Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~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~
-- 1 --
~.
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
-- 6 --
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/~ . . __
- 12 -
