Addition of Magnesium Hydroxide and Oxygen to D1 Stage Bleaching, 2009 TAPPI Engineering, Pulping, Environmental Conference

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Most commercial bleaching processes appear to selectively degrade phenolic hydroxyl groups (PhOH) in kraft residual lignin (i.e. lignin trapped in or bound to the fibers). After O₂ delignification, approximately 80% of the phenylpropane (C₉) units in both softwood and hardwood kraft residual lignin are non-phenolic. These units can consume many ClO₂ oxidizing equivalents without generating much carboxylic acid (COOH) end groups. In this investigation we added O₂ as a co-oxidant to ClO₂ under D₁ and D₂ stage brightening conditions. It was observed that the ClO₂/O₂ combination (D/O stage) afforded higher brightness than ClO₂ alone. This was observed for D₁ stage brightening of hardwood and softwood kraft pulps with elevated kappa numbers (>6.5) and with conventionally bleached hardwood kraft pulp (OD_OE_P) with kappa number 2.6. Oxygen addition also afforded higher brightness and less brightness reversion for D2 bleaching of a eucalyptus kraft pulp with 85% brightness after the OD_OE_OPD₁E₂ stages. It was further observed that Mg(OH)₂ was a superior buffer to NaOH for D₁ stages with or without oxygen addition.