Drying section deposits : origin identification and influence of the recycling processes - deinking and packaging lines comparison

Mills producing papers (particularly packaging papers) made from recycled fibres very often face an important formation of organic deposits on various equipment of the paper machine (formation wire, presses, drying section cylinders and wires...). Moreover, many problems are connected to these deposit formations: breaks, paper defects (presence of spots, holes...), and reduction of the drying section efficiency due to the felts clogging... As a consequence, these phenomena cause many shutdowns of the machine for the cleaning of the clogged equipment. Based on solvent extract measurements. a study has been conducted at CTP with the aim: - To identify the main deposit potential source (in terms of quantity) among the various stickies species identified (macro, micro and colloidal stickies). - To evaluate the influence of different recycling lines (mainly packaging and deinking) on the removal of the main deposit potential source(s). The results of this study have shown that: - The major source of extractible materials is contained in the fine elements fraction. This means that the micro-stickies certainly represent the main stickies species involved in the deposit formation, which takes place in the paper machine drying section. - The recycling lines used in packaging mills do not induce a selective removal of these fine extractible materials. Consequently the rate of extractible materials, that is to say the deposit potential, contained in the pulp, is directly function of the fine elements proportion of the pulp. - On the contrary, deinking lines enable to selectively remove these detrimental substances contained in the fine elements fraction.This is mainly the consequence of both the flotation and the thickening stage (thickening which induces a washing effect) performed in the first flotation loop. The result is 70 % reduction of the pulp extractible materials content. Finally, these results show that the only way to reduce the deposit potential contained in the pulp, is to turn to physico-chemical treatments. Indeed, on the one hand, in deinking, flotation appears efficient to remove deposit potential. On the other hand, the effect of thickening (more unexpected) is due to the washing effect which takes place during this step and which induces the passage of part of the fine extractible materials in the washing filtrates (disc filter and screw press). Nevertheless, this washing effect would not be sufficient if no physico-chemical treatment (dissolved air flotation) of the filtrates was performed which enables to remove the fine elements fraction (containing a high proportion of deposit potential).