# Production of Porous Ceramic Materials from Spent Fluorescent Lamps
## 1 Introduction  
The sustainability of glass recycling is delicate. In fact, there are significant drawbacks along the production chain to be considered [1]. The manufacturing of 1 kg of glass from conventional feedstock involves an energy consumption of 10 MJ which, although much lower than that for structural metals and polymer (e.g., steel has an energy requirement of 30 MJ/kg, whereas Polyethylene Terephthalate (PET) exceeds 80 MJ/kg), is substantial. More importantly, the saving in embodied energy by using recycled material instead of mineral raw materials is lower than 25%, definitely scarce in comparison with aluminum (nearly 90%) [1]. This is caused by the need to separate the glass fractions from impurities and other materials during sorting [2]. Furthermore, there are several difficulties in re-melting end-of-life glasses to produce the original articles (closed-loop recycling) [3,4]: contaminations may degrade the chemical stability or the optical properties (e.g., in pharmaceutical vials, LCD panels, etc.) or even determine the release of noxious emissions, as in the case of fluorine [5]. In this context, glass up-cycling, i.e., making new valuable products different from the original ones, is fundamental to reduce both depletion of natural resources and disposalof glass waste. In particular, viscous flow sintering is interesting in the perspective of controlling energy consumption and emissions [5]. In fact, the densification of fine glass particles can be obtained at much lower temperatures than those required by remelting. More precisely, sintering can take place 100-150 \({}^{\circ}\)C above the glass transition temperature (Tg), which is about 550 \({}^{\circ}\)C for common glass. The amount of energy and CO\({}_{2}\) emissions involved in this treatment at moderate temperatures can be compensated by the savings obtained in the lifespan of the new sintered products, especially when used for thermal insulation in the form of highly porous foams [2; 5]. Most glass foams actually derive from a delicate balance between viscous flow sintering of glass powders and gas evolution from selected additives, known as 'foaming agents'. The gas release is typically due to thermal decomposition (operating with carbonates and sulphates) or oxidation reactions (operating with C or SiC), which should occur in a temperature interval at which the mass, which is determined by the sintering of softened glass powders, has an adequate viscosity [6]. Glasses with low characteristic points, such as glasses from dismantled cathode ray tubes, are thus coupled with foaming agents releasing gas at relatively low temperatures (e.g., CaCO\({}_{3}\)) [7]. The homogeneity of foaming is not always straightforward, as in the case of oxidation reactions of C-based additives, involving atmospheric oxygen as well oxygen deriving from the reduction of ferric and manganic oxides. The use of C-based additives may be controversial, from an environmental point of view, with glasses or glass/waste mixture involving metal oxides such as PbO and ZnO, according to the reduction to metallic colloids and possible volatilization of the same oxides or metal vapours [7]. The present paper is dedicated to a class of glass-rich residues specifically involving a variety of metal oxides. Spent fluorescent lamps (SFL) are classified as hazardous materials in the European Waste Catalogue (EWC) for the mercury content, despite the EU threshold limit of 5 mg/lamp [8; 9]. SFL industrial processing involves a series of steps: (i) removal of Hg; (ii) crushing and sieving of the SFL to recover metals and glass; (iii) extraction of fluorescent phosphor powders [10]. The latter can be treated to recover Rare Earth Elements (REEs), which are considered critical raw materials used in different hi-tech applications for their chemical, optical, electrical, and magnetic properties [11; 12; 13]. Recovered rare-earth-containing compounds may be used as feedstock for a second generation of luminescent materials, reused in the manufacture of new lamps [14].