Leaching and solvent extraction of rare earth metals from fluorescent lamp waste

Licentiate thesis, 2014

With the rapid advancement of technology, the demand for elements such as rare earth metals (REMs) has increased considerably during the last decade. Many countries are facing problems securing sustainable supplies, a fact acknowledged in many publications. Due to the ever-growing demand and supply problems, REMs are now considered to be some of the most critical elements. This has focused attention towards their recovery from end-of-life products and industrial waste streams, with fluorescent lamps being one of the main targets. However, despite the research published, large scale applications are scarce, mainly due to the lack of sustainable processes. The research presented here is aimed at developing a sustainable hydrometallurgical process for the treatment of fluorescent lamp waste, with the goal of recovering the REMs and mercury that these lamps contain. In comparison to other efforts in this field, these investigations were carried out using real waste samples originating from a discarded lamp processing facility. The complexity of the material (a mercury contaminated mix of glass, metallic and plastic parts, phosphors, remaining electronics and other impurities) and the nature of the initial process (crushing of mixed lamps followed by containment of elemental mercury via oxidation) make already proposed methods a challenge and make additional research necessary. This is due to the fact that many of the methods described in the literature have not been optimized for real or complex waste streams; parts of the published studies were performed on a laboratory scale, using clean lamps, artificial mixtures that resemble real samples or pure commercial phosphors. Cerium, europium, gadolinium, lanthanum, terbium, and yttrium were the REMs identified in the material. Leaching of metals was investigated using different solutions (pure water, ammonium chloride, acetic acid, nitric and hydrochloric acid) and parameters (temperature, ultrasound-assisted digestion, solid-to-liquid ratio, and leaching agent concentration). An increased acid concentration and increased temperature, and ultrasound-assisted digestion improved the leaching efficiency for most of the investigated elements. Solvent extraction experiments were carried out using a commercial mix of trialkylphosphine oxides (Cyanex 923), in order to assess the extraction potential of REMs and the advantages/disadvantages for possible industrial scale-up. Separation of heavier elements (such as terbium, yttrium, europium and gadolinium) from lighter ones (like cerium and lanthanum) is possible due to larger separation factors.