| Sphalerite is the most abundant Zn-bearing mineral, which occupies a zinc output more than 90% in industry. However, compared with other sulphide minerals, sphalerite has an inferior natural floatability and doesn’t respond well to short chain xanthate collectors. Thus, it is usually activated by copper sulfate in practice, prior to the addition of ethyl xanthate (EX) and flotation. The mechanism of copper activation of sphalerite has been extensively studied in the past decades. It is generally accepted for the overall reaction mechanism of copper activation sphalerite at acid flotation system, and a representative ion exchange mechanism was proposed. However, the precise mechanism responsible for the surface reaction in detail and the products associated with the activation and flotation of sphalerite is still a contentious issue. For the alkaline flotation system, the mechanism of copper activation of sphalerite is more complicated due to the hydroxylation of Cu2+, and some experimental phenomena cannot be well explained by simple ion exchange mechanism.To day, the activation mechanism is inconclusive. Thus, many shortcomings still exist in the whole theory system of copper activation flotation of sphalerite, and it’s worthy of in-depth research. The present research differs from the previous one, and according to the surface atom structure, it was innovatively focused on the copper adsorption activation on the S site of sphalerite surface.Through the density functional theory (DFT) calculation, molecular mechanics analysis, AFM atomic scale characterization and etc., the electronic structure, population of atom and bond, and surface atomic relaxation properties of sphalerite were firstly investigated. Then, the chemical equilibrium of Cu2+ and Zn2+ was calculated based on the coordination chemistry theory. From the calculation, the preponderant components of the two ions together with their distribution at various pH were determined. After that, the morphology of fluid inclusions in sphalerite and its intergrown quartz, as well as the components released from the inclusions and their interactions with the surface of sphalerite were investigated, with various analytical and testing methods such as IR-UV microimaging, SEM/EDS surface analysis, HRXMT 3D scan, ICP-MS/AES and IC. Based on the surface atom structure of sphalerite, DFT calculation, Ab initio molecular dynamics (AIMD) simulation, Cu/Zn exchange ratio (E) testing, XPS surface analysis were performed to fully study the problems during the process of copper [Cu2+ and Cu(OH)2] activation of sphalerite, such as the activation mechanism, specific reaction process, activation kinetics, surface products, etc. According to these research results, a new viewpoint of copper adsorption activation was proposed by us. At last, the ternary media interaction of sphalerite-copper-EX at various initial structures was calculated by DFT. From the calculated results, three types of new flotation model of copper adsorption activation were proposed. In addition, the effect of copper activation on the adsorption performance of EX on sphalerite surface was also elaborated based on theory and experiments.The results of electronic structure and surface atomic structure of sphalerite show that the perfect sphalerite is a typical p type semiconductor with a direct band gap of 2.71 eV, and has a weak surface electrical conductivity. The electronic configuration of Zn atom on sphalerite surface after geometry optimization is Zn3p0.85 3d9.884s0.69, which is electron donor; however, the S atom is electron acceptor with an optimized electronic configuration of 3s1.83 3p4.66. The chemical bond of Zn-S in sphalerite is mainly covalent bond, but also has a little ionic bond with a ratio of 17%. The new formed surface of sphalerite undergoes a voluntary surface atom relaxation. In the normal direction, i.e., perpendicular to the surface, the S atoms of the first surface layer move outward from the bulk at a displacement of 0.12 A, whereas that of Zn atoms move toward the bulk at a displacement of 0.44 A. The total displacement of Zn and S atoms on the sphalerite surface is 0.54 A, as a result of forming a relative S-rich surface, which is in good agreement with the 3D atomic scale morphology of the sphalerite surface detected by AFM. This S-rich surface has an important effect on the mechanism of copper interaction with the surface of sphalerite.The chemical equilibrium calculatation of Cu2+ and Zn2+ in sphalerite solution indicates that the existing form and preponderant component of activation and inhibition ions are controlled by the solution pH. In a common flotation pH (6～12), the copper in solution presents mainly in the form of Cu2+(pH≤7.5) and colloid Cu(OH)2 (7.5< pH≤ 12.25). Aside from conventional surface oxidation and dissolution of minerals, the components released from the fluid inclusions of minerals during grinding also present an important source of inevitable metal ions in flotation pulp. The Zn2+ released from the fluid inclusions of sphalerite can be adsorbed on its surface and further affect the surface electrical property, which is also an important reason for its inferior natural floatability. The adsorption of Zn2+on sphalerite surface certainly will decrease the available adsorption sites of the subsequent copper, resuting in adverse effect on copper activation.The DFT calculation of copper interaction with the surface atom of sphalerite shows that, aside from the Cu substituting for surface Zn atom, two types of chemical adsorption of Cu2+exist on the S atoms of sphalerite surface, i.e., Cu adsorption on top site of S atom and on bridge site between the two S atoms, which also can result in the activation of sphalerite. However, the interaction intensity of the two types of Cu2+ adsorption activation is remarkable weaker than that of Cu substituting for surface Zn atom. Transient state (TS) search calculation indicated that the copper adsorbed on the S sites of sphalerite surface can further migrate voluntarily to the Zn vacancy of the surface and then form stable coordination with the surrounding S atoms. According to the AIMD simulation, the interaction nature of Cu(OH)2 and sphalerite surface is that hydroxy in Cu(OH)2 interacts chemically with surface Zn atom, however, that of Cu interacts with surface S. At last, the bond of Cu-OH is fractured due to the difference of motion curve of hydroxy and Cu, and Cu2+ is released from Cu(OH)2. The released Cu2+further interacts with S atom, resulting in the adsorption activation of sphalerite or substitutes for the surface Zn atom.The testing of Cu/Zn exchange ratio during copper activation indicates that the activation process is only controlled by conventional ion exchange mechanism under strong acidic condition, however, in the case of weak acidic and alkaline condition, that is controlled by both adsorption activation and ion exchange activation. In addition, the activation mechanism is transferred from adsorption activation to ion exchange activation as the time increasing, which was consistent with the result derived from theory calculation. The whole copper activation process of sphalerite can be divided into two steps, i.e., the rapid activation step (t< 4 min) and slow activation step (t> 4 min). The adsorption rate equation of copper for the rapid activation step can be described by the equation:ΓCu= K1 ln(t)+Γ1. XPS analysis of the surface products after sphalerite copper activation shows that the activation process is a redox reaction for both acidic and alkaline conditions, in which the Cu2+ on the sphalerite surface is reduced to Cu1+ with resulting oxidation of the surface S2+ to polysulfide(Sn2-,n≥2).The DFT calculation of ternary media interaction of sphalerite-copper-EX at various initial structures shows that four types of stable interaction models exist among sphalerite, copper and EX, i.e., EX, respectively interaction with, substituted Cu, Cu adsorbed on the top site of S, Cu adsorbed on the bridge site of S and Cu(OH)2 adsorbed on the sphalerite surface. The four types of interaction models can result in the activation flotation of sphalerite. The nature that sphalerite responds not well to EX is that the Zn 3d orbit electronic peak in ZnS locates far from Fermi level (EF), which cannot effectively interact with the S 3p orbit electronic peak nearby the EF of the bonding S in EX. In other words, the energy level of the interactional atoms doesn’t match with each other. As a result, the Zn atom in ZnS has a weak reactivity and the interaction between ZnS and EX is very weak. However, after copper activation, the Cu 3d orbit electronic peak and bonding S 3p orbit electronic peak are just maximally overlapped around the EF. After copper activation, the surface of sphalerite is characteristic of faster adsorption rate, larger adsorbing capacity and more stability for EX when compared without copper activation.The results presented in this paper develop and enrich the basic theory of copper activation flotation of sphalerite, and have important theoretical significance and guidance on some important aspects, such as in-depth understanding of the copper activation mechanism at various pH values and activation reaction process, improvement of copper activation efficiency, development of new collectors and etc. |
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