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-----How to dispose of 5% Sodium Dichromate bath?
We are looking for current information on how to neutralize our Sodium Dichromate Seal bath (5% w/v) and DI Water. so we can safely dispose of the solution. I know we had a recipe before but are unable to locate it at this time. Our bath is approx. 315 gals.
Thank you,
Jim
Metal Finishing - Indiana
2007
Treatment of spent electroplating solutions requires someone with hands-on training because the treatment methodology is itself dangerous and environmentally risky, James, so I think you might invite a local supplier or consultant in to refresh your memory. But in general the treatment is to add sodium metabisulfite reduce the chrome to trivalent (quick indication is green-blue solution with no more amber color to it), and then neutralize to precipitate the chrome-bearing sludge. Evacuations of more than one plating shop, with tears in everyone's eyes, has resulted from getting the bisulphite too acidic and releasing toxic sulfur dioxide gas.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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2007
2007
What Ted warns of is very important. If you want to treat this bath yourself, good ventilation, and the use of an air purifying respirator fitted with acid gas cartridges by the person doing the treating is essential. Don't attempt the following unless you have these.
Add sulfuric acid to pH 2 with good mixing. Put your respirator on. Now, add sodium metabisulfite, with plenty of stirring - theoretical amount is 142 lbs, but your waste is likely kind of spent (some chrome in the 3+ form already) so it may take somewhat less. The waste should an emerald green color - not grass green, a deep emerald. After you've added two of the three 50 lb sacks, check the pH, you will probably have to add a bit more sulfuric to get the pH back under 2.
Now, add 50% sodium hydroxide to a pH of 9.5 - 10. The waste should turn an opaque gray-green. It kind of looks "chalky" - you'll see what I mean.
Add some flocculant solution, or, just let it settle for a day of two. All the chromium will settle out, leaving a nice layer of clean water up top. Decant this off with a Little Giant sump pump, or by siphoning, to a clean tank.
Take a sample, send it to a good reliable lab for metals analysis, and release it, if it's clean. Drum up the sludge, and have it hauled. Dewatering it with a filter press will reduce the volume, and the disposal costs, very greatly.
Dave Wichern
Consultant - The Bronx, New York
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