Strong Pulp Fibres with Hyperalkaline Polysulfide Pretreatment, 2002 Fall Technical Conference

Polysulphide is used as an additive in kraft cooking in order to improve the pulp yield. The increase in yield is usually achieved by retaining a higher amount of hemicellulose thus resulting in pulp with lower tear strength. By addition of polysulphide to a hyperalkaline pre-treatment however, the delignification rate can be increased and a pulp with higher yield as well as good strength properties is obtained.

The bleachability and the strength properties of a possible industrial application of a hyperalkaline polysulphide process have been investigated and compared to standard kraft and ITC pulps. It is not straightforward to obtain a polysulphide pre-treatment stage at hyperalkaline conditions in a continuous digester. It can be solved by dividing the polysulphide pre-treatment into two steps. The first step has a lower alkali charge enough for neutralisation of acids. Thus in the next step, it is possible to keep the alkali concentration on a very high level. This is favourable since the stabilisation effect of polysulphide on carbohydrates is enhanced at higher alkali concentrations. It also improves the delignification rate in the subsequent cooking stage performed at moderate alkalinity.

By the hyperalkaline polysulphide pre-treatment, it is possible to get a pulp with higher viscosity and yield, good bleachability and good strength properties.