Study On Recovery Of Rare Earth Elements From Coal Fly Ash Digestion Solution By Modified Silicon-Based Adsorbents

Coal fly ash,as the main solid waste from coal-fired power plants,has been widely studied and attracted much attention in the comprehensive recovery of valuable metals due to its large reserves and high production capacity.Using coal fly ash as a substitute resource for rare earth minerals will mainly face challenges such as its relatively low rare earth grade,and difficult separation due to its complex composition.In recent years,research on the recovery of rare earth elements from coal fly ash has mainly focused on the occurrence state of rare earth elements in fly ash and the calcination and leaching methods of fly ash.These studies aimed to address the problem of relatively efficient and low-energy transfer of rare earth elements from solid particles in coal fly ash to leaching solution.However,few studies have been conducted on how to separate and recover rare earth elements from a leaching solution,which has a complex composition and a very low proportion of rare earth.In this study,two silica-based materials,high-purity silica gel,and an SBA-15 molecular sieve,were used as substrates.They were chemically modified by a surface modification to obtain two different solid adsorbents with amino and carboxyl functional groups,respectively.The modification results of the two adsorbents were evaluated by material characterization analysis,and the optimal use conditions and adsorption selectivity of the two.adsorbents were explored through controlled variable experiments.Finally,based on the characteristics of the fly ash digestion solution,the adsorbent with better performance was selected,and the separation of major metal elements and the recovery of rare earth elements in the fly ash digestion solution was achieved through stepwise precipitation combined with solid-liquid extraction.The main research content and results are as follows:（1）Two adsorbents,amino-functionalized silica gel（NH₂-Silica gel）and carboxylfunctionalized silicon-based molecular sieve（DTPA-NH-SBA-15）,were prepared by modifying the surface of high-purity silica gel and SBA-15 molecular sieves,respectively,using 3-aminopropyltriethoxysilane（APTES）and a two-step method involving APTES and diethylenetriaminepentaacetic dianhydride（DTPADA）.Results from controlled variable experiments showed that the two materials had significant differences in pH adaptability,with NH2-Silica gel being suitable only for weakly acidic to neutral environments,almost completely losing effectiveness under conditions of pH≤4 due to the protonation of amino groups.In contrast,DTPA-NH-SBA-15 demonstrated significant acid adaptability and maintained a stable and reliable high adsorption rate in the pH range of 3-6.In terms of adsorption rate,process,and selective adsorption,the two materials performed similarly,with both showing fast adsorption rates and exceptional performance within the first 10 minutes to 5 minutes of adsorption.Adsorption equilibrium was achieved within 30-60 minutes,with 86.4%and 93%of the maximum adsorption capacity being reached,respectively,and dynamic equilibrium being achieved after 90 minutes.Kinetic modeling results showed that the adsorption process of both materials followed a pseudo-second-order kinetic model,with chemical adsorption being the main mechanism.Isotherm modeling results showed that the adsorption process of both materials was more consistent with the Langmuir isotherm model,indicating monolayer adsorption on a homogeneous surface.The maximum adsorption capacities for neodymium were determined to be 32.59 mg·g^-1 and 34.84 mg·g^-1 for NH2-Silica gel and DTPA-NH-SBA15,respectively,with the latter showing an approximately 7%improvement over the former.Selectivity analysis experiments showed that both adsorbents had certain selectivity for rare earth elements,especially the middle and heavy rare earth components,and that the presence of Al3+ in the solution slightly reduced the adsorption capacity of rare earth elements,while low-valence Ca²⁺ and Mg²⁺ ions had almost no competitive effects.（2）Using DTPA-NH-SBA-15 as the solid phase extraction agent,combined with a stepwise precipitation method to recover rare earth elements from fly ash leachate.Aluminum and iron elements are first precipitated as hydroxides,while low-valence elements such as calcium and magnesium are separated from rare earth elements by solid phase extraction.The experimental results show that the overall recovery rates of rare earth elements are about 60%to 80%,and the total rare earth elements recovery rate is about 71.45%.