Investigação dos fatores e suas interações na floculação de quartzo ultrafino e nas características dos flocos

Abstract

The knowledge of the characteristics of the floc and the sediment compaction is decisive for the appropriate selection of solid-liquid separation operations. The present study addressed the effects of nine factors and their interactions on ultrafine quartz flocculation and floc characteristics. The characterization of the quartz sample was carried out by laser granulometry and specific surface area analyses, X-ray diffraction, and X-ray fluorescence. The results indicated the high degree of purity of the sample in the intended size range between 38 and 10 µm, with specific surface area of 0.496 m2/g. The flocculant used was a non-ionic commercial polyacrylamide (PAM). The non-ionic character was confirmed by reactions with bromophenol blue, methylene blue, and chloroform. The PAM molecular weight, calculated from the viscosity measurement (5.35 dl/g), is 2.0 x 10-6 g/mol, considered adequate for the proposed flocculation system. The PAM analysis by FTIR detected the characteristic vibrations of the functional groups carboxamide, alkylethers, and secondary aromatic amines. The zeta potential values in the presence amine in the pH range above the PIE, pH 2, are in agreement with the less negative surface charge of the mineral. This is explained by the adsorption of the cationic amine species on the quartz surface by an electrostatic attraction mechanism and immobilization by formation of hemimicelles. The PAM adsorbs on the hydrophilic surface of quartz with excess negative charge through hydrogen bonds with the NH2 group, therefore resulting in decreased number of negative sites from the surface and, consequently, rendering the zeta potential value less negative. Back scattered electrons scanning electron microscopy images of flocs formed in the presence of PAM at the concentration 30 g/t showed compact flocs in the size range between 150 and 365 µm, with noticeable sphericity. The nine investigated factors were: flocculant concentration, surfactant concentration, surfactant conditioning time, flocculant addition time, flocculant conditioning time, flocculant addition time, stirring time, suspension pH, flocculation time, solids concentration. These factors and their levels, low (-) and high (+) were selected from a careful literature review. The flocculation experiments were carried out in a 1 L beaker followed by placing the suspension in a 500 mL graduate tube adapted with two side orifices, located at 3/5 and 4/5 height from the base, for the removal of the supernatant. The experiments were planned in a (29) fractioned factorial method with nine variables at two experimental levels, conducted in 32 tests. The results were processed with the use of the Minitab20® statistical software with the supernatant turbidity as experimental response. Regarding the influence of interactions between the variables on the supernatant turbidity, the flocculant concentration presented strong interaction with the surfactant conditioning time and the pH with the solids concentration. The surfactant conditioning time showed strong interaction with the agitation intensity and with the suspension pH. The evaluations resulting from the statistical approach adopted in this work indicated that suspension pH was the most significant variable, followed by flocculant concentration with the second greatest effect on the turbidity of the supernatant. Solids concentration, agitation intensity, and surfactant conditioning time, in descending order, also have significant effects on ultrafine quartz flocculation.