Note: Descriptions are shown in the official language in which they were submitted.
B~CKGROUND OF THE INVENTION
This invention relates to the art of pulp and paper
making, more specifically the art of brightening and bleach-
ing kraft pulp.
Development of ways of brigh-tening and bleaching
kraft or sulphate pulps which result in less chemical use,
fewer treatment cycles, and/or more readily available or less
environmentally burdensome chemicals are important from eco-
nomic and ecological points of view.
It is well known in the inaustry that replacement of
chlorine and chlorine dioxide bleaching processes are of par-
ticular interest because of all these considerations. Oxygen
and oxygen-ozone bleaching processes such as those of
Canadian Patent No. 1,132,760, issued October 5, 1982 of R.B.
Phillips et al; Rothenburgh et al, TAPPI, 58 (8), 182 (1975);
Singh, Canadian Patent No. 966,609 and Soteland, Pulp and
Paper Magazine of Canada, 75 (4): T153-58 (1974) are quite
attractive candidates as replacements for chlorine and chlo-
rine dioxide processes and are potentially able to save on
both chemical and capital costs as well as to substantially
reduce chemical concentrations in the process waste water.
Reaching satisfactory brightness levels and to sta-
bilizing brightness once achieved requires multiple ozona-tion
and alkaline extraction steps. As each of these steps add to
capital, energy, and chemical costs of the project as well
as increasing the chance of chemical particularly oxidative
attack on and consequent weakening of the cellulose fibers of
the pulp being bleached, it is desirable that the number of
sequential steps to reach and maintain a given brightness
level be held to a minimum. The use of an addi-tional bleach-
ing reagent is, therefor, indicated.
.
5~
C~TATION OF RELEVANT LITERAT~RE
Wade in United States Patent No. 3,318,657 proposed
the use of borohydride prior to ozonation as a means for
protecting the cellulose of the pulp ~rom subse~uent ozona-
tion. He reports both improved brightness and increased
tensile strength. sack~ et al. in United States Patent No.
2,963,395 and West in United S-tates Patent No. 3,467,574
disclose the use of hydrosulfite in the reductive bleaching
of groundwood pulp. Rigid exclusion of oxygen is required,
the optimum pH is about 5 and naturally no teaching of the
effects of use in ozone bleaching is made because of the
nature of the substrate being bleached~ Eckert in United
States Patent No. 4,119,486 aiscloses -the use of cationic
surfactan-ts as aids in ozone pulp bleaching. Use of hydro-
sulfite is not suggested.
Tredway, C.M. in Pulp and Paper, March, 1979 p. 73
discusses factors affecting the bleaching of groundwood
with sodium hydrosulfite especially with V-Brite B a stab-
ilized sodium hydrosulfite blend. This study confirmed
that in groundwood bleaching presence of oxidants such as
oxygen in the hydrosulfite bleaching operation was dele~e-
rious and best results were in pH ran~es below 7Ø
Wayman, et al. in Tappi, 48, p. 113 (1965) discuss
peracetic acid bleaching of groundwood hydrosulfite stages
to further improve brightness are examined. The pulp is
washed with aqueous SO2 between stages. p~ of hydrosul-
fite treatment stages is always below 7Ø
Hydrosulfite bleaching of qroundwood pulp is well
known in general. Applicants are unaware of any relevant
literature discussing the effect of a hydrosulfite bright-
ening or bleaching steP in combination with oxidative
bleaching processes other than in combination with perace-
tic acid on qroundwood.
The present invention provides for the use of hydro-
sulfite in conjunction with ozone bleaching steps for kraft
pulp at pH values greater than 8 and without requiring ri-
gid exclusion of oxygen during khe hydrosulfite treatment
step.
Copies of all patents and literature are enclosed
for the Examiner's convenience.
SUMMARY OF THE INVENTION
The invention provides a process for enhancing the
brightness and reverted brightness of ozonated oxygen
bleached kraft pulp which comprises treating said ozonated
oxygen bleached kraft pulp with h~drosulfite solution at
pH values of about 8.0 to about 12Ø
The tangible embodiments produced by this process
aspect of the invention possess the inherent physical pro-
;~ perty of being substantially brighter than pulps run
through a similar bleach Process without hydrosulfite and
having almost as strong sheet tensile and bursting strength.
The tangible embodiments produced by this process o the
invention possess the inherent chemical property of retain-
ing a higher brightness on accelerated aging as hereinafter
described than pulps treated in a similar bleach process
omi-tting hydrosulfite treatment.
The tangible embodiments produced by this process
aspect of the invention possess the inherent applied use
characteristics of being formable into a coherent web to
produce a continuous bright strong sheet of paper, having
usefulness as a substrate for printing and in forming
containers, and the like.
The invention also provides in a process whereby
unbleached kraft pulp is bleached by treatment wi-th oxygen
3 --
t~
and ozone the improvement comprising treating the pulp,
following the treatment of said pulp with ozone, with hydro-
sulfite soluti.on at a pH of about 8.0 to about 12Ø
DESCRIPTION OF THE PREFERRED EMBODIMENT
_
The mannex of practicing -the processes of the inven-
tion will now be specifically illustrated with reference
to a specific embodiment thereof, namely incorporation of
an alkaline hydrosulfite treatmen-t stage in an oxygen,
peracid, ozone bleaching sequence for a southern (Louisiana)
softwood kraft pulP (I). I may be treated in a conventional
oxygen bleaching process to produce a partiall~y bleached
(partiallv delignified over I) pulp (II). II :may then be
treated in a conventional peracid bleaching process to
produce a still more bleached (fur-ther delignified over II)
pulp (III). III may be treated in a conventional ozone
bleaching process to produce a still further bleached (.fur-
ther delignified over III) pulp (IV). In a conventional
bleaching process, IV would now be trea-ted in an alkaline
extraction to further bleach and delianify the pulp. IV
may, however, be treated in an alkaline extraction process
at pH ~.0 to 12.0 to which a soluble hydrosulfite salt has
been added to produce a pulp (V) further brightened over IV
and which is also brighter than the product of IV treated
in a conventional alkaline extraction. V also maintains
higher reverted brightness than IV or the product of IV
treated in a conventional alkaline extraction.
The soluble hydrosulfite salt employed may be a salt
of any metal cation which will not supply residual color on
its own, not react with the hydrosulfite ion and its selec-
tion will he at the option of the operator. Commonly, it
will be sodium hydrosulfite although salts of other alkali
_ ~ _
?i~i
metal cations, alkaline earth metal cations and of other
colorless cations such as, for example, zinc may also be
employed. The hydrosulfite may be generated by Processes
well known in the literature. Sodium hydrosulfite is com-
mercially available in solid and solution form. It may
also be generated from sulfur dioxide and alkaline sodium
boroh~dride solution by the well known "borol" process.
One skilled in the art will recognize that although
oxygen and oxygen ozone bleaching processes are particular-
ly suitable in brightening, bleaching and/or further delia-
nifying lignocellulosic Ful~s which had the major portion
of the lignin content removed through the kraft process
they may also be employed in bleaching, brightening and/or
further delignifying lignocellulosic pulPs which have been
treated by other processes such as the sulfite process to
remove the bulk of their original lignin content. Similar-
ly the particular lignocellulosic material is not especial-
ly critical. In addition to I other northern or southern
hardwoods or softwoods may be employed as well as ~ulps
from such materials as tropical hardwoods and softwoods,
bagasse and the like.
One skilled in the art will recognize that in addi-
tion to the treatment stages described in producing V iE
further bleaching, briqhtening and/or delignification is
desired, additional ozonation and alkaline extraction
stages may be employed. It is further contemplated that
each stage of the process, that is, oxygen bleaching, per-
oxide bleaching, ozone bleaching, alkaline extraction, and
alkaline hydrosulfite treatment will be followed by a wash-
ing of the lignocellulosic pulp. Such washing may be
effected as a wholly separate operation, in the
more usual case, on a portion of the rotating screen or fil-
ter drum surface used to separate the earlier treatment sol-
ution from the bleached pulp in preparation for the next
treatment of the process. As stated herein above, the ble-
aching, extraction stages, except for the alkaline hydrosul-
fite treatment stages and the washing processes of this in-
vention are conventional~ Their reactive conditions, pro-
cedures, process limitations and designs are well known in
the art.
An oxygen bleaching stage may be carried out in about
3 to 30% pulp consisting of 0.2 to abou-t 3% oxygen based on
O.D. (oven dried) pulp. The bleaching is effected at 80 to
120C and pH 9 to 12 for 10 to 60 minutes. Such conditions
are similar to that oxygen bleaching described by Soteland,
I'Bleaching of Chemical Pulps with Oxy~en and Ozone", Pulp and
Paper Maqazine of Canada, 75, T153 to 158 (1974).
The peroxide bleaching stage may be internal or up-
stream of the ozone stage. It may be carried out by any of
the well known alkaline peroxide, acid peroxide or peracid
bleaching processes. Alkaline peroxide bleaching stages
are usually carried out on pulp of 10-25% consis-tency in the
presence of 0.2 to about 2% peroxide, based on O.D. pulp.
The bleaching is performed at 20-90C and pH 7-11 for a
reaction time of about 10 to 300 minutes. Conventional per-
oxide stabilizers are commonly used. Similar conditions are
disclosed by Vartiainen, "Utilization of Peroxide in Pulp
Bleaching", Paperi ia PUU, 51, 277-284 (1969) and N. Hartler,
et al., "Peroxide Bleaching of Kraft Pulps", Tappi, 43, 806-
813 (1960).
Under peracid conditions, the Peroxide bleaching step
is conducted on pulp of 3-25% consistency with 0.2-2% per-
acid, exPressed as H2O2 on O.D. Pulp basis. The bleaching
is per-
~ - 6 -
formed at 20-90 and pEI 3-10 for a reaction time of 10-300
minutes. Acidic peroxide bleaching is usually effected at
pulp consistencies of 10-25% and 0.2-2% peroxide based on O.D.
pulp at 40-80C and pH 3-5. More preferably, 5-100%, based
on the peroxide weight, of metal activators selected from the
group comprising tungsten, titanium, tin, molybdenum, chromium,
osmium, selenium, and vanadium are added to such acid peroxide
bleaching stages.
Following the peroxide bleaching stage the pulp may
then be treated in a conventional ozone bleaching stage. Ozone
bleaching stages are carried out for example as described in
Rothenberg et al, "Bleaching of Oxygen Pulps with Ozone,"
Tappir 58, 182-185 (1975), Canadian Patent No. 966,604 and
Canadian Patent No. 1,090,510, issued December 2, 1980, and
having a common assignee. Such processes are conducted at 1-
40% pulp consistency and 0~2 to 1% ozone (consumed) on an O.D.
pulp basis. The reaction is usually carried out at 15-60% and
pH 2-7 for about 5-30 minutes.
The caustic extraction stage which, as mentioned,
may follow the above-described ozone stage is of the types
also described in the prior art. Such types include those of
Canadian Patent No. 966,6~04, e.g. with about a 2% sodium
hydroxide solution at 65-71C (150-160F) for about 1.5 hours.
Most preferably, the alkaline extraction is effected at pH
about 6-8 and at a temperature of 20-70C for about 10 min-
utes to 2 hours on pulp of 1-15% consistency.
The alkaline extraction stage containing hydrosul-
fite may also be effected at temperatures of about 20 to
70C for about 10 minutes to 2 hours on pulp of 1 to 15% con-
sistency but the p~ range is desirably in the range of about8.0 to about
12.0 preferably from about 8.0 to about 10.0 employing a hy-
arosulfite ion concentration equivalent to from about 0.5 to
3.0~ sodium hydrosulfite on a dry pulp weight basis at tem-
peratures from about 40~ to 70C. Depending on the bright-
ness levels desired to be achieved, ozone and extraction
stages may be rePeated in sequence a number o* times as will
be hereinafter illustrated. If such repetition is effected,
alkaline extraction stages containing hydrosulfite may be
emPloyed with other extraction stages being conventional
alkaline extraction stages. In such instances, it is pre-
ferable that the later, at least the last, extraction stages ~ ;
be alkaline extraction stages containing hydrosulfite. If
a hydrosulfite containing extraction stage is not desired
as a last stage, then lt is preerred that the last stage
. :
be an ozonation stage.
As noted above each bleaching stage and extraction
stage of this process is most~preferably followed by a wash-
ing of the pulp. Such conve~tional washing at about 1.0%
pulp consistency a~fords final removal of the bleaching;or
:
~ ~20 extraction solutions and solubilized compounds therein from
.
; the pulp prior to;further bleaching. While fresh or recyc-
led and recovered wash water ma~ be employed ~or such wash-
ing sta~es, it is more pre~erred to use the effluents from
the next later stage of the bleaching sequence as shower
water for that washing fo,llowing the preceding stage of the
continuous bleaching process. Moreover, such effluents may
be used to dilute the filtered and washed pulp slurry to
the appropriate consistency for the next sequential treat-
ment stage. This counter-current washing and dilutlon
markedly reduces the amount of fresh water needed in the
multistage bleaching process. The effluents from the blea-
ching, washing and extraction ste~s and counter current flow
~ ~ ,
- 8 -
~ .
~l~L5'~
are amenable to recyc]ing through a conventional recoverv fur-
nace to remove contaminants and permit reuse in the substan-
tially closed cycle bleaching process. ~ne advantage of this
se~uential process is that such recycling may be had without
fear of corrosion due to sodium chloride effluent build-up
or explosion due to chloride in the smelt. It should be
un~erstood that while a continuous bleaching process is des-
cribed above, a batchwise sequence would also enable such
counter-current flow.
The following examples further illustrate the best
mode contemplated by the inventors for the practice of their
invention.
For convenience and brevity in the examples, the ste~s
in a bleaching sequence may be designated by a letter se-
quence where Z represents an oæonated oxygen bleaching stage,
or, in common usage, an ozone bleaching stage; Pa is a pera-
cetic acid bleaching stage; E is an alkaline extraction stage
(pH 8.0); and V is an alkaline hydrosulfite extraction stage
(pH 8.0 to 12.0~. Thus, Pa E Z EZ represents a peracetic
acid, alkaline extraction, ozone, alkaline extraction, ozone
bleaching sequence and Pa EZ VZ represents a peracetic acid,
alka]ine extraction, ozone~ hydrosulfite treatment combined
with alkaline extraction, ozone treatment sequence~
Example I
Oxygen bleached southern pine kraft pulp (oermanga-
nate No. 7.5) is further bleached through a PaE2 sequence.
After removal of the filtrate from the ozone treatment
step, portions of the pulp are treated in an alkaline ex-
traction step at pH 8.0, 10.0 and 12.0 in the presence of
and the absence of sodium hydrosulfite(V-Brite trademark
Virginia Chemical Inc.)(2~ on a dry pulp basis). The re-
~:~s~
action times and temperature as well as the permangate num-
ber, pulp viscosity, brightness (TAPPI T525) of handsheets
of the pu]p and reverted brightness of handsheets of the
pulp as measured by an accelerated aging for one hour at
105C following the general procedure of TAPPI CA6064 Draft
#3, January 15, 1979. Both brightness and reverted bright-
ness values illustrate the advantage of -the presence of
hydrosulfite in the alkaline extraction stage.
TABLE 1
Reaction Condition
pH 8.0 3.010.010.0 12.0 12.0
Temperature (~C) 65 65 65 65 65 65
Time (hr.) 0.5 0.50.5 0.5 0.5 0.5
Sodium hydrosulfite - 2.0 - 2.0 - 2.0
(wt% on oven dry pulp)
Bleached Pulp
Properties
Permanganate ~o. 1.1 1.1 - - 0.9 1.0
Viscosity (cp) 11.6 10.7 1~.4 10.6 9.5 10.0
Brightness % GE 72.6 77.0 72.4 77.0 73.5 75.3
Reverted Brightness 67.6 73.5 69.5 73.8 69.2 71.8
% GE
xample 2
Oxygen bleached southern pine kraft pulp is further
bleached through a PaEZEZ bleaching se~uence. After sepa-
ration of the filtrate from the las-t ozonation se~uence, a
final extraction of samples of the pulp under various con-
ditions is effected. Treatment conditions and results which
again illustrate the brightness and reverted brightness im-
~rovement obtained through the presence of hydrosulfite in
the alkaline extraction stage are ~resented in Table 2.
-- 10 --
1~54~US
~ ~ o
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8~ ~ ~:
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, ~ . ~ ~
~
' ' ^~1 ~ ; ' "~ ~ :
,
w W W y~ ~;b ~Ijo
I ~
115~205
'u~ ,
~Wo
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~ Lf) O '` .
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o ~ o ~ ~
ll o~ o~ ~:
~I
il -12
case 2509
-~Lg S~L2~S
. I~ple] , .
Oxygen and peracetic acid b]eached southern pine
kra~t pulp sa~ples are fur-ther bleached throuyh ZEZ (A), ~VZ
(B),-and ZVZV (C) treatmènt sequences at pH 8.0 in both E and V
treatment stages. Yield and significant chemical and physical
properties showing the brightness improve~ent, reverted bright-
ness improvement coupled with only slight lowering of physical
properties as a result of alkaline extraction including hydro-
sul~ite are tabulated in Table 3.
: ,
TABLE 3
Bleach Sequ~nce A B C
___ -_
(Properties)
Ozone Consumption
~ on oven dry pulp)
ls~ Stage 1.801.80 1.80
2nd Stage 0.360.35 0.38
Yield ~Based on Dry ~7eight of 97 0 97.0 97.0
- pulp after Pa stage)
Permanganate No. 0.6 0.3 0.3
Carboxyl (moles/gm)xl0~5 4.9 4.9 -
Carboyxl (moles/gm)xl0-5 12.011.0 -
Viscosity, cp 8.6 7.8 8.4
Brightness % GE - 83.487.1 86.4
Reverted Brightness % ~E 75.3 81.2 80.3
Beating Time,* (Rev) 41004000 3000
Freeness (Canadian S-td) ~rnl) 385 385 385
Basis weight (gm/sc~.) 62.160.9 Gl.5
Caliper (inches) 3.523.41 3.46
Bulk (A. S. V.)** (cc/g) 1.44 1.42 1.43
Tensile (lb/in) 28.226.6 27.5
Stretch (%) 2.562.47 2.30
T E A.*** (in. lb./sq.in.) .fi70 .431 .393
Breaking length (meters) 8103 7816 7990
Mullen (p.s.i.) 48.241.3 44~4
Burst Factor 54.647.7 50.8
__., .
*PFI Mill No. 170 - Beating Performance Adjus-ted by Norwegian
Pulp and Paper Research Cns-titute.
**~pparent Specific ~7Olume
*-~Tensile Energy Absorption
-1 -13-
.
'I ..
~ Case 2509 ~
3~ii
TABLE 3 cont'd
Tear (gms.) 56.0 55.2 53.2
Tear Factor 90.2 90.6 86,5
O~acity (%) 59.2 59.1 58.4
Scattering Coefficient (cmw/9)167.4 167.3 170.7
Elastic ~lodulus x 1014.354.39 4.09
(dynes/sq.cm.)
-14-
