A METHOD AND EQUIPMENT FOR MANUFACTURING PREDETERMINED LOW BARK CONTENT WOOD CHIPS AND A HIGH BARK CONTENT FUEL FRACTION FROM WOOD CHIPS WITH BARK ATTACHED

PROCEDE ET INSTALLATION DE PRODUCTION DE COPEAUX DE BOIS PRESENTANT UNE TENEUR FAIBLE, PREDETERMINEE, EN ECORCE ET D'UNE FRACTION COMBUSTIBLE A TENEUR ELEVEE EN ECORCE, A PARTIR DECOPEAUX DE BOIS COMPORTANT DES MORCEAUX D'ECORCE

English Abstract

The object of the invention is a method and equipment manufacturing
predetermined low bark content wood chips and a high bark content fuel
fraction from wood chips with bark attached, which method includes the
following stages: mechanical bark removal treatment (7), in which the bark is
removed from the chips and their size is reduced; and pre-cleaning (6, 9, 11)
of the flow of chips with bark attached; and final cleaning of the aforesaid
wood chip flow from the pre-cleaning, with the aid of an optical separator
(10). Before the mechanical bark removal process (7) a thin fraction of the
chips is sieved out of the flow of wood chips, and then bypasses the
mechanical process (7), which improves the yield and the piece size
distribution.

French Abstract

Procédé et installation de production de copeaux de bois présentant une teneur faible, prédéterminée, en écorce et une fraction combustible à teneur élevée en écorce, à partir de copeaux de bois comportant des morceaux d'écorce, comportant les étapes suivantes: opération mécanique (7) de séparation de l'écorce, dans laquelle l'écorce est retirée des copeaux et la dimension de ceux-ci est réduite, pré-nettoyage (6, 9, 11) des copeaux comportant des morceaux d'écorce et nettoyage final de ces copeaux de bois en provenance du pré-nettoyage, à l'aide d'un séparateur optique (10). Avant l'opération mécanique (7) de séparation de l'écorce, une petite fraction des copeaux est sélectionnée par tamisage dans le flux de copeaux de bois, et saute l'étape de l'opération mécanique (7), ce qui améliore le rendement et la répartition par dimension des morceaux.

Claims

7
Claims
1. A method for manufacturing predetermined low bark
content wood chips (12) and a high bark content fuel fraction
(13) from wood chips with bark attached, which method includes
the following stages:
- mechanical bark removal treatment (7), in which the bark is
removed from the chips and their size is reduced, and
- precleaning (6, 9, 11) of the flow of chips with bark
attached, into a flow of chips with a bark content of less than
10 % and high bark content rejects, with the aid of pneumatic
and mechanical sieving,
- final cleaning of the aforesaid wood chip flow from the
per-cleaning, with the aid of an optical separator (10), to create
low bark content wood chips (12) and a fraction with a higher
bark content,
- collection of the rejects leaving the process, to create the
aforementioned high bark content fuel fraction,
characterized in that before the aforesaid mechanical bark
removal process (7) a predetermined thin fraction of the chips,
for example, pieces less than 6 mm, is sieved (6) out of the
flow of wood chips, and then bypasses the aforesaid debarking
process (7).
2. A method according to Claim 1, characterized in
that the mechanical bark removal process (7) takes place by
means of a grinder, press-rollers, or a crusher.
3. A method according to Claim 1, characterized in
that the mechanical bark removal takes place using a blade ring
chipper, which cuts the wood chips into smaller pieces, while
removing the bark.
4. A method according to Claim 1, characterized in
that 20 - 100 % of the aforementioned fraction with a higher
bark content obtained from the optical separator (10) is

8
directed to the fuel fraction (13), and correspondingly 80 - 0
% is returned to the mechanical bark removal process (7).
5. A method according to Claim 4, characterized in
that the optical separator includes a pneumatic conveyor and a
division plate (18), by means of which the flow of chips is
divided first into the aforesaid wood chips with a low bark
content and wood chips with a higher bark content and further
the latter part is divided with the aid of the division plate
(18) directly into a fuel fraction and the aforesaid part that
is returned.
6. A method according to Claim 1, characterized in
that it includes pre-separation (5) before the aforesaid
thickness sieving (6), in which the oversized wood chips,
stones, and metals are removed from the wood chips with bark
attached.
7. A method according to one of Claims 1 - 5,
characterized in that the flow of wood chips from the
mechanical bark removal process (7) and the aforesaid thin chip
fraction from the thickness sieving (6) are sieved using a
mechanical sieve (9), to remove sawdust as a third reject and
using a pneumatic separator (11) to remove light birch bark,
leaves, needles, and other airborne material as a fourth
reject.
8. Equipment for manufacturing wood chips with a low
bark content from wood chips with bark attached, which
equipment includes mechanical bark removal devices (7) and
pre-separation devices consisting of a mechanical and a pneumatic
separator (9, 11) and an optical sorter (10), characterized in
that the equipment includes a wood chip thickness sieve (6)
located before the mechanical bark removal devices (7) and a
correction line (8) connected to the exit of the thin fraction
obtained, by means of which the thin fraction is taken past the

mechanical bark removal devices (7) to the pre-cleaning devices
(9, 11).
9. Equipment according to Claim 7, characterized in
that the mechanical bark removal devices (7) consist of one or
more blade ring chippers, a grinder (7), a press roller, or a
crusher.

Description

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A hl~il~UL~ AND ~;Q-Il~r4ENT FOR M~ ACTURING PR-.:V:-~ rT1~1;!n LOW BARK
CO~L~N1 WOOD CHIPS AND A HIGH BARR CON1~N1 FUEL FRACTION FROM
WOOD CHIPS WITH BARK AT~
The object of the invention is a method and equipment for
manufacturing predetermined low bark content wood chips and a
high bark content fuel fraction from wood chips with bark
attached, which method incudes the following stages:
- a mechanical bark removal process,
- pre-cleaning of wood chips with bark attached, and
- the final cleaning of the flow of wood chips, with the aid of
an optical separator.
The above-mentioned type of manufacturing method is known from
the international patent publication W0 93/25324. According to
this, the wood chips with bark attached are ground initially
either with a special plate grinder, or with a vibrating cone
crusher, when the bark separates from the wood chips and the
size of the bark particles ~;m;n jshes By means of this, the
bark can be separated more easily during later separation
stages. A grinder or a vibrating cone crusher is followed by
pneumatic separation, especially to separate outer birch bark,
after which sawdust is removed using a vibrating sieve. The
bark content has now dropped to considerably less than 10 ~,
when the ~inal cleaning can be carried out using an optical
sorter. According to the patent, two optical sorters are used
in series, but can, however, be replaced by a single more
powerful device. The accepted fraction ~rom the second optical
sorter is led to grinding and the reject to the fuel fraction.
Equipment that is essentially that described by the application
has been built in Kankaanpaa in Finland, the supplier being BMH
Wood Technology Oy. In the commercial equipment, a magnetic
separator and a pneumatic separator are used in the pre-
cleaning to separate metal scrap and stones. Sawdust is removed
from the commercial equipment before grinding, and after

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grinding the vibrating sieve has been replaced by a so-called
pocket-roll sieve ( a roller sieve developed especially ~or
sieving sawdust). In the commercial equipment, a good yield is
achieved with a bark content of about 1 ~, which is
sufficiently clean for the manu~acture of certain grades of
cellulose. The yield varies between 60 - 70 ~, depending on the
species of timber and other factors. If cleaner chips are
wanted, the yield drops, and correspondingly improves with a
poorer level of cleanliness.
The intention o~ this invention is to achieve a better yield
than previously at each corresponding level of cleanliness. The
characteristic features o~ the method according to the
invention are presented in the accompanying Patent Claim 1 and
the features of corresponding equipment are presented in Patent
Claim 8. The distribution values of the wood chip fractions
according to the accompanying table are, to a great extent, the
point of departure of the invention:
Sieve gap Fraction Clean Wood Bark % of
mm . -;~;~ wood chips wood
in sieve chips in with bark chips
% ~raction with bark
0 5,03
2 16,73 79,03 1,96 3,61
4 37,94 89,98 2,72 8,57
6 25,37 85,54 7,15 16,78
8 8,01 74,54 14,83 19,98
6,92 69,00 25,51 18,30
It is surprising, that in the smallest fractions (sieve gap 4
mm, or less), there is not much bark attached to the chips,
whereas in chips above this size the proportion o~ chips with
bark attached is considerable. Because mechanical processing in
any event reduces the size of the wood chips across the board,

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it is advantageous to separate this fairly clean part of the
wood chips and have it bypass the mechanical processing. There
is bark as such even in the smallest chips, but is it loose,
and most of it leaves during the separation of the sawdust,
which has been moved so that it takes place after the
mechanical processing.
In what follows, the invention is illustrated by reference to
the accompanying Figures, which show schematically one plant
o according to the invention and the method used in it.
Figure 1 shows the method according to the invention, as a
block diagram.
Figure 2 shows equipment according to the invention, as a
plant diagram.
In the diagram in Figure 1, the wood chips with bark attached
can be considered as having been obtained by conventional
methods, using a known cutter. In the first stage of
processing, the wood chips are cleaned in pre-separation 3, 4,
5, which comprises among, other things, magnetic devices for
removing scrap iron and a pneumatic separator 4, 4' ~or
removing stones and sand. In pre-separation, oversize wood
chips are separated by means of disc sieve 5, and in practice
are most advantageously led to the fuel ~raction, but if
necessary they can be led, for example, to a crusher and then
back to pre-separation. The fraction accepted in pre-separation
3, 4, 5 is led to thickness sieving 6, which is intended to
separate the thin wood chip fraction, generally less than 6 mm,
from the part of the flow o~ wood chips travelling through the
thickness sieve. This part is led to the ~ollowing stage, i.e.
past mechanical bark separation 7. Mechanical bark separation
can take place by means o~ either a plate grinder, a crusher,
or in a blade ring chipper, which has been shown to be the most
advantageous, because it does not defibrate the wood material
unnecessarily.

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The mechanically processed fraction, together with the smaller
fraction obtained from the bypass line, is led to pocket-roll
sieve 9, which effectively removes the sawdust from the flow of
material. After this, the accepted fraction is led to a
pneumatic separator, to remove light birch bark, leaves,
needles, and other easily airborne particles from the flow of
material. After this, the pre-cleaned wood chips already have
a bark content of less than 10 ~, when it can be led to optical
sorter 10, from which wood chips with a bark content of about
o 1 ~ can be obtained, even at a large yield. Reject from the
optical sorter is led through selection 15 in a regulated
manner either to the fuel fraction, or back to mechanical
processing. In order to prevent a so-called 'mad cycle', 20 -
100 ~ is always led to the fuel fraction, and only 80 - 0 ~ is
led to the mechanical processing. Certain species of timber or
other conditions will mean that there is not necessarily any
benefit from mechanical processing, in which case it is better
to lead the reject from the optical sorter directly to the fuel
fraction.
The plant diagram, Figure 2, shows cutter 1 and reception
funnel 2, in which the wood chips are collected, or into which
wood chips from external sources are poured. From here the flow
o~ wood chips is led through magnetic separator 3 to disc sieve
5. Between these there is a pneumatic separator 4', which
removes stones and a second pneumatic separator 4, which sucks
light birch bark, leaves, and fine dust into the airflow, see
also stone store 3.1. A disc sieve 5 removes oversized pieces
from the wood chips, which are led to the fuel fraction 13,
together with reject from the pneumatic separator. Beneath the
disc sieve there is thickness sieve 6, which separates the
smallest fraction and the sawdust in it, these being led to
mechanical bark separation, going past grinder 7, whereas the
rest of this part is led to grinder 7. Experiments have,
however, shown that bark can be e~:Eectively separated by
cutting the chips. The mechanically processed chip fraction

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obtained from the plate grinder and the thin fraction obtained
from the bypass line 8 are led to the Pocket-Roll sieve 9,
which removes sawdust and fine bark material from the flow of
chips. After the Pocket-Roll sieve, there is a pneumatic
separator 11, which removes light birch bark, leaves, needles,
and other easily airborne material.
The accepted fraction is led to optical sorter 10, from which
a good yield of good quality cellulose wood chips 12 are
o obtained. The reject is led through selection member 15 either
to the fuel fraction 13 or to the grinder 7. Using selection
member 15, 0 - 100 ~ can be taken from the flow of materials
into the fuel fraction. It is advantageous to take at least 20
~, to prevent a so-called 'mad cycle' in the process. With
certain grades of wood chips part of the material would remain
to circulate continuously through the process, unless part of
the reject is removed to the fuel fraction. All of the removed
rejects are led in the Figure to fuel fraction 13, by means of
conveyor 16.
Because the optical sorter uses a pneumatic conveyor, the
reject flow is not homogeneous. A divider plate 18 set in the
exit area can separate the part with the greatest bark content
from the rest of the reject, it then being advantageously led
directly to the fuel fraction with the aid of conveyor 19. The
figure shows this alternative by broken lines.
The pulverization of the wood is reduced with the aid of the
method according to the invention, which increases the
cellulose chip yield from the previous yield by 5 - 10
percentage units, while share of the fraction of cellulose
chips remaining in the 13 mm particle size perforated sieve
increases substantially (by 15 - 35 percentage units).
In equipment according to the invention, a thickness sieve is
used to separate the fraction with the desired value, 4 - 8 mm,

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for example, thinner than 6 mm, which is led past the barking
separation and the thicker wood chips are fed to the mechanical
bark separation. These fractions are combined and the sawdust
and light birch bark is sieved out and sorted optically, the
accepted fraction of which is clean wood chips and the reject
is returned to mechanical bark separation, in which the bark is
ground to a smaller size, so that it can be distinguished in
the sawdust discharge.
o Improvement of the cellulose wood chip yield by 5 percentage
units improves the profitability of the plant by about FIM
20/m3, which represents FIM 3 000 000 p.a. in a plant producing
150 000 m3 p.a.. The effect of the chip size of cellulose wood
chips on the sales price varies from case to cases, but at its
greatest it too is about FIM 25/m3. Together these correspond,
in magnitude, to the operating and capital costs of the plant,
so that the economy of the method improves substantially.

Representative Drawing