Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR BLEACHING WOOD FIBERS
This invention relates generally to a method for bleaching wood fibers
with mixture of sodium borohydride and sodium bisulfate followed by peroxide
bleaching.
Combinations of reductive and oxidative bleaching have been used to
bleach wood fibers. For example, WO 01/59205 discloses a method for
bleaching mechanical and chemithermomechanical pulp by pre-bleaching the
pulp with a reductive bleaching agent, followed by peroxide bleaching
reinforced with borohydride. The reductive agents mentioned in this reference
are hydrosulfite, borohydride, hydrazine and formamidine sulfinic acid (FAS).
However, there are hazards associated with handling these reductive agents,
alone and/or in combination with peroxide.
The problem addressed by this invention is to find a safer and more
effective process for reductive bleaching followed by peroxide bleaching.
STATEMENT OF THE INVENTION
This invention is directed to a method for bleaching wood fibers by pre-
treating the fibers with a mixture of sodium borohydride and sodium bisulfate,
followed by peroxide bleaching.
The method comprises the steps of:
(a) adding to a vessel containing a wood fiber slurry a mixture produced by
combining: (i) an aqueous solution comprising sodium borohydride and sodium
hydroxide and (ii) an aqueous solution comprising sodium bisulfate, to produce
a treated slurry and (b) adding an aqueous solution comprising hydrogen
peroxide to the treated slurry in said vessel.
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DETAILED DESCRIPTION OF THE INVENTION
All percentages are expressed as weight percentages, unless specified
otherwise. The term "Pre-mix" (PMX) refers to a process in which borohydride
and bisulfate are mixed prior to addition to the wood fibers.
Dithionite ion, also referred to as hydrosulfite, can be produced by the
reaction between bisulfite and borohydride ions, according to the following
theoretical equation:
BH4 + 8 HS03 '+' H+ --~ 4 5204 2 ~' B(OH)3 "~ 5H20
The yield is somewhat less than I00°/ due to competing reactions,
including
that of borohydride with water, but is most often better than 85%. Since the
exact mechanism of the reaction has not been fully characterized, this
invention is not limited to reduction by dithionite ion, and other species
present
in the reaction mixture also may act as reducing agents. When the amount of
bisulfite is at 8 moles per mole of borohydride, the theoretical reaction
proceeds
to completion. Without wishing to be bound by theory, it is believed that use
of
more than the theoretical amount of bisulfate or less than the theoretical
amount of bisulfate results in a mixture containing hydrosulfite, sodium
bisulfite, borohydride and possibly other species.
In a preferred embodiment of the invention, in which borohydride is
mixed with aqueous sodium bisulfate solution in the form of an aqueous
solution containing sodium borohydride and sodium hydroxide, some of the
bisulfate is consumed in a neutralization reaction with the hydroxide ion. In
some applications, hydroxide ion present in borohydride solutions is
neutralized by acid added to the bisulfate solution. In such a case, to the
extent
that the hydroxide initially present in the borohydride solution has been
neutralized, it will not consume bisulfate, and will not be included in the
ratio
calculation. As described above, the theoretical reaction of borohydride and
bisulfate requires 8 moles of unconsumed bisulfate per mole of borohydride,
i.e.,
the ratio (moles bisulfite - moles hydroxide)/moles borohydride is
approximately 8. The present invention uses a ratio from 1 to 25. Preferably,
the ratio is no more than 21, more preferably no more than 18, and most
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preferably no more than 16. Preferably, the ratio is at least 3, more
preferably
at least 5, and most preferably at least 6.
In the embodiment of the invention, in which the fiber slurry is pre-
treated with borohydride and bisulfate solutions which are combined and then
added to the slurry, the ratio (moles bisulfate - moles hydroxide)/moles
borohydride combined for addition to the pulp (i.e., excluding the bisulfate
used
for pre-treatment) preferably is at least 3, more preferably at least 5, and
most
preferably at least 6~ preferably the ratio is no more than 21, more
preferably
no more than 18, more preferably no more than 16, more preferably no more
than 14, and most preferably no more than 10. The time interval between
addition of the premixed solutions and addition of chemicals used in peroxide
stage is not believed to be critical. Preferably, a time interval sufficient
to
ensure thorough mixing of the premixed solution and the pulp is allowed,
preferably at least 5 seconds, more preferably at Ieast 20 seconds, and most
preferably at least 30 seconds. Preferably, the time interval is no longer
than 2
hours, more preferably no longer than 1 hour, more preferably no longer than
30 minutes, and most preferably no longer than 15 minutes.
In one embodiment of the invention, combining water and sodium
metabisulfite, NazS20s, generates bisulfate. The aqueous sodium bisulfate
preferably is about 2% to about 45% active by weight. A preferred borahydride
composition for use in accordance with the methods of the invention is in
liquid
form and comprises about 1% to about 36% active sodium borohydride and
about 30 to about 40% NaOH or Na2COs (also known as soda ash), all by
weight. A particularly preferred borohydride composition comprises
approximately 12% active sodium borohydride and 40% NaOH.
In one embodiment of this invention, preferably, the addition of pre-
mixed solution for pretreatment is carried out by adding premixed solution to
the pulp slurry after the slurry has been screened, cleaned, thickened and
made ready for papermaking i.e., after the deckers in a typical pulp mill. In
one preferred embodiment of the invention, the premixed solution is added to
the disperser followed by adding chemicals for the peroxide stage at the
conveying screw that leads to a down-flow tower or chest.
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The method of this invention is a single-stage bleaching process, i.e., a
process in which the reductive and oxidative bleaching are done in the same
vessel, without a washing or contamination removal step between the reductive
and oxidative bleaching. In contrast, two-stage bleaching typically involves
sending the pulp slurry to another vessel for washing, pressing, or another
purification step prior to another bleaching step, e.g., oxidative bleaching.
In the embodiments of this invention, a borohydride solution and a
bisulfate solution are combined to produce a mixture which is then added to
the
pulp slurry. Preferably, the solutions are combined by mixing them in an, in-
line static mixer, a T-pipe mixer, a pump recirculator, an agitator, the
suction
side of a pump impeller, a high-shear mixer, or a low-shear mixer. Most
preferably, the mixer is an in-line static mixer. Typical in-line static
mixers
have from 2 to 24 internal elements, preferably from 2 to 6 internal elements.
The length of the piping from the mixer to the point of addition to the pulp
slurry also may affect the mixing preferably this length is at least 3.28 ft
(1 m),
more preferably at least 4.92 ft (1.5 m). The number of elements, the diameter
of the mixer and the length of piping required to achieve good mixing, i.e.,
to
produce a substantially homogeneous mixture, can be determined easily from
the flow parameters and fluid properties of each particular system. For
example, in one method dye is added to one of the solutions and good mixing is
assessed by visible determination that the color of the output is uniform. In
another method, the pH of the pulp slurry after addition of the mixed
borohydride and bisulfate solutions is measured a stable pH value is an
indication of good mixing, as are consistent bleaching results. Preferably, if
mixing is insufficient, the borohydride and bisulfate solutions are diluted.
Preferably, the borohydride and bisulfate solutions are mixed at a temperature
in the range from 4°C (39.2°F) to 50°C (122°F),
more preferably from 10°C
(50°F) to 35°C (95°F).
Preferably, the mixed borohydride and bisulfate solutions are added to
the pulp slurry directly, or by storing the output in a vessel for later
addition to
the pulp slurry. In one preferred embodiment, the output of the mixer is
stored
in a vessel and added to the pulp slurry within 12 hours of mixing, more
CA 02522144 2005-10-03
preferably within 6 hours, more preferably within 3 hours, more preferably
within 1 hour, and most preferably within 1/2 hour of mixing. In another
preferred embodiment, the mixer output is added directly through piping which
carries the output to the pulp slurry in less than 15 minutes, more preferably
less than 10 minutes, and most preferably less than 5 minutes.
Preferably, the amount of borohydride combined with sodium bisulfate,
measured as the percentage of sodium borohydride relative to the dried fiber
content of the pulp, is at least 0.0006%, more preferably at least 0.0009%,
and
most preferably at Ieast 0.0015%. Preferably, the amount of borohydride,
measured as the percentage of sodium borohydride relative to the dried fiber
content of the pulp, is no more than 0.12%, more preferably no more than
0.08%, more preferably no more than 0.045%, and most preferably no more
than 0.03%. In a preferred embodiment of the invention, a 12% aqueous
sodium borohydride solution stabilized with 40% NaOH is used. In this
embodiment, the weight of the solution used, measured as a percentage of the
dried fiber content of the pulp, is at least 0.005%, more preferably at least
0.0075%, and most preferably at least 0.0125%. Preferably, the weight of
solution used, measured as a percentage of the dried fiber content of the
pulp,
is no more than 1%, more preferably no more than 0.67%, and most preferably
no more than 0.25°/.
Preferably, the amount of sodium bisulfite combined with borohydride,
measured as the percentage of sodium bisulfite relative to the dried fiber
content of the pulp, is at least 0.02%, more preferably at least 0.03%, and
most
preferably at least 0.05%~ preferably the total amount is no more than 4%,
more preferably no more than 2.8%, more preferably no more than 1.5% and
most preferably no more than 1%.
The peroxide bleaching stage can carried out conventionally. The
preferred peroxide agent is hydrogen peroxide. Preferably, hydrogen peroxide
is provided in the form of an aqueous solution, most preferably approximately
50% aqueous hydrogen peroxide. The initial aqueous hydrogen peroxide
solution may be diluted with water to facilitate addition to the pulp slurry.
The
amount of hydrogen peroxide measured as the percentage of pure hydrogen
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peroxide relative to the dried fiber content of the pulp is 0.5 to 5%.
Preferably,
the bleaching time is 30 minutes to 4 hours. More preferably, the bleaching
time is at least 1 hour, and no more than 2 hours. Other peroxide
enhancement agents can also be added with peroxide. Examples of such agents
are sodium hydroxide (1 to 2.5%), sodium silicate (1 to 5%), and chelating
agents, e.g., DTPA (diethylenetriamine pentaacetic acid) and EDTA (ethylene
diamine tetraacetic acid, at 0.1 to 0.5%.
Preferably, the wood fibers used in this invention are fibers derived from
recycled paper, mechanical pulp or chemical pulp. Most preferably, the fibers
are derived from recycled paper. The initial (prior to bleaching) consistency
of
the pulp slurry preferably is from 2% to 50%, and more preferably from 8% to
30%. In general, the initial pH of the pulp slurry preferably is from 2 to 13.
For mechanical pulp, the pH preferably is from 4 to 8~ for chemical pulp, from
3
to 12, more preferably from 7 to 12~ and for recycled fibers, from 5 to 9. The
temperature for the bleaching stages preferably is from 20°C to
120°C, more
preferably from 50°C to 90°C.
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EXAMPLES
Table 1 shows the results of a comparison between (PMX)-P (pre-mix
followed by peroxide bleaching) and FAS-P processes on a mixed office waste
furnish from mill 1. Premix solution was generated using a (bisulfite - NaOH)
to borohydride molar ratio of 8.9:1. Reductive pretreatments were conducted at
various amounts of Premix and FAS, followed immediately after thorough
mixing of PMX or FAS into the pulp (ca. 1 minute) by peroxide addition to the
pre-treated pulp. Bleaching conditions are listed in Table 1. It is clear from
the table that PMX-P process performed significantly better than FAS-P
process. Table 2 shows the results of a comparison among (PMX)-P, FAS-P,
and Y-P processes on a recycled fiber furnish from mill 2. Bleaching was
conducted in the same way as for mill 1 pulp. Bleaching conditions can be seen
in Table 2. The results on this pulp also demonstrate that (PMX)-P process
performs significantly better than FAS-P and Y-P processes.
Table 1. Laboratory bleaching response of (PMX)-P and (FAS)-P processes on
pulp sample from mill 1.
Process FAS NaBH4 BisulfitepH Bright.~ L* a* b*
% ~ ~~ final%ISO
Unbleached0 0 0 -- 64.9 86.9 0.37 4.33
FAS-P 0.125 10.0274.7 91.1 0.82 3.i2
0.25 10.0774_.6 91.3 0.43 3.58
0.375 10.1175.1 91.6 0.17 3.7_2_
0.5 10.1675.5 91.8 0.15 3.66
(PMX)-P 0.00450.15 9.98 75.6 91.4 0.60 2.93
0.009 0.3 9.94 76.3 91.7 0.45 2.89
0.01350.45 9.87 75.8 91.5 0.49 2.94
0.018 0.6 9.83 75.9 91.6 0.47 2.97
Notes
The initial pH of the pulp slurry was 7.71 in all cases.
The unbleached pulp sample had a consistency of 25.96%.
Bleaching conditions= 77°C, 10% consistency, 1 minute for FAS and
premix stages, 60
minutes for P stage.
FAS stage commercial product, caustic to FAS weight ratio 1~1.
Peroxide stage 1.75% peroxide, 0.75% caustic, I% silicate (as 27% Si02
solution).
"PMX" means mixture solution of borohydride and bisulfite. The molar ratio of
(bisulfite - NaOH) to borohydride is 8.9 ~ 1.
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Table 2. Laboratory bleaching response of (PMX)-P, (FAS)-P, and Y-P processes
on
pulp sample from mill 2.
Process FAS/Y NaBH4 BisulfatepH Bright.L* a* b*
% % final % ISO
Unbleached0 0 0 -- 71.6 89.4 0.80 2.75
~ 0.1 10.02 79.3 92.1 1.51 1.31
(FAS)-P
~ 0,2 79.4 92.3 1.44 1.46
0.3 10.07 79.6 92.6 0.99 2.0
0.4 10.11 80.5 93.3 0.61 2.34
0.5 10.16 80.8 93.4 0.39 2.43
Y-P 0.05 9.98 79.4 92.4 1.11 1.53
0.1 80.0 92.7 0.96 1.56
0.15 9.94 80.4 92.9 0.82 1.65
0.2 9.87 80.5 92.9 0.79 1.64
0.3 9.83 80.8 93.1 0.68 1.73
(PMX)-P 0.003 0.1 9.98 81.0 92.9 1.38 1.42
0.006 0.2 81.7 93.4 1.06 1.65
0.009 0.3 9.94 82.0 93.5 0.99_ 1.60
0.012 0.4 9.87 81.8 93.4 0.98__ i.65
0.018 0.6 9.83 81.7 93.5 0.86 1.84
Notes
The initial pH of the pulp slurry was 8.15 in all cases.
The unbleached pulp sample had a consistency of 32.21%.
Bleaching_,conditions: 69°C (155°F), 10% consistency, 1 minute
for FAS, BGH, and PMX
stage, 60 minutes for P stage.
FAS stage: commercial product, caustic to FAS weight ratio
1:1.
Peroxide stage: 1.5% peroxide, 1% caustic, 1% silicate (as 27% SiOz solution).
"PMX" means a mixture of solutions of borohydride and bisulfate. The molar
ratio of
(bisulfate - NaOH) to borohydride was 8.9:1.
"Y" means sodium hydrosulfite generated by 1 mole of 12% SBH & 40% NaOH
solution, 8 moles of sodium bisulfate, and 1.7 moles of sulfuric acid. The pH
of
hydrosulfite solution is 6.8.
Laboratory bleaching procedures
Based on consistency, 7 g O.D. pulp was placed in heavy gauge polyethylene
I5 bags. The bags were sealed under nitrogen, shaken vigorously to disperse
the
pulp fiber, and preheated in a constant temperature bath to get to the desired
bleach temperature. Premix solutions were generated from an aqueous
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solution containing 12% sodium borohydride and 40% sodium hydroxide, and
NaHSOa (SBS). In the method of generating the premix solution, sodium
bisulfite powder was added to water in a round bottom flask and stirred until
the sodium bisulfite powder had completely dissolved. The sodium borohydride
solution was then immediately added under an inert atmosphere and under
rapid stirring to generate a completely formed premix solution. Based on the
borohydride concentration of each solution, the required volume of premix
solutions was calculated. Bleaching was carried out by adding the pre-mix
solution or FAS solution combined with caustic or hydrosulfite solution under
nitrogen and keeping the pipette below the surface of the pulp. Each bag was
resealed, kneaded thoroughly to mix for 1 minute. Peroxide together with
caustic and silicate were then added to the pulp and kneaded thoroughly to mix
and returned to the constant temperature bath for the desired time. At the end
of the bleaching period each bag was removed from the bath and the pH was
measured and recorded. The pulp was then diluted to 1% using deionized
water and dewatered using a 100-mess screen. The pulp was diluted to 1%
consistency again with deionized water and the brightness pad was made using
a Buchner funnel. One pad was made from each run and air dried overnight at
50% relative humidity. Brightness readings were measured using a
TechnibriteTM ERIC 950 and are the average of five readings from each
brightness pad.
