My phosphogypsum research notes. Part 8

Year 2025. I was considering two ways:
1. Dissolve CaCO3 using acid.
2. Dissolve CaCO3 using ammonium salts.

If acid is used then acid and base should be regenerated. I knew about three ways – salt splitting, pyrohydrolysis and thermal decompostion.

1. Salt splitting using bipolar-membrane electrodialysis. The full process of gypsum purification can be described:
CaSO4 + 2NaOH + CO2 --> CaCO3 + Na2SO4 + H2O
CaCO3 + 2HCl --> CaCl2 + CO2 + H2O
CaCl2 + Na2SO4 --> CaSO4 + 2NaCl
2NaCl + 2H2O --> 2NaOH + 2HCl
The price of bipolar membrane is high. It means high CAPEX. If the process runs from solar energy, it means some 20% capacity factor, and CAPEX will be some 5 times higher. It might be too expensive for gypsum.

2. Pyrohydrolysis of MgCl2. The process of purification can be described:
CaSO4 + MgO + CO2 --> CaCO3 + MgSO4
CaCO3 + 2HCl --> CaCl2 + CO2 + H2O
CaCl2 + MgSO4 --> CaSO4 + MgCl2
MgCl2 + H2O --> MgO + 2HCl
I didn't try to convert gypsum into calcium carbonate using magnesium oxide. But I tried with magnesium carbonate, and it worked. Carbonated water was used as a catalyst. Looks feasible.
What if split magnesium acetate instead of magnesium chloride? It might be simpler. I tried to heat magnesium acetate solution to some 100 degrees expecting to obtain MgO precipitate. But the solution turns into kind of syrup and nothing interesting obtained.

3. Decomposing some salt that consist of volatile and non volatile parts. For example, ammonium salt of non volatile acid. The purification process can be described:
CaSO4 + 2NH3 + CO2 + H2O --> CaCO3 + (NH4)2SO4
CaCO3 + H2-anion --> Ca-anion + CO2 + H2O
Ca-anion + (NH4)2SO4 --> CaSO4 + (NH4)2-anion
(NH4)2-anion --> 2NH3 + H2-anion
I prepared ammonium citrate solution. Then I boil it for a while, ammonia smell appeared. Then I add calcium carbonate to the solution and there were bubbles. Reaction occurred. This approach looks feasible. Just need to select proper organic acid – non volatile and soluble calcium salt.

So, recovering base and acid is possible. But I was also considering another way – dissolving CaCO3 in ammonium salts.