Glass production
From Wikipedia, the free encyclopedia
 |
This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (December 2007) |
Glass is common in everyday life, from glass windows to glass containers. The manufacture of glass for everyday purposes may involve complexity and automation. This article deals with the mass production of glass.
Contents |
[edit] Glass container production
[edit] Glass container factories
Modern glass container factories are broadly divided into three parts: the batch house, the hot end and the cold end. The batch house is concerned with raw materials. In the hot end are furnaces, machines that produce the containers (forming machines) and annealing ovens. In the cold end there are the inspection and packaging equipment.
[edit] Batch house
The batch house holds the raw materials for glass, primarily sand, soda ash, limestone, feldspar (as well as others). These materials are received (typically by truck or rail transport) and elevated into storage silos. From the silos they are weighed out into a batch of several tonnes, using common glass batch calculation procedures. The batch is mixed and sent to silos over the furnace.
[edit] Hot end
The following table lists common viscosity fixpoints, applicable to large-scale glass production and experimental glass melting in the laboratory:[1]
| log10(η, Pa·s) | log10(η, P) | Description |
|---|---|---|
| 1 | 2 | Melting Point (glass melt homogenization and fining) |
| 3 | 4 | Working Point (pressing, blowing, gob forming) |
| 4 | 5 | Flow Point |
| 6.6 | 7.6 | Littleton Softening Point (Glass deforms visibly under its own weight. Standard procedures ASTM C338, ISO 7884-3) |
| 8-10 | 9-11 | Dilatometric Softing Point, Td, depending on load[2] |
| 10.5 | 11.5 | Deformation Point (Glass deforms under its own weight on the μm-scale within a few hours.) |
| 11-12.3 | 12-13.3 | Glass Transition Temperature, Tg |
| 12 | 13 | Annealing Point (Stress is relieved within a several minutes.) |
| 13.5 | 14.5 | Strain Point (Stress is relieved within several hours.) |
[edit] Furnace
The hot end of a glassworks is where the molten glass is formed into containers, beginning when the batch is fed at a slow controlled rate into the furnace. The furnaces are natural gas or fuel oil fired and operate at temperatures up to 1675°C.[3] The temperature is limited by the quality of the furnace superstructure material and by the glass composition. Glass furnaces typically operate an energy recovery scheme known as regeneration. The hot exhaust gas flow back over one of two piles of loosely packed bricks, called regenerators. These bricks become hot and every 20-30 minutes the flow of the combustion system is changed over so that the combustion air, which is mixed with the gas, is drawn through the heated bricks, and the combustion exhaust flows through the other pile of bricks. The batch melts inside the furnace which is maintained as a pool of molten glass, perhaps 1200mm deep by 50 to 150 m². The molten glass flows from a subducted channel known as the furnace throat into the refiner and forehearth channels. These channels, 1200mm wide and 400-150mm deep transport the glass to the glass bottle forming machines. These channels cool the glass very precisely so that the glass at the forming machine is of a uniform and exact temperature.
[edit] Forming process
There are currently two primary methods of making a glass container - the blow and blow method and the press and blow method. In all cases a stream of molten glass at its plastic temperature (1050°C-1200°C) is cut by a shearing blade to form a cylinder of glass called a gob. Both of the processes start with this gob falling by gravity and guided by troughs and chutes into the blank moulds. In the blow and blow process, the glass first is blown from below into the blank moulds to create a parison or pre-container. This parison is then flipped over into a final mould, where a final blow blows the glass out in to the mould to make the final container shape. In the case of press and blow, the parison is formed by a metal plunger which pushes the glass out into the blank mould. The process then continues as before, with the parison being transferred to the mould, and the glass being blown out into the mould.
[edit] Forming machines
The forming machines hold and move the parts that form the container. Generally powered by compressed air, the mechanisms are timed to coordinate the movement of all these parts so that containers are made.
The most widely used forming machine arrangement is the individual section machine (or IS machine), invented in 1903 by Michael J Owens in Illinois. This machine has a bank of 5-16 identical sections, each of which contains one complete set of mechanisms to make containers. The sections are in a row, and the gobs feed into each section via a moving chute, called the gob distributor. Sections make either one, two, three or four containers simultaneously. (Referred to as single, double, triple and quad gob). In the case of multiple gobs, the shears cut the gobs simultaneously, and they fall into the blank moulds in parallel.
[edit] Internal treatment
After the forming process, some containers—particularly those intended for alcoholic spirits—undergo a treatment to improve the chemical resistance of the inside, called internal treatment or dealkalization. This is usually accomplished through the injection of a sulfur- or fluorine-containing gas mixture into bottles at high temperatures. The gas is typically delivered to the container either in the air used in the forming process (that is, during the final blow of the container), or through a nozzle directing a stream of the gas into the mouth of the bottle after forming. The treatment renders the container more resistant to alkali extraction, which can cause increases in product pH, and in some cases container degradation.
[edit] Annealing
As glass cools it shrinks and solidifies. Uneven cooling causes weak glass due to stress. Even cooling is achieved by annealing. An annealing oven (known in the industry as a Lehr) heats the container to about 580°C then cools it, depending on the glass thickness, over a 20 – 60 minute period.
[edit] Cold end
The role of the cold end is to inspect the containers for defects, package the containers for shipment and label the containers.
[edit] Inspection equipment
Glass containers are 100% inspected; every container is inspected. Automatic machines inspect for a variety of faults. Typical faults include small cracks in the glass called checks, foreign inclusions called stones, bubbles in the glass called blisters and excessively thin walls. In addition to rejecting faulty containers, inspection equipment gathers statistical information and relays it to the forming machine operators in the hot end. Computer systems collect fault information to the mould that produced the container. This is done by reading the mould number on the container, which is encoded (as a numeral, or a binary code of dots) on the container by the mould that made it. Operators carry out a range of checks manually on samples of containers, usually visual and dimensional checks.
[edit] Secondary processing
Sometimes container factories will offer services such as labelling. Several labelling technologies are available. Unique to glass is the Applied Ceramic Labelling process (ACL). This is screen-printing of the decoration onto the container with a vitreous enamel paint, which is then baked on. An example of this is the original Coca-Cola bottle. The Absolut Bottles have various added services such as: Etching ( Absolut Citron/) Coating (Absolut Raspberry/Ruby Red)and Applied Ceramic Labelling ( Absolut Blue/Pears/Red/Black)
[edit] Packaging
Glass containers are packaged in various ways. Popular in Europe are bulk pallets with between 1000 and 4000 containers each. This is carried out by automatic machines (palletisers) which arrange and stack containers separated by layer sheets. Other possibilities include boxes and even hand sewn sacks. Once packed the new "stock units" are labelled and warehoused.
[edit] Coatings
Glass containers typically receive two surface coatings, one at the hot end, just before annealing and one at the cold end just after annealing. At the hot end a very thin layer of tin oxide is applied either using a safe organic compound or inorganic stannic chloride. Tin based systems are not the only ones used, although the most popular. Titanium tetrachloride or organo titanates can also be used. In all cases the coating renders the surface of the glass more adhesive to the cold end coating. At the cold end a layer of typically, polyethylene wax, is applied via a water based emulsion. This makes the glass slippery, protecting it from scratching and stopping containers from sticking together when they are moved on a conveyor. The resultant invisible combined coating gives a virually unscratchable surface to the glass. Due to reduction of in-service surface damage the coatings often are described as strengtheners, however a more correct definition might be strength retaining coatings.
[edit] Ancillary processes – compressors & cooling
Forming machines are largely powered by compressed air and a typical glass works will have several large compressors (totaling 30k-60k cfm) to provide the needed compressed air. Furnaces, compressors and forming machine generate quantities of waste heat which is generally cooled by water. Hot glass which is not used in the forming machine is diverted and this diverted glass (called cullet) is generally cooled by water, and sometime even processed and crushed in a water bath arrangement. Often cooling requirements are shared over banks of cooling towers arranged to allow for backup during maintenance.
[edit] Marketing
Glass container manufacture in the developed world is a mature market business. Annual growth in total industry sales generally follows population growth. Glass container manufacture is also a geographical business; the product is heavy and large in volume, and the major raw materials (sand, soda ash and limestone) are generally readily available, therefore production facilities need to be located close to their markets. A typical glass furnace holds hundreds of tonnes of molten glass, and so it is simply not practical to shut it down every night, or in fact in any period short of a month. Factories therefore run 24 hours a day 7 days a week. This means that there is little opportunity to either increase or decrease production rates by more than a few percent. New furnaces and forming machines cost tens of millions of dollars and require at least 18 months of planning. Given this fact, and the fact that there are usually more products than machine lines means that products are sold from stock. The marketing/production challenge is therefore to be able to predict demand both in the short 4-12 week term and over the 24-48 month long term. Factories are generally sized to service the requirements of a city; in developed countries there is usually a factory per 1-2 million people. A typical factory will produce 1-3 million containers a day. Despite it’s positioning as a mature market product, glass does enjoy a high level of consumer acceptance and is perceived as a “premium” quality packaging format.
[edit] Lifecycle impact
Glass containers are wholly recyclable and the industry in many countries retains a policy (or is forced to by Government) of maintaining a high price on cullet to ensure high return rates. Return rates of 95% are not uncommon in the Nordic countries (Sweden, Norway, Denmark and Finland). Return rates of less than 50% are usual in other countries. Of course glass containers can also be reused, and in developing countries this is common, however the environmental impact of washing the container as against remelting them is uncertain. Factors to consider here are the chemicals and fresh water used in the washing, and the fact that a single use container can be made much lighter, using less than half the glass (and therefore energy content) of a multiuse container. Also, a significant factor in the developed world's consideration of reuse are producer concerns over the risk and consequential product liability of using a component (the reused container) of unknown and unqualified safety. How glass containers compare to other packaging types (plastic, cardboard, aluminium) is hard to say, conclusive lifecycle studies are yet to be produced.
[edit] Float glass process
[edit] Environmental impacts
[edit] Local environmental impacts
As with all highly concentrated industries, glassworks suffer from moderately high local environmental impacts. Compounding this is that because they are mature market businesses they often have been located on the same site for a long time and this has resulted in residential encroachment. The main impacts on residential housing and cities are noise, fresh water use, water pollution, NOx and SOx air pollution, and dust.
Noise is created by the forming machines. Operated by compressed air, they can produce noise levels of up to 106dBA. How this noise is carried into the local neighbourhood depends heavily on the layout of the factory. Another factor in noise production is truck movements. A typical factory will process 600T of material a day. This means that some 600T of raw material has to come onto the site and the same off the site again as finished product.
Water is used to cool the furnace, compressor and unused molten glass. Water use in factories varies widely, it can be as little as one tonne water used per melted tonne of glass. Of the one tonne roughly half is evaporated to provide cooling, the rest forms a wastewater stream.
Most factories use water containing an emulsified oil to cool and lubricate the gob cutting shear blades. This oil laden water mixes with the water outflow stream thus polluting it. Factories usually have some kind water processing equipment that removes this emulsified oil to various degrees of effectiveness.
The oxides of nitrogen are a natural product of the burning of gas in air and are produced in large quantities by gas fired furnaces. Some factories in cities with particular air pollution problems will mitigate this by using liquid oxygen, however the logic of this given the cost in carbon of (1) not using regenerators and (2) having to liquefy and transport oxygen is highly questionable. The oxides of sulphur are produced as a result of the glass melting process. Manipulating the batch formula can effect some limited mitigation of this; alternatively exhaust plume scrubbing can be used.
The raw materials for glass making are all dusty material and are delivered either as a powder or as a fine-grained material. Systems for controlling dusty materials tend to be difficult to maintain, and given the large amounts of material moved each day, only a small amount has to escape for there to be a dust problem. Cullet is also moved about in a glass factory and tends to produce fine glass particles when shovelled or broken.
[edit] Global environmental impact
The main global impact factor is the production of CO2 due to the burning of fossil fuels in the heating of the furnace and production of electricity to supply the compressors. Typically a tonne of glass packed will liberate between 500 and 900kg of CO2, assuming a gas fired furnace and coal fired electricity usage.
[edit] Colors
Colors in glass may be obtained by addition of coloring ions and by precipitation of finely dispersed colloides (such as in "ruby gold",[4] white tin oxide glass, red "selenium ruby").[1] Ordinary soda-lime glass appears colorless to the naked eye when it is thin, although iron oxide impurities produce a green tint which can be viewed in thick pieces or with the aid of scientific instruments. Further metals and metal oxides can be added to glass during its manufacture to change its color which can enhance its aesthetic appeal. Examples of these additives are listed below:
- Iron(II) oxide may be added to glass resulting in bluish-green glass which is frequently used in beer bottles. Together with chromium it gives a richer green color, used for wine bottles.
- Sulphur, together with carbon and iron salts, is used to form iron polysulphides and produce amber glass ranging from yellowish to almost black. In borosilicate glasses rich in boron, sulphur imparts a blue color. With calcium it yields a deep yellow color. [5]
- Manganese can be added in small amounts to remove the green tint given by iron, or in higher concentrations to give glass an amethyst color. Manganese is one of the oldest glass additives, and purple manganese glass was used since early Egyptian history.
- Manganese dioxide, which is black, is used to remove the green color from the glass; in a very slow process this is converted to sodium permanganate, a dark purple compound. In New England some houses built more than 300 years ago have window glass which is lightly tinted violet because of this chemical change; and such glass panes are prized as antiques.
- Selenium, like manganese, can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish color, caused by selenium atoms dispersed in glass. It is a very important agent to make pink and red glass. When used together with cadmium sulfide [6], it yields a brilliant red color known as "Selenium Ruby".
- Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. The best results are achieved when using glass containing potash. Very small amounts can be used for decolorizing.
- Tin oxide with antimony and arsenic oxides produce an opaque white glass, first used in Venice to produce an imitation porcelain.
- 2 to 3% of copper oxide produces a turquoise color.
- Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production of ruby-colored glass.
- Nickel, depending on the concentration, produces blue, or violet, or even black glass. Lead crystal with added nickel acquires purplish color. Nickel together with small amount of cobalt was used for decolorizing of lead glass.
- Chromium is a very powerful colorizing agent, yielding dark green [7] or in higher concentrations even black color. Together with tin oxide and arsenic it yields emerald green glass. Chromium aventurine, in which aventurescence was achieved by growth of large parallel chromium(III) oxide plates, was also made from glass with added chromium.
- Cadmium together with sulphur results in deep yellow color, often used in glazes. However, cadmium is toxic.
- Adding titanium produces yellowish-brown glass. Titanium is rarely used on its own, is more often employed to intensify and brighten other colorizing additives.
- Metallic gold, in very small concentrations (around 0.001%), produces a rich ruby-colored glass ("Ruby Gold"), while lower concentrations produces a less intense red, often marketed as "cranberry". The color is caused by the size and dispersion of gold particles. Ruby gold glass is usually made of lead glass with added tin.
- Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green color [8]. Uranium glass is typically not radioactive enough to be dangerous, but if ground into a powder, such as by polishing with sandpaper, and inhaled, it can be carcinogenic. When used with lead glass with very high proportion of lead, produces a deep red color.
- Silver compounds (notably silver nitrate) can produce a range of colors from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colors produced by these compounds. The chemistry involved is complex and not well understood.
[edit] See also
- Borosilicate glass
- Float glass
- Wine bottles
- Drinkware
- Blow moulding
- Packaging and labelling
- Irving Wightman Colburn
|
|||||||||||
[edit] References
- ^ a b Werner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg GmbH & Co. K; 2nd revised edition (November 1994), ISBN 3540575723
- ^ The dilatometric softening point is not identical with the deformation point as sometimes assumed. For reference see experimental data for Td and viscosity in: "High temperature glass melt property database for process modeling"; Eds.: Thomas P. Seward III and Terese Vascott; The American Ceramic Society, Westerville, Ohio, 2005, ISBN 1-57498-225-7
- ^ B. H. W. S. de Jong, "Glass"; in "Ullmann's Encyclopedia of Industrial Chemistry"; 5th edition, vol. A12, VCH Publishers, Weinheim, Germany, 1989, ISBN 3-527-20112-5, p 365-432.
- ^ Formation of Gold Nanoparticles in Gold Ruby Glass: The influence of Tin
- ^ Substances Used in the Making of Coloured Glass 1st.glassman.com (David M Issitt). Retrieved 3 August 2006
- ^ Illustrated Glass Dictionary www.glassonline.com. Retrieved 3 August 2006
- ^ Chemical Fact Sheet - Chromium www.speclab.com. Retrieved 3 August 2006
- ^ Uranium Glass www.glassassociation.org.uk (Barrie Skelcher). Retrieved 3 August 2006
