Metallurgy for Nitric acid 2

[dhdsvr]

Can anyone please tell me what is the mettallurgy of a container for Nitric acid ? The nitric acid is to be kept in a tank with heating coil (for heating upto 80 deg C) for cleaning of diesel filter elements.

 

[btrueblood]

Poly-lined, or teflon-lined.

And well ventilated.

 

[unclesyd]

I'll go along with the well ventilated idea but I have to disagree with the lined vessel. A lined tank is this type service is very susceptible to damage.

We have HNO3 in 304L SS tanks at higher temperature and concentrations that have been around for years.

How do you propose to heat the tank?

Adding to the ventilation requirements, you may get a very violent reaction with the generation of a copious amount of NOX fumes that will overcome most ventilation systems.

Are you precleaning the elements or is there residual fuel oil in the elements?

Are you talking quantities?

How do you propose to dispose of the spent acid?

 

[dhdsvr]

Thank you for the reply.

The tank will be heated by steam coils inside the container. The filter elements will be steamed for removing oil, first and immersed in alkali and then immersed in nitric acid.

Spent acid and alkali will be drained to effluent system after neutralisation.

 

[quark]

I am not good at metals but we used to have aluminium tanks for handling nitric acid. I hope Unclesyd will throw some light on it.

Regards,

 

[unclesyd]

quark,
There are concentrations of HNO3 that can be handled in Al vessels though not commonly done.
We manufacture and use HNO3 and Al has never been considered for any service. We make 65% and use it around 55%-60% @ 120°C.

I have to defer on the particulars because I don't have an iso corrosion chart available.


General comments:
Impurities in HNO3 have a pronounced effect on the MOC, notably with AL.
Some Al tanks are used by the rocket scientists but that is normally for fuming nitric acid either RFNA or WFNA, above 82%. Most of the fuming acids are used with an inhibitor, mainly HF

Iwould not consider Al for any nitric acid service without very specific information on the exact conditions of use. 304l SS is the way to go.

Titanium good in nearly all grades of industrial HNO3 gets Pyrophoric in RFNA.

 

[unclesyd]

Watch out for caustic carry over from the alkaline bath as the reaction could get violent. Been there done that.

Same goes for the naturalization.

Watch out for heavy metals in the neutralized acid.

Watch the temperature of the heating coils do let them get to hot in respect to the bath.


What quantities are you talking about?

I would do a HAZOP or Safety Evaluation on this operation

 

[btrueblood]

Unclesyd,

You are right about linings, please disregard my prior post. Nitric acid and vapors of same can permeate through just about any polymer you care to name. The resulting entrapped acid between lining and metal wall could be a nasty situation.

An unlined poly tank might be okay though?

Not sure about the temperature, and fumes will still be an issue.

 

[Montemayor]

The proper MOC for Nitric Acid, as UncleSyd points out, is very dependent on the purity (or impurities) and the strength of the acid itself. It makes a big difference as to corrosion rate and temperature swings as well. I did a large Nitric Acid recovery project for DuPont some years back and I agree with the first step taken: insist and demand a special, field proven Stainless grade; and provide expert metal inspection at the mill (this was a special mill run) to ensure metal quality. That's how particular and careful the "experts" are in applying the correct MOC.

Aluminum may work well for some concentrations and applications. I would not subject it to an external, industrial application. Parent metal integrity and "toughness" is a characteristic that should, in my opinion, be thoroughly applied to Nitric Acid applications. Liners and coatings are not applicable; they do not even enter into serious consideration. The logic applied in industrial, exposed applications is that the parent metal must carry the chemical AND mechanical loads in a safe, predictable, and controllable manner through the entire range of potential concentrations. Special Stainless alloy met these specifications.

Simply identifying the acid and stating that it will be used to clean filter elements is not a correct or engineering description of the operation. As UncleSyd expounds, you must always take into careful considration the range of concentrations, purities, as well as temperatures and pressures. Nitric acid should never be taken lightly or handled in a "routine" manner. It's terribly hazardous stuff (as are the associated NOx fumes) and it is to be expected that any operation dealing with it is going to be very expensive - if it is going to be SAFE.

I hope this experience helps.

 

[dhdsvr]

The quantity involved is 1.5 m3 of 20% Nitric acid.

The elements will be washed with potash solution before acid wash.

We are going to do a Hazop study on this cleaning operation. It will be helpful for us if any specific issues to be considered in the Hazop are given.

 

[unclesyd]

A HAZOP is the way to go. Glad to see a 20% HNO3 solution.
You had several people worried that the acid was concentrated.

If possible use a commercial Potash (K) based cleaner where you have the benefits of additives to assist in wetting out the part and the major benefit of a rinse aid that will help prevent carryover of caustic to the HNO3 tank.

Watch your temperatures on the Potash tank. I know they will have to be high, but you have to watch for flash drying prior to the rinse tank. Flash drying can increase the chance of carryover to the acid bath from the rinse bath. Watch the mixing of the caustic cleaning solution.

20% HNO3 is still a hazardous material to work with though the visible fuming will be lower unless there is a reaction. You still need good ventilation. Make sure you have good control and limits on the acid tank heating system

Make provisions for the change out of the acid bath especially if you are going to dilute stronger acid. Things like acid into water are sometimes overlooked. If you have to raise the concentration of HNO3 make sure you reevaluate your setup.

 

[NickelMet]

Some literature info:

From the Nickel Institute: (

http://www.nickelinstitute.org/)

a) Report #10075 - SELECTION AND USE OF STAINLESS STEEL AND NICKEL-BEARING ALLOYS IN NITRIC ACID (1975)
b) The following link has a good bibliography:

http://www.nickelinstitute.org/index.cfm/ci_id/10626/la_id/1.htm

From NACE: (

http://www.nace.org)

a) Materials Selector for Hazardous Chemicals--Volume 2: Formic, Acetic and Other Organic Acids, MTI Publication No. MS-2
b) Corrosion Handbook, Volume 2: Hydrochloric Acid, Nitric Acid (Edited by G. Kreysa and M. Schutze)

~NiM