Note: Descriptions are shown in the official language in which they were submitted.
-` l 2 ~
I
Chlorine-Pree Chlorine Dio~ide
Field of the Invention
. . _
This invention relates to the preparation of
substantially chlorine-free chlorine dioxide, apparatus
therefor, and particularly to the preparation of
substantially chlorine-free chlorine dioxide for use in
pulp bleaching~
Background of the Invention
Chlorine dioxide is extensively used in aqueous
solution as a reagent for the bleaching of fibrous
material, particularly wood pulp. Serious environmental
concerns now exist in the bleaching of wood pulps whenever
chlorinated organic compounds are produced, even in
relatively small amounts, by the action of chlorine per se
or chlorine when usually present in chlorine dioxide
bleaching agent mixtures, upon the wood fiber. It is,
therefore, becoming increasingly necessary that chlorine
dioxide when used as a pulp bleaching agent be
substantially chlorine contaminant free.
Chlorine dioxide is produced on a commercial
scale by the acid reduction of a chlorate anion in aqueous
solution according to the following general equation, viz:
C103 + 2H+ + Cl -> C102 + 1/2C12 + H20
2 2 0 ~
Typically, the reaction is achieved by the
direct reduction of chlorate by hydrochloric acid as
practised in, for example, the so-called "Integrated
Chlorine Dioxide Process", viz:
NaC103 + 2HCl -> C102 + 1/2C12 + H20 + NaCl
or by the so called R2 and related R series processes
using sulphuric acid, viz:
NaC103 + NaCl + H2S04 C102 + 1/2C12 + ~2 + Na2S4
The integrated chlorine dioxide process utilizes
3 main process systems, namely, (a) a chlorate
electrolysis unit, wherein sodium chloride is electrolized
to produce sodium chlorate according to the following
chemical reaction:
6e
NaCl + 3H20 --~ NaC103 + 3~2~
(b) a chlorine dioxide generator unit, using hydrochloric
acid as the chlorate-reducing agent as described
hereinabove; and (c) a hydrochloric acid producing unit
wherein the resultant hydrogen gas from the chlorate
electrolysis unit is burned in chlorine to produce
hydrochloric acid.
3 2 0 ~
The integrated process produces chlorine dioxide
from chlorine, water, and electricity. Such a process,
typically, produces 8-10 g/L chlorine dioxide solution
strength containing 0.9-1.8 g/L chlorine.
Methods of producing chlorine dioxide from
chlorate systems having minimal or no amounts of chlorine
are known.
United States patent No. 2,881,052, discloses
reduction of sodium chlorate by an acid in the presence of
methanol according to the following equation, viz:
3 2H2S4 + CH30H 3 2C102 + 2NaHS04 + HCHO + 2H20
In an alternative process for the reduction of
sodium chlorate by methanol in sulphuric acid, known in
the industry as the non-integrated, methanol base chlorine
dioxide process, sodium sesqui-sulfate is produced as a
by-product according to the following equation:
18NaC103 + 4CH30H + 1282S04 ~18C102 + 6Na3H(S04)3
+ C2 + 3HCOOH + 14H20
The gas leaving the chlorine dioxide generator
is a mixture of chlorine dioxide, water vapour and trace
amounts of chlorine. The gaseous mixture is, optionally,
4 2 ~ 61 ~ ~ ~
sent to a contact cooler where the water vapour is
condensed and the resultant gaseous mixture passed into a
packed absorption tower where chilled water is forced into
intimate contact with the gas. The resultant chlorine
dioxide solution is pumped to storage for subsequent use
in a pulp bleaching process. Such a typical ~ethanol-
reductant system provides a chlorine dioxide solution
strength of 10 g/L and a chlorine concentration of 0.1 g/L
in 10 g/L chlorine dioxide solution.
British patent No. 687099 discloses reduction of
sodium chlorate by nitrogen dioxide to produce chlorine
dioxide according to the following equation, viz:
NaC103 + N02- ~ NaN03 + C102
U.S. Patent No. 2,866,682, discloses a related process
using sodium nitrate according to the following equation,
v i z :
3 NaNO2 + H2SO4 ~ 2ClO2 + Na2SO4 + NaNO3 + H2O
Alternative processes are disclosed in U.S.
Patent No. 2,866,683 wherein sodium bisulphite is used in
conjunction with sulphuric acid according to the equation,
viz:
2 ~
4NaC103 + 2NaHS03 + H2S04 >4C102 + 3Na2S 4 2
and the related process using sulphur dioxide, viz:
2NaC103 + H2S04 + S2 2C102 ~ 2NaH 4
2NaC103 + S02 >2Clo2 + Na2S04
French patent No. 991614 discloses the reduction
of a chlorate by manganic salts and other oxidating metal
salts.
It is understood by those skilled in the art
that there are significant deleterious side reactions,
often resulting in significant inefficiencies, in using
reducing agents other than chloride for the production of
chlorine dioxide from a chlorate solution for the purpose
of bleaching wood pulp. However, the process of
generating chlorine dioxide from chlorate using chloride
as a reductant, which process is alternative to those
outlined hereinabove, suffers from the increasingly
disadvantageous side reaction of producing chlorine by-
product.
When chloride is used as a reductant, the yield
of chlorine dioxide is reduced and chlorine is produced by
the following overall reaction, viz:
.. . .
0~ 3
NaC103 + 5HCl ,3C12 + 3H20
The desired overall reaction according to the
equation, viz:
5NaC103 + 6HC1 6C102 + 5NaCl + 3H20
is of insignificant application.
Where a non-fully oxidised sulphur compound is
used as a reductant, inefficiencies resulting in total
reduction to chloride occur, for example, by the following
equation:
2NaC103 + 6S02 + 6H20 - >2NaHS04 + 4H2S04 + 2HCl
In this particular case the extent of the
inefficiency is heightened as it reflects the effective
reduction of the co-produced chlorine, which could
otherwise be reduced with hydrogen and recycled in the
generation of chlorine dioxide in the "Integrated Chlorine
Dioxide Process". Chlorine is, however, also produced
when sodium chlorate is reduced with reducing agents other
than chloride; the extent of which is a function of the
reaction conditions, e.g.
"` 7 2 0 ~ 5
2NaC103 + 5SO2 + 4H2O -> 2NaHSO4 + 3H2So4 + C12
In this case there is some chlorine production though
reduction to chloride also occurs, viz:
C12 + S2 + 2H2 -~ 2HCl + H2SO4
Similarily, use of other reducing agents, i.e.
methanol, as disclosed in U.S. Patent No. 2,881,052, does
not, in itself, prevent the production of some chlorine or
reduction of chlorate to chloride.
Accordingly, while the use of chloride as a
reductant is the preferred manufacturing process at
present, the presence of by-product chlorine is causing
reconsideration of this process as an ongoing viable
commercial process for the production of chlorine dioxide
for the bleaching of wood pulp.
Methods of removing chlorine from chlorine
dioxide are known but, to-date, do not offer practical
manufacturing processes.
U.S. Patent No. 3,854,900, discloses a means of
separating chlorine dioxide from chlorine using a twin
absorption column. Unfortunately, this process leaves the
production of significantly dilute chlorine dioxide
solutions.
8 2 ~ 5
Japanese patent No. 4466(1956), discloses the
selective removal of chlorine dioxide from chlorine
contaminated chlorine dioxide gaseous mixtures by passing
the mixed gases obtained from the production of sodium
chlorate over metals such as iron or aluminum. The
chlorine contaminant forms the respective metal chloride
while the chlorine dioxide gas passes through the system.
U.S. Patent No. 2,481,241, teaches the selective
reduction of chlorine by sulphur dioxide, Tappi 48:110
(1965) discloses the selective removal of chlorine by
means of an alkali and Japanese patent No. 2663(1956)
discloses the oxidation of chlorite with chlorine for
selective removal.
All of the above chloride reductant processes
cause significant inefficiencies by either loss of
chlorine to the system, production of weak chlorine
dioxide solutions, chlorine dioxide reduction, the
formation of chlocites or for the need for additional
chemical reagents.
Accordingly, it can be seen that there is an
increased demand for a manufacturing process for producing
chlorine-free chlorine dioxide for use as a bleaching
reagent which suffers from none of the above
disadvantages.
Summary of the Invention
9 2 ~
It is a primary object of the present invention
to provide chlorine dioxide substantially chlorine-free
and acceptable for use in a pulp bleaching process.
It is a further object of the present invention
to provide a continuous substantially-closed chlorine
dioxide purification process that provides for the
recovery of the chlorine contaminant for subsequent use.
In a yet further object, the invention provides
a pulp bleaching process which uses substantially
chlorine-free chlorine dioxide wherein substantially all
of the chlorine contaminant is removed by selective
desorption rather than by reduction to chloride.
Applicant has surprisingly discovered that
chlorine dioxide may be beneficially separated from
chlorine in a gaseous mixture as is typically produced in
a chlorine dioxide generator from sodium chlorate and
acid, preferably hydrochloric acid, or methanol/acid,
preferably sulphuric acid, mixtures as practised in the
art.
Althoug~. it is known that chlorine and chlorine
dioxide are soluble to some degree in suitable chlorine
dioxide absorbent media, applicant has discovered that the
relative degrees of solubility of these respective gases
may be sufficiently different and dependent upon the
nature of the absorbent medium that practical separations
of chlorine from chlorine dioxide can be achieved.
.. . :
~ o 2 ~
Accordingly, the invention provides a process
for the production of a substantially chlorine-free
aqueous solution of chlorine dioxide which process
comprises generating a chlorine-contaminated gaseous
mixture comprising chlorine dioxide and chlorine by a
chlorine dioxide generator; feeding said gaseous mixture
to an absorption tower in counter-current flow to a
chlorine-dioxide selective absorbent liquid to
substantially selectively absorb said chlorine-dioxide
therein to produce a chlorine-dioxide solution and a
resultant chlorine contaminated gas stream; feeding said
chlorine-dioxide solution to a stripping tower in counter
current flow to a stripping gas to produce a stripped
gaseous mixture comprising chlorine-dioxide and a depleted
chlorine-dioxide solution; feeding said stripped gaseous
mixture to a re-absorption tower in counter-current flow
to a chilled aqueous solution to produce a chilled said
substantially chlorine-free aqueous chlorine-dioxide
solution and a weak gas stream.
In yet a further aspect, the invention provides
a process as hereinabove defined for use in the bleaching
of a fibrous material, particularly wood pulp, in a
fibrous material bleaching system, further comprising
feeding said substantially chlorine-free aqueous solution
of chlorine dioxide to said system and bleaching said
fibrous material with said chlorine dioxide.
11 2~&~
Preferably, the process may be operated as a
continuous process.
More preferably, the chlorine-dioxide selective
absorbent liquid is selected from aqueous sulphuric acid
and acetic acid.
We have surprisingly discovered that although
dilute sulphuric acid having a strength of less than 15%
H2SO4 can be used in the process according to the
invention to produce sufficiently low levels of chlorine
in the substantially chlorine-free aqueous chlorine-
dioxide, that most advantageously reduced levels of
chlorine-contaminant can be obtained with stronger
sulphuric acid strengths. Accordingly, acid strengths
of > 15% H2SO4 is preferred, 25-50% H2S04 more preferred,
and 50-95% yet more preferred. It will be appreciated by
the man skilled in the art that the sulphuric acid
strength may be chosen so as to not only provide the
substantially chlorine-free chlorine dioxide solution by
the process according to the invention, but be selected,
also, as to maintain a water balance in the process and
avoid the need for sulphuric acid "make-up" during the
operation of the process. Water vapour enters the system
in the chlorine-contaminated gaseous mixture comprising
chlorine dioxide, chlorine and water vapour from the
chlorine dioxide generator.
12 2~
Thus, in one embodiment, the sulphuric acid
strength and operating temperature thereof in the chlorine
dioxide absorption tower is selected such that the water
in the form of vapour carried into the system from the
chlorine dioxide generating unit balances the water
carried out of the system with the resultant chlorine
contaminated gas stream.
In yet a still further aspect, the invention
provides apparatus for the production of substantially
chlorine-free chlorine dioxide comprising: a chlorine
dioxide generator for producing a chlorine-contaminated
gaseous mixture comprising chlorine dioxide and said
chlorine-contaminant; an absorption tower; a stripping
tower; a re-absorption tower; means for feeding said
gaseous mixture to said absorption tower; means for
feeding a chlorine-dioxide selective absorbent liquid to
said absorption tower in counter-current flow to said
gaseous mixture to produce a first chlorine-dioxide
solution; means for feeding said first chlorine-dioxide
solution to said stripping tower; means for feeding a
stripping gas to said stripping tower in counter-current
flow to said first chlorine-dioxide solution to produce a
stripped gaseous mixture comprising chlorine dioxide and a
depleted first chlorine-dioxide solution; means for
feeding said stripped gaseous mixture to said re-
:
.~, .
13 2 B ~
absorption tower; means for feeding an aqueous solution tosaid re-absorption tower in counter current flow to said
stripped gaseous mixture to produce an aqueous second
chlorine dioxide solution and a weak gas stream; and
collecting said second chlorine-dioxide solution
constituting said substantially chlorine-free chlorine
dioxide.
Preferably, the apparatus as hereinabove defined
further comprises a fibrous material bleaching unit,
particularly a wood pulp bleaching unit and means for
feeding said second chlorine-dioxide solution to said
bleaching unit.
In a preferred embodiment of the invention the
absorbent medium is an aqueous solution of sulphuric acid.
By the term "substantially chlorine-free" is
meant levels of chlorine in admixture with chlorine
dioxide, either as a gaseous mixture or as a solution,
lower than the ratio of chlorine:chlorine dioxide of 1:20
on a W/W basis.
By the term "chlorine dioxide selective
absorbent liquid" is meant a liquid that selectively
absorbs chlorine dioxide therein to a greater degree than
chlorine under the absorbent conditions practised of
temperature, gaseous mixture flow rates, relative
concentrations of chlorine dioxide and chlorine, and the
14
like, such that the ratio of chlorine:chlorine dioxide in
the liquid is lower than in the gaseous mixture from which
the chlorine dioxide is removed.
We have found that chlorine dioxide and chlorine
in the ratio typically found in the gaseous mixture
generated from a chlorine dioxide generator using sodium
chlorate and hydrochloric acid, when fed to a selective
absorbent, for example, sulphuric acid, water, or acetic
acid, produces a chlorine dioxide solution containing
substantially all of the chlorine dioxide originally
contained in the gaseous mixture, but with significantly
reduced amounts of chlorine. The remaining chlorine
remains in the gaseous stream. The chlorine dioxide can
be liberated from the absorbent, preferably by a suitable
gas purge, such as a warm nitrogen, helium or, most
preferably, an air stream; with re-absorption of the
chlorine dioxide, if required, in a second liquid,
preferably, water.
In those processes where further reduced levels
of chlorine contaminant in the aqueous chlorine dioxide
bleaching solution are desired, which chlorine levels are
produced by operation of the process according to the
invention at desired sulphuric acid strengths and
temperatures as hereinbefore described, the chlorine
contaminant level may be further reduced by treatment of
2 ~ ~ ~ & ~ ~
the chilled aqueous chlorine-dioxide solution with a
reducing agent to chloride anion. Such reducing agent may
be selected from the group consisting of hydrogen peroxide
and a non-fully oxidised sulphur species selected from
sulphur dioxide and metal salts of sulphite, bisulphite,
dithionite and persulphate.
We have found that the process and apparatus
according to the invention is a highly desirable addition
to the typical Integrated Chlorine Dioxide Process having
relatively high levels of contaminant chlorine for the
production of chlorine contaminant-free chlorine dioxide
solution. However, the process and apparatus according to
the invention is also of value in providing an improved
chlorine dioxide solution as generated from non-integrated
chlorine dioxide generating systems.
Other and further objects and advantages of this
invention will become apparent upon reading the following
specification taken in conjunction with the accompanying
drawings.
Brief Description of the Drawings
In order that the invention may be better
understood a preferred embodiment will now be described by
way of example, only, with reference to the accompanying
drawing wherein the Figure is a diagramatic sketch of a
preferred process and apparatus according to the
invention.
16
Description of a Preferred Embodiment
The apparatus and process shown in the Figure
generally as 10 has a chlorine dioxide generator 12, an
absorption tower 14, a stripping tower 16, a re-absorption
tower 18 and a heat exchanger 20.
Generator 12 is of the type well-known in the
art used to produce chlorine dioxide for use as a
bleaching agent of a fibrous material, such as a wood pulp
by the reaction of sodium chlorate with hydrochloric acid
as practised in the so-called "Integrated Chlorine Dioxide
Processn, as hereinbefore described.
Connecting generator 12 to the base of
absorption tower 14 is a conduit means 22. Exiting from
the top of tower 14 is exit gas conduit 24. Connected
from a lower part of tower 14 to heat exchanger 20 is
conduit 26 and from an upper part of heat exchanger 20 to
an upper part of tower 14 is conduit 28. Also exiting
from an upper part of exchanger 20 to an upper part of
stripping tower 16 is conduit 30 and exiting a lower part
of tower 16 to a lower part of heat exchanger 20 is
conduit 32. Connecting tower 16 to reabsorption tower 18
is conduit 34 and entering tower 16 at a lower part is
conduit 36. Entering tower 18 at a top is conduit 36 and
exiting ~ower 18 at a lower part is conduit 38 having
17 2~64~
connected thereto conduit 40. Tower 18 at an upper part
` has a return conduit 42 connected to generator 12.
In operation, circulating through absorption
tower 14 and stripping tower 16, via heat exchanger 20, is
a 75% aqueous sulphuric acid solution at a temperature of
about 15C in tower 14 and a temperature of about 70C in
tower 16.
With reference now to the gas and sulphuric acid
flow systems of this embodiment, in operation a gaseous
mixture of chlorine dioxide, chlorine, water vapour and
air obtained from chlorine dioxide generator 12 is passed
through conduit 22 to the base of absorption tower 14
where it passes in counter-current flow through tower 14
with the sulphuric acid entering the top of tower 14 from
heat exchanger 20 from conduit 28. The impure chlorine
dioxide gaseous feed typically comprises chlorine dioxide
and chlorine in about a 4:1 ratioO Substantially all of
the chlorine dioxide is absorbed in the sulphuric acid in
tower 14 to provide a concentration of Ca 10-llg chlorine
dioxide/litre, while the sulphuric acid absorbs chlorine
to produce c~ O.lg chlorine/litre concentration at the
gaseous mixture and sulphuric acid flow rates and
concentrations used.
The resultant contaminant gas stream,
substantially free of chlorine dioxide, exits from the top
18 2~g~
of tower 14 via conduit 24, and, optionally, sent to a
hydrochloric acid synthesis unit where the chlorine gas is
burned to form hydrochloric acid. The chlorine dioxide
containing sulphuric acid solution exits from the base of
tower 14 and is transferred via conduit 26 to heat
exchanger 20, which warms the solution prior to transfer
via conduit 30 to the top of stripping tower 16 wherein it
is purged in counter-current flow by a stripping hot air
flow having a temperature of about 70C entering tower 16
via conduit 36 to provide a stripped gaseous mixture
comprising chlorine dioxide. Depleted chlorine dioxide
sulphuric acid solution exits tower 16 and is recycled
back to the top of absorption tower 14 through heat
exchanger 20 via conduits 32 and 28.
The stripped chlorine dioxide air gaseous
mixture exits stripping tower 16 via conduit 34 and enters
the base of re-absorption tower 18 where it mixes in
counter-current flow with chilled water having a
temperature of about 8C entering the top of tower 18 via
conduit 37. Chlorine dioxide dissolves in the chilled
water and exits the base of tower 18 via conduit 38 for
transfer directly or after storage in a storage tank (not
shown) to a pulp bleaching plant (not shown). The chilled
chlorine dioxide solution contains between 9 to 10 g
chlorine dioxide/litre and ~ 0.1 g chlorine/litre.
19 20Sl~
The resultant weak gas stream exiting tower 18
is recycled via conduit 42 to chlorine dioxide generator
12.
The absorption tower and re-absorption tower may
be of any suitable and convenient construction to allow
intimate gas-liquid contact. Most preferably, the
respective gaseous mixtures rise through the respective
gas/liquid contact zones under conditions of temperature,
flow rates, and chlorine dioxide and chlorine
concentration, as deemed sufficient to provide the
selective absorption. The choice of such conditions lies
well within the ability of the skilled man in the art.
Further, such parameters may be readily selected to
maintain constant water balance between water entering the
system as vapour via conduit 22 and that lost via conduit
24.
We have found that the lower the temperature of
the circulating liquid in the absorption tower and the re-
absorption tower the greater the absorption of chlorine
dioxide therein, whereas the hotter the stripping tower
circulating acid the geeater the amount of chlorine
dioxide purged therefrom. We have found that liquid
temperatures of 10 to 15C in the absorption tower, 5 to
15C in the re-absorption tower and 50 to 80 C in the
stripping tower of the respective liquid absorbent
2~~
provides satisfactory absorption and de-sorption, as the
case may be.
In addition to the beneficial utilisation of the
heat exchanger as described in the preferred embodiment,
additional heating and cooling means may be desired as
determined by the man skilled in the art, either, for
example, to the heat exchanger 20, re-absorption tower or
stripping gas temperatures and flow rates.
Table 1 shows the experimental solubilities of
chlorine dioxide and chlorine in sulphuric acid as
provided by the process of the present invention in an
air-diluted mixture for the same ratio of gaseous mixture
feed at 15~C. The results have been corrected to a
chlorine dioxide gas strength of 11 mole ~. In each case,
a water blank was used to facilitate correction to some
standard condition to compensate for charging off gas
composition.
Table 1 gives the chlorine concentration in g/L
in approximately 10 g/L chlorine dioxide solution.
21
Table 1
Experimental solubilities of air-diluted chlorine dioxide
and chlorine in sulphuric acid for the same ratio of feed
gases at 15C.
H2S04 C102 C12 C102-Kepenski*
(%) (g/l~ (g/l) (g/l)
0 11.3 1.6 --
10.6 0.7 10.7
13.3 0.7 10.3
10.1 0.4 10.0
; 20 11.4 0.3 9.8
~ 25 12.0 0.2 9.7
; 30 10.5 0.2 9.7
11.8 0.1 10.8
10.7 0.1 10.4
98 10.5 0.5 --
*J. Kepenski and J. Trzeszczynski, Roczniki Chemii, Ann.
Soc. Chim. Polongrum, 38, 201-211 (1964).
22
Table 2 shows the experimental solubilities
obtained from the stripped gas purging effected in tower
18 in distilled water at 5C.
Table 2
Experimental solubilities of the stripped gas of the
solutions shown in Table 1, in distilled water at 5C.
Initial circulating H2SO4 Composition of
Solution Strength final solution (g/l)
(wt%) ClO~ C12
0 10 1.4
9.4 0.7
9.5 0.6
9.5 0.4
10.2 0.3
Accordingly, it can be seen that the chlorine to
chlorine dioxide ratio has fallen from 1:4 of the chlorine
dioxide generated gaseous mixture to a 1:30 ratio in the
chlorine dioxide aqueous solution product.
Optionally, but preferably, the reduced level of
chlorine contaminant in the chlorine dioxide aqueous
product can be readily removed by the use of a reducing
23 20~
agent, foc example, a non-fully oxidised sulphur species,
such as sulphur dioxide and metal salts of sulphite,
bisulphite, dithionite and persulphate, and preferably
hydrogen peroxide.
The above described process provides a virtually
self-contained recycling sulphuric acid system whereby the
level of chlorine as a contaminant in a gaseous chlorine
dioxide mixture is either adequately removed or
significantly reduced to a level such that the resultant
amounts of chlorine in the chlorine dioxide product can be
readily removed by a reducing agent, while the majority of
chlorine contaminant from the chlorine dioxide generator
can be advantageously utilised in the production of
valuable hydrochloric acid. Further, the above process
preferably provides a weak gas recycle option to return
any uncollected chlorine dioxide back to the chlorine
dioxide generator for recycling.
The above embodiment of the process and
apparatus according to the invention illustrates the
feasibility of substantially removing chlorine contaminant
from a chlorine dioxide mixture. Clearly, alternative
recycling chlorine dioxide selective absorbent liquids may
be used without departing from the spirit of the invention
where selective absorption occurs and is of
practicability.
24 20~ S
The dissolution of chlorine dioxide into
sulphuric acid has been reported by Kepenski et al,
Roczniki Chemii, Ann. Soc. Chim. Polongrum, 38, 201-211
(1964), to be dependent upon the sulphuric acid
concentration whereby the chlorine dioxide solubility
decreases by 13% as the sulphuric acid strength is
increased from 0 to 30~, and thereafter increase over the
range 30 to 60%. However, we have found that, as shown in
Table l, the solubility of chlorine dioxide in aqueous
sulphuric acids of various strength of 0-98% to be
substantially constant. We have found, also, that the
solubility of chlorine in sulphuric acid of various
strength decreases significantly with increasing acid
strength, as shown in Table 1. We have further found that
chlorine saturation is reached in about 1/5 of the time
taken for chlorine dioxide saturation in water and 20~
sulphuric acid. Most importantly, however, we have found
that sulphuric acid effects a most beneficial partial
separation of chlorine from admixtures of chlorine and
- chlorine dioxide, and that increasing the sulphuric acid
concentration to at least 20 weight ~ results in
increasing separation. For typical chlorine dioxide
generator gas streams, a change in molar ratio of circa
4:1 chlorine dioxide to chlorine can be increased to circa
30:1. Contrary to the findings of Kepenski et al, the
`
.
2s 2~
solubility of chlorine dioxide in sulphuric acid is
essentially independent of aqueous sulphuric acid strength
over the range 0 to 20% sulphuric acid, whereas the
solubility of chlorine decreases as the sulphuric acid
strength increases.
As hereinbefore described, with appropriate
selection of sulphuric acid strength and/or operating
temperature and/or pressure, there may be no net change in
the water content of the sulphuric acid in a sulphuric
acid absorption/de-sorption process. This latter aspect
is of importance when water balance factors for the
overall process are important. Thus, under the following
conditions wherein: the chlorine dioxide generator air-
gaseous mixture enters the sulphuric acid absorber at
70C, the sulphuric acid absorber operates at 15C, the
resulting solution is stripped at 70C, constant air flow
throughout the system, and near atmospheric pressure
throughout, there will be a small net water loss from the
sulphuric acid if the sulphuric acid strength is less than
30~ but there will be a water gain in the recycling
sulphuric acid stream if the acid strength is higher than
30%.
Optionally, the gaseous mixture containing water
vapour may be sent to an indirect contact cooler where the
water vapour is condensed and removed to provide the
26 2 Q ~ ~ ~3~.~
resultant gaseous mixture with a relatively higher
concentration of chlorine dioxide.
The process according to the invention has
particular utility by removing adequate or significant
amounts of chlorine in admixture with chlorine dioxide,
such that the residual chlorine contained in the chlorine
dioxide product may be readily removed by the addition of
a reductant. We have found that hydrogen peroxide reduces
chlorine essentially exclusively when the peroxide is
added in an amount not greater than that required to
reduce the chlorine present. We have found that the
addition of sodium sulphite (and by inference, sulphur
dioxide) as a reductant to chlorine containing chlorine
dioxide solutions reduces both chlorine and chlorine
dioxide at approximately equal rates. Thus, the
additional step of removing residual chlorine by a
reductant as mentioned above is now an optional, but,
economically feasible step to provide a very useful pulp
bleaching reagent.
In this specification, a chlorine dioxide
generator means a reaction vessel having feed streams
which comprise sodium chlorate feed means and acid feed
means. In the non-integrated methanol based generating
system a methanol feed stream means is also required. In
the integrated chlorine dioxide process, the chlorine
. .
27 20~
dioxide generator is provided with means for feeding
hydrochloric acid and means for feeding sodium chlorate to
the generator. The sodium chlorate is made from the
electrolysis of sodium chloride in a standard chlorate
electrolysis unit. Hydrogen gas produced as a by-product
is cycled to a hydrochloric acid synthesis unit comprising
chlorine feed means and hydrogen feed means, combustion
reactor and hydrochloric acid product separation means.
While a preferred embodiment of the apparatus
and process of the invention has been described using
specific terms, it is to be understood that variations may
be made without departing from or scope of the following
claims. Thus, although this disclosure has described and
illustrated certain preferred embodiments of the
invention, it is to be understood that the invention is
not restricted to these particular embodiments. Rather,
the invention includes all embodiments which are
functional or mechanical equivalents of the specific
embodiments and features that have been described and
illustrated herein.
