Étude des mécanismes et microprocessus régissant les interactions entre les espèces minérales et les modulateurs de flottation en vue de la valorisation des minerais de phosphates complexes

Abstract : Because of the depletion of high grade phosphate deposits, usually associated with silicate gangue minerals, which are easy to treat by grinding/classification and/or by flotation, the industry has to rely more on carbonate gangue phosphate ores which are more abundant but more difficult to beneficiate by flotation. Most of the minerals contained in these phosphate ores are semi soluble, which results in a similar chemistry of their surface and a poor separation by flotation between the phosphate minerals such as apatite (calcium phosphate) and the gangue minerals such as calcite (calcium carbonate). The reverse flotation method in acid medium has been found to produce a better contrast of separation between calcite and apatite. This method relies on the addition of an acid such as phosphoric acid which acts as a selective depressant of apatite. However, the mechanisms governing this mineral separation and in particular the reactions occurring at the surface of these minerals are not very well understood. The objective of this thesis is to study the reactivity of calcite and apatite in aqueous solution as a function of phosphoric acid concentration and reaction time. For this, several analytical techniques (Raman, infrared and X-ray photoelectron spectroscopies, and X-ray diffraction and scanning electron microscopy) are used to measure the nature of the species adsorbed or formed at the surface of calcite and apatite. In particular, with Raman spectroscopy it was possible to measure directly in-situ the changes occurring in solution and at the mineral surface in the presence of phosphoric acid, which should eliminate any possible artefact caused by the drying of the samples when using the other analytical techniques. The first major result was on the reactivity of the two minerals measured after 15 min of contact with an acid solution reveals the phenomenon of passivation of the surface deduced from the variation of the consumption of the protons with the concentration of acid. Indeed, all the in-situ and ex-situ analytical techniques used in this study have shown the presence of calcium phosphate (brushite) on calcite and of calcium carbonate on apatite after contact with phosphoric acid, and with a concentration which increases with acid concentration. Moreover, the acid concentration for the onset of detection of calcium phosphate on calcite and for the start of passivation (decrease in acid consumption) coincides, which implies that the presence of calcium phosphate is responsible for the passivation. For apatite, although the acid concentration for the onset of detection of calcium carbonate on apatite and for the start of passivation coincides, calcium carbonate was removed at higher phosphoric acid concentrations. However, kinetic in-situ studies using Raman spectroscopy performed at several phosphoric acid concentrations and at a fixed pH of 4.5 all confirmed the presence of calcium carbonate on apatite but also revealed that a new phase of calcium phosphate was also present after only 2 min of contact with phosphoric acid and its concentration increased with contact time. Surprisingly, the presence of calcium phosphate and its growth on apatite were transient, lasting 5-6 min before quickly vanishing and then reappearing again for several cycles. These cycles of calcium phosphate growth and disappearance on apatite may be the result of precipitation of calcium phosphate followed by their removal in the chemical and / or mechanical conditions. Similar kinetic in-situ studies of calcite using Raman spectroscopy under the same conditions as with apatite confirmed the presence of calcium phosphate (brushite) on calcite but only after 4 minutes of contact with phosphoric acid, indicating a slower kinetic of calcium phosphate formation than on apatite [...]
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Submitted on : Tuesday, April 3, 2018 - 2:50:33 PM
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  • HAL Id : tel-01757175, version 1

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Kaba Oumar Barou. Étude des mécanismes et microprocessus régissant les interactions entre les espèces minérales et les modulateurs de flottation en vue de la valorisation des minerais de phosphates complexes. Sciences de la Terre. Université de Lorraine, 2017. Français. ⟨NNT : 2017LORR0038⟩. ⟨tel-01757175⟩

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