| The reserves of light rare earth in Bayan Obo Mining District rank first in the world,and there are a number of valuable resources including Ca,Sc,Th,K,P,F,etc.The relationship concerning its rare earth supply and demand directly affects the orderly development of the rare earth industry chain in China and even that of the world.At present,the processes for processing Bayan Obo mixed rare earth ore are mainly concentrated on sulfuric acid roasting method and caustic soda method,but the two methods have practical problems such as associated resource waste,serious environmental pollution,or high energy consumption in some process links.All of these problems need to be solved urgently.Therefore,it is of great strategic significance to fundamentally break through the key technical bottlenecks restricting the extraction of rare earths in Bayan Obo,which invovles the improvement of rare earth leaching efficiency,the selective separation of valuable components,and the comprehensive utilization of resources.This article discusses some key technical issues in the clean and efficient extraction process of Bayan Obo mixed rare earth concentrate and associated resources.To fully understand the target mineral and to predict and construct the roasting reaction model of bastnasite,a series of characterization means were utilized including XRD,BPMA,laser particle size analyzer,XPS,and chemical analysis,etc.Based on the theoretical basis,three kinds of treatment processes for sodiumizing roasting to decompose mixed rare earth concentrates are proposed,and their key scientific problems are further studied.After the study,the main research results are as follows:(1)Bayan Obo mixed rare earth concentrate is mainly composed of rare earth minerals,gangue minerals,iron minerals,niobium minerals,titanium minerals,manganese minerals,etc.It exists in the form of fine-grained minerals or mineral aggregates with the range of single particle size from+0.005 to-0.038mm,and the particle occupancy rate reaches 89.87%.The embedding relationship among the minerals is more complicated.Except for the concatenation of rare earth minerals and gangue mineral aggregates which exists in the form of rich conglomerates,the rest of the minerals all exist in the form of lean intergrowth existing.Bastnaesite and monazite are not only the main target minerals for extracting rare earths,but also important carriers of F-,PO43-,CO32-and Th4+resources.Further analysis by SEM-EDS shows that a mineral whose chemical composition corresponds to the characteristics of carnallite appears.Since fewer specimens were found,a lot of work is still needed to confirm it.(2)The mixture of rare earth concentrate was decomposed by roasting in O2/Ar atmosphere.By using the products of roasting in Ar atmosphere,oxidizing roasting was carried out again.Finally,three kinds of roasting products were obtained.Through the analysis of the phase composition of the calcined product,the characteristics of functional groups,the breaking of chemical bonds,and many other aspects,a number of scientific issues were deeply explored,such as the thermal decomposition characteristics of the main rare earth minerals,the oxidation-reduction law of Ce3+/Ce4+ and the fluorine escape mechanism.The results show that monazite does not change significantly under different roasting atmospheres,while the thermal decomposition process of bastnaesite was carried out at two stages.The first stage is that bastnaesite is thermally decomposed into REOF and CO2 that will adsorb a large amount of free radical O·.The decomposition accelerates the transformation of Ce3+ to Ce4+at the same time,and a certain amount of heat is released,thereby reducing the decomposition temperature of bastnaesite.The second stage is that,with the gradual increase of the reaction temperature(?1000?),a small amount of F begins to escape in the form of OF2 and continues to decompose into O2 and F2.When the roasting temperature rises to the range of 1100??1500?,the F content is significantly reduced,and the La/Ce fractionation also appears.When Na3PO4 is involved,the separation efficiency of La/Ce can reach more than 99%.(3)Based on results above,the process route of NaOH direct roasting to decompose mixed rare earth concentrate is studied.Research on the evolution of its mineral phases and the leaching rules of rare earth elements shows that due to the absence of a large amount of H2O,some of the disadvantages of traditional alkaline decomposition have been fundamentally solved.Not only the reaction temperature is greatly increased,but also the decomposition process changes from liquid-solid reaction to solid-solid reaction transformation,and the target product obtained is RE2O3(CeO2).This is also one of the important characteristics different from traditional alkali decomposition.Research and analysis on kinetics finds that as the leaching temperature continues to rise,the speed control mechanism of the chemical reaction process will also change accordingly.When the leaching temperature is in the range of 40??70?,the chemical reaction process is controlled by the diffusion of the product through the residual layer of inert substances.When the leaching temperature is in the range of 75??90?,the chemical reaction process is under the joint control of interface migration across diffusion and the product layer(mass transfer at the product layer interface).In the end,the rare earth leaching rate can reach 92.5%under the optimal test conditions.(4)Taking the further reduction of the dosage of NaOH as the starting point,the two-stage roasting of oxidation and sodiumization is used to selectively decompose the mixed rare earth concentrate,and in-depth research is carried out around its key technical links such as mineral phase transformation,washing mechanism,and acid leaching mechanism.The experimental results show that compared with the liquid alkali decomposition and NaOH direct roasting decomposition process,the usage of NaOH in this process can be reduced by 73.2%and 55.4%separately,the number of washing times can be shortened to 3,and the washing removal rate of F and P can reach 63.21%and 17.24%separately.Combined with the results of XRD,SEM-EDS and hydrophobicity test,it is confirmed that the surface of the mineral after roasting by this process is not covered by needle-like sodium salt.Its hydrophilicity is strong,so it has high washing efficiency and less water dosage.The HCl leaching and roasting process conforms to the product layer diffusion kinetic model,and the reaction rate is controlled by the interface migration and diffusion across the product layer.(5)In view of the difficulty and high energy consumption of rare earth alkaline wastewater treatment,the decomposition mechanism of NaOH-Al(OH)3 mixed roasting and the technical route of wastewater recycling were explored,and the migration law and mechanism of each valuable element in the process of roasting,acid leaching,and washing were systematically studied and revealed.The whole process made full use of the strong complexing ability of Al3+ and F-,which results in the selective conversion of F and Al resources and the efficient leaching of rare earths in multiple aspects and angles.It not only reduces F and CO2 pollution from the source,but also eventually realizes transformation from Al3+,F-,CO32-into NaF,Na3AlF6,Na2CO3 which are raw materials for aluminum industrial production.The treated waste liquid can be recycled and reused.The whole process provides another idea for realizing the promotion of the liquid alkali decomposition of Bayan Obo rare earth resources,NaOH roasting decomposition and other processes as well as the transformation of alkaline wastewater resources. |
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