Caustic Soda Recovery
Caustic Soda Recovery
(OP)
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
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





RE: Caustic Soda Recovery
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?
RE: Caustic Soda Recovery
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.
RE: Caustic Soda Recovery
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.
RE: Caustic Soda Recovery
RE: Caustic Soda Recovery
The major equipment manufacturer is Eco-Tec. Their website gives further information:
http://www.eco-tec.com/main/aluminum.htm
RE: Caustic Soda Recovery
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 160oF, 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.
RE: Caustic Soda Recovery