The recent interest in printing and writing papers made of hemp fibres requires a different kind of pulp. The speciality pulps coming from those mills are over-qualified for the production of printing and writing grade paper. The main reason for being over-qualified is the extremely long fibre length. A specialty papermaking process has been designed to handle these kinds of fibres. This imposes two problems for potential printing and writing paper production.
The first problem is that the production of such papers can only be carried out in mills equipped with a hollander beater or comparable beating equipment. Traditionally these are mills producing specialty papers, not "regular" printing and writing grade papers. The second problem is that beating chemical hemp pulp with a hollander will drastically worsen the pulp's drainage properties, so that the paper machine will have difficulty in dewatering the pulp. One consequence of slow dewatering is limitation of the maximum weight of the paper produced, another is the fact that the paper machine will have to run slower in order to have enough time to dewater the paper. Paper production in traditional specialty paper mills will therefore be considerably more expensive than using state-of-the-art paper machines designed especially for printing and writing grade paper. By means of a chemimechanical pulping process it is possible to produce bleached hemp bast fibre pulps allowing production of paper on a fast printing and writing paper machine.
In "classical" chemical pulping of hemp the fibres are separated by cooking under high pressure at a high load of alkaline and sulfuric chemicals. This process is suffering the disadvantages mentioned earlier. The principle of mechanical pulping is to separate the fibres by exposing them to mechanical energy rather than using chemical energy. By adding small amounts of alkaline and possibly some catalysts the process performance is improved.
The elementary difference between chemical and mechanical pulping is that in the first lignin is removed from the fibre, in the latter lignin remains in the fibres. Hemp bast fibre contains at the most 4% lignin, therefore the quality difference between chemical and chemimechanical hemp pulp is not very pronounced. In chemical processes, lignin is removed from the hemp fibres at the cost of more than 50% of the raw material. Chemimechanical processing yields more than 75% of the raw material.
A French group developed a mechanical process based on twin screw extrusion (De Choudens; Kurdin). Through the years this principle proved particularly suitable to process long agricultural fibres like hemp, flax and cotton. The main advantage being that the extrusion machine can cut the long fibres to a length "regular" papermakers can handle, without affecting the draining properties too much.
The Extrusion Pulping Process
In the following a short description of the process for production of bleached pulp from hemp bast fibre is given:
- Dry hemp bast fibre from a decortication facility is cut into 5 centimeter lengths and dry cleaned to remove metal, dirt and remaining shives.
- The hemp is soaked for 2 hours in cold alkaline process water, using added sodium hydroxide to correct alkalinity. The soaked hemp is allowed to drain and is briefly washed with cold water to partly remove the compounds dissolved during soaking; this is where the most solid waste emerges from the process.
- The washed hemp is steamed at atmospheric pressure.
- The hemp is introduced into the twin-screw extrusion machine to cut the fibre to the desired quality; washing water is fed semi-countercurrent to further wash the pulp; in the last zone the first peroxide bleaching liquor is mixed into the pulp; the pulp is allowed to react with the bleaching liquor for 1 to 2 hours.
- The pulp is fed to a continuous disc refining system where it is further cut to the desired quality; the second peroxide bleaching liquor is introduced into the pulp; the pulp is again allowed to bleach for 1 to 2 hours.
- Finally, the pulp is washed, fractionated and dried.
Printing and writing grade paper is generally produced from blends of bleached softwood and hardwood pulps. In order to determine the suitability of hemp pulps produced with the process described, a typical bleached hemp bast fibre pulp is compared to common bleached softwood and hardwood pulps, and also to a "classical" hemp market pulp.
When comparing the chemimechanical pulp to the chemical pulp it is shown that strength properties are comparable at a lower beating degree. It is expected that chemimechanical pulp can be fed to a regular paper machine at about 50 degrees SR, whereas chemical pulps often have to be beaten to as far as 85 degrees to remove all remaining long fibres, thereby considerably increasing dewatering problems. The brightness of the chemimechanical paper is sufficient for use in slightly off-white paper. The opacity is much better than that of chemical hemp pulp. The pulp density of both hemp pulps is lower than that of wood pulp, so the paper will have a thicker texture and "feel". This property is excellent for letterhead and presentation papers and even offset printing, but will need some sizing in order to give good office paper quality for laser or inkjet printers. A thicker paper can also be translated in the ability to yield a comparable paper thickness with less pulp, as compared to using a chemical wood pulp.
The production of chemimechanical pulps yields a minimum of 75% of pulp from each tonne of dry hemp fibre raw material. It is expected that more than 50% of the waste can be separated with centrifugal separation. The remainder can be treated in a two-stage biological waste water treatment. Waste water from chemimechanical pulping trials do not contain toxic elements, and that all components can be degraded biologically, except for a fraction of the lignin (Kortekaas). The mills presently producing chemical hemp pulp do not recover their cooking chemicals because their production scale does not allow them to. Compared to these mills the chemimechanical technology can definitely be considered cleaner. A major environmental advantage of this process is that it uses hydrogen peroxide to bleach to a 82 point brightness. The pulp can therefore be classified as totally chlorine free (TCF).
A chemical pulping mill with a full recovery system and waste water treatment cannot be operated economically feasible under 250.000 tons per year capacity. This implies long transport distances, which is especially costly when processing bulky agricultural fibres like hemp. A chemimechanical process using extrusion pulping produces 12.000 tons per year at its maximum. For a larger capacity multiple processing lines can be switched parallel. The economies of scale are of much less influence and can be optimized with the transport distance of hemp coming to the mill from the surrounding growing areas. This is particularly important for the principle of bio-regional production. Calculations based on bench-scale studies at ATO-DLO resulted in an estimated production cost of US$ 530,- per bone dry metric tonne of TCF bleached hemp bast fibre pulp for applications in printing and writing grade papers. This operational cost is including capital load from equipment investment and depreciation. This cost figure is very competitive as compared to present-day chemical hemp market pulp. Written by: Gertjan van Roekel
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