2.5 Kraft pulping problem with increased calcium content in Eucalyptus dunni

Eucalyptus dunni, cultivated in plantations in various locations in Uruguay, have shown slow delignification during kraft pulping. The investigations in the both lab scale and industrial scale have suggested that the calcium plays an important role with this phenomenon. The calcium in wood appears to be present in the form of calcium oxalate crystals in the lumen. From previous studies, it is known that depending on the growth site, the calcium content in wood varies highly, and in some cases, pulping is difficult. In our study, the problem was investigated using wood with different calcium contents. Conventional kraft cooking experiments on wood were performed and supported with preliminary characterisation of wood and obtained pulps. The higher calcium content in wood resulted in higher yield losses in kraft pulping and increased degradation cellulose. A wood chemical analysis of the wood samples indicated that the only detectable significant difference between the sample was the calcium content. The higher pulp yield and pulp viscosity losses for wood with a higher calcium content is most likely explained by increasing alkaline hydrolysis, and the lower delignification rate might be explained by interaction of calcium with lignin, or hydrogen sulfide ions