Glass International February 2021

Page 51


Lipstones and Spout/Lip Casings Valerie Weber* discusses how one fused cast refractory block plays an essential role in the float glass process.


until the sheet can be lifted from the tin onto rollers. Variations in the flow speed and the roller speed control the thickness of the glass. Once the glass has exited the bath, it is gradually cooled in a kiln (annealed). When the glass flows through the canal onto the tin bath, it goes over the lipstone. It is the last piece of fused cast refractory that the glass touches during the manufacturing process. With its excellent resistance to corrosion and low defect potential, alpha-beta alumina fused cast refractory, developed by Monofrax in 1947, has become the standard for this integral part of a float glass furnace for over 50 years. Why isn’t fused cast AZS (aluminazirconia-silica) used in this critical area? While AZS is very resistant to corrosion due to its zirconia content, the zirconia also produces the most stubborn defects in the glass. It is difficult to dissolve zirconia once it gets into the glass batch. If an alpha-beta alumina lipstone loses refractory material due to corrosion, it is most likely alumina or soda. Both are abundant in glass, making them unlikely to cause defects at this critical point in the furnace. Alumina materials are also resistant to alkali corrosion, which is present in this application. However, in rare cases, high zirconia fused cast has been used for lipstones in demanding

applications. Why is high zirconia an option when AZS is not? In AZS, the zirconia is suspended in the silica and alumina. As they wear, they take the suspended zirconia with them. In high zirconia, the wear rate is slow enough that it doesn’t cause issues. Depending upon furnace conditions, this essential piece of fused cast refractory has a lifespan of three to eight years. The type of glass manufactured, furnace temperature, and tonnage all affect the refractory’s longevity. The volume of glass flowing over the lip is the most critical factor. More glass equals more wear. Replacing a lipstone is usually a hot repair undertaken when the furnace is still in operation. An assembled spout/ lip casing makes this possible. What is a spout/lip casing? It’s a steel casing containing the lipstone, jambs, bottom blocks, underlayment, and insulation. It’s a turnkey solution to lipstone replacement. Remove one spent casing and replace it with a new unit. Spout/lip casings may be purchased from a single vendor or assembled onsite. Buying a complete casing has many advantages: � It ships as a complete assembly with a protective layer of steel.



f you work with glass furnace design or refractories, you’re familiar with the furnace regions: breastwall, sidewall, forehearth, crown, canals, pavers, and ports. However, unless you work with the float glass process, you may not be familiar with lipstones. Although the glass that flows over a lipstone is more than 1000 degrees C, lipstones are in no way related to the Rolling Stones’ Hot Lips logo. This single piece of fused cast refractory is an integral part of the float glass process. In the early 1960s, Sir Alastair Pilkington and Kenneth Bickerstaff of Pilkington Brothers in Lancashire, UK, revolutionised the glass industry with the float glass process where a continuous flow of glass moves from a melting furnace and floats on a bath of molten tin. It replaced both the plate and sheet glass processes as it was less expensive and produced a higher quality product. Pilkington Brothers began licensing this new technology in 1962. According to Pilkington’s website, there are now 370 float glass plants in operation worldwide. They produce everything from architectural to automotive glass. In Pilkington’s process, glass flows from the conditioning section through a narrow canal onto molten tin. The surface flows until it becomes flat and parallel. As the glass floats along the tin bath, the temperature gradually reduces

� Lip-spout in casing.

49 Glass International February 2021

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