Analysing Iron Ores as Pressed Pellets Using EDXRF

Iron ore is one of the key minerals present on Earth owing to ever-increasing demand in the steel industry. However, the production process or the final product quality can be negatively affected due to the interference of some of the components in the iron ore.

Hence, it is essential to monitor the chemical composition of iron ores for controlling the mining process (Figure 1) as well as optimizing the feeding process of melting furnaces during the manufacture of steels.

Loading of base metal ore

Figure 1. Loading of base metal ore

It is necessary to monitor both the major elements such as Fe as well as other elements in the iron ore, including P, S, Mn, Mg, Ca, Ti, and Si. Although the concentration of some of these elements is very low, their influence on the steel production process and the final product quality is significant.

A fast feedback of the chemical composition is often required to effectively control the mining process. Hence, the time taken between the sample collection and acquisition of the analytical results must be as short as possible.

This can be achieved by preparing the samples as pressed pellets, which is a simple method and consumes less time than the method of fused bead sample preparation. Consequently, results can be obtained sooner after the collection of samples.

In this article, the energy dispersive X-Ray fluorescence (EDXRF) spectrometer S2 RANGER was used to determine all the major and minor elements of interest, thus providing a rapid and simple measure of controlling the mining process.

Instrumentation

The S2 RANGER EDXRF system is an all-in-one benchtop system equipped with a user-friendly TouchControlTM interface, an XFlash® silicon drift detector (SDD) and a Pd target X-ray tube. In principle, measuring pressed pellets under various environmental conditions is possible. Hence, the measurement chamber is either flushed with helium, nitrogen, or air or is evacuated by the integrated vacuum pump.

However, air or nitrogen must be used in the measurement of light elements such as P or Mg due to absorption of the low energy radiation by these gases. This problem can be addressed with helium, but its increasing price is a concern due to the increased measurement cost per sample albeit consuming only small volumes.

To overcome these drawbacks, performing measurements under vacuum is preferred as it not only provides the best measurement conditions for all elements, but also avoids the consumption of high-cost helium. Since the S2 RANGER features an integrated vacuum pump, optimum analytical results can be obtained at a lower cost of ownership.

Experimental Procedure

Sample Preparation

The preparation of iron ore samples involved milling of 10g sample with three grinding aid pellets from Polysius, followed by pressing them using the automatic press APM from Polysius at 150kN in pressed pellets of 40mm diameter.

Measurement Parameters

After defining two measurement regions, the tube current was optimized and fixed so that maximum count rate can be gained for the different elements. The detailed measurement parameters are given in Table 1. All measurements were carried out under vacuum.

Table 1. Measurement parameters for the different elements

Elements Tube voltage [kV] Tube current [µA] Filter Measurement time [s]
Mg, Si, P, S 10 680 None 100
Ca, Ti, Mn, Fe 40 170 500 µm Al 100

Calibration

The calibration for Fe, Si, Mn, Mg, Ti, Ca, S and P was performed using a set of 10 in-house standards. An independent analytical technique was used to verify the chemical composition of the standards. The concentration ranges of the various iron ore standards used to perform the calibration are presented in Table 2.

Table 2. Concentration ranges used for the iron ore calibration

  Minimum Concentration [%] Maximum Concentration [%] Standard deviation of calibration curve [%]
Fe2O3 57.2 97.4 0.35
SiO2 0.73 42.1 0.30
MnO 0.022 6.88 0.008
MgO 1.27 5.62 0.03
TiO2 0.007 0.24 0.01
CaO 0.014 7.01 0.11
S 0.14 1.32 0.01
P 0.36 0.77 0.01

The standard deviation (SD) for the calibration curve obtained for the different elements are also presented in Table 2, providing a measure of the accuracy of the calibration. The low SD values in Table 2 prove the superior analytical performance of the S2 RANGER EDXRF system.

The calibration curves for the major oxides such as Fe2O3 and SiO2 are shown in Figures 2 and 3, respectively. The calibration curve for MnO in the lower concentration range is shown in Figure 4. It is essential to select standards to match the mineralogy of the materials of interest to obtain the optimum results.

Calibration curve for Fe2O3

Figure 2. Calibration curve for Fe2O3

Calibration curve for SiO2

Figure 3. Calibration curve for SiO2

It is possible to prepare secondary standards in the case of absence of suitable standards by analysing mine site samples with a fusion calibration. Then, pressed pellets of these materials can be used as the calibration standards for regular analysis at the mine site.

Extract of the low concentration range of the calibration curve for MnO (from 0.02 to 0.45 % MnO)

Figure 4. Extract of the low concentration range of the calibration curve for MnO (from 0.02 to 0.45 % MnO)

Experimental Results

The iron ore sample was measured for 10 times to prove the precision of the system. For each measurement, the sample was unloaded from and reloaded into the measurement chamber. Some characteristic results obtained from these measurements and the precision achieved on an iron ore sample are summarised in Table 3.

Table 3. Precision test of ten repetitive measurements of an iron ore sample

CRM SX56-16 Fe2O3 [%] SiO2 [%] MgO [%] MnO [%] TiO2 [%] CaO [%] P [%] S [%]
18.07.2013 09:01 83.26 10.67 4.14 0.417 0.188 0.094 0.448 0.230
18.07.2013 09:10 83.25 10.69 4.11 0.420 0.196 0.085 0.450 0.234
18.07.2013 09:20 83.31 10.67 4.10 0.426 0.182 0.088 0.450 0.240
18.07.2013 09:29 83.30 10.73 4.00 0.413 0.188 0.093 0.452 0.236
18.07.2013 09:38 83.29 10.72 4.00 0.419 0.188 0.087 0.453 0.240
18.07.2013 09:47 83.23 10.70 4.07 0.414 0.173 0.086 0.452 0.239
18.07.2013 09:55 83.29 10.69 4.03 0.416 0.182 0.087 0.452 0.233
18.07.2013 10:04 83.40 10.74 4.00 0.416 0.213 0.097 0.452 0.238
18.07.2013 10:13 83.21 10.72 4.14 0.417 0.183 0.079 0.451 0.237
18.07.2013 10:22 83.17 10.72 4.04 0.413 0.203 0.088 0.453 0.239
Mean value [%] 83.27 10.70 4.06 0.420 0.190 0.090 0.451 0.240
Abs. std. dev. [%] 0.06 0.02 0.06 0.00 0.01 0.01 0.00 0.00
Rel. std. dev. [%] 0.08 0.23 1.44 1.06 6.12 5.91 0.35 1.41

The for P2O5 for an iron ore sample is graphically represented in Figure 5. The red lines represent the three times of SD of the measurements. It is possible to define the threshold values within the instrument software for each element, thereby helping users to determine ‘out-of-spec’ samples.

Repeatability for a typical iron ore sample shown for P2O5

Figure 5. Repeatability for a typical iron ore sample shown for P2O5

Conclusion

The results clearly demonstrate the superior performance of the S2 RANGER EDXRF spectrometer equipped with XFlash® detector. In this experiment, eight most important elements present in iron ores were measured in the required concentration ranges, using a set of 10 standards.

The preparation of samples as pressed pellets makes the sample preparation method simple, rapid and straightforward by avoiding any time-intensive steps. As a result, analytical feedback can be immediately obtained for the mining process. The accuracy and precision achieved corroborate the applicability of the instrument for monitoring the chemical composition of iron ores.

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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