# End-of-Life Liquid Crystal Display Recovery: Toward a Zero-Waste Approach
## 1 Introduction  
Management of end-of-life liquid crystal displays (LCDs), classified as waste from electrical and electronic equipment (WEEE), in the category of screens, monitors, and equipment containing screens having a surface greater than 100 cm\({}^{2}\), represents a critical issue for modern society because significant quantities of waste are reaching worldwide collection centres, often illegally [1; 2; 3; 4; 5]. The reason is there are several applications of LCD technology (e.g., PC monitors, televisions, notebooks, etc.) combined with short lifespans, between three and eight years [6; 7]. Currently, this end-of-life equipment is dismantled to separate possible hazardous components, (e.g., brominated flame retardants and backlight fluorescent lamp systems that contain mercury), which varies on the basis of the specific brand and generation [8; 9; 10; 11]. Thereafter, the remaining panel is disposed of as nonhazardous waste [12]. Nevertheless, the panel's composition includes different fractions with a high recovery potential, including the glass and an indium and tin oxide (ITO) film, composed of 90% of indium oxide and 10% of tin oxide. This material shows characteristics of transparency to visible light, electric conduction, and thermal reflection [13; 14]. The presence of indium, with concentrations around 150 ppm, higher than that in the ones [6; 15; 16; 17; 18], has pushed research towards the development of high-efficiency recovery processes since the metal was classified as critical raw material (CRM) bythe European Commission [19; 20]. Currently, China is the main global producer (around 57% of the entire amount), followed by South Korea and Japan. The metal's significant importance is also confirmed by a substitution index of 0.94/0.97, which proves the difficulty in substituting the material due to its excellent properties [19]. This value, scored and weighted across all applications in a range between zero and one, is calculated for both economic importance and supply risk parameters [21]. The same European source reports that there is no significant secondary production, at the moment, and recycling should be considered as an effective tool to decrease demand of primary indium and CRM in general [19; 22]. In this regard, [PERSON] et al., estimated an indium in-use stock of around 500 tons/year that is able to respond to the global market, which is around 15 times higher than the current European demand, proving the possibility to develop a circular indium production. The study included different kinds of waste with a metal content (e.g., motor cars, alkaline batteries), but it identified LCDs and its equipment with flat panels as potential secondary sources with the highest indium content [20]. The value of the end-of-life LCD is also confirmed by [PERSON] et al. who reported an LCD production higher than 370 million in 2014 (only in China), and they predicted that the development of an LCD recycling strategy could cover around 48% of the indium demand by 2035 [23]. In this context, the literature is rich in treatments focused on waste recycling using different techniques. Nevertheless, many weaknesses could be identified such as a low metal concentration in the starting waste and a high water consumption [14]. For this reason, indium recovery is often combined with the recycling of additional fractions to increase both the environmental and the economic sustainability of the processes, which is in the perspective of a circular economy [8; 24]. Indeed, as reported by [PERSON] et al., this economic strategy is a \"regenerative system in which resource input and waste, emission, and energy leakage are minimized by slowing, closing, and narrowing material and energy loops. This can be achieved through long-lasting design, maintenance, repair, reuse, remanufacturing, refurbishing, and recycling\" [25]. The strong interest for this topic is also confirmed by technological innovation changes, based on the number of published patents, chosen as the indicator of promising new markets [26].