Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention is concerned with a method for the production of
wet process wood fiber board for the early separation of dissolved wood sub-
stances from the process water with the object of benefitting economically from
it. It is an object of the invention to avoid quality problems on the end
product, e.g. the wood fiber board. It is a further object of the invention
to produce an ecologically acceptable waste water for direct discharge into
a receiving body of water.
During conventional production of wet process wood fiber board, the
wood fiber is freed firstly by preheating wooden chips and/or saw dust and
secondly by grinding between two grinding discs, one of which rotates and the
other of which is stationary. The grinding can be carried out in one or several
steps (of which the finishing ones are called "refining steps"). The preheat-
ing takes place in a special preheater by direct injection of steam, usually
below 1 mpa and 200 degrees C. During the preheating and the grinding, wood
substances are dissolved, which later follow the process water to the
Fourdrinier machine and the final pressing of the fiber board, where the water
phase is removed as vapor or liquid. The process water is subsequently puri-
fied before any discharge from the mill is made. A common purification method
is the treatment of the process water in a so called "aeration tank" to lower
the B.O.D.7 content, for an average size factory, the investment sum for an
aeration tank is about 15 million Swedish crowns. There is also an annual
cost of about 4 million Swedish crowns. Despite these formidable expenscs
there is no financial return on the investment made in this equipment. Tn
order to decrease the water flow some mills have installed closed water systems
for recirculating the water. In so doing, the continuously fed pulp will enricl
the process water with respect to the B.O.D.7 content. The drawback with these
systams are the quality problems on the finished fiber board in form of
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indentations caused by contamination of tne caul plates during the consolidat-
ing final hot pressing. Furthermore, susoe ptible parts of the production line
will be exFosed to significant corr~sion problems.
With the method according to this invention a major part of the
water is taken care of already after the dissolving and leachLng of wood sub-
stanoe s from the ground wood fiber pulp by means of pressing to at least 60%
dry content. The substan oes, which are dissolved during the hydrolizing grind-
ing, are separated from the prooess water in an evaporation plant. Part of the
process water can, with advantage, be recirculated into the defibrator and
become enriched with respect to dissolved wood substan oes.
The method according to the invention inclu~es, in sucoe ssive order
a preheater, a defibrator, a cyclone, a leaching devioe and a dewatering devioe
for the ground wood fiber pulp. Ihe invention is characterized by the fact
that the pnocess water from the dewatering device is led back partly to the
defibrator and partly to an evaporation plant, from which the generated water
vapor is led to the preheater, thus creating a closed circulating water system
which includes two paths.
The leading of surplus water vapor from the cyclone to the evapora-
tion plant to be used as a souroe of energy, adds a third path to the closed
water sysb~m.
According to the present invention, there is provided an improvement
in the wet prooess production of wcod fiber board in which dissolved wood
substances are separated f mm process water comprising heating wood source
material selected from the group consisting of wood chips and sawdust in a
preheater wi~l steam, passing the heated wood source material to a grinding
zone of a difibrator wherein the wood source material is formed into wet fiber
pulp comprising wood fibers and dissolved wood substan oes in water, passing
the wet fiber pulp to a cyclone separator wherein steam is removed and
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passing the fiber pulp to a leaching device where m the fiber pulp is leached
and passing the leached fiher pulp to a dewatering device wherein a portion of
the water containing dissolved wood substances in the leached fiber pulp is
removed and passing the dewatered leached pulp to a pulp holding device in
preparation for further grinding and dewatering and hot pressing into fiber
board, the improvement comprising taking off sufficient water in the dewatering
device to provide a dewatered leached fiber pulp having a dry content of at
least 60% by weight, and passing a portion of said water containing dissolved
wcod substances taken off at the dewatering device back through the defibrator,
and passing another portion of said water containing dissolved wccd substanoe s
taken off at the dewatering device to an evaporator wherein it is heated to
evaporate the water as steam which is passed to the preheater and to separate
therefrom the dissolved wood substances as an evaporation residue, and passing
the steam remDved in the cyclone separator to the evaporator and utilizing
said steam as a Source of heat therein, thereby creating three closed loops
in the prooe ss water system.
The method according to the invention is easily visualized by the
attached schematic drawing which is in the nature of a flow sheet. Ihe parts
of the apparatus used are numbered as follows:-
1. Preheater
2. Defibrator
3. Cyclone
4. Leaching device
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5. Dewatering device ~pTess)
6. Evaporation plant
7. Pulp chest
Wood chips and/or saw dust are fed by a conventional screw-feeder
into the preheater 1, in which water vapor is injected. After the preheating,
the wood fibers are fed into the grinding zone of the defibrator 2 and the
fibers are defibrated to pulp. The pulp is led to the cyclone 3, and, there-
after, to the leaching device ~ and the dewatering device 5, which preferably
is a screw-press, and, finally, to the pulp chest 7. After the pulp chest 7,
the pulp is further ground in a refiner and led to the Fourdrinier machine and
the hot press, in which the finished wood fiber board is produced. The latter
steps of production are not included in Figure 1. Since they are conventional
and well-known to those skilled in the pulp mill art. The water that is
squeezed out of dewatering device 5 is, depending on the circumstances, divided
between the defibrator 2 and the evaporation plant 6. The generated water
vapor from evaporation tank 6 is led to the preheater 1. The cyclone 3 is
connected with the evaporator 6, to which the excess water vapor can be fed and
used as a source of energy.
The method according to the invention has scveral advantages.
(a) The total amount of contamination in the process water will be
low because the contaminants have been concentrated in the water that has al-
ready been separated in the dewatering device. The contamination is then fod
to the evaporation plant.
(b) Excessive quantities of water into the defibrator can be
reduced by recirculating the water from the dewatering device to the defibrator.
The amount of recirculated water is settled by the concentration of dissolved
wood substances, which in turn are settled by the water vapor pressure and
temperature, plus the type of wood.
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~ c) The yield of the process is affected positively since the
defibrator is fed with water which already contains dissolved wood substances.
In the reaction
wood (in the defibrator) ~ wood fiber + dissolved wood substances
the dissolving of wood is retarded, which increases the yield of wood fiber
in relation to wood. The system is economically advantageous, as the amount
of water, containing the main part of in the process dissolved and suspended
substances, is only a few percent of the total amount of process water. A fur-
ther enrichment by evaporation is therefore economically interesting. The
evaporation residue can be used either as a fuel or sold as feedstuff for cattle.
(d) The surplus energy from the cyclone can be utili~ed for evapora-
tion, and the excess energy from the evaporation plant can, in turn, be used
for preheating of the raw material or the process water.
(e) The economy of the process can be further improved by allowing
the subsequent refining step to be made under optimum dry content, which posi-
tively influences the energy construction.
(f) By separating dissolved wood substances from the process water
in an early stage of the process, a relatively pure water is released from later
steps in the process, which adds to higher product quality at the hot pressing
to finished boards.
(g) An important condition in obtaining the preclictccl result is thilt
the press, after the defibrator, is about to givc a dry contcnt of the fibcr
pulp, that is about that which is to be present when, in the hot press, the
final pressing and evaporation step to a finished fiber boarcl is to begin.
The following numerical Examples are mcant to provide a betlter uncler-
standing of the enviromnental value of the method of the invention:-
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EXAMPLE 1
Production of 400 metric tons hardboard, utilizing the system
according to the invention.
Dry content of the fiber pulp after the screw-press ~dewater device
5) - 60%.
The final process water for discharge to the receiving body of water
contains about 6.5 kgs B.O.D.7 per metric ton of board.
The corresponding discharge. without utilizing the system according
to the invention, would be about 27 kgs B.O.D.7 per metric ton board.
EXAMPLE 2
Production of 365 metric tons hardboard and 35 metric tons of
semi-hardboard, utilizing the system according to the invention.
Dry content of the fiber pulp after the screw-press (dewatering
device 5) - 60%.
The final process water for discharge in the receiving body of water
contains about 7 kgs B.O.D.7 per metric ton board.
The corresponding discharge, without utilizing the system according
to the invention, would be about 30 kgs B.O.D.7 per metric ton board.
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