New process for bark utilisation

Materials World magazine
28 Nov 2018

A new process for extracting tannins from softwood bark has been discovered, making it possible to use the full material. Idha Valeur reports.

Every year, approximately three million tonnes of softwood bark is produced in the Finnish mechanical and chemical forestry industries. At the moment, it is mainly used for the production of energy. Scientists at VTT Technical Research Centre of Finland Ltd developed a new method where a high yield of pure tannins can be extracted for use as a raw material for resins.

The rest of the fibre can be used to produce sugar, which serves as a basic ingredient in infermentation products and might even be used for material applications.

Commonly, hot water extraction is used to extract tannins from the bark. The yield from this is determined by the species of tree, origin and processing history of the raw material. With a yield of, at most, 10% of bark weight from Scandinavian spruce and pine, there are possibilities for better use as much of the bark tannin and carbohydrates have stayed untouched.

Dr Sami Alakurtti, Senior Scientist of chemical synthesis and polymerisation technologies at VTT, told Materials World, ‘Currently, softwood bark is combusted for energy at pulp and saw mills. However, bark contains around 50% carbohydrates and 40% tannin and lignin, which could be used as valuable feedstock.’

The new method uses higher alkaline conditions and temperature than the original extraction method. ‘The developed method for bark fractionation is close to typical soda or kraft cooking of wood. So, high alkaline and pH content is used at high temperature to yield bark black liquor and bark pulp. In the process, tannins may be recovered from black liquor after acidic precipitation in 30% yield. These tannins are highly reactive towards cross-linkers, such as formaldehyde, and they can replace fossil-based phenol used in phenolic resins,’ Alakurtti explained. According to Alakurtti, the main applications for phenolic resins are plywood, laminates and particleboard, while bark pulp which yields about 40% of bark can be used in several ways, such as being hydrolysed enzymatically to form glucose hydrolysate. Glucose hydrolysate can be further fermented to ethanol or other chemical building blocks.

‘Another possibility is to use bark pulp to produce dissolving pulp after bleaching or use it such as reinforcement fiber in composites. Overall 60-70% of bark can be upgraded to higher value products, thus generating much more value for the bark,’ Alakurtti said.

Developing the process has so far been conducted in REHAP, an EU project.

Alakurtti said, ‘In brief, REHAP aims to strengthen the European bio-economy industry by creating novel materials from agricultural and forestry waste, and considering how they can be used commercially in the green building sector,’

The team is now looking for industrial partners in the value chain, such as raw material owners, process equipment suppliers and relevant application companies, so they can develop the concept further to reach a technology readiness level of 6-7.

Future process

Going forward, the researchers have already filed a patent application for the concept and are looking to undertake more research to optimise the process.

‘The process has been demonstrated in 100kg scale of bark tannin and pulp. However, more research and development is required to optimise the cooking process and evaluate tannin and bark pulp in more detail in the applications. Furthermore, more detailed concept design and techno-economic assessment are needed,’ Alakurtti said.

The properties of bark pulp and its applicability for fibre-based materials or cellulose derivatives, depending on the bark raw material and processing details, is one element the team would like to study in more detail, as well as the possibility of recovering several minor components from bark, like extractives and inorganic components.