Title
Improving the bleaching process of hardwood chemi-mechanical pulp
Authors
FARHAD ZEINALY MEHRAN KARIMI JALAL SHAKHES and HOSEIN MOHAMMADI

Received December 31, 2013
Published Volume 50 Issue 2 February
Keywords bleaching, hydrogen peroxide, sodium hydroxide, magnesium hydroxide, Chemical Oxygen Demand, bleaching effluent

Abstract
In the first phase, the effect of different NaOH to H2O2 ratios on bleaching chemi-mechanical pulp (CMP) was investigated. The pH of unbleached pulp was 5.25 and its brightness and freeness were 37.5 and 745 ml respectively. Results indicated that the brightness increased by growing the peroxide charge, but the increase in the NaOH percentage had inhomogeneous effects. As data showed, the best brightness values were found with 0.75 NaOH/H2O2 ratios. High ratios led to a noticeable upward trend in pulp yellowness, while ratios below 0.75 resulted in lower brightness. In the second phase, the effects of replacing NaOH by Mg(OH)2 were examined. Results showed that this replacement determined an improvement in the optical properties and pulp yield. However, the mechanical properties were similar in both processes. Also, this replacement led to a decrease in the Chemical Oxygen Demand (COD) and conductivity in the bleaching effluent. The residual peroxide in the Mg(OH)2 bleaching process was higher than in the NaOH bleaching process.