Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
s~
The present invention relates to a method for
mechanically raising the wood content of chipped wood
material, that is to purify the chipped material by
mechanical means of bark and green matter and of other
fines such as sand, by separatory screeniny.
It has nowadays become increasingly necessary
to utilize nature's raw material resources to the
highest possible degree. This has also been done in
the case of wood intended -to be used in producing
cellulose. It has been found that after tree felling and
harvesting there often remain felling residues in the
forest which contain quite much wood material appropriate
for digesting. This has led to the practice of chipping
down trees in their entirety above the stump cut and
processing the wood chips thus obtained to cellulose by
digesting.
The most serious obstacle to using chips in the
cellulose pulp industry has been the high proportion of
bark and greenstuff in the chips, and their sand content.
The low wood content causes difficulties as well as a
remarkable added load, for instance in the digesting
step, in the circulation of chemicals and in the washing
process. Owing to these factors, the chances of
employing wood chips more extensively than heretofore can
be significantly increased by raising the wood content of
the chipped wood.
For the barking of wood which is presented in
chip form, a few methodshave been developed. In the
so-called squeeze barking method, which is one of -them,
the chips are passed through between several pairs of
rolls, whereby the pressure effect of the rolls crumbles
thè bark and thus detaches bark from the wood. sut this
method has its weak point therein that acceptable bark
contents are only attained with high wood losses because
under pressure wood is also crushed and part of the wood
~~.
5~Z
goes along with the bark, which is screened off.
Another method is based on -the circumstance
that wood and bark have different water absorptions.
In this method, the chips and bark produced in the
S chipping process are vacuum-treated and thereafter
released to float in a water tank. The wood chips tend
to absorb water and to go under, while the bark particles
tend to keep on floating. The submerged chips are
recovered with a drag chain, and the material on the
surface is skimmed off. This method has two basic
shortcomings, however: it implies that the bark has all
come off during the chipping process and, secondly, it is
not selective enough when used with different timber
species.
It is possible, according to a certain method,
to obtain easier detachment of bark from wood chips
if the chips are stored in heaps, whereupon the chips
are agitated in water and screened in wet condition.
The mixing is accomplished in a pulper, where the bark
is detached from the wood part and is ground fine,
whereupon it is removed through the pulper's screen plate,
the wood part of the chips remaining behind. In this
method no chemicals are used and therefore it re~uires
indeed a fairly prolonged storage of the chips with
intact bark so that biological phenomena might put the
bark detachment under way. This is in fact the weak
point of this method in that when chips with intact bark
are being kept in storage the biological activities
taking place in the bark also start the decay of the
wood part, and the quality of the chips deteriorates, as
can be seen by impairment in paper technology
characteristics of the fibres which can be made from the
chips. Paper made of such chips has a tearing strength
considerably inferior to that of paper made of fresh
wood.
-- 2 --
5~i;2
There are, furthermore, methods which are
based on the use of chemicals. The problem is then in
the first place a high consumption of chemicals, and
effluent problems.
The object of the present invention is to
provide a method for raisiny the wood content of wood
chips, that is for puriying wood chips from bark and
greenstuff constituents, and which is free of the draw-
backs mentioned.
Accordingly, -the method of the invention, as
herein broadly claimed, includes the following essential
steps: initially screening dry wood material in a disk
sieve for removing oversize bodies therefrom; introducing
the material into a grinder drum having grinder bodies,
such as balls; rotating the drum with the dry wood material
to thereby detach bark and green ma-tter therefrom to
yield dry wood chips; comminuting the bark and green
matter; screening of the dry chips by thickness sorting;
directing excessively thick chips to a disk chopper for
crushing and subsequent thickness sorting; directing dry
chips passing through the subsequent thickness sorting
to a separation sifter; and further separating the dry
chips from the bark and green matter by additional
screening.
The method of the invention presents a number
of advantages, owing to which it possesses excellent
chances to effect an improvement in the quality of the
chipped wood. Tha-t part of the material which is not
acceptable as product for use in the cellulose
digesting process proper may be utilized in another way.
As a second advantage, the capacity of the installation
may be raised to be high, merely by increasing the various
components of the installation. Moreover, one of the
advantages is the method's low labour requirement. S-till
another advantage is -the low energy consumption of the
. installation.
In a preferred embodiment, the method of the
invention includes the steps of: separating wood chips
by sifting the bark and green matter that has been
detached and cornminuted during grinder treatment.
According to s-till another preferred embodiment,
the method includes the steps of: separating said thick
particles during thickness sorting, cornminuting with a
disk chopper and reintroducing comminuted product for
separation sifting.
Advantageously, the grinder treatment is carried
out in dry condition. Since no water or chemicals are
used there are no effluent problems, and the waste
fraction retains a high calorific value.
Advantageously also, detachment of bark and
greenstuff is effected in the grinder drum. The
detaching effect is even further enhanced if the grinder
drum contains suitable grinding bodies, such as balls.
Still another advantage is that subsequent to
the grinder treatment is carried out thickness sorting
of the wood chips by disk sieving. A highly uniform
quality is thus obtained. The particles separated in
the thickness sorting step may be minced with a disk
chopper and reintroduced into the process.
Yet another advantage is that af-ter thickness
sorting there is separated from the wood chips by
screening, the bark and green matter which was detached
in the grinder treatment. Hereby the reject fraction can
be separated from the wood chips.
A preferred embodiment of the invention is
described in detail in the following example, reference
being made to the attached schematic diagram.
The reference numerals in the diagram represent
the following:
- 3a -
~ ~5~5~Z 11
1 - Wood chips
2 - Prescreening
3 - Grinding
4 - Thickness sorting
- Separation sifting
6 - Comminution of thick particles
7 - Bark, greenstuff, other fines
8 - Product
9 - Stones, oversize pieces
10 - Iron objects.
To begin with the wood chips, which have 65 to 85% wood content, are screened
in a disk sieve 2, this step removing from the chips the stones and other
bodies 9 which are oversize or would damaee the equipment. The chips are then
conducted to the dry grinder drum treatment 3. In the rotating drum, by means
of attrition--and;-impact treatment with grinding bodies of appropriate shapeand weight in each particular case, such as balls for in~tance, the bark and
green matter are detached from the wood material, and at the same time the
bark pieces, leaves and needles are comminuted prior to the screening taking
place in two steps.
. .
As the~ first step in the screening, the thickness sorting 4 of the chips is
carried out. Chips ~ith excessive thickness are here conducted to the disk
chopper 6, where they are crushed to small size and returned to the thickness
sorting step 4. The chip material that has pas~ed the thickness sorting screen
goes to the plane sifter 5, where separation ~ifting is accomplished. In this
step, the bark and green matter that has been ground fine in the grinder drum
treatment, as well as other fines such as sand 7, are separated from the chip
material intended to be incorporated in the product proper, 8, this product
presenting a wood content up to and over 95~ after separation.
It is obvious to a person skilled in the art that the invention is not
confined to the example presented above, and that it may be modified within
the scope of the claims presented hereinbelow.
.
