Note: Descriptions are shown in the official language in which they were submitted.
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Description
Composition and process for bleaching of mechanical wood pulp
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of Canadian Patent No. 2576882.
BACKGROUND OF THE INVENTION
Chemical pulps have good strength properties and a high brightness value.
These
attributes, however, are obtained at the cost of low yields and the highly
negative effect
produced on the environment by the effluent from the bleaching process.
This has led in recent years to intensive development work aimed at producing
mechanical pulps in high yields to about 90%, and high brightness values, and
with
strength properties approaching those of the chemical pulps, while at the same
time
retaining the opacity and bulk properties unique to the mechanical pulps. The
resulting
pulps, while quite strong, are highly coloured probably due to the coloured
chromophores
in lignin.
Bleaching of mechanical wood pulps; such as ground pulp (GP), refiner ground
pulp
(RGP), thermo-mechanical pulp (TMP), bleached chemical thermo-mechanical pulp
(BCTMP), chemical ground pulp (CGP) and deinked recycled newspaper pulp has
been
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in wide use, as have peroxides, such as hydrogen peroxide, sodium percarbonate
and so
on.
Hydrogen peroxide has environmental benefits over chlorine-based bleaches and
it is
effective for changing chromophores to non-coloured products in bleaching
mechanical
pulp. But the effectiveness of the bleaching liquor is reduced in the absence
of stabilisers
against decomposition of the peroxide by transition metal ions, e.g. iron and
manganese
in the cellulosic material or water. In order to avoid the detrimental effect
of these
ions, a chelating agent is often introduced into the bleaching process or the
pulp is
pretreated with a chelating agent. The most common chelating agents are EDTA
and
DTPA. Sodium silicate solutions normally called water glass have been used in
stabilizing hydrogen peroxide solutions, which are used in alkaline peroxide
bleaching of
mechanical pulps, but this gives rise to other problems e.g. corrosion and
scaling.
Recently alternative processes to improve brightness of mechanical pulp during
subsequent tower bleaching, by treating with oxidizing and reducing agents,
have been
suggested to reduce energy cost and improve optical properties of pulp. For a
number of
reasons, well known to those in the art, hydrogen peroxide and peracetic acid
have
proven to be of particular interest, which are intended to brighten pulp
efficiently in the
presence of an alkali. Also there is a need to partly or totally replace
sodium silicate in
alkaline peroxide bleaching processes
JP05186989 proposes an alkaline process using a bleach activator such as TAED
to give
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a brighter pulp when it is used with oxygen and optionally also with hydrogen
peroxide.
Very few details of the process steps are given and hydrogen peroxide is not
used in the
examples. It appears that the TAED is mixed as a solid with dry pulp at the
start of the
bleaching step.
In W09521290 there is described a process in which per-acid is produced in
situ by
reaction of a bleach activator such as tetraacetylethylenediamine and hydrogen
peroxide
at a pH less than the pKa of peracetic acid formed from the reaction of these
chemicals. It
is stated that in a preferred process, the TAED is first dissolved in hot
water and then
added to the hydrogen peroxide before the reacting mixture is dosed to the
pulp. A
sequestrant may be added to the pulp before the dosing takes place. It is also
stated that
conditions must be optimized to ensure that all of the TAED is consumed. The
chemistry
must be carefully controlled to achieve consistent results when using such
powerful
bleach as peracetic acid.
In EP456032 there is described a similar pulp bleaching process using alkaline
TAED
and hydrogen peroxide. Bleaching of the pulp is done in plastic bags and no
detail is
given of how a scaled up process should be operated.
CA2041468 proposes use of TAED activated hydrogen peroxide to bleach
mechanical
wood pulps at lower temperatures within a short period of time.
W09418298 describes a bleaching process where an N-acyl bleach activator is
reacted
with a source of hydrogen peroxide under acid conditions. The product of this
reaction
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may be used in many bleaching and disinfection applications, including pulp
and paper
bleaching. The activator and other components may be in the form of particles
and these
particles may be provided by techniques similar to those used in the laundry
detergent
industry. For instance by spray drying liquid slurries; by granulation
techniques using
binders, for instance synthetic or natural polymers (or derivatives); or by
melt blending
followed by extrusion or other techniques. A composite product including a
bleach
activator may also include other additives, especially heavy metal
sequestrants, and it
may include surfactants to act as wetting agents and inorganic salts to act as
a diluent or
to increase the rate of disintegration or dissolution of the product. The
composite product
should also include the source of the hydrogen peroxide as well as the bleach
activator
when it includes the wetting agent. Only two granulated activator particles
are
exemplified in this document; both contain carboxymethyl cellulose as a binder
and
neither is used for pulp bleaching.
W09725402 proposes the use of bleach activators such as TAED for various
applications
including pulp bleaching. The preferred form of the TAED is a granule, but no
details are
given of the composition of the granule.
CA2230315 describes a refiner bleaching process where the TAED as a bleach
activator
is reacted with sodium perborate or hydrogen peroxide at elevated pressure.
The pulp
after refining has improved brightness and pulp brightness after optional
tower bleaching
has exceeded 75 ISO% points.
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It is known that hydrogen peroxide will decompose very rapidly in an alkaline
milieu in
the presence of transition metal ions. The most abundant of these ions in
pulps are iron
and manganese. The copper ion is also very detrimental for alkaline hydrogen
peroxide,
but normally it can enter the process only via used process waters.
The theory of the function of sodium silicate varies, but one theory is that
sodium silicate
will deactivate the catalytic surface of iron and other heavy metal ion
"precipitates".
If the silicates, e.g. in a white-water closed loop system, enter the paper
making process,
they will disturb the papermaking process, e.g. by precipitating on hot
surfaces, causing
holes in the paper reel, etc. Another disadvantage with sodium silicate is
that when the
bleaching liquors are recycled and ultimately fed into the recovery boiler,
where the so-
called black liquor from the cooking process after concentration is burned,
the silicate
will cause severe scaling and thus decrease the heat transfer in the recovery
boiler, which
in the worst case scenario can cause an explosion of the recovery boiler.
One solution to stabilize alkaline hydrogen peroxide solutions or to avoid
sodium silicate
is based on the use of antiscalants. U.S. Pat. No. 4614646 discloses the
employing of
peroxide in a silicate-free system in the presence of alkyleneaminephosphonic
acids and
polyalkylenepolycarboxylic acids.
Another patent, U.S. Pat. No. 5145558, describes a pulp bleaching system
employing
peroxide which uses a quaternary amine compound in the stabilized bleach
solution.
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It is disclosed in EP 0369711B1 a stabilised peroxide composition consisting
of
hydrogen peroxide and a phosphate stabilising additive.
U.S. Patent Application No. 20080264584 Al discloses the use of a copolymer of
AHPS
and an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid,
maleic acid or
itaconic acid, together with a chelating agent and an alkaline earth metal
compound, such
as magnesium sulphate, either mixed together or added separately, to achieve
very good
bleaching performance and total replacement of sodium silicate.
Other U.S. patents which employ such copolymers of acrylic acid in a peroxide
bleaching
system, include U.S. Pat. No.7867357B2 and U.S. Pat. No.4363699.
Based upon the limitations of the above patents, there is an ongoing need for
improved
but inexpensive mechanical pulp having increased brightness; through efficient
bleaching
performance with the use of a stabilized peroxide bleaching system, and
through
enhanced production and utilization of peroxide residuals.
SUMMARY OF THE INVENTION
The present invention relates to a novel, low-energy method of producing high
yield
mechanical pulp having a final brightness value not previously achieved, and
reducing
peroxide decomposition.
Theoretically calculated, one mole of TAED will be required to react with two
moles of
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hydrogen peroxide. Since the molecular weight of hydrogen peroxide is 34 and
that of
TAED is 228; 3.35 grams of TAED will react with 1 gram, calculated as pure
substance,
of hydrogen peroxide employed for bleaching the pulp. However, it may be
reasonable to
employ hydrogen peroxide, which has a lower price than TAED, in an excess
amount, in
order to further increase the rate of reaction of hydrogen peroxide with TAED
and to
improve the bleaching effect. In addition, more hydrogen peroxide becomes
effectively
active in the bleaching of pulps by the addition of TAED, so that the amount
of hydrogen
peroxide actually employed can be reduced.
In the pulp and paper industry where chelants are added to enhance peroxide
bleaching
systems, levels of chelant from 1-6 kg/tonne of pulp are typically used. The
chelants
referred to above are the carboxylic acid derivatives of amines, e.g.
diethylenetriaminepentaacetic acid (DTPA), which are added at the pretreatment
(prebleaching) stage to take metals out of the pulp. The chelant is partially
removed in the
subsequent dewatering step, but that which remains is rapidly destroyed in the
bleaching
step when contacted with the peroxide (See U.S. Pat. No.4614646). Bleaching of
mechanical pulp has in the past been conducted with hydrogen peroxide,
employing
sodium silicate as a stabilizer, but this system results in problems when
insoluble silicates
are deposited upon the fibres and the machinery employed. When deposited on
kraft
paper fibres the result is a harsher feel of the paper. The fouling of
equipment can cause
down-time and shortened life of the equipment. Because of this, silicate-free
systems
have been suggested.
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One of the simplest and most effective means of peroxide solution
stabilization is by the
addition of organophosphoric acid or polycarboxylic acid. Organophosphoric
acids and
polycarboxylic acids have excellent chelating abilities, low threshold
inhibition and
lattice distortionability; they can prevent scale formation, calcium carbonate
in particular,
in water systems. They can chelate with Fe, Cu, and Mn ions to form stable
chelating
compounds and show excellent scale and corrosion inhibition effects at
temperatures of
250 C or below. They also have good chemical stability under high pH values,
limit
hydrolyzation and limit decomposition under ordinary light and heat conditions
.
According to the present invention there is provided a method for bleaching
mechanical
pulp. The method comprises treating (digesting) said pulp in an aqueous
hydrogen
peroxide solution containing about 1% - 6% hydrogen peroxide by weight based
on the
pulp, 1% - 5% caustic soda by weight based on the pulp, 0% - 5% sodium
silicate by
weight based on the pulp, 0.01% - 1% TAED by weight based on the pulp, which
liberates nascent oxygen upon reaction with hydrogen peroxide to increase the
peroxide's
efficiency, and 0.1 % - 1 % chelating agent by weight based on the pulp
wherein the
chelating agent is selected from the organophosphonic acids group consisting
of, A)
amino trimethylenephosphonic acid (CAS No. 6419-19-8), B) 1-hydroxy ethylidene-
1,1-
diphosphonic acid (CAS No. 2809-21-4), C) ethylene diamine tetra(methylene
phosphonic acid) (CAS No. 1429-50- 1), D) diethylenetriaminepenta(methylene
phosphonic acid) (CAS No. 15827-60-8), E) 2-phosphonobutane-1,2,4-
tricarboxylic acid
(CAS No. 37971-36-1), F) 2-hydroxyphosphonocarboxylic (CAS No. 23783-26-8),
G) hexamethylenediaminetetra(methylenephosphonic acid) (CAS No. 23605-74-5),
and
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H) bis(hexamethylenetriaminepenta(methylenephosphonic acid)) (CAS No. 34690-00-
1).
Or, the chelating agent is selected from the organophosphoric salts group
consisting of,
A) tetra sodium salt of amino trimethylenephosphonic acid (CAS No. 20592-85-
2),
B) penta sodium salt of amino trimethylenephosphonic acid (CAS No. 2235-43-0),
C) potassium salt of amino trimethylenephosphonic acid (CAS No. 27794-93-0),
D) monosodium of 1-hydroxy ethylidene-1,l-diphosphonic acid (CAS No. 29329-71-
3),
E) ethylenediamine tetra(methylenephosphonic acid) sodium (CAS No. 1429-50-
1),
F) disodium of 1-hydroxy ethylidene- 1, 1 -diphosphonic acid (CAS No. 7417-83-
7),
G) tetra sodium of 1-hydroxy ethylidene-l,1-diphosphonic acid (CAS No. 3794-83-
0),
H) potassium salt of 1-hydroxy ethylidene-1,1-diphosphonic acid (CAS No. 67953-
76-8),
I) pentasodium salt of ethylene diaminetetra(methylenephosphonic acid) (CAS
No. 7651-99-2), J) hepta sodium salt of
diethylenetriaminepenta(methylenephosphonic
acid) (CAS No. 68155-78-2), K) sodium salt of
diethylenetriaminepenta(methylene
phosphonic acid) (CAS No. 22042-96-2), L) sodium salt of 2-phosphonobutane-
1,2,4-
tricarboxylic acid (CAS No. 40372-66-5), M) potassium salt of
hexamethylenediaminetetra(methylenephosphonic acid) (CAS No. 53473-28-2), N)
partially neutralized sodium salt of bis(hexamethylenetriaminepenta(methylene
phosphonic acid)) (CAS No. 35657-77-3).
Or the chelating agent is selected from the polycarboxylic antiscalants group
consisting
of, A) polyacrylic acid (CAS No. 9003-01-4), B) polyacrylic acid sodium (CAS
No.
9003-04-7), C) hydrolyzed polymaleic anhydride (CAS No. 26099-09-2), D)
copolymer
of maleic and acrylic acid (CAS No. 26677-99-6), E) acrylic acid-2-acrylamido-
2-
methylpropane sulfonic acid copolymer (CAS No. 40623-75-4), F) acrylic acid-2-
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hydroxypropyl acrylate copolymer (CAS No. 55719-33-0) at a temperature in the
range
of 65 - 80 C., and preferably in the range of 70 - 75 C. At such
temperatures, the
reaction may proceed for about one to three hours. The pH range of the method
is
preferably about 10 - 12.
We have found that residual hydrogen peroxide, after the bleaching tower, can
be
decreased from 8.5 kg to 4.5 kg by using 10.0 kg TAED per tonne of dry pulp in
a pre-reaction with 50 kg hydrogen peroxide (calculated at 100% hydrogen
peroxide) per
tonne of dry pulp, 37.5 kg caustic soda per tonne of dry pulp and 6.0 kg
chelating agent
per tonne of dry pulp to form a silicate-free bleaching solution for use in a
mechanical
pulp bleaching process.
By using the inventive process, an ISO brightness value of at least 84 is
preferably
obtained, still more preferably, an ISO brightness value of at least 85.6 is
obtained.
The invention provides several advantages over conventional hydrogen peroxide
bleaching: 1. Lower cost benefits due to the amount of hydrogen peroxide,
alkali and
silicate actually employed that can be reduced while still achieving the same,
or greater,
ISO brightness values. 2. Increased brightness values for mechanical pulp over
conventional bleaching, which increases the number and variety of applications
for the
product, significantly expanding its marketability. 3. Environmental benefits
due to the
amount of hydrogen peroxide, alkali and silicate actually employed that can be
reduced.
The invention and its advantages will be illustrated in more detail by the
examples below
which, however, are only intended to illustrate the invention without limiting
the same.
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The percentages and parts stated in the description, claims and examples,
refer to percent
by weight and parts by weight, respectively, unless otherwise stated.
DETAILED DESCRIPTION OF THE INVENTION
The invention will be further described with reference to the following non-
limiting
examples:
Pulp brightness (ISO brightness) is measured with a brightness meter, which
determines
the brightness of a split sheet at a wavelength of 457 nm (ISO D65 Standard
Method)
Using a bleaching solution of hydrogen peroxide, TAED is made to a 0.5%
solution and
chelant is made to a 5% solution.
EXAMPLE 1
Laboratory Studies
A mechanical softwood pulp sample (from a pulp mill in British Columbia,
Canada) had
a brightness of 58.6% ISO, and a concentration of 24.51%, treated with DTPA.
Laboratory bleached at 20% concentration pulp, 70 C and 150 minutes of
retention time.
From the result shown in TABLE 1, it is evident that TAED and chelant
bleaching with
hydrogen peroxide provides maximum brightness gain and minimum residual
hydrogen
peroxide, and is superior to conventional hydrogen peroxide bleaching.
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TABLE 1
Residual
Unbleached Hydrogen ISO
ISO Peroxide Caustic Silicate TAED Chelant Peroxide Brightness
Brightness (%) (%) (%) (%) (%) (%)
58.6 5.0 3.75 3.0 0.00 0.00 0.85 79.6
58.6 5.0 3.75 3.0 0.01 0.10 1.05 80.5
58.6 5.0 3.75 2.0 0.50 0.20 0.99 82.3
58.6 5.0 3.75 0.0 1.00 0.60 0.45 85.6
58.6 5.0 3.75 1.0 0.40 0.30 0.78 84.0
58.6 5.0 3.75 0.5 0.30 0.50 0.99 83.5
58.6 5.0 3.75 0.0 0.01 1.00 1.75 81.3
EXAMPLE 2
Laboratory Studies
A mechanical softwood pulp sample (from a pulp mill in British Columbia,
Canada) had
a brightness of 52% ISO, and a concentration of 31%, treated with DTPA.
Laboratory
bleached at 24% concentration pulp, 70 C and 150 minutes of retention time
using a
single-stage process.
It is evident from TABLE 2 that increasing the amount of chelant does have a
resultant
positive effect on pulp brightness. Bleached pulp with ISO brightness of more
than 80.5
points have been obtained.
TABLE 2
Incoming
ISO Peroxide Caustic (%) Silicate (%) TAED Chelant ISO
Brightness %) Brightness
52 4.0 3.6 3.0 0.00 0.00 71
52 4.0 3.6 3.0 0.01 0.30 73
52 4.0 3.6 3.0 0.02 0.50 74
52 5.8 4.2 3.2 0.00 0.00 76
52 5.8 4.2 3.2 0.01 0.30 79
52 5.8 4.2 3.2 0.02 0.50 80.5
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EXAMPLE 3
Laboratory Studies
Bleaching studies were performed on partial two-stage process bleached
mechanical
softwood pulp samples, which had been bleached in the first stage to have a
brightness of
66% ISO, and a concentration of 39.5%, treated with conventional hydrogen
peroxide
bleaching. Laboratory bleached at 24% concentration pulp, 70 C and 150
minutes
of retention time.
About 20.0 points of brightness were gained with TAED, chelant and hydrogen
peroxide,
compared to about 14.0 points with hydrogen peroxide. It is further evident
that TAED
and chelant with hydrogen peroxide bleaching is more effective in pulp
brightening
during the second-stage compared to conventional hydrogen peroxide bleaching
during
the second bleaching stage.
TABLE 3
Incoming
ISO Peroxide Caustic (%) Silicate (%) TAED Chelant ISO
Brightness M) (W Brightness
66 3.5 2.0 2.5 0.00 0.00 80.0
66 3.5 2.0 1.0 0.50 0.50 86.0
66 3.5 2.0 0.0 0.40 0.60 85.5
EXAMPLE 4
Mill-Trial
A plant test was performed at a pulp and paper mill in British Columbia,
Canada.
Mechanical softwood pulp was bleached using a single-stage bleaching process.
The
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pulp, after the bleaching tower, was diluted and neutralized at pH 5.6 with a
sulphur
dioxide solution in preparation for papermaking stock, was then run through a
pulp
refiner and finally made into paper at the paper machine.
TABLE 4
Unbleached Bleach Paper
ISO Peroxide Caustic Silicate TAED Chelant Tower Machine
Brightness (%) (%) (%) (%) (%) ISO ISO
Brightness Brightness
50 6.0 4.5 3.5 0.0 0.0 74.0 72.0
50 6.0 4.5 3.5 0.1 0.2 75.0 74.0
50 6.0 4.5 2.0 0.3 0.5 78.0 78.0
50 6.0 4.5 0.0 0.3 0.6 77.5 77.3
Evidently, TAED, chelant and hydrogen peroxide bleaching gives the best
optical
properties with about a six point increase in the ISO brightness after the
paper machine.
The brightness does not change between the bleaching tower and paper machine,
or even
after pulp refining, in preparation for papermaking stock. Therefore, it is
clear that this
new composition and process for bleaching of mechanical wood pulp is more
effective in
stabilizing pulp or paper when compared with conventional hydrogen peroxide
bleaching
of pulp, which typically will undergo a brightness reversion of two or more
ISO points
between the hydrogen peroxide tower and paper machine in a closed white-water
system.
