Dry Pulping – A waterless preparation process for difficult-to-recycle paper and board products
RPM and Dresden University of Technology are searching for innovative and more energy-efficient solutions for the most energy-intensive sub-processes in recovered paper stock preparation. Having been developed over decades it is highly unlikely that simply further developing such technologies will provide the energy savings required to achieve the objectives of CEPI’s roadmap 2050. What is needed instead are entirely new processes of a superior energy efficiency.
One of the main reasons for the inadequate energy efficiency of many stock preparation processes are low consistencies and the accompanying large amounts of water which have to be transported and thus render the processes uneconomical. This also applies to the defibration process at the beginning of any stock preparation line. Therefore, in a research project called “dry defibration” we examined in how far and for which qualities waterless processes are able to defibre recovered paper in a more energy efficient way.
The defibration takes place with an aggregate which was originally developed for the gentle fine grinding of temperature-sensitive and difficult to be ground products. This aggregate is generally classified as a mechanical airflow impact mill with a peripheral grinding track. The grinding stock is comminuted due to high-impact collisions with the walls or special installations of the grinding chamber induced by a highly turbulent air flow as well as by the shear forces acting in the grinding gap (fig. 1).
Figure 1: Scheme of the aggregate and forces acting in the grinding gap
Numerous dry defibration trials, however, showed that not only standard grades of recovered paper can be broken down into individual fibers, but – even more importantly – also difficult-to-recycle products such as board cores, release papers, composite materials such as beverage board or wet-strength filter papers. This would allow to turn hitherto entirely or at least partially unused raw materials into secondary sources for paper production. Especially against the background of increasing scarcity of and rising costs for fibre resources, this process might have the potential to become an attractive technology for the industry not only improving their economics but also increasing the security of raw materials supply.
The technology has already been tested successfully in pilot scale and a recently developed test method allows to verify the fitness of the process for the treatment of a broad range of conceivable paper products and to provide a comprehensive and reliable paper technological and energetic assessment.
Figure 2: Dry Pulping aggregate (2) with material inlet (1), cyclone separator (3) and ventilator (4) and product samples (inlet / outlet)
Figure 2 shows the dry defibration aggregate and different specialty paper products before and after dry defibration. The product fineness can be varied according to customer requirements. Both, higher rotor frequencies and longer dwell times in the aggregate would reduce the content of flakes (fig. 3). The remaining flake content could also be reduced directly after dry defibration, e.g. by air classification with subsequent reintroduction of the reject to the process chain. Air classification can also be used to remove unwanted components. After dry defibration the dry fiber stock can be added to the pulper at the beginning of the conventional process chain. This post-treatment of the dry fiber stock with a suitable wet process also leads to a significant reduction of the flake content. Flakes not disintegrated after dry and wet pulping are eventually discharged with the help of standard separation processes.
Figure 3: Reduction of flake content by increasing rotor frequencies and post-treatment with wet process
The dry pulping process presented here has the capability to recycle difficult to recycle paper products at a far higher energy-efficiency than standard wet processes. Because of their properties wet strength papers and numerous specialty papers – products which are produced in large quantities – can only be recovered and recycled unsatisfactorily or not at all. Due to the lack of suitable recycling technologies they often become – already after their first use – waste, the disposal of which is very complex and expensive. In many cases, however, they contain high quality fibres. And this motivated us to search for methods which would allow to make such material part of the paper cycle.