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Waste glass, recovered glass

Recovered glass is packaging glass including glass bottles which is collected and recycelt. After the waste glass collection the material is processed to secondary raw material which can be added to the glass melt mixture in glass factories. Some glass factories operate own waste glass processing plants.

Using recovered glass plenty of energy and raw materials can be saved. Usually up to 65% of waste glass are used in the glass production process, for green glass it can be up to 95%. In the case of 65% waste glass the energy consumption is reduced by 20%.

The waste glass has to be separated mono-fraction as accurately as possible. First step to reach this is to collect it separately. Due to common impurities of approx. 7% in the collected glass before smelting an optical sorting process in fully automated optical sorting plants is necessary. These plants classify the broken glass by optical criteria (colours). The broken glass normally is produced by means of a crusher and should be as homogenous as possible regarding the particle size. To reduce the dust content which downgrades the efficiency of the sorting process the undersize particles are separated by screening.

Also after sorting single colour fractions may contain a certain amount of wrong colour particles to be melted together with the fresh raw materials. For green glass this can be up to 15%, for brown glass up to 8% but for clear glass maximum up to 0.5%. Glass with higher impurities is traded as olive green glass (mixture of green and brown) or natural white glass (white with colour impurities).

Glass is coloured by means of following materials and substances: ferric oxide, copper oxide, cobalt oxide, nickel oxide, manganese oxide (pyrolusite), selenium oxide, silver, indium oxide, neodymium, praseodymium, europium and at least gold (aqua regia).

In the glass melt process problems are caused by ceramics, stone ware and porcelain, they produce imperfections in the glass structure. Furthermore window glass, mirror glass, laboratory glass, monitor glass and other kinds of special glass are not recyclable.

The main important glass are quartz glass, float glass, crystalline glass and borosilicate glass.