IMPACT OF ACID WASHING AND CHELATION ON Mg(OH)2- BASED HYDROGEN PEROXIDE BLEACHING OF MIXED HARDWOODS CMP AT A HIGH CONSISTENCY

The removal of transition metal ions is crucial for improving the efficiency of subsequent peroxide bleaching. Acid-washing and chelation have been proposed for such a purpose. However, their influences on the Mg(OH)2-based peroxide bleaching of hardwood pulps at a high consistency have not been well documented in the literature. In this work, we studied the influence of acid-washing using sulfuric acid or chelation using diethylenetriaminepentaacetic (DTPA) on the Mg(OH)2or NaOHbased hydrogen peroxide bleaching efficiency, effluent properties of bleaching filtrates, and paper properties. The results showed that for Mg(OH)2-based peroxide bleaching, the pulp yield and water retention value of acid-washed pulp were higher than those of the chelated pulp; the chemical oxygen demand (COD) and turbidity of the bleaching filtrates for the acid-washed pulp were lower than those of the chelated pulp. The bleached acid-washed pulp had lower strength properties than bleached chelated pulp did. Additionally, at a high pulp consistency (25%), the Mg(OH)2-based process had a higher bleaching efficiency and superior bleaching effluent properties, but a lower strength properties, in comparison with the NaOH-based process.