Characterization of organic deposits produced in the kraft pulping of Eucalyptus globulus wood

Abstract

The composition of the organic deposits (the so-called pitch deposits) accumulated in different parts of the mills during the kraft pulping of Eucalyptus globulus wood using an ECF (elementary chlorine free) bleaching sequence has been studied. Three pitch deposits were selected in the bleaching sequence. The first one was taken just after the kraft pulping and the oxygen prebleaching step, while the other two were taken from different parts of the mill after the chlorine dioxide bleaching step. The pitch deposits were Soxhlet extracted with acetone, and the extracts were redissolved in chloroform and subsequently analyzed by gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). The composition of E. globulus wood extractives was also analyzed for comparison. High temperature short capillary columns were used for the GC and GC–MS analyses, which enabled the elution and separation of compounds with a wide range of molecular mass, with no prior derivatization nor fractionation. Sterols and sterol esters were major compounds in the lipidic extractives of E. globulus wood, and hence, contribute to pitch deposition during pulping. The composition of the pitch deposits varied along the bleaching sequence. The kraft cooking and oxygen prebleaching had a minor influence on the composition of the extract of the pitch deposits. These extracts were very similar to those of E. globulus wood extractives, but with a higher content of waxes. No structural changes of sterols and sterol esters were observed. In contrast, after chlorine dioxide bleaching (ECF), the deposits were composed of saturated sterols and sterol esters, with the remarkable absence of unsaturated sterols and sterol esters, mainly the β-sitosterol and β-sitosterol esters, which were completely degraded. The insoluble residues left after acetone extraction of the pitch deposits were analyzed by Curie-point flash pyrolysis–GC–MS and by pyrolysis-methylation–GC–MS. Whereas conventional pyrolysis released series of n-alkanes/ n-alkenes, a series of fatty acid methyl esters were released after pyrolysis-methylation, suggesting that the residues left after acetone extraction might be made up of fatty acids salts. Very minor amounts of ellagic acid were also found in all the pitch deposits, probably deposited as magnesium complex.