Stabilization Of Heavy Metals In Municipal Solid Waste Incineration Fly Ash Using Inorganic Chemicals

Compared with other waste disposal technologies,incineration has the advantage of occupied area,volume reduction,detoxification and energy recovery.Therefore,it is developing rapidly in China.However,municipal solid waste incineration fly ash(MSWIFA),which is classified as hazardous waste due to the accumulated soluble heavy metals,will be generated after incineration.Therefore,MSWIFA must be treated effectively before landfill.At present,the main ways of fly ash treatment are cement solidification,chemical stabilization and heat treatment.Cement solidification is mature,easy to operate and cost low,but the weight and volume gain is high.In addition,the generated cement is easy to rupture due to the high salt content in MSWIFA.Chemical stabilization has a nice effect of heavy metal stabilization and a little weight and volume gain,but it is difficult to stabilize a variety of heavy metals simultaneously and the production cost of chemicals is high.After the heat treatment,the leaching toxicity of fly ash is low and the treated fly ash has a nice chemical and mechanical properties.However,heat treatment consumes a lot of energy due to the high temperature.At present,the main problem is to find out a way which has a nice effect of heavy metal stabilization,small cost,low energy consumption and can reuse the MSWIFA.In this study,a new method for stabilizing heavy metals from fly ash was proposed by heat treatment with a small amount of amorphous nano-SiO2 materials such as silica fume or silica gel.The new method was compared with conventional NaH2PO4 treatment and silica fume/silica gel curing treatment,which is a popular research topic recently.The effects of the relevant process parameters on the leaching of heavy metals were studied by leaching experiments on the three kinds of treated fly ash with two kinds of leaching standards,such as standard HJ/T 300-2007 and CEN EN 12457-2.The changes of chemical speciation of heavy metals,crystal structure,microstructure and pore distribution during treatment were observed by BCR continuous extraction experiment,XRD,SEM and BET to analyze the mechanism of the stabilization of heavy metals.The main conclusions of this paper are as follows:NaH2PO4 treatment could effectively reduce the concentration of Pb in leaching solution.With the increase of the amount of NaH2PO4,the concentration of heavy metals in Chinese and EU standard leaching solution was decreased.When the addition amount of NaH2PO4 reached 4%,the decrease trend was slowed down.The higher the reaction temperature,the higher leaching concentration of heavy metals in Chinese and EU standard leaching solution.The best temperature for heavy metals stabilization was 20 ℃.With the increase of curing time,the concentration of heavy metals in Chinese and EU standard leaching solution decreased.For NaH2PO4 treated fly ash,the 3-day curing time was sufficient.With the increase of the initial liquid-solid ratio,under the HJ/T 300-2007 standard,the leaching concentration of Pb,Cd,Cu and Zn decreased,while the European standard is the opposite.The XRD results showed that NaH2PO4 reacted with CaClOH to form Ca5(PO4)3Cl and Ca5(PO4)3OH,changing the chemical speciation of heavy metals and stabilizing heavy metals in the phosphate lattice,which reduced the leaching toxicity.Silica fume or colloidal silica treatment could slightly reduce the leaching concentration of Pb,Cd,Cu and Zn in EU standard leaching solution if natural curing conditions are used.Silica fume or colloidal silica treatment could significantly reduce the leaching concentration of heavy metals in EU standard leaching solution if steam curing conditions are used.Under the conditions of adding 10%silica fume or 25%JN-40 colloidal silica and steam curing for 30d,the concentration of Pb in EU standard leaching solution could be reduced to 0.04 mg/L or 0.05 mg/L,the concentration of Cd could be reduced to 0.049 mg/L or 0.045 mg/L,the concentration of Cr could be reduced to 0.58 mg/L or 0.48 mg/L,the concentration of Cu could be reduced to 0.03 mg/L or 0.01 mg/L,the concentration of Zn could be reduced to 0.04 mg/L and 0.05 mg/L.If appropriate parameters were selected to properly adjust the leaching concentration of Cr,silica fume/colloidal silica steam samples could meet the EU landfill standards.XRD analysis showed that colloidal silica and silica fume could react with calcium and heavy metals in fly ash to produce CSH colloids or silicates such as CaSiO3 and PbSiO3 under steam curing conditions.In addition,alkaline substances CaClOH in fly ash could react with heavy metals and CO2 to form CaCO3,Cu2(OH)2CO3,ZnCO3,Zn(OH)2 and other substances.Under the high pH condition,such as EU standard,CSH colloids,silicates,heavy metal bases and carbonates were relatively stable,which could effectively inhibit the leaching of heavy metals.Under the low pH condition,such as HJ/T300-2007 standard,CSH colloid was destroyed by acid corrosion,heavy metal bases and carbonates completely dissolved,resulting the poor stabilization effects.During the medium heat treatment,silica fume and colloidal silica could react with calcium-based materials in fly ash to form silicate phases such as CaSiO3、Ca2SiO4、Ca3SiO4Cl2、Ca3Fe2(SiO4)3 and hydroxyl-ellestadite during the heat treatment,changing the chemical speciation of heavy metals and stabilizing heavy metals in the silicate lattice,which reduced the leaching toxicity.Under the conditions of adding 10%silica fume or 25%JN-40 colloidal silica and heat treatment at 600℃ for 1h,the concentration of Pb in Chinese standard leaching solution decreased from 11.91 mg/L to 0.79 mg/L and 0.78 mg/L,the concentration of Cd decreased from 3.18 mg/L to 0.09 mg/L and 0.10 mg/L,the concentration of Zn decreased from 9.48 mg/L to 0.28 mg/L and 0.74 mg/L,the concentration of Cu decreased from 7.45 mg/L to 0.65 mg/L and 0.69 mg/L.During the medium heat treatment,the concentration of Cr in Chinese standard leaching solution increased but did not exceed the Chinese standard for pollution control on the landfill site of municipal solid waste.The heat treatment temperature of the method was 600℃,which was lower than the melting and sintering temperature,and could suppress the evaporation of heavy metals.The heat treatment process could be carried out in a moving bed,introducing the incinerator flue gas to heat the fly ash directly.The produced gas entered the incinerator gas treatment system.This method saves energy and does not require additional flue gas treatment equipment.