UBC Theses and Dissertations
The flotation of apatite and dolomite in orthophosphate solution Johnston, David Lawrence
A study has been conducted on the loss of phosphate ions and excessive oleic acid consumption encountered in the selective flotation of dolomite from apatite. An attempt has been made to evaluate the mechanism by which orthophosphate ions depress apatite flotation. Replacement of SO₄⁻² ions on gypsum by HFO₄⁻² ions is found to occur rapidly in solution. The presence of CaHPO₄⁻² 2H₂O on gypsum surfaces is shown using infrared reflectance spectroscopy. CaHPO₄⁻²H₂O is isomorphous with gypsum and has identical lattice parameters. Experiments show that the addition of sulfate ion to the system suppresses the reaction of orthophosphate ions with gypsum by the common ion effect. The proposed reversible reaction in the system is: CaSO₄∙2H₂O + HPO₄⁻²↼⇁ CaHPO₄ ∙2H₂O + SO₄⁻² A proposed mechanism by which orthophosphate ions selectively depress apatite flotation is shown to fit all experimental observations. Orthophosphate ions are known to be potential determining for calcite, dolomite and apatite. It is observed that in the presence of orthophosphate ions calcite and dolomite recovery is higher at pH 6.0 than at pH 8.5. Apatite and gypsum exhibit opposite behavior. It is proposed that the adsorption of strongly hydrogen bonding H₃PO⁴, H₂PO₄⁻² and HPO₄⁻² in the electrical double layer results in collector species being slow to penetrate and adsorb on the minerals. Acid attack on calcite and dolomite results in disruption of the hydrogen bonded layer and allows rapid collector adsorption. Two equally important effects of adsorbed phosphates are to decrease collector adsorption due to the large negative zeta potential generated and to impede fruitful particle-bubble interactions. Brittle froths encountered in flotation at pH 6.0 are related to condensation of surface films formed by oleic acid -sodium oleate complexes. The froth stabilizing effects of hydrophobic dolomite particles is noted.
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