The Transformation Of Key Components During Coal Combustion And Controls On Occurrence And Leaching Properties Of Uranium

Although the proportion of coal in China’s energy supplement will decrease from60.6%in 2015 to 33.7%in 2030,China is the largest coal producer and consumer globally,with～2.7 billion tons of coal consumed in 2017.The coal combustion and the following ash deposition release a range of contaminations to the ecosystem.Thus,developing non-coal energy is an inevitable choice for China’s energy strategy.Compared with other clean energy sources,nuclear energy shows many incomparable advantages.According to the"13th Five-Year Plan"energy instructions,the development of China’s nuclear power has been restarted since 2015,and the proportion of nuclear energy supplement in total energy supplement will increase from 1.6%in 2015 to 5.5%in 2030.However,in China,the uranium（U）resource is imported mainly from overseas,and the external dependence remains>70%all year around.The utilization of U in coal-uranium interbedded basins is receiving increasing attention due to the decreasing storage of standard U reserves.In some regions with the Late Permian substrata,e.g.,Guiding in Guizhou,Lincang and Yanshan in Yunnan,Heshan and Fusui in Guangxi,Yili in Xinjiang,etc.,the U-rich coal is found with U content>1000 mg/kg.From the discussion above,the efficient recovery of U and other trace metals from U-rich coal ashes（CAs）can（1）display the economic effect of coal combustion to produce electricity,supply the demand for nuclear power stations,and reduce the environmental risk during CA storage.Based on U-rich coals from the Lincang,Yunnan,and lab-synthesized U-rich ashes（SAs）,this study includes three aspects,（1）the morphology and distribution characteristics of U and other trace metals in coals and CAs,（2）the determination of U mobility by the complex interaction among key components during combustion,and（3）the investigation of high-efficiency leaching methods for U and other trace metals in CAs,by combing with the thermochemical activations,aqueous extraction experiments,solid characterization technologies,and thermodynamic/hydrochemical simulations.The findings are as follows:（1）The U occurrences involve 3 evident changes with increasing the combustion temperature.At～500℃,the content of carbonate-bound U（S2-U）in CAs is enhanced.But with the temperature>700℃,some carbonates are decomposed to metal oxides and combine with acidic minerals,e.g.,iron oxides and aluminosilicates,which contributes to the transfer of S2-U to Fe-Mn oxide-bound U（S3-U）and/or to aluminosilicate-bound U（S5-U）.When the temperature increases to 1100°C,iron oxides can bridge crystalline and amorphous aluminosilicates with the formation of large-size aggregations,making>90%of active U transfer to S5-U.With increasing the combustion temperature,there is a similar change between the leaching efficiency of U and the proportion of active U.Considering high acidic solubility of most U oxides and uranates,some U is immobilized in inert minerals.（2）The（NH₄）₂SO₄ thermochemical treatment can activate amorphous Al phases in CAs.Additionally,the changes in（NH₄）₂SO₄ dosage,incineration temperature,and time influence the release of trace metals.With the treatment,the leaching amounts of U,Cs,Rb,and Sr in（NH₄）₂SO₄-treated 900CA（NS-900CA）are1.89,2.31,8.39,and 1.74 times higher than directly leaching CA by H₂SO₄.Thus,the formation of amorphous Al-bearing phases can conduce to the immobilization of some trace metals during combustion.However,the amorphous Al-bearing phases are recrystallized when the combustion temperature increases to 1100℃,inhibiting the（NH₄）₂SO₄ activation.The release of U from NS-1100CA rarely increases.After NaOH-based thermochemical activation（Na-CA）,the distributions of V,Cs,Rb,and Sr are mainly found in aluminosilicate glass,instead of 47%,45%,and 8%of U in aluminosilicate glass,alkaline eutectic,and iron oxide,respectively.However,about 59%of Ga remains in Fe-rich silicates even after a series of treatments,including NaOH activation and leaching.Because the mobility of U and other trace metals in Na-CA is largely improved,the solid waste that is formed after synthesizing function sorption materials,e.g.,zeolite,needs to be appropriately disposed of due to the potentially environmental risk.（3）Based on the SA synthesized by SiO₂-Al₂O₃-Fe₂O₃-CaO systems,the determination of U mobility by the complex interaction among key components during combustion is comprehensively discussed.Instead of the evident but opposite effects of U mobility with adding CaO and Fe₂O₃,the effect of SiO₂-Al₂O₃ mixtures is in-between and subject to the crystallization of amorphous aluminosilicate.Moreover,the values of x and y in x SiO₂·y Al₂O₃ mixtures are significant to the crystallization.Since the contents of active U in SA and CA are significantly reduced after combusting at 1100°C,showing the low mobility of U,this work only discusses the environmental risk of U in 900SA.Before combusting,increasing SiO₂ content and adjusting 0<Ca/Si<0.5,Al/Si>1.25and Fe/Si molar ratio>0.75 in the fuel can effectively reduce the environmental risk of U in combustion solid wastes.As for 1100SA,the parameterα（as a function of ash composition）is submitted,and a nonlinear regression equation(y=-17.056+117.284×^（.）/.,R²=0.88)is established to describe the relationship betweenα（x）and S5-U（y）,which achieves a relatively accurate output of U leachability from the input of SA composition.（4）The oxidant-H₂SO₄ synergistic leaching and（NH₄）₂SO₄ incineration-water leaching systems are studied to improve the leaching of some valuable metals（including U）from CA.The shrinking core model describes the H₂SO₄ leaching process of CA,in which the leaching is controlled by chemical reaction or mixing diffusion and chemical reaction.The factors,such as H₂SO₄ concentration and temperature,can change the leaching efficiency of U significantly.Under the conditions including 0.3 M of H₂SO₄,1:20 mg/L of the solid-liquid ratio,60℃and 12 h,the leaching efficiency can be up to45.2%.The efficiency of U increases to 55.0%with adding the proper amount of oxidants（e.g.,4%of KMn O₄）.Additionally,the（NH₄）₂SO₄-based thermal treatment is able to activate the amorphous Al-bearing minerals in CAs and to promote the release of some immobilized U and other trace metals.Under the conditions of wt.（（NH₄）₂SO₄）:wt.（CA）=2,400℃,and 2 h,increasing the leaching temperature can improve the release of U by water,due to the more dissolution of Al³⁺and Ca²⁺.At temperatures>40°C,the hydrolysis of Fe and Ti ions can form co-precipitation with V and Ga ions,thus increasing the proportion of S3-V/Ga and S5-V/Ga in leaching residues.However,the formation of co-precipitation is inhibited under 0.1 M H₂SO₄,and leaching efficiencies of U,V,and Ga from NS-CA are 1.9,1.3,and 5.0 times higher than those by leaching CA with 0.4 M H₂SO₄.