E. Isaza Ferro: Catalyzed and non-catalyzed hypochlorous acid bleaching of kraft pulps

Abstract: 

Pulp bleaching is an energy intensive process because removing residual lignin and HexA, while preserving carbohydrates, requires multistage bleaching sequences which operate at high temperatures and have long residence times. Hence, creating faster bleaching technologies could help to decrease the energy consumption of pulp industry.

Catalytic pulp bleaching (Hcat) is a new technology that uses hypochlorous acid (HOCl) and a tertiary amine (DABCO or its derivatives) to selectively oxidize lignin and HexA. Indeed, fully bleached pulp can be obtained in just 1 h using catalytic a bleaching sequence (HcatZP). But there are still several incognitas regarding the efficiency of Hcat stage and its environmental impact. Furthermore, bleaching with HOCl has been discouraged in the past for quality and environmental concerns; even thought HOCl is produced in situ in chlorine dioxide bleaching (D), the most common bleaching stage nowadays.

This dissertation tested several catalytic bleaching sequences and identified that the best position for Hcat is at the start of the bleaching sequence. Here, it is most efficient for delignification, and the produced organic halogen bound to the pulp (OX) can be degraded in posterior ozone (Z) and peroxide (P) bleaching stages, producing fully blechached pulps with industrially acceptable OX levels.

Several DABCO derivated catalysts were tested in Hcat stage, with an alkyl substituted DABCO derivative called CEM DABCO being the most stable with HOCl while forming a highly active chloroamonium cation. In an attempt towards a catalytic D stage, CEM DABCO was used the reaction of HOCl with chlorite ions, which regenerates chlorine dioxide in-situ regeneration in D stage. CEM DABCO catalyzed the reaction and increased chlorine dioxide stability. However, further studies are required for achieving a catalytic D stage.

The role of the catalyst on bleaching with HOCl was assessed in catalyzed (Hcat) and uncatalyzed (Hmild) bleaching by measuring their stoichiometry for lignin and HexA degradation, and their pulps and filtrates properties. The similar properties of the pulps, filtrates, and stoichiometric ratios (for lignin and HexA degradation) confirmed that the catalyst only increased the speed of reactions and did not modify their stoichiometries.

Hence, the bleaching sequence without the catalyst HmildZP was tested producing fully bleached pulps with low levels of OX in the final pulp. The filtrates from Hmild stage were digested to european acceptable levels for liquid emissions from pulp mills.

Overall, pulp bleaching with HOCl is a feasible alternative due to its efficiency, selectivity, and low cost (compared to chlorine dioxide) but its use is limited to low HOCl dosages at the start of bleaching sequences.

Follow the remote defence: https://aalto.zoom.us/j/68539594512

Opponent: Professor Thomas Rosenau, University of Natural Resources and Life Sciences, Vienna (BOKU), Austria 

Supervisor: Professor Tapani Vuorinen,  Aalto University, Department of Bioproducts and Biosystems

Link to electronic thesis: Catalyzed and non-catalyzed hypochlorous acid bleaching of kraft pulps