| The Fe-RE-Nb polymetallic symbiotic mine in Bayan Obo is the world's largest mixed light rare earth mine and of stratigic importance in China.At present,the processes for treating mixed light rare earth minerals are concentrated sulfuric acid roasting method and sodium hydroxide decomposition method,but both of them have serious problems in waste pollution in the form of gas,liquid and gas and associated resource underutilization.The research team has long been committed to the green development and comprehensive utilization of the rare earth resources in Bayan Obo.Based on the source management and interdisciplinary research,our team proposed a new technology for the selection and smelting of the rare earth resources of Bayan Obo.The new technology achieved the industrialization of the beneficiation process of high-grade rare earth concentrate,and developed a new technology for clean and efficient extraction of high-grade mixed rare earth concentrates by acid leaching and alkali dissolution,and carried out industrial demonstration.This research is to study the mechanism of key technologies for the problems existing in the demonstration process of new technology industry,in order to better guide the process production and realize the green extraction of rare earth resources.The specific research results are as follows:Aiming to solve the problems of cerium oxidation and acid mist leaching in the roasting-acid leaching process of rare earth concentrate,the roasting reaction mechanism of mixed rare earth concentrate under different roasting atmosphere(oxidation/non-oxidation)and the synergistic leaching rule of RE and F in the leaching process were studied.It has been found that the bastnaesite in the mixed rare earth concentrate is decomposed into(Ce,La)OF at 600? in either oxidative or non-oxidative roasting.The cerium in the oxidized roasting ore(Ce,La)OF phase is mainly composed of Ce4+,while the non-oxidized roasting ore is mainly Ce3+.The changes of acid leaching temperature,hydrochloric acid concentration,liquid-solid ratio and zeta potential with acid leaching time were studied,and the synergistic leaching behavior between Ce3+,Ce4+,F-and H+during leaching was confirmed.Because of the different content ratio of Ce3+/Ce4+between oxidized roasted ore and non-oxidized roasted ore,the surface of oxidized roasted ore is more repulsive to H+than that of non-oxidized roasted ore at the initial stage of reaction.Therefore,at low temperature and low acidity,Ce4+is more difficult to leach than Ce3+and non-cerium rare earth.However,at high temperature and high acidity,the movement of H+ions in the solution is intensified,which breaks through the repulsive force of Ce4+on H+in oxidative roasted ore.As a result,a large number of H+ions start to be absorbed on the surface of oxidative roasted ore.Ce4+will form a[CeFx]4-x complex with F-in a short time,and promote the dissolution of rare earth in the(Ce,La)OF phase.Moreover,Ce4+is easily reduced to Ce3+by Cl-and simultaneously oxidizes Cl-to chlorine gas,which eventually forms a CeF3 precipitate with F'.Finally,the best leaching rate of oxidative roasted ore is Ce 51.28%,non-Ce rare earth 45.66%.In order to improve the leaching rate of non-oxidized roasting ore,ultrasonic field strengthening means is used to promote the leaching of rare earth,and the rare earth leaching rate is greatly improved.The optimal leaching rate of ultrasonic assisted leaching of non-oxidative roasted ore is finally obtained:Ce 87.54%,non-Ce rare earth is 86.43%.This is mainly because ultrasonic has physical and chemical effects on the leaching process of rare earth.In terms of mechanism in physics,ultrasonic wave can cause great shear stress on the solid surface of bastnaesite and facilitate the removal of passivation film,thus promoting the leaching of rare earth.In the chemical mechanism,it was found that in the ultrasonic assisted leaching process,Ce3+on the mineral surface was oxidized to Ce4+by hydroxy oxidation,and Ce4+formed a complex with F-and promoted the leaching of rare earth.At the same time,the reduction of Ce4+to Ce3+by H2O2 in the solution prevents the chloride ions from being oxidized to chlorine gas.In view of the low efficiency of alkali decomposition of acid leaching slag and low recovery rate of lye,sodium hydroxide sub-molten salt system is used to decompose acid leaching residue.Through the study of temperature,concentration,solid-base ratio,time and other conditions,the optimal process conditions were determined:reaction temperature 180?,alkali concentration 70%,liquid-solid ratio 5:1,reaction time 90 minutes,conversion rate of rare earth is 98.02%,which greatly improves the efficiency of alkali decomposition.The kinetic analysis of the reaction process was carried out,and the kinetic equation of the stage was obtained,which confirmed the feasibility of the process of decomposing the acid leaching residue by the sub-molten salt system.The apparent activation energy of short-term phase(0?20min)is 40821.74J/mol.The reaction rate is controlled by interface mass transfer and product layer diffusion.The kinetic equation of acid leaching slag decomposition is determined by calculation combined with empirical formula:The apparent activation energy for a long period of time(20?90min)is 10475.64)/mol,and the reaction rate is only affected by the diffusion of the product layer.The kinetic equation is:Aiming to solve the problem of high impurity content in mixed rare earth chloride solution,the rare earth compound was used as a fluorine removal agent to remove trace fluorine in solution.By studying the related parameters of rare earth compound type,content,reaction time,reaction temperature,rare earth concentration and values of solution pH,the content of strontium carbonate is 8%,the concentration of rare earth is 240g/L,the pH of solution is?1,the temperature is 90?,and the reaction time is 2h and 1h incubation,the efficiency of fluorine removal of the solution can reach 99.10%.The analysis of the filter residue shows that after adding rare earth carbonate to the solution,on the one hand,the basic rare earth carbonate RECO3(OH)with high specific surface area can be formed,which will strongly adsorb fluorine.On the other hand,the fluoride ion in the solution and the rare earth carbonate form a stable fluorocarbon rare earth compound REFCO3 by forming a polar covalent bond to achieve the goal of removing fluorine.At the same time,the process of removing calcium from mixed rare earth chloride solution by sulfuric acid precipitation method was studied,and the optimum process conditions were as follows:rare earth concentration was 350g/L,reaction temperature was 25?,reaction time was 2h,and sulfuric acid dosage was 1.0 times of calcium content.Under this condition,the removal rate of calcium is 80.35%,and the loss rate of rare earth is 16.86%.The Ca/REO in the liquid phase is 1.67,which meets the requirements of the extraction index.The rare earth and calcium elements in the calcium residue are recovered by the countercurrent circulation washing process to obtain the best recovery process conditions:liquid-solid ratio 5:1,water washing temperature 25?,water washing time 2 h,washing stage 3,rare earth recovery rate reaches 97.03%.The final calcium residue is prepared into calcium sulfate whiskers toachieve the conversion and high utilization of resources. |
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