Effect of Magnesium Dihydroxide on Bleaching of Thermomechanical Pulp Made from Pinus densiflora

Abstract

The conventional NaOH-based peroxide bleaching process has been regarded as an effective option in increasing pulp brightness for thermomechanical pulp (TMP). However, as a strong base, NaOH can cause the dissolution of hemicelluloses into the bleaching effluents and is responsible for the increased COD load and the formation of a significant amount of anionic trash. In recognition of several drawbacks associated with the NaOH-based peroxide bleaching process, magnesium dihydroxide as a weak alkaline source was used to replace sodium hydroxide used for peroxide bleaching of TMP made from domestic pine, Pinus densiflora . Brightness gain of BTMP with Mg(OH)₂ as the sole alkali source was comparable to the NaOH-based bleaching process. The Mg(OH)₂-based bleaching generated less anionic trash as well as lower COD in the bleaching effluents. In conclusion, the change of alkali to Mg(OH)₂ may well prove to be an efficient solution to bleach TMP from the softwood without sacrificing brightness.