Possibilities to Increase the Pulp Yield in a Kraft Cook of Itc-Type, 1998 Breaking the Pulp Yield Barrier Symposium Proceedings

To clarify the technological potential and consequences of the introduction of black liquor gasification into the pulp mill, polysulphide and hydrogen sulphide cooking have been investigated. The addition of 2% polysulphide to the white liquor in a modified kraft cook of ITC-type increases the pulp yield by 2 percentage points on wood at a kappa number of 24. The simultaneous addition of polysulphide and anthraquinone (0.1 % AQ on wood), increases the pulp yield by a further one percentage point. Hydrogen sulphide cooking (IX-type) with green liquor pretreatment in the presence of carbon dioxide or hydrogen sulphide gives a yield increase of 2 or 5 percentage points on wood, respectively, compared with the reference cook. The yield increase in polysulphide cooking remains after ECF- and TCF-bleaching. The consumption of bleaching agent to obtain pulps with full brightness in ECF- and TCF-bleaching is very similar for the reference pulp and for all the polysulphide pulps. The reactivity with ozone tends, however, to be better for the hydrogen sulphide pulps than for the other pulps and this results in a higher final brightness for a given bleaching chemical charge. All the unbleached pulps studied have the same physical properties as the unbleached reference pulp, except for the tear index at a given tensile index. No differences were observed in the beatability of the unbleached pulps. The ECF- and TCF-bleached polysulphide pulps also had a lower tear index but a slightly better beatability than the fully bleached reference pulps.