| Bayan Obo ore deposit is a rare multimetallic mine comprising of iron,rare earth,niobium and other metallic elements in the world.There are 15 kinds of rare earth minerals,which are mainly mixed light rare earth ore including bastnaesite and monazite.In Baotou steel large numbers of rare earth tailings are produced after iron ore dressing,which was based on Bayan Obo ore.In the weak magnetic-intensity magnetic-flotation process,the content of iron and rare earth in weak magnetic tailings is 13%,7%?9%,respectively.However,large quantities of tailings including rare earth was discarded in the tailings dam during iron ore dressing,which resulted in seriously environmental pollution and complete waste of iron,rare earth,niobium,etc.Bayan Obo ore possesses many complicated mineral characteristics:fine-grained dissemination,mutual inclusion,low grade,similar physical and chemical properties,which make it very difficult to get efficient separation with the conventional beneficiation methods.In this work,the rare earth was extracted from low-grade and preprocessed rare earth tailings by the technological process of roasting with(NH4)2SO4-water bath leaching.Rare earth tailings were transformed into soluble ammonium sulfates or sulfates,in which realized the efficient separation and extraction of rare earth elements were realized.In the meantime,the roasting of single mineral with ammonium sulfate was studied.The existing state and the morphologies of rare earth after direct reduction at high temperatures,the phase transformation of rare earth after roasting with(NH4)2SO4 at different temperatures,the chemical composition of leaching solution and residue were characterized by chemistry quantitative analysis,XRD,SEM-EDS,and TG-DSC,etc.This work puts forward a new rational utilization way to reuse Baotou iron tailings.The technological process including high temperature direct reduction,magnetic separation,roasting with(NH4)2SO4 and leaching by water bath successfully retrieved iron from the tailings and rare elements.Rare elements in the slags existed mainly in the form of Ca2RE8(SiO4)6O2 and CaRE2(SiO4)2 which were transformed into soluble sulfates or ammonium sulfates.Separating rare earths from gangue minerals(Ca4Si2O7F2,CaF2,CaSO4,etc.)by means of water-soluble difference,the leaching solution with high leaching rate was obtained.The target is 91.43%of powdered iron with metallization rate,87.85%of grade and 89.47%of recovery rate after magnetic separation.Under the conditions of roasting for 2h at 1150? and adding 10%CaO,rare earth slags with 14.35%of rare earth grade(REO)increases by 5.71%compared with untreated tailings.The leaching rate of La,Ce and Nd can respectively reach 96.13%,98.88%and 97.1%under the following optimized experimental conditions:8:1 of the mass ratio of rare earth tailings and(NH4)2S04,roasting for 60min at 400?.conducting the water bath leaching at 80? for 1h with 10mL/g of leaching liquid-solid ratio.The addition of CaO did promote the reduction of iron ore.Separating magnetic iron concentrate avoided the negative effects on the subsequent extraction and separation of rare earth elements.In addition,CaO also participated in the decomposition of bastnaesite and monazite of tailings.The fluoride cuspidine generated has certain effects on "fluorine fixation".The technological process of coal-based moderate reduction-magnetic separation-roasting with(NH4)2SO4-water bath leaching was implemented under relatively low temperatures.In addition,bastnaesite is decomposed by roasting at 570?,which is beneficial for transforming rare earth residue into soluble sulfates or ammonium sulfates by ammonium sulfate roasting process.Rare earth roasting with ammonium sulfate could effectively decompose(La,Ce,Nd)C03F,(La,Ce,Nd)OF and turn them into soluble ammonium sulfates,which are coexisted with the decomposed products of soluble sulfates.Water-bath leaching was carried out to separate RE from insoluble minerals(BaSO4,CaSO4,CaF2,(La,Ce,Nd)PO4).Ammonium sulfate could not decompose monazite,which resulted in the low leaching rate compared with high temperature process.The optimized experimental conditions were as follows:6:1 of mass ratio of(NH4)2SO4 to slag,roasting at 350? for 80min,water bath leaching for 1h at 80? with 10mL/g of leaching liquid-solid ratio.The leaching rate of rare earth(La,Ce,Nd)were 83.08%,76.55%,77.16%,respectively.Adding NH3·H2O and CO2 to leaching liquid to prepare rare earth oxides was also exploringly studied.After removing impurity,crystallization precipitation,aging,filtration,drying,roasting and other processes,58.56%of the rare earth oxides grade and 48.32%of recovery rate were obtained.Rare earth elements were separated and purified.Considering the leaching rate of rare earth treated with high temperature is relatively higher than low temperature processing,this work puts forward the method of bastnaesite oxidizing roasting-ammonium sulfate roasting-leaching of water bath.Monomineral study revealed that bastnaesite's surface density became loose and porous,and bastnaesite was turned into(La,Ce,Nd)OF,Ce0.75Nd0.25O1.875,Ce11O20,such as rare earth fluoride and oxide by oxidizing roasting.The leaching rate of rare earth increased significantly after ammonium sulfate roasting.Under conditions of bastnaesite with 850? roasting for 1h,subsequent ammonium sulfate roasting at 400? for 60min,water bath leaching at 80? for 1h with 20mL/g of leaching liquid-solid ratio,the leaching rate of Ce reachs as high as 90.68%.Raw material particle sizes have certain influence on ammonium sulfate roasting at a specific range.Both ammonium sulfate and pretreated bastnaesite have small particle sizes which can rapidly promote generation of liquid and formation of embedded bastnaesite,increase the contact area between reactants and speed of mass transfer of liquid-solid.Small particle sizes also promote transformation of(La,Ce,Nd)CO3F or(La,Ce,Nd)OF into soluble ammonium sulfates,reaction with ammonium sulfate and increase of leaching rate.The technological process of rare earth mine roasting with(NH4)2SO4-water bath leaching rare earth breaks through the traditional ideas of mine roasting and leaching with strong acid or alkali,reduces the discharge of wastewater containing acid and alkali.High concentration ammonium sulfate leaching liquor after rare earth extraction can be used for the effective constituent to leach ion-absorbed rare earth mineral.The recovery of mother liquor is relatively easier than traditional sulfuric acid liquor. |
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