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Float glass is a type of glass sheet produced by floating molten glass on the surface of molten metal with a lower melting point. Tin is commonly used, although lead has been used in the past. This method ensures that the glass sheet has uniform thickness and a very flat surface. The float glass process is also known as the Pilkington process, named after the British glass manufacturer Pilkington, who pioneered this technology at their production base in St Helens, Merseyside, in the 1950s. The chemical composition of float glass mainly includes silicon dioxide (SiO2), sodium oxide (Na2O), calcium oxide (CaO), magnesium oxide (MgO), aluminum oxide (Al2O3), iron oxide (Fe2O3), and sulfur dioxide (SO2). After long-term testing, the optimal chemical composition characteristics of high-quality float glass are "high calcium, medium magnesium, low aluminum, and trace iron." Raw materials for float glass can be divided into main materials and auxiliary materials based on their usage. Main materials include: 1. Silica sand or borax: Used to provide silicon or boron dioxide for glass composition, resulting in silicate or borate glass. 2. Soda ash or saltpeter: Primarily supplying sodium oxide for the glass, aiding in fusion and facilitating glass shaping. Excessive amounts can increase thermal expansion and decrease tensile strength. 3. Limestone, dolomite, feldspar: Limestone contributes calcium oxide, enhancing glass chemical stability and mechanical strength; dolomite provides magnesium oxide, improving transparency and reducing thermal expansion; feldspar introduces aluminum oxide and potassium oxide, controlling melting temperature and enhancing durability. 4. Cullet: Typically, 15%-30% of cullet is added to reduce the glass melting temperature. Auxiliary materials include: 1. Decolorizers: Used to remove impurity coloration, such as soda ash, sodium carbonate, cobalt oxide, etc. 2. Colorants: Various metal oxides dissolve in the glass solution to impart color, e.g., iron oxide (yellow or green), manganese oxide (purple), cobalt oxide (blue), nickel oxide (brown), copper oxide, and chromium oxide (green). 3. Clarifiers: Compounds like arsenic trioxide, sodium sulfate, sodium nitrate, manganese dioxide are used to reduce glass viscosity and clarify the glass melt. 4. Opacifiers: Materials like cryolite, sodium fluosilicate, tin phosphate form particles suspended in the glass, creating a milky semi-transparent appearance. The float glass process is as follows: The principal ingredients in soda-lime-silica float glass include sand, soda ash, dolomite, and limestone. The quality of the produced glass depends on the purity and grain size of the input material. The float glass production process begins with the raw materials being melted in a furnace at approximately 2,900°F or 1,600°C. The continuous strip of glass exiting the furnace is known as the “ribbon”. Forming takes place on a tin bath. The liquid glass floats upon the tin bath at approximately 2,000°F or 1,100°C, to achieve a flat, smooth surface without polishing. On exiting the tin bath, the glass is progressively cooled in the annealing lehr, which releases stress from the glass ribbon. A uniform stress profile supports the reliability of subsequent glass cutting operations. Laser sensors or high-resolution cameras installed along the line capture production data which is evaluated by yield-optimization software to direct a robotic cutting system. The ribbon is cut into stock sheets which are then prepared for storage. The sheets are sprayed with a separation material which helps to prevent scratches and sticking as the lites are arranged into packs. Size and thickness Standard float production dimensions can vary slightly depending on the region. In North America for example, the ribbon thickness can range from approximately 3/32” to 19/32”. The typical width of the float glass ribbon is 102 inches, and the maximum length is 240 inches. In Europe, the ribbon thickness can range from approximately 2 mm up to 19 mm. The typical width is 3,21 m and the typical length 6 m. Color options Standard float glass, normally called clear float glass, has a slightly greenish appearance that is particularly visible when looking at the edges of the glass. This slightly greenish tint is due to the presence of iron in the raw materials used to produce float glass. The iron content can be reduced to produce a more neutral glass, known as “low-iron glass”, which can be a good choice for interior design, but also for architects who wish to create very crisp and clean glazed façades. If a tint to the glass is desired, metal oxides can also be added to the raw materials to provide a grey or green tint. It will not only affect the appearance of the glass but also its visible light and solar heat transmission. Tinted glass can be combined with low-E coatings to allow more aesthetic options of substrate/coating combinations, to further improve the solar performance of the glass, as well as selecting the right level of light transmission and transparency to help mitigate glare and maintain some privacy. China is the largest manufacturer of float glass globally. With a significant presence in the glass industry, China boasts numerous glass production enterprises and advanced production technologies. Chinese float glass production accounts for over one-third of the global total, meeting both domestic demand and international export requirements. Other major float glass-producing countries include the United States, India, Japan, and various European nations. As of November 23, 2023, China's domestic float glass production lines totaled 308 (with a daily output of 206,045 tons), including 255 in operation and 53 undergoing cold repairs. The increase in float glass production capacity primarily comes from the commissioning of new production lines, resumption of cold-repaired lines, and production line conversions, while capacity reductions mainly result from cold repairs and conversions (e.g., architectural ultra-clear glass to photovoltaic back panels).
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