Note: Descriptions are shown in the official language in which they were submitted.
WO92/20855 2 ~ PcrlsEg2~00348
A METHOD FOR THE MANUP'AC`TURE OF
CHhrMIq~HERMOMECHANICAL PULP
[
The Field and Backqround of_the Tnvention
The present invention relates to a method for the manu-
facture of chemithermomechanical pulp (hereinafter
referred to as CTMP-pulps), and par~icularly to a method
for avoiding pulp discolouration and/or for counteract-
ing a reduction in pulp brightness in the manufacture of
a CTMP-pulp at elevated temperature, and also to an
impregnating liquid used when practicing the method.
Paper pulp produced from lignocellulosic material is
manufactured by subjecting raw material in the form of
wood chips to a mechanical, a chemical and a thermal
treatment process, these processes either being effected
simultaneously or separàtely. CTMP-pulp is manufactured
conventionally by preheating and refining the wood chips
at a temperature of about 100-130-C~ It has now been
found that energy consumption can be reduced to about
half the energy that is consumed when refining CTMP-
pulps conventionally at 130-C, by preheating and refin-
ing the pulps at an elevated temperature of 160-180 C.
A pulp refining process which is 80 lean in energy is
highly desirable and has also arrived at the right time,
: 80 to speak, and, furthermore, will probably become a
necessity in the future, since energy prices are expect-
ed to increase considerably.
It has earlier been considered impossible to manufacture
bright mechanical pulp subssquent to preheating and
refining at temperatu~es higher than about 130-C, since
pulp brightness is impaired at higher temperatures. It
has also been considered impo6sible to bleach pulps
W092J20855 PCT/SE92/00
which have been refined at higher temperatures in subse-
quent bleaching stages at reasonable costs and at rea-
sonable chemical consumptions. Furthermore, when refin-
ing at said higher temperatures, the fibres are liable
to become covered with lignin, which may result in
impaired strength and absorption properties. These
latter properties are particularly important in the case
of absorbent pulps. Conse~uently, refinement of the
pulp has normally been effected at a temperature of at
most 140-C and often at a temperature beneath 130-C. In
the manufacture of highly absorbent pulps intended for
the production of fluff and tissue, a high freeness and
low shive content is desired, i.e. the pulp freeness
index will preferably lie above 600 ml CSF, preferably
above 700 ml CSF, and the shive content will not exceed
10%, preferably will lie beneath 6%.
Disclosure of the Invention and the Ob~ect Thereof
The inventive method enables pulp to be refined at an
elevated temperature,- with the intention of saving
energy - without impairing pulp brightness. The pulp
produced can also be bleached with peroxide in a subse-
quent bleaching stage. The inventive method is charac-
terized by pre-impregnating the wood chips with an
t impregnating liquid that contains borohydride, at least
one compound chosen from the group sodium sulphite,
sodium hydrosulphate and sodium disulphite, and option-
ally one or more sequestering agents, wherein the im-
pregnating li~uid has a neutral, a near neutral or aslight ~acid p~-value, and by preheating the impregnat-
ed chips to a temperature of 16~-180'C and carrying out
the refining process while maintaining said temperature.
The borohydride is suitably added in the for,m of an
alkaline aqueous solution, for instance the borohydride
W092/20855 2 ~ 0 9 ?~ PCT/SE92/00
solution retailed commercially under the trade name
BOROL~ and which contains 12 percent by weight NaBH4.
The borohydride is added in the form of said aqueous
solution in an amount of about 5-15 kg BOROL~ per tonne
of chips, preferably about 10 kg per tonne of chips.
Thus, the amount of active borohydride added should -
correspond to about 0.6-1.8 kg NaBH4 per tonne of chips,
preferably about 1.2 kg per tonne of chips.
The sulphite-containing compound consists of sodium
sulphite, sodium hydrosulphite, sodium disulphite, or a
mixture of two or more of these compounds. Calculated
as the amount of sodium sulphite, Na2SO3, added to the
system the total sulphite charge is roughly 20-40 kg per
tonne of chips, preferably about 30 kg per tonne of
chips, or correspondin~ stoichiometric quantities of
S02, HSO3 and/or S2O~ . In order to achieve maximum
pulp brightness, the impregnating liquid should be qiven
a neutral pH, a near neutral pH or a slightly acid pH,
i.e. the pH of the liquid should lie between about 5 and
7, preferably from 5.5-6.5. This pH-value can be ob-
tained with the aid of an acid, for instance sulphuric
acid, H2SO4, or with the aid of sulphur dioxide, S02,
which is often available in the pulp mill. When S02 is
used to acidify the impregnating liquid, or when HS03
or S2O5 is used instead of SO3 , the total sulphite
charge must, however, be adapted so as to lie within the
aforesaid pH-range. The manner in which this sulphite
charge and the recommended pH-value are achieved is,
however, unimportant to the invention and may be carried
, out in any appropriate manner when practicing the
invention.
One or more se~uestering agents are also preferably
added to the impregnating liquid, in order to bind in
the pulp any metal ion~ that may be whiah are liable to
~ ~ ~ v
W092J2085S PCT/SE92/00
cause discolouration. A suitable sequestering agent in
this regard is the sodium salt of diethylenetriamine
pentaacetic acid (DTPA), or ethylenediamine tetraacetic
acid (EDTA), said sequestering agent being added in an
amount of about 2-4 kg per tonne of chips.
The invention also relates to an impregnating liquid
intended for t~e impregnation of lignocellulosic mate-
rial in the manufacture of chemithermomechanical pulp.
The impregnating liquid includes borohydride, at least
one compound selected fro~ the group sodium sulphite,
sodium hydrosulphite and sodium disulphite, and option~
ally one or more se~uestering agents. The weight ratio
between the active components borohydride and total
sulphite concentration of the liquid is between 20:80
and 30:70, preferably about 2S:75. The pH of the im-
pregnating liquid is adjusted to a neutral value, a near
neutral value or a weakly acid value, in other words the
~pH-value of the liquid will preferably lie between 5 and
7, more preferably between 5.5-6.5.
When producing wood pulp in accordance with the present
invention, wood chips are impregnated with an impregnat-
ing liquid that contains borohydride and at least one
compound selected from the group sodium sulphite, sodium
hydrosulphite and sodium disulphite, and optionally one
or more sequestering agents. The impregnated chips are
then preheated over a period of from 1-3 minutes, pre-
ferably over a period of about 1 minute, at a tempera-
ture of 160-180'C, preferably about 165-170 C, where-
after the wood chips are defibred/refined to pulp form
while essentially maintaining pressure and temperature
from the preheating stage. The pulp is suitably washed
under pressure and high temperature, preferably without
the ingress of air, in immediate connection with the
refining process, the pulp being dewatered to a
W092/20855 ~ ~9 ~ 1 ~ PCT/SE92/~
consistency of 25-40%, and then bleached with peroxide.
The pulp can be rewashed after the bleaching stage, if
necessary.
- 5 The pulp is refined to a freenQss value in excess of
700 ml CSF, at an energy consumption of about 500 kWh/
tonne.
The aforedescribed method enables a pulp of unbleached
brightness to be produced by high temperature refinement
prior to the peroXide bleaching stage, this brightness
being comparable to the brightness of conventionally-
produced CTMP-pulps. When the pulp produced in accor-
dance with the invention is subjected to a following
peroxide bleaching stage there is also obtained an
equivalent bleaching response and a maximum brightness
of the same order of magnitude as that achieved with
pulps produced conventionally at lower temperatures.
Working Example
Example 1. ~High temperature refinement with boro-
hydride).
.
~25 ~ ~ Spruce chips were impregnated with impregnating liguids
containing 30 kg sodium sulphite, 10 kg BOROL~ t12%
a~ueous solution of sodium borohydride) and 3 kg DTPA
per tonne of chips. The impregnating liquid had a
pH = 11, or alternatlvely was neutralized or acidified
with H ~ 04 to p~ = 8 and pH = 6 respectively. The chips
~; were preheated at a temperature of 165-C for a period of
l minute. The chips were then refined while maintaining
the aforesaid temperature.
Subsequent to this refinement, the pulp was washed and
dewatered in immediate connection with the refining
W092/20855 ~ PCT~SE92/~
process under pressure and at high temperature, about
1~0-180C, without the ingress of air into the pulp.
Reference is made to the following Table 1 with regard
to other process conditions.
~ The brightness of the unbleached pulps produced was
measured in accordance with SCAN-C 11 at a wavelength of
4S7 nm.
The produced pulps were then bleached in a conventional
manner, using 40 kg of peroxide per tonne of pulp. The
brightness of the pulps was measured in t~e same manner
as that used to measure the brightness of the un~leached
pulps. The brightnes~ values measured are set forth in
lS Table 1 below.
Example 2. (High temperature refinement without boro-
hydride)~
Spruce chips were impregnated with an impregnating
liquid having a pH = 9.5 and containing 30 kg Na2S03 and
3 kg of DTPA per tonne of chips. The chips were heated
to 165 C over a period of 1 minute and were treated, in --
other respects, in accordance with Example 1. The
brightness of the unbleached pulp and the brightness of
the pulp bleached with peroxide were measured in accor-
dance with SCAN-C 11 and the results are set forth in
Table 1 below.
Example 3. (Conventional refinement).
'
Spruce chips were impregnated with impregnating liquid
similar to that used in Example 2. ~he chips were
preheated to 130-C over a period of 3 minutes and
refined while maintaining this temperature. Subsequent
to refinement, the pulp was washed and dewatered in a
W092/208S5 - ~ J~;~ PCTtSE92/00
conventional manner at about 130 C, without the ingress
of air into the pulp.
The brightness of the unbleached pulp and the brightness
of the pulp bleached with peroxide were measured and ~he
results obtained are set forth in Table 1 below.
The results obtained with the three methods described in
Examples 1-3 above and the process conditions applicable
to said method~ are collected in Table 1.
Table 1
Ex. 1 Ex. 2 ~x. 3
Process conditions: -
Preheat temp. C 165 165 130
Time min.
Energy consumption kWh/t 550 600 900
~0
Chip impreanation:
Na2S3 kg/t 30 30 30
BOROL~ (12 percent by
weight NaBH4) kg/t 10 - - -
DTPA kg/t 3 3 3
pH-value 6 8 119.5 9.5
Result:
Freeness CSF ml 740 740 730
Unbleached brightness
after refinement ISO % 59 57.5 55 54 57
Bleached brightness
after peroxide bleach-
ing (H2O2, 4~ kg/t)
ISO % 78.5 77 7573 78
W092/20855 ~ ~ PCT/SE92/~
It is evident from the results obtained wit~ the bright-
ness measurements made on the produced pulps that the
high temperature refinement carried out with the inten-
tion of reducing the amount of energy consumed by the
refinement process can be achieved while maintaining the
brightness of the pulps produced provided that the chips
are pre-impregnated with an impregnating liquid in
accordance with the invention. The measured brightness
values also show that the pH of the impregnating li~uor
has an effect on the result.
