Method development for the quantification of selected early rare earth elements

[摘要] English: The aim of this study was to develop an analytical procedure to accurately quantifythe early REE (La, Ce and Nd) in pure REE metal, in inorganic compounds as well asin a mixture containing all three elements and finally in organometallic complexes.The study involved the use of different inorganic acids such as H2SO4, HNO3 andHCI as dissolution reagents and determination of their influence on the La, Ce andNd recoveries. Both bench top and microwave dissolution were used in this study.Different analytical techniques such as inductively coupled plasma optical emissionspectrometry (ICP-OES), CHNS-micro analyser and infrared spectroscopy (IR) wereused to characterise and quantify the REE in the different samples. Validationparameters such as accuracy, precision, linearity, sensitivity, etc., using ICP-OESanalysis were also evaluated (Chapter 4) to determine the suitability of the digestionand quantifying methods for REE analyses. Analytical lines (wavelengths) werecarefully selected to minimize or prevent any spectral overlap with the other rareearth elements investigated in this study. Secondly, the acid matrix was strictlymatched throughout the analytical process to ensure accurate analyticalmeasurements and well controlled experimental conditions.Bench top dissolution with 98 % H2SO4, 65 % HNO3 or 32 % HCI yielded excellent Laand Nd recoveries from the pure REE metal, inorganic compounds as well as in amixture containing all three elements, ranging from 97.9(5) to 100.6(1) %. Cerecovery in the pure metal only ranged from 81.37(9) to 95.30(8) %. Microwaveassisted acid digestion was employed to improve the recovery of Ce metal. The Cerecoveries improved to 96.5 and 99.6 % after microwave digestion. The efficiency ofthe acids in dissolving and recovering Ce in the pure metal sample was in the orderHCI < HNO3 < H2SO4. Excellent metal recoveries for the synthesised organometalliccomplexes ranging from 98.0(2) to 103.6(4) % for acac, imda and nta were obtainedfor the different organometallic complexes. ([Ln(dap)(NO3)3] Ln = La, Ce) yieldedmetal recoveries ranging from 88.1(5) to 95.5(3) % using bench top dissolution. Themetal recoveries of TPPO complexes ranged between 98.45(5) and 99.4(2) % aftermicrowave digestion. The method validation done in Section 5.8 was considered satisfactory. Allcalibration curves showed good linearity with excellent r2 values which rangebetween 0.9997 to 1 and fairly constant slopes. The elemental LODs rangedbetween 0.0030 - 0.0188 ppm and LOQ between 0.0289 - 0.1881 ppm in thedifferent acid matrices used in this study. It can be concluded that the La, Ce and Ndanalyses in pure REE metal, inorganic compounds and organometallic complexeswere successful and the results met most of the guidelines within acceptable criteriaas set out in ISO 17025.The successful metal recoveries in acac, imda, nta and TPPO complexes were alsodue to the fact that these complexes have been crystallographically characterisedand their chemical structures are well known, whereas the metal-dap complexes hasnot yet been fully characterised. Characterisation of synthesised organometalliccomplexes and ligands using IR also played a huge role in illustrating the possiblecoordination of the metal and ligands. The IR spectra were analysed by simplecomparisons of the stretching frequencies between the unreacted samples and thereaction products. The shifting to low or high wavenumbers and disappearance orappearance of peaks was used as a measure for the possible formation of a newproduct.

[发布机构] University of the Free State