Structure And Surface Adsorption Behavior Of Apatite:a DFT Investigation

As a vital non-renewable resource, comprehensive recycling of apatite is important to the phosphorus cycle in the nature. Natural phosphate rock often mixed with other gangue mineral association, such as quartz, dolomite, calcite, etc. These minerals have the same chemical and physical properties with the phosphate and often entangled together which cause difficulties for separation. Flotation is considered to be the valuable separation of phosphate rock resources, however the flotation reagents can severe affect the result and often lead to some environmental problems. Therefore, it’s important to explore the new high efficiency, environmental flotation regents. The electronic structure and surface flotation behavior of different apatite were studied using first–principles method based on density functional theory(DFT). The structure and electronic properties with the bulk as well as the(001) surface of the fluorapatite and chlorapatite are investigated. Meanwhile the behavior mechanism of the water and collector on the two kinds of apatite give more attention. The main results are as follows: 1. Calculation shows that both the fluorapatite and chlorapatite are insulator and the band gap are 5.667 eV and 5.154 eV. The charge of the F 2s and 2p are 1.96 e and 5.72 e, which means that he is stronger than the Cl( with 3s, 3p are 1.94 e and 5.65e). The length of the O—P in both the fluorapatite and the chlorapatite are almost equal, but the F—Ca is stronger than the Cl—Ca and the O—Ca in the fluorapatite is longer than that in the chlorapatite. Further study shows that the O—P is more covalent, while the F—Ca, O—Ca and Cl—Ca are more ionic and their bonds are longer. 2. The Ca termination in the fluorapatite and the Cl-Ca termination in the chlorapatite are smaller than the other two terminations, with their surface energy are 3.86 eV/cell and 3.73 eV/cell, which mean that they are more stable. The relaxation analysis shows that these Ca in the independent layers have the larger displacement, even though they are not enough to reconstruct. Further analysis shows that the Ca in the fluorapatite and the Cl-Ca,Ca terminations in the chlorapatite have more similar with the bulk in the properties.3. In the fluorapatite system, the F-A structure has the largest adsorption energy and the F-B has the smallest, with their values are-69.541 kJ/mol and-123.817 kJ/mol respectively. For the chlorapatite, the Cl-A is-67.031 kJ/mol and the Cl-D is-118.996 kJ/mol and they are the two extremes. The DOS show that both the two apatite are affected by the water and all the adsorptions on the apatite(001) surface are physical process. The water molecules show a kind of regularly disorderly arranged, when a layer of water on the two apatite(001) surface. 4. There is only one kind of adsorption structure for each aqueous apatite with the collector(CH3(CH)2COO), with the adsorption energy of the fluorapatite and the chlorapatite are-137.515 kJ/mol and-134.733 kJ/mol. The Mulliken charge analysis shows that more electrons transfer at the the O86 and the Ca6. The Mulliken bond analysis shows that these O of the water around the Ca6 are pushed away and the length of these O—Ca are all increased. Further analysis shows the interaction of the Ca6 with the O75, O77, O78 are all weaker, meanwhile there is strong bond between the O86 and Ca6 near the Fermi level.