Bauxite - Phase Analysis Using Quantitative XRD

TOPAS Rietveld analysis is being used as a routine tool in quantitative phase analysis of powder samples. XRD data measured in a quick time-span with the LynxEye detector, the use of co-radiation and the TOPAS software enables a quick and accurate quantitative investigation of multi-phased samples containing iron. In this experiment, 10 mineral phases in bauxite have been quantified together with the determination of the crystallite sizes. Also the precision of the technique is evaluated.

Reason for Studying the Phase Content of Bauxite

The primary compounds in bauxite include hydrous aluminum oxides, iron and titanium oxides, quartz, and other silicate species. The name “bauxite” is commonly used for hydrous clay rocks of varied composition and geological origins. Bauxite is the major source for industrial aluminum production.

Preparation of Aluminum

Alumina (Al2O3) is prepared from raw bauxite by a hydrometallurgical technique named after its inventor the “Bayer process”. Bauxite is leached under high temperature and pressure with sodium hydroxide solution and insoluble material also termed as red mud is separated to form a solution that is seeded to precipitate aluminum hydroxide that finally is calcined to alumina. After that, alumina is subjected to electrolytic reduction to obtain pure aluminum.

It is important to know the composition and mineralogy of bauxite deposits in order to evaluate the ‘processability’ of the material. This characterization is required for alumina producers and companies who want to develop bauxite deposits. Detailed mineral characterization of bauxites can lead to the creation of effective processing options and can also be used to evaluate bauxite beneficiation techniques for ore quality enhancement before processing.

DIFFRACplus TOPAS Quantitative Rietveld Phase Analysis

XRD is the most precise and direct analytical technique for ascertaining the presence and the absolute amounts of mineral species in a sample.

There are several benefits of the Rietveld phase analysis over conventional methodologies. They are listed below:

  • Full pattern quantitative phase analysis with the Rietveld method does not need time consuming calibration.
  • Multi-phase samples are easily studied without being constrained by peak overlap.
  • The adding of new phases present in qualitative XRD is straightforward.
  • Additionally, crystallinity and crystallite size that affect the reactivity of the mineral components can be derived from the peak profiles simultaneously.

Quick and dependable Rietveld-based quantitative analysis became routinely possible by integrating rapid advanced computer technology and optimized mathematical algorithms with basic parame ters approach in the DIFFRACplus TOPAS software.

Accuracy: the CPD Round-Robin Data

Synthetic bauxite XRD data provided by the commission on powder diffraction (CPD) of the IUCr were tested using TOPAS. The outstanding performance of TOPAS and the high quality of the analysis directly follows from agreement of the computed composition with the anticipated results (straight line). The results obtained are listed below:

  • The variations between the TOPAS values and data obtained from weighing are less than 1 % for the minor phases.
  • The highest deviations are 1.8 and 2.5 % for boehmite and gibbsite, respectively.
  • Other participants of this round robin analysis show systematic deviations of 1 – 5 wt-% for the minor components and about 15 wt-% for gibbsite for the CPD data set.
  • TOPAS may partially compensates for deficiencies in the preparation of the powder sample by its advanced micro-absorption, texture and peak shape abilities.

The result of the round robin analysis is shown in Figure 1.

Comparison of the TOPAS quantitative results with the expected values from weighing. The straight line (1:1 relation) corresponds to perfect agreement.

Figure 1. Comparison of the TOPAS quantitative results with the expected values from weighing. The straight line (1:1 relation) corresponds to perfect agreement.

Bauxite from Turkey

Experiment

Samples obtained from ETI Aluminium (Turkey) were examined with a D8 ADVANCE diffractometer equipped with an automatic sample changer.

Measurement devices include the following:

  • Co Ká-radiation, Fe K&946;-filter
  • LynxEye detector
  • 4° Soller collimators
  • Counting time 0.5 s/step, range 5 – 100°, step 0.02°, total measuring time per sample about 50 min.

Instead of copper radiation, cobalt radiation was selected to avoid microabsorption occurring due to Fe-bearing phases shown in Table 1

Table 1. Mass absorption coefficients in cm2/g of selected compounds for Cu and Co Ká- radiation.

Composition Cu Ká Co Ká
Fe2O3 224.1 44.5
FeO(OH) 202.3 41.5
AlO(OH) 27.2 42.4
Al(OH)3 23.1 36.1
TiO2 127.2 191.7
CaCO3 69.8 105.4
SiO2 33.6 52.2
Al2Si2O5(OH)4 28.9 45.1

Results

In Figure 2, the results of the quantitative analysis with TOPAS is displayed.

Quantitative phase analysis of bauxite using TOPAS4. The intensity is given in sqrt(I) units, the fit converged to

Figure 2. Quantitative phase analysis of bauxite using TOPAS4. The intensity is given in sqrt(I) units, the fit converged to Rwp=4.1, GoF=1.6

The results obtained are listed below:

  • Totally, 11 phases were used in the analysis. For all the minerals examined the scale factors and cell parameters were refined and the comparative intensities of the Bragg reflections were computed from the crystal structures.
  • The instrument contribution to the peak shape of all phases was modeled by the fundamental parameters approach, while the individual contributions from each phase were taken into account by a single crystallite-size parameter per phase.
  • Preferred orientation of kaolinite was rectified by 4th order spherical harmonics.
  • Quartz was detected in other samples from that commodity and hence included in the refinement.
  • The sample evaluated did not contain quartz (0 wt-%).
  • Major phases are quantified with high precession (better 0.5 wt-%), minor phases below 1% are also quantified.
  • The peak shape of the boehmite reflections showed the presence of various sizes of crystallites.
  • Two independent boehmite phases with common cell parameters were therefore refined resulting in crystallite-size fractions of about 35 and 400 nm covering 11.5% and 88.5% of boehmite, respectively.

All phases quantified for the example shown in Figure 2 are listed in Table 2.

Table 2. Quantitative TOPAS refinement results of bauxite from Turkey.

Phase name Composition Phase amount / wt-% Crystallite size / nm
Anatase TiO2 1.9(1) 139(8)
Bayerite AlO(OH) 1.2(1) 45(8)
Boehmite AlO(OH) 7.4(7) 35(3)
    57.4(8) 401(35)
Calcite CaCO3 1.2(1) 155(22)
Diaspore AlO(OH) 2.9(1) 58(12)
Gypsum CaSO4 · 2H20 0.6(1) 56(16)
Haematite Fe2O3 15.9(3) 35(1)
Kaolinite Al2Si2O5(OH)4 10.7(1) 30(1)
Rutile TiO2 0.7(1) 99(18)
Quartz SiO2 0.0 -

Conclusions

The use of co-radiation and the LynxEye detector is essential for quick measurement of XRD data suitable for very precise and accurate Rietveld refinement of iron-bearing samples. Convolution-based Rietveld profile fitting in combination with instrument function constraints enable complex phase mixtures to be quantified with TOPAS and to obtain additional microstructure information. As a result, TOPAS permits the routine analysis of complicated material from the Minerals and Mining industry.

This information has been sourced, reviewed and adapted from materials provided by Bruker AXS Inc.

For more information on this source, please visit Bruker AXS Inc.

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