Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR BLEACHING MECHANICAL PULP
FIELD OF INVENTION
This invention relates to the manufacture of paper and, more particularly, to
the
bleaching of mechanically-produced pulps containing lignin, including both
softwood and
hardwood, so as to reduce yellowness and improve brightness.
BACKGROUND OF INVENTION
Mechanically-produced pulps, softwood and thermomechanical pulps containing
lignin, as opposed to chemically produced wood pulps, used for the production
of paper,
have traditionally been bleached to improve the whiteness thereof. Newsprint,
which
contains a relatively high lignin content, is either not bleached or only
mildly bleached,
with the result that it is usually of a darker quality than paper produced
from fully
bleached pulp, and tends to darken further when exposed to light, a phenomenon
known
as "reversion".
Whiteness or "brightness" of paper is conventionally measured by brightness
measurements based on the reflectance of light at a wavelength of 457 nm,
using such
instruments as an Elrepho brightness meter. There is, however, another measure
of
whiteness which is even more significant and that is the degree of yellowness
(CIE
yellow colour coordinate b*). Bleached softwood pulps usually have a
brightness of
about 70-90% Elrepho and a yellowness b* of 8 or more (often 9-10 at a
brightness of
76-78 points ISO), and there are several known methods for achieving these
results,
using either an oxidative process using hydrogen peroxide under strongly basic
conditions or reductive processes using hydrosulfite (dithionite) or
combinations thereof.
Currently, hydrogen peroxide bleaching of pulps is generally performed at an
alkaline pH of -10-11 in order to facilitate the ionization of hydrogen
peroxide to HOO-.
This is accomplished using a mixture of a water soluble alkali (usually sodium
hydroxide)
and a buffer (usually sodium silicate). This allows maintenance of the pH at a
level that
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permits effective bleaching, and also buffers the solution to minimize
peroxide
decomposition, which increases with increasing alkalinity. Often a small
amount of
magnesium ion is added to form a colloidal suspension of magnesium silicate,
which
may help, by adsorption, to inactivate metallic catalysts of peroxide
decomposition
such as Mn and Fe ions.
Silicates, however, can result in buildup of silica scale, especially when
bleach
process water is recycled. The alkali (NaOH) is expensive and must be
neutralized
(soured) and/or washed out after the bleaching process. In addition, alkali is
known to
have a darkening effect on pulps. Alkali-induced darkening reactions in the
pulp
compete with bleaching and effectively limit the brightness to ISO -84. Alkali-
induced
depolymerization and "peeling" reactions of cellulose are detrimental to fiber
length and
strength. Rates of the various alkali-induced reactions increase at higher
temperatures
and limit practical bleaching temperatures to -80 C.
There is an ongoing need for improved but inexpensive mechanical and
thermo-mechanical pulps having improved brightness of the order of 85 and
decreased
b* values of the order of less than 5; and with greater stability of the
optical properties,
i.e., decreased reversion.
OBJECT OF INVENTION
An object of the present invention is to provide improved mechanical and
thermo-mechanical pulps having a brightness of 80 (ISO) or more and a
yellowness
(b*) value of 6 or lower. A further object is to provide an improved process
for
producing such pulps, wherein added alkali and/or silicate is not employed but
equivalent optical characteristics to processes employing alkali, optionally
with silicate,
are obtained. A still further object is to provide means to break the
"brightness ceiling"
of -84-85 ISO common in alkali-based processes.
BRIEF DESCRIPTION OF INVENTION
By one aspect of this invention, a method for bleaching an unbleached
softwood or hardwood pulp is provided, using hydrogen peroxide without added
alkali
for activation. The method comprises treating (digesting) said pulp in an
aqueous
hydrogen peroxide solution containing up to about 10% carbonate at a
temperature in
the range of 15 to 80 C, and preferably 15 - 60 C. The solution may be up to
about
30% by weight of pulp. The carbonate is preferably magnesium or calcium
carbonate.
The pH range of the method is preferably about 6 to about 7.
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By a second aspect of the invention, a method for bleaching an unbleached
softwood or hardwood pulp is provided, comprising treating (digesting) said
mechanical
pulp in an aqueous hydrogen peroxide solution containing at least 4% H202 and
at least
6% alkaline earth metal carbonate by weight of pulp at a temperature of at
least 15 C,
more preferably at least 40 C, still more preferably at least 60 C. Still
more preferably,
the temperature range is 750 to 120 C, more preferably 80 to 115 C, still
more
preferably, 80 to 110 C. At such temperatures, the reaction may proceed for
about one
to three hours, with shorter times required at higher temperatures. No alkali
(NaOH) is
added to the reaction mixture. The alkaline earth metal carbonate is
optionally calcium
carbonate and preferably magnesium carbonate. Preferably the alkaline earth
metal
carbonate is at least 10% by weight of pulp, more preferably at least 20%, at
least 25%,
at least 30%, at least 35% or at least 40%. Preferably, the H202 is at least
5%. But, the
H202 may be at least 10%, at least 15% or at least 20%. The invention provides
the
advantage of employing temperatures greater than the practical "ceiling" of -
80 C for
alkali bleaching reactions, without alkali induced darkening of the pulp.
According to this aspect of the invention, a b* value of less than 6 is
preferably
obtained. An ISO brightness value of at least 85 is preferably obtained, still
more
preferably an ISO brightness value of at least 90 obtained. In a particularly
preferred
embodiment, the wood pulp is a mechanical hardwood pulp, the ISO brightness
value
obtained is in the range of 90-96 and the b* value is less than 2.6.
If desired, the bleaching method of the present invention can be coupled with
a
formaldehyde/carbonate process as described in our U.S. Application Serial No.
08/936,200 (1997). In said process, a wood pulp containing lignin, such as a
bleached
hardwood or softwood pulp, is digested in an aqueous formaldehyde solution
containing
carbonate, preferably an alkali or alkaline earth metal carbonate, and
preferably at
ambient temperature. Yellowness b* is preferably reduced to less than 8, more
preferably to less than 6, still more preferably to less than 5. This coupled
method of a
peroxide/carbonate treatment and a formaldehyde/carbonate treatment is also
encompassed by the invention.
Preferably, the invention provides improved b* without significant
delignification.
Delignification commonly occurs under strongly acidic or strongly basic
conditions and
can result in significantly reduced yields.
The invention provides several advantages over conventional hydrogen peroxide
bleaching:
1. Lower cost. An alkaline earth metal carbonate such as magnesium
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carbonate is very inexpensive as compared to sodium hydroxide and
sodium silicate.
2. Increased brightness values and reduced b* values for both softwood and
hardwood mechanical pulps over conventional bleaching. This increases
the number and variety of applications for the product, significantly
expanding its marketability.
3. Much less iight sensitive pulps. This is indicated by their lower b*
values,
limiting light induced reversion processes.
4. Avoidance of problems with silica scale and alkaline corrosiveness on
equipment.
5. Environmental benefits due to the avoidance of the use of soluble alkali,
silicate and acid souring agents.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The inventors have found the inclusion of carbonate, particularly alkaline
earth
metal carbonate, in a hydrogen peroxide bleaching process to be beneficial and
effective
without need for added alkali (sodium hydroxide). The method of the invention
is carried
out at approximately neutral pH, and less than about pH 8, thus avoiding the
disadvantages and drawbacks of commonly used alkali bleaching processes.
Although it is known to use sodium carbonate at fairly high concentrations in
some bleaching reactions, the use of substantial levels of alkaline earth
metal
carbonates, e.g., calcium carbonate or magnesium carbonate, has heretofore
been
unknown. Previously, low levels (~ 1%) of magnesium salts, such as, e.g.,
magnesium
silicate, have been employed in bleaching reactions optionally containing
sodium
carbonate, to adsorb and inactivate metallic catalysts of peroxide
decomposition such as
Mn and Fe ions. However, prior to the present invention, the advantages of
employing
levels of magnesium carbonate of 10-fold or greater magnitude have been
unrecognized.
According to the invention, a method for bleaching an unbleached softwood or
hardwood pulp is provided, comprising digesting said mechanical pulp in an
aqueous
hydrogen peroxide solution containing at least 4% H202 and at least 6%
alkaline earth
metal carbonate by weight of pulp at a temperature of at least 60 C. No alkali
(NaOH) is
added to the reaction mixture. The alkaline earth metal carbonate is
optionally calcium
carbonate and preferably magnesium carbonate. Preferably the alkaline earth
metal
carbonate is at least 10% by weight of pulp, more preferably at least 20%.
Preferably,
the H202 is 5-15%. The temperature range is preferably 75 to 120 C, more
preferably
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80 to 115 C, still more preferably, 800 to 110 C. At such temperatures, the
reaction
may proceed for about one to three hours, with shorter times required at
higher
temperatures.
Because of their low level of solubility in water (compared to that of e.g.,
sodium carbonate) an alkaline earth metal carbonate employed according to the
method of the invention generally does not completely dissolve. Thus,
bleaching
mixtures containing, for example 10% magnesium carbonate, appear as a white
suspension. Hardwood mechanical pulps have been found to be bleached by this
new
heterogeneous process to ISO brightness values of about 90 and b* values from
< 5 to
as low as -2. Softwood mechanical pulps have been found to reach brightness of
about 85-87 and b* values of about 5-8.
Although the mechanisms by which this novel hydrogen peroxide bleaching
process accomplishes the significant enhancements in b* and brightness values
are
incompletely understood, and without wishing to be bound by this explanation,
it is
believed that the process is initiated due to heterogeneous activation of the
hydrogen
peroxide by the insoluble alkaline earth carbonate. For example:
MgCO3 + H202 - Mg2+ + HC03 + HOO'
In contrast to conventional processes it is not necessary to have any soluble
alkali
(OH-) to ionize the peroxide, and the process takes place at neutral or mildly
basic pH
(6-8). As a result bleaching can be accomplished at temperatures ranging from
about
15 C to about 120 C. No alkali induced darkening reactions compete with
bleaching
with the result that the "brightness ceiling" of 84-85 which limits the
conventional
processes can be broken. Preferably, no silicate, e.g., sodium silicate is
used.
Any alkaline earth metal may be employed according to the invention, but
magnesium has the added beneficial effect of replacing manganese in the pulp,
allowing its easier removal by chelation. As noted above manganese and iron
ions are
the main cations responsible for decreasing the activity of hydrogen peroxide
solutions
in pulp bleaching.
According to one embodiment of the invention, using a 6% charge of both
hydrogen peroxide and alkaline earth metal carbonate, an aqueous suspension of
pulp
(about 10-20% consistency), and a reaction time of 1 h at 50 to 60 C, effects
good pulp
bleaching for both unbleached hardwood and softwood mechanical pulps. Values
of
brightness and b* at least as good, and in some cases better, than those
obtained by
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conventional alkali peroxide bleaching have been obtained. The pH of the
mixture
remains about 6-7 throughout the treatment.
The significant drop in b* values renders the pulps visibly much whiter in
appearance and they are accordingly believed to be less susceptible to light
induced
reversion processes. This is tested by accelerated photoreversion experiments
done in
a photoreactor under several 350 nm ultraviolet lamps. Papers made from pulps
treated according to the invention are expected to photoyellow more slowly
than papers
made from control pulps, especially during initial irradiation.
The inventors believe that a further decrease in the reversion rate can be
obtained if the pulps treated according to the invention are further treated
by a reversion
inhibitor, such as, for example, the polyethylene glycol bisthiols described
in our paper
(Wan et al., "Some mechanistic insights in the behaviour of thiol containing
antioxidant
polymers in lignin oxidation processes", Res. Chem. Inter. 22: 241-253
(1996)).
Suitable reversion inhibitors include polyethylene glycol bisthiolactate and
polyethylene
glycol bisthioglycolate.
The following Examples further illustrate the present invention and are not
intended to be limiting in any respect. Those skilled in the art will
recognize, or be able
to ascertain using no more than routine experimentation, numerous equivalents
to the
specific procedures described herein. Such equivalents are considered to be
within the
scope of this invention and are covered by the claims.
Example 1
Aqueous suspensions of unbleached hardwood and softwood mechanical pulps
(about 10-20% consistency) were subjected to the method of the invention,
employing
6% hydrogen peroxide and 6% magnesium carbonate, and a reaction time of about
1 h
at 50 to 60 C.
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TABLE 1
Peroxide/Magnesium Carbonate Bleaching of Mechanical Pulps
Sample ISO brightness b* AISO Ob*
Hardwood
(Aspen)
Control 63.5 17.2
Treated 83.6 8.9 20.1 -8.3
(Maple)
Control 53.8 17.2
Treated 83 10.16 28.2 -7.1
Softwood
Control 57 16.3
Treated 80 8.2 23 -8.1
Example 2
5 g of birch pulp, unbleached, were mixed with an aqueous solution containing
4.5% H202 and 6% MgCO3 and treated for 1 hour at a temperature of 110 C. The
peroxide was added in three equal amounts at 20 minute intervals (i.e., 0, 20
and 40
minutes). This procedure was repeated with 5 g of an aspen pulp, unbleached,
using a
solution containing 8% H202 and 10% of MgCO3. The results are tabulated below
in Table
2.
TABLE 2
Peroxide/Magnesium Carbonate Bleaching of Mechanical Pulps
Pulp MgCO3 H202 ISO b*
Brightness
Birch 6% 4.5% >90 <2
Aspen 10% 8% 96 2.6
It has been found that softwood pulps are more difficult to bleach than
hardwood
pulps, requiring proportionately more peroxide and carbonate (approximately
twice as
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much as for the hardwood pulps) but also yield ISO brightness values >90 with
yellowness b* values in the range of 5-6 which is substantially lower than
those attainable
for conventional alkali peroxide bleaching. The bleached puips produced
according to
this process are much less photosensitive than conventionally bleached pulps
as
indicated by their extremely low b* values, and may, therefore, be expected to
undergo
reversion at much reduced rates as compared to conventionally bleached
mechanical
pulps.
Example 3
Mechanical pulps (5-40% consistency) were mixed with alkaline earth carbonate
(usually magnesium carbonate, 6-40%) and heated. Temperatures for the
bleaching
process ranged from 25-110 C with contact times of 1-3h, shorter reaction
times being
employable as temperature increases. On reaching the desired reaction
temperature,
hydrogen peroxide (4-16%) was added either in one addition, or as two or three
aliquots
spaced by approximately %2 to 1/3 of the total reaction time. At the end of
the bleaching
process the pulp is washed with water; no neutralization is necessary.
Brightness measurements from handsheets made from pulps bleached in this
manner are shown in Table 3.
TABLE 3
Peroxide/Magnesium Carbonate Bleaching of Mechanical Pulps
Sample Temp. ( C) MgCO3 H202 Brightness b*
Aspen 85 10% 6% 86
Aspen 85 20% 10% 90
Aspen 110 25% 10% 89.9 3.2
Birch 110 25% 10% 90 2.5
Softwood 110 30% 15% 87 6.8
Softwood 85 20% 10% 84
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Example 4
Softwood pulp, treated 3 h at 85 C with 30% MgCO3 and 15%H202 yielded
handsheets of 90 brightness and b*<3.
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