CA1184006A - Method of processing digesting chemicals for cellulose production - Google Patents
Method of processing digesting chemicals for cellulose productionInfo
- Publication number
- CA1184006A CA1184006A CA000413062A CA413062A CA1184006A CA 1184006 A CA1184006 A CA 1184006A CA 000413062 A CA000413062 A CA 000413062A CA 413062 A CA413062 A CA 413062A CA 1184006 A CA1184006 A CA 1184006A
- Authority
- CA
- Canada
- Prior art keywords
- lime
- aqueous solution
- improved method
- milk
- liquor
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F11/00—Compounds of calcium, strontium, or barium
- C01F11/02—Oxides or hydroxides
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Life Sciences & Earth Sciences (AREA)
- Geology (AREA)
- Inorganic Chemistry (AREA)
- Paper (AREA)
- Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A method of processing digesting chemicals for cellulose production. In this method unslaked lime from the lime kiln is slaked and the slaked lime thereafter mixed with green liquor (sodium carbonate) and causticized into white liquor.
For the purpose of allowing bleeding-off and separation of non-desirable elememts such as e.g. Si, Al, Mg, Mn, Fe, and P obtained in the processes the unslaked lime is slaked in accordance with the teachings of the invention in a separate process step in an aqueous solution, whereupon the milk of lime (Ca(OH2) thus formed is separated from the aqueous solution and the causticizing thereafter effected.
Fig. 2
A method of processing digesting chemicals for cellulose production. In this method unslaked lime from the lime kiln is slaked and the slaked lime thereafter mixed with green liquor (sodium carbonate) and causticized into white liquor.
For the purpose of allowing bleeding-off and separation of non-desirable elememts such as e.g. Si, Al, Mg, Mn, Fe, and P obtained in the processes the unslaked lime is slaked in accordance with the teachings of the invention in a separate process step in an aqueous solution, whereupon the milk of lime (Ca(OH2) thus formed is separated from the aqueous solution and the causticizing thereafter effected.
Fig. 2
Description
~ACKGROUND OF T11E INVENTION
The subject inventinn concerns a ~0thod oF
wa~hing lime and bleeding-off non-de~irable ~ub~tance obtainad in th8 processe~ of cellulDse-production plants, for instance in the mar-u~actura of sulphate pulp. The msthod i8 partiGularly applicable in plsnts ~herein solid fuels that are rioh in ashes ~re usad to re~burn lime or wherein the fibrous raw materlal ha~ a high content of mineral~A
In the production o~ cellulose in accordancs with the process known as the sulphate procesR3 wooden chip~
are dlgested in a digasting liquor tha active ingradients o~ which are sodiu~ hydroxide (NaOH) and sodium ~ulphide (Na~S)c During the dige~tion; the hydroxide and sulphide ox~diza and form carbonate~ and sulphates. Al~o during the digestion, approximately 50~ of the wood are dissolued, primarily the lignin and the hami-cellulo~a but also the minarals contained in the wood.
A~ter concentration o~ the dige.ting liquor in 2n ~aporisers the ha~t cDntent o~ the lîquor i9 used in the ~oda pan wherein liquor combustion i~ effected.
The melt of sodium salt a~hes flows ~rom the soda pan to a ~elt dissolving unit where it is dis~olved with the aid of weak liquor~ The green liquor thu3 ~orMed i~
~.
separated from ~olid~ purities)j as a rul~ tbroegh sedim~ntation in an apparatus known as a graen liqunr clarifi3r~ The green liquor is then used to slake c~lcium oxid~ which i~ addsd i~ 8urplug amnunt~ to ~ slaker unit. In thi~ apparatus li~e partiole~ which have not been brought into ~u~pension state in the gr0~n liquor but ~ettle st the bottom are ~eparated of~ and re~ovedJ
During the sl~king proce~Q calcium hydroxide 9 Ca(OH)2, i~ formed and together with ~odiu~ carbonate 10 Na2C03 (green liquer)9 it for~s ~dium hydroxide, NaOtl (dige~ting liquor), and c~lcium carbonatea CcC03 (lLms sludgs). This reaction ia ~enerally knn~n as causticizing .
A certain time i8 requirad to achie~e oquilibriu~
in the reaction of sodiu~ carbonate with unslaked lime and the reaction i8 thsre~or0 ellowed tQ progress in slaking and caustioizing vs~sel~ for periods e~ bstween 1 and 1~ hours and even up to 2 hour Tha digesting liqu~r and the calcium carbonate, that i~3 the li~a sludge9 are separated efter the raactinn, and this ~eparation take~ place in a white 1iguor clarifier or through filtrationO After separation of the lime ~ludge the digesting liquor i~ stored and for thi~ purpo~e it i~ pu~pHd into tanks from which i$ 31ay 25 later be withdrawn in dosed quantities and used For digssting .
~ Q~
Tha llme sludge is th~r~aftsr subjsctsd to a re-causticizing step whbrsin remaining surplu6 nf li~8 i3 ~llowed to react with green liquor. In this reaotioll waak liquor is obtalned which is u~d to solve the ~lt o~ ashas~ as m~ntioned abovec The lime 81udge l~ de~tered in uacuum ~ilt~rs~
which inoreases the dry content~ to bstween 6D and 70%~
and is ~ashed on the filtsrs by wat~r bafor~ being transfarrsd to the rear ~cold) and of the lime kiln.
1a In thi3 kiln, the li~e sludge i9 dried and burnad to li~e while giving of~ watar vapour and calciu~ dioxide.
Tha kiln i8 U ually heated by oil or ~as with the aid of ~ burner positionad in ths kiln proper. The ch~mic~l proc~s i~ a~ follows~
Reaction in the soda pans
The subject inventinn concerns a ~0thod oF
wa~hing lime and bleeding-off non-de~irable ~ub~tance obtainad in th8 processe~ of cellulDse-production plants, for instance in the mar-u~actura of sulphate pulp. The msthod i8 partiGularly applicable in plsnts ~herein solid fuels that are rioh in ashes ~re usad to re~burn lime or wherein the fibrous raw materlal ha~ a high content of mineral~A
In the production o~ cellulose in accordancs with the process known as the sulphate procesR3 wooden chip~
are dlgested in a digasting liquor tha active ingradients o~ which are sodiu~ hydroxide (NaOH) and sodium ~ulphide (Na~S)c During the dige~tion; the hydroxide and sulphide ox~diza and form carbonate~ and sulphates. Al~o during the digestion, approximately 50~ of the wood are dissolued, primarily the lignin and the hami-cellulo~a but also the minarals contained in the wood.
A~ter concentration o~ the dige.ting liquor in 2n ~aporisers the ha~t cDntent o~ the lîquor i9 used in the ~oda pan wherein liquor combustion i~ effected.
The melt of sodium salt a~hes flows ~rom the soda pan to a ~elt dissolving unit where it is dis~olved with the aid of weak liquor~ The green liquor thu3 ~orMed i~
~.
separated from ~olid~ purities)j as a rul~ tbroegh sedim~ntation in an apparatus known as a graen liqunr clarifi3r~ The green liquor is then used to slake c~lcium oxid~ which i~ addsd i~ 8urplug amnunt~ to ~ slaker unit. In thi~ apparatus li~e partiole~ which have not been brought into ~u~pension state in the gr0~n liquor but ~ettle st the bottom are ~eparated of~ and re~ovedJ
During the sl~king proce~Q calcium hydroxide 9 Ca(OH)2, i~ formed and together with ~odiu~ carbonate 10 Na2C03 (green liquer)9 it for~s ~dium hydroxide, NaOtl (dige~ting liquor), and c~lcium carbonatea CcC03 (lLms sludgs). This reaction ia ~enerally knn~n as causticizing .
A certain time i8 requirad to achie~e oquilibriu~
in the reaction of sodiu~ carbonate with unslaked lime and the reaction i8 thsre~or0 ellowed tQ progress in slaking and caustioizing vs~sel~ for periods e~ bstween 1 and 1~ hours and even up to 2 hour Tha digesting liqu~r and the calcium carbonate, that i~3 the li~a sludge9 are separated efter the raactinn, and this ~eparation take~ place in a white 1iguor clarifier or through filtrationO After separation of the lime ~ludge the digesting liquor i~ stored and for thi~ purpo~e it i~ pu~pHd into tanks from which i$ 31ay 25 later be withdrawn in dosed quantities and used For digssting .
~ Q~
Tha llme sludge is th~r~aftsr subjsctsd to a re-causticizing step whbrsin remaining surplu6 nf li~8 i3 ~llowed to react with green liquor. In this reaotioll waak liquor is obtalned which is u~d to solve the ~lt o~ ashas~ as m~ntioned abovec The lime 81udge l~ de~tered in uacuum ~ilt~rs~
which inoreases the dry content~ to bstween 6D and 70%~
and is ~ashed on the filtsrs by wat~r bafor~ being transfarrsd to the rear ~cold) and of the lime kiln.
1a In thi3 kiln, the li~e sludge i9 dried and burnad to li~e while giving of~ watar vapour and calciu~ dioxide.
Tha kiln i8 U ually heated by oil or ~as with the aid of ~ burner positionad in ths kiln proper. The ch~mic~l proc~s i~ a~ follows~
Reaction in the soda pans
2 o4 ra~uced t~osphere Na25 + 202 Reaction in the slaker and causticizing vss alss C;~10(8) ~ H20 (~ Ca(Otl)2~5) Ca~OH~2(s) ~ C03 ( = ) CaC03(3) ~ 20H
20 Re~ction in the li~e kilns 1 oonc CaC03(8) ( ~ ) CaO(s) ~ Cû2(3) In the methods u3ed tod~y, non-deslra~le ~ubstancos are entrainsd with ths lime ~ks-up7 the wood (unh~n ~ 3~
timbar~ carefully gr~dad chipsi and ths proce ~ water.
In plant~ using perenni~l~ (gra~s, reed~ str3w, baga4~e;~, ba~boo or other kinds o~ ~ood Gontaining lar~a quantitie~ o~ non-daeirabl0 substancas, it is difficult ta r~-u~e tha digesting che~icals and c~nse~uenkly they ars dispo~ed o~ s 5nlid-fuel h~ating i~ becoming an attrQCtiv~
alternative in lime kilne a8 the energy price~ are rising and ~tilization o~ the antire tr~e i increa~in~.
1~ All- tree u~e has al80 re~ulted in a ~ecreass of tha quolity of the chip8~ th~t ie, their contents ~f non-dssir~wble substancos h~ve incrsased. When wolid fuel~
and/or ohip~ from the antiro tree are used ln th~ lime kiln problems similar to those met when peronnial~ are u3ed sro encountere~.
The non~desirable elements~ ~uch as Si, Al, Mn, M~, Fejand P are concentrated in all khe proce~s flow~, dspending on how closed the syste~ is. The conoentrations incra~e until equilibrium is achieved in the 8y8tem.
U~ually, there are no other blesding-off po~ibLlities than with the Gel1ulose~ the green liquor ~ludge and the calcareou~ gravel, The concantration o~ nonwdasirable subskancss in~olve~ a number n~ problem s - Increas~ of tha incru~tation tendencie~ o~
partioularly sodium aluminiu~ silio3tes on heat -transfar surfaces~ ~uch as in th~ evaporator, tha soda pan and the digester. The elu~iniu~ content i the decieive ~actor in tbe pre2ipitat$0n of sodiu~
alu~inium 8ili cat~s.
- Incra~se of depoaits in lo~ ~lo~ ~psed pi pe8 ~nd appar~tu9e67 pri~rily due to the reduced ~sdi~
~entatiOn and filtratiQn cepacity o~ the lims eludge~
- Incre~ed anergy consu~ption, both becau~s inert6 ln eag. li.me ~ust be heated unneces~arily and becau~e the dry cnntsnt h~ua been lo~ered in the li~e sludge ~ilterv as explained abovs~
- Lo~ered capacity of the plant, which i8 the result ef tha effects outlined above~
During the slaking of ths calciu~ oxide ~c~rbonata solution (green liquor) in required n~ounts i9 ~upplied in do~ed quantities to the slaker only or, alternatively, in the ~orm of several ~lows to e.g. the causticlzing st2tion and/or .re-Gausticizing station.
ConsiderablH amount~ of energy are released in 2D ths sl~king process. For rapid reactions temperatures batween 90 and '1~0C are ~eguired. With the u~s o~
today's technology it i9 diPflcul.t to control the ~laking conditions. Too low tanlperatures result in incomplste ~laking and there~ore impaired capacity~ Toe high t~mperature~ (energy gsneration) ~akes the liquld in the slak~r bnil ou~r and ~evere du~ting problems oceur during the li~e ~upply opsrationsO An attempt t~
overcome the practical problsms caused by wncontrolle~
slaking has rs~ulted in th~ advent of a pressurized ~lak~r9 built into a closed system.
Th~ ~ub~ject inventiorl has ~or its purpo~e to 801Ve the pr~blem~ outlined abovs.
~L~
The invention conc~rns a methDd primarily spplicable ln plants wherein soli~ ~uels9 3uch a~ bark, wood, chips~ peat and coal are used in a lime kiln and/or wherqln ~ibrous and chemical raw material~ h~ve a high content of non-de~irable ~ubstancas. Raw mat~rial~
o~ thiq kind m2ly be chips ~rom the entirs trse9 lime ~tone, watsr, bagasse, bamboo9 reed7straw and th~ like.
In the consumption of the above-~entioner~ funls in lime kilns, non-d0sirable sub~tarlces ocrurring in the ashss such as Si, Al, Mg, Mn, FB~ P and others are added to the li~eO
Ths ~ethod in accordr~nce with the invention i8 ch~racterised by ~l~king un~lak~d lilRe in a sepsrat~
process step in an aqueous ~olution to produce a ~usp~nsion of calcium hydroxide in an aqueou~ solution.
From the ~9U~OU8 ~olution is then separated the ~ilk of 2S lioe thus formed before causticizing is effected by p~
addition of eodium carbonate. The aqueous solution coold bs pure ~ter9 ~crubber /wa~hing~ watera purified condensate, alkaline w~t~r or othsr pxooass water. The alkalinity may, if reqL!ired, by adju~ted by a S~all ~d~ition o~ e~,g, N~OH, whit~ liquor"
Na2C03" that i~ çlreen licluor. In the two la~tter cases Na2C03 reacts with CaO~ fDrming NaOH in ~uf~ioient ~ount3 to adJust the p~-valLIe to th~ de~ired level, th2t is 12-15.
~0 ~RIEF DESCRIPTION OF THE DRAWINGS
In the ~ttached drawings Fig. 1 illustrate~ She prior art procese described ~bovn, and Fig. 2 S~ho~ ths ~ethod in accordance with the sub jeot invention .
DETAILED DESCRXPTION OF A PREFERRED EM~ODIMENT
. Ths particlns which have ~ettled in the slaking~
that i8 hard unslaksd lime, stones from the kiln bri~k-work and from the lime mske-up are re~oved ~rom the slaker with the aid of scrapers and similar tool~, In accordonce with the teachings of the ~ubject in~Jention the homogeno~ suepensiQn i~ ther~after eubjeGted to a ~eparation 3tep, which pref~rably i9 ef~ectnd throu~h filtration but could likeL~ise ba ef~ected through 3edi~entation in a eedimentation plant or throu~h a - ? -corRbination of both ~ethod~0 In thi~ ~eparakion step a ~urplu~ ~f the aqueou~ 301ution i8 separated fro~
the calcium hydroxide which i8 then wa~hedJpreferably with cl~an w2tar (altornatively purified pl~nt conden- -~ate~ A1RO the alkaline solution~ ~r N~zCO3-sol~tion ~ay be used as the wa~h watsr to adju3t th~ p~value.
The res~lting slaked ~nd ~a~hed lime is ~uppliod in dosed quantitiss together with green liquor to the cau~ticizin~
unit. The strongly alkaline aq~aous solutlon ~ay be 1D ~holly or partly recirculated to the 31aker or ba discharged to the plant sewage drain~ or to ~ther ~ater-~rocessing systems. When lsrg~ a~ounts are re-circulated it i~ neoesss~ry to puri~y th~ aqueous ~olution fro~
sub~tances dissol~ed therein befors the ~olution is re-turned to the slaker. As e~ample o~ puri~ying proce~e~may be mentioned reduction of the pH-value followed by separation of precipitated suhstances by ~ean~ o~
csntrifuging, ~iltration, sedi~entation or cyclone--separation. Preferably~ agueou~ solution i~ added, in eXOe~9J partioularly in the form o~ wa~h water, ~nd bleoding-o~f oF di3solved chemical~ occurs bnfore t,he re~aining aqueous ~olution i~ ~8 circulated.
The ~athod in accordanc~ with the subject inuention provides n number o~ aclvantages over the tschnology u~ed hitherto. For in~tance, one has found that ths majority of non-de~irable subst~nce~ in unslaked lime ~ppear in ~uch a for~ (compounds3 that they will lar~ely dissolve in the slaking process ~ugg9sted in accordanc~ ~ith the teachings of the swbject invsntion. P~rticularly valuable c~pounds containing Al and Si therefore ~ay be removed and the iocru~tation of Na~ ilicatee in the incru~tation of black liquor ~ay be avoid0dO A1BO
other tra~er substance~ ~ay be bled off and in thi~
1~ ~anner their cDncsntrati~n in the plant di~e~ting llquor and lime cycle i9 prevented. 5urprisingly~ one ha~
al80 ~ound that the sedimentation and filtration ~roper-ties of white liquor/weak liquor CaC03 (limc sludg0) ara improved. A~ a re~ult, the capacity nf the ~hit0 liquor clsrifying incraases and ths li~e sludge w~shlng and dewatering may be improved. As a sDnsequence of the higher dry conl;ents of the lim0 sludga the fuel consump-tion in the lime kiln is reduced. In addition, the slaking process bncome~ easier to control, which ansures that constant te~peratures are maintained, resulting in maximum capacity without ov~rboiling. Owing to the improved control also the conditions prevailing during the causticizing process may he held constantly at cpti~um lavels~ which give~ maximum white liquor yialds and 25 quali ty.
EXAMPLE
-In plant operation7 powder~d bark was ueed instead of oil a ths lime kiln heating ~u91. The ashs~
content~ in powder~d bark wsre appr. 3~. The ashes ~onsist of several unorgsnic eubstances. For exaQple, the contents ef ~ on and aluminium wBre respectively 14 and 1~.2 g/kg ashee~ In ~ddition, lime make-up was a~dad to ths lime sludge flow be~ore the li~e sludge is reburned. The lime ~ake-up contain~ 7.3 9 Al/kg and 1~4 9 Si~kg.
When ueing the prior-art procese~ miniu~
~ilicon and other non-de~irable chemicals in the digesting liquor and lime sludge cycls are concentrated.
Compared wlth oi1 hsating in the lime kiln the a1uminiu~
and ~ilicon contents in white liquour increased from re~pectively 2n and 90 to raspectivaly 180 and 22U.
APter thle procsss suggested herein wa8 adopted, see FigO 2, contents of 20 mg aluminium/l white liquor and 70 mg silicon/l white liquour have been found in the white liquorO In the lime kiln ~3.3 tons calcium car-bonate/h (Th = 69%) and D.04 ton ~ake~up li~e/h were reburned into 9 tons unslaked lime. For the reburning
20 Re~ction in the li~e kilns 1 oonc CaC03(8) ( ~ ) CaO(s) ~ Cû2(3) In the methods u3ed tod~y, non-deslra~le ~ubstancos are entrainsd with ths lime ~ks-up7 the wood (unh~n ~ 3~
timbar~ carefully gr~dad chipsi and ths proce ~ water.
In plant~ using perenni~l~ (gra~s, reed~ str3w, baga4~e;~, ba~boo or other kinds o~ ~ood Gontaining lar~a quantitie~ o~ non-daeirabl0 substancas, it is difficult ta r~-u~e tha digesting che~icals and c~nse~uenkly they ars dispo~ed o~ s 5nlid-fuel h~ating i~ becoming an attrQCtiv~
alternative in lime kilne a8 the energy price~ are rising and ~tilization o~ the antire tr~e i increa~in~.
1~ All- tree u~e has al80 re~ulted in a ~ecreass of tha quolity of the chip8~ th~t ie, their contents ~f non-dssir~wble substancos h~ve incrsased. When wolid fuel~
and/or ohip~ from the antiro tree are used ln th~ lime kiln problems similar to those met when peronnial~ are u3ed sro encountere~.
The non~desirable elements~ ~uch as Si, Al, Mn, M~, Fejand P are concentrated in all khe proce~s flow~, dspending on how closed the syste~ is. The conoentrations incra~e until equilibrium is achieved in the 8y8tem.
U~ually, there are no other blesding-off po~ibLlities than with the Gel1ulose~ the green liquor ~ludge and the calcareou~ gravel, The concantration o~ nonwdasirable subskancss in~olve~ a number n~ problem s - Increas~ of tha incru~tation tendencie~ o~
partioularly sodium aluminiu~ silio3tes on heat -transfar surfaces~ ~uch as in th~ evaporator, tha soda pan and the digester. The elu~iniu~ content i the decieive ~actor in tbe pre2ipitat$0n of sodiu~
alu~inium 8ili cat~s.
- Incra~se of depoaits in lo~ ~lo~ ~psed pi pe8 ~nd appar~tu9e67 pri~rily due to the reduced ~sdi~
~entatiOn and filtratiQn cepacity o~ the lims eludge~
- Incre~ed anergy consu~ption, both becau~s inert6 ln eag. li.me ~ust be heated unneces~arily and becau~e the dry cnntsnt h~ua been lo~ered in the li~e sludge ~ilterv as explained abovs~
- Lo~ered capacity of the plant, which i8 the result ef tha effects outlined above~
During the slaking of ths calciu~ oxide ~c~rbonata solution (green liquor) in required n~ounts i9 ~upplied in do~ed quantities to the slaker only or, alternatively, in the ~orm of several ~lows to e.g. the causticlzing st2tion and/or .re-Gausticizing station.
ConsiderablH amount~ of energy are released in 2D ths sl~king process. For rapid reactions temperatures batween 90 and '1~0C are ~eguired. With the u~s o~
today's technology it i9 diPflcul.t to control the ~laking conditions. Too low tanlperatures result in incomplste ~laking and there~ore impaired capacity~ Toe high t~mperature~ (energy gsneration) ~akes the liquld in the slak~r bnil ou~r and ~evere du~ting problems oceur during the li~e ~upply opsrationsO An attempt t~
overcome the practical problsms caused by wncontrolle~
slaking has rs~ulted in th~ advent of a pressurized ~lak~r9 built into a closed system.
Th~ ~ub~ject inventiorl has ~or its purpo~e to 801Ve the pr~blem~ outlined abovs.
~L~
The invention conc~rns a methDd primarily spplicable ln plants wherein soli~ ~uels9 3uch a~ bark, wood, chips~ peat and coal are used in a lime kiln and/or wherqln ~ibrous and chemical raw material~ h~ve a high content of non-de~irable ~ubstancas. Raw mat~rial~
o~ thiq kind m2ly be chips ~rom the entirs trse9 lime ~tone, watsr, bagasse, bamboo9 reed7straw and th~ like.
In the consumption of the above-~entioner~ funls in lime kilns, non-d0sirable sub~tarlces ocrurring in the ashss such as Si, Al, Mg, Mn, FB~ P and others are added to the li~eO
Ths ~ethod in accordr~nce with the invention i8 ch~racterised by ~l~king un~lak~d lilRe in a sepsrat~
process step in an aqueous ~olution to produce a ~usp~nsion of calcium hydroxide in an aqueou~ solution.
From the ~9U~OU8 ~olution is then separated the ~ilk of 2S lioe thus formed before causticizing is effected by p~
addition of eodium carbonate. The aqueous solution coold bs pure ~ter9 ~crubber /wa~hing~ watera purified condensate, alkaline w~t~r or othsr pxooass water. The alkalinity may, if reqL!ired, by adju~ted by a S~all ~d~ition o~ e~,g, N~OH, whit~ liquor"
Na2C03" that i~ çlreen licluor. In the two la~tter cases Na2C03 reacts with CaO~ fDrming NaOH in ~uf~ioient ~ount3 to adJust the p~-valLIe to th~ de~ired level, th2t is 12-15.
~0 ~RIEF DESCRIPTION OF THE DRAWINGS
In the ~ttached drawings Fig. 1 illustrate~ She prior art procese described ~bovn, and Fig. 2 S~ho~ ths ~ethod in accordance with the sub jeot invention .
DETAILED DESCRXPTION OF A PREFERRED EM~ODIMENT
. Ths particlns which have ~ettled in the slaking~
that i8 hard unslaksd lime, stones from the kiln bri~k-work and from the lime mske-up are re~oved ~rom the slaker with the aid of scrapers and similar tool~, In accordonce with the teachings of the ~ubject in~Jention the homogeno~ suepensiQn i~ ther~after eubjeGted to a ~eparation 3tep, which pref~rably i9 ef~ectnd throu~h filtration but could likeL~ise ba ef~ected through 3edi~entation in a eedimentation plant or throu~h a - ? -corRbination of both ~ethod~0 In thi~ ~eparakion step a ~urplu~ ~f the aqueou~ 301ution i8 separated fro~
the calcium hydroxide which i8 then wa~hedJpreferably with cl~an w2tar (altornatively purified pl~nt conden- -~ate~ A1RO the alkaline solution~ ~r N~zCO3-sol~tion ~ay be used as the wa~h watsr to adju3t th~ p~value.
The res~lting slaked ~nd ~a~hed lime is ~uppliod in dosed quantitiss together with green liquor to the cau~ticizin~
unit. The strongly alkaline aq~aous solutlon ~ay be 1D ~holly or partly recirculated to the 31aker or ba discharged to the plant sewage drain~ or to ~ther ~ater-~rocessing systems. When lsrg~ a~ounts are re-circulated it i~ neoesss~ry to puri~y th~ aqueous ~olution fro~
sub~tances dissol~ed therein befors the ~olution is re-turned to the slaker. As e~ample o~ puri~ying proce~e~may be mentioned reduction of the pH-value followed by separation of precipitated suhstances by ~ean~ o~
csntrifuging, ~iltration, sedi~entation or cyclone--separation. Preferably~ agueou~ solution i~ added, in eXOe~9J partioularly in the form o~ wa~h water, ~nd bleoding-o~f oF di3solved chemical~ occurs bnfore t,he re~aining aqueous ~olution i~ ~8 circulated.
The ~athod in accordanc~ with the subject inuention provides n number o~ aclvantages over the tschnology u~ed hitherto. For in~tance, one has found that ths majority of non-de~irable subst~nce~ in unslaked lime ~ppear in ~uch a for~ (compounds3 that they will lar~ely dissolve in the slaking process ~ugg9sted in accordanc~ ~ith the teachings of the swbject invsntion. P~rticularly valuable c~pounds containing Al and Si therefore ~ay be removed and the iocru~tation of Na~ ilicatee in the incru~tation of black liquor ~ay be avoid0dO A1BO
other tra~er substance~ ~ay be bled off and in thi~
1~ ~anner their cDncsntrati~n in the plant di~e~ting llquor and lime cycle i9 prevented. 5urprisingly~ one ha~
al80 ~ound that the sedimentation and filtration ~roper-ties of white liquor/weak liquor CaC03 (limc sludg0) ara improved. A~ a re~ult, the capacity nf the ~hit0 liquor clsrifying incraases and ths li~e sludge w~shlng and dewatering may be improved. As a sDnsequence of the higher dry conl;ents of the lim0 sludga the fuel consump-tion in the lime kiln is reduced. In addition, the slaking process bncome~ easier to control, which ansures that constant te~peratures are maintained, resulting in maximum capacity without ov~rboiling. Owing to the improved control also the conditions prevailing during the causticizing process may he held constantly at cpti~um lavels~ which give~ maximum white liquor yialds and 25 quali ty.
EXAMPLE
-In plant operation7 powder~d bark was ueed instead of oil a ths lime kiln heating ~u91. The ashs~
content~ in powder~d bark wsre appr. 3~. The ashes ~onsist of several unorgsnic eubstances. For exaQple, the contents ef ~ on and aluminium wBre respectively 14 and 1~.2 g/kg ashee~ In ~ddition, lime make-up was a~dad to ths lime sludge flow be~ore the li~e sludge is reburned. The lime ~ake-up contain~ 7.3 9 Al/kg and 1~4 9 Si~kg.
When ueing the prior-art procese~ miniu~
~ilicon and other non-de~irable chemicals in the digesting liquor and lime sludge cycls are concentrated.
Compared wlth oi1 hsating in the lime kiln the a1uminiu~
and ~ilicon contents in white liquour increased from re~pectively 2n and 90 to raspectivaly 180 and 22U.
APter thle procsss suggested herein wa8 adopted, see FigO 2, contents of 20 mg aluminium/l white liquor and 70 mg silicon/l white liquour have been found in the white liquorO In the lime kiln ~3.3 tons calcium car-bonate/h (Th = 69%) and D.04 ton ~ake~up li~e/h were reburned into 9 tons unslaked lime. For the reburning
3~6 tons bark/h were required. The unelaked li~e was tr~nsportsd via a mill tn the slaker, wherJin the li~e wa~ mixed under vigoroue agitation with the filtrate (7a m /h)obtainad from a li~e milk filter. Tha li~9 was thu~ slaked while giving off heat, and the lime nodules were fragmentized irlto a fine sludg34 Calcareous gravel, that i5 nodula~ not ~o fragmen-tizad(unreactiva lime, ~tones, and se on) eettledin the ~lakar and were removed by means of a ecraper conveyor, This amount waa found to be about 0.~30 tons/hO The re~idence time in the ~laksr was 15 min.
From the slak~r the milk o~ lime at appr~ 100~C (slak~d li~e and liquid) wa~ transported to tha milk li~e ~ilter, ~hich is a drum type o~ filter having a diameter of 2.5 m snd a width o~ 4 m.In the filter, the slaked li~e ~as filtered from the liquid and Wa9 dewatered in vacuu~.
Tha li~e wa~ washed in the filter with the aid o~ an aqu~oue solution (14 m /h), consisting oF 1 part NaOH
and 10 part~ pur0 water. From the Pilter the lime~ no~
having a dry contant of 7~, was tran~ported on a conveyor belt to the csusticizing unit in which green liquor was added. The filtrate (75 lR Jh) frem tha drum filtsr W~5 pumpad through a heat exchanger in which the wa~hing liquid was heated -to 95C~ wheraafter tha Piltrats wa~ divided inko two flows, one of which at a rate of 5 ~ /h wa9 di~charged to the sewage and tha other at a rate of 70 ~ /h flowed to the slaker.
On on9 s~mpling occasion the ~ollowing resU ~ ts were obtained: Al Si ashes from bark 14000 mg/kg 1220D ~g/kg unsl,3ked lims 2DOU ~g/kg 250a mg/kg liquid fraction o~ lime ~ilk 364 mg/l 32~ ~9/l solid fraction of lir~e railk 1~40 mg/kg 1574 ~g/kg ~iltrata efter filtering 305 r~g/l 264 mg~l A cor~parison between oil heatingt bark heating end bark heating ~ accor~ance with the new rnethod a8 taught herein gave tha followirlg rs~ultsa bsrk heating bark oil using new method heating heating ~ . . ~ .. . -, __.
Fuel con~umption G J/t on li~ 7 . 9 9 . 2 7 . 7 dry cnntent after lime ~ilter" in % 68 70 60-65 6~70 . ~___ ~ __ reactivity o~ lime in ~econde to achieve raaxir~um tumperature, pro-vided a predeter- sn 1 on 180 mined amount of water at 85C is added _ _ _ __ . . ~ .
inerts in lime, in % 1 2.5 1.5 . . , . _ _ _ _ _ sludge cont9nts in clari~ied wh~te 80-120 00~1 oao 1 sn-2no r, ~D/~ . . __
From the slak~r the milk o~ lime at appr~ 100~C (slak~d li~e and liquid) wa~ transported to tha milk li~e ~ilter, ~hich is a drum type o~ filter having a diameter of 2.5 m snd a width o~ 4 m.In the filter, the slaked li~e ~as filtered from the liquid and Wa9 dewatered in vacuu~.
Tha li~e wa~ washed in the filter with the aid o~ an aqu~oue solution (14 m /h), consisting oF 1 part NaOH
and 10 part~ pur0 water. From the Pilter the lime~ no~
having a dry contant of 7~, was tran~ported on a conveyor belt to the csusticizing unit in which green liquor was added. The filtrate (75 lR Jh) frem tha drum filtsr W~5 pumpad through a heat exchanger in which the wa~hing liquid was heated -to 95C~ wheraafter tha Piltrats wa~ divided inko two flows, one of which at a rate of 5 ~ /h wa9 di~charged to the sewage and tha other at a rate of 70 ~ /h flowed to the slaker.
On on9 s~mpling occasion the ~ollowing resU ~ ts were obtained: Al Si ashes from bark 14000 mg/kg 1220D ~g/kg unsl,3ked lims 2DOU ~g/kg 250a mg/kg liquid fraction o~ lime ~ilk 364 mg/l 32~ ~9/l solid fraction of lir~e railk 1~40 mg/kg 1574 ~g/kg ~iltrata efter filtering 305 r~g/l 264 mg~l A cor~parison between oil heatingt bark heating end bark heating ~ accor~ance with the new rnethod a8 taught herein gave tha followirlg rs~ultsa bsrk heating bark oil using new method heating heating ~ . . ~ .. . -, __.
Fuel con~umption G J/t on li~ 7 . 9 9 . 2 7 . 7 dry cnntent after lime ~ilter" in % 68 70 60-65 6~70 . ~___ ~ __ reactivity o~ lime in ~econde to achieve raaxir~um tumperature, pro-vided a predeter- sn 1 on 180 mined amount of water at 85C is added _ _ _ __ . . ~ .
inerts in lime, in % 1 2.5 1.5 . . , . _ _ _ _ _ sludge cont9nts in clari~ied wh~te 80-120 00~1 oao 1 sn-2no r, ~D/~ . . __
Claims (7)
1. An improved method of processing digesting chemicals for cellulose production, wherein unslaked lime from the lime kiln is slaked and the lima thus slaked is thereafter mixed with green liquor (sodium carbonate), which is causticized into white liquor, the improvement comprising slaking said lime (calcium oxide) in a separate process step in an aqueous solution, and there-after separating the milk of lime (Ca(OH)2) thus formed from said aqueous solution before causticizing.
2. An improved method according to claim 1, wherein the pH-value of said aqueous solution is between 12 and 15.
3. An improved method according to claim 1, comprising separating said milk of lime from said aqueous solution and washing said separated milk of lime with water before causticizing.
4. An improved method according to claim 3, comprising washing said milk of lima separated from said aqueous solution in an alkaline aqueous solution before causticizing.
5. An improved method according to claim 1, comprising recirculating to a slaker said aqueous solution obtained from said step of separating milk of lime from said aqueous solution,
6. An improved method according to claim 5, comprising adding acids at a pH-value lower than between 12 and 15 to purify said recirculated aqueous solution, and separating precipitated substances from said solution.
7. An improved method according to claim 1, comprising maintaining the temperature between 60° and 120°C during said slaking process by cooling or heating said aqueous solution.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| SE8105886-9 | 1981-10-13 | ||
| SE8105886A SE448102B (en) | 1981-10-13 | 1981-10-13 | PROCEDURE FOR PREPARING COOKING CHEMICALS FOR CELLULOSAMAS PREPARATION |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1184006A true CA1184006A (en) | 1985-03-19 |
Family
ID=20344708
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000413062A Expired CA1184006A (en) | 1981-10-13 | 1982-10-07 | Method of processing digesting chemicals for cellulose production |
Country Status (5)
| Country | Link |
|---|---|
| AT (1) | AT378381B (en) |
| CA (1) | CA1184006A (en) |
| FI (1) | FI73754C (en) |
| FR (1) | FR2514383B1 (en) |
| SE (1) | SE448102B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1997003245A1 (en) * | 1995-07-11 | 1997-01-30 | Ahlstrom Machinery Oy | Method of separating impurities from lime and lime sludge and a method of causticizing green liquor containing impurities, such as silicon, in two stages |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FI49331C (en) * | 1973-06-27 | 1975-05-12 | Ahlstroem Oy | The method of removing paint from the pulp mill effluent and the device for applying the method. |
| US3120464A (en) * | 1960-11-29 | 1964-02-04 | Nat Council For Stream Improve | Decolorizing kraft waste liquors |
| GB1010296A (en) * | 1961-08-25 | 1965-11-17 | Eimco Corp | Improvements in or relating to aqueous caustic soda production |
| US3635670A (en) * | 1969-06-11 | 1972-01-18 | Allied Chem | Recovery of dilute caustic soda solutions from spent liquors containing hemicellulose |
| FI53728C (en) * | 1974-03-12 | 1978-07-10 | Ahlstroem Oy | FOERFARANDE FOER AOTERVINNING AV KEMIKALIER UR AVLUTAR FRAON SULFATCELLULOSAKOK OCH AVFALLSVATTEN FRAON BLEKNING |
-
1981
- 1981-10-13 SE SE8105886A patent/SE448102B/en not_active IP Right Cessation
-
1982
- 1982-10-07 CA CA000413062A patent/CA1184006A/en not_active Expired
- 1982-10-08 FI FI823417A patent/FI73754C/en not_active IP Right Cessation
- 1982-10-11 AT AT374982A patent/AT378381B/en not_active IP Right Cessation
- 1982-10-12 FR FR8217338A patent/FR2514383B1/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1997003245A1 (en) * | 1995-07-11 | 1997-01-30 | Ahlstrom Machinery Oy | Method of separating impurities from lime and lime sludge and a method of causticizing green liquor containing impurities, such as silicon, in two stages |
| US6074521A (en) * | 1995-07-11 | 2000-06-13 | Ahlstrom Machinery Oy | Method of separating impurities from lime and lime sludge |
| CN1108412C (en) * | 1995-07-11 | 2003-05-14 | 安德里兹公司 | Method of separating impurities from lime and lime sludge and method of causticizing green liquor containing impurities, such as silicon in two stages |
Also Published As
| Publication number | Publication date |
|---|---|
| FI823417A0 (en) | 1982-10-08 |
| AT378381B (en) | 1985-07-25 |
| FI73754C (en) | 1987-11-09 |
| FR2514383B1 (en) | 1986-08-22 |
| FI73754B (en) | 1987-07-31 |
| SE448102B (en) | 1987-01-19 |
| FI823417L (en) | 1983-04-14 |
| ATA374982A (en) | 1984-12-15 |
| SE8105886L (en) | 1983-04-14 |
| FR2514383A1 (en) | 1983-04-15 |
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