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.

   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.

   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
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
Kappa number
% 50.1
Physical properties of 75 g/m 2 hand sheets refined to 60 + 1 0SR
Breaking length
Tensile elongation
Folding endurance, double folds
Burst index kPam2/g 3.56 4.17 5.58 5.58 6.24 3.05 2.59 4.07 4.42
Tear index mNm2/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.

   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.


Staminate inflorescence
(courtesy of VIR)