H2O2 bleaching of mechanical pulps.: Part III:: Thermodynamics

The thermodynamics of H2O2 bleaching was investigated using a mechanistic model and mathematical expressions developed in the earlier parts of this series. The model suggests the thermodynamic limit of any given peroxide bleaching is determined by the extent to which the active chromophore sites, or cages, on the fibre wall can be reached by the bleaching chemicals. The mechanism and expressions are able to explain the thermodynamic effects of H2O2 concentration, pH and temperature observed in the present investigation and those commonly observed in industrial practice. The effects were found within a certain range, to be positive - the higher the variable, the higher the thermodynamic limit. All variables had their limitations in improving the thermodynamics, or the brightness ceiling. Because of this, as shown in this investigation, a multistage bleaching system appears to have the potential to reach a higher brightness ceiling than single-stage bleaching.