A simple, precise and accurate solvent extraction method is described for the separation and estimation of Indium in geological materials. Solvent extraction of Indium from 6 M HCl medium using tri-n-butyl phosphate, selectively separates Indium from accompanying elements in different type of geological samples. Acid hydrolysis of Nb/Ta samples separates Indium from major matrix elements like Nb and Ta and the remaining elements do not influence the selective extraction and preconcentration of In and its subsequent determination by ICP-AES or flame AAS. The silica rich geological samples are decomposed by HF-H2SO4-HCl treatment followed by dissolution in 6M HCl before applying solvent extraction procedure. In Nb/Ta type of samples, Indium was separated from Nb and Ta by acid hydrolysis, involving fusion with Na2O2, dissolution in HCl followed by NH4OH precipitation and hydrolysis in HCl. The oxychloride precipitates of Nb and Ta are filtered off and subjected to solvent extraction using TBP. The proposed method has been applied to some international reference standards (IGS-33 and ASK-3) and to some Nb/Ta type samples and the results are compared by ICP-AES as well flame AAS techniques. The method is simple, rapid and accurate showing a relative standard deviation of 2% (at 170 μg/g) to 7.0% (at 16 μg/g ) and the method can be applied down to 1 μg/g and above. ---------------------------------------------------------------------------------------------------------------------
Key words : Indium; Geological materials; Niobate-Tantalate; acid hydrolysis; solvent extraction; ICP-AES.
Indium is a highly dispersed element in nature with a crustal abundance of 0.1 μg/g(1). Though indium is present in minerals like Jallindite In(OH)3, Indite FeIn2s4, Roquesite CuInS2, Sakurayite (CuZnFe)3InS4, (2,3) their occurances are very rare and they are not important as a source of indium. Due to the similarity in the ionic radii of In3+(0.81A0) with Fe2++(0.74A0), it mainly occurs in igneous iron bearing minerals. Because of its chalcophilic nature indium is present in sulphide minerals of Zn, Cu, Sn and Pb(1) It is highly expensive due to its scarcity. Indium, as an alloy of Ag, Cd and In is used as control rods in pressurised water reactors and as an alloy of In, Ge, Ga, Zn, Cd and Pb is used in fire alarm systems and as an alloy with other metals it is used in dentistry, Jewellery and as catalysts in hydrogenation reactions. Indium phosphides, arsenides and antimonides are used as semiconductors in electronic industry(4). Indium oxide is used in liquid crystal displays (5). Because of its increasing demands it is mainly recovered from the dust arising out of pyro metallurgical treatment of Zn, Pb and Sn sulphide ores and as well from raffinates of Nb-Ta metallurgical extractions(6). Determination of indium in Nb/Ta bearing minerals is complicated due to the spectral and matrix interferences. Therefore, separation of Indium is required from the matrix elements prior to its quantification. The methods which have been reported for the separation and determination of indium in different type of samples are solid phase adsorption on chromsorb108 resin (7), adsorption of indium on nanometer TiO2 (8) and extraction of indium to supercritical carbon dioxide from acidic solution (9). Several methods have been reported (10-15) for the determination of indium in various types of samples including soil and geological materials. Determination of In as an impurity in Niobium Carbide, by ICP-AES has also been reported in recent times (17). Solvent extraction separation (10-12) coupled to graphite furnace - atomic absorption spectrometry (GF-AAS) (7,14), or neutron activation analysis (NAA) (13,16) or inductively coupled plasma atomic emission spectrometry (ICP – AES) (17) or inductively coupled plasma-mass spectrometry, (ICP-MS) (15), have been applied for separation and...