Estimation of gallium in a bauxite-ore deposit using an energy-dispersive X-ray fluorescence technique
C.K. Bhat
Bhabha Atomic Research Centre, Nuclear Research Laboratory, Mumbai 400 085, India Received 18 March 2002; accepted 7 May 2002
Abstract
Energy-dispersive X-ray fluorescence (EDXRF) technique was used to determine the gallium content of bauxite ore samples collected from several sites located in the Jammuregion of North India. To achieve an optimum detection efficiency, mono-energetic X-rays of a molybdenum-X-ray tube were used and, for quantitative estimation of the gallium content, the fundamental parameter method. Our results indicate the presence of commercially exploitable gallium deposits. In addition, they show that the gallium content of bauxites strongly depends on depth. This latter finding is assumed to be caused by long-term chemical weathering effects. The present investigation shows that the EDXRF technique can easily be used to search for new sources of gallium. r2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
Gallium (Ga) was discovered as a new element more than a hundred years ago. It is only during the past three decades, however, that this element has acquired a significant commercial importance, notably due to its use in the manufacture of solid-state devices, including gallium-based compound semi-conductors for the electronic industry. This strategic metal is also finding ever-increasing application in the manufacture of microwave devices, bubble memories, light-emitting diodes and photo-voltaic cells, etc.
In order to meet future demands for this high-tech metal, a search is on for finding new resources of bauxites of commercially exploitable gallium content.
Bauxites are believed to be formed due to intense chemical weathering under strong oxidising and leaching conditions (Maynard, 1983; Gribble, 1988). They are enriched in aluminium and titanium but are strongly depleted in soluble elements such as potassium, calcium, sodium and magnesium (Maynard, 1983; Krishnaswa- my, 1979). The detailed behaviour of these elements
during the formation stage of bauxites is not well established (Giaugue et al., 1977).
Our aim here is to estimate the concentration of gallium from high quality diaspore bauxite samples of the Jammuregion by making use of the energy- dispersive X-ray fluorescence (EDXRF) technique. Our results may also help to understand the cause of variation of gallium concentration in different layers of bauxite deposits at different location sites.
The trace-elemental determination in bauxite ores, using wavelength-dispersive X-ray fluorescence (WDXRF) spectrometry, was reported previously (Schorin, 1982). Schorin used the standard-condition method for the determination of various trace elements.
This method requires a large number of standards in matrices similar to that of the unknown samples. An adequate attention has been paid towards employing the EDXRF spectrometry for making a comparative estimation of the gallium concentration from bauxite ores.
Several techniques have been applied for the determi- nation of trace elements (Schorin, 1982; Saxena et al., 1994) such as the neutron-activation method or the inductively coupled plasma atomic emission spectro- metry. But these techniques, although fairly sensitive, E-mail address:[email protected] (C.K. Bhat).
0969-806X/02/$ - see front matterr2002 Elsevier Science Ltd. All rights reserved.
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are comparatively more time consuming and have some other disadvantages such as the need for using sophisticated equipment, extensive chemical pre-treat- ment of samples and other manipulations. Because of this, we preferred to use the EDXRF technique because its sensitivity range corresponds to that of the principal scientific objective of the present investigation,
2. Sample-site description
The bauxite samples used in this work were collected from several locations spread over a distance ofB2 km in Sukhwalgali and Jangalgali sites, situated in the Jammuregion of the Jammu& Kashmir state in Northern India. These sites are located near Udhampur town, which is situated about 60 km in the North-East from Jammucity. The ore samples were collected at different depths from several places around these sites.
Each sample comprises three different layers viz., Pissolitic (top), semi-Pissolitic (middle) and Kaolinite (bottom) as sketched in Fig. 1. The top layer (Pissolitic bauxite) varies in thickness from 0.5 to 1 m and is hard diasporic bauxite, the middle layer (semi-Pissolitic) is a gray clay of 1 m–1.5 m thickness and the bottom layer (Kaolinite) 1.5 m–2 m thickness, is rich in aluminium clay. To minimize such effects as biological weathering and other biochemical processes, the samples were taken from that area which was free from plantation.
3. Experimental
For the present study, the samples were dried at 501C and then crushed to a fine powder with the help of a mortar and a pestle. A known weight of sample was mixed with a pre-measured quantity of the cellulose and several specimens of the samples were made in the form of pellets at 3.94107kg/m2pressure using a hydraulic
press. The weight of the pellet was kept around 20–30 mg/cm2. The samples were subjected to desired elemental analysis using the EDXRF spectrometer as shown in Fig. 2. The heart of this instrument is the Si(Li) detector which was always kept cooled to the liquid nitrogen temperature. The detector assembly is coupled to a pulse height multi-channel analyser with a personal computer (PC)-based data-acquisition system. The energy-resolution of the spectrometer is 160 eV at 5.9 keV, which is sufficient to resolve the K/L shell characteristic X-rays from the elements of interest.
Molybdenum-anode X-rays of 17.4 keV energy from the X-ray tube have been used for the excitation of gallium characteristic X-rays.
A fundamental parameter and Compton scattered method has been invoked in practice for a quantitative estimation of the gallium concentration (Van Dyck and Van Grieken, 1980; Giaugue et al., 1977).
The detected fluorescence X-ray intensity of the element of unknown mass (mj) is given by
Ij¼I0GKjmjA; ð1Þ where I0 is the incident excitation photon flux, Gis a geometrical factor,Ais the absorption correction, and Kj is the relative excitation efficiency and detection sensitivity of the element of interest (gallium) and is given by
Kj¼tð11=Jk;lÞokeZT; ð2Þ wheretis the photoelectric mass absorption coefficient, Jk;lis the jump ratio,okis theK-shell fluorescence yield, eis detector efficiency at the fluorescence energy,Zis the fractional intensity of the X-ray line under analysis e.g.
ðKa=KaþKbÞ;andT is the transmission of fluorescence radiation in the air path and Be window of the detector.
The absorption correction term for the intermediate thick samples is given by
A¼1exp½ðmicosecfþmfcoseccÞM
ðmicosecfþmfcoseccÞM ; ð3Þ
Fig. 1. Depth description of bauxite ore samples.
whereM is the total mass of the sample,miis the total mass absorption coefficient at the incident energy,mf is the total mass absorption coefficient at the fluorescence energy, f is the mean angle formed by the exciting radiation and c is the mean angle formed by the fluorescence radiation with the sample surface, respec- tively.
Ideally, the incident photon flux I0 should remain constant during the analysis of the whole set of samples.
However, in practice, there are minor variations in the photon flux from the X-ray tube. These variations have been accounted for by using the Compton normalized incident and fluorescent X-ray peak intensities.
In the fundamental parameter approach, the tabu- lated values of the photoelectric absorption cross-section
(McMaster et al., 1969), Jump ratio (Bambynek et al., 1972) and fluorescence yields (Krause et al., 1978) have been used for the evaluation of Eq. (1). FinallyI0Gwas determined using a single element synthetic standard prepared in an alumina (Al2O3) and cellulose matrix.
4. Results and discussion
Fig. 3 represents a typical energy spectrum of a sample obtained for 4000 s using the molybdenum- anode X-ray tube excitation. Fig. 4 shows a representa- tive plot of the inferred gallium concentrations of samples belonging to different depths of various sites.
Concentration values of gallium of different layers at various sites are given in Table 1. It is clear from the Table 1, that the concentration of gallium is low in all Fig. 2. Block diagram of the EDXRF spectrometer.
Fig. 3. A typical EDXRF spectrum using low power transmis- sion type molybdenum X-ray tube excitation.
Fig. 4. Variation of gallium concentration at different depths w.r.t. several sites (1–10).
the three layers of the samples from sites 1–6 and in the top layers of sites 7–10, in contrast the middle and bottom layers of sites 7–10 show higher concentrations.
When the total extent of the sites is taken into consideration, the overall concentration of gallium estimated in the area is well within the reach of commercial exploitation. The possible reason for the low level gallium concentration in all layers of sites 1–6 and in the top-layer of the sites 7–10 may be the occurrence of different weathering and chemical con- version processes at these places over millions of years.
This may have led to an erosion and depletion of gallium from these layers. The high concentration of gallium in the middle and bottom layers of the sites 7–10 clearly indicate that weathering processes had only a small effect at these places. Samples obtained from sites 7–10 belong to a higher slope terrain than the samples from the sites 1–6. Weathering process as due to chemical conversion and precipitation can occur at any point of any site but from our existing experimental data it can be assumed that the weathering effect might strongly depend on depth and also slope of terrain.
5. Conclusion
This study demonstrates the relative ease and accuracy with which the EDXRF technique may be used for estimating the gallium concentration in bauxite ore deposits. The analysis enables us to correlate the gallium concentration in bauxite deposits of different location sites. The main result of this exploratory study indicates that the concentration of gallium is around 0.015% in the bauxite ore samples which were extracted from a depth of 1–2 m from several sites surveyed in the course of this work and possibly can be commercially exploited. Most of the developing countries, where commercial gallium is being met by imports, can make use of less expensive and most powerful (EDXRF) technique for commercial exploitation of Gallium content from its ore.
Acknowledgements
The author wishes to express his gratitude to Dr. C.L.
Bhat, and Dr R.K. Koul, NRL, BARC, Mumbai for constant encouragement and support provided during the course of this work. and also to Dr. B.S. Negi of the Environmental Assessment Division of BARC for the help rendered during the course of experimentation and sample preparation.
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Table 1
Gallium concentration of different sites at various depths S.No. Sample No./
Site No.
Concentration+s (ppm)
Depth (m)
1 SBT1 }1 2074 1.1
SBT2 } 2274 3
2 P2/S1 } 2275 0.5
P2/S2 }2 2775 1.5
P2/S3 } 4076 2.5
3 P3/S1 } 2474 0.5
P3/S2 }3 2874 1.5
P3/S3 } 3075 2.5
4 JBT4/1 } 1676 0.5
JBT4/2 }4 2175 1.5
JBT4/3 } 2275 2.0
JBT4/4 } 2774 2.5
5 JBT3/1 } 2274 0.5
JBT3/2 }5 2574 1.5
JBT3/3 } 3075 2.0
JBT3/4 } 3675 2.5
6 JBT2/1 } 1676 0.5
JBT2/2 }6 3174 1.5
JBT2/3 } 3775 2.0
JBT2/4 } 4075 2.5
7 SBP/1 } 5775 2
SBP/2 }7 7775 4
8 P1/1 } 5475 0.5
P1/2 }8 9374 1.5
P1/3 } 10173 2.5
9 JBT1/1 } 7074 0.5
JBT1/2 }9 9774 1.5
JBT1/3 } 12073 2.0
JBT1/4 } 12573 2.5
10 KBT6/1 } 5475 0.5
KBT6/2 }10 5875 1.5
KBT6/3} 14673 2.5
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