A Partial Magnesium Hydroxide Substitution for Sodium Hydroxide in Peroxide Bleaching of an Aspen CTMP

A partial substitution of sodium hydroxide (NaOH) by magnesium hydroxide (Mg(OH)2) as the alkali source was studied in peroxide bleaching of an aspen chemi-thermomechanical pulp (CTMP). The effects on pulp properties, including pulp strength, optical property and carboxylic groups content, and process characteristics, such as oxalate and anionic trash formation, were studied. The X-ray photoelectron spectroscopy (XPS) technique was used to determine the lignin and extractives concentrations on the fiber surface. With the increased Mg(OH)2 substitution ratio, the peroxide bleaching process gave a higher pulp yield and peroxide bleaching efficiency, produced pulps with higher light scattering coefficient and bulk, while the total oxalate, anionic trash, and chemical oxygen demand load (COD) were less. The total carboxylic groups content and the surface charge of pulp fibers were decreased. The underlying mechanism responsible for the decreased strength properties, when the Mg(OH)2 substitution ratio was increased, is discussed.