16
Use of AAS pulping for flax and hemp shives
V.S. Krotov
Ukrainian Pulp and Paper Research Institute, Kiev, Ukraine
Krotov, V.S., 1995. Use of AAS pulping for
flax and hemp shives. Journal of the International Hemp Association 3 (1) :
16-18.
Some plants such as hemp, flax and kenaf produce two types of fibres:
long bast fibres and short woody core (shive) fibres. The bast fibres are used in
the pulp and paper industry; however, the short core fibres do not produce good quality
pulp using conventional pulping technology. Test results have shown that the
alcohol-based ammonia-sulphite (AAS) pulping process has produced good quality unbleached
pulp from flax and hemp shive; pulp which has a high yield, low Kappa number and good
physical properties for papermaking.
Introduction
When flax and hemp stocks are processed, 70-80% of
the initial raw materials are rejected as shive. Flax and hemp shive are not very
suitable for utilization; however, sometimes it can be used for low-quality building
materials (shive-containing fibreboards, gypsum, concrete); in agriculture as bedding for
livestock and for soil mulching; and it can be processed into combustible briquettes.
Of the 56 mills processing flax and hemp in Ukraine, only 20 process a
part of the shive. Of this number, 7 mills produce shive-containing fibreboards or
building gypsum blocks and the rest make briquettes. A large part of shive is used
as fuel for the boiler houses of the mills themselves but greater amounts are thrown out
forming dumps which become a source of environmental problems and also create fire hazards
near the factories.
The pulp and paper industry does not use flax and hemp shive, mainly
due to the following factors:
the difficulty of delignification which is determined by the
composition of the raw materials i.e there are high lignin contents in fibre cell walls
and, in particular, the lignin content of hemp shive approaches the lignin content in
softwood and that of flax shive exceeds the lignin content in softwood;
unbleached flax and hemp shive pulps produced using conventional
pulping processes have lower yields and higher Kappa numbers as compared to hardwood
pulps;
pulps produced from flax and hemp shive have small length fibres
which are 2 to 3 times shorter than hardwood pulp fibres;
extremely low drainage rate of pulp produced from flax and hemp
shive by conventional methods;
low papermaking and mechanical pulp properties of pulp produced
from flax and hemp shive by conventional methods;
possible recovery problems due to higher ash content and silica
compounds present in the ash.
All the above factors make the pulping of flax and hemp shive difficult
if conventional or modified sulphite, kraft, soda or other known methods are used, but
they are not of vital importance in the case of our pulping method based on water-organic
solutions of ammonia (NH3) and sulfur dioxide (SO2) (Krotov & Lavrinenko 1984, Krotov
& Lavrinenko 1985, Krotov 1994).
Alcohol-based Ammonia-sulphite (AAS) pulping
For AAS pulping, a wide range of organic solvents
of different classes and their mixtures could be used including, but not limited to:
monobasic alcohols (methanol, ethanol, etc.)
polyols (diethylene glycol, glycerine, etc.)
cellosolves (ethylene glycol monoethyl ether)
The major requirements of the organic solvents used is that they should
mix with water at the pulping temperature and they should not degrade or undergo
condensation reactions with lignin contained in the raw material. From economic and
technical points of view, in the AAS pulping process preference should be given to ethanol
or methanol.
A remarkable feature of AAS pulping is the possibility of the combined
pulping of different raw materials, including those with considerably different
morphological characteristics, structure and chemical composition. This feature
would allow the development of the most efficient technology for pulping bast-fibre plant
stalks such as hemp, kenaf and flax without their separation into bast (fibre) and woody
(shive) fractions, as is the suggested current practice for kenaf pulping described in the
technology of Ankal Proprietary Ltd. (Australia) (Kaldor et al. 1990).
Previous testing shows that AAS pulping results in high yield for long
bast fibres with very low lignin contents. Thus, our studies in this paper of the
processing of disintegrated bast-fibre plant stalks are directed at the delignification of
the woody (shive) core part.
Methods
Raw materials were pulped using aqueous-alcohol
(65:35 vol%) ammonia and sulfur dioxide solutions. Cooks were made in a 1 litre
stainless-steel digester placed in a heated glycerine bath. All cooks were made at
near isothermal conditions. To simulate a digester screw feeder, shive with a dry
weight of 140-145 g was placed in a hydraulic press and compressed rapidly (5-10 seconds)
using pressures of up to 8.5 Mpa. The shive was transferred to the digester, cooking
liquor was added, and the digester was hermetically sealed. The digester was placed
immediately in the heated glycerine bath and the contents were raised to cooking
temperature within 2-3 minutes. The digester remained in the heated glycerine bath
at the cooking temperatures and for the cooking times described in Table 2. After
cooking for the allotted time, the digester was cooled using cold water. Then, the
pulp was removed from the digester, placed on a 100 mesh screen and washed using warm
water. The washed pulp was then analyzed. No shive and mass sorting was
carried out. In the first run of experiments, the wastes of primary hemp and flax
whole stalk processing mills were used. Soda and soda-anthraquinone pulping tests
also were conducted with the same raw materials for comparison purposes with the AAS
pulping tests. Soda pulping tests were performed under conditions typical for
processing the nonwood raw materials in a Pandia digester apparatus.
The analysis of raw materials, preparation and refining of fibrous
stock, making hand sheets and determination of physical-mechanical characteristics of pulp
were performed to Ukrainian standards which mainly conform to ISO standards. Hand
sheets of 75 g/m were made of the various fibrous stocks refined to 60 + 1 0SR.
Table 1. Chemical composition of hemp and flax shive as % of oven-dry raw material weight. | ||||
Characteristics | Hemp shive | Flax shive | ||
Ash | 1.00 | 2.55 | ||
Extractable substances (in a 1:2 ethanol-benzene mixture) | 3.16 | 2.76 | ||
Lignin | 25.52 | 30.11 |
Table 2. Pulping conditions and test results for three pulping methods (Soda, Soda-AQ and AAS). | ||||||||||
Hemp shive | Flax shive | |||||||||
----------------------------- | ----------------------------- | |||||||||
Characteristics | Units | Soda | Soda-AQ | AAS | AAS | AAS | Soda | Soda-AQ | AAS | AAS |
Pulping conditions | ||||||||||
Chemical consumption,
as % of oven-dry raw material weight: |
||||||||||
active alkali, Na2O anthraquinone, AQ ammonia, NH3 1-- sulfur dioxide, SO2 |
|
18.0 - - - |
18.0 0.1 - - |
- - 15 10 |
- - 15 28 |
- - 15 18 |
18.0 - - |
18.0 0.1 - |
- - |
- - |
Water to alcohol ratio | - | - | 65:35 | 65:35 | 65:35 | - | - | 65:35 | 65:35 | |
Liquor to shive ratio | 4.0:1 | 4.0:1 | 4.0:1 | 4.5:1 | 4.5:1 | 4.5:1 | 4.5:1 | 4.5:1 | 4.5:1 | |
Temperature | °C | 170 | 170 | 170 | 150 | 170 | 175 | 175 | 160 | 170 |
Time | min | 45 | 45 | 75 | 180 | 180 | 45 | 45 | 180 | 180 |
Pulping results | ||||||||||
Yield Kappa number |
% | 50.1 64.4 |
46.5 25.2 |
62.2 48.1 |
67.9 56.5 |
54.5 17.3 |
42.1 90.0 |
44.2 78.4 |
55.2 78.9 |
47.9 42.7 |
Physical properties of 75 g/m 2 hand sheets refined to 60 + 1 0SR | ||||||||||
Density Breaking length Tensile elongation Folding endurance, double folds |
g/cm3 m % |
0.62 6310 3.4 320 |
0.62 7150 3.4 680 |
0.67 8550 3.4 1250 |
0.68 10140 3.0 140 |
0.75 12000 3.0 280 |
0.59 5130 2.7 26 |
0.58 5860 2.8 26 |
0.64 7860 3.3 70 |
0.64 8710 3.4 140 |
Burst index | kPa·m2/g | 3.56 | 4.17 | 5.58 | 5.58 | 6.24 | 3.05 | 2.59 | 4.07 | 4.42 |
Tear index | mN·m2/g | 3.19 | 3.28 | 3.33 | 2.65 | 3.71 | 4.10 | 4.28 | 3.78 | 4.47 |
Results and discussion
Characteristics of the raw materials are given in
Table 1, and experimental conditions and results are summarized in Table 2. As is
seen in Table 1, flax shive contains much more lignin and ash than hemp shive which means
that hemp shive is the more preferable raw material for pulping.
The data in Table 2 demonstrate that shive pulp produced using the soda
process is suitable only for low-grade papers and boards and that soda pulping is not very
suitable for making bleachable shive pulp. The addition of anthraquinone (AQ) to
soda liquor considerably improves the process giving bleachable pulp with a 46% yield for
hemp shive and strength properties higher than those of soda pulp. However,
soda-anthraquinone pulping did not produce hemp pulp which could compete with hardwood
pulps.
As compared to hemp shive, flax shive is weakly responsive to the
addition to anthraquinone to the soda pulping liquor. Soda-anthraquinone flax shive
pulp displays low strength properties (excluding tear) which do not differ considerably
from those of soda pulp. Despite higher yield and low Kappa number,
soda-anthraquinone pulp made of flax shive will be difficult to bleach.
Based on the test conditions used, the test results of soda and
soda-anthraquinone processes confirm that hemp shive is of little use and that flax shive
is practically unsuitable for the production of pulp and paper if traditional pulping
technologies are used. The AAS pulping process, however, offers entirely new
potential for the use of hemp and flax shive in the pulp and paper industry.
The data in Table 2 show that AAS pulping can produce shive pulp which
is not inferior in its properties to hardwood kraft pulps. In fact, AAS hemp shive
pulp displays especially high yield and physical characteristics. The yield of
easily bleachable AAS hemp shive pulp is at least 10% higher than that of the
soda-anthraquinone pulp, and it is at the same level or exceeds the yield level of aspen
kraft pulp. Of physical characteristics, the very high breaking length of AAS pulp
deserves special attention. In the case of AAS hemp shive pulp, the breaking length
is 12,000 m which is comparable with breaking length of high-strength softwood kraft
pulps.
The high breaking length of AAS pulps may be explained by a higher
selectivity of AAS pulping than conventional soda or soda-AQ pulping with the result that,
in AAS pulping, the hemicelluloses are retained in the pulped stock. Also, it is
especially valuable for AAS pulps that high breaking length is combined with high tear
strength.
The characteristics of AAS flax shive pulps are somewhat lower than
those of hemp shive pulps. However, an appropriate choice of pulping conditions can
give stock comparable with conventional hardwood pulps, and such pulps can be used in the
furnish of high-grade papers and boards.
Conclusions
The studies have demonstrated that our pulping
method with aqueous-organic, in particular aqueous-alcohol ammonia and sulfur dioxide
solutions, offers the potential to make hemp and flax shive valuable raw materials for
pulps of different purposes, including possibly high-grade bleachable pulp which could
compete with hardwood kraft pulps.
References
Staminate inflorescence
(courtesy of VIR)