Calcium sulfate can be converted into calcium carbonate. Then calcium carbonate can be dissolved using acid. As a result there will be solution of sulfate and solution of calcium. After mixing these two solutions calcium sulfate will precipitate. So, it means dissolution and precipitation of gypsum.
In February-March 2024 I tried this approach. I used baking soda (sodium bicarbonate) to convert gypsum into calcium carbonate. Then I used vinegar 9% to dissolve calcium carbonate. Two clear solutions were obtained. Then I mixed them and calcium sulfate precipitated. The approach works.
In this way sodium bicarbonate acts as base and acetic acid acts as acid. Let's call this method base-acid purification of gypsum – this term is unofficial and non-academic and might differ from used meaning.
As we can see base and acid neutralize each other forming a salt. The issue is they can be used only once. It means high cost of processing and large amounts of waste. There are two possible ways to overcome this issue:
1. Exploit existing industrial process of neutralization.
2. Recover base and acid from their salt.
One of existing processes of neutralization is producing ammonium nitrate. Nitrogen fertilizer plants just mix ammonia and nitric acid to produce ammonium nitrate. This process can be modified so it will additionally dissolve and precipitate gypsum.
Base and acid can be recovered by these ways:
1. Salt splitting a.k.a. bipolar-membrane electrodialysis a.k.a. electrolytic hydrolysis. For example,
NaCl + H2O --> NaOH + HCl
2. Thermohydrolysis a.k.a. pyrohydrolysis. For example,
MgCl2 + H2O --> MgO + 2HCl
3. Thermal decomposition. For example,
Ammonium citrate --> ammonia + citric acid
I studied these ways later in 2025.
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