Using XRF to Grade Bauxite for Aluminium and Refractory Production

Aluminium has become very important for packaging, construction, aircraft and car manufacture especially due to its light weight. The most important aluminium ore is bauxite and comprises several minerals.

The Bayer process uses a typical bauxite grade that has 50 – 55 % Al2O3, SiO2 and Fe2O3 at maximum concentrations of 1.5% and 30% respectively. Bauxite having a higher concentration of SiO2 can be used only for the production of abrasives, cement, refractories and chemicals. Hence its commercial value does not depend on the aluminium concentration alone but on the full elemental composition.

Wavelength Dispersive X-Ray Fluorescence (WDXRF) Spectrometry

Wavelength dispersive X-ray fluorescence (WDXRF) spectrometry is an ideal method for monitoring mining operations and grade control because of the simple integration in industrial processes and the advanced analytical performance, especially for the determination of major and minor light elements.

Advantages of WDXRF Spectrometry

The key advantages of WDXRF Spectrometry are listed below:

  • The fusion method is used with WDXRF for sample preparation enabling high accuracy and precision, which is far better than other conventional technologies based on digestion preparation methods.
  • The instrument is calibrated only once during installation, hence it perfectly fits in industrial processes with a standardized quality
  • Only during maintenance intervals quick drift monitoring procedures are needed whereas other techniques run complete calibration on a routine basis
  • WDXRF can be run in daily routine by non analytical experts

This report illustrates the analytical performance of the WDXRF spectrometer S8 TIGER in combination with the GEO-QUANT M solution for the grade control of bauxite.

S8 TIGER WDXRF Spectrometer

The S8 TIGER WDXRF spectrometer is provided in three different power versions 1, 3 and 4 kW. The excitation power finally ascertains the time-to-result and maximum sample throughput. The shortest measurement time for a specified analytical precision is achieved by using 4 kW excitation power. The high intensity end window X-ray tube with Rhodium target has a very close coupling between anode, tube and sample resulting in optimal excitation of all elements in the specimen. The S8 TIGER can be fitted with up to eight analyzer crystals and four collimators so that every element and concentration range has the best sensitivity and resolution.

Sample Preparation and Measurement Parameters

The calibration of the S8 TIGER was performed with the GEO-QUANT M solution. The details of the method are enumerated below:

  • This method is dedicated for the optimum determination of major and minor elements as oxides in geological materials.
  • It provides optimized measurement parameters for tube voltage, crystals and collimators.
  • It is based on more than 20 certified geological reference materials (CRM) GEO-QUANT M covers a wide concentration range for the elements Na, Mg, Al, Si, P, S, K, Ca, Ti, Mn and Fe. The specimens and samples were prepared as fused beads by using 0.8 g sample with 8.0 g lithium tetraborate as flux.
  • Generally fusion furnaces are used with gas, heat or electricity. Modern furnaces having induction heating are ideal for preparing bauxite samples. T
  • The specific flux recipe and the temperature programs for various fusion models are provided with GEO-QUANT M.
  • The measurements for this report have been performed on the S8 TIGER at 4 kW. The total measurement time at peak height and background positions for 11 elements was 6.5 minutes.

Results

Five bauxite materials with certified values GBW 07178, GBW 07180, GBW 07181, GBW 07182 and BA-H have been prepared as fused beads and are used to study the GEO-QUANT M calibration. Figure 1 shows the linking between the calibration standards and the five reference materials.

Calibration curve of Al2O3

Figure 1. Calibration curve of Al2O3

The results of the accuracy test for all five samples are compared with the certified concentrations in Table 1.

Table 1. Results of the accuracy trial of GEO-QUANT M for five samples

  GBW 07178 GBW 07180 GBW 07181 GBW 07182 BA-H
Meas. Cert. Meas. Cert. Meas. Cert. Meas. Cert. Meas. Cert.
Na2O 0.10 0.07 0.04 0.04 0.09 0.05 0.09 0.06 0.11 0.04
MgO 0.31 0.26 0.33 0.31 0.21 0.08 0.21 0.10 0.62 0.52
Al2O3 54.70 54.94 42.90 42.97 91.07 90.63 74.69 75.13 50.52 50.72
SiO2 15.18 15.24 38.56 39.03 3.60 3.20 19.22 19.44 6.46 6.63
P2O5 0.23 0.21 0.15 0.14 0.20 0.19 0.17 0.16 0.11 0.09
SO3 0.16 0.11 0.05 0.07 0.03 T 0.02 T 0.21 T
K2O 0.33 0.31 0.22 0.19 0.07 0.06 0.18 0.17 0.05 0.04
CaO 2.30 2.22 0.09 0.12 0.09 0.14 0.12 0.16 0.64 0.67
TiO2 2.47 2.46 2.06 2.06 3.88 3.80 3.28 3.22 2.39 2.49
MnO 0.03 0.03 T T T T T T 0.12 0.13
Fe2O3 8.92 9.04 0.37 0.41 1.29 1.31 1.22 1.24 22.65 22.59

The typical absolute deviation is below 0.05% for minor oxides and the relative deviation is typically below 0.1 % for major oxides Al2O3, SiO2 and Fe2O3

To demonstrate the short time stability of the method, the certified standard GBW 07178 had been measured 20 times. Results are summarized in Table 2.

Table 2. Short term precision trial for sample GBW 07178, all values shown in % 20 measurements, alternated with another sample

  Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO Fe2O3
Certified concentrations 0.07 0.26 54.94 15.24 0.21 0.11 0.31 2.22 2.46 0.03 9.04
Average 0.10 0.31 54.70 15.17 0.23 0.17 0.33 2.31 2.47 0.03 8.93
Min. 0.08 0.31 54.60 15.10 0.23 0.16 0.33 2.30 2.46 0.03 8.92
Max. 0.10 0.31 54.78 15.22 0.24 0.17 0.33 2.31 2.47 0.03 8.94
Std. dev. 0.01 0.00 0.04 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.01
Relative SD 5.81 0.00 0.08 0.20 1.41 1.18 0.00 0.18 0.15 0.00 0.10

This was also repeated 18 times for a period of 30 days to show the repeatability and reproducibility of the method and the long time stability of the S8 TIGER. Results are summarized in Table 3.

Table 3. Long-term precision trial for sample GBW 07178, all values shown in % 18 measurements, alternated with another sample

  Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO Fe2O3
Certified concentrations 0.07 0.26 54.94 15.24 0.21 0.11 0.31 2.22 2.46 0.03 9.04
Average 0.09 0.31 54.62 15.20 0.23 0.17 0.33 2.31 2.47 0.03 8.92
Min. 0.08 0.30 54.56 15.15 0.23 0.17 0.33 2.30 2.46 0.03 8.89
Max. 0.10 0.31 54.70 15.23 0.24 0.18 0.33 2.31 2.47 0.03 8.99
Std. dev. 0.01 0.00 0.04 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.03
Relative SD 6.13 1.01 0.07 0.16 1.69 2.52 0.11 0.16 0.11 0.53 0.28

Conclusion

The analytical performance of the WDXRF spectrometer S8 TIGER in combination with the GEO-QUANT M solution for the grade control of bauxite has been demonstrated. From the results, it is clear that the method displays short-term and long-term stability as well as reproducibility and repeatability.

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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