Peroxymonosulfuric Acid Treatment as an Alternative to Ozone for Totally Chlorine-free and Elementary Chlorine-free Bleaching of Hardwoods Prehydrolysis-kraft Pulp

This study is focused on improving totally chlorine-free（TCF）and elementary chlorine-free（ECF）bleaching by using peroxymonosulfuric acid（Psa）instead of ozone（Z）. Bleaching was conducted by a combination of oxygen（O）with Psa, followed by alkali extraction with hydrogen peroxide（Ep）and chlorine dioxide（D）stages in accordance with proposed sequences O-Psa-Ep-Psa-Ep and O-Psa-Ep-D. Increments in pulp consistency during the O stage and an addition of MgSO4 during the Ep stage were performed to improve pulp brightness and viscosity. The O-Psa-Ep-Psa-Ep sequence was compared with the O-Psa-Z-Ep sequence, and 87.3-88.3％ ISO brightness was achieved by TCF bleaching corresponding to the O-Psa-Ep-Psa-Ep sequence. However, the viscosity obtained after the final stage of the TCF bleaching was 5.1-6.0cP, while the O-Psa-Z-Ep sequence resulted in an 83.1％ ISO brightness and viscosity of 5.4cP. It was difficult to reach brightness above 88％ ISO with viscosities greater than 6.2cP, which are the minimum standard levels for dissolving pulp, using the TCF bleaching sequence. Further studies with better qualities of raw materials are required to investigate this possibility. Meanwhile, brightness 88.1％ ISO and viscosity 7.3cP were achieved by the ECF bleaching corresponding to the O-Psa-Ep-D sequence with only a 0.5％ ClO2 dosage, which suggests a potential reduction in the usage of chlorine-containing chemicals during the ECF bleaching.