De-icing is the process of removingsnow,ice orfrost from a surface.Anti-icing is the application of chemicals that not only de-ice but also remain on a surface and continue to delay the reformation of ice for a certain period of time, or prevent adhesion of ice to make mechanical removal easier.
De-icing can be accomplished by mechanical methods (scraping, pushing); through the application ofheat; by use of dry or liquid chemicals designed to lower thefreezing point of water (varioussalts orbrines,alcohols,glycols); or by a combination of these different techniques.
In 2013, an estimated 14 million tons of salt were used for de-icing roads in North America.[1]
De-icing of roads has traditionally been done withsalt, spread bysnowplows ordump trucks designed to spread it, often mixed withsand andgravel, on slick roads.Sodium chloride (rock salt) is normally used, as it is inexpensive and readily available in large quantities. However, sincesalt water still freezes at −18 °C (0 °F), it is of no help when the temperature falls below this point. It also has a tendency to causecorrosion,rusting thesteel used in most vehicles and therebar in concrete bridges. Depending on the concentration, it can be toxic to some plants and animals,[2] and some urban areas have moved away from it as a result. More recent snowmelters use other salts, such ascalcium chloride andmagnesium chloride, which not only depress the freezing point of water to a much lower temperature, but also produce anexothermic reaction. They are somewhat safer forsidewalks, but excess should still be removed.
More recently, organic compounds have been developed that reduce the environmental issues connected with salts and have longer residual effects when spread on roadways, usually in conjunction with salt brines or solids. These compounds are often generated as byproducts of agricultural operations such assugar beet refining or thedistillation process that producesethanol.[3][4] Other organic compounds arewood ash and a de-icing salt calledcalcium magnesium acetate made from roadside grass or even kitchen waste.[5] Additionally, mixing common rock salt with some of the organic compounds and magnesium chloride results in spreadable materials that are both effective to much colder temperatures (−34 °C (−29 °F)) as well as at lower overall rates of spreading per unit area.[6]
Several of these new compounds release very small amounts of gases into the air, which are known to be able to cause irritation of the throat and the respiratory tract in humans and animals. The majority of the human population do not experience problems although long-term effects have not been studied. People with sensitive airways, especially infants, may experience serious respiratory problems. Broader scientific studies of the respiratory health problems specifically for people with sensitive airways are lacking (in general, scientific studies have focused on non-respiratory health issues and environmental issues).
Solar road systems have been used to maintain the surface of roads above the freezing point of water. An array of pipes embedded in the road surface is used to collect solar energy in summer, transfer the heat to thermal banks and return the heat to the road in winter to maintain the surface above 0 °C (32 °F).[7] This automated form of renewable energy collection, storage and delivery avoids the environmental issues of using chemical contaminants.
It was suggested in 2012 thatsuperhydrophobic surfaces capable of repelling water can also be used to prevent ice accumulation leading toicephobicity. However, not every superhydrophobic surface is icephobic[8] and the method is still under development.[9]
Trains andrail switches in Arctic regions can have significant problems with snow and ice build up. They need a constant heat source on cold days to ensure functionality. On trains it is primarily thebrakes,suspension, andcouplers that require heaters for de-icing. On the rails it is primarily track switches that are sensitive to ice. High-powered electrical heaters prevent ice formation and rapidly melt any ice that forms.
The heaters are preferably made of PTC material, for examplePTC rubber, to avoid overheating and potentially destroying the heaters. These heaters are self-limiting and require no regulating electronics; they cannot overheat and require no overheat protection.[10]
On the ground, when there are freezing conditions andprecipitation, de-icing an aircraft is commonly practiced. Frozen contaminants interfere with the aerodynamic properties of the vehicle. Furthermore, dislodged ice can damage the engines.
Ground de-icing methods include:
Ice can build up on aircraft in flight due to atmospheric conditions, causing potential degradation of flight performance. Large commercial aircraft almost always have in-flight ice protections systems to shed ice buildup and prevent reformation. Ice protection systems arebecoming increasingly common in smaller general aviation aircraft as well.
Ice protection systems typically use one or more of the following approaches:
De-icing operations for airport pavement (runways,taxiways,aprons,taxiway bridges) may involve several types of liquid and solid chemical products, includingpropylene glycol,ethylene glycol and other organic compounds. Chloride-based compounds (e.g.salt) are not used at airports, due to their corrosive effect on aircraft and other equipment.[11]: 34–35
Urea mixtures have also been used for pavement de-icing, due to their low cost. However, urea is a significant pollutant in waterways and wildlife, as it degrades toammonia after application, and it has largely been phased out at U.S. airports. In 2012 theU.S. Environmental Protection Agency (EPA) prohibited use of urea-based de-icers at most commercial airports.[12]
Water agitators are electric motors put under water that propel up warmer water andagitate the surface with it to de-ice aquatic structures on rivers and lakes in freezing temperatures. There are also agitator bubblers that usecompressed air, run through ahose, and released to agitate the water.[13]
All chemical de-icers share a common working mechanism: they chemically prevent water molecules from binding above a certain temperature that depends on the concentration. This temperature is below 0 °C, the freezing point of pure water (freezing point depression). Sometimes, there is anexothermicdissolution reaction that allows for an even stronger melting power. The following lists contains the most-commonly used de-icing chemicals and their typicalchemical formula.
De-icing salts such assodium chloride orcalcium chloride leach into natural waters, strongly affecting their salinity.[1]
Ethylene glycol and propylene glycol are known to exert high levels ofbiochemical oxygen demand (BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen needed by aquatic organisms for survival. Large quantities ofdissolved oxygen (DO) in thewater column are consumed when microbial populations decompose propylene glycol.[14]: 2–23
Some airports recycle used de-icing fluid, separating water and solid contaminants, enabling reuse of the fluid in other applications. Other airports have an on-site wastewater treatment facility, and/or send collected fluid to a municipalsewage treatment plant or a commercial wastewater treatment facility.[11]: 68–80 [15]
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