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How clean is your rinse water?

Q. In powdercoating (and other) processes, involving a liquid based pretreatment, where rinsing is done between active phases, water is commonly used.
The water rinses the previous chemical off by dilution, and there's usually some mechanism for cleaning that water. This can be as simple as a filter, removing particulates, or as often the case in powdercoating, a demin plant, sometimes an RO plant.

In order to measure effectiveness of the cleaning system, a common measure used is conductivity. Often stated in mS/cm (micro Siemens per centimeter) this measure assumes that increasing conductivity of that rinse water means dirtier, while low conductivity means clean. Often conductivities of under 100 mS/cm are considered "clean" with <40 mS/cm very clean. <20mS/cm might be "ultra clean"?

But is this the best measure?
Can there be contaminants that do not affect the conductivity?
Surely only dissolved ions will change the conductivity? Organics say, which are in suspension, or perhaps colloidal form might not affect the conductivity, but could be a major cause of contamination?

Should a plant using rinses, and using only conductivity as a measure of cleanliness consider using something else as a measure alongside?

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
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June 12, 2012

simultaneous replies

A. Dissolved non-ionic solids would likely be indicated by monitoring the specific gravity and/or refractive index.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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A. Funny you should bring that up.

Not too long ago, I worked at an anodizing shop where the local water authority insisted that we install conductivity controllers on the Nickel-bearing seal, and the chromate coating rinses.

I did some preliminary studies, comparing conductivity vs. dissolved solids, measured gravimetrically. The results were great for the chromate rinse, but just terrible for the seal rinse.

Not surprising, because the seal is an acetic acid [on eBay or Amazon affil link] based solution with no strong electrolytes, and HOAc is largely non-ionized at the control pH.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

A. How true.
In OZ we have a Standard AS3715 for powdercoating of Architectural Aluminium.
This standard requires that the conductivity of rinse water running off product from the final rinse cannot exceed 25 micro Siemens.
Organic contaminants by definition cannot register in this test.
Time and again I have found that Beeswax which Aluminium Fabricators favour as a lubricant will defeat the strictest test regimes.
What is needed in addition to the Conductivity test which can only be expected to detect dissolved salts is a further test to detect carbonaceous materials.
I am unaware of such a test but really, like you, invite further input.
I fear our friends in the EAST may still have the advantage of a "lack of knowledge" about things like Trike?

William Doherty
Trainer - Salamander Bay, Australia

A. Conductivity is the best parameter to measure when all of the contaminants are ionic. However, when the contaminants are not ionic, as the case may be when there are non-ionic organic materials or particulate materials present, conductivity is of limited usefulness and other parameters such as turbidity or refractive index may be better indicators of rinse water purity.

Lyle Kirman

Lyle Kirman
consultant - Cleveland Heights, Ohio

A. Test for carbonaceous materials = Chemical Oxygen Demand. It can be done on a continuous flow basis.

dave wichern
Dave Wichern
Consultant - The Bronx, New York
June 18, 2012

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