Whiteness process of tile ceramics: using a synthetic flow as a modifier agent of color firing

Transcription

1 Hyperfine Interact DOI /s Whiteness process of tile ceramics: using a synthetic flow as a modifier agent of color firing G. R. dos Santos M. C. Pereira M. Olzon-Dionysio S. D. de Souza M. R. Morelli Springer Science+Business Media Dordrecht 2013 Abstract Synthetic flow is proposed as a modifier agent of color firing in tile ceramic mass during the sinterization process, turning the red color firing into whiteness. Therefore, the 57 Fe Mössbauer spectroscopy was used to understand how the interaction of the iron element in the mechanism of color firing mass occurs in this system. The results suggest that the change of color firing can be alternatively due to two main factors: (i) diluting the hematite content in the sample because of the use of synthetic flow and (ii) part of the hematite is converted in other uncolored crystal structures, which makes the final color firing lighter. Keywords Synthetic flow Whiteness Ceramic materials Mössbauer spectroscopy 1 Introduction Ceramic tile industries in Brazil are the greatest producers and consumers in the world competing with countries like China, Italy, Spain and others [1, 2]. The typology of higher consumption in the domestic market is the ceramic tiles manufactured by the dry process in Santa Gertrudes, São Paulo, Brazil, where Proceedings of the Thirteenth Latin American Conference on the Applications of the Mössbauer Effect, (LACAME 2012), Medellín, Columbia, Novermber G. R. dos Santos (B) M. R. Morelli Departamento de Engenharia dos Materiais, Universidade Federal de São Carlos, , São Carlos, SP, Brazil geocris.rodrigues@gmail.com M. C. Pereira Instituto de Ciência, Engenharia e Tecnologia, Universidade Federal dos Vales do Jequitinhonha e Mucuri, , Teófilo Otoni, MG, Brazil M. Olzon-Dionysio S. D. de Souza Departamento de Física, Universidade Federal de São Carlos, , São Carlos, SP, Brazil maristolzon@hotmail.com

2 G. R. dos Santos et al. the industries use only red color firing clay. However, the fastest growing typology globally is Brazilian porcelain tiles (or stoneware tiles), a production which reached 60 million square meters in 2010 [1]. The reason for this growth is directly related to its highly technical and aesthetic performance, and consequently, with higher added value products [1, 2]. Among the basic requirements for the masses of porcelain tile is the white color firing, which restricts the use of raw materials containing high levels of chromophore oxides (mainly Fe 2 O 3 and TiO 2 ) so that the feldspars are used as the main fluxes of these compositions. Furthermore, clay found widely in Brazil is also not used in the porcelain tile masses due to the high content of iron and titanium oxides, thus causing a massive dependence on foreign markets where the large deposits of white clay is [3]. Knowledge in the area of developing whitening materials in ceramic is extremely limited in databases of papers, books or patents, and at the moment, there is no product on the market showing similar properties. To the best of our knowledge, only one study about the whitening process of ceramic materials has been reported [4]. In this study, Yatsenko et al. reported that the use of wt.% of calcite combined with clays containing large amounts of iron oxide after firing leads to the production of white ceramic. The whitening phenomenon occurs due to the crystallization of the okermanite ferrous phase, dicalcium ferrite, wollastonite and anorthite, mainly melilite, i.e., depending on the proportion of Fe 2+ or Fe 3+ present in the crystalline phases, the color effect performs differently. Thus, the increase in the concentration of calcium carbonate in the composition helps the growth of these crystals. On the other hand, it also increases the formation of free CaO and porosity, which damages the structure of the ceramic because it is less resistant to the compression [4]. In this short description of the state of art, it is important to reaffirm that the publications on this topic are extremely limited, and most of the knowledge that has been applied directly to traditional ceramic comes from this work [5, 6]. Thus, there is continuing research for new materials that can replace or aid the porcelain tile compositions. Therefore, this work proposes the development of a novel process to obtain white ceramic using a synthetic flow as a mediator agent of color firing. This process can partially replace feldspars and phyllites in these compositions and change the coloring of a ceramic mass during sinterization. The synthetic flow proposed herein consists of a viscous flow (glassy phase), which has a characteristic of an amorphous solid. In general, the chemical composition of the synthetic flow is formed by SiO 2 Al 2 O 3 M 2 O M O, where M is selected from the elements Na, K or a combination of them, and M is selected from among the elements Mg, Ca or a combination of them. This chemical composition promotes the devitrification of the flow during the firing cycle in crystals from the clinopyroxenes family [7 11]. 2 Materials and methods The ceramic sample used in this work, labeled SG, was obtained from Santa Gertrudes, São Paulo, Brazil. The iron oxide contents present in this sample are approximately 5 wt.%, which depreciates the value of these ceramics.

3 Whiteness process of tile ceramics In order to produce an added value material, 45 wt.% of synthetic flow (SF) was added to the 55 wt.% original ceramic sample and fired at 1,100 C for 5 min in air atmosphere. This sample was labeled 45SF/55SG. The chemical composition of the synthetic flow used in this work was wt.% SiO 2, wt.% Al 2 O 3, wt.% K 2 O, wt.% Na 2 O, 5 12 wt.% CaO, 1 5 wt.% MgO. X-ray diffraction (XRD) in powder form was carried out using CuKα with a Rigaku Geigerflex diffractometer equipped with a graphite diffracted-beam monochromator. Data were collected from 10 to 80 2θ in steps of 0.02 per 5 s iteration. Silicon was used as an external standard. Mössbauer spectra were collected in a constant acceleration transmission mode with a 10 mci 57 Co/Rh source. The spectra of all samples were taken at 298 K and Doppler velocities ranged between ±11 mm s 1. The data were stored in a 512- channel MCS memory unit and were fitted using the NORMOS program. Isomer shifts were quoted relative to α-fe. 3 Results and discussion Chemical analysis shows the SG sample to consist essentially of 68.2 wt.% SiO 2, 14.2 wt.% Al 2 O 3, 5.3 wt.% Fe 2 O 3, 3.6 wt.% K 2 O, 2.7 wt.% Na 2 O, 1.9 wt.% MgO, 0.7 wt.% CaO and 0.6 wt.% TiO wt.% synthetic flow were mixed to 55 wt.% SG sample and heated at 1,100 C for 5 min to produce a white ceramic (Fig. 1). A preliminary test showed that the whitening effect is clearly observed when the synthetic flow is added in levels above 30 wt.% in the SG sample. Moreover, the fusibility of the compositions increases as the proportion of the synthetic flow increases. Consequently, this leads to a densification, reduction in porosity and mechanical strength increasing the ceramic tiles, properties which are very favorable for manufacturing ceramic tiles because of their highly technical performance, such as porcelain tiles [12 19]. To fully understand the origin of the color change in the ceramics, XRD and 57 Fe Mössbauer measurements were carried out. The powder XRD patterns of all samples are shown in Fig. 2. The SF sample showed an amorphous region between 15 to 40 2θ, which is characteristic of the synthetic flow produced in this work. The SG sample basically consists of quartz. A minor contribution of illite and albite was also observed. The XRD pattern of the SG sample after heating at 1,100 C, labeled SG1100, is very similar to the XRD pattern of the SG sample before the thermal treatment. This sample is still dominated for quartz. However, illite should be converted into cristobalite at 1,100 C. After heating the mixture of synthetic flow and SG ceramic at 1,100 C, the XRD pattern of the 45SF/55SG sample (Fig. 2) was completely different from the other samples, indicating that the use of synthetic flow can promote structural changes in the original ceramic sample. The reflections of quartz decreased and new reflections due to diopside appeared in the diffraction pattern of the 45SF/55SG sample, suggesting that the synthetic flow can promote the migration of Si from quartz structure to form a new crystal, i.e. diopside. Although the XRD data were fitted assuming these dominant phases, contributions of other minor crystalline species cannot be excluded.

4 G. R. dos Santos et al. Fig. 1 Digital image of samples (a) SG1100 and (b) 45SF/55SG, after firing at 1,100 C Fig. 2 X-ray diffraction patternsofsamplessf (synthetic flow), SG (original ceramic, Santa Gertrudes), SG1100 (original ceramic heated at 1,100 C) and 45SF/55SG (mixture of 45 wt.% synthetic flow +55 wt.% original ceramic heated at 1,100 C). Legend: δ = albite, +=quartz, *= illite, = diopside Therefore the XRD data shows that the thermal treatment of the original ceramic sample (red color) in the presence of a synthetic flow induces structural changes that lead to the formation of new crystals (lighter color). In spite of this, the origin of this color change could not be completely understood based only on these data. The red color observed in the SG sample is mainly due to the presence of iron. However, it was not possible to specify iron in these samples in view of its low concentration in them. In order to understand the interaction of iron and different matrixes during the sinterization process at 1,100 C, 57 Fe Mössbauer spectroscopy measurements were carried out. The spectrum of SG1100 sample (Fig. 3, Table1) indicated that 62 % of the total iron is in the hematite structure and this is the main reason of the red color of that sample. Approximately another 13 % is due to Fe 3+ located in octahedral sites and about 25 % of Fe 3+ in tetrahedral sites. However, further studies are needed to define unequivocally the Fe sites in this sample.

5 Whiteness process of tile ceramics Fig K Mössbauer spectra of SG1100 and 45SF/55SG samples Table 1 Mössbauer parameters of the samples SG1100 and 45SF/55SG at room temperature Sample δ/mm s 1, ε/mm s 1 B hf /T Ɣ/mm s 1 RA/% 57 Fe site SG (2) 0.19(2) 49.7(3) 0.37(1) 62(1) Hematite 0.38(1) 0.88(3) 0.39(1) 13(1) VI Fe (2) 0.67(3) 0.34(1) 25(1) IV Fe 3+ 45SF/55SG 0.38(1) 0.15(2) 50.6(4) 0.35* 45(1) Hematite 1.27(2) 1.85(2) 0.30(1) 2(1) VI Fe (1) 0.76(2) 0.36(1) 52(1) IV Fe 3+ *Parameter fixed during the fit After the thermal treatment in the presence of the synthetic flow, the hematite content decreases from 75 % in the SG1100 sample to 45 % in the 45SF/55SG sample. Another 53 % of Fe was found in the tetrahedral site and only 2 % is due to Fe 2+ in octahedral coordination. Thus, we suggest two alternative ways that can contribute to the whitening process of ceramics: (i) (ii) Fe can migrate from the hematite structure to other crystal structures; The addition of synthetic flow causes a dilution effect of hematite, thus decreasing the intensity of the red color. 4 Conclusions In this work, a novel process for ceramic whitening was developed based on thermal treatment at 1,100 C of a mixture of red ceramic and synthetic flow. The thermal treatment in the presence of the synthetic flow suggests the reduction in the hematite content in the ceramic, which is mainly responsible for the red color in these samples. This process led to the production of a white ceramic, which has a market

6 G. R. dos Santos et al. added value. Nevertheless, further studies are needed to completely understand the mechanism of whiteness of ceramics promoted by the synthetic flow. Because of the huge gap of knowledge in the area of ceramic whitening, this work could bring a significant scientific-technological contribution to future studies. Acknowledgement We would like to thank FAPEMIG, FAPESP and CNPq for the financial support of this work. References 1. Anfacer.: Setor. (2012). Acessed 31 Oct Junior, M.C., et al.: Panorama e Perspectivas da Indústria de Revestimentos Cerâmicos no Brasil. Ceram. Ind. 15(3), 7 18 (2010) 3. Motta, J.F.M., Zanardo, A., Junior, M.C.: As Matérias-Primas Cerâmicas. Parte I: O Perfil das Principais Indústrias Cerâmicas e Seus Produtos. Ceram. Ind. 6(2), (2001) 4. Yatsenko, N., et al.: Regularities of tinting ceramics based on low-melting clays. Glass Ceram. 63(7), (2006) 5. Santos, G.R., Salvetti, A.R., Morelli, M.R.: Estudo de um aditivo descolorante de massas para revestimentos cerâmicos. Anais 54 Congresso Brasileiro de Cerâmica: Foz do Iguaçu PR (2010) 6. Santos, G.R., et al.: Composições de vidro soda-cal com massas cerâmicas vermelhas, processo de obtenção de artefatos cerâmicos, artefatos cerâmicos assim obtidos e uso dos mesmos. INPI. PI (2011) 7. Zanotto, E.D.: Crystallization of liquids and glasses. Braz. J. Phys. 22(2), (1992) 8. Barbieri, L., et al.: Crystallization of (Na 2 O MgO) CaO Al 2 O 3 SiO 2 glassy systems formulated from waste products. J. Am. Ceram. Soc. 83(10), (2000) 9. Saiello, S., et al.: Crystallization of glasses in the 2 Na 2 O.CaO.3SiO 2 -Na 2 O.2CaO.3SiO 2 Composition Range. Thermochim. Acta 52(1 3), (1982) 10. Poirier, T., et al.: Formation of crystalline phases in (SiO 2 -CaO-Fe 2 O 3 )-TiO 2 based glasses. Mater. Lett. 59(2 3), (2005) 11. Navarro, J.M.F.: El vidro. CSIC, España (2003) 12. Matteucci, F., Dondi, M., Guarini, G.: Effect of soda-lime glass on sintering and technological properties of porcelain stoneware tiles. Ceram. Int.28(8), (2002) 13. Martín-Márquez, J., Rincón, J.M., Romero, M.: Effect of microstructure on mechanical properties of porcelain stoneware. J. Eur. Ceram. Soc. 30(15), (2010) 14. Suvaci, E., Tamsu, N.: The role of viscosity on microstructure development and stain resistance in porcelain stoneware tiles. J. Eur. Ceram. Soc. 30(15), (2010) 15. Martín-Márquez, J., Rincón, J.M., Romero, M.: Effect of firing temperature on sintering of porcelain stoneware tiles. Ceram. Int. 34(8), (2008) 16. Rambaldi, E., et al.: Using waste glass as a partial flux substitution and pyroplastic deformation of a porcelain stoneware tile body. Ceram. Int. 33(5), (2007) 17. Zanelli, C., et al.: Glass-ceramic frits for porcelain stoneware bodies: effects on sintering, phase composition and technological properties. Ceram. Int. 34(3), (2008) 18. Luz, A.P., Ribeiro, S.: Use of glass waste as a raw material in porcelain stoneware tile mixtures. Ceram. Int. 33(5), (2007) 19. Tucci, A., et al.: Use of soda-lime scrap-glass as a fluxing agent in a porcelain stoneware tile mix. J. Eur. Ceram. Soc. 24(1), (2004)