Analyzing HF in Titanium pickle
Could anyone give me a simple safe way to analyze (WET METHOD) the HF content in a Ti pickle solution of HF and HNO3.Graham Best
engineering - Llay, Wrexham, U.K.
You can analyze for HF with an ion selective electrode. It is not exactly what you call a wet analysis, but probably the only way.
Now I have a question for you, what do you do for the waste treatment of this solution? Do you have a proven method for its destruction?
There is a thorium nitrate titration for HF described in various metal finishing handbooks that used to be standard. I never performed it because of the problem of disposing of the thorium waste, which is quite toxic and mildly radioactive.
Sara is quite right in saying that the ion selective electrode method is the most practical method by far. You will find colorimetric methods (all based on the inhibition by fluoride of the formation of colored "lakes" resulting from the reaction of various dyes with a metal, generally zirconium) in the literature, but these are all plagued by interferences and are very unwieldy.
The way I used to do this test is by using the fluoride electrode as an endpoint detector in a lanthanum nitrate titration. The procedure is given in the manual that comes with the fluoride electrode manufactured by Orion.
As for treating it after it's spent, I would dilute it by a factor of 3 - 5 with water, depending on its strength, and add hydrated lime to a pH of 9.5 - 10. It is best to add the lime as a powder until the pH reaches 4- 5, then start adding it as a water slurry, slowing the rate of addition greatly towards the end. Otherwise, you will wind up with an even larger volume of sludge than necessary, from undissolved lime particles. Even when the treatment is performed perfectly, expect very large sludge volumes. The remaining dissolved fluoride in the free liquid and filter press return water generally runs from 5 - 10 ppm.
If you do not have a discharge limit for fluoride, the waste can be neutralized with sodium hydroxide instead. This greatly cuts the solids generation, but leaves the dissolved fluoride in the 500 - 1000 ppm range.
Be careful with the initial lime additions, because the neutralization generates a great deal of heat, and NOx and HF fumes are released. The treatment needs to be performed in a very well ventilated area, and the operator should wear an air purifying respirator fitted with acid gas cartridges. The treatment tank needs to be made of a material that will stand up to heat as well.
- The Bronx, New York
We pay for a disposal company to remove it, have you any other alternatives?Graham Best
engineering - Llay, Wrexham, U.K.
Sara is correct by saying you can determine HF concentration of a solution using a fluoride probe. However, in my experience, this only works with the initial setup of a nitric/HF bath. After parts (titanium) have been introduced into the processing bath the fluoride ions will begin to form complexes in solution. The fluoride probe with give you a measurement of free fluoride, but not complexed fluoride. So if you continue to use the probe for HF determination you will see a decrease in fluoride concentration, but only because the conc. of free fluoride in solution has changed, not the total fluoride. Testing the HF in this etching bath is a bit tricky, but there are several ways to make this determination. The best method I have found, and the one that I use, is to run an etch rate test on a titanium coupon. If you continue to monitor the etch rate of the bath you will see etch rate decreases over time and thus indications to add some HF to your solution. Depending on what specification your working off of, it should include a range for your etch rate test. Keep your rate in this level and your HF conc. should be up to par.Steven Smithers
- Kansas City, Missouri
This analysis method can be found in the Metal Finishing Guidebook, Vol. 96, No. 1, 1988, I've been told...I've been meaning to look it up myself.Douglas A. Hahn
- Mason, Ohio
take 5 ml solution in a round bottle make a slurry with
cab-o-seal titrate with 1N KOH until red to phenol phtalein. Add 40 ml of water, boil and continue titration again.
HNO3 70% =(A-B/2)*12.8
HF 70%= B*6.8
This procedure gives you free nitric acid and free hydrofluoric acid with the above procedure you get the ion concentration.
Of course from practical point of view get a reliable system of etch rate depend on your alloy and thickness you want to remove.
I use only 30 seconds for etch rate with air mixing of the solution.
- Lod, Israel
February 11, 2009
If you etching solution is for example 4%HF and 18% HNO3 and the rate decrease, how you correct it. suppose 100 lts of solution.If you ad HF 70% how much is enough?
- Chiahuahua, Mexico