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Author: Subject: Preparing CaO from carbonate or hydroxide?
Fyndium
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Preparing CaO from carbonate or hydroxide?

Which would be better feedstock for Ca oxide, carbonate or hydroxide? Both are apparently available otc for less than € per kg.

The major difference is decomp temp, 500+ for hydroxide and 800+ for carbonate. The former is much more easily achievable, but are there some unforeseen caveats in this way?

Calcium oxide would be used as an effective desiccant.
Maurice VD 37
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CaO is obtained by heating Calcium carbonate, because calcium carbonate is natural, and can be obtained from marble, calcite, calcareous substances, which are all present in minerals all over the planet. CaO could be obtained from hydroxide of course. But there is no natural source of calcium hydroxide. Calcium hydroxyde is always obtained by heating CaCO3 till it is transformed into CaO, then after cooling down, this CaO is treated by water. This the only way of obtaining Ca(OH)2.
VancouverBackyardChemist
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I've found some sellers on Ebay, I see one from Spain and another from Portugal, selling CaO, seems a bit questionable, but the prices are much more reasonable.
https://www.ebay.ca/sch/i.html?_sacat=0&_nkw=calcium+oxi...
Depending on what access you have to what appliances, 500-800 C temperatures are only possible with kilns and lab furnaces so if you're a home chemist without access to those, it might be better to simply buy it. As Nurdrage almost always says, Crushing your expecatations buying the thing directly usually is cheaper than producing it yourself. Kitchen ovens only reach max 500 F, which is around 300 C, nowhere close to the temperatures needed to decompose CaCO3 or even Ca(OH)2.
Fery
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Fyndium also CaO is OTC if you are satisfied with technical purity (CaO content somewhat slightly more than 90%). In my country the price is approx 6 EUR per 25 kg CaO in shops selling materials for construction workers. If you would like to prepare it in sake of the experiment itself, decomposition of Ca(OH)2 occurs at 512 C and CaCO3 at 840 C.

If there is a heaven, it seems not to be materially based. Does chemistry exist there and if yes, how does it look like? Are there good souls well supplied with laboratory equipment, glass, chemicals and information?
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I can imaging the carbonate is preferred because the decomposition temperature of the hydroxide is not far below the melting point. The carbonate would stay solid during decomposition.

Bubbling molten hydroxides are not the most fun to work with.
Fyndium
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Options for every situation applies, and it's good to point them out. I'm lucky to have access to high pressure gas burner and DIY ceramic fiber raku kiln, so I can easily achieve temps up to about 1200C, but not in an inert environment, for the sake of it. Anything a stainless steel or ceramic kiln can handle, is within my reach. For the matter of splashing anything corrosive, I do these high temp things outdoors so fumes or splashes are not of a big concern.

I find it easier to work with lower temps because the reaction is more controllable and easier to carry out quantitively, hence the hydroxide route was an option. I have ceramic grade carbonate on hand actually, so a straightforward method would be to just calcine it. I was not able to find CaO for sale in any of my country's construction supplies, except for the ones that sell only large amounts, starting from 600kg to 1000kg.
S.C. Wack
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It would be nothing to not do this, but it wouldn't take much more effort to do 100 g in a small saucepan on a gas stove, or in anything that can be made red or preferably orange. 450C is supposed to be sufficient. Or pile up some decorative white marble rock.

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macckone
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 Quote: Originally posted by S.C. Wack It would be nothing to not do this, but it wouldn't take much more effort to do 100 g in a small saucepan on a gas stove, or in anything that can be made red or preferably orange. 450C is supposed to be sufficient. Or pile up some decorative white marble rock.

550C is the point where the outgassing exceeds normal atmospheric CO2 partial pressure. But practically it needs to be above 800C to happen at an appreciable rate. At 900C the pp of release is higher than atmospheric pressure and it rapidly proceeds to completion.

However a gas flame can easily exceed 800C if a small amount of material is heated.
S.C. Wack
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 Quote: Originally posted by macckone 550C is the point where the outgassing exceeds normal atmospheric CO2 partial pressure.

The part not talking about marble referred to the hydroxide.

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macckone
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SC Wack,
Yes the hydroxide is very easy.
As long as the temperature is kept below 520C, it won't melt but will decompose to the CaO in a CO2 free environment.

If anyone is having trouble getting calcium hydroxide it is easily made by combining a lye solution with calcium chloride and letting it settle with minimal exposure to CO2.
Excess moisture can be removed by heating in a reasonably air free environment (water or oil trap) then desiccating with calcium chloride.
S.C. Wack
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The directions I suggested are for use with a fancy contraption known as a lid. At max heat I'd think that CO2 would be crowded out by water, and it doesn't take long. Perhaps a thin layer of sand or talc would be better. No idea what y'all are talking about melting.

A flowerpot furnace will decompose the crushed dolomite used as a parking lot surface here, but IDK about proper limestone (if there's any left) and especially marble (no problem in an ordinary electric lab furnace). No need for inert atmosphere.

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macckone
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SC Wack,
Someone earlier suggested the hydroxide would melt, it won't as long as the temp doesn't go too high.
Impurities can lower the melting point but 520C seems to be high enough to get rid of H2O but not melt it.

Melting of the pure substance happens at a higher temp (580C).

A lid will work if it fits right, not too loose to allow CO2 in or too tight to pop off.

As for the carbonate, if you are decomposing carbonate you don't need to exclude CO2 since you are getting a flood of it from the decomposition although flushing it out will help the reaction along.
S.C. Wack
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 Quote: Originally posted by macckone SC Wack, Someone earlier suggested the hydroxide would melt, it won't as long as the temp doesn't go too high. Impurities can lower the melting point but 520C seems to be high enough to get rid of H2O but not melt it. Melting of the pure substance happens at a higher temp (580C).

I still don't know what you're talking about and might not be alone. IME with a thin layer of several OTC brands over the years (only Hi-Yield seems to be available now, and only at one store locally), your crucible will melt before it does.

"You're going to be all right, kid...Everything's under control." Yossarian, to Snowden
macckone
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SC Wack,
580C is not close to the melting point of a crucible.
Ca(OH)2 melts at 580C.
The food grade melts considerably below that depending on impurities.
I suspect either NaOH or CaCl2 in the pickling lime, possibly both if it is precipitated from calcium chloride with sodium hydroxide.
Tsjerk mentioned this earlier in the thread.

The agricultural hydrated lime is calcined limestone that is then slaked.
It is going to have whatever impurities came over from the limestone since there is minimal processing.

S.C. Wack
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Perhaps in supercritical water it does.

It's funny how the conservative talkers complained about the new English (ebonics) and "new math" while bringing us this new science that everyone seems so eager to follow.

You didn't even google the analysis for the brand (over 95%) that was mentioned.

> The food grade melts considerably below that depending on impurities.

I will give you $10,000 if you can pull that off. "You're going to be all right, kid...Everything's under control." Yossarian, to Snowden macckone International Hazard Posts: 1647 Registered: 1-3-2013 Location: Over a mile high Member Is Offline Mood: Electrical  Quote: Originally posted by S.C. Wack Perhaps in supercritical water it does. It's funny how the conservative talkers complained about the new English (ebonics) and "new math" while bringing us this new science that everyone seems so eager to follow. You didn't even google the analysis for the brand (over 95%) that was mentioned. > The food grade melts considerably below that depending on impurities. I will give you$10,000 if you can pull that off.

Sodium hydroxide as an impurity is an effective flux, melting at 318C or lower depending on water content.

Calcium Chloride melts at a higher temperature than Calcium Hydroxide but the fluxing action lowers the melting point of the mixture.
Calcium Hydroxide in pure form melts at 580C but when contaminated it can easily drop 20C.
Freezing point depression is a real thing.
Tsjerk
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 Quote: Originally posted by macckone SC Wack, 580C is not close to the melting point of a crucible. Ca(OH)2 melts at 580C. The food grade melts considerably below that depending on impurities. I suspect either NaOH or CaCl2 in the pickling lime, possibly both if it is precipitated from calcium chloride with sodium hydroxide. Tsjerk mentioned this earlier in the thread.

I mentioned what? I just said I wouldn't want to work with molten Ca(OH)2 if I could work with a solid just a couple hundred degrees higher.

@macckone; how would you keep the temperature above 520 but below 580 degrees?

[Edited on 11-9-2020 by Tsjerk]
macckone
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Tsjerk,
variac on a high temp electric burner with mineral wool insulation (MP 1000C)
S.C. Wack
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 Quote: Originally posted by macckone Sodium hydroxide as an impurity is an effective flux, melting at 318C or lower depending on water content. Calcium Chloride melts at a higher temperature than Calcium Hydroxide but the fluxing action lowers the melting point of the mixture. Calcium Hydroxide in pure form melts at 580C but when contaminated it can easily drop 20C.

IDK what your trip is trolling bullshit or why the decline of the board is encouraged, but this ends my participation.

PS "Food grade" chemicals don't have NaOH and CaCl2 in them.

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macckone
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Quote: Originally posted by S.C. Wack
 Quote: Originally posted by macckone Sodium hydroxide as an impurity is an effective flux, melting at 318C or lower depending on water content. Calcium Chloride melts at a higher temperature than Calcium Hydroxide but the fluxing action lowers the melting point of the mixture. Calcium Hydroxide in pure form melts at 580C but when contaminated it can easily drop 20C.

IDK what your trip is trolling bullshit or why the decline of the board is encouraged, but this ends my participation.

PS "Food grade" chemicals don't have NaOH and CaCl2 in them.

That is not true at all.
CaCl2 and NaOH are not prohibited in food grade chemicals.
The AMOUNT of them is limited depending on intended use.
And in the case of pickling lime, remaining hydroxides are expected to be neutralized by the vinegar used in the next step of the process.
CaCl2 is used for a similar purpose and is added with the vinegar.

NaOH is used in pretzel making and CaCl2 is used as a pickle crisp.
3% of mrs wages pickling lime is impurities.
So those are the expected impurities.

https://www.amazon.com/Mrs-Wages-Pickle-Crunch-Ounce/dp/B01M...

MidLifeChemist
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They sell food grade NaOH right here