Peracetic Acid in Low AOX and High Brightness Pulp Production.

The effects of chelating agents, peracetic acid and alkaline peroxide on the replacement of chlorine dioxide and hypochlorite in D-Eo-H-D bleaching of oxygen delignified hardwood kraft pulp were studied in a laboratory scale. Since an increased use of peroxygen chemicals necessitates a use of a chelating agent, both DTPA and a new chelating agent with low nitrogen content (DEAS) were tested. Peracetic acid was used here as a bleaching agent in combination with chlorine dioxide in a D2 stage. Also the effect of peracetic acid on post-bleaching of O-D-EOP-D bleached softwood kraft pulp was studied.The bleaching experiments indicate that the importance of a chelation prior to Eo stage increases with the decreasing active chlorine charge in the D1 stage and increasing peroxide charge in the E. stage. With a sequence of Q/D-EOP-Q/D/PAA the total active chlorine charge in D-Eo-H-D dropped from 48 kg act. C1/tp to 20 kg act. C1/tp by using 10 kg H2O2/tp in the E0 stage and 5 kg peracetic acid/tp in the D2 stage. Also 8 kg hypochlorite/ tp were eliminated at the same time. The use of DEAS in the Q/D stage gave more improved pulp properties for bleached pulp than the use of DTPA correspondingly.