Precipitation of uranium from alkaline liqueurs

ABSTRACT In Argentina there is uranium with abundant organic matter and variety in quantity of sulfides and carbonates which act as cement or sealing joints. The latter minerals modify directly the choice of metal leaching process for the concentration of uranium. The most beneficial selected process turns out to be the alkaline leaching. Once this process has been finished, using the lixiviante agent such as Na2CO3 NaHCO3 in presence of oxidizing agent, the hexavalent state of uranium as U3O8 has been achieved. In this research the experimental design is proposed at the laboratory level, in order to obtain the precipitation of uranium from alkaline leach liquors using two different precipitation mechanisms. One of them uses a strong oxidizing agent such as excessive hydrogen peroxide. Stoichiometrically, 0.9 g of H2O2 were used, obtaining the uranium precipitation as UO2*2H2O. The working variables are lightly acidic pH values, for values between 2.5, 4 to 5 and reaction times with mineral exposure of 1, 2, 3 and 4 hours. Working temperatures are 25 °C, 60 °C and 80 °C for a constant stirring speed of 400 rpm. In the other mechanism, a precipitating agent such as NaF in alkaline medium is added, without modifying the pH of leached liquors, generating an insoluble compound such as NaUF6. For this purpose we worked with different: concentrations of the precipitating agent, stirring speed and precipitation times. However, the first mechanism proved to be the best. Uranium recovery values were found for working conditions of pH: 2.5; temperature of 60 ºC and reaction times higher than 3 h until 4h.摘要：阿根廷境内赋存铀矿床，伴生丰富的有机质，且硫化物与碳酸盐的含量分布不均，上述矿物可作为胶结剂或封堵节理的填充物。其中碳酸盐类矿物会直接影响铀富集工序中金属浸出工艺的选型，经筛选最优工艺为碱性浸出（alkaline leaching）。该工艺以碳酸钠（Na₂CO₃）、碳酸氢钠（NaHCO₃）作为浸出剂，搭配氧化剂使用，最终可得到以六价态存在的铀产物八氧化三铀（U₃O₈）。 本研究在实验室尺度开展实验设计，旨在通过两种不同的沉淀机制，实现从碱性浸出液中沉淀铀。第一种机制采用过量过氧化氢（hydrogen peroxide）作为强氧化剂，按化学计量比投入0.9g过氧化氢，可将铀以二水合二氧化铀（UO₂·2H₂O）的形式沉淀析出。该工艺的控制变量包括弱酸性pH值（设置为2.5、4~5）、反应时长（1、2、3、4小时），反应温度分别为25℃、60℃、80℃，搅拌转速恒定为400rpm。 第二种机制则在碱性介质中添加氟化钠（NaF）作为沉淀剂，无需调整浸出液的原始pH值，即可生成不溶化合物六氟铀酸钠（NaUF₆）。该工艺的考察变量涵盖沉淀剂浓度、搅拌转速与沉淀反应时长。 实验结果显示，第一种沉淀机制的综合效果更优。当pH值为2.5、反应温度为60℃、反应时长处于3~4小时区间时，铀的回收率可达最优水平。