Effect of pulping processes on bleachability with ECF, Z-ECF and TCF bleaching

The bleachability of various hardwood pulps produced by the kraft, kraft-anthraquinone (kraft-AQ), kraft-polysulfide (kraft-Sn), kraft-anthraquinone-polysulfide (kraft-AQ-Sn), modified continuous kraft (MCK) and modified batch kraft (MBK) pulping processes were compared by bleaching each pulp type using the sequences ODEopDD, Oq(OP)(ZE)DD and Oq(OP)(ZQ)(PO). The kraft-AQ-Sn process resulted in the highest pulping yield whereas the MCK produced the pulp with highest viscosity. The MCK and MBK pulps were slightly lower in pentosan content than the other pulps. Pulp bleachabilities with ECF, Z-ECF and TCF processes were similar with exception of the kraft and kraft-AQ pulps which showed poorer results. The kraft-AQ-Sn pulp had the highest overall yield and the lowest and lowest recovery load. The different cooking procedures had no significant impact on pulp quality and effluent loads for a given bleaching sequence. The pulps produced by the MCK and MBK processes are the easiest to bleach, followed by the kraft-Sn and kraft-AQ-Sn, regardless of the bleaching sequence. For a given pulp, the ECF sequence had the lowest operating cost followed by Z-ECF. The Z-ECF sequence exhibited the lowest effluent COD load. Regardless of the pulping process, the quality of the ECF and Z-ECF pulps were similar whereas the TCF pulps showed slightly higher tensile and slightly lower tearing resistance. Suggestions for a preferred pulping-bleaching combination to obtain high brightness, high quality pulp with minimal cost and environmental impact are made.