Oxygen Delignification Performance, Caustic Soda vs. Unoxidized White Liquor – A Mill Study, 2018 PEERS

Pulp mills commonly replace caustic soda with unoxidized or oxidized white liquor as the alkali source in oxygen delignification. The impacts of using white liquor in oxygen delignification have been previously studied but the topic is still somewhat controversial. Accordingly, the present work investigated the impacts of using unoxidized white liquor as a partial alkali source in the oxygen delignification stage of a Brazilian eucalyptus pulp mill. 

Mill data was used to compare oxygen delignification performance between the use of caustic soda as the only alkali source (i.e., 0% WL) and the replacement of 20% and 30% of caustic with unoxidized white liquor (WL). Data analysis indicated that the use of 30% white liquor decreased delignification performance by 4.5% while the use of 20% WL performed equally to solely using caustic. 

Oxygen consumption in the oxygen stage reactors was estimated based on energy balances and literature review. The values were used in mass transfer simulations to further investigate the performance results. Simulations revealed a decrease in the dissolved oxygen concentration when using white liquor. The decrease is likely related to the oxygen consumption rate and gas-liquid mass transfer coefficient. Dissolved oxygen concentration appears to correlate well with actual mill performance.

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