Mechanism Study On The Transfer And Pollution Control Of Heavy Metals And Fluorine In The Incineration Of Hazardous Wastes

With the rapid growth of economy,the output of hazardous waste in China has increased rapidly.Harmless disposal of hazardous waste must be carried out as hazardous waste has causticity,toxicity,flammability,reactivity and infection.Incineration has become one of the most preferred disposal technology in China because it can effectively reduce the volume and weight of waste,decompose hazardous materials and recover the energy to generate heat or electricity.However,secondary pollution of fluorine and heavy metals will be caused during the incineration of the hazardous wastes containing fluorine and heavy metals.It is of great significance to study the migration,transformation and occurrence distribution of heavy metals and fluorine in the incineration of hazardous wastes for the secondary pollution control of heavy metals and fluorine.Thus,this thesis took hazardous wastes from pharmaceutical chemical industry and high fluorine-containing organic wastes from fluorine industry as the research objects,and studied the transfer and pollution control of heavy metals（As,Cd,Cr,Cu,Pb,Se,Zn,and Hg）and fluorine during the incineration of hazardous wastes.The main research contents and conclusions are as follows.（1）The migration and distribution characteristics of heavy metal during the incineration of hazardous waste from the pharmaceutical chemical industry were studied in a hazardous waste incinerator with a treatment capacity of 100 t/d.The results showed that the relative enrichment indexes of heavy metals Cd,Pb,Zn and Hg in the fly ashes were higher than that in the bottom slag.Cd,Pb,Zn and Hg were concentrated in the fly ashes during the incineration,whereas Cr was enriched in the bottom slag.The relative enrichment of Cu and As in the incineration residues were affected by the chlorine and calcium/iron contents in the hazardous waste,respectively.Hg,Cd,As,Pb,and Zn were transmitted into the fly ashes mainly by evaporation,condensation,and adsorption.The transmission route of Se and Cr into the fly ashes were mainly entrainment.The transmission route of Cu to the fly ashes was affected by the incineration condition and chlorine content in the waste.The mass balance of heavy metals in the incineration system was calculated.Most of the Cu,Cr,As and Zn mainly remained in the bottom slag,whereas Cd was retained in the fly ashes and the absorbent solution of the WFGD.And most of Pb was found in the fly ashes,whereas more than 74.7%of Hg remained in the absorbent solution of the WFGD.The relative mass distribution proportion of the studied heavy metals in the exhaust gas was below 5%.Additionally,the leaching characteristics of heavy metals in the incineration residues were studied using TCLP method.The bottom slag was sintered in the rotary kiln,and the studied heavy metals,with the exception of Zn,in the bottom slag showed low leachability.For each studied heavy metal,the leaching ratios in the bag filter ash were the highest,as the bag filter ash contained amounts of soluble salt and heavy metals mainly existed on the surface of bag filter ash.（2）In order to assess the ecological harmfulness of heavy metals in the incineration residues comprehensively,the chemical speciations of heavy metals were investigated using BCR leaching method.Meanwhile,the thermal stability of heavy metals during the heat treatment was studied in view of the secondary volatilization of heavy metals during the heat treatment of incineration residues.The transformation of heavy metal speciation was investigated under different heat treatment temperatures.The results showed that As and Cr in the raw incineration residues mainly fractionated as the residual form,whereas most of Se was associated with the oxidizable fraction and residual fraction.Cd and Cu in the bottom slag was mainly found in the oxidizable and residual form,while the acid soluble fraction of Cd was the most dominant in fly ashes.The chemical speciation of Cu in fly ashes was related to the chlorine content in the waste.Pb in the bottom slag,burt-out ash and waste heat boiler ash was mainly in the residual form,whereas less of Zn was associated with the oxidizable fraction.Zn in the bag filter ash was mainly found in the acid soluble fraction and residual fraction.The residual fraction of Hg in the bag filter ash had the highest proportion.The chemical speciation of Pb in the bag filter ash was affected the chlorine content in the waste.The evaporation rate of the studied heavy metals rised up with the increase of heat treatment temperature,and the evaporation rate in the bag filter ash was the highest at the same temperature.As,Cr and Se had high thermal stability,and heat treatment would promote the transformation from the transferable fraction into the residual fraction.Hg and Cd had the worst thermal stability,and heat treatment would cause the volatilization from the incineration residues.Cu,Pb and Zn had moderate thermal stability,the heat treatment process would promote volatilization of the transferable fractions or transformation from the transferable fractions into residua fraction.（3）The evolution of fluorine during the incineration of low fluorine-containing waste（F content<5 wt.%）from the pharmaceutical chemical industry,as well as the distribution characteristics and chemical speciations of fluorine in the incineration system were investigated in a hazardous waste incinerator with a treatment capacity of 100 t/d.The results showed that hydrogen fluoride（HF）and silicon fluoride（Si F₄）were detected as the initial fluorine form to be released during pyrolysis.As Si F₄ was hydrolyzed at high temperature,HF was the main form of fluorine gas in the flue gas after the secondary combustion chamber.In the incineration process,the relative enrichment indexes of fluorine in the bottom slag and fly ash were 0.161-0.163 and 0.104-0.144 respectively,and fluorine did not show significant enrichment in the incineration residues.The mass balance of fluorine in the incineration system was calculated.Less than 20.73%of the fluorine was retained in the incineration residues and over 79.17%of fluorine was absorbed into the absorbent solution of WFGD system.The amount of fluorine discharged into the atmosphere was below 0.12%.It was found that fluorine almost entirely existed in the water vapor or in the droplets in the downstream flue of the WFGD system.A mist eliminator was needed for the further removal of fluorine in the exhaust gas.The fluorine in the bottom slag and burn-out chamber ash mainly existed in the residual form.The proportions of transferable fraction（water-soluble,exchangeable,and Fe/Mn bound）fluorine in the boiler ash and bag filter ash were both over 80%.The leaching fluorine concentration exceeded the control limit of landfill.（4）The experiments of fixing fluorine using calcium absorbent during the pyrolysis of fluorine-containing waste from the pharmaceutical chemical industry were carried out in a tube furnace in order to reduce the release of fluorine during incineration and solve the problem of salt formation in the WFGD system.The effects of temperature,addition amount and type of calcium absorbent,particle size and pore structure on the fluorine fixation were studied.The recommended conditions for fluorine fixation were as follows:when Ca（OH）₂ was used as the calcium absorbent and the molar ratio of calcium to fluorine was 1:1,the defluorination rate could reach more than 68%at 900°C.Meanwhile,a field test was carried out in a hazardous waste incinerator with a treatment capacity of 100 t/d.The results showed that adding Ca O in the rotary kiln could effectively reduce HF concentration in the flue gas.When 3.87%Ca O was added,HF in the flue gas decreased from 304.5 mg/m³ to 99.1 mg/m³ with a decreasing amplitude of 67.4%.（5）In view of waste heat hard to be recovered,serious problem of HF corrosion and larger output of secondary solid waste in the WFGD during the incineration of high fluorine-containing organic waste,the method of fluidized incineration and water washing to recover HF was proposed to dispose the high fluorine-containing organic waste from the fluorine chemical industry.Meanwhile,a fluidized bed incinerator with a treatment capacity of 10 t/d was designed to study the incineration and resource utilization of the high fluorine-containing organic waste.The weight-loss process and the release form of gaseous fluorine during the combustion of high fluorine-containing organic waste were analyzed using TG-FTIR.The weight-loss process of the waste sample basically ended at 770°C,and the incineration temperature of fluidized bed incinerator could ensure complete incineration of the waste.HF and fluorocarbons were detected as the initial released fluorine form during the combustion.Fluorocarbon compounds could be destroyed through reactions with free radical H,O and OH,or through single molecule decomposition.During the incineration of high fluorine-containing organic waste,enhancing temperature and oxygen content in the furnace and increasing hydrogen content in the incineration materials were conducive to reducing the generation of fluorocarbons.By sampling and analyzing of the bottom slag,bag filter ash and exhaust gas during the field test,the relevant pollutant discharge could meet the discharge requirements.And the waste heat utilization in high fluorine-containing flue gas and the recovery of hydrogen fluoride and hydrochloric acid were realized.In the recovery of HF by water washing,the HF absorption process in the 1#and 2#packed absorbing towers was mainly controlled by gas film,and the total absorption coefficients of gas phase were 52.38 kg/（h·m²）and 39.96kg/（h·m²）,respectively.The HF absorption process in the 3#and 4#packed absorbing towers was mainly controlled by liquid film,and the total absorption coefficients of liquid phase were5.98 kg/（h·m²）and 3.89 kg/（h·m²）,respectively.Alumina has good HF high-temperature corrosion resistance and could be used as bed materials or refractory.In actual operation,HF low-temperature corrosion occured to the quenching heat exchanger.Meanwhile,as the quenching heat exchanger adopted coil form,ash wear and ash deposition would accelerate HF corrosion process.The quenching heat exchanger was damaged due to corrosion after half a year of continuous operation of the incineration system.HF high-temperature corrosion occurred to the waste heat boiler.And large flux membrane wall was adopted.No significant corrosion was observed in the 24 months of operation of the waste heat boiler...