Note: Descriptions are shown in the official language in which they were submitted.
WO ~U16139 2 1 5 1 3 6 q PCT1SE93101058
A method of producin~ mechanical and chemi-mechanical ~ul~
This invention relates to the production of mechanical and
chemi-mechanical pulp with a yield of above 85% from ligno-
cellulose-containing materia~ for the Inaking of paper or
board products.
Mechanical pulp (e.g. TMP) or chemi-mechanical ~ulp (e.g.
CTMP) is today produced in several different process vari-
ations, where steamed chips are refined in disc refiners
of various types. At the production of pulps for different
printing papers or wrapping materiais, of board type, the
refining usua~ly is carried out in one or more steps. The
first step normally is pressurized, i.e. the refining takes
place a~ temperatures exceeding 100C, usually immediately
below or at the so-called softening temperature (Tg) of the
lignin. Heretofore, it was chosen to hold the pressure and
temperature in subsequent refining steps on the same level
as in the first refining step, or to carry out later refin-
ing steps in systems not pressurized, i.e. at a temperature
lower than in the initial step, usually at tAe sof~ening
temperature of the lignin or below the same.
The softening temperature of the lignin, which has proved
to be an imporlant variable at the refining of chips in
mechanical and chemi-mechanical pulp processes, has been
determined in the most recent decades in a number of
scien~ific investigations for a plurality of wood types
concerned. At the investigations standard equipment and
conventional measuring principies for the determination
of viscoelastic parameters have been used. For wood, as
for other viscoelastic materials, the softening temperalure
varies with the load frequency at the measurements. At a
higher load frequency, the softening temperature increases.
At the processing frequencies normally applied in refiners
the softening temperalure of coniferous wood was deler~.ined
to be bet~een 125C and 145C, ~.~hile it proved ~o be so~ewhat
W094116139 PCTlSEg3101058
2~S~364
lower for our most usual hardwood types. The softening
temperature can be shifted by the addition cf different
chemicals. It can be lowered, for example, after impregnation
by usual iignin softening chemicals, of the type sulphite.
Relatively high total electric energy amounts are required
for producing the aforementioned types of puip. The production
of pulp for newsprint from coniferous wood, for example,
can require up lo 2000kWh/ton pulp. In many studies carried
out recently wi~h the object of trying to lower the electric
energy consumption in the TMP-process, it W2S found that
the initial processing phase seemed to be quite essentiai
for the total energy consumption in different process
variants and for the character of the resulting pulp.
This seems to apply in spite of the fact that only a small
part of the total electric energy consumption in the refining
process is used for the fiber separation proper, i.e. for
the conversion from chips to free individual fibers (also
called defibering).
A fiber separation energy-effective per se 2S a result of
an effective thermal or chemical softening of the chip areas rich
in lignin, however, does not ;crove to be a garanty that
the total energy consumption wiil be low. On the contrary,
it was proved that TMP-process variants, which were initiated
with a mild fiber separation poor in energy, often require
a high total energy input.
This circumstance seems to be caused by the fact, that mildly
separated but unprocessed fibers, which were obtained by
carrying out the defibering at temperatures above the soften-
ing temperature of the wood lignin, are difIicult to fibrillate
during ~he continued working in the refining process. This
fibrillation is necessary for increasing the flexibility of
the fibers to a desired level and bringing acout the fine
material characterizing a gocd T~P-au21ity. An intensive
l'~2l5l364
processlng below the softenlng temperature of the wood llgnln
lnltlally and durlng the contlnued reflnlng process, on the
other hand, easlly leads to a deterloratlon of the long flber
content and thereby of the strength propertles of the pulp.
Thls 18 ln many cases unacceptable from a quallty polnt of
vlew. A decrease ln the energy consumptlon from an
establlshed level ln the TMP-process, as a rule, has been
assoclated wlth a deterloration of certaln quallty propertles
of the resultlng pulp, for example lower long flber content,
lower tear strength, lower tenslle strength and hlgher shlves
content. The present hlgh energy consumptlon ln the TMP- and
CTMP- process, therefore, has been necessary for achlevlng
the deslred pulp propertles.
It was now found by surprlse, that lt 18 posslble
to comblne low energy consumptlon ln a mechanlcal or cheml-
mechanlcal pulp maklng process wlth malntalned quallty
propertles. The present lnventlon relates to such a method,
where the mechanlcal processlng, for example reflnlng, takes
place ln at least two steps. Accordlng to the lnventlon, the
materlal at lts feed lnto the flrst processlng step has a
temperature below the softenlng temperature of the llgnln,
and at lts feed lnto at least one subsequent processlng step
has a temperature exceedlng the softenlng temperature of the
llgnin. The lnventlon ls descrlbed ln greater detall ln the
followlng, wlth reference to some expedlent embodlments and
examples.
The lnventlon wlll be further descrlbed wlth
reference to the accompanylng drawlngs ln whlch 2
Flgure 1 ls a graph showlng freeness as a functlon
28229-54
A
~_ 3a ~ 215 ~ 364
of energy consumptlon;
Flgure 2 18 a graph showlng shlves content as a
functlon of energy consumptlon~
Flgure 3 18 a graph showlng long flber content as a
functlon of freeness~
Flgure 4 18 a graph showlng tear lndex as a
functlon of freeness~
Flgures 5 and 6 are graphs showlng tenslle lndex
and llght scatterlng as a functlon of freeness~
Flgure 7 18 a graph showlng energy consumptlon as a
functlon of freeness; and
Flgure 8 18 a graph showlng freeness as a functlon
of llght scatterlng.
In a TMP-process accordlng to the lnventlon,
reflnlng take~ place ln at least two steps. In the flrst
step the chlps are fed lnto the reflner at a temperature
below the softenlng temperature of the llgnln and then are
processed under relatlvely lntenslve condltlons, for example
ln a double-dlsc reflner wlth a speed of at least 1200 rpm or
ln a slngle-dlsc reflner wlth hlgh relatlve speed between the
reflner dlscs ~at least 1800 rpm, preferably at lea~t 2400
rpm). The energy lnput
'A- 28229-54
WO ~/16139 PCTlSEg3/01058
3 6 ~ 4
in the first step is chosen to be on such a low level, that
the long fiber content of the pulp which a.o. yields the
potential for the later strength development at the refining,
is not deteriorated appreciably. The freenes (CSF) of the
pulp after the first step, therefore, shall be high, preferably
~ 500 ml. A subsequent refining step is carried out under
conditions where the lignin of the fi~er materia-l is well
softened. The fiber material then is fed into the refiner
at a temperature exceeding the softening temperature of the
lignin. In cases when the material consists of coniferous
wood not treated with chemicals, the temperalure should exceed
150C, suitably 160C and preferably 170C. When the material
is treated with chemicals, the temperalure should exceed
135C, suitably 150C and preferably 160C. As regards an
upper temperature limit, temperatures over 200C should
be avoided, a.o. with regard to dark colouring of the fiber
material. The processing frequency preferably can be high
(relative speed at least 2400 rpm) at the processing of the
well-~oftened fiber material, which has proved especially
favourable from an energy point of view.
The temperature difference between the temperatures of the
material at its feed into the first and, respectively, a
subsequent processing step should be at ieast 15C, suitably
at ieast 25C and preferably at ieast 35C.
In a process according to the invention fractures and fracture
indications in the material initially are guided to layers
in the fiber wali not rich in lignin. During the final refining
the known fact then can be utilized, that the fiber material
can be separated with low energy inputs in areas rich in
iignin at temperatures above the softening temperature of
the wood iignin. The fractures initially Aaving been guided
to areas not rich in lignin, it is thereby avoided to obtain
a fiber material with only lignin-~overed surfaces which are
difficult to fibrillate. Tnis has previously been the great
:
WO94/1613g 2 I 513 6 4 PCT1SE~3/01058
problem when it was tried to utiiize refining temperatures
above the softening temperature of the lignin a-t the production
of mechanical pulps for printing paper or board products.
Fine material from areas between the initial fracture zone
and the middle lamina of the fiber rich in lignin also is
easily released at temperatures above the softening temperature
of the lignin in the later refining step, which can explain
the low total energy consumption to a certain freeness (C,~F)
in this process step and in the entire process according
to the invention. The production of fine material otherwise
is the most energy requiring part of the mechanical pulp
process using conventional technique.
EXAMPLE
Thermomechanical pulp from ~echips was produced after
refining in two steps in a 20" single-disc refiner of a
well-equipped test plant. The first refining step (defibering)
was carried out after preheating the chips at 115C for
about 3 minutes, i.e. at a temperature below the softening
temperature of the lignin. The refiner was driven by a 3000 rpm
motor, in order to ensure that the initial defibering should
not take place under too mild conditions. The effect input
in the first step was 640 kWh/t, which yielded a pulp with
freeness (CSF) 518 ml. In the second refining step the
conditions were varied according to the following Table:
Test Preheatin~ t,ime .~efining temperature ~otor speed
min C ) r~m
A about 1 115 ( ~ Tg) 1500
B " 1 160 ( ~ Tg) 1500
C " 1 160 ( ~ Tg) 3000
D " 1 170 ( ~ Tg) 3000
.
x) Temperature at preheating and at feed into the refiner
The effect of the varying conditions is shown in Figs. 1-6
where the most essential pulp ~roperties have been valued,
2nd aI e commen-ted cn as follows:
21S 13 6 ~ PCT1~ 3/01058
Fi~. 1
shows freeness as a function of energy consumption. It
appears that by carrying out the second refining step at
temperatures above the sof-tening temperature of the lignin
the energy input at refining to a certain freeness can be
reduced considerably compared to conventional second step
refining at temperatures below the softening temperature
of the lignin (compare Tests A and B).The energy reduction
will be still greater when, in addition, the speed is
increased from 1500 to ~oon rpm (compare Test B with Tests
C and D).
Fi~. 2
shows the shives content as a function of the energy
consumption. It appears that second step refining at temper-
atures above the softening temperature of the lignin yields
a clearly lower shives content at a certain energy input
than refining at a temp~rature below the softening temper-
ature of the lignin (compare Test A with Tests B-D). Also
in this case the higher speed yields the most favourable
values. This proves to be a further advantage by using the
conditions according to the invention.
Fi~. 3
shows the long fiber content as a function of freeness.
It appears that the long fiber content of the pulp generally
can be maintained all the way down to the freeness range
150-200 ml, in spite of the heavy energy reduction at refining
according to the conditions of the invention.
Fi~. ~
show~ the tear index as a function of freeness. It appears
that the tear index of the pulp can be maintained all the
way down to the freeness range 150-200 ml, in spite of the
large energy reduction at refining at the conditions o
the in~rention.
WO ~116~g 2 1 5 1 3 6 ~ PCT1~ 3101058
Fi~s. 5 and 6
show the tensile index and, respectively, light scattering
as a function of freeness. It appears that all tested pulps
develop tensile index and, respectively, light scattering
coefficient in a similar way when they are ~alued conventionally
against freeness.
In parallel with the tests described and with reference
thereto it also was investigated, how energy consumption
and pulp quality are affected when a refining process contrary
to the conditions of the invention was started with a refining
step where the iemperature at the feed to the first step
refiner is higher than the softening temperature of the lignin.
Also in this case the thermomechanical pulp was made from
spruce chips after refining in two steps by single-disc refiners.
The first refining step was carried out at temperatures
above the softening temperature of the lignin in the same
equipment which was used previously in the test. The conditions
in the first refining step and the freeness after refining
with a certain energy input are described in the following
Table:
Test Preheating Refining ) Motor Energy Freeness
time, min temp. C speed rpm input kWh/t ml
E about 1 150 (> Tg) 3000 940 450
~ " 1 160 ( ~Tg) 1500 900 580
G " 1 160 ~Tg) 3000 790 415
x) Temperature at preheating and feed into the refiner
In a second refining step which was carried out under
atmospheric conditions in a 2~" refiner, i.e. at temperatures
below the softening temperature of lignin, the freeness
was lowered to an interesting range for printing paper pulps.
The refiner speed in this case was 1500 rpm.
WO ~/16139 ~ PCT1SE93/01058
2 15 13 6 4 8
The effect of the varying conditions on energy consumption
and light scattering capacity appears from Figs. 7 and ~,
which show freeness as a function of energy consumption
and, respectively, light scattering as a function of freeness.
Fi~. 7
shows that the energy consumption is considerably higher
when the TMP-process is initiated with a refining step
at a temperature above the softening temperature of lignin
than that obtained with conditions according to the
invention (compare Fig. 1).
Fi~. 8
shows that the light scattering coefficient is considerably
lower when the TMP-process is initiated with a refining
step at temperatures above the softening temperature of
lignin than that obtained with conditions according to
the invention (compare Fig. 6). The pulps produced according
to the invention, therefore, are clearly most suitable
for use as printing paper pulps, where just the light
scattering coefficient must be sufficiently high for
achieving the desired optical properties.
The example described proves cleariy, that mechanical pulp
can~be produced with the conditions of the invention at
low energy consumption, at the same time as essential properties
~ike shives content, long fiber content,tear strength,
tensile strength and light scattering meet high requirements
on this type of pulps. The energy consumption at the production
of newsprint, for example, can be reduced by about 40%
- compared with conventional manufac-turing methods.
In the process according to the invention, chemicals can be
added advantageously after or during the first refining step,
in order to avoid dark colouring at the high temperatures
above the sof-tening cemperature o~ lignin in subsequent
refining sieps. The chemicals also c2n have & bleaching effect.
WO ~/lC139 2 1 5 1 ~ 6 4 PCT~93101058
9 ;
Examples of such chemicals are sodium sulphite, sodium
bisulphite, sodium ditionite, peroxide etc.
According to the invention, the initial processing can be
carried out, besides in refiners, also in grinders,
compressing screws or other mechanical processing equipment.
In cases when a reject fraction separated from the processed
material is subjected to additional mechanical processing,
this reject with a temperature abo~e the softening
temperature of lignin shall be fed into at least one subsequent
processing step.
The invention, of course, is not restricted to the examples
shown, but can be varied within the scope of the invention
idea.
.
