含矿含水层中低含量黄铁矿对CO2+O2地浸采铀过程中铀浸出与迁移的影响机理研究

It is an unquestioned fact that CO2+O2 in-situ leaching deposits of uranium in China have the problem such as low leaching concentration of U(VI) and long leaching cycle. What specific reasons causing such problem, have been puzzling the technical personnel for long time. In all the in-situ leachable sandstone type uranium deposits in China, there are more or less contents of pyrite in ore-bearing aquifers and it is commonly accepted that the oxidation of pyrite has significant influences on the process of CO2+O2 in-situ leaching of uranium. However, there are few studies direct at this question. In this project, we aim at studying the influence and mechanism of the low-content pyrite in ore-bearing aquifer on the leaching and migration of uranium (VI) during the process of CO2+O2 in-situ leaching of uranium by combining multiple methods including site investigation, laboratory experiment as well as numerical simulation. The contents will focus on: quantifying the influence of pyrite oxidation on the release and migration of U(VI) absorbed on the surface of minerals; determining the control mechanism of ferric ions on the leaching and transport of U(VI); revealing the change of porosity and permeability caused by the mineral dissolution and precipitation enhanced by pyrite oxidation; building the reactive transport model for CO2+O2 in-situ leaching of uranium. The achievements of this project will provide scientific guidance for the improvement of leaching concentration of uranium (VI) and the adjustment and control of the formation permeability as well as the determination of the distance between pumping and injection wells for the industrial production of CO2+O2 in-situ leaching of uranium.

我国CO2+O2地浸采铀矿山，浸出液铀浓度低、浸出周期长是不争的事实，什么具体原因造成的，一直困扰着铀矿开采技术人员。我国所有的可地浸的砂岩型铀矿床中或多或少存在黄铁矿，黄铁矿氧化对CO2+O2地浸采铀有较大影响，这也是共识。然而目前针对该问题的研究甚少。本项目以含矿含水层中低含量黄铁矿共生的砂岩铀矿床为研究对象，融野外调查、科学实验与数值模拟为一体，研究黄铁矿氧化产酸对吸附态铀的迁移释放的影响，确定铁离子的赋存形态及对U(VI)迁移的控制机制，探索黄铁矿氧化作用下水岩反应及矿层孔渗性的演变规律，建立浸出液铀在矿层中的时空演化数值模型，揭示含矿含水层中低含量的黄铁矿对CO2+O2地浸采铀过程中铀浸出和迁移的影响与机理。该项目的研究成果对于CO2+O2地浸采铀工业化生产中铀浸出浓度的提高、矿层渗透性能的调控以及抽注孔间距的确定具有重要的理论指导意义。

中性地浸采铀适用于砂岩型铀矿床，我国存在较多可地浸的砂岩型铀矿床中或多或少存在黄铁矿，而黄铁矿在有氧环境中容易发生氧化释放出大量的Fe2+，在Fe2+氧化生成Fe(OH)3胶体或沉淀过程中会对U(VI)产生吸附、还原、共沉淀等作用，对铀的迁移与浸出产生较大影响。本项目以含矿含水层中低含量黄铁矿共生的砂岩铀矿床为研究对象，通过批反应实验、酸溶实验以及X射线衍射（XRD）、傅里叶变换红外（FT-IR）、扫描电子显微镜（SEM）、X射线能谱（EDS）和X射线光电子能谱分析（XPS）等表征分析手段，研究了单独通O2和CO2+O2共通条件下黄铁矿氧化对U(VI)的吸附与浸出的影响，考察了Fe/U摩尔比、初始pH、HCO3-离子浓度以及CO2分压等不同因素对Fe(II)诱导Fe-U共沉淀的影响，并开展了Fe(II)与U(VI)共沉淀的机理与稳定性研究。实验结果表明，黄铁矿氧化形成氢氧化铁胶体、针铁矿和纤铁矿。在无CO2分压体系中，少量的黄铁矿氧化可以增强U(VI)的固定，去除率达到95%，推测黄铁矿氧化除U(VI)机理主要为吸附、还原和共沉淀。黄铁矿对U(VI)的吸附行为符合伪二次吸附模型，主要为化学吸附。在CO2+O2条件下，约32%的U(VI)被黄铁矿氧化固定；另外方解石的溶解能有效抑制黄铁矿氧化对U(VI)的固定，去除率仅为2%。在中性氧化环境中少量Fe(II)的氧化可诱导Fe-U共沉淀的发生，对U(VI)的沉降效率达到96%以上，在酸性条件下，当Fe/U摩尔比为2左右时，U(VI)的固定效率最高。Fe(II)与U(VI)在中性富氧条件下发生共沉淀作用，主要是形成了低结晶度的铁氧化氢氧化物FeO(OH)，其在弱酸性和富氧环境中都具有较强的稳定性。少量Fe(II)氧化固铀机理主要为吸附、还原、共沉淀、晶格嵌入等，其中27-29%的U以U(V)和U(VI)的形式被嵌入FeO(OH)晶体结构中。此外HCO3-离子浓度和CO2分压会降低Fe(II)氧化固铀的效率。该项目的研究成果对于定量评价中性地浸采铀过程中铁铀界面作用对铀迁移与浸出的影响具有重要的理论指导意义，同时有助于评估铀元素在复杂地球化学环境中的迁移性和稳定性。