CA2116974A1 - A method in ozone bleaching of pulp - Google Patents

Abstract

When closing the process of manufacturing cellulose pulp of high brightness, i.e. including the recovery of essentially all waste liquor, there is an untenable enrichment of certain basic elements, such as sodium and sulphur. The present invention provides a partial solution to this problem and is concerned with a method in bleaching with ozone lignocellulosic material (cellulose pulp) which has been at least partially manufactured chemically, in the presence of water at a degree of acidity, expressed as pH, of 2-5. The method is characterized in that the acidity is maintained during the ozone bleaching process by adding a sulphuric acid solution or an acid sodium sulphate solution produced by electrolysis (or electrodialysis) of an essentially neutral sodium sulphate solution obtained by recycling chemicals in a system to which rest solution (waste liquor) from the ozone bleaching process is added.

Description

W0~3/~6297 PCT/SE92/0063~

A method in ozone bleachlnq of pul~

.
Technl~al field The presen~ t nvention relates to a method ~n bleaching of llgn~celluloslc material ~cellulose pulp) wlth ozone.
The ~ellulose pul~ can ~e produced both chemically:and chem~mechanlcally. Chemical eellulose pUlpB are a clearly defined category of eellulo~e pulps a~d are comprised, for ln~;tdnce, of ~ulphite pulp, sulphate pulp and ;po1ysulphid~
pulp. Particularly suita~le chemlcal cellulose ~pulps are tho~e tha~ have a hl~h vi5cosity at relatively low Kappa 1~ numbers. ~e~onglng to ~th1s group~ o~ cel~lulose pulps: are sulpha~e ~u~ps ~hieh~have~been ~cooked or dig~sted at a hlgh sulphldity, ~nd modlfled sulphate pulps that have~ been cooked o~ di~e~ted:in accordance w~th a counterflow method in whicb ~hite l~quor is added also durlng an advanced stage of the co~king proces~, and alka~lne~ cellulose: pulps:which a~e eooked in the presence of one Dr more catalysts, for i~s~ance ~ ~he p~ese~ce of a qu~noQe compound:, such as anthr~quinooe. Other sulta~bl~ celluloBe pulps are those having the d~signations M55-AQ ~mlnl - sulphlde :sulph1te -anthcaquinone), Si-Sa-S1 (sulphite-sulphate~-sulph1te~ and PS-Si (poly~ulphlde-sulphlte~ aIl of which ar~ r~eferred ~o ln the ~our~al ~Pape~i ~a Puu, 5Jl9~B9, ~p. 509-5l3.

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WO 93/06297 PCT/SE92/0063~

211~9 1 4 Particularly sultable chemimechan~cal pulps are those in which the ~igest~on or cook~ng process ls relatlvely far reaching and is followed by a mechanical defibering process.
The ozone bleachlng proces~ can be applied to unbl~ached 5 ~ellulose p~lp oJ~ celllllo~e -pulp w~ich has been delig~lf ied/
bleached $n one or more stages.

Pre~ent st~ndpoi~ of teçhniques In recç~nt ti~nes, ths use ~f chlo~lne as a bl2aching agent ~n ~he delign~fication of cellulose pulp has de~rea~ed 1~ dra~;tl~a~ly ~or ens~ron~nen~al rea~oQs. In3t ad, chlorine-fre~ blea~hlng chemicals',~ ~uch a~ o~yg~n and hydrogen p~roxlde, have bee~ u~ed ~o bleach cellulo~e pulp. Ozone bleachiny ha~ also become re1 e~rant ~Ol appllcation on a full s~ale, 1 . e . f D~ use i n practlce . The use of ozone has 15 pre~iously ~een inhibited amo~g other ~h~ngs because ozone as a bleachln~ agent ha~ ~ot be~en ~uf f ~cien~ly selectlve, i.e. the carbohydra~es of ~he cellulose pulp have been seriou~ly damaged from the at~ac~c of free radicals, parti-cularly at th~ large ozone additions ~10-15 kg o~one per 20 tonne of pulp), that are of ten requlr~d by ~uch processes .
It has been reallzed that an ad~ranltage ls af f o~ded in the manugactute of ~ e.-g~, chs~ical eellulose pulp when the pulp is sub~ected to an extensitre, or ar reachl~g. d1~estlon process, i . e . are digested to an e:ctent such that the lignin 25 co~tents, ~xple~ed in ~appa numbess, wlll ~e much lower tharl has earlier been usual and~or to greatly reduce the lig~ln content of the cellulo~e pulp in a delignifying stage, for instance an oxygen blea~hlrlg stage following the ~ooking process. Sir;~e the c~llulo~e pulp to be ozone 3~ ble~ched has a relatlvely low li~nl~ content~ it ls posslble .... ..
to use a compara~lvely ~mall ozorle charge. Fur~hermo~e, 1~ ~
ha3 been found po~lble to ~ontrol the fo~mation of free ~:
rad~cals by the ozone, by ~aintalnin~ the ozone t~eatment ~ ~
p~oce~s a~ a low ~H" low te~perature a~d a rslatiYely high ~-35 pulp ~o~istency ( Por instance, a co~siBtency w~thin the rang~ of 10-35%3 and by addiny radical cap~urers.
The desire ~o ma1ntain a low pH in the ozone bleaching 3 211~97~1 process, i.e. a high de~ree of acidi~y, particularly follow-lng an alkaline ~ooking process, w~ll put hlgh dema~ds on the wa~h~ng equipment for removlng of cooking liquor from the cellulose pulp and optlonally for removing of oxygen 5 blea~hlng ~pent liquor from the cellulose pulp. Despl'ce thorough washlng of ~he cellulose pulpo ghere i5 a certain amourlt of chemlcally bound alkall in the ~ellulose pulp and that alkali consumes hydrogen lo~ls when acidifylng the cellulose pulp priCI to the ozone treat~ent process. In 10 addition comes the n~ed of acid ~ydrogen iOI~S~ in ~rder to ~bta~n the hydrogen i~n co~entration deslred when bleachlng the cellulose p~}lp wit~ ozone. Normally, only sulphuElc acid ls ~on~ldered for~acid~f~cat~on of th~ ~ellulo~e pulp. The u~e of, for ~ns~an~e, hydro~hlorlc acld may r~sult ln en-15 l~chment of chloride in ~he chem~cal recovery system, there-by creatlng eorrosion probl~ms.

D~ sclosure o~ ~he inventlon i The waste li~uors derlving from the f illal pulp bleaching 20 stage have hitherto not g~nerally been returned ~o the c~emlc~ r~o-.rery sy~t~m, but they have ~een dlscharged to the reclplent. The ever ir~creaslng demands pl~ced on emissions to the environment have meant that the release of thQ~e liquors to tle recl-pierlt ~ust be re~uced to a mt nimum .
25' This ~lso applles to relatively harlale~s compound~ ln the form, for ln~tan~e, of non chlorlnated but oxygen consuming substances.
When ~ub~tantially all waste li~uors from the bleaching proces~ a~.e ~eturned to the che~lcal recovery ~y5t ~
30 ~um~er of basic substances, ~uch a~ sodium alld sulphuE, become e~riched. Some of the~e ~ub~tance~ must then be e~ected from ~he sy~em, elther to alr or water, in ~ome form. In the worst of cases, thls ~ result in harm to ~he en~iro~ment, for ~rlstarl~e when sul~hu~ 1~ e)ected in t~e 35 form of sulphur dloxide and/or hydrogerl sulphide. In other ca~e, for lns~ance, sod~um sulphate is obtained for whlch a .. .. .. ... .. .. . .. .... ..... ........ .

WO 93/06297 pcr/sE92/oo63s 21~9~ ~ 4 ~arket ~anrlot be readl ly f ound .

Solutio~
The pre3en~ inYentlon ~olves this prob~em pa~tially and relates ~o a ~eth0d in bl~aching wi~h ozone lignoc~llulosic mater~al (celi~ e pulp3 ~rhlch h~s been p~oduced at least partlally by chemical proce~es, in ~he pre~ence of water and at an ac~d~ty, expres~d as pH, of 2-5, The method ls characte~lz~ in tha~ the acldlty ls sustained du~ing tbe ozone b~eachlng proce~s by addl~g to ~he system elther a 1~ ~u~E~hu~ acid ~olution o~ an 2cl~ ~od~u~ ~u~phate ~olutlon prepared by e~e-ctrolys~s (or elec~rodialy~s) of a ~ubstan-tlally n~utral ~odium sulphate 801utlon obtained by recyc-limg ~eD~a~ a ~y$tem to wh~ ~h ~e~t solutlon (wa~te liqu~r) de~lved from the ozone bleachil~g process is pa~s~d.
A~ord~ ng to the i~ven~on, ~odium sulphate ls taken fr~Dm the ~hemlcal recovery ~ystem at some appropria~e loca-tloh. An exar~ple of an appropriate location ls the electr~c p~ecip~ta~oE in the recovery boi:Ler lnclud2d in the ehemical r~overy sy~i~e~, whele the separated -du~t conta~ns essen-~lally ~odiu~ ~ulphate and m~noT quantitles of sod~um car-bo~at.e and sodium chlorlde.
Ac~ording to o~e pr~ferred ~bodiment of the inventior the waste l~quor f~om the oz~ne bleaching process i~ mixed ~r~th o~heE was~e liquor f rom the pulp manuf actur~ng process, th~ waste ~iquor be~ng re~overed and re~u~n~d to the evapo-ratio~ a~d ~o~abustio~ pla~t ~the recovery boiler) of the che~ al ~y~e~n. Par~ of the ~odlum sulphake colle~ted 'I n the electric precipltator is removed therefrom and dlssolved ln ~a~r and the ~bsequent solution is then sub)e~ted to an elect~lysl~ or an elec~rodlalysis process.
~ ccordlng to another preerred embodiment o~ the ~nven-~ion, ozo~e bl¢achin~ ~a~te llquor 1~ mixed with waste liquv~ f~om an alkaline tceat~ent stage, for instance in the ~leachl~g depar~ent, so a~ . o obta~n an ess~ntlally neu~al ~olutio~, whlch is then ~ooled ~o that ~odium sulp~a~e (Na2S0~ ) ~ 5 preeip~tated. this 50dium sulphate precip~-~a~ belng ~p~at~d and ~ed awa~ and dlssolved ~n water and WO 93/06~97 pcr/sE92/oo635 5 211~74 the result1ng ~olutio~ i8 then ~ub~ected to an Qlectrolysis or an elec~rodlalysis p~ocess.
The remaln~ng mothe~ 1 lquor, which con~alns essential ly organlc materlals, i~ tra~sferred to the ehemical rscovery system. ~hen ~che sodlum sulphate 50~ iOn contains ~etals, su~h as cal~ium, magneslum or ~angarle~e, in harmful qua~ltl-tles, it ls approprlate to treat the solution with a hydro-gen satu~ated cation exchange~ prior to the ele~trolysis or electrodialy~s stsge.
?0 The sodlum sulphate solut~ons obtained, for instance, in the aforedescribed ~anner, are ~ub~cted to electrolysis or electrodialysis in a cell whiCh is pro~rlde~ wi~h anode, cathode and orle or more membranes.
The treatment process ~an be ca~ried out ln at least three ways. Ac~ording to the ~implest alternative, the el~troly~ls ls carried out ln a cell that ls provlded with anode, cathode, and a catlon ~elect ~ ve membrane, whe~ein ~odium sulpha~ ~oîution is del~vered to the anode chamber and ~ater is deli~rered to the cathode chamber 50 that an ~ ac~d solution i~ for~ed and oxy~ean gas is generated at the anode and so that a ~od~um hydr~s:ide solutlon is f ormed and hydrogen gas i5 generated at the cathode.
hccording to anogher alternatlve embodiment, the electrolysls is car~led out in a ~ell that is provided with ~node, cathoàe an anion selective and a catlon selectlve mem~rane, wherein sodlum sulphate solution ls introduced between the two membranes and water is introduced to the an~de chamber and the cathode chamber respectively, so that a sulphuric acld solution i~ formed and oxy~en gas generated 39 at the an~de and so that a sodium hydroxlde solution ls forr~ed and hyd~oge~ gas gerlerated at the cathode.
accord~ n~ to a thl~d, more advanced alternative, ~he treatment ls carrled out in a mult~ chamber cell that is p~o~ided with anode, cathode and anion selective, cation ~electlve and b~polar membranes O whereln the anode chamber ~5 delimited by an anlon sel~ctltre membrane and thQ ca~hode cha~ber is del~ d by a cation ~elective membrarle, and whereln b1 polar membranes arP d~ ~posed between sald WO 93/06297 PCr/SE92/00635 21iL Ç) g ~ ~ 6 ~e~branes, and w~ere~n 30dium sulphate ~olution 1~ deliYered to tho~e cha~bers that are deliraited by blpolar membranes, ~o tha~ a ~ hu~ cid ~olutlon i~ :for~ed in at least the ~aembra~e chamber ~hich 1~ pla~ed nearest the cathode and 5 hydEogen ga~ is ge~erated at said cathode, and ~o ~hat a ~odlu~ hydrox~ de ~olutior~ i5 f ormed at least in ~che membrane cha~ber which 1~ plaeed nea~est the an~de, arld oxygen gas 1 generated at sald anod~.
Accordlng to ~s thlrd al~ernati~le, oxygen gas and 10 h~lrog~n ya~ a~forla~d ln ~alle~ qua~ti~es ~han in the tsdo preeedl~g ~ e~atives, slnee the~e ~lectrode reactlons do ~ot ~alce pla~e at the bipola~ membrane~ Thls thl~d method ~5 re~e.rr~d to a~ el~ctrodialys~s~
The f ir~t ~nd ~he second alternative methods, 1. e . ~he 1~ laeth~às which des~rlbe conventio~al electrolysis, are par~l-cularly preferred because of t~e~ r simpliclty.
The acld ~lution (sulphurle acid solution) and the sodlllm hydrox~Lde ~olution respec ~ ely f ormed in the af ore-~ald cell~ ar~ ~e~oved f~o~ the cell indlvidually and the 20 acid ~olut~on (~e sul-~huric acid ~olutlon~ is delive~ed to the c~llulo~e pulp B0 as to o~ta~n the acid~ty ~equ~ed and desired ~or the ~one bleaGhing process . Sodlum hydrsxlde ~ s a}so a valuable chemi~l ~hl~h can be ~edJ fo~ i~stance, in the alkallne stage of the ~ellulose pul~ trea~ment p~ocess, 25 this stage cften foll8~ing the ozone ble~chlng stage. If desîred, the ~ntrinsically ~aluable chemicals oxygen and s hyd~ogen can al~o ~e collected.

AdvantaqQs The in~ ntion enables the con~umption of ex~ernally 30 produ~ed ~purcha~ed) a~ld and ext~rnally produc~d (pur-cha~2d) alkall to be greatly r~duced ln the ~anufa~ture of cellul4~e pulp that ls bl~ached with ozone. The 5econdary p~odue~s o~ygen ~as and hyd~o~en ga5 can be used in the cellul~e ~ulp ~allufa~tur~ng p~OCeB5, fOI ~X~IIple for oxygen 35 ga~ each~llg aad ~tQam ~ erat~on ~e~ectlv~ly. The inven-tlon ~ri~lly l~ad~ to the po5~ibility of ~a5tering the e~v~orlmental problems in a praeticall~ ~o~plete clos~ng of WO 93/06297 PCI /SE92/0063~
7 2 ~ 7 ~

the pulp manufacturing process, l.e. also lncluding îinal bleachlrlg waste liquo~s. When ~ommerclal sulphuric acld is u~ed to ach~e~re the neces~ary ac~dlty of the cellulose plllp ln an ozone blea~hlng proeess, i~ would mea~, fo~ lnstance, that 12,000-18~,000 tonnes of s~dium sulpha~e must be e~ected per year f rom the process in asl average sized sulphate pulp ~111, when thls alternative 15 chosen f or maintalning a ~hemi~al b~lance ~ n the ~ys~em .

~ '.
1 D Flg~lre 1 15 a f low sheet ~hlch lllus~rates a pulp manu-fa~turlng proc:ess ln whlch a 1rst embod~m~nt of ~che inven-tion ~ luded ~nd descrlbed, ~hl~., e~bodlr~ent includlng m~x~Lng toge~her ozone ~leachiny waste 1 iquor and some other alkaline waste liquor.
~igure 2 iïlustrate~ a flow ~,,heet for a pulp manufac~ur~
ing p~oces~ in which a second embodiment of the lnvention is lncluded arld described, thls em~odllaent including returning ozone bleachlng waste 1 i~uor to the chemical recover system of the pul p ml 11 .
~igu~es 3, 4 and 5 illust ate in detail a partial stage of the 1nvention, namely in Flgure~ 3 ~nd 4 the electrolys~s of the sodium sulphate solution produced, and in ~igure 5, ~he electrodialysis of ~he so~1um sulphate solution produced.

Bes~ modes of ~arryin~ out the inve~tion 25~ Preferred embodiments of the lnvention are desc~ibed in the following with reference to the drawings. Two exempll-fying embodiments are also descrlbed. In connection here-th, the descr~ptlon include~ ~o~e detailed informatlon as ~o how the ~nventive ~hod 15 c~rried out.
Ac~ordlrlg i:o Figure 1 plne chi~s are dellvered throug~
the condult 1 ~o the dlge~ter ~, whlch contsin~ a cooklng liquo~ composed essentially of sodium hyd~oxide and sodlum sl31phlde. Subsequent ~o d~gest~ny the wGod a~d ex~oslng the f lbles thereof, thQ exposed f lbres - the pulp - ls transpor-ted through the ~ulp cor~duit 3 (whlch extends through the ent~re pulp mlll3 to the washlnq ~ection 4, where ~he ma~or WO 9~/06~97 PCr/SE~2/00635 2~ 6 9~ 4 B

part of ~he consumed cooking 1 ~qu~r ( ~he cooklng waste llquor or the black llquor) is extra~ted f~om ~he pulp. The ~ook~ng wast~ llquor ls tran~ported through col~duits 5 and 6 to the chemical recoveEy sys~em of the mlll, this system 5 lncludlng, a~nong other thlngs, an eYaporation plant 7 and a combu~tlon plant ~r~co~rery boiler ) ~, wh~ch is provided with an eleetrlc ~reciplt~or ~.
Subsaqllent to washint3 of the pulp in the washing depart-~erlt 4 and ~ubseq~en~ to op~ional screeni~g of the pulp (not 1 0 sho~n ~n th~ dràwlng), the pulp 1~ pas~ed o an oxygen ~leachlng stage whe~e the pulp is ~upplied ~lth oxyge~ gas, through the conduit 11, arld also wlth alkali, normally sodlum hydroxlde, ~nd optionally al~o with a protector. The oxygen bleached pulp i5 pas~ed to a w~shl ng stage 12 ~n 15 ~hich the pulp ~s freed from the ma~or part of the waste liquor derivi~lg from the oxygen bleachtng s~age. The afore~
said t~eatment p~oce~s i5 ca~r~ed ou~ a~vantageously ln a ~onventlonal press or in a wa~hlng press ~o wh~ eh washing liquid ls added. The eulp is brt)ught to the deslred con-2~ slstency for the ~ollo~ing ~one ble~ching process ~f ~hepulp, by ad~ustlng th~ pr~ssure at which ~he pulp is pressed. The oxygen b~eachiny wastP ~iquor pressed from the pulp ls pa3sed throllgh the condults 13 and 6 to the traàl-ti onal ch~m~cal recov~y system . Part of the oxygen bleach~
25 lng waste liquor can be re~urned to the bottom of the oxygen b~each~ng reactor 10 through the conduit 14. The pulp9 which - ~ has a compara~ively lligh pulp cDn~ls~e~cy is then dellvered ~ the raixer 15 ln which an acid sodlum sulphate solution is added to the stlll alkaline pulp, this sodium sulphate solu-3~ ~ion belng p~epared ln the electrolysis ~ell 16 and tran-~ported through the co~du~t 17. The acid ~olution i5 added ~n a quan~i~y whlch will ensure that the pH value of thP
pulp/water mi~ture will ~e ~uitable for the o~one bleachlng of the pulp, i . e . within the range 2-5 .
5~hen the pulp leaves the mixer 15, it may, for lnstance, have a medlum pulp ~ons~s~ency (10-15%) or a high pulp conslstency (30-35%), depe~dlng on requiremerlts. The pulp is then delivered to the ozone b}eaching rea~tor lB to which 9 21 1 6g 7~

oxygen gas containing a given low ozone content is deliver-ed, ln addition to pulp, through the co~dult 19. The ozone blea~hlng s~age is maintalned at a comparatively low tem-perature, for lnstance a temperature wlthin the range of 40-60~C. The pulp reacts co~plet~ly wlth the ozone ~n this ~age, and resldual oxygen gas 15 led from the reactor thr~ugh the conduit 20, ~OE tra~6p~rtat~n to an ~ppa~atus fol generating fresh ozone (no$ show~ ln the Flgure). The ~ulp ls then t~anspo~ted ~o a further washlng ~tage 21, for 1~ ~nstance in the form of a wa~hlng pre~s. The OZO~Q, bleaching was~e ~lquor recovered ls pas~d back to the system, through the condult 22. The waste li~uor i~ dlvlded i~to two flows, of whi~h on2 flow ls pa~sed through the conduit ~3 back to the ozone bleachlng reactor 18 for the purpose of flushing pulp f~om the rea~tor. The other flow is passed to a mixing and ~rystalltzing vessel 25, through the conduit 24.
The pulp is then pas~ed to a m~xer 26 in which alkali, essentially sodlum hydroxide, i5 added to the pulp. A ~igni-f~cant part of t~e alkall ls recovered ln the form of a ~odium hydroxide solution ln the electrolysis cell 16, and the solution 1s added to the pulp ~n the mixer 26 through the condult 27~ The remainder of the sodlum hydroxide required ls supplied externally (pu~chased alkali) through the co~dult 43. The pulp is then passed to an alkalizing or extraction tower 2~. In addltion to using a plain alkali stage ln this stage of the process, it ls possible, and even ad~antageous, to supply al~o oxygen or ~erox~de o~ both of these chemicals. Flnally, the pulp is washed ln the washlng stage 29. It is quite posslble to termlnate the pulp manu-facturing proces5 at this stage a~, for instan~e, to tran-~port ~he..pul~ to a ~aper ~ill and~or to a dewate~ing and drylng machine for the ~anufacture of comme~cial pulp. How-e~er, it is often desired to increa5e both the cleanliness and th~ brightness of the pulp by unde~takin~ one or more addltional bleaching stages wh~le uslng bleachi~g chemical~
such as, e-gD ~ dithio~ite, peroxlde, chlorlne dloxlde or additional ozone.
~ hen the bleaching process is terminated at the afore-WO ~3J062g7 PCI`/SE92/0063~
2~.~69 1 4 lo ~ald alkal~ stage, clean water ls normally added to the pulpthrough the eondult 30. The ~lkali~e wa~te llquo~ ls return~d in the ~y~tem through the conduit 31. The reco~ered waste llquor ~ s dlvid~d into three parts . A first part is 5 added through t~e ~ondult 32 to the mixer 26, where lt is mixed wi~h the pulp so as to increase it~ alkal i content . A
~econd part is delivered through the condult 33 to the con-dult 24 where the alkallrle wa~te liquor ls mlxed wlth the acid ozone bleachlng was~e liquor ~o that 1;he waste liquor 10 m1x~ure deli~rereq ~ to ~he crystallization ves~el 25 is generally ~eutEal~i i.e. ha~ a pH of ~ or a value in the vi~inlty thereof . The re~aining quantlty ~f waste liquor is pa~f~ed t~ough the ~onduit 31 to the pulp washer 4, im~e-dlately downstream of the dlgester 2.
The waste liquor mixture in the vessel 25 is cooled to a temperature of, e.g., 15C or therebelow. A ma~or part of the sodlum sulpbate presen~ ln the wa~te liquor will then precipi~ate in the forr~ of crysl:als, whlch ~ettle on the bottom o~ the ves~el 25. The cr~st~ls are kherl passed ~ through the eo~duit 34 to a washing filt2r 35, wlth the aid of an appropriate fe~ding out device. Th~ waste llguor -mother liquor - freed from the sodlum sulphate crystals ln the vessel 25 is discharged through tbe ~o~duit 36 and mixed ~ith oxygen bleachlng waste liquor and co~klng waste liquor 2s (bla~k liquor~ for transportat~on lnto the e~aporator plant 7.
The sodium sulphate ~rystals are washed and cleaned on the f llter 35 with a ~mall quant~ty of liquld, and are thereaf~er passed to ~he ~is~ol~ y vessel 3B through the 30 eondult 37. The llquld rer~ove~ from the sodlum sulphate crys~als on ~he filter 35 ~ for lnstarl~e, be introdu~ed ln ~he conduit 36 (~ot 5hown in the Fi~ure), Preferably, cl~an wa~er iL5 pas~ed through the condult 39 to the dls~olv-ln~ ves~el 38 in a quantity sueh that es~entially all sodlum 35 ~ul~hate cry~kals ar~ dls~ol-~red. The dl~solving process ls f~cllitated by usin~ a t@mpe~atuEe which i5 sllghtly higher than room tempera~ure and whlch, for instancev reaches 35C.
The solutio~ eor~talrl~ng a lar~e quantlty of dissolved sod~tlm WO ~3/0629'7 PCI`/SE92/0063!;
ll 211697~ . ~
fiulphate ls tran~p~rted ~hrough ~he - conduit 39 to the anode chalaber of the elect~oly~is ~ell 16. Preferably cl~an water i~ dellvered to the cathode chambe~ in the electrolysis cell 16 through the c~nduit 40. During the proce3s o~ ele tro-5 lysis, there ls f or~d at the anode an acld solution whlch1~ recove~ed ln the ma~ner earller de~c~ibed, whlle at the cathode there ~s fo~med a ~odium hy~roxlde solution ~h~ch is r~covered tn tAe aforedescrlbed manner. Oxygen gas is al50 g~erated ln the anode cha~nber, this gas being led away 1~ throug~ the conduit 41, blhlle hydrogen gas ls gsQerated ln the cathode chalnber a~d l~d away ~hrough th~ conduit 42.
In ~igure 2 is a f lo~ ~h~et shown, ~hich coincides totally wlth ~he f low ~heet of ~igure 1 wlth regard to the pulp manufacturlng (and pu~p reflnir~g) process. A l~rge part 5 of t~e liquid and waste liquol transportation 15 also ef ~ected in dlrect ag~eeme~t with the two f low sheets Con~eguently, those reference siyns u~ed in F~ yure 2 tha~
are ln agreement with t~e FigurP 1 illustration are used with the addltion of ~50 except w~ th re~pect to those few 20 d~vlations whi~h exi~t when ~aking a ~ompari~on ~etween the two embodiments of the invention.
In order to avoid unnece~sary repetition, only those par~ stag~s ~n the flow shee~ of ~lgure 2 which differ from what is shown i~ the f~ heet of E`igure 1, will here be ~5 descr ibed and co~mented on .
~ cc~rdlng ~o this embodiment of the invention, the waste lt quor 1~ conducted f~om the ~ashing stage 71, which is loca~ed i~medlately ~ownst~eam of the 020ne ~leachlng stage 68, through the condui~ 72 and 74 to the condult 63, where 3~ ~he mentio~ed ~zorle bleaching waste li~uor ls mixed with oxygen bleachi~g ~aste 1~ r f rom the ~ash~ng ~ pres~ ) stage 62. $~e mixture ~o~c~rrled i~ trarlsported to the ~vaporation pl~nt S~ through the condult 56. The ~a~te ïlquor concentLa-ted by evaporation, ~he thick l~uor, ~s then passed to the 35 r~covery boiler 58 in which lt is burned. The organ~c ma~erlal cont~nt of the thick llquor is now conveEted to carbon dloxide asld ~rater ~ whereas its inorganic content i8 e~entially recovered as sodium carbonate and sodlum WO 93/06297 PCr/SE92/00635 ~llG9~ ~ 12 ~ulphlde ln the bottorQ part of the recovery boller. Howeve~, a signlf ican~ part of the inorganic material accompanies the f lue gases and is separated a~ 30dium ~ulphate ~n the elect~lc precipltator ~g. All or a part of the sodlum sulphate recovered is pa~sed in powder f orm to the dissolv-ing tan3c 83, through the condult 75 . Pref erably, clean ~rater ls added through the condult 8~1 ln an amount such ~hat all ~odium ~ulphate will be dissolved. If it ls nece~sary to ~urlfy ~he ~olution~ from 3Eoreign chemicals, this can be effected ~n a ~ollow~ng treatDIent ~tage ~ot ~hown ~n the Figure). The ~esultant ~odlum ~ulphal~e ~olution i5 transpor-ted through the conduit 85 to ~he anode ~ha~ber of the ele~-trolysl ~ ~ell 66 . Pr~ferably eiQan ~ra~er is introduced to the ca~h~de char~ber Qf ~he electrolysi~ cell 66, thcough the ~onduit 86. Oxygen gas for~e~ ln the anode chamber is led away through the ~ondui~ 87 and hydrogen gas f o~med in the cat,hode ~hamber ls led away t~rollyh the condult 88. The acld ~olut~on genera~ed in ~he proces~ of electrolysis ls tran-seorted ~hrough ~he cn~duit 67 to the mlxer 65" for 2~ nece~sary acidif lcation of the pt~lp prio~ to the pulp comlng into ~ontact with ozone in the blea~h~n~ stage 68. The ~od~um hydroxid~ ~olutlon produ~ed in the process of electrolysl~ is tran~ported through the conduit 77 to the mixer 76~ in wh~eh the pulp is made alkaline.
These two flow sheets illustrate applications of the inYe~t~on when bleaching sulphate pulp wlth ~ne, thls pulp ha~ing b~en ~b~ted to an oxygen bleaching process prior to the ozffne bleaching stage. Reference has earlier been ~ade to sultable fi~al bleachtng ~tage or stage~ when desir-~g to further incr~a~e the c~eanllne5~ and brlghtness ofthe ~ulp. Sim~arly, it i5 posslble to in*rodu~e further de-lignifylng and/or bleacht~g stage~ between the dige~lon ~tage, i.e. ~he ~a~ufacture of the original pulp, and the 020~e bleachl~g ~age~ Advantageously, an aeid treatment ~tage c~n be in~lu~ed 9 for ln~ta~ce by allowi~g the ~ulp to reset ~ith a ga~ that eontalns ~itrogen dloxide (thls pro~
ce3s is ~nown as PR~NOX),- immediately prior to the oxygen ble~chi~g stage. In addltion ~4 an oxy~en bleaching ~tage in WO 93/06297 pcrtsE92/oo63~ :
1 3 :
21I 6!~7~
the mentioned posltion, there can be applied a pla n alkall stage, or a lperoxide reinforced alkall stage, or an alkall st~ge whlch ~ reinforced w~t~ both p~roxide and oxygen.
Irre~pecti~r~ of whet~er an a~ld treatment stage, f or ln 5 stance in accordance wi~h the PRENOX method, i5 introduced ln the mentio~d po~itlon or rlo~, it may be benef icial to treat (blea~h) the pulp ~ith ~hlorine dio~lde4 wil:h or without interrQedia~e wa~;hing of the pulp, immediately prior ~o ~he 020ne ~leachlng stage.
A~ pr~viously ~nention~d, the ~nventiv~ method 15 in no ~ay li~ ed to the 020~1e blea~h~ng of su~phate pulp, but can be appl~ed to the ozorle ~lsacil~ng of any c~emlcal pulp ~ha~-soever and al~o ~o the o~one bl~aching of chemlme~hanlcal pulp. ~hen bleaching such pulps, ~he bleaehing se~uen~es ean be quite dlffer2nt to the aforede~crlbed and mentloned blea~hing sequences. ~7lt~ regard, for il stance, ~o sulph~te pulp, thi~ pulp will have after the dige~tion stage a much lower lignlr~ conten~ and a mui h h~gher ~rightness than sulphate pulp, which m~aIls ~hat only t~o bleachlng `s~ages and at most three ~leachlng stage!;, of whlch one ls an ozone blea~hlng stage, - are requi~ed in order to ~btain a h~ ghly clean and brt ght pulp .
In flgure 3 ~how~ in more detail the construci:lon of an ele~troly~is c211 - simllar to that shown in f igures 1 and 2 ~5 ~nd lllu~rates how deco~nposition of the ~odium sulphate takes place.
The el~troly~is cell 100 is comprised of two chambers~
the anode chamber lOl and the cathode chamber 102. An anode 103 ls arrange~ ln the f irst m~n~ioned chalabe~ and a ~athode 3~ 104 ls arranged ln the other cha~be~. The two ch~mbers are mutually separated by a ~ation s~lective membrane 105, whlch wlll only allow sodium losls to paE;5 through. Sodlum ~ulphate ~olut~on is added to the anode chamber 101 through the con-duit 106 snd ~ater i~ added to the eathode chambeI 102 through the conduit 107. ~ a re~ult of applying an elect~ic voltage a~ros~ the cel 1 0 hydr~gen ion~ are f ormed at the a~ode 103 whlle g~nerating ox~rgen gas at the ~ame tlme, this ~as bein~ led away from the cell through the conduit lOBo Hydro~e~de ions are for~ed at the cathode 104 at ~he same WO 93/06297 PCI ~SE~2/0063 2~69~
e afi hydrogen gas is gene~ated, this gas belng led ~way from the cell th~ollgh the ~ondu~t 109. The ~u~rent effi~
ciency in the electrolr3is proces~ ls greatly depe~dent on the concentEatlons of hydro~en ions and hydroxlde ions 5 re~pectively in respective chambers. Consequently, i~ is not po~slble ~n this ~ype of electroly~ls cell to produce ~wo hydrogen lo~s = H+, for each sulphate ion = S0,~ and norm~lly it is neces~ary to be satisf ied wlth a yleld of one hydrogen ~on or slightly more w~h each ~ulphate lon. A part 10 of the sod1u~a ~ulpha~e is- the~efore still not decompo~ed.
Co~equen~ly, the ~olutlon r~aoved $rom the anode chamber 101 through the condult 110 ls desig~a~d an acid sodlum ~ulpha~e ~olu~lon. A sodlum hydroxlde solutloIl is remoYed f ~om the ca~hode cha~ber 1~2 through the condult 111 . The ~5 destinatlon of these solutlons wlll be apparent from the earlier desc~ptlon.
&ahen d~sirlng to produce tlwo hyd~ogen ions for each sulphate lo~,, i.e. to ploduce ~;ulphuric acld, an electro-ly~is cell 120 having thrae chai~be~s can be used, as shown 20 ln ~lgu~e 40 ~
Thls cell also ~ncludes an anode chamber 121 and a eathode chamber 122 . An anode 12~ is ar~arlged ln the anode chamb~r while a cathode 124 ~s arranged in the c2thode - chamber. ~ocated ce~ltrally between t~ese two chambers is a 25 further cha~ber 125 which is encircled by a cation ~elective me~brane 126 whlch allo~rs sodium ions to pass through, and an anion ~elective mernbrane 127 whlch allows sulphate io~s to pas~ through. Sodium ~ulphate ~olutlon is dellY*red to ~he cell through the co~duit 12B and ~5 re~ycled back to the 30 ~onduit ( in a lower ~on~entration3 through the condult 129 .
at~r i~ dellvered to ~he cell th~ou~h the co~duit 130, to bo~h th~ cathode chamber 122 and the a~ode chamber 121.
~ hen an el-ectric voltage is a~pl~ed over the ~ell l~û, oxygen gas is gene~at~d at the anode 123, thls ~as belng led 35 a~ay through the conduit 131, at the ~ame time as sulphu~ic a~id (H2S04 ~ 2H~ ~ So2 ) ls f ors~ed ln the anode ~hamber 121. Hydrogen gas is g~n~rated at ~he cathode 12~
and ls lad away from the cell through the co~duit 132, at ,~

21~ ~97~

the ~ame tlme as sodium hydrox~de (NaOH or Na+ ~ OH ) is for~ed in the cathode chamber 122. Respectlve solut~on is led away from the cell 120 through the conduits 133 and 134 and the chemicals concerned are used ln the manner earlier mentioned.
~ hen desirlng to ~educe the amount of oxygen gas and hydrog~n gas for~ed in relation ~o the ~a~or products acld and alkali ln co~pari~on wi~h tho~e electrolysis processes de~cribed with reference to Flgures 3 and 4, a sell 140 of ~he type illustrated in Flgure 5 can be used.
This cell also ha~ an ~n~de chamber 141 and ~ cathode chamb~r 142 havl~g re~pectively an ano~e 143 a~d ~ cathode 144. The anode ~ham~er 141 is del~mited vn one slde by an anlon selective membrane 145, while the cathode chamber 142 t5 is delimlted on one slde by a catlon selecti~e membra~e 146.
Arranged between these two membranes are two blpolar mem-branes 147 and between these a further catlon selective mem-brane 146 and an anion ~elective membrane 145 are arranged.
Ssdium sulp~ate solutlon is added to the anode chamber 141 and the ca~hod~ chamber 142 re~pe~tively through the ~onduit 14B; and also to each third chamber between the bip~lar mem ~ranes, which can be in~luded in a greater or smaller ~umber. ~ater ls added to remainlng cell chambers through the condu~t 149. ~hen an electr~c vvltage is applied across the cell, o~ygen gas is generated at the anode 143 and led ~way through the conduit 150, and hydrogen gas ~ 5 generated at the cat~ode 144 ahd led away through the conduit 151.
Furthe~more, sodium ions mlgrate ln a directlon towards the ~athode 144 and sulphate ions ~lgrate ~n a directlon towards ~he anode 143. The water ls dissoclated to hydrogen lons and hydroxide ions at the same tl~e. Sulphuric acid (H2S04 or ~H~ ~ So2 ) and sodium hydroxlde re5pectlvel~ are formed in thls way. Sulphuric ac~d 501ution is led away from two of the me~brane ~hambers and tran5port~d though ~he conduit 152 fo~ u~e ln ae~orda~ce with the earl~er descrip-tlon. Sodiu~ hydroxide s~lution i~ l~d away from two other membrane cha~bers and tran~ported through the conduit 153 for use in accordance ~ith the earlier desc~iption. Sodium ~ .

WO 93/06297 PClr/SE92/0063 21169 i ~ 16 sulphate ~olut~on (of lower concentration) is ~ed away from the bottom of the three chambe~s where fre~h such solut~on is added to the upper part of khe ~ell and re~y~led back to the conduit 14B, through ~he condui~ 154. The decomposition S of ~odium sulp~a~e and wate~ illustrated ~n Figure 5 is normally d~ nated ele~trodlalysls.
It wi~ll be understood ~hat other pro~esses of elec~ro-lysls re~p~ctively ele~trodialysis can al~o be applled when carrying out the in~tentlve r~ethod.
A nu~ber of experlments have been ~arried out wlth the method ~cording to the pres~nt ~nventlon. The m~Qner ~n which the~e experlment~ were car~ied out and ~he results o~talned will ~e ev1dent fr~m ~he followlng worklng examples. The experlments concerned were carried out on a 15 pilot-plant scale.

E~
The experiment was carr~ed out wlth a pine sulphate pulp having a ~appa ~umber of ~9 .0 and a ~ cosity of 1, 250 dm3/kg. The pulp was oxygen bleached at a pulp conslstency 20 ~f 12% and a~ an oxygen pressure of ~ kg/cm2~ 2% sodlum hydroxid~ and 0.3% magnesium in the form of magneslu~
sulphate ~fgS0,~ ~ h3d earller been suppl~ed to the pulp .
Upon ~ompletion of the ~xygen ~leachlng proce~s, the pulp had a ~appa number of 15.0 and a v~scosity of 1,010 25 dm3/kg. The pulp was then pressed to a pulp consistency of 40%. Inorganic chemi~ls (washing losses) corresponding ~o 12 kg sodlum ~ulphate per tollne of pulp were found to rema~n ln the pulp. An a~id sodium s~lphate solutlon was then added t~ a plllp f low of 100 kg/min . in an amount of 10 l ltre/mln .
3~ The liquid. ~ntained 80 g~l of sodlum sul~hate and hydrogen io~s, exp~e~ed as su~phurlc acld, in an amou~t of 160 g/l.
The pulp concentrat~ on fell therewlth to 33% and the pH o~
the pulp -b~came 2 . 5 .
The pulp was flu~fed a~d introduced lnto an ozone 35 bleach~ny reactor, where 'che pulp wa5 allowed to react with ~20ne added ln a ~uantlty of 0.4 k~/mln in a flow of 5~7 kg/~in oxygen g~5. The temperature wa~ 50C and 'che trea~-WO 93~'062g7 PCr/SE92/0063~
17 21~974 Dlent time 30 minu~es . The oxygen gas f re~d f rom ozone was removed f rom the rea~toL . At the end of the ~reatme~t tlme, l~quid was added ~o as to flush a~ay the pulp. Dilutlon liquid in the form of water wa~ add~d in an amount of 540 l;~min. The pulp wa~ then pre~ed to a pulp consl~tency of 30% . The re~ultant o~one bleachl ~g waste l~quor was thereby reco~rered. The pulp wa~ ~hen transfe~ed to a mlxer, to whl~h 2 . 7 Icg of sodium hydro~ide~min we~e added . The pulp ~as all~wed to lea~ w~th the alkali for 123 minutes at a 1~ teJnperature of 65C and a pulp concentra~ion of 14%. TAe pulp wa~ then wash~d with clean wate~ ~o as to obtain an alkaline ~asts liquor.
The Kapp~ numbe~ of the pulp af ter th~s t reatment ~tage ~a~ 7 . 3, its viscoslty 903 kgfdm~ and its brlghtness 53%
IS0.
The Kappa ~umbers, v~scoslti~es and ~ightnesses rec~ ted in ~hls pa~ent applicat~on l~ve been deteLmined in accor-dance ~lth SCAN-C 1:77, SCAN-(:M 15:8~ a~d SCAN-C 11:75 ~espe~tlvely .
The ozone bleaching waste liquor in an amount of 15 l/mln~, contalnlng 310 g~l sodlum sulphate, hydrogen lo~st calculated as H2SO,4, ln an amount of 40 g/l and organic ~aterial in an amount of 90 g/l, W2S mixed wlth the alkalîne waste liquor ln arl amount o~ 12 l/~in so that the resultant pH of the mlxture was 7 .1. The mixtllre was transf er~ed to a c~ystallizatlon vessel, ln wh~ch the mlxture was ~ooled to 10C. Sodlllm ~ulphate crystals ln an amount of 3.3 kg~min were separated from the mixture (the mother llquor) and the sodlum sulpha~e contenk thereof fell to 90 g/l.
These crystals we~e then dlssolve~ in a ves~el ln clean water in a quantlty such that the content of dissolved sod~um ~ulphate became 430 g/l. Thl~ ~olutlon had a tempera-tUEe of 35C and ~as added to the anode chamber of an eleet~olysls cell of the type 1 llustrated ln Flgure 3 . Clean w~ter ~a~ added ~o the cathode chamber of the cel ~ . The c~
t~np~rature -was 50C~ the voltage was 3 . 8 V, the current densi~y s~a~ 25 R~dm arld the power consumptlorl was 240 1~
Durlng the electroly5is process, oxygen gas was genera~

2l~69~ ~ 18 ed ln t~e anode chamber ln an amoun~ of l9O l/mln (0.27 kg/mln3 at the ~ame tlme as an aci~ ~odlum sulphate solutlon was forr~ed ln whlch the hydrogen lons, calculated as sulphuric acid, rose ~o 1.7 kg/mln ~75% co~ver~ion). Hydro-5 gen ga~ developed 1 n the cathode chamber 11l an amount of 38ûl/lain (0~034 kg~mln) at t~e same ttme as a sodium hydroxide solution was f Qsmed in an a7il0unt of 14 l/min at a conce~tra-tion of 10%. :~
As prevlou~ly me~tloned, the ~esultant acid s~dium 10 sulpha~e ~olution cont~ined 80 ~/1 ~odiLum sulphate andbydrog~n ion~ expres~ed as sulphu~ic ac~d, in an amount of 160 g~l. The ~olutinn ~oncerned was -used fully to acidify ~:
the pulp prior to the ozone bleaching stage, as earlier describ@d, so that the pulp had a pH of 2 . 5 . The resultant ~odlum hydroxide ~olution was added ~o the pulp, as earlier mentioned, in the mixer pr~or to the alkalization treatment process. Tl~te sodlum hydroxide content of this solutlon c~ered 50% o~ the ~odiur~ hydroxide addl~on to the pulp.
The following ~he~ical savings were ach~sved ln- this experiment ~
Acidlfytng ch~mi~al, ~alculated as sulphurlc acid =
17 kg per to~ne of pulp;
AlkallziElg chemlcal, sodlum hyd~ox~de = 14 kg per tonne ~:
o~ pulp;
Oxygen gas = 2 . 7 kg per tonn~ of pulp .

As ea~lier mentioned, ~hen prac~lcillg the lnventlve me~hod on a full scale, the oxygen ~as can be used i n an .-alklal~ne treatloent stage, for ~nstance in an introductory oxygen bleaching stage.
~ ., . , :
3~ ExamPl e~2 Thi~ ~xperlm~nt was carrled out with a pine sulphate pulp havl~g a Kappa number of 26 .0 asld a ~riscoslty of 1, 202 d~ ~kg. The pulp wa5 blea~hed wi'ch oxygen gas at a pulp co~sl tency of 12% ~d ~t an oxygen gas p~essuLe of 6 35 kpJcm . 1 . 5% ~od~um hydroxide and O . 3% magnesium ln the forrQ of magneslum sulphate (~gS04 ) had earlier been added WO 93/06297 pcr/sE92/oo63 21~697~
to the pulp. Upon complet~ion of the oxygen blea~hing p~o-~ess, the pulp had a ~appa number of 13.0 and a viscoslty ~f 990 dm3/kg. The pulp was then pressed to a s~onsi~tency of 40~ . Inorganlc chemleals (wa~hing losses ) corresponding to 9 5 kg ~odi~m sulphate per tonne o~ pulp were then found ~o re~ain ln the pulp. A sulphuric a~id ~olut~on of 10% concen-tration was then added to the pulp ln a f low of 100 kg/mln .
The pulp consistency then fell to 38% and the pulp was found to have a pH - ~ . 7 .
The pulp was fluffed and introduced into an ozone bleach~ng reactor, where ~he pulp was allowed to react wi~h ozone, whi~h ~*s added in a quantl~y of 0 . 3 kg~in in an oxygen gas f low of 4 . 3 kg~min . The tenlperature was 50C and the treatment time 30 minutes.
The oxygen gas freed from ozone was withdrawn from the rea~or. At the end ~f the treatment t~me, llquld was added so as to flush away the pulp. Dilution li~uld in the form of water was added in an amount of 500 l/mln. The pulp was then pres~ed ~o a pulp consis~ency o~E 30%. The resultan~ ozone 20 blea~h~ng ~aste l~guor was recovered. The pulp was then trans~erred to a ml~er to which 2 . 5 kg sodium hydro~cide~mln was added. The pulp rea~ted with the alkali for 130 minutes at a temperature of 60C and a pulp consistency of 1496. The pulp wa~ then washed wlth clean water so as to obtain an ~5 alkallne waste liquor. Subsequent to this treatment stage, the pulp was found to have a Kappa number of 6.7, a vi~cosi~
ty oiE 900 kg/dm arld a brightness of 55% IS0.
Precipitator dust from the sulphate m~ll from wh1 ch the experl~ent pulp was taken was dissolved in an amount of 1. B
3~ kg/~i~ in clean water. The amount of water u~ed was such tha~: the resultant solu~iorl had a ~odlum sulphate c~ntent of 360 g/l. The dls~ol~ng took place at a temperature of 55C.
In thl~ experiment 0 there was u~ed an ele~troly~is cell ~rhich had three chambers of the type illustrated ~n Flgure 35 4. The cell lncluded two r~embranes. The sodium ~ulphate 801Util)n ~as added t~ the ~entre ~h~mbe~, i.e. the chamber ~ocated between the two membranes. Clean water was added to the anode chamber and the cath~de chamber ~e~pectively. The .. . . .

WO 93~(~ti2g7 PCr/SE92/00635 69~cell te~pera~ure was 55~C, the voltage 4.2 V, the current - de~sity 20 AJàm and ~he power consuDIption 190 k~.
Duriny the elect~oly~;is process, oxygen gas developed in the anode chamber ~n an ~moun~ of 135 l~mln (0.19 kg/min) at 5 the same ti~e as sulphu~l~ ac~d ~as formed in an amount of 1. 2 kg~mln in a coQcentrat1on of 10% . Hydrogen gas was deve}oped in the . cathode cha~ber ln an amount of 270 ltmin ~0.024 kg/3ain) ~ at the samP tllne as a sodlum hydroxide solu-tion ~ras f ~r~ed in an amount of 1 0 0 kg/~in a~ a co~centra-10 tion of 10~. The ele~trclyzed so~ium ~ulpha~e solutioll wasre~o~red ~om the bo~t~m of the ~en~re chamber and sras f ound to ~till ~ont~in a given cont2nt of ~odlum ~u~pha~e, this ~olu~ on be~ng ~cycled alld m~xed ~lth fre~h sodium sulpha~e 801ution, which wa~ added to the cell at t~e upper part o~
15 the centre chamber.
Ths ~ulphuri~ acld solut~on obta~ned was u~ed fully to ac~dify the pulp p~lor to he o~one ~leaching stage, as pre-vlou~ly ~a~ntioned , su~h ~hat the pH of the pulp wa~ 2 . 7 ~ The ~odium hydroxide ~olut~on obtainled wa~ added to the pulp ln 20 th~ mlxer, as ea~lier de~cribed, prlo~ to the alkallzing treatmen~ stage. The sodium hydroxlde solution obtained was added to the pulp in the mlxer, as earlier described, prior to the alkalizing treatraent sl:age. The sodium hydroxide conteE~t of this solutioQ covered 40% of the sodium hydroxide ~5 chalged to the pulp.
The following chemical ~avings were a~hieved wlth this exper lment:
.~ .
Sulphuric acid = 12 kg per tonne of pulp Sodium hydroxlde = 10 ~ a ;
Oxyge~ gas = 1 . 9 ~ u R ~ ~
'~:

Claims (7)

1. A method in bleaching with ozone lignocellulosic material (cellulose pulp) which has been at least partially manufactured chemically, in the presence of water at a degree of acidity, expressed as pH, of 2-5, c h a r a c t e r i z e d in that the acidity is maintained during the ozone bleaching process by adding a sulphuric acid solution or an acid sodium sulphate solution prepared by electrolysis (or electrodialysis) of an essentially neutral sodium sulphate solution obtained by recycling chemicals in 2 system to which rest solution (waste liquor) from the ozone bleaching process is added.

2. A method according to Claim 1.
c h a r a c t e r i z e d in that the waste liquor from the ozone bleaching process is added to the evaporation and com-bustion plant of the chemical system; and in that essen-tially neutral sodium sulphate is taken from an electric precipitator belonging to the combustion plant; and in that said sodium sulphate is dissolved in essentially clean water and then subjected to electrolysis (or electrodialysis).

3. A method according to Claim 1, c h a r a c t e r i z e d in that the waste liquor from the ozone bleaching process is mixed with waste liquor from an alkaline treatment stage so as to obtain an essentially neutral solution; and in that this solution is cooled so as to precipitate sodium sulphate (Na2SO4) therefrom; and in that the sodium sulphate precipitate is removed and dissolved in essentially clean water and thereafter subject-ed to electrolysis (or electrodialysis).

4. A method according to Claim 3, c h a r a c t e r i z e d in that another liquor obtained from the crystallization of sodium sulphate is transferred to the chemical system which includes the evaporation and combustion plant.

5. A method according to Claims 1-4, c h a r a c t e r i z e d in that the electrolysis is carried out in a cell which comprises anode, cathode and a cation selective membrane, wherein sodium sulphate solution is added to the anode chamber and water is added to the cathode chamber so that an acid solution is formed and oxygen gas is generated at the anode and so that a sodium hydroxide solution is formed and hydrogen gas is generated at the cathode; and in that respective solution is removed from the cell and in that the acid solution is added to the ozone bleaching process.

6. A method according to Claims 1-4, c h a r a c t e r i z e d in that the electrolysis is carried out in a cell which comprises anode, cathode, an anion selective membrane and a cation selective membrane, wherein sodium sulphate solution is added between the two membranes and water is added to the anode chamber and the cathode chamber respectively so that a sulphuric acid solu-tion is formed and oxygen gas is generated at the anode and so that a sodium hydroxide solution is formed and hydrogen gas is generated at the cathode; in that respective solution is removed from the cell; and in that the sulphuric acid solution is added to the ozone bleaching process.

7. A method according to Claims 1-4, c h a r a c t e r i z e d in that the treatment process (the electrodialysis) is carried in a multi chamber cell which comprises anode, cathode, anion selective, cation selective and bipolar membranes, wherein the anode chamber is delimit-ed by an anion selective membrane and the cathode chamber is delimited by a cation selective membrane, and wherein the sodium sulphate solution is added to the anode chamber and the cathode chamber, and wherein bipolar membranes are dis-posed between said anion selective membrane and said cation selective membrane; and in that water is added to those chambers which are delimited by bipolar membranes so that a sulphuric acid solution is formed at least in the membrane chamber which is situated nearest the cathode and hydrogen gas is generated at said cathode, and so that a sodium hydroxide solution is formed at least in the membrane chamber which is situated nearest the anode and oxygen gas is generated at said anode; in that respective solution is removed from the cell; and in that the sulphuric acid solu-tion is delivered to the ozone bleaching process.