Study On Mechanism Of PCDD/Fs Formation And Inhibition During Steel Manufacture Process

With the sustained economic development,the amount of crude steel production in China increases rapidly over the past 40 years since its reform and opening-up.However,many hazardous pollutants are emitted from iron and steel production,including exhaust fumes,waster water,smelting slag and fly ash.With the implementation of the new national standards about integrated iron and steel industry,dioxin(polychlorinated dibenzo-p-dioxins(PCDDs)and dibenzofurans(PCDFs))emissions and control has attracted more and more attention.In order to verify in how far the different process of steel production could contribute to such emissions,and develop practical and efficient inhibitors as well as study the mechanism of PCDD/Fs formation,a lot of research work have been done during past five years.Several valuable conclusions are as following:(1)The sintering fly ash generates far greater amounts of these pollutants than other fly ash(including blast furnace fly ash,basic oxygen furnace fly ash and electric arc furnace fly ash),especially in terms of PCDD/Fs and dioxin-like PCBs,which are 2-3 orders of magnitude higher.The yield of PCBs,CBzs and PAHs of sintering fly ash is 2-4 times than that of other fly ash.The higher PCDD/Fs generation and higher Cl-PCDD/Fs of sintering fly ash should be ascribed to higher chloride content than other four fly ash samples.The concentration of organic pollutants emitted from all fly ash samples are in decreasing order of PAHs>>CBzs>PCBs>PCDFs>PCDDs>dl-PCBs,with small shifts in individual sample.(2)The concentration of PCDD/Fs formation of sintering fly ash is more than three orders higher than that of sintering raw material.Within the range of temperatures tested,the yield of PCDD/Fs and I-TEQ peak at 350 ?,both for sintering fly ash and sintering raw material,designating that 350 ? is the optimum temperature for PCDD/Fs-formation.The PCDD/Fs concentration of sintering raw material peak at a granule size of 2 to 4 mm.The yield of PCDD/Fs and I-TEQ show a growing pathway with rising O2 content both for sintering fly ash and sintering raw material.The log-linear regression analysis on PCDD/Fs concentration and oxygen concentration of sintering fly ash sample is built and the correlation coefficients R2 is up to 0.98.The yield of PCDD/Fs of sintering fly ash could increase as the time goes on and then stabilize after 2 hours.High temperature is the main effect on PCDD/Fs signature of sintering raw material,however,low temperature and oxygen-free is the main effect on that of sintering fly ash.The PCDD/Fs signature of the original sintering fly ash is more similar to heating sintering fly ash,and the PCDD/Fs signatures of sintering fly ash in low temperature and oxygen-free condition are more similar to that of sintering raw material.(3)The reduction efficiency of PCDD/Fs by various inhibitiors(2 wt.%)have great difference,the order of the reduction efficiency are as follow:calcium hydroxide(77.28%)>Monoethanolamine(49.77%)>calcium oxide(46.93%)>thiourea(43.25%)>disodium hydrogen phosphate(43.16%)>urea(41.09%)>triethanolamine(36.06%)>ammonium dihydrogen phosphate(29.61%)>ammonium sulfate(22.81%)>spent anion exchange resins(9.24%).The special PCDD/Fs signature and low Cl-PCDD/Fs of sintering fly ash with alkaline suppressant addition suggest that acid gas inclucing Cl2 and HCl consumd by alkaline suppressant could effectively affect the Deacon reaction and then reduce the PCDD/Fs-yield.The result of principal component analysis based on PCDD/Fs signature suggests that 10 inhibitors could be divided into two catagories:nitrogenous suppressant and alkaline suppressant.Alkaline suppressant include calcium hydroxide,calcium oxide and disodium hydrogen phosphate,and nitrogenous suppressant contain all the rest.(4)There is significant difference between three steel rolling sludges on the yield of PCDD/Fs,the order of that from higest to lowest is hot rolling sludge(127.82 ng/g)>cold rolling sludge(24.46 ng/g)>continous casting sludge(4.05 ng/g).The calciferous compounds,especially alkaline additive,have low suppression for PCDD/Fs formation during steel rolling sludge incineration,even could promote the dioxin formation.Spent anion exchange resin(SAER)appeared as a dioxins suppressant during combustion tests of hot rolling sludge in a bench-scale tubular furnace.The reduction efficiency(RE)of PCDD/Fs and I-TEQ generation attains 91.1%and 90.2%,respectively,with the addition of 2.5 wt.%resin,and the highest dioxins RE reachs 97.8%with 10 wt.%resin addition.According to high S-content in residue during co-combustion and high SO2 concentration in the off-gas,suppression of PCDD/Fs formation by SAER should be ascribed to the poisoning of catalytic metal by S-containing compounds.(5)The concentration of PCDD/Fs generated from various steel materials are in decreasing order of sintering fly ash>>hot rolling sludge>cold rolling sludge>continuous casting sludge ? electric arc furnace fly ash ? primary blast furnace fly ash ? secondary blast furnace fly ash ? basic oxygen furnace fly ash>>sintering raw material.Within each homologue group,the isomer signature has been further scrutinized,with special emphasis on the seventeen 2,3,7,8-substituted PCDD/F,as well as on the seven PCDD-congeners and two TCDF usually associated with chlorophenol precursor routes.The results suggest a distinctive PCDDs-congener pattern along the CP route,but similar consensus fails regrading characteristic PCDFs-patterns from the CP route.The contribution degree of CP route for PCDDs formation range from 6.3%to 65.0%in all samples.The contribution degree of CP route for PCDDs formation in sintering fly ash is much lower than other fly ash.CP route relative selectivity of sintering fly ash shrinks strongly with temperature rising,becoming minimal at a temperature of 350?,later regaining some of their relative importance at higher temperature(>450?).The contribution degree of CP route for PCDDs formation increases when oxygen is inlet,but there is no notable effect with oxygen content increasing.With spent anion exchange resins addition,the contribution degree of CP route for PCDDs formation during hot rolling incineration rises vigorously,attaining to the highest value(65.0%).