Extractives in unbleached birch kraft pulp and filtrate : Focus on analytical method development

Abstract

Extractives cause various problems in pulp and paper mills, so the removal of extractives from the process is desired. To assess the efficiency of the removal of extractives, accurate analysis techniques need to be employed, and the analyses should also preferably be fast and easy to perform. Calcium causes additional problems because it decreases the washing efficiency. This work aimed at finding the best analytical method for unbleached birch kraft pulp and wash filtrate, especially at high calcium levels. This was done by performing several analysis procedures on mill samples, and from these methods the three most promising methods for pulps and the three most promising methods for wash filtrates were chosen for further analysis on laboratory samples cooked specifically for this work.

Six laboratory cooks were made, three with added betulinol and fatty acids to the cooks. Three different calcium contents were used; low with acid-soaked chips, normal with water-soaked chips, and high with water-soaked chips and added calcium. The three cooks without added extractives and the three cooks with added extractives had different contents of calcium. The raw materials were analysed, as were all the fractions from the cooks, i.e., pulp, black liquor, and wash filtrate. This resulted in a complete picture of the extractives, i.e., if everything that was added to the process could also be detected after the process, and in which fractions the different extractives were detected.

With low calcium content, most extractives were found in the wash filtrate. When the calcium content was increased, more extractives were found in the black liquor, and at high calcium content, more extractives were also found in the pulp, showing that calcium decreased the washing efficiency. There were some discrepancies between the detected extractives before and after the cooks.

Regarding the recommendations of analysis methods, the most accurate, fast, and easy method for pulp was ASE with a solvent mixture of acetone, water, and acetic acid in the ratio 95:4:1, and for the wash filtrate the most accurate, fast and easy method was acidification of the wash filtrate to pH 3.5, followed by liquid-liquid extraction with MTBE. The extracts should then be silylated and analysed with GC.