Beneficiation of Donganshan Iron Ore Using Magnetic Pre-enrichment and Fluidized Magnetization Roasting
Author:Yu Jian Wen
Supervisor:han yue xin
Donganshan iron ore is a typical refractory hematite ore in China,and it is difficult to achieve efficient utilization by traditional mineral processing techniques including magnetic separation,gravity concentration,flotation or their combined beneficiation methods due to its low grade,complex mineralogy,fine mineral dissemination and hard-controlled grinding fineness.Therefore,it is of great significance to carry out the fundamental theory and key technology research on the efficient utilization of low-grade Donganshan hematite ore.Pre-enrichment followed by magnetization roasting and magnetic separation is the typical and most effective method for dealing with the complex refractory iron ores.In particular,the fluidized roasting has the advantages of high heat and mass transfer efficiency as well as low calcination energy,which has become a research hotspot in recent years.In this paper,low-grade Donganshan hematite ore was selected as studied materials.Based on the process mineralogy,the magnetic pre-enrichment behavior of Donganshan hematite ore was firstly investigated by using the magnetic difference between iron minerals and gangue minerals.Second,the feasibility of selective reduction magnetization for mixed magnetic concentrates at reducing atmosphere as well as of the conversion of magnetite to maghematite at oxidizing atmosphere was analysed by thermodynamic calculation.Third,the kinetics of reduction of hematite to magnetite,as well as the formation and growth behavior of magnetite were studied by micro-fluidized bed reactor,optical microscopy and scaning electron microscope(SEM).Finally,the magnetic properties and phase transition of magnetite ore after low-temperature oxidation roasting were also discussed by Mossbauer spectrum and vibrating sample magnetometer(VSM).Then,the theoretical basis for the selective reduction magnetizing-reoxidation of hematite ore was established,and a new process of preenrichment-fluidized magnetization roasting-magnetic separation for efficient utilization of Anshan-type low-grade hematite ores was developed.Some scientific and practical research results were obtained as following:(1)The magnetic pre-enrichment behavior of low-grade Donganshan hematite ore was revealed.Through the analysis of XRD,iron mineral phase and SEM for different products produced by procentration,it is found that the magnetite particles and some hematite particles associated with magnetite are mainly enriched in the weakly magnetic concentrate,coarse hematite and siderite particles are mainly enriched in high-gradient magnetic concentrate,while some fine(<10μm)hematite particles are lost in tailing due to its weak magnetic force.(2)The kinetics mechanism of reduction of hematite to magnetite was established.The removal of oxygen occurs on the outer surface of magnetite layer during the reduction of hematite to magnetite.The kinetics of the reduction of hematite to magnetite in a fluidized bed can be described by the Avrami-Erofeev equation F(a)=[-In(1-α)]l/n(n=1.50-1.58),that is,the reduction process is controlled by the random nucleation and 1D growth of the newly-generated magnetite phase.By fitting the reaction results,the reaction activation energy Ea=48.70-49.64 kJ/mol,the pre-exponential factor A=5.58～6.55s-1,and the reaction rate constant is:k=5.58exp(-48700/R·T)or k=6.55exp(-49640/R·T)(3)The formation and growth mechanism model of magnetite phase nuclei were determined.The magnetite nuclei are preferentially generated at the edge of hematite particle with a shape of needle,forming the growth core of magnetite phase.Fe2+ ions formed by the reaction are responsible for the growth of magnetite phase by means of the diffusion of Fe2+ions to the magnetite-hematite phase interface.In the early stage of reduction,the formation of the new-phase magnetite nuclei is difficult,the pre-exponential factor k0=1.95×105μm2/min and the reaction activation energy Ea=60.86 kJ/mol.In the later stage of the reduction,the formation of magnetite nuclei contributes to the growth of the magnetite phase,the pre-exponential factor k0=9.77×104 μm2/min and the reaction activation energy Ea=35.71 kJ/mol.(4)The magnetic properties and phase transition of magnetite ore after low-temperature oxidation roasting were identified.The oxidation degree of magnetite at 300～400℃ increases with the increase of temperature and the prolonging of time,and the temperature has a greater effect on the oxidation degree of magnetite.The oxidation rate of magnetite in the early stage is much faster than that in the later stage.The oxidation process of magnetite can be divided into two different stages:the oxidation product of magnetite is maghemite(γ-Fe2O3),and its content gradually increases with the temperature increasing if the temperature is lower than 350℃.When the oxidation temperature is higher than 350℃,the magnetite is unstable,and the conversion of γ-Fe2O3→α-Fe2O3 occurs.Meanwhile,the residual magnetization(Mr)of the magnetite oxidation product begins to decrease.(5)A new process of preenrichment-fluidized magnetization roasting-magnetic separation for the efficient utilization of low-grade Donganshan hematite ores was developed.Under the condition of the grinding fineness of 60 wt.%passing 0.074 mm,a mixed magnetic pre-concentrate containing 42.02 wt.%Fe with an iron recovery rate of 90.02 wt.%is obtained from the low-grade(TFe 31.74 wt.%)Donganshan hematite ore with the process of low-intensity magnetic separation followed by high-gradient magnetic scavenging.Meanwhile,the tailings yield is up to 32 wt.%.Finally,a magnetic concentrate containing 66.06 wt.%Fe with an iron recovery of 81.22 wt.%and a yield of 39.02 wt.%was obtained under optimal conditions:roasting temperature of 540～560℃,CO flow rate of 4 m3/h,N2 flow rate of 2 m3/h,grinding fineness of 95 wt.%passing 0.038 mm,and the magnetic intensity of 100 mT.The research results of this paper have enriched the fluidized magnetization roasting theory of refractory iron ores,which not only provides a theoretical support for improving the fluidized magnetization roasting of low-grade Donganshan hematite ores,but also be instructive for the efficient utilization of other complex refractory iron ores.