Investigation of Pulp Fractionation and its Influence on the Production of Dissolving Grade Pulp

Description

Wood represents one of the key renewable resources in Austria and its efficient and sustainable utilization across the whole value chain is one of the major requirements for the transformation towards a renewable bioeconomy. Regarding pulp, one of the main products of the wood-value chain, it is an important goal of research to improve current production processes in terms of overall yield and ecological compatibility.
One of the purification steps in pulp production – alkali extraction, which targets the removal of short chain alkali-soluble cellulose fragments and hemicellulose – necessitates high chemical charge and elaborate process efforts for the treatment of the bleaching effluents. In this study we demonstrate that the rarely utilized mechanical fiber fractionation, which comprises the removal of the short fibers (fines) before the alkali extraction, is an economically feasible option to design the extraction stage more efficiently and environmentally friendlier.
Fiber fractionation trials were performed using various screening parameters (flow rates, feed consistencies and reject rates) and equipment parameters (screen basket designs) to determine the conditions for the best separation efficiency. The influence of the hole diameter on the separation efficiency has been explored in the past, while the separate consideration of hole diameter and hole spacing was often neglected. With the usage of different screen basket designs – same hole diameter but different hole spacing and same hole spacing but different hole diameter – the separate influence of hole diameter and hole spacing on the fractionation performance was investigated. As expected, the hole spacing has a certain influence on the separation efficiency, but the hole diameter is the equipment parameter which decisively influences the fractionation performance. Production parameters are always a compromise between separation efficiency and drivability/clogging and, above all, throughput. Therefore, a lot of attention was paid to comparing the production parameters required for a certain improvement in quality in terms of throughput.
While running the strictest screening conditions – low reject rate and high feed consistency – we identified that the length weighted fines content was reduced more than 50% via one stage fiber fractionation. In theory, this should correspond to a replacement of the chemical charge and improve the pulp quality accordingly. This assumption shall be confirmed by separate laboratory extraction trials. Further laboratory trials shall allow the rough estimation of chemical savings as well as quality improvements in the alkali extraction through fiber fractionation.
Fiber fractionation shall offer two main advantages compared to the established extraction approach. First, the ecological footprint will be reduced by the adaption of the alkali extraction and second the overall production yield shall be increased as the fines are separated but not chemically destructed and thus can be used separately. Further investigations on the following process steps – dewatering & drying of the fine fraction – with respect to application routes are planned.

Period	25 May 2023
Event title	Paper & Biorefinery Conference 2023: Shine a Light on Energy Solutions
Event type	Conference
Location	Graz, AustriaShow on map
Degree of Recognition	International