Determination of niobium in rocks, ores and alloys by atomic-absorption spectrophotometry

Talanta,

1972. Vol. 19, pp. 863 to 869.

Pasamoa

Pmaa.

Printed

itt Northem

Inland

DETERMINATION OF NIOBIUM IN ROCKS, ORES AND ALLOYS BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY Department

JOHN HUSLER of Geology, University of New Mexico, Albuquerque,

New Mexico 87106, U.S.A.

(Received 7 October 1971. Accepted 14 November 1971) Summary-Niobium, in concentrations as low as 0.02% Nb106, is determined in a variety of materials without separation or enrichment. Chemical and ionization interferences are controlled, and sensitivity is increased, by maintaining the iron, aluminium, hydrofluoric acid and potassium content within certain broad concentration limits. There is close agreement with the results of analyses by emission spectrographic, electron microprobe and X-ray fluorescence methods. THE DETERMINATION of niobium suming because of the invariable

by classical wet methods is involved and time-conneed for separations or extractions. Nearly all the commonly used calorimetric methods require the prior separation of tungsten, molybdenum and titanium.l Atomic absorption has not been widely used for the determination of niobium because of the low inherent sensitivity, e.g., Schiller advocated the use of as much as a 60-g sample to analyse steels containing 0.01% niobium.2 Fortunately, however, there are several chemical species which, over a wide concentration range, enhance the absorption of niobium. This property can be used to advantage to lower the niobium detection limit, because these species provide an “enhancement buffer” over reasonably broad concentration limits. They include aluminium, iron, hydrofluoric acid and the easily ionized alkali metals. Initially, attempts were made to analyse pandaite ores from Brazil and pyrochlore ores from Canada by the often recommended method of fusion with potassium bisulphate followed by leaching of the melt with hot 4 % ammonium oxalate solution or hot 20-30% tartaric acid solution. Most of the Brazilian ores contained pandaite, approximate formula Ba,_ENb20,(H,0)1+,, with large amounts of titanium in the form of ilmenite, FeTiO,; also present were aluminium phosphate, the rare earths, quartz, and apatite, along with small amounts of titania.3*4 Most of the Canadian ores carried very fine-grained uranium-bearing pyrochlore in a silicate-rich matrix such as acmite and potash feldspar. Many other minerals were present, such as calcite, pyrite, pyrrhotite, fluorite, apatite, biotite and magnetite.5 The bisulphate fusion of these types of ore proved to be unreliable, presumably owing to the presence of rare earths in the Brazilian ores1 and uranium in the pyrochlore. Faye reported that uranium in pyrochlore, betafite, euxenite and other niobium-bearing ores led to low niobium results with associated low precision.6 Faye further found by thorough study of the hydrofluoric acid dissolution method that a mixture of that acid with hydrochloric acid readily dissolved the niobium along with the associated minerals such as ilmenite and magnetite, and that sulphides such as pyrite and molybdenite remained unattacked. The hydrofluoric acid, which interfered with the subsequent ether-thiocyanate spectrophotometric estimation as prescribed by Ward and Marranzino,7 was removed by evaporation (to a paste) in the presence of phosphoric acid. Faye’s hydrofluoric-hydrochloric-phosphoric acid 863

864

JOHN HUSLER

approach was used successfully for the atomic-absorption method in this laboratory, but since hydrofluoric acid greatly enhances the niobium absorption, it was decided to use this to advantage and to eliminate the volatilization of hydrofluoric acid in the presence of phosphoric acid, if possible. This proved to be feasible because the effect of hydrofluoric acid is constant when between 10 and 20% of concentrated hydrofluoric acid are present, the resulting increase in absorption being about 30 % (Table I). Teflon beakers eliminate the need for platinum ware, and loss of hydrofluoric acid by its reaction with glassware is circumvented by utilizing polypropylene volumetric flasks. Silica, in the amounts which might remain from incomplete volatilization of silicon tetrafluoride, does not interfere. TABLE I.-EFFECT OF ALKALI METALS, ALU~WJM, TITAMUM IRON AND HYDROFLUORIC 125-~g/ml~OBIvMPENToXIDEABSORPnON Added species

K, 500 &ml 1000 2000 Li, 1000 ,ug/ml Na, 1000 ,@ml Rb , 500 ,ug/ml Cs, 500 pg/ml Al, 200 pg/ml 500 1000 2000 3000 Ti, 500 Pg/ml 1500 500 + AI, 500 Fe, 500 pg/ml 1000 2000 3000 HF, 5 ml conc./lOO ml 10 15 Fe, 1500 pg/ml 1500 + HF, 1 ml conc./lOO ml 1500 + 2 1500 + 10 1500 + 15

Absorption,

ACID ON

chart divisions (10 x scale)

Without enhancement buffer

With buffer*

23 23.5 24 24 40 34 40 34 53 60 60 61 63 37.5 -

65 64 45 40 65 65 65 65 -

33 36 36 36 26 31 31 36 45 47 55 55

64 65 65 63 65 65 63 -

65 64 -

* 20 ml per 100 ml of solution.

In a thorough investigation of other potential interfering ions in the atomicabsorption determination, it was found that aluminium, iron and the alkali metals also produce enhancement effects, but that these effects can also be controlled (Table I). Aluminium also negates potential interference from titanium, which otherwise would produce low results (if the titanium is not eliminated in the acid attack). The enhancement from the alkali metals is attributed mainly to the ionization effects of the nitrous oxide-acetylene flame. Manning observed that the best sensitivity for niobium, about 20 pg/ml for 1% absorption, was obtained at 3343.7 A, and that when 0.1% of potassium was added, a detection limit of 5 ,ug/ml was achieved.8 This

Determination

of niobium in rocks

865

sensitivity was not attainable on the instrument in this laboratory, but by addition of aluminium, iron, hydrofluoric acid and potassium, a sensitivity of 15 ,ug/ml could be achieved. In this laboratory the effect of potassium was not observed to be as great as that reported by Manning, but this is probably because of different burner conditions and because Manning used niobium as the chloride, whereas in this work studies were made with stock solutions of niobium pentoxide which had been dissolved in hydrofluoric and hydrochloric acids. The effects of other ions were studied, with solutions containing 500 rug of aluminium, 1000 ,ug of iron and 500 pug of potassium per ml and 5% v/v concentrated hydrofluoric acid. These quantities of additives cause nearly a threefold enhancement of the niobium absorption, and are not critical, since at least twice these amounts can be present without significant change in the enhancement. The absorption of a niobium solution containing between 500 and 3000 ,ug of aluminium per ml appears to be only 3-5x less than that of a similar solution plus hydrofluoric acid, iron and potassium. The concentrations chosen for these reagents in the enhancement buffer are near the middle of the tolerance ranges, so their presence in samples is without further influence. Table I indicates that large amounts of potassium in the sample begin to reduce the enhancing effect of the combined additives. In the presence of 20 ml of enhancement buffer per 100 ml of sample solution, there was no significant interference with 125 rg of niobium per ml from the following concentrations of other species : TiO, or Na,HPO,-12H,O, 3-Omg/ml; Mg, 2.5 mg/ml; Na,SiO,*9H,O, CaCO,, SrCO,, BaCO,, 1.0 mg/ml; Co, Cr, Cu, Mn, MO, Ni, Ta, U, V, W, or Zn, 200 pg/ml. These were added as solutions of the species named, in hydrochloric, nitric or hydrofluoric acid, or mixtures of these acids. No interference was observed from a mixture of yttrium plus the rare earths La, Ce, Pr, Nd, Sm and Gd totalling 800 pg/ml. The composition of the mixture was based on the ratio of the abundances of these elements in the earth’s crust.n No interference was noted from 5 % v/v nitric or sulphuric acid, or from as much as 15 % v/v hydrochloric or phosphoric acid, except for the slight reduction in absorbance expected as a result of the change in viscosity. EXPERIMENTAL Instrument parameters The conditions used to achieve optimum sensitivity are listed in Table II. One of the most important factors is the use of wide diameter (0.023 in. bore) tubing to allow a sample uptake rate of 10-12 ml/mm. It is felt that many of the conflicting reports of sensitivities, chemical and ionization interferences, etc, could be explained if contributors would report aspiration rates, burner heights and flame characteristics. For example, the best niobium sensitivity is obtained with a large uptake rate and the burner head l-l.5 cm below the light-path. The acetylene and nitrous oxide flow-rates should be set to yield a l-2 cm high blue flame just above the burner, with an 8-9 cm high “red feather.” There is a large reduction in sensitivity for niobium (and tungsten) when the acetylene pressure drops below SO-100 psig. Similar effects, causing erratic results, and attributed to acetone in the acetylene cylinder, have been reported by the Perkin-Elmer Corporation, as for example in the determination of gold with an acetylene-air flame. lo Niobium sensitivities obtained with other burner and flame types are given by Slavinlr

soIutions Niobium pentoxide, 2QOO&ml stock solution. Dissolve 1.000 g of pure NbtOs by warming with 30 ml of 48-50x hydrofluoric acid and 20 ml of concentrated hydrochloric acid in a Teflon beaker. 3

866

JOHN HUSLER

Instrument Wavelength Slit Cathode current Scale Fuel Oxidant Sample uptake rate Burner height

Perkin-Elmer, Model 303 with Model 165 recorder 3343.7 A 3 (2 A) 38 mA (40 max) 10 x (for less than 1% Nb,O,) Acetylene (9 psig) Nitrous oxide (40 psig) 10-12 ml/min l-l .5 cm below light-path

After dissolution is complete, dilute to 500 ml with water. Transfer immediately to a polyethylene bottle. This solution is stable for at least 6 months. Iron solution, 2Og/l. Dissolve 20.0 g of pure iron powder in 200 ml of hydrochloric acid (1 + 1) and dilute to 1 litre with water. Aluminium solution, log/l. Dissolve 10.0 g of aluminium powder or fine wire in 200 ml of hydrochloric acid (1 + 1) plus a few drops of 30% hydrogen peroxide and dilute to 1 litre with water. Potassium solution, log/l. Dissolve 19.1 g of reagent-grade potassium chloride in water and dilute to 1 litre. Enhancement buffer solution. Mix 500 ml each of the iron, aluminium and potassium solutions with 500 ml of 48-50% v/v hydrofluoric acid. Store in a 2-litre polyethylene bottle. This is enough solution for 100 samples and standards. Standardsolutions ofNb,O,, 50, 100, 150 and 200 Z&ml. Using a polyethylene pipette, transfer 25, 5*0,7.5 and 10 ml of the 2000~,og/ml NbsOs stock solution into 100~ml polypropylene volumetric flasks. Add 20 ml of enhancement buffer solution and dilute to 100 ml with water. Prepare a blank containing 20 ml of the buffer solution. These solutions are stable for at least 6 months. Dissolution of sample Place an accurately weighed l-2 g sample in a 100~ml Teflon beaker. Rinse the walls with 5-10 ml of water. Add 10 ml of concentrated hydrochloric acid and 10 ml of concentrated hydrofluoric acid and heat the solution for 2 hr on a hot-plate at a temperature low enough to prevent the beaker from sticking to the hot-plate. The solution should not evaporate to dryness, but if it does, repeat the addition of acid and heat to redissolve the niobium. Add 20 ml of the enhancement buffer solution and filter through a Whatman No. 1 paper in a polyethylene funnel into a 100~ml polypropylene volumetric flask. Rinse the beaker and paper with 2-3 ‘A v/v hydrochloric acid and dilute to 100 ml with water. Analyse for niobium, using the parameters listed in Table II. Samples containing more than 1% niobium pentoxide can be analysed by preparing more concentrated standards with a corresponding attenuation of the instrument scale. CONCLUSIONS The comparison between the atomic-absorption (AA) and two independent X-ray analyses of Canadian niobium-bearing ores is presented in Table III. The two sets of samples, those analysed by AA and laboratory A and those analysed by laboratories A and B are from different drill holes but from the same deposit. The AA results of samples containing less than 0.8 % niobium pentoxide agree with the X-ray results from laboratory A within about 4 %, but at higher concentrations there is a difference of up to -14%. Results from laboratory B for >0*8% Nb2 are lower than those from laboratory A by about 6%. This implies that the AA results would agree with those from laboratory B to within 3 %. The results in Table IV show that there is also excellent agreement between the AA and spectrographic analyses for low-grade Brazilian ores. Tables V and VI, respectively, show the comparative results of microprobe analyses and replicate AA analyses of other ores. The results for duplicate analyses of BCS Steel Standard No. 365 were O-56 and O-58%. Nb (certificate value 0.57%). The only procedural difference in

Determination

867

of niobium in rocks

TABLE III.-COMPARI~~N OF AA AND INDEPENDENT X-RAY FLUORESCENCE ANALYSES(LABORATORIES A AND B) OF CANADIANNIOBIUM-BEARING ORES

Sample

14 15 16 17 18 19 20 21 22 23 24

NWs,

%

AA

Lab. A

0.24 0.71 0.82 0.84 1.41 1.76 1.72 l-91 2.15 2.28 2.50

0.25 0.77 0.82 0.88 1.64 1.85 1.87 2.12 2.44 2.62 2.64

Relative difference,* %

Nb&,

Sample

25 26 27 28 29 30

-4 -8 0 -5 -14 -5 -8 -10 -12 -13 -5

3’: 33 34 35 36 37 38 39 40 41

%

Lab. A

Lab. B

0.14 0.26 0.39 0.45 0.55 0.79 0.79 0.87 0.95 0.95 1.05 1.63 1.80 1.81 1.95 1.96 2.08

0.16 0.27 0.40 044 0.56 0.79 0.77 @83 0.90 0.92 1.03 1.58 1.73 1.74 1.83 1.83 1.87

Relative differenqt % +14 -c-4 +3 -2 +2 0 -3 -5 -5 -3 -2 -3 -4 -4 -6 -8 -10

* AA relative to Lab. A. t Lab. B relative to Lab. A.

TABLE IV.--COMPARISON OF AA AND EMISSION SPECTROORAPHIC ANALYSESOF BRAZILIAN NIOBIUMBEARING

Nb&,

Sample

1 2 3 4 5 6 7

ORES

%

AA

Spectrographic*

0.14 o-12 0.13 0.25 0.31 0.31 0.12

0.14 0.16 0.13 0.28 0.28 0.32 0.12

Nbdh, %

Sample

8 9 10 11 12 13

* Emission spectrographic analyses were performed Horizonte, Minas Gerais, Brazil.

AA

Spectrographic*

0.40 0.28 0.42 0.22 0.06 0.23

0.29 0.28 0.42 022 0.12 0.24

by Geologia

e Sondagens

Ltda.,

Belo

TABLE V.--COMPARISON OP AA AND ELECTRONMICROPROBE ANALYSESOF BRAZILIANORE NbA Sample AA 42 43

+ 100 Mesh Probe

0.47 2.40

0.50 26

-100 AA 0.98 2.95

%

+ 200 Mesh Probe 0.81 2.3

-200 AA 1.92 3.40

Mesh Probe 2.9* 2.4*

+ Large deviation attributed to insufficient statistical sampling by the electron microprobe analysis.*

868

JOHN HUSLER TABLEVI.--RESULTSOFREPLICATEANALYSESOFNATURA~ANDSYNTHE~CNIOBIUMORES

Canadian pyrochlore standard (03% Nb,OJ: NbB06 0.50, 0.45, 0.47, 054, 0.50, @50, o-51, 0.50 %. Mean 0.496 %, standard deviation = 0.026 % (1-2 g samples). Synthetic rock standards prepared by addition of pure NbaO, to syenite containing less than 0.02% Nb,O,. 1% Nb,O,: 0.99,0.98, 1.02, 1.03 % (l-2 g samples) 10% Nb,O,: 10.2% (0.2 g sample), 9.7 % (0.1 g sample) TABLE VII.-EFFECT OF NICKEL (ADDED AS NiCI,*6HIO) ON ABSORPTIONBYAN SO+g/mlNb,O, SOLUTION

Enhancement ml

buffer,

Ni added, mglml

Absorption (chart divisions)

0

2: ;: 20 20 20

0 0.1 0.5 2.5 5.0 7.5

:: 31 33 33 23 20

analysing alloys is the addition of 5 ml of concentrated nitric acid per gram of sample to aid in the dissolution. In an attempt to determine the niobium content of an Inconel nickel sample, it was found that concentrations of nickel much greater than 25OO&ml begin to reduce drastically the absorption of niobium (Table VII). In such analyses, when the niobium content is low enough to require a dissolved sample with a nickel concentration greater than O-25g/l00 ml, a prior chemical separation is recommended. Acknowledgements-The author wishes to thank Paul-Emile Hamelin of the St. Lawrence Columbium and Metals Corporation for the analysed Canadian ore sample, and the University of New Mexico Department of Chemistry for the steel standard. Thanks, also, to Klaus Keil for information on the microprobe and microscopic studies, E. F. Cruft and F. Dubuc for additional information on the ore samples, and R. A. Nash for providing a copy of the spectrographic results. Zusammenfassung-Niob wird in Konzentrationen bis herunter zu 0,02x NbpOs in einer Reihe von Materialien ohne Abtrennung oder Anreicherung bestimmt. Man behat chemische und Ionisierungsstarungen im Griff und erhiiht die Empfindlichkeit, wenn mann die Gehalte an Eisen, Aluminium, Flu%iure und Kalium innerhalb bestimmter breiter Konzentrationsgrenzen h&lilt. Die Analysenergebnisse stimmen gut iiberein mit solchen der Emissionsspektrographie, der ElektronenMikrosonde und der Riintgenfluoreszenz. R&urn&On d&ermine le niobium B des concentrations aussi faibles que 0,02x de Nb,O, dans diverses substances sans sdparation ou enrichissement. On contr8le les interfdrences chimiques et d’ionisation, et la sensibilitb est accrue, en maintenant les teneurs en fer, aluminium, acide fluorhydrique et potassium & l’intbrieur de certaines larges limites de concentration. 11 y a un accord Ctroit entre les r&ultats d’analyses par les m&hodes de spectrographic d’kmission, de microessai klectronique et de fluorescence de rayons X. REFERENCES 1. A. R. Powell, Niobium and Tantalum, in Standard Metho& of Chemical Anafysis (N. Ed.), 6th Ed., Vol. 1, pp. 714-735. Van Nostrand, Princeton, 1962. 2. R. Schiller, At. Absorption Newsletter, 1970, 9, 111.

H. Furman,

Determination

869

of niobium in rocks

3. K. Keil, Report on Mineralogic, Microscopic, and Electron Microprobe Study of Nb-Bearing Brazilian Soil Samples, Private investigation, Nov. 28, 1970, 7 pp. 4. E. F. Cruft, personal communication, 1970. 5. F. Dubuc, personal communication, 1971. 6. G. H. Faye, Chem. in Canada, 1958, 10, (4) 90. 7. F. N. Ward and A. P. Marrauzino, Anal. Chem., 1955,27, 1325. 8. D. C. Manning, At. Absorption Newsletter, 1967, 6, 35. 9. L. H. Ahrens and S. R. Taylor, Spectrochemical Analysis, 2nd Ed., p. 218. Addison-Wesley, London, 1961. 10. Perk&Elmer Corp., Analytical Methods for Atomic Absorption Spectroscopy, 1964. Norwalk, COM.

11. W. Slavin, Atomic Absorption Spectroscopy, pp. 137-138. Interscience,

New York, 1968.