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

Steel etching Byproducts

2005

Hi.

I am doing a waste survey for a cutlery finishing company in Sheffield. The company use oxalic acid (5%) solution to etch martensitic steel cutlery. This produces a mystery waste sludge. Does anyone know what it is as I think it might be relevant for waste disposal purposes.
Thanks

Tom Cullingford
- UK

---

Ferric hydroxide, with significant amounts of nickel and chromium. Try dissolving it in HCl.

Dave Wichern
Consultant - The Bronx, New York
2005

What valency is the chromium in and what is the resultant end product from using HCl?

Tom Cullingford
- Sheffield, UK
2005

---

Trivalent; precipitable trivalent chromium. More can be found on one of the longest, most rambling, but most entertaining threads on finishing.com, letter 12044.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005

---

2005

I know very little about haz waste regs in the UK. If you were in the USA:

1) First, see if the solids generated are a categorical hazardous waste. Sludges resulting from a particular industrial process, for example, electroplating, are presumed to be hazardous regardless of what they may or may not contain. Even if you were plating chemically pure iron parts, the iron hydroxide sludge resulting from neutralization of the pickling HCl would be considered hazardous waste. There is the option of "delisting" such a solid, but this is a very exhaustive process that requires much documentation and lab work.

2) A sludge that does not fall into an established Federal category may still be hazardous if it possesses one of four characteristics: ignitability, corrosivity, toxicity, or reactivity. Your sludge, depending on the pH, might be corrosive. If it was, that would not be hard to fix. It might also be toxic - this depends on whether it can pass a test, the Toxicity Characteristic Leaching Procedure. In this test a sample of the solid is tumbled for 18 hours in a liquid that is buffered at about pH 5. The mixture is then filtered, and the filtrate analyzed for various metals and organic compounds. There are established limits which, if exceeded, make the solid a hazardous waste.

I hope this has been of some help. Good luck.

Dave Wichern
Consultant - The Bronx, New York

---

I think your referring to the sludge that is created in the oxalic etching bath, not a waste sludge that results from treatment, right? That is an oxalate sludge that forms from the reduction of oxalic acid by the dissolution of the steel. Its also prevalant in valve metal alloys as well. The good news is that it is stable and would pass your equivalent of a TCLP test, so it's usually not considered hazardous at all. It does not redissolve in water, nor will it dissolve in acids or caustics either. It is similar to a metal oxide powder that sometimes forms in other etch baths that contain high levels of oxidizers.

Tom Baker
wastewater treatment specialist - Warminster, Pennsylvania
2005

---

Ha! Good one, Ted Mooney! Yes, it is trivalent Cr, as illustrated by one bored chemist in letter 12044. Ted, it was truly a great pleasure meeting you at the show in Charleston, SC. Let's see if we can fire up another one of those threads!

Randall Fowler - Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA
2005