Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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RECYCLING OF LIGNOCELLULOSE BASED BOARD MATERIALS
The present invention relates to the recycling of
lignocellulose based board (or panel) material comprised of
a matrix of adhesively bonded lignocellulosic elements so as
to permit recovery of constituents of the board material,
particularly but not exclusively of the lignocellulose.
It is well-known that various board materials comprise
a matrix of lignocellulosic elements (e.g. in the form of
chips, particles or fibres) bonded together by means of an
adhesive such as, for example, a polyurethane,
urea/formaldehyde, melamine-urea or phenolic resin.
Examples of board materials produced in this way include MDF
(Medium Density Fibreboard), particle board and chip board.
Board materials of the type described above are used
extensively for producing finished articles such as, for
example, furniture. For this purpose, the board materials
are entirely satisfactory. However there is a substantial
amount of waste material for which disposal poses a problem.
To illustrate the point, the UK furniture manufacturing
industry generates over 170,000 tonnes of MDF waste every
year. This does not include rejected and damaged furniture
items. Ideally the waste material would be recycled to
recover constituents thereof, particularly the
lignocellulose for reuse. However, no satisfactory
recycling process is currently available. The problem is
made worse by the fact that the waste board material may be
laminated to a surface layer such as, for example, paper
foil or plastics (e. g. for decorative purposes) or may have,
for example, plastic or metal inserts. As such, any
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recycling process will need to remove the laminates and/or
inserts. In the absence of any suitable recycling process,
most of the waste board material will be dumped in landfill
site, which is becoming more difficult and very expensive.
It is an object of the present invention to obviate or
mitigate the above mentioned disadvantages.
According to the present invention there is provieled a
method of recovering a constituent of a board material
comprised of a matrix of adhesively bonded lignocellulosic
elements, the method comprising subjecting the material to a
combination of (i) electromagnetic radiation and (ii)
soaking or immersion in a liquid medium, and recovering the
constituent.
The constituent to be recovered will generally comprise
lignocellulose, which may, however, incorporate residual
resin, for example urea-formaldehyde resin.
The electromagnetic radiation will typically have a
frequency in the range of from 100 kHz to 300 GHz, more
typically from 10 MHz to 300 GHz.
The liquid medium will typically comprises water or an
aqueous solution. The liquid medium could', however,
comprise any suitable organic or inorganic solvent capable
of swelling the material so that the constituent can be
recovered. Possible other examples include ethyl alcohol,
ethyl alcohol/water mixtures, and dilute sodium hydroxide
(for example 0.1-9o by volume). This latter example has
been found to improve fibre texture.
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The invention has been based in part on our discovery
that treatment of board materials comprised of an adhesively
bonded matrix of lignocellulosic elements, for example
particles or fibres, by exposure to electromagnetic energy
in the frequency range of from 10 MHz to 2500 MHz and
soaking with a liquid medium such as water produces
substantial swelling of the board material, which, we
believe, mechanically disrupts and possibly at least -
partially hydrolyses the adhesive bonding the
lignocellulosic elements together so that these elements can
now be readily separated from each other. The degree of
swelling achieved is considerably more that that which is
obtained simply by soaking the board material in the liquid
medium.
Steps (i) and (ii) may be effected simultaneously or
sequentially. The degree of swelling achieved in the
thickness dimension of the board should generally be in the
range of from 3 to 6 times the original thickness.
Separation of the lignocellulosic elements from each
other may be achieved using a relatively low degree of
mechanical agitation while the treated material is in the
liquid medium, for example water. Once the elements have
been separated, it is possible to recover a desired
constituent of the board, which will usually comprise the
lignocellulose. Thus, for example, the resultant dispersion
of fibres may be dried, for example by press-drying (if the
fibres are to be transported) or by a fan-assisted blowing
system (if the fibres are to be re-used on site). Moreover,
surface laminates, for example paper, foil, melamine, veneer
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or other finishes commonly used on board materials to which
the invention relates, can readily be separated from the
treated board prior to recovery of the fibres, for example
by agitation, as may inserts or other bodies included in the
panels.
The board material will typically have a density of
from 200 Kg m 3 to 1200 Kg m 3.
The invention is applicable to a wide variety of wood
based boards, including particle boards and fibre boards.
Specific examples of board materials to which the present
invention is applicable include MDF, chip board, hard board,
soft board, orientated strand board, flax board and wood
chip board, and combination of any two or more thereof.
The invention is applicable both to industrial- and
consumer- waste board material.
The electromagnetic radiation used in the process of
the invention preferably has a frequency in the range of
from 10 MHz to 300 GHz, more preferably from 10 MHz to 2500
MHz. These frequencies have been found to result in
substantial swelling of the board material in the liquid
medium. The power is preferably~in the range of from 500 W
to 30 kW, more preferably from 3 kW to 15 kW, although
certain values in these ranges may be more applicable to
some materials rather than others. Thus, for example, the
power used should preferably not be so high as to cause
charring of the board material. The optimum parameters may
readily be determined by a person skilled in the art.
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It is particularly preferred that microwaves in the
frequency range from 896 + 20 MHz to 2450 + 25 MHz are
employed (such as generated by a magnetron). Thus, for
example, the electromagnetic radiation used may be 896 + 20
MHz or 2450 + 25 MHz, both of which are frequencies reserved
for domestic/industrial microwave use. These frequencies
have been found to result in substantial swelling of the
board material in the liquid medium.
Alternatively the electromagnetic energy may have a
frequency in the range of from 10 MHz to 50 MHz.
For all embodiments the microwaves may be generated by
means of a magnetron in a conventional way. The power
output to the cavity, which may, for example, be in the form
of a metallic vessel or pipe, is preferably in the range 500
W to 30 kW. The microwaves propagate through the cavity,
which contains the wood-based panels immersed in the liquid
medium such as water.
Electromagnetic radiation having a frequency in the
range of from 100 kHz to 100 MHz may also be used, which is
typical of radio frequency (RF) waves. Thus, the process
according to the present invention is not restricted to the
use of microwaves. RF may also be utilised due to the lower
frequencies of operation resulting in greater penetration
through the board, which is often advantageous. RF may be
defined as all frequencies used for communication,
corresponding to 100 kHz to 300 GHz. Further details may be
found in Kitchen, R. (2001) RF and Microwave Radiation
Safety. Newnes ppl-2.
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The invention may be practised in a number of ways. Tn
a particularly preferred embodiment, the board material is
initially subjected to the electromagnetic radiation and is
then immersed substantially immediately into the liquid
medium, for example within 5 to 15 seconds. The liquid
medium, for example water, is preferably at an elevated
temperature, for example 60°-90°C, preferably about 80°C.
For this embodiment, it is desirable that the board material
does have internal moisture content, preferably a minimum of
80, which may, if necessary, be enhanced prior to the
treatment with electromagnetic radiation. Immersion of the
board material that has been subjected to electromagnetic
radiation into the liquid medium causes substantial swelling
to occur. Typically the exposure time to the
electromagnetic radiation will be in the range of from 30 to
90 seconds. Subsequently the material is soaked in the
liquid medium to swell the material. Typically the degree
of swelling may be to 3 to 6 times the original degree of
thickness, for which an immersion time of typically 10-25
minutes in, for example, water may be required, although the
exact time will depend on factors such as the nature of the
board, the.parameters employed (for example frequency and
power) employed during the treatment with electromagnetic
energy, and the temperature of the water.
Any surface laminate applied to the board may easily be
removed from the swollen board (and in fact the laminate may
start to peel-off during the treatment with electromagnetic
radiation). Similarly any inserts may also be removed
easily. The swollen material may then readily be converted
to a fibrous suspension using, for example, a moderate
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degree of agitation such as provided a low power mechanical
blender, for example.
The fibrous suspension may then be dried, for example
by press-drying or by means of a fan-assisted blowing system
as described previously.
This embodiment of the invention may be effected on a
continuous or semi-continuous basis by, for example, passing
the material to be treated through or passed a microwave
source (with the material, for example, being on a conveyer
belt) and then introducing the material into a tank of
liquid medium (for example water) for the desired residence
time therein.
In an alternative, but less preferred, embodiment of
the invention, the board materials is immersed in the liquid
medium (for example water) and subjected to electromagnetic
radiation as discussed previously followed by soaking in the
liquid medium without irradiation. All other conditions
being equal, this embodiment of the invention tends to
produce a lower degree of swelling of the board than the
above described preferred embodiment and does not lend
itself as readily to continuous or semi-continuous operation
as the above described preferred embodiment. It may however
be possible to recover heat from the liquid medium using,
for example, a heat exchanger to assist in drying of the
board. Moreover, this embodiment involves not only heating
of the board material by the electromagnetic radiation but
also the liquid medium in which it is immersed, thus
reducing energy efficiency. Furthermore, it is envisaged
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that this embodiment may provide more problems with effluent
disposal than the above described more preferred embodiment.
For all embodiments of the invention, the board
material may initially be subjected to a vacuum impregnation
so as to increase its moisture content, for example up to
50o by weight. Alternatively, or additionally, the liquid
medium in which the board is immersed may incorporate an
additive such as, for example a surface active agent or
surfactant, to assist penetration of the water into the
board.
Furthermore, for all embodiments of the invention, the
board material may be "turned°' during treatment with the
electromagnetic radiation to ensure uniform exposure.
It will be appreciated that the invention is able to
provide clean recycled fibre for a number of possible uses,
for example production of other board products, wood plastic
components, fillers and insulating materials.
The process according to the present invention can be
conducted on a mobile basis if desired.
The invention will be further described, by way of
example only, with reference to the following non-limiting
Examples and accompanying drawing (Figure 1), which
illustrates the result of Example 1.
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Example 1
Two experiments were conducted as detailed under (a)
and (b) below.
(a) Samples of MDF measuring (approx. 50 x 50 x 18) mm
were immersed (individually) in approximately 1000
ml of water in a non-metallic container and
subjected to microwave radiation at a frequency. of
2450 + 25 MHz at power levels of 3 kW to 15 kW for a
period of approximately one minute. The samples
were allowed to stand in the water for approximately
10 to 15 minutes. The procedure was carried out a
total of three times at each power level using fresh
MDF samples each time. The thickness of the samples
was measured after this treatment and the results
plotted in Figure l, which is a graph of the mean of
the three thicknesses of the MDF samples (after the
treatment) at each power level vs. power level
employed.
(b) The procedure of (a) was repeated but using samples
of MDF measuring (approx. 150 x 150 x 18) mm using
power levels of approximately 12 kW and 15 kW for a
period of approximately 60 seconds. The results are
also plotted on Figure 1.
For the experiments of Parts (a) and (b), the water
temperature was monitored and was found not to exceed 90°C.
It can be seen from the results presented in Figure 1
that all samples swelled as a result of the combination of
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microwave treatment with simultaneous immersion in water.
For the (50 x 50 x 18) mm samples, best results were
obtained at power levels greater than approximately 5 kW,
with the samples swelling to a thickness of 60 mm or
greater. The (150 x 150 x 18) mm samples provided even
greater degrees of swelling. This finding could be
indicative of "greater cavity loading'° at higher power
levels. Although not illustrated on the graph, a further
sample of (approx. 150 x 150 x 18) mm board, which was;
treated for about 45 seconds at 12 kW power, recorded a mean
thickness swell of 92.11 mm.
All samples of the swollen material could easily be
converted to a fibrous suspension in either approximately
two minutes using a pulp disintegrator rated at
approximately 1.5 kW or in approximately four minutes using
a 700 W laboratory stirrer.
Example 2
A (approx. 150 x 150 x 18) mm sample of MDF was
subjected to microwave radiation at a frequency of
approximately 2450 + 25 MHz at a power level of
approximately 12 kW for a period of about 45 seconds and
then added immediately to water at a temperature of above
about 60°C and allowed to stand for approximately 10 to 15
minutes.
The MDF was found to have swollen in thickness to
approximately 90.42 mm. The swollen material could easily
be converted to a fibrous suspension in either approximately
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2 minutes using a pulp disintegrator rated at 1.5 kW or in
approximately 4 minutes using a 700 W laboratory stirrer.
10
20
30
