Oxidized white liquor as an alkali source in kraft hardwood and softwood bleaching sequences

Increasing environmental regulations for today's sulfate mills have resulted in an increase of oxygen delignification as well as the elimination of elemental chlorine and reduction of chlorine-based bleaching sequences. As a result, many mills are faced with a chemical imbalance. The use of oxidized white liquor as an alkali source in bleaching sequences can serve to maintain a chlor-alkali balance while at the same time utilize any excess white liquor. If the oxidized white liquor is of sufficiently high quality (i.e. no sodium sulfide and limited thiosulfate present), it can be used as an alkali source in any bleaching stage which would normally require sodium hydroxide. The oxidized white liquor used in this study (Q-OWL(TM)) was prepared at either a 3 gpm pilot plant located at a kraft mill using a stream of the mill's white liquor or at a 50 gpm plant trial unit which operated at a different kraft mill. Analysis of the oxidized white liquor produced by both units gave results indicating no sodium sulfide present and greater than 95% conversion to sodium sulfate. The oxidized white liquor was used in laboratory bleaching in O, E-O, E-OP, E-2, and high temperature peroxide stages of kraft hardwood and softwood pulps. The OWL was compared to both NaOH and unoxidized white liquor as alkali sources. Additionally, the OWL produced with the 50 gpm plant trial unit was used in the mill's E-2 stage. In all cases, the Q-OWL(TM) resulted in equivalent or higher brightness and viscosity values than obtained using caustic and both the OWLand NaOH gave better results than those obtained using white liquor.