Primary Study On Supergene Weathering Of Skarn-type Molybdenite Ore And Microbial Oxidation Of Molybdenite

Tailings are generally disposed around the mine as wastes due to low content of target minerals.It not only takes up a lot of land resources,but also imposes severe risks of heavy metal contamination as sulfide minerals in tailings are weathered.China is one of the countries which have the most molybdenum ores in the world,and thus the molybdenum contamination is a crucial environmental issue in China.For all the molybdenum-bearing minerals,molybdenite(MoS2)is the most abundant one,so it is of great importance to investigate its weathering process,and further reveal the migration and valance change of molybdenum in supergene environments.Microbe-mineral interactions affect the geochemical behavior of many elements in supergene environments.A cidi th i abacillus ferrooxidans(A.ferrooxidans),a representative microbe in sulfide mines,plays important roles in the formation of acid mine drainage(AMD).Molybdenite is one of the main kind of metal sulfides in the skarn-type molybdenum ore,and can be oxidized by the microbes in mining environments and therefore molybdenum would be released and mineralized,which probably alters the local environmental quality.So,investigating the interactions between A.ferrooxidans and molybdenite can provide a basis for disclosing the weathering mechanism and developing control technologies of molybdenum contamination.In this thesis,the surface changes of skarn-type molybdenum ore sampled from the Nannihu distict and the secondary minerals formed during weathering have been investigated.On the other hand,experiments were designed to study the interaction mechanism of A.ferrooxidans and molybdenite.To investigate the influence of iron ions,commonly found ions in surface waters,three systems were designed:one with 4.43g/L FeSO4·7H2O initially,one with pyrite as source of Fe2+ and the last one without iron ions.The experiment reveals the microbial oxidation mechanism of molybdenite.The ions concentration in the experiment are tested through ICP-OES and spectrophotometer;with the assistance of XRD,Diffuse Reflection Spectrum(DRS),SEM and EDS,EPMA,the types of secondary minerals in natural-weathered ore and experiment systems were identified.Through the study of natural-weathered molybdenum ore and the experiment of molybdenite oxidation by bacteria,we drew the following conclusions:(1)The primary minerals in molybdenum ore are dominantly molybdenite,calcite,pyrite and chlorite.Powellite,gypsum,goethite and hematite are formed in the weathering process as secondary minerals.The water and air play leading roles in the oxidation process.During the process,molybdenite breaks and curles,and such physical changes break the stability of molybdenite structure and futher accelerate the molybdenite oxidation.There exist different secondary mineral assemblages on the interfaces between molybdenite and other minerals.Powellite and gypsum mainly form on the molybdenite-calcite interface,while iron oxide is the main secondary mineral formed on the interface of molybdenite and pyrite,but no ferrimolybdite formed in the two cases.Based on the Eh-pH diagram of molybdenum-bearing and iron-bearing minerals reported before,an Eh-pH diagram of weathering products of molybdenite ore is presented in this paper.It reveals that in neutural environment,molybdenum is fixed in powellite or adsorbed onto iron-oxides,thus reducing the effect of the releasing element Mo to the environment.SEM observation shows that there are hyphae and fungal spores on the surface of weathered molybdenite ore,which indicates that the organism might make some contributions to the weathering process.(2)The simulation experiment studies the interaction of molybdenite and A.ferrooxidans under acid conditions.The results show that the added-FeSO4 initially system has the highest dissolving-out amount of Mo.While there is pyrite in the system,the bacteria tended to adhere onto the surface of pyrite and clear erosion pits on pyrite formed.Mo-bearing iron-sulfate,Mo-bearing iron-phosphate and gypsum form in the system containing FeSO4 and pyrite,while only a little gypsum is observed in the iron-free system.All the results reveal that the initial Fe in the system is in favor of the growth of bacteria and the increase of redox potential in the system,which offers the favorable condition for molybdenite oxidation.The indirect machenist(effect of Fend)makes more contribution to molybdenite oxidation.And in the system with pyrite,a competitive oxidation between pyrite and molybdenite exists,pyrite oxidizes preferentially.