Caustic Soda Recovery 1

[capral]

I am currently researching methods to recover a spent sodium hydroxide solution used to dissolve aluminium from extrusion dies.
We have two 900L tanks of 25% NaOH and one 900l rinse tank. When the aluminium concentration reaches about 20-30g/L, the spent solution is dumped (handled by independent disposal company). The pH of the spent solution is about 13.

I have been researching methods such as electrodialysis and the use of membranes but have not found much information in relation to recovery of spent caustic.

Our focus is on recovering the caustic so that it can be re-used - recovery of the aluminium is not a priority.

Chemical engineering is not my field so any help on this topic would be greatly appreciated.

thankyou

Jim

 

[vintage70]

If you remove the Al is the caustic solution usable or are there other contaminants you have to deal with?
You dissolve Al in pH 13, you need to maintain that pH level? I'm not familiar with the chemistry but doesn't aluminum hydroxide preciptate under these or similar conditions?

 

[capral]

To my knowledge, aluminium hydroxide best precipitates at pH between 8 and 9 - at a pH of 13, a lot of the aluminium would be redesolved.

The other major contaminant would be sodium carbonate which I believe would also be removed if using membranes - requiring a top up of the caustic solution.

But as I said I don't have any sort of chemistry background, so any technical help on the topic would be greatly appreciated.

Thanks
Jim.

 

[DeltaCascade]

What is the %NaOH of the solution when it is dumped? That will determine it's value and economics of recovery. P-Alk and M-Alk test results will show amount of NaOH, and also the amount carbonates (much more infomrative than pH). Once CO2 contaminates the caustic to make carbonates, for example, you don't have caustic left to recover...you have less valuable "soda ash" solution which can also have high pH.

At the very least, that existing highly alkaline aluminum hydroxide solution could find a valuable use in another process. An option is to work through a waste exchange to find use for the material, as is. Collect $ instead of paying out $ for disposal.

Use which comes to mind is in cold lime softening of water, replacing lime and alum use there.

 

[Stevecort]

We have been investigating sources of waste caustic from the alumunum extrusion industry. We can add it to our products for wastewater treatment. I would like to know the availability and your location. I can be reached at 919-678-0000 if you are interested. Also, if you are not interested, do you know other sources of this material. It is our experience that it is not cost effective to treat this material and recycling to a beneficial use is the best approach. Steve Cort

 

[kenvlach]

There is a well-known method (at least, in the aluminum finishing industry) of crystallizing out aluminum oxide from caustic etch solutions. Basically, the etch operates at an elevated temperature, and a crystallizer operates at lower temperature.
The major equipment manufacturer is Eco-Tec. Their website gives further information:

http://www.eco-tec.com/main/aluminum.htm

 

[kenvlach]

Another idea: Maybe you can change your operating conditions so that you have a ‘never-dump’ bath.

Why are you using such a high NaOH concentration (25%)? Is this at ambient T?

I operate a caustic etch at 2.5% NaOH and 160^oF, and it can handle a higher disssolved Al concentration w/o dumping. I use sodium gluconate as a sequestering agent for the Al to prevent scale formation (NaAlO₂ and Al₂O₂), and never have to dump: eventually you reach a steady state concentration of Al where the loss via drag-out equals the new Al dissolution. Other metal finishers operate as high as 5-6 wt% NaOH with the same basic procedure. Many commercial caustic etch products use sodium gluconate or other chelant, and contain fume suppressant as well.

 

[amugoth]

Can anyone suggest to remove H2S libetated from oily water system in a refinery by using caustic solution.And the disposal methods for the spent caustic please.