Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVE~TION
Semi-chemical pulp is produced generally by the so-called
cold soda process wherein wood chips are impregnated with sodium
hydroxide and after storage for some time in a reaction vessel,
they are defibrated, which usually is effected in a disc-equipped
grinding apparatus, whereupon the ground pulp is relieved from resi-
dues of chemicals by washing.
Sometimes in the production of mechanical pulp, the wood is
impregnated with a weak alkali solution in order to swell the wood
fibres and to increase the strength of the fibre coherence in the
final product. ~owever, all alkali treatment results in a deterior-
ation of the optical properties of the pulp such as brightness and
light-scattering properties thereof. These changes are particular-
ly conspicuous when deciduous wood is used for making pulp in this
manner,
SUMM~RY OF ~IE INVENTION
:
One purpose of the invention, among others, is to reduce the
deterioration of the brightness and light-scattering properties of
the pulp. According to this invention, alkaline chemicals are ad-
mixed with the fibrous starting material prior to its introduction
into a pressure vessel which forms part of the grinding apparatus,
and the pressure in the grinding apparatus is wholly or partly pro-
duced by compressed air.
Another way to reduce discoloration from the alkali treatment
is to use alkaline chemicals with lower alkalinity, such as alkali
silicate, alkali carbonate or alkali bicarbonate, instead of sodium
hydroxide.
The penetration of chemicals into the starting material can
be enhanced in conventional manner by means of pressure impregnation
or by steaming the starting material directly before the intro-
duction into the chemical solution.
In the subsequent defibration in a disc-equipped grinding ap-
paratus, the temperature, due to the friction heat developed at
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least near the end of the grinding process, normally will run up
to at least about 100C. In a closed grinding apparatus subjected
to steam overpressure, the temperature will exceed 100C. Grind-
ing alkalized starting material at such high temperature has proved
to have a deleterious effect on the brightness and opacity of the
ground pulp. According to the present invention, it has been
found advantageous to lower the temperature during the grinding
operation in a pressurized grinding apparatus by supplying com-
pressed air to the pressure vessel through which the starting mat-
erial is fed to the grinding members. In this connection, it is
suitable to supply such an amount of compressed air that the start-
ing material at the inlet to the grinding space will have a tem-
perature of not more than 90C.
In order to obtain a better and more uniform impregnation,
it has proved necessary to retain the impregnated chips in the
impregnation solution, or after the impregnation, for at least 10
minutes, preferably 20 minutes. A retention period exceeding 2
hours appears not to result in any further improvement.
Alkaline defibration is especially advantageous in the pro-
duction of pulp from various inferior raw or starting materials
such as greenchips (which means chips obrained by grinding all cel-
lulose-containing material from trees including fresh bough and
leaves), sawdust, straw, bagasse or the like. These starting mater-
ials yield, however, a rather dark-coloured pulp, which is also the
case when pulp is produced by alkaline defibration of deciduous wood.
These facts render bleaching of the pulp still more essential.
According to the invention, it is particularly advantageous in this
connection to treat the starting material with bleaching agent prior
to the defibration with peroxide and alkali, and, of course, in case
of need, with usual chemical adjuvants such as e.g. water glass,
magnesium salt or complex forming substances. This renders possible
an alkali treatment highly advantageous per se for the strength pro-
perties of the pulp prior to the defibration without imparting too
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low brightness to the pulp.
In order to obtain full bleaching effect, it is necessary
to retain the impregnated starting material for some time together
with bleaching agents at increased temperature. It has been es~
tablished that the temperature must not exceed 80C, since at
higher temperatures, the peroxide is decomposed rapidly. Optimal
temperature is generally regarded to be about 60C to 70C. A
temperature down to about 50C may also be maintained, but then
the time of retention must be closer to 2 hours. At 80C, a re-
tention time of 10 minutes has proved to be sufficient, but a more ~ ;~
effective utilization of the peroxide is obtained by operating at
a temperature of about 70C and with a period of retention of at
least 20 minutes.
According to the present invention, a consiberably lesser
impairment of brightness and light-scattering properties of the re-
sultant pulp can be obtained when the chips are compressed and de-
watered to a high dry content prior to their introduction into the
grinding apparatus. Thus, residues of chemicals and dissolved wood
substance or lignocellulose which have a deleterious effect on -
brightness and light-scattering properties of the final pulp are
removed.
When all peroxide has been consumed, the optical properties
of the fibres will deteriorate by continued storage of alkalized
fibre raw material, especially if this takes place at increased
temperature.
Therefore, temperature and time of retention prior to the de-
fibration must be chosen so that not all peroxide is consumed, but
so that a certain content of peroxide remains at the moment the
material to be disintegrated is introduced into the grinding ap-
paratus.
Therefore, the temperature of defibration must be restricted
so as to insure that some amount of peroxide is retained also dur-
` ing the defibration. According to the invention, this is accom-
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plished by supplying compressed air to the g~indin~ apparatus in
the same manner as described hereinbefore for treating fibrous
starting material impregnated solely with alkali. Furthermore,
according to the method of the present invention, so much air
must be added that the temperature does not exceed 90C when the
starting material is fed into the grinding zone between the oppos-
ed grinding discs. Normally, however, the alkalinity of the fib-
rous material in the grinding apparatus is still so high that the
temperature must be restricted to 80C, since otherwise the alkali
would cause the properties of the pulp to deteriorate.
The process of the invention can be programmed for optimum
results in various manners, e.g., for varying starting materials,
and, in this connection, one also can choose to operate with such
high amounts of added peroxide that considerable quantities of per-
oxide residues still are present after the defibration. Under the
hereln indicated conditions of temperature and retention time, con-
tinued bleaching can then, according to the invention, be effected
by maintaining the pulp after the defibration at a high concen-
tration level and at a suitable temperature, namely, 50C to 80C,
before it is diluted further to low concentration and/or cooled
down.
The method according to the invention of pretreating the fib-
rous starting material with alkali and peroxide and then defibrat-
ing it at low temperature by supply of air, has proved to be par-
; 25 ticularly advantageous when it is desired to produce pulp of high
brightness from starting materials of deciduous wood species such
as birch, aspen, alder or poplar.
In the manufacture of bleached mechanical pulp, it is desir-
able in most cases wholly or partly to remove consumed alkali or
bleaching chemicals and dissolved wood substance or lignocellulose
from the produced pulp. This is effected usually by dilution and
subsequent dewatering of the produced pulp.
~ccording to the invention, a great portion of the bleaching
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chemicals and dissolved wood substance can be removed prior to the
defibration by compressing the impregnated fibrous starting mater-
ial. Prior to the defibration, the fibrous starting material is
considerably easier to compress to a higher dry content than is
the finally defibrated pulp. Furthermore, the equipment used for ,
dewatering chips is much cheaper than similar equipment for de-
watering finally defibrated pulp to correspondingly high dry con-
tents.
If a more effective removal and possible re-use of bleaching
chemicals should be desired, it may be suitable to carry out the
impregnation and liquid removal in two or more successive steps.
This procedure gives the further great advantage in that it makes
it easier during the pretreating period to maintain an alkalinity
suitable for the bleaching process. The alkali which is supplied
in the first impregnation step is consumed to a large extent for
neutralixation of acid wood constituents. If the starting material
should consigt of wood chips, the outer portions of each chip will
during the first impregnation step take up a great portion of the
alkali when the bleaching liquid penetrates into the chip. Thus,
the alkali in the bleaching liquid will have been consumed to a
large extent when the liquid reaches the centre of the ch~p. If
one should try to avoid this drawback by increasing the alkali con- .
tent of the bleaching liquid, the outer portions of the chip will
be treated with too strong an alkali solution which impairs the
brightness and light-scattering properties of the produced pulp.
Due to these circumstances, it is most suitable, according
to the invention, to perform the impregnation in two or more steps
and therebetween to squeeze out chemicals. The bleaching chemicals
which are supplied in the first impregnation step will be consumed
rather rapidly by the fresh starting material which has not been
greated before with chemicals, This holds especially true with
that alkali which is contained in the solution of bleaching chem-
icals. This squeezed-out liquid is consequently not required to
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be reused, but can be discharged from the plant into an outlet
leading to a natural receiver or be destroyed,
Bleaching liquid added in a subsequent step is introduced
by impregnation into wood material which has already been pretreated.
Hence, the peroxide content must be higher in order to impart max-
imum final brightness to the pulp, whereas the content of alkali
adjuvant can be relatively minor, since the wood has already been
alkalinized in the preceding impregnation step.
The liquid which is squeezed out after such a subsequent
bleaching step will contain more non-consumed bleaching chemicals
and ought wholly or partly to be recycled and used for impregnation,
preferably in a preceding impregnation step. Prior to its reuse,
fresh bleaching chemicals may be added to the squeezed-out liquid,
in order to increase the concentration thereof in the bleaching
liquid
It will be understood from ~he above explanation that bleach-
ing liquid, including eventually recycled squeezed-out liquid, which
is added to the material in a preceding bleaching step, must have
a higher ratio of alkali/peroxide than the bleaching liquid which
is added thereto in a subsequent step.
In connection with chemical impregnation in two or more suc-
cessive steps with intervening retention time and removal of chem-
ical solution between the impregnation steps, it has been shown to
be advantageous to use in the first impregnation step a chemical
solution with low peroxide content, possibly no peroxide at all,
and subsequently in a following impregnation step to add the main
,;:
portion of the peroxide.
In order to counteract discoloration in such first impreg-
nation step with little or no peroxide present~ a weaker alkaline
chemical may be used, e.g., carbonate or bicarbonate. This weaker
alkali will also neutralize wood acids and promote the elimination
of wood components such as resin which are detrimental to the
bleaching process or which cause the properties of the resultant
pulp to deteriorate.
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If the bleaching according to the invention is effected by
pretreating the starting material with peroxide and alkali, the
defibra~ion may be performed at that degree of alkalinity which
the starting material has acquired after the treatment. Grinding
of strongly alkaline starting material results, however, in a pulp
having low light-scattering properties, which is a disadvantage for
many fields of application.
If the peroxide in the wood has been consumed for the most
part after the last compressing operation, a higher degree of
opacity can be obtained according to the invention by totally or `~
partly neutralizing or even slightly acidifying the fibrous raw
material prior to the defibration. This acidification has a posi-
tive effect on the grindability of the pulp and can to advantage be
effected with an acid sulphite or with sulphur dioxide, which, in
addition, has a reducing effect. By such a final reducing treat-
ment with chemicals, some additional increase of brightness is at-
tained. A further reinforcement of this increase of brightness
can be obtained by adding a solution of a suitable hydrosulphite
instead of acid suplhite of SO2.
The pulp produced according to the invention is characterized
by long fibres and good water-absorbing capacity. Thus, it is well
suited for making tissues and other products where high absorption
is wanted, such as fill for diapers.
DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
A specific apparatus for carrying out the method according
to the invention with impregnation in several steps with bleaching
` agent is shown diagrammatically, by way of example, in the accom-
.
panylng drawlng.
Starting material, e.g., such as wood chips, is introduced in-
to the apparatus through a hopper 10, the lower portion of which is
connected to a suitably inclined screw conveyor 12. While the wood
chips are carried upwards by the conveyor 12, steam is supplied
through a pipe 14 to steam heat the chips to about 100C. The chips
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then drop through a vertical tube 16 into a pressure dewaterer 18,
within which the chips are in known manner compressed, and water
and air present in their pores are removed to a substantial ex-
tent. This water is collected in a cup 20 opening into a pipe 22
through which the water is discharged from the apparatus. As chem-
icals have not yet been supplied to the chips, the squeezed-out
water can be allowed to escape into a receiver or be recovered for
utilization in another phase of the treatment of the fibrous mater-
ial. The pressure dewaterer 18 may be of the screw type and act
against a throttling means such as a valve body (not shown) in
front of the outlet, as can be seen in the French Patent Specifi-
cations Nos. 1,235,833, 1,254,243 and 1,378,710.
Connected to the discharge side of the pressure dewaterer
is a first bleaching vessel 24 which contains a bath of bleaching
liquid having a suitable temperature ranging between 55C and 61C. -
The bleaching liquid may consist of an aqueous solution of perox-
ide and alkali as main constituents. The mixture of these chemi-
cals is in~roduced into the vessel 24 through a pipe 26 from a stor- ~
age tank 28 for the chemical solution, the discharge duct 30 from -
said tank housing a pump means 32, if desired. Liquid-containing
alkali only may in addition be supplied to the vessel 24 through
the pipe 26 from a separate storage tank 34 via a pipe 38 housing a
pump means 36. The vessel 24 is equipped with an agitator member
40, in order to keep the distribution of the chips in the chemical-
containing liquid as uniform as possible. From the base portion of
the vessel 24, extends a duct 42 which houses a pump means 44, and
through which the low-consistency chips can be pumped up into a de-
waterer 46 of the screw type, for example, and chemicals collected
on the bottom thereof can be returned through a duct 48 to the
vessel 24. The chips, which now are relieved of a substantial por-
tion of the liquid required for their conveyance, fall down into a
vertical tube 50, the bottom portion of which houses one end of a
pressure dewaterer 18. The liquid now removed from the interior
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pores ofthe chips is collected in a cup 54 and is discharged from
the apparatus through a pipe 56. In this liquid, which thus has
been squeezed out of the inner pores of the chips. the major part
of the chemicals has been consumed, for which reason said liquid
may understandably be discharged into the environment.
Thereafter, the compressed mass of chips is allowed to ex-
pand in a receptacle 58 below the surface of bleaching liquid
present therein, which results in a fresh quantity of bleaching
liquid being sucked into the pores of the chips. This liquid is
fed into the receptacle 58 through a pipe 60 from the tank 28. The
chips are conveyed upwards by a screw conveyor 62 and discharged
through a vertical tube 64 into a second impregnation vessel 66 ~ .
equipped with an agitator member 68. As in the vessel 24, an in-
creased temperature of e.g~ 55C-65C also prevails in the vessel
66. After a predetermined period of stay in the second bleaching
step, the chips are pumped through a duct 70 by a pump means 72
; into a dewaterer 74 which removes the liquid serving as propellant
for the chips and which is recycled through a pipe 76 to the
vessel 68. The thus dewatered chips fall down through a vertical
tube 78 at the bottom of which they again are subjected to strong
compression in a pressure dewaterer 80 of the same type as the
pressure dewaterers 18 and 52. The squeezed-out chemical-contain-
ing liquid is collected in a cup 81 and returned therefrom through
a duct 82 to the first impregnation vessel 24.
In each of the three pressure dewaterers 18, 52 and 80, re-
spectively, so much liquid is squeezed out that the chips will have
a dry content exceeding 35%, and preferably, 40%.
After the double treatment with bleaching agent, the chips
are discharged into a pressure receptacle 82 within which a super-
atmospheric pressure is maintained by means of a gas andtor steam.
From said vessel, the chips are fed by a screw conveyor 84 into a
grinding apparatus 86, preferably of the type equipped with two
opposed grinding discs. In the pressure receptacle 83, chemicals
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exercising a reducing or neutralizing effect on the starting mater-
ial as explained above are supplied through a duct ~8. Such chem-
icals may be hydrosulphite or sulphur dioxide.
Through the duct 88, compressed air also may be fed into
the pressure receptacle 83 of the grinding apparatus, in accord-
ance with the present invention.
In order to maintain desired temperature in the two bleach-
ing vessels 24 and 66, heat can be supplied to the same through
ducts or pipes associated with the apparatus, and, in addition,
the vessels may be heat-insulated.
The vessel 24 contains an aqueous mixture of peroxide and
alkali in such composition that the peroxide preponderates. Thus,
the liquid supplied to the vessel 5~ may contain 2% of peroxide
and 0.5% of alkali, e.g., sodium hydroxide, calculated on the
bone dry weight of the chips. In the second bleaching step effect-
ed in the vessel 66, the bleaching chemicals have approximately
the same composition as in the vessel 58. In the bleaching ves-
sel 24, however, the amount of alkali (for example, sodium hy-
droxide) preponderates, and to this end, alkali only is supplied
from the tank 34 simultaneously with the supply of some quantity
of chemical-containing liquid from the tank 28 to the vessel 24.
Thus, the content of alkali calculated on the bone dry weight of
the chips in the vessel 24 may amount to 1.5%, while the content
of peroxide is 1.0% at the utmost. The total time of stay dur-
ing the bleaching operation shall be kept within the limits stated
hereinbefore.
Obviously, the invention is not limited to the shown and de-
scribed embodiment) but may be varied in the widest sense within
the scope of the basic idea thereof as defined in the appended
claims.
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