Evaluation Of Rare-Earth In Coal Fly Ashes And Environmental Indication Of Rare-Earth-Containing Micro-Nanoparticles

Rare-earth（REY）is an important mineral resource as well as geochemical indicator.Rare-earth is being widely used in many fields including military,defense and civil construction due to its unique physical and chemical properties,and has been classified as"strategically critical resource"and"key raw material",with its global demand growing year by year.Facing the shortage of traditional rare-earth mineral resources,exploring additional sources of rare-earth is one of the important strategies to fill the rare-earth demand gap.Coal fly ash（CFA）is widely considered as one of the most promising additional sources of rare-earth.Mountains of CFAs discharged every year provide a huge potential source of rare-earth.In order to explore the resource potential of rare-earth in CFAs,it is important to screen the CFAs with economic recovery value of rare-earth,and identify their spatial distribution and reserves.Several countries have conducted nationwide studies on rare-earth potential in CFAs from their domestic coal-fired power plants（CFPPs）to date,while it is still lacking in China,who is the largest CFA producer in the world.REY is an important geochemical indicator,and the media formed under different environmental conditions usually have their unique REY distribution patterns to be distinguished.CFPP is a typical anthropogenic pollution source.However,due to the diverse chemical compositions of both coal combustion products and supergene environmental media,traditional REY source apportionment approaches can hardly perform effective quantitative traceability.Coal combustion activities can continuously release large amounts of micro-nanoparticles（MNPs,particle size<1μm）.MNPs is a source tracer due to its strong penetration and mobility,and the ability to inherit its source characteristics to some extent.REY can present in coal combustion products in the form of MNPs,while quantitative source traceability analysis of the MNPs containing REY（REY-MNPs）is still lacking.To investigate rare-earth potential in CFAs from China’s CFPPs and environmental indication of REY-MNPs,this study focused on the CFAs from 186 typical CFPPs in China（including 8 long-term monitoring CFPPs,4 ash dams with a total time span of exceeding 60 years）and road dusts around a typical CFPP,applied electron paramagnetic resonance spectroscopy（EPR）,inductively coupled plasma mass spectrometry,single-particle inductively coupled plasma time-of-flight mass spectrometry（spICP-TOF-MS）,electron microscopy（EM）technologies,machine learning and storage inventory to conduct a comprehensive assessment of rare-earth potential in CFAs from China’s CFPPs,and explore the environmental indication of REY-MNPs.The main results are as follows:（1）CFAs in China’s CFPPs is a promising source of rare-earth.Results showed the distribution characteristics of REY concentrations and light to heavy REY ratios reflected the coal transportation patterns of China,indicating these samples were nationally representative.REY concentrations fluctuated in the short term,while relatively stable in the long run,with deviations of REY concentrations generally below 16%.This could be attributed to the huge and continuous demands of coals in CFPPs and relatively fixed transportation networks,making the coal sources in CFPPs remain relatively fixed over long periods.According to the potential studies based on REY_def-K_outl,all the CFAs in this study had rare-earth recovery potential,with REY_defranged from 27.3%to 42.7%and K_outl ranged from 0.62 to 1.35.（2）Considering REY concentration determined whether the CFAs had economic recovery value,while traditional approaches of REY concentration determination were generally time-consuming and labor-exhausting,this study established a fast screening method of the CFAs with economic recovery value of rare-earth by EPR.Results showed the CFAs without significant six-fold resonances had economic recovery value of rare-earth,as REY concentrations in these CFAs were higher than the cut-off grade of 300 mg/kg,while the CFAs with significant six-fold resonances were the opposite.This could be attributed to the homogeneity replacement of Mn²⁺and REY³⁺by Ca²⁺,resulting in the reduction of Mn²⁺and REY³⁺in Ca-rich CFAs.Considering the main coal-forming periods in China are consistent with the global ones,making Chinese coals representative globally,and the consistency of the major components of global CFAs,this study suggested this fast screening method is potentially globally applicable.（3）CFAs in China’s CFPPs is a huge storehouse of rare-earth.Based on random forest algorithm,this study established a prediction model,with prediction deviation<16%,to predict REY concentrations in CFAs from exceeding 1,000 typical CFPPs in China.Results showed the average REY concentration in CFAs from China’s CFPPs was 345 mg/kg.Raw coal characteristics and coal combustion technologies affected REY concentrations in CFAs together,among which,REY concentration and ash content in raw coal,and boiler type were the 3 governing factors.Storage inventory was applied to calculate rare-earth reserves in the CFAs with economic recovery value（REY≥300 mg/kg）.Results showed there were 74,000 tons of rare-earth in the unutilized CFAs in China annually,which could meet the global demand for about half a year and bring in gross profits of 4.2 billion dollars.In addition,CFAs from the CFPPs located in 11 regions in China,including Inner Mongolia,Anhui and Guizhou,et al.,contributed exceeding 70%,which was closely related to the background of REY mineralization in coal,cross-regional coal transportation and CFAs production.（4）CFPPs are typical anthropogenic pollution sources,focusing on the road dusts around a typical CFPP,this study,with the help of source indication of REY and MNPs,found that REY-MNPs could be used to conduct quantitative traceability analysis for complex environmental samples.After establishing a multi-element simultaneous testing method in spICP-TOF-MS,optimizing a quantitative extraction algorithm of MNPs fingerprints,and establishing and optimizing a quantitative traceability model,this study found that CFPP emissions contributed 63.99±8.55%to the surrounding road dusts.In the relative importance ranking of all elements,3 of the top 5 key drivers were REY,and Ce ranked top.