Agriculture Reference
In-Depth Information
conflict would interrupt its supply of cotton,
restarted hemp production.
It was at this time that the process of
steam explosion was used on a grand scale, as
it was of particular interest for the production
of textile fibres. Where thread was concerned,
it allowed the production of single fibres (as
opposed to fibres in bundles), which could then
be used to produce thread, and ultimately cloth.
The technique used needed to reproduce on an
industrial scale all the characteristics and quali-
ties of the fibre: length, tension resistance, sup-
pleness and fineness, etc.
This process will be described briefly, for,
given the cost of exploitation, it is now used for
laboratory research onl y 6 (Kessler and Kohler,
1996).
The machinery consists of a compression
chamber and an extrusion system. The fibre
(bundles of single fibres that are still attached to
each other), having already been subjected to a
mechanical fibre extraction process, is placed
in the reactor at a pressure of 2.9 mPa of satu-
rated vapour and a temperature of 150-220°C.
The fibres are impregnated with caustic soda in
order to break down the pectins. The fibre
stays in the reactor for 5-15 min, during which
time the pressure starts to break up the fibres.
At the end of the treatment, the material is
sent to a cyclone, where it literally explodes,
allowing the fibres to be separated individually.
This mechanical and chemical treatment pro-
duces extremely fine fibres.
The experiment reported shows that the
key point is the diffusion of steam within the
bundles of fibres at the start of the process.
A minimum time of 5-10 min gives even bet-
ter results.
This time guarantees a uniform penetra-
tion of the material and yields fibres of excel-
lent quality. By contrast, longer treatments
(>20 min) at high temperatures break down
the cellulose components of the fibre.
It would appear that with this procedure,
and the resulting adaptation of the chemical
treatment, the qualitative differences found in
the raw materials could be reduced, thereby
producing fibre that corresponds to the specifi-
cations of the end user. For this to happen, the
length of steam treatment and the dose of soda
must be adjusted according to the material
being used.
In this way, a uniform batch of fibres can
be produced, even though the raw material is
far from uniform.
SIFTING AND CLEANING . A number of methods
are used to separate the fibre from the hurds.
Essentially, there are two common processes:
shakers and rotating sieves (most commonly
made by Duvex). These will knock the sepa-
rated hurds from the fibres; the hurds fall into a
drainage channel that delivers them to storage.
RECRUSHING - PASSAGE THROUGH A SECOND CRUSHING
DEVICE . After passing through the first crusher,
and regardless of the extent of retting, a large
amount of hurds will still be attached to the
fibres. In order to complete the breakdown of
these two elements, a second mechanical treat-
ment is required. This follows the same princi-
ples as the first, will often use the same machinery
and subjects the straw to a new trituration.
CLEANING AND PURIFICATION . In the same way
and with the same technologies as described
earlier, the fibre is subjected to a second siev-
ing. Separators, shakers and sieves, moving
vertically, laterally and rotationally, are all
added to the mix. The combination of these
movements deposits the hurds gradually on the
belt that takes it away to storage.
OPENER CARDER . At the end of this process, an
opener carder is used to arrange the cleaned
fibres into a uniform bed.
TRIMMING . Where the fibre needs to respect a
dimensional standard, it is fed continually
through a trimmer.
CONDITIONING . Finally, the fibre is conditioned
by high-density presses into bales weighing in
excess of 400 kg, with dimensions of the order
of 1.5 × 0.8 × 0.8 m.
Results obtained
At the end of this process - a process that lasts
only a matter of minutes from the time that the
bale enters the system to the time the bales of
processed fibre emerge - fibre is obtained con-
taining between 5 and 35% of hurds.
 
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