Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Brief Summary o the Invention
Variou~ process~s have been developed or pulping of
wood employing alkali metal polysulfide as a primary pulping
agent. For exaraple, a procQss for forming a poly~ulfide pulping
li~auor is described and claimed in our U.S. Patenk ~o. 3,470,061,
gra~ted septemb0r 30, 1969. In ~hat patent, various ~angan~se
compounds are employed for oxidation of alkali mQtal sulfide to
alk~li m~3tal polysulfide.
A ~urther proc~ss for forming polysul~ide pulping
li~uors is described in Per .~ndreas Landmark U.S0 Patent No
3,216,887, granted ~iovember 9, 1965, where t~e polysul~idll3 i3
form~3d by t~le air olcidatioll of sodium sulfide in a mixture of
white liquor and black li~uor. Sub~e~auent to ~ormatior~ of the
polysulfide, the liquor contacts the w~od to be pulpedO Becau5e
of this me~hod of operation, a sul~idity level of at least 40%
is neces~ary in order to obtain any significant yield illcrease
and, b~c:ause o~ this, there are sigi~iicant sulfur lo~es. The
sulfur losses provide a signi~ ant level oP pollution.
White liguor, recirculated ~rom wood impragnation, is
treated so as to form polysul~ida according to Canadian patent
No. 815, 432, i3sued on June 17, 196g to Domtar Limited. EiEic-
i~cy lev~ls operating in accordance with thi~ patent are relati-re-
ly low.
In accordance with the present invention, it ha~ been
de~ermined that a mix~ure of black liquor and white liguor ~:an
be oxidized to produce alkali metal polysulide, while the mixed
li~uor is in ~he presence of ~he wDod to be pulpedO Operating in
thi~ manner, the relativcly unstable poly3ulfide act~ upon the
wsod in~ediately after ~ormationO TllU5, nol: only ara there
i~proved pulping yields, but, in addition, a lower level o~
sulfidity can be e~ployed. q~hi9 lower sulfidity r~sults in
significantly dec~reasqd amount~ of sulfur loss, w~ich significant~
ly reduces pollution problems.
Brief Description o~ ~he Draw ng
In the ac~companying drawing, ths pulp yields obtained
according to t~e pre~ent process are compared with those obtainQd
according to a ~tandard ~aft process, at varying, co~parable
lignin levels.
D~tail~d De~cri tion of ~he Inv~ntion
_ _ p _
~ n accordance wi~h the present invention, wood chips
are impregna~ed wi~h a miacture of white liquor and bla~:k lisluo~.,
Air, or anot~er oxygen bearing gas, i5 int~oduced into the li~uor
in the digester, du:ring the impregnationr in ordsr to oxidize
the alkali metal sulfi~e p~esent in the mixed liquors to alkali
~etal polysulfide. In th~ oxidation o~ alkali metal sulfide,
not only is alkali metal polysulfide prod7lced, but~ additionally,
sulfur, sulfites, and thios~lfate~ are produced. ~he ~ulfur
coD~bines with a sulfid~ to yield t~e desired polysul~ide~ ~w-
~0 ever, ~ecau~e o~ the instability o~ the polysulfid~J it ~ayreact with the sulfite to produce thiosulate. As the thio-
sulfate is not effective in pulping, the result of thi~ reaction
i~ los~ of e~f~ctiv~ pulping p~wer, ~o tha~ higher ~ulfur levels
mu~ be maintained in.order to provide a given yield lev~l.
When thege higher sulfur levels are maintained, there is an
increased loss of sulfur products to the atmosphere, m2aning
increased pollution.
Aocording to t~e present process, w~ere th0 mixture
of ~lack liquor and white liquor is oxi~ized, in the presens~e
~ o~
of wood to be pulped, the polysulide generated iæ i~mediately
available to react with t~e wood. For thi~ reason, the poly-
~ulfide is effectively employed before any ~ubstantial amount
react~ with the sulfite, resulting in the ineffectual thio~ul-
fate and loS5 of the yield improvement provided by the poly-
sulfide~ ~hus, a given yield level can be obtained with a
lower total sul~ur content.
~ he alkali me~al sul~ide employed i~, preferably,
sodium sul~ide. This is the preferred material becau~e o~ i~5
signiicantly lowar cost and easier recovery.
In accordance with the present inven~ion, the sulfidity
level of the mixed liquor need not be above 30%. ~he mixed
liquor generally contains be~w~en 30 and 75% white liquor, with
the remainder black liguor~ Pre~erably, betw~en 50 and 60%
white liquor is employed~ The black liguor which i9 used i3 the
spent liquor from ~he dige~ting step, as is ~ormally obtained
with Kraft processing.
It wa~ praviou~ly no~ed tha~ the afoæemen~ioned
Landmark paten~ aid disclose an oxidation o~ a black liquor-white
liquor mixture to form a polysulfide for pulping~ ~owever, as
also indicated, the polysulfide is unstable and t~us react~ with
the ~ulfit~ to form thiosulate, before the polysulfide can
effectively b~ used. The prese~t pro~ess avoids this prc~blem,
by carrying out the oxidation in the pres3nce o the wood chip,
thus allowing for imrnediate utilization o~ the poly~ulfide and
Fninimizing the formation of the in~ffectual thiosulfate.
~ s also indicated, the aforem~ntior~ed ~omtar Limi~ed
pat~nt teaches the oxidation of white liquor, but thi~ oxidation
i5 not carried out in the pres~nce c~f the wood chips. ~!he
~8C~4~
oxidation, according to Domtar, i~ carried out in a ressel
3eparate from tha chips, so that the inunediate reac~ion of the
polysulfide with the w~od i~ not pos3ible and, therefore, a
higher total ~ulur level i~ re~auired in order ~o obtain a given
yield level, some of ths banefits o~ ~he polysulfide being lost.
~!he amount of oxygen bearing gas w~ich is erllployed, according
~o ~he process o$ the pre~ent invention, s~ould be suficient
to oxidize a significant guantity o~ the sulfide pre~ent in the
mixed liquor to polysulfide. Generally, ~ufficient polysul~id~s
10 is pxoduced tc significa~tly i~slprove the yield~ employing air
in an am~unt o fro~ 200 to 2,000 standard cubic feet for each
ton of pulp bei~g proces~ed. ~h~ gas being employed in this
oxidation can b~ any havillg an oxygen conten~ of from 15 to
lQ0%, ~ubstantially the remaind~r of the gas t~eam being made
up o che~ica~ly inactive gases.
During impregnation of the wood, the temperature
~hould be maintained below 120C~ and, preerablyO th~ wnod
chips are i~pregna~ed wi~h the ~ixed liquor at a tempera~ur~ of
abo~t 90C. f~r about 45 min~te~ while maintaining the flow
of the oxygen bearing gas. The ~low o~ oxygen bearing ga~ i~
th~ terminated and the tamperature of the contents are raised
~o n~r~al pulping temperatures of about 160 to 175C. in order
to compl~te th~ pulping stage.
A primary advantage of the present p~ocess is that
the formation of. polysulfide, with incr~a~ed yields~ can be
carried ~u~ empl~ying pulping ~olu~ion having ~u~fidity levels
no greate~ than 30%. Thu~, the white liquor employed con~ains
between 5 and 60 grams per liter o~ sulfur, in all o~ its com-
p~und and elemsntal forms. Preferably, the amount of sulfur ;~.
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,,~ , ... . ..
contained in the whi~e liquor i~ be~ween 10 and 30 grams per
liter~ e amoun~ of alkali contained in the pulping solu~ion
i5 not critical, but should range from 15 to 400 grams per liter,
as l~a20. More E~referably, the total alkali c~onten~ is from
70 to 120 gramsl per liter. The alkali i9 ~3nerally derived
from the group consisting o:E ~odium hydroxide, sodium carbonate,
and sodium ~ulf ide .
Paper p~oduced accordi~g to the proc:ess of tha pr~sen
învention, compar~d with paper produced by the standard Kra~t
process at eyuiv~lent Kappa numbsr~, doe.~ ~ot show any di:~erence
with regard to bur~t or teæ strength. Th~re is a sligh~
increase in th~ luminous reflectance o~ tllese paper produat,
~rh0n ~:ompareld with lthe ~ra~t process using a black liquor re-
cyde .
The irnprovements accomE~lished by the process o ~he
pre~nt invention will be described in greater detail in th~3
following exam~ s. q~hese examples ~hould be con~idered as
illu~trative onlyO and not as limiting in any way tha ~ull
~cope of the invention as ~overed in the append0d claims.
Exa~E~e 1
Two 20 lb. ch~rges (ov~n dry weigllt) of the sam~
batch of slash pine w~r~3 pulp~d according to this e~can~ple.
Ona ~3f t~e chargeY wa~ pulped according to a s~a~dard Kraft
process, with black liguor recycle, ~ile the other batch wa~
pulped according to the present pro~ess where tlle mixed li~auor
contained 55% white liquorl. In each case, the ~c)dium sulide
level of the whi~e~ liquor wa~ 40 grams pe3r liter. A guantity
of 5 cubic: feet of air per hour wa.~ introduced into the wood
chip-mixed pulping liguor of th~ charge p~lped a~cording to
-- S --
the present invention during the 90 minute impregnation,~ while ~ !
maintaining ~he li~auor at a terap~ratur~ of 90C~ The dige~tion~
were oltherwise carried out under ~e ~ame condil~ion~ including ::
heating to 173Co~ after impregnation, over a perio~ of 30
minutes, and then holding the ~atarial at 173Co for 90 minute~.
T~e ~ctive alkali content of l:he wl~ite liguor er~ployed wa~ about
106 grams E~er liter, exE~re~sed as P~a20. ~rhe rç!~ults o~ pulping
thes~ two batche:3 according ~o the proces~3es indicat0d ar~ ~hown
be low in Tabla I:
q~able I
T~ of PxOcess ~Field ~%) Screerled 5rield (O ~CapE~a ~umber :~
Rraft Process 57.0 45O3 83.2
Oxidation of 5~ . 2 50 . 2 78 . 7
Liquor in the
Pre~ence- of
Wood Chips
As can be seen from ~h~se resul~3, a ~3ignificantly improved yield
ia; obtainea employing th~ oxidation p,roces~ of the p~e~ent
invention, over a normal t~a~t proce-~, and thi~ increased yield
i~; ob~ain2d wi~hou~ an increa~Q in ~he sulfur level of the liquor.
l~hus, ~he increasad yield~ come with no inereas~ in pollution.
Exampl~ 2
~ rhe proce~ oiE ~he present invention was repeated,
employing ~he same ~a~erials and conditions a~ noted in Exa~pl~3 :
1, but with certain proce~s variakion~. In particular, the mixed
liglaor contain~ 60% white liquor and ~he air was introduced at
a rate of 3 cubi~ fee~ per hour for a 45 minu~e impregna~ion
period at 80C. Following dige~;tion, a yi~3ld o 57.,3% and a
~creen~d yield o 51.5% wexe realized at a Kappa number of 72.8. ~:
Thu~, the economic advan~age of ~he process of the pre~an~
invention, this advantage being obtained without an increase in
pollution level, is agaln demonstrated.
The figure shows yield increases obtained employing the
process of the present invention, when compared with standard Kraft
pulping. As can be seen rom the figure, at Kappa numbers of ~rom
50 to 80, yield increases of ~rom 1 to 3% are obtained employing
the process o~ the present invention. These yield increases are
obtained without an increase in pollution level.
Mhile slash pine has been indicated as the wood pulped
in the specific examples, various other wood species may also
be used. In general, any wood pulpable according to a Kraft
process can be pulped accord~bo the present invention, with
improved yields~
The various streams in the pulping process according
to the present invention are recoverecl by the same methods
normally em~loyed in Kraft processing~ The sulfide level of
the black liquor leaving the digester according to the present
invention is no higher than that found in the blac]c liquor
from a standard ~raft process. Thus, the present process
provides a significant economic advantage over a standard
~raft process in that, while essentially the same equipment
and processing are employed, increased yields are obtained.
The white liquor may be added to the wood to produce
a white liquor-wood mixture in which the wood is impregnated
with the white liquor, the black liquor then being added to
the white liquor-wood mixture with the result that the wood
is inevitably impregnated also with the black liquox. Thus,
the impregnation of the wood is still with the white liquor
and black liquor.
~8~40~3
The i.nvention has been described in detail with
particular reference made to various prefexred embodiments
thereof, but it will be understood that variations and modi-
fications can be effected within the spirit and scope of the
invention as described hereinabove and as defined in the
appended claims.
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