METAL MANAGEMENT IMPROVES PEROXIDE BLEACHING OF TMP

Several pretreatments of a commercial thermomechanical pulp (TMP) were carried out to achieve improved brightening and reduce peroxide decom position by sequestering metal ions with a series of chelants prior to bleaching with hydrogen peroxide. Pulp consistency as well as pretrea tment temperature and time have no effect on the chelation of manganes e within the experimental scope of this study. Diethylenetriaminepenta methylene-phosphonic acid (DTPMPA) shows best chelation of manganese i n a pH ranging from 7 to 10. Among all tested chelants, hydroxyethylen ediaminetetraacetic acid (HEDTA) is an excellent chelant in the pH ran ge from 5 to 7. Chelation effect of TMP with dibenzo-18, crown-6 ether is poor. Thus, a HEDTA pretreatment followed by hydrogen peroxide ble aching of TMP gives about an additional two-point ISO brightness gain over conventional DTPA pretreated pulp under the same conditions. For nonpretreated TMP, DTPMPA is the best chelant to be added in the bleac h liquor. A combined effect of HEDTA pretreatment at pH 6 and peroxide bleaching with addition of DTPMPA in the bleach liquor yields the bes t brightness gain coupled with the lowest peroxide consumption.