Note: Descriptions are shown in the official language in which they were submitted.
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TREATMENT OF WOOD-CHIPS
TECHNICAL FIELD
The present invention relates to the treatment of wood-chips for the
manufacture of chemi-thermomechanical pulp, known as CTMP.
BACKGRO~ND ART
Chemi-thermomechanical pulp, known as CTMP, has begun increasingly to
be used in place of chemical pulps in many products in order to
improve the yield from limited raw material resources. Before
refining, the wood-chips are usually treated in a series of processes
consisting of washing the wood-chips, pre-steaming, pressure-impreg-
nation and pre-heating. The usual impregation medium is a sodium-
based neutral sulphite, although ammonium-based sulphite may also be
used. Also previously disclosed is the process of two-stage impreg-
nation, whereby pre-impregnation is carried out after pre-steaming
but before the wood-chips are washed, in this case followed by
draining prior to final impregation.
By comparison with the thermo-mechanical pulps, T~, which have not
been exposed to chemical modification prior to refining, CT~
exhibits certain improved characteristics. This has the effect of
reducing the shive content and increasing the content of long fibres~
which manifests itself in a higher tensile and tearing index. It is
desirable, however, to be able further to reduce the shive content
and to increase the content of long fibres. This is particularly true
when a high pulp yield is required which is suitable for liquid
packaging board or as fluff pulp. Excellent translucence is also
required in the latter case, although this is of less significance
to liquid packaging board. If it is wished to be able to use one and
the same plant for the production of pulps for different purposes,
then it is also desirable to be able to control the individual
treatment in such a way that the pulp for each application will
acquire the desired combination of features. It is also desirable to
be able to treat the wood-chips in a uniform fashion, thereby
enabling a more homogeneous refined pulp to be obtained after the
refining operation.
DISCLOSURE OF THE INVENTION
The above and other objects can be achieved to a very great extent
through more effective impregnation and through the chemical treat-
ment of the pulp before refining, which may be achieved in accordance
with the invention by keeping the wood-chips at an intermediate stage
at a temperature of not less than 50C and for a period of at least
10 minutes after pre-steaming, pre-impregnation ~Jith H20 and/or a
sulphite solution, washing and drying, but before final impregnation,
pre-heating and refining. The sulphite solution, which normally is a
sodium sulphite solution, preferably has a pH of between 4 and 11.
This treatment will produce a significant increase in the content of
long fibres, which is beneficial both to fluff and board and to all
papers which require to exhibit high resistance to tearing. A simul-
taneous reduction occurs in the shive content which, like the higher
long fibre content, appears to be attributable to the fact that
keeping the pulp hot prior to the final impregnation process faci-
litates the exposure of the fibres in such a way that they are
subjected to less damage during refining. The pulp also becomes more
tough, which is an additional desirable property of board pulps.
Moreover the resin content of ~he pulp is also reduced by this
treatment.
Experiments have been conducted in which the quantity of sulphite
added during the pre-impregnation and final impregnation stages is
varied, in addition to which a study was made of the effects produced
when no chemicals are added prior to ~ashing the wood-chips. The
findings of these investigations suggest that the shive content
is dependent on the quantity of chemicals added, but that the stage
of the treatment at which the chemicals are added is less important.
Never~heless, a eertain risk of reducen translucence does appear to
exist if all the sulphite is added during the final impregnation
stage, with none being added in conjunction with the washing of the
wood-chips.
The manner in which the wood-chips are kept hot in accordance with
the invention during the actual intermediate stage between the
washing of the wood-chips and the final impregnation may take one
of a number of different forms. In accordance with a preferred
embodiment the wood-chips are treated with steam in a steam-
treatment vessel at atmospheric pressure. The treatment may also
take place slightly above atmospheric pressure. One other conceiv-
able alternative is to heat the wood-chips during or after washing
and before they are transferred to the intermediate reaction vessel,
where the wood-chips are maintained at the temperature in question.
Therefore the vessel may be insulated, if necessary. Combinations
of the two methods are also conceivable. As has already been men-
tioned, the temperature inside the intermediate reaction vessel
is maintained at a minimum of 50C. The temperature may rise to a
maximum of 99C at atmospheric pressure, but may be increased to
about 105 C at a maximum pressure above atmospheric of 0.2 bar. The
optimum temperature appears to lie within the range between 60 and
85 C at atmospheric pressure.
The period spent in the aforementioned vessel may also be varied.
The optimum period is between 30 and 70 minutes9 during which time
the sulphite will be able to penetrate deep into the wood-chips
and to react with the wood at the same time as this is softened
by the heat.
Other characteristic features and advantages of the present invention
will become clear from the following description of the tests which
have been made.
DESCRIPTION OF DRAWINGS
In the following description of the tests which have been made,
reference will be made to the following drawings, in which:
ig. 1 is a diagrammatic representation of a plant for the
manufacture of CTMP to which the treatment of the
wood-chips in accordance with the invention has been
applied;
s
Fig. 2 is a graph illustrating the penetration of the sulphite
into the wood-chips both before and after the treatment
in accordance with the invention,
0 Fig. 3 is a graph illustrating the long fibre fraction of the
pulp as a function of the density for different treatment
processes;
Fig. 4 is a graph illustrating the shive content as a function
of freeness for pulps treated in various ways.
DESCRIPTION OF THE TESTS CARRI~D OUT
A plant of the nature illustrated in graphic form in Fig. 1 was
used for the execution of the tests. The plant comprises an pre-
steaming vessel 1, a screw conveyor 2, a wood-chip washer 3 with
a sand trap 4 and a drier 5 which feeds into a holding or inter-
mediate reaction vessel 6 with a feed screw 7. The feed screw 7 is
connected to a line 8 for breaking up the wood-chips prior to
impregnation, pre-heating and refining. A return line and a return
hopper are identified respectively by the reference designations
9 and 10. Two or more refiners are normally included in the
installation, although the figure shows only a single refiner 11,
to which is connected a pre-heater 12 and an impregnator 13. The
wood-chips are caused to be introduced into the impregnator 13 by
means of a 'Prex' screw conveyor 14 ('Prex' is a trade mark).
The i~,lpregnation liquid can be added to the wood-chips on the one
hand via a line 15 to the feed screw 2 ahead of the wood-chip
washer 3, and on the other hand ~ia a line 16 to the impregnator 13.
The pre-steaming vessel 1 in this case had a usefui volume of 20 m3
and a wood-chip requirement of 15 tonnes/h. The holding vessel 6
was in the form of a container known under its trade name of an
'Enso' vessel having a useful volume of 44 m , in the base of which
were fitted steam inlet organs. The wood-chip requirement of the
holding vessel, at 15 tonnes/h, was matched to the capacity of the
pre-steaming vessel.
Spruce wood-chips were the substance treated in the tests. The
period spent in the alkaline cooking vessel 1 was 16 minutes and in
the holding vessel 6 37 minutes in each test. The impregnation
liquids which were added prior to the washing of the wood-chips,
that is to say via the screw 2 and via the impregnator 13, consisted
of sodium sulphite Na2S03 with a pH of about 9. Varying quantities
of the impregnation medium were added at both points. Zero addi-
tions were also tested for comparative purposes. Washing was done
using a cold washing liquid in order to achieve the best possible
penetration of the impregnation medium in a manner known per se.
Any excess fluid was removed in the drying screw 5 and was recircu-
lated. The pressure inside the pre-heater 12 was varied within the
range of 1.0-1.8 bar. The temperatures inside the pre-steaming vessel
1 and inside the holding vessel 6 were varied within the approximate
range of 75-95 C. The temperatures, the pressures and the quantities
of chemicals added appear in Table 1. The left-hand part of the
first column9 headed 'Test', indicates the nominal quantities of
added chemicals, with the first figure indicating the specified
q~lantity of chemical added during the pre-impregnation phase, i~e.
to the screw 2, whereas the second figure indicates the specified
quantity of chemical added to the impregnator 13. The right-hand
part of the first column indicates the nominal pressure inside
the pre-heater 12.
Table 1
Test Pre- Holding Pre- Sulphite Pre- Final
steaming vessel heater pH impregnation impregnation
C C bar 9 2 9 2
0/0-1.Q 98 95 1.0 9.0 0 0
0/10-1.0 96 91 1.0 9.1 0 10.1
0/20-1.0 98 89 1.0 8.8 0 20.2
0/30-1.0 98 1.0 0 30.3
0/0-108 81 91 1.8 9.2 0 0
0/10-1.8 91 78 1.8 8.9 0 10.2
0/20-1~8 98 77 1.8 9.0 0 20.h
0/30-1.8 97 76 1.8 8.8 0 29.1
5/0-1.0 93 86 1.0 9.0 4.9 0
5/10-1.0 68 90 1.0 9.1 4.9 9.9
5/20-1.0 79 91 1.0 9.0 4.9 19.8
5/30-1.0 82 89 1.0 7.3 5.0 29.8
5/10-1.3 95 36 1.27 8.1 2.45 5.5
5/20-1.3 94 93 1.31 8.2 2.60 10.2
S/30-1.3 98 32 1.23 7.1 2u6 16.7
5/0-1.8 92 88 1.8 8.7 5.2 0
5/10-1.8 95 90 1.8 9.7 5.0 10.0
S/20-1.8 95 87 1.8 9.5 5.1 20.6
; 20 5/30-1.~ 97 94 1.8 7.3 5.1 30.9
15/0-1.0 87 89 1.0 7.0 12.0
15/5-1.0 90 85 1.0 9.0 12.0 5.2
15/15-1.0 66 84 1.0 7~7 12.0 15.5
15~0-1.3 94 86 1.3 9.2 12.0 0
15/5-1.3 93 93 1.3 9.5 11.6 5.1
25 15/15-1.3 79 87 1.3 7.8 11.6 15.0
l S/0-1.8 78 88 1.8 9.0 11.6 0
lS/5-1.8 85 95 1.8 9.1 11.6 5.1
15/15-1.8 83 81 1.8 9.0 11.6 15.0
.
It was found that the treatment applied in the holding vessel
produced a dramatic improvement in the penetration into the wood
of the chemicals added prior to the washing of the wood-chips.
This may be appreciated from Table 2 below and is also illustrated
in the form of a graph in Figure 2.
TABLE 2
Sulphur content in small and large pieces of wood-chips before and
after the holding vessel. Addition of sulphite equivalent to 1.2 kg
of S02 prior to washing the wood-chips.
Dry matter Total S
% g S/kg
Lar~e ~ieces of wood-chi~s
___ __ _________________ _
Before holding vessel 38.Q 0.9
After holding vessel 37.0 1.2
Small 2ieces of wood-chi~s
______ _________________ _
Before holding vessel 34.4 1.5
After holding vessel 35.3 1.7
It is obvious that the effective penetration of the chemicals into
the wood in combination with the holding period and the temperature
helped in the softening of the wood-chips and in their chemical
modification. This was also manifested in the form of the higher long
fibre fraction and the reduced shive content and resin content after
refining, as may be appreciated from Table 3 below and from the
graphs contained in Figures 3 and 4. Figure 4 also clearly shows that
the addition of the chemicals should take place in two stages, that
is to say both before the washing of the wood-chips and in the
impregnator, if the lowest possible shive content is to be achieved~
whereas Figure 3 indicates that even the penetration of the water
into the wood in conjunction with the heat-treatment in the holding
vessel 6 has the effect of improving the long fibre fraction. The
addition of the sulphite at the pre heating stage has the effect,
on the other hand, of reducing the translucence. Graphs 3 and 4
also reveal that the best values are achieved when the total
quantity of added chemical is equivalent to 25-35 kg of S02 per
tonne of wood-chips, but that very good values can be achieved
from a quantity of chemical equivalent to as little as 10-20 kg of
S2 per tonne.
The experiments which were carried out did not, however, clearly
indicate the manner in which the chemicals should be apportioned
between the pre-impregnation stage ahead of the wood-chip washer
3 and the final impregnator 13. Table 3 below compares a number of
characteristic data relating to sieved CTMP for board manufacture
treated in accordance with the invention and in accordance with
a treatment process known in the art involving pre-impregnation
prior to washing but without being subjected to a period of heat-
treatment in the holding vessel 6. The treatment in the holding
vessel in accordance with the invention took place at atmospheric
pressure using steam.
9 TABL~ 3
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