Glass-ceramics obtained by the recycling of end of life cathode ray tubes glasses

Transcription

1 Waste Management 25 (2005) Glass-ceramics obtained by the recycling of end of life cathode ray tubes glasses F. Andreola a, L. Barbieri a, *, A. Corradi a, I. Lancellotti a, R. Falcone b, S. Hreglich b a Dipartimento di Ingegneria dei Materiali e dellõambiente, Facoltà di Ingegneria, Università di Modena e Reggio Emilia, Modena, Italia b Stazione Sperimentale del Vetro, Murano, Italia Accepted 17 December 2004 Abstract This work is concerned with open-loop recycling of end of life Cathode Ray Tubes glass (an unsolved problem when considering that in Europe almost 90% of EOL electronic goods is disposed of in landfills), focusing on the development of glass-ceramics from panel or funnel glass with dolomite and alumina, and the evaluation of the tendency towards crystallisation with particular attention on composition and thermal treatment. Glasses were melted at a temperature of about 1500 C and transformed into glass-ceramics by different thermal treatments (900 C to 1100 C temperature range and 0.5 to 8 h soaking time). By using the evaluation of thermal, mineralogical and microstructural data it has been pointed out that a good degree of crystallisation is reached at about 1000 C and with a high proportion of waste glass (50 75%) if 40 45% of CaO and MgO bearer (dolomite) is introduced. In this way alkaline and alkaline-earth silicate and aluminosilicate mainly develop probably with a surface mechanism. Ó 2005 Elsevier Ltd. All rights reserved. 1. Introduction A recent problem regarding the industrialised countries is represented by the management of the electronic wastes, 80% of which comes from TVs and computers. The electronic products are one of the segments of consumer goods in rapid expansion, and both the technological innovation and the increased availability on the market facilitate the substitution process of the existing models (average life of 10 and 4 y for TV and computers, respectively). The EU produces around 7.5 million tons of electrical waste every year (Marshall and Henderson, 2001) and this trend is increasing. In fact, an increase of 3 5%/y is expected, corresponding to a percentage of 16 28% in 5 y (CEC, 2000). In Europe, almost 90% of the end-of-life (EOL) electronic goods is disposed of in landfills causing pollution problems due to the hazardous elements contained. However, some * Corresponding author. Tel.: ; fax: address: luisab@mail.unimo.it (L. Barbieri). industrial operations for dismantling of such products have been initiated in Europe. This operation yields a wide range of sorted materials such as metals, circuit boards, plastics and, in the case of television sets or computers, Cathode Ray Tubes (CRT), which represent about two thirds of the weight of a television or a computer monitor, which is composed of about 85% glass. In particular, for this kind of waste, the amount produced in Western Europe is 300,000 ton/y (CEC, 2000). While recycling techniques and outlets for the other materials already exist, those for the CRT glass are still at the beginning and are being developed in countries such as the UK, Spain, and Italy (Doring, 2002). On the other hand, in the Scandinavian countries and Benelux, the acceptance of TV glass recycling is high. In order to solve this problem, from 1997 to 1999 the Recytube Project financed by the European Commission, set up a technology able to produce economically a high-quality Secondary Raw Material (SRM) from the processing of EOL CRT glass. In this way all contaminations, metal, plastics, silicon, as well X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi: /j.wasman

2 184 F. Andreola et al. / Waste Management 25 (2005) as functional coatings (fluorescent layers, graphite, iron oxide) are removed. This SRM should be usable both in the CRT glass making process (closed-loop recycling), and in other applications (open-loop recycling). CRTs are made up of four different glass components: panel, funnel, frit and neck, each one having a particular chemical composition very different from the others. For this reason the CRT glass crushed and mixed together cannot be recycled as cullet, for example in the production of industrial glass (container glass, tableware glass, TV glass, etc.). The front part (panel) of a CRT is made of bariumstrontium glass, now free of lead. It is a very homogeneous glass, like an optical glass and is quite thick in order to absorb the UV and X-rays produced by the electron gun. The inside part of the TV set (funnel) is a lead glass, thinner than the panel, which blocks all of the UV and X-rays that escape laterally from the TV set. Regarding a European experience (Doring, 2002), it seems that it is possible to reuse the complete glass fraction (panel, funnel and mixed glass from panel and funnel glass) in the production of new TV glass. Due to their lead content, funnel and mixed glass can only be reused in the funnel glass production; only perfectly separated and definitely lead free panel glass can be reused in the panel glass production. By considering an open loop-recycling system in the glass industry (container glass, tableware glass and glass fibers), where it is forbidden to introduce toxic elements such as Pb and As, only the panel glass could be employed as a SRM. On the contrary, in the ceramic industry the recycling of CRT glasses seems to be more widespread and panel and funnel glass could be used as SRM, for example in the production of glazing (Hreglich et al., 2001; Siikamaki and Hupa, 2001). With the aim of valorising these waste glasses, in this paper the preliminary results reached on the possibility of transforming EOL CRT glass into glass-ceramics are presented. Glass-ceramics are very attractive for material developers; by composing a glass composition and designing a glass-ceramics microstructure by means of well chosen heat treatment for ceramization, developers have high flexibility in tailoring materials with specific properties. In this point of view, panel and funnel glasses were mixed with other raw materials and were subjected to different heat treatments to promote the devitrification process. 2. Experimental The panel and funnel cullet glass used in this work are representative of EOL CRT from coloured TVs and comes from the separation process set up by the VALLONE plant (Anagni, Italy) within the Recytube project. These glasses have been characterised by a chemical (ICP Varian Liberty 200), thermal (Netzsch DSC 404 from 20 to 1400 C at the heating rate of 10 C/min in static air on powdered sample at 30 lm and Optical Dilatometer HT Misura 3 on bars cm in size with a gradient of 10 C/min) and mineralogical (XRD powder diffraction in the 2h =5 60 range by a Philips PW 3710 equipment) point of view. Optical heating microscopy (Misura 3.32 Microscope) was used on pressed glassy powders by recording the images of the profile of the compact at regular temperature intervals of 5 C from sintering to melting using a heating rate of 10 C/min. Due to the particular stability of the amorphous material towards crystallisation (underlined by thermal and diffractometric results reported below), addition of alumina and dolomite (see Table 1 regarding the chemical analysis of the raw materials) has been necessary in order to formulate the ternary compositions reported in Table 2, trying to use high amounts of starting glass (50 75wt%). These glasses were prepared by melting the raw materials in refractory crucibles around 1500 C. The melt was poured into a graphite mould and quenched in air. The crystallisation was induced by thermal treatments in the C temperature Table 1 Chemical composition (oxides in weight %) of the starting raw materials Oxide Panel Funnel Alumina Dolomite SiO Al 2 O CaO MgO BaO SrO Na 2 O K 2 O Fe 2 O TiO ZrO ZnO PbO NiO CoO MnO 0.01 P 2 O Table 2 Weight % ratio of the component of the ternary glass-ceramic formulations Composition code Panel Funnel Alumina Dolomite P P P P F F

3 F. Andreola et al. / Waste Management 25 (2005) range with soaking time from 30 min to 8 h. The tendency of the quenched glass specimens to crystallise was evidenced by differential thermal analysis and the crystalline phases developed were detected by X-ray powder diffractometer in the same experimental conditions reported above. Microstructural analyses were performed on fresh fractured glass-ceramics embedded in epoxy resin, polished with alumina paste and coated with Pd Au film. Scanning electron microscopy (Philips SEM XL 40) coupled with X-ray microanalysis was used for these analyses. 3. Results and discussion The values obtained by the chemical analysis applied on the panel and funnel glass (Table 1) are in agreement with Hreglich et al. (2001); underlining that panel is a Ba and Sr rich glass, while in the funnel there is a high Pb percentage making the use of this glass in the glass industry difficult, as discussed in Section 1. The characterization of the parent glasses evidenced that these EOL CRT glasses are very stable towards devitrification (essential requirement in the glass industry) as confirmed by the XRD broad band and the thermograms recorded on the glassy powders. Furthermore, the high thermal dilatability of the examined materials is imputable to the high percentage of alkaline oxides present in the glasses. In particular, the higher PbO and the lower SiO 2 content present in the funnel glass with respect to the panel, translatable in a structural relaxation, is probably responsible for a light increase in thermal expansion coefficient of funnel and in a decrease of the temperatures corresponding to the main transformations of the amorphous structure such as glass transition, softening and melting temperatures. From the temperatures detected by Optical Heating Microscope the low melting nature of both the glasses is underlined as well. By considering the glass composition, due to the high alkaline oxides content and the alkaline-earth oxides and alumina deficiency, the necessity to adjust the composition by adding Ca, Mg and Al oxides to obtain a glass-ceramic with high properties appears evident. The prepared compositions have been formulated on the basis of the role played by these ions. In particular, dolomite has been used to introduce in the composition alkaline-earth Ca and Mg oxides, which can favour the crystallisation process, because of their high field strength, unlike the still present stabilising Sr and Ba oxides which, due to the higher dimension atoms, have lower field strength. On the other hand, alumina has been used to introduce Al oxide, which improves the chemical resistance and hardness of the glass. Further alumina, when the composition has the tendency to crystallise, can favour the formation of aluminosilicates in the glass-ceramic. The formation of this kind of crystalline phase is a positive effect because these phases are contained in many commercial glass-ceramics, such as Pyroceram, Sitalls, CerVit, Slagceram, etc., conferring good technical properties to the material (Strnad, 1986a,b). The behaviour of the formulated compositions on the base of the role played by the introduced cations was followed by the Ginsberg method (Garcia Verduch, 1980) usually adopted for studies on natural stones. Thus, the Ginsberg diagram (Sal = SiO 2 +A1 2 O 3 ; Cafem = CaO + (FeO and Fe 2 O 3 ) + MgO; Alk = Na 2 O+ K 2 O) reported in Fig. 1 shows the starting glass (P, F) and mixtures with dolomite and alumina (P1, P2, P5, P6, F1, F2) compositions. All compositions are located near 60 70% Sal band, within which Ginsberg predicts optimized glass-ceramic formation. Below this zone, the high content of modifier oxides weakens the glass network, making it very mobile. Above 70% SiO 2, however, the network becomes such a rigid structure, making the processing operations too difficult. The oxides included in the three diagram vertexes consider wt% of the glass composition, but the trend is representative of the glass-ceramic behaviour because the excluded oxides, Sr, Ba and Pb, have a low tendency for crystallization. In fact Pb also presents low field strength and large radius (1.32 Å) comparable to those of Ba and Sr ions. While considering the EOL CRT glass composition effect, from Fig. 2 it appears evident that in the funnel-containing compositions the crystallisation capacity is higher than in panel-containing ones. In particular with a CRT glass/alumina/dolomite ratio corresponding to a 75/5/20 (P1 and F1) the exothermic peak is evident for the funnel-containing composition only, moreover the crystallisation is favoured when the dolomite amount increases (glass/alumina/dolomite Cafem F Sal P5 P P1. F P2 F1 P6 Fig. 1. Starting glass (P, F) and mixtures (P1, P2, P5, P6, F1, F2) compositions in the Gingsberg diagram Alk

4 186 F. Andreola et al. / Waste Management 25 (2005) Fig. 2. (a) DTA curves of (1) P1 and (2) P2 glasses; (b) DTA curves of (1) F1 and (2) F2 glasses. Fig. 3. SEM micrograph of P6 glass-ceramic treated at 900 C for 2 h. ratio corresponding to 50/10/40 in P2 and F2 compositions). These observations support the role played by Ca and Mg cations on the crystallisation. By the evaluation of DTA data, the ratio between glass transition and melting absolute temperature, T g /T m, was obtained. This ratio is, according to Zanotto (1987) and James and Jones (1992), a useful parameter in identifying the crystallisation process. These authors observed for glasses which crystallise internally (homogeneous nucleation) T g /T m < 0.58, whereas for glasses which crystallise only from the surface (heterogeneous nucleation) T g /T m > 0.58: in the studied glasses the ratio always resulted in being >0.60 and therefore surface crystallisation is probably favoured with respect to internal crystal nucleation. The crystallization mechanism is supported by SEM analysis (Fig. 3), which shows that the crystal growth starts from the surface and proceeds towards the internal zone. This information could be important from a technological point of view, i.e., in the perspective of obtaining glass-ceramics by the sintering of powders and the authors are investigating this option. As a consequence of the different thermal treatments performed on the glassy samples, the XRD patterns showed a progressive increase of the reflections, attributed to different crystalline phases, dependent

5 F. Andreola et al. / Waste Management 25 (2005) upon the composition more than on the thermal treatment. The positive role played by Ca and Mg on the crystallisation is evident from the comparison of the XRD patterns of Figs. 4 and 5; the resolution and intensity peaks increase by increasing the dolomite content. With the panel-containing formulations, akermanite, Ca 2 MgSi 2 O 7 (JCPDS file ) is the common phase and it is present as prevalent crystals in P2 and P6 formulations independently from the thermal treatment. Nepheline, K(Na, K) 3 Al 4 Si 4 O 16 (JCPDS file ) is also present in P2 and forms as the main phase in P5, because the decrease of the CaO and MgO bearer Fig. 4. XRD pattern of P5 glass-ceramic heat treated at 1000 C for 2h.(d) nepheline; ($) celsian; (r) akermanite. Fig. 5. XRD pattern of F1 glass-ceramic heat treated at 1000 C for 2h.(s) diopside; ($) akermanite. (dolomite) in the batches and the corresponding increase of alkaline oxides and alumina present in the starting CRT glasses favour its formation. Furthermore, the increased amount of alumina in P5 also favours the crystallization of celsian, BaAl 2 Si 2 O 8, (JCPDS file 19-90) aluminosilicatic phase. Moreover, in the P2 compositions only, as a less important secondary phase, crystals of Ba Sr aluminosilicate are identified. In P6 some crystals of calcium and magnesium silicates (akermanite Ca 2 MgSi 2 O 7 and monticellite CaMgSiO 4 (JCPDS file )), are present as main phases because of the higher amount of Mg and Ca oxides introduced. A small amount of a Ba-silicate has been identified, instead of celsian, at a low treatment temperature (900 C for Fig. 6. (a) EDS spectra and (b) SEM micrograph of F1 glass-ceramic treated at 1000 C for 2 h.

6 188 F. Andreola et al. / Waste Management 25 (2005) Fig. 7. SEM micrograph and EDS spectra of P5 glass-ceramic treated at 1100 C for 2 h. 2 h), which disappears at 1000 and 1100 C; this formation is probably due to the low aluminum content in the P6 glass, which is not enough for the formation of the Ba-aluminumsilicate (celsian). By considering the F glass-ceramics, similar crystalline phases to those identified in P glass-ceramics are identified when considering the main silicates and silicoaluminates (akermanite and nepheline). This crystallization behaviour derives from the raw material ratio utilized. The differences evidenced are caused by the different chemical composition of the parent glass (absence of Ba and Sr oxides). In the F2 glass-ceramics akermanite and nepheline, as prevalent phases, are present, together with monticellite; while in F1 the main crystalline phase present is diopside Ca (Mg,Al)(Si,Al) 2 O 6 (JCPDS file ) followed by akermanite. Notwithstanding in the F1 composition 15 wt% of PbO is present; any Pb-containing crystalline phase has not been identified, confirming that this element remains in the glassy matrix and does not take part in the devitrification process, in agreement with the EDS spectra reported in Fig. 6. SEM analysis supported XRD results. The samples P glass-containing show different kinds of crystals: grey crystals rich in Ca and Mg imputable to akermanite, dark grey crystals enriched in Na and K characteristic of nepheline, and Ba-containing acicular white crystals of celsian (Fig. 7). The obtainment of a glass-ceramic material with a high crystallisation degree is reached at 1000 C after 4 h, as evidenced by the time temperature-transformation TTT curves (Fig. 8) drawn corresponding to the time/temperature coordinates for which the XRD patterns, performed on the powder of the bulk treated samples, showed sharp peaks at first, indicating crystalline phase formation. The shift between the crystallization temperature interval detected by DTA ( C) and the experimental complete devitrification which occurred in the samples is probably due to the fact that in DTA the samples were pulverized (30 40 lm) and in a low amount (40 mg), while the ther- Fig. 8. TTT diagram of the P2 composition showing (1) start, (2) partial, (3) total and (4) absent crystallisation.

7 F. Andreola et al. / Waste Management 25 (2005) mal treatments were performed on bulk samples of a few grams. 4. Conclusions SRMs originating from the separation and purification of the main glasses (panel and funnel) constituting the EOL CRT of a TV seems a valuable raw material. The results obtained indicate that these kinds of waste glasses are very versatile because if they are mixed with suitable raw materials (dolomite and alumina) and subjected to adequate thermal treatments they can be used in glass-ceramic manufacture. The different crystalline phases identified in the glass-ceramic formed is a consequence of the different SiO 2 /Al 2 O 3 ratio in the formulations. The prevalent formation of silicate or aluminosilicate is related to the major amount of dolomite or alumina, respectively, in the formulated compositions. This study demonstrates that the reutilization of these kinds of glasses allows technologically interesting crystalline phases, i.e., akermanite and nepheline, which are contained in semicrystalline materials with high mechanical strength, in tableware (Strnad, 1986a,b) and in dielectrical substrates (Francis, 1989) to be obtained. References Commission of the European Communities, Proposal for a Directive of the European Parliament and of the Council on WEEE. Doring, E., TV glass recycling in Europe - description of the situation and possibilities, 1-2 ( kimokelatv_glass_summary.pdf). Francis, F.W., Glass-ceramics suitable for dielectric substrates. Corning, Inc. USA Patent. Garcia Verduch, A., Materiales Obtenidos a Partir da Rocas y Escorias Fundidas, , in Proceedings of the Jornadas Cientificas Sobre Ceramica y Vidrio, Oviedo. Hreglich, S., Falcone, R., Vallotto, M., The recycling of end of life panel glass from TV sets in glass fibres and ceramic products. In: Telford, T. (Ed.), Recycling and reuse of glass cullet, pp Marshall, M., Henderson, J., New approaches to the challenge of CRT recycling. In: Telford, T. (Ed.), Recycling and reuse of glass cullet, pp James, P.F., Jones, R.W., In: Cable, M., Parker, J.M. (Eds.), High-Performance Glasses. New York, pp Siikamaki, R., Hupa, L., In: Telford, T. (Ed.), Utilisation of EOL CRT-glass as a glaze raw material, pp Strnad, Z., 1986a. Glass-ceramic Materials, Glass Science and Technology, vol. 8. Elsevier, Amsterdam, pp Strnad, Z., 1986b. Glass-ceramic Materials, Glass Science and Technology, vol. 8. Elsevier, Amsterdam, pp Zanotto, E.D., Isothermal and adiabatic nucleation in glass. J. Non-Cryst. Solids 89,