soda regeneration from sodium sulphate 1

[fvincent]

Can we expect that a solution of 20%w of Na2SO4 to which lime is added produces a precipitate of gypsum plus regenerated soda ? Or the reaction is in the other direction?

The regenerated soda after filtration - if the reaction in the desired direction - would be contaminated with soluble gypsum. If we add barium carbonate or barium hydroxide is it possible to reduce sulphate contamination to very low levels?

Thanks a lot



fvincent

 

[fvincent]

Can anyone tell me at least where I can find such information?
Regards

fvincent

 

[StoneCold]

fvincent
I would guess that nobody does this because soda is cheap. Maybe you could fill us in on why you want to do it?

Regards
StoneCold

 

[fvincent]

Well, soda is not that cheap when compared to lime.

My client is an alumina refinery that intends to use bauxite residue (red mud) as sorbent in the calciners flue gas de-sulfurization systems.

After reaction of red mud with SO2 + O2 a solution of sodium sulphate is obtained. Sodium sulphate has no use and should be discharged to the sea after intense mixing with sea water.

As an alternative, lime milk could be added provided the fore mentioned reaction does occur, that is, regeneration of soda. Formed gypsum could then be filtered and sent to landfill. The filtrate containing soda is sent back to the process.

Important to say: the bauxite residue contains Na2O as sodalita, being such Na2O enprisioned for ever and so representing a soda loss. The reaction with flue gas can release part of the Na2O provided the pH is low.

CaO costs 60US$/t
current price of soda is 460US$/t 100%NaOH

The question so remains: is the reaction possible?

Regards

fvincent

 

[cub3bead]

You would be creating a double alkali FGD system.

This link should give you some of the basics involved:

http://www.paramountlimited.com/fgdmerits.htm

Good luck.

 

[fvincent]

In fact after the oxidation (aeration) and excess red mud injection in the circulation tank of the FGD we will find Na2SO4 and not NaHSO3. In a double alkali FGD we obtain NaHSO3 which is converted to Na2SO3 after lime injection.

So the point still is 2Na2SO4 + Ca(OH)2 -> 2NaOH + CaSO4???

Regards

fvincent

 

[RGCook]

To determine if an aqueous phase inorganic precipitation reaction will proceed, look at the solubility of the reagents vs. the products (as a rule). In this example, you have sodium sulfate that is very soluble and lime (very, very insoluble)...can't recall the solubility off the top of my head. Anyone?

If the products are NaOH, well, we know that has tremendous solubility and CaSO4, which has a solubility of about 2000 mg/L as the dihydrate. That is higher than the lime I suspect. Check the CRC to be sure.

So the reaction isn't going to proceed to the right. You might try cooling the solution to increase the solubility of lime (it has an inverse solubility with Temp), to try to drive it, but might be wishful thinking.

 

[cub3bead]

In the double alkali process when the scubbing liquor is regenrated with the addition of lime or limestone, CaSO3 * 1/2 H2O precipitates. You didn't take your equation far enough. Natural oxidation of the scrubbing liqour will yeild about 20-25% CaSO4 * 2 H2O also.

If you are force oxidizing your scrubbing liquor prior to regeneration you will precipitate CaSO4 * 2 H2O.

 

[29palms]

thanks cub3bead for noting the reaction. This confirms an opinion from an engineer who told me the reaction proceeds but did not remember the equilibrium concentration of the sodium hydroxide other than that is was not very high.
we have great quantities of natural sodium sulfate in a surface bed and can use dilute sodium hydroxide even if contaminated with sodium sulfate.
have you any idea as to the concentration possible?

 

[Ferl]

fvincent I'd like to know more about your desulfurisation with red mud, do you have an e-mail that to reach you?

Thanks

Ferl

 

[fvincent]

ferl,

you can write to fabio_vincent@figener.com.br

hope I can help you and you're welcome

regards

fvincent