Development of an Axial Fractionator for Hemp Shive Cleaning and Industrial Applications of Shives

Due to increasing shortage of raw materials, traditionally used for production of wood based panels and other composite materials, there is a growing call for the use of raw materials from agricultural production. Hemp shive, a by-product of hemp straw processing, is in eager demand due to its wood-like material properties. In order to be suitable for industrial use, respective agricultural raw material must be competitive in price, as well as be available in respective quantity and quality. ATB has developed a new processing technology for the cleaning of shive-fibre mixes for better accommodation such requirements. Ultimate goal is the generation of dust and fibre free shives as well as the recovering of high quality short fibres from shive fractionation. This is facilitated by a new type of axial fractionators, which utilizes a multi-stage paddle screw and a screen drum, corresponding with the screw. This allows classification and cleaning of the shives as well as recovering of the short fibres in only one processing step. Finished products are widely dust and fibre-free, high value shives; and shive-free short fibres. The shives can be used for the production of particle boards without further preparation. The recovered short fibres are mainly suitable for the use as reinforcing fibres in composite materials due to their short fibre length (<50 mm). In cooperation with a machine engineering company, the design of the developed axial fractionator shall be adapted for industrial application. Owed to its simple and flexible design, common by-products of hemp processing can be processed at reasonable costs into high-quality market-going products.


Introduction
For a considerable period of time, growing shortage and a cost increase of raw materials for the wood based panel and composites industry has been evident.The use of wood as a fuel additionally boosts the demand in cellulose raw materials (Pecenka et al., 2010).Wood-like raw materials, e.g. from agricultural production, can be understood as alternative or supplement to the classical range of raw materials.Besides cereal straw, hemp and flax could play an important role as wood substitute in future.Some of the biggest hemp producers are Europe (approx.8,000 ha/year 2011), Canada (approx.16,000 ha/year 2011) and China (more than 80,000 ha/year 2008) (Kruse, 2012;Müssig, 2010).Especially in Canada hemp is mainly cultivated for seed production.Therefore, the straw could be used at reduced raw material costs for industrial applications if adequate processing technologies are available.Furthermore, in year 2012 more than 430,000 ha of flax have been cultivated for seed production in Canada alone (Agriculture and Agri-Food Canada, 2012).Because modern processing facilities for bast fibre production are able to process hemp and flax with the same equipment, an interesting additional income for farmers could be generated from flax straw.
A large proportion of the produced natural fibre from hemp and flax is used for the production of insulation material and fibre fleeces.Substitution of synthetic fibres (e.g.glass fibre) in composite materials by natural fibres, yet maintaining similar properties of the composite, is also considered feasible (Graupner & Müssig, 2009).Natural fibres and glass fibres show comparable specific strengths and Young's moduli (Bledzki, Gassan, Fink, & Kleinholz, 1999).The demand for shive, which at 50-60 mass-% make up for the significantly larger

Curren
The decort mass-% (F mass-% of and fibre by-produc     On the one hand, as in material flow a) the mix flows over the paddle and the newly aligned shives can pass through the cleared screen behind the paddle.On the other hand, as in material flow b) the mix is moved forward over the screen by pitching the paddle axially, thus causing the shives to pass through the screen as well.By dropping the flake from the paddle, as in material flow c) and simultaneously crushing the fibre flake by a successive paddle, the shives are threshed from the flake and can thus pass through the screen.

Results
Figure 6 shows selected fractionation examples, generated by cleaning of shive-fibre mixes in an axial fractionator.Figure 7 shows the detailed proportionate content of end products from the shive flow, as collected after the cleaning of the shive-fibre mix in the axial fractionator.Moreover, the currently existing test plant was fitted with a close-meshed screen in the first section of the paddle screw for the separation of dust and sand particles.The utilization of dust separation via gravitation and the respective load reduction in the further cleaning process has also proved to be very effective (Figure 9).By variation of the first screen mesh-width, the composition of the separated dust ratio can be controlled and the quality of cleaned shives can be adapted to market requirements.The cleaned shives are the largest proportion of the generated materials and have an insignificant content of dust and short fibre.
The processed shives were recovered to fibre and subsequently pressed to medium density fibre boards (MDF) and particle boards.Figure 10 shows the compared bending strengths of the boards made from different raw materials under addition of 10 mass-% of PF-resin.
Figure 10.Bending strength of medium density fibre and particle boards from different raw materials Due to their high purity, the shives (free of dust and short fibres) could be directly processed to particle boards (Figure 11).Potential particle board densities, dependent on the shive properties, are currently examined in detail.This is supplemented by research about the suitability of the separated short fibres and the ultra-short fibres for the use in injection moulding materials.Current findings allow expectations for production potential of composites based on these fibres, featuring similar properties as composites produced from conventional hemp fibres.Using these fibres for injection moulding processes has advantages due to the improved agglomeration properties of the recovered ultra-short fibres and low costs for raw-material or granulates respectively.

Conclusions
Farmers need reliable partners in the processing industry for profitable cropping of bast fibre plants for industrial applications.Efficient technologies for fibre processing have been developed in the last decade.Additionally, the implementation of efficient shive cleaning technologies in new processing lines plays an important role for the overall economy of hemp and flax processing.
The research to the axial fractionator has shown that hemp shives can be effectively cleaned with the developed plant concept and consequently be used in wood based panels due to their high quality.The manufacturing cost on the basis of dried chips or shives for producing shive boards are similar to particle boards.The quality hemp shives as a raw material can be offered for a price from 150 to 180 € t -1 .The recovering of short fibres and ultra-short fibres in the axial fractionator can result in an increased fibre generation for the overall process by up to 10 mass-% (relative to the fibre content) and thus improve the efficiency of plant operation.Due to the shorter fibre length of the recovered fibre compared to the conventional technical hemp fibre, they are well suitable for the use in injection moulding materials.Currently, detailed research is carried out for this application with respect to processing and product properties compared to conventional reinforcing fibres.
According to experience with the test plant, operated under practice conditions, a reviewed plant concept for industrial application of a shive cleaning plant has been developed with a machine engineering company.Advantages of the new axial fractionator will be a simpler design of the screen path, higher mass flow (2 t/h shive-fibre mix) and relatively higher quality of recovered fibres (rest shive content <6 mass-%).
a flexible frac straw types, b mass flow of 2 cient fibre pro short fibres w fibres (5-20 m cing fibres, e.g ds shive-fibre mi types require a cations of the c s, and short fib ially enclosed the shives.Fur e a form closu especial notic mix as accumul separate the m t weight and f excess of bindi are sti duction, and m pes th the usual cla ds there is a la d fibres).Only the axial fractionator (Figure4), can accommodate a reliable separation of the mix into its main components.

Figure 5 .
Figure 5. Material flow and straining process of a shive-fibre mix in the axial fractionator a) flow over the paddle; b) flow along the paddle; c) dropping from paddle.

Figure 9 .
Figure 9. Particle size of cleaned and unclean shives, using different mesh sizes for dust separation in the axial fractionator (shown without fibre)

Figure 11 .
Figure 11.Shive particle board with HPL-surface and 2 mm front liner, shavings completely substituted by shives