Study On Environment And Resource Properties Of Coal Gasification Wastes

Coal gasification is an effective way of clean and efficient utilization of coal,which is of great significance to the development of national economy and society.However,in addition to all kinds of coal chemical products,the process will inevitably produce a large number of coal gasification wastes（such as gasification ash,gasification slag,gasification residue）.A large amount of coal gasification wastes are produced and can not be used effectively,causing serious environmental pollution and ecological damage,which restricts the sustainable development of coal gasification industry.In this paper,the gasification ash,gasification slag and gasification residue produced in typical coal gasification process are taken as the research objects,through carries on the apparent morphology,particle size distribution,specific surface area,melting characteristics such as physical properties,proximate analysis,element composition,mineral composition and chemical properties,carbon crystallite structure,the characteristics of the groups,such as element valence,the structure of the ligand structure feature analysis,clear the resource properties of typical coal gasification ash;At the same time,the leaching characteristics of toxic and harmful elements in gasification ash,gasification slag and gasification residue,as well as the static leaching characteristics and dynamic leaching characteristics of heavy metal elements were compared and studied to reveal the environmental properties of typical coal gasification ash and slag.The main research conclusions are as follows:（1）The resource properties of typical coal gasification ash are clearly analyzed.（ⅰ）The physical properties of coal gasification wastes were quite different.The macroscopic appearance of coal gasification ash was irregular block and powdery particles,and the microscopic morphology was dense and smooth with flocculation porous block or spherical.The particle size of gasification ash and residue was smaller in micron level,and the particle size of gasification slag was larger in millimeter level.By comparing the specific surface area and pore diameter of the three,it is found that the specific surface area and pore diameter of the gasification ash and slag were small,and the gasification residue was large,mainly due to the dense and smooth surfaces of gasification ash and gasification slag,and the gasification residue was mainly porous and spherical.The melting point of gasification ash has obvious regularity.The melting temperature of gasification ash was the highest,followed by that of gasification ash and the lowest of residue,which was mainly related to its chemical composition and mineral composition.（ⅱ）The industrial composition analysis of gasification ash,gasification slag and gasification residue mainly includes ash,volatile matter,fixed carbon and calorific value.The gasification ash and gasification slag has a larger ash content,while the gasification residue has the largest fixed carbon content and a higher calorific value.The main chemical components of ash in gasification slag were inorganic components Si O₂and Al₂O₃,and the contents of alkali metals and alkaline earth metals were different.On the whole,the three minerals of gasification ash were vitreous body and residual carbon,mainly because the ash of coal gasification was rapidly cooled by water or air cooling after high temperature and does not form crystalline minerals.（ⅲ）The morphology of carbon in coal gasification wastes were different.The carbon in gasification residue was graphene-like aromatic layer carbon,while the gasification ash and gasification slag were amorphous carbon.The characteristic group analysis results of coal gasification ash and slag were consistent with the industrial composition analysis results.The Si-O and Al-O strength of gasification ash and slag were higher,and the aromatic C-H group strength of gasification residue is higher.The valence states of Al and Si in coal gasification wastes are+3 and+4,respectively.The coordination structure of Al was mainly[Al O₄],and the coordination structure of Si was mainly Q⁴（2Al）and Q⁴（1Al）.（2）The environmental properties of typical coal gasification ash are revealed.（ⅰ）The Leaching behavior of As,Ba,Cd,Cr,Cu,Mn,Ni,Pb,Ti and Zn in coal gasification wastes were studied by acetic acid buffer solution method,sulphuric acid and nitric acid method,horizontal vibration method and Toxicity Characteristic Leaching Procedure（TCLP）.The results showed that the coal gasification ash had the largest leaching amount in the method of acetic acid buffer solution,but the leaching amount was lower than the limit specified in the"Identification of Leaching Toxicity of Hazardous Wastes",so it did not belong to hazardous waste.In particular,the leaching amount of Zn（3.9mg/L）in gasification ash by horizontal oscillation method exceeds the prescribed limit of"Comprehensive Wastewater Discharge Standard"（2.0mg/L）,and it belongs to the general industrial solid waste（Ⅱ）.（ⅱ）During the static leaching process,the leaching concentration of heavy metal elements such as As,Cd,Cr,Cu,Mn,Ni,Pb,and Zn in the coal gasification wastes showed fluctuating changes.Under the same p H value,the leaching concentration of Mn,Ni,Pb in the gasification ash increases slowly and then becomes stable with the extension of the leaching time;The leaching concentration of Zn first increases rapidly with the extension of the leaching time,then decreases to a certain concentration and finally stabilizes;the leaching concentration of As,Cd,and Cu has been fluctuating during the experiment time;the leaching concentration of Cr has not reached the detection limit.The leaching concentration of As in the gasification residue increases slowly with time and then becomes stable;the leaching concentration of Mn and Ni has been in a stable state;the leaching concentration of Cd,Cr,Cu,Pb,and Zn has not reached the detection limit.The leaching concentration of As in the gasification slag increased slowly with time and then stabilized;the leaching concentration of Mn,Ni,and Pb had been fluctuating with time;the leaching concentration of Cd,Cr,Cu,and Zn did not reach the detection limit.In the same leaching time,the difference in the leaching concentration of heavy metals in coal gasification eastes under different p H values is small,and the leaching concentration is generally larger when p H=4.5.（ⅲ）The leaching concentration of heavy metals in the dynamic leaching liquid of gasification slag is affected by time and p H.Under the same p H conditions,the leached concentration of As in the gasification slag gradually increased with time;at the same time,the leached concentration was greater at p H=4.5.Under the same p H condition,the leached concentration of Cu in the gasification slag gradually increased with time;in the same time,the leached concentration was greater at p H=5.6.The leaching concentration of Ni varies greatly.At p H=4.5,the leaching concentration gradually increases with time;at p H=5.6 and p H=7.0,the leaching concentration of Ni first decreases rapidly and then stabilizes.The leaching concentration of Pb at p H=7.0 gradually increased with time;the leaching concentration at p H=4.5 and p H=5.6,that is,under acidic conditions,did not reach the detection limit.