My phosphogypsum research notes. Part 5

Calcium carbonate can be dissolved with acid. What are another ways?

It is known that carbon dioxide dissolves calcium carbonate. So, the idea is to add CO2, it dissolves CaCO3, then mix this solution with sulfate solution, and gypsum will precipitate.

I tried to dissolve CaCO3 powder in carbonated water. It dissolves very slowly and very little. I also tried egg shell with the same result. I would say that CaCO3 solubility in the presence of CO2 is quite low and this approach is not feasible.
Previously I tried to dissolve MgCO3 in carbonated water, and it dissolves much more readily compared to CaCO3.

What if replace carbon dioxide with sulfur dioxide? Convert gypsum into calcium sulfite. Then dissolve calcium sulfite with SO2 gas. Mix two solutions and obtain gypsum. This might work.
Sulfur dioxide is hazardous. It is more difficult to purchase and more difficult to work with compared to CO2. Maybe I will try this approach later. 

Another possible way to dissolve CaCO3 is ammonium salts, for example:
CaCO3 + 2NH4Cl --> CaCl2 + 2NH3 + CO2 + H20

I mixed calcium carbonate and ammonium chloride powders and add hot water to the mixture. Almost immedeately ammonia smell appeared. So, this reaction is possible.

I take CaCO3 from gypsum conversion, add stoichiometric amount of NH4Cl and some water. Then I boil the mixture. Ammonia smell appeared. The smell was getting weaker and then it almost disappear, but CaCO3 was not dissolved yet. Looks like Ca ions in the solution suppress the reaction.
I added more NH4Cl to the solution, so the amount became some 5 times of stoichiometry. The reactions continued, then ammonia smell disappeared, but CaCO3 is still not dissolved.
All rigth, let's add ammonium nitrate. It has higher solubility than ammonium chloride, so there will be more ammonium ions in the solution. And it looks like that nitrate works better than chloride. Finally I dissolved CaCO3 in ammonium salts. I mixed two solutions and obtained gypsum. The approach works.
Interesting observation: saturated NH4NO3 solution doesn't dissolve CaCO3 – no ammonia smell. After adding some water ammonia smell appears.

The process can be described as:
1. Convert gypsum using NH3 and CO2 gases from step 2
CaSO4 + 2NH3 + CO2 + H2O --> CaCO3 + (NH4)2SO4
2. Dissolve CaCO3 in ammonium salts, gases go into step 1
CaCO3 + 2NH4NO3 --> Ca(NO3)2 + 2NH3 + CO2 + H2O
3. Mix two solutions to precipitate gypsum
(NH4)2SO4 + Ca(NO3)2 --> CaSO4 + 2NH4NO3
Ammonium nitrate is in mother liquor.

I made 3 or 4 cycles of dissolution and precipitation of gypsum. What I noticed is that water evaporates along with NH3 and CO2. The amount of evaporated water is some 1 liter per 10 grams of CaCO3. It means very low effiency of the process – most of energy is spent just to distill water. This proportion could be improved by stirring, varying geometry and temperature, trying to lock water vapor inside.