| The mixed rare earth ore from Baotou is the most abundant rare earth resource in the world.Due to its complex composition and stable structure,the conditions of process are trenchant for processing the mixed rare earth ore.At present,the sulfuric acid roasting and decomposition with NaOH are the main methods in the industrial application.With the increasing environmental protection requirements,the problems of environmental pollution and waste of resources are becoming more and more serious obstacles to its development for the existing processes.Therefore,the reasonable development,comprehensive utilization and green-pollution free process for mixed rare earth ore from Baotou are important for the rapid development of rare earth industry.Inthis paper,a new method is proposed for processing mixed rare earth ore by microwave heating technology.On the foundation,this research focuses on the transformation of mixed rare earth ore to give a detail description of mechanisms of the microwave heating and acid leaching process,which is to provide a valuable reference for the treatment of mixed rare earth ore.The mixed rare earth concentrate was characterized by chemical analysis,X-ray diffraction(XRD)and scanning electron microscope(SEM).The results showed that the content of rare earth elements consisting of cerium,lanthanum and neodymium was 56.08 wt%,and the contents of fluorine and phosphorus were 8.63 wt%and 3.26 wt%,respectively.The main phases of mixed rare earth concentrate consisted of monazite,fluorocarbonite and fluorite.Three rare earth elements were distributed evenly and were intertwined with impurity elements,such as fluorine,phosphorus and calcium,which resulted in the absence of enrichment of rare earth elements.The absorbing performance of mixed rare earth concentrate was studied by thermogravimetric analysis and complex permittivity analysis.The results showed that the complex permittivtiy of mixed concentrate changed obviously at the temperature of 200,600 and 800 ?,respectively.At 200 ?,the mixed concentrate was dehumidified and there was no change in the mine phase.With the increasing of temperature,the fluorocarbonite was decomposed to produce rare earth oxides at 600 ?.Although the minerals before and after decomposition belonged to the hexagonal system,the lattice structure of mineral before decomposition was more compact.When the temperature was higher than 800 ?,the complex rare earth oxide was generated with the combination of rare earth oxides,and the hexagonal system was transformed into the cubic crystal system,a highest symmetry crystal system.The changes of the complex permittivity of mixed concentrate were caused by the phase transformation of mixed concentrae and changes of the lattice structure at these temperatures.The reflectivity,attenuation factor and penetration depth of mixed concentrate were calculated under microwave irradiation at 2.45 GHz.The results showed that the reflectivity was basically stable at about 13%when the temperature was below 800 ?,and the penetration depth was above 0.25 m.Therefore,the method of decomposing the mixed concentrate by microwave heating was feasible.But the absorption performance of mixed concentrate needed to improve because of its poor absorptive capacity.The mixed concentrate could be heated quickly with sodium hydroxide employed as a microwave absorber.And compared to activated carbon,sodium hydroxide could also play a role in promoting the decmposition of mixed concentrate and solid fluoride.So,the sodium hydroxide was chosen to improve the absorption performance of mixed concentrate as an absorbing agent.The decomposition mechanism of mixed concentrate in the microwave heating process was discussed by the thermogravimetric analysis and characterized by X-ray diffraction.The results showed that the existing forms of the REE were transfromed from bastnaesite and monazite into rare earth hydroxides firstly with the increase of temperature,and then the rare earth hydroxide was decomposed into rare earth oxides continuously.Finally,the existing form of rare earth elements was the complex rare earth oxide.The elements of fluoirne and phosphate were forming eventually sodium fluoride and calcium phosphate,respectively.Under the condition of sufficient heating time,the above phase transition could be achieved at the temperature of 350 ?.Under the conditions of constant temperature 450 ?,heating time 40 min,the addition amount of sodium hydroxide 45 wt%and microwave power 1500 W,the decomposition rate of mixed concentrate and the conversion rate of fluoride were respectively 89.33%and 88.73%,which indicated that the mixed concentrate could be decomposed effectively and that the separation of fluorine from rare earth elements and phosphorus can be achieved through water washing process.What's more,the specific surface area of mixed concentrate increased after microwave heating,and the surface of mineral particles presented a porous structure,which was conductive to the leaching process of rare earth elements.The leaching process was mainly affected by the leaching temperature,the hydrochloric acid concentration and the ratio of liquid to solid.And the stirring speed had little effect on the rare earth leaching.With high leaching efficiency,the leaching process had reached the chemical equilibrium in about 30min.Under the conditions of leaching temperature 80 ?,time 60 min,liquid-solid ratio 12:1,initial hydrochloric acid concentration 3 mol/L and stirring speed 500 rpm,the leaching rate of rare earth elements reached 88.31%.However,the rare earth elements with cerium-based will still react with the phosphate and fluoride to form needle-like crystals in millimicron.The macroscopic reaction kinetic of the leaching process was studied.The results showed that the leaching reaction was basically equilibrium as the leaching time was more than 25 min.At the temperature range from 45 to 67.5 ?,the apparent activation energy of the leaching process was 27.01 kJ/mol,and the leaching process of mixed concentrate after decomposed was controlled by chemical reaction and diffusion.In the early stage,the leaching process was controlled by the rate of chemical reaction.Subsequently,a part of rare earth combined with the phosphate and fluoride ions formed a solid product layer in the surface of mineral particles,which resulted in the leaching process controlled by diffusion.Based on the above results,the decomposition rate of mixed concentrate and leaching rate were compared when the mixed concentrate was processed respectively by microwave heating and conventional heating under the same conditions.The results showed that the decomposition rate of mixed concentrate and conversion rate of fluorine were 89.73%and 88.73%respectively when microwave heating was employed,which were 67.56%and 45.84%respectively when conventional heating was employed.In addition,the leaching rate were 88.31%and 71.82%with microwave and conventional heating.The above results indicated that the microwave heating had a strengthening effect on the decomposition of mixed concentrate.The trends of main elements in the mixed concentrate in the whole process were analyzed.The results showed that the fluorine was mainly into the washing liquid,and the content of fluorine in the mixed concentrate after water washing was 1.09%,which was 11.36%to the total amount of fluorine.The phosphorus was mainly leached into the leaching solution,and a small amount of phosphorus was into the washing liquid.For rare earth elements,11.67%of the rare earth elements was into the leaching residue so that the mass content of rare earth elements in the leaching residue was up to 43.16%,which can be returned to the microwave heating process for decomposing secondly. |
PDF Full Text Request