Original Research PaperSurface analyses of calcite particles reactivity in the presence of phosphoric acid
Graphical abstract
Introduction
The reactivity of calcium carbonate and phosphate materials has received considerable attention because of the abundance of these materials in the environment, their importance in the living organism and their wide range applications in the industry [3], [4], [33], [36]. The reactivity of calcium carbonate and phosphate minerals in solution, including their dissolution, is also a key factor for a successful separation of these minerals by flotation [2], [14]. The crystal structure and the presence of impurities in these sparingly soluble minerals (eg., calcite and apatite) have been found to affect their dissolution and reactivity [7], [28]. Dissolution of ions from these different minerals, followed by their interaction in solution, and their indiscriminate adsorption/precipitation on the surface of all minerals present in the ore have resulted in a poor separation of these minerals by flotation [16], [21], [40], [52]. Thompson and Pownall [41] proposed that the formation of a new surface phase modifies the physical and electrical properties of the calcite surface. Numerous investigations devoted to the dissolution of calcite in acidic media can be found in the literature [25], [31], [38], [47]. Their results indicate the formation of a passivation layer on the calcite surface which is associated to a decrease in calcite dissolution. This phenomenon has been observed for other minerals, such as chalcopyrite where its leaching is significantly reduced as a result of passivation of its surface with formation of a layer of polysulfide/elemental sulphur and/or precipitation of jarosite [22]. The nature of this layer depends on the type and strength of the acid used for the dissolution (e.g. H3PO4, H2SO4, KH2PO4, C2H2O4, H2SiF6, HF, tannic acid). Among these acids, phosphoric acid is the most used for the separation of carbonate minerals (calcite, dolomite) from phosphate by reverse flotation, mainly in acidic flotation [1], [50]. The reaction between calcite and phosphoric acid gives essentially Ca-phosphate compounds as products (e.g., dicalcium phosphate dihydrate, CaHPO4·2H2O; dicalcium phosphate anhydrous, CaHPO4; octacalcium phosphate, Ca8H2(PO4)6·5H2O; tricalcium phosphate, Ca3(PO4)2; amorphous calcium phosphate (Ca9(PO4)6xH2O); hydroxyapatite, Ca10(PO4)6(OH)2) which have all different structures, compositions, solubilities and stabilities [43]. Several studies [10], [31] have suggested that the initial uptake of phosphate onto calcite occurs via chemisorption, which is then followed by a slow transformation of amorphous calcium phosphate to crystalline apatite. The formation of a protection coating on the calcite surface through pretreatment is aimed at reducing the reactivity of calcite [29], [47].
Although the reactivity and dissolution of calcite in phosphoric acid solution has been largely investigated there is still a lack of understanding about the exact species formed at the calcite surface, especially at low phosphoric acid concentrations, and its kinetic of formation. Therefore, in the present study, the reactivity of calcite in the presence of phosphoric acid and the species formed at its surface were measured with complementary analytical techniques as a function of reaction time and acid concentration. In particular, Raman spectroscopy was used to measure in-situ the kinetic of the reaction between calcite and phosphoric acid.
Section snippets
Materials
The calcite sample used in this investigation is from Madagascar. Its chemical and mineralogical compositions were determined to check its purity. The XRD analysis shows that the sample is relatively pure and does not contain other minerals in significant proportion except for a small amount of quartz and tremolite (Fig. 1).
The chemical analysis in Table 1 reveals the presence of Mg which can be attributed to tremolite (Fig. 1) or the substitution of Ca by Mg in the calcite structure. Its BET
Raman spectra of phosphoric acid in water
The Raman spectra of the solutions containing phosphoric acid (without mineral present) at various concentrations are shown in Fig. 2. Three peaks are present in the spectra which intensity increases with the phosphoric acid concentration: the major one occurs at 890 cm−1 while the other two of weaker intensity are found at 1075 cm−1 and 1174 cm−1. These peaks are only observed in solution of high phosphoric acid concentrations above 0.1 M; at lower concentrations these peaks are
Discussion
The SEM analysis has shown that after treatment with phosphoric acid the calcite surface was covered with micron size calcium phosphate particles which were identified by XRD as brushite (CaHPO4·2H2O). Results of the infrared and Raman spectroscopic analyses are in agreement with this finding because they showed a decrease in intensity of the IR peaks associated with carbonate and an increase of those associated with phosphate and hydroxyl after phosphoric acid treatment, which can be
Conclusion
The changes in solution and at the surface of calcite after contact with various concentrations of phosphoric acid have been monitored using in-situ Raman spectroscopy and completed with complementary analytical techniques. The trends observed in the Raman, IR and XRD analyses with increasing phosphoric acid concentration are similar with the apparition of characteristic peaks of PO4 and the disappearance of CO3 peaks all occurring at a phosphoric acid concentration between 10−2 and 10−1 M, and
Acknowledgements
Financial support from LabEx RESSOURCES21 is acknowledged (contract Investissements d’Avenir no. ANR–10–LABX–0021).
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