Browsing by Author "Bumhira, Levie"
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- ItemRecovery of rare earth elements from fluorescent lamp phosphors(Stellenbosch : Stellenbosch University, 2019-04) Bumhira, Levie; Dorfling, C.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: In recent decades, the production and usage of fluorescent lamps has rapidly increased. This is attributed to several benefits they offer in comparison to incandescent bulbs. The lamps are known to have lower energy consumption (about 75 % less than incandescent bulbs) and longer life expectancy. The rapid growth in production and usage has resulted in large volumes of waste fluorescent lamps being discarded every year. Recycling of spent fluorescent lamps offers a number of economic benefits apart from the well-known environmental benefits. The primary objective of this study is to identify a viable process for rare earths recovery from end of life fluorescent powders in the South African context. The study focused on the use of hydrometallurgical unit operations in the recovery of four rare earth metals namely yttrium (Y), europium (Eu), cerium (Ce) and terbium (Tb). A two-staged leaching process was employed followed by solvent extraction to recover the metals from solution. Leaching tests indicated that the red phosphor component (Y2O3:Eu3+) could be easily dissolved into solution during the first step of acid leaching. Over 98% Y and 89 % Eu recoveries were achieved using sulphuric acid as the lixiviant. Ce and Tb were not leached at this stage. The effect of ultrasound and alkali fusion on Tb and Ce leaching was then investigated. Alkali fusion followed by acid leaching produced the best recoveries with Ce and Tb recoveries exceeding 96 % and 99 % respectively. Poor recoveries of the rare earths (<10 %) were obtained using ultrasound assisted digestion. Solvent extraction tests showed that DEHPA could be used to recover rare earths from aqueous solutions obtained after the first and second leaching stages. The results showed that about 11 countercurrent equilibrium stages are required to achieve more than 95 % yttrium extraction at pH -0.25, O/A ratio of 1 and a temperature of 25 ºC. Eu and residual Y could only be extracted after pH adjustment to 0.5 using 5 M sodium hydroxide. More than 92 % Eu and 99 % Y extraction was achieved using an O/A ratio of 1.5 and 1M extractant concentration. A multiple stage stripping process can then be used to recover more than 95% of the rare earths from the organic phase prior to precipitation and calcination. Ce and Tb solvent extraction results showed that all the targeted rare earths could only be recovered as a mixed product at this stage. A mixed rare earth product was produced using 1 M DEHPA, a pH of 0.5, O/A ratio of 1 and a temperature of 25 °C. Complete extraction of Y and Eu was assumed at these conditions since both rare earths could not be detected in the remaining aqueous solution. More than 95 % Ce and 98 % Tb were extracted in a single stage solvent extraction process. The rare earths can undergo stripping using 5 M sulphuric acid prior to precipitation and calcination to recover the metals as rare earth oxides.