Topic of the Month in January

The Processing of Used Glass - Cullet as a Raw Material

Recycling does not only mean leading back waste materials into the production cycle, but also reusing them as raw materials for qualitatively equivalent end products. If park benches or bags are made from yogurt beakers, this should rather be called downcycling, since the end products make lower demands to the raw material and should rather be classified as of inferior value.

For the manufacture of identical products from valuable materials collected - such as glass, for glass containers can be produced from glass again and again - the processed used glass has to be of very high purity. Only then can the large share of cullet (used glass) be maintained during the production of new glass containers. Taking into consideration the tendency of glass industry to produce ever lighter glass containers with thinner walls, even the slightest impurities by foreign material in the melt lead to problems in production and to waste.

Among the most critical impurities are ceramics, china, stones and highly temperature-resistant glass. These components are not molten or burnt in the melting process and can bring about problems in production as they lead to inclusions in the glass containers.

A similarly high importance is to be attributed to the color purity of the recycled used glass - in order to be able to process more used glass still, the glass collected has to be consequently separated according to color from the beginning. Here you are requested to throw your used glass containers separated by color into the collection bins for glass to be found everywhere.


The Süddeutsche Altglas Rohstoff GmbH in Bad Wurzach

The recycling process in the different enterprises like the Süddeutsche Altglas Rohstoff GmbH at Bad Wurzach runs as follows:

Coarse impurities, entire ceramics or plastic bottles, garbage bags, metal cans, or other foreign substances are sorted out at a manual pre-selection site. Thereafter the glass flow is seperated into the desired grain sizes by means of a resonance screen and a distribution machine.

Uniform grain fractions assure optimum performance of the subsequently placed sorting aggregates. The sieved major debris, bottle necks or bottoms are broken up to the desired grain fraction in an impact vessel removing also the caps and closures from the glass. Magnets located above the conveyor sort out all impurities containing iron.


Iron containing impurities are sticking to a magnet above the conveyor

Suction by means of air collects all light, flat parts like paper from labels, plastic and aluminum foil, closure capsules and closure rings as well as organic pollutants and dust.
Separating out the other foreign material particles is done by opto-mechanical means.

The isolated used glass stream runs across a sloping plane; transparent glass particles and stones or ceramics not permeable to light are detected as dissimilar.

All non-transparent parts are blown through an air nozzle. A separation performance of up to 95% in the case of one separation course and up to 99% with a subsequent second separation stage can be reached.




Opto-mechanical separation of non-transparent particles

In order to further reduce the share of foreign colors, in particular in white (flint) glass, special color sorting devices can be used further down the line when processing flint glass. These devices work according to a similar principle as the separation devices for the non-transparent parts in the used glass stream.


The product: homogene cullets without impurities

At the end of the sorting process the cullet then falls onto a big heap and can be used as raw material during melting.

It is remarkable that, with a rate of 80% of cullet used in the melt - which is perfectly normal nowadays - approx. 20% of energy can be saved, which would be necessary for melting lime, sand and "soda".


Source: OGIS GmbH, glassglobal.com in cooperation with: Dr. Martin Buss & Partner GmbH and complimentary information by Süddeutsche Altglas Rohstoff GmbH at Bad Wurzach