium to impart
heat of reaction thereto, to form hydrochloric acid and
sulphuric acid for feed to the reaction medium in sufficient
quantity to provide all the sulphate ions required at the
prevailing efficiency of the chlorine dioxide-producing reaction
to form neutral sodium sulphate from sodium ions introduced
to the reaction medium with the sodium chlorate The remain-
der of the hydrogen ion and chloride ion feed requirements of
the reaction medium are provided either by hydrogen chloride
or hydrochloric acid, preferably formed by reaction of
chlorine, including recovered chlorine, and hydrogen, or by a
mixture of 2 moles of sodium chloride and 1 mole of sulphuric
acid.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. A method for the steady state continuous production
of chlorine dioxide from an acid aqueous reaction medium by
reduction of chlorate ions with chloride ions, which comprises:
establishing an aqueous acid reaction medium
containing hydrogen ions, chlorate ions, chloride ions,
sodium ions and sulphate ions in a reaction zone capable
of forming chlorine dioxide and chlorine and having a total
acid normality of between about 2 and about 4.8 normal,
maintaining said reaction zone under a subatmospheric
pressure of about 20 to about 400 mm, maintaining said
reaction medium at its boiling point at an elevated temperature
in the range of about 25° to about 90°C and below that above
which substantial decomposition of chlorine dioxide occurs,
continuously generating chlorine dioxide and chlorine
from said aqueous reaction medium in accordance with the
equations:
ClO3- + Cl- + 2H+ ? ClO2 + 1/2Cl2 + H2O .... (1)
ClO3- + 5Cl- + 6H+ ? 3Cl2 + 3H2O .... (2)
the proportion of chlorate ions converted to chlorine
dioxide in accordance with-equation (1) being designated
the efficiency (E) of the chlorine dioxide and chlorine
generating reaction, said efficiency being less than 1.00,
continuously feeding to said reaction medium hydrogen
ions in a quantity of <IMG> moles per mole of chlorine
dioxide formed and chloride ions in a quantity of <IMG> moles
per mole of chlorine dioxide formed,
continuously feeding to said reaction medium sodium
chlorate in a quantity of <IMG> moles per mole of chlorine dioxide
formed and sulphate ions in a quantity of <IMG> moles per mole
of chlorine dioxide formed,
- 24 -

continuously evaporating water from said reaction
medium to form in said reaction zone with said generated
chlorine dioxide and chlorine a gaseous mixture of chlorine
dioxide, chlorine and water vapour,
continuously depositing from said aqueous reaction
medium <IMG> moles of anhydrous neutral sodium sulphate per
mole of chlorine dioxide formed,
continuously removing said gaseous mixture from
said reaction zone, continuously condensing the water
vapour of said removed gaseous mixture, continuously
forming an aqueous solution of substantially all the
chlorine dioxide content of said gaseous mixture and
additionally optionally containing a minor proportion of
the chlorine content of said gaseous mixture,
continuously recovering said aqueous solution
of chlorine dioxide,
reacting sulphur dioxide and water with chlorine
including at least part of substantially all the remainder
in said gaseous mixture not dissolved in said aqueous
chlorine dioxide solution to form hydrogen chloride and
sulphuric acid in just sufficient quantity to provide <IMG>
moles of sulphate ion in sulphuric acid,
continuously forwarding said hydrochloric acid
and sulphuric acid formed in the latter reaction to said
reaction medium to provide part only of both said fed
hydrogen ions and chloride ions and all of said fed
sulphate ions,
continuously supplementing the hydrogen ion and
chloride ion quantities of said forwarded stream to provide
said feed of hydrogen ions of <IMG> moles per mole of chlorine
dioxide formed and said feed of chlorine ions of <IMG> moles
- 25 -

per mole of chlorine dioxide formed, and
removing said deposited anhydrous neutral sodium
sulphate from said reaction zone.
2. The method of claim 1, wherein said continuous
supplementing of the hydrogen ion and chloride ion quantities
of said forwarded stream is achieved using <IMG> moles of
hydrogen chloride per mole of chlorine dioxide formed.
3. The method of claim 2 wherein said hydrogen chloride
is continuously formed by reaction of hydrogen and chlorine,
said chlorine being constituted at least in part by the
portion of the remainder of said chlorine in said gaseous
mixture not dissolved in said aqueous chlorine dioxide
solution.
4. The method of claim 3, wherein part of said chlorine
in said gaseous mixture is dissolved in said chlorine dioxide
solution, and all said remainder of said chlorine not
dissolved in said chlorine dioxide solution together with any
chlorine required from an external source is reacted with
said sulphur dioxide and said hydrogen to produce from said
reaction with sulphur dioxide said hydrochloric and sulphuric
acid in just sufficient quantity to provide <IMG> moles of
sulphate ion in said sulphuric acid and from said reaction
with hydrogen said <IMG> moles of hydrogen chloride.
5. The method of claim 2, wherein the hydrogen chloride
is in the form of hydrochloric acid.
6. The method of claim 1, wherein said continuous
supplementing of the hydrogen ion and chloride ion quantities
of said forwarded stream is achieved using sodium chloride
and sulphuric acid in the molar ratio of 2:1 and in quantities
- 26 -

continuously to provide <IMG> moles of both hydrogen ions and
chloride ions per mole of chlorine dioxide formed.
7. The method of claim 1 wherein said continuous
supplementing of the hydrogen ion and chloride ion quantities
of said forwarded stream is achieved using hydrogen chloride,
sodium chloride and sulphuric acid in quantities continuously
to provide <IMG> moles of both hydrogen ions and chloride
ions per mole of chlorine dioxide formed and 2 moles of
sodium ions for each mole of sulphate ions.
- 27 -

8. The method of claim 1 including providing said
reaction of chlorine and sulphur dioxide in heat exchange
relationship with said reaction medium to provide continuously
at least part of the heat required to maintain said reaction
medium at said boiling temperature.
9. The method of claim 8 wherein said heat exchange
relationship is achieved by establishing a second reaction
zone in which said sulphur dioxide, chlorine and water react
in exothermic manner to form said hydrochloric acid and said
sulphuric acid, said second reaction zone being physically
separate from said reaction zone, carrying out said removal
of said deposited solid material from said reaction zone in
a slurry with spent reaction medium, separating substantially
completely said deposited solid material from a part only of
said slurry, continuously incorporating said chlorate feed
for said reaction medium in the remainder of said slurry,
continuously passing the resulting combined aqueous solution
into heat exchange relationship with said second reaction zone
thereby to heat said resulting aqueous solution with the heat
of reaction provided in said second reaction zone, continuously
adding said forwarded stream of hydrochloric acid and sulphuric
acid to said heated aqueous solution, continuously supplementing
the hydrogen ion and chloride ion quantities of the resulting
solution at a stage subsequent to said separation of solid
material, and continuously cycling the feed solution resulting
from said adding and supplementing to said reaction zone
as said feed stream and to provide said continuous feed of
chlorate ions to said reaction zone.
10. The method of claim 8 wherein said heat exchange
relationship is achieved by establishing a second reaction
zone in which said sulphur dioxide, chlorine and water react
tin exothermic manner to form said hydrochloric acid and said
sulphuric acid, said second reaction zone being located within
- 28 -

said first reaction zone in direct or indirect heat exchange
relationship with said reaction medium.
- 29 -

11. The method of claim 1 wherein said reaction
of chlorine and sulphur dioxide occurs in a reactant feed
line for said reaction zone wherein substantially complete
reaction between said chlorine, sulphur dioxide and steam
occurs prior to injection of the reaction products directly
into the generator.
CLAIM SUPPORTED BY SUPPLEMENTARY DISCLOSURE
12. A method for the steady state continuous production
of chlorine dioxide from an acid aqueous reaction medium by
reduction of chlorate ions with chloride ions, which
comprises:
establishing an aqueous acid reaction medium
containing hydrogen ions, chlorate ions, chloride ions,
sodium ions and sulphate ions in a reaction zone capable
of forming chlorine dioxide and chlorine and having a
total acid normality of between about 2 and about 4.8
normal,
maintaining said reaction zone under a subatmospheric
pressure of about 20 to about 400 mm Hg, maintaining said
reaction medium at its boiling point at an elevated temperature
of about 25° to about 90°C and below that above which
substantial decomposition of chlorine dioxide occurs,
continuously generating chlorine dioxide and
chlorine from said aqueous reaction medium in accordance
with the equations:
ClO3- + Cl- + 2H+ ? ClO2 + 1/2Cl2 + H2O ....(1)
ClO3- + 5Cl- + 6H+ ? 3Cl2 + 3H2O ,...(2)
the proportion of chlorate ions connected to chlorine
dioxide in accordance with equation (1) being designated
the efficiency (E) of the chlorine dioxide and chlorine
generating reaction, said efficiency being less than 1.00,
- 30 -

continuously feeding to said reaction medium
hydrogen ions in a quantity of <IMG> moles per mole of
chlorine dioxide formed and chloride ions in a quantity
of <IMG> moles per mole of chlorine dioxide formed,
continuously feeding to said reaction medium sodium
chlorate in a quantity of <IMG> moles per mole of chlorine dioxide
- 30a -

formed and sulphate ions in a quantity of at least <IMG>
moles per mole of chlorine dioxide formed,
continuously evaporating water from said reaction
medium to form in said reaction zone with said generated
chlorine dioxide and chlorine a gaseous mixture of chlorine
dioxide, chlorine and water vapor,
continuously depositing from said aqueous
reaction medium at least <IMG> moles of anhydrous neutral
sodium sulphate per mole of chlorine dioxide formed,
continuously removing said gaseous mixture from
said reaction zone, continuously condensing the water
vapour from said gaseous mixture, continuously forming an
aqueous solution of substantially all the chlorine dioxide
content of said gaseous mixture and additionally optionally
containing a minor proportion of the chlorine content of
said gaseous mixture,
continuously recovering said aqueous solution of
chlorine dioxide,
reacting sulphur dioxide and water with chlorine
including at least part of substantially all the remainder
in said gaseous mixture not dissolved in said aqueous
chlorine dioxide solution to form hydrochloric acid and
sulphuric acid in no more than just sufficient quantity to
provide <IMG> moles of sulphate ion in said sulphuric acid per
mole of chlorine dioxide formed,
continuously forwarding said hydrochloric acid and
sulphuric acid formed in the latter reaction to said reaction
medium to provide part only of both said fed hydrogen ions
and chloride ions and <IMG> moles of fed sulphate ion,
continuously supplementing the hydrogen ion and
chloride ion quantities of said forwarded stream to provide
- 31 -

said feed of hydrogen ions of <IMG> moles per mole of chlorine
dioxide formed and said feed of chloride ions of <IMG> moles
per mole of chlorine dioxide formed, and
removing said deposited anhydrous neutral sodium
sulphate from said reaction zone.
- 32 -

Description

1053877
The present invention is directed to the produc-tion
of chlorine dioxide.
Chlorine dioxide is a valuable chemical, typically
used in aqueous solution form in the bleaching of cellulosic -
fibrous material pulps, and is ~nown to be formed in a number
of ways, generally involving the reduction of a chlorate by
chloride in an acid medium.
The basic reaction involved in such processes is ;
summarized by the equation: -
C103 + Cl + 2H > C102 + 1/2 C12 + H20 ............. (1)
Commonly, the chlorate ions are provided by sodium chlorate,
the chloride ions by sodium chloride and/or hydrogen chloride
and the hydrogen ions by sulphuric acid and/or hydrochloric
acid. Along with the reaction depicted by equation (1), there
is a competing reaction which decreases the efficiency of the
conversion o~ chlorate ions to chlorine dioxide, represented
by the following equation 2:
C103 + 6H + 5Cl > 3C12 + 3H20 .................. (2~
In one known manner of producing chlorine dioxide, ;~- -
as set forth in more detail in Canadian Patent 826,577, the
aqueous reaction medium containing sodium chlorate, sodium
chloride and/or hydrogen chloride and sulphuric acid at an -
acidity of about 2 to 4.8N, is maintained at its boiling point
while the reaction vessel is maintained under a subatmospheric
pressure, resulting in evaporation of water from the reaction -
medium, removal of the chlorine dioxide and chlorine in gaseous
admixture with the steam and deposition of solid by-products,
mainly anhydrous sodium sulphate, in the reaction vessel. The
reaction is carried out at an`elevated temperature below that
,~.
- 2 -
_, , ,,, , ~ : :

-~ 105~77
above which substantial decomposition of chlorine dioxide
would occur. The gaseous mixture of chlorine dioxide, chlorine
and steam then is processed to condense the steam and recover
an aqueous solution of chlorine dioxide, which usually
contains some dissolved chlorine.
The sodium sulphate deposited as by-product is
removed from the reaction vessel, continuously or inter-
mittently, and is utilized as make-up chemical in the
recovery cycle of a pulp mill of any convenient type, typic-
ally a Kraft pulp mill
In a Kraft pulp mill operation, wood chips or other
raw cellulosic fibrous material are digested with a white
liquor containing sodium sulphide and sodium hydroxide as
the active pulping chemicals to form a wood pulp which is
separated from spent pulping liquor, otherwise known as black
liquor.
The pulp thereafter usually is subjected to ,
bleaching and purification operations in a bleach plant to
form a pulp of desired brightness, the bleaching steps
typically utilizing the chlorine dioxide solution produced ~-~
in the chlorine dioxide-producing system. -
The black liquor is subjected to recovery and re- -
generation operations to form fresh white liguor for recycle
to the digestion stage to provide at least part of the white
liquor used therein. The recovery and regeneration operations
generally involve an initial concentration of the black liquor
followed by burning in a furnace to form a smelt containing
sodium sulphide and sodium carbonate. The smelt is dissolved
in water to form an aqueous solution thereof, known as green
liquor, which, after clarification to remove undissolved
solids, is causticized with slaked lime resulting in the
- ~:
-- 3 --
. . . . .

L053877
conversion of carbonate to hydroxide, with consequent deposi~ `
tion of calcium carbonate. The calcium carbonate is
recovered and usually is used to form further slaked lime.
The aqueous solution resulting from separation of the
calcium carbonate therefrom is the rec~cled white liquor.
The sodium sulphate recovered from the chlorine
dioxide generator is used as a source of make-up chemicals
for this system, to make up losses of soda and sulphur
values. The quantity of make-up chemical required varies
from mill to mill, although in most cases it is less than the
chlorine dioxide requirement, due to tightening up of the
mill recovery system, with less losses thus being experienced
The above-mentioned chlorine dioxide-producing
procedure provides about 1 mole of sodium sulphate and
1/2 to l mole of chlorine for each mole of chlorine dioxide
formed. With the decrease in losses of sodium and sulphur
values, the prior art process typically produces more sodium
sulphate than can be used by the mill and hence the excess
accumulates.
Further, with the tendency to use less quantities `~ -
of chlorine in bleaching, excess quantities of chlorine gas
may be produced.
In accordance with the present invention, chlorine,
including the by-product chlorine is reacted in a known
` exothermic reaction with sulphur dioxide, in accordance with
the equation:
2 2 + 2H2O ~ H2SO4 + 2HCl .. (3)
The h~drogen ions and chloride ions formed in this reaction
are utili~ed to provide at least part of the hydrogen ion
and/or chloride ion requirements of the chlorine dioxide-
, .
: :
.~

` l~S3~7
proaucing reaction.
Due to the inefficiencies introduced to the chlorine
dioxide-producing process by the competing reaction according
to equation ~2~, imbalances result from feed of the products
of equation (3) and these must be compensated for if a stable
stead~ state chlorine dioxide- forming procedure is to be ;
maintained. The present invention achieves such compensation
while at the same time decreasing the quanti~y of sodium
sulphate produced per mole of chlorine dioxide.
:-
The extent to which equation (1) predominates over
equation (2) in the reduction of chlorate ions represents the
efficienc~ of chlorine dioxide production, which may be desig-
nated E. E is expressed in a decimal less than 1.00, corres-
ponding to the % conversion of chlorate to chlorine dioxide
~y equation (1), the value 1.00 representing 100% conversion or
efficiency, 0.90 representing 90~ conversion or efficiency, !
and so on. The efficiency E may be controlled by controlling
the chloride to chlorate mole ratio in the generator or by the -
use of chlorine dioxide formation-promoting catalysts.
Thus, it is well known that the lower the chloride
to chlorate mole ratio in the feed, the more efficient is
the chlorine dioxide-producing process, up to the inherent
limits of efficiency of the particular system.
Several chlorine dioxide formation-promoting
catalysts are ]cnown which increase the efficiency of production
of the chlorine dioxide. Typical of the catalysts is silver
ions.
The hydrogen ion requirement for a chlorine dioxide-
producing system to produce 1 mole of chlorine dioxide depends
on the overall efficiency (E), and from equations (1) and (2),
_ 5 _
.- - .. ~ . .. ... . . . .

- 1053877
is represented as follows:
H require~ent = 6-4E moles H /mole C102 formed
E
Similarly, the chloride ion requirement per mole of chlorine
dioxide formed is represented as follows:
Cl re~uirement = 5-4E moles Cl /mole C102 formed
~ E
Further, the quantity of chlorine produced from the reaction
medium is represented as follows:
C12 produced = 6-5E moles C12/mole C102
The chlorate feed requirement is 1 moles/mole C102
produced and hence the mole ratio of Cl :C103 in the feed
is 5-~E:l. Hence, for all efficiencies les~ than 1, the
mole ratio of Cl :C103 is always greater than 1:1. It is
preferred to operate as close as possible to a 1:1 mole ratio
in the feed. :
Since the chlorate is added typically as the sodium
salt and sodium sulphate is the sole salt precipitated from
~he reaction medium, the quantity of sulphate ion fed to the
reaction medium should be controlled to provide :he 1 moles
2E
of sulphate ions required for reaction with the 1 moles of
sodium ions introduced with sodium chlorate.
There are thus a number a constraints on the
chlorlne dioxide-generating system which must be carefully
observed if a steady state continuous production of chlorine
dioxide is to be maintained at any given efficiency. It
would normally be preferred, of course, to operate the system
at the highest possible efficiency to obtain as high a
conversion as possible of chlorate to chlorine dioxide by
the reaction of equation (1~. Nevertheless, the constraints
apply irrespective of the efficiency (E), and are as follows
per mole of chlorine dioxide formed: -
-- 6 --
- , , , . . . -
.
- ~ ,
: . . .
..

1053l~77
H feed = 6-4E moles
Cl feed = 5-4E moles
C10 feed = 1 moles
3 E :
H :Cl mole ratio = 6-4E : 1
_ _ 5-4~
Cl :C103 mole ratio in the feed = 5-gE : 1
S0~ feed = 1 moles
Feeds in these quantities produce 1 mole C102 and
6-5E moles C12.
2E
The reaction of chlorine and sulphur dioxide by
equation ~3~ produces Hf and Cl ions in a mole ratio o~ 2 to 1.
It will be seen from the above equations that the mole ratio
feed n~irement of H :Cl to the reaction medium is always
less than 2 to 1 whenever the efficiency (E) is less than 1,
which is the usual circumstance.- It also follows that the
lower the efficiency, the lower is the H :Cl mole ratio
required.
Thus, direct feed of the products of the reaction
of sulphur dioxide and chlorine by equation ~3) to the
reaction medium introduces imbalances to the system and
upsets its steady state operation. Thus, either ~he qùantity of
hydrogen lon is too high while the chloride ion quantity is
correct or quantity of chlorine ion is too low while the
~uantity of hydrogen ion is correct. Either condition is -
unsatisfactory. In the first instances, the acidity of the
reaction medium will increase leading to an increased rate of
reaction which in turn leads to a more rapidly increasing
acidity which itself leads to an even faster reaction rate.
The reaction thus will tend to get out of control and possibly
lead to explosion. The acidity of the reaction ~edium also
will achieve a value at which acid sulphate is deposited.
While the acid sulphate deposition removes acid from the
.. ' ' ' .
-- 7

i(~S3~
reaction medium and hence would help to control the acid
level, nevertheless the use of sodium acid sulphate in a pulp
mill is not desirable because of the loss of acid with the
salt.
In the second instance, ~hile the decreased chloride
to chlorate mole ratio initially leads to an increased
efficiency, the necessity to ~eep the acid feed level correct
leads to decreasing feeds of chloride ion which in turn leads
to a slowing down and eventual stopping of the chlorine dioxide~
producing reaction.
Additionally, if all the chlorine which is produced
by the reaction of equations (1) and (2) along with the
chlorine dioxide is recovered and reaction of that recovered
chlorine with sulphur dioxide in equation (3~ proceeds to com-
pletion, then the quantity of hydrogen ion available from that
reaction per mole of chlorine dioxide produced is as follows:
H available = 12-lOE moles - per mole of C102
and similarly the chloride ion availability i5 as follows:
Cl available = 6-5E moles - per mole of C102
Thus, the availability of hydrogen ions and chloride ions
provided by the reaction of the recovered chlorine with sulphur
dioxide exceed both the hydrogen ion and chloride ion require-
ments for the production of chlorine dioxide, except in the
remote case when the efficiency of conversion of chlorate to
chlorine aioxide is 100~.
However, some of the chlorine produced by equations
(1~ and (2) usually is dissolved in the chlorine dioxide
solution and therefore is not normally recoverable. This
usually amounts to 20 to ~0% of the chlorine present in the
gaseous reactants stream removed from the chlorine dioxide
generator.
- ; . . ...

1~538~7
The loss of chlorine in ~he chlorine dioxide
solution thus considerably decreases the chlorine available
from the chlorine ~ioxide-producing reaction to react with
sulphur dioxide and this may result in an insufficient
quantity of recovered chlorine, necessitating a feed of
external chlorine, as discussed in more detail below.
Another important consideration is that the quantity
- of chlorine reacted with sulphur dioxide must be restricted
to the quantity which produces just sufficient sulphate ion
to combine with the sodium ions introduced to the reaction
medium with the sodium chlorate.
In addition to ~he many constraints on the parameters
of the chlorine dioxide producing reaction, therefore, there
are constraints on the sulphur dioxide and chlorine reaction
to provide hydrochloric acid and sulphuric acid which must be
observed if a steady state chlorine dioxide-generating process
is to result.
These constraints are:
i~ produce just sufficient sulphate ion to compen-
sate for the quantity of sodium ions introduced to the reaction
medium,
ii) react less than all the chlorine produced in
the chlorine dioxide-producing reaction with sulphur dioxide
to compensate for the excess quantities o~ hydrogen ion and
chloride ion otherwise provided,
~ iii) compensate for losses of produced chlorine
in the chlorine dioxide solution ~o provide the required
quanti~ies of acid and chloride, and
iv) compensate for the incorrect mole ratio of
hydrogen ion to chloride ion to provide tha~ required in the
feed to the chlorine dioxide-producing reaction medium at its
prevailing efficiency.
.
.. . . ~ - .; ~ - , . , - ~.......... .

~053~77
Thus, in order to provide a chlorine dioxide
generating system in which chlorine produced in the generation
reaction is reacted with sulphur dioxide and the products of
the reaction are fed to the generator and in which steady
state conditions are maintained throughout there are a large
number of parameters which have to be balanced at the
prevailing efficiency of the chlorine dioxide-producing
reaction.
Because of the numerous constraints on the system
as discussed fully above, the options open for achieving the
suitable adjustment of the feeds are strictly limited. If
the options discussed in more detail below are not adopted,
then the system is imbalanced and non-steady state operations
result with consequently non-bene~icial results.
In accordance with this invention, the reaction
of sulphur dioxide with chlorine produced by the chlorine
dioxide generation reaction is used to produce just sufficient
sulphate ions equivalent to the quantity of sodium ions
introduced with the sodium chlorate to produce neutral sodium
sulphate. As mentioned above, the quantity of sodium ion
introduced with sodium chlorate is 1 moles/mole of chlorine
dioxide and hence the quantity of sulphate ions required is
1 moles/mole of chlorine dioxide.
2E
It follows from equation (3) above that if just
1 moles o~ sulphate ion are to be formed, then the following
2E
molar quantities of hydrogen ion and chloride ion only are
formed by the reaction:
H = 2 moles/mole ClO2
Cl = 1 moles/mole ClO2
From the recitation of hydrogen ion and chloride ion require-
ments above, it will be seen that-the following additional
-- 10 --
i
, .. . .

~` lOS38~7
quantities of hydrogen ion and chloride ion need to be
provided to supplement those provided by the reaction of
sulphur dioxide with chlorine if the feed requirements of
these species to the chlorine dio~ide-generating reaction
medium are to be met,
H required = 6-4E 2 - 4-4E moles/mole ClO
_ _ 2
E E E
Cl required = 5-4 _ 1 = 4-4E moles/mole C1O2
E E E
The supplemental requirements of hydrogen ion and
chloride ions, therefore, are the same, and, in accordance
with one of the options provided in this invention, are
provided by hydrogen chloride or hydrochloric acid, which
provides 1 mole of hydrogen ion for each mole of chloride
ion.
In a preferred aspect of this option, the hydrogen
chloride or hydrochloric acid used in the supplementation of
the feed is formed by reaction of a further portion of the
chlorine formed in the chlorine dioxide generator with hydrogen
to provide the required quantity of hydrogen chloride. :~
The total quantity of chlorine reacted in accordance
with this preferred aspect of the invention in the reactions
with sulphur dioxide and hydrogen thus is: -
4 4E = ~-4E moles/mole ClO2
However, the quantity of chlorine formed in the
chlorine dioxide-producing reaction is 6-5E moles/mole ~lO :
2E
and hence the quantity of chlorine produced always exceeds
the chlorine requirement by a quantity of l-E moles/mole ClO2,
- except in the unlikely condition of 100% efficiency.
As mentioned above, however, usually between about
20 and 40% of the chlorine produced is dissolved in the
chlorine dioxide solution during the recovery of chlorine
dioxide and hence is not available for reaction with sulphur
-- 11 --

105387'7
dioxide or hydrogen. Where this quantity exceeds l-E moles/
2E
mole ClO2, therefore, an external feed of chlorine is
required to compensate for such loss. Since substantially
complete separation of chlorine dioxide and chlorine usuall~
is not achieved, an e~ternal source of chlorine normally is
required in this preferred aspect of the invention.
The hydrogen chloride or hydrochloric acid may be
mixed with the products of the reaction of sulphur dioxide
and chlorine prior to feed to the chlorine dioxiae-producing
reaction medium or the various materials may bè separately
fed to the reaction medium.
The second option which may be used to provide the
required adjustment is to use sodium chloride, as a solid
or as an aqueous solution thereof, to provide the supplemental
chloride ion requirement. Since 1 mole of sodium ions is
introduced to the reaction medium for each mole of chloride ion,
equivalent adjustment of the sulphate ion feed is required to
provide sufficient sulphate ions to form sodium sulphate
from the sodium ions introduced with the sodium chloride.
There is also required suitable adjustment of the hydrogen
ion feed and, in this option, both the hydrogen ion adjustment
and the additional sulphate ion re~uirement are provided by
external sulphuric acid.
Since 1 mole of sulphate ion is required for each
mole of sodium ion added and 1 mole of chloride ion supplement
is required for each mole of hydrogen ion supplement, it
follows that, in this second option, the external sodium
chloride and sulphuric acid feeds are used in a molar ratio
of 2:1.
The external feeds each may be added to the products
of reaction of sulphur dioxidé and chlorine prior to feed
thereof to the chlorine dioxide-producing reaction medium.
- - 12 -
: . . . ~ .

lil~S3877
Alternatively, one only of the external feeds may be added to
the products of reaction of sulphur dioxide and chlorine
prior to f~ed thereof to the chlorine dioxide-producing
reaction medium, while the other of the external feeds is
fed directly to the reaction medium. Further, each o the
feeds to the reaction medium may be fed separately thereto.
As a third option, part of the required hydrogen
and chloride ion may be provided by hydrogen chloride or hy-
drochloric acid, and part by a mixture of sodium chloride
and sulphuric acid.
These three options are the only ones available
if a steady state reaction is to be maintained. Attempts to
use other superficially-obvious alternatives leads to imbalance.
For example, in the second option above, if it
were attempted to produce the additional sulphate ion require-
ment by the reaction of sulphur dioxide and chlorine, failure
would result since the reaction also produces chloride ions,
thereby requiring a suitable decrease in the quantity of
supplemental sodium chloride, which in turn would lead to
a decrease in the quantity of sodium ions, so that the
quantity of sulphate ions then would exceed the required
quantity.
Therefore, it is essential in the present invention,
to react just sufficient chlorine with sulphur dioxide to
produce sulphate ions in a quantity to form neutral sodium
sulphate (Na2SO4~ from the sodium ion fed to the chlorine
dioxide-producing reaction medium with sodium chlorate, and~
to provide supplementary equimolar quantities of hydrogen and
chloride ions to the products of the sulphur dioxide and
chlorine reaction to give the total feed requirements of these~
ions, such supplementary equimolar quantities being provided
- 13 -
. .

1~53~77
either by hydrogen chloride or by a mixture of 2 molar
proportions of sodium chloride to each molar proportion of
sulphuric acid.
As mentioned previously, the reaction of sulphur
dioxide and chlorine is an exothermic one and, in accordance
with a further aspect of the present invention, the heat
generated in the exothermic reaction is utilized to provide
at least part of the heat required to maintain the chlorine
dioxide-producing reaction medium at its boiling temperature.
This may be achieved by providing the reactor in which the
sulphur dioxide and chlorine react in heat exchange relation-
ship with the reaction medium.
Where the exothermic reaction of chlorine and sulphur
dioxide is insufficient to provide all the heat req~irement of
the chlorine dioxide-producing reaction medium, addltional
quantities of the total heat requirement may be provided,
for exàmple, from the heat value of steam from an ejector used
to maintain the generator under a subatmospheric pressure
where such an operation is used, from the heat of dilution
of the acid produce~ by the reaction of the sulphur dioxide
and chlorine when such acid is added to the reaction medium,
and from the heat of reaction of hydrogen and chlorine where
such reaction occurs.
The invention is described further by way of illu5-
tration with reference to the accompanying drawings, in which:
Figure 1 is a schematic flow sheet of one embodiment
of the invention; and
Figure 2 is a schem~tic flow sheet of a second
emboaiment of the invention.
Referring first to Figure 1, there is illustrated
a chlorine dioxide producing system 10 including a generator
- 14
'
':

1053877
12. In the generator 12, there is present a chlorine
dioxide-producing reaction medium containing sodium, chloride,
chlorate, sulphate and hydrogen ionic species. The reaction
medium is maintained at its boiling point while the vessel is
maintainea under a vacuum. The temperature and vacuum may
vary widely, typically from 25 t:o 90C at a pressure of 20
to 400 mm mercury absolute. Air is bled into the generator
- by line 13 to provide the absolute pressure in the generator.
The boiling temperature of the reaction medium
causes water to be evaporated therefrom which dilutes the ~-
chlorine dioxide and chlorine formed from the reaction medium.
The resultant gaseous mixture is removed from the reaction
zone by line 14. When after initial start-up the reaction
medium becomes saturated with sodium sulphate, anhydrous
neutral sodium sulphate precipitates from the reaction medium.
The generator 12 is run in a substantially continu-
ous manner with the spent chemicals of the reaction medium -
being replenished continuously, in this embodiment, by an
aqueous feed solution in line 16 containing sodium chlorate,
sodium chloride, hydrochloric acid and sulphuric acid, and the
liquid input being balanced by liquid and steam output so
that the liquid level in the generator 12 remains substantially~
constant.
The precipitated sodium sulphate is removed, contin-
uously or intermittently, from the generator 12, as a slurry
with some spent reaction medium by line 18. The removed slurr~
is partly passed by line 19 to a separator 20 wherein the
crystalline sodium sulphate is separated from the mother
liquor and is recovered therefrom by line 22 as one of the
products of the system 10 for use in pulp mill operations.
The remainder of the slurry in line 19 is recycled to the

~1053877
generator 12 in a recirculation loop, described in more
detail below.
The mother liquor from the separation of the solid
phase in the separator 20 is combined with the remainder of
the slurry in line 21 and is pas~ed by lir.es 24 and 26, after
- the addition of an aqueous solution of sodium chlorate and
sodium chloride by line 28 containing sufficient quantities
of these materials to make up the quantities consumed in the
generator 12, to a heat exchanger 30 in the recirculation
loop and associated with a reactor 32.
The heat exchanger 30 and the reactor 32 may be
constituted by the shell side and tube side respectively of
a conventional heat exchange apparatus, or any other con-
venient apparatus.
From the heat exchanger 30, the heated solution
passes by line 34 to a reboiler 36 wherein any additional
heat required to increase the temperature of the aqueous l i
solution in line 34 to that required in the generator 12 may
be provided. me reboiler may be omitted in Qses where sufficient
heat is available from other sources to provide a recirculating
feed liquor of required temperature.
The solution then passes by lines 38, 40 and 16,
which complete the recirculation loop, to the generator 12,
after addition of a mixture of hydrochloric and sulphuric acids
by line 42 and hydrochloric acid by line 44. The heat of
dilution of the acids in lines 42 and 44 also increases the
heat content of the feed solution in line 16.
The gaseous mixture of chlorine dioxide, chlorine
and steam in line 14 is passed, after partial condensation of
tfie steam, if desired, to a chlorine dioxide absorption towex
46 to which water is fed by line 48 to condense the remaining
- 16 -

~1~538~7
steam and to form an aqueous solution of chlorine dioxide
which is removed fro~ the absorption tower 46 by line 50.
Usually a certain quantity of the chlorine present
in the gaseous mixture in line 14 is dissolved in the chlorine
dioxide solution during the absorption procedure. The concen-
tration of dissolved chlorine present in the chlorine dioxide
solution may be decreased by air stripping chlorine from the
aqueous solution of chlorine dioxide in line 50 by air fed . ~-
by line 52 to a chlorine stripper 54. The chlorine stripper
may be omitted if the concentration of chlorine in the
aqueous solution in line 50 may be tolerated in the end use of
the chlorine dioxide solution.
The aqueous solution of chlorine dioxide resulting
from the stripping operation is recovered by line 56 as the
other product of the system 10. The mixture of air and
chlorine also resulting from the stripping, together with any
chlorine dioxide stripped from the aqueous solution line
50, is forwarded by line 58 to the absorption tower 46.
The gaseous mixture of chlorine and air resulting
from the separation of the chlorine dioxide in the absorption
tower 46 is passed to an ejector 60 by line 62, with an air
bleed thereto, if desired, by line 64. Steam is fed to the
ejector 60 by line 66 to maintain the generator 12 under the
desired vacuum and to form with the chlorine in line 62 a
gaseous mixture in line 68 of chlorine, steam and air. Any
- other convenient vacuum-inducing means may be used to maintain
the vacuum in the generator 12.
A portion of the gaseous mixture of chlorine, steam~
and air passes by line 70 to the reactor 32 in the quantity
desired to react with sulphur dioxide to provide the
sulphate ion requirement of the generator 12. The remainder:
- 17 -

S38~7
of the gaseous mixture may be vented by line 72 for absorption
in water or sodium hydroxide solution.
Sulphur dioxide also is fed to the reactor 32 by
line 73 and reacts with the chlorine and the steam to provide
a mixture of sulphuric acid, hydrochloric acid, air and
steam. The reaction between the sulphur dioxide and chlorine
is exothermic and the heat generated heats the solution
passing through the heat exchanger 30 to provide at least
part of the heat requirement of the generator 12. The water
fed to the reactor 32 in the form of steam generally is
insufficient to absorb all the hydrogen chloride and hence
a mixture of gaseous and liquid products result
The gaseous and liquid phase mixture formed in the
reactor 32 is passed by line 74 to a phase separator 75
wherein the liquid phase of sulphuric acid and hydrochloric
acid is separated and passed by lines 76 and 42 to the hot
aqueous recirculating generator feed liquor in line 38. '
Additional quantities of sulphuric acid are added
to the acid solution in line 76 by line 78 in the m~lar ratio
of 1:2 with respect to the sodium chloride fed by line 28,
the quantities of sodium chloride and sulphuric acid in this
way being iust sufficient to provide the hydrogen ion and
chloride ion requirements of the generator 12 not provided - -
by the reaction in the reactor 32.
The gaseous phase from the phase separation, con-
sisting of steam, hydrogen chloride and air, is passed
from the phase separator 75 by line 80 to an absorber 82, to
which cooling water is fed by line 84. The cooling of the
gaseous phase causes condensation of the steam and the forma-
tion of hydrochloric acid by absorption of hydrogen chloride,
allowing the air to be vented by line 86.
: - 18 -
,
.. . . . .

- lOS38~7
The hydrochloric acid formed by absorption of hydro-
gen chloride in the absorber 82 then is passed by line 44 to
the hot a~ueous solution in line 40. '
~lternatively, the hydrochloric acid in line 44 may
be mixed with the acid in line 76 or line ~2 to provide a
single acid feed line to the hot aqueous solution in line 38.
In a further alternative, the mixed acid stream in
line 76 may be combined with the hydrochloric acid stream in
line 44 to again provide a single acid feed streamto the hot
recirculating generator feed liquor in line 40. The
supplemental sulphuric acid fed in line 78 may be provided
-prior to or after combination with the hydrochloric,acid feed
line.
- As an alternative to the use of a reactor 32 in
heat exchange relationship with the recycle stream 26, the
reaction between the sulphur dioxide fed by line 73 and the
chlorine fed by line 70 may be carried out in a suitable device
positioned in a generator feed line. The latter feed line is
designed so that substantially complete reaction between
the chlorine, sulphur dioxide and steam occurs prior to the
injection of the reaction products directly into the
generator. The feed line is positioned in heat exchange
relationship with the generator liquor so that the heat of, ,
reaction is provided to the liquor. ' -'
Any other convenient manner of obtaining heat
exchange between the chlorine'dioxide-producing reaction
medium and the reaction vessel wherein the sulphur dioxide~ ~ '
reacts ~Jith chlorine may be utilized.
Turn:ing now to the embodiment of Figure 2, there is ' ,,
shown therein a similar operation to that illustrated in
Figure 1 with important modifications. Since many elements
- are common to the embodiments of Figures 1 and 2, the same
- 19 ~
.. , .. . , . . ... .. , ... - , ~ . .. . - .. , .. - ., . . . -. .. ...

lOS38'77
reference numerals are used to denote such common elements.
As compared with Figure 1, feed of sodium chloride
in the solution in line 28 is omitted, the chlorine vent in
line 72 is omitted and the supplementary sulphuric acid feed
in line 78 is omitted. Included, however, is a second
reactor 88 to which.a gaseous mi~ture of chlorine, steam and
air is fed by line 90 for reaction with hydrogen ~ed by line
92 to form hydrogen chloride.
The chlorine feed stream in line 90 is provided
by part of the chlorine stream in line 68 supplemented as
required by an external source of chlorine in line 94. The
proportion of the mixed chlorine feed in line 96 passing to
the reactor 32, as in the embodiment of Figure 1, is just
sufficient to form the sulphate ion required to form neutral
sodium sulphate from the sodium ions fed to the generator 12 .
in the sodium chlorate while the proportion of chlorine fed i
by line 90 to the reactor is just sufficient to form hydrogen . - ~ :
chloride in the quantity required to provide the supplemen- .
tary hydrogen ion and chloride ion feeds for the generator 12. .~
The gaseous mixture of hydrogen chloride, àir and ~- :
steam resulting from the reactor 88 is passed by line 98 to
the absorber 82 for absorption of the hydrogen chloride and
steam and venting of the air along with the gas stream in ;
line 80. . ~:
If desired, a combination of the embodiments of
Figures 1 and 2 may be used, with part of the supplementary
hydrogen ions and chloride ions being provided by sulphuric
acid fed by line 78 and sodium chloride fed by line 28 while .
the remainder of the supplementary hydrogen ions and chloride
ions is provided by hydrogen chloride in line 98.
The present invention, therefore, provides a con-~
.
- 20 -

:~053~77
tinuous chlorine dioxide ~enerating system which is inteyrated
with the reaction of sulphur dioxide and chlorine and which
achieves continuously steady state conditions. Modifications
are possible within the scope of the invention.
- 21 -
.. ~................................................................. ~
, . ~ . . . .

~1~53877
SUPPLEMENTARY DISCLOSURE
In the principal disclosure, there is described an
integrated 5 ~ tantially steady state chlorine dioxide
producing process in which chlorine formed in the generator
with the chlorine dioxide is reacted with sulphur dioxide
to form hydrochloric acid and sulphuric acid which are fed
to the generator.
- Sodium sulphate is the sole salt precipitated from
the reaction medium. Where sodium chlorate provides the sole
source of sodium ions to the reaction medium, then 1 moles
2E
of sulphuric acid and hence sulphate ions are required to
form sodium sulphate with the sodium lons of the sodium `
chlorate.
This latter condition is the one that pertains
when the reaction products of sulphur dioxide and chlorine
are intended to provide all the sulphate ion requirement -~of the reaction medium, as is the case of the system ~
described in the principal disclosure. The quantity of
sodium sulphate produced per mole of chlorine dioxide -
formed may, however, be increased, in accordance with this
supplementary disclosure, by introducing additional sodium
ions in the form of sodium chloride and sulphate ions in
the form of sulphuric acid.
When sodium chloride and sulphuric acid are added
in this way, the proportion of the chloride and hydrogen
ion requirements supplied from the reaction products of
sulphur dioxidle and chlorine diminish.
While, therefore, the minimum quantity of sodium
sulphate produced and the maximum amount of sulphuric acid
required to be produced by reaction of sulphur dioxide and
.
- 22 -
~, . . , ' ,.

1053~7
chlorine result when sodium c;llorate is the sole source of
sodium ions for chlorine dioxide generator and hence
when 1 moles of sulphuric acid are produced and fed to
2E
the chlorine dioxide generator, the quantity of sodium
sulphate produced by the chlorine dioxide generator may be
varied independently of the quantity of chlorine dioxide
produced, in accordance with this supplementary disclosure.
23 -
.