Note: Descriptions are shown in the official language in which they were submitted.
~1~96Pl
,
POTASSIUM-BASED PULP MILL PROCESS
FIELD OF INVENTION
The p~esent invention is directed to the production
of cellulosic ~ibrous material pulp using potassium based
pulping chemicals, to, the recovery of pulping chemicals and
to the removal of potassium chloride contaminants.
BACKGROUND TO THE INVE:NTION
Generally in the pxoduction of pulp suitable for
formation into paper, wood or other raw cellulosic fibrous
material is subjected to chemical digestion in a pulping
liquor to form a pulp of the cellulosic fibrous material.
The pulp thereafter is subjected to bleaching and purifi-
cation operations ina bleach plant.
The spent pulping liquor usually is subjected to
a series of recovery and regeneration operations to recover
pulping chemicals and to provide fresh pulping liquor.
Sodium-based chemicals conventionally have been used as
the pulping liquor, such as, sodium hydroxidej possibly
along with other sodium salts.
One widely used pulping process is the kraft
process wherein raw cellulosic fibrous material, generally
wood chips, is digested in white liquor containing sodium
.
: . : . , , , " . , .; ; . . . , ~ .~
1~$~
hydroxide and sodium sulphide as the active pulping chemicals
to provide a pulp and 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 bleaching
5 and purification operations.
The black liquor then is passed to the recovery
and regeneration systém in which the black liquor is
evaporated and the concentrated black liquor is burned in
a furnace to yield a smelt containing sodium carbonate and
10 sodium sulphide. Sodium sulphate is usually added to the
black liquor 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 is clarified to remove undissolved
solids and the green liquor is causticized with reburned
lime to convert the sodium carbonate to sodium hydroxide.
The resulting white liquor then is at least partially re-
cycled to the pulping step to provide at least part of the
pulping liquor.
Conventional bleach plant operations generally
; involve a sequence of bleaching and purification steps,
~ogether with washing steps. The bleaching steps usually
involve the use of at least one chlorine-containing bleaching
agent, such as, chlorine, chlorine dioxide, chlorine
25 monoxide and sodium hypochlorite. The purification steps
conventionally involve treatment with sodium hydroxide
solution.
In some instances, the bleaching and extraction
steps may be combined, for example, using an oxygen bleaching
.
3 l~S6~
operation. Where such oxygen bleaching is used, it usually
is supplemented by further bleaching operations using
chlorine-based bleaching chemicals.
The aqueous effluents of bleach plants including
5 spent bleaching chemicals, spent extraction chemicals and
spent wash water conventionally have been discharged to
convenient water bodies but the effluents are environment-
ally hazardous, possessing objectionable colour and high
toxicity towards a~uatic and marine biota, and containing
10 fibres and oxygen-consuming materials. To avoid the dis-
charge of such materials and thereby eliminate the harmful
and polluting effects of the effluents, it has been suggested
in U.S`. Patents No. 3,698,995 and No. 4,039,372 to forward
~ the e~fluents as a single stream or in a plurality of streams
- -15 to the chemical recovery system after utilizing at least
part thereof in washing the unbleached pulp in the brown
stock washer. When this procedure is adopted, the organic
contaminants of the bleach plant effluents are consumed
in the recovery furnace.
As a result of the use of chlorine-containing
bleaching chemicals and sodium-containing purification
agents, the aqueous effluents from conventional bleach plant
operations contain sodium chloride which then enters the
recovery and regeneration cycle when the effluent handling
25 process disclosed in the above patents is used.
Sodium chloride passes without change through the
recovery and regeneration system and hence constitutes a
dead load which, in continuous operation, may build up to
intolerable levels. Prior suggestions have been made to
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. . .: :;: : ~ .... , :
combat this problem by precipitation of the sodium chloride
" from white liquor, as described in detail in U.S. Patents
Nos. 3,746,612 and 3,950,217, or by separation from smelt
components, as described in detail in U.S. Patents Nos.
5 3,986,923, 3,909,344, 3,954,552 and 3,945,880.
It has previously been suggested to utilize
potassium-based chemicals in plaee of sodium-based chemieals
in pulping proeesses and bleach plant extractions. The use
of potassium-based pulping chemicals as ccmpared with sodiunr
based chemieals lS advantageous, in that a greater pulp
yield is attained with consequently increased pulp pro- ;
duetion and decreased wood costs. The rate of pulping
using potassium-based ehemieals is faster than for
,
sodium-based ehemicals, thereby inereasing the overall mill
pulping eapaeity. While the inereased eost of potassium
sulphate make-up as eompared with sodium sulphate make-up
partly offset these benefits, make-up chemieal requirement
ean be minimized.
The use of potassium-based ehemieals in eonjunetion
with a bleaeh plant effluent disposal proeedure by way of the
pulping llquor reeovery and regeneration cyele would result
in an analogous build up of potassium ehloride to that
. . .
deseribed above for the sodium-based proeess.
SUMMARY OF INVENTION
:6
The present invention, in its broadest aspect, is
direeted to the separation of potassium ehloride from a
potassium-based pulp mill spent chemical recovery and re-
generation proeess by the concentration of liquors formed
in that proeess. The source of the potassium-chloride
- .~
;
contamination -s not significant to the broadest aspects
of the invention, but in the preferred embodiment of the
invention the potassium chloride arises mainly from the
forwarding of bleach plant effluents to the recovery
and Xegeneration system, thereby providing a liquid effluent-
free closed cycle potassium-based pulp mill.
It has been discovered that the solubility charac-
teristics of thesystems KOH-K2CO3-K2SO4-KCl-H20, KOH-K2S-
K2cO3 K2SO4 Cl 2 2 2 3 2 4 2somewhat different from the corresponding sodium salt systems
and these differences permit potassium chloride removal from
the recovery and regeneration cycle to be effected in a
si~plified manner, as compared to the prior processes
.,
utilized in soda-based pulping systems and disclosed in
lS the above-recited patents.
The simplicity of the potassium chloride removal
processes adds to the benefits of utilization of potassium-
based chemicals outlined above. The simplicity arises
from the pronounced tendency for potassium carbonate to
remain in the aqueous phase upon concentration of either
the green liquor or white liquor, in complete contrast to
the tendency of sodium carbonate to precipitate from green
liquor or white liquor in the sodium-based systems discussed
above.
:
The evaporative procedures used in this invention
enable an additional economic saving to be achieved, as com-
pared with the sodium-based processes mentioned above. Thus,
less evaporation is necessary, with consequent heat savings,
.
6 ~ 6~1
usually in the form of steam, in the case of the potassium-
based process to achieve the required purge of potassium
chloride as compared with the sodium-based process to
achieve the required purge of sodium chloride. For example,
5 a 3.5 molal active alkali solution (OH ~S ~ requires about
65% evaporation to crystallize the requisite potassium
chloride as compared with about 80~ for sodium chloride
crystallization ~rom sodium-based white liquor.
The present invention is primarily concerned with
the pulping of cellulosic fibrous material using pulping
liquors containing potassium hydroxide, and also preferably
potassium sulphide, as at least the major proportion of the
pulping chemicals. However, sodium ions may be present in
the system, arising from a variety of sources, such as,
deliberate inclusion of sodium-based pulping chemicals,
; such as, sodium hydroxide and sodium sulpbide, in the pulping
liquor and/or from the cellulosic fibrous material which is
. ~ .
pulped.
When such sodium ionic species are present in
significant amounts, the present invention is concerned
generally with smelt solutions and white liquors formed in
the regeneration cycle having a potassium molar ratio, i.e.,
(K/K+Na), ~reater than about 0.5, preferably greater than
about 0.6. The potassium molar ratio present in these
liquors influences solid phases which can be precipitated
therefrom.
Generally, the solid phases differ depending on the
potassium molar ratio, different phases oeing experienced in
the molar ratio ranges of about 0.5 to about 0.7, greater
_! .
,
~IDS6~1
than a~out 0.7 to about 0.9 and greater than about 0.9 to 1.0,
in the case of white liquor and the molar ratio ranges of
about 0.5 to a~out 0.8, greater than 0.8 to about 0.9 and
greater than about 0.9 to 1.0 in the case of smelt solution.
s The build up of excess sodium values within the
system, such as may occur when some sodium-containing wood
- . species are pulped and a wholly potassium-based pulping
liquor is used, may be prevented ~y purging the excess sodium
values in the form of solid sodium carbonate.
BRIEF DESCRIPTION OF DR~WINGS
:~ ~ Figure 1 is a schematic flow sheet of a liquid
effluent-free potassium based Kraft pulp mill wherein
potassium chloride is removed;
Figure 2 is a schematic flo~ sheet of a white
liquor evaporation process for use as an alternative to the
process of Figure 1 for removal of potassium chloride;
Figure 3 is a schematic flow sheet of a green
- liquor evaporation process for the removal of potassium
chloride in an effluent-free potassium-based Kraft pulp
2Q mill;
Figure 4 is a schematic flow sheet of an alterna-
tive green liquor evaporation process to that illustrated
in Figure 3;
Figure 5 is a schematic flow sheet of a smelt
. 25 leaching and white liquor evaporation process for the
removal of potassium chloride in an effluent-free potassium-
:~ based Kraft pulp mill;
Figure 6 is a schematic flow sheet of a smeltleaching and potassium sulphide solution evaporation process
~ .
:
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S6~1
for the removal.of potassium chloride as an alternative to
the process of Figure 5;
Figure 7 is a schematic flow sheet of a modifica-
tion of the process of Figure 2 wherein a sodium purge also
is effected;
Figure 8 is a schematic flow sheet of an alterna-
tive sodium purge process from that of Figure 7; and
Figures 9 to 14 are graphical representations of
the solubility characteristics of a multi-ionic aqueous
10 system. . .... . .. . . . . ..
DESCRIPTION OF PREFERRED EMBODIMENTS
.Referring to the drawings, Figure 1 is a schematic
flow sheet of a liquid effluent-free or "closed cycle" pulp
mill which includes a digester 10 to which wood chips are
- 15 fed by line 12 and pulping liquor by line 14. The pulping
liquor contains potassium hydroxide and potassium sulphide
: as the active pulping chemicals. The pulp passes by line
. 16 to brown stock washer 18 for separation from black liquor
and for washing by bleach plant effluents are possibly by
additional w~sh watex fed by line 20.
The washed and unbleached pulp from the brown
stock washers 18 passes by line 22 to a bleach plant 24
wherein the pulp is subjected to a series of bleaching,
; caustic extraction and washing operations to result in
bleached pulp of the required brightness and purity in
line 26.
The actual sequence of operations which is effec-
ted on the pulp in the bleach plant 24 is not significant,
although it is preferred to effect an initial bleaching
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.: .. .. :~ . : : .
~` 9 ~ 6~1
with an aqueous solution of chlorine dioxide and chlorine
in which chlorine dioxide provides about 20 to about 95
of the available chlorine of the solution or with an
a~ueous solution of chlorine dioxide and chlorine followed,
without an intermediate wash, by an aqueous solution-of
chlorine, the overall quantity of chlorine dioxide used
being about 20 to about 95~ of the available chlorine of
- the solution.
After the initial bleaching step, the pulp may
be washed and extracted with potassium hydroxide solution.
Further washing, bleaching with chlorine dioxide solution,
washing, extraction with potassium hydroxide, washing,
bleaching with chlorine dioxide solution and final washing
completes thls preferred sequence of operations.
The chlorine dioxide and chlorine feeds to the ~. ,
bleach plant 24 are indicated by line 28, the potassium .
. hydroxide solution feeds are indicated by line 30 and the
wash water feeds are indicated by line 32. The filtrates
from the various operations are passed countercurrently to
20. pulp flow within the bleach plant 24, to form an alkaline
(El filtrate1 effluent in line 34 and an acid (D/C filtrate)
effluent in line 36, as described in more detail in U.S.
Patent No. 4,039,372. Following a procedure analogous to
.. ~ that described in U.S. Patent No. 4,039,372, part of the
D/C filtrate is neutralized with potassium hydroxide solution
fed by line 38 and then is fed by line 40 for use in the
~ brown stock washer 18 after washing by part of the E
filtrate fed by line 42. The remainder of the D/C and El
filtrates are used elsewhere, as described in more detail
.
below and entex the recovery system by those routes.
The black liquor and the bleach plant effluents
and water used as wash water pass by line 44 to black liquor
evaporators 46 wherein the dilute black liquor is concen-
trated, the concentrated black liquor passing by line 48 toa recovery furnace 50. In the recovery furnace 50, the
organic materials in the concentrated blac~ liquor are
.
burned off. Since all the bleach plant effluents in lines
34 and 36 enter the recovery system, the organic materials
`; lO contained therein are removed by the furnacing. The bleach
plant effluents also introduce potassium chloride to the
recovery system so that the liquid smelt exiting the
recovery furnace by line 52 contains not only potassium
sulphide and potassium carbonate but also potassium chloride.
; 15 Losses of potassium and sulphur values from the
system are made up by introducing potassium sulphate to
the furnace by line 54, typically by introduction to the
dilute black liquor in line 44. The furnacing operation
generally is less than 100% efficient in converting
20 potassium sulphate to potassium sulphide, so that the
smelt in line 52 also usually contains potassium sulphate,
the proportion depending on the efficiency of the furnace.
Potassium sulphate in the liquors formed from the smelt
may also arise from oxidation of potassium sulphide therein.
The smelt in line 52 is forwarded to a smelt
dissolving tank 55 for dissolving in an aqueous medium fed
by line 56 to form green liquor thereby.
The green liquor preferably has the following
composition:
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... , .. ,,,, . . .. , . . . . . ., . -
.: ,............. . : : . : : : ..
$~
Sulphide 0.25 to 0.75 molality
Carbonate 1.2 to 1.8 molality
Chloride 0.3 to 1.0 molality
Sulphate 0.01 to 0.1 molality
Potassium 3.2 to 6.2 molality
After filtration or settling to remove un-
dissolved dregs, the green liquor is forwarded by line 58
to a recausticizer 60 wherein the potassium carbonate is
converted to potassium hydroxide by reaction with reburned
10 lime fed by line 62. The lime mud (calcium carbonate)
precipitated in the recausticization is remo~ed by line 64
for kilning to reform reburned lime for feed by line 62.
The remainder of the D/C filtrate in line 66 may be used
for kiln gas scrubbing and/or lime mud washing, as
` ~ 15 described in U.S. Patent No. 4,039,372, and hence enters
the recovery system in this way. The weak wash water from
lime mud washing usually is u~ed as at least part of the
aqueous medium in line 56 to prevent loss of the chemicals
contained in the weak wash water.
In the event that the furnace 54 is so inefficient
that more potassium sulphate is produced than can be
dissolved in the green liquor, loss of the excess undissolved
potassium sulphate with the dregs may be avoided by leaching
the dregs to dissolve the potassium sulphate, such as, by
;~ 25 use of the aqueous medium in line 56 and/or D/C filtrate in
line 66, before or after use thereof in lime kiln scrubbing
- and/or lime mud washing.
~ .
'-
The white liquor resulting from the recausticiza-
tion in line 68 is an aqueous solution of potassium hydroxide,
potassium sulphide, potassium sulphate, potassium chloride
and potassium carbonate, the potassium carbonate arising from
incomplete causticization of all the potassium values of
the green liquor, and preferably has the following composi-
tion:
hydroxide 2.2 to 2.8 molality
sulphide 0.25 to 0.75 molality
carbonate 0.15 to 0.4 molality
chloride 0.30 to 1.0 molality
sulphate 0.01 to 0.1 molality
potassium 3.2 to 6.2 molality
It has been found that the solubility characteris-
tics of this system are quite different from the corres-
ponding sodium system and these differences enable potassium
chloride to be separated by a simpler procedure than the
procedures for the corresponding sodium system. In
particular, it has been found as a result of extensive
investigation of the solubility characteristics of the
system that potassium carbonate is very much more soluble
in the potassium based white liquor at elevated tempera-
tures than sodium carbonate and burkeite are in the so~ium
based white liquor. In addition, potassium sulphate is less
soluble in the potassium based white liquor than sodium
sulphate and burkeite are in the sodium based white liquor.
As a result of the solubility of potassium
carbonate in the system, potassium sulphate and potassium
chloride can be precipitated from the white liquor by con-
.
- : -
~ ,; ' i . . ` ::::, . ~ ' :''.:. ' :
13 ~ 6~ -
centration thereof, usually by boiling, without contamination by
potassium carbonate. This result contrasts markedly with
the sodium based systems, described in U.S. Patents Nos.
3,746,612 and 3,950,217, wherein sodium carbonate is always
5 precipitated. In the two-stage evaporative procedure
described in U.S. Patent No. 3,950,217, for example, sodium
sulphate and sodium carbonate are coprecipitated in the
first stage, with sodium carbonate normally being the
dominant phase. Since it is required to recycle this
10 precipitate to the furnace for consumption of the sodium
sulphate and the sodium carbonate xepresents a dead load
on the furnace which decreases furnace capacity, split
; recycle procedures have to be adopted to increase the
relative proportion of sodium sulphate in the precipitate
15 to decrease thereby the dead load effect of sodium carbonate _
on the furnace. Such operations are not required in this
; invention and relatively uncontaminated potassium sulphate
can be recycled to the furnace.
In Figure 1 there is illustrated one procedure
for the separation of potassium chloride wherein the white
liquor in line 68 is evaporated by boiling in a white
liquor evaporator 70 to precipitate a mixture of potassium
chloride and potassium sulphate whic~ is removed
from the evaporator 70 by line 72. Generally, the evapora-
tion of the white liquor is effected in such a way that thequantity of potassium chloride precipitated and removed
corresponds to the quantity entering the system, usually
about 90 to about300 lbs. KCl/ton of pulp.
The evaporation of the white liquor in the
. ~' ' .
..~.
il~g6~
14
evaporator 70 may be effected over a wide range of tempera-
tures, the lower limit of which is dictated by carbonate
solubility while the upper limit is dictated by the
atmospheric pressure boiling point of the white liquor,
although superatmospherlc pressure may be adopted to in-
crease the boiling temperature further. Generally, the
evaporation of the white liquor in the evaporator 70 is ~:
effected by boiling at a temperature of about 40 to about
120C, preferably about 40~ to about 70C.
The concentrated white liquor resulting from the
one-stage evaporation is removed from the evaporator 70 by
line 74 and is diluted to the required concentration for
pulping by the remainder of the ~1 fîltrate in line 76, so
that in this way the remainder of the El filtrate enters
15 the recovery system. The diluted white liquor then passes ~ '
by line 78 for feed to the digester 14, after supplementa-
tion, if required, by an additional external source of
pulping liquor by line 80.
The solid phase removed from the evaporator 70 by .
line 72 preferably is treated to remove potassium sulphate
therefrom to avoid losses of this expensive chemical with
the potassium chloride. As illustrated in Figure 1, the
solid mixture in line 72 is leached at an elevated temperature
in a leacher 82 by leach liquor fed by line 84 to dissolve
all the potassium chloride and some of the potassium sulphate,
leaving substantially pure solid potassium sulphate, which
is recycled by line 86 to the furnace 50, with the potassium
sulphate feed in line 54, or by any other convenient route.
While it is preferred to forward substantially pure potassium
~ . .
:, . . . ::
~¢~96~
sulphate by line 86 to the furnace 50, some potassium
chloride contamination of the potassium sulphate may be
tolerated.
The hot leach liquor from the leacher 82 is
S passed by line 88 to a cooler 90 wherein the hot liquor is
cooled to crystallize potassium chloride which is recovered
by line 92, the cooled mother liquor being recycled, after
heating, to the leacher 82 by line 84.
The solid potassium chloride in line 92 usually -
is contaminated with a small quantity of potassium sulphate,
which represents a loss from the system. The relative
solubilities of potassium sulphate and potassium chloride
do not permit their complete separation by crystallization
techniques. The loss of potassium sulphate values in this
way may be avoided by using the procedures outlined below
with respect to Figures 5 and 6 described in detall below.
The proportion of potassium sulphate in the
mixture by line 72 depends on the efficiency of the furnace
50 in converting potassium sulphate to potassium sulphide.
As the efficiency of the furnace increases, the ability to
separate pure potassium sulphate from the potassium chloride
decreases until, in the limiting condition, the proportion
of potassium sulphate decreases to that which is inseparable
from potassium chloride by crystallization techniques. Under
typical operating conditions, this limitation arises at
about 98% efficiency.
The hot leach of the solid mixture generally is
effected at a temperature of about 70 to about 100C, pre-
ferably about 80 to about 100C. The hot leach liquor
'` ' - .
16 ~ 6 ~ ~
generally is cooled to a temperature of about 20to about
~C, preferably about 20 to about 40C.
The potassium chloride removed by line 92 may be
; purified to remove the contaminating potassium sulphate by
5 precipitating the sulphate as the barium salt. The potassium
; chloride may be used, if desired, for the formation of
potassium hydroxide by electrolysis for use elsewhere in
the system.
Turning now to the embodiment of Figure 2, there is
&hown therein a two-stage evaporative white liquor pro-
... . .
cedure for the separation of potassium chloride, in place
~` of the single evaporative process of Figure 1, followed by
- leaching. The other portions of the mill remain the same
and are not illustrated in Figure 2.
lSAs shown therein, the white liquor in line 68 is --
forwarded to a first white liquor evaporator 100 wherein the
white liquor is subjected to a first evaporation by boiling
to precipitate substantially pure potassium sulphate, which
is removed from the ~irst evaporator 100 by line 102, for
passage to the furnace S0 in analogous manner to that
described above with respect to the potassium sulphate
in line 86 in Figure 1. The evaporation of the white liquor
in the first evaporator 100 is effected until the white
liquor becomes substantially saturated with potassium
chloride.
The partially concentrated white liquor is passed
.by line 104 to a second white liquor evaporator 106 wherein
~",!,the white liquor is boiled further to precipitate potassium
chloride which is removed by line 108. The concentrated
~ , , .r
~.. ",. .
17
white liquor in line 74 then is diluted and recycled as
described in connection with Figure 1. The potassium
chloride usually is contaminated by small quantities of
potassium sulphate which are thereby lost from the system,
since it cannot be separated by simple crystallization
techniques. Further purification of the potassium chloride
may be effected by precipitation of the sulphate as barium
sulphate.
In place of two separate evaporators 100 and 106,
a single evaporator may be used wherein two evaporation
steps are effected respectively to crystallize potassium
sulphate and potassium chloride.
The white liquor may be boiled over a wide range
of temperatures to effect the crystallization in the evapora-
tors 100 and 106. Generally, temperatures of about 40to about 120C are used in each of the evaporators,
preferably about 40to about 70C. ~
; The embodiment shown in Figure 3 is directed to
removal of potassium chloride from green liquor in place
Of the white liquor evaporative procedures of Figures 1 and
; 2. In this embodiment, the green liquor in line 58 is
.
concentrated by boiling in a green liquor evaporator 110
to precipitate a mixture of potassium sulphate and potassium
;~ chloride, which is removed from the evaporator 110 by line
112.
The precipitation of potassium sulphate and
- potassium chloride from the green liquor wherein the pre-
:,
dominant dissolved salt is potassium carbonate without
contamination by potassium carbonate is markedly different
:;.
.. . .
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~396~
18
from the sm21t manipulation procedures used in sodium-based
mills, as outlined, for example, in U.S. Patents Nos.
3,986,923, 3,909,344 and 3,954,552, wherein sodium carbonate
; must first be removed before recovery of sodium chloride
5 can be effected.
There is a limiting condition on the operability
of this embodiment below which potassium carbonate contamina-
tion occurs, such precipitation occurring when the chloride
to carbonate molar ratio falls below about 0.3 in the green
10 liquor. However, the ratio generally is well in excess of
this value and hence carbonate contamination is unlikely to
occur.
The precipitation of the mixture of potassium
salts may be effected over a wide range of temperatures,
15 the lower limit being dictated by potassium carbonate solu-
bility and the upper limit being dictated by the atmos-
pheric pressure boiling point of the green liquor, although
superatmospheric pressure may be utilized, if desired, to
achieve higher boiling temperatures. Usually, the
20 evaporation of the green liquor is effected by boiling at
a temperature of about 40 to about l20C, preferably
about 4Qo to about 70 C
The concentrated green liquor resulting from the
eVaporation step is removed from the evaporator 110 by line
.
114, is diluted to the required concentration by the El
- filtrate in line 76 and forwarded to the recausticizer 60
by line 116. The white liquor resulting from the recaustici-
zer is recycled by line 68 to the pulping liquor feed in
line 14.
~ .
.: .
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.;. .
... ~ .. .. .. . . .
.. , ~ , . : ,, -- : ,:: : , -
6~
19 :,
The mixture of salts in line 112 contains a
variable proportion of potassium sulphate which depends on
the efficiency of the furnace, as discussed above in
connection with the mixture in line 72 of Figure 1.
5 Separable potassium sulphate may be removed from the -
mixture in analogous manner to that described above with
respect to Figure 1 by hot leaching the mixture in a
leacher 118 with aqueous medium fed by line 120 to dissolve
all the potassium chloride values and some of the potassium
sulphate to leave substantially pure potassium sulphate for
-
forwarding by line 122 to the furnace 50.
The hot leach liquor is forwarded by line 124 to
a cooler 126 wherein the leach liquor is cooled to precipitate
potassium chloride for recovery by line 128. As in the
15 case of the Figure 1 embodiment, the solid potassium -- ,~
chloride recovered is contaminated with small quantities
of potassium sulphate. The mother liquor from the potassium
chloride crystallization is recycled, after heating, to the
leacher 118 by line 120.
The leaching and cooling steps effected in leacher
118 and cooler 126 respectively may be carried out under the
same temperature conditions as recited above for the leacher
82 and cooler 90 in the embodiment of Figure 1, and reference
may be had thereto for the temperature conditions.
~` 25 Figure 4 illustrates a modification of the embodi-
ment of Figure 3 wherein two separate e~ap~rative steps
` are effected for separation of potassium sulphate and
potassium chloride. The procedure is analogous to the white
liquor process described above with respect to Figure 2.
' ''' .
2Q
Green~liquor in line 58 is forwarded to a first
green liquor evaporator 130 wherein the green liquor is boiled
to deposit potassium sulphate, which is removed by line 132.
The partially concentrated green liquor is forwarded by
5 line 134 to a second green liquor evaporator 136 wherein
- .. .
the partially concentrated green liquor is boiled to deposit
potassium chloride, which is removed by line 138. The con-
centrated green liquor is forwarded by line 114 for further
processing as described above with reference to Figure 3.
1~ The concentration of the green liquor in the two
evaporators 130 and 136 is effected at the same temperatures
as the two evaporators in the two-stage white liquor process
of Figure 2 and reference may be had thereto for the res-
pective temperatures.
Turning now to the embodiment of Figure 5, there
is illustrated therein a potassium chloride removal system
wherein potassium sulphate losses with the potassium
chloride are substantially eliminated. In place of the
smelt being dissolved in an aqueous medium to form green
2a liquor, as is the case in the embodiments of Figures 1 to
4, the smelt in line 52, which first may be solidified, if
,.. .
desired, is subjected to leaching in a leacher 140 by an
aqueous medium fed by line 142 to dissolve only potassium
sulphide, potassium carbonate and potassium chloride from
the smelt and leave the potassium sulphate with the smelt
dregs in the solid phase.
The smelt leaching step may be effected over a
wide temperature range, generally about 30 to about 110C,
, . . .
,,. ~ ~ .
6~ -
21
although the lGwer end of the temperature range is pre~erred
to inhibit potassium sulphate solubility. The preferred
temperature range is about 30Oto about 70C.
The aqueous solution of potassium sulphide,
potassium carbonate and potassium chloride resulting from
the smelt leaching is forwarded by line 144 to a re-
causticizer 146 wherein the potassium carbonate values
are mainly converted to potassium hydro~ide. The white
liquor formed in the recausticizer 146 contains potassium
hydroxide, potassium sulphide, potassium chloride and
potassium carbonate, the latter arising from inefficien-
cies in the recausticization process. The white liquor
is vlrtually free from potassium sulphate as a result of
the initial smelt leaching step.
lS The white liquor is passed by line 148 to a
white liquor evaporator 150 wherein the white liquor is
concentrated by boiling to precipitate potassium chloride
in substantially pure form, the potassium chloride being
removed by line 152. The evaporation of the white liquor
~; 20 may be effected over a wide temperature range, generally
about 40 to about 120C. It is preferred to operate at
the upper end of this range to inhibit crystallization of
any potassium sulphate present and the preferred tempera-;:
ture range is about 70to about 110C.
.~
~; 25 Any potassium sulphate contamination in the
. ~ . . .
~ potassium chloride may be removed by precipitation as
~ .
barium sulphate and, as in the other cases, represents a
loss of chemicals from the system. Utilization of the
smelt leaching procecure, however, minimizes such losses,
"'~;'
:: "
, . , - : , . - , :. . .:
~ $~
. 22
as compared with the procedures described above with respect
: to Figures 1 to 4.
: The solids remaining from the smelt leaching
are forwarded by line 154 to a dregs leacher 156 wherein
5 the potassium sulphate is dissolved by wash water fed by
line 158 to form a potassium sulphate solution and leave
dregs solids for recovery by line 160.
: The potassium sulphate solution in line 162
resulting from the dregs washing is recycled to the furnace,
such as, by dilution of the concentrated white liquor in
line 164 to form diluted white liquor in line 166 for re-
cycle to the digester. Alternatively, the potassium sulphate
solution may be forwarded directly to the black liquor
evaporators 46.
Figure 6 illustrates a modification of the embodi- -
ment of Figure S which also utilizes the smelt leaching
operation and the same reference numerals are used to
` denote the same items. In the embodiment of Figure 6, the
'! aqueous solution in line 144 containing potassium sulphide,
,;
20 potassium carbonate and potassium chloride is concentrated
`,.~! by boiling in an evaporator 170 to precipitate potassium
chloride which is removed by line 172.
r'~' The temperature of operation of the evaporator
170 may vary widely, generally about 40 to about120C,
25 preferably towards the upper end of this range to inhibit
the tendency for any potassium sulphate present to precipi-
~` tate, and in the range of about70 to about llOoc.
. .; .
~. The concentrated liquor in line 174 and formed in
``~ the evaporator 170 may be diluted with potassium sulphate
,',,,~ ' .
,,
" . ~-, .,:, ?. ., ,:
23 11~611
solution in line l62, or El filtrate, before passage by
line 176 to the recausticizer 146 from which the white
liquor in line 148 is removed for recycle to the digester
10. Alternatively, the potassium sulphate solution in
5 line 162 may be forwarded directly to the black liquor -~
evapOrators 46, or to the white liquor in line 148.
The processes for potassium chloride removal
described above with reference to Figures 1 to 6 have
assumed that sodium does not enter the system, or is present
1~ in such minor quantities that it has no effect on the
system. The embodiments of Figures 7 and 8 are directed
to processes for the removal of potassium chloride when
significant concentrations of sodium salts are present to
- the extent that a purge of sodium values is required.
The relative solubility characteristics of sodium -
salts in the presence of large concentrations of potassium
; salts,i.e.at high K/K+Na molar ratios, have been determined.
Assuming that the màjor source of the sodium is the wood
which is pulped and this corresponds to about one-quarter
to about one-half of the potassium content thereof, then
the green liquor and white liquor compositions preferably
are as follows:
Green liquor:
. . ,;. .
~ sulphide 0.25 to 0.75 molality
i 25 carbonate 1.2 to 1.8 molality
-,~
chloride 0.3 to 1.0 molality
sulphate 0.01 to 0.1 molality
K + Na 3.2 to 6.2 molality
K/K+Na 0.50 to 0.75 molar ratio
.:',
` 24 ~ 6~
White liquor:
hydroxide 2.2 to 2.8 molality
sulphide 0.25 to 0.75 molality
carbonate 0.15 to 0.4 molality
chloride 0.30 to 1.0 molality
sulphate 0.01 to 0.1 molality
K + Na 3.2 to 0.2 molality
K/K + Na 0.50 to 0.75 molar ratio
It is possible to effect recovery of potassium
chloride over a wide range of K/K+Na molar ratios but simple
procedures involving precipitation of potassium sulphate
and potassium chloride without the precipitation of
potassium carbonate, while maintaining the desired purge
~` of potassium chloride,can only be effected under limited
: ''`
; 15 conditions, the ranges depending on the temperature and _
.~ carbonate concentrations.
Referring now to Figure 7, there is shown therein
-~ a two-stage white liquor evaporation process similar to that
i.9
; described above with respect to Figure 2, except that in
`l 2Q this instance, sodium carbonate coprecipitates with potassium
chloride in the second white liquor evaporator 106. The
sodium carbonate present in the potassium chloride removed
by line 108 represents a purge of sodium values from the
i :.
` system. If desired, the potassium chloride and sodium
carbonate may be separated by leaching. The conditions of
operation of the two evaporators 100 and 106 are as
described above with respect to the embodiment of Figure 2,
f`;`` and reference may be had thereto for appropriate conditions.
, .- --.
~ In Figure 8, a three-stage white liquor evaporation
..
, ~ .... .
, . ~
, ~
2s 1~96~1
process is illustrated whereby potassium chloride and sodium
carbonate are independently separated from the white l~quor.
In this process, the second stage evaporator 106 is opera-
ted to deposit potassium chloride, with a small degree of
5 contamination by potassium sulphate.
The concentrated white liquor from the second
stage white liquor evaporator 106 is passed by line 180
to a third stage white liquor evaporator 182 wherein the
white liquor is evapcrated to deposit sodîum carbonate
which is removed by line 184. The concentrated white
liquor in line 74 is then recycled as described above.
The first stage evaporation is e~fected as
described above in connection with Figure 2 and reference
may be had thereto for details thereof. The second stage
. ~, .
- 15 evaporation is effected at a low temperature, generally
about 40 to about 60C, in order to favour potassium
i .
chloride crystallization. The deposition of potassium
chloride is effected until substantial saturation of the
i,..;:
white liquor by sodium carbonate is reached.
In the third stage evaporator 182, the white
liquor is boiled at a high temperature, generally about
~; 90 to 110C, to cause crystallization of sodium carbonate
but to keep the potassium chloride in solution. The sodium
~'!" carbonate remove by line 184 may be contaminated with traces
., .
25 of potassium sulphate.
.~
The amount of sodium carbonate which may be
~ ~ removed by the third stage evaporation in the process of
- ~igure 8 exceeds the expected sodium inputs to the
mill and hence need be operated only intermittently, if
:~ , -
: . - - .: , , . , . , , ~ . . ., -
- . . .
26 ~S~l
desired, with a two-sta~e operation being effected for the
remainder of the time.
The processes of Figures 7 to 8 are applicable
to white liquor in line 68 having a K/K~Na ratio of about
5 0.6 to about 0.7. Attempts to control sodium levels at
higher ratios, for example, about 0.7 to about 0.9, produce
a carbonate phase which is KNaCO3, which would represent
a loss of potassium from the system when the sodium purge
is effected, which may be considered undesirable. However,
10 since the quantities of sodium required to be removed are
small, the potassium losses resulting from the removal of
KNaCO3 also are small, and hence may be tolerable by some
mills.
It is also possible to limit sodium build-up by
15 removal from green liquor but at a typical R/K~Na ratio of
about 0.8, the sodium is removed as KNaCO3, which may be
considered undesirable, as noted above.
EXAMPLES
Example 1
The solubility characteristics of an aqueous
~; system containing the ionic species K , Na , OH , S , Cl ,
~ CO3 , and SO4 were studied at various potassium molar ratios
;; of K/K+Na up to 1.0 (i.e. the case where sodium is absent).
Figures 9 to 11 are graphical representations of
;~1 2S part of this study, illustrating, respectively, the solu-
`, bility of chloride at 100C whe~ the system is saturated with
CO3 and SO4 , the solubility of carbonate at 100C when
the system is saturated with Cl and SO4 and the solu-
bility of sulphate at 100C when the system is saturated
~,.~" .
'
27
with Cl and CO3 .
Figures 12 to 14 are also graphical representa-
tions of part of this study showing the stable solid phase
transitions upon variation of potassium molar ratio and
total alkali concentration (S ~ OH ) at 50C, 75C and 100C
respectively.
This solubility data was used to determine mass
balances for the various pulp mill procedures outlined above
, . __ _ . ............. . .
; in Figures 1 to 8. The determinations are outlined in the
; io following Examples II to IX.
: Example II
:
Utilizing the developed solubility data, a mass
balance was calculated for the white liquor evaporation
: process of Figure 1, omitting the cooling crystallization
: 15 of KCl from leach liquor and mother liquor recycle. In --!
this process, the evaporation temperature in evaporator 70 ~-
: was assumed to be 100C while the leach temperature in
. leacher 82 was assumed to be 30C.
: The results are reproduced in the following
~: 20 Table I:
,~ ' .' .
,. .
.
. . .
- -
28 ~56~1
TABLE I
Stream Water Solution Composition-m~lality Solid Phaæs
No. lb/ton lb.l/ton pulp
OH 3 4 K Cl K2S04
.
5 68 6000 2.5 0.5 0.65 0.30.05
71 4320
74 1680 9.01 1.8 1.04 1.08. ~.02
72 2.18 0.27
84 450
10 88 450 0 0 4.. 84 0 0.09
.~ 86 0.0 0.23
. Example III
A mass balance was calculated for the procedure of
Example II in which the cooling crystallization of KCl from .
15 mother liquor is effected and mother liquor recycle occurs.
~: The tempexature in evaporator 70 was assumed to be 100C,
that in leacher 82 was assumea to De 100C and that in-cool~r --
~: 90 was assumed to be 30C. -
The results are reproduced in the following
'~i: : 20 Table II:
;.~`3: .
~'
.', ~ , .
' .
-
,
~'''' .
..,
", ...
-: : : . .::: .::: : : . . : : ,: : . ::: :,: . ; ,, : -
29
TABLE II
Stream Water Solution ~sition Solid Phases
: No. lb/ton - m~lality lb.-mol/ton of pulp
. . . . . QH . S Cl CO3 . SO4 KCl K2SO4
68 6000 2.5 0.5 0.65 0.3 0.05
71 4320
-` 74 1680 9.01 1.80 1.04 1.08 0.02
,~ 72 2.18 0.27
-~ 84 876 . O O 4.84 O 0.09
- . 10 86 o 0.22
88 876 O O 7.33 O 0.148 -
. - 92 . 2.18 0.05::
:, .: . ...... ...
~; ' 2 ~ ,
Example IV
. ,. . ~
... . A mass balance was calculated for the two-stage :.
; 15 white liquor evaporation process of Figure 2. The tempera- _ :
ture of each white liquor evaporation stage was assumed to be
.~' - 50C. The results are reproduced in the following Table III:
I TABLE III
S~m Water Solution Cbmposition Solid Phases.
.~ 20 No.lb/bon - molality lb.-m~Vbon o~ pulp
OH 3 4 KCl. K2SO4 .
68 6000 2.50.5 0.65 0.3 0,05
,
1013222
102 0.234
1042778 5.4 1.08 1.40 0.65 0.025
105 630
:~ 108 2.130.036
742148 6.97 1.39 0.82 0.~4 0.015
'; ' ' '
'
~ .
- ., , - :: ~ . .
3n 1~}963Ll
Example V
A mass balance was calculated for the process of
Figure 3, with the exception that the leach liquor cooling and
mother liquor recycle were omitted. The green Iiquor evapora-
tion temperature was assumed to be 100C while the leachingtemperature was assumed to be 30C. The results are repro-
duced in the following Table IV:
TABLE IV
-;
Stream Water Solution ~osition Solid Phases
10 No. lb/ton - mDlality lb.-m~l/ton of pulp
OH S Cl Co3 SO4 KCl K2S04
;:. . . ___ __. _ . _ . . . . __ .. . . .
~j 58 6000 0.1 0.5 0.65 1.5 0.05
,'~ 111 4440
, 114 1560 0.38 1.90 1.12 5.69 0.029~."~
- 15 112 2.140.25
120 442
124 44Z 0 04.84 0 0.09
;~ .
122 0.00.21
- . .
i~ Example VI
;: .
,. ........................................... .
For the process of Figure 4, a mass balance was
calculated with the evaporatlon temperatures in the two
evaporators being assumed to be 50C. The results are
reproduced in the following Table V:
,
~ .
.~ ~ .
..... .
31
TABLE V
Stream Water Solution ~x~osition Solid Phases
No. lb/ton - molality Ib.-mol/ton of pulp
OH S Cl Co3 so4 KCl K2S04
- - - ,~
58 6000 0.11 0.53 0.68 1.58 0.053
1313222
132 0.20
1342778 0.22 1.08 1.40 3.24 0.037
137668
138 2.15 0.06
114 2110 0.2g 1.47 0.86 4.4 0.021
. .
Exam~le VII
:A mass balance was calculated for the embodiment
of Figure 5. A temperature of 50C was assumed for the
smelt leaching, a temperature of 100C was assumed for the _
dregs leaching and a temperature of 50 C was assumed for
~",
the white liquor evapoxation. The results are reproduced
in the following Table VI:
TABLE VI
Stream Water Solution C~osition Solid Phases
No. lb/ton - m~lality lb.-m~l/ton of pulp
OH S o3 SO4 KClK2S04
,
1422640
1442640 0.23 1.141.48 3.41 0.041
154 0.19
158 140
162 140 0 0 0 0 1.36
1482640 5~68 1.141.48 0.68 0.041
1642148 6.97 1.390.82 0.84 0.015
152 2.130.08
151 492
.
.. ~ ,
32 ~ 6~
Example VII r
A mass balance for the embodiment of Figure 6
. was calculated, with the smelt leach temperature being
. .:
assumed at 50C, the dregs leach temperature at 100C and
the green liquor evaporation temperature at 100C. The
results are reproduced in the following Table VII:
TABLE VII
S~tre~m H O Solution C~osition Solid Phases
!~r~ Nb. lb~ton - m~lality
i?~'~ I0 OH S Cl CD3 SO4 KCl K2S04
, . - - .
-. 142 2640
,, .
1442640 0.23 1.14 1.48 3.41 0.041
154 0.19
158 140
15 162 140 j
1742154 0.38 1.90 1.10 5.69 0.029 . :-.
~,~ 172 . 2.15 0.062 -:
171 486
, : ~ Example IX
Based on the experimentally-attained solubility
data, a mass balance was calculated for the potassium chloride
~ and sodium carbonate removal process of Figure 8, with the
: first two white liquor evaporations being assumed to be
effected at 50C and the third white liquor evaporation
.25 being assumed to be effected at 100C. The results are
reproduced.in the following Table VIII:
~ ~ .
~ .
.
:, , , -~
: - . . ~ . : ,: ,.. ,:. , .: . :-
- . , :: , .: :. :::. . , -. ~ . : . ,. :
33
TABLE VIII
Stream H20 Solution G~sit~on Solid Phases
No. lb/ton - molality lb,mol/ton of pulp r
OH S Cl Co3 so4 KCl Na2003 K2S04
68 6000 2.5 0.5 0.65 0.30 0.005 0.7
1013324
102 0.24
104~676 5.60 1.12 1.46 0.67 0.022 0.698
105 576
108 2.10 0.03
1802100 7.15 1.43 0.85 0.86 0.015 0.68
..... .
~81 438
184 0.01
,. . .
74 1662 9.01 1.80 1.07 0.63 0.018 0.72 0.76
15 *R is the molar ratio K/X+Na
The quantity of sodium carbonate removed by
this procedure represents the maximum sodium recovery under
the assumed operating conditions and greatly exceeds the
expected sodium input from wood sources, indicating that
the third stage evaporation need be operated only inter-
mittently.
SUMMARY
The present invention, therefore, provides a
potassium based pulp mill procedure which is able to achieve
potassium chloride recovery and further is able to prevent
the build up of sodium values within the system. Modifica-
tions are possible within the scope of the invention.
.
.. . .: : .: : ,
