Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to the production of
bleached cellulosic fibrous pulp. More particularly, the
present invention relates to a method of producing bleached
fibrous cellulosic pulp in which the chemical compounds used
and formed in the various steps and stages are utilized to
maximum efficiency with a minimum of loss and resulting pol-
lution by the recovery and reuse of products previously passed
as waste from pulp bleaching processes.
In the conventional Kraft process, raw cellulosic
fibrous material, generally wood chips, is digested, by heat-
ing,in a pulping liquor, (white liquor) which contains sodium
sulfide and sodium hydroxide as the active pulping chemicals.
The digestion provides a pulp and spent pulping liquor (black
liquor). The black liquor is separated from the pulp by
washing in a brown stock washer and the pulp utilized as
feedstock in a bleach plant for brightening and purification
operations.
The black liquor is then concentrated, usually by
evaporating, and the concentrated black liquor burned in a
reducing furnace to yield a smelt containing primarily sodium
carbonate and sodium sulfide.
The smelt is then dissolved in water to yield a raw
green liquor which may be clari~ied to remove undissolved
solids. The green liquor is then causticized, usually by
treatment with Ca~ to convert sodium carbonate to sodium
hydroxide. The resulting liquor is white liquor and is use-
ful in the initial digestion step to provide at least a part
of the pulping liquor.
The foregoing sequence or cycle is well known and
30 i5 referred to herein as the digestor liquor cycle.
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Bleach plant operations generally involve a sequence
of briyhtening and purification steps which may be combined
with washing steps. The brightening steps generally involve
the use of bleaching agents, such as chlorine or chlorine di-
oxide. The purification steps invol~e washings and treatment
with sodium hydroxide solution, caustic extraction.
A particular bleaching sequence which finds use in
one mode of the present invention involves an initial bleach-
ing of the pulp with an aqueous solution containing 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.
These are the so~called DCEDED or D/C EDED sequences.
A common source of chlorine dioxide for the bleach-
ing operation is a chlorine dioxide generator which produces
chlorine dioxide, usually as an aqueous solution of chlorine
dioxide and chlorine by the reduction of a chlorate salt,
e.g., sodium chlorate. Such chlorine dioxide generators
utilize a feed of H2S04 which is reacted with a mixture of
sodium chlorate with some sodium chloride to produce a chlor-
ine dioxide bleaching product and a sodium sulfate (saltcake)
by-product.
The present invention utilizes the digestor liquor
cycle, as described in the foregoing, in tandem with a sulfur
cycle. The sulfur cycle utilizes a portion of the green
liquor of the digestor cycle and has in common the furnacing
and smelt dissolving steps of the digestor liquor cycleO The
green liquor utilized in the sulfur cycle is carbonated,
using stack gas, or other suitable sources of carbon dioxide,
producing three phases. One is a gas phase of substantially
pure hydrogen sul~ide. One is a solid phase of sodium bi-
carbonate. ~he remaining phase is an aqueous liquid phase
containing the soluble impurities of the green liquor, mainly
chlorides. The hydrogen sulfide phase is converted into
H2SO4 by oxidation and the product utilized as feedstock into
a chlorine dioxide generator. The solid phase of NaHCO3 may
be converted by causticizing to yield a relatively pure feed-
stock for caustic extraction steps in a bleaching sequence ormay be furnaced to yield additional smelt. The aqueous phase,
high in chlorides, is suitably used as feed into the chlorine
dioxide generator as a source of chlorides or may be fed to
a salt extractor, in either case, removing chlorides from the
operationO
The alkali metal suifate ~y-product o~ the chlorine
dioxide generator, for example sodium sulfate, is furnaced
in common with the evaporated or concentrated black liquor
to form a common smelt and subsequently additional green
liquor, this completing the sulfur cycle.
It is contemplated that the carbonation reactions
of the present invention proceed as follows; however, it is
to be understood that the reactions are shown for clarity of
understanding and are not limiting to the invention as it is
to be considered that the green liquor and flue gas reactants
are highly impure products.
Na2S + H20 + CO2-~ NaHS + ~aHC03~
Na2S + 2H20 + C2-~ 2~aHC03~ -t H2S
~a2S + H2S-~ 2NaHS
2~3 + H20 ~ C02~2NaHC0
3~2
The present invention also contemplates a balanced
mill operation utilizing the digestor liquor and sulfur cycles
in tandem.
The present inv~ntion will be described more in
detail by reference to and discussion of the drawings~
Figure 1 is a schematic flow sheet illustrating
a typical pulp mill operation utilizing a digestor liquor
cycle, a bleach plant, and a chlorine di~xide generator to
produce bleaching solutions.
Figure 2 is a schematic flow sheet illustrating a
pulp mill operation utilizing a digestor liquor cycle in
combination with a sulfur cycle, and a bleach plant. In this
mode a portion of the sodium bicarbonate produced is causti- -
cized and utilized in the bleaching operation and effluent
from the carbonating tower is utilized as feed to the chlorine
dioxide generator.
The chemlcal consumptions and requirements given in
the following descriptions are given in pound equivalents
required to produce one ton of air-dried pulp.
Looking now to Figure 1, the basic digestor liquor
cycle is shown in combination with a chlorine dioxide genera-
tor. Thus, fibrous cellulosic material, e~g., wood chips,
are fed by line 10 into the digestor 12 wherein the wood
chips are digested with a pulp liquor fed by line 14 contain-
ing NaOH and Na2S as the main pulping chemicals. Miscel-
laneous losses in the digestor amount to about 24~5 pounds
of ~a20 equivalent, about 4.5 pounds of sulfur and about
0.6 pounds of sodium hydroxide, based upon one ton of air-
dried pulp product from the plant. The pulp liquor fed by
line 14 contains about 286 pounds sodium sulfide, about 684
pounds sodium hydroxide, about 7 pounds sodium sulfate, about
~34~;~
70 pounds sodium carbonate, about 0.1 pound sodium chloride
and about 5,700 pounds of water.
The resulting pulp and spent pulping liquor are fed
through line 15, and the pulp washed and separated from the
waste pulping liquor in brown stock washer 16. The washed
but unbleached pulp is fed by line 20 to bleach plant 22
wherein the pulp is subjected to a series of bleaching and
purification processes invol~ing, in at least one stage, the
use of chlorine dioxide. These processes may include ad-
ditonal bleaching or purification steps utilizing a causticextraction wherein aqueous solutions of sodium hydroxide are
fed through line 26. Generally, the pulp is washed during
the ble~ch plant operation, typically after each bleaching
or caustic extraction step by water fed through line 28. The
spent wash water from the bleach plant washing operations,
together ~ith the spent chemicals from the bleaching and
caustic extraction steps, provide the plant bleaching ef~l-
uent in line 18.
The bleach plant effluent in line 18, a potential
loss andpollutant,might be considered to be added to the
spent pulp liquor in line 36; however, this procedure is not
feasible because substantial amounts of chlorides would be
added to the pulp digestion system which are a cause of
e~uipment erosion and, since chlorides do not react in this
cycle, they accumulate as large amounts of non-useful mater-
ials in the cycle. As shown in the remaining Figures, ~he
present invention alleviates the problem of build~up of
unusable chlorides in the digestor liquor cycle and further
produces a sodium bicarbonate product which may readily be
converted into sodium hydroxide and returned as a useful
component to either the digestor liquor cycle or to the
bleach plant operation.
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A chlorine dioxide generating unit 24 utilizes
H2S0~, NaC103 and NaCl as feedstock and produces chlorine
dioxide which is fed through line 30 into bleach plant 22.
Typically, about 24 pounds of NaCl, about 40.7 pounds of
NaC103 and about 95 pounds of H3S04 are utilized to produce
about 24.5 pounds of chlorine dioxide and about 15.2 pounds
of chlorine. About 56.3 pounds of salt cake, Na2S04 are
produced and are fed through line 32 to furnace 34. Although
furnace 34 is equipped with precipitator 36, there is stack
loss of about 2.90 pounds Na20 equivalent, 3.70 pounds of
sulfur and about 0.3 pound NaCl.
The waste pulp liquor is fed by line 36 to evapora-
~or 38, and the evaporator materials fed through line 40 to
furnace 34. The smelt produced by furnace 34 is fed into
smelt dissolver 42 to produce green liquor which is butted
by the addition of soda ash, in the amount of about 116
pounds, and recausticized in recausticizer 48~ The recausti-
cized solution is fed by line 46 into digestor 12, thus
completing the digestor liquor cycle.
Looking now to Figure 2, this figure shows the
digestor liquor cycle of Figure 1 in tandem with a sulfur
cycle. A quantity of green liquor containing ~a2S equivalent
to about ~4 pounds H2S04 (the requirement for chlorine dioxide
generator 24 based upon one ton of air-dried pulp mill pro-
duct) is withdrawn from smelt dissolver 42 through line 50
and fed into precarbonation vessel 52. The precarbonation
vessel receives feed of C02 gas through line 54 from bicar-
bonate tower 56. The precarbonated material is fed through
line 58 into ~2S stripping tower 60 where additional bicar~
bonate solution cr slurry is fed through line 62 and
exhausted solution is returned through line 64 to bicarbonate
~ 39~
tower 56. Substantially pure H2S in gaseous form is fed by
line 66 into oxidizer 68 to produce M2SO4 or H2SO3. The
suIfuric acid thus produced is fed through line 70 as feed-
stock into chlorine dioxide generator 24, thus completing
the sulfur cycle.
Looking now in more particular to bicarbonating
tower 56, an integral part of the present invention, this
tower, as a source of CO2, receives stack gas through line
72, although it will be understood that CO2 from any source
may be used. The carbonation reaction precipitates a rela-
tively pure NaHC03 product which may be easily separated by
elutriation and recausticized by treatment with CaO to yield
a substantially pure NaOH product for use in bleach plant
22 or to butt or produce additional pulping liquor. A por-
tion of the carbonated product is utilized H2S stripper 60 -
to liberate H2S. The mother liquor from elutriation, mainly
chlorides, is utilized at leaat in part as a source of
chlorides for chlorine dioxide generator 24.
It may be noted at this point that bleach plant
e~fluent, especially extraction effluent containing large
amounts of chlorides may now be returned to the digestor
liquor cycle as the chlorides are removed in the sulfur
cycle and utilized as feed into the chlorine dioxide genera-
tor. Thus extraction effluent is returned from bleach plant
22 through line 19 to brown stock washer 16 and utilized as
wash make-up.
The stack gases fed into bicarbonating tower 56
by line 72 precipitator 36 contain about 15 percent by
weight of CO2. The entering CO2 reacts with Na2S and NaCO3
to produce NaHCO3. Although bicarbonating tower 56 may
operate at pressures from atmosphere to about 100 p.s.i.a.
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and at temperatures of from about 25~ to about 100 C, a
more practical range is atmospheric pressure to about 30
p.s.i.a. and about 50 to about 80r C. The main products of
bicarbonating tower 56 are ~aHC03 and a chloride feedstock
for chlorine dioxide generator 24. In precarbonization vessel
52, green liquor from line 50 is treated with a solution of
~aHC03 containing some C02 from bicarbonating tower 56. The
desired reaction is to convert ~a2S to HaHS without libera-
tion of H2S. This treatment is preferably carried out in a
closed vessel at atmospheric pressure to minimize formation
and loss of H2S. The treated solution from precarbonator
vessel 52 is fed by line 58 to stripping column 60 where the
mixture is stripped, preferably by use of steam to release
H2S in concentrated and substantially pure form. H2S is
removed from the solution preferably by flash or vacuum
stripping in which is fed solution containing NaHC03 by line
62 and exhausted solution containing Na2C03 returned to
bicarbonating tower 56 through line 64. Steam is preferabl~
used as the stripping agent since a recovery of concentrated
~2S gas can be effected simply by condensing the steam from
the mixture of steam and H2S.
The bicarbonating and stripping operations may be
carried out in any type of equipment conventionally employed
for gas absorptiorl or stripping operatiorls. Thus pacXed
columns, plate columns, spray columns and continuous liquid
phase columns are aptly suited to use. Agitated gas disper-
sion equipment rnay be advantageously used in the bicarbon-
ating tower.
The liberated H2S is fed by line 66 to sulfuric acid
pla~t 68 where the H2S is oxidized to produce S03 which is
dissolved in a H20 solution to yield a H2SO4 product.
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The H2SO~ produced is fed into chlorine dioxide
generator 24 as feedstock. The salt-cake produced as a by~
product from chlorine dio~ide generator 24 is fed by line 32
to furnace _ where it is furnaced in common with evaporated
waste pulping liquor from evaporator 38.
In this mode of the invention, the excess ~aHCO3,
about 200 pounds, is fed to a recausticizer 76 which converts
the ~a2CO3 into about 95 pounds of ~aOH which is suitably fed
through line 78 to bleach plant 22 and utilized in the pulp
bleaching operation, although, if required, can be utilized
to butt or produce pulp liquorO
Because of losses in the various stages of a Kraft
mill operation, additonal alkali soda (~aOH) and, in some
cases, additional sulfur make up are required. As much as
1 possible, this is done by the utilizatlon of salt cake, but,
because of its sulfur content, salt cake usually cannot be
fully utilized, and additional alkali soda will need to be
acquired. It will be noted that, in the process of Figure 2,
the alkali soda in excess is produced from salt cake without
the addition of excessive amounts of sul-Eur being added to
the digestor liquor.
It will be understood that many modifications to
the present invention may be made, for example, various
bleach sequences will yield effluents of varying compositions,
or other paper making activities which have chemical require-
ments or by-products, which may be provided for by modifica-
tion of the present process.
Economic or environmental factors may also be-pro-
vided for by modifications, for example, in one mode of
operation of the present invention a pulp bleaching sequence
of DCEDED is utilized in bleach plant 22. ~hen this sequence
_ g _
is used, the entire effluent from the bleach plant may be
returned to brown stock washer 16, thus utilizing and there-
by eliminating all effluent from the bleach plant operation.
If desired, a salt crystallizer may be installed in connection
with bicarbonating tower 56 and sodium chloride in substan-
tially pure form recovered.
The following table gives comparative.chemical
requirements and credit balances for the process of the
present invention when operated under the conditions of
Figure 2 (Column A) and under a total recycle of bleach plant
effluent (Column B) as compared with a plant operating under
a normal digestor liquor cycle (Column C). The requirements
and credits are given in pounds of chemicals required to
produce one ton of air-dried pulp.
TABLE 1
Requirements A B C
H2SO4 ~ - 38,8
NaClO3 96.8 96.8 40,7
NaCl - - 24.0
Na2C3 94.2 - 116.0 40.8
NaOH _ _ 95 0
Na2S4 46.'7 43.9
Credits
Na2SO~ - - 6.3
~aCl - 39.0
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