Note: Descriptions are shown in the official language in which they were submitted.
10';'04~ ;J
The present invention is directed to the removal
of sodium chloride, from pulping operations, more particu-
larly from chemical wood pulping operations including bleaching
procedures for the pulp, with enhanced efficiency of pulp
mill operations.
,',
Generally in the production of pulp suitable for
formation into paper, wood or other raw cellulosic fibrous
materials, is subjected to chemical digestion in a pulping
liquor to form a pulp of the cellulosic fibrous material.
The pulp thereafter is subjected to brightening and purifi-
i cation operations in a bleach plant.
~. .
The spent pulping liquor usually is subjected to
~, a series of recovery and regeneration operations to recover
:, . . . ........... . ..... .. ..
j pulping chemicals and to provide fresh pulping liquor. Gen-
erally, the pulping liquor contains alkali metal salts, usually
sodium salts, although potassium or lithium salts could be
~, employed.
Two well-known pulping processes are the Kraft
and soda processes. While the present invention will ~e
described hereinafter with particular reference to Kraft
and soda mills, the process is applicable also to other pulp
mill operations involving the use of alkali metal salts as
pulping chemicals and which utilize spent chemical recovery
and pulping chemical regeneration operations. Many such
.,~
operations involve the use of sulphur-containing chemicals,
including the Krat process, high yield pretreatment Kraft
process, alkaline sulfite process, neutral sulfite process,
i - 2 -
,~'`
107045~ -
acid sul~ite process, bisulfite process, acid bisulfite
process, polysulfide process, alkafide process. other
pulping operations which may be employed are the soda-
oxygen proccss and the holopulping process. The present
invention also is applicable to cross-recovery operations
used in mixed systems.
In the conventional Kraft process, raw cellulosic
fibrous material, generally wood chips, is digested, by
heating, in a pulping liquor, known as white liquor and
containing sodium sulphide and sodium hydroxide as the active
pulping chemicals, to provide a pulp and spent pulping liquor,
-~ known as black liquor. The black liquor is separated from
the pulp by washing in a brown stock washer and the pulp
then is passed to the bleach plant for brightening and
purification operations.
The black liquor then is passed to the recovery and
regeneration system in which the black liquor first is con-
centrated, usually by evaporation, and the concentrated black
liquor is burned in a furnace to yield a smelt containing primarily
sodium carbonate and sodium sulphide. A sodium-and sulphur-
containing compound, generally sodium sulphate, is added to
the black liquor generally prior to feed of the concentrated
black li~uor to the ~urnace, although such sodium-and sulphur-
containing compound may be added at any other convenient
point, such as to the white liquor prior to the digestion
:~.
~ step, to make up sodium and sulphur values lost from the
.~
; recovery system.
The smelt is dissolved in water to yield a raw green
liquor which then is clarified to remove undissolved solids.
,i
-- 3
107045';'
The clarified gre~n liquor is causticized with slaked lime
whereby the sodium carbonate is converted to sodiùm hydroxide-
The resulting liquor is white liquor which then may be re-
cycled to the digestion step to provide at least part of
the pulping liquor.
As mentioned above another pulping process to which
the present invention may be applied is the soda process.
In this process, the pulping liquor consists predominantly
of aqueous sodium hydroxide solution. Spent pulping liquor
is subiected to a recovery and regeneration procedure as in
the Kraft process. The'smelt which results from furnacing in the
soda process contains primarily sodium carbonate which, after
formation into an aqueous solution, is recausticized to
regenerate sodium hydroxide solution for recycle to provide
at least part of the pulping liquor.
Bleach plant operations generally involve a se~uence
of brightening and purification steps, together with washing
' steps. The brightening steps generally involve the use of
bleaching agents. At least one of the brightening steps
usually invol~es the use pf at least one chlorine-containing
bleaching agent. Such chlorine-containing bleach-ng
I agents include chlorine, chlorine dioxide, chlorine
monoxide and sodium hypochlorite.
The purification steps generally involve treatment
with sodium hydroxide solution, and usually is known as a
~, caustic extraction step. In some instances, the bleaching
and caustic extraction steps may be combined, for example,
using the so-called "oxygen bleaching" operation.
~ ' .
,~,
10~0457
A particular bleaching operation which has been
employed involves an initial bleaching of the pulp with an
aqueous solution containing chlorine or a mixture of chlorine
dioxide and chlorine, an intermediate washing, a caustic
extraction using aqueous sodium hydroxide solution, a further
washing, a bleaching with an aqueous solution of chlorine
dioxide, another washing, a further caustic extraction using
aqueous sodium hydroxide, an additional washing, a final
bleaching with chlorine dioxide solution and a final washing.
This is the so-called OE DED operation. The present invention
will be described with particular reference to this procedure,
although other procedures may be employed, such as, the use
of an aqueous solution containing approximately 100% chlorine
dioxide in the first bleaching step.
; The above-described CEDED operation may be carried
out using the so-called "Dynamic Bleaching" process outlined
in Canadian Patent 783,483. In this process, pulping treating
solutions are passed successively through a mat of fibers in
which the fibers are maintained relatively stationary with
respect to each other. Washing steps, except for a washing
after the last step of the bleaching and purification sequence,
may be omitted.
The spent wash waters from bleach plants generally
have been discharged to water ~odies, such as streams, rivers,
.
lak~s and oceans, without any attempt to recover chemicals
therefrom, although in some instances solid particle recovery
107()457
operations have ~en made. One of the main reasons that no
attempt has been made to recover these chemicals is because
they are very dilute and of small value. The bleach plant
also produces spent bleaching liquor effluents and spent
caustic extraction effluents. These effluents have objec-
tionable colour and are toxic and harmful to aquatic and
marine biota and polluting since they contain fibers and
materials consuming oxygen present in the water. It is
desira~le to avoid such environmental pollution, and hence
avoid the discharge of these effluents from the mill.
Due to the use of chlorine-containing bleaching
chemicals and sodium-containing purification agents, the
spent wash water contains substantial quantities of sodium
chloride. In addition, when the spent bleaching liquor
effluents and the spent caustic extraction liquor effluents
are mixed at least part of the residual chlorine and soda
values combine to form sodium chloride. The normally dis-
chaxged effluents, namely, the spent wash water, the spent
bleaching chemicals and the spent caustic extraction liquor
may be mixed to provide a bleach plant effluent stream,
known as BPE.
The quantity of the chlorine containing bleaching
agents and the quantity of sodium hydroxide used as caustic
extraction li~uor preferably are balanced to provide about one
atom of sodium for each atom of chlorine, whereby these chemica~
form sodium chloride. In practice, the quantlty of sodium
hydroxide solution is in a slight stoichiometric excess to en-
sure the conversion of all the chlorine values to sodium chloride
The equivalence of sodium and chlorine atoms in the ~leach plant
-- 6 --
~070457
effluent also is preferred so that the overall sodium inventory
of the mill thereby remains unchanged. In the CEDED sequence
when chlorine or mixtures of chlorine dioxide and chlorine in
which the proportions of available chlorine provided by chlorine
dioxide is low, is used in the first stage an amount of sodium
hydroxide solution in excess of that required for extraction
must be added to match the chlorine atoms present. If no excess
is added, only about 40 to 50% of the first chlorination stage
filtrate can be recovered to match the stoichiometric equivalent
of sodium atoms used in the extraction. Where, however, the
available chlorine is provided predominantly by chlorine dioxide,
- typically above about 70%, the quantities of sodium and chlorine
atoms are substantially equivalent and hence it is preferred
to employ the latter sequence.
Sodium chloride present in the bleach plant effluent
also may arise from sodium chloride present in the pulp when
it is introduced to the bleach plant. Such sodium chloride
may be present where the logs are floated in sea water prior
to formation of wood chips therefrom. In addition, if brackish
water is used to provide the bleach plant wash water, sodium
chloride again is present in the bleach plant effluent.
Sodium salts ~ay be introduced to the pulp mill
system, from other sources, such as from the cellulosic
fibrous material itself.
In the present invention, the bleach plant effluent
preferably is added to the spent pulping liquor recovery and
regeneration operation, and in this way this effluent is
retained within the mill. It has been proposed previously in
Canadian Patent No. 832,347 and U.S. Patent No. 3,698,995 to
-- 7 --
1070~57
reduce the environmental problems of bleach plant effluent
by utilizing the spent wash waters to wash the pulp in the
brown stock washer The use of the spent wash water in this
manner reduces the overall water requirement of the mill. In
the present invention, it is preferred to use the bleach plant
effluent, consisting of a mixture of spent wash waters, pre-
ferably provided by countercurrent washing as described in
Canadian Patent No. 832,347 and U.S. Patent No. 3,698,995,
spent bleaching chemicals and spent caustic extraction liquor,
to wash the pulp in the brown stock washer and provide thereby
an "effluen.-free" pulp mill and a lowering of water requirement.
The bleach plant effluent may be introduced at
other stages of the recovery and regeneration operations.
Further, the bleach plant effluent may be split into two or
more streams which are introduced at different locations of
the recovery and regeneration operations, for example, to wash
calcium carbonate mud to provide the "weak wash" water or to
dilute concentrated white liquor.
The quantity of sodium chloride present in the
bleach plant effluent varies depending on the bleaching se-
quence which is employed. In a typical procedure where a
mixture of chlorine dioxide and chlorine are utilized in the
first stage of a CEDED sequence the quantity of sodium
chloride may vary between about 120 and 160 lbs/ton of pulp
depending on the proportion of chlorine dioxide used. Typi-
cally, when the total available chlorine in the first stage
- is provided 70% by chlorine dioxide and 30% by chlorine, the
guantity is about 120 lbs/ton of pulp.
The introduction of the bleach plant effluent to
10'7(~45';'
the pulping liquor recovery and regeneration operations closes
the whole system and sodium chloride is not purged by way of
discarded bleach plant effluent. The sodium chloride remains
unconverted by the black liquor recovery steps and hence
would build up in the system. In order to prevent such build
up and at the same time utilize the concepts of an effluent-
free pulp mill, it is essential to remove sodium chloride from
the system. Such removal of sodium chloride should be such
that the other valuable components utilizable as or convertible
into pulping chemicals are not removed from and thereby lost
to the system along with the sodium chloride. In addition,
it is preferred to remove a quantity of sodium chloride from
the mill equivalent to the amount introduced to and/or pro-
duced within the mill, typically about 120 lbs/ton pulp.
In instances where bleach plant effluent is not
` introduced to the recovery and regeneration operations,
but i8 discharged in the normal way, sodium chloride may
still be present in the mill recovery and regeneration
':
cycle which is required to be removed to prevent build up.
Such sodium chloride may arise from sea-borne logs which
are pulped and washed, thereby removing some sodium chloride
from the pulp and introducing it to the b~ck liquor, prior
to passage to the bleach plant. Further, where wash water
,
i for washing the pulp prior to passage to the bleach plant is
contaminated with sodium chloride, at least part of this
sodium chloride is introduced to the black li~uor.
- ~he present invention is concerned broadly, there-
fore, with the remo~al of sodium chloride from pulp mill
_ g
1~70~5~
recovery and regeneration systems irrespective of the source
of such sodium chloride, and is preferably concerned with
the removal of sodim chloride from effluent-free pulp mills.
The process described and claimed in Canadian Patent
No. 915,361 and U.S. Patent No. 3,746,612 makes it feasible
to remove from the mill on a continuous basis an amount o~
sodium chloride equivalent to the amount of chloride intro-
duced to the recovery and regeneration system. This is
achieved by concentrating, preferahly by evaporation, the
white liquor prior to recycle to the digestion stage in order
to precipitate and remc>ve sodium chloride from the white liquor.
The white liquor evaporation process is outlined in
the above-mentioned Canadian Patent No. 915,361 and U.S.
Patent No. 3,746,612, while effective in removing sodium
chloride from the system in the desired quantities, neverthe- ?
less involves consid~rable energy to evaporate the required
amount of water to achieve precipitation of the requisite
quantity of sodium chloride and further involves considerable
capital investments in special equipment in view of the high
temperature and corrosive nature of the solutions. Further,
in the process of Canadian Patent No. 915,361 and U.S. Patent
3,746,612, some sodium chloride remains in the recycled white
liquor and cycles through the system as a dead load.
;` ' , .
In accordance with the present invention, an alter-
native sodium chloride separation procedure is provided which,
in its preferred embodiments, enables sodium chloride to be - --
removed from the system at a lower cost.
-- 10 --
45,
In accordance with the present invention, a smelt
containing sodium carbonate and sodium chloride is dissolved
in an aqueous material to form hot aqueous solution thereof,
the hot aqueous solution is cooled to crystallize hydrated
~odium carbonate therefrom while avoiding precipitation of
sodium chloride and until the cooled solution is substantially
saturated with respect to sodium chloride, the crystallized
hydrated sodium carbonate is recovered from the resulting
~queous solution substantially saturated with sodium chloride,
the sodium chloride solution is evaporated with heating to
crystallize sodium chloride therefrom, and the ctystallized
sodium chloride is removed from the resulting mother liquor.
The hot aqueous solution which is cooled by the
procedure of the invention therefore contains dissolved
; quantities of the material of the smelt, including sodium
carbonate and sodium chloride. In some instances, the
8melt may be substantially sodium chloride-free but the hot
aqueous solution nevertheless contains dissolved quantities
of sodium chloride as a result of additions of a sodium
chloride_containing aqueous solution, such as bleach plant
effluent, to the aqueous solution of the smelt. Such proce-
dures are within the scope of this invention.
. , .
Further, it may be desired, under certain circum-
- stances to form a hot a~ueous solution for treatment in accor-
dance with the present invention from only part of a sodium
chloride-containing smelt and such procedures are within the
scope of this invention.
The hydrated sodium carbonate which is crystallized
and recovered in this way then may be recausticized, after
making up into an aqueous solution thereof, or otherwise
107()457
reconverted into pulping chemicals. Since the sodium carbon-
ate crystallization substantially avoids the crystallization
of sodium chloride, little or no sodium chloride is recycled
with the pulping liquor as a dead load.
The mother liquor remaining after removal of sodium
chloride usually still contains dissolved quantities of sodium
chloride and sodium carbonate, and this mother liquor may be
recycled to the aqueous solution formed from the materials
of the smelt.
The quantity of sodium chloride recovered by the
process of the invention preferably is substantially the same
quantity as is originally present in the smelt, and this may
be achieved by judicious choice of conditions.
Chemicals other than sodium carbonate and sodium
chloride may be present in the smelt, depending on the pulp
mill system which is used. For example, where the Kraft
process is used, the smelt also may contain sodium sulfide
and small quantities of various sodium and sulfur compounds
including sodium sulfate, sodium sulfite, sodium thiosulfate
and sodium polysulfide. Sodium sulfate is present in the most
significant amount. In one embodiment of the invention as
applied to the Kraft process, at least the sodium sulfate and
optionally the sodium dulfide are crystallized along with
t~e sodium carbonate.
~he cooling of the hot aqueous solution may be
carried out in any convenient manner to a temperature which
wi~l result in crystallization of hydrated sodium carbonate,
; optionally together with any other crystallizable salts, with
the exception of sodium chloride. The form of the sodium
~; 30 carbonate which is crystallized depends on a number of
- 12 -
~0'70457
factors, including the composition of the cooled solution
and the temperature to which the hot aqueous solution is
cooled to cause crystallization. The sodium carbonate is
crystallized in a hydrated form, which may be the
septahydrate or decahydrate. Preferably the de~ahydrate
is crystallized The decahydrate form is crystallized at tempera-
tures below about 25C. Crystallization of the sodium carbonate
in a hydrated form in the invention removes water from the
system, thereby decreasing the quantity of water which must
subsequently be evaporated to remove water to deposit sodium
chloride.
During the cooling step, water preferably is evapora-
ted from the aqueous so~ution to increase the concentration of
the dissolved salts and this evaporation may constitute the
sole form of cooling the hot aqueous solution. The cooling
effect of the evaporation, carried out under vacuum, may be
enhanced in any desired manner.
The smelt may be fractionated during its formation
into the hot aqueous solution to dissolve substantially all
the sodium chloride therefrom along with part of the sodium
carbonate and part or all of other salts present in the smelt,
depending on the conditions of the fractionation. The un- -
dissol~ed portion of the smelt is substantially free from
sodium chloride and may be ~orwarded directly to the recaus-
ticization stage. Fractionation of the smelt in this way
decreases the energy required to deposit the sodium carbonate
from the hot aqueous solution.
The invention is described further, by way of illustra-
tion, ~ith re~erence to the accompanying drawings, in which:
107l)457
Figure 1 is a schematic flow sheet illustrating
one embodiment of the invention, as applied to a soda pulp
mill; and
Figure 2 is a schematic flow sheet illustrating a
further embodiment of the invention, as applied to a Kraft
pulp mill.
Referring first to Figure 1, wood chips, or other
raw 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 hydroxide as the
active pulping chemicalin the soda process.
- 14 -
1070457
The resultin~ pulp and sp~nt pulping liquor are
separated and the pulp is washed in a brown stoc~ washer 16.
- The pulp is washed, in the embodiment illustrated, with aqueous
bleach plant effluent fed by line 18. Alternatively, the
pulp may be washed with water or ~'contaminated condensate",
and the bleach plant effluent may be utilized elsewhere in
the system, as described in more detail below, or discharged
as desired.
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 puriflcation processes involving the use of
one or more chlorine-containing bleaching agents. Generally,
the bleaching and purification processes involve bleaching
with chlorine, chlorine dioxide or mixtures thereof fed ~y
line 24 and purification by caustic extraction, using aqueous
eodium hydroxide solution fed by line 26, typically in a OEDED
DCEDED or DEDED sequence. The pulp is washed during the
bleach plant operations, typically after each bleaching or
caustic extraction operation, by water fed by line 28. The
spent wash water from the bleach plant washing operations
together with the spent chemicals from the bleaching and
- caustic extraction steps provide the bleach plant effluent
in line 18.
Preferably, the washing operations in~olve counter-
current flow of pulp and wash water through the bleach plant.
; Prefe~ably, the quantity of sodium hydroxide used in the
caustic extraction of the pulp corresponds to the
stolchiometric requirement of one sodium atom for each chlorine
atom in the bleaching chemicals, whereby the bleach plant
1070457
effluent in ~ e 18 has a neutral p~. The bleached and
purified pulp of required brightness is recovered from the
bleach plant 22 by line 30 and is sold as such, or passed to
paper making procedures.
If desired, the bleach plant effluent in lire18 may
be added directly to the spent pulping liquor in line 32,
although this procedure is less preferred, since the overall
water requirement thereby is increased.
The bleach plant effluent in line18 contains con-
siderable quantities of sodium chloride which are transferred
to the spent pulping liquor in line 32. The spent pulpingliquor is evaporated in an evaporator 34 prior to passage by
line 36 to a furnace 38 of any convenient construction. The
water recovered from the evaporator 34 by line 40 may be used
to provide at least part of the water requirement of the
system, for example, as at least part of the water fed to the
bleach plant in line 28, after suitable cleaning, if required.
The spent pulping liquor forms in the furnace 38 a
smelt containing sodium carbonate possibly along with some sodi~-
sulfide and ~dium sulphate and addi~onally unreacted o~x~ents consistingsubstantially of sodium chloride. Typically, the smelt contains
about 70 to about 90~ by weight of sodium carbonate, about 1~
to about 25% by weight of sodium chloride, ~ to about 3% by weiS~t
o~ sodium sulfate and 0 to about 4~ by weight of sodium sulfide.
Thus, there is obtained from the furnacing operation
a smelt containing sodium carbonate and sodium chloride. In
accordance with this embodiment of the present invention, the
smelt is dissolved in a smelt dissolver 42 in water fed by
line 44. The water ~iay ~e constituted by BPE, evaporator
14S,
condensate or ~weak wash" water, if available after dissolv-
ing hydrated sodium carbonatein ~ssolver 54. The water passed by
line 44 to the smelt dissolver 42 may be partially bleach
plant effluent fxom line 18, especially in the case where
fresh water or contaminated condensate is fed to the brown
stock washer 16.
The resulting hot aqueous solution in li~ 46 also
contains-recycled chemicals as described in more detail ~elow.
If desired, all the smelt need not be dissolved in
the aqueous material fed by line 44, but instead the smelt
may be leached with the aqueous material fed by line 44 to
dissolve substantially all the sodium chloride values and
part of the sodium carbonate values of the smelt, leaving
substantially pure sodium carbonate. Operation in this manner
reduces the quantity of water which requires evaporation.
' lhe res~ ~ng substantially pure ~um ca~x~abe ~y beused to form
pulping liquor along with the sodium carbonate later separated
' from the hot aqueous solution resulting from the leaching.
s The hot aqueous solution is cooled in a crystal-
lizer 48 to crystallize hydrated sodium carbonate, preferably
sodium carbonate decahydrate, while avoiding substantially
the crystallization of sodium chloride. The cooling prefer-
~ ably is achieved by evaporation o~ water from the hot solu-
i' - tion under vacuum, the evaporated water being removed by line
50.
The hot aqueous solution in line 46, which generally
has a very high temperature, typically a~out 180F to 21~F,
may be suhjected to aninitial cooling, with or without evapora-
~ tion o~ water, to reduce the cooling load to be borne by the
¦ 3~ sodium carbonate crystallizer, prior to further e~aporative
_ I7 -
107~}45~
cc>olin~ to cause cr~s.allization of the sodium carbonate.
More than one such initial cooling steps may be used, i
desired. Th~ initial cooling may be omitted, if desired.
In a typical operation, the hot aqueous solution
at an initial temperature of about 200F is cooled to abcut
120F with some evaporation of water and then the solution
is evaporatively cooled to below about 70F, preferably to
about 60F, causing crystallization of sodium carbonate deca-
hydrate. Since the sodium carbonate is crystallized in the
form of the decahydrate, thereby more water i5 removed from
the solution.
The evaporative cooling and crystallization o~-sodium
carbonate are continued-until the mother liquor-
i8 substantially saturated with respect to sodium chloride.
The resulting slurry preferably is diluted with recycled
mother liquor to improve the workability thereof and sodium
carbonate, substantially free from sodium chloride, is separated
from the mother liquor by line 52. While the crystallization
of sodium carbonate and its separation from the mother liquor
are illustrated as occurring in the same vessel in Figure 1,
this is for ease of illustration and normally separate vessels
are used.
The sodium carbonate is passed by line 52 to a dis-
solving tank 54 wherein it is dissolved in water, or any other
suitable aqueous medium, preferably "weak wash" water, fed by line
56 to form an aqueous sodium carbonate solution which i5
passed to a causticizer 58 by line 60. The sodium car~onate
in the aqueous solution is converted substantially comple.ely
to sodium hydroxide by the action of lime fed by line 62 from a
lime kiln 64. The calcium car~onate mud precipitated in the
1~70~57
caustlcizer 58, after suitable washing to remove entrained
alkali values, is returned to the lime kiln 64 by line 66 for
reconversion to lime. The weak wash water resulting from the
calcium carbonate mud washing may, in part be used as the aqueous
material fed by line 44 to dissolve the smelt.
The aqueous sodium hydroxide solution resulting
from the recausticization process is recycled by line 68 to
provide at least part of the pulping liquor fed by line 14
to the digester 12.
The mother li~uor resulting from separation of the
sodium carbonate, containing dissolved quantities of sodium
chloride and sodium carbonate, and saturated at least with
respect to sodium carbonate, is forwarded by line 70 to an
evaporator 72 wherein the mother liquor is heated and water is
evaporated therefrom to deposit sodium chloride, while avoiding
subs~antially the crystallization of sodium carbonate.
Generally, the evaporation of water in the evaporator 72 is
achieved by boiling the solution under a reduced pressure,
preferably until the solution is substantially saturated with
respect to sodium carbonate. The water evaporated in the
evaporator 72 is removed by line 74.
The temperature to which the mother liquor is heated
to achieve the desired evaporation and crystallization of
sodi~m chloride depends on the temperature at which the sodium
carbonate crystallization occurred in the crystallizer 48 and
the relative concentrations of sodium carbonate and sodium
chloride in the mother liquor. The temperature must be higher
than a temperature at which sodium carbonate will crystallize,
if substantially pure sodium chloride is to be recovered,
5 ,~
and temperatur~s as hi~h as 220F may ~e used. ~t these
temperatures, the temperature to ~hich the hot aqueous
solution is cooled to crystallize hydra~ed sodium carbonate
must be quite low, typically about 32 to 50F. Tempera.ures
in the range of about 100 to 140~ have proved to be the
desixable range. In the typical procedure outlined above
wherein the hot aqueous solution ~as cooled to about 60F to
achieve crystallization of hydrated sodium carbonate, the
mother liquor may be heated with boiling to about 120F to
achieve crystallization of sodium chloride.
The sodium chloride which is crystallized is
removed by line 76 in substantially pure form. Preferably,
the quantity of sodium chloride removed in this manner is
substantially the same as the quan.ity of sodium chloride
present in the hot aqueous solution which is subsequently cooled
to crystallize hydrated sodium carbonate, less any sodium
chloride present in the recycle stream in line 78.
The sodium chloride removed by line 76 may be put
to a ~ariety of uses. Typically, the sodium chloride is
utilized to regenerate bleach plant chemicals. For example,
the 80aium chloride may be used to generate sodium hydroxide
by electrolysis of an aqueous solution thereof, the sodium
hydroxide ~eing used in the bleach plant in line26. Alter-
natively, the sodium chloride may be used to generate chlorine
di~xide and chlorine ~y reaction wi~h sodium chlorate and
sulphuric acid, the chlorine dioxide and chlorine being fed
to the bleach plant by line 24. Further, the sodium chloride
may be electrolyzed as an aqueous solution to sodium chlorate
for use in a chlorine dioxide producing reaction involving
reduction of the sodium chlorate in an acid medium.
1(~70457
~ evaporation Ol tAe mother liquor from the
separation of sodium carbonate and the separation of the
sodium chloride crystallized therefrom is illustrated as
occurring in a single vessel 72. In pr~ctice, usually t~o
separate vessels are used, the slurry resulting from the
evaporation in one vessel being passed to a second vessel
for separation of .he solid sodium chloride.
The mother liquor resulting from the sodium chloride
separation usually will still contain quantities of sodium
chlor;de and sodium carbonate and is recycled by line 78 to
the smelt dissolver 42 along with the aqueous material in
line 44, or, alternatively, is recycled to hot a~ueous solu-
tion in line 46.
The mother liquor in iine 78 may contain small
quantitites of sodium hydroxide, which would not be removed
from the system by the illustrated procedure. The sodium
hydroxide may arise from weak wash water from the washinq of
the calcium carbonate mud where such water is used to pro-
vide at least part of the aqueous material fed by line 44 to
the smelt dissolver 42. The sodium hydroxide also may arise
from small quantities of sodium oxide values in the smelt.
The mother liquor in line 78 may be carbonated using
carbon dioxide,or in any other conve~ient manner, to convert
i``4 such sodium hydroxide values to sodium car~onate, prior to
passage of the mother liquor in line 78 to the hot aqueous
solution in line 46. In this way, any build up of sodium
hydroxide values in the recycled mother liquor in line 78 is
avoided.
I~ desired, followin~ the separation of the sodium
- 2~ -
107(~5,
chloride ~ erefrom, the resulting mother liquor ma~ ~e cooled
again to precipitate further quanti.ies of hydrated sodium
carbonate.
It will be apparent, therefore, that in the process
of the embodiment of Figure 1, the sodium carbonate and
sodium chlori~e in the smelt are separated one from another
into substantially pure product streams which contain o~uanti-
ties of the hydrated sodium car~onate and sodium chloride
which are substantially equal to the quantities of these
materials p~esent-in the smelt.
Turning now to consideration of Figure 2, there is
illustrated the application of the present invention to a
Kraft mill operation. A smelt, consisting predominan.ly of
sodium sulfide, sodium carbonate and sodium chloride, and
also containing minor quantities of sodium sulfate and other
sodium and sulfur-compounds due to inef~iciences of the furnace,
typically containing about 7 to about 22% by weight of sodium
sulfidel about 60 to about 78% by ~Jeight of sodium carbonate,
about 10 to about 25% by weight of sodium chloride and about 1
to about 6% by weight of sodium sulfate, is passe~ by line 110
to a smelt dissolver 112.
As mentioned previously, in the Kraft mill process
sodium hydroxide and sodium sulfide are the active pulping
chem.icals and sodium sulphate, or o~er source o~ soda and
sulfur values, is added to the bl~ck liguor to provide ma~e
up for soda and sul~ur losses.
The smelt is dissolved in the smelt ~issolver 112
in an aqueous material ~ed by line 114, which may ~e water
or,in part, ~eak wash water, to form a hot aqueous solu.ion,
commonly known as green liquor, in line 116. The ~reen li~uor
22
1~7Q457
in line lL6 ~ay al~o contain recycled liq~id, as ~ill beco o
more a~parent hereinafter.
The hot green liquor, typically having a tempera-
ture of about 200F, is cooled in a precooler 118, with
evaporation of some water therefrom, if desired. The water
evaporated in the precooler is removed by line 120. Typi-
cally, the green liquor is cooled to about 100F in the
precooler 118.
The precooled green liquor is passed by line 122
to an evaporative cooler 124 wherein the precooled green
liquor is subjected to evaporative cooling. Water is evapor2-
ted from the precooled green li~uor, to cause cooling of the
liquor and thereby bring about crystallization of a mixture
of ~odium carbonate, sodium sulfide and sodium sulfate, together
with minor quantities of other precipitated sodium and sulfu~
salts which may be present while avoiding crystallization
of sodium chloride. The water evaporated is removed by line
126. The evaporative cooling i~ carried out
until the mother liquor is substantial~y saturated with res-
2~ pect to sodium chloride.
~ue to the rapid increase in solubility of sodium
sulfide with increasing temperature, coo~ing of the green
liquor to a temperature of less than about ~F, prefer-
a~ly about ~0 F, is necessary to ensure crystallization of
sodium carbonate, sodium sulfide and sodium sulfate. At these
te~peratures the sodium carbonate is crystallized predominan~y
as the decahydrate and sodium sulfide is crystallized pre-
dominantly as the nonahydrate, thereby removing further
water from the solution.
In the illustrated embcdiment, the hot green li~uor
is subjected to a two-stage evaporative cooling to achieve crys-
1070~57
tallization f a mixture of sodium carbonate, sodium sul-
fide and sodium sulfate in the second stage. If desired, the
cooling of the green liquor to achieve the desired precipita-
tion may be carried out in a single step, and may be carried
out in any convenient manner other than evaporative cooling,
if desired. Further, the cooling achieved by evaporation
may be enhanced by suitable additional cooling.
It may be desired, depending on the initial concen- -
tration of the hot green liquor, to crystallize only sodium
carbonate and sodium sulfate, leaving the sodium sulfide in
the aqueous phase along with the sodium chloride. This
particular embodiment will be discussed in more detail below.
The slurry resulting from the evaporative cooling
is diluted with recycle mother liquor in line 128 and the
diluted slurry passes by line 130 to a separator 132 of any con-
venient construction. In the separator 132, the mixture of sodium -
carbonate, sodium sulfide and sodium sulfate is separated
from the mother liquor and is passed by line 134 to a recaus-
ticizer wherein the sodium carbonate is converted to sodium
hydroxide, the resulting white liquor being subsequently
used as pulping liquor. The sodium sulfate passes to the
furnace in the subsequently formed black li~uor.
, The quantity of sodium sulfide removed by line 134 is
~ substantially the same as that contained in the smelt in line
,-~, 110.
The quantity of sodium carbonate and sodium sulfate
removed in line 134 may be less than the quantity present in
I
10704S,
the smelt, and, in the case of sodium sulfate, formed in
the green liquor and, hence, a further separation of sodium
carbonate and sodium sulfate may be necessary. In the illus-
trated embodiment, this is carried out by passing the mother
liquor from the separator 132 by lines 136 and 138 to an
evaporator 140, with`a portion of the mother liquor
being recycled by line ~28 as diluent for the slurry formed
in the evaporative cooler 124.
The mother liquor is heated and evaporated in the
evaporator 140 to cause the crystallization of sodium carbcn-
ate together with the dou~le salt of sodium carbonate and
sodium sulfate known as burkeite water vapor is
removed by line 141. Typica~ly, the mother liquor may be
evaporated at a temperature of about 220F, while avoiding
substantially the precipitation of sodium chloride, the evap-
oration being continued until the solution i5 substantially
saturated with sodium chloride.
The slurry of burkeite and aqueous solution is
passed by line 142 to athickener 144 of any convenient con-
struction wherein the sodium carbonzte and burkeite is
separated from the mother liquor and removed by line 146.
The sodium carbonate and ~urkeite separated in this way may
be passed to the recausticizer along with the solids in line
134. Alternatively, a separator may be employed instead
of the thickener 144. The thickener underflow is
forwarded to the smelt dissolver 112, to the precooler 118
or to any other convenient location.
The mother liquor from the ~urkeite separation is
passed hy line 148 to an evaporator 150 wherein water is
evaporated to deposit substantially pure sodium chloride, which,
1a7~)45 ,'
after separation from the resulting mother liquor, is removed
by line 152. The quantity of sodium chloride removed by line
152 preferably is substantially that contained in the smelt
in line 110. The sodium chloride removed in this way may be
utilized in any convenient manner, typically to form bleaching
chemicals.
The evaporation in evaporator lS0 results in water
vapor which is removed by line 154 and may be carried out at
any convenient temperature. The temperature chosen depends
on the initial concentration of the dissolved components of
the mother liquor and the quantity of sodium chloride required
~ to ~e removed. The evaporation of the solution in the
evaporator lS0 to precipitate sodium chloride generally is
carried out until the solution is su~stantially saturated
with respect to sodium carbonate and/or sodium sulfide and/or
60dium sulfate, and typically is carried out at a temperature
of about 120 F.
The mother liquor resulting from the sodium chloride
crysta~iza~n contains residual quantitites of sodium carbonate,
sodium sulfide, sodium sulfate and sodium chloride and is
recycled by line 156 to a car~onator 158. The mother liquor
is contacted with carbon dioxide fed ~y line 160, in order to
convert any sodium hydroxide present in the mother liquor in
line 156 to sodium car~onate, so that a ~uild up of sodium
hydroxide upon repeated recycle is prevented. As mentioned
above in connection with the embodiment of Figure 1, such
sodium hydroxide may arise rom wea~ wash water and/or from
materials in the smelt.
The carbonated mother li~uor is forwarded by line
107~)~5~
162 to the hot green liquor in line116. This may ~e achie-~ed
by feeding ,he mother liquor in line 162 along with the aqueous
material in line 114 to dissolve the smelt. Alternatively,
the mother liquor may be added to the hot green liquor in
line 116 after formation thereof.
By recycling the mother liquor from the sodium chlo-
ride crystallization in this way, residual quantities of
material, which, in a continuously operating procedure, pre-
ferably will be substantially constant, loss of chemicals is
2voided.
It may be desirable, in certain instances, to cool
the mother liquor in line 156 once more to crysta`llize fur-
ther quantities of sodium c~rbonate and/or sodium sulfide
and/or sodium sulfate, which may be forwarded to the recaus-
ticizer along with the material in line 134.
In the embodiment discussed above, wherein the pre-
cipitation of ~odium sulfide is a~oided when the green liquor
is cooled, following evaporation to precipitate sodium
chloride, the mother liquor resulting after separation of the
sodium chloride consists mainly of sodium sulfide solution,
which may be evaporated to crystallize therefrom the remaining
quantities of sodium carbonate, sodium sulfate, and sodium
chloride. The sodium sulfide solution then may be added to
the sodium hydroxide solution resulting from the recaustici-
- zation to provide white liquor for the recycle as pulping
liquor. Alternatively, the sodium sulfide solution ~ay be
recycled as the pulping liquor, while part o~ the sodium hydro~ide
~olution resu7ting from the recausticization is forwarded to
the bleach plant for use therein as caustic ~xtraction liquor,
10'704S,
or in o~ygen-~leaching processes. As another alternative,
the sodium sulfide solu~ion may be used in imprecnatlng wood
chips. The crystallized solids may be recycled to the hot
green liquor.
The invention is illustrated by the following e~amples:
Example 1
A mass balance for the em~odiment of Figure 1 pro-
ducing 500 tons/day of pulp was calculated from ~nown solu-
bility data for the system Na2C03-~aCl-H20. 25,209 lbs/hr.
of a smelt having a con~titution of 87.5% Na2C03 and 12.5%
NaCl is dissolved in 66,774 lbs/hr. of water to provide
91,667 lbs/hr. of green liquor havlng a temperature of about
200 F.
The green liquor is mixed with 43,372 lbs/hr. of
recycle mother liquor containing sodium carbonate in a
concentration of 12.4% and sodium chloride in a concentration
of 18.4% and having a temperature of 120F, to provide a
combined liquid stream which is passed to an evaporative
precooler wherein 6,034 lbs of water are removed while the
temperature is cooled to about 120F, resulting in 129,005
l~s/hr. of precooled liquor.
The precooled liquor then is evaporatively cooled
; to 59F resulting in crystallization of sodium carbonate
decahydrate. 11,36a lbs/hr. of ~m are evaporated from
the precooled liq~or in this cooling step, and 52,085 l~s/hr.
of recycle mother liquor, containing 9.24% sodium carbonate
and 19.10% sodium chloride, is recycled to the evaporat~ve
cooler to dilute the slurry of sodium carbonate decahydrate
formed therein.
- 28 _
107045,
' 169,722 lbs/hr. of slurry is forwarded to a fi3ter
wherein 59,~93 lbs/hr. of sodium carbonate decahydrate are
recovered, leaving 110,229 lbs/hr. of filtrate, which is
split into the mother liquor recycle stream to dilute the
slurry in the evaporative cooler and into 58,144 lbs/hr. feed
stream to an evaporator. In the evaporator, the feed stream,
containing 9.25% sodium carbonate and 19.10% sodium chloride,
is heated under a reduced pressure to evaporate 11,929 lbs/hr.
of water from the solution at 120F, resulting in crystalli-
zation of sodium chloride from the s~lution.
46,215 lbs/hr. of slurry from the evaporator is
passed to a filter whereon 3,15g lbs/hr. of sodium chloride
are recovered, resulting in 43,372 lbs/hr. of filtrate stream
containing 12.4% sodium carbonate and 18.4% sodium chloride.
After contact with carbon dioxide to convert any sodium
hydroxide values to sodium carbonate, the filtrate stream is
recycled to the green liquor.
Example 2
A mass balance ~or the ~d~nt of Figure 2 utilized in a p~p
~ill producing 500 ~s/day of pulp was ~lcula~ed ~rom expe~ntal
solubilit~ data for the system Na2S-~Ta2C~3-~aCl-H2O. 95,750
lbs/hr of green li~uor containing 4.2% sodium sulfide, 17.6%
sodium car~onate, 3.3% sodium chloride and 0.6S% sodium sul-
fate and ha~ing a temperature of 20~F is mixed with 28,4Q8
lbs/hr. of recycled mother liquor from sodium chloride removal
containing 10.0% sodium sulfide, 6~2% sodium carbonate, 12.0%
sodium chloride and 0 65% sodium sulfate and 7,755 lbs/hr. of
~urkeite ~hickener underflow at 225F containing 2,713 lbs/hr.
of burkeite and 5,043 lbs/hr of liquor containing 7.3% sodium
107045 ,~
sulfide, 4.6% sodiu~ carbonate, lG.8% sodium chloride and
0.5% sodium sulfate, and the mixture of the three streams
is precooled to 120F with evaporation of 6,397 lbs/hr. of
water.
The resulting 125,516 lbs/hr. of precooled liquor
containing 5.8% sodium sulfide, 15.6% sodium carbonate, 5.9%
sodium chloride and 2.2% sodium sulfate then is evaporatively
cooled to 50F, with evaporation of ~,70S lbs/hr. of water.
The resulting slurry containing a solid phase consisting of
a mixture of sodium carbonate, sodium sulfide and sodium
sul~ate is diluted with 56,695 lbs/hr. of recycled filtrate
from the solid phase separation containing 6.0% sodium sulfide,
5.3% sodium carbonate, 13.8% sodium chloride and 4.1% sodium
sulfate.
The 169!506 lbs/hr of slurry is passed to a filter
whereon 59,327 lbs/hr. of solid is separated from the liquid
phase. The separated solids consists of 6.8 % sodium sulide,
28.4% sodium carbonate and 1.1 % sodium sulfate. The 110,179
; lbs/hr. of filtrate from this solid phase separation is
divided into two streams, one of which is recycled to dilute
the slurry in the evaporative cooler. The other stream, in a
quantity- of 53,484 l~s/hr. and containing 6.0% sodium sulfid~,
; 5.3% sodium carbonate, 13.~/~ sodium chloride and 4.1% sodium
sulfateris passed to an evaporator wherein ~,814 ~bs/hr.
of water is evaporated at a temperature of 225F, resulting
in crystallization ofbur~eite. The solid phase bur~eite is
recovered in the form of a thickener under~low which contains
738 lbs/hr. of sodium carbonate and 1,974 l~s/hr. of sodium
- 30 -
,~ .
1070'~57
sulfate, as solid phase and the thickener underflow is recycled
to the green liquor. Thc ~other liquor from this separation,
in a quantity of 38,914 lbs/hr. and containing 7.3% sodium
sulfide, 4.6% sodium car~onate, 16.8% sodium chloride and
0.5% sodium sulfate, then is evaporated at 120F to crystal-
lize sodium chloride therefrom. 31,533 lbs/hr. of slurry
from the evaporator is passed to a filter whereon 3,125
; lbs/hr. of sodium chloride are recovered, resulting in
28,408 l~s/hr. of filtrate stream containing 10.0h sodium
sulfide, 6.2% sodium carbonate, 12.0% sodium chloride and
0.68% sodium sulfate.
After contact with carbon dioxide to convert any -
sodium hydroxide values to sodium carbonate, the filtrate
stream is recycled to the green liquor.
. ' ' .
It will be seen that the process of the present
invention provides in its preferred embodiments, a salt
removal process in which the need for high temperature-
resistant equipment is avoided, and the quantity of water
requiring evaporation is decreased, and in which a aead load
of sodium chloride does not recycle in the white liquor. In
these respects, the present invention represents an improve-
ment on the white 11quor evaporation procedures outlined in
the aforementioned Canadian Patent ~o. 915,361 and its equiva-
lent U.S. Patent No. 3,746,612.
- 3I -
