New technology for the production of high-purity dissolving pulps

The world's most modern dissolving pulp factory, Bacell S.A., was constructed near Salvador in Brazil in a joint-venture between Klabin, the largest pulp and paper manufacturer in South America, and Lenzing AG. High-quality pulps have been produced there since the beginning of 1996 from locally grown eucalyptus species applying the prehydrolysis Kraft process. The pulping technology used, for which a world-wide patent has been filed as the Visbatch process, represents a joint development between the research groups of Klabin, Lenzing AG and the former pulp group of VAI. The process combines the advantages of displacement technology and steam prehydrolysis and is characterised by short conversion times, low energy requirements, a very homogeneous and high product quality as well as a simple and precise process control. The pulp is bleached according to a very effective four-stage TCF-bleaching sequence, OO-A-ZQ-P, developed at Lenzing. During the two-stage oxygen delignification the main part of the residual lignin is removed and fluctuations in brownstock quality are levelled out. The viscosity and the brightness of the final product are set during subsequent ZQ-P bleaching. The dissolving pulps, known as Solucell, are characterised by a narrow distribution of the molecular weight, greater resistance to degradation reactions and a high and homogeneous reactivity during viscose production which is unusual for prehydrolysis Kraft pulps. The high product quality of the Solucell pulps is also manifested in the superior properties of the viscose fibres made from these pulps. Using this new pulping technology in combination with low-hemicellulose eucalyptus species, acetate pulps with a high level of purity can be produced without subjecting the pulp to expensive cold caustic extraction.