Using Mg(OH)2 in peroxide bleaching of wheat straw soda-AQ pulp

Abstract

The peroxide bleaching of high yield pulps from wood with Mg(OH)2 has been developing recently in the pulp and paper industry. However, there is still a lack of data on the application of Mg(OH)2 in peroxide bleaching of non-wood fibres. In this work, our purpose was to study the effect of Mg(OH)2 on peroxide bleaching of wheat straw soda-AQ pulp. The results showed that Mg(OH)2 significantly improved peroxide bleaching efficiency (expressed as the ratio between the brightness gain and the H2O2 consumption) and selectivity (expressed as the ratio between the brightness gain and the viscosity losses) of wheat straw soda-AQ pulp. The brightness, viscosity, and yield of bleached pulp can be significantly enhanced by increasing the replacement ratio of Mg(OH)2. However, at 100% replacement of NaOH with Mg(OH)2, the brightness of bleached pulp was much lower than that of the bleached pulp with NaOH as the sole alkaline source. When 24 to 73% of the NaOH was replaced with Mg(OH)2, the COD of the bleaching filtrate was 11 to 38% lower than that of the NaOH as the sole alkaline source. The lower solubility and alkalinity of Mg(OH)2, as well as the reduction of Cu ion content in bleached pulp were proposed as accounting for the favorable effect of Mg(OH)2 on peroxide bleaching of wheat straw soda-AQ pulp.