Alternative technique for evaluating yield loss in modern bleach plants

Abstract

In this study an alternative technique is presented and discussed for measuring yield loss based on the analysis of total organic carbon (TOC) in bleaching filtrates. Oxygen predelignified eucalyptus kraft pulp was subjected to the DHT(EP)D, D(EP)DD and D(EP)DP bleaching sequences and the gravimetric yield of each stage was determined, with 10 replicates. Then the filtrates were analyzed for TOC and COD. Correlations were established among the three measurement methods (gravimetric, COD and TOC). Yield losses in the DHT stage were more accentuated than in the D0 stage, these losses being recovered in subsequent stages of the bleaching sequence. Yield loss decreased with each advancing stage of the bleaching sequences (D0,DHT>(EP)>D1>P>D2). Yield loss increased with higher kappa factors in the first bleaching stage and with higher temperature in the (EP) stage. The final P stage resulted in greater yield losses than the final D stage. Yield loss intensifies with gain in brightness from bleaching. Yield losses as measured by the TOC method revealed a narrower range of variation in relation to the gravimetric and COD methods, and the linear equation derived for the TOC vs. gravimetric yield relationship provided the best fit (R2=97.27%). Overall yield losses throughout were in the range of 4-5%, depending on the bleaching sequence and operating conditions.