Surface reactions in aqueous suspensions of fluorapatite and iron oxides
Abstract: The main objectives have been to study and model surface reactions of aqueous suspen-sions of minerals with relevance to the dephosphorization of iron oxides by reverse froth flotation. Synthetic hematite, maghemite and fluorapatite in colloidal forms were prepared and thoroughly characterized using XRD, SEM, BET, FT-IR and FT-Raman. The protolytic surface reactions were studied by means of high precision potentiometric titrations at two different ionic strengths. The æ-potentials were measured as a function of pH for maghemite and fluorapatite in aqueous suspensions at 0.10 mol dm-3 ionic strength. Surface complex equilibria according to the constant capacitance model (CCM) were established for aqueous single minerals as well as for a mixed mineral suspension of fluorapatite-maghemite. Two different models were adopted for the mixed system. It was found that a model based on the results from the subsystems of fluorapatite and maghemite interpreted titration data extremely well and information about the interac-tions between the two minerals was obtained by this model as well. These interactions were confirmed by SEM/X-ray mapping and FT-Raman spectroscopy. Solid state nu-clear magnetic resonance (NMR) spectroscopy was used to achieve information about the surface reactions of fluorapatite on the molecular level and merge this knowledge with the results from the surface complex model calculations. By means of 1H and 31P MAS NMR the phosphorus and calcium hydroxyl surface sites of fluorapatite were as- signed and their composition and mutual ratio were studied as a function of pH. The adsorption of maghemite and Fe2+-ions as well as the adsorption of the flotation re-agent ATRAC on the fluorapatite surface were studied using 1H and 31P MAS NMR. Maghemite particles and Fe2+-ions were found to be adsorbed in a close vicinity of the 31P nuclei. This was indicated by an increasing broad spinning side band manifold with increasing iron adsorption in the 31P MAS NMR spectra, caused by the influence on the chemical shift anisotropy (CSA) of the 31P nuclei due to the paramagnetic properties of the adsorbed iron species. The adsorption of ATRAC was found to be depending on both pH and concentration. The results from FT-IR and NIR-Raman spectroscopy of ATRAC in ethanol solutions including added calcium nitrate displayed that ATRAC contains four carbonyl functions which are clearly affected by the presence of Ca2+-ions, which indicate their importance on the adsorption of ATRAC at the fluorapatite surface.
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