Efficiency of colloidal pitch adsorption onto phyllosilicates : Comparing talc , chlorite and pyrophyllite

Talc is often used to control unwanted deposition of resins and wood processing derivatives in the wet end circuit of a paper mill. These compounds, called pitch, originate from the raw forest material and are brought into a semi-stable colloidal dispersion during the pulping process. In preceding work we found that the efficiency of different talcs in reducing the level of pitch present in a thermo mechanical pulp (TMP) was proportional to the particulate specific surface area. To a lesser, but nonetheless significant, extent, the efficiency also depends on the mineralogical composition. This paper compares talc with its closest structural relatives in the phyllosilicate family, namely its di-octahedral counterpart pyrophyllite and the magnesium-rich chlorite, which is often present in significant quantities in talc ores, studying both the impact of mineralogy and generated specific surface area on the adsorption efficiency of pitch. The minerals chosen are typical commercially available grades. Removal of colloidal pitch by the various minerals from a filtered liquor, obtained by filtration from TMP pulp, the filter pore size being chosen carefully to remove fibrils but not the colloidal pitch, was evaluated by centrifugation of a mix of the mineral and the liquor. According to the results given by chemical oxygen demand (COD), turbidity, gravimetry, thermo-gravimetric analysis and microscopic analysis, no significant difference between talc and chlorite was observed. Pyrophyllite performs similarly to the other tested phyllosilicates but the measurements were affected by pyrophyllite fines, which in combination with wood resin material could not be removed by the applied centrifuge conditions. These fines may become further stabilised in suspension in the presence of dissolved and colloidal substances (DCS).