Treating acid rinse water?

[Oak1]

We evaporate about 200 gal a day of rinse water from an electropolish line. The untreated water has a ph of about 1. We have been adjusting ph to about 5 with caustic soda. it does not take to long to build up a sludge in the bottom of the evaporator that is a problem in a few ways.

Sludge built up to high in the last evaporator and caused the burn tube to buckle. $15,000-Pooffff!

My question. Is there another way to adjust ph that could at least create less sludge?

Thanks
Oak

 

[CMfgE1]

I would look into Soda Ash (Na2CO3). It is not cost effective to transport this in solution form. Most sites Ship in dry soda ash then mix the solutionon site. This is feasable if you have most of the support already in place. And budget of course. But this has better benefits and should not sludge as much.

Do some searching based on your requirements. Contact your supplier and see if they porvide soda ash instead of caustic soda.


Good Luck!!

Quote: "Its not what you know, its who you know"
Everythings a learning experience-Everything

 

[hotdipper]

Yout electropolish rinse water has metallic ions in it. When you add the caustic you create insoluble metal hydroxides. This is a classic old hat metal finishers water trearment process to drop out the metals. You might be able to add the caustic and allow the hydroxides to settle and then evaporate.

 

[Eltron]

I could be off base here, but you could also run your water through a centrifugal separator. They are fairly inexpensive, and it seems like it would spin your sludge out for you. You could also try KOH or NaOH pellets to raise the pH of your solution, but if hotdipper is right you'll still have to get rid of the metals.

Hope that helps.

 

[Oak1]

Thanks for the replys.

We understand about the metals. We have delt with that with our waste hauler. There is not much but enough to classify the waste as hazardous.

I believe the sludge that we get is a result of the inert solid materials in the caustic soda. Our polishing process is not producing anywhere near the solid that we develop in the evaporator. I was hoping there would be a neutralizing agent that would not precipatate solids.

Thanks
Oak

 

[Zoobie]

I somehow doubt that the caustic soda is contributing anything that could be classified as 'inert solid materials'. Insoluble content is generally very low and usually the biggest impurity is salt (sodium chloride). There may be sulphates which will precipitate with calcium and barium. I'm not sure about barium but calcium sulphate will precipitate as gypsum in acidic conditions. Is your caustic soda added as a solid (flake or pellet) or is it added as a solution? Is that solution diluted? If so, have you looked at the water quality?

I used to work in an electrochemical plant. It sounds like what you have is a mixed bag of 'cats and dogs' (thats what my boss used to call cations and anions). Metals (including alkali earths)plus caustic is a great recipe for sludge (precipitation). In fact this is intentionally done in the chlor-alkali industy.

You say you are developing solids in your evaporator...it sound like your evaporator is acting like a crystallizer

Have you analyzed your sludge? This really should be the first thing that you do. A full ICP metals scan plus analysis for common anions (carbonates, sulphates, etc) would be a start. I would also include silica (silica tends to make any precipitation problem a bit more complicated).