Geotechnical Properties And Leaching Characteristics Of Refuse Derived Fuel Incineration Fly Ash

As a new waste incineration technology, refuse derived fuel (RDF) can reduce the content of pollutants in incineration residues by controlling the incineration temperature, mixed ratio and the kinds of additives and their content. Fly ash holds 5% of total mass in RDF incineration residues, due to the cementitious properties, has the potential of resource utilization. At present,few research was performed on RDF fly ash, which contants lower pollutants, this dissertation aims to study on the geotechnical properties and leaching characteristics and leaching control mechanism of RDF fly ash from landfill as road construction material, and provide references on the engineering properties and potential environmental effects of RDF fly ash for recycle applications.Two kinds of fly ash from landfill: fresh ash (stacking time < 10 days) and aged ash(stacking time ? 6 months) are the research object in this dissertation. Through the study of geotechnical properties, leaching characteristics under different conditions, long-term leaching migration of minor elements and control mechanism of leaching under different different pH value, draws the following main results:The basic physical properties, microscopic characteristics and chemical properties of RDF fly ash were studied by particle size analysis, SEM (scanning electron microscope) and SEM-EDS (energy dispersive X-ray Spectroscopy scanning), and comparative analysis were carried on with MSW (municipal solid waste) incineration fly ash. Results show that particle size of RDF fly ash sample are mainly distributed in between 0.075 and 4.75 mm, and contains lots of larger particle polymer which are formed by tiny particles; Fly ash particles present cotton texture, rough surface, looser accumulation, obvious gap between particles, particle surface of aged ash presents amorphous structure due to erosion effect; The content of CaO in RDF fly ash samples is higher than those of MSW incineration fly ash and CCP (coal combustion product) fly ash, rendering cement properties, and other ingredients are similar to MSW fly ash.The geotechnical properties of RDF fly ash samples were studied by compaction test,permeability test, unconfined compressive strength test, CBR (California bearing ratio) test and freeze-thaw test. Rresults show that the particle polymer of RDF fly ash contained more space, and dry bulk density value is lower than the gravel materials; Permeability coefficient of RDF fly ash samples range from 1.4x10-6m/s to 1.4x10-8m/s, and place the permeability to"poor" level; CBR value of aged RDF fly ash samples ranges in 9.5 to 16.4, can be rated"moderate to good" level in subgrade material; Breakage mode of Aged and fresh fly ash samples are brittleness and ductile damage respectively; Aged ash samples present high freezing-thawing resisting ability.Water leach test (WLT), Synthetic Precipitation Leaching Procedure (SPLP) and pH dependent leaching test were performed respectively to study the potential recycle possibility of RDF fly ash under different leaching liquor, the leaching characteristics of major elements(Al, Ca) and trace elements (Ag, As, B, Ba, Cd, Cr, Cu, Pb, Mn, Se, Zn) under different pH condition. Results show that: leaching concentration of B, Cd, Cr, Se, Ag and SO42-from RDF fly ash by water leach tests meet the requirement of industrial by-product utilization application standard. Leaching concentration of As and Ba from WLT and SPLP exceed the EPA MCL limit, which may produce potential environmental risk. Acid neutralization capacity curve analysis shows that pH of RDF fly ash is easily affected by acid and alkali conditions.Ion leaching pattern analysis show that leaching behavior of Cu, Mn, Zn, Ag, Cd and Pb is similar to the cation leaching pattern, leaching concentration of elements decreased with the increase of pH value; Leaching behavior of Al, Cr and As is similar to that of amphoteric leaching pattern, where the leaching concentration reaches minimum value in natural pH condition (pH=7), while increases in acidic and alkaline conditions; leaching behavior of Ca,Ba and Se is independent on pH condition.Leaching process major elements (Al, Ca, K, Na) and minor elements (Ag, Ba, As, Cr,Cu, Cd, Pb) for a long time in the flow system and migration process of minor elemtns (As,Ba) in the groundwater of RDF fly ash were studied by column leach test and numerical simulation. Research shows that: cumulative mass release of major elements including Al and Ca present a steady increase with the increase of the cumulative liquid-solid ratio (L/S), while cumulative mass release of K and Na decrese with increased L/S; Leaching of Cr and Cu occurs only at the beginning of the column leach test, while leaching of As stay stable and continuous during the whole experiment process, the cumulative mass release of Ag, Cd and Pb is low; Fresh RDF fly ash samples release more more minor elements compare to aged ash sample. In groundwater leaching process for a long time, leaching concentration of As reaches the maximum after 79 years, and predict concentration of As from fresh ash samples exceed the EPA MCL requirements, which has potential risk.Leaching control mechanism of the four major leaching elements (Al, Ca, Mg, Fe) and six trace elements (As, Ba, Cd, Cr, Cu and Zn) from RDF fly ash were studied by geochemical equilibrium model. Results show that in the process of leaching, As, Cr, Cu and Fe belongs to the redox reaction sensitive elements; Al, Ba, Ca, Cd, Mg and zinc belong to low sensitivity or no redox sensitive elements; Al and Fe main exist in reductant form such as Al (?) and Fe(?), and Cr main present as reductant Cr (?), which As, Cu and Zn exist by oxidation state As (?), Cu (?) and Zn (?) respectively. Major elements (Al, Fe, Mg, Ca) and trace elements(Ba, Cd, Cr, Cu, zinc) belong solubility control mechanism, and the main controlling mineral are amorphous Al (OH)3, water iron ore [Fe (OH) 3],dolomite [Mg,Ca(C03)2], calcite (CaCO3),barite (BaSO4), otavite [CdCO3],amorphous Cr(OH)3, black copper mine (CuO) and red zinc(ZnO) respectively, while leaching of As belongs to non-solubility control mechanism, and tend to exists as [AsO43-] and [(AsO4) 2]6- etc.,which can combine with metals to form relatively insoluble metal arsenates.The results from this dissertation show that RDF fly ash after the aging process present better geotechnical properties, and could be potentially beneficial use as light bone material resources in road construction, while the leaching of pollutants content is easily influenced by the environmental conditions, which may produce potential environmental risks.