Properties of hyperalkaline polysulphide pulps

The term hyperalkaline polysulphide pulping is used to denote a process utilising a polysulphide pretreatment step at high alkalinity. The stabilising effect of polysulphide on carbohydrates is enhanced by higher alkali concentrations. The high alkali concentration in the stabilisation stage also increases the delignification rate in the subsequent cooking stage. In order to keep the alkali at a high level, hyperalkaline polysulphide pulping employs two pretreatment stages. In the first stage, the alkali charge is aimed to neutralise the acids formed of the wood components. The succeeding stage is the actual hyperalkaline polysulphide stage. It has a noticeably higher alkali concentration, thus promoting an efficient carbohydrate stabilisation by the polysulphide. The hyperalkaline polysulphide pretreatment makes it possible to obtain a pulp with high viscosity and yields good bleachability and good strength properties. The pulps however, need more beating energy to reach a high tensile strength than kraft and normal polysulphide pulps.