My phosphogypsum research notes. Part 10. Ammonium lactate

Acetic acid is one of myriad organic acids. What about other acids? As I found before ammonium citrate increases gypsum solubility as well. I searched for possible organic acids on Wikipedia. I preffered monocarboxylic acids because they form only one type of salts, and this is simpler to understand. For example, tartaric acid is dicarboxylic, it can form tartrate and bitartrate, and this is more difficult to understand.

I discovered lactic acid on Wikipedia. It is pretty safe, not toxic, high boiling, calcium lactate is soluble, and it can be purchased. So, I decided to experiment with ammonium lactate.

The first experiment is dissolving gypsum in ammonium lactate in order to estimate its chelating properties. The second experiment is dissolving CaCO3 in ammonium lactate.

My phosphogypsum research notes. Part 9. Ammonium acetate can dissolve gypsum

Ammonium acetate readily dissolves CaCO3. Is it due to ammonium acetate decomposition or due to chelating properties of ammonium acetate? How to check?

In case of chelating properties it should increase the solubility of calcium salts. Good salt to check is gypsum.

I dissolved some ammonium acetate in 50 ml of water. Then I added gypsum into it. Some 2 grams of gypsum were dissolved. Solubility of gypsum is 0.2 grams per 100 ml of water. It means that ammonium acetate increased the solubility of gypsum in 20 times. Ammonium acetate definetely has chelating properties.

My phosphogypsum research notes. Part 8. The opposite of neutralization reaction

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 brown syrup and nothing interesting obtained. The dark color might be due to iron presence or due to solution degradation.

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.
When dissolving CaCO3 there was some odor new for me. I suspect it could be hydrogen fluoride, but I don't know how it smells.

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

My phosphogypsum research notes. Part 7. Ammonium acetate

Calcium carbonate can be dissolved by ammonium salts. But Ca ions accumulate and slow down the reaction. To increase the effiency it is necessary to increase ammonium ions concentration. What if use several ammonium salts together? I thought they together could make higher ammonium ions concentration than if used alone.

Ammonium acetate has good solubility similar to ammonium nitrate. The mixture of both could give higher ammonium content than these individual salts. So, I added some ammonium acetate to the solution.

The effect was surprising. CaCO3 dissolved much more faster. By the way, the cup corroded making the solution brown. What happened?

At first, ammonium acetate decomposes. Ammonia gas escapes and acetic acid stays. Then acetic acid dissolves CaCO3.

Secondly, acetate ion is a chelating agent. It makes complexes with Ca ions. Ca ions are blocked and do not suppress the reaction of dissolving CaCO3.

But if ammonium acetate accelerates the dissolution of CaCO3 it might slow the precipitation of gypsum. I checked and it is so.

My phosphogypsum research notes. Part 6. Calcium carbonate purification

Dissolving of calcium carbonate is going according to equation
CaCO3 + 2NH4NO3 --> Ca(NO3)2 + 2NH3 + CO2 + H2O
During the dissolution Ca ions accumulate in the solution and they stall the reaction.

Gases NH3 and CO2 should go to react with gypsum. And Ca ion concentration affects both reactions. Gypsum solution will have low Ca ion concentration and might not absorb NH3 and CO2. If it is not possible, then another idea might be possible.

The idea is to purify CaCO3. The process can be described:
1. Dissolve impure CaCO3
CaCO3(impure) + 2NH4NO3 --> Ca(NO3)2 + 2NH3 + CO2 + H2O
2. Precipitate pure CaCO3
Ca(NO3)2 + 2NH3 + CO2 + H2O --> CaCO3(pure) + 2NH4NO3

So, the reuse of phosphogypsum can be to process it into pure (NH4)2SO4 and pure CaCO3 instead of pure gypsum. This process will consume NH3 and CO2 or (NH4)2CO3. This means additional logistics costs.
Pure (NH4)2SO4 can be precipitated from mother liquor by adding (NH4)2CO3. I checked and it works.

But if we have pure ammonium sulfate and pure calcium carbonate can we obtain pure gypsum? Yes, it is possible by reaction
CaCO3 + (NH4)2SO4 --> CaSO4 + 2NH3 + CO2 + H2O
In this case extra logistics will be avoided.
The reaction is slow and a catalyst should be used, e.g. ammonium acetate.

My phosphogypsum research notes. Part 5. Reversible dissolving of calcium carbonate

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. 

My phosphogypsum research notes. Part 4. Converting gypsum into calcium carbonate and restore it back

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.

My phosphogypsum research notes. Part 3. First steps

January 2024. It is time to start doing something. I don't need phosphogypsum to research – experiments can be done with usual gypsum. The goal is to dissolve and precipitate gypsum. At first I need to dissolve it. I put 0.1 gram of gypsum hemihydrate (plaster of Paris) into some 30 ml of liquid.

Water – doesn't dissolve.

Carbonated water – doesn't dissolve.

Vinegar 9% – doesn't dissolve.

NaCl solution – dissolve. They say NaCl increases solubility of gypsum.

(NH4)2SO4 – doesn't dissolve. But they say ammonium sulfate increase solubility of gypsum.

10% ammonia – doesn't dissolve. I expected it will form complex with Ca, but it seems it doesn't. I wanted to try ethylenediamine as well, it also creates complexes. But I am not sure about its safety, at least it is flammable. Not this time, maybe later.

Citric acid – dissolve (not so true). I was lazy to prepare a new sample, so I put citric acid into ammonia solution. Several months later I realized that it was ammonium citrate to dissolve gypsum. I put more gypsum into solution, it didn't dissolve. I was lazy and just put the bottle on a shelf in order to wash it later. I didn't want gypsum to go to the sewer, so it would not be just washing a bottle. A few weeks later I noticed that gypsum becomes more white and it didn't lie on a bottom but was floating in the solution. Some year later I made a conclusion that gypsum had been converted into calcium citrate.

Phenibut (possible hydrochloride) – dissolve.

Glycerol – doesn't dissolve.

DMSO – doesn't dissolve.

Acetone – doesn't dissolve.

Vodka (40% ethanol), glycine – I don't remember.

The conclusion:
The solubility of gypsum can be increased. This increase did not look very high. I decided to search for another ways. I heard about converting gypsum into calcium carbonate. Let's try it.

My phosphogypsum research notes. Part 2. Considerations

I learnt about phosphogypsum in some 2011. After that I thought and read about it occasionally. How it can be reused? In January 2024 I realized that thinking without doing makes no progress and decided to do something.

What changed since 2011?

REE prices decreased some 10 times. 
Solar energy increased its share in the energy pie. Solar electricity is intermittent energy source. Dealing with intermittency is a challenge, and reusing phosphogypsum could be a good opportunity to use solar energy.

How pure gypsum can be extracted from phosphogypsum?

Gypsum has solubility of 2 grams per liter of water – not too much. Dissolving gypsum and evaporating water will need 2,26 MJ of energy per 2 grams of gypsum, or 314 MWh per ton. This amount can be reduced some 50 times by distilation using mechanical vapor recompression, it means 6,28 MWh of energy per ton of gypsum. 1 MWh costs some $100, so this way is not economically viable.

What if change the temperature of the solution? Heating a liter of water by 100 degrees needs 420 kJ of energy. It means 58,3 MWh per ton of gypsum. And counter-flow heat exchangers can reduce this amount by some 50 times, so it could be 1,2 MWh per ton. It is better but still high.

Energy required per ton is not only energy cost but also equipment cost (CAPEX). And another problem with low solubility is huge amounts of the solution to pump, filter, store, etc. I needed to find another way to dissolve and precipitate gypsum.

My phosphogypsum research notes. Part 1. The purpose

First time I heard about phosphogypsum was in some 2011. I read an article, and it said phosphogypsum is unused industrial waste and also it contains rare-earth elements. I thought that REE sounds interesting and I would like to extract REE from phosphogypsum. I was thinking for several days about such extraction and was reading about phosphogypsum. Phosphogypsum mostly contains gypsum, and I realized that the optimal way is extracting pure gypsum. Pure gypsum can be sold easily, and after extracting it from phosphogypsum REE extraction becomes more simple.

So, the goal is to extract pure gypsum from phosphogypsum.

But how to extract gypsum? I assumed that dissolution and precipitation can do it. While gypsum is dissolved it can be purified.

So, the goal is to dissolve gypsum and then precipitate it.