Container Glass

Commercial container glass is made by melting silica sand, soda ash and limestone. Various secondary ingredients are added to control colour, provide ultra-violet protection and enhance the working properties of the molten glass. Blastrite supplies a blended, refined form of blast furnace slag called Magflux® to the container glass industry.

The process of making container glass

The raw materials used for making container glass need to meet stringent specifications. In line with the drive to steward our earth's resources carefully, recycled glass (or cullet) is an important raw material in the production of glass containers worldwide. In some cases, as much as 50 per cent of ecology cullet is included in each batch of container glass.

From the holding bin, the batch is continuously fed to a furnace where it is converted to molten glass and maintained at temperatures in excess of 1500ºC. Molten glass is continuously withdrawn through a submerged throat where it proceeds to the refiner area of the furnace and cooled to approximately 1200ºC, before being delivered to the individual bottle-making machines via the forehearths. The furnaces are fully computerised and critical parameters are controlled to very close tolerances.

The molten glass enters the feeder to the bottle-making machine where the streams of glass are cut into pieces of a pre-determined weight required to make a single bottle. The gobs of molten glass are then individually fed into the moulds of the bottle-making machine. The bottle is formed in two stages. Firstly, the gob of glass falls into a blank mould to produce a parison. Here the finish (neck) of the bottle is formed and a long narrow cavity is blown within the center of the parison. All blowing is done by means of compressed air.

The parison is transferred to the main mould where the bottle is given its final shape. Air is forced under pressure into the hollow cavity to expand the glass to its final shape inside the mould. The newly formed bottle is coated with a thin layer of tin oxide to strengthen it before entering the annealing lehr. In the lehr, the bottle is cooled from 600°C to 100°C in a controlled manner to remove the stresses caused by uneven cooling and to ensure the bottle is stable and safe to handle.

The advantages of using Magflux® in container glass

Magflux® is a blended, refined form of blast furnace slag which predominantly consists of CaO, SiO2, Al2O3 and MgO.Its mineral composition is a mixture of akermenite (2CaO, MgO, 2SiO2), gehlenite (2CaO, Al2O3 , SiO2) and small amounts of anorthite (CaO, Al2O3, 2SiO2).

Magflux® is added to a glass batch as a fining agent and its reducing action helps liberate retained gases from the glass batch which may cause defects such as seeds and blisters. Also, the addition of Magflux® forms a lower eutectic with silica resulting in a lower melting temperature and lower energy costs. Furthermore, a more active melt is produced thus improving homogeneity in the batch temperature and composition.