Note: Descriptions are shown in the official language in which they were submitted.
CA 02285148 1999-10-OS
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CHEMIMECHANICAL OR MECHANICAL PULP MANUFACTURE WITH REDUCED
ENERGY CONSUMPTION
The present invention relates to a method of reducing energy
consumption in the chemimechanical or mechanical manufacture
of pulp from pieces of pulp raw material, such as wood chips,
straw chaff or bagasse chaff, in refiners.
A large number methods of compressing pulp raw materials with
the intention of lowering energy consumption in the
manufacture of chemimechanical and mechanical pulp have been
proposed earlier in the art. For instance, axial
precompression of wood has been proposed (Frazier, W.C. and
. Williams, G.J., (1982), "Reduction of specific energy in
mechanical pulping by axial precompression of wood", Pulp and
Paper Can., 86:6, 87) and screw compression of chips
(Hartler, N., (1995), "Aspects on curled and microcompressed
fibres", Nordic Pulp Paper Res. J., 1,4). The energy gains
achieved, however, have been moderate in relation to
investment costs and in relation to the energy consumed by
screw compression. With the intention of facilitating suction
of chemical solution into chips, it has also been suggested
that the chips are compressed between rolls and the
compressed chips immersed directly into the chemical solution
for expansion therein.
An object of the present invention is to provide a novel and
advantageous method of reducing energy consumption in
chemimechanical (e. g. CTMP) or mechanical (e. g. TMP) pulp
manufacturing processes in refiners.
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To this end, it is proposed in accordance with the invention
that when proceeding in the manner described in the
introduction, the pieces of pulp raw material are subjected
to bulk-density reducing plastic deformation between
compression rolls prior to refining the material, this
plastic deformation being continued until a reduction in bulk
density of 35-65% is achieved. It has been found quite
surprisingly that the compression treatment proposed in
accordance with the invention, this compression treatment
having a low energy-input requirement (e.g. in the order of
30 kWh per tonne of pulp raw material), can result in energy
savings of several hundred kWh in the subsequent refining
process.
Co-acting rolls are suitably driven at different peripheral
speeds, conveniently with a speed difference of 5-50%, e.g.
30-50%, between the rolls. The latter may advantageously
present a shallow pattern for establishing a certain amount
of friction against the pulp raw material, so as to
facilitate introduction of the raw material into the roll
nip. When the moisture content of the pulp raw material is
very low or uneven, it may be beneficial to steam said
material prior to compression treatment, to obtain a more
uniform product.
In order to obtain uniform quality, it is also advantageous
to orientate the pieces of pulp raw material in a similar
fashion, preferably so that the fibre direction will define
only small angles with the axial direction of the rolls. For
instance, it is desirable that the fibre direction of at
least the major part of the pieces of pulp raw material
CA 02285148 1999-10-OS
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defines an angle of less than 45° with the roll axes prior to
said material passing between the rolls.
From an energy aspect, the inventive method of procedure will
preferably be independent of whether or not the refining
stages that follow compression of the material are effected
at a temperature above or below the lignin softening
temperature, although a particular advantage is afforded when
the first refining stage is carried out at a temperature
above the lignin softening temperature, since this enables
fibre shortening that accompanies refining of the compressed
chips at temperature beneath the lignin softening temperature
to be avoided. This is thought to be due to the fact that
compression of the material between the rolls causes
microcracks to form in the fibre walls in the wood, meaning
that separation of the fibres in the disc refiner takes place
in the fibre walls and not in the lignin-rich centre
lamellae, even at temperatures above the lignin softening
temperature (above 150°C). Thus, the invention enables
significant advantages to be achieved when refining is
carried out in at least two stages, wherein the first and the
second stage are both carried out at temperatures above the
lignin softening temperature, or wherein the first stage is
carried out at a temperature above the lignin softening
temperature and the second stage is carried out at a
temperature beneath the lignin softening temperature.
Significant advantages can be achieved with a roll-
precompression of the kind concerned, even when refining in
one stage at high temperature, roughly in accordance with the
original Asplund method. The invention also enables
significant advantages to be achieved when the first stage is
carried out at a temperature beneath the lignin softening
temperature and the second stage is carried out at a
CA 02285148 1999-10-OS
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temperature above the lignin softening temperature in
accordance with SE-B-470 555, or the second stage is also
carried out at a temperature beneath the lignin softening
temperature as in the case of traditional TMP production.
The aforesaid advantages will be evident from the following
Tables, which illustrate results obtained when applying the
invention on a laboratory scale on pine chips. Precompression
was carried out on chips of normal size, between generally
smooth rolls having a diameter of about 300 mm and a roll nip
'smaller than 0.5 mm. The rolls rotated at respective speeds
of 20 and 40 rpm and gave a bulk density reduction of 60% at
an energy input of less than 30 kWh/tonne chips.
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CA 02285148 1999-10-OS
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Column No. 1 in the Table shows in the left sub-column a
conventional chemimechanical process, a CTMP process, in
which two refining stages were both carried out at 130°C,
i.e. beneath the lignin softening temperature. At a total
refining energy of 1850 kWH/tonne chips, there was obtained a
freeness or dewatering capacity of 350, expressed as Canadian
Standard Freeness. It will be seen from the centre column in
column 1 that only 1150 kWh/tonne was required to achieve the
same dewatering capacity with chips pretreated by roll-
compression, in other words energy corresponding to almost
700 kWh/tonne can be saved when producing a corresponding
pulp when the chips have been precompressed in the manner
proposed by the present invention. A certain amount of fibre
shortening occurs, as made evident by a lower tear index,
although this need not always imply a drawback. With a total
refining energy input of 1500 kWh (right sub-column) there
was obtained with an unchanged tear index a lower dewatering
capacity and significantly increased tensile index, at an
energy saving of above 300 kWh/tonne.
In Column No. 2, the left sub-column and the centre sub-
column show refining of uncompressed chips at a temperature
of 175°C, i.e. a temperature well above the lignin softening
temperature, at a respective energy input of 1620 and 1800
kWh/tonne. These two tests can be considered to illustrate a
conventional Asplund method. The pulp obtained in the left
sub-column cannot be used for paper manufacture. Subsequent
to precompression of the chips in accordance with the
invention, there is obtained according to column 3 a pulp
which is fully comparable with modern TMP at a low total
refining energy. Division of the refining step into two
mutually sequential steps at 175°C and an unchanged total
CA 02285148 1999-10-OS
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refiner energy input gives the same results as those shown in
column No. 2.
The left sub-column of column No. 3 illustrates the
manufacture of TMP in accordance with SE-B-470 555. As
evident from the centre sub-column after precompression of
the chips in accordance with the invention corresponding
results are obtained with a 200 kWh/tonne lower total refiner
energy input, with the exception of a certain degree of fibre
shortening. According to sub-column 3, an essentially
unchanged refiner energy input results in a lower dewatering
capacity and higher tensile index. As evident from column No.
' 2, right sub-column, an increase in the temperature in the
first refining stage results in lower energy consumption,
higher tensile index and higher index.
The left sub-column of column No. 4 illustrates conventional
two-stage TMP manufacture in refiners at temperatures beneath
the lignin softening temperature. Subsequent to
precompression in accordance with the present invention,
there is obtained according to the centre sub-column the same
dewatering capacity at a refiner energy saving of more than
600 kWh/tonne pulp. The tensile index and tear index values
are lower, however. According to the right sub-column,
unchanged refiner energy results in a decreased dewatering
capacity and increased tensile index.
The two left sub-columns of column No. 5 illustrate TMP
manufacture in two refiner stages, of which the first stage
is carried out at a temperature in the region of or
immediately above the lignin softening temperature, whereas
the second stage is carried out at temperatures that lie
CA 02285148 1999-10-OS
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beneath the lignin softening temperature. The two right sub-
columns illustrate the same TMP manufacture subsequent to
precompression of the chips in accordance with the invention.
It is evident that significant savings in refiner energy can
be made while retaining the dewatering capacity essentially
unchanged and obtaining improved tensile index and tear index
values.
It will be understood that the invention is not restricted to
the aforedescribed exemplifying embodiments thereof and that
it can be implemented in any desired manner within the scope
of the inventive concept as defined in the following Claims.
It can be mentioned in this connection that relative movement
between the compression rolls can also be achieved by mutual
axial displacement of the rolls and can replace or be
combined with the relative movement that is achieved by
rotating the rolls at different speeds, as described above.
