Note: Descriptions are shown in the official language in which they were submitted.
3907
A METHOD FOR TREATMENT OF SPENT PEROXIDE E~LEACHING LIQUORS
Technical Fleld
The present invention relates to a method for treatment
of spent liquors obtalned when bleachlng hlgh yield pulps
and waste-paper pulps wlthln the cellulose industry. By hlgh
yield pulp is meant here pulp produced at yields above 60%,
such as varlous types of groundwood pulp, thermomechanical
pulp and chemlmechanlcal pulp for example. The method can
also be applled to spent liquors obtalned when bleachlng
textiles.
Backqround Prlor Art
Normally no water g]ass ls added when bleachlng chemlcal
pulp with peroxlde. When bleachlng groundwood pulp and
chemlmechanical pulp wlth peroxlde~ however, signlflcant
quantities of alkall sillcate, such as water glass, are add-
ed as a stabilizer agalnst the decompositlon of peroxide.
For example, as much as up to 80 kg of water glass
(38-54Be) may be used with each ton of pulp. Large quanti-
ties of water glass are also used when delnking waste-paper
; and when bleachlng textiles.
Subsequent to bleachlng wlth peroxlde ln the presence of
large quantltles of water glass, the pulp ~uspenslon or
stock ls normally dewatered to a dry solids content of
20-60~. The resultant filtrate ls not normally recovered,
but ls discharged to an outlet ln the form of some klnd of
recipient or another, elther directly or vla a purlflcation
plant. In those cases where the filtrate is discharged to an
outlet vla a purlflcation plant, the contamlnant present ln
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the filtrate are normally extracted therefzom by sedimenta-
tionO subsequent to addlng a suitable flocculant to the
flltrate.
In the case of a plant whlch has an annual production
of, for example, 100,000 metric tons ancl consumes 40 kg of
water glass (Na2SiO3) with each ton of pulp produced,
the amount of water glass consumed annually reaches 4,000
tons, whlch represents a hlgh market value.
It is kno~n when producing chemical pulp from slllca-
-rich starting materlals, such as bamboo and bagasse in
accordance with the soda or alkali methods, that silica
present in the resultant black liquor can be removed there-
from by supplying carbon-dloxide containlng gas to the
llquor and, at the same time, lowering the pH of the liquor
to beneath 10, over a perlod of less than 30 mlnutes. As a
result the sllica preclpitates and can be removed from the
black liquor, elther by bolllng (pressure heating), followed
by filtration, or by evaporatlon, followed by sedlmentation-
centrlfugation. A similar two-stage method ls known ln which
the black liquor ls treated in a flrst stage wlth carbon~
dioxide gas until a given pH ls reached and at least 75% of
the sllica has precipltated, whereafter the liquor is pro-
cessed in a se~ond stage wlth milk of lime, untll a de-
sired level of purity has been reached and calclum sillcate
has been separated.
In the case of closed ~leaching systems, lt i5 also
known to fllter the spent liquor through membranes prlor to
returning the llguor to the process, in order to separate t
undesirable constituents from tha liguor.
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Summary of the Invention
Technical Problems
The use of water glass as a stabilizing agent is en-
cumbered with disadvantages of both a technlcal and an
economical nature, such as the undesirable and uncon-
S trollable precipitation of substances in conduits andvalves, which can result in stoppages in productlon. The
addit~on of water glass also makes it impossible to recover
the sodlum hydroxide charged to the system by evaporating
and combusting the spent liquor, since the water glass will
therewith precipitate and form an insoluble slag, which also
constitutes a drawback from a~ environmental aspect, since
the organlc substance released ln the manufactllre of pulp
cannot be burned. The~e drawbacks are particularly ~nanlfest
when produclng high yleld pulps from bamboo and grasses,
such as straw and esparto.
The known silica extraction methods can only be a~plied
when producing chemical pulp by the alkaline delignification
of raw materials other than wood. The known methods have
also been found to be highly uneconomical, since they are
particularly t~me consuming and require a high energy input
and considerable equlpment.
It is not possible ln the cellu}ose lndustry to extract
sillcon compounds from spent liguors containing water glass
with the aid of membrane filters, since such compounds cause
the membrane to become blocked.
Consequently, there is conceivably a need of a method
~apable of being applled industrially for separatlng and
optlonally recovering soluble silicates prssent in process
waste liquors and deriving from ~ater glass.
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Solutlon
The present inventlon solves the aforledescribed problems
and relates to a method for treatment of spent liquors ob-
talned when bleaching high yield pulps and waste-paper
s pulps. The method is characterized in that the soluble sili-
cates present in the spent liquors and d~eriving from added
water glass are precipitated by adding an inorganic compound
containing a multi-valent cation and/or an organic floccu-
lant, while simultaneously ad~usting the pH of the spent
liquor to a value of between 3.5 and 7.5, preferably between
4.0 and 6.0; in that the resultant silicate precipitate is
caused to settle to form a sediment, in that the sediment
thus obtained is separated and recovered; and in that the
spent llquor f~eed from sllicate is Leturned to the process.
It has been found that a particular advantage is afford-
ed when the silicate present ln the spent llquors is precl-
pl~ated by addlng an aluminium compound, such as alumlnium
sulphate or alum. Iron and calclum compounds are other sult-
able inorganic compounds containing multi-valent catlons and
which also provide well-sedlmenting and readily separable
precipitates with the sllicates present in the spent
liquors. Ferrlc chlorlde or ferric sulphate and calcium
hydroxide or calcium o~ide have been found to be partlcu-
larly beneficlal in this context.
2S It has been found that in certain cases an advantage is
gained by combining the catlonic inorganlc precipitating
chemical with an organic flocculant in the form o~ a high
molecular weight polyelectrolyte, preferably a polyacryl-
amlde. In certain cases an organlc flocculant can also be
used alone, and therewith effec~ flocculation and precipi-
tation of the ~ cates perferctly satisfactorily.
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When precipitating sillcates with the ald of the afore-
mentioned chemicals and flocculating agents, it is lmportant
in all cases to ad~ust the pH of the spent liquor to between
3.5 and 7.5, preferably to between 4 ancl 6, if the need is
found, i.e. 1f the pH of the spent liquor lies outside the
given range. The pH is suitably ad~usted with the aid of
sulphuric acld, sodlum hydroxide or soda.
The slllcate precipltate formed subseguent to adding a
precipitatlng and/or flocculatlng agent ls allowed to settle
to form a sediment, which is then passed to a device in
which it is concentrated, for example a centrifuge or clouble-
wire belt-press. The concentrated preclpltate is then drled,
suitably in a spray drler or a paddle drler of the kind
manu~actured by Goudsche Machlnefabrlek B.V., Holland, and
incorporatlng two mutually co-acting staam-heated screw~
provided with wedge-shaped paddles. It has been found that a
partlcular advantage ls afforded when the drying process is
carried out so that the product obtalned in each particular
case achleves a dry solids content of at least 70% by; . 20 weight. If desired, the dried preclpltate can also be heated
(roasted), so as to destroy any organlc materlal whlch may
;have precipitated together with the silicate precipitate.
;It has ~lso been found to advant~ge to further reflne or
process the sllicate-precipltate sedlment ln a manner to
impart certain desired properties thereto. For example, the
~llicate precipltate can be given lncreased flameprooflng
propert1es, by adding an lnorganlc boron or phosphorous
compound, preferably borax or potasslum dihydrogen phos-
phate. The mixture thus obtained is then concentrated and
drled in accordance with one of the aforedescrlbed methods.
Alternatively, when further refining the sllicate preclpl-
; tate, lnorganic ammonium or halogen compounds can ~e added,
depending wholly upon how and for which purpose ~he resul-
tant product i~ to be ~sed.
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It has been found that a particular advantage is affor-
ded when the silicate precipitate further processed by add-
ing a boron or phosphorous compound thereto is admixed wlth
a cellulose-pulp suspension and the resultant ~ixture ls
dewatered and dried. Thls provides a special-purpose flame-
reslstant cellulose material.
Advantaqes
Several advantages are afforded when processing ln
accordance with the invention spent liquors that contaln
water glass. For example, the invention enables the recovery
of combustible organic substances and chemicals, and also
enables spent liquors to be evaporated and burned wlthout
forming slags fro~ precipitated sillcate compounds.
The inventlon also ~nables sillcate-containing spent
bleach llquor obtalned from a peroxide bleaching ~tage to be
.filtered through a membrane fllter in order to separate
extractive substances from the spent llquor, wlthout the
me~brane beco~lng blocked with sllicate preciplta~es. In
additlon, the inventlon makes it possible to return purified
~pent bleach liquor to the bleachlng department or to some
other location in the process. A further advantage ls that
smaller quantitles of environmentally harmful ~ubstances are
released to the reciplent due to the re~oval of the extrac-
tive substances.
Corresponding advantages are obtalned when spent liquors
obtalned from wa~te-paper pulp bleaching processes are
treated ln accordance with the lnvention.
Brief DescriPtion-of the Drawinqs
Figure 1 i5 a flow dlagram lllustrating a te~t plant
-^~ 30 lntended for the treatment of ~pent liquor ln accordance
~ ~ with the invention.
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Prefer~ed Embodiment
The invention will be described in more detall herein-
after wlth reference to examples representatlve of prefer-
red embodlments of the method according to the inventlon.
Example 1
Hot spent liquor having a temperature of 60 was extrac-
ted from the bleaching stage of a plant used for the manu-
facture of peroxide-bleached groundwood pulp. The extracted
spent bleach liquor had the following properties:
Chemlcal oxygen demand
(COD) mg O2/litre 3050
Water-glass content (Na2SiO3)
gJlltre 0 6
Acidity pH B.8
Temperature C 60.0
Three samples, designated A, B and C, each of one litre were
taken from the spent bleach liquor.
To sample A there were added 2.0 g aluminium sulphate
~0 (A12(SO4)3) and to sample B 2.0 g aluminium sulphate
; and also sulphuric acid, untll obtalning a pH = 4.5. 2.~ g
of aluminium sulphate were also added to sample C and the pH
of the solution was ad~usted to 4.5, whereafter 0.25 g
potassium dlhydrate phosphate (KH2PO4) was added as a
flame retardlng agent. Upon carefully stirring the samples
there ~as obtalned, ln all cases, a flaky precipltate which
s~ettled relatively qulckly onto the respective bottoms of ,
the test vessel~. The temperature ln respective vessels was
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measu~ed and found to be 57C. 5ubsequent to allowing the
precipitate to settle during some hours to form a sedlment,
the liquid above the sediment was drawn-off by suction, so
as to leave solely the sediment on the bottom of respective
vessels. The COD of the llquid withdrawn from respective
vessels was then determined, the followlng values being
obtained:
COD(mq 02~1itre)
Sample A 1950
Sample B 1~00
Sample C 1790
The various samples thus contained far less oxygen demanding
substances than the untreated spent bleach liquor. As shown
by the above results, the COD of the samples had decreased
on average by 39.5%. This ls a surprising positive effect,
which remarkably enough was achieved wlth a method ~or re-
covering water glass from the spent bleach liquor. It ls '~
thus quite clear that in this case organic substances
accompanied the precipitation of the sllicates, which posl-
tively influenced the extent to which the treated spent
bleach llquor was purlfied.
The sediments from samples A and B were then evaporated
and dried to a water-free state. The sediment from sample C,
on the other hand, was ~ixed with unblea~hed groundwood pulp
having a pulp consistency of 6%. Subsequent to mixing-in the
sediment, the pulp suspension was thickened to 42% dry r1 '
solids conten~, whereafter the thickened pulp was drled to a
dry solids content of 94S.
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When drying the sediments from samples A and B, there
was obtalned a powderous product comprised of small crys-
tals. These crystals were weighed in order to determlne
their quantlty. The results are given below.
Sample A 2.8 g crystal powder
Sample B 3.l g crystal powder
When totalllng up the amount of water glass contained ln
the spent bleached liquor and the amount of chemicals ~alu-
mi~ium sulphate) added to respectlve samples, there is ob-
tained a theoretical total quantlty of 2.6 grams for each
sample. Thus, the quantlties recovered ln both instances
were greater than the theoretlcal quantitles, probably
because the crystalline powder~ contalned co-precipltated
organlc substance. ThiR is conflr~ed by the afore~hown `
reduction in the chemical oxyyen demand of the purifled
spent bleach liquor samples.
The pulp impregna~ed with the sediment from sa~ple C was
tested with respect to lts flame resistance. It was impossi- ¦
ble to ignite the pulp, despite frequent attempts being
made. Similar tests carrled out on an unimpregnated pulp
resulted in lmmediate ignltion of the pulp.
Example 2
The tests of this example were carried out in a test
plant according to the ~low diagram of Flgure l, for treat-
~ent of spent bleach llguor in order to r0cover lnltlally
added water glass. The test plant was installed in a factory
which produced peroxide-bleached gIoundwood pulp.
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The pulp suspension obtained ln the factory subsequent
to bleaching was pressed to a dry solids content of 40% and
part of the thus dralned spent bleach liquor was passed to
the test plant through a pipe 1 and ~nto a tank 2, in which
each lltre of spent liquor was mlxed with 2.5 g aluminium
sulphate supplied through a pipe 3. The pH of the spent
liquor was ad~usted to 4.5 with sulphuric acld supplied
through a pipe 4. The spent liquor was mlxed effectivQly
with the chemicals supplied with the aid of a stlrrer 5
installed on that side 6 of the tank on whlch the spen~
bleach llquor was supplled. Arranged on the opposite slde 7 ,~
of the tank along the bottom thereof was a discharge screw
8, which contlnuously discharged the on the tank bottom
se~tled preclpitate to a pipe 9. Clear spent bleach llquor
was dlscharged to an outlet 11 through a plpe 10 lnstalled
1n the top of the tank. The clear spent bleach liquor, how-
ever, can be returned to the pulp manufacturing plant
through the pipe 10, if so deslred. In thls latter case the
spent bleach llquor can be returned beneficlally ~o ~he pulp
manufacturlng plant via a membrane filter, since the risk of
the membrane becoming blocked has been ellminated by remov-
ing sllicate from the spent liquor. I
Part of the settled precipitate discharged through the j-
pipe 9 was passed through a pipe 12 to a centrlfuge 13,
whlle a further part of the precipltate was pa~sed to a tank
; 15 ~hrough a plpe 14. The settled preclpltate was concen-
trated ln the centrlfuge 13 and then passed to a spray drier .
16, in whlch all water present was evaporat~d off, so as to
obtain a completely dry product.
To that part of the precipltate passed to the tank 15,
0.3 g potasslum dlhydrogen ~phosphate was ~upplied t~rough a
pipe 17 for each li~re of precipitate present. In addltion ~
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hereto pulp suspension, or stock, was supplled to the tank
through a pipe lB in a quantity such that the chemlcal son-
tent (precipitate + KH2P04j per ton of pulp reached
60 kg. The resultant pulp suspension admixed w~th che~lcals
5 was passed through a ~ipe 19 to a screw press ~0, ln which
the pulp was dewatered from a pulp consistency of 6% to a
dry solids content of 45%. The dewatered pulp was then
passed through a pipe 21 to a flash drier 2~, in ~hich the
pulp was dr1ed to a dry solids content of 92%. The flltrate
obta~ned from the screw press 20 was passed to an outlet 24,
~hrough a pipe 23. Analysls of the filtrate showed that lt
contained only traces of sllicate.
~n analysis of the sllica content of the clear spent
bleach liquor obtalned ln the pipe 10 showed only traces of
sillcate in thls spent liquor. When testing the pulp
obtained from the flash drle~ 22 lt was establlshed that the
pulp was totally flame resistant. Pulp of this nature can be
used beneficially, for example, for lnsulating purposes. r
Many fields of use are to be found for the products re~ove-
red with the aid of the lnventlon, and the dried sillcate
sedi~ent can, for example, be used as a filler when manu-
facturing cardboard and paper, or as an addltive in the '~
cement and concrete lndustry. ~ -
ExamPle 3
When carrying out the tests of thls example ther0 was
used a test plant complemented with a membrane-filter system
and having a flow diagram according to the Flgure for re- !
coveri~g w~ter glass from the bleachlng department. The test
plant was installed in a factory ln which pero~lde-bleached
chemlme~hanical pulp ls produced.
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The pulp suspension or stock obtained in the factory
subsequent to the bleaching stage was pressed to a dry
solids content of 40%, whereafter the spent bleached liquor
obtained was passed to the test plant through the plpe 1.
The spent bleach liguor entering the plant through the pipe
1 was passed to the tank 2, to which 2.5 g aluminium sul-
phate were charged through the pipe 3 for each litre of
spent liquor present in the tank, and mixed with said
liquor. The pH of the spent liguor was ad~usted to ~.5 wlth
sulphuric acld introduced through the pipe 4. The spent
liquor was mixed effectlvely with the chemlcals supplied
thereto wlth the aid of the stlrrer 5, which was installed
on that side 6 of the tank on which the spent bleach liquor
was introduced lnto the tank. A dlscharge sc~ew 8 was pro~
vided on the opposit~ side 7 of the tank along t.he bottom
thereof, this screw belng arranged to pass ~he precipltate
sediment formed on the bottom of the tank contlnuously to
the plpe 9. The clear spent bleach llquor, separated from
the precipitate by sedlmentatlon was passed to a membrane
filter system 26, through the pipe 10 provided in ~he top of
the tank and through a newly installed pipe 25. Excess
extractive substances were separated ln the membrane filter
system and, ln this test, were passed to the outlet 11
through a plpe 27. In the case of continuous operation, the
extractive substances are ~uitably used as fuel, since this
substance has a fuel value in parlty with conventlonal fuel
oil. The spent bleach llquor freed from sillcate and extrac- I
tive substances w~s returned to the pulp manufacturing plant ,~
through a pipe 28. The peroxide-containing spent bleach ,~
liquor/ which contalns on~y traces of 511ic2te, can be used
in the pulp manufacturing plant for, for example, washing
th~ pu}p sub~equent to screenlng the pulp, or for wa~hing
bleached pulp.
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The settled precipitate discharged through the pipe 9
was passed to the centrifuge 13, through the pipe 12. The
precipltate was concentrated in the centrifuge 13 and then
passed to the fan drier 16, where all water present was
evaporated off, to provide a completely dry product.
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