How to Conduct Moisture Testing for Rock Salt Quality

December 13th, 2012

A Rock Salt Storage Bin

A Rock Salt Storage Bin

With winter storms already impacting much of the North state Departments of Transportation, businesses and homeowners are relying on rock salt to melt snow and ice for safer passage of traffic and pedestrians.  Whether dispensed from DOT salt trucks, hand operated equipment or simply by hand, an essential characteristic of rock salt quality is that it is free flowing.  That’s why specifications for rock salt used in deicing include recommended moisture content.  Field and lab testing for or by DOT personnel as well as testing by vendors provide a moisture content quality control check.

As an example, the Minnesota Department of Transportation provides instructions on salt testing for moisture that include a moisture analyzer as one of the recommended testing procedures.

Why Rock Salt Moisture Content is Important

The Minnesota DOT notes that excess moisture content impacts the flow ability of rock salt, making it hard to handle.  This is also evident when homeowners open last winter’s bag of rock salt only to find it has changed from granules to a “rock” in fact.  Improper storage in DOT salt storage bins can yield similar results.  The transformation from granules to “rocks” is due to the product being hygroscopic:  it likes to absorb moisture.  Proper storage in the off season is important for efficient DOT winter road clearing operations.

As another point the product is generally sold by weight.  Excess moisture means transportation officials are paying for water weight, not product weight.  Vendors that supply rock salt with too high a moisture content will likely have to make a price adjustment with state departments of transportation.  This is the case in Connecticut, where tests are performed by the ConnDOT materials testing lab.

Use a Moisture Analyzer for Accurate Measurement

There are several techniques for conducting moisture content calculation, all of which require adhering to strict testing procedures.   In general they calculate moisture content by the thermogravimetric method, also termed loss of weight on drying (LOD).  The sample is weighed before and after the test, the difference in weight being the amount of moisture removed during the test.  We describe here the use of a halogen moisture analyzer such as the 50g capacity Kern MLB model offered by Tovatech to determine rock salt moisture content following Minnesota DOT recommendations as a guideline.  This moisture balance has 0.001g readout, well within the MN DOT minimum of 0.005g.

Before testing, set up the Kern moisture balance in a level and protected area according to the operating instructions.  If it has been moved or environmental conditions change significantly it will have to be recalibrated, which also can be accomplished by following simple instructions.

Sample Prep:   

  • Take 3 rock salt samples totaling 500 grams at different locations from 1 to 6 inches below the surface of the salt stockpile.
  • Thoroughly blend the samples to produce a representation of the salt stockpile.
  • Keep the sample in an airtight container to prevent it from absorbing ambient moisture.

Rock Salt Moisture Content Testing:

Testing procedures can be programmed into the Kern MLB moisture analyzer so that they do not have to be reset each time a moisture check is conducted.

  • Place a sample plate on the sample plate holder and tare the balance.
  • Set the drying profile for standard drying.
  • Set the temperature at 160˚C.
  • Carefully and evenly spread approximately 25g of rock salt on the sample pan.
  • Closing the moisture analyzer lid will start the analysis.

The halogen heater quickly drives off moisture while the balance displays the status of each stage of the process.  Moisture analysis automatically terminates when the dry weight is stable or after a specified time.  The final moisture content is posted on the unit’s display panel along with the heating mode, temperature and time. Data can be transferred via an RS232 interface for record keeping.

The target moisture content can be set by the operating authority or by an industry standard.  For example, the Connecticut DOT specifies the maximum moisture content at 3%. The British Standard Institute in 1991 established BS 3247 concerning rock salt moisture, setting a target at less than 4%.

Please contact the scientists at Tovatech for unbiased advice on selecting a moisture analyzer to meet your specific requirements, and for information on its operation and maintenance.


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