End-of-Life Liquid Crystal Display Recovery: Toward a Zero-Waste Approach

Abstract

End-of-life liquid crystal displays (LCD) represent a possible source of secondary raw materials, mainly glass and an optoelectronic film composed of indium (90%) and tin (10%) oxides. A strong interest for indium, classified as critical raw material, pushed research towards the development of high-efficiency recycling processes. Nevertheless, a deepened study of the technological innovation highlighted that only a small number of treatments included use of whole waste. Furthermore, these processes often need high temperatures, long times, and raw materials that have a significant environmental impact. In this context, this article shows an approach developed in accordance with the “zero waste” principles for whole, end-of-life LCD panel recycling. This process includes preliminary grinding, followed by cross-current acid leaching and indium recovery by zinc cementation, with efficiencies greater than 90%. A recirculation system further increases sustainability of the process. To enhance all waste fractions, glass cullets from leaching are used for concrete production, avoiding their disposal in landfill sites. Considering the achieved efficiencies, combined the simple design suitable for real-scale application (as confirmed by the related patent pending), this process represents an excellent example of implementing circular economy pillars.

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 cm², 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 ores [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) by the 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, Ciacci 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 Wang 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 Geissdoerfer 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]. As highlighted in Figure 1, several fractions have attracted research attention with temporal modification of the target materials, passing from the most traditional glass to the most innovative indium recovery [26]. Many patents focus on only waste pretreatment, and both scientific literature and the technological innovation field still lack whole processes able to combine efficiency and sustainability aspects. Indeed, the few examples identified need extreme time and temperature conditions, and they often involve the use of high-impact organic agents for the oxidative action [27].

In this context, the present research shows a treatment for end-of-life LCD panels aimed at indium and glass recovery. The process combines four essential characteristics: high efficiency, saving raw material, simple design, and an almost complete absence of final scraps to manage. The strategy follows zero-waste principles with complete recycling, reducing harmful streams for our environment [28,29]. Relevance of this process, also confirmed by the related pending patent, is further related to the European interest of recovering critical raw materials, which is identified as one of the challenges that must be addressed to move toward a consolidated circular economy [30]. In accordance with this economic schema, injection of both secondary indium and glass into the economy as new materials, avoiding their disposal, closes the loop and reduces the traditional supply of primary raw materials [30].

2. Research Methodology

The present process included all recycling steps of the panel, from grinding to both indium and glass recovery, in accordance with circular economy principles. Hydrometallurgical treatment development started from the operative conditions optimized by Toro et al. [27], Rocchetti et. al [12,17], and Amato et al. [14] within the FP7 European Project Hydro WEEE-DEMO (grant agreement No. 308549). Overall, the main process’ innovation is the combination of known techniques, already used for metal extraction and recovery for both waste and minerals, for LCD treatment [15,31,32,33]. Starting from the previous achievements, the present research aimed to further improve weaknesses in the process to increase the entire sustainability level. This aspect was already confirmed by preliminary evaluations carried out by the life cycle assessment tool, which also identified the main process’ weaknesses (e.g., waste pretreatment) [12,16,34]. In more detail, preliminary grinding to obtain a particle size < 1 mm allowed degradation of the organic components, due to the liquid crystal, without the use of high-impact oxidizing agents. Furthermore, the mechanical process, carried out in the absence of water, with an electricity consumption of 0.015 kWh/kg of treated panel avoids the production of hazardous wastewater flows that will need to be managed [17]. This pretreatment was followed by a cross-current acid leaching (2 or 3 steps, under stirring) by sulfuric acid, which ensured both reduction in consumption of the raw material and the increase in metal concentration in the solution. A final zinc cementation allowed indium recovery after pH adjustment by sodium hydroxide. The treatment produced indium, in the form of metal, with an efficiency higher than 90%. The cementation agent represents around 10% of the process’ economic cost, and it ensures a high process effectiveness. Furthermore, economic potential of the process was further increased by the indium features in the waste, compared to the element in the primary ores (e.g., bond geometries, mechanical properties), which improved the extraction efficiency [35,36]. A system of wastewater recirculation increased the sustainability of the entire process using the flow from cementation for the following leaching treatments. Treatment of the manufactured mortar prevented the glass cullet from leaching. The glass used for LCD panels is generally alkali-free; nevertheless, the content of alkali was assessed and had a very low result (Ca 68.9, Na 28.5, K 4.2, and Mg 0.5 mg/kg as reported by Ruello et al. [24]). In any case, concerning the risk of an alkali–silica reaction (ASR), it is possible to fully replace traditional calcareous gravel with waste glass as recycled aggregate for the production of mortars, without any particular addition or admixture when glass is finely grinded, as in this case [24,37]. Preliminary washing removes sulphate from the scraps, and they are mixed with water, Portland cement, sand and lime, or calcium carbonate to produce mortars with different qualities [24]. With the aim of reducing acid flow from the scraps, washing stream is used to produce the sulfuric acid leaching solution mixed with cementation waste flow.

3. Results and Discussion

Figure 2 shows the entire developed process, including all necessary phases for end-of-life LCD use.

As described in the previous paragraph, improvement of the process included the addition of a grinding step to obtain a particle size smaller than 1 mm. This mechanical treatment, which substitutes high-impact washing reported in the literature, allowed the removal of organic compounds. In this regard, Figure 3 shows the liquid crystal presence in the panel scraps with size around 10 mm (highlighted by iridescence in Figure 3a) and the complete absence at the end of grinding (Figure 3b).

The second block includes an extraction carried out by a 1–2 M sulfuric acid solution, with a solid concentration of 20%, at 60–80 °C for about 20 min, under stirring. To implement the cross-current design, the solution from the first leaching step was used for the following indium extraction, after refreshing about 10% of the volume to ensure an acidic pH. Wastewater recirculation can be repeated up to three times without a significant decrease in efficiency. The extraction treatment produced two output flows, the solid scraps and a leaching solution, to send to further recovery. The solid was observed by scanning electron microscopy (SEM), equipped with EDS EDAX detecting unit (PHILIPS ELECTRONICS N.V., Eindhoven, The Netherlands, 1992), to know the indium content before (Figure 4a,b) and after cross-current acid leaching (Figure 4c,d). Microstructural investigation identified two different morphologies within the same sample. It was possible to recognize agglomerates of small particles (10–20 µm) and single particles (0.5–1 mm). This description is common for both samples before and after treatments. Elemental analysis, performed on both morphologies (on three different points of each sample), showed a higher indium concentration before leaching (2.2%, wt%) than after the treatment (0.5%, wt%), without differences between single particles and agglomerates. The following recovery step included a preliminary pH adjustment up to 2.5–3 by sodium hydroxide at 60 °C under stirring. When the operative conditions were fixed, zinc powder was added at a concentration around 3 g/L (0.005 kg of Zn/kg of treated panel and 30 kg of Zn/kg of recovered indium), and the reaction was carried out for 20 minutes. The treatment produced indium, in the form of metal, with an efficiency higher than 90%. Among the extracted metals, aluminum represented the main impurity in the product, despite its low extraction yield [16]. Nevertheless, its presence is not critical since the literature describes different applications of alloys with indium and aluminum content [38,39]. Concerning tin, the other ITO component, a recovery lower than 10% of the waste content was observed [16]. However, this element was not chosen as the process target because of both its low concentration [13] and market price compared to indium (290 €/kg of indium vs 18 €/kg of tin) [40,41].

Additional production of mortar, classified as compressive strength CS II and CS III (suitable for general works or as a plaster for indoor/outdoor), allows overcoming the process’ criticality connected to leaching solid waste, around 99% of the entire amount [42]. The best selected conditions for mortar production include a two-step washing of the leaching solid waste, as already discussed in [23], at room temperature for 20 min with a solid:liquid ratio of 1:3 by weight. Both the produced mortars included a quantity of solid waste (glass cullet) around 40%, mixed with additional components, as reported in

Table 1. Best composition by weight selected for CS II and CS III mortar production.

Mortar CS II	Mortar CS III
40% glass cullet	40% glass cullet
18% water	20% water
4% Portland cement	8% Portland cement
28% sand	24% sand
10% lime	8% calcium carbonate

. Considering implementing the process in a real-world context, and the related pending patent, the agent’s ratio can be slightly adjusted based on the required compressive strength.

Though a complete “zero waste” approach is relatively hard to achieve [28,43], the high effectiveness of a wastewater recirculation system ensures an almost complete recovery of the liquid flows, avoiding a big quantity of streams to manage. In this regard, the cross-current design allows a solution savings around 60%, which could significantly increase thanks to the possibility of using the resulting solution from cementation, after a volume refresh of 50% to ensure the acid conditions. Furthermore, the acid solution for extraction can be prepared by the stream from glass cullet washing (before the mortar production), considering its sulfuric acid content. Overall, this recirculation design produces and almost completes water and sulfuric acid savings that, combined with LCD use as secondary raw material, further increases the process’ sustainability. This aspect is confirmed by the determination of a sustainability index as a ratio of output waste and input flows, following the method proposed by Nelen et al. [44]. Indeed, the achieved index value was 0.10 (in a 1–0 range, where 0 is complete zero waste), thanks to almost complete flow recirculation and a discharge of only 50% of the cementation solution. In the perspective of scaling up the process, the avoided waste disposal and the minimum raw material consumption reflect the three sustainability spheres: environmental, economic, and social. As reported by Geissdoerfer et al., less scraps are good for the environment, organizational profits, and consumer prices [25].

4. Conclusions

Considering the current number of end-of-life LCDs to manage, development of highly efficient and sustainable processes, within the urban mining field, represents a priority. In this context, this paper aims at combining known treatments, overcoming the identified weaknesses, to achieve a whole innovative process for panel use. The simple design process allows an indium recovery efficiency higher than 90% and makes it suitable for fulfilling an entire management system for the enhancement of LCDs [45]. Considering the strategic role of indium, recycling further fractions (e.g., plastic, printed circuit boards) could have a significant impact from an economic point of view [14,45,46]. Compared to the current literature, the treatment solves one of the main panel recycling bottle-necks due to the relatively low indium concentration. Indeed, optimized grinding pretreatment allows the metal to be concentrated and combined with degradation of the organic component. The process’ economic value is further enhanced by the almost avoided disposal of both solid and liquid streams, in accordance with the “zero-waste approach”. Indeed, mortar production allows complete use of the glass cullet from leaching. On the other hand, the saved leaching solution, achieved thanks to the cross-current design (around 60%), can be added to a further 50% of wastewater recirculation from the recovery step to avoid discharge of the flow from washing the scraps. The simple design of the process represents its main strength, since it makes the treatment suitable for scale-up at different levels. Compared to treatments showed in the literature, the possibility to add value to all treatment streams can be translated into an economic advantage thanks to multiple aspects: the possibility to obtain two products that will be placed on the market as well as further enhancement by minimized long-term costs for the environment and human health. Indeed, the use of a high-efficiency recirculation system, with a reduction in stream discharge, is combined with the use of low-toxicity agents. Considering the results, the present work represents an excellent example of circular economy implementation where the avoided waste disposal in landfill sites is combined with its entire enhancement.

5. Patents

Beolchini F., Amato A., Mariani P., Carducci F., Ruello M.L., Monosi S. A zero waste method for the treatment and the enhancement of end-of-life liquid crystal displays. Italian Patent No. 102018000008207.