A Novel Approach to Fast Digestion of Rare Earth Element Ores

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The mining industry relies greatly on the data provided by analytical chemistry where the elemental concentrations of exploration samples are concerned. Exploration companies require analytical results to be turned around in good time so as to reduce the number and any potential costs, of drilling cores beyond the extent of ore deposits (“dry holes”).

A Novel Approach to Fast Digestion of Rare Earth Element Ores

1. Environmental health and safety – The use of acids that are particularly hazardous, such as hydrofluoric acid (toxic by skin absorption), and perchloric acid (can produce explosive salts, especially when taken to dryness). These two acids, along with nitric and hydrochloric acids, constitute the basis of the mixture that is typically used for “total” or 4-acid digestions. Moreover, some fusions require reactive fluxes, such as sodium peroxide and sodium hydroxide, to make ores soluble in any successive acid dissolution step. These conditions, as well as the handling of high-temperature furnaces and the transfer of molten samples, create a hazardous environment for the operator. Progress in the speed of spectroscopic methodologies has left sample digestion as the rate-determining step in ore analysis. When it comes to the case of rare earth element (REE) ores, digestions are often only partially completed because REE oxides and accessory minerals, such as zircon, tend to have a refractory nature¹. Zircon is a recurrent accessory mineral in many rocks that frequently incorporates P, Hf, REEs, Th, and U into its crystal lattice². Therefore, determinations of these elements often yield low recoveries. Recently, beyond the necessity to produce fast, accurate results of ore compositions, the operations of analytical laboratories have been scrutinized on two significant issues:

2. Capital costs of equipment and infrastructure – Significant costs are incurred when using perchloric acid due to ensuring the analytical laboratory is adequately equipped with fume hoods that have wash-down capabilities (“perchloric acid hoods”). In addition, high-temperature furnaces for fusion work and the requisite platinum and zirconium crucibles are inherently expensive. While automation means that fusion is now safer than in previous times, it comes at a considerable cost.

Thus, providing precise, safe, and high-speed methods for the digestion of samples, while ensuring workers are safe in analytical laboratories, is fast becoming a vital consideration in the development of analytical facilities. These measures coupled with comparatively low-cost spectroscopic techniques, such as nitrogen-based plasma atomic emission spectrometers, will be crucial in supporting the ongoing viability of mining mineral resources in extreme and remote environments.

Over the last three decades, the digestion of rare earth ores by 4-acid treatment has waned in favor of fusion techniques. Fusion is currently the go-to method when digesting refractory mineral phases. In 2001, Yu et al. detailed a Li2B4O7 fusion method that was effective in digesting many geological materials containing REEs such as granites and basalts. Furthermore, they advanced a sodium peroxide sinter technique that successfully digested ironstone matrices containing REEs. Yet, alkaline fusion increases the total dissolved solids (TDS) in samples that are delivered to the analytical instrument, an unpreventable pitfall that results in a decrease of instrumental sensitivity³.

Commercial laboratories favor acid-based hotplate digestion due to the fact they introduce the minimal background and produce high throughput, results achieved by their compact design. Hotplate digestions of non-refractory rocks containing REEs, such as basalts, achieve precise elemental determinations. However, refractory phases require extended digestion times and are typically unable to accomplish the complete dissolution of refractory minerals such as chromite, rutile, or zircon. This deficiency of hotplate digestions often concludes with low recoveries of REEs³.

The shortcomings of hotplates can be improved upon using microwave systems, since sealed digestion containers mean higher temperatures can be achieved: digestion times are also reduced, between 20–60 min. Furthermore, microwave digestion techniques tend to deliver much better REE recoveries when compared to hotplate digestions⁴. Recently, Wang et al. noted the use of infrared radiation to expedite acid digestions of environmental and geological materials^5–8. The novel methodologies chronicled in their work improve on the deficiencies of other acid digestion techniques, particularly where the time is taken and completeness of digestions are concerned.

Positively, the results accomplished in this work, especially where the refractory mineral chromite is concerned, suggested that infrared digestions could be employed to prepare samples of other refractory minerals, such as REE ores. Therefore, the aim of the research, as outlined here, was to analyze the acceleration of acid digestions of well-known refractory mineral phases using infrared radiation. A group of typical reference ores, including REE-1, OREAS-465, and OKA-2, all of which are geological materials containing copious amounts of REEs, were tried for investigation. These materials would be typically vulnerable to determination by microwave plasma atomic emission spectrometry (MP-AES).

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References and Further Reading

1. F.G. Pinto, R. Escalfoni, T.D. Saint’Pierre, Sample preparation for determination of rare earth elements in geological samples by ICP-MS: a critical review, Anal. Lett. 45 (2012) 1537–1556.
2. J.M. Hanchar, P.W.O. Hoskin (Eds.), Zircon, In: Mineralogical Society of America, Geochemical Society, Washington DC, 2003, pp. 19.
3. Z. Yu, P. Robinson, P. McGoldrick, An evaluation of methods for the decomposition of geological materials for trace element determination using ICP-MS, Geostand. Newsl. 25 (2001) 199–217.
4. M. Totland, I. Jarvis, K.E. Jarvis, An assessment of dissolution techniques for the analysis of geological samples by plasma spectrometry, Chem. Geol. 95 (1992) 35–62.
5. Y. Wang, R. Kanipayor, I.D. Brindle, Rapid high-performance sample digestion for ICP determination by ColdBlock™ digestion: part 1 environmental samples, J. Anal. At. Spectrom. 29 (2014) 162–168.
6. Y. Wang, I.D. Brindle, Rapid high-performance sample digestion for ICP determination by ColdBlock™ digestion: part 2 gold determination in geological samples with memory effect elimination, J. Anal. At. Spectrom. 29 (2014) 1904–1911. 
7. Y. Wang, L.A. Baker, I.D. Brindle, Determination of gold and silver in geological samples by focused infrared digestion: A re-investigation of aqua regia digestion, Talanta 148 (2016) 419–426.
8. Y. Wang, L.A. Baker, E. Helmeczi, I.D. Brindle, Rapid high-performance sample digestion of base metal ores using high-intensity infrared radiation with determination by nitrogen-based microwave plasma optical spectrometry, Anal. Chem. Res. 7 (2016) 17–22.

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