Migration And Evolution Mechanisms For Heavy Metals During Residual Carbon Enrichment And Oxidation/activation Processes Of Ningdong Coal Gasification Fine Slag

Due to the continuous and rapid development of the coal gasification industry in China over recent years,the large amount of coal gasification fine slag（CGFS）generated has brought great pressure to the ecological environment.The unique physicochemical properties of carbon and ash components in CGFS are the basis for its resource utilization.However,there is no mature utilization pathway for CGFS due to the high carbon content/hybrid/toxic as well as the low activity.The migration and transformation patterns of harmful heavy metals in the typical resource utilization pathway of CGFS are still unclear,which restricts the development of its clean resource utilization.This thesis focuses on typical heavy metals（V,Cr,Mn,Ni,Cu,Zn,Ba,and Pb）in the CGFS from the Ningdong region.The leaching characteristics of heavy metals in different environments were studied,besides,the migration and evolution mechanisms of heavy metals in the residual carbon enrichment and oxidation/activation processes of the CGFS were also explored,which could provide the theoretical basis for the clean,green and efficient resource utilization of the CGFS.The main research results achieved in this thesis are as follows:（1）The leaching features of typical heavy metals from the CGFS in different environments were investigated.The leaching toxicity analysis of TCLP showed that the leaching concentrations of Cr,Mn,Ni,Ba,and Pb in different particle size fractions of the CGFS exceed the upper limit of GB/T 14848-2017 for Class III groundwater quality limit value.The leaching rate of V,Cr,Mn,Ni,Cu,Zn,and Ba decreased with the increase in p H value.The leaching concentrations of heavy metals were higher in the small-size particle fractions than the large-size particle fractions in either the acidic or alkaline p H range（2-13）.The leaching rates of Cr,Ni,Zn,and Pb in the CGFS increased continuously with the rising of leaching time and temperature,in addition,the leaching rates of heavy metals changed less after the leaching time exceeded 1080min.Moreover,the leaching kinetic results showed that the reaction rate control step of the leaching process of Cr,Ni,Zn,and Pb in gasification fine slag in an acidic solution with p H 2 was diffusion control,and the leaching activation energy decreased gradually with the decline of particle size fractions.（2）The distribution pattern and occurrence feature of heavy metals in the physical enrichment process of residual carbon were revealed.It was found that there were differences in the degree of residual carbon enrichment among the physical separation products obtained by froth flotation,density separation,and particle size separation.The carbon content of the flotation carbon-rich fraction can reach 54.90%（226.20%higher compared to the raw CGFS）,making it a high carbon resource with promising energy/carbon material applications.Inorganic mineral fractions and skeletal structural properties jointly influenced the distribution of heavy metals in the CGFS.The concentration of typical heavy metals had a strong positive correlation with the mineral content of the physical separation products.However,porous carbon structures with high specific surface area and particle size effects can attenuate the dependence between some volatile heavy metals and mineral content,resulting in the enrichment of Ni,Cu,Zn,and Pb in the flotation carbon-rich fraction,while Zn and Pb were enriched in the small particle size products obtained by particle size separation.The proportion of heavy metals in the acid extractable fraction was higher in the flotation carbon-rich fraction than in the flotation mineral-rich fraction,and the proportion of heavy metals bound in the unstable non-residue fractions was higher in the physical separation products with high residual carbon content and small particle size.In addition,different environmental risk assessment methods indicated that heavy metals in high carbon fractions and small particle size fractions had higher potential environmental risks.（3）Based on the analysis for the properties and mechanisms in the oxidation reaction of carbon-rich fraction,the migration and chemical form transformation patterns of heavy metals during the oxidation reaction at different temperatures/times were clarified.The kinetic results of Coats-Redfern and Doyle equations confirmed that the secondary chemical reaction model（O2）is the most effective reaction rate control mechanism in the oxidation combustion process of the carbon-rich fraction.The activation energy of the oxidation reaction reached the peak（E=174 k J/mol）when the carbon conversion rate was 30%-40%,and the complexity of the oxidation reaction process influenced the conversion behavior of heavy metals.A high correlation between the volatility of heavy metals and the rate of oxidation reaction was found,and the volatility of heavy metals basically tended to increase first and then decrease during the oxidation reaction,among which the volatility of V,Ni,Cu,Zn,and Pb was higher than that of Cr,Mn,and Ba.The alkali and alkaline earth metal compounds were effective carriers of Cr,Mn,and Ba in the bottom slag of the oxidation reaction.As the oxidation reaction proceeds,the type and number of crystal minerals in the bottom slag gradually increased,and heavy metals bound to unstable carbonate minerals as well as sulfides were gradually converted to the reducible and residue states.Two different transformation modes existed for the gradual stabilization of heavy metals in the residue state during the carbon-rich fraction oxidation reaction:heavy metals can enter the stable residue state by binding to crystal minerals（e.g.,Ni are bonded to Ca Ni（Si₂O₆）and Ni Fe₂O₄ crystal minerals）;heavy metals bonded with stable silica-aluminate oxides and then entered the stable residue state.（4）Through investigating the reconstruction pattern of the carbon ash skeleton phase structure during the activation reaction of carbon-rich fraction,the coupling evolution mechanism between heavy metals and the carbon-ash skeleton structure was elucidated.It was found that the raw silica-aluminate-dominated mineral fraction was destroyed during the activation reaction of the carbon-rich fraction,and a new Si-Al-K-Fe skeletal mineral structure was formed.Furthermore,the proportion of V,Cr,Ni,Mn,Cu,and Ba in the stable residue state gradually decreased,while the proportion of heavy metals combined with Fe oxides increased and thus transformed into the reducible state.In addition,V,Cr,Ni,Mn,Cu,and Ba in the non-residue state during the activation reaction were mainly bound to oxides with non-bridging oxygen silicon Si-O₂and aluminum-oxygen tetrahedral structure[Al O₄].While the Zn and Pb in the non-residue state were mainly bound in oxides with bridging oxygen silicon Si-O and aluminum-oxygen octahedral structural units[Al O₆].With the increasing of activation time,more Si-Al skeleton minerals were fused in the carbon skeleton pores,which provided more active sites and carriers for the conversion of heavy metals and thus promoted the chemical form transformation of heavy metals.The silica-aluminate mineral system tended to react with oxygen to form more stable oxides and crystalline minerals during the oxidation reaction,which led to a higher proportion of heavy metals in the residue state.In contrast,the silica-aluminate mineral skeleton tended to transform into the more unstable Si-Al-K-Fe amorphous mineral system during the activation process,while the increased porosity of the carbon skeleton synergistically contributes to the activation of heavy metals to the more unstable chemical forms.Moreover,the risk assessment results suggested that the potential environmental risk of heavy metals in the activation solid products was higher than that of the oxidation reaction bottom slag.