| Niobium has broad applications in steel,chemical,aerospace,and nuclear industries,which is regarded as a strategic metal by many countries.Over 90% of the global niobium resources occur as pyrochlore mineral.Currently,the beneficiation of pyrochlore ores is mainly carried out by flotation,and the smelting method is mainly by aluminothermic reduction.The existing benefication and smelting processes have problems of low recovery of niobium,low availability of valuable elements and high cost of aluminothermic reduction,which were caused by the brittle and fragile properties of pyrochlore mineral,the difficulty of flotation of microfine minerals and the difficulty of reduction of niobium minerals.Thus,it’s of great significance to develop new technology of high efficiency enrichment of pyrochlore ore.On the basis of process mineralogy of a typical pyrochlore ore in Brazil,the calcination behavior of the ore was investigated,and a novel process consisting of calcining-slaking followed by gravity separation was developed.The carbothermal reduction behavior and reaction path of pyrochlore mineral was studied,and the phase transformation and structure evolution of the gravity concentrate during the carbothermal reduction was further investigated.A novel process for the preparation of crude niobiumtitanium-iron alloy via carbonization roasting followed by magnetic separation from pyrochlore ore was developed,and the utilization method of crude alloy was analyzed.Finally,the principle process of coenrichment of niobium-titanium-iron from pyrochlore ore based on carbonization roasting is constructed.The main conclusions and innovations of this thesis are shown as follows:(1)The process mineralogy of pyrochlore ore was investigated,the raw ore had a complex minerals composition,consisting of carbonate minerals,silicate minerals,magnetite,apatite,barite and pyrochlore.The grades of niobium,titanium and iron are 0.85%,1.25% and 11.57%,respectively.Niobium mainly occurs in pyrochlore,titanium mainly occurs in ilmenite and rutile,and iron mainly occurs in magnetite and mica.The pyrochlore mineral in the raw ore is well crystallized,but it’s brittle and fragile,so it is easy to be slimed during grinding,which is easy to cause niobium losses in the beneficiation operations.(2)Based on the investigation of calcination behavior of pyrochlore ore,a new technology of calcination followed by gravity separation was formed.The properties of pyrochlore,apatite and barite are stable,and no phase transformation occurs below 1200 ℃.Carbonate minerals will be decomposed at 700～1000 ℃,but they are easy to react with silicate minerals at higher temperatures.The carbonate minerals in the ore will be transformed into porous lime after calcined at suitable temperatures,and the lime converts into fine slaked lime after digestion,and efficient dissociation of main minerals in the ore is achieved.Under the conditions of calcining at 900 ℃ for 60 min,approximately 40% of tailings can be removed via gravity separation,and the recoveries of Nb,Ti and Fe attained 94.7%,91.0% and 88.4%,and their enrichment ratios were 1.61,1.43 and 1.46,respectively.Grinding is not necessary in this new technology,thus the sliming phenomenon of pyrochlore can be avoided effectively.The recovery of niobium via the novel method is more than 15 percentage points higher than that of the existing desliming and decalcification procedures.(3)The carbothermic reduction behavior and reaction path of pyrochlore minerals were investigated.Niobium oxides and titanium oxides in the pyrochlore minerals can be reduced to Nb C and Ti C,respectively,and the carbonization of niobium is prior to that of titanium.The carbonization rate of niobium is over 98% when the pyrochlore is reduced at 1400 ℃.Sodium oxides in the minerals can be reduced and volatilized under this condition,calcium and fluorine will react with the impurity component silicon to form cuspidine.Latrappite and perovskite are intermediate products during the reduction,and the reduction reaction is basically completed at 1400 ℃.(4)Phase transformation during the carbothermic reduction of pyrochlore ore was investigated,and the mechanism of Nb-Ti-Fe migration and growth of their grains was revealed.The roasting product of raw ore is mainly akermanite,while that of gravity concentrate is mainly diopside.The melting point of slag reduced over 130 ℃ by reducing Ca O content of the ore.Iron minerals can be reduced to metallic iron,and niobium and titanium minerals can be reduced to carbides,these carbides gather around the metallic iron grains,providing a favourable condition for the coenrichment of niobium-titanium-iron.(5)An innotivative process for the co-enrichment of niobiumtitanium-iron from pyrochlore ore based on carbonization roasting was developed.After reduction at 1400 ℃ for 120 min,yields a crude alloy of4.18% niobium grade,4.60% titanium grade and 70.57% iron grade from pyrochlore gravity concentrate in the presence of 6% coke under conditions of grinding fineness of 88.4% less than 0.074 mm and magnetic field intensity of 1400 Gs.The recoveries of niobium,titanium and iron by magnetic separation can reach 70.5%,59.8% and 92.6%,respectively.The valuable elements in the magnetic separation tailings can be further recovered by strong magnetic separation,and the distribution of niobium,titanium and iron in the strong magnetic separation tailings is only 13.5%,19.9% and 3.0%.Niobium,titanium and iron in the crude alloy can be further separated by “low-temperature selective chlorination” method and their chloride products can be obtained.The niobium-titanium-iron coenrichment technology is characterized by high recovery of niobium,high utilization of valuable elements and low smelting costs.117 Figures,34 Tables and 204 References. |
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