[kennymcmack]

We have about 1000 gallons of a 40% NaOH solution. We need to get rid of it, and our policy says that we need to get it down to a pH of 11 before we dump it down the drain. We mixed a small representative sample of it with water to determine how much we will need to get to pH 11. We determined that it will take way too much water to dilute the entire 1000 gallons. We figure that a strong acid will neautralize it faster. Is there a cheap acid out there that we can use to neutralize this 1000 gallons of NaOH? Whats the most economical way to neutralize 1000 gallons of 40% NaOH? Thanks, Kenny. 

[408]

You sound like an industry, so I will recommend concentrated sulfuric which can be obtained as a waste product.

Of course dilute both and monitor the temperature during the reaction.

....Either that or ebay....

[info.che]

why drain? sell to someone who need it.

[billnotgatez]

 info.che --
What if they can not guarantee purity -- would you still buy it?

408 --
Is muriatic acid (HCl) not cheaper and easier to use and easier to acquire.

Kennymcm  ---
I assume you understand the safety issues when mixing a strong acid with a strong base.

[DrCMS]

I'd not recommend using HCl as it causes more issues with handling fumes etc and chloride causes more corrosion problems.

You could use Sodium Bicarbonate i think that would take the pH down to less than 11 with less heat output and acid handling issues but it would take a lot more material to do it.

[408]

I am under the impression that conc. H2SO4 is cheaper industrially than conc. HCl.  

While I understand it is debatable, I consider H2SO4 safer simply due to being less volatile.  While conc. H2SO4 does behave worse on contact than conc. HCl, when each is diluted by say 50% so ~9M H2SO4 vs ~6M HCl, the sulfuric looses most of its nastyness, while the HCl will still have a significant vapour pressure.  This will be magnified on an industrial scale.  

[billnotgatez]

DrCMS --

Thank you about HCl

It is interesting that sodium bicarbonate is your choice -- I had always learned it was a weak base.
My real initial suggestion was to use vinegar

But I am not well versed in large neutralization projects.

408 --- Thank you for your response

[Borek]

Left alone open for long enough it will neutralize itself by absorbing CO₂

Bicarbonate is amphoteric, so it would work.

[kennymcmack]

Thank you guys for all the help.

About selling it. I'm not sure how that process goes. We are a very large government agency and I don't think we can't just sell it to someone without a ton of paperwork, which will probably cost more than getting rid of it. By the way, it's about $8k to get rid of it at our facility, plus it adds to our annual allowable wastes. My boss said that our facility is only allotted so much allowance to get rid of waste chemicals.(I don't know what the official term was for this.)  We are trying to what we can to get that price down and also trying not to add to our facilities allowance.

About the purity. The reason we are getting rid of it, is because the manufacturer recommends it has useful life of about 3 years. And I think we have had it for about 5. We use it too clean components, that have paint, rust scale, whatever on it.  I don't know why there is a life span if it's just NaOH. The pH is still registering a 13. But maybe it has to do with the additives in it. Anyway, it probably has a whole bunch of gunk in it that it has collected over the years and it probably not too pure anymore. Although, it still might be useful to someone who doesn't care about purity too much. Does anyone know any industry that might want this?

About the acid-base reaction. I'm not a chemist, but I did stay at a Holiday Inn and read a chemistry textbook last night. It looks like it will create a lot of heat and a dissolved salt solution. Maybe mixing with an acid is not a good idea. I wouldn't even know where to begin. How much should I mix at a time? What's the mixing process? What's the PPE? etc.

I see someone said CO2 will neutralize it. Can we get a tank of CO2 and create bubbles with it? How much CO2 will that take? How long will that take?

Also, I am learning how incredibly alkaline this solution is. I see the pH scale is logarithmic. I believe pH 13 is 1 million times basic than pH 7. Where as pH 11 is only 10,000 times as basic than 7. So, when looking at the big picture, trying to get a pH 13 solution down to pH 11, is almost like trying to get it to pH 7. It's very hard.

Also, we had a little explosion in the caustic tank in the past, and some of the NaOH solution got on the floor and a nearby table. No one was any where near at the time thankfully. The employees told me when the fire dept came, they sprayed vinegar all over. I got to researching vinegar and it looks like typical vinegar is a 5% solution of acetic acid, which is about pH 3. It looks like since NaOH is 13 and vinegar is 3, does it take 10 times more vinegar than NaOH to neutralize them? If so, then we need 10,000 gallons of vinegar to neutralize 1000 gallons of NaOH. ?? 

We also performed a little test yesterday to see how much water we need to neutralize it. We put a couple teaspoons of NaOH in a 5 gallon pail and filled the rest with water. We filled it completely and it was still saying pH 13. Wow thats  amazing. It would take a whole bunch of water to dilute the amount of NaOH we have.

Thanks, Kenny.

[DrCMS]

Kenny my advice is to pay the $8K.  

Acid/base neutralisations can get hot enough to boil the solution very easily if not done properly.  On the scale you have that could kill someone.  

The fact that you are asking the questions here and the other info you've given suggests to me that you should not attempt to do this yourselves.   I'm not trying to put you down or belittle your lack of chemical training but I would warn against doing it yourselves. I have carried out large scale neutralisation in our stirred stainless reactors against full cooling from a 4MW cooling tower and the amount of energy involved is huge.  

[Borek]

I see someone said CO2 will neutralize it. Can we get a tank of CO2 and create bubbles with it? How much CO2 will that take? How long will that take?

Don't treat as a viable solution - it'll take eons. It was rather a joke, albeit not without serious chemical background.

typical vinegar is a 5% solution of acetic acid, which is about pH 3. It looks like since NaOH is 13 and vinegar is 3, does it take 10 times more vinegar than NaOH to neutralize them? If so, then we need 10,000 gallons of vinegar to neutralize 1000 gallons of NaOH.

Much less than that. pH is not a good way of measuring concentration of acetic acid. It is not fully dissociated, so its pH is much higher than one may expect.

[408]

Start a business selling drain opener!

Draino is mostly solid sodium hydroxide.

Concentrated sulfuric acid is(was?  I have not checked in the last couple years) sold as a drain unplugger, as it was impure waste industrial acid.

[Gerard]

why not scrape the NaOH slurry instead of diluting and neutralizing it,recall you will need extra cost for the neutralizing agent, plus the labor for the cleaning and the payment for disposing an industrial waste as diluted NaOH.
Scraping it will just be with the labor cost(Saftey management required NaOH can burn the skin), Then you can sell the slurry as "Industrial Grade NaOH" or have it for bidding. I think there are industries who will buy "impure NaOH" especially those that have small scale businesses that utilizes NaOH, soap or drain cleaner, even paper industries.(This is my opinion on the economic side, the chemical side has been more or less answered)

[typhoon2028]

Pay the $8k.  You have already labeled the material as waste.  I am not 100% sure about environmental laws, but it may be against the law to perform waste treatment without a permit.  You have already referenced your "allowable" limit.  This probably refers to your hazardous waste permit.  I understand wasting this amount of material will put you over the "limit" probably labeling you a "large generator." 

If I was you, I would find out how close you are to your waste limit.  Then I would waste some of the NaOH up to the permit limit, then hold the remainder until next year (1.5 months from now) then waste the remainder in Jan & Feb.

[Gerard]

Please do some depreciation analysis on your reactor....
3 years time and you used it for 5 years. I think it has become more of a liability to you than an asset. Normally you can have it as a scrap with a corresponding scrap value. But now you need to pay to dispose the waste and the reactor as well. Better dispose of it now because the more it will age the rate you will pay also increases with time.