Acta Chim. Slov. 2003, 50, 335-342. 335 DIRECT ANALYSIS OF TUNGSTEN IN CRUDE NIOBIUM PENTOXIDE USING MICROWAVE DIGESTION AND ICP-OES Gopala Anil, Sivasankaran N. R. Kumar, Arbind Kumar, Manda R. P. Reddy,* and Tirthalli L. Prakash Centre for Materials for Electronics Technology (C-MET), IDA, Phase-II, HCL Post, Cherlapally, Hyderabad' — 500 051, INDIA, Email: mrpreddy@yahoo.com. Received 09-08-2002 Abstract Niobium Pentoxide (Nb205) is prepared from feed raffinate solution of tantalum solvent extraction. The major impurities, which are associated with niobium pentoxide, are tantalum, tungsten, titanium and iron. While analyzing tungsten by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) niobium interference free lines has to be selected first and this line should also be free from the interference of tantalum, titanium and iron. The interference free line thus selected (224.875nm) has been analyzed for its accuracy and preciseness by analyzing 1 mg/L tungsten doped into 1000 mg/L Nb, 100 mg/L to 500 mg/L Ta, Ti and Fe the results are presented along with a pictorial depiction of spectral interferences by Nb, Ta, Ti and Fe for interference free line selection of tungsten. Nb205 being ceramic in nature dissolving in open digestion is very difficult and tirne consuming, here a simple and rapid method of dissolution by microwave digestion (MDS) is adopted. Introduction Niobium and tantalum occur together in the nature in the form of mixed oxides. In India, columbite-tantalite mineral occurs in pegmatite found in mica belt of Bihar, Rajastan and Madhya Pradesh. A good stockpile of this ore is accumulated mainly as a by-product of the mica mining operations in Bihar. " The solvent extraction process carries out extraction and separation of niobium and tantalum from the ore. Niobium and tantalum are extracted from a solution of HF using an organic solvent namely tributyl phosphate (TBP). Tantalum is extracted from a solution of 0.5N HF and 2N H2SO4 containing tantalum, niobium, iron, tungsten, titanium and other minor constituent elements. Niobium is further extracted from the raffinate solution of tantalum extraction at a concentration of 5N HF and 9N H2SO4. Niobium in the aqueous solution is precipitated as niobium hydroxide, which is then calcined to get NbaOs. Attempts have been made to prepare niobium electrolvtic capacitors by the alkaline earth metal reduction of NbiOs.3"4 One of the prerequisites for niobium capacitors is the purity of niobium powder, which is prepared after reduction of M^Os. Hence total metallic impurities put together in NbiOs should be less than 1000 ppm and each elements Fe, Cr, G. Anil, S. N. R. Kumar, A. Kumar, M. R. P. Reddy, and T. L. Prakash: Direct Analysis of Tungsten... 336 Acta Chim. Slov. 2003, 50, 335-342. Ni, K, Na and W should be less than 100 ppm. If these impurities are in excess the dielectric film quality deteriorates and DC leakage current increases. Indian tantalite ores contains appreciable amounts of tungsten. During the solvent extraction of niobium feed, a part of tungsten also gets extracted. Therefore, the final M^Os contains appreciable amount of tungsten as impurity. Analysis of tungsten in the M^Os resulted in erroneous results because of interferences from matrix element and associated impurities like tantalum, titanium and iron. Tungsten analysis in M^Os is carried mostly on lm-monochromator ICP-OES,5 direct solid sampling electrothermal atomic absorption spectrometer6 and also some classical techniques involving separation of matrix 7 1 1 element. " A systematic study of line profiling was not reported for the analysis of tungsten in niobium matrix using 0.64m monochromator ICP-OES. Due to the complexity of interference spectra's (Nb, Ta, Fe and Ti) an attempt has been made to identify interference free line to analyze accurately the content of tungsten in presence of tantalum, titanium and iron in M^Os with the help of microwave digestion system and ICP-OES. Microwave digestion methods have become a popular technique and are more reproducible, accurate and less tirne consuming, than conventional digestions on hot plates in open beakers. Microwave digestion keeps blank values low because small volumes of reagent required and allows more samples to be processed per hour than conventional digestion system. Experimental Instrumentation Sample digestion was performed using a microwave digestion system (MDS) Model MLS 1200, Milestone, Italy. The Instrument used for the analysis of tungsten in niobium pentoxide was model JY-24R inductively coupled plasma optical emission spectrometer (ICP-OES), Jobin Yvon, France. A cross flow type of nebulizer (Teflon®) was used for nebulization in the ICP-OES instrument. The instrumental parameters of the ICP-OES and the microwave digestion system with dissolution parameters are given in Table 1 and 2. Reagents Ali chemical used were of suprapure reagent grade from Merck, Germany. The single elemental standard solutions were procured from Merck, Germany. The Nb20.• HBF4 + H20) and 2 mL of hydrogen peroxide in a Teflon vessel. Eight different sequential steps (Table 2) of microwave heating with intermittent power were applied to control the reaction of the G. Anil, S. N. R. Kumar, A. Kumar, M. R. P. Reddy, and T. L. Prakash: Direct Analysis ofTungsten... 338 Acta Chim. Slov. 2003, 50, 335-342. sample during dissolution. After completing the digestion, the solution was made upto 50 mL in a polypropylene standard flask from Gradplex, Azlon, England before aspirating into the plasma. Ali samples were prepared in five replicates. Results and Discussion Due to the complexity of interference spectra's from Nb, Ta, Ti, and Fe a careful line selection has to be conducted to avoid spectral interferences. A 0.64 m monochromator used here had difficulty in handling complex line rich spectrum of niobium, tantalum, titanium and iron. Spectral scans were performed for each wavelength with 1 mg/L single elemental solutions of tungsten and 1 mg/mL niobium solution. Seven sensitive lines13"14 of tungsten were selected (207.911, 209.475, 218.935, 220.448, 222.589, 224.875 and 239.709 nm). Out of seven lines only two lines (207.911 and 224.875 nm) showed no overlap/interference of niobium line. The spectral overlaps are shown in (Figure 1). The rest of the lines showed either direct overlap or wing overlap of niobium line. These two lines (207.911 nm and 224.875 nm) were checked for interference of tantalum, titanium and iron line by profiling with 100 mg/L single elemental standard solution. The line 207.911 nm had no titanium and iron interference but a small left wing overlap and a right big wing overlap of tantalum were found hence it is not suitable for analysis of tungsten. It was found that 224.875 nm line was free from titanium and tantalum line interference but iron had a left wing overlap only hence this line was used for analysis of trace tungsten. The spectral overlaps are shown in (Figure 2). In order to analyze tungsten, the line 224.875 nm was chosen, and it was found that upto 500 mg/L of iron with right background subtraction gave accurate results. To validate the ICP-OES methodology a standard addition of iron tantalum and titanium of different concentrations (100, 200, 300, 400 and 500 mg/L) added into 1000 mg/L niobium solution, this was doped with 1 mg/L tungsten. The results of tungsten analyzed were found to be well within the limit of accuracy, which is given in Table 3. A typical analysis of crude and purified Nb205 prepared at our laboratory is given in Table 4. NbiOs being ceramic in nature, it was very difficult to achieve complete dissolution in open hot plate condition. A simplified microwave digestion procedure with intermittent cooling was designed so that the pressure build up did not arise very fast. The dissolution was achieved in eight steps of G. Anil, S. N. R. Kumar, A. Kumar, M. R. P. Reddy, and T. L. Prakash: Direct Analysis of Tungsten... Acta Chim. Slov. 2003, 50, 335-342. 339 W - 207.911 nm No overlap W- 209.475 nm Direct overlap W-218.935 nm Direct overlap Nb W- 220.448 nm Direct overlap W-222.589 nm Direct overlap W - 224.875 nm No overlap w- 239.709 nm Direct overlap Figure 1. 1 mg/mL of niobium overlapped with 1 mg/L tungsten. W - 207.911 nm Right & Left wing overlap of Tantalum i—i—i—i—i—i—i—i—i—|—i—i—i—i—i—i—i—r W- 224.875 nm Left wing overlap of Iron overlap Figure 2. Spectral overlaps of tantalum and iron 100 mg/L with added 1 mg/L of tungsten in it. G. Anil, S. N. R. Kumar, A. Kumar, M. R. P. Reddy, and T. L. Prakash: Direct Analysis of Tungsten... 340 Acta Chim. Slov. 2003, 50, 335-342. cooling and microwave heating as detailed in Table 2. To validate the microwave digestion of NbaOs and the analysis of tungsten, 0.500 g of Nb20s (Lot -03861R), procured from Glen Spectra Reference materials, England was taken in triplicate. Sample 1 was used as blank, Sample 2 was doped with 500 mg/L of Ta, Ti and Fe and 0.5 mg/L of tungsten and Sample 3 was doped with 500 mg/L of Ta, Ti and Fe and 1.0 mg/L of tungsten and subjected to microwave dissolution as in Table 2. After dissolution the solutions was made upto 50 mL. The results of tungsten show a recovery in the range of 99 to 101% is given in Table 5. The detection limit of tungsten thus calculated as 3 s of the blank was found to be 0.031 mg/L for pure tungsten solution and 0.221 mg/L for tungsten in presence matrix (0.5 g/50 mL of NbaOs) and 0.234 mg/L in presence of 500 mg/L of Ta, Ti, Fe and matrix. Table 3. Standard addition and recovery of tungsten. S. No. Standard Addition Recovery of tungsten (224.875 mn) in mg/L (SD) 1. 1 mg/L Tungsten + Niobium lOOOmg/L +100 mg/L 1.02±0.008 Tantalum, Titanium and Iron 2. 1 mg/L Tungsten + Niobium lOOOmg/L +200 mg/L 1.01+0.007 Tantalum, Titanium and Iron 3. 1 mg/L Tungsten + Niobium lOOOmg/L +300 mg/L 1.03+0.011 Tantalum, Titanium and Iron 4. 1 mg/L Tungsten + Niobium lOOOmg/L +400 mg/L 1.02+0.009 Tantalum, Titanium and Iron 5. 1 mg/L Tungsten + Niobium lOOOmg/L +500 mg/L 1.02+0.008 Tantalum, Titanium and Iron SD: Standard Deviation of five readings. Table 4. Typical analysis of Nb205 prepared at C-MET, Hyderabad. S. No. ELEMENTS Crude Nb205 (in %) Purified Nb205 (in %) (SD) (SD) 1. Tantalum 2.240+0.121 0.070+0.007 2. Iron 1.535+0.078 0.005+0.0003 3. Titanium 0.585+0.015 0.002+0.0001 4. Tungsten 0.621+0.035 0.071+0.008 SD: Standard Deviation of five readings. G. Anil, S. N. R. Kumar, A. Kumar, M. R. P. Reddy, and T. L. Prakash: Direct Analysis of Tungsten... Acta Chim. Slov. 2003, 50, 335-342. 341 Table 5. Microwave digestion of doped Nb205 and recovery. S.No. Doped Samples ofNb205 Recovery%of Tungsten (SD) 1 Nb205 doped with 0.5mg/L 98.86±0.32 of W and 500mg/L of Ta, Fe and Ti 2. Nb205 doped with l.Omg/L 101.34±0.85 of W and 500mg/L of Ta, Fe and Ti (S.D): Standard Deviation of five readings. Conclusion Generally direct analysis of tungsten in M^Os was difficult without matrix/interferent separation using a 0.64 m monochromator with ICP-OES. The present method developed with a systematic line profiling can be applied successfully in analyzing accurately the content of tungsten in presence of tantalum, titanium and iron in Nb205. The complete dissolution of NbiOs was achieved very easily by microwave digestion procedure. The method is rapid accurate and could be successfully applied for the analysis of Nb20s. Acknowledgements Authors would like to thank the department of information technology (DIT), Govt. of India, New Delhi, for sponsoring tantalum technology mission Project. Authors are also thankful to Dr. B. K. Das, Executive Director, C-MET, and Dr. K. S. K. Sai, Senior Director, and Dr.V.C.Sethi, Director, DIT for their encouragement. References and Notes 1. K. Satyanarayana and M. A. Nayeem, Atomic Spectroscopy 1993,14, 180-186. 2. C. K. Pillai, S. Natrajan, and Ch.Venketeshwarulu, Atomic Spectroscopy 1985, 6, 53-56. 3. V. G. Maiorov, A. I .Nikolaev, and V. K. Kopkov, Russ. J. Appl. Chem 1994, 67, 296-298. 4. V. G. Maiorov, A. I. Nikolaev, and V. K. Kopkov, Russ. J. Appl. Chem 2001, 74, 363-367. 5. R. A. Conte, J.-M. Mermet, J. De Anchieta Rodrigues and J.- L. Di Martino, J. Anal. At. Spectrom. 1997, 12, 1215-1220. 6. M. D. Huang, V. Krivan, Fresenius J. Anal. Chem 2000, 368, 227-234. 7. J. J. Furey and T. R. Cunningham, Anal. Chem. 1948, 20, 563-570. 8. W. T. Ehvell and D. F. Wood, Anal. Chim. Acta 1962, 26, 1-31. 9. R. H. Atkinson, J. Stiegman, and C. F. Hisley, Anal. Chem. 1952, 24, 477-480. 10. V. Kanicky, V. Otruba, and J.-M Mermet, Fresenius' J. Anal. Chem. 2001, 371, 934-938. 11. F. Fairbrother, The Chemstry ofNiobium and Tantalum, 1967, Elsevier publications, New York. 12. S. Mann, D. Geilenberg, J. A. C. Broekaert, and M. Jansen, J. Anal. At. Spectrom. 1997, 12, 975-979. G. Anil, S. N. R. Kumar, A. Kumar, M. R. P. Reddy, and T. L. Prakash: Direct Analysis of Tungsten... 342 Acta Chim. Slov. 2003, 50, 335-342. 13. A. Montaser and D. W. Golightly, Inductively CoupledPlasmas in Analytical Spectrometry, 2nd ed., 1992, VCH publishers Inc. 14. R. Payling and P. Larkins, Optical Emission Lines ofthe elements, 2000, John Willey and Sons Ltd. New York. Povzetek Niobijev pentoksid (Nb^Os), dobljen iz rafinata pri ekstrakciji tantala, vsebuje nečistoče kot so tantal, volfram, titan in železo. Zato moramo pri analizi volframa z ICP-OES izbrati takšno spektralno črto, kjer so spektralne interference elementov, ki so prisotni v analizirani raztopini, zanemarljive. Ugotovili smo, daje takšna spektralna črta pri 224.875 nm. Pri tej valovni dolžini smo tudi določili točnost ter natančnost postopka z analizo volframa, kije bil prisoten v koncentraciji 1000 mg/L v raztopini, ki je vsebovala še interferenčne elemente Nb, Ta, Ti in Fe v koncentracijskem območju od 100 do 500 mg/L. Ker se Nb2Os počasi in težko raztaplja s kislinami v odprtem sistemu, smo razvili postopek za njegovo raztapljanje v mikrovalovni peči. G. Anil, S. N. R. Kumar, A. Kumar, M. R. P. Reddy, and T. L. Prakash: Direct Analysis ofTungsten...