Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a process for prepar-
ing hot groundwood from wood chips, wherein a chip stream is
passed into a pressurized steaming chamber, where the chips res-
ide fora few minutes and are heated to a temperature higher than
lQ0C, and are passed from the steaming chamber through a sluice
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feeder into a hot grinder of closed construction, i.e., stage I
grinder, the pre-ground groundwood so obtained is then conducted
through a separating cyclone into a stage II grinder and the
groundwood produced there is passed to further treatment steps.
~` 10 In particular the present invention relates to such a process in
which the pre-ground groundwood is passed from the steam separ- ~`
~` ating cyclone through a sluice feeder into the stage II grinder r
and the stage II grinding is carried out in a hot grinder of ;~
closed construction.
Hot groundwood is made from wood chips by a continuous
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process in which the chips are first passed into a steaming ~ ¦
chamber, where they are heated to a temperature higher than 100C,
preferably a temperature between 120 and 135C. From the steaming
chamber the chips stream is conducted through a sluicei feeder into
a hot grinder, at which stage the dry solids content of the chips
is about 20 to 30%. The hot groundwood obtained from the grinder
is passed into a steam separating cyclone and thereafter usually
further into a second grinder, which consists of a grinder of the
~- open type operating under atmospheric pressure and at ambient
temperature.
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A hot groundwood process of this type has a number of
j advantages. The heating of the chips prior to grinding promotes
j the detachment of the fibres so that the energy required in the
, grinding process is reduced. Due to the heating of the chips even ~-
wood species can be processed, such as pine and aspen, which can~
not otherwise possibly be ground. Deciduous wood produced in short r
rotation cultivation may also be used as raw material. The
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saving in grinding energy is about 20 to 30%. The strength
characteristics of the groundwood are improved. The procedure
is favourable in view of environment protection since the use
of water is minimal. The process is a so-called semi-dry method.
The conventional hot groundwood processes however still
have drawbacks. It is true that the energy requirements are less
than in earlier grinder methods, but they are still higher, so
far, than in the normal grinding method ~or instance. Part of
the energy is dissipa-ted with the steam. It is also a drawback
that wood slivers remain in the groundwood. If attempts are made
to increase the degree of grinding, the proportion of the long
fibre fraction decreases substantially.
According to the present invention there is provided
a process for preparing hot groundwood from wood chips, wherein
a stream of said chips is passed into a pressurized steaming
chamber in which chamber the chips are heated to a temperature
higher than 100C over a period of a few minutes, from the
steaming chamber the chips are passed through a sluice feeder
into a hot stage I grinder of closed construction where said
chips are pre-ground into groundwood, the pre-ground groundwood ~ ~
is conducted from said grinder through a separating cyclone into ~ `
a stage II grinder where said pre-ground groundwood is further
ground to form said hot groundwood and the groundwood produced
in said stage II grinder is passed to further treatment steps,
the improvement in which the pre-ground groundwood is conducted
from the steam separating cyclone through a sluice feeder into
the stage II grinder and the stage II grinder is a hot grinder
of closed construction.
The present invention provides a process for preparing
hot groundwood from wood chips in which the chip stream is con-
ducted into a pressurized steaming chamber, where the chips dweil -
a few minutes and are heated to a temperature higher than 100C,
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and from the steamin~J chamber through a sluice feeder int~ a ~ot
grinder of closed construction, i.e., the stage I grinder, whence
the pre-ground groundwood is passed through a steam separating
cyclone into a stage Il grinder, the groundwood obtained therefrom
being carried towards furt:her treatment steps. The process of the
invention-is characterized in that the pre-ground groundwood is
conducted fro~i the steam separating cyclone through a sluice feeder
into the stage II grinder and that the stage II grinding is carr-
ied out in a hot grinder of closed construction.
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In the process of the invention the energy requirement
of the stage II grinding is reduced because the grinding takes
place under hot condition and the wood splinters therefore are
softer. Since the stage II grinding is effected in a hot grinder,
~j the quality of the resulting groundwood is improved. For instance,
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' the long fibre fraction is retained because the grinding is not
-1 violent.
In an advantageous embodiment of the process of the
invention the steam generated by the stage II grinder is conducted
into a steam separating cyclone. It is thus understood that all
~ 20 the excess steam both from the stage I grinder and from the stage
:- II grinder ls collected at one point, whence the energy contained
in the steam can be withdrawn for use in controlled manner at
other consumption points, such as the prewashing of the wood chips. ~ ~
i~ ~ favourable energy efficiency is thus attained. Energy is only ~ ;~
introduced into the process as rotational energy for the grlnders,
,:~3 and is removed from the process in the form of steam energy only.
`;1 Thus, the energy introduced into the process is also used, in
-, addition to the grinding, for chip washiny, chip heating and the
transporting of chips or groundwood in the process. The trans- ~
3n port is by means of pressure through pipelines. No separate ~ ~ -
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; screw conveyors or equivalent are needed. Recovery of solid
residues from the steam of the steam separating cyclone is
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?ossible, and thereEore the process also meets high re~uirementS
as regards environment pro-tection.
In another advantageous embodiment of the invention
part of the groundwood produced by the stage II grinder is passed
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back to the steam separating cyclone. From here the groundwood
passes once more into the stage II grinder. In this manner the
stage II grinder will always operate under full load. No equival-
ent recycling is applied in the stage I grinding because the ~-
material would be highly unhomogeneous and because it is possible
to dimension the stage I grinder for the process in such manner
- that it will always operate under optimum load.
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~: The invention also provides an apparatus for carrying r -
out the methodj comprising a steaming chamber, a sluice feeder, ~
a hot grinder of closed construction, i.e., a stage I grinder,
a steam separating cyclone, and a st~ge II grinder and means for
passing a chip stream consecutively therethrough for being ground
into groundwood. In the apparatus between the steam separating ~;
cyclone and the stage II grinder there is a sluice feeder, and
the stage II grinder is a hot grinder of closed construction.
The present invention will be further illustrated by 1'
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way of the accompanying drawing, wherein~
Fig. 1 is a diagrammatic illustration of an apparatus
for use in the groundwood preparing process according to one '-
~ embodiment of the present invention; and
;~j Fig. 2 is a block diagram illustrating the process of
Fig. 1.
In Fig. 1, prewashed wood chips are supplied in a L:
continuous stream through the pipeline 1 into the sluice feeder
2 and thence into the steaming chamber 3. In this chamber 3 a
pressure about 2 kg/cm2 gauge prevails, the temperature being
between 120 and 135C. The dry solids content of the chips is
about 20~. From the steaming chamber 3 the chip stream is
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; conducted through the sluice feeder 4 via the duct 5 into the
- hot grinder 6 of closed construction, which is the stage I
grinder. The grinder generates steam, the main portion thereof
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; discharging in the direction against the chip stream into the
- duct 5. A portion of the steam passes through the sluice feeder
4 via the conduit 7 into the steaming chamber 3. The quality ~ -
of steam arriving in the chamber 3 is regulated so that the "~
-` chips attain the required temperature within 2 to 4 minutes,
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~i which is the time which the chips stay in the steaming chamber 3. - "
Part of the steam entering the duct 5 is conducted back to the
grinder 6 and to the outer periphery thereof, through the conduit ~`
8. In the duct S and on the outer periphery of the grinder 6 ,1
` pressure transmitters are provided, which give information to the ; ~
pressure controller 9, which in its turn controls the quantity ~ -
, of steam flowing in the line 8. Part of the steam generated by ~ `
, the stage I grinder 6 escapes by the pipeline 10 along with the ~ ~`
groundwood to the steam separating cyclone 11. Hence, the steam F~
escapes into the pipeline 12, and the groundwood settles
Form the lower end of the steam separating cyclone 11
o~ ~ the groundwood passes through the sluice feeder 13 via the duct
14 to the hot grinder 15 of closed design, which is the stage II h~
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, grinder. The stage II grinder 15 with its associated apparatus
`', is identical in operation with the stage I grinder 6. It has as
additionaL equipment the pipeline 16, which starts on the outer
periphery of the stage II grinder 15 and terminates at the steam
~, separating cyclone 11 and which conducts off any excess steam.
~`~ Thus the excess steam is all collected in the steam separating
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~i cyclone 11. The hot groundwood leaves the stage II grinder lS,
pushed by steam pressure, through the pipeline 17, towards further
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treatment steps. This line 17 has branch line 18 connected thereto
through which part of the hot groundwood is recycled to the steam
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separating cyclone ll, whence the groundwood passes once more
through the stage II grinder 15. As a result, the stage II
grinder is continuously opera-ting under full load.
In Fig. 2, the process has been shown in the form of
a block diagram. As in Fig. l, the steaming chamber 3, the stage
I grinder 6, the steam separating cyclone ll and the stage II
grinder 15. Furthermore a condensate separator 19 is present,
the condensate accruing therefrom being used in the washer 20 to
;` wash the wood chips. The chip flow and the groundwood flow are -
lO represented with double lines, the steam flow with single lines,
and the energy flow required for rotation of the grinders 6
and 15 with dashed lines. The design and construction of the r-
various pieces of equipment, such as the sluice feeders and the
grinders may be varied as desired. ~;
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