Brightness reversion of hardwood and softwood kraft pulps during bleaching

Brightness reversion may occur if the bleaching chemicals become exhausted part way in the tower. When this happens, pulp brightness may drop significantly as the pulp remains at high temperature until it exits the tower. This phenomenon has been observed with most bleaching chemicals but few, if any, attempts have been made to quantify its extent as a function of bleaching operating conditions. We used experimental data from softwood and hardwood pulps to derive empirical equations on the effect of tower residence time, kappa factor, and temperature on the maximum achievable brightness, reversion time, and brightness reversion for different bleaching sequences. Bleach plant engineers and operators can use the equations to calculate the maximum brightness achievable and the time at which brightness reversion occurs. Application: This report will help mill staff better understand brightness reversion and improve bleach plant operations.