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SnailsAttack
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[*] posted on 6-6-2023 at 20:16
Accidental pyrolysis of magnesium chloride


A couple weeks back I tried pyrolyzing some magnesium chloride at around 200°C to remove an organic impurity, which worked, but I ended up with several grams of an off-white insoluble precipitate. Confusion ensued, and by the time I figured out what was happening I probably pyrolyzed like a third of my magnesium chloride.

What I found was that, according to this study, magnesium chloride starts to decompose at temperatures as low as 167°C by the following formula:

MgCl₂·2H₂O(s) -> MgOHCl(s) + H₂O(g) + HCl(g)

This behavior seems to be enabled by the stability of the dihydrate at high temperatures, which provides a source of water for hydrolysis; however, analogous behavior doesn't seem to be exhibited by calcium chloride despite the monohydrate remaining up to 260°C according to wikipedia.

Wikipedia also erroneously suggests that magnesium chloride dihydrate is stable up to 300°C, however.




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[*] posted on 26-10-2023 at 19:47


Quote: Originally posted by SnailsAttack  
A couple weeks back I tried pyrolyzing some magnesium chloride at around 200°C to remove an organic impurity, which worked, but I ended up with several grams of an off-white insoluble precipitate. Confusion ensued, and by the time I figured out what was happening I probably pyrolyzed like a third of my magnesium chloride.

What I found was that, according to this study, magnesium chloride starts to decompose at temperatures as low as 167°C by the following formula:

MgCl₂·2H₂O(s) -> MgOHCl(s) + H₂O(g) + HCl(g)

This behavior seems to be enabled by the stability of the dihydrate at high temperatures, which provides a source of water for hydrolysis; however, analogous behavior doesn't seem to be exhibited by calcium chloride despite the monohydrate remaining up to 260°C according to wikipedia.

Wikipedia also erroneously suggests that magnesium chloride dihydrate is stable up to 300°C, however.




how did you know the organic impurity was truly gone?
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[*] posted on 26-10-2023 at 19:59


Is this a possible route to hydrochloric acid for those who cannot get it?
Roasting at 200C and collecting the escaping gas is not seriously out of reach. MgCl2 is not terribly difficult to obtain. This would seem to be a lot simpler than pyrolysis of PVC for example.
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[*] posted on 26-10-2023 at 23:14


If you Add MnCl you can make Chlorine. There is the Secound Weldon process i have it only in German maybe someone can translate it.

Ueber W. Weldon's neues Verfahren der Chlorentwickelung (mit Anwendung von Magnesia); von Georges Lemoine.
Der Genannte theilt über dieses Verfahren, über welches im Jahrgang 1872 des polytechn. Journals Bd. CCIII S. 501 schon berichtet wurde, Folgendes mit:
Man denke sich einen Apparat, in welchem durch Einwirkung von Salzsäure auf mit Magnesia gemischtes Manganbioxyd Chlor entwickelt worden ist. Man hat in demselben ein Gemenge von Manganchlorür und Chlormagnesium. Man dampft diese Lösung ab und fährt mit dem Erhitzen fort, indem man Luft zu der Masse hinzu treten läßt. Wenn das Chlormagnesium allein da wäre, so würde es (durch Zersetzung mit dem noch vorhandenen Wasser) Magnesia und Salzsäure geben; aber das zugleich vorhandene Manganchlorür verhindert zum Theil diese Zersetzung und sucht mit dem Chlormagnesium ein Doppelsalz zu bilden. Das Mangan oxydirt sich unter dem Einflusse der Luft um so leichter, als das Manganbioxyd mit der Magnesia eine Verbindung bildet, welche Weldon Magnesiamanganit nennt. Zugleich zersetzt das entstandene Manganbioxyd die Salzsäure und gibt Chlor. Kurz, die Reaction welche während der Abdampfung zur Trockne und der Röstung der beiden Chlorüre eintritt, ist wenigstens zum Theil die durch die Gleichung
MnCl + 2 MgCl + 4 O = 2 MgO, MnO² + 3 Cl
ausgedrückte.
Das (mit unzersetzt gebliebenem Manganchlorür gemengte) Magnesiamanganit wird pulverisirt und dann wieder in dem gewöhnlichen Apparat mit Salzsäure zusammengebracht. Es liefert nun wieder Chlor, und als Rückstand bleibt ein Gemenge von Manganchlorür und Chlormagnesium, welches wieder in der beschriebenen Weise behandelt wird.
Das bei diesem Verfahren entwickelte Chlor hat hiernach zweierlei Ursprung; es rührt theils von der Einwirkung der Salzsäure auf das mit Magnesia verbundene Manganbioxyd, theils vom Abdampfen und Rösten des Gemenges der beiden Chlorüre her. Das bei dem letzteren Proceß entwickelte Chlor, welches erheblich mehr beträgt, als das Chlor des ersteren Ursprunges, ist mit Luft und Stickstoffgas vermischt; aber dieser Uebelstand findet auch bei dem Deacon'schen Verfahren statt, und zwar hier nicht bloß bei einem Theile des Chlors, sondern bei der ganzen Menge desselben. Weldon hilft diesem Uebelstande dadurch
ab, daß er dieses verdünnte Chlor durch Absorption mittelst Kalkmilch und nachherige Wiederaustreibung durch Salzsäure in concentrirtes Chlor verwandelt. Die Verdünnung des Chlors bietet übrigens für gewisse Verwendungen, wie für die Darstellung von Chlorkalklösung und von chlorsaurem Kali, keine erheblichen Uebelstände dar.
Das Abdampfen und die Oxydation des Gemenges der beiden Chlorüre bewirkt man in drei besonderen Apparaten. Das Abdampfen beginnt in einer offenen Pfanne. Es wird beendet in einer Art von Muffel, die von oben erhitzt wird, und in welcher die Lösung nicht mit der Flamme in directe Berührung kommt. Das Rösten endlich geschieht auf dem Herd eines zweiten Ofens, wohin die Masse, wenn sie eine hinreichende Consistenz erlangt hat, mittelst eines Rechens geschoben wird.
Die Salzsäure, welche sich am Ende des Abdampfens in Folge der theilweisen Zersetzung des wasserhaltigen Chlormagnesiums entwickelt, wird in einem Thurm, durch welchen man Wasser fließen läßt, verdichtet. In Folge dessen gewinnt man fast die ganze Menge des Chlors, welche in Form von Salzsäure in den Apparat gebracht wurde, wieder; der Verlust beträgt in der Praxis höchstens 5 Proc.
Weldon nimmt an, daß die verbrauchte Säure sich in folgender Weise vertheilt: 25 Proc. liefern concentrirtes Chlor. 75 Proc. gelangen in Form von Manganchlorür und Chlormagnesium zur Abdampfung. Während des Röstens liefert die Hälfte davon verdünntes Chlor, die andere Hälfte aber geht wieder in Salzsäure über, welche man verdichtet. Die so wiedergewonnene Säure reicht hin, um das verdünnte Chlor durch Vermittelung von Kalk in concentrirtes Chlor zu verwandeln. Hiernach hält man also im Ganzen circa 62 Proc. des in der angewendeten Salzsäure enthaltenen Chlors in concentrirtem Zustande.
Man verwendet das Manganbioxyd und die Magnesia gewöhnlich in Mengenverhältnissen welche gleichen Aequivalenten entsprechen.
In Summa, das neue Verfahren des Hrn. Weldon bietet die Möglichkeit dar, in continuirlicher Weise Chlor zu erhalten, indem man bloß Salzsäure und Wärme aufwendet; bei dem alten Weldon'schen Verfahren ist außerdem noch Kalk nöthig. Der in der Praxis durch das neue Verfahren zu erzielende Vortheil hängt hauptsächlich von den relativen Preisen des Kalkes und der Kohle ab.82) In theoretischer Beziehung
bietet dasselbe die vollständige Lösung des gestellten Problems dar; das Manganbioxyd und die Magnesia sind beide nur Vermittler; sie erleiden beide nur Umwandlungen, die sich fortwährend wiederholen, und dienen immer auf's Neue wieder.
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[*] posted on 27-10-2023 at 09:48


Quote: Originally posted by j_sum1  
Is this a possible route to hydrochloric acid for those who cannot get it?
Roasting at 200C and collecting the escaping gas is not seriously out of reach. MgCl2 is not terribly difficult to obtain. This would seem to be a lot simpler than pyrolysis of PVC for example.


This is for Magnesium recovery:
"The magnesium chloride is hydrolyzed by steam to produce magnesium hydroxide, from which magnesium metal is produced by carbon reduction of the oxide.

MgCl2 + H2O → Mg(OH)2+2HCl

Mg(OH)2→MgO+H2O

MgO+C→Mg+CO

(Resource Recovery and Recycling from Metallurgical Wastes
S. Ramachandra Rao, in Waste Management Series, 2006
10.3.6.1 Hydrometallurgical Process)

I dont know if by roasting, but maybe with steam (or heating + steam)?





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[*] posted on 27-10-2023 at 15:56


Quick search for carbothermal reduction of magnesium oxide has mentioned 1600°C, cooling Mg vapor and carbon monoxide gas, and doesn't sound friendly at all?! Magnesium vapor... that sounds like a brilliant idea if you developed a leaky reaction chamber to ambient atmosphere
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[*] posted on 27-10-2023 at 16:32


By "brilliant" I assume you mean blinding white light.


(All of my imagination of that scenario is rather scary.)
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[*] posted on 27-10-2023 at 17:09


Quote: Originally posted by j_sum1  
Is this a possible route to hydrochloric acid for those who cannot get it?
Roasting at 200C and collecting the escaping gas is not seriously out of reach. MgCl2 is not terribly difficult to obtain. This would seem to be a lot simpler than pyrolysis of PVC for example.

I would think so, yes. But who can't get HCl?

This happens because MgO is just very stable, i.e. it has a high lattice energy. This is also why it has a very high melting point and can be used as a refractory. Unfortunately, it also means that the reaction
Mg(OH)2 + Na2CO3 >/> MgCO3 + 2 NaOH
does not proceed under ordinary conditions, unlike with lime (which works).




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 28-10-2023 at 02:14


I was meaning the first part. Maybe a small tube furnace in which a boat of MgCl2 is heated and then a steam of water is passed to possible collect HCl on the other side after a condenser. (of course with a great amount of water). or a steam destilation apparatus.



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[*] posted on 28-10-2023 at 05:51


Quote: Originally posted by clearly_not_atara  
Quote: Originally posted by j_sum1  
Is this a possible route to hydrochloric acid for those who cannot get it?
Roasting at 200C and collecting the escaping gas is not seriously out of reach. MgCl2 is not terribly difficult to obtain. This would seem to be a lot simpler than pyrolysis of PVC for example.

I would think so, yes. But who can't get HCl?


Oh, I can't! Anything above 10% is monitored, and buying them can be troublesome here!

This method is pretty interesting. Might try it in the future.
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[*] posted on 31-10-2023 at 05:08


On further consideration, one thing that makes this a particularly attractive option is that you can just use the metathesis between MgSO4 and CaCl2, both of which are very cheap and easy to find. But make sure you don't use any containers that you like, since you will be coating it with plaster. Consider a dollar store plastic tub.

Decant the mixture carefully after letting it stand to remove most of the precipitate without filtering, then you can try to filter out the rest. MgCl2 remains in the solution.




[Edited on 04-20-1969 by clearly_not_atara]
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