| China is the world’s largest producer and consumer of raw coal,primary aluminum and calcium carbide.Hundreds of millions of tons of coal gangue,millions of tons of hazardous waste aluminum dross and spent cathode carbon blocks and tens of millions of tons of carbide slag are discharged every year in China.But these solid/hazardous wastes is hard to be recycled and reused as resources at present,and only some of them is used by the simple and low-value utilization technologies,such as using coal gangue for bricks making,using aluminum dross for producing water purifiers or refining agents in steelmaking,the processes of which usually consume lots of energy,cause serious secondary pollution for low added value of products.Most solid/hazardous wastes are still landfill disposal or stored on site,causing serious environmental pollution and waste of resources as well.Therefore,it is urgent goal to research and develop green resource utilization technologies to change these solid/hazardous wastes to high value products by a green and clean technical route.Aluminum dross,coal gangue and spent cathode carbon block contain a large amount of valuable components such as alumina,silica and carbon,as well as harmful substances such as aluminum nitride and fluorides.The research targets are proposed to synergistically detoxify and to process a variety of industrial solid and hazardous wastes such as aluminum dross,coal gangue and spent cathode carbon block for the high value added alumina-silica based materials.A series of key scientific issues have been studied in this paper such as enhanced separation of alumina and silica components in coal gangue,deep denitrification of aluminum dross and simultaneous efficient alumina digestion,synergistic and coupling detoxification of the multiple solid/hazardous wastes and efficient alumina extraction,and synthesis of alumina-silica based materials from silica and alumina sources.An innovative technology route and relative key technologies have been developed for high value alumina-silica-based materials such as activating silica minerals in the coal gangue by catalytic combustion to promote caustic silica extraction from its residue,deep denitrifying the aluminum dross and efficient extracting alumina by caustic solution,synergistic sintering of desilicated coal gangue and denitrified aluminum dross residues with carbide slag and spent cathode carbon blocks to extract alumina,and synthesizing high value alumina-silica based materials by using silica source and alumina source made by the technologies mentioned above.An innovative concept of high efficiency separation of alumina and silica minerals by activating silica components and inerting alumina components in coal gangue during combustion is proposed.The enhanced minerals inversion catalysis mechanism in coal gangue combustion activation process was revealed.Based on study results of the properties of aluminosilicate minerals in coal gangue,it was found that the minerals inversion reaction can be enhanced by aluminum dross addition.As a result,a new process of coal gangue activation combustion was invented,including the optimal combustion temperature and activation additives.The minerals transformation process of metakaolinite(Al2O3·2SiO2)to mullite(3Al2O3·2SiO2)by releasing active silica(SiO2)were described.The carbonaceous materials in the coal gangue are burnt for high temperatures,so that the kaolinite releases as much amorphous silica as possible by aluminum dross activation during the reaction process.The amorphous silica can be separated from residues in the subsequent caustic leaching reaction.The silica extraction rate is significantly improved from the traditional 40% to about 60% by the activation process.The silica source(sodium silicate)and the desilicated coal gangue residue with high alumina content were obtained,which laid a good foundation for the further efficient extraction of the alumina source.The high-concentration caustic denitrification and simultaneous alumina digestion process of aluminum dross is investigated,as a result of which the self-heating deep denitrification and simultaneous alumina digestion are realized.Factsage was used for the reaction thermodynamics study for aluminum nitride,which is a harmful substance in aluminum dross under caustic conditions.As the results of thermodynamics study the reactions are feasible and the process is exothermic.When the reaction temperature was 90℃ and the holding time was1.5 h,about 99.26% of aluminum nitride is catalytically reacted and removed,while 38.74% of alumina digestion rate is achieved.All the aluminum hydroxide produced by denitrification was dissolved out to sodium aluminate.Not only the nitrogen element is changed to useful ammonia solution,but also part of the alumina source can be digested in a short process with a lower energy consumption.In this process a desired caustic ratio of sodium aluminate solution can be obtained by adjusting the aluminum dross addition.A reaction system is built and investigated for synergistic and coupling detoxification and alumina extraction from desilicated coal gangue and denitrified aluminum dross residues,carbide slag and spent cathode carbon block.Thermodynamic calculations and synergistic reaction mechanism studies for this complex process are carried out.The alumina components in the wastes are directionally reacted with sodium additives to generate sodium aluminate soluble in caustic solution by the synergistic and coupling reactions to obtain high-quality alumina source,which can be separated from all other solid impurity minerals.By use of carbide slag,the major component in which is Ca(OH)2,the fluoride salt can be converted into harmless calcium fluoride,and the silica minerals can be converted into insoluble calcium silicate during the high temperature treatment.The spent cathode carbon block is used as a reduction agent for variable valence elements,which promotes directional reactions of various elements and achieves the purpose of efficient separation of various impurity elements.As a result,the precise detoxification and impurity removal and the efficient extraction of alumina components from coal gangue and aluminum dross etc.are realized.The alumina extraction rate greater than 92% is achieved,and the recovery rate of sodium oxide is more than 94%.By optimized synthesis process,a series of high-quality zeolite,molecular sieve and high-white aluminum hydroxide and other alumina-silica based materials are synthesized using clean silica and alumina sources extracted from various solid/hazardous wastes.In summary,an innovative research concept is proposed for the efficient separation of alumina and silica minerals from coal gangue by activating silica components and inerting alumina components in this paper and the enhanced minerals inversion catalysis mechanism of silica components is revealed during the coal gangue heat treatment process.By activating agent addition and temperature adjusting,the caustic extraction rate of silica from the heat treated residue was greatly improved.By exploring a high-temperature directional reaction system,the synergistic and coupling reaction mechanism of alumina,silica and other components in multiple solid/hazardous wastes with additives has been revealed,and the high-temperature co-processing technologies for multiple solid and hazardous wastes have been developed.The alumina component in the wastes is efficiently extracted as well as the accurate detoxification and impurity removal is realized in the same process.Based on the high efficiency processes,more than 85% of the valuable resources in coal gangue and aluminum dross are extracted and recovered.Not only the serious environmental pollution by the wastes is eliminated,but also a series of high value alumina-silica based materials are produced to realize resource utilization from the major valuable substances in the wastes for a remarkable social and economic benefit.This study has opened up a green,efficient and large-scale industrialized application route for the harmless disposal and resource utilization of multiple industrial solid and hazardous wastes in China. |
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