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 estimation of indium in different types of materials.
In the present study, the estimation of indium in Nb-Ta bearing minerals were carried out by acid hydrolysis followed by solvent extraction separation. The indium present in Nb-Ta bearing samples were quantitatively separated from Nb and Ta by acid hydrolysis by prior fusion of the samples with sodium peroxide, dissolution in HCl followed by ammonia precipitation, dissolution of precipitate in HCl 10% (v/v) to carry out the acid hydrolysis in presence of added sulphurous acid (18). The hydrolysed Nb and Ta are filtered off and the filtrate in 6M HCl is extracted with TBP. The extracted Indium is stripped back with water and indium is estimated using ICP-AES. The method has been applied to some synthetic standards as well as a few international reference standards like IGS-33 (Tantalite from International Geological Sciences, U.K.) and ASK-3 (Sulphide ore, Analytisu Spoelement, Kanite, Norway) including 10 niobate – tantalite samples. The procedure show excellent agreement with the certified/usable values and where (standard reference material) values are not available, the obtained values are prescribed as the usable values.
A sequential JOBIN YVON (France) model Jobin Yvon Horiba JY 2000(2) spectrometer was used for all ICP-AES measurements. Optimum Instrumental parameters and operating conditions are given in Table-1. Reagents
The standard indium stock solution (1mg/ml) is prepared by dissolving 0.25gm of indium foil in HCl and making up the solution to 250 ml in 10% (v/v) HCl. The working standards for the calibration were single element solutions in 10% (v/v) HCl prepared by dilution of stock solution. Acids and all other chemicals used are of analytical grade reagent. Acid hydrolysis (Nb/Ta – bearing samples)
A 0.5g sample (150-200mesh) was fused with 10g Na2O2 in a nickel crucible and cooled melt was dissolved in dilute 5% (v/v) HCl and boiled for 15 minutes. Ammonia precipitation was carried out in presence of ammonium chloride; R2O3 precipitate was filtered through a whatman 540 filter paper. The precipitate was washed with 5% (v/v) NH4OH in 1% (w/v) NH4Cl and transferred to the same beaker with a fine jet of 10% (v/v) HCl. The solution was boiled gently to complete the hydrolysis of Nb and Ta. Few drops of sulfurous acid, a quarter of a Whatman ashless tablet were added into the solution for making it easily filterable which was cooled and filtered through the same filter paper with thorough washing with 10% (v/v) HCl. The filtrate was evaporated and the residue is dissolved in 6M HCl and made up to 100ml volume.
However, the reference standards like ASK-3 can be dissolved directly by treating with acid mixture of HF, H2SO4 and HCl and with filnal dissolution in 100ml of 6M HCl, prior to the extraction of Indium using TBP. Solvent Extraction
The sample solution in 6M HCl was extracted with 10 ml tri-n- butyl phosphate ( twice), a contact time of 5 minutes, using a separating funnel.; the organic extract containing indium was back extracted with 20 ml of water (thrice); the back extracted aqueous portions were combined and evaporated to dryness. The dried residue was dissolved in 5% (v/v) HNO3 and made up to 10 ml and aspirated to plasma for estimating In concentration values at the most sensitive Emission line of Indium 230.606 nm, after prior calibration with the standards.
RESULTS AND DISCUSSION
Selection of emission line
The five most sensitive emission lines of indium as listed in the Atlas of spectral lines(19), were scanned thoroughly and the most sensitive ionic emission line at 230.606 nm was selected for estimation of the element after separation from matrix elements. The line gives a detection limit of 0.9 ng/ml with a net line to back ground intensity 306 for In , 10 μg/ml. The detection limits and the background equivalent concentrations studied for the five emission lines are presented in Table-2 and the spectral interference studied are given in Table-3. The only interference at the line 230.606 nm are due to the direct overlap of Th which gives interfrent equivalent concentration (IEC) of 3% and that of Dy and Mn 0.5%. However, these three elements are eliminated during the solvent extraction stage of the procedure. Solvent extraction of Indium
The optimum acid concentration for the extraction of indium using TBP was selected by extracting 100 μg of indium at different concentrations of(1M –10 M)HCl. From Table-4 it is observed that indium was quantitatively extracted into TBP from 3M –10M HCl. Hence 6M HCl is selected as optimum acid concentration for the sake of convenient operation. Ratio of the volume of aqueous layer to the organic layer was optimised as 10:1.Indium was quantitatively stripped back from TBP with 20ml water (thrice).
Determination of Indium in Nb/Ta – bearing samples
Three synthetic Nb/Ta samples prepared by doping with known amount of Indium (100 ug) were sutdied and the results obtained are given in Table – 5. The percent recovery of Indium varies from 95.0% to 98.5%. Quantitative recoveries were obtained in all the three synthetic standards, indicating no hold up of indium alongwith the precipitated niobic or tantalic acids during acid hydrolysis stage. Similarly, the method has been applied to 10 niobate/tantalate samples and two International Reference Standards, IGS – 33 and ASK – 3 and the values are presented in Table – 6. In the case of ASK – 3, the obtained value by the proposed method agrees well with the certified value whereas in case of IGS – 33, a value of 170 ug/g is proposed as the usable values as there is no certified value for this standard. The % RSD varies from 2.2% to 7.0% in the case of all the studied samples. Further studies are under progress for application Cu – Zn metallurgical samples.
The acid hydrolysis sepration (for Nb/Ta – bearing samples) followed by solvent extraction – stripping method described is rapid, precise, accurate, and highly selective for the extraction of Indium in different types of geological samples including Nb/Ta – bearing samples. The method offers a determination limit of 1 ug/g and above in a variety of samples. The proposed acid hydrolysis method for the separation and determination of Indium in Nb/Ta bearing samples is simple and offers better results in view of the hydrolyzable nature of Nb and Ta.
The authors thank Dr. D.S.R. Murty, Head, Chemistry Group, AMD, Hyderabad and Sri A.K. Rai, regional Director, SR, AMD, Bangalore for their constant encouragement in the R & D acitivities during the course of the work. Further, the authors offer their sincere thanks and gratitude to Dr. Anjan Chaki, Director, AMD, for his kind permission to publish these findings.
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