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Surface properties of crandallite in relation to froth flotation Kaushik, Sarthak


The surface properties of crandallite relevant to the anionic flotation of salt-type minerals were investigated through electrokinetic, turbidity, and wettability techniques. All the tests were performed on fine crandallite particles as a function of pH and reagent concentration. The selected reagents included starch and oleic acid. From the zeta potential measurements, the iso-electric point of crandallite was found to be at pH 5.5 and the mineral surface can be expected to be negatively charged under typical flotation conditions (pH 9-11). The value of the iso-electric point correlated well with the stability of crandallite particles towards aggregation. Suspensions of fine crandallite showed minimum turbidity at pH 5.5 suggesting that the aggregation of the mineral was most pronounced at the iso-electric point. Calcium ions behaved as potential determining ions by increasing the iso-electric point to pH 6.5. In addition, calcium ions also acted as specifically adsorbing ions at higher pH. In the presence of starch, crandallite particles were strongly flocculated producing supernatants with the clarity of tap water. At the same time, the effect of starch on the zeta potential of crandallite was rather weak, consistent with the non-ionic character of the polysaccharide. The stability of crandallite towards aggregation was not strongly affected by oleic acid suggesting that the surfactant did not interact with the mineral. Wettability measurements on crandallite particles showed that oleic acid did not render the mineral particles hydrophobic at pH 10.5 (under normal flotation conditions) or at pH 7, which was in strong contrast to the wettability response of apatite. Under these conditions, crandallite surface was negatively charged and the adsorption of the anionic surfactant seemed to involve physical electrostatic forces as opposed to chemical interactions known to occur between apatite and oleic acid. Overall, the experimental results strongly suggest that crandallite remains hydrophilic during apatite flotation at pH 10.5 and the main mechanism of contamination of apatite concentrates by crandallite is through mechanical entrainment in the froth rather than by true flotation. In this respect, the role of starch as a flocculant should be beneficial in reducing the amount of fine crandallite reporting to the apatite concentrate.

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