Study On The Solidification Behavior And Incorporation Mechanism Of Heavy Metal Ions In Cement Clinker

In recent years,as an important material foundation to support the rapid development of China's economic construction,the cement industry is now facing serious problems,such as shortage of high quality raw materials and huge fossil energy consumption.At the same time,the amount of China's solid waste?such as domestic waste,municipal sludge,industrial hazardous waste?and industrial waste residue,slag are massive and increasing rapidly.The inorganic components,such as Ca Si Al and Fe,and organic components in these substances can be used as alternative raw materials and fuel in cement industry.The use of alternative fuels/materials and the road of sustainable development are the key for China's cement industry to achieve strategic transformation,by which the national urbanization construction and energy conservation and emission reduction will be continuely supported.However,heavy metal ions in alternative fuels and materials can affect the cement calcination,the structure and hydration properties of clinker minerals,and have the environmental risks,which restricts its application and popularization.Based on The 12th Five-Year National Key Technology R&D Support Program?2012BAC15B04?,Cr,Cd,Ti,Mn,Zn,Cu and Co heavy metals were selected according to the type and content of heavy metal ions in China's solid waste and industrial waste residue.The migration process and ion replacement of these 7 kinds of heavy matels were investigated and discussed.The main work and the main innovations carried out in this paper are as follows:Concerning the limitations of free lime content?f-CaO?in characterizing the burnability of raw meal,EDS and XRD were also used to investigate the mechanism of the influence of heavy metal ions on burnability,which could provide theoretical guidance for cement raw material proportioning calculation.This mechanism consist of three kinds of mechanisms:?1?Forming new phases:this kind of ion could effectively reduce the content of f-CaO in clinker.However,when it exceeded a certain value?TiO₂ 1.5 wt.%,CuO 1 wt.%?,it would absorb Ca²⁺to form a new phase which didn't have hydration activity and reduced the quality of clinker;?2?mainly acting on silicate phase:a small amount of heavy metal ions could promoted clinker's burnability.After exceeding the limit value?Cr₂O₃ 1.5 wt.%?,the ions would result in decomposition of C₃S,which could reduce the burnability and the property of clinker.?3?Mineralization agent:the liquid content of clinker could be improved,the viscosity of liquid phase could be reduced,and the content of f-CaO in clinker was decreased,which meant the increasement of burnability for cement.Cd,Mn,Zn and Co belonged to this kind of ions.Based on the multi-phase composition of cement clinker,the unequal content of each phase and the difference of the structure,the principle and limitation of the test method were analyzed,and the test method and data processing method were optimized to reveal the solid solution priority and partition in clinker mineral phases for heavy metal ions,which improved the theoretical system of heavy metal ions distribution in cement clinker.The combination of ESEM low vacuum mode and EDS were used to effectively avoid the shielding effect of coating conductive elements,and the concentration differences of the heavy metal ions in crystalline grain of the clinker mineral phase,i.e.solid solution priority,were detected.The distribution partitions of heavy metal ions in the silicate and interstitial phases,i.e.solid solution partition,were determined by chemical extraction and ion concentration test.At the same time,partition coefficient?K_f?was used to calculate the solid solution attraction of different mineral phases to heavy metal ions,which could explain the differences between solid solution priority and partition and provide a theoretical guidance for the different proportion of clinker mineral phase difference disposal of different kinds of heavy metal ions.The changes of crystal structure of clinker and four kinds of single mineral with different dosages of heavy metal ions were studied systematically.The ion substitution formula,Hume-Rothery formula,was optimized,and the first principle calculation was used to be bridge between experimental results,such as distribution characteristics of the heavy metal ions,ion valence and clinker hydration characteristics,and experience of theoretical.The migration process and substitution mechanism of heavy metal ions in the process of clinker calcination were investigated and divided into three types.?1?Substitution in silicate phase:The difficulty order of substitution of Cr ions in silicate phase C₂S(Si⁴⁺)>C₂S(Ca²⁺)>C₃S(Si⁴⁺)>C₃S(Ca²⁺).It was difficult for chromium to be immobilized in Belite when its grains firstly formed?¹250oC?.As the temperature rose to 1450?,two kinds of substitution pattern,Cr³⁺to Ca²⁺and Cr⁶⁺to Si⁴⁺,occurred in C₃S,which caused its crystal structure distortion and decomposition,and leaded to partial decomposition of Alite to f-CaO and Belite.The chromium ions were retained in C₂S and delayed the hydration of clinker.?2?Substitution in interstitial phase:Mn,Ti,Zn,Cu and Co were present in the valence state of Mn³⁺,Ti⁴⁺,Zn²⁺,Cu²⁺and Co²⁺respectively,and the structural characteristics of the five ions were more similar to those of Al³⁺and Fe³⁺,especially Fe³⁺.In addition,the simulation results shew that the formation energy of Mn ions to replace Fe ions into the C₄AF crystal was only-0.77eV,which was lower than that of the other three mineral phases.The substitution and migration process of these five kinds of ion mainly occurred in the interstitial phase,especially in C₄AF,and changed the composition,crystallization and content of interstitial phases,which accelerated the reaction rate of clinker initial hydration period,but inhibited the hydration accelerated phase reaction.?3?Equal substitution:Cd²⁺had high similarity with original clinker ion Ca²⁺in ion migration process and structure.It could equivalently substitute Ca²⁺in silicate and interstitial phases,and had little effect on the crystal structure of them.However,the substitution could significantly prolong the induction period,and reduce the maximum heat release rate.