Study On Crystal Defects And Surface Adsorption Characteristics Of Pyrite

Pyrite (FeS2) is one of the most widely distributed sulfide minerals in the Earth’s crust, and it often grows with non-ferrous metal sulfides. In addition, the natural pyrite is the most main mineral of sulfur and iron resource and contains some precious or rare metals. Therefore, mineral-processing technology used for pyrite has been the subject of intense investigation. The flotation practice presents that the floatability of pyrite from various deposits is very different, which often leads to unsatisfactory effectiveness of treatment of the pyrite ore by using froth flotation. However, there is no unitive and good understanding for this phenomenon. This is attributed to the facts that the flotation procedure is extremely complicated and the modern analysis determination techniques are limited, which results in many unresolved problems. These results restrict the improvement of separation between the pyrite and other minerals, as well for the comprehensive utilization of pyrite resources.Much of crystal chemistry knowledge represent that the pyrite bulk cannot grow in accordance with ideal lattice, i.e. there are various lattice defects in pyrite bulk. In microcosmic view, the presence of micro-defects such as atomic substitution, interstitial atoms and other atomic symmetry in pyrite bulk would change the crystalline nature and impact the flotation behavior of the mineral. In macroscopic aspect, the macro-defects including hole, dislocation, micro-cracks have formed in pyrite bulk, and they have trapped the ore-forming fluid passing through small porous or fractured rocks in diagenetic process. During the ores crushing and grinding processing, the macro-defects in pyrite are dilapidated and release their components which may constitute the "unavoidable ions" in the flotation pulp and affect pyrite floatability. So far, however, there is little investigation conducted on these issues. In addition, an in-depth understanding of the surface properties of pyrite and the interaction between the pyrite surface and flotation reagents will aid in to learn the flotation behavior of pyrite. At present, sulfide flotation theory has been developed well, and it can provided a good macroscopic explanation for floatability of sulfides. However, it cannot provide atomic-scale details of the interaction between the mineral surface and flotation reagents, which is very important to deep know the flotation behavior of sulfide. Due to the aforementioned reasons, the aim of the present study is to deep investigate the crystal defects and surface adsorption characterization of pyrite by using combination of experimental methods and density functional theory (DFT) calculation, and fully discusses the causation of floatability difference of various deposits pyrite from a new point of view.In this paper, the effect of micro-defects on the crystalline nature of pyrite was investigated by using DFT, and it on the pyrite floatability was fully discussed. The IR-UV microscopic imaging, SEM-EDS analysis and HRXMT analysis were used to characterize the macro-defects in the nature pure pyrite, and ICP-MS and IC were adopted to detect the chemistry component of filler in macro-defects. Methods such as DFT calculation and AFM atomic scale characterization were used to study the pyrite surface state, including the surface characterization and the electronic structures of surface atoms. Finally, the adsorption characterization of active component on pyrite surface and its effect on the floatability of pyrite were investigated by using experimental and quantum chemistry calculation methods, including Zeta potential, EDTA extraction, adsorbing capacity determination, flotation tests and DFT calculation. The following conclusions can be stated as:The results indicate that the micro-defects have significant influence on crystalline nature of pyrite. In comparison to ideal pyrite, the semiconductivity and stability of pyrite with As-substituted decreases; while the presence of Co-substituted or intercrystalline-Au significantly increases the semiconductivity and stability of pyrite. The difficulty of pyrite depression and the positivity of dixanthogen formation on pyrite surface increases in the following order:As-substituted<ideal<Co-substituted<intercrystalline-Au.The semiconductivity and stability of pyrite with two or up two defects was investigated. Combined with the effect of single defect, the difficulty of pyrite depression and the positivity of dixanthogen formation on pyrite surface increased in the following order:As-substituted<ideal <As-substituted and Co-substituted<Co-substituted<As-substituted and intercrystalline-Au< intercrystalline-Au<As-substituted, Co-substituted and intercrystalline-Au<Co-substituted and intercrystalline-Au. It can be seen that the up to one micro-defects have interaction effect on the crystalline nature of pyrite.The calculated results indicate that the atomic symmetry have negligible influence on the electronic structure of pyrite. However, it significantly affects the stability of pyrite. The experimental results show that the pyrite with the spatial symmetry of Pa3 has a relatively lower stability and is easier to be oxidized and depressed, while the pyrite with the spatial symmetry of Pca21 has a relatively higher stability and is difficult to be oxidized and depressed.The test results demonstrate that there are many closed macro-defects including holes, dislocations and micro-cracks existing in the pyrite bulk. These defects assume a variety of shapes, including oval shapes, long strips and irregular shapes, with sizes that range from a few microns to dozens of microns, and filled by ore-forming fluid. During the crushing and grinding of ores in mineral processing, the closed macro-defects are dilapidated and release their ore-forming fluid. The determination of fluid shows that the fluid contain many chlorine, sulfate and metallogenic element ions, and represent the dominant sources of these ions in the aqueous solution as compared to the dissolution of pyrite sample. In addition, the quartz associated with pyrite also has many macro-defects with various sizes, shapes and distribution, in which were filled by gas-liquid mixing phase. The fluid in macro-defects of quartz contains same species of chemical component with associated pyrite. By comparison the components released from fluid in macro-defects in pyrite and quartz from Weixin and Dapingzhang area, it can be concluded that the fluid in macro-defects of components have significant relationship with the minerogenetic condition (metallogenic element).To stabilize the surface, surface relaxation processes occur at the newly formed surface and are likely driven by electrostatics due to the chemical bond rupture of surface atoms. Generally, the relaxations of S atoms tend to displace outwards the surface, while Fe atoms tend to displace inwards the bulk. The relaxations contribute to the sulphur-rich surface of pyrite. The electronic structure of pyrite surfaces indicates that the tow new surface states formed in the upper part of valence band and the conducting band, which were primarily attributed to Fe 3d states further splitting. In combination the molecular orbital theory, it can be concluded that Fe atoms have high activity in redox interactions. Moreover, the pyrite surface possessed very small band gaps and close to the conductor nature, unlike that of bulk pyrite. The surface properties have a close relationship to the action mechanism with flotation reagents.The experimental results indicate that positive ion released from the fluid in macro-defects can change the pH of pulp via hydrolysis, and can change the surface electrical potential of pyrite after adsorption. The fluid in macro-defects of pyrites from various deposits contain different species and amount of chemical component, which results in the different effect on the pulp pH and surface electrical potential of pyrite.DFT calculations show that the adsorption of water molecule accords to oxygen-end adsorption on the top iron atom of pyrite (100) surface, and results in a light oxidation to the vicinal sulfur atom; The adsorption of hydroxyl ion also accords to oxygen-end adsorption on the top iron atom of pyrite (100) surface, and significantly contribute to the oxidation of iron atom.The investigation results indicate that the copper, lead or calcium ions have interaction with pyrite surface, respectively. These ions cannot substitute iron atom on pyrite surface. The adsorption of copper ions on pyrite surface is considered to be an electrochemical process involving the formation of new copper sulfide and copper hydroxide species. Unlike that the adsorption of copper ions, the lead ion adsorption is difficult to form lead sulfide but favor to the formation of lead hydroxide species. The calcium ion cannot bond with surface atom, which suggests that the absorption of calcium ion on pyrite surface is very weak and belong to physical absorption.The interaction between copper, lead or calcium ions and pyrite surface has significant influence on the flotation behavior of pyrite, especially in alkali condition. The absorbed copper or lead ions oxidize the sulfur atom and results in the quality decrease of iron hydroxide, and enhance the stability of xanthate adsorption layer on pyrite surface which activates pyrite; The presence of calcium ions leads to increase the quality of iron hydroxide on the pyrite surface and weaken the adsorbing capacity xanthate and the stability of xanthate adsorption layer and thus enhances the depression of pyrite.