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Adsorption of oleate on magnesite and its influence on flotation Branda̧o, Paulo Roberto Gomes


A highly sensitive reflection-absorption infrared spectrophotometric technique was used to investigate the adsorption of sodium oleate from aqueous solutions onto magnesium carbonate (magnesite) thin film substrates. Valuable qualitative results were obtained, regarding the nature of the adsorbed species and their molecular orientation. However, this orientation effect strongly influenced the relative intensities of the infrared absorption bands; this prevented precise quantitative determinations. Under moderately acidic conditions, the original surface charge of magnesite was positive and near zero. The anionic collector was chemisorbed, forming only one type of complex, very similar to the bulk magnesium oleate. The hydrocarbon chains were oriented nearly parallel to the substrate, forming a homogeneous partially polymerized epoxide-polyether layer, strongly laterally bound, at the sites of the former C=C double bonds. The C-0 bonds were formed following a partial oxidation of the oleate in solution, by oxygen from the air. This adsorbed film converted the carbonate surface to a strongly hydrophobic condition, which was responsible for the high flotation recoveries obtained. Under basic conditions, the original magnesite surface was negatively charged. This limited the formation of the chemisorbed collector complex to a lower amount; different complexes, probably physically adsorbed, joined in forming a film. However, this was a more heterogenous film, showing different molecular orientations - - from nearly parallel, to closer to the normal to the adsorbent's surface. These aspects caused a net decrease in the degree of hydrophobicity, in comparison to the acidic conditions. The same oxidation and polymerization effects were observed, although to a lower extent. The film was also very stable. These properties of the adsorbed film were consistent with the lower flotation response observed under basic conditions. Therefore, more concentrated collector solutions were needed, to reach the same high recoveries obtained under moderately acidic conditions.

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