Section 7: Chemical TypesAnchor: #i1001042
Sodium Chloride, NaCl
Sodium chloride has been used as an ice-control chemical on roads since early in the previous century. It is produced by three processes:
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- rock salt is mined by conventional hard rock mining equipment and techniques Anchor: #PWKWFRKG
- solar salt is produced by the evaporation of sea water and may contain only a small amount of impurities Anchor: #EBWGHYTG
- evaporated, solution or vacuum salt is a very pure form made by drying under vacuum the solution resulting from injection of water into deep underground deposits.
Most salt used for highway applications in the U.S. is rock salt, though some solar salt is produced in several western states and some is imported into the eastern states. Naturally occurring rock salt is the mineral halite, which usually contains between 1 percent and 4 percent impurities, mostly gypsum, shale, dolomite and quartz.
Brine is a solution of salt (Sodium Chloride-NaCl) in water, which will be covered in Section 8 of this chapter.Anchor: #i1001073
The principal source of this ice control chemical is brines from the Great Salt Lake. Though it is available in solid (flake) form, it is used in liquid form for ice control. The lowest temperature at which MgCl2 can melt snow or ice (eutectic temperature) is about -28 degrees Fahrenheit at a concentration of 21.6 percent. Its ice melting capacity is about 40 percent greater than Calcium Chloride CaCl2. Proprietary mixtures are available containing 20 percent to 25 percent MgCl2 with various corrosion inhibitor additives. One proprietary compound reportedly has an eutectic temperature of -4 degrees Fahrenheit. These solutions are effective ice-melting agents at temperatures above 19 degrees Fahrenheit.
With its competitive price and low freezing point, magnesium chloride works well as both a de-icer and anti-icer. It contains a corrosion inhibitor making it less damaging to concrete and steel than other products and it is less harmful to the environment than calcium chloride and sodium chloride. Manufacturer recommends no pre-treatment should occur unless the roads are already wet.Anchor: #i1001091
Calcium Magnesium Acetate (CMA)
Currently there is only one commercial source for CMA, using the reaction of acetic acid with dolomitic limestone for production. Acetic acid, the costly component of the compound, is manufactured from natural gas or petroleum, though small quantities have been produced by biodegradation of agricultural wastes.
The compound is available as pellets or powder. Though not as soluble in water as NaCl and CaCl2, solutions can be made for use as a pre-wetting agent or straight chemical application.
It is not a highly effective de-icing chemical in solid form because of its affinity for water and its light particle mass. Its benefit is that it makes snow mealy so that it doesn’t compact.
CMA is primarily a mixture of calcium and magnesium acetates, produced with a 3/7 Ca/Mg ratio which was found to be optimum in previous FHWA studies. The eutectic temperature is about -18 degrees Fahrenheit at a concentration of 32.5 percent.
When CMA degrades, the calcium and magnesium elements are said to actually improve the water and air permeability of the soil by restoring sodium-compacted soils.
Since acetate degrades into carbon dioxide and water and is a natural component of plant decay, CMA is appropriate where roadside vegetation, crops, or ground water are especially vulnerable. Because it is less corrosive than salt, some agencies prefer CMA for use on bridges, parking structures, sidewalks, and certain road surfaces (caution: it does cause major scaling).
The cost of CMA is approximately $600 per ton whereas salt generally costs $20 to $40 per ton. Some advocates of CMA argue that the initial costs may be misleading because replacement costs for roads and bridges damaged by chloride-related corrosion should be factored into the overall figures.
The pellet form of CMA is usually preferable to the powdered form, since the powder dust is less controllable. The pellet form of CMA does not bounce off the road before melting and its residual action can reduce reallocation frequency.Anchor: #i1001140
Potassium acetate, or KAc as it is commonly known, is produced by the reaction of acetic acid with potassium carbonate.
The sources of acetic acid are the same as in the production of CMA.
Potassium carbonate is one of the groups of salts commercially known as potash. Potassium carbonate was originally obtained by running water through wood ashes and boiling the resulting solution in large iron pots. The substance that formed was called potash.
Potassium carbonate is currently produced by one of several processes that use potassium chloride, another salt of the potash family. The compound, potassium acetate, is a white, crystalline, deliquescent powder that has a saline taste. It is soluble in water and alcohol. Solutions are alkaline under a litmus test.
The dry compound is combustible but is used as a dehydrating agent, a reagent in analytical chemistry, and in the production of synthetic flavors, in addition to other uses. The eutectic temperature of a KAc and water solution is -76 degrees Fahrenheit at a concentration of 49 percent.
A commercial form of liquid KAc, containing a 50 percent concentration by weight plus corrosion inhibitors, has been used as a pre-wetting agent with dry salt or as a straight chemical application.