Structural characterization of some new glass ceramics from industrial wastes.

Ghiban, Alexandru ; Moldovan, Petru

1. INTRODUCTION

Glass-ceramics are fine-grained polycrystalline materials formed when glasses of suitable compositions are heat treated and thus undergo controlled crystallisation to the lower energy, crystalline state. The mechanical properties of glass ceramics are superior to those of parent glass. In addition, glass ceramics may exihibit other distinct properties which are beneficial for particular applications. Many scientists demonstrated the potential of turning silicate wastes into useful glass-ceramic products (Bernardo, 2008), (Jae-Myung Kim, 2004), (McMillan, 1979), (Rimon, 1999), (Yang J. Xiao, 2009). The general process involves the vitrification of a silicates waste, or a mixture of wastes, followed by crystallisation. Pilot plants have been successfully operated for the manufacture of these glass-ceramics, but unlike the situation with technical glass-ceramics produced from high purity raw materials for specific applications, industrially produced glass-ceramics from wastes are not yet widely commercially available. Indeed, if a wide application and commercial exploitation of the products is to be expected, concerns related to the toxic potential of products made from industrial wastes will have to be fully addressed and clarified, in order to ensure their acceptance by the public.

2. MATERIALS AND PROCEDURES

The investigated waste materials consisted of fly ash from lignite combustion, labelled as FA, electric furnace metallurgical slag after Fe-Ni production, and mixed quality soda-lime-silica glass, SLS. he industrial wastes were milled in a ring mill for 2min, homogenised, and then mixed in the weight proportion: FA/MBW/EF/SLS=30/25/30/15, which is labelled as BLD. The resulting blend was mixed with analytical grade quality CaO and two more mixtures were prepared, in weight proportions, BLD5: FA/MBW/EF/SLS/CaO =28.50/23.75/28.50/14.25/5.00 and BLD10: FA/MBW/ EF/ SLS/ CaO =27.00/22.50/27.00/13.50/10.00. Each final mixture was further milled in a ball mill (Spex, USA) for 45min in order to achieve a fine particle size.

Melting was performed in an induction furnace for 5min at 1410-1440[degrees]C in a platinum crucible. The melting temperature was measured with a double beam radiation pyrometer. After the completion of the melting, glass casting was performed centrifugally in a steel target. A glass layer is formed of maximum 4mm thickness. The cooling rate is estimated at 10[degrees]C/min, approximately. The glass was subsequently milled again in the aforementioned ring mill for 2 min (in batches of 15g) in order to attain even better homogenization. The resulting glass powder was melted again for 10 min (conditions as described above). Glassiness was checked by optical microscopy.

3. RESULTS AND INTERPRETATIONS

3.1 Thermal analysis of experimental glass ceramics

Results concerning thermal analysis are illustrated in Fig.1. The DTA curves of the experimental glass ceramics are as it follows: black for 0% CaO, red for 5% CaO and blue for 10% CaO. As one may see, for normal composition, respectively with no CaO, the crystallization temperature during heating was detected at 750[degrees]C. By increasing of CaO amount, the crystallization temperature may be increased in the range of 765[degrees] + 770[degrees]C. So for all three experimental glass ceramics the crystallization peak temperature is between 750[degrees]C + 800[degrees]C. Crystallization temperature during cooling may be observed in the range of 1000-1100[degrees]C. For 5% CaO and 10% CaO, the crystallization temperature is in range of 1080-1090[degrees]C, and for 0% CaO the crystallization temperature is at 1050[degrees]C. So, by increasing of CaO amount in the experimental glass ceramics, the crystallization temperature during cooling may also increase.

3.2 Microstructure of glass--ceramics. SEM analysis

Results concerning the structural analysis by scanning electron microscopy of the experimental glass ceramics are given in Fig. 2, Fig. 3, and Fig. 4. As one may remark in normal composition glass ceramic (respectively 0%CaO) a gradually crystallization may take place from the edge of the sample to the center. Also, a distribution of spherical crystals may be seen in the glass matrix. This behavior may be explained by the manner of spin cooling, which may push the crystals to the glass edge.

[FIGURE 1 OMITTED]

As is illustrated in Fig. 2, one may observe two types of crystals due to specific manner of crystallization. Dendritic shape crystals may have a linear disposal, in ordered and compact configuration. Also, new generation of crystals may nucleate and quickly grow on preexisted crystals.

[FIGURE 2 OMITTED]

The aspects of crystals in the field of nucleation illustrated in Fig. 3 shows that crystals may nucleate and agglomerate on preexisted crystals. At higher magnification, Fig.4, one may remark either dendritic shape of crystals, or well defined grown crystals in the vitreous matrix.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. CONCLUSIONS

From present paper the following conclusions may be drawn. By increasing of CaO amount, the crystallization temperature may be increased in the range of 765[degrees] = 770[degrees]C. So for all three experimental glass ceramics the crystallization peak temperature is between 750[degrees]C=800[degrees]C. Crystallization tempe-rature during cooling may be observed in the range of 1000-1100[degrees]C. For 5% CaO and 10% CaO, the crystallization temperature is in range of 1080-1090[degrees]C, and for 0% CaO the crystallization temperature is at 1050[degrees]C. So, by increasing of CaO amount in the experimental glass ceramics, the crystallization temperature during cooling may also increase. At normal composition glass ceramic (respectively 0%CaO) a gradually crystallization may take place from the edge of the sample to the center. This behavior may be explained by the manner of spin cooling, which may push the crystals to the glass edge. Two types of crystals due to specific manner of crystallization may be seen at higher magnification. Dendritic shape crystals may have a linear disposal, in ordered and compact configuration. Also, new generation of crystals may nucleate and quickly grow on preexisted crystals. Crystals in the field of nucleation may agglomerate on preexisted crystals, having either a dendritic shape, or a well defined shape.

5. ACKNOWLEDGMENTS

The work has been performed in the laboratory of Materials and Metallurgy, Department of Chemical Engineering, University of Patras-Greece, within the framework of Erasmus Socrates project in the year 2009.

6. REFERENCES

Bernardo E., Esposito L., Rambaldi E., A, Tucci, S. (2008). Hreglich-Recycle of Waste Glass into "Glass-Ceramic Stoneware", J. Am. Ceram. Soc., 91 [7] 2156-2162 (2008)

Jae-Myung Kim, Hyung-Sun Kim. (2004). Glass-ceramic produced from a municipal waste incinerator fly ash with high Cl content. J Eur Ceram Soc (2004);24:2373-82

McMillan P. W. (1979). Glass-Ceramics. 2nd edn. "Non-Metallic Solids", edited by J. P. Roberts. Vol. 1, 1979

Olland W. H & Beall G. (2002). Glass-Ceramic Technology, The American Ceramic Society, Westerville, OH, 2002

Rimon J.M. & Romero M. (1999). Boccaccini-"Microstructural characterization of glass ceramic obtained from municipal incineration fly ash", J. Mater. Sci, (1999). 18,4413-23

Yang J. & Xiao B. (2009). Boccaccini A.R.--"Preparation of low melting temperature glass-ceramics from municipal waste incineration fly ash", Fuel 88 (2009), 1275-1280 A.R.

Tab. 1. Calculated chemical composition of the glasses

 Glass Si[O.sub.2] [B.sub.2] [Al.sub.2] [Na.sub.2]O
 [O.sub.3] [O.sub.3]

GL0 46.93 5.99 10.21 6.61
GL5 44.54 5.69 9.69 6.27
GL10 42.23 5.39 9.19 5.95

 Glass [K.sub.2]O [Fe.sub.2] FeO [Cr.sub.2]
 [O.sub.3] [O.sub.3]

GL0 0.67 3.16 10.41 0.90
GL5 0.64 2.99 9.88 0.86
GL10 0.60 2.84 9.37 0.81

 Glass MgO CaO MnO

GL0 4.56 10.26 0.11
GL5 4.33 14.83 0.11
GL10 4.10 19.24 0.10