Investigation of pitch deposits throughout the fiber line of softwood pulp mill

Abstract

The formation of detrimental pitch deposits can cause various problems at pulp and paper mills, as they can contribute to unscheduled shutdowns and even paralyze the operation of the entire mills. To understand, avoid and control of pitch deposits formation in pulp mills, we investigated the overall chemical composition of 16 pitch deposit samples collected from the fiber line of a softwood kraft pulp mill. Sodium salts were tentatively speculated as the dominant fractions of the pitch deposits in addition to the minor fractions of the extractives and carbohydrates. Low molar mass extractives from dichloromethane, acetone and water extracts were analysed by gas chromatography–mass spectrometry. The extractable fraction is chemically heterogeneous, representing the degradation products of carbohydrates, lignin, and woody extractives. Furthermore, new chlorinated stilbenes (monochloropinosylvin, dichloropinosylvin, trichloropinosylvin dimethyl ether, and dichlorinated pinosylvin monomethyl ether) and other chlorinated components (chloro-manoyl/epimanoyl oxide, 12- and 14-chlorodehydroabietic acids) were for the first time identified as part of pitch deposits in the fiber line. Moreover, significant amount of 3-hydroxy fatty acids was found from the drying stage, also representing novel pulp mill deposit compounds. Pinostilbene and pterostilbene are rarely identified from the industrial woody chips of Scotch pine. Overall, the investigation of extractive profile from pitch deposits may provide us with a new perspective to understand the formation mechanism of pitch deposits and study the binding behavior between the extractive and surfaces of the process equipment. The new findings may provide guidance for the pulp mills to design optimal strategies to avoid and control pitch deposits.