提取风化壳淋积型稀土矿中稀土元素的前处理方法探讨

Discussion on Pretreatment Method for Extracting Rare Earth Elements from Weathered Crust Elution-deposited Rare Earth Ores

传统观点认为,风化壳淋积型稀土矿床(即离子吸附型稀土矿床)中,稀土主要以吸附态赋存于风化壳黏土矿物表面,其他赋存形式占比较少。但近年来的同步辐射研究显示,稀土元素也同时以内层络合物形式存在,而内层络合物的存在可能会抑制稀土元素的离子交换。如何将风化壳淋积型稀土矿中各种形态的稀土元素有效地溶出,对于提高稀土资源利用率十分重要。本文选取南岭地区风化壳淋积型稀土矿石样品,选择混合酸(五酸)消解、盐酸消解、硝酸消解、硫酸铵浸提的前处理方式,采用电感耦合等离子体质谱(ICP-MS)测定,探讨样品中稀土元素的溶出情况。结果表明,采用混合酸(五酸)能够溶出样品中的全相稀土;盐酸或硝酸能够溶出以离子状态吸附于黏土矿物或铁锰氧化物中的稀土元素,以及以碳酸盐、磷酸盐等形式存在的稀土元素;硫酸铵浸提则只能将离子相的稀土元素置换出来。采用硝酸、盐酸溶出的稀土量,占混合酸(五酸)消解溶出稀土量(全相稀土)的71.7%~97.5%,硫酸铵浸提溶出的稀土量(离子相稀土)最低,仅是全相稀土量的9.1%~75.5%。Sc^(3+)的离子半径明显小于其他稀土元素,其提取率也明显低于其他稀土元素;铈(Ce)不同于其他稀土元素的分异-富集特性,使其在硫酸铵浸提中提取率与其他稀土元素不一致。

BACKGROUND:Weathered crust elution-deposited rare earth ores are the dominant mineral resources in China[1-5].The occurrence of rare earth elements(REEs)in weathering crust elution-deposited is very complicated.At present,the“ionic”rare earth extraction process can only use the“exchangeable adsorption state”rare earth elements,that is,the“ionic phase”rare earth elements,and other phase rare earth elements cannot be effectively recycled[6].How to effectively dissolve various forms of rare earth elements in ion-adsorbed rare earth ore is very important to improve the utilization rate of rare earth resources.OBJECTIVES:To compare the extraction efficiency of REEs in weathered crust elution-deposited rare earth ores by different pretreatment methods and discuss the influencing factors.METHODS:Five pretreatment methods,being open mixed acid digestion(HCl+HNO3+HF+HClO4+H2SO4),nitric acid digestion with or without hydrogen peroxide,hydrochloric acid digestion with hydrogen peroxide and ammonium sulfate leaching,were used to extract REEs in six rare earth samples from the Nanling area.The content of REEs in samples was determined by ICP-MS.For ion-adsorbed rare earth minerals,the open digestion method of five acids can replace the complicated alkali melting method for the determination of REEs in samples under certain conditions.Nitric acid,hydrochloric acid digestion or ammonium sulfate leaching can only dissolve part of REEs in ion-adsorbed rare earth samples.RESULTS:The amount of REE extracted by different pretreatment methods is quite different.The results of mixed acid digestion(five acids)are the highest;the results of nitric acid with or without hydrogen peroxide digestion,hydrochloric acid and hydrogen peroxide digestion are similar.The amount of REEs dissolved by nitric acid or hydrochloric acid digestion is slightly lower than mixed acid digestion.The amount of REEs dissolved by ammonium sulfate leaching is the lowest.The amount of REEs dissolved by 50%nitric acid,hydrochloric acid and other digestion methods accounts for 75.3%-91.7%of the total phase REEs,and the amount of REEs dissolved by ammonium sulfate leaching(ionic phase rare earth)only accounts for 9.1%-5.5%of the total phase REEs.The extraction rate of scandium(Sc)is much lower than that of total REEs in nitric acid and hydrochloric acid digestion,and Sc can not be dissolved by ammonium sulfate leaching.There is no correlation between the extraction rate of cerium(Ce)and other REE,the total extraction rate of light REE or the total extraction rate of REEs.The difference of the results by different pretreatments is closely related to the occurrence state of REEs in the samples.Mixed acid digestion can completely destroy the structure of the sample,and all the REEs in the sample can be dissolved.The REEs are differentiated and enriched in weathering crust layers.Some REEs still exist in the form of mineral facies after weathering.The content of REEs in ionic phase is closely related to weathering degree and mineral composition of each layer of weathering crust.Proportion of REEs content in different parts of weathering crust is not the same.Hydrochloric acid or nitric acid can dissolve REEs in the ionic state adsorbed in clay minerals or iron and manganese oxides,as well as REEs in the form of carbonates,phosphates,etc.Some REEs exist stably in silicate mineral lattices that cannot be completely dissolved by nitric or hydrochloric acid.Ammonium sulfate leaching can only dissolve the ionic REEs in the sample,so the digestion result of hydrochloric acid and nitric acid is lower than that of the whole phase REEs,and higher than that of ammonium sulfate leaching.CONCLUTIONS:The differences and influencing factors of REEs extracted from ion-adsorbed rare earth samples by different pretreatment methods are discussed,which can provide reference for further research on the extraction methods of REEs from elution-deposited rare earth ores.Mixed acid digestion can extract the full phase REEs in the weathered crust eluviated rare earth ore sample,which can be used to evaluate the total amount of REEs in the weathered crust eluviated rare earth ore.Nitric acid digestion,nitric acid and hydrogen peroxide digestion,hydrochloric acid and hydrogen peroxide digestion cannot dissolve REEs in the silicate structure completely.Therefore,this method is suitable for evaluating the content of REEs in the form of ionic states,oxides,carbonates and phosphates in samples.Ammonium sulfate leaching can be used to evaluate the content of ionic phase REEs in weathering crust leaching type rare earth ores.The extraction rate of REEs is greatly affected by the chemical characteristics and occurrence state of REEs.Because the ionic radius of Sc3+is obviously smaller than that of other REEs,Sc3+can enter the crystal lattices of many rock-forming minerals in the form of homogeneity,which results in that ammonium sulfate leaching cannot dissolve Sc.Only a small amount of Sc can be dissolved by hydrochloric or nitric acid digestion.The different enrichment-differentiation characteristics of Ce and other REEs also make the extraction rate of Ce by ammonium sulfate leaching inconsistent with other REEs.The REEs with similar ionic radii often have similar extraction efficiency in the same pretreatment.