Controlling of the Grade of Bauxite for Aluminum and Refractory Production by XRF

Aluminum is very important in construction, car and aircraft manufacturing, as well as in packaging due to its lightweight. The main ore of aluminum is bauxite, which is a combination of a number of minerals. The typical bauxite grade useable in the Bayer process consists of 50 - 55 % Al2O3, up to 30% of Fe2O3, and up to 1.5% of SiO2.

Bauxite with a concentration of SiO2 higher than 50% is suitable only for the manufacture of refractories, chemicals, abrasives, and cement. Hence, the commercial value of bauxite ores largely relies on both the concentration of aluminum and the complete elemental composition.

Wavelength dispersive X-ray fluorescence (WDXRF) spectrometry is the ideal technique to monitor mining operations and grade control due to its ability to be integrated easily into industrial processes, and its advanced analytical performance, especially in detecting major and minor light elements.

Advantages of WDXRF Spectrometry

The use of the fusion method for sample preparation enables WDXRF to provide more accurate results than other analytical techniques based on digestion preparation methods. The lifetime of the calibration is another key advantage of WDXRF. Once calibrated during the installation, WDXRF is perfectly suited for industrial processes with a standardized quality regime.

Quick drift monitoring procedures are needed only in maintenance intervals compared to the requirement of running complete calibration sets in other techniques on a daily basis, thereby enabling even non-analytical experts to run WDXRF in daily routines. This article discusses the grade control of bauxite using the WDXRF spectrometer S8 TIGER in conjunction with the GEO-QUANT M solution.

The S8 TIGER WDXRF Spectrometer

The S8 TIGER WDXRF spectrometer is offered in three different power versions: 1, 3 and 4kW. The time-to-result and maximum sample throughput are determined by the excitation power. The use of 4kW excitation power yields the shortest measurement time for a given analytical precision.

The high intensity end window X-ray tube with Rhodium target has a very close coupling between the anode, tube and sample, causing optimal excitation of all elements in the sample. It is possible to equip the S8 TIGER with up to eight analyzer crystals and four collimators to achieve the best sensitivity and resolution for each element and concentration range.

Experimental Procedure

The GEO-QUANT M solution was used to calibrate the S8 TIGER to effectively determine major and minor elements present as oxides in geological materials. This method yields optimized measurement parameters for tube voltage, collimators, and crystals. Based on over 20 certified geological reference materials (CRM), GEO-QUANT M encompasses a wide concentration range for the elements Fe, Mn, ti, Ca, K, S, P, Si, Al, Mg, and Na.

Taking 0.8g sample with 8.0g lithium tetraborate as flux, samples were prepared as fused beads. For this purpose, fusion furnaces are typically used in conjunction with gas or electrical heating.

Bauxite samples can also be prepared using modern furnaces with induction heating. The specific recipe of the flux and the temperature programs for different fusion models were supplied with GEO-QUANT M. The S8 TIGER 4kW was used to perform the measurements. The total measurement time at peak height and background positions for 11 elements was 6 minutes and 30 seconds.

Experimental Results

Five bauxite materials with certified values GBW 07178, GBW 07180, GBW 07181, GBW 07182 and BA-H were prepared as fused beads for evaluating the GEO-QUANT M calibration. The correlation of the calibration standards and the five reference materials is illustrated in Figure 1. Table 1 provides the comparison of the results of the accuracy test for all five samples.

Calibration curve of Al2O3: calibration standards evaluation samples.

Figure 1. Calibration curve of Al2O3: calibration standards evaluation samples

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 typical relative deviation for the major oxides (Al2O3, SiO2 and Fe2O3) is below 0.1%. The certified standard GBW 07178 was measured 20 times to illustrate the short time stability of the method.

This was also repeated 18 times for 30 days to demonstrate the repeatability and reproducibility of the method and the long time stability of the S8 TIGER. The corresponding results are given in Table 2 and Table 3, respectively.

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

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

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

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

Using the GEO-QUANT M solution, the S8 TIGER WDXRF spectrometer facilitates the analysis of Bauxite for grade control. The calibration with selected standard samples and the optimized measurement parameters enabled rapid and efficient setup of the S8 TIGER, saving significant time and money for method development.

The accuracy and precision trial results obtained with certified reference materials demonstrate that the combination of both products meets the prerequisites of the industry and service labs.

Relative standards deviations of better than 0.1 % for major oxides are outstanding. A diverse range of materials can be controlled, thanks to the wide calibration range of GEO-QUANT M, enabling faster grade control of bauxite. In addition, sorting of different grades can be done quickly due to the ability to obtain results immediately.

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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