Application
Note: 41801
Analysis of Bauxite by X-Ray Diffraction
ARL X’TRA Powder X-Ray Diffraction System
Introduction
Key Words
• ARL X’TRA
• Bauxite
• X-Ray Diffraction
• XRD
Four steps in aluminum process can be clearly distinguished:
mining the ore, refining the bauxite, smelting and reducing
alumina to metal and basic products process (aluminium
sheets, alloys, ingots, roll, wires, cans, …).
Bauxite is the starting raw material in the aluminum
industry; this ore contains mainly hydrated forms of
aluminun and iron oxides, such as gibbsite (Al(OH)3),
diaspore (AlO(OH)), boehmite (AlO(OH)) and goethite
FeO(OH).
Figure 1: Bauxite is a general term for a rock composed of hydrated
aluminum oxides; it is the main ore of alumina to make aluminum; also used
in the production of synthetic corundum and aluminous refractories
It must first be refined into aluminum oxide, or
alumina. This is realized via the Bayer refining process
(digestion, clarification, precipitation and calcinations).
To turn bauxite into alumina, the ore is ground and mixed
with lime (CaO) and caustic soda (NaOH), this mix is
pumped into high-pressure containers, and heated. Bauxite
is dissolved by the caustic soda in order to form NaAlO2.
Then, this solution is filtered to remove impurities, such as
iron, and precipitated out of this solution, washed, and
heated at 1300 °C to drive off water. The left white
powder is called alumina, which is transformed into
aluminum metal in the smelting process through
electrolytic reduction. The fundamental component of a
smelting operation is the electrolytic cell, or "pot" in
which this reaction takes place. During smelting, large
amounts of current pass through molten alumina
dissolved in a 920 - 980 °C cryolite (Na3AlF6) bath thanks
to graphite anode.
Then, full line of commodity can be found such as
grade aluminum ingot, high-purity ingot, aluminum billet
-for extrusion, forging, impact extrusion applications-,
casting alloys, cast rod - electrical cable, magnet wire and
steel industries-, and rolling ingot.
During this complete process, several parameters have
to be controlled and sometimes adjusted to provide the
best performance of aluminum refined products.
Instrumentation
All samples have been analyzed with the Thermo Scientific
ARL X'TRA in Θ:Θ geometry (see Fig. 2), with a Cu
ceramic X-ray tube and with a sealed Peltier cooled
detector. Due to a high-energy
resolution of this type of detectors,
beta filters and diffracted beam
monochromators have been
removed resulting in a gain of
intensity; this “extra” intensity
allows collecting data more rapidly.
An increasing of intensity can still
be obtained by adjusting the optic.
Source
Detector
θ
θ
Sample
Figure 2: Θ:Θ geometry of ARL X’TRA diffractometer
Bauxite identification and quantification
Commonly, bauxite is composed of the aluminumcontaining minerals gibbsite (or hydragillite), boehmite
and diaspore; iron-containing minerals hematite, goethite,
magnetite, siderite and ilmenite; titanium-containing
minerals anatase, rutile and brookite; and the siliconcontaining minerals halloysite, kaolinite and quartz.
Figure 3 shows how various bauxite rocks are in
different mining region.
use just a few of the most intense diffraction peaks of
phases; they are based on the intensity/area ratios of each
phase, but require calibration curve. This is the case of
Internal standard method, External standard method, and
Standard addition method; the RIR method (Relative
Intensity Ratio) is a kind of “standarless” method.
Another one, the so-called “standarless” or semiquantitative method, based on Rietveld refinement
technique is more and more used because it has a higher
accuracy and partially compensates for overlapped peaks,
preferred orientation and extinction. Rietveld technique
corresponds in a whole-pattern-fitting least-squares
technique. It uses the entire pattern rather than a limited
number of reflections, and a model that includes the
crystal structure for each phase, the pattern background,
and peak width and shape parameters. Because the
methods uses all lines, severely overlapping reflections are
not a problem, and errors due to orientation and
extinction are minimized.
An example is shown on raw bauxite materials in
figure 5 and table 1.
Figure 3: Continuous 5 min scan of different bauxite extracted from the same
mining region in Australia
Depending on the proportional mineral content of the
bauxite ore, the dissolution and extraction of alumina in
the Bayer process is appropriately adjusted. The clay and
diaspore contents of the bauxite need to be low if the
resource is to be economically viable because it is expensive
to separate from alumina in the refining process. For this
reason, quantitative analysis has also to be performed.
An example of phase identification with WinXRD
software is reported in figure 4. For a more efficient
search/match, bauxite database can be created with the
software.
Figure 5: Examples of phase quantification in bauxite sample: Siroquant
calculates a theoretical XRD profile (red curve) and fits it (second box: Fit
result) to the measured pattern (yellow curve) by full-matrix least-squares
refinement of the following Rietveld parameters: phase scales, line
asymmetry, phase preferred orientation, phase line widths (U,V,W),
instrument zero, the line shape parameter for each phase, and the phase
unit cell dimensions
BAUXITE 1
Figure 4: Identification of bauxites: blue curve is the raw scan and sticks
correspond to identified phases via the search/match procedure in WinXRD
As soon as you have identified phases, quantitative
XRD analyses can be carried out. Four different methods
are available in basic WinXRD software. Some of them
BAUXITE 2
Clay
Gibbsite
Hematite
Goethite
Diaspore
Boehmite
Anatase
Rutile
%wt.
6.7
42.3
21.1
30
-
Error
0.67
0.54
0.26
0.37
-
%wt.
18.7
4.1
56
16.6
2.6
0.6
Error
0.17
0.23
0.28
0.19
0.11
0.11
Calcite
-
-
1.4
0.2
Table 1
Residue analysis is often used to estimate the efficiency of
the process from bauxite to α-alumina change and phase
identification can be carried out with the ARL X’TRA
powder diffraction system (see figures 6 and 7).
Two different detectors -scintillation and solid-state
detector- has been used to evaluate the best configuration
for this kind of application.
Once scans were obtained (see figure 8), peak areas at
25.58° 2Θ after background subtraction have been
measured.
Figure 6: Continuous scan of different bauxite residues
a)
Analysis of bauxite residues and alumina after the
Bayer process
b)
Figure 8: Angular range analysis for α alumina quantification in a) unknown
sample and b) in standard sample.
Table 2 below summarizes α-alumina content obtained in
test sample and measurement reproducibility for 7 analyses.
DETECTOR
MEAN α ALUMINA CONTENT %
ACCURACY %
4.64
4.19
3.73
0.92
0.69
0.43
Scintillation + Ni
Scintillation
Solid state
Table 2
Bath ratio indication
Figure 7: Identification of bauxite residues: blue curve is the raw scan and
sticks correspond to identified phases via the search/match procedure in
WinXRD
Regarding α-alumina quality, quantification based on
Australian standard norm, AS 2879.3, has been performed
with the ARL X’TRA and WinXRD software. This norm
is related to intensity measurement for the calculation of
the peak area of the (012) reflection (~25.58° 2Θ for
CuKα). α − alumina content will be calculated by the
following equation:
The electrolyte used is cryolite (Na3AlF6), which is the
best solvent for alumina. To improve the performance of
the cells, various other compounds are added including
aluminium fluoride and calcium fluoride (used to lower
the electrolyte's freezing point). The ratio of sodium
fluoride to aluminum fluoride in the cryolite bath changes
over time and corrective additions are added based on
laboratory analyses, which measure LiF, CaF2 and excess
AlF3 in samples. However, some other phases (weberite,
neyborite, simmonsite, ..) can be found in certain
condition and can influence the bath ratio (see figures 9
and 10). Another point is sometimes the spinel presence,
which does not influence the bath ratio but has to be taken
into account for total Mg and Al content estimation.
In addition to these
offices, Thermo Fisher
Scientific maintains
a network of representative organizations
throughout the world.
Figure 9: Continuous scan of different bath.
Figure 10: Identification of bath: blue curve is the raw scan and sticks
correspond to identified phases via the search/match procedure in WinXRD
Via a standarless method it is easy to quantify different
phases, in one step, as shown in table 3.
PHASES
Chiolite Na5Al3F14
Corundum Al2O3
Cryolite Na3AlF6
Lithium Fluoride LiF
Sodium Calcium
Aluminiun Fluoride
AlF3, etc..
Fluorite CaF2
WEIGHT %
#15
ERROR
WEIGHT %
#11
ERROR
27
2.1
55.2
4.5
11.1
0.27
0.21
0.41
0.39
0.42
2.1
84.7
1.1
5
0.27
0.44
0.25
0.3
-
-
7.2
0.1
Table 3
Conclusion
Recent developments in X-Ray diffraction in optic
flexibility and especially in detector technology allow fast
scan acquisition with a high accuracy. High quality of data
acquisition allows high performance of data analyses by
using phase identification and quantification tools.
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