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Author: Subject: separation of carbonates
International Hazard

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[*] posted on 23-4-2016 at 01:59

Quote: Originally posted by JJay  
There is an industrial process for preparing magnesia from dolomite.

Indeed there is. First step is to calcinate the carbonates into oxides which is then reacted with sea water. Magnesium oxide then forms insoluble hydroxide while the calcium undergoes a double displacement with the magnesium in sea water to produce even more magnesium hydroxide.

We're not banging rocks together here. We know how to put a man back together.
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Radically Dubious

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[*] posted on 23-4-2016 at 10:14

Quote: Originally posted by unionised  
Dissolve it in vinegar to get the mixed acetates
Add dilute sulphuric acid to ppt the CaSO4.2H2O
Filter it off and boil it with dilute Na2CO3 to get CaCO3
Add Na2CO3 to the Mg sluphate/acetate mixture to get Mg CO3

The big problem is that CaSO4 doesn't ppt well- it's often sludgy and hard to filter.

More problems then CaSO4. The reaction between MgSO4 (aq) and Na2CO3 (aq) is one of my recommended reaction that everyone should try!

It forms a massive water loving basic magnesium carbonate hydrate that may consume all the water in your vessel!

Here is a reference ( ):

"Magnesia alba is commercially prepared as a white powder by precipitating magnesium sulphate or chloride with sodium carbonate. Magnesia alba levis results from precipitation in the cold, and magnesia alba ponderosa from precipitation in the heat. Both the light, bulky form and the heavier variety are used as laxatives in medicine, in dentistry, and for toilet purposes. They ignite respectively to the light and heavy oxides of magnesium. The light carbonate, " when examined under the microscope, is found to consist of amorphous particles with numerous slender prisms intermixed."
Magnesia alba is only slightly soluble in water (the solution has an alkaline reaction), but is soluble in acids and in solutions of ammonium salts.
Its composition is usually said to approximate to that of hydromagnesite, Mg(OH)2.3MgCO3.3H2O, but analyses of its composition were never very consistent, and it seems quite certain that its composition is very variable. "

In agreement with the last variable composition comment, the product is sold by one vendor ( ) with the formula, MgCO3·Mg(OH)2·nH2O, which agrees with what I have observed. See also

[Edit] Based on this patent, , I would try to separate the carbonates by treating with vinegar and then NaOH. Treat the suspension with CO2 under pressure forming CaCO3 and Mg(HCO3)2. Filter out the CaCO3 and mildly heat the clear filtered solution until white MgCO3 is precipitated.

[Edited on 23-4-2016 by AJKOER]
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