QUICK NON-DESTRUCTIVE ANALYSIS OF LIGNIN CONDENSATION AND PRECIPITATION BY FTIR

Kraft mills with conventional batch cooking systems sometimes encounter problems related to lignin condensation and its subsequent precipitation. Precipitated lignin increases bleaching chemical consumption and has an adverse effect on pulp quality. Pulp mills must address this issue by improving their digester operation, but it is also important to rapidly analyze and quantify the condensed lignin in brownstock pulp. This paper introduces and discusses a quick method based on Fourier transform infrared (FTIR) spectroscopy to study lignin condensation and precipitation from brownstock pulp. The samples represented two different cooking results of Eucalyptus camaldulensis as feedstock: no lignin precipitation on fibers, and apparent condensed approximately lignin precipitation on fibers. The infrared spectra revealed that the sample with condensed lignin had four times more lignin on the fiber surface than the sample without lignin precipitation. Scanning electron microscopy (SEM) images of the fiber surfaces confirmed the lignin precipitation on the fibers. This study therefore proved that the extent of condensed lignin precipitation can be successfully measured using FTIR spectroscopy.