砂岩型铀矿床硫化物还原富集铀的机制

Mechanism of U-reduction and concentration by sulphides at sandstone type uranium deposits

在层间氧化带砂岩型铀矿床中,铀在氧化还原前锋线附近的富集通常被解释为地下水中的U6+被还原为U4+形成铀矿物并沉淀,铀的还原剂则是保存在砂岩中的炭化植物碎屑和硫化物,特别是黄铁矿。作者通过大量的相关文献的调研,结合国内外在砂岩型铀矿研究方面取得的成果,对硫化物,特别是黄铁矿在砂岩型铀矿氧化还原前锋线附近铀还原、沉淀和富集过程中所起的作用提出了自己的认识。氧化还原前锋线富矿区的试验数据和地质观察显示只有在介质中缺少自由氧时黄铁矿才能够作为U6+的良好的还原剂,其还原机制是黄铁矿和水反应生成的H2S气体还原了铀。然而,磁黄铁矿是比黄铁矿更好的铀的还原剂,因为磁黄铁矿和水反应能产生氢气(H2),一种更有效的U6+还原剂。平常观察到的铀矿物(沥青铀矿和铀石等)围绕新生的黄铁矿颗粒沉淀的现象在很多情况下反映的是结晶顺序,实际上是Eh值降低时或酸化时黄铁矿早于铀矿物发生沉淀的结果。

The concentration of uranium at the redox front of sandstone type U-deposits are commonly interpreted as a result of the reduction of U^6＋ in ground water to U^4＋ to form uranium minerals and their precipitation, and reductants of uranium are considered to be carbonized plant debris remained in sandstone and sulphides, of them, particularly the pyrite. Having reviewed a large number of relevant literature in detail, combined with some research achievements at sandstone type uranium deposits in China and abroad, The authors try to make an overview of the role of sulphides, particularly the pyrite, in the reduction, precipitation and concentration of uranium at the redox front of sandstone type uranium deposits. Experimental data and geologic observation of the ore-concentrated zone at the redox front indicate that pyrite could be a good reductant of U^6＋ only in case when the medium is lacking of free oxygen, and the reduction mechanism of pyrite lies in the formation of H2S as a result of the reaction between FeS2 and H2O. However, the FeS is a stronger reductant as the reaction between FeS and H2O could produce H2 which is more effective reductant of U^6＋. The commonly observed phenomenon-uranium minerals （UO2 and USiO4） which precipitate around newly-formed pyrite grains in many cases shows a precipitation sequence of the Eh value or pH value dropping, and the pyrite is always formed earlier than uranium minerals.