Preparation Of Red Mud Based Ceramsite And Study On Its Solidification Effect And Occurrence State Of Heavy Metals

Red mud is a strong alkaline solid waste discharged during the industrial production of alumina.China produces an average of 1-1.5 tons of red mud per ton of alumina produced.Red mud has a high alkali content and contains small amounts of heavy metal and rare earth elements,which are classified as hazardous industrial waste.China is a major producer of alumina,with a production capacity of 77.475 million tons in 2021.Based on this data,if 1.5tons of red mud are emitted per ton of alumina,the emissions of red mud in 2021 alone will reach around 100 million tons.Due to the lack of feasible technologies for the application of red mud in China,the current utilization rate of red mud in China is only 7%.Most of the red mud cannot be effectively utilized and can only be stored in the form of open-air stacking in yards.This storage method not only occupies a large amount of land,but also pollutes groundwater and soil,causing a great burden on the ecological environment in China,and also poses great safety hazards.Therefore,the comprehensive,efficient,and harmless utilization of red mud has become an urgent problem to be solved.Improving the utilization of red mud without generating secondary pollution will directly affect the sustainable development of future alumina industry production and the protection of the ecological environment.This article uses red mud as the main raw material,compounded with slag and steel slag to prepare red mud based ceramic particles.The effects of different mix ratios,sintering temperature,and holding time on the properties of red mud based ceramic particles were studied through XRD,SEM,DSC,infrared analysis,XPS,etc.And ceramic mortar was prepared using red mud based ceramic particles to explore the effect of different amounts of ceramic particles on the performance of the mortar.The effects of different p H values,liquid-solid ratios,and sintering temperatures on the leaching of four heavy metals,Cr,Ni,Pb,and As,were studied.On this basis,the occurrence states of Cr and Ni elements in the sintering process of ceramic particles were studied.The results indicate that:（1）When the ratio is red mud:slag:steel slag=4:5:1,and the sintering temperature is kept at 1150℃for 15 minutes,the prepared ceramic particles have the best performance.Under these conditions,the single strength of the ceramic particle is 16.8MPa,the stacking density is696kg/m³,the apparent density is 1922kg/m³,the porosity is 63.79%,and the water absorption rate is 0.81%.XRD analysis shows that the phase formed by ceramsite includes quartz,microcline,mullite,and kyanite.By using SEM and optical microscopy to observe the microstructure of ceramic particles at different sintering temperatures,it was found that the pore structure of ceramic particles prepared under these conditions is relatively rich,with sufficient liquid phase generation and high crystallinity.（2）Replacing river sand with a volume of red mud ceramic particles can effectively improve the flexural and compressive strength of ceramic mortar.The larger the replacement ratio,the more the strength of the mortar is improved.When the curing period is 7 days,the compressive strength of the mortar without adding or replacing 20%,40%,60%,80%,and 100%is 29.2 MPa,30.6 MPa,32.7 MPa,34 MPa,35.1 MPa,and 36.3 MPa,respectively.The flexural strength is 6.5 MPa,6.75 MPa,7.14 MPa,7.3 Mpa,7.46 MPa,and 7.6 MPa,respectively.（3）The existing forms of the four heavy metals Cr,Ni,Pb,and As vary in different p H environments.As the sintering temperature increases,the leaching concentration of Cr and Pb elements decreases,the leaching concentration of As increases,and the leaching concentration of Ni slightly decreases.The leaching amount of all four heavy metals increases with the increase of the liquid-solid ratio.In the effective release test under extreme environments,the leaching amounts of four heavy metals were all within the standard range,indicating that this ceramic particle can be safely used for production.（4）Using ICP to study the content of heavy metal elements in ceramic particles before and after sintering and leaching,the results showed that high-temperature sintering can solidify 99.3%of Cr element and 96.6%of Ni element.Analyze the occurrence status of heavy metals using XRD,XPS,and infrared spectroscopy.The results show that the occurrence state of Cr element before and after sintering of ceramic particles and after acid leaching is:Cr Si₂/Ca Cr O₄/Ca Cr₂O₇→Fe Cr₂O₄→Cu Cr₂O₇;The occurrence state of Ni element is:Ni SO₄→Ca Ni Si₂O₆/Ni Fe₂O₄→Ni Si O₄.In summary,using red mud,slag soil,and steel slag can prepare high-performance ceramic particles and use them to prepare ceramic mortar.High-temperature sintering can effectively solidify the heavy metal elements inside and can be safely put into production and use.To lay a theoretical foundation for the application of red mud as a building material.