Note: Descriptions are shown in the official language in which they were submitted.
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Process for recovering chemicals from the waste li~uors of
sulfate cellulose digestion and the waste waters of bleaching
The present invention relates to a process for the
recovery and reuse of sodium and sulfur chemicals in connection
with sulfate cellulose production, whereby the chlorine chemicals
are simultaneously removed from the liquor system in the form of
sodium chloride.
There are advanced processes for the recovery and
reuse of digestion chemicals. So far, bleaching chemicals have
not been recovered to a noteworthy extent. This is unfortunate,
considering environmental pollution caused by the chemicals
used for bleaching and by dissolved organic materials when they
are discharged into the sewage system.
It is possible to use the waste water flow from the
bleaching apparatus either entirely or partially for washing
the digestion chemicals from cellulose. Besides, the waste flow
- from the bleaching apparatus can be used for dissolving the melt
coming from the soda-ash roaster. The waste water coming from
the bleaching apparatus can, furthermore, be used instead of
condensation water and clean water at several other stages of
the recovery system for digestion chemicals, not mentioned here.
The waste water flow coming from the bleaching system
can be purified and then released into the seware p~e. The
purification can be performed by, for example, an ion exchange
or calcium hydroxide treatment. In this case, however, there is
the problem of what could be done with the chemicals and organic
compounds removed from the waste water. In both of the methods
for purifying waste water mentioned above, the purpose is to feed
these chemicals and organic compounds into the recovery system
for digestion chemicals. The waste water flows of the bleaching
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apparatus can be purified in other wa~s as well, and -the removed
substance can be led into the digestion chemical system.
In the recovery and purification systems for the
bleaching system waste water described above, organic and
inorganic substances are fed into the digestion chemical system.
Various substances can be fed into the digestion chemical system
even in other ways. The combustible substances are burned either
in a soda-ash roaster or in a caustic-sludge furnace and then
~; removed from the system. The incombustible substances, such
as sodium chloride, remain mainly in the liquor system, unless
they are removed or unless they are eliminated from the system
in the form of losses incurred during the process.
When cellulose is bleached by means of oxygen gas,
sodium is fed into the digestion chemical system along with the
bleaching residues at either the recovery or the purification
stage.
In chlori~e bleaching, chlorine and possibly sodium
are fed into the system at either the recovery or the purifica-
- tion stage. The present invention relates to a process for
removing chlorine, sodium and sulfur from the system, whereby
sulfur-free and chlorine-free sodium hydroxide is simultaneously
produced for bleaching or the other unit processes. The
invention makes it possible to control the sulfide content of the
. digestion solution in a simple manner.
There are also other processes for removing chlorides
from the liquor system. Four processes are described below,
bu-t cnly in Process 3 has the sodium and sulfur balance control
been taken into consideration.
1. Leaching of the chloride-containing dust from
the soda-ash roaster. This process produces, however, a white
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liquor with a chloride content man,y times that produced by
the process according to the invention.
2. Removal of sodium chloride from the white liquor
by means of evaporation crystallization. The sodium chloride
content obtained for the white liquor by the process is about
3 times that obtained by the process now proposed. The
evaporation requirement is, however, more than twofold. The
greatest disadvantage of the process if the great carbonate
, quantity which is precipitated together with the sodium chloride
- 10 crystals and which must be recovered by leaching.
3. Separation of the carbonate and chloride from
the green liquor by evaporation crystallization. After
separation, the crystals are dissolved and the solution is
,' causticized, whereafter the chlorides are removed by evaporation
crystallization. The process has the same disadvantage as
Process 2. Besides, the evaporation requirement is even greater
' than in Process 2.
4. Separation of the carbonate and chloride from
the melt by leaching. This process is hardly practicable for
' 20 the reason that the leaching is very difficult to perform.
The present invention resides in the improvement in
a process for the recovery of chemicals from the waste liquors
of sulfate cellulose digestion and from the waste waters of
bleaching, wherein the waste liquor from the digestion is
concentrated by evaporation and the concentrated waste liquor
is burned in a recovery boiler to produce a melt mainly contain-
ing sodium sulfide, sodium carbonate and sodium chloride, the
melt is dissolved in the waste waters of bleaching and is
clarified to produce green liquor containing sodium carbonate,
sodium sulfide ancl sodium chloride and the carbonate and the
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chlorlde are separated fro~ the ~reen liquor. The improYement
in question consists in (1) separating sodium carbonate from
the green liquor by crystallization under conditions to retain
all the sodium chloride in the sodium sulfide-containing mother
liquor said mother liquor being saturated with respect to sodium
chloride, dissolving the crystallized sodium carbonate caustizing
at least a portion of the sodium carbonate solution with a
caustizing agent which is calcium hydroxide to form sodium
hydroxide, (2) caustizing the sodium carbonate-depleted sodium
sulfide containing moth~r liquor with calcium hydroxide, to
convert residual sodium carbonate to sodium hydroxide (3)
concentrating by evaporating said mother liquor to precipitate
sodium chloride, (4) separating sodium chloride therefrom and
combining the resultant mother liquor containing sodium sulfide
with the sodium hydroxide-containing solution in a ratio suitable
to form a digestion solution with a desired sulfide content.
The invention is described below in more detail with
reference to the enclosed drawings, in which Figs. 1 and 2
depict the flow diagrams of two systems meant for applying
. 20 the process according to the invention. Fig. 1 depicts a flow
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diagram in which the waste liquor is burned in the soda-ash
roaster 1 and the obtained melt, which mainly consists of
sodium carbonate and sodium sulfide, is dissolved in the dis-
solving unit 2, thereby producing green liquor. The waste
- water flow 20 from the bleaching unit is fed into the dissolving
~ unit. The green liquor is cleared in the settler 3, whereafter
- it is fed into the sodium carbonate separator 4.
` The sodium carbonate is separated by crystallization
from the green liquor 10 fed into the separator; the crystal-
30 lization is performed in such a manner that the chlorides
remain in the mother liquor. The crystallization can best be
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performed by cooling, since the car~onate is then precipitated
in the form of a decahydrate, which reduces the evaporation re-
quirements. The flow 11 from the soda separator, containing a
great quantity of sulfide, is fed into the causticization appara-
tus 5 for the removal of the remaining carbonate, although this
is not always necessary. The causticization can best be per-
formed by slaking, in a sulfide solution, the entire quantity of
lime required in the liquor system, whereby water is combined
with the lime, which reduces the evaporation requirement in the
crystallization apparatus 7. Simultaneously, the large quantity
of lime produces a rapid, high-degree causticization. The crys~
talline sodium carbonate 17 obtained from the soda separator 4 is
dissolved in the dissolving apparatus 6 and fed into the caustici-
zation apparatus 9. Thereby a solution 18 is obtained which mainly
contains sodium hydroxide, although certain impurities may be
present. The soaium hydroxide solution is used for preparing the
digestion liquor, but it can also be used in any unit processes
in which sodium hydroxide is required.
The flow 11 consists of a sulfide solution in which
certain impurities are present; above all sodium chloride is
significant among the impurities. The flow 1i contains only a
small part of the sodium carbonate because a great part of it has
been separated in the separator 4. If the flow 11 is causticized,
most of the sodium carbonate present in it is converted into
sodium hydroxide, in which case it can be assumed that the flow
-~ 12 leaving the causticization apparatus contains very little
carbonate. The sulfide flow 12 is directed into the evaporation ~-
crystallization apparatus 7, in which the sodium chloride is
precipitated in a crystalline form. It is possible to obtain a
sodium chloride concentra~ion of less than 4 percent by weight in
the sulfide solution 14 leaving the crystallization apparatus,
while the sodium sulfide concentration may rise to more than 25
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percen-t by weight without the sodium sulfide precipitatiny in
a crystalline form.
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The crystalline material 13 contains certain im-
purities, above all sodium carbonate and sodi~n sulfide.
m eir quantities are, however, so small that they can usually
be removed from the process together with the sodium chloridet
If the losses incurred during the process grow too
great, the sodium carbonate and sodium sulfide can be re-
covered by leaching the crystalline material in the apparatus
8. The recovered chemicals 15 can be returned to some stage
of the process, prior to the soda separator 4.
The digestion solution is prepared by mixing the
flows 14 and 18 in a suitable ratio so that the correct sulfide
content of the diges~ion liquor is obtained~ Excess sodium
:: . present in this system can be easily removed in the form of
crystalline sodium carbonate from the process flow 17. The
excess sulfur can be removed in the form of a concen~rated
sodium sulfids solution from the process flow 14. The excess
chlorine is removed in the form of crystalline sodium chloride
~ from the process flows 13 and 16.
; Fig. 2 depicts an embodiment in which the entire :
: 20 lime amount required for the caustici~ation is slaked in a
sulfide solutionO In the figure, 23 indicates the caustic
:~ sludge flow from the causticization apparatus, mainly CaC03,
24 indicates the caustic sludge storage, 25 the caustic sludge
furnace in which the caustic sludge is burned into calcium
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. oxide, 27 indicates ~he lime storage, and 28 the flow from
there into the lime slaking apparatus 22. In the lim~ slaki~g
unit, the carbonate present in the sulfide solution i5 con-
~ verted into hydroxide. The sulfide solution 12, $he flow 21
containing calcium hydroxide, and the purified water 32 to be
33 fed into the dissolving apparatus 6 are separated fr~m each
other in the filter 29, into which the waste water flow 31
from the bleaching unit i~ fed in addition to the sulfide
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solution 30 containing calcium hydroxide.
The following example illustrate~ the process accor~-
ing to the invention. The green liquor being studied has a
sulfide content of approximately 27%, which is very no~nal.
The quantity of chloride fed into the liquor system is 20 kg
of sodium chloride per one metric ton of green llquor. This
chloride quantity must be removed.
Process Flow Na2C~3 Na2S NaOH NaCl H2o
kg kg kg kg kg
Green liquor 10180 50 -- 30 740
Crystallization 11 27 50 -- 30 ~80
cooling 17 153 ~ 2~0
~; Caustici~ation 122 50 19 30 ~50
Crystallization 14 1.1 50 19 10 143
evaporation 130.9 -- -- 20 0.3
By the process according to the invention, it i9 thus
possible to remove 20 kg of sodium chloride per one metric ton
of green liquor from a green liquor which contains 30 kg of
sodium chloride per one metric ton of green liquor. The purity
of the chloride crystals is over 95%.
By starting with the sodium carbonate flow 17 and by
adding or removing sodium carbonate, the desired quantity of
sodium hydroxide, devoid of both sulfur and chlorine, can be
produced by causticization.
- By starting with the flows 18 and 14, white liquor
` with ~he desired sulfide content can be produced. The mixing
ratio of the flows 18 and 14 determines ~he concentration of
sulfide. If excess sodium sulfide is thereby produced, the
excess sulfur is removed i~ the form of a 23% sodium sulfide
solution from the flow 14.
The solubility data of ~he example above are based
on those given in the followi~g publications:
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l~ Seidell et al., "Solubilities of Inorganic and
Metal Organic Compounds", 3rd Ed.~ D. va~
Nostrand ~o. Inc., N.Y. (1944~;
20 Itkina, J. A~. Chem. U.S.S.R., 22,278 (1949); and
3. Rydholm, "Pulping Processes", Interscience
Publishers (196l5).
In no previously known processes (1-4) can 20 kg o~
sodium chloride be removed from a green liquor which contains
only 30 kg of sodium chloride. This means that it is necessary
to take a proportionately rather great sodium chloride amount
from the liquor system before obtaining a removed chloride
quantity equal to that fed into it. Thus, more and more sodium
chloride accumulates in the liquor system. For example,
Process 2 yields an approximately three-fold NaCl content in
the white liquor. Thereby the NaCl content in the green liquor
must rise as high as 50 kg before the necessary 20 kg can be
removed. The sodium chloride content of the green liquor being
less, eDg. 40 kg, only 10 kg can be removed and the remaining
10 kg remain in the liquor system, gradually raising the sodium
chloride concentration until the removed quantity is equal to
; that fed into it. Since in the process according to the in-
vention, the sodium chloride crystals need not be washed, the
evaporation requirement i5 less than in the previously known `;~
processes 2-4. Furthermore, the evaporation re~uirement is
reduced by the fact that the carbonate is separated in the form
o~ a decahydrate and that the calcium oxide is slaked in a
sulfide solution, according to preferred aspects of the present
invention.
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