Effect of Acid Leaching of Eucalyptus Wood on Kraft Pulping and Pulp Bleachability

Abstract

This work evaluated the impact of chip acid leaching prior to kraft cooking on pulp yield bleachability and strength properties. Optimal leaching time, temperature, and pH (3 h, 70°C, and pH 2.0) were used to leach wood chips prior to kraft cooking, followed by oxygen delignification, ECF, and light-ECF bleaching with the sequences AZDP (0.19% ClO₂), AZDP (0.76% ClO₂), and A/Z/EDP (1.06% ClO₂). Acid leaching significantly reduced the metals content of the wood chips. Time and pH greatly influenced the metals removal rate, whereas temperature did not. Acid treatment had no effect on cooking yield but improved delignification rate. The pulp from acid leached chips showed improved performance during oxygen delignification and bleaching over the reference pulp. For light-ECF bleaching, chip leaching resulted in H₂O₂ and NaOH savings of 18 and 14.5 kg/t pulp, respectively. No significant differences in strength properties between the reference and leached pulps were observed.

Keywords:

• Acid leaching
• eucalyptus
• kraft pulping
• metals
• pulp properties

Notes

∗ Adjusted to desirable end pH.

∗∗ Adjusted to desirable final brightness.

∗ Total active chlorine: 2.63∗(ClO₂, kg/t) + 4.44∗(O₃, kg/t) + 2.09∗(H₂O₂, kg/t).