Effect of pH on the retention of inorganic fillers

The effect of pH on the retention of various inorganic fillers has been studied. It is found that the isoelectric point of the filler has no effect on its retention characteristics with our particular sulphate bleached pulp. This is due to the fact that the surface of the filler is modified by either the adsorption of or the flocculation with the fines and colloidal material originating from the fibre surface. This surface modification produces a shift of the apparent isoelectric point of the fillers and in many cases gives rise to heteroflocculation of the fibres and filler particles only when the pH is at more acid values than 4.0.