Immobilization Of Low Radioactive Rare Earth Acid Soluble Slag By Fly Ash Based Geopolymer

The ionic rare earth smelting process generates acid-soluble slag,mainly silica and barium sulfate,which is included in the regulatory scope because it contains radioactivity and is a hazardous waste slag.Due to the lack of suitable treatment,the acid-soluble slag of rare earths accumulates in large quantities for a long time,and there are hazards such as radiation exposure,environmental pollution and potential health risks.Therefore,how to make the rare earth acid soluble slag harmless has become a problem faced by rare earth producers.Geo-polymer is a new type of silica-aluminate inorganic polymer material that can play an important role in solidifying heavy metal fractions.In this paper,to address the radioactivity problem of rare earth acid-soluble slag,geopolymer curing body was prepared with rare earth acid-soluble slag and fly ash as raw materials and sodium hydroxide mixed with water glass as exciter,and the radionuclides in rare earth acid-soluble slag were effectively cured.This paper investigates the optimal preparation process of fly ash-based geopolymer and the effect of rare earth acid soluble slag addition on the leaching of radionuclides from the rare earth acid soluble slag curing body.The physical phase,morphology,structure and toxic leaching of the cured bodies were characterized by XRD,SEM,FT-IR and ICP-OES analyses.On this basis,the curing mechanism of radionuclides in geopolymers was investigated.The main research contents and results are as follows:（1）The physical phase and morphology of the rare earth acid soluble slag were studied by XRF,SEM and XRD.The study showed that although the rare earth acid soluble slag contains a high content of Si and a small amount of Al,they are crystalline substances,which do not have the corresponding activity to participate in geopolymerization reactions and cannot provide Si and Al components for the synthesis of geopolymer skeleton.（2）Using fly ash as raw material and compressive strength as the index of investigation,the effects of alkali exciter type,Na/Al molar ratio,calcium ion content,liquid-solid ratio,curing temperature,curing time,and Si/Al molar ratio on the compressive strength of geopolymer were investigated.The optimal geopolymer was produced when sodium hydroxide mixed with water glass as the alkali exciter,Na/Al molar ratio of 0.9,Si/Al molar ratio of 2.0,calcium oxide addition of 4wt%,liquid-solid ratio of 0.28,maintenance temperature of 75°C,and maintenance time of 24 h were used.The geopolymer prepared under these conditions has an amorphous Si/Al gel phase with a dense and uniform internal structure,and the compressive strength of the geopolymer produced reaches 45 MPa,which has good mechanical properties.The compressive strength of the geopolymer was tested by adding 5wt%～25wt%of rare earth acid slag to the geopolymer,and it was found that the addition of slag had a negative effect on the compressive strength of the geopolymer.The compressive strength of the geopolymer cured body was 12 MPa when 25wt%of rare earth acid soluble slag was added.（3）The geopolymer curing body was subjected to particle leaching test and static leaching test,and the results of both leaching tests proved that the geopolymer curing body had a strong curing effect on radionuclides in the rare earth acid slag.In the particle leaching test,the curing rate of the geopolymer curing body was greater than 95%for thorium and 85%for uranium.In the static leaching test,the 7 d leaching rate of thorium in the geopolymer cured body with 25wt%slag addition was 3.77×10^-5cm·d^-1,and the 7 d cumulative leaching fraction was 4.42×10^-5cm.The XRD,SEM-EDS,and FT-IR analyses showed that the reaction products of both the geopolymer and the geopolymer cured body were mainly amorphous gel phase materials.The incorporation of rare earth acid soluble slag makes the structure of the geopolymer loose.Thorium mainly exists in the geopolymer as thorium oxide and thorium phosphate precipitates,and uranium mainly exists in the geopolymer as heavy uranate and uranyl hydroxide precipitates;a few thorium and uranium produce covalent bonds with non-bridging oxygen at the end of the geopolymer skeleton,thus stabilizing the radionuclides in the cured body.