Study On The Migration And Environmental Influence Of Trace Elements In Fly Ash And Its Utilization

Coal fly ash is the most by-product of coal-fired power plants,and the fly ash from the thermal power generation have become the largest single source of industrial solid waste in China.Some heavy metals in fly ash are considered as the environmentally sensitive elements,if not properly disposed of,fly ash can cause potential hazards to the environment.Therefore,it has important theoretical significance and application prospects to study the migration and transformation rules of harmful and valuable trace elements in fly ash.Firstly,the distribution of trace elements in high-arsenic coal ash from a power plant was studied in this paper.The enrichment of trace elements in different particle size samples and different components of raw coal,fly ash,slag and flue gas desulfurization gypsum were discussed.Environmental impact about land treatment of coal ash was also discussed.In addition to the Mo,Sb,W in the original coal,most of the trace elements have a trend enriching in the finer coal samples.Most elements in fly ash are close to the average values in the world lignite coal ash,and the elements of Hf shows a significant enrichment with a concentration factor of 20.As a result that the old fly ash was piled up for a long time,some of the elements migrated out,causing most of the content of trace elements in the old fly ash to be lower the fly new ash.The enriched elements in the soil around the ash dump include Zn,Cd,Sb,Cs,and W,especially the concentration coefficient of Hf is 30.The fly ash can increase the trace elements in the soil around the ash dump after long-term environment piled up.Secondly,the arsenic distribution and arsenic speciation in typical high-arsenic coal and coal ash in Inner Mongolia were studied,and the influence mechanism of arsenic in high-arsenic fly ash on soil was established.As content in the coal is up to 40μg/g,and that in the high arsenic fly ash is up to 110μg/g.The As elements will be enriched in the fine particles of fly ash,and As content of ferrospheres in fly ash is much lower than that in the maternal fly ash.By analyzing the As contents in the fly ash and slag by microstructure analysis,it was found that the As content in the particle of fly ash is the range of 0¹.9%with an average value of 0.44%.As content of slag particle is 0⁰.94%with an average content of 0.43%.The newly discharged fly ash mainly contains As（Ⅴ）,with the proportion of As（Ⅲ）being only 1.22%.The proportion of As（Ⅲ）in the fly ash increases with the decrease of the particle size,which is up to 2.27%.As（Ⅲ）is enriched the ferrospheres of fly ash.After the fly ash was picked up in the atmospheric environment stacked for some time,the As（Ⅲ）in fresh fly ash will be oxidized to As（Ⅴ）.The As（Ⅲ）content in the slag is higher than that of in the fly ash.Also,the As（Ⅲ）in slag will be converted into As（Ⅴ）due to the storage of slag in the environment.Arsenic species in the flue gas desulfurization gypsum is only As（Ⅴ）.The proportion of As（Ⅲ）in soil is higher than that in fly ash.Because of the impact of environmental,the As（Ⅲ）and As（Ⅴ）in the soil can be transformed into each other.Thirdly,the pre-desilication and lime-soda sinter combination method was used to extract Al₂O₃ and SiO₂ from fly ash in a laboratory scale study,the influence of reaction conditions on the extraction of Si and Al and the heavy metal migration during the extraction process was analyzed.The increase of reaction temperature and time can improve the extraction of SiO₂.The best reaction condition was 95?C/4h.With the increase of sintering temperature,the extraction rate of Al₂O₃ will increase,while the extraction rate may decrease in the excessively high temperature,and 1150?C/120min was the optimal reaction condition for the extraction of Al2O3.In the pre-desilication section,the contents of the heavy metals Cr,Mn,Cd and Pb were more likely to be distributed in the pre-desilcation solution as the dissolution temperature and time increase,while the sintering temperature has little effect on Pb.Fourthly,the mineralogical evolution and trace element distribution in various process sections of the industrial production line for extraction alumina by the high aluminum fly ash in Inner Mongolia,China were studied.The Al₂O₃ content of the high aluminum fly ash from Inner Mongolia reached 47.3%.The minerals in high-alumina fly ash include mullite,corundum,and quartz,while the minerals in the pre-desilication fly ash are mullite,corundum,quartz,and nosean.After pre-desilcation pretreatment of high-aluminum fly ash by NaOH,the amorphous phase SiO₂ is removed,and the mullite particles in the fly ash are exposed.The minerals in the active calcium silicate are tobermorite and a small amount of sodium hydroxide.The main minerals in the sintered fly ash are sodium aluminate and larnite,and the mullite in the pre-desilcation fly ash basically was completely reacted after sintering.The main minerals in the silicon-calcium residue are larnite and calcite.In the deep desilication section,the minerals in the sodium-silicon residue and the calcium-silicon residue are hibschite and nosean.The gibbsite is the main mineral in aluminum hydroxide,whose particles are 200μm spherical particles with hexagonal prisms exposed on the surface.High aluminum fly ash is slightly enriched in Li,Ga,Zr,Nb,Hf,Pb and Th,while the major by-product of silicon-calcium residue is slightly enriched in Li,Zr,Nb,Hf,Ta and Th.In the industrial process of Al₂O₃ extraction,Ga element is mainly concentrated in the Al-bearing material,and Ga content in the aluminum hydroxide product reaches 97μg/g.At last,the reaction mechanism of the main valuable elements Si,Al,Li,and Ga during coal utilization and the Al₂O₃ extraction from fly ash was revealed,and the relative distribution of the main and trace elements in the industrial extraction process was quantified.The main impurities generated in deep-desilication process are Na₂O·Al₂O₃·1.7SiO₂·nH₂O and 3CaO·Al₂O₃·xSiO₂·（6-2x）H₂O.The relative total emission flow of Li in silicon-calcium residue is 21,522 g/h,and that of Ga in aluminum hydroxide is the largest part.The trace elements with the emission flow exceeded 5000g/h in silicon-calcium residue were Li,V,Zn,Sr,Zr,Ba,La,Ce,Nd,and Pb.The relative proportions of trace elements released at high temperature ranged from 10 to 50%,while the release rates of Mo,Hg and Tl at high temperatures are very high.The Ga element is mainly concentrated in the aluminum hydroxide product,and the relative distribution ratio of Ga reaches 83.7%.