ISO 9516-1 Simplified borate fusion & WDXRF analytical method for iron ores analysis including exploration samples

Introduction

Mining companies that sell their products by the millions of tons could incur millions of dollars in revenue losses, if a slight bias related to the total iron analysis process were to be found. The profitability of any mining company depends on the assays that are run on exploration samples, concentrates, internal standards and/or reference materials. The current International Standard Method, which remains widely accepted among peers, 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.

The industry needs a single calibration for both the iron ore products and the exploration samples. This necessity forced us to rethink the calibration methodology that is described in the standard method. Good Certified Reference Materials (CRMs) from this industry are available all over the planet and allow the valuation of a simplified calibration strategy to the usual mixes of pure oxides. Due to the complexity of the CRMs matrix, using the borate fusion preparation allows for a more accurate analysis and requires less calibration curves because the technique removes particle size and mineralogy effects[1, 2] that would  be encountered if pressed pellets were used. To reach these objectives, a robust analytical method using an automated fusion instrument as sample preparation tool and a WDXRF spectrometer has been optimized from the methodology described in the ISO 9516-1 standard method for the quantification of all elements of interest in the iron ore industry. This single method was used to prepare fused disks from more than 150 types of materials (CRMs and various samples) covering a very vast range of compositions. A set of CRMs from more than 10 suppliers of different origins were selected as calibration standards which allowed a matrix match for worldwide origin iron ores. The evaluation of performance was executed using the new calibration approach and according to the instructions provided by the International Organization for Standardization (ISO), through the standard method for analysis of iron ores by X-ray fluorescence: ISO 9516-1^[3]. To better evaluate the accuracy of the CRM based calibration, a pure oxide calibration was prepared according to the ISO 9516-1 guidelines and served as a reference.

Experimental

Apparatus and instrumental conditions

A Claisse^® M4^TM propane fired automatic Fluxer was used to generate all calibration standard fusion glass disks, but the precision evaluation was run using both the M4^TM and the TheOx^® Fluxers; The M4^TM with its auto-regulating gas system and the TheOx^® with its stable electric heating system have both been designed with pre-set fusion programs allowing for the most repeatable and reproducible fusion conditions as well as the capability to retain the volatile elements perfectly.

A Fisher Scientific^® drying oven was used for moisture determinations. This method was used for all the samples and consisted in drying at 105°C in a clean ceramic/porcelain crucible for 120 minutes.

A Fisher Scientific Isotemp^® programmable muffle furnace was used for the LOI determinations. The LOI method used for all the samples was an ignition at 1000°C in a clean ceramic/porcelain crucible for 60 minutes. This method was executed after the moisture determination, using the same crucible and dried sample.

A sequential WDXRF spectrometer with a rhodium end-window X-ray tube was used for data generation. A 34 mm collimator mask and vacuum were used for all the measurements. Spectrometer analytical conditions, peakline, background measurements, background position, pulseheight, counting time and others were selected and optimized following the ISO 9516-1 recommendation and by wavelength step-scanning of selected standard disks. The spectrometer setup and performance evaluation guide also included in the ISO 9516-1 were used to verify proper spectrometer operation.

Sample preparation method

The optimisation of the sample preparation was performed on both the iron ore and the exploration samples using a majority of parameters as described in ISO 9516-1, but still some parameters had to be modified to achieve such a wide calibration.

The platinumware used for this project was made from 95 % platinum and 5 % gold as accepted by the ISO standard. It was proven that our optimized fusion method can be used to prepare glass disks with a diameter ranging between 32 to 40 mm. Nevertheless, all glass disks produced for this application had a 40 mm diameter. The sample to flux ratio was as described in the ISO standard and kept at 1: 10.3. Samples can be fused on an as-received basis or on a dry basis. The «Catch Weight» correction used for weighing was applied as described in the standard method. The standard method recommends using one of the three following fluxes: Pure Sodium Tetraborate, pure Lithium Tetraborate or a mix of 35 % Lithium Tetraborate with 65 % of Lithium Metaborate. The flux used for this study was 50 % Lithium Tetraborate with 50 % of Lithium Metaborate which made the preparation of exploration samples easier without decreasing the success rate of iron ore preparation. The oxidizer recommended by the standard method is the Sodium Nitrate (NaNO₃), but the one used for this project was Ammonium Nitrate (NH₄NO₃) which allowed analyzing Sodium (Na).

This substitution was made because Na is now considered as an element of interest in the iron ore related materials. The Ammonium Iodine can be used as a Non-Wetting Agent (NWA) when needed as stated in the standard method. However, the methodology described in this paper does not recommend using NWA. A VortexMixer^TM was used to mix the sample and flux homogeneously prior to the fusion and its speed was controlled to avoid any loss of material that could cause error in the results^[4].

The temperature range of the fusion process was kept between 1000 and 1050°C. It is well known that over the critical temperature of 1050°C, flux^[5] and other compounds like SO₃^[2] begin to volatilize without consistency and can change the sample to flux ratio. Finally, the fusion process had pre-programmed steps with fixed times in order to obtain the highest level of precision and accuracy. The study presented in the following paper proves that a high level of precision is attainable for this sample preparation using fully automatic fusion instruments.

Preparation for the calibration with CRMs and the selection of control samples

More than 80 CRM preparations were produced and evaluated only to select the best set of standards for the calibration of the borate fusion and XRF analytical application for iron ores and the related exploration samples. Out of all the evaluated CRMs, 16 were selected as iron ore calibration standards and 12 were selected as exploration calibration standards. Table 1 shows the element concentrations for the two separate CRM sets as well as for the global application. The CRMs originated from 13 different producers.

For the calibration of the WDXRF instrument and further evaluation of precision and accuracy of this borate fusion and XRF analytical method, selected CRMs were prepared in duplicates using an M4^TM gas Fluxer to verify the precision of the sample preparation over the wide range of composition that was covered.

This analytical method for iron ore products and exploration samples showed good efficiency to prepare homogenous and stable lithium borate glass disks with all of the materials; it had limitations only for the preparation of iron ore related materials that contained relatively high level of Copper (Cu). When the content of this element was higher than 500 ppm, the glass disk had a tendency to stick to the mold and often lead to disk cracking.

For this reason, it was determined that high Copper iron ores needed a different sample preparation fusion methodology that includes a NWA to avoid having the disk stick to the mold.

Results and discussion

Precision and accuracy

For the precision evaluation, 12 glass disks were produced using both the M4^TM gas Fluxer and the TheOx® electric Fluxer. All fusion positions of both instruments were used to produce the complete set of glass disks of the precision evaluation. The sample selected for the precision evaluation was a high iron magnetite known to be relatively difficult to prepare using fusion. The accuracy evaluation was conducted using 4 control samples which were CRMs that were not included in the calibration. Two of these results are presented in this paper.

It is important to note that the ISO 9516-1 limits for precision are not fixed. The ISO limits are variable as a function of the concentration of the element in the sample analyzed. The ISO precision test was applied as described in the method^[3]. The standard deviation limit calculated for all the elements is shown in Table 2. The precision results of the spectrometer are presented as well as the precision of the M4^TM gas Fluxer and the TheOx^® electric Fluxer. These results were compared to the ISO precision limits. The values obtained for all the elements met the specified limits.

The ISO 9516-1 method refers to a trueness test for the accuracy evaluation. This test is complex even for experienced analysts. For this reason the accuracy results are presented in the following tables as the maximum deviation between certified values and results. Accuracy validation was examined on the 2 different calibration curves: a first calibration based on pure oxides fused into calibration glass disks acting as the reference methodology as described in the ISO test method and a second calibration based on CRMs fused into glass disks. This allowed evaluating the performance of the CRM based calibration against the reference methodology using pure oxides as standards. The maximum deviation was calculated over two different preparations.

The accuracy results presented in Table 3 and in Table 4 demonstrate that both calibration strategies allow for similar accuracy levels for all the evaluated elements and over the different types of iron ore related material. The great benefit of using a CRM based calibration is the simplified calibration steps: The CRMs are directly fused in glass disks on an as-received basis, and in parallel the moisture content and the LOI are determined.

The pure oxide calibration strategy as described in the ISO test method necessitates many steps in order to prepare all the mixes of the different oxide powders to produce the various calibration points (drying of all powders, ignition of some powders, weighing of all powders to produce all the mixes in the exact ratio, etc.). It is time consuming as well as expensive to buy all the oxide powders in analytical grade and also opens the door to many manipulation errors that can bring bias in the analysis.

Conclusions

To match the actual needs and expectations of the iron ore industry regarding the elemental characterization of iron ore materials and exploration minerals, a new borate fusion and XRF application was evaluated through this study.

Consisting of a single method of preparation by fusion, it allows fusing the various iron ore types and the exploration samples normally found in this industry in a lithium borate glass disk. A CRM based calibration proved to provide the same level of precision and accuracy for iron ore samples when compared to the traditional pure oxide calibration as recommended in the ISO 9516-1 international test method. In addition to providing high analytical performance in line with the ISO analytical targets, the CRM based calibration allows for extended calibration ranges that cover both the iron ores and the exploration samples and also makes the analysis of Sodium (Na) possible. Finally, this alternative CRM based application significantly simplifies the preparation of the calibration standard fused disks which allows for time savings and minimizes the work needed to prepare the application. It was also proven that both the M4^TM gas Fluxer and TheOX^® electric Fluxer offer the same versatility and ability to fuse the full range of samples while offering similar analytical performance.

References

1. ANZELMO, J. A., «The Role of XRF, Inter-Element Corrections, and Sample Preparation Effects in the 100-Year Evolution of ASTM Standard Test Method C114», Journal of ASTM International, Vol. 6, No. 2, Paper ID JAI101730, available online at www.astm.org, 2009, pp 1-10.
2. SPANGENBERG, J. and FONTBOTÉ, L., «X-Ray Fluorescence Analysis of Base Metal Sulphide and Iron-Manganese Oxide Ore Samples in Fused Glass Disk», X-Ray Spectrometry, Vol. 23, 1994, pp 83-90.
3. ISO 9516-1:2003 (First edition, 2003-04-01), Iron ores -- Determination of various elements by X-ray fluorescence spectrometry -- Part 1: Comprehensive procedure (ISO 9516-1:2003), 72 pp.
4. BÉRUBÉ, L., RIVARD, S., ANZELMO, J. A., «XRF Fusion Precision with TheAnt», International Cement Review, March, 2008, 4 pp.
5. LOUBSER, M., STRYDOM, C., and POTGIETER, H., «A Thermogravimetric Analysis Study of Volatilization of Flux Mixtures Used in XRF Sample Preparation», X-Ray Spectrom. 2004; 33: 212–215, Published online 29 January 2004 in Wiley InterScience (https://onlinelibrary.wiley.com/). DOI: 10.1002/xrs.700