Occurrence And Transformation Of Lithium And Gallium In Coal And Coal Ash From Typical Mining Areas Of Shanxi Province

Lithium（Li）and gallium（Ga）are important strategic metal resources in China.Due to the growing demand and the tight supply of traditional ore resources for them,the secondary utilization of critical elements from alternative resources has become a research hotspot.As an organic rock,coal is enriched with Li and Ga under a certain geological condition,reaching the content and scale of industrial extraction.Moreover,the content in coal ash even is higher than the grade of conventional deposits for these elements.The distribution and occurrence of Li and Ga in coal and coal combustion products has become one of the frontier directions in the field of coal geology.However,insufficient understanding of the occurrence of Li and Ga in coal and their transformation during combustion restricts their utilization.Samples of feed coal,fly ash and bottom ash were collected from seven typical power plants in the mining areas of Pingshuo,Xishan,Yangquan and Jincheng in Shanxi Province,as well as the raw coal（Antaibao No.11 coal seam）used for the feed coal of the power plants in the Pingshuo mining area.The samples mineralogical characteristics were characterized by X-ray diffraction（XRD）,scanning electron microscopy with energy spectrum（SEM-EDS）and micro-Raman spectroscopy（Micro-Raman）.The content of major and trace elements in the samples were measured by X-ray fluorescence spectrometer（XRF）and inductively coupled plasma mass spectrometer（ICP-MS）.The occurrence of Li and Ga in coal and coal fly ash were discussed by combining the method of low-temperature ashing,correlation analysis,cluster analysis,classification experiments,and time-of-flight secondary ion mass spectrometry（TOF-SIMS）.The transformation of Li and Ga during combustion were analyzed based on the distribution of elements in coal and coal combustion products.The following results were obtained.It firstly investigated the geochemical characteristics of the Antaibao No.11 coal,which was used for the feed coal of power plants in Pingshuo mining area.Visualization of elemental distribution in micro-regions was achieved by means of TOF-SIMS and SEM-EDS.The results show that No.11 coal is bituminous coal with medium ash,high volatile matter,and medium to high sulfur content.Mineralogical characteristics and major elements index（K）indicate that the upper coal seam is more influenced by marine water.The average content of Li and Ga in the upper seam are 81.09μg/g and 21.50μg/g,while 145.84μg/g and 20.83μg/g for the lower part samples.Correlation analysis and cluster analysis show that Li is associated with aluminosilicate.In the micro area of the Li-enriched coal bench of the upper part,Li is associated with Si,Al,Na,K,Ca,Mg,and Fe identified by TOF-SIMS.The carries of Li are kaolinite,montmorillonite,illite,and chlorite according to the mineral composition.In another Li-enriched sample in the lower part,only Si and Al are enriched in the Li rich areas.Lithium only occurs in kaolinite.Gallium is more uniformly distributed in the coal seam.The correlation analysis shows that there is no obvious correlation between Ga and aluminosilicate,but Ga is clustered with Stannum（Sn）related to sulfide.Gallium is enriched in kaolinite,boehmite,pyrite,halite,and sylvite in micro area identified by SEM-EDS.For the feed coal of power plants,the correlation between Li and Ga with other elements were quantified according to the element distribution identified by TOF-SIMS.Combing the mineralogical composition,the occurrence of Li and Ga was obtained.The results show that Li and Ga mainly occur in kaolinite for each feed coal.In addition,there are differences for the samples in each mining area.The feed coal of the power plant in Pingshuo mining area is high volatile bituminous coal.The carriers of Li and Ga are mainly kaolinite and boehmite.Little Li and Ga are associated with organic matter.The feed coal in Xishan mining area is volatile bituminous coal.66.67%of the Li-enriched area has the enrichment of Mg and the substitution of Li for Mg in montmorillonite is the main occurrence of Li.Moreover,part of Ga occurs in sulfide for the coal in this mining area.The feed coal of power plant in Yangquan mining area is low volatile bituminous coal.Illite can host part of Li and Ga due to the obvious K enrichment in the Li and Ga rich area.The feed coal in Jincheng mining area is low volatile bituminous coal.Lithium and Ga can occur in ammonium-rich mica and paragonite,which makes a high proportion of Na enrichment in the Li and Ga-enriched area.Lithium ions and other alkali and alkaline earth metals are easily retained in solution during weathering and transportation,then they can form clay minerals or adsorb on the surface of clay minerals.Gallium shows a certain degree of sulfur affinity and iron affinity besides the lithophile affinity,which makes the enrichment of Si,Al,Na,K,Mg,Fe,and S in the Ga-enriched area.The composition of pulverized coal furnace（PC）fly ash and circulating fluidized bed（CFB）fly ash was characterized by XRD,SEM-EDS and Micro-Raman.The structure of Si and Al were analyzed by X-ray photoelectron spectroscopy（XPS）and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy（MAS-NMR）.The occurrence of Li and Ga in fly ash was analyzed by combining sieving,phase classification,and element distribution identified by TOF-SIMS.The results show that the content of Li in all size fractions for PC fly ash particles increases with the increasing Si O₂content.95.14%of the Li in PC ash occurs in glass according to the phase separation.The relatively high content of bridging-oxygen Si and less substitution of aluminum tetrahedra for silica tetrahedra allow Li ions to break the bridging silica and bind to oxygen.This leads to a higher enrichment proportion of Si in the Li-enriched region.Gallium is physically adsorbed to the particles below 300 mesh in PC fly ash and is independent of the Si and Al content.For the particles over 300 mesh,Ga content increases with increasing Al content.The substitution for Al is the main occurrence of Ga.Lithium and Ga in CFB ash particles of all size fractions increase with increasing content of Al.Lithium is confined in the Si–O–Al bond,and Ga exists in the form of substitution for Al.In addition,TOF-SIMS element distribution map shows that there is Na enrichment and Ca enrichment in the Li-enriched area in PC ash,and K enrichment in the Li-enriched area in CFB ash.Moreover,there is Fe enrichment in the Ga rich area for PC ash and CFB ash.The replacement of Fe ions by Ga ions makes the distribution proportion of Ga in magnetic phase higher than that of Li.The volatilization and transformation of Li and Ga during coal combustion were analyzed by comparing the element contents in the feed coal,fly ash and bottom ash.The relative enrichment index indices show that Li and Ga are moderately volatile during coal combustion in pulverized coal furnace and circulating fluidized bed.Most of the volatilized elements are present in fly ash.The enrichment multiplier shows that the enrichment of Li in fly ash decreases with the increase of feed coal ash content.The results of emission rate show that the emission rate of Ga decreases with the increase of feed coal ash content.The reducing phase formed by organic matter in the high temperature reduction zone reduce the oxides of gallium to low-valent oxides which are easy to volatilize.The weak relationship between Ga and aluminosilicate in the feed coal and the promotion of reducing substances make the proportion of Ga escaping to the gas phase higher than the proportion of Li escaping to the gas phase in the same plant.Lower combustion temperature（900°C）and longer residence time of coal particles in CFB furnace decrease the volatilization and release of Li and Ga.By comparing the relationships between Li and Ga with other elements in the feed coal and fly ash,it is found that Li,Na and K in the water-soluble and organic bound states in the feed coal at the Pingshuo mine migrate together and are enriched in the same region in fly ash after volatilization,condensation,and physical adsorption.Lithium and Ga,occurring in aluminosilicates in the feed coal of power plants in Xishan,Yangquan and Jincheng mining areas,migrate with Na and K then enter in glass in PC ash or aluminosilicates in CFB ash.The addition of desulfurized lime during combustion promotes the capturing of volatile Li and Ga,resulting in the enrichment of Ca in the Li and Ga-enriched areas in the TOF-SIMS map of fly ash.