Glass Recycling

(Glass smelter at Kremsmuenste, Austria)

Flashback: In prehistoric times objects were fashioned from natural glass such as obsidian (a volcanic substance) and rock crystal (a transparent quartz). The oldest extant manufactured glass is from Egypt, dated around 2000 B.C. Many types were made in Roman times, but little is known of European glassmaking from the fall of Rome until the 10th century when stained glass appeared. Methods have changed little since ancient times. The materials are fused at high temperatures in seasoned fireclay containers, boiled down, skimmed, and cooled several degrees; then the molten glass is ladled or poured into moulds and pressed, blown or drawn. As a result of temperature fluctuations during this manipulation, the shaped glass is annealed to relieve internal stress; i.e. slowly cooled down to room temperature. Until the 17th century the finest glass was made in Venice; later France and England became centres of glassmaking. In the 20th century many new types have been developed, including fibreglass and safety glass.

Glass is a hard, usually brittle and transparent substance, composed chiefly of silicates and an alkali metal fused at high temperatures. Metallic oxides impart colour. Glass is resistant to attack by most chemicals (except for hydrofluoric acid). The inert properties make it ideal for the storage and transport of most food and liquids. Even though glass is a lot heavier than HDE-plastic bottles, glass extends the shelf-life of foodstuffs considerably. Most glass is nowadays used in a one-way pattern (disposed off after single use). But glassware (e.g. bottles) is probably the most suitable items when talking about reusing materials. Unfortunately, and in ever-wider use, glass is been substituted with either plastic containers or aluminium cans. But still, many central European brewers, manufacturers of spring water and wine makers use glass as their most preferable choice to maintain the qualitative properties of their product for weeks, months or even years - something that the plastic industry just can dream of. These companies rotate their glass inventory in a closed loop system; i.e. the bottles are sterilized, filled, caped, sold, collected and reintegrated in the production line.

Collection: Glass is best recycled on the bases of its colour. In central Europe separation occurs primarily in two categories; i.e. of clear and coloured glass and is ideally done at the collection site. Colour separation is necessary, as the introduction of recycled glass different in colour into the smelter would change the basic recipe and thus the visual and physical properties of the final product. It is important to note that ceramics and window glass should not be introduced into this collection system as their varying chemical composition would seriously interfere with the chemical reaction taking place in the smelter.
Glass recycling stations in Austrian are not yet available for every household. And still, the network of collection station covers an area large enough, to render recycling acceptable for the public; so far an equivalent density of about 300 individuals per collection site is guaranteed.
Similar to the "split-truck" concept of Brisbane city council, modified pick-up trucks collect the two glass fraction (separated by colour) and deliver them to the glass smelter. Having visited the plant at Kremsmuenster, this plant will be used as a general reference for this page.
Þ For collection guidelines, refer to the

Glass recycling container - separation based on colour

Preselection: The delivered material is first pre-selected manually to remove unsuitable constituents (metal caps, ceramic parts, etc.). Thus ceramics, window glass, crystals still have to be eliminated because these materials do not have the same physical and chemical properties of bottle glass (different melting points, and a wide array of different metal-oxide inclusions making them unsuitable for bottle recycling). By sabotaging the entire recycling philosophy, these foreign composites cause major damage as the interfere and corrode the furnace's cost intensive insulation, resulting in a major financial loss for the company utilizing such raw material.
Before the glass is introduced into the smelter, it has to pass through a crasher and breaks it into pieces not larger than 15mm. An attached a metal separator and an aeration tank extract any non-glass particles that have bee overlooked so far.

Preselection by hand

Glass is an amorphous solid, revealing an ionic character, with properties resembling a very viscous liquid. Its non-metallic network structure is based on silica (SiO2), that is bonded together with metal oxides that act as "network modifiers".
Silica in the form of sand is heated to about 1600C. Metal oxides are added to the silica. As the mixture melts, many of the Silica-bonds break apart destroying the orderly structure of the solid crystal. When the "liquid" cools, the Silicium-Oxygen (Si-O) bonds of silica start to rebuild; the metal oxides present, partly inhibit this lattice-forming process, by forcing some of the silica-molecules to bond with the O of the metal oxide.
Without becoming too technical, some basic aspects should be added to explain why different types (applications) of glass should not be mixed at all:
Almost 90% of all manufactured glass combines sodium and calcium oxides with silica in soda-lime glass. Glass, which is used for windows and bottles, contains about 12% soda and 12% lime. Tinted glass is obtained by adding small amounts of other substances; e.g. cadmium sulfide and selenide result in ruby glass, which is red. Ordinary soda-lime glass is usually very pale green due to iron impurities. Brown beer-bottle glass is coloured with iron sulfides. Amber glass, such as that used for medicine bottles, is blended with a mixture of sulfur and iron oxides that give hues from pale yellow to amber.

Sketch of reaction chamber

Smelter: The smelter is kept operational for as long as the interior insulating sheath protects the reaction chamber. The molten liquid is squeezed out at the bottom and is primarily regulated via the gravitational pull of the molten glass column within the oven itself. To maintain a regular flow of glass droplets at the base, the oven has to be fed at regular intervals with raw and recycled materials from above. Each droplet contains the equivalent amount of molten glass required to shape for example a bottle with 1L volumetric capacity.

Image: Bottle production
(1) emerging droplet from furnace; (2) glass droplet slipping into the mold; (3) IR-image of shaped bottles; (4) bottles before the annealling process;

The energetic requirements of the plant at Kremsmuenster are covered by natural gas. Being of non-renewable origin, the operators have to imply with the strict environmental guidelines set up by the Austrian Environmental Protection Agency. A series of devices (baghouse and electrofilter) are used to purify the exhaust gases.
Þ For exhaust gas treatment, refer to the residual (incinerator) waste-section.

Processing: The emerging glass-droplet at the base of the smelter's feeder is cut off at a precise stage (volume of droplet is proportional to the orifice of the feeder and the time it takes to form a droplet). Each single droplet is routed downwards into a bottle shaping mould. Preheated compressed air squeezed it into proper shape with.
Automated optical sensors detect whether certain parameters (such as wall thickness, etc.) match preset values. Deviations of these parameters immediately automatically convey the bottle to a separation unit where it will be derogated back up into the smelter.

Annealing: Before cooling off completely, the bottles are annealed to compensate tensions within the glass that result from such manipulation. The endless flow of bottles is conveyed into a tempered remuneration chamber. In this stage, a furnace, generally electrically heated, attached a control system capable of performing a definite temperature profile, exposes the glassware for a few minutes to very elevated temperatures. Glass has a softening temperature above which it can be easily bent or moulded to shape, and an annealing temperature below which the properties of the glass remain fixed down to room temperature. These two set-points characterize the annealing range in which it is essential to cool the glass very slowly to remove all strain and to drive the refractive index up to its stable maximum value.

Annealing procedure (left) and Production flaws (right)

Packaging: Once cooled down to room temperature, automated packaging machines pile up the bottles on panels, sandwiching each layer with a protective stratum, and finally wrap the entire unit with PE coating, before it is ready for shipment.

Final product ready for shipping

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