Analysis of Phosphate Rocks Using EDXRF

Phosphate rocks are the major source of phosphorous containing materials for various industrial applications. One such phosphorous source is apatite, which contains a high proportion of calcium phosphate. This mineral is widely used in the manufacture of phosphoric acid, which is the raw material for the manufacture of phosphate fertilizers.

The continuously declining trend of available high-grade phosphate sources increases the importance of the beneficiation of lower grade ores by washing, flotation and calcining. The growing demand for economical process optimization also makes the analysis of major and minor elements in phosphate rocks essential (Figure 1).

Open pit of a phosphate mine

Figure 1. Open pit of a phosphate mine

X-ray Fluorescence Spectroscopy (XRF)

XRF is the ideal method for such elemental analysis as it is rapid, accurate, straightforward, and requires simple sample preparation. Of the two key types of XRF spectrometer, energy dispersive X-ray fluorescence spectrometry (EDXRF) is widely used for such analysis.

In apatite, the most common impurity is fluorapatite, which releases hydrogen fluoride as a byproduct during phosphoric acid production. The hydrogen fluoride is later used in the production of hydrofluoric acid.

In that regard, gaining information about the fluorine concentration in phosphate rocks is helpful for an optimized process control. However, conventional EDXRF spectrometers show very poor sensitivity for light elements such as magnesium (Mg, Ka1 = 1.25keV) and fluorine in particular, due to absorption of low energy fluorescence from such elements by their thick detector windows.

Conversely, the combination of modern detector technology, thin stabilized entrance windows, and high power direct excitation provides far superior capabilities to the EDXRF spectrometer S2 RANGER equipped with the XFlash® LE detector compared to traditional EDXRF spectrometers. This article discusses the analysis of major and minor elements in phosphate rocks, fluorine in particular, using the S2 RANGER with XFlash® LE detector and Ag target X-ray tube.

Instrumentation

An S2 RANGER with XFlash® LE silicon drift detector (SDD) was used to perform the measurements. It is an all-in-one benchtop system featuring a user-friendly touchscreen interface, TouchControlTM. The ultrathin, high transmission entrance window equipped with the XFlash® LE SSD substantially improves light element sensitivity.

The use of an Ag target X-ray tube in place of the standard Pd target X-ray tube prevented overlays with tube lines, helping obtaining optimum results for these light elements. The spectrum of the fluorine standards acquired with the Ag target X-ray tube is presented in Figure 2. The spectrum reveals the clearly resolved F signal at 0.677keV, demonstrating the ability to the setup to evaluate even very low concentrations of fluorine.

Spectrum of different CRM standards for fluorine In the low energy range of 0.45 to 1.25 keV, concentration range shown from 0.51 to 4.04 % F

Figure 2. Spectrum of different CRM standards for fluorine In the low energy range of 0.45 to 1.25 keV, concentration range shown from 0.51 to 4.04 % F

Experimental Procedure

Sample Preparation

The sample preparation for each sample involved careful mixing of 10g sample with 1g of Hoechst Wax C, followed by pressing the mixture in a semi-automatic press from Herzog at 150kN to produce a 40-mm-diameter pellet. The high energy and the low energy lines were excited using the two measurement regions.

Measurement Parameters

The detailed measurement parameters are summarized in Table 1. All measurements were carried out under vacuum. It took 7 minutes to complete for the overall processing per sample, including sample handling, evacuating the sample chamber and actual counting time for the measurement.

Table 1. Measurement parameters for the different elements

Elements Tube voltage [kV] Tube current [µA] Filter Measurement time [s]
F, Na, Mg, Al, Si, P 10 600 None 200
K, Ca, Ti, Fe, Sr 40 900 500 µm Al 200

Calibration

A set of 13 internationally certified standard reference materials (CRM) have been employed to set up the calibration for the different major and minor elements. The wide element concentration ranges for the different major and minor elements in phosphate rocks are shown in Table 2.

The calibration curve for P2O5, Ca, and F is shown in Figures 3, 4, and 5, respectively. The established calibration curve for fluorine enables performing concentration measurements down to 0.5% F.

Table 2. Concentration ranges of elements in certified reference materials used for the calibration

Element Minimum concentration [%] Maximum concentration [%]
F 0.50 5.45
Na2O 0.06 0.52
MgO 0.03 20.4
Al2O3 0.05 19.5
SiO2 0.04 71.5
P2O5 0.01 40.0
K2O 0.10 2.60
CaO 0.08 52.6
TiO2 0.02 2.50
Fe2O3 0.23 25.7
SrO 0.06 0.48

Calibration curve for P2O5

Figure 3. Calibration curve for P2O5

Calibration curve for CaO

Figure 4. Calibration curve for CaO

Calibration curve for F

Figure 5. Calibration curve for F

Experimental Results

Once 13 CRM standards were calibrated, measurement of different apatite samples was performed against the calibration. This was followed by the determination of the loss on ignition (LOI) of the samples in a muffle furnace at 950°C. During routine analysis, the SpectraEDX software prompts measuring the LOI value of the sample automatically. Table 3 and 4 summarize the chemical composition for some samples.

Table 3. Chemical composition for some selected apatite samples for the major elements

Sample CaO [%] P2O5 [%] SiO2 [%] Al2O3 [%] F [%] Fe2O3 [%]
Apatite 1 42.4 33.3 10.3 3.81 3.42 1.98
Apatite 2 42.6 35.4 7.04 4.29 3.75 2.12
Apatite 3 42.1 35.2 7.75 4.32 3.63 2.11
Apatite 4 42.3 33.3 10.2 3.86 3.13 2.01

Table 4. Chemical composition for some selected apatite samples for the minor elements

Sample MgO [%] TiO2 [%] K2O [%] SrO [%] LOI [%] Sum [%]
Apatite 1 0.75 0.486 0.317 0.141 2.56 99.5
Apatite 2 0.66 0.468 0.269 0.151 3.32 100.1
Apatite 3 0.65 0.453 0.275 0.151 3.34 100.0
Apatite 4 0.78 0.453 0.313 0.140 3.41 99.8

The samples were measured for 10 times in order to demonstrate the precision of the system. Table 5 shows the measurements of apatite sample 1 are presented for some selected elements. The precision data demonstrates the outstanding repeatability of the instrument, for major elements as well as minor elements in lower concentrations.

Table 5. . Precision test of ten repetitive measurements for the major elements of the apatite sample 1

Sample CaO [%] P2O5 [%] SiO2 [%] Al2O3 [%] F [%] Fe2O3 [%] MgO [%] TTiO2 [%]
Sample 1, rep. 1 42.79 33.33 10.26 3.80 3.35 1.99 0.76 0.481
Sample 1, rep. 2 42.71 33.38 10.31 3.82 3.42 1.99 0.78 0.479
Sample 1, rep. 3 42.44 33.41 10.29 3.83 3.36 1.97 0.78 0.474
Sample 1, rep. 4 42.55 33.41 10.34 3.81 3.30 1.99 0.77 0.473
Sample 1, rep. 5 42.48 33.41 10.31 3.80 3.24 1.99 0.77 0.478
Sample 1, rep. 6 42.69 33.40 10.31 3.82 3.42 1.99 0.76 0.483
Sample 1, rep. 7 42.80 33.42 10.32 3.83 3.38 1.98 0.77 0.486
Sample 1, rep. 8 42.79 33.40 10.32 3.82 3.35 1.99 0.78 0.486
Sample 1, rep. 9 42.63 33.40 10.32 3.83 3.50 1.98 0.76 0.481
Sample 1, rep. 10 42.75 33.40 10.33 3.81 3.31 1.99 0.77 0.487
Mean value [%] 42.66 33.40 10.31 3.82 3.36 1.99 0.77 0.48
Abs. std. dev. [%] 0.13 0.03 0.02 0.01 0.07 0.01 0.01 0.00
Rel. std. dev. [%] 0.31 0.08 0.22 0.30 2.17 0.35 1.06 1.02

Conclusions

EDXRF is a valuable tool to determine major and minor elements in phosphate rocks rapidly and reliably. However, conventional EDXRF finds difficulty in determining fluorine, which is a very light element.

Conversely, S2 RANGER determines light elements very accurately and precisely, thanks to the direct excitation technique, the excellent resolution of the SDD, and the ultrathin high transmission window.

The results presented in this article demonstrate the superior analytical performance of the S2 RANGER equipped with XFlash LE and its applicability for monitoring phosphate rocks in process and quality control.

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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