Study On The Enrichment,Extraction,and Mechanism Of Occurrence Of Rare Earth Elements In Coal Fly Ash

Rare earth elements(REE),including scandium,yttrium,and,lanthanide,are strategic metal resources.Due to the increasing market demand and the shortage of traditional ore resources,the recovery of rare earth elements from secondary resources has become a research hotspot.Coal is considered as a future source of rare earth elements,and the levels of rare earth elements in coal or coal ash in some areas are close to or up to those found in traditional rare earth mines.This paper studied the occurrence state of REE in CFA,discussed the transformation law of REE during the process of coal combustion,compared the enrichment effect of various methods on REE,and developed the roasting-washing-acid leaching recovery process,which provides a reference for the resource utilization of REE in CFA.The geochemical characteristics and occurrence modes of REE in CFA are analyzed.The sequence chemical extraction procedure showed that modes of occurrence of REE contain ion-exchange,acid soluble,metal oxide,organic/sulfide,and aluminosilicate(the dominant one).REE is enriched in non-magnetic fine particles with a density of 2.4-2.8 g/cm³ after the research on the REE distribution using sieving,magnetic separation,and sink-float test.The fact was found that REE have a positive correlation with major elements and their linear regression prediction model was established using the mathematical analysis method.The occurrence carrier of REE in CFA was observed by scanning electron microscope,which includes rare earth minerals,rare earth oxides,existing in or on glass phase,and combining with iron oxides.The model minerals for constructing REE in coal include monazite(rare earth phosphate resources),fluorite(rare earth carbonate resources),ion-adsorbed rare earth ores(rare earth aluminosilicate resources),and modified lignite(rare earth in organic matter).Using the drop tube furnace combustion platform,the coal combustion process was simulated,and the transformation of rare earth in the formation of CFA was studied.The rare earth phosphate ore after combustion still exists in the form of phosphate ores.REE in carbonate ore and organic matter exists in the form of rare earth oxides.Rare earth elements in aluminosilicate are most likely to oxidized and still bond to aluminosilicate,though the different changes happened at different temperatures.The dehydrogenation reaction happened for halloysite and kaolinite at800?,with the morphology of particles is mainly clastic and higher leaching rate of REE.Whereas,the aluminosilicate minerals mainly were transformed to mullite at1200?,with the globular and flake particles,and lower leaching rate of REE.It was found that the leaching rate of REE in CFA from circulating fluidized bed is higher than that in CFA from pulverized coal furnace.the effects of particle size and magnetic strength classification on the enrichment of REE in CFA were investigated.REE content increased from 782ppm to 1025ppm in Guang'an CFA,as a proxy.The leaching rate of REE in the enriched products also increases due the touch area of acid and particles.The effect of flotation parameters on the removal of unburned carbon in CFA was evaluated.It was found that REE didn't enrich after flotation and the leaching rate didn't increase too.The kinetic of desilication of CFA using alkali solution was studied and the process meets the unreacted core model.The first stage was controlled by chemical reaction and the second stage was controlled by inner diffusion.Based on the chemical reaction rate equation,the reaction order was calculated using the differentiation for NaOH concentration and Si removal.They are 1.02 and 0.73 for the first and second stage,respectively.Based on the Arrhenius equation and Si removal at different temperatures,activation energy and pre-exponential factor were obtained,resulting in the kinetic equation for the first and second stage:1-(1-X)^1/3=72.15×10⁵exp[-70890/RT]t,and1+2(1-X)-3(1-X)^2/3=42.72×10^-5exp[-3.21/RT]t,respectively.The third stage existed in which Al-Si zeolite was generated when temperature and NaOH concentration are proper.The multistep desilication was developed to decrease the content of Al and Si in the solution,avoiding the generation of zeolite.REE increased from 1025ppm to 2135ppm using this method.The difference of Al and Si behavior during desilication support the possibility of comprehensive recovery.The bond energy decrease of Si(2p)and Al(2p),the results of X-ray Photoelectron Spectrometer indicated the breakage of the bond of Si-O and Al-O and the dissolution of aluminosilicate glassy phase which is helpful for the leachability of REE.The higher recovery of REE was obtained using the multistep desilication because the generation of zeolite and silicic is less than one-step desilication.The experimental investigation on roasting and leaching for the recovery of REE from CFA was performed.Based on references and chemical knowledge,the hypothesis of chemical reactions between CFA and additives during the roasting.The Gibbs free energy at different temperatures was calculated using the software of HSC chemistry.The hypothesis was verified by the phase in roasting production as the results of X-ray diffraction.Based on the results of washing and acid leaching,the optimal roasting temperature should be around melting points of additives and NaOH/Na2CO3 was chosen as the optimal additive.The roasting product contains Na2Si O3and Na Al Si O4 which dissolve in washing,and acid leaching,respectively.The residue of washing is porous and benefits the leachability of REE.Silica generated from silicic after heating is the main component of acid leaching residue.The model compounds of CFA were established using pure silica,mullite,and ferric oxide powder.The theoretical requirements of NaOH for the complete reaction with CFA were calculated which 1g of CFA required 0.65g of NaOH.The ratio was verified by the fitting of the thermogravimetric curve.The D-Optimal design and Box-Behnken experiment design were applied for response surface analysis to study the effect of process parameters on REE recovery and get the optimal conditions.Thus,a combined process of roasting-washing-acid leaching was formed to recover rare earth elements from fly ash.This paper contains the figures of 108,the tables of 30,and the references of 235.