Study On Solidification Effect Of Heavy Metals In Waste Incineration Fly Ash

Waste incineration fly ash belongs to hazardous solid waste,and its utilization value is minimal.Because it contains a lot of harmful substances including water-soluble salts,heavy metals and dioxins,it has become the research object of many researchers.In this paper,the municipal solid waste incineration fly ash produced by a waste incineration plant in Jiangxi Province was used as the raw material and mixed with cement,red mud and mineral powder to prepare cement-based cementitious material（hereinafter referred to as cement solidified body）,red mud based cementitious material（hereinafter referred to as red mud solidified body）and mineral powder based cementitious material（hereinafter referred to as mineral powder solidified body）.The incineration fly ash should be consolidated.The effects of cementitious material content,water-solid ratio and curing time on the compressive strength and heavy metal leaching toxicity of fly ash solidified body were explored,through the morphological analysis of heavy metals in fly ash solidified body under the best preparation conditions,the distribution law of heavy metals in incineration fly ash and solidified body were compared and analyzed through SEM XRD and FTIR were used to further analyze the solidified structure of the solidified body compared with the original fly ash.The results were summarized as follows:（1）Incineration fly ash has low moisture content and was a strong alkaline substance mainly rich in Ca,O,Cl,S and Si.The content of heavy metal Zn in fly ash was the highest,followed by Ba,Pb and Cu,and the content of Be was the lowest.The leaching test of fly ash showed that the leaching values of Cr,Cd,Pb and Ni were higher than the specified concentration limit that can enter the landfill.（2）Through the compressive strength test and heavy metal leaching test on the solidified body,under the condition that the compressive strength of the solidified body could reach a certain strength and the leaching concentration of most heavy metals was low,considering that red mud and mineral powder belong to solid waste,plus factors such as cement cost,water consumption,curing age and alkali activator content,the cement content was determined to be 30%.The water-solid ratio of 0.2 and curing time of 14d were the best preparation scheme of cement solidified body.The red mud content of 40%,water-solid ratio of 0.35,curing time of 14d and adding 3%alkali activator were the best preparation scheme of red mud solidified body.The mineral powder contented of 30%,water-solid ratio of 0.3,curing time of 14d and adding 3%alkali activator were the best preparation scheme of mineral powder solidified body;（3）The compressive strength of solidified body under the experimental conditions of all different factors was higher than the standard of 0.5 MPa specified for safe landfill of domestic waste;（4）Comparing the form distribution of heavy metals in the three solidified bodies,it was found that the weak acid content of heavy metals in all solidified bodies was basically low,basically no more than 20%,and only Cd in red mud solidified bodies accounts for 24.2%of the total;Cu was the main existing form of reducible state of the three solidified bodies,and the heavy metal content of most elements was more than 30%;As was the main existing form in the oxidizable state of the three solidified bodies,all accounting for more than 50%among them;The residual content of most heavy metals in the three solidified bodies was high,indicating that most heavy metals were relatively stable,meaning that the solidified bodies had played a different degree of curing effect on heavy metals.（5）By comparing and analyzing the morphological changes of heavy metals under the best preparation conditions in waste incineration fly ash and each solidified body,it was found that fly ash and solidified body were mainly reducible,Cr was mainly fixed in fly ash in the form of precipitation,and the mineral powder solidified body had the highest residual Cr than the other two solidification methods;Acid soluble Cd decreased after solidification,and the content of acid soluble Cd was cement solidification<mineral powder solidification<red mud solidification,and the proportion of reducible Cd and residual Cd increased;The content of residual Ni was less,and the content of residual Ni in the three solidified bodies was about twice that of incineration fly ash;The solidified form of mineral powder produced more residual Pb,and the acid soluble Pb basically did not change,while the acid soluble Pb basically did not exist after cement solidification and red mud solidification of incineration fly ash.（6）Through SEM analysis,it was found that the surface of fly ash was coarse and porous,the specific surface area was larger,the water absorption was stronger,and it was easier to leach.After solidification treatment,the compactness of the three solidified bodies was increased,and the minerals such as Ca CO₃crystal and C-S-H gel layer were produced,which played a good role in fixing heavy metals in fly ash.（7）Through XRD and FTIR analysis,it was found that several solidified bodies contained minerals such as Ca CO₃,(Mg_0.03Ca_0.97)（CO₃）,ettringite and C-S-H gel,indicating that the active groups such as O-H,O-C-O and Si-O-Al（Si）played a significant role in the solidification of heavy metals.The existence of these substances indicated that the solidified bodies played a good role in curing heavy metals.（8）By changing the material content,water-solid ratio and curing time,the curing experiments were carried out respectively.After obtaining the data of the compressive strength of the solidified body and the leaching concentration of heavy metal Cr,the polynomial fitting modeling was carried out.It was found that the content of red mud had relatively little effect on the curing effect of fly ash;The water-solid ratio of solidified red mud had a great influence on the solidification effect of fly ash;The curing time of mineral powder solidified body had a relatively small impact on the curing effect of fly ash,and the curing time of cement solidified body had a relatively large impact on the curing effect of fly ash.