Effect of particle size on the flocculation of sub-micron titanium dioxide by polyacrylic acid

This paper reports the results of a study on the effect of particle size on the flocculation of ultrafine titanium dioxide (TiO2) by polyacrylic acid. Three TiO2 samples of a narrow size distribution, with median size at 38, 186 and 357 nm, respectively, were used in this work. The flocculation process was monitored in real-time by focused beam reflectance measurement and photometric dispersion analyses, coupled with settling tests using graduated cylinders, and adsorption analysis of the polyacrylic acid on the TiO2 by an elemental analyzer. The results demonstrated that the optimum polyacrylic acid dosage, when expressed as mass of the polymer per unit mass of the particles, increased with decreasing particle size. However, when the optimum dosage was divided by the specific surface area of the ultrafine particle samples, a uniform value was obtained. Therefore, it was proposed that the appropriate method to express the dosages of the polymer flocculants should be based on the quantity of the polymer flocculant per unit surface area of the particles to be flocculated, rather than the traditional method which expressed the quantity of flocculant per unit mass of the particles. In fact, it was found that by using this new dosage designation, the finer TiO2 particles had faster flocculation, floc growth, and settling rates than the two coarser TiO2 samples under the optimum dosage. This observation was in contrast to the conventional observations/claims that fine particles flocculate poorly. The reason behind the discrepancy is the insufficient dosage of the polymer flocculant to the fine particles using the traditional way of dosage designation.