Study on the Effects of Chlorine,sulfur and Alkali on the Migration of Heavy Metals and Related Mechanisms during Co-processing Solid Waste in Cement Kiln
Supervisor:wei jiang xiong
With the rapid development of cities and industries in China,a huge amount of municipal and industrial solid wastes(SW)are generated.According to the statistics,more than 300 million tons of municipal SW and 3.3 billion tons of industrial SW were discharged in China in 2017.Meanwhile,the cement manufacture in China is at a large scale,which consumes a lot of non-renewable resources such as limestone and coal each year.Using the SW as raw fuels and/or cement raw materials in cement kiln,i.e.,the co-processing of SW,not only can be used to solve the environmental problems caused by SW,but also can be applied to reduce the consumption of non-renewable resources such as limestone and coal.Therefore,it becomes an important direction for the sustainable development of cement industry.However,most of the SW contain trace elements such as chlorine,sulfur,alkali,and heavy metals(HMs).The volatilization characteristics of HMs and the distribution of volatilized HMs in gas phase and solid phase will be affected by the presence of chlorine,sulfur and alkali under the high calcination temperature during clinkerization,which could cause secondary pollution to the environment.This thesis focused on the volatilization and solidification of HMs during clinkerization of co-processing of SW in cement kiln.The effects of chlorine,sulfur,and alkali on the volatilization ratio,volatilization morphology,gas-solid content,distribution in clinker phases of HMs,and the clinkering process and mineral composition of the clinker were systematically studied.The effects of temperature and chlorine/sulfur/alkali type and content on the volatilization ratio of HMs were also investigated based on the content of volatilized HMs.Combining with the chemical forms and morphology of HMs and their distribution in gas and solid phases,the regularity and mechanism of the effects of chlorine,sulfur,and alkali effects on the volatilization of HMs were explored.At the same time,the effect rules of clinkering process,and mineral composition of the clinker on solidified ability of clinker to HMs were determined by clarifying the influence of chlorine,sulfur,and alkali on clinkering process,mineral composition of clinker,and distribution of HMs in clinker phases.Finally,based on the above results and analysis,and the practical example of co-processing sweage sludge(SS)in Yuebao cement kiln,the regularity and mechanism of HMs solidification and migration during the co-combustion of SS were clarified,as well as the methods to control the environmental impact of HMs during co-processing of SW in cement kiln were put forward.These research works are presented in detail as follows:(1)The effects of chlorine,sulfur,and alkali on the volatilization ratio of Cu,Pb,and Cd at 950~1450℃ were studied.The content of HMs in digestion solution were determined by atomic absorption spectrometry after digesting the samples calcined at different temperatures.Then,the volatilization ratios of Cu,Pb,and Cd of the samples added with different types and contents of chlorine/sulfur/alkali and calcined at different temperatures were calculated.The results showed that the volatilization ratios of Cu,Pb,and Cd increased by 17%,71%,and 44% respectively during heating from 950℃ to 1450℃.The addition of chlorides transformed the HM oxides into HM chlorides,which significantly increased the volatilization of HMs.With the addition of 1.6% chlorine in raw meal as Al Cl3·6H2O,Fe Cl3·6H2O,or Ca Cl2,the volatilization ratios of Cu,Pb and Cd increased by 24~42%,14~48%,and 18~48% respectively at 950~1450℃.Sulfur and alkali also increased the volatilization of Cu in clinkerization.2.0% of sulfur as Ca SO4·2H2O or 3.2% of Na as Na2CO3 increased the volatilization ratio of Cu by 6~19% at 950~1450℃.However,the sulfur and alkali decreased the volatilization of Pb and Cd at 1350~1450℃.Addition of 2.0% of sulfur as Ca SO4·2H2O or 3.2% of Na as Na2CO3 decreased the volatilization ratio of Pb and Cd by 15~20%.(2)The effects of temperature and chlorine/sulfur/alkali on the volatilization morphology of Cu,Pb,and Cd were investigated,as well as the related mechanism was discussed.The flue gas generated at different temperatures was collected directly during clinkerization.The solid phase of HM powders collected by filter membrane were analyzed with XRD.Combining the formation energy of HM compounds and the influence of chlorine,sulfur and alkali on the clinkering process,the volatilization forms of HMs with the presence of chlorine,sulfur and alkali were predicted.The results showed that without the addition of chlorine,sulfur and alkali,the volatilization forms of Cu,Pb,and Cd were HMs oxides during clinkerization.With the addition of chlorine,HM oxides will be transformed into HM chlorides,and then volatilized with the flue gas.The volatilization forms of Cu and Cd were not affected by temperature,sulfur and alkali.When the temperature was lower than 1150℃,the volatilization form of Pb was not affected by sulfate;when the temperature rose to 1250~1350℃,Pb mainly volatilized as the form of solid solution with Pb O and Pb SO4;when the temperature was higher than 1350℃,Pb mainly volatilized as the form of Pb O,accompanied by a small amount of Pb O·Pb SO4 and Pb SO4.(3)The effects of chlorine,sulfur and alkali on the distribution of Cu,Pb,and Cd in gas and solid phases of flue gas were studied.The results show that the formation of gaseous Cu,Pb,and Cd was promoted by high temperature.Although the formation of HM chlorides increased the volatilization of HMs,it decreased the ratio of gaseous HMs in the volatilized HMs.Therefore,chlorine has little effect on the content of gaseous HMs in flue gas.Sulfur and alkali affected the morphology of volatilized HMs during clinkerization,reduced the agglomeration and mutual adsorption of HM compounds,and then promoted the generation of gaseous Cu,Pb,and Cd.The content of Cu,Pb,and Cd in flue gas will be increased with increasing the sulfur and alkali content in raw meal during the co-processing of SW in cement kiln.(4)The effects of chlorine,sulfur and alkali on the clinkering process and mineral composition of the clinker were investigated by XRD,EPMA,SEM,TG and lithofacies analysis.The distribution and existing forms of Cu,Pb,and Cd in the mineral phases of clinker were analyzed,and then the regularity and mechanism of chlorine,sulfur,and alkali effects on the distribution of HMs in the mineral phases of clinker and the solidification ability of clinker to HMs were discussed.The results show that Cu and Pb were mainly solidified in the interstitial phase of the clinker,while Cd was solidified in both the silicate and interstitial phases.Chlorine in raw meal promoted the solid-state reaction during clinkerization,increased the content of C3 S,decreased the content of C3 A and C4 AF,and then decreased the solidification ability of clinker to Cu and Pb.Sulfur and alkali(Na)mainly existed in the interstitial phase of clinker.During clinkerization,sulfur increased the solidification ability of interstitial phase of the clinker to HMs.(5)The physical and chemical properties of SS and the migration and solidification of Cu,Pb and Cd during co-combustion of SS were studied.Combining the above results and analysis and the practical example of co-processing of SS in Yuebao cement kiln,the solidification mechanism of HMs during clinkerization with SS was clarified,and the relationship between the content of chlorine/sulfur/alkali and HMs fed into kiln was established.The results showed that the main chemical compositions of SS were calcium-siliceous compounds such as Si O2 and Ca CO3.The contents of chlorine,sulfur,alkali and Cu,Pb,Cd in the SS were much higher than those in raw meal.Trace elements such as chlorine,sulfur,and alkali improved the solidification of Cu,Pb,and Cd by affecting the clinkering process of the clinker.To satisfy the regular production of cement,the contents of Cu,Pb,and Cd in the raw materials fed into kiln should be controlled less than 630,30 and 260 mg/kg respectively during the co-processing of SW in cement kiln.