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Author: Subject: Recrystallizing molten sulphur in a calcium chloride brine
SnailsAttack
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[*] posted on 7-7-2023 at 02:44
Recrystallizing molten sulphur in a calcium chloride brine


In many commercial fertilizers containing elemental sulphur, the sulphur is fused with a large amount of dirt, necessitating further purification. The typical methods cited on these forums are dissolution of the sulphur in hot, highly flammable aromatic solvents (such as toluene or xylene) or distillation of the sulphur at high temperatures in range of its 445°C boiling point.

These procedures can be difficult or even dangerous to attempt if you don't have the right equipment, so I looked into a safer, more accessible method of purifying elemental sulphur which was eluded to in a late 19th century paper that I can no longer seem to find.

The TLDR is you can purify sulphur by melting it in a pot with some concentrated calcium chloride solution and collecting the beads of molten sulphur once they've resolidified. A brief video is available here. I've got more to say though.

Sulphur_comparison.png - 373kB
Sulphur purified from garden fertilizer by the method in this thread.

The melting of the sulphur is mediated by a calcium chloride brine with a concentration in excess of 35-40% CaCl₂ by mass, which is required to elevate the boiling point of water above 115-120°C, which is the temperature range quoted for the melting point of the alpha and beta sulphur allotropes.


Calcium_chloride_solution_graph.png - 34kB

According to this phase diagram, the maximum boiling point of a calcium chloride brine could reach an astounding 175°C at a concentration of 78% CaCl₂ by mass, beyond which it collapses to the dihydrate.

Aside from raising the boiling point of water, the brine serves several other purposes, which are that it:
1. lowers the exposure of the sulphur to air, which reduces fuming and prevents the sulphur from catching fire.
2. agitates the molten sulphur globs, separating them from any entrained dirt.
3. floats the sulphur above any impurities that are denser than the brine.

The density of molten sulphur is around 1.82 g/cm³, which is quite a lot lighter than silica and clay minerals (~2.65 g/cm³). I'm not sure what concentration of calcium chloride is required to exceed the density of molten sulphur, but it's probably in excess of 70% CaCl₂ by mass.


Melting_pot.png - 138kB Molten_sulphur.png - 196kB Sulphur_patty.png - 92kB

A beaker of melting sulphur, a large red globule of molten sulphur floating atop the brine, and the same glob, crystallized as a shiny yellow 'patty'. Brief video available here.

The sulphur globs, which are surprisingly fluid, typically don't entrain the calcium chloride brine as they solidify due to their high cohesion, which I'm guessing is due to the difference in molecular polarity between the polar brine and the non-polar orthorhombic octasulphur (S₈) molecules. Nevertheless, the sulphur beads can be broken apart and washed clean to ensure their purity.

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The offgassing of sulphur fumes is low enough that this process is safe to perform indoors with a few windows cracked open.

I suspect there may be other specialty applications for the calcium chloride brine regarding its extremely high density, boiling point and inertness with respect to other common liquids.

Tags: Purifying molten sulphur in a saltwater brine, melting sulphur in saltwater, separating sulphur from dirt impurities by melting, purifying sulphur by fusion in hot water




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SnailsAttack
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[*] posted on 7-7-2023 at 02:45
Alternative brines


This is a speculative, untested list of alternatives to the calcium chloride brine, which get progressively worse and ill-advised.

A suitable substitute has to have a boiling point of at least ~120°C and ideally should be chemically inert and easy to obtain. I think the best option would be to use some sort of pressure vessel (such as an ordinary pressure cooker) to elevate the boiling point of pure water above 120°C, which would only require around 2 atmospheres of pressure. I believe this is by far the best alternative.

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Aside from calcium chloride, magnesium chloride may also work, as well as magnesium acetate and sodium acetate. Phosphoric acid should work, as well as the triphosphate salts (NaH₂PO₄, KH₂PO₄ or NH₄H₂PO₄).

Sugar (sucrose) might be a viable option, if a bit sticky, flammable and prone to decomposition. Concentrations in range of 90% sugar by mass may be required to hit 120°C. It's worth noting that if the sugar hydrolyzes (this typically occurs under acidic conditions) you may have to be concerned about the production of hydrogen sulphide gas caused by reduction of sulphur via the aldehyde functional group in the monosaccharides. This is very speculative.

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If for some reason you're not concerned with using hot concentrated alkalis, potassium carbonate or hydroxide (the sodium analogs likely aren't soluble enough) might work, although the latter may react with elemental sulphur to form potassium sulphides, sulphites and thiosulphates, perhaps by the following formulas:

6KOH(aq) + (5+n)S(l) -> K₂Sₙ(aq) + 2K₂SO₃(aq) + 3H₂S(aq/g)
6KOH(aq) + (7+n)S(l) -> K₂Sₙ(aq) + 2K₂S₂O₃(aq) + 3H₂S(aq/g)

I'm not familiar with alkaline sulphur chemistry, however.

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Sulphuric acid can reach a boiling point 120°C at a surprisingly high concentration of at least 50%, though aside from being corrosive, I believe sulphuric acid and elemental sulphur tend to redox to water and sulphur dioxide, like so:

2H₂SO₄(aq) + S(l) -> 2H₂O(l) + 3SO₂(g/aq)

I'm not sure what temperatures are required for this reaction to become a problem.

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Vegetable oils can easily reach temperatures in range of 300°C and I believe should be fairly non-flammable at 120°C, but I don't recommend using them because of the absolutely abysmal mess they make.

Aliphatic hydrocarbons such as kerosene and mineral spirits with a boiling point between 150-225°C are less greasy and will evaporate within a reasonable amount of time if spilled, but are extremely highly flammable near their boiling point.

Additionally, I think that both vegetable oil and hydrocarbons have a slight tendency to dissolve elemental sulphur, and may react to form hydrogen sulphide gas:

CₙH₂ₙ₊₂(l) + S(l) -> CₙH₂ₙ(l) + H₂S(g)

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Many (if not all) of the nitrate salts (NH₄NO₃, KNO₃, Ca(NO₃)₂, Cu(NO₃)₂ etc.) exhibit the solubility required to reach a boiling point elevation well above 120°C, but are, of course, oxidizers, which have the potential to react with sulphur by some combination of the following formulas:

2KNO₃(aq) + 2S(l) -> K₂SO₄(aq) + SO₂(g/aq) + N₂(g)
2KNO₃(aq) + 4S(l) -> K₂S(aq) + 3SO₂(g/aq) + N₂(g)

This could be a gradual reaction or it could be comparable to an explosion.

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Similarly to the nitrates, a number of chromates and dichromate salts are soluble enough to effect a boiling point elevation of at least 120°C, particularly the sodium, ammonium and magnesium di/chromates. However, besides being carcinogenic, they're also oxidizing agents which may react with sulphur, possibly by the following formulas:

4Na₂CrO₄(aq) + 9S(l) -> 4Na₂S(aq) + 5SO₂(g/aq) + 2Cr₂O₃(s)
Na₂Cr₂O₇(aq) + 3S(l) -> Na₂S(aq) + 2SO₂(g/aq) + Cr₂O₃(s)

I'm not sure why you would have enough chromate salts to do this project, but what kind of scientist would I be if I didn't mention them.

Corrections and other suggestions are appreciated. If anyone wants to test these, I'd love to hear how it goes. Unless you have a good lawyer.

[Edited on 7/7/2023 by SnailsAttack]




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[*] posted on 7-7-2023 at 03:19


Very cool, I like what you've done here. Nice looking sulfur.



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[*] posted on 7-7-2023 at 03:42


Quote: Originally posted by BromicAcid  
Very cool, I like what you've done here. Nice looking sulfur.
Thanks. molten sulphur has such a beautiful amber color, I wish it didn't fade as it crystallizes.



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[*] posted on 15-7-2023 at 16:50


That is a really cool experiment, never would have thought of using aq CaCl2 for that. But I have always been able to flours of sulfer in almost any pharmacy, just in case you need a purer source of it, It should also be readily available from any pyro chemical source.
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[*] posted on 15-7-2023 at 22:45


A very interesting experiment.
The principles have generally useful potential
but in the case of sulphur
for as long as crude oil is refined
sulphur will be available in huge quantities




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[*] posted on 16-7-2023 at 18:28


Quote: Originally posted by Sulaiman  
A very interesting experiment.
The principles have generally useful potential but in the case of sulphur
for as long as crude oil is refined[,] sulphur will be available in huge quantities
Yeah. Even in absence of Claus process sulphur from the oil industry, I think this method of refining sulphur is only of amateur interest.

Quote: Originally posted by Dr.Bob  
That is a really cool experiment, never would have thought of using aq CaCl2 for that. But I have always been able to [find] flours of sulfer in almost any pharmacy, just in case you need a purer source of it, It should also be readily available from any pyro chemical source.
Thanks. You probably can't beat flowers of sulphur as far as purity goes. I got a pound of fertilizer-grade sulphur on sale at Lowes for like 3 bucks though, so I decided to refine it myself.



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[*] posted on 18-7-2023 at 14:19


Nice work!
A pressure cooker would maybe be an alternative. At least you could reach the required temperature for melting sulfur without adding a salt.
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[*] posted on 18-7-2023 at 19:31


Quote: Originally posted by FrecherChemiker  
Nice work!
A pressure cooker would maybe be an alternative. At least you could reach the required temperature for melting sulfur without adding a salt.

Thank you. Yes, a pressure cooker would (in theory) work awesome for this, it takes forever to recycle the brine.

[Edited on 7/19/2023 by SnailsAttack]




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