Changes in a eucalyptus kraft pulp during a mild acid treatment at high temperature

Authors: Leonardo Clavijo, María Noel Cabrera, Susanna Kuitunen, Sirje Liukko,Tiina Rauhala, Tapani Vuorinen

Modern eucalyptus kraft pulp bleaching includes a hot acid step
for removal of hexenuronic acid. This step is typically run at pH
3.0-3.5 at 90-95ºC with a delay of 2 to 4 hours. In this study the
target was to find out if similar or better results could be obtained
at higher temperatures with a shorter delay. A commercial oxygen
delignified eucalyptus kraft pulp was treated at pH 3.0-3.2 at 80,
100, 120 and 140ºC for varying times. The decrease in kappa
number could be modeled with a single component first order
kinetics. The correlation between kappa number and 2-furoic acid
content of the filtrates was significant (R2=0.9969). The difference
UV-visible reflectance absorption spectra (Δ(k/s)) were similar
shape showing mostly decrease in content of hexenuronic acid.
At the harshest condition, the baseline of the difference spectra
at >270 nm was shifted upwards indicating partial degradation of
lignin. UV Raman measurements showed that the residual aromatic
(lignin) content of the pulp was reduced by one third under the
most extreme conditions. Quite surprisingly, the selectivity of the
treatment was almost independent of temperature. This study
gives additional information for designing mild acid stages for
eucalyptus kraft pulp bleaching lines. The treatment could be
carried out at a temperature much higher than applied today
without any significant loss in the quality of the pulp.