Impact of Ozone and Oxygen, 1999 Pulping Conference Proceedings

Patricia Hurter, Ph.D.

Hurter Associates

Two fiber line changes are discussed: a modification from a chlorine to an ozone bleached sequence on a softwood pulp, and the change from chlorine to oxygen bleaching on a hardwood pulp.

Standard handsheet tests comparing the quality of the ozone and chlorine bleached softwoods showed no shift in the tear tensile curve after the new ozone process was introduced. Despite this laboratory data, mill perception was that the ozone softwood had significantly inferior quality. A closer examination of the laboratory data showed that in fact the tensile freeness relationship had shifted significantly, and that the new ozone pulp showed lower tensile strength. Further studies also showed that using a standard Valley Beater or PFI mill to refine this ozone pulp did not give a realistic picture of mill performance. A pilot refining study investigating the effect of refining intensity on this ozone softwood showed high levels of fiber cutting at normal mill intensities. Subsequent to this work, new lower intensity refiners were installed at the mill, which improved the refined ozone softwood pulp quality significantly. The poor performance of this pulp appeared to result from a low fully bleached pulp viscosity, rather than anything inherent in the ozone process itself.

The second case presented here is a comparison of the pulp quality of two hardwood bleach lines, one an oxygen bleached hardwood pulp (OC/DED) and the other a chlorine bleached hardwood pulp (CEDED). These two fiber lines had a number of differences apart from the bleach sequence (digesters and wood source for example) resulting in different final pulp viscosity, and a different fiber curl index. A study was done to determine whether the differences in pulp quality between the two lines could be explained by these known differences in fiber and pulp characteristics, or whether the bleach sequence itself was an additional factor. A factorial refining trial was executed, with varying levels of fiber curl and pulp viscosity, with pulp samples from the two hardwood bleach lines. This study showed that the papermaking properties, such as tensile strength, could be predicted from refining energy, pulp viscosity, and fiber curl, without introducing any other factors to account for unknown differences between the bleach lines.