Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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wo 94JOlGI7 2 1 3 9 7 6 ~
kSE~D FOR ~ILTERING A PRO~T FORII~ IN C~USTICIZ~TION
The pre~ent invention rel~tes to a me~od according ~o
the preauible of claim 1 for filteri~g a product ~o~med in
causticization.
The cooking lic~uor reqrlired in the dige~io~ proces-~ of a
Q~fate cellulo3e plant is mos~ generally prepa~e~
accord~ng to the following ~che~ne.
g~Qe~en~ to the cookang o~ the cellulos~ c raw m~terial
wit~h ~hite liquor, t~le pulp i-~ washed and the ~ookin~
liquor with waqh waters is recovered ~co~u~only called
weak hl ~çk lic~lor) . The weak b~ ack liquor i~ dewater~d by
~5 e~apora~ion ~d thus obt~i~eA he~y bla~ li~or is
bur~ed in ~ ~o~ recovery ~ n ~ ce . The molte~n ~melt
formed onto ~he -~oda I ~_ov~ry ~u~nn~oe ~ottom, consis~ing
p~nn~r~ly o~ sodi~m sutfide ~N~S) and ~odsum car~on~te
(~a2C03~, is taken out in a ~ontrolled m~r~ and di.q-
~ol~ed lnto wa5h wa~ters recovered ~rom s~sequent s~ages
of the proce~. The ~lution thu~ obtained is called
green or fresh liquor, also fre~h liquor. The green
liquor is clarified a~d the sludge ~hus o~tained iQ
~ e~ to re~over con~ained chemicals. The ~pent w~; ng
liquid i~ routed ~o the wea~ l~quor stora~e t~nk for use
ln the dissolut~on of the ~melt obtained ~rom the furna~e
ou~u~ .
The clarified green liquor is prep~red into white li~u~r
3~ for digestion through a ~wo-stage process.
~ir-~t, s1~kin~ of lime ls ~erforme~ in a unit ~alled ~ime
~laker w~ere lime (~aO1 is a~ded to the clarified gree~
liquor, whereby ~he lime rea~t~ wi~h th~ water contained
in the ~een liquor there~y ~orming slaked lime (Ca(0~)2)
a~ordin~ to ~he follow~ n~ reaction formula:
DEC-20-1994 12: 35 FROM OY HEIN~NEN R~ TO 990161~ 8821 P. 09/28
WO ~4~01617 2 1 3 9 7 6 ~ Pcr~93~002g~
C:aO + H20 ----> ~a ~OH~ 2
~rl the c~usticization pr~ce~s the calt:i~m hydroxide
(Ca (0~) 2~ fo~med as descri~ed abo~e i rea~ed ~ith
S sodium c~hon~te ~Na2C:03) con~in~ in t~e green li~or,
where~y 80~um hydroxide ~NaO~I) and calcium ca~ ~ona~e
~caCO3), al~o called slake lime, are formed al:cording t:o
thë following ~o~
C:a ~0~) 2 I Na2C:03 ~ 0~ I CaC03
~rhe Gausticization reac~-io~ starts irl the lime slaleer a~d
is con~ ci in mul~ple ~-3 pcs.~ causticiz~ion
c:ontA; ~r~ wherefrom the reaction products are routed to
15 a sock-type wh:~te liquor filter or arlother type of
clarifier. ~e filtra~e from the fil~er is the cooking
~ iquor (i . e ., white li ~uor) which is roul~ed to a s~orage
tan~, aLnd furth~r, to the cligester. The o}~tA~ ned white
liquor has a ca ~y o~er of ~ ~ 30 ~ of ~he a~ti~e
~odium in t~e form of ~a2S which, ho~e~er, h~-~ no
detri~ental e~fe~t on the -~u~equent reactions.
The filter ~otto~ di~char~e normRlly o~ n~rl as a
30-40 % solids cont~ning -~ludge i8 filtered in the lime
~S s~udge wns~i~g filter who~e filtrate is ~he weak ~hite
liquor tha~ is routed ~ia a storage ~ank to a dis~olution
tank. The l~e slud~e fonming the bot~o~ di~rharge is
routed via a storage tank to, e.g., a lime slu~ge ~ilter
o$ the crum ~ er type w~re ~he lime sludge is ~ashed
and r~ied. The filtrate proper and the was~ filtrate are
u~ed for, e.g., diluting the ~ludge entering the lime
sludge w~ch;ng ~ilter, whi~e the ~ried l~me sludge is
route~ to the rotary li~e kiln.
~he l~me -~l~dge is incinerated wit~ the help of an
~ iary fuel ~oil, gas, et~.) in a xotary lime kiln,
DEC-20-1994 12:35 FROM OY HEIN~NEN RB TO 9g0161.~ 8W1 P.10/28
w~ ~u~1617 P~T/~3/00287
213~7~ S
whereby calcium oxide and carbon dioxide are fon~ed
according ~o the follow~ng formula:
C:a¢03 ~~~~ ¢~ + C2
s
The bur~t l~e (CaO~, ~ith makeup lime added to compen-
sa~e for ~ l lo8-~es~ ig u~ed in ~austic~zation.
In the above-de~cri~ed method the ~eak white li~or
0 rou~ed to t~e di~so}ution tank con~ains ~aO~ whi~h i5
there~rom c~ eyed to the caus~icizatio~ reaction. As
known in the art (~f. e.g. Sympo ium ~or ~eco~ery of
Pulping Ch~mical~; K. ~in~n~ Unter~l~rhl~n~en zur
Kaustizierung ~on Gr~nl~ en~, the in~oduc~ion of
1~ existing ~dium hydroxide into the caus~i~iza~on
rea~tion is detr; ent~l to the rea~ion.
It is an object of the present in~ention to simplify the
causticiza~ion proce~s in a m~n~_ in which the weak
20 cooking liquor, u~h resul~s from concurrent caus~ici-
zation proc:e . ses and i~ known ~o be detriment~l to the
cau~tic:iza~ion proc:ess, need not be returned ~o the dis-
so}ution tank of t~e soda reco~rery furnace cixcui~ for
weak alkali ,eco~,~ry, ~ut rather, ~he ~ ; ne .~olution
resulting from WA~h; ng wi~h a smal~ amount of warm water
in a f~ er can be c:om~ined with the main circulation o~
the coo3c~ng li~uor, i.e., the pr~mary li~id cont~
u~eful white li~uor at a ~oncen~r~tion o~ approx. g0-
110 ~Jl (Na~O~.
~o
The me~hod a~cor~ing to the prese~t in~en~ion aims at
o~er~oming the a~ove-described dr~w~acks and to sim~lift~
~he pro~ess in a m~nner whic:h pe~i~s omicsion of a
~ignifi~ant portion of equipment needed in conçurrent
pro~esses. Separation o~ white liquor, as wel~ as wasning
and ~ub~equent dry~ng of ~he lime sludge, take ~l~ce in a
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WO 94/01617 PCI~/FI93/l~
2139766
single pro~e-~s st~ye. The characteri~ing propertie~ o~
the method accor~i~g ~o the in~en~ion are di~clo-~ed in
the Anne~ed c~ c.
Accordi~g to the i~ention, the white liquor and l~me
sl~dge recei~red ~rom ~he causticization tank~ are ~outed
~ a filter, most adv~ntageo~sly a pr~surize~ filter~
wherefrom the ~hite liquo~ o~tA; n~ as ~iltrate is routed
to a sto age tank an~ fur~her to the digester. The solids
content of the lime slud~e cake -;n;ng in t~e ~ilter
is high, which means a low d~O~ o~ white liquor to be
washed away from the cake.
~ter filtering i5 completed, the lime ~udge c:ake still
15 rer-~n;n~ in the filter is w~ . Ow:~g to the high
-qoli~ co~tent ~f t~e l~e -~lu~ge cake, a good w~in~
re~ult c~n be at~ using or~ly ~ ~mall ~ -L of ~arm
water . Thus, the 3mall ~ou~t of filtrate from we~ ; n~,
that i_, of the weak w~ite liquor c:~ be rou~ed ~ nto the
20 stora~e tank of white liquor alre~dy sepàrated in the
riltration s~-a~e.
Ater f~lterin~ and w~-~hi~, the lime slud~ cake ~an ~e
~rther dried by, e.g., blowing co~preQQe~ air throu~h
2~ . it. Subse~uent to t~e-ee steps the lime sludge cakes are
~ ed from the ~i~ter and ~onveyed to ~n~ineration in
the rctary lime kiln.
By virt~e of the above-de~ribed method, the filterin~,
w~shing an~ dryin~ of the lime sludge can achieve
complete el~miA2tion of the pre-exi~ting sodium
hydroxide, which otherwi~e ~ould ~e c~rried over to ~he
~issolution of the smelt and subse~uent aetrime~tal
transfer i~to the cau~tirization pro~ess.
3~
DEC-20-1994 12:36 FROM OY HEINRNEN RB TO 990161.~ U8f2~1 P.1~/28
~ U~OI / PC~/~ JO~1287
~1397~
The ~ethod according to ~he inventio~ achie~es a simpler
and more e~fective causticization process o~r conven-
tional techniques, ~in~e unnece~axy circul~ions o~
liquids are ~im;n~ted and the number of re~ining ~ir-
S cula~ions is thus &~nifi~antly s~ller than tho-~e nee~ed
in pr~or art. Moreover, ~e abo~e-discusqed benefitQ also
o~fer essential ~pro~ements in the form of reA~C~A
en~ironment~
1~ T~e invention i~ next ~inO~ in sreater de ail with the
help of an excmp~ify~ng em~odiment ~y ~yking reference 'o
the attached draw~g (Fig . 1 ) ~howing ~he ~chematic
prin~iple of a plant acco~ding to t~e pre~ent in~ention.
Smelt recei~ecl ~rom a Qoda recovery furnace ~1 ) is routed
~o a di:3~olution tank ~2~ where it i~ cliRQol~red in wa~er
or any suitable rec:ycled proc~eQ~ wat-er o~ the f ac~o~ .
Therefrom the -~olution i5 routed via an ec~salizin~ ~a~k
(3) to a green liquor ~:larifier ~4). The slu~ge obtained
~o as t~le bottom disçharge of tb,e clarifier (4) is fil~erecl
ar~d washed in a green lic~uor filter (~)~ The liquid
~iltrate t.hus ob~inf~ can ~e u~ed from~ e g. ~ a ~uffer
storage ~ 6) for dis~solYin~ the Qmelt, while t:he ~olids
are removed 2g waste ( 7 )
2s
The ~:larii~ie~ green lit~u~r i routed toge~her with 1~ me
(8~ O~t~neA from the rotary l~me kiln, po-~2sibly
c:o~le~ented with makeup }ime ~9), ~n~o a l:Lme slakin~
tan3c (10 ) . The calc:ium hydroxide ~ormed in the lime
30 slaking rea~:tion reacts wit~ sodium c:ar~onate to forn
sodium hydroxide and ~alc~ arbonate, ~hat is, lime
sludge. The latter reaction takes place in causticization
tanks ~11) loc:ated after the lime sludge slaker (lO).
In the next pro~e~ ~teps accor~ing to the in~ention, the
calcium ~ar~onate and sodiu~ hydroxide recei~ed from the
DEC-20-1994 lZ:37 FROM OY HEINRNEN ~ TO 990161~ 8~1 P.13/28
W O 94/01617 2 1 3 9 7 ~ ~
cau~ticization t~ks are routed to a f~ t:er (1~), where
the solids cc~nten~ of ~he c~e ~o~med durin~ filtr2tion
is increa~ed ~y compre-~8ing the cake with the help of a
pressurizec} med~ act:Lng via a ~er membrane. The fil-
trate obtained from the filter is white liquor, which is
rollted ~ia a white liquor stora~7e tank (13) to t;he
dis~e~er ~
S~seque~t to the co~p~ession ~3~age, the calce 801id~ con-
10 tent i~ approX . 75 wt-S~ at the star~ of the W:~ ch; '''~ step .
Con~e~uen~ly, t~e wa~hing 8tep can ~e c~rried out usi~g a
very 8m~11 amOl~l't: of wash ~Tater whe~ comparecl to conven-
tional tech~iques. ~n the me~hod ac~ording to the inver~-
tion, the ~mount of wa--h water requi~ed is in the ra~s~e
of 0 . B-l m3 w~s~t ng water per ton of d~y llrne sludge .
After w:~3h ~ng, the -~olids calce i~ dxied by repea~ l;he
compre~Qion st~p via ~he r~be~ mç~ra~e in the ~o~re-
deQcri~ed ~ n~er, complemented ~ith add~ tional d~y:~g by
20 blowing com~resQed gas ~ypicall~ cG~r e-~-Qed air) through
the cake. The ~ma11 amount of riltrate o~tained from the
w:~qhin~ ~tage and ~3ub~equent drying !3t;a~s, c:om~rised of
w~ite liquor~ is routed into t.~e ~hite liquor cirCul atio~
( 13 ) and the~e~rom ~urther into the di~ester .
After drying, the 1ime sluclge cak~s are r ~ov~d from the
~i~ter into a !3torage tank ~14), wherefrom t~ey are
further conveyed into a rotary l ime kiln (15) . In ~he
rotary lime ~ciln (15) the c:al~ m carbonate ~ s in~::iner-
30 a~ed a~d ~he ca1cium oxide ~-b,~s formed is routed to a
lime storage t~nk (8) for u~e in the cauctici~ation
rea~tior~ ~
In practice, the a}~ove-de~cri~ed method entirely prevents
35 pre-exis1~ing sodium hydroxide ~rom reach~ ng the c;austici-
zation reaction, thus si~nificantly impro~ring the
DEC~ 1994 12:37 FROM OY HEINRNEN RB TO 990161 ~;~.sl~8~3~1 P. 14/28
WO g4/01617 2 13 9 7 ~ ~ ~C~/F1~3/00~87
reaction conditions. Owing tc: t~e e~iciency of th~
er empl oyed, most advan~ageouLly a pressurized
fi}ter, re~uc~e-i water cor~ tion is at Pine~ and wAshing
can be per~o~ed using proce~ waters obt~ ~A from,
e . g ., ~he e~aporatio~ stage .
For those ver ed in th~ art it i~ e~icleslt that the dif~e-
rent ~le~entations oi~ the invent~ c~n are no~ ted by
~he exem~lifying em}~otiment descri~ed a~ove, bu~ instead,
10 it can be varied within the c:ope of the ~n- r~ed cla~ms .
