RECYCLING OF CRT WASTE GLASS IN PORCELAIN STONEWARE PRODUCTION

Intensity Ratio) method. Data have been recorded in the 5-140° 2 range (step size 0.02° and 6s counting time). The phase fractions extracted by the Rietveld-R.I.R. refinements, using GSAS software and EXPGUI as graphical interface [5] have been rescaled on the basis of the absolute weight of corundum originally added to the mixtures as an internal standard, and therefore internally renormalized. Finally, microstructural studies of the samples were conducted by a scanning electron microscopy, SEM, (Philips, XL 40) on superficially polished gold-coated specimens.

RESULTS AND DISCUSSION The dilatometric sintering plots are presented in Fig. 2. The traces of plant C-35/0 composition highlight a beginning of the sintering at 1100-1120 °C (i.e. after the melting of feldspars) and end of the densification corresponding to a shrinkage, ΔL/L o, of about 8 % at 1260-1270 °C. At about 1280 °C, due to the load of the dilatometer push-rod, the increase of the amount of liquid phase and the decrease of its viscosity starts the deformation of the sample. -9% -7% -5% -3% -1% 1%

950 1000 1050 1100 1150 1200 1250 1300 T ( o C)s h r i n k a g e ( % ) C-0/10 C-0/20 C-0/25 C-0/15 C-0/35 C-35/0 C-0/0 Fig. 2 Sintering dilatometric curves In C-0/0 composition, where the total amount of feldspars is very low, the sintering is negligible: the densification starts at 1170-1190 °C and at 1300 °C ΔL/L o reaches only 3 %. The addition of CRT glass in other compositions significantly improves the densification and up to 1050-1100 °C the degree of sintering increases as a function of CRT percentage in the batch. At higher temperatures C-0/20, C-0/25 and especially C- 0/35 show a significant decrease of the shrinkage rate, while C-0/15 and C-0/10 have sintering curves somewhat similar to C-35/0 standard. The deformation in C-0/35, C- 0/25, C-0/20 starts at about 1250, 1260 and 1270 °C, respectively (i.e. at lower temperature than in the plant ceramic), while in C-0/15 and C-0/10 the deformation takes place at >1300 °C. The porosity variations of C-0/0, C-0/10, C-0/15, C-0/20, C-0/25 and C-0/35 after

min sintering at 1200 °C are presented in Fig. 3. The densification of C-0/0 is insignificant, while the addition of only 10% G in C-0/10 decreases the total porosity from 21 to 15 %. In the other compositions PO decreases but PC increases as a function of the CRT addition; as a result, PT remains always high, reaching 18 % in C-0/35. The formation of high % of closed porosity in C-0/20, C-0/25 and C-0/35 may be explained by the presence of elevate amount of low viscosity liquid phase, formed by the CRT glass; this melt traps the CO2 gassy phase, developed at 900-1000 °C by the decomposition of carbonates (~ 1%), presented in the raw materials. Similar phenomenon is not observed in the usual ceramic batches. In fact, the C-35/0 sample, sintered at 1200 °C, shows about 6 % PO and only about 2 % PC . 0 5 10 15 20

10 20 30 CRT %p o r o s i t y ( % ) total porosity open porosity closed porosity Fig. 3 PT, PC and PO 1200 °C vs. CRT % in the batch In view of the obtained results it was concluded that a sintering behaviour, similar to the plant composition, may be obtained if F is only partially substituted by G. Different compositions with reduced amount of feldspars were sintered at 1200 °C and the degree of densification was studied by pycnometry. Then four batches (C-31/5, C- 27/10, C-25/7 and C-21/9 were selected and sintered in the industrial kiln, producing C- 35/0 composition. The phase compositions of obtained ceramics, evaluated by combined Rietveld- R.I.R method, are shown in Table 3; the technological and mechanical properties are summarized in Table 4. The phase analysis show that the addition of CRT glass accelerates the quartz dissolution, which increases the amount of formed amorphous phase. In fact, in C-31/5 and C-27/10 the sands amounts in the batches are similar to C-35/0, while the residual quartz is significantly reduced. In C-25/7 and C-21/9 the sands amounts are higher by

and 5 %, respectively, whereas the percentages of residual quartz are lower and similar to one of the plant ceramic. At the same time, nevertheless of the decreased percentages of sodium feldspar in the new batches, the amount of un-react albite in new ceramics is higher than inthe plant batch. This phenomenon may be explained by G chemical composition, which modifies the equilibrium between plagioclases and melts at the sintering temperature. The presence of Na2O and K2O in the CRT glass leads to formation of a melt, partially saturated of alkali oxides. Additionally, due to their big radius Ba2+ and Sr2+ decrease the diffusion coefficient of Na+, thus reducing the rate of albite melting. Table 3. Phase compositions of the final ceramics C-35/0 C-31/5 C-27/10 C-25/7 C-21/9 quartz 22.7 19.8 18.5 20.6 23.4 mullite 4.8 4.2 2.6 3.7 3.2 albite 3.6 4.7 6.1 4.2 6.7 glassy phase 68.9 71.3 72.7 71.4 66.7 Table 4. Sintering parameters, mechanical properties ( E, G, v) and calculated total (PT), open (PO) and closed (PC) porosities for the studied samples C-35/0 C-31/5 C-27/10 C-25/7 C-21/9 LS (%) 7.46 7.60 7.34 7.29 6.85 WA (%) 0.015 0.004 0.007 0.028 0.027 PT (%) 5.3 5.1 5.4 5.9 7.0 PC (%) 4.1 4.2 4.6 4.9 5.7 PC (%) 1.2 0.9 0.7 1.0 1.3 E (GPa) 70.6 71.7 68.6 68.7 67.7 G (GPa) 29.5 29.7 28.5 28.7 28.4 v 0.198 0.201 0.208 0.200 0.196 Table 4 shows that the degree of sintering and the properties of new compositions correspond to the standards for porcelain stoneware production [2]. C-31/5 and C- 27/10, which contain higher % of fluxes (i.e. F +G), show improved sintering than the plant ceramic, while C-25/7 and C-21/9, where the total amount of fluxes is lower, have a little bit higher WA and closed porosity. According to the dilatometric sintering curves (Fig. 2) of these two ceramics some improvement of the sintering, and as a consequence of the mechanical properties, may be obtained at higher with 10-20 °C sintering temperature. The similar structures of the final ceramics were confirmed by SEM studies. Fig. 4 shows images of C-35/0 and C-27/10, highlighting the good degree of the sintering and the improvement of the densification after the addition of CRT glass. C-35/0 C-27/10 Fig. 4 SEM images of the structure of C-35/0 and C-27/10 The proposed new compositions give possibility to re-cycle significant amount of CRT glass and to diminish the used feldspars. Considering that the market price of feldspars, which natural reserves are limited, is 2-3 times higher than one of the other raw materials was estimated that the batch cost may be reduce by 10-15 %. CONCLUSIONS The possibility to substitute feldspar raw materials in a porcelain stoneware body with waste panel Cathode Ray Tube (CRT) glass was demonstrated. The low viscosity CRT glass has a positive effect on the quartz dissolution and on the formation of liquid phase, which gives a possibility to reduce the total amount of fluxes in the batch and to decrease its cost price. It was shown that the simple replacement of Na-feldspar with more than 10 % CRT glass leads to an abnormal formation of closed porosity. Compositions with technological and mechanical characteristics, similar to the plan ceramic, were obtained by substitution of 5-14 % feldspar with 5-10 % CRT glass. REFERENCES:

T. Manfredini, G. Pellacani, M. Romagnoli, L. Pennisi, Porcelainized Stoneware

Tile, Am. Ceram. Soc. Bulletin, 74 (5) (1995) 76-79.

G. Biffi, Porcelain stoneware, Ed. Gruppo Editoriale Faenza, 1997.

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K. Heritage, C. Frisby, and M.Wolfender, Impulse excitation technique for dynamic flexural measurements at moderate temperature, Rev. Sci. Instrum. 59(1988) 973-

A.C. Larson and R.B. Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 2000, pp86-748. m m