Interaction between AQ and sulfidity on yield and pulp strength in kraft cooking of mixed northern hardwoods

The DOMTAR mill in Cornwall, Ontario is an integrated fine paper operation producing most of the hardwood, softwood and recycled fiber required by its three paper machines. Mixed HW chips are cooked by the kraft-AQ process. The main economic driver for using AQ is incremental production. Furthermore, the mill's urban setting and an old recovery boiler necessitates maintaining a relatively low sulfidity level of 16%. Whether and to what degree AQ could be benefiting pulp strength when cooking at 16% sulfidity was the subject of a laboratory investigation. Experimental results confirmed the two main benefits of AQ, i.e. cooking acceleration and yield gain. An increase in viscosity was also observed, but only in the 0-15% sulfidity range. However, AQ had no effect on physical strength, and in a few cases, it actually caused some strength deterioration. While the absence of any benefit from AQ at higher sulfidity (e.g. 25%) might have been predicted, and was also indicated by the results, it was surprising that no benefit at either intermediate (15%) or low (0%) sulfidity was found.