Research On The Solidification And Its Mechanism Of Heavy Metals In MSWI Fly Ash-cement Clinker

With the advantages of high efficiency, high volume and weight reduction rate and secondary energy recovery, Municipal solid waste (MSW) can be effectively disposed by the means of incineration, and Municipal solid waste incineration (MSWI) has become an important development direction of MSW treatment technology. However, MSWI-fly ash which is rich in heavy metals and other harmful substances will be generated by municipal solid waste under high temperature incineration. So MSWI-fly ash needs to be disposed prior to landfill. MSWI-fly ash containing large amounts of silicon and aluminum and can be utilized as raw materials in the production of cement. However, the high content of chlorine and alkali metals in MSWI-fly ash will bring a negative influence on cement kilns.The effect of different pretreatment methods on the content of chlorine and alkali metal in MSWI-fly ash was studied. The experiment rexults showed that pretreatment using water and sulfuric acid, phosphoric acid can effectively reduce chlorine and alkali metal content in MSWI-fly ash, while hydrochloric acid pretreatment showed a poor performance. Additionally, the effect of different processing time and liquid-solid ratio on the properties of MSWI-fly ash was also studied. Research results showed that, the best processing time and solid-liquid ratio were10min and10:1respectively. Increasing processing time and liquid-solid ratio did not bring an obvious improvement in efficiency of pretreatment.In addition, high content of heavy metals in MSWI-fly ash has some impact on the formation of cement clinker and the subsequent hydration process. The influence of heavy metals on the formation and hydration properties of C3S and C3A, the two major mineral phases in cement clinker, were studied by means of XRD,calorimetry testing,EPMA. The research results will supply a theoretical basis for revealing the effects of heavy metals on cement clinker sintering and hydration property.Research on the single mine (C3S and C3A) was divided into two parts. Part one was focused on the effect of heavy metal ion on the hydration performance of single mines and their solidification in hydration products. Part two was focused on the effect of heavy metal doping on the formation and the subsequent hydration process of single mine minerals, discussing solidification of heavy metals during high temperature firing process and hydration process.Research results showed that heavy metal ion Cr and Cu could accelerate C3S hydration, and Ca2Cr(OH)7·3H2O and2Ca(OH)2·4Cu(OH)2·H2O were formed during hyration. Early hydration of C3S was retarded by Pb and Zn ions. After7d hydration, Pb ions showed a promotion of hydration of C3S, while the Zn ion still showed inhibition the hydration of C3S. After curing for28d, hydration products of pure C3S showed a good performance in solidification of heavy metals of Cr, Cu, Zn and Ni. For Pb ions, the solidification performance was poor.As for C3A, with the exist of dihydrate gypsum, heavy metal ion Cr and Cu showed a promotion of hydration of C3A-dihydrate gypsum, while Pb and Zn ion exhibited a retardation effect at an early period time of C3A-dihydrate gypsum. After curing for28d, hydration products of pure C3A-dihydrate gypsum showed a good performance in solidification of all the five heavy metals of Cr, Cu, Zn, Niand Pb.During the preparation of heavy metals doped single mine C3S and C3A, it was found that heavy metal element Pb in basically all escape into the air, indicating that strict control of Pb content in waste was required when using Pb-containing waste in the production of cement clinker. As for the other heavy metals, Cr and the Ni doping caused a gradual transformation of C3S from T1to T2, and the addition of Cr caused a decomposition of C3S into C2S and CaO. Cu doping can prompted C3S from T1to M1. And Zn doping can prompted C3S from T1to M1, and then to R. No obvious effect of heavy metal doping on the crystal structure of C3A.During the hydration process of heavy metal-doped single mine C3S and C3A, different heavy metals showed different influences. After curing for28d, hydration C3S and C3A showed good solidification of Cu, Zn, Ni. As for Cr-doped single mines, leaching value exceeds the limits required by GB5085.3-2007after hydration for28d. It was indicated that, special attention should be payed on the Cr content during the utilization of Cr-containing waste in the production of cement clinker.