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I read calcium carbonate is the main component of seashells and other animals' shells (such as snails), and also limestone. How can I prepare pure $\ce{CaCO_3}$?

Could someone describe the process and the chemical equations involved? I saw this video and apparently that's one way. Are there others? For example, is there a way to obtain calcium carbonate from bones?

Please do tell me about the risks, but don't worry about my safety, since this is not going to be done in real life, but in a Minecraft mod instead. It is important to know the risks though, because I would like to make those risks part of the game, to make very clear for my players that, if its risky, the same stuff that happens to Steve/Alex will happen to them. (I intend to make it realistic gruesome, meaning I won't exagerate a bit, but either will I minimize the risks because it's "just a game". I'm 14, so I know better, but I know Minecraft is played for kids as young as 7, so I really don't wan't little toddlers thinking Chemistry is all fun and games).

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  • $\begingroup$ What with heating seashells to high temperature and putting to water to dissolve wouldn't it create "quicklime" and then "lime" ? Shells are made up from CaC03 and few percent of proteins if i remember correctly $\endgroup$
    – Chirou
    Commented Nov 29, 2019 at 20:27

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Most of the $\ce{CaCO3}$ in commercial use today is mined not synthesized. The one exception I know a bit about is the ultra-small particle sized materials. However, few of those are pure, they have surface treatments which constitute a significant portion of their total weight. Pharmaceutical and perhaps food grades are also synthesized.

The major synthetic pathway is circular: limestone is mined, then it is heated which produces lime and carbon dioxide. The lime is then mixed with water and $\ce{CO2}$ is added (bubbled in/through) and $\ce{CaCO3}$ precipitates. It is then dried.

Purification depends on the source; how impure it is to start with. It is generally difficult to separate magnesium carbonate from calcium carbonate, and given that limestone and aragonite as well as dolomite ($\ce{CaMg(CO3)2}$) are taken out of the ground (i.e. are 'dirt' cheap) there's usually no need.

As far as the risks of making it, well you could burn yourself, adding lime too fast to water could splash and it's infamous for dissolving bodies in murder mysteries, as well as being used in real life to help in outhouses and gardens where organic material is desired to rapidly decompose. Meaning you don't want to get it in your eyes, nor do you want it left on your skin for very long. It can, theoretically, fume, while being decomposed and that wouldn't be good to breathe. And if a centrifuge or mechanical press is used to squeeze the water out of it (first step in drying), then you have the assorted mechanical dangers. Its synthesis is probably one of the safest chemical syntheses you could undertake.

$\ce{CaCO3}$ decomposes with fairly mild heating (I think around 840 °C), so any material which is predominantly that (like seashells, bones) could be used to make the lime.

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  • $\begingroup$ Thank you very much! so could you expand on the equations and chemical reactions to obtain CaCO₃ from bones, specifially? $\endgroup$
    – FinnTheHuman
    Commented Jul 31, 2016 at 4:56
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Bones are mainly made up of Collagen and Calcium, in the form of Hydroxyapatite (Ca10(PO4)6(OH)2), though there's traces of other minerals, such as Mg, Bo, Si, Al, etc.

If you put bones in Hydrochloric acid (HCl), you'd get a solution predominantly composed of Calcium Chloride, via the same reaction occurring in nature (which is the way osteoclast dissolve Calcium from our bones in the context of bone remodelling).

Be careful as (1) HCl is dangerous to handle and can cause severe burns to eyes, skin, and lungs, especially if using a concentrated solutions thereof / in a closed environment, (2) The reaction will release H2 which makes up for an explosive mixture with Oxygen in air.

From that solution you could precipitate calcium carbonate by adding Na2CO3, via the following reaction:

Na2CO3(s) + CaCl2(s) + H2O(l) ⤑ CaCO3(s) + 2 NaCl(aq)

NaCl is soluble so you could just rinse it away (poor your solution & precipitate through a coffee filter for instance and rinse with water).

The result would be CaCO3, probably at min >98% purity, though you'd have traces of a few other carbonates in there.

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  • $\begingroup$ 1. "Bones are mainly made up of Collagen and Calcium": this is a rough approximation, also in this context calcium appears as metal (you probably mean calcium(II) salts), plus Mg, Bo, Si, Al are not minerals, those are elements; 2. Unless the first word of the sentence, chemical names are not capitalized; 3. "Ca + 2HCl → CaCl2 + H2": again, there is no calcium metal in bone tissue, and it's not clear what this reaction is supposed to demonstrate; $\endgroup$
    – andselisk ♦
    Commented Jul 1, 2019 at 16:03
  • $\begingroup$ 4. For some reason the first equation doesn't include states of aggregation (as the second one does), also the listed precautions are relevant for a highly concentrated HCl solution or HCl gas; 5. "Na2CO3(s) + CaCl2(s) + H2O(l) ⤑ CaCO3(s) + 2 NaCl(aq)": what water on the left is reacting with? 6. ">98% purity": any reference to the published procedure to back this number up? $\endgroup$
    – andselisk ♦
    Commented Jul 1, 2019 at 16:03

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