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Geochemical characterization of uranium mill tailings (Bois Noirs Limouzat, France) highlighting the U and 226Ra retention.
Journal of Environmental Radioactivity ( IF 2.3 ) Pub Date : 2020-03-30 , DOI: 10.1016/j.jenvrad.2020.106251
Camille Chautard 1 , Catherine Beaucaire 2 , Martine Gerard 3 , Régis Roy 4 , Sébastien Savoye 2 , Michael Descostes 1
Affiliation  

As with other metals, the management of tailings from former uranium (U) mines requires a good knowledge of the geochemical mechanisms governing the retention of radioelements of interest: U and 226Ra. This article presents the results of the study of the bearing phases featuring these two radioelements within the Bois Noirs Limouzat tailings storage facility (Loire), the only site in France where the tailings (a sandy silt facies and a clayey silt facies) are currently stored only under water. The aim is to gain a better understanding of their respective mobility under current storage conditions. For this purpose, a multi-scale approach was adopted combining historical research and airborne image analysis to select the core location, chemical and radiological analyses, mineralogical characterizations supplemented by sequential extractions (two specifically developed protocols). The results show that U and 226Ra are mainly found in the clayey silt facies with an average U concentration of 243.3 ppm (132.3 ppm in the sandy silt facies) and an average 226Ra mass activity of 64.7Bq/g (18.0Bq/g in the sandy silt facies). These results are in accordance with the initial U grade of the ore (2‰), the extraction efficiency of the ore processing plant (95%) and the age of mineralization (305 Ma). The approach adopted made it possible to highlight several mineralogical traps available for each radioelement, regardless of the facies type. Thus, a significant part of the U is still trapped within the primary phases, resistant to treatment and therefore relatively immobile under current storage conditions (49.6%-77.8% for the sandy silt facies and 27.2%-36% for the clayey silt facies). Most of the leached U is mainly associated with weakly crystalised iron oxyhydroxides (8.7%-42.4% for the sandy silt facies and 50.9%-71.8% for the clayey silt facies) and to a lesser extent with clay minerals (5%-12.3% for the sandy silt facies and 0.8%-11.5% for the clayey silt facies). For the 226Ra, irrespective of the facies type, a significant part remains trapped within phosphate phases, resistant to the leaching process and therefore also relatively immobile under storage conditions (24.4%-38.9% for the silty sandy silt facies and 39.9%-98.9% for the clayey silt facies). Sequential extractions revealed a different geochemistry of 226Ra depending on the facies. For the silty sandy silt facies, most of the 226Ra is mainly associated with the clay minerals (6.4%-69.2%) and to a lesser extent with iron oxyhydroxides, barite or aluminum phosphate sulphate minerals (APS) (6.4%-33.9%). For the clayey silt facies, most of the 226Ra is mainly associated with iron oxyhydroxides, barite or APS (6.4%-53.3%) and to lesser extent clay minerals (0.4%-6.8%). The leaching process did not allow the differentiation between the contributions of each of these phases to the retention of 226Ra. At last, all the identified bearing phases demonstrate that the U is relatively immobile under the current storage conditions, irrespective of the facies. For the 226Ra, the bearing phases differ according to the facies. Within the sandy silt facies, the 226Ra is mainly borne by clay minerals and can be mobilised more easily. However, the sandy silt facies represents only one third of the tailings currently.

中文翻译:

铀厂尾矿的地球化学特征(法国的Bois Noirs Limouzat)突出了U和226Ra的保留。

与其他金属一样,对前铀矿的尾矿的管理也需要对控制感兴趣的放射性元素(U和226Ra)保留的地球化学机制有充分的了解。本文介绍了Bois Noirs Limouzat尾矿存储设施(卢瓦尔河)中这两种放射性元素的方位相研究结果,这是法国目前唯一存储尾矿(砂质粉砂岩相和粘土质粉砂岩相)的场所。只在水下。目的是更好地了解它们在当前存储条件下的移动性。为此,采用了多尺度方法,将历史研究和机载图像分析相结合,以选择核心位置,化学和放射学分析,矿物学特征,辅以顺序提取(两种专门开发的方案)。结果表明,U和226Ra主要存在于黏土粉砂岩相中,平均U浓度为243.3 ppm(沙质粉砂岩相中为132.3 ppm),平均226Ra质量活度为64.7Bq / g(在砂土中为18.0Bq / g)。沙质粉砂相)这些结果与矿石的初始U品位(2‰),矿石加工厂的提取效率(95%)和成矿年龄(305 Ma)相符。所采用的方法使得有可能突出显示每种放射性元素都有几个矿物学陷阱,而与相类型无关。因此,U的很大一部分仍被困在初级相中,对治疗有抵抗力,因此在当前的储存条件下相对固定(49.6%-77。砂质粉砂岩相为8%,粘土质粉砂岩相为27.2%-36%。大部分浸出的铀主要与弱结晶的羟基氧化铁有关(沙质粉砂岩相为8.7%-42.4%,黏土粉砂岩相为50.9%-71​​.8%),而与粘土矿物质的含量较小(5%-12.3%)含沙质粉砂岩相为0.8%-11.5%)。对于226Ra,无论相类型如何,大部分都保留在磷酸盐相中,对浸出过程具有抵抗力,因此在储存条件下也相对不动(粉质砂质粉砂相为24.4%-38.9%,39.9%-98.9%黏土淤泥相)。顺序提取显示出不同的226Ra地球化学,取决于相。对于粉质砂质粉砂相,226Ra的大部分主要与粘土矿物有关(6.4%-69。2%),并在较小程度上使用氢氧化铁,重晶石或磷酸铝硫酸盐矿物(APS)(6.4%-33.9%)。对于粘土质粉砂岩相,大部分226Ra主要与羟基氧化铁,重晶石或APS(6.4%-53.3%)有关,在较小程度上与粘土矿物(0.4%-6.8%)有关。浸出过程不允许区分这些阶段中每个阶段对226Ra保留的贡献。最后,所有确定的方位相表明,U在当前存储条件下相对固定,而与相无关。对于226Ra,轴承相根据相的不同而不同。在沙质粉砂相中,226Ra主要由黏土矿物携带,并且更容易动员。但是,砂质粉砂岩相目前仅占尾矿的三分之一。
更新日期:2020-04-21
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