CSC Scientific Blog

A blog about test equipment

Static Electricity and Sieves: No Problem -- Maybe

Posted by

Art Gatenby on Apr 21, 2010 2:02:00 PM

I'll bet that most of you have experienced the shock of walking onstatic electricity shock a thick wool rug on a dry winter's day and getting zapped out of your reverie when reaching for the metal door knob.

The same process of static electricity generation can play havoc with your sieving process.

"Static electricity" (also called charge separation) occurs whenever four conditions exist:

1. Humidity is fairly low
2. Two surfaces are touched together, then separated
3. The surfaces are made of two different materials
4. Both surfaces are electrically insulating

For many products -- particularly hydrocarbon-based materials, plastics, reactive metalsTest sieves, paint pigments and powders with a large fraction of fines -- the sieve action provides a fertile environment for charges to build up on the particles and sieve components. This causes clinging, agglomeration and blinding. In other words, as material bumps and bounces around in the sieve stack, they generate that same type of electrical charge that zapped you last winter.

So what to do?

There is a sure, but messy (and a pain-in-the-butt) solution.

Wet Sieving

In wet-sieving, the separation of fines wet wash test sievesfrom the coarse portion of a sample is done while it is suspended in an aqueous solution introduced to each test sieve.

The liquid is used to negate static charges, break down agglomerates and lubricate near-size particles.

After the fines have been washed through the sieve, the residue is oven-dried and re-weighed.

---- Difficult, but effective.

Anything Easier?

A material that makes the particles slippery can be added to the sample, such as silica, activated charcoal, talc, magnesium carbonate, tricalcium phosphate or silicon dioxide.

If you make thehigh voltage power lines optimal selection, the sample stops being an electrical generating plant as well as an agglomerator. It goes through the sieves and gets the desired result.

The problem is, you have to know the additive's size and adjust your results by the amount of this material that is retained.

----- This is another fix but it is still a pain.

A simple approach that might work.

Perhaps strips of anti-static sheeting , such as those used in your clothes dryer will work. This is as easy as putting a few strips in at least the small mesh sieves.

If this is effective, it is the simplest solution we know.

Static Electricity and Sieves

Yet again -- one of Nature's ways to make a simple process difficult.

Please let me know if you've found any other effective solutions to a static problem in your sieve testing.




Topics: Sieve Blinding, Agglomeration, Sieve Testing, Sieves

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