pH is stuck, better caustic?

[Wraevyn]

Question: I have a 1000 Gallon waste treatment tank that has spent nitric acid, hydrofluoric acid, ammonia hydroxide, hydrogen peroxide and lots DI water. It is a sludge tank being treated to go out to the state. The problems which I am having are: the pH has not raisen much at all. I have been adding Sodium Hydroxide to bring it up, but it doesn't seem to be working very well. I was sitting at a pH of 2.5 for the better half of the day. Adding the caustic at a regular rate, but slow enough not to over heat. I got the tempurature around 33C and the pH started to climb but now that I am at a pH of 4.5, it is stuck and holding. Every time I add more Sodium Hydroxide, the pH drops a little. If I leave it to mix, it comes back up to 4.5 and holds. The temp. however goes up. So I let it sit over night to cool off. Starting again at the pH of 4.5 and 19C its stuck. I have gotten the temp. to 26C and still holds at 4.5. So the problem is the same. So does the acid take the caustic better at a higher temp? Is there a better base to neutralize this mix better? And is the heat a factor I should be taking into consideration?

 

[Latexman]

Open up your college chemistry textbook and look for a titration curve. It looks like an S curve. It stays on the acid side seems like forever, then all of a sudden it'll pass thru neutral (pH = 7) real quick and stays in the basic.

Yes, pH is affected by temperature. The reaction that is going on is essentially instantaneous, so it shouldn't be affected by temperature that you would notice.

Good luck,
Latexman

 

[moltenmetal]

Still getting heat rise? You're still neutralizing significant amounts of acidic stuff. Does the pH rise, then fall again? If so, you're hydrolyzing stuff in your "sludge". Strong acids neutralize quickly, but weak acids and acid salts can hydrolyze rather slowly, releasing acid all the while.

Using a hand-held pH meter? It's probably not all that well compensated for temperature. Make sure you take the readings when the tank is cool prior to discharge.

If by "the state" you mean a publicly owned treatment works, remember that they tend to like pH in the range of 6-9: they'd prefer it a little alkaline to a little acidic.

If you value your time more than your money, you can use magnesium hydroxide for the last bit- it's tough to over-dose. If you value the latter more than the former, calcium hydroxide (lime) is cheaper than NaOH.

 

[Wraevyn]

Thanks for the info. It worked out. Just took alot more caustic then I thought, and yes I checked out a friends old chemistry book. HA HA. I also found that the Magnesium Hydroxide is actually cheaper then the Sodium here, so I think I will order a few barrels and see how it goes. If I work the numbers it should cut the caustic use in half. Which should save me time and money.

Thanks

 

[yank38]

There is no better base available for the nuetralization of acids than caustic soda. You do have the downside of heat generation. Is this a metal bearing waste? Mg(OH)2 is typically used in wastewater treatment when heavy metals are present. It alters the sludge formation in a favorable fashion. But Mg(OH)2 is insoluble and your reactions will generally be slower. In a situation like you described where you have an ovious buffering issue the Mg(OH)2 may not be very effective.

 

[moltenmetal]

Mg(OH)2 is slower, but it's also difficult to over-dose- and a h*ll of a lot less hazardous to handle. In his case it's also cheaper- go figure. It's EASY to over-dose NaOH, requiring yet more time and money to get pH back below 9. He has to wait anyway due to heat generation. I stand by the recommendation.

 

[Wraevyn]

The tank is a metal bearing "sludge". Metals of aluminum, tantalum, copper, iron, lead titanium, chromium 3+, possible but very slim to none chance of chromium 6+. I had been making the sludge tanks before by using a few drums of spent caustic. Lowering the pH was easy, one drum of HF and Nitric mix would bring it down to a pH comfortable to start using the metal grabbers and coagulant. I hadn't thought about the reversal side. A couple barrels of HF/Nitric would mean I would have to almost double my caustic that I had on hand. I will order caustic soda and Mg(OH)2 and play around a little to find the best over all.

 

[unclesyd]

Back when we had several large HNO3/HF pickling vats we used a system where we ran the spent acids through a bed of dolomitic limestone or marble chips. The contactors were fiberglass and later changed to a lined pit on the side of a hill were there was gravity drainage.
I can't recall what the secondary treatment was but will try to get the information