Limestone is an important component in glassmaking and is used as a stabilizer which improves the mechanical properties and physical appearance of the glass.
Specifications for Limestone in Glassmaking
Glass industry required high limestone (CaCO3: 94.5%) combined CaCO3 and MgCO3 should be 97.5%. Iron and other colouring matter like carbon are regarded as objectionable. Fe2O3 should be up to 0.20% (max.). For colourless glass, limestone should not be more than 0.04%; and for bottle glass Fe2O3 up to 0.05% is used.
McGrath’s Limestone is ideal for the glass industry as it has a high purity with very low levels of undesirables and contaminants that would be detrimental to the glass making process. It has a very low Iron content typically < 0.03% Fe2O3 with high Calcium content in the order of 98.5% CaCO3 and with the added advantage of consistent Magnesium content typically of 1% MgCO3. The Limestone that is produced can have both economic and environmental benefits to the glass producer.
McGrath’s Limestone Works Ltd currently produces high quality limestone for the manufacture of container glassware in both the UK and mainland Europe.
Commercial Glass Making
The majority of commercial glasses, bottles and jars consist of Soda-Lime Glass. Soda-Lime glass is the most common type of glass produced for glass containers (bottles and jars) for beverages, food, and some commodity items. Silica sand, soda ash and calcium limestone or dolomitic limestone represent together a majority of the raw materials used in the production of glass.
Soda-Lime glass is generally prepared by melting the raw materials or batch, such as Silica Sand (Silicon Dioxide, SiO2) with Soda Ash (Sodium carbonate, Na2CO3), Limestone or Dolomite (Calcium Carbonate, CaCO3) plus a small percentage of other materials can be added for specific properties such as colouring. The composition of soda-lime glass is normally 60-75% silica, 12-15% soda, and 5-12% lime.
The glass batch once produced is fed into the glass furnaces where temperatures can be in excess of 1500 °C. Green and brown bottles are obtained from raw materials containing iron oxide, whereas clear glass bottles are obtained from the lack of colouring agents.
Normally, sand melts and softens at temperatures of up to 2000°C, by adding soda this will act as a flux and will lower the melting point to 1000°C making it more manageable. The soda makes the glass water-soluble, soft and not very durable. Therefore the limestone is added to act as a stabilizer and increases the hardness and chemical durability of the glass.
Soda-lime glass is relatively inexpensive, chemically stable, reasonably hard, and extremely workable, because it is capable of being re-softened and re-melted numerous times, ideal for recycling. These qualities make it suitable for manufacturing a wide array of glass products, including bottles. All glass bottles and jars are now made automatically by one of two methods – ‘Press and Blow’ or ‘Blow and Blow’ using compressed air.
Recycling of Glass
- Glass accounts for 8% of waste in the household bin.
- Glass can be recycled repeatedly as glass does not degrade or wear out.
- Recycling 1 glass jar saves enough energy to power a light bulb for nearly 1 hour or run a TV for 15 minutes.
- Glass cullet (crushed and sorted used glass) can be recycled into new glass jars and bottles; as well as other products, i.e. concrete, fibreglass, ceramic tiles, picture frames and it can also be used in roadbeds.
- Using recycled glass requires 40% less energy than making glass from new raw materials.
- In Ireland almost 75% of all glass packaging is recycled.