Glass is made from raw materials which are combined in a specific ratio and melted at 1 700°C. The raw materials used are:
An equally important ingredient is recovered glass, known as cullet. The amount of cullet used in the glass making process is very important as it determines the quantity of raw materials needed. By adding more cullet, which melts at a lower temperature, less emissions are released into the atmosphere and energy is saved.
The raw materials are stored in silos where they are measured and delivered to batch mixers according to specific recipes. Batches are continuously fed into furnaces or the “hot end”. It takes 24 hours for a batch of raw material to be converted into molten glass (red hot liquid glass). Molten glass is drawn through to the refiner where it is cooled but remains liquid. Maintaining the correct temperature to keep the flow of the molten glass is important as it affects the quality of the end product. The glass is now delivered to the individual bottle-making machines.
The molten glass is cut into gobs of a predetermined weight, enough to make a single bottle. “Forming” now takes place as the gobs are guided into the moulds of the bottle-making machine. During the first stage of moulding, the bottle opening is made by allowing a gob of glass to fall into a blank mould, forming what is called a bubble or cavity. Although the bottle opening is completely finished in this stage, the body of the bottle is smaller than its final stage.
Depending on what type of glass container is being produced, there are two primary methods used. The Blow-Blow Method and the Press-Blow Method.
The Blow-Blow method is used to make narrow-neck containers.
The Press-Blow method is used for jars and tapered narrow-neck containers.
The new bottle is then removed from the mould and transferred to the annealing oven or Lehr. The outer surface of the bottle is coated with a thin layer of tin oxide to strengthen it. It is cooled to 100°C in a controlled manner inside the Lehr. By doing so it relieves stresses within the glass and ensures it is safe to handle. The bottle exterior is now coated with a polyethylene wax to protect the glass and prevent scratching between bottles.
The bottle exists the Lehr and cools to air temperature. This is called the “cold end” of the plant.
All glass containers are then inspected. Bottles are individually coded with the production date and time, packed on pallets, wrapped in shrink wrap and distributed to customers.
Source: Consol Glass
Recycling of glass starts with the consumer, in your home or business, by collecting glass bottles and containers and returning them to the manufacturer via a recycling centre.
The manufacturer then sorts the glass into different colours as each colour contains a different ingredient with varying melting points. The glass is melted down to produce a new glass product. Some glass bottles, once empty, may be returned to the retailer, supermarket or liquor outlet where you will received a refund. The retailer then returns the bottles to the beverage manufacturer where they are sterilised and refilled.
By recycling, you are preventing South Africa’s landfill sights from filling up with glass which takes millions of years to degrade.
Recycling reduces the need for raw materials to be quarried thus saving non-renewable, natural resources.
Interesting resources: www.theglassrecyclingcompany.co.za