Note: Descriptions are shown in the official language in which they were submitted.
1057~09
This invention relates to the production of
chlorine dioxide, more particularly to the integration of
a chlorine dioxide producing procedure with a pulp mill oFeration.
A Kraft pulp mill operation, which is the pulp
mill operation to which the present invention is particularly
directed, although any other convenient pulp mill operation
may be employed, involves digestion of wood chips, or other
. ~
cellulosic- fibrous material, with white liquor containing
æodium 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 the desired brightness. The black liquor is subjected to
recovery and regeneration operations to form fresh white liquor
which then is recycled to the digestion-~stage to provide at
least part of the white liquor digesting further wood chips.
The recovery and regeneration operations generally
involve an initial concentration of the black liquor followed
: .
by burning in a furnace to provide a solid mass, otherwise
known as 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 conversion of carbonate to hydroxide, with
consequent deposition of calcium carbonate. The calcium
carbonate is recovered and usually is used to form further
slaked lime. The aqueous solution resulting from separation
- 2 -
. ' , ' ~
105790~
of the calcium carbonate therefrom is the recycled white
liquor.
Sodium sulphate commonly is used as a source of
make-up chemical for the system, to- make-up losses of soda
and sulphur values. Various sources of sodium sulphate may
be used and typically, the sodium sulphate may be formed in
the production of chlorine dioxide and chlorine for use in
the bleaching operations in the bleach plant.
A typical operation for the production of chlorine
dioxide involves reduction of sodium chlorate with chloride
ions in an acid medium containing sulphuric acid. Sodium
sulphate is recovered as a by-product of this process. In
such a process the basic reaction involves the production of
chlorine dioxide, chlorine and water in accordance with the
equation:
C103 + Cl + 2H+ - ? C102 + ~Cl2 + H20 -(1)
The quantity of make-up chemical required for the
mill recovery system has been falling with tightening up of
the recovery system while the demand for chlorine dioxide
and chlorine has remained at the same level. The quantity of
sodium sulphate formed in a chlorine dioxide generation
operation may be lowered while maintaining the chlorine dioxide
output at the same level by utilizing mixtures of hydrochloric
acid and sulphuric acid to provide the acid requirement. The
hydrochloric acid at the same time may provide part or all of
the chloride reducing agent, the remainder of the reducing
agent, where required, being provided by sodium chloride,
The quantity of sodium sulphate formed depends on
the relative concentrations of the hydrochloric acid and
- 3 -
.. . . :
10575~9
sulphuric acid in the feed. Up to the ~oint where the hydr~oric
acid provides half the acid requirement, sodium sulphate is
recoverable in substantially pure form; however, when the
quantity of hydrochloric acid exceeds half the acid require-
ment, the sodium sulphate is recoverable only in admixture
with sodium chloride.
The processes may be represented by the following
equation:
NaC103 + (l-x) NaCl + xHCl + (2-x) H2S0
C12 + ~Cl2 + H20 + (2-x) Na2So4 -(2)
in the case where x is 0 to 1, and
,
NaC103 + x~Cl + (22x) 2 4
2 ~ 2 + H20 + (2-x) ~a2S04 + (x-l) ~aC1 -(3)
in the case where x is 1 to 2.
Thus, the mill requirement for sodium sulphate and
; chlorine dioxide may be such that the chlorine dioxide
generator must be operated in a manner which produces mixtures
of sodium sulphate and sodium chloride if stoichiometry is
to be maintained.
It has been considered undesirable to introduce
sodium chloride to pulp mill recovery and regeneration systems
since this material is not affected by the recovery and
regeneration operations and hence builds up as a dead load.
Hence, where the quantity of pure sodium sulphate recoverable
from the chlorine dioxide generation has exceeded the
stoichiometric requirements, the excess must be disposed of,
with consequential loss of soda and sulphur values.
- 4 -
.' ' , , ~ . . .
.
579(~
In accordance with the present invention, there is
provided an improvement in a pulp mill process including the
steps of contacting a cellulosic fibrous material with a
pulping liquor containing sodium sulphide and sodium hydroxide,
separating pulped material from spent pulping liquor, sub-
jecting the spent pulping liquor to recovery and regeneration
steps to provide a white liquor, recycling the white liquor
as at least part of the pulping liquor, providing sodium
sulphate in the spent pulping liquor to make up soda- and
sulphur-losses from the process, and subjecting the pulp to a
series of brightening and purification steps to provide a
bleached pulp, the improvement comprising forming chlorine
dioxide by reaction of sodium chlorate with hydrochloric
acid in the presence of sulphuric acid, recovering a mixture
of sodium chloride and sodium sulphate from the reaction,
separating substantially pure sodium sulphate from the
mixture, utilizing the sodium sulphate as the make-up sodium
sulphate and utilizing the chlorine dioxide in at least one
- of the brightening steps.
In acco~dance with the present invention,-therefore, the
sodium sulphate obtained in admixture with sodium chloride
~m thechlorine dioxide generating process is separated to
provide a feed of substantially pure sodium sulphate for
. . .
the pulp mill.
The invention is described further, by way of
.. - . .:
illustration, with reference to the accompanying drawings,
in which
Figure 1 is a phase diagram for the system
Na2SO4 - NaCl - H2O at temperatures of 50C, 75 C, and 100 C;
Figure 2 is a schematic flow sheet of one embodiment
of one aspect of the invention; and
Figure 3 is a schematic flow sheet of a secon~
embodiment of the one aspect of the invention.
.
- ."~ ,
,
10579(~9
It is known that the solubility characteristics
of sodium chloride and sodium sulphate vary with temperature,
the solubility of sodium chloride increasing with increasing
temperature while the solubility of sodium sulphate falls
at certain relative concentrations of sodium sulphate to
sodium chloride, while the reverse is true at other relative
concentrations, namely, that the solubility of sodium chloride
decreases with temperature while the solubility of sodium
sulphate increases. These facts may be utilized in the
present invention to ~eve the separation of the sodium sulphate
and sodium chloride.
The phase diagramfor the system Na2S04 - NaCl - H20
is shown as Figure 1 of the accompanying drawings, the
dat~ ~or the phase diagram being-computed from:
.. . .. . . . . .
Chretien A, Annalen der Chemie 10 serie 11-12,
pp 9 to 155 (1929) for the 50 C and 75C data; and
Pylkova,E.V. and Akhimov,E.I., Russian Journal of
Inorg.Chem. 7(1), 104 (1962).
,. . . . .
, ... .. , , . . .. .. . . , . , . . . -- -- -- -- -- -- -- ----
In accordance with one aspect of the present invent-
ion, chlorine dioxide and chlorine are formed by reduction
of an aqueous solution of sodium chlorate using chloride ions
in the presence of a mixture of hydrochloric acid and sulphuric
acid in accordance with equation 3 above, a mixture of sodium
sulphate and sodium chloride is recovered from the reaction
medium, the mixture is dissolved in water, sodium chloride
and sodium sulphate are crystallized separately from the
solution, their order of separation depending on the
solubility relationships which are, in turn, dependent on
the initial weight ratio of sodium sulphate to sodium chloride
-- 6 --
, - ' :,
1057~09
in the recovered mixture, and the sodium sulphate is used as
make-up chemical in the recovery and regeneration of spent
pulping chemical in a pulp mill system.
In accordance with one embodiment of this aspect
of the invention, the separation of the salts independently t
from the aqueous solution may be achieved by concentrating
the aqueous solution to deposit sodium chloride therefrom
until the substantial saturation point of sodium sulphate is
reached. The aqueous solution thereafter is heated to render
the solutlon undersaturated with respect to sodium chloride,
whereupon sodium chloride is added thereto to deposit sub-
stantially pure sodium sulphate from the solution. This
procedure may be used at high relative concentrations of
sodium chloride, as will be apparent from the phase diagram
shown in Figure 1. At lower relative concentrations, however,
an alternative procedure may be adopted.
In accordance with a second embodiment of this
- aspect of the present invention, the separation of the salts
independently from the aqueous solution may be achieved by
concentrating by evaporation the aqueous solution to deposit
sodium sulphate therefrom until the substantial saturation
point of sodium chloride is reached. The aqueous solution
thereafter is cooled to deposit some sodium chloride therefrom.
Sodium sulphate may be added to-tne cooled solution to
deposit further quantities of sodium chloride. ;
In a second aspect of the present invention, the
separation of the sodium sulphate and sodium chloride re-
covered from the chlorine dioxide generator is achieved
using a classification of the crystal sizes. Thus, the
10575~09
sodium sulphate and sodium chloride mixture may be slurried
in an aqueous solution which is saturated with respect to
sodium sulphate and sodium chloride and then passed through
a classifier, which may take the form of a vertical tube
through which the slurry is upwardly passed at a flow rate
which allows the heavier crystals to settle in the tube
while the less heavy crystals pass upwardly and out of the
tube to be separated from the aqueous phase in any convenient
manner. Alternatively, the slurry may be passed through a
sieve in which the openings allow one of the crystal sizes
to pass therethrough but prevent the other crystal size from
passing therethrough.
The sodium chloride crystals are larger and heavier
than those of sodium sulphate and hence separation of the
components by the above-described physical separation methods
may be used in the present invention.
- Separation of the sodium sulphate and sodium
chloride may be achieved in accordance with a third aspect
of the present invention. me solid mixture of sodium sulphate
and sodium chloride obtalned from the chlorine dioxide gener-
ator is contacted with an aqueous solution of sodium sulphate
and sodium chloride in a ratio that upon saturation of the
aqueous solution, sodium sulphate will be the only solid phase
present. In order to minimize the quantity of sodium
sulphate leached from the mixture, the ratio should be as
close to the eutecticcomposition, as seen in Figure 1, as
possible. In practising this aspect of the invention,
it is preferred to carry out the leaching at higher temperatures,
to decrease further the quantity of sodium sulphate dissolved
lVS~9(~
from the mixture. Following the leaching, the solid sodium
sulphate is forwarded to the furnace, while the mother liquor
is partly recycled, after dilution, to leach further mixture.
This leaching procedure to recover pure sodium
sulphate is utilizable when the weight ratio of sodium sulphate
to sodium chloride is less than that at the eutectic defined
by a set operating temperature. For example, at 100C, the
limiting weight ratio of sodium sulphate to sodium chloride
is 0.17.
In accordance with a further aspect of the invention,
the separation is achievable by a flotation technique. For
example, the mixture of sodium sulphate and sodium chloride
may be ground to fine particle size, such as 36 to 170 mesh
and formed into a pulp with a brine saturated with sodium
chloride and sodium sulphate. A solution of sodium octyl
sulphate is added to the slurry and stirred, after which the
pulp is diluted with further brine until the weight ratio
of solids to brine is about 20 to 30:100.
Thereafter, the slurry is introduced to a flotation
cell of any convenient construction and air is passed through
the cell as a fine stream of bubbles. Sodium sulphate then
separates out and the crystals may be collected~
. . .
Referring to Figure 2 of the accompanying drawings,
wood chips, or other cellulosic fibrous material, are fed by
line 10 to a digester 12 wherein the wood chips are digested
with a pulping liquor fed by line 14 and containing sodium
sulphide and sodium hydroxide as the active pulping chemicals,
and hence utilizing the Kraft process.
-` lOS790~
The resulting pulp and black liquor or spent pulp-
ing liquor are separated and the pulp is washed in a brown
stock washer 16 with water fed by line 18. The wash water
used in line 18 may be fresh water or "contaminated condensate".
The washed and unbleached pulp is fed by line 20
to a bleach plant 22 wherein the pulp is subjected to a
series of bleaching and purification processes. Generally
the bleaching and purification processes involve bleaching
with chlorine, chlorine dioxide or mixtures thereof. The
chlorine dioxide is fed as a solution by line 24 and the
chlorine is fed by line 26. The purification generally is
_ carried out by caustic extraction, using aqueous sodium hydr-
oxide solution fed by line 28. Any desired sequence of steps
may be used in the bleach plant 22 to achieve the desired pulp
brightness. For example, the so-called CEDED, C/DEDED or
DEDED sequences may be used, wherein C represents bleaching
with chlorine, C/D represents bleaching with mixtures of
chlorine and chlorine dioxide, D represents bleaching with
chlorine dioxide and E represents caustic extraction. In the
DEDED sequence, chlorine is not required and hence feed line
26 may be omitted. - -
The pulp is washed during the bleach plant operations,
typically after each bleaching or caustic extraction operation,
by water fed by line 30. The washing operations may involve
countercurrent flow of pulp and wash water through the
bleach plant 22. The bleached and purified pulp of required
brightness is recovered from the bleach plant 22 by line 32
and is sold as such or is passed to further operation to be
converted into paper or other pulp products.
-- 10 --
909
The chlorine dioxide which is used in the bleach
plant 22 is formed in a chlorine dioxide generator 34 by
reduction of sodium chlorate fed by line 36 with hydrogen
chloride fed by line 38 in the presence of hydrochloric acid
provided by the hydrogen chloride and sulphuric acid fed by
line 40.
The generation of chlorine dioxide may take place
in any convenient manner, typically by carrying out the process
at the boiling point of the reaction medium, typically with
the generator 34 being maintained under a reduced pressure.
- When the generator 34 is operated in this manner,
the water vapour evaporated from the aqueous reaction medium
, ~
dilutes the chlorine dioxide and chlorine and a gaseous
mixture of steam, chlorine dioxide and chlorine is removed
from the generator 34. t
- The gaseous mixture removed from the generator
generally is forwarded by line 42 to a separator 44 to condense
the steam and recover an aqueous solution of chlorine dioxide
containing some dissolved chlorine which then is forwarded
to the bleach plant 22 by line 24~for use in the bleaching
steps designated D. The chlorine separated from the gaseous
mixture may be used in the bleaching steps designated C in
the bleach plant 22 as at least part of the chlorine feed in
line 26.
Alternatively, the separated chlorine is forwarded
by line 46 to form part of a chlorine feed 48 to a hydrogen
chloride reactor 50 wherein the chlorine reacts with hydrogen
fed by line 52 to form hydrogen chloride, which is forwarded ~ . .
by line 38 to the chlorine dioxide generator 34 as such or
as hydrochloric acid.
-- 11 -- .
. .
.
1057909
The quantity of chlorine fed by line 48 and hydrogen
fed by line 52 should be substantially the stoichiometric
quantities to form the amount of hydrogen chloride required
in the generator 34, so that a balance of chemicals may be
achieved in a continuous operation.
In addition, operation of the generator 34 at the
substantial boiling point of the reaction medium causes the
deposition of product sodium salt in the generator 34. The
generator 34 is operated with the hydrochloric acid providing
more than half the acid requirement, i.e. in accordance with
equation 3 above, so that a mixture of sodium chIoride and
.. .... . ..
sodium sulphate is deposited in the generator 34 and is
removed therefrom by line 54.
The form of the sodium sulphate present in the
mixture in line 54 depends on the acidity of the reaction
mixture in the generator 34, and may be sodium bisulphate
if the reaction is carried out at high acidity typically at ~-
above 9N, sodium sesquisulphate at acidities around 5 to 7
and neutral sodium sulphate at acidities below about 4N.
The sodium chlorate solution fed to the generator
34 by line 36 may be provided by electrolysis in a suitable
chlorate cell 56 of an aqueous sodium chloride solution fed
thereto by line 58. The by-product hydrogen formed in the
latter electrolysis is removed by line 60 from the cell 56
and is used partially to provide the hydrogen feed in line
; 52 to the hydrogen chloride reactor 50 and partially is
discharged by line 62. The discarded hydrogen in line 62
- may be used to form part of the water requirement of the
system.
- 12 -
-` lQ57~
The black liquor in line 64 is evaporated in an
evaporator 66, prior ~o passage by lines 68 and 70 to a
furnace 72 of any convenient construction. The water re-
covered from the evaporator 66 by line 74 may be used to
provide at least part of the water required of the system,
for example, as at least part of the water fed to the bleach
plant 22 in line 30, after suitable cleaning, if required.
Sodium sulphate is added to the black liquor,
either in solid or slurry form or as an aqueous solution
thereof, either directly, or indirectly, typically by line
76 to the concentrated black liquor in line 68. The sodium
sulphate is used to make up the sodium and sulphur values
lost from the system in the chemical recovery and regeneration
processing steps.
The black liquor forms in the furnace 72 a smelt
containing sodium sulphide and sodium carbonate. The smelt
is dissolved in a smelt dissolver 78 by water, generally
weak wash water from the washing of calcium carbonate mud,
fed by line 80, to form a green liquor in line 82.
2~ The green liquor, after clarification to remove
dregs, is passed to a causticizer 84 of conventional con-
struction wherein the sodium carbonate values in the green
liquor are to a large extent converted to sodium hydroxide by
lime fed by line 86 from a lime kiln 88. The calcium
carbonate precipitated from the resulting white liquor is
separated and after washing to remove entrained white liquor
(not shown) is returned to the lime kiln 88 by line 90. The
aqueous solution resulting from this washing is known as
"weak wash water" which may be used, as mentioned above, to
form the green liquor.
- 13 -
., , ~ :
- , . ., : :
lOS79(19
The white liquor resulting from the causticization
step in line 92 is recycled to provide at least part of the
pulping liquor in line 14.
The mixture of sodium chloride and sodium sulphate
removed from the chlorine dioxide generator 34 by line 54 is
passed to a dissolver 94 wherein the mixture is dissolved in
water fed by line 96.
The relative proportions of the sodium chloride and
sodium sulphate in the mixture depends on the relative pro-
portions of hydrochloric acid and sulphuric acid present in
... . ~ , . . .
the reaction medium.
The quantity of chlorine dioxide and quantity of
sodium sulphate produced in the generator 34 depend on the
individual requirements of the mill for these products. The
present invention is concerned with a pulp mill operation
where the requirements of chlorine dioxide and sodium sulphate
are such that the generator 34 must be operated with an acid
mixture of hydrochloric and sulphuric acids such that it is
possible only to produce the sodium sulphate in admixture with
sodium chloride.
In those instances where the sodium sulphate require-
mentis such that equation 2 above applies, then the sodium
sulphate is recovered in substantially pure form and may
be forwarded directly by line 76 to the black liquor.
In thls embodiment, the weight ratio of sodium
sulphate to sodium chloride is less than 0.17 and hence the
phase diagram of Figure l to the left of the eutectic points
applies.
.
1~)57~V9
The aqueous solution of sodium sulphate and sodium
chloride resulting from the dissolver 94 is forwarded by line
98 to a first evaporator 100, which is the first stage of
three multiple effect evaporators 100, 102 and 104. Water
is evaporated from the solution in each of the evaporators
100, 102 and 104, sodium chloride being deposited from the
solution in each evaporator and being removed by lines 106,
108, 110 respectively, to provide a combined sodi~m chloride
product stream 112.
The concentration of the solution in the evaporators
100, 102 and 104 continues until the solution is substantially
saturated with sodium sulphate. The evaporators 100, 102
and 104 operate at successively lower temperatures, typically
about 100 C, about 75C and about 50C, the evaporators being
indirectly heated by steam, the steam for evaporators 102
and 104 being provided by the steam formed in evaporators
100 and 102 respectively.
The aqueous solution resulting from evaporator 104
has a reduced sodium chloride content as compared to the
solution in line 98 and is substantially saturated with respect
both to sodium sulphate and sodium chloride.
The sodium chloride recovered from the evaporators
100, 102 and 104 in line 112 may be put to a variety of
uses. For example, it may be used to provide part of the
sodium chloride solution fed by line 58 to the chlorate cell
56 and/or part of the sodium chloride solution which is fed
by line 114 to a caustic-chlorine cell 116 wherein there is
formed the sodium hydroxide feed solution for the bleach plant
22 in line 28. The caustic-chlorine cell 116 also produces
- 15 -
.,: .
~ .
1057909
chlorine which is removed by line 118. The chlorine gas in
line 118 may provide the chlorine requirement of the bleach
plant in line 26, with any excess chlorine being recovered by
line 120. The excess chlorine in line 120 may be used to
provide part of the chlorine feed in line 48 to the hydrogen
chloride reactor 50. Alternatively, the sodium chloride may
be sold as such.
The aqueous solution o sodium chloride and sodium
sulphate resulting from the third evaporator 104 is passed
by line 122 through a heater 124 wherein the solution is
heated, typically to about 100C, so that it becomes under-
saturated with respect to sodium chloride and by line 126
to a precipitator 128.
,, ,
To the heated solution in the precipitator 128 is
added solid sodium chloride by line 130 causing precipitation
of sodium sulphate from the solution. The sodium chloride
fed by line 130 may be provided by part of the sodium chloride
recovered from the evaporators 100, 102 and 104 in line 112.
The sodium sulphate which is precipitated in the
precipitator 128 is forwarded by line 76 to the concentrated
black liquor in line 68. The mother liquor is recycled by
line 132 to the evaporators 100, 102 and 104, typically by
feeding, as illustrated, to the liquid entering the third
evaporator 104.
The manner of concentration of the aqueous solution
of sodium sulphate and sodium chloride in line 98 to the
substantial saturation point of the solution with sodium
sulphate and precipitating sodium chloride therefrom may be
varied from that illustrated.
- 16 -
, ~057909
Similarly, the manner of achieving deposition of
~26
sodium sulphate from the heated solution in line ~ may be
varied, as desired. For example, the heated solution in
line 126 may be evaporated to render it more concentrated
with respect to sodium chloride, thereby resulting in pre-
cipitation of sodium sulphate from the solution.
Figure 3 illustrates an alternative embodiment to
the embodiment outlined in Figure 2, applicable when the
weight ratio of sodium sulphate to sodium chloride in the
mixture formed in the chlorine dioxide generator is greater
than 0.17, and hence the portion of the phase diagram of
Figure 1 to the right of the eutectic point applies.
In the embodiment of Figure 3, a mixture of sodium
sulphate and sodium chloride, recovered from a chlorine
dioxide generator operating as described above with reference
to chlorine dioxide generator 34, but with a lesser proportion
of the acid requirement being provided by hydrochloric acid, -
is passed by line 200 to a dissolver 202 which the mixture
P is dissolved in water-, preferably the minimum quantity, fed
by line 204.
The resulting solution is passed to a first evaporator
206, which is the first stage of three multiple effect evaporat-
ors 206, 208 and 210. Water is evaporated from the solution
in each of the evaporators 206, 208 and 210, sodium sulphate
being deposited from the solution in each evaporator and being
removed by llnes 212, 214 and 216 respectively, to provide a
combined sodium sulphate product stream 218.
The concentration of the solution in the evaporators
206, 208 and 210 is continued until the solution is substantial-
- 17 -
lOS7gO5~ .
ly saturated with sodium chloride. The evaporators 206, 208
and 210 operate at successively higher temperatures, typically
about 50C, about 75C and about 100C, the evaporators being
indirectly heated by steam, the steam for evaporators 206 and
208 being provided by the steam formed in the evaporators 208
and 210 respectively. Ifdesired, all the evaporators may be
carried out in a single evaporator, typically at about 100 C.
The aqueous solution resulting from the evaporator
210 has a reduced sodium sulphate content as compared to the
solution in line 205 and is substantially saturated with
respect both to sodium sulphate and sodium chloride.
The sodium sulphate recovered from the evaporators
206, 208 and 210 in line 218 is forwarded as make-up chemical
to the concentrated black liquor prior to passage thereof
to the furnace, in analogous manner to the passage of sodium
sulphate by line 76 to the concentrated black liquor in line
68 as described above with reference to Figure 2.
The aqueous solution of sodium chloride and sodium
sulphate resulting from the third evaporator 210 is passed
by line 220 through a cooler 222, wherein the solution is
cooled, typically to a temperature of about 50 C, so that
it becomes undersaturated with respect to sodium sulphate,
and by line 224 to a precipitator 226. Some precipitation
of sodium chloride may occur during the cooling operation,
and this precipitated sodium chloride may be separated from
the cooled solution.
To the cooled solution in the precipitator 226 is
added solid sodium sulphate by line 228 causing precipitation
of sodium chloride from the solution. The sodium sulphate
fed by line 228 may be provided by part of the sodium sulphate
- 18
1057909
recovered from the evaporators 206, 20~ and 210 in line 218.
The sodium chloride which is precipi-tated in the
precipitator 226 is recovered by line 230 and may be put to
a variety of uses, as discussed above in connection with the
sodium chloride recovered in line 112 in Figure 2. The
mother liauor is recycled by line 232 to the evaporators 206,
208 and 210, typically by feeding, as illustrated, to the
first evaporator 206.
The manner of concentration of the aqueous solution
of sodium sulphate and sodium chloride in line 205 to the ~ -
substantial saturation point of the solution with sodium
chloride and precipitating sodium sulphate therefrom may be
varied from that illustrated.
Similarly, the manner of achieving deposition of
sodium chloride from the cooled solution in line 224 may
be varied as desired. For example, the cooled solution in
line 224 may be evaporated to render it more concentrated
with respect to sodium sulphate, thereby resulting in
precipitation of sodium chloride from the solution.
The invention is illustrated further by the
following Example:
EX~MPLE
A chlorine dioxide generator, maintained under a
reduced pressure, is operated at the boiling point of an
aaueous solution containing 1.4 moles of hydrochloric acid
and 0.3 moles of sulphuric acid per mole of sodium chlorate.
- This results in the deposition of a mixture of 0.3 mole of
sodium sulphate and 0.4 mole of sodium chloride per mole of
chlorine dioxide.
-- 19 --
1057909
A mixture of 23.38g of sodium chloride and 42.6g
of sodium sulphate is leached with water at 100C and operat-
ing at the eutectic point. The amount of water required per
gram of sodium chloride leached is 2 69g, i.e. a total of
62.89g for 23.38g of sodium chloride and the quantity of
sodium sulphate leached per gram of sodium chloride is 0.169g,
and hence 3.95g of sodium sulphate is leached from the mixture
38,~:r
along with the 23.38g of sodium chloride, leaving 19.13g of
pure sodium sulphate for forwarding to the mill per mole of
chlorine dioxide producedO
,.... ..
The present invention, therefore, is able to
provide an integrated pulp mill system in which the sodium
sulphate and chlorine dioxide requirements are provided
while avoiding the feeding of sodium chloride contamination
to the pulp mill.
Modifications are possible within the scope of
the inventionO
~ . .
- 20 -
