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What conductivity setting for Caustic Etch Rinse Tank?

January 21, 2009

We are using Chemetall Oakite 360L for Caustic Etch of our Aluminum parts in our Chem-Film Line. It is 4% to 8% Concentration at Ambient Temp. and has a pH of approx. 12 and is in a 112 Gallon tank. Right after this tank we have a Clear Water Rinse that we measure Conductivity in. The Cond. Sensor is set at 32 us/cm so when it reaches anything above that it turns on a solenoid to start rinsing/adding RO water into the tank to get the conductivity back down. This generally take 10 minutes and uses 100 gallons.

So what I am asking...
Is there a better setting we should use instead of 32 micron/cm?
What is the Conductivity of the 360L (Sodium Hydroxide) that it takes so much water to bring the rinse tank down?
Any other ideas that would help us minimize the amount of water needed to bring the Rinse Tank conductivity down?

Thanks,
Dan

Dan Brinkman
Job Shop - Yankton, SD

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Hi, Dan. A very general rule of thumb traditionally alluded to by the authors who studied rinsing is to shoot for 500 to 1000:1 dilution for your first cut at it. So if you are dragging out 0.10 to 0.20 gallons of etch solution, then 100 gallons of rinse water sounds about right for a starting point.

You could get by with 10 gallons if you had two rinse stages.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 22, 2009

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Dilute sodium hydroxide is highly conductive. Each mg/L adds about 4.9 uS of conductivity. So 32 uS is only about 6.5 mg/L as NaOH. This is very clean water. Depending upon what type of bath you are dragging this into, you may be over-rinsing, and could possibly use a higher set point.

Lyle Kirman
consultant - Cleveland Heights, Ohio
January 30, 2009

With Ted's 2 tank system, you could drop it to 1 - 2 gals if you used tank one to mist spray over the process tank if you can live with it.

James Watts
- Navarre, Florida
February 2, 2009