Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1064206
The presen~ invention relates to the recovery of
substances in the extraction of spent liquors from delig-
nification in order to reduce water pollution from pulp
mills employing some of the previously known systems for
digestion and subsequent delignification of cèllulose
pulp in a sequence of bleaching stages of which the final
stage of said delignification is effected with bleaching
agents which form chlorides during the combustion of the
effluent in the chemical regeneration process of the pulp
mill.
According to this invention of the extracted spent
liquor, the chloride-forming bleaching effluent is sub- -
jected to a thermal oxidation or combustion, but averting -
build-up of chloride in the recovery system for the
digestion chemicals.
In cellulose pulp technology it is known to combust
bleaching effluents in the alkali recovery system of a
kraft (sulphate) mill, and then to separate ceystallized
sodium chloride. The total amount of liquor which is to
be used for alkaline delignification such as digestion, `
may be used for such chloride separation. The crystal-
lization of the sodium chloride in kraft process liquors ~ ~
containing sodium sulfide, sodium carbonate and sodium `
hydroxide is also known. An embodiment relates to kraft ` ~-
mills comprising two or more recovery units and to con- ~ -
~; centrating the chloride to one single unit. A chloride-
leaner fraction of the effluent from the bleach plant
alkaline extraction stages is, for this embodiment pre-
ferred to serve as a kraft process chemical make-up
~- 30 because this particular effluent is better converted
for digestion to useful sodium compounds.
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~ 1064206
Another known pulping procedure comprises delignifica-
tion of cellulose fiber raw materials with caustic soda
or, as an alternative soda, and oxidative chemicals in the
form of either elementary chlorine or chlorine compounds.
The regeneration of these delignification chemicals in the
absence of sulphur compounds simpler than the dominant
kraft- and sulfite pulping methods comprising digestion in
one or more stages.
Recently an intermediate delignification stage between
digestion and bleaching with chlorine compounds has been
developed. -
; According to the invention there is provided in the -~
manufacture of bleached cellulose pulp from lignocellu~
losic raw material a method to reduce water pollution from
bleaching effluent comprising the steps of delignifying
the raw material with useful non-chlorine delignification
agents to form spent delignification liquor and unbleached
stock, bleaching said stock with a chlorine-compound
: containing bleaching agent to form a bleached stock and
chlorine-compound substances containing bleaching effluent -
and applying bleaching effluent for displacement of the
spent delignification liquor from said unbleached stock,
and thereby employ means for counteracting chloride :~
accumulation in plants for utilization of substances
dissolved from the lignocellulosic raw material by said ~ -
non-chlorine delignification agents of which agents on
the most only a minor portion is generated from bleaching - :
effluent, the improvement comprising:
. (a) removing the major portion of the spent delignifi- -:~
cation liquor from the unbleached stock to form thereof
a lye with on the most a minor portion of the chlorine-
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compound containing bleaching effluent entrained into thelye;
(b) removing in the unbleached stock remaining portion of
the spent delignification liquor by displacement washing
with the addition of a major portion of on said unbleached
stock applied bleaching effluent in a counter-cuerent
washing system, to form brine of the applied bleaching
effluent and which brine contains a higher concentration
of chlorine compounds than the lye does;
(c) passing the brine, discharged from step (b), directly
to a separate brine treatment system which comprises a
step for thermally oxidating organic substances of the
major portion of said brine by combustion to such degree
that therein contained chlorine compounds form chloride
and that any together with said chloride occurring sulphur
compounds of lower oxidation numbers than IV are oxidized
to at least oxidation number IV; and
(d) subjecting the lye, discharged from step (a), to ~- -
utilization of the substances which have been dissolved by
the delignification and which are contained in said lye.
In the following description of the invention all
digestion methods employing sulphur compounds in some
stage of digestion will be considered. Therefore it is
found useful to give an introductory review of the sig-
nificant properties of various digestion spent liquors.
The established course of delignification of cellulose
material, e.g. wood chips, runs in two or more stages.
First brown stock (pulp) is produced and the brown stock
is subsequently further delignified by means of oxidizing
bleaching agents.
The brown stock digestion encompasses digestion with
;' ~ ' ' . .'
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solutions containing a base which supplies cations
selected from the group of sodium, ammonium, magnesium and
calcium. The cations exist in the digestion solution in
various combinations with sulfite, bisulfite, sulfide,
hydrogen sulfide ions etc. at certain hydroxide ion and
hydrogen ion concentrations. Common digestion methods
comprise 1) kraft pulping with essentially hydroxide and
2) sulfite pulping employing acid, neutral or even alka-
line solutions. For common sulfite pulping SO2 is used
in the presence of said cations but an extreme type of
sulfite pulping using only SO2 and water (hydrogen
serves as cation) is also known. Another extreme is
sulphur-free caustic soda pulping with solely sodium
hydroxide and possible residuals of non-causticized soda
(sodium carbonate). Caustic soda pulping may be applied
subsequently to pretreatment of the lignocellulose
material with sodium carbonate liquor. Optionally,
sulphur compounds, e.g. sulfite, sulfide, polysulfide or -~
hydrogen sulfide, may be used in a pretreatment stage of
caustic soda pulping. Caustic soda digestion or sodium
carbonate pretreatment is used for semi-chemical pulping,
e.g. of hardwood, followed by mechanical defibration and
optional bleaching. -~
The brown stock is obtained at a yield in the range
of from about 35 to 90~ calculated on the moisture-free
weight lignocellulose material. In the range above 90
yield, the pulp is usually not considered as brown stock
but a sort of mechanical pulp, nevertheless the ligno-
cellulose material, i.a. wood chips, is treated by 8ulfite
or soda liquor.
The various brown stock pulping methods discharge
B
.
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- . . ' ... .
, .......... . . . ;
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spent pulping liquors, which in the following description
are denoted black liquor. Thus, even sulfite spent liquor
is denoted as black liquor.
Brown stock is obtained in general at a yield in the
range of same type of reducing recovery furnacès as used
in the kraft process while Mg-, Ca- and NH4-base sulfite
black liquors are combusted under entirely oxidative con-
ditions. Mg-base sulfite pulping, e.g. according to the
Magnefite method, gives a black liquor that on combustion
produces MgO and Mg(OH)2 respectively from which chlor-
ides and alkali salts may be leached. In the leaching
some losses of other Mg-compounds occur. This loss has
to be compensated for by a make-up of other Mg-compounds.
Ca-base sulfite pulping gives a black liquor which on
~` combustion produces an ash from which calcium cannot be
regenerated except in a certain known cross-recovery
system with the kraft process.
Corrosion caused by chlorides creates problems in
combustion plants for sulfite black liquor and regenera-
tion of sulfite. There are, however, systems availablefor flue gas scrubbing in which absorbed hydrogen chloride
and sulphur dioxide is separated from regenerated S02
and sulfite, respectively. Therefore it was suggested in
the Inventor's U.S. Patent No. 3,248,169 (col. 5, lines
5-7) and Canadian Patent No. 593,787 to, in a sodium base
sulfite process, recover liquor combustion products for
regeneration of chlorine dioxide to be used in brown stock
bleaching.
Many problems arise in chloride accumulation in closed
chemical recovery systems for all various pulp digestion
methods. Most investigations in this field have been
_ 5 _
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:`~ . .
1064Z06
applied on the kraft process.
Kraft mills supplied entirely with fresh water have
only approx. one percent of their sodium as sodium
chloride. Ocean-floated wood for pulping as well as use
of brackish water at some mills has proven that it is
indeed possible to run kraft mill recovery processes at
an equilibrium with 20% of the sodium as sodium chloride.
But when such chloride levels are further raised, the
costs of corrosion and its prevention increase. There-
fore a build-up of chloride would have to be counteracted
or removed according to known methods, e.g. comprising
separation of crystallized sodium chloride from white
liquor or leaching of precipitated flue gas carryover dust.
The brown stock delignification is carried out in suc-
cessive stages using oxidative bleaching agents and the
resulting bleached stock is ultimately obtained at a yield ~-
amounting to between 30 and 65 percent of the original
lignocellulosic material.
By using chlorine based bleaching agents in lenient
and especially well controlled delignification stages, it
is possible to produce bleached stock wherein the cellu~
lose components, as for instance holocellulose, are
preserved, whereby a yield of about 70~, calculated on
original material, is obtained.
Oxygen is another oxidative bleaching agent which in
recent years has been put into practice and it may be
applied on the stock preferably under alkaline conditions
as either concentrated oxygen gas or air, Oxygen deligni-
fication stages, here denoted oxystages, convert brown
stock to oxystock. An oxystage is used in sequences
either prior to or subsequent to a stage using a chlorine
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,W
~, . ' .. :.. , , ' ~
1064206
bleaching agent. The great advantage of oxygen is that it
replaces chloride-forming bleaching agents. Brown stock
as well as oxystock releases, during its bleaching, a
bleaching effluent containing dissolved colored substances
and degraded and hydrolyzed cellulose components.
Alkaline extraction stages follow in sequences com-
prising chlorine compound stages which do not terminate
the sequence. The chloride compound is preferably
selected from the group of chlorine dioxide or chlorine
monoxide. The stock to be treated with chlorine compounds
is chosen from brown stock and oxystock. The alkaline
extraction may either involve pure alkali or alkali to-
gether with an oxidant. With regard to the recovery of
alkali from bleaching effluent it is preferred to choose
said oxidant from oxygen or oxygen compounds such as ozone
and peroxide. However, addition of hypochlorite to alka-
line stages is still practised. The alkalinity in an
alkaline extraction stage is accomplished by supply of
either sodium hydroxide or, for gas phase operation, - ~
ammonia. To shorten a bleaching sequence to a few stages -~ -
it is known that an alkaline oxystage following an initial
chlorine/chlorine dioxide stage is highly efficient in ~ ~-
making bleached stock of high brightness in combination
with well-preserved paper sheet physical strength. To - -
minimize the chlorine consumption in such a short
sequence, this sequence may be preceded by an additional
oxystage, alternatively using air with optional charge of ~ -
alkali.
The bleaching effluents that have been dealt with
here have different properties which determine their
manner of utilization together with black liquor recovery
- 7 -
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~,., . . . . . . .. . . ~ .. ,
. ~ . . . . .
.. . . . ..
1064206
in a closed system. Environmental requirements these
days not only demand a closed system with regard to black
liquor recovery but also a similar treatment of bleaching
effluents, at least of the fractions deriving from the
heavily polluting stages. Thus it has been proposed to
use bleaching effluent either for displacing black liquor
in the brown stock washing or for preparing digestion
liquors, kraft or sulfite, from bleaching effluent, con-
taining sodium, which is of value as make-up. However,
previously mentioned difficulties caused by chlorides
arise. To avoid such difficulties, it has been proposed ~ ;
(N. Mannbro, Svensk Papperstidning 1963, page 34) to
extract only effluent fractions having low contents of
chloride. On the basis of this concept, methods have
later been developed for discharge of NaCl from the kraft
process regeneration of NaOH and Na2S. Rapson and co-
workers have made such great contributions to the tech-
nique for crystallization of NaCl from the kraft process
; effluents that it is possible to direct almost all former
bleaching effluents into the kraft mill black liquor
system. The sodium deriving from bleach plant effluent -~
will be included in the soda smelt produced together with ~-
Na2S from one or more of the black liquor furnaces. The
presence of Na2S (sulphur with oxidation number -II) in
kraft process liquors, however, necessitates particular
procedures for the sodium chloride separation. When
separated, the sodium chloride can be electrolyzed for
manufacture of chlorine dioxide and chlorine according
to Rapson's momentous methods.
In the pulp industry, the major portion of brown
stock is kraft pulp. Kraft pulping also includes use of
, ~
~, ~ . ' " 1 ' :
1064206
predigestion to improve the pulping yield. These pre-
digestion methods utilize sulfide compounds. One such
compound is hydrogen sulfide and another is polysulfide
which constitutes Na2S (wherein sulphur has oxidation
number -II) and sulphur (with oxidation number 0). Kraft
pulping is the most common form of pulping. Sulfite
pulping twherein the sulphur has oxidation number IV), is
common also and particularly magnesium base (Magnefite)
pulping and neutral sulfite pulping (NSSC) using either
sodium or ammonium as base. However, a number of calcium
base sulfite mills still operate.
Combinations of kraft and sulfite digestion methods
are also known. For special purposes, an acid prehydro-
lysis is used fpr sulfate (kraft) pulping. In other
instances, sulfite digestion is combined with a subsequent
alkaline digestion. Modifications of alkaline digestion
in the presence of either sulfite alone or sulfite ~
sulfide are also known. Brown stock obtained according
to any of the methods described herein may be bleached in
sequence featuring one or more of the oxidation agents
chlorine, chlorine dioxide, chlorine monoxide, oxygen
(optionally as air), peroxides, ozone etc.
Of prime importance to system closing in practice
is whether the recovered black liquors and bleaching
effluents are compatible in the various combustion and
chemical regeneration systems. Offhand it is clear that
kraft digestion as well as sodium base sulfite digestion
(possibly mixed Ca- and Na-base digestion) are favorably
matched with effluent from oxynation ~oxygen deligni-
fication) in the presence of alkali, e.g. NaOH and/orNa2CO3. Oxynation with NH3 is, however, adaptable
:
_ g _
. ~ .
..
1064206
to all digestion methods because ammonia in oxynation
effluent as well as ammonia in other spent liquors is
combusted with no solid residue. Alternatively, ammonia
may be expelled from a mixture made alkaline by a base.
Consequently, sul~ite digestion using a base selected from
a group consisting of Mg-, NH4- and Ca- base should be
considered together with oxynation using NH3 as alkali
supply.
Preferred embodiments of the invention are described
in the following with reference to the accompanying
drawings, in which:-
Figure 1 is a schematic representation of a Rapsonsystem of countercurrent washing;
Figure 2 is a schematic representation of an oxygen
delignification system;
Figure 3 is a schematic representation of an
embodiment of the invention applied to conventional
chlorine bleaching;
Figure 4 is a schematic representation of a further
development of oxygen delignification;
Figure A is a diagrammatic scheme of a kraft mill
employing the invention; and
Figure B is a diagrammatic scheme of a sulfite pulping
process using MgtHSO3)2 for digestion according to the
Magnefite method.
The distinguishing features of some fundamental
effluent systems are schematically shown in Figs. 1 to 4.
These schemes refer to kraft brown stock bleaching and
also apply to a sodium base sulfite process.
The following symbols are used:
::
- 10 - .:
1 _ ,
:~
: . ; . ,,
1064Z06
K = dig~sting stage
Bl; B2 = bleaching stages in sequences employing
chloride-forming bleaching agents
OXY = oxynation (oxygen delignification) stage ;_
T = stages for recovering black liquor by brown stock
washing
Bf = combustion of chloride-forming bleaching effluent
R = regeneration of white liquor for digestion
NaCl = separation of chloride.
The state of the art is shown in Figs. 1 and 2. Fig.
1 illustrates a Rapson system of countercurrent washing
along the bleaching stages, and mixed black liquor and -
bleaching effluent are combusted together in a black
liquor furnace of the chemical regeneration system
(R + Bf). This regeneration involves separation of NaCl
from which chlorine is generated by electrolysis. If
black liquor combustion involves a pyrolysis step
according to the SCA-Billerud method, it is possible in
step R to convert almost sulfide-free soda liquor to -
caustic soda for bleaching.
Fig. 2 shows an oxynation system (oxygen delignifica-
tion) which half replaces delignification with chloride
forming bleaching chemicals. Products of these chemicals
(Cl-residues) are in a traditional way discharged as
bleaching effluent at the arrow marked "Cl-REST" in Fig. 2
while only alkali passed through "T" stages is regenerated
for digestion and, optionally, for the oxynation(oxy).
The method according to this invention is disclosed
with reference to Figs. 3 and 4.
Fig. 3 shows an embodiment of the invention applied
on conventional chlorine bleaching and the effluent thus
: .;
-- 11 --
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. .
.. . . . .... . . .. . .. .
.. . . .. .. . .
. . , ~. . i,
. . . .. .
1064Z06
obtainecl, denoted brine, is subjected to separate
combustion (Bf) which produces a combustion residue
enriched on NaCl.
Fig. 4 represents a further development of oxynation
succeeded by bleaching with chlorine compounds which are
contained in the brine which is combusted separately (Bf)
with no chloride contamination of the alkali regeneration
system (R). The split of countercurrent washing is
indicated by the dashed slope in the middle "T" washing
stage.
To facilitate the description of the invention, the
following terminology has been applied to the digestion ~.i
spent liquors and bleaching effluents of various origins:
Black liquor = spent digestion liquor from brown stock,
discharged from a pulp digester;
Oxyliquor = effluent recovered from oxystock (oxygen
bleached stock) from an oxystage, where
oxynation (oxygen delignification) is
carried out. Oxyliquor also denotes
effluent from a bleaching stage using a i
peroxide or ozone; ~ -
Lye = black liquor and mixture of black liquor
and regenerable bleaching effluents, i.e.
either oxyliquor or bleaching effluents ~-
containing either no chloride forming
compounds or containing said compounds at a
concentration that is lower than in brine;
Brine = bleaching effluent recovered from various
bleach plant stock washing stages, said
effluent containing substances formed by
reactions of a spent liquor with chlorine
. ~ .
. : : ,- '
. ~ . I
1064206
based bleaching agents and which substances
by treatment comprising a combustion step,
would produce chloride.
This invention relates to the bleaching of unbleached
stock, such as brown stock and oxystock in sequences,
where a chlorinating agent is used in at least one of the
stages and brine, as indicated above a current mixture of
bleaching effluents, is recovered from the bleach plant
washers. The target is environmental protection and it is
achieved by the effluent being recycled in a closed system
wherein the organic delignification products are elimi-
nated by oxidation. Such a system may be either preceded
by or comprise an oxystage from which oxyliquor is dis-
charged. In the case addition of alkali is made to the
oxystage this alkali will be included in the oxyliquor.
The oxynation alkali is usually supplied by a hydroxide-
forming liquor, i.e. NaOH or possibly Na2C03-liquor,
but there is an increasing use of NH3 which forms
NH40H in the liquid.
Particularly gas phase delignification with oxygen, is
favorably combined with gas-phase application of NH3 as ~ s
a supply of hydroxide to alkaline stages. It is possible
to combine oxygen delignification and gas phase deligni- -~
fication with chlorine, chlorine dioxide, etc.
According to the invention, oxyliquor is used for
displacement of black liquor from brown stock, and
residues of lye remaining in the stock are displaced by
means of bleaching effluents whereby, if desired, the
addition of brine terminates the succession of stock
washing steps. For most efficient recovery of the lye,
a limited penetration of brine into the regeneration
- 13 -
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.~.~ , ~.
. -
,
1064206
system may be tolerat~d in practical application of the
invention. This tolerance for chloride as previously
staged, has been demonstrated in coastal kraft mills
with wood containing so much salt that soda smelt from
the black liquor combustion comprise a substantial part
of NaCl.
The tolerable input of chloride formers to the lye
which as previously stated is obtained by black liquor
or bleaching effluent displacement differs for various
pulping methods and it is also dependent on the degree
to which the entire process is closed. As a rule, in
the kraft process not more than 20% of the volume of lye ~ -
should comprise brine and preferably less than 10%. The
actual limits will be influenced by other sources of
chloride, e.g. wood and water.
The invention is primarily directed towards pulp
digestion methods which produce black liquors containing
residues and reaction products of the particular sulphur
compounds in the digesting liquor which, during digestion,
promote the delignification and prevent the degradation of - i;
cellulose- and hemicellulose compounds. This degradation
lowers the yield of cellulose pulp.
The sulphur compounds contained in the fraction of
black liquor which is left in the brown stock after
washing, are transferred to the brine oxidation step. `- -
According to this invention they are oxidized to products ~-
with oxidation number IV or, alternatively, VI. For ~ -
example sulphur in Na2SO4, SO3 and CaSO4 has oxidation
number VI and sulphur in SO2 and thereof prepared Mg(HS03)
(NH4)2SO3, NA2SO3 has oxidation number IV. Thus,
according to the invention, no Na2S, i.e. sulphur with oxi- ;
dation number -II, can be produced in smelt containing NaCl
B - ~
,. .... .. . ... . . . ............ . . . .
;..... . . ~ , . ., ~ . . ..... ...
1064206
deriving from brine, This is an important advantage of
the invention, because it is otherwise hazardous to handle
Na2S and s2 which readily emit toxic H2S from
steps in generation of chlorine compounds as well as from
purification of effluents discharged to a recipient. The
termic oxidation of brine may be carried out in liquid
phase with supply of oxygen to a more or less complete
oxidation of the effluent's combined content of organic
carbon. The oxidized effluent may subsequently be dis-
charged or recycled to the process. In case oxygen isused for bleaching in oxystages, an additional quantity
of oxygen for oxidation of brine may be produced at
acceptable economic terms.
In case the combustion products of the brine can
be processed in electrolytic plants for regeneration of
oxidizing (active) chlorine, it would be of advantage to
also supply the brine system with sodium chloride that
has been separated from the kraft process according to
Rapson. An optimized operation of an electrolytic plant
in connection with this invention could utilize a chloride
make-up to compensate for some acid chlorine and chlorine
dioxide stage effluent and also less sodium-containing
extraction stage effluents which may be discharged to the
recipient either directly or after cleaning treatment.
In those cases where the kraft process is operated
at the tolerable chloride level in the liquors, sodium
chloride is enriched in the flue gas carry-over which
ordinarily comprises what is called precipitator salt
cake. This chloride can be extracted, e.g. leached, from
the salt cake. Extracted chloride is, according to the
invention, trans~erred to the brine system and treated
together with the combustion products of the brine.
.
v.
Y, 1064Z06
Brine evaporation according to the invention, separate
from lye evaporation, allows water introduced with bleach-
ing effluents to be recycled with no contamination from
black liquor vaporized digestion products and compounds
of sulphur with oxidation number -II. Thus evàporation
condensate from brine may be utilized for washing of
finally bleached pulp, whereas the previous known closed
systems produced one common evaporation condensate con-
taminated by volatilized black liquor substances. These
volatile substances effect the various digestion processes
differently. Thus they produce evil odors in the kraft
process while condensate for sulfite digestion acid pre-
paration may jeopardize the stability of the digestion
liquor to the degree that delignification is inhibited.
The utilization of brine evaporation condensate in said
` processes according to the invention will overcome such
problems.
Treatment of condensates, e.g. by stripping or similar
purification steps, will be considerably simplified when i
only a limited amount of vapor condensate from black
liquor or lye has to be treated. The common rules for
connecting a series of evaporator units with regard to
steam flow between brine and lye lines are applied to
recover the largest possible amount of non-contaminated
condensate.
Because all or almost all sulphur from digestion is
contained in the lye, the brine combustion gas will emit
only a small quantity of sulphur compounds. The forma-
tion of HCl in the flue gas increases with decreasing
content of ash forming base in the brine and simultao
neously increases the input of HCl to flue gas scrubbers
~,'' -, :,' , .
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. - ........ . . . . .
,~, , . . . . . ~ - ; ~ .
' 1064206
in systems for separation of chloride.
A brine recovery system combined with a sulfite
pulping MgO-recovery plant will prevent corrosion in the
lye chemical recovery system. Chlorides from ocean water
floated wood introduced into the lye through the diges-
tion system may also be discharged by means of the brine
system. Thus, the passing of lye combustion flue gas
containing chloride and/or chloride extracted thereof to
a single flue gas cleaning system shared with the brine
combustion lies within the scope of the present invention.
As examples on the application of the invention are
a kraft mill with C/D E D-bleaching of the brown stock
(Example A) and a magnesium base sulfite mill with O C/D E
D-bleaching (Example B) are described.
Example A
Fig. A on the attached drawing comprises a diagram-
matic scheme of a kraft mill using the invention. In the
digestion plant 1 produced brown stock and black liquor
are separated by a disc filter 2. About 2/3 to 4/5 of the
black liquor 3 is first drained off from the brown stock,
and of remaining black liquor somewhat more than 50% is in
the stock layer removed by displacement with bleaching
effluent 4 and with essentially no intermixing of the
liquids in the pulp layer.
Lye is by this means obtained from about 85 to 90
of the black liquor with no dilution caused by bleaching
effluent. This lye is drawn off by pipe 3. Then more
bleaching effluent 4b is used for displacement washing.
The brine produced by this washing is discharged at 5 and
amounts to between 3 and 5 cubic meters per metric ton of
brown stock, of which about 2 to 4 cubic meters derives
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~064206
from bleaching e~1uent.
Prior to stock discharge from the disc filter,
bleaching alkaline effluent remaining in the stock layer
is removed by displacement with an acid bleaching effluent
supplied through lines 9 and 6 from the acid bieaching
stages 7 and 13, respectively. Chlorinated stock from
a hot chlorine/chlorine dioxide stage 7 (C/D-stage) is,
on filter 8, washed free from dissolved, mainly oxidized
delignification products by supply of acid bleaching ef-
fluent and is then neutralized on the filter by applying
alkaline bleaching effluent 4 to reduce the consumption
of caustic 10. The caustic may be either a make-up
or regenerated from NaCl 10b. The alkaline bleaching
effluent 4 is recovered by washing stock from the alkaline
extraction stage 11 on filter 12 with chlorine dioxide
stage 13 effluent 9. The chlorine dioxide bleached stock
is washed on the filter with process vapor condensate
15 or alternatively with stripped or otherwise treated
process vapor condensate and optionally a supply of water
16 as white water or fresh water. ~
BOD-causing residues of black liquor are oxidized in ?~ -`
the C/D-stage 7 as stated in the Inventor's U.S. Patent
No. 3,830,688 (col. 7, lines 13-19) and Canadian Patent
No. 828,653. Effluent 17 from C/D-stage 7 may be dis-
charged to the recipient while bleaching effluent passed
through the brown stock washing 2 constitutes brine that
is introduced to a plant 18 designed for its evaporation `~
and in turn cooperating with a fluid bed combustion furnace
19. This furnace is of the type which is manufactured by
Copeland or Dorr-Oliver (Fluo-Solid) and its design makes
it possible to perform a substantial part of the brine
- 18 -
.
~V ~.
1064206
evaporation directly witl- hot ~lue gas l9b.
The fluid bed delivers solid combustion products
as ash in the form of a pelletized salt residue 20. It
contains about 40% NaC1 by weight. The NaCl is in stage
21 separated from Na2CO3 and Na2SO4 alternatively
aEter causticizing of Na2CO3 to NaOH accocding to
methods recently published by Rapson et al. The NaCl is
regenerated in an electrolytical plant such as a chlorine -
alkali plant or chlorate plant 21b and the products 21c
are used for bleaching. Electrolytic caustic 21d is used
for delignification such as bleaching 10b or as make-up
for the digester with white liquor 23b.
In case diaphragm cells are used to make caustic soda
for the bleach plant, a partial recycling of chloride,
contained in the caustic, to the bleach departments 10
may be possible because, according to the invention, this
chloride will not burden the regeneration of alkali for
digestion of pulp. This novel unloading of the chloride
from black liquor and alkali regeneration system is con-
sidered to be a major feature of the invention.
Subsequent to the separation of NaCl the residue of
sodium compounds 22, containing i.a. Na2SO4, Na2CO3
and double salts etc., is passed to the alkali regeneration
system 23 of the kraft mill. Concentrated lye 24b from
the normal black liquor evaporation plant 24 is combusted ~
in the recovery furnace 25 and the soda smelt 25b, which ~ -
contains sulfide, is in the regeneration plant 23 converted
to white liquor 23b for the digestion 1.
The evaporation lines 24 and 18 are interconnected in
a manner known in the art for multiple use of steam and
vapour. The invention may be operated in such a way that
'` '''
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~ ' ' :~' ,' .-
1064Z06
the combined amount o~ evaporated water not exceed that of
the previous kraft process with its discharge to sewer of
all bleach plant effluents.
The recycled bleaching effluent contributes make-up
Na and a substantial amount of combustible solids to the
kraft process while chloride is controlled according to
the present invention. The combustible solids comprise
organic substance amounting to about 5 to 10% of the
weight of the brown stock and corresponds to about 5 to
25% of the steam generated by the boilers of the furnaces
19 and 25. The actual amounts of steam depend on the
relations between wood solids dissolved by digestion 1
and solids from delignification of the brown stock in the
primary bleaching stages 7, 11.
The additional capacity of furnace 19 is of particular
interest to kraft mills which otherwise would have their -
capacities for evaporation 24 and combustion 25 overloaded. --
Thus the amount of black liquor left with the brown stock
to the brine 5 can be optimized for the economy of the `
process as a whole. - -
Prior art combustion of bleaching effluent in the -~
black liquor furnace, e.g. according to Rapson, implies
problems in the flue gas cleaning system caused by
hydrogen chloride and sodium chloride.
Such problems are eliminated according to the present
invention because chloride compounds in the flue gas l9b
are treated by contact with just brine 5. Chlorides
recovered from black liquor recovery 25 flue gas also may
be passed to the chloride separation system 21. The
suphuric acid that is used for acidulation of by-product -~
tall oil contibutes sulphur to the emission of odorous
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sulphur compounds from the conventional kraft process, but
according to the present invention, hydrogen chloride can
replace sulphuric acid. This change is made possible by
passing also the soap splitting brine to the brine combut-
tion plant.
Example B
Fig. B exhibits a scheme of a sulfite pulping pro-
cess with Mg(HSO3)2 for digestion according to the
Magnefite method. Brown stock from a digester 1 is blown
to a dual blow and washing (diffuser) tank 2, whence lye
2a is recovered by displacement washing with oxyliquor
Sa. Brown stock 3 is also flushed with oxyliquor to the
screening operation 4. Coarse material in the stock is
disintegrated in the screening department to facilitate
the use of a press filter 5 for washing. Black liquor `
in the brown stock layer is on the filter removed by dis-
placement with oxyliquor 11 and the operation concludes
with pressing 5b.
. . .
Lye of mixed black liquor and oxyliquor 5a is passed
through the vessel 5c to said washing tank 2, whence lye
2a is drawn off. (The diffuser washer, the filter and/or
the press with their interconnections may advantageously
be arranged as described by the Inventor in his Swedish
patents 127,315 and 139,301 and in his article in Paper
Trade Journal, February 24, 1958). ~
Brown stock is fed from the press 5b to the oxystage ~ -
6 where it is reacted with oxygen 7 and NH3 8. Oxystock
9 is diluted with recycled oxyliquor lla and the stock
suspension is passed to a multiple-zone wash filter 10
on which the oxyliquor 11 is successively removed by dis- ;
placement bleaching effluent 12 from subsequent stages of
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bleaching 1~, 18, 21. Oxyliquor 11 is recovered by only
a negligible dilution of the bleaching effluent 12 which
contains chloride forming substance.
The liquid phase of the oxystock at discharge from the
first zone 10a of the washing filter constitutès a mixture
of black liquor, oxyliquor and bleaching effluent. This
liquid phase is in a following zone 10b gradually dis-
placed by bleaching effluent 12 and finally discharged
as brine 13. Oxystock 10c is pressed 14 to a consistency
required for gas phase reaction in the vessel 15, in which
the oxystock is further delignified by a mixture of chlor-
ine and chlorine dioxide 15b. The press liquor 14a is ~ -~
passed to the brine 13. Chlorinated stock suspended in
bleaching effluent 15a is washed in the diffuser 16 with
chlorine dioxide stage effluent 17 and advantageously
neutralized slightly with an alkaline bleaching effluent
18a from the ammoniacal extraction stage 18. For gas -
phase alkaline extraction a suitable stock consistency is
obtained by the concentrator 18b and then the stock is
reacted with ammonia 19. At the discharge zone 18c of
the extraction vessel the stock is diluted with recycled
bleaching effluent 18a from the screening plant decker 18d.
Bleaching effluent, made up from vapor condensate from
the process, is used in the wash apparatus 20 and the pulp
is finally bleached in the chlorine dioxide tower 21.
Caustic bleaching effluents precipitate Mg-compounds
in magnesium base sulfite black liquor, but the fact that
ammonium hydroxide does not precipitate Mg-compounds makes
this application of the invention possible. The brine 13
is passed to an evaporation plant 25, and the concentrated
brine 13a is combusted in a fluid bed furnace 26, e.g. of
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~ 064206
Copeland design, which furnace on demand may admit
various combustible wastes 26a, e.g. slude containing
waste sediment from settlers etc. The ammonia compounds
are combusted in the furnace and release chlorides with
the flue gas which, optionally after separatioh of parti-
culate matter, can be recovered by scrubbing 26b with
water. The ash 26c is optionally discharged. (In a gas
scrubbing stage 26b produced chloride solution, e.g.
hydrogen chloride, can be utilized for production of
chlorine/chlorine dioxide for the bleaching.)
Lye 2a withdrawn from washing tank 2 is evaporated j--
in a sulfite black liquor type evaporation plant 31 and
concentrated liquor 2b is combusted in another fluid bed
- furnace 32. Hereby Mg0-ash 33 is produced which is
utilized for preparation of magnesium base digestion
liquor. In this preparation, the ash passes a leaching
stage 33a which separates easily soluble salts 33b and
thereby causes some loss of, i.a., MgC12. But this loss
of Mg will be low according to the invention because
chloride, which makes magnesium soluble, is essentially
kept away from the recovery plant for digestion chemicals
30. The plants for regeneration of Mg(HSO3)2 30 and
S2 from flue gas 32b are considerably more complicated
but well-known to those skilled in the art and therefore
they are not included in detail in the drawings.
Evaporation condensates 31b and 25b are passed to pulp
washing 24, 20 and to the sulfite acid making system 31c,
25c, 30.
A useful modification of Fig. B would involve sewering
of the chlorine/chlorine dioxide stage (C/D-stage) 15
effluent 12 with low BOD as compared to the extraction
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1064206
stage 18. Therefore also countercurrent washing with
chlorine dioxide efluent 17 from stage 21 through stages
16 and 10 to sewer is another useful application of the
invention. Simultaneously, extraction stage effluent 18a
is passed directly to the oxystock washing 10 and from
there is conveyed to the brine combustion 13, 25, 26.
In sulfite pulping technology it has been common
practice to dissolve or prepare the sulfites in digestion
spent liquor (sulfite black liquor) which allows recycle
of used liquor to the digester. Thereby the solids con- ~
centration of the black liquor reaches an equilibrium -
level that is expedient for evaporation. Similar re-use
may, according to the invention, include oxyliquor 11 from ~ -
which ammonium bisulfite or monosulfite is prepared also -~ -
for other digestion stages than those indicated here. One
such stage may use oxyliquor 11 at higher pH than what
the solubility of Mg-sulfite would allow. This higher pH
stage may be applied either prior to or subsequent to a - - ~
main digestion stage with Mg-bisulfite. - -
By-product recovery from sulfite black liquors has -
attracted renewed interest and is indicated 31a on the -
figure. When coniferous woods are digested, the liquor
may be fermented so as to produce ethanol and independent
of the raw material, i.e. wood species, protein producing
microorganisms, e.g. certain types of yeast, can be cul-
tivated in sulfite black liquor. These fermentation
processes require addition of nitrogen nutrients. Thus
the ammonia is first utilized for bleaching and possibly
also digestion and the resulting nitrogen compounds are
ultimately consumed by fermentation of the carbohydrate
content of the black liquor in the by-product plant 31a.
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1064206
Expul~ion of N~3 from successive stages of evapora-
tion in the plants 31 and optionally 25 can be effected
by adding alkaline substance to the black liquor. This
substance is preferably Mg0-ash or a slurry thereof. The
regenerated NH3 is then used for either bleaching or
other purposes. The ammonia compounds will also serve as
nutrient in the fermentation of hydrolyzates.
Hydrolysis of cellulosic material, similar to that
which takes place in acid sulfite pulping, can also be
applied in alkaline pulping process by a prehydrolysis of
the cellulosic raw material with an acid, e.g. sulphuric
acid, sulphurous acid tsulphur dioxide) and so-called-
wood acids released by water or steam hydrolysis. Total
- hydrolysis of wood waste may also be considered. Hydro-
lysis of hardwood employing hydrogen chloride produces
furfural that is separated in connection with brine
evaporation.
Sulphite pulping methods other than that dealt with
in Example B will in the following be reviewed in view `~ -
of the invention. Many sulfite mills still operate the -~
previously most common of the sulfite methods, using a `;
digestion acid containing calcium bisulfite and sulphur
dioxide. These calcium base sulfite mills are often
equipped with plants for spent liquor evaporation and
manufacture of by-products from the sulfite spent (black)
liquor and several show promise of profibability in the
future.
Sulfite digestion acid containing both calcium and
ammonium ions may be produced by using ammonia oxyliquor
for absorption of SO2 from combustion of calcium base `
spent sulfite liquor. Operations according to the present
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1064206
invention can b~ supplemented with a calcium preclpitation
of the digestion spent liquor to make it suitable for
manufacture of various by-products, e.g. tanning, binding
and dispergating agents.
All known methods for fermentaion of the carbohydrates
of sulfite black liquor can be used within the scope of
the invention. Any ammoniacal bleaching effluent, e.g.
oxyliquor, can be utilized for fermentation of Ca-base
sulfite black liquor. Thus, according to the invention,
either ammoniacal oxyliquor or a chloride-lean fraction
of ammoniacal bleaching effluent can serve as nitrogen
nutrient for fermentation of any sulfite black liquor
which is combusted subsequent to the separation of the
fermentation products. Alternatively brine may be sub-
jected to separate fermentation, or its ammonia content `
can be expelled prior to separate combustion.
It has been pointed out that Ca- and NH4-ions in
certain proportions can be utilized for single and multi-
stage sulfite digestion followed by combustion of sulfite
black liquor as a component of the lye according to the
invention. However, combination of Ca- and Na-ions pro- ~`
duces an ash upon combustion of the lye, the properties
of which ash make it difficult to handle. This ash pro-
blem can be overcome by precipitation of Ca-ions by added
Na2C03 followed by separation of the CaC03-precipitate prior
to combustion. By this means the combustion of sodium
hydroxide-containing oxy- and bleaching effluents as lye
can be incorporated with a calcium base sulfite process
that includes combustion of brine which thereby produces
Na2C03 amd NaCl etc. According to established methods
the NaCl can be separated and electrolyzed to chlorine
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bleaching chemiclls.
Cross-recovery between two processes, e.g. sulfite
and kraft mills, with bleaching of some or all brown stock
produced in the processes, can be arranged according to
the principle of this invention while considering the
chloride content of the various liquors or fractions
thereof. Consequently, bleaching effluent, e.g. oxy-
liquor, from one type of pulp can be utilized for black
liquor displacement from another type of pulp while
considering which combinations of treatment steps for
chloride forming versus easily regenerable chemicals
will be found to be most economical.
..'
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