Anionic Trash in Mechanical Pulping Systems, 1999 International Mechanical Pulping Conference Proceedings

Peter Bräuer, Johannes Kappel & Manfred Holler

Andritz AG

Stephen Munro

Andritz Inc.

Detrimental substances, generally referred to as "anionic trash", are a major source of reduced paper machine runability. The problems in all production stages become even more severe if the water loops are closed to a large extent and these impurities build up in the different process steps.

A laboratory study has shown that the bleaching reaction accounts for approx. 80 – 90% of the anionic trash released during alkaline peroxide bleaching. Sodium silicate accounts for the remaining 10 – 20% of the detrimental substances. The different measuring methods like COD, cationic demand and conductivity show a good correlation and in many cases the measurement of one or two values is sufficient for process control in a mill.

In many mills cationic papermaking chemicals are recirculated to the mechanical pulping process in the filtrate loops. If these chemicals are added to the pulp prior to bleaching they may have a negative influence on the bleachability of the pulp. If the pulp is dewatered to high consistency before peroxide bleaching, detrimental substances are removed from the pulp and bleachability is improved.

Plant measurements have shown that the COD and anionic trash accumulate in the filtrate loops and that the level of anionic trash in a mechanical pulp mill may be 50 – 100 times higher than in a paper machine filtrate. The filtrate that has the highest concentration of COD and cationic demand has to be led to the effluent.