Occurrence Characteristics,Migration And Transformation Of Critical Elements During Coal Combustion

With the development of new technological industries,China is gradually in short supply of strategic critical metal resources.Recent researches show that amounts of critical elements are associated with coal in many areas of China,and there exists obvious element enrichment in coal ashes after combustion,which makes it possible that coal fly ash becomes a new alternative resource for critical metals.In this thesis,feed coal samples and combustion products containing abundant critical metal elements in Chongqing power plant are investigated.The enrichment factors,distribution pattern and occurrence modes of critical elements in coal and coal ash are characterized,using technique methods such as ICP-MS,XRD,XRF,SEM-EDS,TG-DSC,sequential chemical extraction and so on.In addition,the migration law and influencing factors of critical elements in the process of coal combustion are discussed,and the results will provide a scientific reference for the extraction of critical elements from coal and coal ash.The critical elements Li,Ga,Sr,Zr,Nb,Hf,Ta and REY are enriched in the feed coal of the power plant,while the contents of Rb,Cs,U and Ge are at the normal level.Li and Ga mainly occur in clay minerals such as kaolinite and illite,and part of Ga is related to pyrite.The occurrence state of Sr is multiple,which is made up of calcite,organic matter,clay minerals,and ion free form in coal.The elements Nb,Ta,Zr and Hf host in clay minerals,zircon and anatase.REY is mainly related to silicate minerals,and its independent minerals（i.e.,REE-phosphate mineral）have also been found in coal.The behavior change of critical elements during coal combustion is mainly affected by temperature.As the temperature is elevated,the contents of Li,Sr,Zr,Nb,Cs,Hf,Ta,U,Ge and REY first increase and then decrease,while the Ga content increases and the Rb content decreases.Among them,in terms of possible extraction,Ga,Sr and REY have the best activation effect at 700℃,and Li has the best activation effect at 500℃.However,Zr,Nb,Hf and Ta are constrained by their carrier minerals and could not be activated within the full temperature range in this experiment.Combined with the results from thermodynamic calculation simulation,the main forms of some critical elements at 700℃are inferred as following:Li mainly exists as silicate aluminates(Li Al Si O₄,Li Al Si₂O₆,Li₂Al₂Si₈O₂₀ and Li Al Si₂O₆（β）);Ga is present in the form of oxide（Ga₂O₃）;Sr mainly cccurs in Sr Si O₃;Nb compounds are stable,mainly in the form of oxides（Nb O₂ and Nb₂O₅）;Ce in LREY primarily exists in the form of Ce O₂,Ce O_1.83,Ce O_1.72 and Ce Al O₃.Y in MREY mainly occurs as Y₂Si₂O₇ and a small amount of YFe O₃.Er in HREY is mainly in the form of Er Fe O₃ and Er₂O₃.The contents of most critical elements in different coal combustion products display the following order:fly ash>slag>bottom ash,and the critical elements Li,Ga,Sr,Zr,Hf,Nb,Ta and REY in fly ash are enriched.Among them,Li,Ga,Nb,Ta and REY are more enriched in small particle size and non-magnetic fly ash components.Taking fly ash as an example,most critical elements mainly occur in silicate glass phase,followed by mullite&quartz phase.