De-icing is the process of removing ice from a surface.
Anti-icing is the process of preventing ice from forming on a surface.
Deicing can be accomplished by mechanical methods (scraping); through the application of heat; by use of chemicals, known as icemelters, designed to lower the freezing point of water (various salts or alcohols); or by a combination of these different techniques.
When there are freezing conditions and precipitation, deicing an aircraft is critical. Ice causes the surface of the wings to be rough, disrupting smooth air flow and greatly degrading the ability of the wing to generate lift. This situation can cause a crash. If large pieces of ice separate when the aircraft is in motion, they can be ingested in engines or hit propellers and cause catastrophic failure. Thick ice can jam control surfaces, preventing them from moving properly. Because of this potentially severe consequence, de-icing is performed at airports where temperatures are likely to dip below the freezing point.
De-icing techniques are also employed to ensure that engine inlets and various sensors on the outside of the aircraft are clear of ice or snow.
De-icing on the ground is usually done by spraying aircraft with a deicing fluid such as monopropylene glycol, similar to ethylene glycol antifreeze used in some automobile engine coolants. Ethylene glycol is still in use for aircraft deicing in some parts of the world, but Monopropylene glycol is more common because it is classified as non-toxic, unlike ethylene glycol. Nevertheless, it still must be used with a containment system to capture the used liquid, so that it cannot seep into the ground and streams. Even though classified as non-toxic, it has negative effects in nature, as it uses oxygen during breakdown, causing life to suffocate. (In one case, a significant snow in Atlanta in early January 2002 caused an overflow of such a system, briefly contaminating the Flint River downstream of the Atlanta airport.) Many airports recycle used deicing fluid, separating water and solid contaminants, enabling reuse of the fluid.
Infrared is the transmission of energy by means of electromagnetic waves or rays. Infrared is invisible and travels in straight lines from the heat source to surfaces and objects without significantly heating the space (air) it passes through. When infrared waves strike an object, they release their energy as heat. This heat is either absorbed or reflected by the cooler surface. Infrared energy is continually exchanged between "hot" and "cold" surfaces until all surfaces have reached the same temperature (equilibrium). The colder the surfaces, the more effective the infrared transfer from the emitter. This heat transfer mechanism is substantially faster than conventional heat transfer modes used by conventional deicing (convection and conduction) due to the cooling effect of the air on the deicing fluid spray.
De-icing of roads has traditionally been done with salt, spread by snowplows or dump trucks designed to spread it, along with sand and gravel, on slick roads. Sodium chloride (rock salt) is normally used, as it is inexpensive and readily available in large quantities. However, since salt water still freezes at -18°C or 0°F, it is of no help when the temperature falls below this point. It also has a strong tendency to cause corrosion, rusting the steel used in most vehicles and the rebar in concrete bridges. More recent snowmelters use other salts, such as calcium chloride and magnesium chloride, which not only depress the freezing point of water to a much lower temperature, but also produce an exothermic reaction. They are somewhat safer for concrete sidewalks, 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 generated as byproducts of agricultural operations such as sugar beet refining or the distillation process that produces ethanol.
Since the 1990s, use of liquid chemical melters has been increasing, sprayed on roads by nozzles instead of a spinning spreader. Liquid melters are more effective at preventing the ice from bonding to the surface than melting through existing ice.
In Nagano, Japan, relatively inexpensive hot water bubbles up through holes in the pavement to melt snow, though this solution is only practical within a city or town. Some individual buildings may melt snow and ice with electric heating elements buried in the pavement, or even on a roof to prevent ice dams under the shingles, or to keep massive chunks of snow and dangerous icicles from collapsing on anyone below. Small areas of pavement can be kept ice-free by circulating heated liquids in embedded piping systems.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)