Empirical Analysis of Limestone Using EDXRF

Limestone has found use as construction material, in the production of glass, mortar, cement and quicklime, as a basic aggregate in road construction, as a reagent in blast furnaces for iron extraction from ore, and as a scrubber stone in the desulfurization of flue gas. Purified limestone is used as a whitener in cosmetics and as a calcium supplement in foods and animal feeds.

Figure 1.

Since the applications of limestone are vast, precise characterization and quality control is a key element in the mining and processing of limestone. This article demonstrates the application of the NEX CG energy dispersive X-ray fluorescence (EDXRF) analyzer from Rigaku to analyze limestone using the empirical approach.

Experimental Setup

The Rigaku NEX CG EDXRF Analyzer

Figure 2. The Rigaku NEX CG EDXRF Analyzer

Model: Rigaku NEX CG
Detector: High performance SDD
X-ray tube: 50 W Pd-anode
Excitation: Indirect with polarization
Analysis Time: 140 sec
Environment: Helium Purge (optional vacuum available)
Standard: 15-position Sample Tray (32mm)

Sample Preparation

Using a ball mill or ring-and-puck shatterbox, samples were ground to a fine, dry, homogeneous powder of <200 mesh (<75 µm particle size). The powdered samples were then pressed into pellets using a hydraulic press at a pressure of 20 tons for 30 seconds. This allows for the optimal measurement of light elements, particularly Mg.

Calibration

A suite of eight assayed standards, including NIST SRM 1d (Limestone, Argillaceous), was used to develop empirical calibrations. The standards represent the limestone ores that can be obtained from the mine site. Variations in X-ray absorption and enhancement effects caused by independent variations in element concentration were compensated by employing ‘alpha corrections’ through the use of the empirical approach. This yielded a highly precise model that characterizes the ore type. The results of each empirical calibration are listed in Table 1.

Table 1. Eight Standards

Component Concentration Range (%) RMS Deviation R2 Confidence
MgCO3 0.53-8.66 0.344 0.9937
Al2O3 0.19-2.35 0.031 0.9991
SiO2 0.95-8.67 0.168 0.9981
CaCO3 84.95-95.12 0.294 0.9980
Fe2O3 0.12-0.86 0.006 0.9998

Instrument Precision

The repeatability or precision of the instrument was demonstrated by selecting one representative sample from the suite of calibration standards. Then, the measurement of each sample was performed in static position for ten repeat analyses utilizing a measurement time of 140 seconds per analysis. The repeatability test results are listed in Table 2 and Table 3.

Table 2. Sample: NIST SRM 1d

Component Assay Value (%) NEX CG Average Result Standard Deviation % Relative
MgCO3 0.63 0.64 0.047 7.5
Al2O3 0.526 0.535 0.010 1.9
SiO2 4.08 3.99 0.035 0.9
CaCO3 94.32 94.57 0.23 0.2
Fe2O3 0.136 0.138 0.002 1.5

Table 3. Sample: Quarry Sample 7A

Component Assay Value (%) NEX CG Average Result Standard Deviation % Relative
MgCO3 1.64 1.69 0.022 1.3
Al2O3 0.29 0.26 0.004 1.4
SiO2 2.58 2.50 0.020 0.8
CaCO3 95.12 95.32 0.19 0.2
Fe2O3 0.120 0.127 0.002 1.7

Typical Detection Limits

Using the empirical approach, the lower limit of detection (LLD) was determined by measuring ten repeat analyses of a ‘blank’ sample and calculating the standard deviations. Ti simulates a blank sample of limestone having high calcium content. CaF2 powder was selected as the blank and pressed into a pellet. The LLD is set as three folds the standard deviation. This method ensures that analyses above the defined LLD are determining signal above background. Employing the same measurement time utilized for calibration and repeatability, the LLDs for the components were determined, as shown in Table 4. Actual detection limits may differ with combinations of elements present, concentration levels of elements, and the analysis time employed.

Table 4. The LLDs shown here represent detection limits in a matrix contain very high calcium using an analysis time of 140 sec.

Component LLD (%)
MgCO3 0.12
Al2O3 0.007
SiO2 0.009
Fe2O3 0.001

Results

A typical method to create the most accurate empirical calibrations involves the development of a suite of calibrations from actual quarry samples. It is possible to expand and augment the initial calibrations utilizing additional assayed samples in order to optimize the model of the matrix and to compensate for changes in limestone composition. The use of longer analysis times can improve limits of detection, precision and performance.

The results demonstrated in this article represent performance in a real-world condition that requires a short analysis time of 140 seconds. The elements were measured using two secondary targets. The RX9 polarizer that was set to 100 seconds is suitable for light elements (Na-Cl) and a count time of 40 seconds was used to employ the Cu target. Precision and detection limits can be improved by a factor of two by quadrupling the analysis time to 560 seconds.

Sodium content in limestone is rarely analyzed for a few specialty limestone products. If sodium is the element to be analyzed, then an optional secondary target can be employed with the Rigaku NEX CG to measure Na and Mg.

Conclusion

With a high performance SDD detector, polarization targets, and indirect excitation with secondary targets, the Rigaku NEX CG analyzer is a powerful tool to perform elemental analysis of limestone. It is essential to reliably monitor the oxide composition of the limestone material during the quarrying and processing cycle in order to obtain optimal process control and profitability. The Rigaku NEX CG analyzer is the suitable solution that can handle these analyses throughout the quality control process. Moreover, the Rigaku UltraCarry can be utilized for monitoring process effluents to ppb range detection limits, and NEX CG can be utilized for monitoring air filters for air quality emission control. This makes the NEX CG a versatile and robust tool for numerous limestone mining and processing applications.

This information has been sourced, reviewed and adapted from materials provided by Rigaku Corporation.

For more information on this source, please visit Rigaku Corporation.

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