P-D Refractories - Publikation Refractories Worldforum, No. 1/2009 - Tin Bath Bottom Blocks

TECHNOLOGY TRENDS refractories WORLDFORUM 1 (2009) [1] 1 brick by reacting with the brick's SiO 2 glassy phase. This reaction is accompanied by a re- duction in brick material which is shown by the greyish colour of used bricks. The trans- formation leads to the formation of albite (Na 2 O · Al 2 O 3 · 2 SiO 2 ) or nepheline (Na 2 O · Al 2 O 3 · 6 SiO 2 ) in the contact area of liquid tin and brick material. The significant difference between the thermal expansion coefficients of nepheline and the brick mate- rial gives rise to the so-called flaking pheno- mena which can greatly affect the quality of flat glass production. This was the reason to look for new refrac- tory qualities that are not subject to this at- tack mechanism. Routschka has written a report about the absorption of alkaline com- positions in refractory materials under tin bath bottom conditions in float glass pro- duction [6]. Strohm also reported about brick qualities resistant to alkaline composi- tions at the meeting of technical committees II and III of the DGG/HVG on 17–18 October 2007 in Puschwitz ( P-D refractories , Wetro plant) [7]. The studies of Routschka were carried out at the institute of the German re- fractory industry in Bonn and were transfer- red to the industry afterwards. It took a lon- ger development period for the industry to improve calcium aluminate bricks to techni- cal perfection before taking this material into the production process [8]. Until now only the chemical reactions of the alkaline containing tin with the tin bath bot- tom blocks were studied but the influence of the float bath atmosphere on these reac- tions has been neglected. In this study, the corrosion behaviour of brik- ks of the system SiO 2 – Al 2 O 3 and CaO – Al 2 O 3 in the presence of alkaline composi- tions as well as in float atmosphere was in- vestigated. Besides nitrogen and hydrogen the atmosphere also contains water vapour and tin sulphide as well as alkali com- pounds, hydrogen sulphide and elemental tin vapour. Tin Bath Bottom Blocks – a Comparison of Refractories of the Systems SiO 2 – Al 2 O 3 and CaO – Al 2 O 3 This paper deals with the corro- sion behaviour of bricks of the system SiO 2 – Al 2 O 3 and CaO – Al 2 O 3 against alkaline composi- tions of the glass as well as flo- at atmosphere. The results show that the float atmosphere affects the calcium aluminate brick whi- le the chamotte brick remains unaffected. Introduction The first production line for flat glass by the float process on the European continent was built in Köln-Porz in 1965. The then new technical procedure yielded many advanta- ges for the production of flat glass. Meanti- me this process has been described extensi- vely [1; 2; 3; 4, 5] and is well known among experts, which is why we will not discuss it in detail here. Figure 1 shows a vertical section of a float bath to remember the construction. As can be seen, the liquid tin lies on fireclay bricks. The main producers of these bricks in Euro- pe are RHI (formerly Didier ), P-D refractories GmbH (formerly VGT-DYKO ) and DSF Re- fractories & Minerals Limited (UK). As described in the literature mentioned ab- ove, the slagging of the bricks is controlled by a diffusion process: The sodium oxide in the glass dissolves into metallic sodium in the tin and oxidizes as it diffuses into the Testing program The properties investigated to characterize the bricks are discussed below. Table 1 shows the chemical, physical, technical and mineral data of the two brick systems. Pore Size Distribution The pore size distribution is very important for the absorption of smallest solid particles, liquids (smelting) and gases into refractory materials. The absorbing capacity of the mentioned materials rises with increasing pore-size values. Žagar defined the intrusion velocity of a melt into the pores of a brick as the quotient of surface tension and viscosity of the melt and named this factor capillary mobility [9]. Concerning tin bath bottom bricks Busby showed that the thermal tran- spiration which leads to the formation of H2 bubbles at the tin side of the glass sheet be- gins at a pore diameter of < 10 µm [10]. The diagram of the pore-size distribution measu- red in an Al 2 O 3 – SiO 2 brick shows a bimodal configuration. The pore-size distribution of the calcium aluminate brick leads to diffe- rent diagrams for the interior and exterior zone. While the interior area shows a mono modal configuration, the exterior part is more bimodal. Figure 2 gives a comparison of all curves. Fig 1 Vertical section of a float tank G. Boymanns, F. Gebhardt, M. Schilling P-D refractories GmbH DYKO-GLASS, Wiesenstr. 61, 40549 Düsseldorf, Ger- many Corresponding author: M. Schilling, michaela.schilling@pd-group.com