Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the removal of sodium
chloride from pulp mill systems, especially from Kraft mill
operations.
In the Kraft system, cellulosic fibrous material,
generally wood chips, is subjected to pulping with a pulping
liquor containing sodium hydroxide and sodium sulphide, known
as white liquor, and the resulting pulp is separated from the
spent pulping liquox, known as black liquor. Usually the pulp
is washed and subjected to brightening and purification steps
to provide a pulp having the required brightness.
The black liquor is subjected to recovery and re-
generation steps to form white liquor for use in contacting
further pulp. The recovery and regeneration steps involve
concentration of the black liquor and furnacing of the con-
centrated black liquor ~o form a smelt containing sodium sulphide
and sodium carbona~e. After the smelt is dissolved in water
to form green liquor, the sodium carbonate values thereof are
subjected to causticization to sodium hydroxide, thereby forming
white liquor for recycle to the pulping step.
To make up sodium and sulphur values lost from the
system during the recovery and regeneration operation, a sodium-
and sulphur- containing material generally is added to the
black liquor in the required quantity. Generally, the sodium-
and sulphur- compound used is sodium sulphate, which may be
provided from any convenient source.
The furnacing operation results in the sublimation of
solid materials from the furnace. The solid materials are
removed, usually by electrostatic precipitators, from the flue
gases, the removed material being known as precipitator dust
or fume deposit. The precipitator dust may contain a variety
of materials depending on the constitution of the black liquor
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and the conditions of furnacing. Usually, the precipitator
dust consists mainly of sodium sulphate and is returned to
the black liquor.
In coastal regions where logs are floated in sea water,
sodium chloride is introduced to the mill with the wood chips.
The presence of sodium chloride in the balck liquor results
in the presence of sodium chloride in the precipitator dust.
If the precipitator dust therefore is recycled directly to
the black liquor, the sodium chloride, which is unaffected
chemically by the furnacing operation, will build up in the
furnace as a dead load, causing considerable problems in the
furnacing operation.
The precipitator dust usually contains small amounts
of sodium carbonate, in addition to sodium sulphate and sodium
chloride. It has previously been suggested in Canadian Patent
No. 958,158 to purge sodium chloride from the precipitator dust
in an amount approximately equal to the amount present therein,
followed by recycle of the purified sodium sulphate to the
black liquor. This prior art procedure involves preferentially
leaching the sodium chloride from the precipitator dust with
water or an acid medium including recycled leach liquor con-
taining dissolved sodium sulphate and sodium chloride to obtain
a dilute readily filtrable slurry. The undissolved portion
of the precipitator dust, which is mostly sodium sulphate, is
recovered by filtration and forwarded to the black liquor.
The aqueous solution containing dissolved sodium chloride par-
tially is recycled as leaching liquor and partially is removed
from the system by sewering.
Operation in this manner has the drawback that
quantities of sodium sulphate, and hence soda and sulphur
values, are lost by the sewering steps. The aqueous phase
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contains dissolved quantities of sodium sulphate along with
the sodium chloride, and hence, when the sodium chloride
solution is sewered, the sodium sulphate values present in
the solution also are sewered.
In addition, since the sodium chloride is sewered as
an aqueous solution thereof, the potentially valuable sodium
chloride is not recovered. Further, in this prior art system,
in order to avoid loss of the sodium carbonate values con-
tained in the precipitator dust and which normally would be
present in the aqueous phase recycled and sewered, although
some sodium carbonate values may remain in the solid phase,
the sodium carbonate values are converted to sodium sulphate
by the use of the acid leach liquor or by addition of acid to
the slurry. Spent sulphuric acid from a chlorine dioxide
generator, wherein sodium chlorate is reduced with sodium
chloride in the presence of sulphuric acid typically is used
to provide the acidity.
In certain chlorine dioxide producing systems, such
as described in Canadian Patents Nos. 825,084 and 826,577
liquid effluents containing spent sulphuric acid are not formed.
Therefore, in a mill using one of the latter chlorine
dioxide-forming processes to furnish its chlorine dioxide
requirement, in order to utilize the prior art proposal of
Canadian Patent No. 958,158 for the n~val of sodium chloride,
an additional sulphuric acid requirement is necessary if loss
of the sodium carbonate values is to be avoided.
In accordance with the present invention, the loss
of sodium sulphate values experienced by the prior art process
through sewering is eliminated, sodium chloride is recovered
in substantially pure solid form for use in a number of ways,
liquid effluents are avoided and the necessity for neutraliza-
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tion of sodium carbonate is eliminated.
The process of the present invention utilizes the
leaching procedure used in the above described prior art
process with the optional exception of the addition of acid.
Thus, the precipitator dust is slurried with water and recycled
mother liquor to dissolve all the sodium chloride values
therein to provide a slurry of workable consistency, typically
containing about 10 to 20% solids, allowing readily separation
of the solid and liquid phases by filtration. Some sodium
sulphate also is dissolved, along with any sodium carbonate
present.
After filtration, the solid phase, consisting of
sodium sulphate substantially free from sodium chloride,
possibly together with some sodium carbonate, is recycled to
the black liquor. In the prior art system of Canadian Patent
No. 958,158 prior to filtration, the sodium carbonate values
are converted to sodium sulphate values using spent sulphuric
acid contained in the Ieach liquor and are precipitated from
the aqueous phase and hence, after filtration, the recycled
solid contains sodium sulphate from the sodium carbonate
values.
In the process of the present invention this step
may be omitted since the sodium carbonate values are not lost
as will become apparent hereinafter. However, it may be
utilized, if desired. Since the neutralization of the sodium
carbonate step adds to the processing steps and is unnecessary,
it is preferred in the present invention to omit the step.
In order to increase the proportion of sodium sulphate
in the solid phase and hence reduce the quantity in the
aqueous phase, the leaching operations may be carried out at an
elevated temperature, up to 212 F, but typically about 140 to
160F
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The mother liquor resulting from the filtration is
a saturated solution of sodium chloride, sodium sulphate and
usually sodium carbonate. As mentioned above, in the prior
art system, this solution partially is recycled to dilute slurry
and partially is discharged to sewer.
In accordance with the present invention, the aqueous
solution of sodium chloride, sodium sulphate and sodium car-
bonate is manipulated to deposit sodium chloride thereform
and substantially pure sodium chloride is recovered from the
deposited materials. The sodium sulphate and sodium carbonate
values par~ially are recycled as dilutent solution and,
partially are returned to the recovery and regeneration steps,
after concentration, if desired, to recover the same or solids.
Hence, liquid effluents from the- system are eliminated and
no sodium sulphate or sodium carbonate values are lost
from the system. Alternatively, all the mother liquor may be
recycled to the leaching step.
The invention is described further with reference to
the accompanying drawing, which is a schematic flow sheet of
one embodiment of the invention.
~ In the drawing, 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.
The resulting pulp and ~lack liquor or spent pulping
liquor are separated and the pulp is washed in a brown stock
washer 16. The pulp may be washed with water fed by line 18
or "contaminated condensate", or aqueous bleach plant effluent.
The washed and unbleached pulp is fed by line 20 to
a bleach plant 22 wherein the pulp is subjected to a series of
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bleaching and purification processes, at least one of the
bleaching steps generally involving the use of one or more
chlorine-containing bleaching agents. Generally, the bleaching
and purification processes involve bleaching with chlorine
dioxide or mixtures thereof, and purification by caustic -
extraction, using aqueous sodium hydroxide solution, typically
in the well known CEDED sequence. The pulp is washed during
the bleach plant operations, typically after each bleaching or
caustic extraction operation, by fresh water or white water.
The spent wash water from the bleach plant washing operations
together with the spent chemicals from the bleaching and
caustic extraction steps proviae ~leach plant effluent. Preferably,
the washing operations involve countercurrent flow of pulp
and wash water. The bleach plant effluent contains sodium
chloride values resulting from reactions between the soda and
chlorine materials used in the bleaching and purification
steps. The bleached and purified pulp of required brightness
is recovered from t~e bleach plant 22 ~y line 24 and is sold
as such or passed to paper-making procedures.
The black liquor from the brown stock washer 16 in
line 26 is passed to a recovery and regeneration zone 28 for
recovery of spent pulping chemicals and regeneration of pulping
liquor. In the recovery and regeneration zone 28 the black
liquor first is evaporated in an evaporator 30, prior to
passage by lines 32 and 34 to a furnace 36 of any convenient
construction. The water recovered from the evaporator 30 by
line 38 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 after suitable cleaning, if
required.
Sodium sulp~ate or other source of sodium and sulphur
values, such as, spent sulphuric acid, is added to the recovery
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and regeneration zone, typically by addition to the black
liquor, either in solid form or as an aqueous solution thereof
by line 40, for example, after evaporation in the evaporator
30. The sodium sulphate fed by line 40 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 36 a smelt
containing sodium sulphide and sodium carbonate. The smelt is
dissolved in a smelt dissolver 42 by water, generally weak
wash water from the washing of calcium carbonate mud, fed by
line 44, to form a green liquor.
If desired, the smelt may be fractionated to remove
the sodium sulphide values therefrom and thereby provide a
sulphide-lean green liquor, substantially free from sodium
sulphide values.
The green liquor after clarification to remove dregs,
is passed by line 46 to a causticizer 48 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 50 from a lime kiln 52. 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 52 by line 54. 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.
The white liquor resulting from the causticization step
in line 56 represents the effluent from the recovery and re-
generation zone 28 and contains the active pulping chemicals
sodium sulphide and sodium hydroxide. The white liquor is
recycled by line 56 to provide at least part of the pulping
liquor in line 14.
The furnace 36 has a flue gas stream 58 which contains
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ent~ained solids. Where the wood chips fed by line 10 contain
entrained quantities of sodium chloride, for example, when the
logs from which they are formed are sea-water borne, the black
liquor fed to the recovery and regeneration zone 28 by line
26 contains dissolved quantities of sodium chloride. The
presence of sodium chloride in the black liquor in line 26 may
arise from other sources, such as the bleach plant effluent.
The presence of sodium chloride in the black liquor
in line 26 gives rise to quantities of sodium chloride in the
flue gas in line 58. The bulk of the entrained solid matter
in line 58 is sodium sulphate, together with some sodium car-
bonate.
Where the quantity of sodium chloride introduced to the
system is low, i.e. less than 60 lbs/ton of pulp, the quantity
of sodiu~ chloride removed from the system with the pulp in
line 20 and in the flue gas in line 58 maintains the sodium
chloride content of the recycled white liqucr in line 56 at
a steady state value. Where larger amounts of sodium chloride
are involved, it is necessary to utilize the procedures outlined
in Canadian Patent No. 915,361, copending Canadian application
Serial No. 187,714 or copending Canadian application Serial
No. 206,076 in order to remove sufficient sodium chloride to
prevent build up in the white liquor in line 56.
The flue gas in line 58 is passed to a precipitator 60
wherein at least part, preferably substantially all, the en-
trained solids are removed from t~e flue gas stream, the remain-
ing gas being vented by line 62 to atmosphere, after scrubbing
to remove noxious gases, if required. The precipitator dust
collected in the precipitator 60 consists mainly of a mixture
of sodium sulphate, sodium car~onate and sodium chloride the
bulk of the solid being provided by sodium sulphate.
The precipitator dust in line 64 is slurried in a leach
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tank 67 with water and recycled mother liquor fed by lines 66
and 68 to form a slurry of consistency about 10 to 20% solids
in line 70.
The resulting slurry in line 70 is passed to a filter
72 wherein the solid phase, consisting substantially of sodium
sulphate, is separated. The separated sodium sulphate is passed
by line 74 to form at least part of the sodium sulphate fed
to the black liquor by line 40. The remainder of the sodium
sulphate fed by line 40 to the recovery and regeneration zone
28 may be fed by line 76 from any convenient source thereof.
The sodium sulphate in line 74 may be forwarded to the black
liquor in line 32 in any other convenient manner. For example,
the sodium sulphate may be forwarded to the green liquor in
line 46 or to the white liquor in line 56.
Preferably, the slurrying operation is carried out at
an elevated temperature, preferably above about 150F, so that
the mother liquor in line 78 resulting from the filtration is
a hot aqueous solution of sodium chloride, sodium carbonate and
sodium sulphate.
The mother liquor is cooled in a crystallizer 80, with
evaporation, if desired, to result in precipitation of sub-
stantially pure sodium chloride which is recovered by line R2.
After separation of the pure sodium chloride, the
resulting saturated solution in line 84 is divicled into two
streams, one constituting the recycle stream in line 68, typic-
ally heated to the leach temperature prior to feed to the
leach tank 67, and the other passing by line 86 to the recovery
and regeneration zone 28. The passage of the saturated solution
to the zone 28 may be to any convenient location. For example,
the saturated solution in line 86 may be included in the green
liquor in line 46 by direct addition thereto by line 88.
Alternatively, the solution may be added to the white liquor in
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line 56 by line 90. Further, the saturated solution may be
added to the concentrated black liquor in line 32 by line 92.
If desired, the saturated solution in line 86 may be partially
fed to two or more different locations.
The solution in line 86 contains the sodium sulphate
values which otherwise would be discarded in the prior art
process of Canadian Patent No. 958,158. In addition, the
solution in line 86 contains sodium carbonate values, preferably
in an amount substantially equal to that contained in the
precipitator dust and hence these values are recycled within
the system.
Other manners of manipulation of the mother liquor are
pos~ible to deposit sodium chloride. For example, the mother
liquor may be evaporated to deposit pure sodium chIoride which
is recovered by line 82.
Alternatively, the sodium chloride may coprecipitate
with sodium carbonate and sodium sulphate by evaporatlon of the
mother liquor. After separation of the deposited solids, the
resulting solution is forwarded as the saturated solution of
line 84.
The quantity of sodium chloride precipitated from the
mother liquor and recovered by line 82, whether deposited in
pure form or in admixture with other salts, preferably is
substantially equal to the quantity of sodium chloride in the
precipitator dust in line 64, so that a balanced system is
provided.
Where the sodium chloride is precipitated and recovered
as a mixture, the sodium chloride is recovered in substantially
pure form and the sodium sulphate and sodium carbonate is
returned to the mill. The separation may be achieved in any
desired manner. For example, the mixture may be leached to
dissolve the sodium sulphate and sodium carbonate valu~s from
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the mixture together with some of the sodium chloride values,
leaving substantially pure sodi~m chloride. The aqueous
solution of sodium sulphate, sodium carbonate and sodium chlor-
ide obtained in this leaching may be forwarded to the system.
Therefore, it is possible by the present invention to
achieve separation of substantially pure sodium chloride from
precipitator dust without discard of sodium sulphate or sodium
carbonate values, and hence, the required removal of sodium
chloride is achieved without the necessity of sewering liquids.
The sodium chloride, recovered in substantially pure
form in line 82, may be sold as such or put to a variety of
uses in the mill. For example, it may be used to form bleach
plant chemicals, such as, chlorine dioxide and chlorine by use
as a reducing agent for sodium chlorate, or sodium hydroxide
for use as caustic extraction liquor by electrol~ysis of an
aqueous solution thereof. Alternatively, the sodium chloride
may be used to form sodium chlorate by electrolysis of an
aqueous solution thereof, the sodium chlorate being used to
form chlorine dioxide.
Modifications are possible within the scope of the
invention.
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