U.S. patent application number 09/841606 was filed with the patent office on 2001-09-13 for process for manufacturing high-yield, high-strength pulp at low energy.
Invention is credited to Browne, Thomas C., Chagaev, Oleg V., Heitner, Cyril, Karnis, Alcibiadis, McDonald, J. David, Miles, Keith B., Stationwala, Mustafa I..
Application Number | 20010020522 09/841606 |
Document ID | / |
Family ID | 29547871 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010020522 |
Kind Code |
A1 |
Karnis, Alcibiadis ; et
al. |
September 13, 2001 |
Process for manufacturing high-yield, high-strength pulp at low
energy
Abstract
A low energy process for the manufacture of high yield pulp that
involves the processing of chemically-treated chips or wood fiber
at high stresses or intensity.
Inventors: |
Karnis, Alcibiadis;
(Dollard-des-Ormeaux, CA) ; Heitner, Cyril;
(Pierrefonds, CA) ; McDonald, J. David;
(Vaudreuil-Dorion, CA) ; Miles, Keith B.;
(Montreal, CA) ; Chagaev, Oleg V.; (Pierrefonds,
CA) ; Stationwala, Mustafa I.; (Etobicoke, CA)
; Browne, Thomas C.; (Montreal, CA) |
Correspondence
Address: |
SWABEY OGILVY RENAULT
SUITE 1600
1981 MCGILL COLLEGE AVENUE
MONTREAL
QC
H3A2Y3
CA
|
Family ID: |
29547871 |
Appl. No.: |
09/841606 |
Filed: |
April 25, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09841606 |
Apr 25, 2001 |
|
|
|
09192294 |
Nov 17, 1998 |
|
|
|
60066259 |
Nov 20, 1997 |
|
|
|
Current U.S.
Class: |
162/25 ; 162/83;
162/90 |
Current CPC
Class: |
D21C 3/12 20130101; D21B
1/16 20130101; D21C 3/06 20130101 |
Class at
Publication: |
162/25 ; 162/83;
162/90 |
International
Class: |
D21B 001/16; D21C
003/04; D21C 003/02 |
Claims
We claim:
1. A process for producing a high yield pulp comprising: chemically
treating a pulp precursor selected from wood chips or wood fibre to
develop wood fibre of said precursor while maintaining fibre
integrity, and refining the chemically treated pulp precursor at a
high refining intensity.
2. A process according to claim 1 wherein said refining intensity
is in a range of 200 to 2,000 J/kg/impact.
3. A process according to claim 1 wherein said intensity is in a
range of 300 to 700 J/kg/impact.
4. A process according to claim 1 wherein said high refining
intensity is one which establishes a high stress in said chemically
treated pulp precursor in a range of 0.1 to 30 MPa at peak strain
rates ranging from 50 to 100 s.sup.-1.
5. A process according to claim 1 wherein the chemical treatment
comprises sulfonating the precursor to provide a content of
SO.sub.3 on wood fibre of 1 to 3%, by weight, based on weight of
oven dry wood fibre.
6. A process according to claim 5 wherein said content is 1 to 2.5%
by weight.
7. A process according to claim 3 wherein the chemical treatment
comprises sulfonating the precursor to provide a content of
SO.sub.3 on wood fibre of 1 to 2.5%, by weight, based on weight of
oven dry wood fibre.
8. A process according to claim 2 wherein the chemical treatment
comprises sulfonating the precursor with charge of alkali metal
sulphite at a charge of 6 to 25%, by weight of sulphite, based on
the oven dry wood fibre.
9. A process according to claim 2 wherein the chemical treatment
comprises contacting said precursor with a charge of alkali metal
carbonate or alkali metal hyroxide at a charge of 0.5 to 10%, by
weight, based on weight of oven dry wood fibre.
10. A process according to claim 2 wherein said precursor comprises
wood chips and the chemical treatment softens said wood chips for
separation of wood fibres in said wood chips, and develops surface
characteristics of the wood fibres of said wood chips, while
maintaining fibre integrity.
11. A process according to claim 2 wherein said precursor comprises
wood fibres and the chemical treatment and the refining develop
surface characteristics of the wood fibres, while maintaining fibre
integrity.
12. A process according to claim 2 wherein said precursor comprises
wood chips and the chemical treatment swells the chips to
facilitate separation of wood fibres in the chips, and develops
surface characteristics of the wood fibres.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is related to U.S. Ser. No. 60/066,259
filed Nov. 20, 1997.
BACKGROUND OF THE INVENTION
[0002] i) Field of the Invention
[0003] This invention relates to a process for manufacture of high
yield pulp, more especially a low energy process.
[0004] ii) Description of Prior Art
[0005] Although refining of wood chips into pulp has been applied
commercially since the 1960's, the mechanism of refining is not
completely understood. A major step in this understanding was made
by Miles and May in their articles in the Journal of Pulp and Paper
Science 16(2):J63 (1990) and Paperi ja Puu 73(9):852 (1991). They
developed a set of equations based on a mass and energy balance to
calculate the consistency and velocity of pulp within a refiner.
From this approach the residence time and specific energy per bar
impact or refining intensity can be determined. Experimental and
empirical relationships can be developed between pulp properties
and specific energy and refining intensity. However, because it is
not known how the wood fibres react to the stresses imposed in
refining, it is not possible to make these relationships analytical
or predictive.
[0006] Earlier work on untreated chips in RMP and TMP refining
suggested that an energy savings of approximately 15% to 20% was
possible by increasing refining intensity. (See Tappi Journal
74(3):221 (1991), Journal of Pulp and Paper Science 19(1):J12
(1993) and U.S. Pat. No. 5,167,373). The higher intensity treatment
was implemented by either increasing the disc rotational speed from
1200 to 1800 RPM or reducing the inlet consistency to the refiner
from 20 to 10%. This approach was most effective when the first
refining stage was operated at high intensity and the second stage
was operated with conventional conditions. The optimum energy
saving which maintained pulp and fibre properties was obtained by
putting a smaller portion of the total specific energy in the
first, high intensity stage. A typical split in specific energy
between the first and second stages of refining would be 40/60.
Increasing the refining intensity in the first stage or the
proportion of the specific energy applied in the first stage
further would lower the total energy to reach a given freeness. It
would also lower the average fibre length and pulp strength,
limiting the potential for additional energy savings.
[0007] The RTS Process, developed by Andritz, claims a reduction of
at least 10% in specific energy over conventional TMP (U.S. Pat.
No. 5,776,305). In this process, the chips are pre-treated above
their glass transition temperature for 5 to 30 seconds before
refining at higher than conventional intensities by means of higher
disc speeds. Further pulp and fibre development takes place in a
second refiner operating under conventional conditions. Pulp
strength and optical properties are the same or better than pulp
produced by a conventional TMP Process.
[0008] A mild bisulphite pretreatment of chips at pH, 4.2 combined
with high intensity refining gives an energy savings of 33% over
conventional TMP as shown by Stationwala in Tappi Journal
77(2):113(1994). The energy savings by chemical treatment with
sodium sulphite, 0.20 to 0.45% SO.sub.3 content on wood, and
refining, are additive.
[0009] Broderick et al. (U.S. Pat. No. 5,540,392) have shown that
it is possible to reduce energy by up to 18% in a two-stage
refining system. At least 65% of the total energy is applied in a
low intensity first stage refiner. The remaining energy is applied
in a high intensity second stage refiner. The pulp properties are
reported to be at least as good as or better than that produced by
conventional refiners. Using high intensity refining in the second
stage seems to be in direct contrast to Paprican's approach.
However, in the final analysis both strategies appear to be limited
to energy savings in the order of 15 to 18%. Greater energy savings
would lead to an unacceptable deterioration in pulp properties.
[0010] There are, however, certain chip pre-treatment processes
which produce substantial changes in the properties of the raw
material. Under these conditions high intensity refining results in
greater energy savings without unacceptable compromises in pulp
quality.
SUMMARY OF THE INVENTION
[0011] It is an object of this invention to provide a process for
the manufacture of high yield pulp, especially a low energy
process.
[0012] In accordance with the invention, there is provided a
process for producing a high yield pulp comprising:
[0013] chemically treating a pulp precursor selected from wood
chips or wood fibre to develop wood fibre of said precursor while
maintaining fibre integrity, and
[0014] refining the chemically treated pulp precursor at a high
refining intensity.
DETAILED DESCRIPTION OF THE INVENTION
[0015] A low energy process has been discovered for the manufacture
of high yield pulp which comprises treating wood chips or wood
fibre with sodium sulphite or other chemicals then processing the
material mechanically using higher stress levels than are currently
used in commercial applications.
[0016] In the case of treatment with sulphite, the sulphonate
content on the chips is determined as %, by weight SO.sub.3 is
between 0.1 and 3%, more especially 1 to 3% and preferably 1 to
2.5%, by weight, on an O.D. (over dried) basis.
[0017] The compressive forces in this process are in the range of
0.1 to 30 MPa more especially 0.5 to 30 mPa which is several orders
of magnitude greater than that in conventional refining. This
greater compressive force can be applied in a variety of devices
including rolling mills, vibratory inertial refiners, and refiners
that have been modified to deliver higher stresses or intensities
than is conventional. Up to 70% less energy is consumed in this
process giving the same pulp and fibre properties as conventional
refining.
[0018] The materials or precursor used in this pulping process can
be either softwoods or hardwoods in the form of chips, fibres or
pulp. This material is impregnated with sodium sulphite or other
chemicals that soften the chips and develop the wood fibres. This
material is then processed into pulp by mechanical action in
devices that deliver higher stresses or intensities than are now
used conventionally. The key to this discovery is that wood fibres
can be separated and developed at very high stresses without being
broken if they are sufficiently softened. The advantage of
operating at these high levels of stress is that the pulp develops
with fewer impacts which significantly lowers the energy
requirement.
[0019] High refining intensities in a range of 200 to 2,000,
preferably 250 to 1,000 and more preferably 300 to 700 J/kg per
impact are applied to wood chips, fibres or pulp at stresses of the
order of 0.1 to 30 MPa and at peak strain rates ranging from 50 to
1000 s.sup.-1.
[0020] In contrast with the high refining intensities of the
invention, conventional disc refiners operate at a refining
intensity of 150 to 200 J/kg/impact.
[0021] This can be done in a vibratory inertial refiner (VIR) which
consists of a conical rotor rolling on the surface of a conical
stator. The force exerted by the rotor is concentrated along the
line of contact with the stator which gives a higher compressive
stress on the material than a conventional disc refiner, and which
leads to higher levels of specific energy per impact.
[0022] Similarly, a rolling mill consisting of a series of rollers
rotating within a cylindrical surface which exert line loads on the
material that are in excess of compressive stresses exerted in
conventional disc refiners and which lead to higher levels of
specific energy per impact may be employed. This process can also
be implemented with a disc refiner by increasing refining intensity
by either increasing the rotational speed of the disc(s) or
lowering the outlet consistency.
[0023] These levels of refining intensity compare with 150 J/kg per
impact for a double-disc operating at 1200 RPM and an outlet
consistency of 25%. This process can also be implemented by
increasing refining intensity in a disc refiner by either
increasing the rotational speed of the disc(s) or by lowering the
outlet consistency. For instance, for a disc speed of 1800 RPM and
an outlet consistency of 25%, the refining intensity was 330 J/kg
per impact. Lowering the outlet consistency to 12.5% and
maintaining the disc rotational speed at 1800 RPM, increased the
refining intensity to 660 J/kg per impact.
[0024] Chemicals which may be employed to treat the pulp precursor
prior to the high intensity refining include alkali metal sulphite
or bisulphite, alkali metal hydroxide and alkali metal carbonate,
especially sodium sulphite, sodium bisulphite, sodium hydroxide and
sodium carbonate. The alkali metal hydroxide is preferably employed
in conjunction with hydrogen peroxide to improve pulp
brightness.
[0025] The alkali metal hydroxide and carbonate have the effect of
swelling wood chips.
[0026] The sulphite is typically employed in a charge of 6 to 25%,
by weight, based on the oven dry weight of the wood fibre, whereas
the alkali metal hydroxide or carbonate is typically employed in a
charge of 0.5 to 10%, preferably, 0.5 to 7%, by weight, based on
the oven dry weight of the wood fibre.
[0027] Surprisingly, it was found that the chemical treatment which
in turn softens the wood chips permits refining at high refining
intensities, which develops the surface characteristics of the wood
fibres, collapses the fibre walls, and renders the fibres more
flexible with reduced energy consumption but without loss of fibre
length and pulp quality, as compared with conventional processes
which employ lower refining intensities and consume higher
energy.
[0028] It might have been expected that the chemical treatment
would render the wood fibres fragile to high intensity
refining.
[0029] This new process could be implemented as a single stage
followed by subsequent stages of conventional disk refining. It
could also be implemented as several consecutive stages to take
full advantage of the energy savings over conventional processes.
Because of the high stress levels, this approach would also be
effective in treating chemimechanical pulp (CMP) screen and cleaner
rejects.
EXAMPLES
[0030] The following examples illustrate the nature of the
invention.
[0031] Refining Equipment and Procedures:
[0032] The roller mill consisted of four, 160 mm diameter rollers
with helical grooves and ridges, rolling inside a cylindrical shell
with an inside diameter of 500 mm and a height of 0.9 m. The
helical grooves and ridges were both 10 mm wide and the groove
depths were 10 mm. For these experiments, the rotational speed of
the shaft was 250 RPM and the stress exerted on the material was
estimated to be in the range of 300 to 600 kPa with peak strain
rates of 200 to 500 s.sup.-1.
[0033] The vibratory inertia refiner (VIR) consists of a housing
with a spherical support. Mounted on the spherical support is a
shaft with a conical rotor; the conical rotor is 155 mm in height
with a base of 275 mm and rolls within a conical stator. A
debalance weight, installed on bearings within the conical rotor,
and is driven independently at 1,000 rpm. The centrifugal force
generated by the rotation of the debalancer is transmitted to the
conical rotor (by inertia). For experiments with the VIR, the
measured stress exerted on the material was between 3 and 30 MPa
with strain rates from 50 to 300 s.sup.-1, and the average specific
energy per impact would be greater than generated by the disc
refiner.
[0034] Refiner trials were conducted with an atmospheric 36 inch
Bauer Model 400 double rotating disc refiner equipped with
conventional Bauer pattern 36104 plates. Variable frequency AC
drives permitted each of the machine's motors to operate over a
range of 1200 to 1800 RPM. Normal discharge consistency is 25
percent, but lower levels were also explored in the trials
described herein.
[0035] The sulphonation in the Examples was carried out with sodium
sulphate at pH 9.
Example 1
[0036] Highly sulphonated black spruce chips (sulphonate content
.about.2.0% on chips OD basis) were refined in a roller mill and
for comparison in an unpressurized refiner operating at rotational
speeds of 1200 rpm to produce a chemimechanical pulp (CMP).
[0037] As shown in FIGS. 1 and 2, the specific energy required to
produce CMP with a Canadian Standard Freeness of 400 mL and a
tensile index of 35 N.m/g with the roller mill was about one third
of the disc refining energy.
[0038] At a tensile index of 35 N.m/g this process gives a tear
index equal to that of conventional refining (FIG. 3) and a low
debris level as measured by means of Somerville shive (FIG. 4).
Overall this pulp meets conventional quality specifications but is
produced with significantly less energy.
Example 2
[0039] Highly sulphonated black spruce chips (sulphonate content
.about.2.0% on chips OD basis) were refined in an unpressurized
double-rotating disc refiner at a rotational speed of 1800 rpm. For
comparison, these wood chips were refined in the same disc refiner
at the conventional disc rotational speed of 1200 rpm.
[0040] As shown in FIG. 5, the specific energy required to produce
CMP with a Canadian Standard Freeness of 400 mL with the disc
refiner at 1800 rpm was about 33% less than that required at 1200
rpm. The higher intensity provided at the elevated speed permitted
a given quality to be obtained at lower specific energy. A tensile
index of 35 N.m/g could be reached at 20% lower energy (FIG. 6),
with no associated compromise in tear index (FIG. 7).
Example 3
[0041] Highly sulphonated black spruce chips (sulphonate content
.about.2.0% on chips OD basis) were refined in an unpressurized
double-rotating disc refiner operating at a very high level of
intensity by simultaneously rotating the discs at 1800 rpm and
lowering the inlet consistency. For comparison, the wood chips were
also refined in the novel disc refiner at its conventional
intensity.
[0042] As shown in FIG. 8, the specific energy required to produce
CMP with a burst index of 2.0 at very high intensity was 52% less
than that needed at conventional intensity. It is also evident from
FIG. 9 that refining at very high intensity did not compromise tear
strength.
Example 4
[0043] Highly sulphonated black spruce chips (sulphonate content
.about.2.0% on chips OD basis) were refined in a vibratory inertia
refiner (VIR) using either 75 or 100% of the debalancer weight.
[0044] As shown in FIG. 10, the specific energy required to produce
CMP with a Canadian Standard Freeness of 400 mL was 33% less with
the VIR than the disc refiner. At a tensile index of 55 N.m/g, the
VIR required 55% less energy than the disc refiner operating at
1200 rpm (FIG. 11).
Example 5
[0045] Aspen chips treated with alkaline sodium sulphite (10%
sodium sulphite charge and 1.4% sodium hydroxide charge on chips,
OD basis) were refined in a roller mill and for comparison in a
unpressurized refiner operating at rotational speeds of 1200
rpm.
[0046] FIG. 12 shows that the specific energy required to produce
aspen CMP with a Canadian Standard Freeness of 200 mL was about 75%
less than that required using the disc refiner at 1200 rpm. As seen
in FIG. 13, the specific energy required to produce aspen CMP with
a tensile index of 50 N.m/g in the roller mill was 70% lower than
that used in the disc refiner.
Example 6
[0047] Highly sulphonated black spruce chips (sulphonate content
.about.2.0% on chips OD basis) were refined in a roller mill at low
specific energy (0.53 GJ/t) followed by an unpressurized disc
refiner. These results were compared to experiments on an
unpressurized disc refiner operating at rotational speeds of 1200
rpm.
[0048] The specific energy required to produce CMP with a Canadian
Standard Freeness of 400 mL using the two stage roller mill/disc
refiner process was 40% lower than that required to produce the
same pulp with a disc refiner alone, as shown in FIG. 14, The
specific energy required to produce CMP with a tensile index of 40
N.m/g, with the roller mill/disc refiner process, shown in FIG. 15,
was 25% lower than the specific energy required using the disc
refiner alone.
Example 7
[0049] Mixed hardwood chips treated with sodium carbonate were
refined using either VIR (2 passes) or the disc refiner (1 pass) to
produce a carbonate medium pulp. The refining temperature in the
VIR was 20-25.degree. C. FIG. 16 shows that the VIR was
significantly more efficient in decreasing the freeness of the pulp
than the disc refiner. To produce the medium pulp with a freeness
of 300 mL CSF the VIR consumed about 40% less energy than the disc
refiner. The concora stiffness of carbonate medium pulp produced in
the VIR at a specific energy of about 1 GJ/t was 2-3 times higher
than that of pulp produced in the disc refiner, as seen in FIG.
17.
* * * * *