Removal of dissolved and colloidal substances in water from compressive pre-treatment of chips using dissolved air flotation. Pilot trial

Abstract

The main abjectives of this work were to evaluate the extent to which extractives in wood chips for mechanical pulping are released into process water during compressive pre-treatment before refining and to develop a method to remove the released extractives from the process water. In thermomechanical pulping, pre-treatments based on a high compression ratio and high temperature, are used to reduce the amount of extractives in wood chips of different raw materials before refining. One objective of this study was to evaluate the removal of extractives from the wood chips during compressive pre-treatment, in an Impressafiner by determining mass balance of extractives around a mill scale Impressafiner installation. The mass balance showed that it was possible to remove up to 15% of extractives before the refining process. Thus one advantage of using an Impressafiner is that removal of dispersed extractives can be done with little loss of fibres, from a small concentrated water stream. Handling of this water is important in order to avoid that the extractives enter the effluent treatment or reach undesirable levels in the process water. Thus the solubility, colloidal stability and flocculation of the extractives in the pressate water was studied. The objective was to obtain a better understanding of the effects of cationic polyelectrolytes on the stability of colloidal extractives in the pressate water from Impressafiner. A high charge density and low molecular mass polymer (poly-DADMAC), a high molecular mass and low charge density polymer (C-PAM) and combination of the polymers (mass ratio 1:1) were used. It was found that these polymers efficiently flocculate colloidal extractives present in the process water via two different flocculation mechanisms: charge neutralization and bridging flocculation. Extractives which are removed from the process water have to be taken care of in an economical way. They have to be brought into a form making it possible to remove them from the process water before the water is sent to the effluent treatment or before the water is reused in the mill. In that respect, the extent to which the extractives are present in either dissolved, colloidal or bound to fibres is important. In this work, Dissolved Air Flotation (DAF) was used to remove the flocculated extractives from Impressafiner process water. For comparison, laboratory experiments were also conducted on water from chip washer. It was shown in both laboratory and pilot scale that by using DAF in combination with flocculation of the extractives with polyelectrolyte the colloidal and fibre bound extractives could be removed from the process water with a removal efficiency of 80-90%, with equal efficiency for the Impressafiner and chip washer waters. The results attained in the thesis strongly suggest that by using a compressive pretreatment method of the wood chips is possible to remove a substantial fraction of extractives before the refining process. A larger fraction of the more toxic extractive components, resin acids, than of fatty acids were removed. Further, this study emphasizes the importance of the polymer properties for their floccculation efficiency of colloidal extractives present in process water. The results also show that Dissolved Air Flotation is a convenient method to remove flocculated extractives from process water.