MadSci Network: Chemistry
Query:

Re: USING SALT TO MELT ICE

Date: Mon Nov 9 12:38:11 1998
Posted By: Bob Peeples, PE, Chemical Engineer, Environmental Program Management, U S Postal Service
Area of science: Chemistry
ID: 909868014.Ch
Message:

The United States Environmental Protection Agency studied this subject about 25 years ago and produced Water Pollution and Associated Effects From Street Salting available from the National Technical Information Service (NTIS# PB.222795/7BA). Many types of chemical ice melters are used on streets, driveways, parking lots, and sidewalks. With more than 200,000 vehicles driving more than 850 million miles annually, the United States Postal Service drives on all of them (and our letter carriers have to walk on all of them). With nearly forty thousand customer service facilities to manage, environmentally responsible selection of ice melting chemicals is a very important issue for the USPS, for the continued safety of our customers and our employees.

Ice and snow melting chemistry is pretty simple. It works on a colligative property known as freezing point depression. Colligative means that the property depends only on the number of particles present, and not on chemical properties of the particles. We look for a chemical that dissolves in water easily and quickly, dissociates into ions, and is safe for application. All ice melting salts dissociate into ions as they dissolve into the melting ice and snow. This multiplies the molar quantity, and multiplies the effect of freezing point depression. Rock salt for instance, releases a ratio of one sodium ion (Na+) to one chloride ion (Cl-) for twice the effect. Calcium chloride releases one calcium ion (Ca+) for every two chloride ions for three times the effect. However, calcium and magnesium chlorides pose a greater risk than potassium and sodium chlorides because they release twice the number of damaging chloride ions. Plants vary in sensitivity to these salts. Excessive application of ice melting salts in a small area will kill vegetation. Indirect damage, resulting from a build-up of salt in the soil, is longer lasting and more difficult to remedy. The build-up can prevent existing plants from absorbing water and other required nutrients, and inhibit new plant growth. High sodium concentrations can harden soil, leaving a layer impermeable to water. (While ions such as calcium, magnesium, sulfate, ammonium, and potassium are necessary for plant vigor, excess amounts will also result in plant damage.)

Three common ice melting ingredients can chemically attack concrete: ammonium sulfate, magnesium chloride and calcium chloride. However, most pavement surface damage results from a natural process called the freeze-thaw cycle, rather than direct chemical attack. The freeze-thaw cycle involves moisture seeping into the cracks and surface pores and freezing. As the moisture changes to ice, it expands which puts stress on surfaces. Ice melting chemicals increase the number of freeze-thaw cycles and can also double the rate of expansion during freezing. Weak pavements may crack or pit under this added stress. Concrete less than one year old, masonry, stone, and asphalt are all particularly vulnerable to freeze-thaw cycles. For these surfaces, the end user may wish to consider using a pure traction aid such as garnet sand, rather than ice melting chemicals. However, if the maintenance of a clear sidewalk or driveway is critical, the end user must weigh the risk of surface damage against the potential liability. Careful selection of ice melting chemical blends and the application of a pavement sealer prior to any snow will help protect the pavement.

Halite (rock salt) is the most common ice melting salt. Halite is mined throughout the world. The primary chemical in rock salt is NaCl, which causes damage to vegetation in concentrated form. Halite is usually medium to dark gray in color if mined from shaft or pit mines. Purer forms of sodium chloride can be solution mined (forcing water into an underground salt dome and evaporating the brine that is forced out to recover the dissolved salt), but these methods are rather expensive for ice melting.

Calcium chloride (CaCl2) and magnesium chloride (MgCl2) can be manufactured or evaporated from naturally occurring brines like the Great Salt Lake in Utah. Both chlorides release heat (exothermic) as they dissolve, which helps it melt ice at very low temperatures. Both chemicals can be hazardous to human health. Both magnesium chloride and calcium chloride can leave a slippery residue that is difficult to clean. Both tend to refreeze quickly and may require frequent reapplication. In addition, both are hygroscopic, which can cause them to clump, harden or even liquefy during storage.

Ammonium Sulphate ([NH4]2SO4) is a fertilizer ingredient that is infrequently used in ice melting salts. It will attack concrete and is not recommended for use on concrete surfaces.

Potassium Chloride (KCl, potash) commonly takes the form of red or white granules. The red grade comes from traditional shaft mines and gets itís color from iron contamination. The pure white grade is solution mined. Potassium chloride is not as effective at very low temperatures, making pure potassium chloride impractical unless used in conjunction with other ingredients. Potassium chloride is a common plant nutrient. This makes it more safe than halite for landscaping, but it is not always a good idea to add nutrients to the environment even though it may sound good. Excess application of potash can be harmful to the same plants that it would normally nourish at lower concentrations and sometimes increasing plant vigor is worse than harming the plants: In northern Arizona, the Highway Department found that the resulting rich, green grass in freeway medians was causing an increase in accidents between vehicles and wildife seeking out new winter grazing pastures between six lanes of speeding traffic.

Urea is also a common fertilizer nutrient. In its pure form, urea is not corrosive making it a good choice for use around airplanes. Urea must meet strict contamination regulations before being approved to use at airports. However, most of the urea sold for melting ice is considered an agricultural grade is not suitable for use in corrosion sensitive environments.

Ethylene glycol is a liquid deicer. It is commonly mixed with liquid urea and applied using bulk sprayers and tanker trucks applied primarily at airports. It is non-corrosive but poses environmental hazards, especially to grass, animals and to our water supply.

Potassium acetate is a biodegradable liquid deicer. It is also primarily used for airports. Because potassium acetate is corrosive it is often mixed with a corrosion inhibitor.

Calcium magnesium acetate (CMA) was developed as an environmentally responsible alternative to road salt. It is the safest of all ice melting chemicals in regards to vegetation, concrete, water sources, and the applicator. CMA is non-corrosive, biodegradable and can effectively prevent the formation of ice-surface bonds when applied prior to precipitation. Research even suggests that CMA's coating action reduces the risk of moisture penetration and surface damage. CMA is more than 30 times the cost of rock salt. The high cost has limited its practical use. CMA is not effective at very low temperatures. Pure CMA is applied in critical areas where corrosion, vegetation, or water contamination is a primary concern. When blended with other ingredients, CMA can significantly reduce the risk of corrosion and surface damage. In fact, tests have found that a minimum of 20% CMA can reduce corrosion and concrete damage by as much as 80%.

Alpha methyl glucoside (MG-104) is a corn by-product that is most effective when combined with other ingredients. MG-104 provides a catalytic affect that speeds melting, helps other chemicals to work at lower temperatures, and assists in the extension of freeze-thaw cycles to reduce surface damage.

To avoid environmental damage, always use an ice melting blend made from a well selected balance of ingredients. This reduces the risk of excessive concentrations of any single ingredient. Remember, over application of any chemical (including fertilizer), can damage vegetation. The best formulations will not cause environmental harm unless misapplied or used under extreme conditions.

Follow recommended instructions prescribed by the manufacturer, and disperse the melted ice and snow over a wide area to reduce the risk of over concentration. Apply the chemicals as the precipitation begins. This concentrates the chemical at the pavement surface to prevent surface bonding and facilitate ice removal. This will reduce the amount of chemical needed and the potential for damage. Chemicals are also more effective at the pavement surface because the pavement is usually warmer than the snow and ice.


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