Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to dry wood pulp suitable for
example for use as fluffing pulp or for dry paper- or board-
making.
In a conventional pulping process logs of wood are stone-
ground to pulp. Cooling water is used to control the temperature
of the grinding process and the product obtained is a dilute
,
slurry of wood pulp in water. The process uses great quantities
of water and produces great quantities of aqueous effluent which
has to be treated.
An alternative known process for comminuting wood into
; pulp is thermomechanical pulplng. In this process wood chips
~; are first pxe-heated and then confined between two opposed
grinding surfaces, generally rotating discs. The pre-heated
wood chips are introduced at or near the centre of the wood
lS pulping machine and have to pass outwards through the grinding
zone of the outlet. The heat generated by the grinding process
converts water present in the wood chips to steam. Water is
,~ added during the thermomechanical pulping generally at such a
rate as to keep the moisture content of the wood chips approxi- ;
mately cons~ant. The wood being pulped is thus constr~ined wïth-
- in a high pressure steam zone at a temperature of 100C or above. ;~
The pulp produced by thermomechanical pulping usually
has a moisture content similar to that of the wood from which
u it was made. If it is to be dried as pulp it is usually screen-
ed to remove shives and also treated to relax the pulp fibres and `~
to make them bond together more readily. The relaxation treat-
men~t involves sus~ension of the wood pulp fibres in~hot water.
To carry out screening satisfactorily the pulp is diluted to
~2-
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1~)5'~3L56
give a ratio of moisture to dry wood Eibre solids of at least
30:1 by weight. The whole production process stlll uses con-
siderable quantities of water.
According to the present invention a process for the
preparation of dry wood pulp comprises thermomechanically pulp-
ing wood chips and drying the pulp so formed by contacting it
with a gas at a temperature of 300~600C in or at the entry of
~- a high turbulence mixer through which the pulp and hot gas pass
and which exerts a shearing action on the pulp to separate fib-
re bundles in the pulp, the ratio of moisutre to dry wood solids
- being maintained at less than 5:1 by weight throughout the
process. While the invention includes processes in which the
ratio of moisture to dry wood fibre solids in the pulp approa-
. :
ches 5:1 it is preferred that this ratio is maintained at less
.. . .
than 3:1 and indeed that the moisture content of the wood is
not substantially increased at any stage~ The ratio of mois-
ture to dry wood fibre solids in a living tree is generally
between 2:1 and 3:1. Wood as supplied to the pulping factory
commonly has a ratio of moisture to dry wood fibre solids of
between 1.5:1 and 2.5:1 and this moisture content is suitable
` for thermomechanical pulping.
We have found that the process of the present inven-
; .:'.
tion produces a dry wood-pulp which is particularly suitable às
`~ fluffing pulp for use in water-absorbing materials such as ln-
continence pads, disposable diapers, sanitary towels and the
`` like. The dried wood pulp produced by the invention is also
`` suitable for use in a dry paper- or board-making process.
.
~35~5~
The thermomechanical pulping process can be carried out
in a single pulping machine (which is generally of the type
known as a double disc refiner) or two or mor0 machines can be
successively used to pulp the wood. The wood chips are pre-
heated before being fed to the double disc refiner for exampleby treating the chips with live steam. The amount of moisture
added to the chips by this steam pre-heating is small, usually
less than 15 per cent. Our preferred procedure for thermo-
mechanical pulping comprises a pre-heating step using live steam
at a temperature of from 105 to 135C and a single pulping
step in a double disc refiner at a temperature lower than that
used for pre-heating and of from 100 to 110C. The steam
generated in the double disc refiner can be used to provide
some of the steam for pre-heating the wood chips. A bleaching
agent can if desired be introduced into the double disc refin-
er to bleach the wood pulp during the pulping process.
The drying process is carried out in a high turbulence
mixture. The wood pulp, which is usually at the moisture con-
tent at which it is discharged from the thermomechanical pulp-
ing machine, and a gas, generally air, at 300 to 600C are
both fed to the mixer and contact each other in or at the
entry to the mixer. The mixer is designed to exert a shearing
action on the pulp. It may for example be fitted with hammers
working against a surface and/or intermeshing and relatively
rotating spikes or pegs. The shearing action breaks up ag-
glomerated bundles of wood fibres in the pulp. One example
of a suitable high turbulence mixer is that sol~ under the -
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trade mark "Atritor".
The high turbulence mixture also disperses the wood
pulp fibres rather evenly in the air flowing throuyh the mix-
ture. It preferably acts so quickly that the temperature of
the stream of hot gas falls to 100C within one second of con-
tacting the pulp. This suspension of fibres in air can if
desired be used in immediate subsequent treatment such as de-
positing the fibres on a screen.
When the process of the invention is used for making
paper or board, the wood can be pulped and dried and the dry
pulp fibres stored for subsequent use, but it is preferred that
the high turbulence mixer be used as a dryer directly feeding
the paper-making or board-making process.
At the paper- or board-making machine a stream of gas,
generally air, carries the fibres through a screen and deposits
them as a ]ayer on a forming surface. Various dry forming paper-
and board-making machines are known. In known processes the
wood fibres are usually supplied to these machines as fluffed
dried pulp which may for example be dried pulp which has been
; 20 treated in a hammer mill to get it in a suitable form for sus-
pension in an air stream. The stream of air containing the -
fibres is passed through a forming surface which is permeable
~; by the air but on which the fibres are deposited as a layer.
The apertures in the screen through which the st-ream of
25~ air must pass before it reaches the forming surface are of a
size to allow passage of separated wood fibres of a size suit-
able in incorporation in the grade of paper or board being made ~;
but will not allow passage of agglomerated fibre bundles or
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shives. The dry forming machine preferably has a stirring orscraping device ensuring that the screen does not become bloc-
ked by fibre bundles and shives and preferably has a waste out-
let designed so that the agglomerated fibre bundles and shives
are removed from the machine without it being necessary to halt
the paper making process. In an integrated operation the shives
~ and similar material collected from the waste outlet can be re-
!i:
~- cycled to the inlet of the thermomechanical pulping apparatus.
The air stream containing the wood fibres after passing
through the perforated screen impinges on the paper forming
surface. This can be a fine mesh screen or a gas permeable web
or fabric.
The stream of gas used to carry the wood fibres through
''~' . .:
r~ the perforated screen to the paper forming surface can be pro-
vided in a known manner, for example by a suction box position-
1 :.
ed beneath each dry forming machine. Alternatively, or addi-
tionally, the stream of air issuing from the dryer can carry the
- wood fibres right through to the pulp forming surface. It may,
;` however, be preferred to separate the fibres from the moisture
laden air issuing from the dryer, for example in a cyclone
~` separator.
The stream of air containing wood fibres which issues
from the dryer feeds one or more dry forming màchines. Usu- 1
: .
~- ally one double disc refiner and high turbulence dryer can feed
several dry forming machines. These can be arranged in sequence
particularly when forming board. When the fibres from one ma-
chine are deposited on the layer of fibres deposited by the pre-
vious machine then board consisting of a number of layers of
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fibres can be formed from the output of one double disc refiner
and dryer.
After being laid on the fibre forming sur~ace the paper
or board undergoes various consolidating treatments. It is
generally treated with a liquid binder, Eor example by spray-
ing. Examples of binders are starch and starch derivatives
and polymer latex emulsions. Subsequently it is usually sub-
jected to pressure to consolidate the web and to drying to re-
move moisture applied in the binder spray. It can also be
sized, coated, embossed and/or calendered.
Paper and board made from wood pulp obtained by the pro-
cess according to the invention need not consist exclusively of
the thermomechanically pulped fibres. If desired one or more
outer layers made from covering stock, for example chemical
pulp, can be used to cover one or more layers formed of wood
- . .
pulp obtained by the process according to the invention. For
example, to make cardboard the first iayer of fibres laid on
, ,:~ - .
the paper forming surface can be formed from chemical pulp fed
to a dry forming machine. Several subsequent layers of board
can be successively laid on top of this outer layer from dif-
ferent dry forming machines which may all be fed from the same
thermomechanical pulping machine and dryer. The last layer
applied can be a further layer formed from covering stock.
~`` The combined board can then be treated with binder and consoli-
dated as described above.
The above described process provides a method of making
paper or board which requires no, or very little, water and
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discharges very little polluted water. It need not be sited
near a large water supply. Moreover it can be used economi-
cally on a much smaller scale than a conventional wet pulping
and paper-making process. It can for example be sited in or
near a forest of moderate size for example lO,000 acres where-
as a conventional paper-making factory of economic scale re-
quires a forest at least ten times as big.
~ When the pulp produced by the thermomechanical pulping
; machine and the high turbulence dryer is to be used as fluf-
fing pulp it can be collected loose to be packed in bales or
sheets. In one preferred procedure, however, the pulp issuing
; from the dryer is deposlted on a gas permeable surface as a
layer and has the form of a coherent sheet of fluffing pulp. ~-
The pulp issues from the dryer as a suspension of wood
fibres in air. This is arranged to impinge on a permeable sur-
face which can be in the form of a rotating drum or a flat mo-
ving belt. A suction device is preferably used to apply vacuum
` to the other side of the permeable surface to ensure that the
wood pulp fibres are firmly held on the permeable surface. In
one preferred procedure the permeable surface is part of a va-
cuum drum. The wood pulp fibres are initially strongly held on ~;
the drùm by suction but as the layer of wood pulp fibres builds
:: . . .
up on the drum the air flow through it decreases. The suctiondevice preferably covers only part of thè drum and the layer of
wood pulp fibres can be carried off the drum, for example onto
a flat conveyor, at a point on the drum where suction is not
applied, for example approximately opposite to that at which
fibres are first deposited on it.
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The layer of wood pulp fibres is preferably arranged to
have a ~sis welght of 500-5000 grams per square metre. It
requires consolidation, for example at a pressure of about
350 kgs per square centimetre, to form a coherent sheet that
can be handled without falling apart. Consolidation can be
by a platen press in which case the fibrous layer is generally
cut into separate sheets before pressing or by pressure rolls
in which case continuous or separate she~ts can be formed.
The dry wood pulp formed by`thermomechanically pulping
and high turbulence drying according to the invention has un-
dergone no xelaxing treatment in water and readily forms a
bulky fluffed pulp. The sheets of consolidated pulp are par-
i .
ticularly easily broken into wood fibres by pulp fluffing appa-
ratus for example a hammer mill. Moreover, the pulp can be
consolidated by pressure to a relative density of 0.5 to 1
and still undergo this easy integration whereas pulp produced
by conventional processes cannot be consolidated to a relative
density above 0.5 or it will not disintegrate satisfactorily~
This is a particular advantage if the sheets of pulp have to
be transported from the pulp mill -to the manufactùrer of absor-
bent products.
The invention will now be described by way of example
with reference to the accompanying drawings in which
Figure 1 is a diagrammatic side elevation partly in sec-
~tion of an apparatus suitable for producing dry wood pulp by
; 25 the process according to the invention and forming this pulp
.~ .
~into paperboard, and
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Figure 2 is a diagrammatlc side elevation partly in sec-
tion of an apparatus suitable for producing fluffing pulp using
the process according to the invention.
The apparatus shown in Figure 1 comprises generally a
pre-heater 1, a thermomechanical pulping machine 2, a dryer 3,
a dispensing unit 4 for the wood fibres, a dry forming machine
5 and a web consolidation unit 6.
Wood chips are fed to the pre-heater 1 through an inlet
11. The pre-heater is in the form of a screw conveyor. The
residence time of the wood chips in the pre-heater can be con-
trollèd by controlling the speed of the screw conveyor and is
preferably from 0.5 to 3 minutes. ~team is fed to the pre-
heater 1 through an inlet 15.
Wood chips emerging from the pre-heater 1 pass through
15 a rotary valve 17 and a chamber 18 to a feeder 19 which is a
screw conveyor. It feeds the wood chips to the thermomechani- -
cal pulping machine 2.
The pulping machine 2 is a double disc refiner comprising
two contra-rotating discs 31, 32 mounted on shafts 33, 34,
: .
respectively, which rotate about a common axis. The discs 31
and 32 have confronting grinding surfaces 35 and 36, respec-
- tively, separated by a narrow gap 41. Wood chips are fed by
the feeder 19 to a chamber 37 behind the disc 31. From this
chamber 37 the wood chips pass through two inlet ports 38 and
25 39 in the disc 31 to the central region 40 of the double disc
refiner. To pass out of the pulping machine 2, the wood chips
must pass through the gap 41 from the central region 40 to the
rim 42 of the discs. The wood chips are ground to pulp in the
gap 41.
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~05'~56
,
Water is supplied to the double disc refiner through an
inlet 43 feeding a chamber 44. The water passes to the central
region 40 of the double disc ref~ner through ports 45 and 46
in the disc 32. The heat generated in the grinding causes some
of the water in the wood chips and added ~hrough the ports 45 and
46 to be released as steam. The steam can vent back through the
ports 38 and 39, the chamber 37 and the feeder l9 to the ohamber 18.
; The chamber 18 is provided with a vent pipe 22 controlled by avalve 23, so that when excess pressure of steam builds up in
chamber 18 it can be vented out of the apparatus. The amount
of water supplied to the inlet 43 is controlled so that the moisture
content o~ the wood chips is kept approximately constant. The
;~ temperature in the thermomechanical pulping machine 2 is thus
preven~ed from becoming too high.
Pulp emerging from the pulping machine 2 falls on to a screw
con~eyor which feeds the pulp to the flash dryer 3 ~ia a pipe 47.
Hot air, for example a temperature from 400 to 500C, is produced
., ~
in an air heater 48, heated by a burner 49, and is supplied to
` the dryer through a pipe 50. The hot air contacts the pulp at the
inlet chute 51 of the dryer.
The dryer 3 is a high turbùle~e mixer in which the ho-t air
and wood pulp are rapidly mixed, agglomerated bundles of wood pulp`
~. . .
~ibres are broken up and the dried wood fibres are carried away in
a stream o~ the hot ~ir. The dryer 3 comprises a rotary disc 52 9
`' 25~ a stationary disc 53 and an extractor fan 540 The rotary disc 53
` carries on one side hammers 55 which work against the wall of ~he~
dryer and on the other side carries pegs 56. The pegs 55 inte~me~
wi~h pegs 57 moùnted on -the stationary disc 53. The wood pulp and
, ` `. - 11 - ~.
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~ 0l5'~
air ent~ring dryer 3 via the inlet chute 51 have to pass the
hammers 559 which serve to break up fibre bundles, and then
between the pegs 56 and 57, which evenly disperse the fibres
in the flow of air. The air and fibres then pass through an
opening 58 at the centre of the stationar~ disc 5~, which opening
is controlled by a ~alve 59. Thence they are forced by the
extractor fan 54, which is mounted on the same shaft 60 as disc 52
and valve 59, to the exit 61 of the dryer 3.
,-. :
The stream of air carrying the wood ~ibres o~ the pulp -
emerges from dryer 3 ~hrough a pipe 62 and conveys them to the
dispensing unit 4. The air pipe 62 feeds six ir~et pipes such
as 6~ feeding six cyclone separators 64a to 64~, respectively.
The entry9 ~or example 65a, to each inlet pipe 6~ is adjusted,
for example by a butterfly valve, so that substantially e~ual !~:
quantities of fibres are supplied to each of the cyclone separators
64a to 64f. Each cyclone separator 64 is a conical rotary ~
separator discharging the moisture laden air through a top outle$, ~`
-i such as 66a9 and the wood fibres through a base outlet such as 67a.
The cyclone separators 64a to 64f are ~unted above~the
dry forming board-making machine 5. Each of the cyclone separators
:- s~
54a to 64f feeds wood fibre through its base ou~let to fibre
dispensers 68a to 68f, respectivel~.
Each of the fibre dispensers 68a to 68f can be of the type
described in British Patent Specification No. 1,207,556. Thus~
as shown in the case of the dispenser 68a, each dispenser conslsts ~;
of a housing 69 whose lo~er surface is a flat screen 70 ha~ing ;~
., . . ::' ~.
; apertures large enough to permit the passage of wood fibres ~ ~`
` suitable for making board but ~ine enough to prevent the passage ~
of agglomerated fibre bundles and shives. Two or more stirrers 7r. .
;, . .:
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~05~ 5~
hover abo~e the screen 70, rotating in a panetary motion.
The wood fibres enter the dispenser 68a through an inlet 72
at one side of the housing 69, and the stirrers 71 serve to
distribute the fibres uniformly across the screen 70. Shives
and fibrous lumps which will not pass through the screen 70
are moved along the screen to an oulet 73 at the opposite
side of the housing from the inlet 72. From the outlet 73 the
shives pass through a pipe 74 to a conveyor 75 where they join the
material rejected from the dispensers 68b to 68f and are
10 returned to the inlet 11 of pre-heater 1. The rate of feed
of wood fibres to each of the dispensers 68a to 68f compared~to
the rate at which fibres pass through the screen 70 is generally such
: that about one-third of the weight of wood fibre is rejected through
. the outlet.73.
~ 15 The fibre dispensers 68a to 68f are mounted above the dry.
:. forming board-making machine 5. So are two more dispensers
79 and 80 of the same design as the fibre dispensers 68a to 68f.
~ The dispensers 79 and 80 receive wood fibres from cyclone separators
.: 81 and 82, respectively. The inlet pipes 83 and 84 of the cyclone
.
~ 20 separator 81 and 82 are not fed from the air pipe 62 but from a
..
different pipe 85. This receives an air suspension of wood ~:
fibres from a separate supply of covering stock ~not shown)
produced from fluffed chemical pulp. The covering stock is used
.~ to make boards having a white surface, and dispensers 79 and/or 80
may not be used if no or only one white surface is desiredO
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The dry forming machine 5 comprises an endless belt 86
of air-permeable fabric passing beneath -the ~ibre dispensers
79, 68a to 68f and 80 and around rollers 87 and 90. The
belt co~prises the paper-forming surface on which the ~ibres
are laid. The first fibres to be laid are those issuing from
dispenser 79. A suction box 91 is mounted behind -the belt 86
beneath the dispenser 79 to ensure a constant flow o~ air
through the belt 86. The fibres ~rom dispenser 79 are laid-
as an entangled layer on the belt 86. The fibres issuing
from the dispenser 68a are then laid in like manner on top
of the layer just formed. The air from the dispenser 68a
passes through the first layer o~ fibres from dispe~ser 79 and through
the belt 86. The fibres from the dispenser 68a a~e entangled both
, ,
with each other and with the fibres deposited from the dispenser 79.
In similar manner successive layers o~ fibres are deposited from
the dispensers 68b to 68~;;and 80. Suc~l-on~boxes 92a to 92f and
93 are posiffDned be~eath dispenser 68a to 68f and 80~ respecti~ely.
After alI the layers of fibres have been deposited on the
belt 86 the~ pass beneath a spray head 94 mounted above a suction
box 95, which applies a binder ~or example an aqueous starch
binder to the ~ibres.
,
At a roller 87 the formed layer o~ paper board 96~ which
co~tains binder but is not consolidated, is separated from the ~-
belt 86. The paper board 96 consists predominantly of fibres -
from the dispensers 68a to 68f, that is to sa~ fibres pulped
and dr~ làid according to t~ invention. It has white coverin~
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layers formed from the chemical pulp supplied to the dispensers 79
an~ 80.
The paper board 96 then passes to the consolidating unit 6,
where it first pass~s around a heated drum 97. This carries out
- 5 some drying of the paper board and pressure rolls 98 and 99 which are
in contact with -the drum 97 consolidate the paper board. From
the drum 97 the paper board passes to a sizing press 100 where size
is applied to the paper board 95 at the nip between two rolls.
The paper board 95 then passes to a final dryer 104 comprising 10 . six heating drums and calender rolls 105 before it is wound on
re~l 106.
; ~he apparatus shown in ~igure 2 comprises generally a pre-
heater 201, a thermomechanical pulping macine 202, a dryer 203 and
a sheet-~orming drum 204. The pre-heater 201, thermomechanical
: 15 pulping machine 202 and dryer 203 are the same as the pre-heater 1thermomechanical pulping machine 2 and dryer 3 shown in Figure 1
.
and described above; the elements designated by the numerals 211 - :
261 in Figure 2 are the same as the elements designated b~ the
. numerals 11 - 61, respectively, in ~igure 1.
:. 20 The stream o~ air carrying the separate wood fibres o~ the
.~
: pulp emerges ~rom the outlet 261 o~ the dryer 203, through a pipe 26?
- leading to the sheet-forming drum 204. The pipe 262 has a oo~vergent
portion 263 o~ Pish-tail shape. For example, the cross-section ma~ -
reduce ~rom 20 x 20 cm in the pipe 262 to 5 x 60 cm at the narrowest
point 264. ~rom 264 the air and fibres enter the headbox 265 of the
sheet-forming drum 204.
The sheet-forming drum 204 comprises a drum 266 ro-ta-ting
: in the direction shown and having its surface in -the Porm oP a
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air permeable surface 267. Mounted inside the drum 266 is a
suction box 268 communicating via a pipe 269 to an exhaust fan
270. The suction box 266 lies behind that portion of the perm-
eable surface 267 which at any one time is passing the headbox
265. The headbox 265 is sealed against the drum 266 at its
edges by resilient sealing rolls 271 and 272.
.: An endless belt apron conveyor 273 passing over rolls
274, 275, 276 is arranged to contact the drum 266 over a por-
tion approximately opposite to the headbox 265 and suction box
268. The wood pulp fibres entering the headbox 265 are deposi-
ted on the permeable surface 267 over that portion behind
: which the suction box 268 lies. The layer of fibres on the
: permeable surface 267 becomes gradually thicker as it progres-
ses from sealing roll 271 to sealing roll 272. The layer of
deposited fibres 277 is then transferred from the permeable
surface 267 to the conveyor 273, no suction being applied to
i; the permeable surface in this portion. ~,
` The conveyor 273 transports the layer of fibres 272 to . :
pressure rolls 278, 279 which app:Ly pressure sufficient to
. 20 consolidate the pulp fibres to a coherent sheet. This sheet
.. ., ~ .
. is carried away from the pressure rolls by a conveyor 280 and
: is collected at 281. ~:
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