Did you know the main ingredient in glass is silica sand? Silica is a mineral and the word “sand” refers to the classification of the texture of the sediment. If sand sized sediment grains are mainly silica or high in silica, it can be classified as silica sand. If feldspar (another mineral) is present in sand sized particles it is sand, but it is not silica sand (it is feldspar sand). In South Africa Silica sand is mined in Delmas, a small maize farming town situated east of Johannesburg in Mpumalanga, South Africa. The town is located some 19 km north-east of Springs and 73 km south-east of Pretoria.
Bearing this in mind it made sense to erect a float glass manufacturing line close to this area in Springs. PFG is part of the PG Glass group and manufactures 260 000 tons of float glass per year with the two float lines at the Springs manufacturing plant. These are the only float lines in South Africa. PG Glass was established in Cape Town in 1897 and Saint Gobain currently owns 20% shares in the company. They in turn, have 35 float lines in 17 different countries.
The process used by PFG to manufacture float glass refers to the method first used by Pilkington Glass in 1959. At the heart of this method of manufacture is the process of floating the molten glass ribbon (1550 ℃) on the surface of a molten bed of tin. The glass is held at a 1000 ℃ long enough for irregularities to melt out and the glass surface to flatten on top of the tin layer, creating a smooth even surface layer. The top of the glass is also flattened and polished by the use of nitrogen under pressure.
It is standard practice for a float line to run 24 hours a day and for approximately 15 years before a cold repair is done. This means that for 15 years the line runs non-stop every day and night. It may sound slightly unbelievable, until you factor in the start-up costs involved in the amount of electricity required to initially heat the furnace to the required 1550 ℃. The only way to at least lower the melting temperature to the used 1550 ℃ is by adding Sodium Oxide or Potash. If this is not added the melting temperature would be even higher. PFG sources Sodium Oxide from Botswana.
When the cold repair is done, huge chunks of glass are removed. This is called cullet. The cullet can be re-used in float glass manufacturing, but only makes up about 6% of the end product. Clear Float glass is the only glass type that can be easily recycled. At the Springs plant there are various huge pyramids of float glass that they can re-use.
The process is as follows:
1. Ingredients are mixed
2. Smelted in furnace at 1550 C
3. Glass gets drawn through onto the tin – speed determines the thickness
4. Liquid glass floated on melted tin
5. Glass Ribbon gets drawn through – speed determines the thickness
6. Annealing Lehr for cooling
Sources: My absolute biggest “Thank you” for the very informative training session this week at our offices goes to Martin Volker from Glass Partners.
PFG Building Glass website
PG Group website