Iron Ore Analysis Using Simplified Borate Fusion and the WDXRF Analytical Method

The profitability of mining firms depends on assays, which are run on concentrates, exploration samples, reference materials, and internal standards. Even a small bias relating to the total iron analysis process can result in significant revenue loss.

The current International Standard Method, is the one entitled Iron ores -- Determination of Various Elements by X-ray Fluorescence Spectrometry -- Part 1: Comprehensive Procedure (ISO 9516-1:2003). However, there are a number of unveiled and corroborated limitations to the predominant version of this standard; it lacks adaptability when coping with recent advances in the fields of sample preparation by fusion and Wavelength Dispersive X-ray Fluorescence (WDXRF) spectrometry. Moreover, the preparation of standards made of pure oxides is a complex and time consuming task and the calibration ranges do not cover for the exploration samples.

Therefore, a single calibration for both the exploration samples and iron ore products is required in the mining industry. The industry’s Good Certified Reference Materials (CRMs) help in assessing a simplified calibration approach to the standard mixes of pure oxides.

However, because of the intricacy of the CRM matrix, the borate fusion preparation enables a more precise analysis and needs less calibration curves, as it capable of removing mineralogy and particle size effects, which are often encountered if pressed pellets were utilized.

In order to realize these goals, a strong analytical method using a WDXRF spectrometer and an automated fusion instrument as sample preparation tool has been optimized from the methodology specified in the ISO 9516-1 standard method for measurement of all reverent elements in the iron ore industry.

This approach was utilized to prepare fused disks from over 150 types of materials, covering a wide range of compositions. For calibration standards, a set of CRMs from over 10 suppliers were selected which facilitated a matrix match for global origin iron ores.

Experimental Framework

Apparatus and Instrumental Conditions

To produce all calibration standard fusion glass disks, M4^TM Claisse Fluxer^® (propane fired automatic Fluxer) was utilized. However, accurate evaluation was done using the M4TM and the TheOx^® Fluxers (Figure 1). The former with its auto-regulating gas system and the latter with its stable electric heating system have been designed with pre-set fusion programs to enable reproducible and repeatable fusion conditions and to retain the volatile elements.

Figure 1. Claisse^® TheOx^® Fluxer

For moisture and LOI determinations, a Fisher Scientific^® drying oven and a Fisher Scientific Isotemp^® programmable muffle furnace were utilized, respectively. For data generation, a Bruker-AXS S4 Explorer sequential WDXRF spectrometer with a rhodium end-window X-ray tube was employed.

A collimator mask and vacuum were utilized for all the measurements. Parameters such as spectrometer analytical conditions, background position, background measurements, peak-line, counting time, pulse-height and so on were chosen and optimized in accordance with the ISO 9516-1 specification and by wavelength step-scanning of the chosen standard disks. The ISO 9516-1 also included a spectrometer setup and performance evaluation guide, which was utilized to validate the correct operation of the spectrometer.

Sample Preparation Method

The optimization of the sample preparation was carried out both on the exploration samples and the iron ore using most of the parameters specified in ISO 9516-1. Nevertheless, some parameters had to be changed to obtain a broad calibration.

The platinum ware employed for this study was made from 5% gold and 95% platinum. It was shown that the optimized fusion approach can be utilized to prepare glass disks. The sample to flux ratio as illustrated in the ISO standard was maintained at 1:10.3. Samples can be combined on a dry basis or as-received basis. Here, the «Catch Weight» correction was applied.

The typical method proposes the use of the following fluxes:

• Pure Lithium Tetraborate
• Pure Sodium Tetraborate
• Mix of 35 % Lithium Tetraborate with 65 % of Lithium Metaborate

The flux utilized was 50% Lithium Tetraborate with 50% of Lithium Metaborate. Although Sodium Nitrate (NaNO₃) was recommended by the standard method, Ammonium Nitrate (NH₄NO₃) was proposed for this study as it allowed Sodium analysis.

Finally, a VortexMixer^TM was utilized to combine the flux and sample before the fusion and its speed was controlled to prevent loss of material which may lead to erroneous results. The temperature range of the fusion process was maintained from 1000 to 1050°C. This process had pre-programmed steps with fixed times to achieve excellent accuracy.

Preparation for the Calibration with CRMs and the Selection of Control Samples

Over 80 CRM preparations were created to choose the optimum set of standards for calibration of borate fusion and XRF analytical application for exploration samples and iron ores. The CRMs thus chosen were prepared in duplicates using an M4^TM gas Fluxer to validate the accuracy of sample preparation across the broad range of composition. Table 1 illustrates the element concentrations for the two CRM sets and also for the worldwide application.

Table 1. CRM sets element composition

This analytical approach for exploration samples and iron ore products demonstrated good efficiency to prepare uniform and stable lithium borate glass disks, although limitations did exist during preparation of iron ore-related materials containing high Copper levels.

Results and Discussion

Precision and Accuracy

For the accuracy evaluation, the M4^TM gas Fluxer and the TheOx^® electric Fluxer were used to produce 12 glass disks. A high iron magnetite was chosen for the accuracy evaluation. The precision evaluation was then performed using four control samples (CRMs). Two of these results are given below. Table 2 shows the standard deviation limit quantified for all the elements. The values achieved for all the elements have met the specified limits.

Table 2. ISO 9516-1 precision test results

Precision validation was assessed on two different calibration curves. This helped in comparing the performance of the CRM-based calibration against the reference method using pure oxides. The precision results illustrated in Tables 3 and 4 show that both calibration approaches enable similar precision levels for all the assessed elements and across different types of iron ore-related materials.

Table 3. Accuracy evaluation for JK 42 control sample

Table 4. Accuracy evaluation for FER-2 control sample

The major advantage of employing a CRM-based calibration is the easy calibration steps. In contrast, the pure oxide calibration approach requires a number of steps to prepare the mixes of different oxide powders to create numerous calibration points. This strategy is also costly, takes significant amount of time, and leads to manipulation errors.

Conclusion

This study assessed a new borate fusion and XRF application to match the exact requirements and anticipations of the iron ore industry with regard to the elemental characterization of exploration minerals and iron ore material.

When compared to the standard pure oxide calibration proposed in the ISO 9516-1 international test method, a CRM-based calibration offers the same level of accuracy for iron ore samples.

Besides providing high analytical performance, the CRM-based calibration enables extended calibration ranges, streamlines the preparation of the calibration standard fused disks, and makes it possible to analyze Sodium. All these factors translate into reduced time and reduced work load.

The study also shows that the TheOX^® electric Fluxer and the M4^TM gas Fluxer offer similar analytical performance and have the ability and versatility to combine the entire range of samples.

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

For more information on this source, please visit Claisse.