Study On Separation And Recovery Of Zinc And Iron From Gossan

Gossan is the name given to a large mass formed by sulfide ore deposits after oxidation and weathering and distributed widely above or near sulfide ore deposits. Its composition is dominated by neoformed minerals bearing iron and manganese, mainly oxidate, secondary sulfate, vitriol and clay. Gossan is a special oxide ore with high contents of iron and other valuable metals and has greatly potential developing value. In order to effectively utilize it, the object of this thesis is the gossan ore derived from zinc sulfate ore deposits in Guangxi Zhuang Autonomous Region of China. It was focused on the problem of separation and recovery of zinc and iron. Mineralogy properties of the gossan ore were studied by means of modern analysis methods such as XRD, SEM, EDS, AAS and ICP and combined with the theories of element geochemistry, metallurgical principle, experiment design, thermodynamics and kinetics. Moreover, the conventional mineral processing methods, the conventional alkaline leaching, mechanical activation leaching, sulfuric acid leaching, the conventional roasting and deep reduction were used to study the feasibility and characteristics, product features, effects of operating factors and process mechanism in mineral processing and metallurgy. The results are shown in followings.(1) Compared with other oxide ores, the difference of origins and properties of gossan ore minerals lead to the difficulty in mineral processing and metallurgy. In this work, the occurrences of iron and zinc in gossan ores were investigated. The results show that iron occurs predominantly in forms of siderite and limonite, and zinc in forms of siderite, limonite and smithsonite. Siderite belongs to primary deposit born in the early stage of sulfide deposit formation, in which zinc exists in the form of isomorphism. Limonite is the product of pyrite oxidation and weathering in the process of gossan formation, and zinc sulfate is intercepted and absorbed by limonite in the course of migration.(2) The grade of rough concentrate is near to that of the raw ore and metal recovery is low in the conventional mineral processing. The results indicate that the methods do not realize the effective separation of zinc and iron and the gossan ore is a refractory oxide ores because independent zinc minerals content is low and zinc is the form of isomorphism and adsorption in siderite and limonite, respectively.(3) In alkaline leaching, the zinc leaching rate of single leaching was less than50%, and that of multi-leaching and mechanical activation leaching could be improved to a certain extent, up to63%maximumly. The results show that zinc in smithsonite could be dissolved in leaching solution, zinc on limonite could be partly desorbed under the action of external forces and zinc in siderite could not be recovered due to its solubility in leaching agent. Therefore, it could be concluded that the alkaline leaching is difficult to recover zinc from the gossan ore for the reasons above mentioned.(4) In sulfuric acid leaching, the zinc leaching rate is greater than90%because smithsonite is dissolved and zinc in limonite is desorbed easily at high concentration and long leaching time. However, iron is also leached with zinc leaching and the leaching rate of iron is more than50%, which results in the following separation of zinc and iron becomes more difficult.(5) The results of alkaline leaching and magnetic separation are not ideal because zinc ferrite which is indissoluble and nonmagnetic generated easily for complex associations between zinc and iron in the conventional roasting.(6) The results show that the process of deep reduction-magnetic separation is most suitable to treat this type of gossan ore which zinc is the form of isomorphism and adsorption. The effective separation and recovery of zinc and iron could be realized because iron minerals could be thoroughly changed into iron particle recovered with magnetic separation easily and zinc could be volatilized into gaseous phase under the atmosphere of strongly reduction and high temperature. The metallization rate, zinc volatile rate, iron grade and iron recovery is greater than92%,97%,90%and90%, respectively.(7) The influence laws of factors and quadratic regression equation with a simple form and high prediction reliability can be obtained easily by analysis of response surface method. The influence sequence is leaching time> leaching agent concentration> leaching temperature, and the interactive effect of leaching time and leaching agent concentration is more obvious than others in alkaline leaching. The influence sequence is sulfuric acid concentration> leaching time> leaching temperature, and the interactive effect of sulfuric acid concentration and the other factors is more obvious than others without sulfuric acid concentration in sulfuric acid leaching.(8) The leaching kinetics of gossan ores could be described with unreacted shrinking core diffusion control model, model parameters is closely related with reaction conditions, and activation energy is less than30kJ/mol. Kinetics of alkaline leaching can be described with unreacted shrinking core diffusion control model in two time scales, and the activation energy of10-120minutes was less than that of120-360minutes. According to the simulation results and the characteristics of sectional activation energy, it could be inferred that the activation energy for smithsonite dissolution was low and the leaching process belonged to fast reaction, while the activation energy of zinc desorption in limonite was high and the leaching process belonged to slow reaction, which was the key step for alkaline leaching. Kinetics of sulfuric acid leaching can be described with unreacted shrinking core diffusion control model in whole time scale, and the activation energy of zinc leaching was less than that of iron leaching. According to the simulation results and the characteristics of activation energy, it could be inferred the activation energy for smithsonite and siderite dissolution was low and the leaching process belonged to fast reaction, while the activation energy for zinc desorption and limonite dissolution was high and the leaching process belonged to slow reaction. So zinc desorption in limonite was still the key step in sulfuric acid leaching.In summary, in this thesis, the relations between gossan formation mechanism and mineral composition were revealed by modern analysis methods combined with the theories involved geology, mineral processing and metallurgy. The main reasons why gossan ores were difficult to be treated with conventional process were found out, and the new process for utilization of this type ores was suggested. The research results in this thesis deserve important academic value and is instructive for gossan resources exploitation and utilization.