Note: Descriptions are shown in the official language in which they were submitted.
-
7~
SPE CIFICATION
In the chemical pulpin~ of lignocellulosic material there is
obtained a chemical pulp which contains inter alia inorganic
chem-cals and dissolved o. ganic su~stances, ~ainly li~nin and
5 mulor quantitie~ of cellulose, toge~er wi~ extractive substances,
such as res3n. The ur~esirable residual material is washed out ~f -
the pulp~ The waste pulping liquor and washing liquids obtained
normally are evaporated and burned, so that ~eir chemicals
content can be recovered. In order to fac~litate recovery of the
10 chemicals, the washing effluent should have the highest possible
~ollds concentratlon a~d ~e least possible amount oE water, which
mea~s that the amowlt of water used during the washing should be
kept to a mLnlmum.
When producing cellulose pulps wi~h a yleld in e~ccess of
15 abo~t 55%, the wo~d chlps are normally mechanically deEibrated
to separate and free the fibers before washing and/or screening
the pulp. During screening the pulp7 the screens collect nondefihrated
wood residues, referred to as rejects or tailings, which are
normall~ defibrated in a separate stage to separate and free ~le
,
~`
., j~ .
fibers which are ~en returned to the pulp flow.
In order to obtain the purest possible pulp, efforts are
made to wash ~e greatest possible amount of contaminants from
the pulp suspension after deligni~ication, i. e., such contaminants as
5 inorganic chem ical residues and dissolved organic substances. The
washing techniques a~plied hitherto, however, leave significant
qua~tities of residual chemicals and undesirable organic substances
in the pulp.
lhere are a number of d~fering washing methods ~at can
10 be used. Tn the oldest method, the pulp suspension is washed in so~
- called diffusers, whivh give a displacement wash which is carried out
batchwise ill large vesse~s. Ano~er method, which is commonly
used nowadays, washes ~e pulp suspension on drum filters,
normally in several stages. The number of drum filters may vary,
15 although normally from three to four filters a~e used. Tn a more
recent method, the washed pulp ls compressed In presses, whlch
remave larger volumes of liquid than can be done with filters,
resultlng ln a more efflcient recovery of chemicals and organlc
substances.
These washing technlques all leave slgnificant quantlties
of resldual chemlcals and unde#lrable organlc substances in the
pulp. When the pulp Is to be subsequently bleached, these chemlcals
and organlc substances compete for and thus glve an unnecessarlly
high consumptl~ of bleaching chemicals, and al80 I-ad to dlscharge
..
1.~61:~7~
o an undesk~bly high quantity of substarlces harmful to the environ-
ment. ~n the waste liquors. An increase ill the reco~ery ol chemicals
used in the cellulose processes is highly desirable, bo~ froxn an
economic and environmental aspect.
In Canadi~n a1pplication Se~ial No. 360, 196, filed ~eptember 12,
1980, to 30nas Arne Irlgvar Lindahl, a process and ~pparaLus are
provided for the deres~ation of cellulose pulp, by a~ljusting th~ pulp
concentration to with~ the range from a~out 15 to about 35~/c; add~
sufficient alkali to the pulp to bri~; -~e amount of alkali, calculated
10 as NaOH, wi~in the range f~om about 2 to about 1~ g/kg of water
accompanying the pUL~; ~ubjecting ~e pulp to a mild, mechanical
working in the bite of twin interdigitated ~otating sc~ews at all
energy input of from 8 to 100 kWh per to~ of pulp, removing and
reacting the pulp with the added alkali for from about 0.1 to about
15 five hoùrs, and then washing out dissolved resirl from the pulp.
In Canadian application Serial No. 391, 674, filed December 7,
1981, to Jonas A~ne Ingva~ Lindahl, a process a~d apparatus are
provided for. the deresillation and brightness improvement of
cellulose p1llp, by adjusting ~e pulp concentration to within ~e
20 range from about 15 to about 35%; adding sufficient allcali to the
pulp to b~ing th~ amount of alkali, calculat~d as NaO~I, within the
range f~om about 0. 5 to about 17 g/kgr of water accvmpan~rinlr the
pulp; adding sufficient o:~idizing bleaching agent to the pulp to bring
the amount of oxidizing bleaching gent to w~in the range Irom
-
7~'~
about 0.2 to about 22 g/kg of water; subjecting the pulp to a mild,
mechanical working in the bite of twin interdigitated rotating screws
at an energy input of from 8 to 100 kWh per ton of pulp; removing
alld reacting the pulp with the added alkali and bleaching agent for
5 from about 0.1 to about five hours; and then washing out dissolYed
resin rom the pulp.
The present invention provides a method for washing
cellulose pulps produced from lignocellulosic material more effective-
ly than lmown washing techniques, so as to improve the recovery of
10 chemicals and combustible organic materials from the wa~te liquors.
The invention is applicable to chemical pulps, semimechanical pulps,
and chemimechanical pulps, produced from both softwood and
hardwood.
In the process of the invention, unbleached cellulose pulp
15 produced from lignocellulosic material in a pulping process including
a chemical pulping stage and, optionally, a mechanLcal defibrating
stage, is washed with water or an aqueous solution In a plurality of
stages, and in between at least two washing stages is subjected to a
mild mechanical working between two interdigitated rotating screws
20 at an énergy input of from 7 to 200 kWh per ton of pulp.
VVhen applying the method according to the InventLon, the
cleansing effect obtained has been found to be partlcularly good when
at least 10~C of the impurities originally present in the pulp sus-
pension are removed in at least one washing stage preceding the
25 mild mechanical working. Tn other words, at least 10~C of the
3-~4~
solids content of the waste pulping liquor should be washed out prior
to subjecting the pulp suspension to the mild mechanical working
It h~s also been found particularly suitable to increase ~e
solids content of the pulp suspension prior to the mild mechanical
5 treatment to a pulp concentration between 10~C and 50~7c, preferably
between 14~C and 40~c, and suitably between 20~C and 35~c- Upon
completion of the mild mechanical worldng, ~e pulp suspension is
diluted to a pulp consistency of between 0. 5 and 13 %, in conjunction
with mixing wi~ wash liquid in the su~sequent washing step. ~en
10 the pulp is washed on filters, the most suitable input pulp consistency
lies between 0. 5 and 6~c, while washing-dewatering in, for example,
a press is suitably per$ormed at an input pulp consistency of
between 2 and 13~c.
The present invention also provides apparatus for improving
15 the washing of unbleached cellulose pulps comprising, in coznbination,
means for adjusting the pulp concentratlon to withln the range from
about 10 to about 50~c, preferably from about 14 to about 40~c;
washlng means for washing cellulose pulp with water or an aqueous
solution, and separating the waste wash water or aqueous solution;
20 means $or subjecting the pulp to a mild mechanical treatment in the
bite of twLn interdigitated rotating screws at an input energy of
from 7 to 200 kWh per ton of pulp; means for adjusting the pulp
concentration to within the range from 0. 5 to 13~c; and means for
washing the cellulose pulp at substantially the same pulp consistency.
~ ~61~71~4
There can be several washing stages before and/or after
the mild mechanical working, and ~ere can be several mild
mechanical working stages, each preceded by and followed by a
washing stage.
In a preferred embodiment, there is means for adjusting
pulp concentration after each washing stage which comprises a
dewatering device preferably including a press; ~e twin interdigitated
rotating screws comprise a screw defibrator; a screw feeder is
inc.luded for transferring the pulp from the dewatering device to the
10 screw defibrator andbetween each washing stage; and there is
means for transferring the pulp from the last washing stage to a pulp
container or to the next processing stage.
The Figure shows schematically apparatus embodying the
meff~od according to the invention applied in the Examples in washing
15 blrch sulfate pulp and chemimechanical spruce pulp in three stages,
each of the washing stages beLng equipped with a press, 4, 14, 18.
Installed between the first and second washing stages is a screw
defibrator 10 of the kind sold under the trade mark FROTAPULPER6'.
The washing process of the invention is preferably carried
20 out on unbleached cellulose pulp, after the lignocelluloslc material
has been digested to cellulose pulp, as Ln a dLgester with digesting
chemlcals recovered from spent digestLon liquor, and then ~e
pulping liquor washed out in a washing stage.
7~
As ~e wash liquid, water is normally used and is preferred.
However, waste aqueous wash liquids such as are obtained in de-
watering stages in ~e washing process of the invention can be used,
for example, waste press liquor used in the Examples herein, as
5 well as acidic and alkaline waste liquors from o~er pulp treating
processes in the plant or pulp mill, such as pulping and bleaching
waste liquors, w~ich may desirably be diluted with water before use.
The pulp can be dewatered in one or more stages to a
relatively high pulp concentration within the range from about 10 to
10 about 50~ in any conventional dewatering device, such as a drum
washer, belt washer, roll press or screw press.
A suitable apparatus for the mild mechanical working
treatment is a screw defibrator (screw refiner)j and especially
sultable is the screw defibrator sold by MoDoMekan AB under the
15 trademark FROTAPULPER~. This screw defibrator has two
rotatlng Interdigitated screws which are arranged in parallel to
each other in a housing provided with an inlet and an outlet for
pulp The screws are interdigitated or engage each other for
kneadlng the pulp and at lea~t some of the screw flights are
20 provided wi~ serrations or indentations on their outer periphery.
Such a screw deflbrator is described In U . S patents Nos 3,054, 532,
patented September 18, 1962, 3,064,908,patented November 20, 1962,
3,533,563, patented October 13, 1970, and 3,724,660 patented
Aprll 3, 1973.
3 ~ 7~
~ nother type of ~crew defibrator that can be used is
described inU.S. patentNo, 4,284,24~, patentedAugust 18, 1981,
to Erik Folke Eriksson.
The pulp mi~ed wi~ water is subjected to shearing and
5 kneading forces in the screw defibrator in the form of pulsating
pressure loads. As a result OI this treatment, a very effective
washing of the pulp with ~e water is obtained. As for the pulp
fibers, the treatment is mild, since the fibers are not shortened
(which is the case in beating or milling) or adversely affected in
10 any other way.
~ e treatment in the screw defibrator usually is carried out
at atmospheric pressure, but it can also be carried out at super-
atmospheric pressures of ùp to 500 kPa. During the mechanical
treatment, the temperature of the pulp increases, due to liberation
15 OI heat, sLnce at least 60~C of the energy input is transformed to
heat. I~e hlgher the input of energy, the greater is the temperature
Increase durLng the work.
The mild mechanical working ac~ording to the invention
can be carried out at any desired temperature, as normally used in
20 practlce. However, a particularly suitable temperature range Is
between about 20 and about 110C, while a preferred range is between
about 35 to about 90C.
By "mild mechanical working" is meant a working in which
pulp suspensions of the aforementioned solids concentration or pulp
7 !r~ 4
consistency are subjected to a repeated pressing, kneading and
shearing at an input of electrical energy for carrying out said
treatment within the range from aloout 7 to about 200 kWh per ton of
bone dry pulp, and prefera~ly wi~in the range from about 10 to
about 100 kWh per ton OI bone dry pulp.
After ~le mild mechanical treatment the pulp is transferred
by means of a suita~le device, such as a pump, screw feeder or belt
conveyor to another washing apparatus for continued washing with
water or an aqueous solution.
Thereafter, the washed pulp may be carried directly to
bleaching and drying or final treatment, for example, to manufacture
of paper of different qualities. The mel~od according to ~e invention
ls prlmarily applicable to the manufacture of washed unbleached or
sllghtly bleached cellulose pulp. However, it is also possi~le to
apply the method to ~e manufacture of pulp which is bleached to a
varylng extent, lncluding bleachLng to a final brI~htness exceedlng
~0~; ISO. Usually the pulp after it has been treated according to the
Inventlon ls bleached In one or more bleachlng stages In any selected
bleachlng sequence.
The novel washing method according to the Inventlon, In
whlch 1he pulp suspenslon Is mechanlcally treated ln a mlld fashlon
between two or more of the stages of a washlng sequence, has been
surprisingly found to provide a cleaner pulp and to result In a
higher chemlcal recovery, a lower dlscharge of Impurlties In the
7~, 4
waste water or aqueous solution effluent, and a higller solids concen-
tration in the effluent than was previously the case, when applying
conventional washing techniques.
Thus, ~e present invention affords the important advantage
5 cf yielding a purer pulp. This is manifested, inter alia, by a
considerable lowering of the extractives content of the p~lp, and of its
Kappa number.
The advantages afforded by the method according to ~e
invention also include the possibility of increasing the solids concen-
10 tration o~ the waste washing liquor from the pulp washing stage, i. e.,~e concentration of organic materials contained in said liquor, and
also the concentration of inorganic chemicals contained therein.
This increases the fuel value of the waste washing liquids,
representlng a valuable savlng In energy. Because more contamlnants
15 are brought to e~aporation and burning, and a greater amount of
chemLcal8 are recovered,the amount of envlronmentally harmful
8ubstances contained in the effluent is less than would otherwlse be
the case, whlch is an important advantage, and also desirabLe from
an envlronmental aspect. Consequently, the cost involved In the
20 treatment and destructlon of envIronmentally harmful substances In
~e effluent can be reduced when applylng the Inventlon.
~ fur~er advantageous and surprLslng effect afforded by the
invention is that the consumption of bleachlng chemi cals, such a8
chlorine, chlorine dioxide, hypochlorite, hydrogen peroxide and
oxygen gas, is markedly reduced when bleaching the washed pulp.
Ano~er surprising effect obtained by the inclusion of the
mild mechanical treatment process according to the invention is
5 that the dewatering of ~e resultant pulp suspension is markedly
improved, a fact which affords decisive advantages when fur~er
treating the pulp.
The advantages afforded by the method according to ~e
invention are fur~er illustrated by ~e Examples which follow.
Ille Examples set ~orth preferred embodiments of ~e
l~lvention carried out in parallel, In which birch sulfate pulp and
chemimechanical spruce pulp were washed in three stages, incorpor-
atlng the mild mechanlcal treatment process between the first and
second washing stages (Examples 1 and 2), in comparison wi~
15 Control8 ~ and 2, In which the pulp suspensions were not subjected
to any form of mechanlcal treatment between the dlferent stages of
the washLng se~uence.
The apparatus illustrated in the Figure was utilized in all
the E~camples, and hence the reference numerals refer to correspond-
20 ing references in the Flgure.
EXAMPLE 1
Unble~ed and un~ashed birch sulfate p~llp was introducedthrough the line 1 into ~e tank 2 of the processing system shown in
the Figure. In the tank 2 the pulp suspension was diluted with press
5 was$e liquor supplied through the line 22, until the pulp consistency
was about 8~c. The pulp suspension was then passed through line 3
to the first washing stage 4, including a press in which the pulp
suspension was compressed to a dry solids content of 33. 3~c,and
then passed through the line 5 to the screw feeder 6, arranged to
10 feed the pulp suspension ~rough the lines 7 and 8 (line 9 was closed)
to the screw defibrator 10 provided with two rotating screws whose
bla~les had concave portions and in which the pulp suspension was
sub3ected to a mild mechanical treatment, after which the sus-
pension passed throu~ line 11 to the second screw feeder 12. The
15 ternperature of the pulp suspension at the inlet to the screw de-
fibrator 10 was 68C, and at the outlet of the defibrator was 71C.
The amount of electrical energy consumed by the mild mechanlcal
treatment in the screw defibrator 10 was 15 kWh per ton o bone-dry
pulp.
The treated pulp suspension was diluted in the screw feeder
12 to,a pulp consistency of 8%, by introduclng press waste liquor
to the feeder through line 21,the wastellquor mixing effectively with
~e palp suspension during its transport through the feeder 12.
The diluted pulp suspension was then passed through the line 13 to a
second washing stage 14, including a press similar to that used in
the first washing stage 4. The pulp suspension was washed alld
agaill thic'~ened in the press 14 to a dry solids content of 33. 3~c-
Waste liquor obtained from the press 14 was recycled through ~e
5 line 22 to the tank 2, where it was used to dilute incoming pulp
suspension. The pulp suspension after being washed and thickened
ill the second washing stage was then passed through the line 15 to
~e screw feeder 16, and then through the line 17 to a ~ird washing
stage 18 equipped with a press similar to the presses used in the two
10 preceding washing stages 4, 14.
The pulp suspension was diluted aIld mi~ed in ~e screw
feeder 16 in a manner similar to that in the preceding screw feeder 12,
although in this case with pure water supplied through the line 20,
to a pulp conslstency of 8%. After washing the pulp suspension and
15 compresslng lt to a dry sollds content of 33. 3~c in ~e press 18, the
wa~hed pulp suspenslon was remo~ted through a llne 19.
Samples of the press waste lLquors were removed from the
Ilne# 21 and 22 for analysis purposes. Samples of the washed pulp
were also taken from the line 19. The analysls results are shown
20 in Table 1.
Waste llquor from the presæ in the flrst washlng stage 4 was
taken through a llne 23 and passed to a recovery plant (not shown In
; the drawîng), where the waste llquor was evaporated and burned.
Because the washlng llquid was passed ln counterflow to the flow
13
7~
of pulp suspension, and because of the mechanicaL treatment to
whi~h the pulp suspension was subjected, the press waste liquor had
a high organic substance content (about 13~c) and7 at the same time~
ahigh fuelvalue.
As a control, a birch sulfate pulp taken from the same batch
as the pulp used in Example 1 was washed a~d treated in ~e same
apparatus, but with ~e dlfference ~hat the pulp suspension was not
subjected to a mUd mechanical treatment between ~e first a~
seccnd washing stages. Thus, all flow of pulp suspension through
10 the line 8 was stopped, and line 9 was opened, so that ~e pulp
suspension passed directly through the line 9 to the second screw
feeder I 2.
~ fter washing and compressing the pulp suspension in the
press 1~ of the second washing stage, the pulp obtained had a dry
15 sollds content of 2B~c. ~fter washing and thickenlng the suspenslon
In the press of the third washing stage 18, the pulp also had a dry
fiolids content of 26/~C. Tt will be noted that the conslstellcy of the
pulp suspenslon at ~e inlets to stages 14 and 18, respectively, was
also 8~c in this test, and that the pressure exerted by the presses
20 was the same as that in Example 1.
Samples for analysls were removed in this control run at the
sa~ne locations as in the run accordlng to Example 1. The analysls
results are shown in Table I below:
3 ~7~4
TABLE I
Exampie
% Dry sol-ids content of pulp suspension Control
.
Subsequent to passulg ~e press of the
second washing stage 14, ~c 26.0 33.3
Subsequent to passing the press of the
thirdwashingstage18, ~c 26.0 33.3
1~ Amount of press waste liquor obtained from:
The press of the second washing stage 14,m3/ton 8. 65 9 50
The press of the thlrd washu~g stage 18, m3/ton 8 . 65 9. 50
Dry solids content of press waste liquor from:
The press of ~e second washing stage l4, 3Zc 3. 503. 70
The press of the third washing stage 18, ~c 1. 621. 89
Sodium content of ~e press waste liquor from:
The press of ~e second washing stage 14, g~l Na 6. 55 7. 02
The press of the thlrd wa~hing stage 18, g/l Na 2. 83 3. 38
Totally washed from the pulp suspenslon ln
20 the second and ~lrd washing staces:
As dry sollds, kg/ton bone dry pulp 450. 0532. 0
As Na, kg/ton bon~ dry pulp 80.9 98. 5
Propertles of finally washed pulp
Extractlves content (according to SCAN-C7: 62) 0. 34 0.18
25 DCM (dlchloromethane), %
Vlscoslty (accordlng to SCAN-C15:62) cm3/g 972 969
Kapp~ number (according to SCAN-C1: 59) 17. 514. 7
'Irg~
As seen from the Tal~le, the method according to the inven-
tion -gives quite surprisingly a considerably better washing ~an can
be obtained when wash-ng in accordance wLth l~own techniques.
Despite the fact that the mild mechanical treatment was carried out
5 on a pulp suspension which had only been partially washed, the
method according to the invention surprisingly resulted in a pulp
with a much lower extractives conte~t than can be obtained when
corlventionally washing pulp without mechanically treating the pulp
l~etween washing stages.
One important advantage with respect to energy afforded
by the increased extent to which organic substances are washed ~rom
the pulp is that the fuel value of the waste liquor is also increased
Fur~er, a higher percentage of the cooking chemicals are recovered.
3 ~ 7;~
EXAMPLE 2
Unl~leached and unwashed chemimechanical spruce pulp
cooked to a yield of 73% based on the dxy wood was introduced
through the line 1 into the tank 2 of the processing system shown in
5 the Fi,?ure. In the tank 2 the pulp suspension was diluted with press
waste liquor supplied ~rou~h the line 22, untll ~e pulp consistency
was about ~c. The pulp suspension was then passed through line 3
to ~e first washing stage 4, including a press in which the pulp
suspension was compressed to a dry solids content of 33 . 3~c ,and
10 then passed ~rough the line 5 to the screw feeder 6, arranged to
feed the pulp suspension through the lines 7 and 8 (line 9 was closed)
to the screw defibrato~ 10 provided with two rotating screws whose
blades had concave portions and in which the pulp suspension was
subjected to a mUd mechanical treatment, after which the sus-
15 penslon paæsed throu~ line 11 tothe second screw feeder 12. Thetemperature of the pulp suspension at the Lnlet to the screw de-
flbrator 10 was 68C, and at the outlet of the defibrator was 71C.
The amount of electrical energy consumed by the mild mechanical
treatment in the screw defibrator 10 was 15 kWh per ton of bone-dry
20 pulp.
The treated pulp suspenslon was diluted In the screw feeder
12 to a pulp consistency of 7%7 by introduclng press waste liquor
to the feeder through line 21, the waste liquor mixing effecti~ely wi~
~e pulp suspensi~n durlng its transport ~rough the feeder 12.
25 The diluted pulp suspension was then passed through the line 13 to a
17
second washing stage 14, including a press similar to that used in
the first washulg stage 4. The pulp suspension was washed and
again thickened in ~e press 14 to a dry sollds content of 34~c
Waste liquor obtained Irom the press 14 was recycled ~rough ~e
5 line 22 to the tank 2, where it was used to dilute incomlng pulp
suspension. The pulp suspension after being washed a~d $hickened
in the second wash~g stage was then passed fflrough the line 15 to
the screw feeder 16, and then ~rough the line 1~ to a third washing
stage 18 equipped with a press similar to the presses used in the two
10 preceding washing stages 4, 14.
The pulp suspension was diluted and mixed in ~e screw
feeder 1~ in a manner similar to that in the preceding screw feeder 12,
althou~h in this case with pure water supplied through ~e line 20,
to a pulp conslstency of 7~c- After washing the pulp suspension and
15 compresslltg It to a dry sollds content of 34~c In the press 18, the
washed pulp suspensLon was remoYed through a line 19.
Samples of the press waste liquors were removed from the
llnes 21 and 22 for analysLs purposes. Samples of the washed pulp
were also taken from the line 19. The analysls results are shown
20 ln Table IT.
Waste liq,uor from the press in ~e flrst washing stage 4 was
taken throug~ a llne 23 and passed to a recovery plant (not shown ln
the drawlng), where the waste llquor was evaporated and burned.
Because the washlng llquid was passed in counterflow to the flow
18
,
7~r~
of pulp suspension, and because of the mechanical treatment to
which ~e pulp suspension was sub3ected, the press waste liqu~r had
a high organic sub~tance content (about 13%) and, at the same time,
a high fuel value.
As a control, a chemimechaJlical spruce pulp taken from
the same batch as the pulp used in Egample 2 was washed and treated
in the same apparatus, but wLth the difference that the pulp sus-
pension was not subjected to a mild mechanical treatment between
1he first and second washing stages. Thus, all flow of pulp sus-
10 pension throu~h the line 8 was stopped, and line 9 was opened, so
that the pulp suspension passed directly through the line 9 to ~e
second screw feeder 1~.
After washing and compressing the.pulp suspension in the
press 14 of the second washing ætage, the pulp obtained had a dry
15 solids content of 30~c After washlng and thickening the suspension
ln the press of the thlrd washing stage 18, the pulp also had a dry
solldfi content of 30%. It will be noted that the consIstency of the
pulp suspension at the Inlets to stages 14 and 18, respectively, was
also 7~c ln thls test, and that the pressur~ exerted by the presses
20 was the same as that in Example 2.
Samples for analysls were removed ln this control run at
the same locatlons as ln the run accordlng to Example 2. The
anal~sls results are shown ln Table II below:
19
3.~ 7~
TABLE: 11
Example
~c Dry solids content of pulp suspension Control 2
Subsequentto passing ~e press of the 30 34
second wash~ng sta~e 14, ~c
Subse~uent to passing ~e press of the 30 34
third washing stage 18, ~c
Amount of press waste liquor obtained from:
.
The press of ~e second washing stage 14,m3/ton 10. 95 11. 34
The press of the third washing stage 18, m3/ton 10. 9G 11. 34
~0 Dry solids content of press waste liquor from:
The press of the second washing stage 15, ~c 0. 77 0. 80 ',
The press of the third ~vashing stage 18, ~c 0. û9 0.12
Sodium content of ~e press waste liquor from:
-
The press of the second washing stage 14, g/l Na 1. 62 1. 87
The press of the thiTd washing stage 18, g/l Na 0. 45 0. 47
Totally washed from the pulp suspension in20 e second and thlrd washin~ sta~es:
As dry solids, kg/ton bone dry pulp 94. 2 104. 3
A~ Na, l~g/ton bone dry pulp 22. 7 24. 3
Properties of finallg washed pulp:
Extractlves content (according to SCAN. C7: 62) 0. 62 0.12
25 DCM (dlchlorome~ane), ~c
2~0
17~J~4
As seèn from the T~le, the method according to the in~ren-
tion gives ~uite surprisingly a considerably better ~ashing than can
be obtained when washing in accordance with ~mown techniques.
Despite the fact ~at ~e mild mechanical treatment was carried out
5 on a pulp suspension which had only been partially washed, ~e
method according to the invention surprisingly resulted in a pulp
with a much lower extractives content ~an can be obtained when
conventionally washing pulp without mechanically treating ~e pulp
between washing stages.
One important advantage with respect to energy afforded
by ~e Increased extent to which organic substances are washed from
the pulp iS that the fuel value of the waste liquor is also increased.
Fur~er, ahigher percentage of the cooking chemicals are recovered.
A comparison of washing the pulp in accordance with known
15 technlques and washing the pulp in accordance with the present
Invention shows that more water is removed and more chemicals
recovered when uslng the method according to the invention This
Is shown by the fact that the volume of waste liquor recovered when
applying the present invention was greater, despite the fact that the
20 consistencies of the pulp suspensions entering the presses, in both
tests, were maintalned constant ~t 7%.
Accordlng to measurements taken, the washlng water in
~cample 2 also contained slightly larger quantities of dry solids than
the wa~hlng water in the Control. Thus, it can be establiæhed that
21
~e fuel values of the liquors obtained when applying the method
according to the invention are higher than ~e fuel Yalues of corres-
ponding liquors obtained when washing chemimechanical pulp in
accordance with conventional techniques.
22
