Hydrometallurgical recovery of rare earth elements from flourescent lamp waste fractions

Doctoral thesis, 2016

Recovery and reuse of materials is important for a circular economy. In recent years the recovery of critical metals from end-of-life products has received increased attention. Various streams, e.g. permanent magnets, nickel metal hydride batteries and fluorescent lamps are considered targets for the recovery of rare earth elements (REEs). The last can be a source of up to six different REEs: europium and yttrium (primarily), as well as lanthanum, cerium, terbium and gadolinium (secondarily). Because fluorescent lamps use mercury to generate light, a decontamination step needs to be carried out prior to processing discarded products for REEs recovery. This is often carried out using thermal treatment (up to 800 °C) but this method has some drawbacks, e.g. energy consumption and the fact that it is not best suited for waste streams containing high amounts of moisture. Hydrometallurgical methods for the decontamination of fluorescent lamp waste fractions and subsequent recovery of the REEs contained are presented in this study. A selective leaching process followed by separation of metals using solvent extraction was developed. Mercury was leached in a first stage using iodine in potassium iodide solutions. Further processing of the mercury in solution was investigated using various techniques, e.g. ion exchange, reduction and solvent extraction. In a second leaching step, impurity metals, e.g. calcium, barium, etc., were selectively leached from the REEs with nitric acid solution by making use of their fast dissolution kinetics. Further leaching, carried out with more concentrated acidic solutions for longer time, led to the dissolution of the REEs. Partial leaching selectivity between yttrium + europium and the other four REEs was achieved by controlling the leaching time, acid concentration and temperature. A group separation of the REE ions in solution was carried out using solvent extraction with Cyanex 923, a commercial mix of trialkyl phosphine oxides. Testing of the process at laboratory pilot scale in mixer-settlers showed promising results, leading to a final product consisting of a yttrium/europium-rich solution. Over 99% of the REEs present in lamp leachates were extracted and stripped, respectively, in a mixer-settler system comprised of three extraction stages and four stripping stages. The metals were then further separated using Cyanex 572, a novel phosphorus-based chelating extractant aimed at the separation of individual REEs. Selective separation of yttrium and europium was achieved by controlling the equilibrium pH (pHeq) during extraction. Yttrium was extracted at pHeq = 0 and europium at pHeq = 1. Rare earth oxides were prepared via oxalic acid precipitation and thermal treatment of the obtained oxalates at 800 °C. A mixed REE oxide (99.96% REEs, with 94.61% yttrium, 5.09% europium and 0.26% others) was synthetized from the strip product after extraction with Cyanex 923. Yttrium oxide (99.82%) and europium oxide (91.6%) were synthetized from the strip products after extraction with Cyanex 572.