| After decades of development,CO2+O2 in-situ uranium leaching technology has been widely recognized worldwide.At present,China's Inner Mongolia and Xinjiang have adopted this technology for large-scale industrial uranium mining.However,in uranium production,the phenomenon of colloid blockage and resin bonding seriously affects uranium extraction efficiency.Therefore,it is necessary to explore the types and formation mechanisms of colloids in the in-situ uranium leaching and colloids'influence on the adsorption and transport of uranyl carbonate(UC(VI)).This is conducive to developing our country's CO2+O2 in-situ leaching uranium technology,provides basic theory and technical support for the sustainable development of the technology,and is of great significance for the prevention and control of uranyl carbonate pollution transport.In this paper,SiO2 colloids and uranyl carbonate are selected as the research objects after analyzing the ore blockage colloids.The adsorption and cotransport law and mechanism of SiO2 colloid and uranyl carbonate are discussed,and the key factors affecting their cotransport are studied.The main contents and conclusions of this thesis are as follows:1.By collecting sandstone uranium ore cores and surrounding rock core samples from CO2+O2 in-situ leaching uranium mines,the core samples were identified.The chemical components of the leaching stock solution,the adsorption tail solution,and the resin hardened substance were analyzed.A small amount of mineral colloid was collected through the CO2+O2 pressurized shake flask experiment and column leaching experiment,and finally,the formation mechanism of the colloid was determined.The results showed that:(1)The aggregation of Ca and Mg carbonate colloids is the main cause of resin hardening,while SiO2 colloids and clay colloids are the main components of the blockage of the ore bed.(2)At the level of physical factors,based on the torque balance model theory,the action of high-speed shear force formed by water pumping and water injection activities promotes the formation of mineral colloids.(3)At the level of chemical factors,the injection of leaching solution causes fluctuations in pH and ionic strength in the groundwater environment.The decrease in pH makes the carbonate minerals and clay minerals such as kaolin dissolve to form colloids;the reduction of ionic strength promotes the fine mineral particles attached to the mineral bed matrix to enter the liquid phase to form colloids.2.Kaolin was selected as the raw material for mineral colloids preparation,and high-purity SiO2 mineral colloids were obtained after high-temperature modification activation,acid leaching,and dialysis bag purification.Through batch adsorption experiments,the effects of pH and adsorption time on uranyl carbonate adsorption by SiO2 mineral colloids were studied.And Fourier infrared spectroscopy results revealed the adsorption mechanism of SiO2 mineral colloids on uranyl carbonate.It was found that(1)SiO2 mineral colloid maintains high stability in the range of pH=5?11 and 1?50 m M ionic strength,which is the basis for its long-distance transport in the underground environment.(2)Uranyl carbonate is sensitive to pH changes,and hydrolysis precipitation occurs when the solution environment changes from alkaline to acidic.(3)Due to electrostatic repulsion,the adsorption rate of SiO2 mineral colloid on uranyl carbonate decreases with the increase of pH value,and pH=7.0 is the best adsorption pH value.(4)Uranyl carbonate adsorption onto SiO2mineral colloid can be described by the pseudo-second-order kinetic model and the Freundlich adsorption isotherm model.The analysis shows that the adsorption is mainly chemical adsorption,but there is physical adsorption simultaneously.The isotherm model calculates that the maximum adsorption capacity is 40.46 mg/g,and the adsorption process tends to be heterogeneous multi-molecular layer adsorption.3.To reduce the influence of impurities in the above-mentioned SiO2mineral colloid on the adsorption and transport of uranyl carbonate,a SiO2colloidal suspension was prepared using high-purity spherical SiO2nanoparticles.Through column transport experiments,the influence of ionic strength,uranyl carbonate concentration,and pH on the cotransport of SiO2 colloid and uranyl carbonate was explored.The results showed that(1)When the flow rate is 0.399 cm/min,the SiO2 colloid always inhibits the transport of relatively low concentrations of uranyl carbonate(i.e.,4.2×10-6 and 2.1×10-5 M)in the saturated quartz sand column.(2)The existence of uranyl carbonate makes the stability of SiO2 colloid more challenging to be affected by the increase of ionic strength.(3)The release of HCO3-and the deprotonation of silanol(?SOH)lead to the opposite change trend of the effluent pH at the initial pH 9 and 5.(4)The theoretical calculation of DLVO shows that whether uranyl carbonate exists or not,ionic strength is more likely to affect the stability of SiO2 colloid than pH.(5)The adsorption and transport behavior of uranyl carbonate and SiO2colloid in the saturated quartz sand column was well described by the two-site chemical non-equilibrium model and the colloidal filtration theoretical model,respectively.4.Natural groundwater environment is often rich in a variety of organic colloidal substances.Therefore,to study the effect of organic matter on the cotransport of SiO2 colloid and uranyl carbonate,a batch adsorption experiment and column transport experiment were carried out using purified humic acid(HA)colloid.This chapter mainly studies the cotransport behavior and mechanism of SiO2 and uranyl carbonate in a saturated quartz sand column in the presence of humic acid colloids.The conclusions could be drawn as follows,(1)The adsorption distribution coefficient of humic acid colloid for uranyl carbonate is close to that of SiO2 colloid,which is 4 to 5 orders of magnitude higher than that of quartz sand.(2)When the flow rate is 0.201 cm/min,SiO2 colloid and humic acid colloid can promote the transport of low-concentration of uranyl carbonate(4.2×10-6 M).(3)The increase of humic acid concentration can promote the co-transport of SiO2 colloid and uranyl carbonate,and humic acid will cause the low concentration of uranyl carbonate adsorbed on the surface of quartz sand to be easily eluted.(4)In the uranyl carbonate+SiO2 binary co-transport system,the transportability of SiO2 and uranyl carbonate decreases with the increase of ionic strength.However,when humic acid exists(that is,in uranyl carbonate(UC(VI))+SiO2+HA ternary co-transport system),the transportability of SiO2 and uranyl carbonate is almost not affected by the increase of ionic strength.(5)The retention profiles in the column of uranyl carbonate individual transport are linear and independent of the transport distance,which indicates that uranyl carbonate does not precipitate during the individual transport process,and adsorption is the mechanism that affects its transport.(6)The hydrodynamic diameter and Zeta potential of the SiO2 colloid in the effluent of the binary co-transport system and the ternary co-transport system show opposite changes,which indicates that the presence of humic acid colloids can promote SiO2 and uranyl carbonate transport.(7)The longitudinal hydrodynamic diffusion coefficient(Dx),irreversible attachment rate coefficient(kirr),reversible detachment rate coefficient(r2),and reversible attachment rate coefficient(r1)can well describe the deposition and transport behavior of SiO2 colloid.(8)With the increase of uranyl carbonate and humic acid concentration and the decrease of ionic strength,the diffusion coefficient of SiO2 colloid increases,which indicates that the change of these conditions can enhance the transportability of SiO2 colloid. |
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