From iPODs to catalytic converters, from wind power generators to computer disc drives and hybrid electric vehicles – Rare Earth applications are ubiquitous and critical from all day’s life to advanced R&D. The Rare Earth Elements (REEs) have become uniquely indispensable in many electronic, optical, magnetic and catalytic applications because of their ability to readily give up and accept electrons. Most of Rare Earth Resources are actually located in China but there is a need of alternative resources in some other countries such as Brazil, Australia, Canada, South Africa, Greenland and USA. Electronic wastes are also a potential alternative source for REEs. Once extracted, REEs-bearing minerals contain as many as 14 Rare Earth Elements (Ce, Dy, Er, Eu, Gd, Ho, Sc, Lu, Nd, Pr, Sm, Tb, Tm, Yb), Lanthanum and Yttrium. Complex extraction and refining should be conducted and the refined material should be analyzed to check for its purity.
ICP-OES is the one of the most adequate technique to analyze traces of Rare Earth Elements in a matrix of another Rare Earth Element. Its major advantages compared to other techniques are the speed of analysis, its ability to analyze all elements in a single run and its tolerance to high concentration of major elements. For Rare Earth Elements, the ICP-OES should also offer high resolution as Rare Earth Elements exhibit line-rich emission spectra.
Rare Earth Elements matrices
In this study, three matrices were studied and according to the matrix, different traces of Rare Earth Elements were determined.
Matrix 1: Rare Earth matrix
Matrix 2: Iron Ore matrix
Matrix 3: Mn Ore matrix
Instrument and Results
To ensure the best resolution available, a Perkin Elmer ICP-OES, Optima 7000 DV was used for this study. The characteristics of the Instrument and of the operating conditions are given in Table 1 below. Fig.1 shows the calibration curves made against the reagent blank, 0.1, 0.5, 1.0 and 2.0 mg/L multielement rare earth standards. The linearity of calibration curve for each parameter is quite good (correlation coefficient >0.999-) for each element as evident from the curve.
Fig.1: Calibration Curve for the rare earth elements using 0.1, 0.5, 1.0 and 2.0 mg/L multielement rare earth CRM solutions
The detection limit for each of the rare earth elements were determined by repeat analysis of a blank test portion and is the analyte concentration whose response is equivalent to the mean blank response plus 3 standard deviations. Its value is likely to be different for different rare earth elements. The LOD determined based on the calibration curves of the individual elements is presented in Table 2.
Synthetic REE samples were prepared by adding REE standard sample solution of strength 0.25 and 0.5 mg/L into Iron Ore (Euronorm 686-1) and Manganese ore (BCS 176/2) solutions. The recovery studies were carried out for these analyses which are also shown in Table 2. The highlighted colors show an enhanced recovery of the respective REEs whereas; colors show suppression of the rare earths in their respective matrices. For the rests, the recovery value obtained is in the range of 85-115 % and is well within the acceptable limit.
To ensure reliability of the results for the low concentration REEs in the samples, the ICP-OES should allow long term stability over hours with such samples. Stability was evaluated on spiked samples on Matrix 2 till 120 minutes for all the rare earth elements. Stability is displayed in Fig.2a) and b). The stability obtained is identical with that in the pure matrix and hence proves the robustness of the method.
Analysis of traces of REEs is not an easy task due to the line-rich emission spectra of these elements. Nevertheless, using a High Resolution ICP-OES spectrometer helps reaching high performance on such matrices. The experimental profiles, the stability obtained on synthetic samples and the consistence between expected results and measured concentrations prove it as a perfect tool for this particular application.