CA2139766A1 - Method for filtering a product formed in causticization - Google Patents

Abstract

The invention concerns a method for filtering a product formed in a causticization process, in which filtration process the product is subjected to separation of the filtrate from the solids. The product received from a causticization tank is routed to a filter, where the filtrate is separated from the product to be filtered and the solids content of the remaining solids cake is increased by drying the cake, after which the cake is washed with water and dried. The invention is characterized in that the filtrate separated from the product in the filter is combined with the washing filtrate obtained from the washing and drying steps of the filtration process, and the primary liquor this obtained is routed into a digester for use as cooking liquor.

Description

D~c-20-1s94 12: 34 FROM OY HE I NRNEN RB TO 99016132308821 P .08/28 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 ~C-20-1~4 12:~ FRO~ OY HEIN~NEN ~ TO 9~161.~ 08R~1 P. 11/28 WO 94/01617 PCI~/FI93/l~

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 .

Claims (6)

Claims:

1. A method for filtering a product formed in a causticization process, in which filtration process the product is subjected to separation of the filtrate from the solids in a manner in which the product received from a causticization tank (11) is routed to a filter (12), where the filtrate is separated from the product to be filtered and the solids content of the remaining solids cake increased by drying the cake, after which the cake is washed with water and dried, c h a r a c t e r -i z e d in that the filtrate separated in the filter (12) from the product is combined with the washing filtrate obtained from the washing and drying steps of the filtration process.

2. A method as defined in claim 1, c h a r a c t e r -i z e d in that the filtrate removed from the product in the filter (12) is white liquor and the washing filtrate obtained from the washing and drying steps of the filtration process is weak white liquor.

3. A method as defined in claim 1 or 2, c h a r a c -t e r i z e d in that the filtrate proper and washing filtrate are routed either separately or combined into a storage tank (13) and therefrom further into a digester for use as cooking liquor.

4. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that the amount of wash water used in the filtration process is in the range of 0.8-1 m3 of wash water per ton of dry lime sludge.

5. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that the primary liquor formed from the filtrate proper and the washing filtrate has a concentration in the range of 90-110 g/l (Na2O).

6. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that said filter (12) is most advantageously a pressurized filter.