First-Principles Study Of The Crystal Structure Of Calcite And Surface Adsorption With Flotation Reagents

Calcite is one of the main gangue minerals in low-medium grade calcareous phosphorite. Calcite and apatite are slightly soluble calcium salt minerals with similar surfacial physical and chemical properties, and are difficult to be separated through flotation. At present, reverse flotation in acidic conditions using fatty acid as collector is an effective method in the flotation separation of calcite and apatite, which depends on different physical and chemical properties which is essentially determined by the minerals' structure. So, the interaction of mineral and flotation reagents and their flotability is closely related to the minerals' crystal structures. Quantum chemistry is an efficient method for study on the electronic structure and properties of solid microscopically. Therefore, in this paper, the first-priciples is used to research the surface structure and properties of calcite, and the micro mechanism of the interaction of flotation reagents and calcite is also studied. Thus, the microscopic mechanism of flotation between apatite and calcite is clarified.First, the bulk of calcite is optimized. When the exchanging correlation functions is generalized gradient approximation?GGA? with modified approximate PW91 gradient, the cutoff energy is 400 eV, k point is 3×3×2, the energy of calcite is the lowest, and a stable crystal structure of calcite was thus obtained, namely, a=5.049 ?,c=17.200 ?, the length of C-O bond is 1.300 ? and the Ca-O bond is 2.383 ?. The density of states and Mulliken population show that in the calcite unit cell, O is the strongest in chemical reaction, followed by C and Ca.The adsorption between calcite surface and water molecules and water molecule clusters has beenstudied. Calcite unit cell was expanded as a 1×1×1 supercell, and planes of {10???4}?{01???8} and {21???4} were set up, among which {10???4} has the lowest surface energy with a total charge of 0, namely, the plane is electrically neutral and is thus the most stable plane of calcite. After plane relaxation, it was found that the activity of calcite atom is stronger than that in the bulk, and the activity order is O > Ca > C. Expanding the relaxed calcite unit to a 2×1×1 supercell, the supercell is used to study the interaction between calcite and water molecules and reagent molecules. When the water molecule absorbed to the site C on calcite surface, the water molecule structure is damaged, the adsorption is thermodynamic instable and cannot occur; When the water molecule adsorbed to site Ca and O, the adsorption energy is-0.75 eV and-1.01 eV respectively, the adsorption occur. Hydrogen bond formed when the water molecule absorbed to site O, the atomic charge changed relatively large after adsorption. The results of water molecule clusters absorption on calcite is similar to that of a single water molecule, the water molecule clusters is more likely to be adsorbed at site O through hydrogen bonds. The site O of calcite is more hydrophilic than site Ca.H₂PO₄^- and SO₄^2- are respectively the main form of phosphoric acid and sulfuric acid in the acidic reverse flotation, the anionic collector oleic acid is mainly C₁₇H₃₃COO^-,while the cationic collector twelve amine is the mainly in the form of C₁₂H₂₅NH₃⁺. When H₂PO₄^- absorbed at the calcite site O and site Ca, the adsorption energy is respectively-0.95 eV and-0.72 eV, the adsorption at site O is more stable than at site Ca. The orbital contribution near Fermi level is bigger when H₂PO₄^-absorbed at the site O, hydrogen bond formed between the atom H of H₂PO₄^- and atom O of calcite surface; When SO₄^2- absorbed at the calcite site O and site Ca, the adsorption energy is respectively-0.20 eV and-0.19 eV. Since the adsorption is relatively weak, and the density of states changed slightly after adsorption.Adsorption between C₁₇H₃₃COO^- and site O on the calcite surface section cannot occur. Interaction between reagent molecules and mineral surface leads to serious structure deformation of them.Adsorption occurred between C₁₇H₃₃COO^- and site Ca on the calcite surface, with an adsorption energy of 0.86 eV. Both of the two O atoms of the C₁₇H₃₃COO^- bond with site Ca on the surface of calcite. And after adsorption density of states of calcite as a whole moved to the right by 1.1 eV, which is beneficial to the adsorption.Adsorption between cation collector C₁₂H₂₅NH₃⁺ and site Ca on the calcite surface section cannot occur. Interaction leads to serious deformation of molecular structure of the reagent, as a result, the density of the states of C₁₂H₂₅NH₃⁺ and calcite moved to the left by 2.5 eV with an adsorption energy of 4.94 eV. Adsorption occurred between C₁₂H₂₅NH₃⁺ and siteOon the calcite surface with an adsorption energy of-0.76 eV, after that the orbit of density of states of the C₁₂H₂₅NH₃⁺ moved as a whole to the left by 0.5 eV, the peak energy near Fermi level is in accordance with the peak energy at Fermi level of calcite, which is conducive to the adsorption.