Simulation of tin penetration in the float glass process

Simulation of tin penetration in the float glass process
Posted by rtyu1yu on 2021/08/24
Simulation of tin penetration in the float glass process

    Simulation of tin penetration in the float glass process 

    The flat glass produced by the float glass process has a tin-rich

surface due to the contact with molten tin. The penetration of tin into the glass surface is assumed to involve coupled

diffusion of stannous (Sn2+) and stannic (Sn4+) ions. The diffusion coefficients of these ions were calculated using the

modified Stocks–Einstein relation with the oxidation velocity of stannous ions depending on the oxygen activity in the

glass. The ion diffusion was analyzed using a coupled diffusion simulation with a modified diffusion coefficient to

compensate for the negative effect of the glass ribbon’s stretching or compressing in the glass forming process. Tin

penetration simulations for both green glass and clear glass show an internal local tin concentration maximum in green glass

which is quite different from that in clear glass. The local maximum in the profile is associated with the accumulation of

stannic ions where the greatest oxygen activity gradient occurs. Since more float time is needed in the manufacture of

thicker glass plate, the tin penetrates to a greater depth with the maximum deeper in the glass and the size of the maximum

larger for thicker glass.

    The float glass process, which was originally developed by Pilkington Brothers in 1959 (Haldimann et al., 2008), is the

most common manufacturing process of flat glass sheets. More than 80–85% of the global production of float glass is used in

the construction industry (Glass for Europe, 2015a). In the float glass process, the ingredients (silica, lime, soda, etc.)

are first blended with cullet (recycled broken glass) and then heated in a furnace to around 1600°C to form molten glass.

The molten glass is then fed onto the top of a molten tin bath. A flat glass ribbon of uniform thickness is produced by

flowing molten glass on the tin bath under controlled heating. At the end of the tin bath, the glass is slowly cooled down,

and is then fed into the annealing lehr for further controlled gradual cooling down. The thickness of the glass ribbon is

controlled by changing the speed at which the glass ribbon moves into the annealing lehr. Typically, glass is cut to large

sheets of 3 m × 6 m. Flat glass sheets of thickness 2–22 mm are commercially produced from this process. Usually, glass of

thickness up to 12 mm is available in the market, and much thicker glass may be available on request. A schematic diagram of

the production process of float glass is shown in Fig. 5.2.The float glass process was invented in the 1950s in response to a

pressing need for an economical method to create flat glass for automotive as well as architectural applications. Existing

flat glass production methods created glass with irregular surfaces; extensive grinding and polishing was needed for many

applications. The float glass process involves floating a glass ribbon on a bath of molten tin and creates a smooth surface

naturally. Floating is possible because the density of a typical soda-lime-silica glass (~2.3 g/cm3) is much less than that

of tin (~6.5 g/cm3) at the process temperature. After cooling and annealing, glass sheets with uniform thicknesses in the ~1

–25 mm range and flat surfaces are produced. The ultra

clear float glass
process is used to produce virtually all window glass as well as mirrors and other items that

originate from flat glass. Since float glass is ordinarily soda-lime-silica, the reference temperatures and behavior of this

glass are used in the discussion below.

    Figure 3.48 shows the basic layout of the clear float

line. The glass furnace is a horizontal type, as described above. For a float line, the glass furnace is

typically on the order of ~150 ft long by 30 ft wide and holds around 1200 tons of glass. To achieve good chemical

homogeneity, the glass is heated to ~1550–1600°C in the furnace, but is then brought to about 1100–1200°C in the

forehearth. From there, the glass flows through a channel over a refractory lipstone or spout onto the tin bath. As it flows,

the glass has a temperature of about 1050°C and viscosity of about 1000 Paradical dots. A device, called a tweel, meters the

flow of the molten glass.Imperfections include bubbles (or ‘seeds’) that may have a number of possible sources, the most

common being gas evolved during firing. Bubbles may contain crystalline materials formed during cooling of the glass that may

provide clues to the origin of the bubbles. Cords are linear features within the glass that may result either from

imperfectly homogenized raw materials, dissolved refractories or devitrified material. Figure 360 shows the appearance of

soda–lime–silica glass that exhibits bubbles and cords. ‘Stones’ are solid crystalline substances occurring in glass that

are regarded as defects. They are usually derived either from the batch material, refractories, or devitrification. Figure

361 shows the appearance of soda–lime–silica glass that contains a devitrification ‘stone’. These may develop as the

result of incomplete mixing of the molten glass constituents and/or too low a firing temperature. The ‘stone’ shown in

Figure 361 contains an aggregation of tridymite crystals (see 362).

    As the floating glass ribbon traverses down the length of the tin bath, its properties change dramatically. The glass

enters as a viscous liquid and exits virtually a solid at a temperature very close to its glass transition temperature. The

details of how the temperature changes and the viscosity builds are complicated. On one side, the free surface of the glass

is exposed the atmosphere; heat can leave this surface by radiation or convection. Cooling and heating apparatuses are

stationed above the glass ribbon down the length of the bath to allow adjustment of the ribbon temperature. On the other

side, the glass is in contact with the tin bath, which can absorb some of the heat and transport it away from the ribbon. The

tin bath is in constant motion due to the moving glass above it as well as the thermal convection currents. Unfortunately, no

simple approximations can be made to make the modeling of the heat transfer.

    The thickness of the tinted float glass sheet is

adjusted by controlling flow onto the tin bath as well as by tension exerted along the length of the bath by rollers in the

annealing lehr and sometimes by rollers in the bath unit itself. In the Pilkington design, the melt enters the bath and

spreads out laterally to a thickness near the equilibrium value. If a sheet thicker than the equilibrium is required, then

this spreading is constrained with physical barriers. If a sheet thinner than equilibrium is needed. then the glass ribbon is

pulled in tension by rollers. In the PPG design, thickness is regulated by the tweel position and by tension from rollers in

the lehr. The thermal profile allows the thinning deformation to take place effectively. A short distance away from the entry

point, the temperature of the ribbon drops and the viscosity rises. Overhead coolers help this process. The glass viscosity

is high enough so that knurled rollers contact the glass ribbon and pull it forward (and in some operations, laterally as

well). Heaters are placed shortly downstream of these edge rollers to raise the temperature of the ribbon and create a

deformable zone. This zone is followed by coolers that again lower the temperature and raise the viscosity. At exit from the

lehr, the ribbon is virtually solid. The main deformation is due to the rollers in the lehr, which pull on the glass ribbon

from the lehr to the edge rollers; extension takes place in the deformation zone. Example 3.15 considers the exit velocity of

glass from the process.

    For many years, however, the glass industry has been trying to solve a problem which affects almost every building in the

world. How do you maintain the fundamental characteristics of glass, such as optical clarity and external esthetics without

constant and costly maintenance? Whether the building is for commercial or residential use, the one constant requirement is

for regular cleaning to be undertaken to ensure the glass maintains its optimum appearance.

    The challenge for the glass industry is increased as a result of architects finding ever more resourceful and novel uses

for glass. The use of glass in atria and overhead glazing can sometimes result in complex areas, which can make maintenance

more difficult.

    In addition to the esthetic issues it is a well-known phenomenon that if glass is not cleaned regularly then over a

period of time the glass can weather, which makes it almost impossible to restore its esthetic properties. In extreme

circumstances this can lead to the glass needing replacement.

    The process of cleaning windows can also lead to safety and environmental issues. Window cleaning generally involves the

use of portable ladders for cleaning windows on ground, first, and second floors. Figures for accidents reported to the

Health and Safety Executive (HSE) and local authorities reveal that unfortunately between two and seven window cleaners have

been killed every year in Great Britain and around 20–30 suffer major injuries due to falls involving ladders. From an

environmental aspect window cleaning can involve the use of harsh chemicals. These are often washed off during the cleaning

process and can ultimately lead to ground contamination.

    Recently, self-cleaning coatings have been developed, which are designed to reduce the amount of maintenance required by

working with the forces of nature to clean dirt from the glass. These coatings are based on a well-known metal

oxide called titanium dioxide, which is regularly used in paints, toothpaste, and sunscreens.

    Tin is an ideal bath material because it has the right set of physical properties. Tin melts at 232°C, has relatively

low volatility, and does not boil until over 2000°C. Molten tin is denser than molten glass and is not miscible or reactive

with molten glass. The gas atmosphere is controlled so that tin does not oxidize at a fast rate. Any oxide that does form is

collected in a dross container on the bath.

        Regulating the flow of the wired glass is

important at this stage, both from the entry point and the lateral flow. The glass flow onto the tin bath is regulated by a

gate, called a tweel, which is located in the canal between the forehearth and spout. The glass flows down the spout or

lipstone onto the tin surface. There is some pressure driving this flow through the gap of the tweel. See Example 3.14.

As the glass flows onto the tin bath, the thickness of the glass sheet depends on how that flow is controlled laterally and

along the length of the bath. The first step to understanding thickness control is to examine the equilibrium thickness.

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