Note: Descriptions are shown in the official language in which they were submitted.
iO5~86V
The p~esent invention relates to the generation
of chlorine dioxide, more particularly, to the integration
of chlorine dioxide generation with bleach plant operations.
This application is a division of application
Serial No. 208,075 filed August 29, 197~.
It is well known to form chlorine dioxide by
reduction of a chlorate in an acid medium in accordance
with the equation:
C103 + Cl + 2H > ClO2 + 1/2 C12 + H2O
Typical processes are disclosed in Canadian Patents Nos. -
825,084 and 826,577, wherein sodium chlorate is reduced
by chloride ions, provided by sodium chloride, hydrogen `
chloride or mixtures thereof, in the presence of sulphuric
acid. Other procedures are disclosed in Canadian Patents
Nos. 913,328 and 908,944 wherein sodium chlorate is
reduced by hydrochloric acid which also provides the
acid requirement. A further procedure is disolosed in
Canadian Patent No. 920,773 wherein sodium chlorate is
reducea by chloride ions in the presence of sulphuric
acid, sodium sulphate is deposited from the reaction
mixture, and an aqueous solution of the sodium sulphate
- is treated with hydrogen chloride to form sodium chloride
ana sulphuric acid, the latter being recycled to the
reaction mixture.
In the production of bleached cellulosic fibrous
material pulp it is known to bleach cellulosic fibrous
material pulp, formed in any convenient manner, typically
by digestion of wood ships using the Kraft process with
sodium hydroxide and sodium sulphide as the active pulping
chemicals, with chlorine-containing bleaching chemicals, -especially chlorine dioxide and chlorine.
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The present invention is directed to a novel
integration of a chlorine dioxide-producing process and a
bleach plant operation to achieve efficient and economic
utilization of chemicals.
In accordance with this invention, there is
provided a continuous pulp mill process to provide
bleached pulp which comprises: continuously digesting wood
chips with a pulping liquor contalning sodium sulphide and
sodium hydroxide as active pulping chemicals to form a wood
pulp, continuously separating the wood pulp from spent ~ .
pulping liquor, continuously washing the separated pulp
free from spent pulping liquor to provide a washed but
otherwise untreated wood pulp, continuously subjecting the
spent pulping liquor to recovery and regeneration opera-
tions to form fresh pulping liquor, continuously rec~cling
the fresh pulping liquor as at least part of the pulping
liquor, continuously incorporating sodium sulphate into
.. ..
the spent pulping liquor to make up chemical losses in -
the recovery and regeneration steps, the recovery and ~ .
regeneration steps including furnacing spent pulping .
liquor to provide a solid mass and a flue gas stream
containing entrained sublimed solids, continuously intro-
ducing sodium chloride into the spent pulping liquor whereby
the entrained solids consist mainly of sodium sulphate
and sodium chloride, continuously removing the entrained
solids ~rom the flue gas stream, continuously leaching -
the removed solids with water to dissolve substantially all
the sodium chloride values thereof along with part of the
sodium sulphate values thereof and leave a solid mass
consisting essentially of sodium sulphate, continuously
recycling the solid mass to the spent pulping liquor,
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continuously generating chlorine dioxide in a reaction
- vessel by r~duction of sodium chlorate using sodium chloride
in an aqueous acid reaction medium containing sulphuric
acid and having an acidity of about 2 to about 4.8N in
accordance with the equation:
NaC103 + NaCl + H2SO~ ~ C12 + 1/2 C12-~ H20 + Na2so4
continuously maintaining the aqueous reaction medium at
its boiling point at a temperature below that above which
substantial decomposition of chlorine dioxide occurs while
maintaining the reaction vessel under a reduced pressure
to form the chlorine dioxide in gaseous admixture with
water vapour and chlorine and to deposit sodium sulphate
produced by the chlorine dioxide generating reaction o~
equation 1 from the reaction medium, continuously removing
the gaseous mixture of chlorine dioxide, chlorine and water
vapour from the reaction vessel, continuously separating
the removed gaseous mixture into an aqueous solution of
chlorine dioxide containing no more than 10~ of the
available chlorine thereof as chlorine and the remainder
of the available chlorine as chlorine dioxide, and gaseous
chlorine, continuously subjecting the washed but otherwise
untreated wood pulp to bleaching utilizing the aqueous
solution of chlorine dioxide to provide bleached pulp
having a predetermined brightness, continuously removing
the deposited sodium sulphate from the reaction vessel,
continuously forwarding part of the removed sodium sulphate
to the spent pulping liquor as at least part of the make up
sodium sulphate, continuously forming a substantially
saturated aqueous solution of the remainder of the sodium
sulphate, mixing the latter saturated aqueous solution of
sodium sulphate with the aqueous solution resulting from
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the le~ching, continuously contacting the latter mixture - -
with hydrogen chloride to deposit therefrom sodium chloride
and to form sulphuric acid in accordance with the
equation: .
Na2 4 ~ 2NaCl + H2SO4 -(2)
continuously separating the sodium chloride from the
sulphuric acid, continuously forwarding the sulphuric
acid containing dissolved sodium chloride to the reaction
vessel ~o provide the sulphuric acid requirement of the
chlorine dioxide generating reaction of equation 1, contin-
uously recovering from the separated sodium chloride an
amount of solid substantially pure sodium chloride
substantially equal to the amount present in the solution
resulting from the leaching, continuously forming an
aqueous solution of the remainder of the separated sodium
chloride, continuously electrolyzing the latter aqueous
solution to form an aqueous solution of sodium chlorate ~ `
containing the stoichiometric requirement of sodium
chlorate of the chlorine dioxide producing reaction of
equation 1, and hydrogen in accordance with the equation:
NaCl + 3H2O ~ NaClO3 + 3H2 ~(3)
continuously forwarding the aqueous solution of sodium ~ -
chlorate containing the stoichiometric requirement of ::
sodium chloricle of the chlorine dioxide producing reaction
of equation 1 not provided by the sodium chloride contained
in the forwarded sulphuric acid, continuously reacting
substantially one-third of the molar amount of hydrogen ~:
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formed in the electrolysis reaction of equation 3 with
chlorine .in accordance with the equatlon:
H2 + C12 ~ 2HCl -~4)
the gaseous chlorine separated from the gaseous mixture of
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chlorine dioxide, chlorine and water vapour providin ;
part of the molar amount of chlorine required by equatio~
4, and continuously utilizing the hydrogen chloride formed
by equation 4 as the hydrogen chloride requirement of
equation 2.
The process of the invention is described
further, by way of illustration, with reference to the
accompan~ing drawing, which is a schematic flow sheet of
one embodiment of the invention.
Referring to the drawing, wood chips, or other
cellulosic fibrous material, are fed by line 110 to a
digester 112 wherein they are cooked to form a pulp by the
action of a pulping liquor fed by line 114 containing sodium
hydroxide and sodium sulphide as the active pulpin~ chemicals.
The pulp is washed and separated from the spent
pulping liquor (black liquor) in a brown stock washer 116
and is passed by line 118 to a bleach plant 120. In the
bleach plant 120, the pulp is bleached to the required
brightness and removed therefrom by line 122.
The bleaching oper~tions occurring in the bleach
plant 120 are carried out by an aqueous solution of chlorine `
dioxide fed by line 124. The precise sequence of steps
utilized in the bleach plant may vary widely, although,
it is preferred to employ~a series of alternate bleaching
and caustic extraction operations in which bleaching is
carried out with the chlorine dioxide solution fed by
line 124 and the caustic extraction is carried out using
aqueous sodium hydroxide solution. Washing commonly is ;
carried out after each bleaching and caustic extraction
step.
The black liquor leaving the brown stock washer
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by line 126 is subjected to recovery and regeneration
operations to form white liquor in line 128 which is
recycled to provide at least part of the pulping liquor in
line 114. The black liquor first is evaporated in an
evaporator 130 and the concentrated black liquor is passed
by lines 132 and 134 to a furnace 136 wherein the black
liquor is burned.to provide a solid smelt which contains
sodium sulphide and sodium carbonate. The smelt is
dissolved in water to form green liquor which is passed
by line 138 to a causticizer 140 wherein the sodium
carbonate values of the green liquor are converted to
sodium hydroxide, generally using lime, and the resulting
white liquor is recycled by line 128.
Sodium sulphate, or other source of sodium-.and ~.
sulphur- values, is introduced to the black liquor ~o
make up losses of sodium and sulphur values during the
recovery and regeneration operations. Typically, sodium
sulphate is added to the concentrated black liquor in line
132 by line 142. .
The furnace 136 produces a flue gas stream which . .-
contains entrained quantities of sublimed solids and the
flue gas in line 144 generally i9 passed through a ..
precipitator 146 wherein the entrained solids are removad
from the flue gas prior to the discharge of the latter to `: .
atmosphere by iine 148, after further treatment to remove .
any noxious or odorous gases, if desired.
The black liquor in line 126 may contain
dissolved quantities of sodium chloride arising from
various sources. For example, in coastal regions where
logs are floated in sea water, sodium chloride is
introduced to the mill with the wood chips in line 110.
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1~3860
The presence of sodium chloride in the black liquor may
arise from other sources, for example, from the bleach
plant 120, if effluents therefrom, such as spent wash
water is introduced to the blac]c liquor.
The entrained solids :in the flue gas in line 144
and hence the fume deposit obta:ined in the precipitator
146 contains mainly sodium sulphate and sodium chloride.
Small amounts of sodium carbonate also may be present.
The fume deposit is passed by line 150 to a leacher 152
wherein the solid is leached with water fed by line 154,
preferably in an amount sufficient to dissolve all the
sodium chloride values of the fume deposit. The resulting
solid mass consisting mainly of sodium sulphate is recycled
to the black liquor by line 156.
The preferably sàturated solution of sodium
chloride and sodium sulphate resulting from the leacher
152 is passed by line 158 to a chlorine dioxide generation
~one 160. In the chlorine dioxide generation zone 160,
there is provided a chlorine dioxide generator 162 contain-
ing an aqueo~s reaction medium which contains sodium
chlorate, sodium chloride and sul~huric acidy The
reaction medium generally is malntained at its boiling -
point while the generator 162 is maintained under a
reduce~ pressure in any convenient manner. The gaseous
mixture of chlorine dioxide, chlorine and steam formed in
the generator 162 is passed by line 164 to a separator
166 of any convenient construction, wherein the chlorine -
dioxide is formed into an aqueous solution which passes
by line 124 to the bleach plant 120. The separation
- 30 achieved in the separator 166 should be such that no more
than about 10% of the available chlorine in the chlorine
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dioxide solution in line 124 is provided by chlorine,
with the remainder of the available chlorine being
provided by chlorine dioxide.
The sodium sulphate which is precipitated in
the generator 162 is removed therefrom by line 168, partly
is passed by lines 169 and 142 to the concentrated black
liquor as the make up chemical and partly by line 170 to
a reaction tank 172. The sodium sulphate may be forwarded
by line 169 in solid or aqueous solution form and is
passed to the reaction tank 172 by-line 170 prQferably as
a substantially saturated aqueous solution thereof.
In the reaction tank 172, the aqueous medium
provided by the feed solutions in line 158 and 170 is
contacted with hydrogen chloride fed by line 174. This
causes reàction of the sodium sulphate to sodium chloride,
which precipitates, and sulphuric acid, in accordance with
the equation:
Na2S4 + 2HC1 ~ 2Nacl + H2SO4 - ~ ~-
The hydrogen chloride fed by line 174 to the
reaction tank 172 may be in gaseous form or as hydrochloric
- acid. In the latter case, the total quantity of water
present in the reaction tank should be insufficient to
dissolve all the sodium chloride. Similarly, the aqueous
sol~tions in lines 158 and 170 may be less th~n saturated
provided the total quantity of water is insufficient
- to dissolve all the ~odium chloride.
The liquid effluènt from the reaction tank 172
contains sulphuric acid and sodium chloride as is passed
by line 176 to the chlorine dioxide generator 162. The
solid sodium chloride deposited in the reaction tank 172
is removed therefrom by line 178, partially is recovered
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1053~6~
in substantially pure form by llne 180 and partially is
made up into an aqueous solution which is passed by line
182 to a chlorate cell 184.
In the chlorate cell :L~4, the aqueous sodium
chloride solution is electrolysed to form a sodium
chlorate solution which passes by line 186 to the chlorine
dioxide generator 162. Gaseous hydrogen also is formed
in the electrolysis and is removed from the cell 184 by :~
line 188. Approximately two-thirds of the mole amount of
hydrogen is vented to atmosphere by line 190 or burned to
form water, if desired. The remaining approximately one-
third mole amount is forwarded by line 192 to a hydrogen -:
chloride reactor 194.
The chlorine separated from the chlorine dioxide
in the separator 166 is passed by line 196 to the hydrogen
chloride reactor 194, wherein, together with chlorine fed
by line 198, the chlorine is reacted with the hydrogen
fed by line 192 to form hydrogen chloride which passes by
line 174 to the reaction tank 172.
To preserve the stoichiometry of the system
described with reference to Figure 2, and where the
chlorine dioxide generation zone 160 constitutes the sole
source of make up sodium sulphate in line 142, additional
sulphuric acid may be required in the chlorine dioxide
generator 162 in addition to that provided from the
reaction tank 172 and such additional sulphuric acid may
be provided by line 200.
- Where, however, an additional external source
of sodium sulphate is available as make up chemical by
30 . line 202 the sodium sulphate in lines 170 and 15~ may be
sufficient to provide the total sulphuric acid r~quirement.
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1~53860
Since it may not be possible to convert all the sodium
sulphate to sodium chloride in the reaction tank 172, an
excess over stoichiometry may be continuously cycled between
the reaction tank 172 and the generator 162.
The amount of sodium chloride present in the
solution forwarded by line 176 to the chlorine dioxide
generator 162 generally is insufficient to provide all the
stoichiometric requirement of sodium chloride for the pro-
duction of chlorine dioxide. The amount of sodium chloride ~-
fed by line 176 depends on the quantity present in the
flue gas 144. To provide the required amount of sodi;um
chloride in the generator 162, part of the sodium chloride
in line 176 may be fed to the gènerator 162, usually as ,-
an aqueous solution thereof. Alternatively, the required
amount is present in the aqueous solution fed to the cell
1~4 by line 182 and hence is present in the sodium chlorate
solution fed by line 186. The latter mode of operation
is the preferred.
The quantity of pure sodium chloride recove~ed
by line 180 corresponds to the quantity of sodium chloride
- present in the solution in line 158 in a stoichiometric
continuous operation. Hence, this embodiment of the
invention provides separation of the sodium chloride present
in the fume deposit in a pure form without discard of the~ ;
other useful chemicals contained in the fume deposit.
The sodium chloride recovered in line 180 may ~e
sold as such or used in a variety of ways. For example,
the sodium chloride may be used to provide the sodium
hydroxide requirement of the bleach plant 120 and the
chlorine requirement of line 198.
Therefore, the process of the invention provides
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a process in which chlorine dioxide is formed in admixture
with chlorine in the chlorine di.oxide genera,ion zone 160,
the chlorine dioxide and chlorine are separated in a
separator 166 to provide an aqueous solution of chlorine
dioxide in line 124 wherein no more than 10~ of the
available chlorine content of the aqueous solution is
provided by chlorine, the aqueous solution of chlorine
dioxide is used in the bleach plant 120 to bleach pulp in
all bleaching stages, the chlorine separated from the
chlorine dioxide is used to form hydrogen chloride in
the reactor 194 by reaction with hydrogen fed by line
192, and the hydrogen chloride formed thereby is used in
the chlorine dioxide generation zone 160.
- Modifications are possible within the scope of
the invention.
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