The authoritative public forum
  for Metal Finishing since 1989
-----

How to calculate waste loadings from HF Etch Bath

We have an etching bath that is composed of 10% v/v HF (49%); 10% HNO# (63%), and 80% H2SO4 (98%). Each bath is 33 gallons, and we dispose of 634 gallons per day. I am afraid that I do not know how to calculate my fluoride and sulphate lading to make sure that I will meet my discharge requirements.

Michele Keane
Employee - Atlanta, Georgia
2007

---

First of two simultaneous responses --

It's not clear to me what you are trying to do. Treat by dilution? Your limits would have to be very liberal to do that.

You can treat that solution by adding it, WITH GREAT CARE (lots of heat will be generated, along with some nasty fumes) to at least 5 times the volume of water, and then adding hydrated lime [affil links] with very vigorous agitation to pH 9 or so. You will get a lot of sludge.

If your fluoride limit is very high, you may be able to get away with simply neutralizing with caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] . It's still necessary to dilute first, a concentrated acid mixture like that would react violently if you did not.

Dave Wichern
Consultant - The Bronx, New York
2007

---

Second of two simultaneous responses --

If you're looking for F levels, you might be able to run a test using an ISE (ion selective electrode). Basically, you titrate with lanthanum nitrate (sp? I think?) with an ISE to determine free fluoride levels. You might be able to also use a method from someone like Hach or another test company. They have many tests that can be used to detect various ions in solution. I suggest giving them a call and telling them what you're looking for. They should also be able to help you with testing for sulphate levels.

Christian Restifo
- Pittsburgh, Pennsylvania
2007