Study On The Occurrence And Extraction Of Rare Earth Elements In Coal And Their Coal Ash

Rare earth elements(REY)play an important role in industrial applications.As unbalanced between supply and demand of REY increases,it has stimulated the global search for alternative non-traditional rare earth sources.Coal and coal by-products(slag and fly ash)are considered potential rare earth sources.Extensive studies have been done on the content,occurrence form,and recovery of REY,but the research on the release of harmful elements in the recovery process of REY is insufficient,which has important guiding significance for the process design of environmentally friendly recovery of REY.Therefore,aiming at the coal,coal gangue,and coal ash of four typical coal-fired power plants in China,the enrichment and occurrence form of REY in the combustion process were studied,and the method of alkali fusion-water leaching-acid leaching recovery of REY in fly ash was established.The mechanism of alkali fusion activation of fly ash and the migration and transformation of harmful elements in the recovery process of REY was revealed.Focusing on the above research contents.Firstly,the contents of REY in coal and coal gangue and their static combustion products in laboratory and dynamic combustion products in power plants were determined by microwave digestion-inductively coupled plasma mass spectrometry.The enrichment ratio,enrichment factor,and mass balance rate were used to evaluate the enrichment effect of fly ash on REY in the dynamic combustion process of power plants.The results show that the outlook coefficient of REY in coal and coal gangue and their combustion products are greater than 0.7,and the content of critical rare earth elements is more than 30%.The total REY content in fly ash of three power plants is higher than the world average,and it has great potential to recover rare earth elements;The enrichment ratio and enrichment factor of REY in fly ash are larger than those in slag,and the enrichment ratio of REY in slag is close to that in static combustion ash of muffle furnace.In addition,during the static combustion process,the release rates of total REY in Diaobingshan coal gangue,Xuzhou bituminous coal,Sanhe bituminous coal,and Shangdu lignite are 10.2%,4.5%,5.2%,and 8.1%,respectively.Meanwhile,the release rates of each rare earth element are between 0.6%and 16.7%.However,during the dynamic combustion process,only Ce,Er,Yb in Diaobingshan coal gangue and Nd,Y,Er,Yb,and Lu in Sanhe bituminous coal were slightly released,and other rare earth elements and total REY in four power plants are almost retained in slag and fly ash.It shows that fly ash has an enrichment effect on REY in the dynamic combustion process.Secondly,sequential chemical extraction,correlation analysis,SEM-EDX,and thermodynamic equilibrium calculation were used to study the occurrence of REY in coal and coal gangue and their combustion products.The results show that the REY mainly occurred as the acidsoluble and aluminosilicate fraction.At the same time,the content of REY in coal and coal gangue is positively correlated with ash content,and negatively correlated with volatile content.It shows that REY in coal and coal gangue mainly exists in inorganic minerals;However,REY in the combustion products was mainly presented in the aluminosilicate fraction regardless of coal or coal gangue,reaching up to approximately 80%of the total REY content.During the combustion,around 36%,23%,5%,and 20%of the other five fractions(water-soluble,ion-exchangeable,acid-soluble,organic,and sulfide)are transformed into the aluminosilicate fraction from Diaobingshan coal gangue,Shangdu lignite,Sanhe and Xuzhou bituminous coal,respectively;In addition,the thermodynamic equilibrium calculation and SEM-EDX characterization show that Al in fly ash plays a major role in the occurrence of REY.At the same time,uncalcined rare-earth phosphate mineral particles are found in fly ash,indicating that the rare earth mineral particles in coal and coal gangue are still dispersed in fly ash as original mineral particles during combustion.Then,REY in fly ash was extracted by NaOH roasting activation-water leaching-acid leaching.The activation mechanism was revealed by thermodynamic equilibrium calculation,TG-MS analysis,and XRD characterization.The effects of different alkali-to-ash ratios and alkali fusion temperatures on the recovery rate of REY were analyzed by ICP-MS.The results show that the reaction temperature range of NaOH roasting activated fly ash was near the melting point of NaOH(318℃).NaOH can convert SiO2,Al2O3,Fe2O3,and mullite in fly ash into Na2SiO3,NaAlO2,NaFeO2,and NaAlSiO4 dissolved in water or acid.Under the conditions of an alkali-to-ash ratio of 1.2:1 and roasting temperature 318℃,the glass phase in fly ash can be effectively decomposed,and the recovery rate of REY in fly ash can reach more than 90%.Finally,the migration and transformation of mercury in alkali fusion activated fly ash were analyzed based on the method of temperature-programmed desorption coupled online mercury analyzer.At the same time,during the extraction of REY from alkali fusion activated fly ash,the hydride generation-atomic fluorescence method was used to determine As and Se,and ICP-MS was used to determine Pb,Cr,and Cd.The results show that mercury compounds,i.e.,Hg2Cl2,HgCl2,and HgSO4 in fly ash were converted into HgO during NaOH roasting activation.At the same time,the generated water reduced part of HgO to elemental Hg during calcination,which increased the release of Hg in fly ash at low temperature;In addition,other harmful elements,such as As,Se,Pb,Cd,Cr,have also undergone a chemical transformation during NaOH roasting.For example,As compounds(As2O5,As2O3,Ca3(AsO4)2,and FeAsO4)were converted into Na3AsO4.Se compounds(SeO2,and CaSeO3)were converted into Na2SeO3.PbSO4 and PbCl2 were converted into PbO.CdCl2 and CdSO4 were converted into CdO.CrO3,CrCl3,and Cr2(SO4)3 were converted into Na2CrO4 and NaCrO2.Thus,the leaching characteristics of each element in water leaching and acid leaching were changed.For example,As,Se and Cr are mainly retained in the water leaching solution,accounting for 75.16%,92.50%,and 61.16%,respectively,while Pb and Cd are the same as REY,and are all retained in the acid leaching solution.Therefore,alkali fusion not only improves the recovery rate of REY but also increases the release of harmful elements and changes the leaching characteristics of harmful elements.The research results of this work not only reveal the enrichment and the occurrence characteristics of REY during the combustion process of coal and coal gangue in power plants but also reveal the reaction mechanism of alkali fusion activation fly ash and the migration and transformation path of harmful elements based on the optimal extraction rate of REY obtained by alkali fusion activation-water leaching-acid leaching,which provides a theoretical basis and guidance for controlling and reducing the emission of harmful elements in the recovery process of REY.