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The use of microfiltration to eliminate colloidal suspended matter (pitch) from different streams in the pulp production process can lead to the increase of pulp quality as well as the reduction of some problems associated to the accumulation of such matter in the process. Previous studies [1] have aimed at the treatment of white waters and diluted black liquors, the results being successful though a large volume would need to be treated. An alternative could be the direct treatment of more concentrated kraft black liquors (KBL). However, permeation fluxes with this liquor are often too low for a feasible industrial application [2]. In this work the microfiltration of KBL (14–15% solids) has been carried out using a ceramic membrane (0.2 µm). Experiments were carried out on site at around 90ºC and at relatively high feed velocities (up to 9 m/s). The introduction of a simple backflushing device (pulses of 15 s every 15 min) allowed to keep a stable flux over 200 L/m2h, while without the backflushing a continuous decay in flux was observed. The selectivity of the membrane was above 99.9%.

Microfiltration is a feasible technique for the removal of colloidal suspended matter in paper pulp mills. Pitch rejection is essentially complete and the membrane can be operated for at least a week without any chemical cleaning. However, the major drawback of the process is related to the high values of the crossflow velocity, which are required in order to achieve stable fluxes. Even with the aid of an effective backpulsing, the pumping costs associated with the high velocities (on the order of 8 m/s), constitute about 70% of the total operating costs.