Magnesium chloride (MgCl2) hydrate, the miracle liquid, has been in the news lately. In fact, it’s in the news every winter and you’ve likely already had some exposure to it today. Do you know what it is or what it does? Magnesium Chloride has many uses, actually. From the production of soy milk and tofu, to uses in cement and fireproofing, to dust and erosion control, this type of salt or liquid salt chemical can be created in the lab or extracted directly from sea water. However, its use as anti-icer is why it’s in the news today.
In the old days rock salt would be spread behind the snowplow. The salt would melt the remaining snow or ice and the salty liquid runoff would melt additional snow as it spread from the road’s crown to its shoulder. Actually this practice is common today and, despite its drawbacks (salt attacks the underbodies of vehicles as well as the rebar in concrete which leads to rusting, and in high enough concentration can damage roadside vegetation), it’s cheap and effective at melting snow and returning traction to miles upon miles of snow- and ice-covered roadways. Rock salt is also the popular choice for homeowners to melt the slick stuff from sidewalks and driveways.
Magnesium chloride, conversely, is applied to roadways before the snow or ice event by a truck-mounted sprayer. Even in a dried state, it can limit the ability of snow or ice to accumulate, while lowering the freeze point of water well below that of rock salt (rock salt becomes useless below 0F). In Southeast Texas, as well as other southern climes, MgCl2 is applied to bridges and other elevated road surfaces subject to freeze when snow or ice enters the immediate forecast.
Since a prolonged sub-freezing cold is rare in these locations, most surface streets are not subject to freezing which allows snow or ice to melt on contact. With that in mind, surface streets rarely get the MgCl2 application. Like rock salt, magnesium chloride is exothermic in that it releases heat (energy): it prohibits accumulation by melting the ice or snow. Unfortunately, MgCl2 can be washed away by rain, so the application must be perfectly timed to coincide with the freezing or frozen precipitation. Many highway departments communicate directly with a private weather service such as ImpactWeather for the most accurate and timely weather information so they can make the most effective last-minute decisions.
Airports also use magnesium chloride to deice not only concrete, but aircraft control surfaces as well, though it is not the first line (or even second line) of defense (salt=rust). Fluids of alcohol and glycol are sprayed directly on airplanes as they queue for departure. These fluids, two mainly, have distinct uses. Anti-icing fluids tend to prohibit accumulation while de-icing fluids (heated, typically) melt ice and/or snow that has already accumulated. Infrared energy is also used to warm aircraft surfaces and melt accumulated snow and prohibit additional accumulation.
Houston drivers may likely notice bridges and overpasses having a slightly darker appearance when driving today. This is because MgCl2 applications have been occurring throughout the Metro over the past 24-36hours.
Last year in a YourWeatherBlog post during an early 2010 Arctic outbreak I mentioned MgCl2 and you can read it here.