Artalyst, February, 1968, Vol. 93, $9. 93-96 93 A Gravimetric Method for the Determination of Mixed Oxides (Niobium and Tantalum Pentoxides) in Niobium - Tantalum Minerals BY J. B. POLLOCK (Geological Survey and Mines Department, P.O. Box 9, Entebbe, Uganda) A method is described in which the sample is dissolved by repeated evaporation with hydrofluoric acid, and the final solution then considerably diluted and filtered. The insoluble residue is reserved for determining tin, uranium and thorium, if required, and the filtrate made up to a fixed volume. A suitable aliquot is evaporated to fumes with sulphuric acid, diluted and the combined oxides precipitated with tannin. A second aliquot may be used, after evaporation to fumes with sulphuric acid, for determining niobium pentoxide by Pickup’s method.HYDROFLUORIC acid was first proposed as a solvent for niobium and tantalum in the analysis of samarskite by Smith1 nearly 90 years ago. His procedure was reviewed in 1888 by Hille- brand,a some of whose observations were incorrect in that he declared iron and manganese to be insoluble in hydrofluoric acid, and again, in 1924, by Wells.3 More recently, a mixture of hydrofluoric and sulphuric acids has been applied to the analysis of niobium-bearing loparites by Cherniltov and Uspenskaya? and Chernikov and Goryu~hina.~ EXPERIMENTAL The method described here can be used in the analysis of columbite - tantalite, microlite and samarskite, whether alone or in mixed concentrates with wolfram and .cassiterite, and is a useful adjunct to the determination of uranium and thorium.Although not original, it offers the following advantages over methods that begin with the more customary potassium (or sodium) hydrogen sulphate fusion. (i) Silica need not be filtered off as it is volatilised by the hydrofluoric acid in the sub- sequent stages of the analysis.8 (ii) Cassiterite is not attacked but remains with the insoluble residue. This is important in the analysis of mixed columbite - cassiterite concentrates. The tin reported by Wells to be present in his hydrofluoric acid solution was derived from the samarskite itself. (iii) Thorium, bismuth (from bismuthotantalite), calcium (from microlite) and rare earths are all, likewise, conveniently separated. Wells’s analysis for his samarskite sample indicated that while the quadrivalent uranium was insoluble in hydrofluoric acid, the sexavalent uranium passed into solution, but experience with radioactive niobium - tantalum minerals in Uganda has shown, from whatever cause, that any uranium present remains entirely in the residue.INTERFERING ELEMENTS TITANIUM INTERFERENCE- According to Schoeller’s work,’ niobium and tantalum can be separated quantitatively from titanium by precipitation with tannin in dilute sulphuric acid (4 + 96), subject to a slight negative error. By inference, therefore, titanium will not interfere in the determination of niobium and tantalum pentoxides by the present method, but, because of its importance, the point was checked experimentally. It was found from the first that precipitation in 3 per cent.sulphuric acid (3 + 97) gave a slightly better recovery of niobium and tantalum pent- oxides than in the 4 per cent. acid recommended by Schoeller, and the former concentration was, therefore, used in the following investigation. Weighed mixtures of niobium and tantalum pentoxides (about 0-2 g for each experiment to simulate the maximum weight to be precipitated when assaying a mineral concentrate) were fused with 3-6 g of sodium hydrogen sulphate in 20-ml Vitreosil crucibles for 2 hours. 0 SAC and the author.94 [Autalyst, vol. 93 When cold, each melt was ejected by gently dropping the crucible, mouth downwards, on to an iron plate. The crucible and lid were placed in a 260-ml beaker containing 40 in1 of water and 15 ml of sulphuric acid (1 + 1).After warming the beaker for 20 minutes, they were removed, washed and well scrubbed with a rubber-tipped rod, and the melt was placed in the solution. At this stage any titanium required was added by transferring into the solution, with a pipette, the necessary volume of a standard solution of 0.25g of titanium dioxide plus 25 ml of sulphuric acid (1 + 1) per 100 ml, the sulphuric acid used for dissolving the nielt being adjusted beforehand to keep the total acid concentration constant. The solution was then evaporated to fumes under an infrared heat lamp. This gave a clear solution with the high niobium mixtures, but to dissolve the high tantalum mixtures com- pletely it was necessary to digest them on the hot-plate for up to 3 hours after the fuming stage had been reached.When cold, the solution was diluted, transferred into an 800-ml beaker, diluted to 250 ml, with addition of paper pulp, and boiled. Four grams of tannin dissolved in 40 nil of hot water were added and the procedure completed, as described below. The results obtained (Table I) show that there is a slight negative error but, contrary to Schoeller’s statement, it is greater when the amount of tantalum exceeds that of niobium. Secondly, part of the titanium is co-precipitated with the mixed oxides, about one fifth or less, varying according to the ratio of niobium to tantalum, and about one tenth in the two experiments in which precipitation was carried out in 4 per cent. sulphuric acid. In the fourth column of Table I, 0.0125 g of titanium dioxide corresponds to 5 per cent.in a sample, 0.0250 g to 10 per cent. and 0.0500 g to 20 per cent. The usual titanium content of a columbite - tantalite or microlite concentrate produced in Uganda does not exceed 1 per cent., and is generally less; 5 per cent. might be found in dirty low grade material, but for practical purposes, titanium interference can be disregarded. TABLE I POLLOCK: A GRAVIMETRIC METHOD FOR THE DETERMINATION RECOVERY OF NIOBIU?tl p l z l S TANTALUM AND TITANIUM 7 Niobium pentoxide 0-1734 0.0290 0.1753 0.0297 A 0.1744 0.1751 0.0266 0.1752 0.0260 0-174G 0-0249 B 0.1751 C 0-0259 Taken, g Tantalum pentoxide 0.0250 0.1760 0.0250 0.1760 0.0265 0-0254 0-1757 0.0267 0.1752 0.0216 0.1736 0.0266 0.1752 Total 0.1984 0.2050 0.2003 0.2057 0.2009 0.2005 0-2023 0.2019 0.20 12 0.1992 0.1985 0.201 7 0.201 1 -7 Titanium dioxide Nil Nil 0.0250 0.0250 Nil 0.0125 0.0125 0.0125 0.0 125 0.0125 0,0125 0.0500 0.0250 Found, g & Niobium and tantalum Titanium pentoxide dioxide 0.1970 Nil 0.2010 Nil 0.1965 0.0054 0.2012 0.0036 0-1978 Nil 0.1945 0.0021 0.1965 0.0015 0.1986 0.0035 0.1965 0.0023 0-1967 0.0031 0.1940 0.0022 0.1953 0-0058 0.1943 0-0022 Recovery, per cent.+---7 Apparent True N.A. 99.29 N.A. 98.05 100.79 98.1 1 99.56 98-12 N.A. 98.47 98.23 07-02 97-87 97.13 100*10 98.40 99.81 97-66 100-31 98.74 98-84 97.73 99-71 96.83 97-71 96.62 N.A. Not applicable. Experiment A included iron equivalent t o 0.05 g of iron(I1) oxide. Experiments B and C were carried out in 4 per cent. v/v sulphuric acid and are shown for comparison. MANGANESE INTERFERENCE- Manganese tends to be precipitated by fuming with sulphuric acid and remains insoluble after dilution.In one mineral deposit examined by the author, the columbite occurred closely associated with wad, and the manganese content of samples ranged up to 40 per cent. of manganese(I1) oxide. These samples were assayed by one of the author’s colleagues (Mr. Is. C. Patel) by the method described here, who found that the addition of 1 ml of ethanol to the hydrofluoric acid solution entirely prevented manganese precipitation, either on fuming with sulphuric acid or on subsequent dilution, and was equally effective in preventing manganese interference when niobium was determined by Pickup’s method8February, 19681 OF MIXED OXIDES (NIOBIUM AXD TANTALUM PENTOXIDES) 95 TUNGSTEN INTERFERENCE- The only significant interference is from wolfram and other tungsten minerals, as they are partly attacked by hydrofluoric acid, and the tungsten dissolved will be carried down in the tannin precipitate.However, it is usually necessary to determine tungsten trioxide (WO,) as well as niobium and tantalum pentoxides in a mixed concentrate, and the sample should first be assayed for tungsten by the aqua regia - ammonium tungstate method.' The niobium - tantalum minerals present will remain in the insoluble residue left after dissolving the tungsten trioxide in ammonia solution, and can be determined as described here. Alterna- tively, a method of dealing with tungsten interference is given below. METHOD PROCEDURE- Weigh 0.5000 g of sample into a 3-inch diameter platinum dish, add 10 ml of concentrated hydrofluoric acid solution and evaporate to dryness on a water-bath or carefully regulated sand-bath.Repeat the evaporation twice, with 15ml and then 20ml of acid, stirring occasionally with a polythene strip held in platinum-tipped tongs. Remove the dish from the water-bath, add 5 ml of hydrofluoric acid, stir and cover with a polythene circle. Leave for 2 or 3 hours, or overnight, as convenient. Remove and wash down the cover and stirrer, dilute the solution considerably and filter it through a 12.5-cm Whatman No. 541 filter-paper, in a polythene funnel, into a polythene beaker. Wash the platinum dish and filter-paper each three times with water. Reserve the residue. Rinse a 500-ml calibrated flask with dilute hydrofluoric acid (1 + 99), place into it 100 to 200 ml of water and pour in the sample solution, washing the polythene beaker three times.Make the volume up to the mark, shake the flask well and measure out 250 ml of the solution in a 250-ml flask that has been first rinsed out with a few millilitres of sample solution. It should be noted that hydrofluoric acid, at this dilution, has no effect on brief contact with glass, except to clean it by stripping off gelatinous silica. Keep the unused sample solution in a polythene bottle. Transfer the contents of the 250-nil flask (equivalent to 0.2500 g of sample) to an 800-ml glass beaker, washing the flask out three times, add 15 ml of sulphuric acid (1 + 1) and evaporate to fumes under an infrared heat lamp.The beaker will be etched, for which reason the same vessels should be kept aside for future analyses to avoid spoiling new glassware, but any silica abstracted is driven off by the fuming. Dilute to 250 ml, add some paper pulp, boil, add 4 g of tannin dissolved in 40 ml of hot water, and boil for 10 minutes. Turn the hot-plate switch to "low," leave for 2 hours and then allow to stand overnight. At this concentration of sulphuric acid, 3 per cent. v/v, tannin gives a clear-cut separation of niobium and tantalum from iron and manganese and an acceptable separation from titanium. Filter through a 15-cm Whatman No. 541 filter-paper, wash the beaker and precipitate three times each with dilute sulpliuric acid (2-5 + 97.5) and place the precipitate in a weighed Vitreosil crucible, which in turn should be put in a cold electric furnace. Switch on the furnace, ignite at 800" C until all carbon is burned off, allow the crucible to cool in a desiccator and re-weigh.If the precipitate is to be ignited over a burner it must be carefully dried at 105" C before ignition, to prevent loss by explosive boiling of the voluminous wet mass in the crucible. Multiply the weight of precipitate by 4 x 100 to obtain the percentage of niobium and tantalum pentoxides. CORRECTION FOR TUNGSTEN INTERFERENCE- If wolfram present in the original sample has not been separated by a preliminary aqua regia - ammonium tungstate treatment the tungsten derived from it will be precipitated as a yellow powder when the hydrofluoric acid solution is evaporated to fumes with sulphuric acid.(NOTE-A white precipitate is iron(II1) sulphate, which can be ignored, but should be re-dissolved on dilution before adding tannin.) In this event, transfer the concentrated sulphuric acid solution into a 25-ml calibrated flask, washing the beaker three times with more concentrated acid. Make up to the mark with concentrated sulphuric acid, cooling if necessary, and allow all of the precipitate to settle. By using a pipette fitted with a pipette filler, measure 15 ml of clear solution into a 1-litre beaker, dilute to 500 ml and precipitate with 4 g of tannin, as directed above.96 POLLOCK According to Pickup, the solubility of tungsten trioxide in concentrated sulphuric acid is about 8.4 mg per 10ml of acid.Therefore, deduct (1.5 x 04084g), i.e., 0.0126g from the weight of the final precipitate to allow for tungsten retained in the sulphuric acid solution. DETERMINATION OF NIOBIUM Returning to the unused part of the hydrofluoric acid solution, measure, with a 100-ml pipette fitted with a pipette filler, 100 ml into a 250-ml beaker, add 50 ml of sulphuric acid (1 + 1) and evaporate to fumes under an infrared heat lamp. The procedure for determining niobium pentoxide is then as described by Pickup, and tantalum pentoxide may be calculated by difference. TREATMENT OF INSOLUBLE RESIDUE DETERMINATION OF TIN- When tin is to be determined in a mixed mineral concentrate, the insoluble residue left after the hydrofluoric acid treatment should be wholly transfened into the filter-paper, which is gently heated in a 50-ml nickel crucible to burn off carbon.Tin may then be determined by the Pearce - Low iodine titration m e t h ~ d , ~ by using 0.002 N iodine for small amounts. DETERMXNATION OF URANIUM AND THORIUM- If uranium and thorium are to be determined in a radioactive niobium - tantalum mineral, the hydrofluoric acid insoluble residue should be washed back off the filter into the platinum dish, where it can be treated for the determination of uranium and thorium by any suitable method,lO for example, by dissolving it in nitric acid as a preliminary to column chromatography. COMPARISON OF RESULTS Several samples of columbite - tantalite that had been assayed by the proposed method were re-assayed by using the standard procedures described by Schoeller and Powell. The comparative figures are given in Table I1 and show closer agreement than would be expected from the experimental results in Table I, probably because recovery is not complete by any method.TABLE I1 COMPARISON OF RESULTS Assay by proposed method, Assay by standard method, Sample niobium and tantalum pentoxides, niobium and taotalum pentoxides, No. per cent. per cent. 1 76.8 77.7 2 74.9 74.9 3 72.1 72.8 4 76.9 77.8 5 73.6 74.5 6 73.1 73-0 7 64.1 64.5 8 60-2 00.4 Samples Nos. 7 and 8 contained 1 per cent. of titanium dioxide. whose 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. The author thanks the Commissioner of Geological Survey and Mines, Entebbe, with permission this paper is published. REFERENCES Smith, J . L.. Amer. J . Sci., 1877, 13, 359. Hillebrand, W. F., Colo. Scient. SOC. Proc., 1888, 3, 38. Wells, R. C., J . Amer. Chem. Soc., 1928, 50, 1017. Chernikhov, Yu. A., and Uspenskaya, T. A,, Nauchno-Tek. Otch. Tyemnye, 1936, No. 83, Ciredment, Chernikov, Yu. A., and Goryushina, U. G., Zav. Lab., 1945, 11, 876. Langmyhr, F. J., and Graff, P. R., Alzatytica Chim. Ada, 1969, 21 (4), 334. Schoeller, W. R., and Powell, A. R., “The Analysis of Minerals and Ores of the Rare Elements,” Third Edition, Charles Griffin & Company Limited, London, 1966. Pickup, R., Colon. Geol. Miner. Resouv., 1966, 5, 174. Weinig, A. J., and Schoder, W. P., “Technical Methods of Ore Analysis for Chemists and Colleges” based upon the text by Albert H. Low, Eleventh Edition, John WiIey 62 Sons Inc., New York, 1939. “The Determination of Uranium and Thorium, Handbook of Chemical Methods for their Deter- mination in Minerals and Ores,” National Chemical Laboratory, H.M. Stationery Office, London, 1963. Received November 28th. 1966 Moscow.