Method development for the quantification of rare earth elements in South African monazite by various spectroscopic techniques

Abstract

The rare earth elements are critical, and their various applications include electronics, the defence sector, manufacturing, renewable energy, technology and the medical sciences. The increasing demand for these critical elements will strain the existing supply chain. There is a major concern that any disruption to the supply chain will negatively impact innovation. Countries worldwide are seeking to secure new reserves, find the material substitutes, and boost the research in recycling electronics that have reached the end of life. Monazite contains critical rare-earth-elements responsible for green energy transition. The bottleneck associated with analysing rare-earth elements in monazite is its refractory nature and associated heavy minerals that do not decompose completely in the sulphuric acid fuming procedure. Furthermore, rare-earth elements have been successfully measured in geological materials employing the sensitive inductively coupled plasma mass spectrometry techniques. Nevertheless, it suffers from a low tolerance of total dissolved solids in matrices such as monazite. While, inductively coupled plasma optical emission spectroscopy suffers from poor sensitivity and matrix removal is often required for accurate rare-earth elements determinations. Extensive research has been conducted on sensitive procedures and efficient sample preparation for trace elements determinations including rare-earth elements. This study aims to develop a method for direct determination of rare-earth elements in monazite. The monazite ore samples were collected from the extractive metallurgy within Mintek for method development for quantification of REEs. Three preparation methods for sample digestion techniques were evaluated. The methods involved the flux fusion method using the lithium metaborate and sodium peroxide respectively and the multi acids digestion with hotplate as the source of heat followed by rare earth elements quantification using both the inductively coupled plasma mass spectrometry and inductively coupled plasma optical emission spectroscopy. Moreover, the complete sample dissolution was achieved with the flux fusion method while the partial sample dissolution was observed via multiple (repetitive) treatment steps by multi acids digestion method. The total dissolved solids were mitigated, and the matrix effect was eliminated when glass beads were treated (dissolved) in a mixture containing nitric and hydrofluoric acids. The analyte spectrum was often completely obscured by wing overlap and direct overlap of emission lines from interfering elements during-inductively coupled plasma optical emission spectroscopy analysis. In addition, the recoveries for some rare earth elements were suppressed with enhanced spikes for rare earth elements concentration were observed from inductively coupled plasma optical emission analysis particularly from the solution samples digested in alkaline fusion. The inductively coupled plasma optical emission spectroscopy results for the rare earth elements suggest that the sample matrix significantly affects the sensitivity. The method’s validity was tested using a monazite ore-certified reference material. In contrast to the optical emission spectroscopy, the accurate analysis of rare earths from all the digested samples was obtained using the inductively coupled plasma mass spectrometry. The results of the rare earth elements obtained for all digested samples analysed using the mass spectrometry strongly agree with the certified values. The excellent accuracy of the method was realised with the rare earth elements data obtained from quality control samples, and recoveries yield within the range of 90-110%, and the relative standard deviation of 0.2-5.4%. The selectivity and sensitivity of the method were realised with individual rare earths accurately measured in the presence of other interfering elements, as indicated by lower detection of limit ~ (0.0004-0.016 ppm) and limit of quantification ~ (0.0001-0.0290 ppmn). Calibration curve linearity for individual rare earth elements is demonstrated by the correlation coefficient in the range of 0.99 to 1.0. Moreover, multi fusion methods followed by mass spectrometry are futuristic analytical approaches for determining rare earth elements in monazite associated minerals and they can be used interchangeably. In South African context, South Africa should prioritise the development of Rare Earth Elements (REE) extraction and recycling technologies in order to maximise yield, purity, and environmental sustainability, notably through enhanced procedures for monazite-rich deposits. Investing in a circular economy paradigm, such as boosting REE recovery from electronic trash, can minimise dependency on virgin resources and help to create a more sustainable future. To further help the country's green energy transition, the government should fund research into REE-based technologies critical to renewable energy systems such as wind turbines, solar panels, and electric cars. Furthermore, developing regulatory frameworks that ensure responsible mining techniques and monitor the entire REE supply chain will assist to mitigate environmental and social consequences. Additionally, South Africa can enhance its role in the global REE market by collaborating with international partners, leveraging external expertise, and promoting sustainable technologies, thereby contributing to a low-carbon economy.