Hot Chips

March 01, 2014
Jerry Byers
Online Only Articles

 

Over the years, manufacturing industries have had to deal with a strange problem described as “hot chips”. This is a situation where a bin of metal chips (the “saw dust” from a metal cutting or grinding operation) suddenly starts generating heat and smoke; under some conditions it can actually bust into flame. In at least one instance, a railcar full of metal grinding “swarf” began burning in a Chicago rail yard. Such an incident can quickly become a concern for governmental agencies such as the EPA, and put manufacturing companies in a very negative light.

Figure 1 – A burning dumpster can be a big problem!

In an earlier article, I wrote about corrosion issues with four common industrial metals, and some of the corrosion test methods used for each. “Hot chips” are a related issue. How is it possible that metal particles can start smoldering on their own, and even burn like a trash fire? The answer is: corrosion.

If you have ever been hunting or at a football game in the winter, you might have purchased little packs, about 2 inches by 3 inches in size, that you activate by pulling a tab off and then putting them into your pocket or gloves. The packet gradually gets warm and keeps you toasty for several hours. These hand warming packs contain little more than cast iron powder mixed with carbon powder, some water, and perhaps a little salt to accelerate the corrosion process. The reaction that generates the heat starts when the tab is pulled, exposing the mixture to oxygen from the air. The oxygen and water slowly oxidizes (rusts) the cast iron, generating a considerable amount of heat. This is a quite useful reaction and easily controlled in little packets, but a large hopper full of tiny metal pieces in warm humid weather is a different matter! Any oily residue remaining on those metal chips will help to generate smoke and flames.

We studied this reaction in the laboratory by placing 20 grams of iron powder in a small insulated beaker with varying amounts of tap water, plus a thermocouple to measure the temperature. We found the following:

  1. We found that chips (cast iron) could go from 64º F to 190º F within 12 minutes. See Figure 2.
  2. Cast iron was the metal most likely to generate heat – much more than steel fines. A mixture of aluminum and cast iron chips seemed to be no worse than cast iron alone. (Figure 2)
  3. The amount of heat generated is affected by the moisture content. The maximum amount of heat is generated when there is about 10 -15% water by weight. (Figure 3).
  4. “Fretting corrosion” exacerbates the amount of heat generated. (Fretting corrosion is caused by vibrations as it rubs off the oxide layer to expose a fresh surface ready to rust. Thus the problem becomes even bigger when the chips were transported by rail car or truck.)
  5. The most effective way of preventing this heat generation is to
    1. Shut off the air supply if possible or
    2. Spray the chips with a corrosion inhibitor.
  6. When a dilute solution (about 0.5%) of an oil-free corrosion inhibitor was applied, the temperature rise reached a maximum of only 84º F (barely warm). See Figure 4. Some inhibitors were more effective than others. A pump-up type garden sprayer would work as the applicator, but it is important to get the rust inhibitor to penetrate down deep into pile of chips; not just on the top surface.

An obvious question is: why doesn’t the metalworking fluid residue on the metal chips prevent oxidation of the chips in the hopper, and thus prevent spontaneous heating? The answer is that it does to a certain degree, but the corrosion inhibitor from the metalworking fluid is eventually depleted by trying to protect the vast surface area represented by the metal fines. This requires additional reinforcement in the form of added rust preventive. 

Figure 2 – Comparison of the heat generated from different powdered metals.

Figure 3 – Graph of the temperature rise for cast iron powder with varying amounts of water.

Figure 4 – This graph demonstrates the beneficial effect of adding two different rust inhibitors.

Jerry P. Byers works at Cimcool Fluid Technology. His contact information can be found in our membership database.

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