Best way to extract calcium carbonate from egg shells?
This is actually a chemistry question more than anything else—specifically chemistry in the context of plants, soil etc. I suppose there may be a few in the hydroponics area that might have the knowledge & answers but I'll start here.
I tend to repurpose/recycle everything. We eat a lot of eggs so I save, dry, grind up and try to make use of a lot of egg shells. Unfortunately, over the years reading, research has revealed that every commonly suggested garden related use (adding calcium, lowering soil pH, repelling snails, etc.) for them has proven false. I’m aware it has repeatedly been discussed here and and elsewhere that egg shells can/should be added directly to the soil or to the compost pile. This can be done, causing no harm, but it seems neither will any benefit be received. I won't go into all the details here (though this covers some), it can be easily found online, but apparently they don't decompose much or very fast, they don't break down but simply get smaller and smaller. I was surprised to find one article about how in one case, chicken & duck eggshells were found to not have decomposed very much after being in the soil 165 years!
Based on whatever I've read, all the research I've done I’ve come to the conclusion that the only way they can be utilized—specifically their calcium carbonate, is to extract it by applying to them something as acidic as they are alkali. I used to mix them with old wet coffee grounds & let them stew awhile but it's unlikely they are nearly acidic enough to make any significant change.
I suspect any common acid (acetic, citric, hydrochloric, lactic, malic, oxalic, phosphoric, sulfuric, etc.) of the right type, amount and concentration would be sufficient to release calcium carbonate but might or could also introduce any number of other unwanted, harmful elements as well.
A frequently recommended method to lower soil pH is the addition of elemental sulfur. Thus far, I've been unable to determine what the actual pH of elemental sulfur is. So, I suppose my questions are:
• What is the pH of elemental sulfur?
• Is elemental sulfur sufficiently acidic that it could be mixed with water and powdered egg shells to render something near neutral, that would add to the soil calcium carbonate, sulfur, possibly other beneficial nutrients but nothing harmful?
• If elemental sulfur is not sufficiently acidic, could the above be accomplished using sulfuric acid?
• …or phosphoric acid instead rendering calcium carbonate, phosphorus (and possibly other beneficial nutrients but nothing harmful)?
• If “no” to all of the above what are your suggestions on how this could be accomplished?