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PostPosted: May 30, 2010 5:54 am 
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KCl - MgCl2 is a good one for you Kirk. Non-toxic, absurdly cheap, and a much lower melting point than NaCl-KCl. Easy to prepare because of the 2:1 eutectic (2 parts KCl for each part MgCl2). We looked at it for thermal storage (PCM or direct liquid tank storage) and it looks promising.

http://nuclear.inl.gov/deliverables/doc ... l_salt.pdf


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PostPosted: May 30, 2010 11:00 am 
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Cyril R wrote:
KCl - MgCl2 is a good one for you Kirk. Non-toxic, absurdly cheap, and a much lower melting point than NaCl-KCl. Easy to prepare because of the 2:1 eutectic (2 parts KCl for each part MgCl2). We looked at it for thermal storage (PCM or direct liquid tank storage) and it looks promising.

http://nuclear.inl.gov/deliverables/doc ... l_salt.pdf


Watch out! MgCl2(hydrate, naturally) emits unpleasant fumes when heated. I still remember that nasty whiff of HCl when I tried this eutectic. Looks good on paper, but learn to respect even the safest-looking chemicals and read their MSDS carefully. Inside all that boilerplate and derriere-covering there is also some useful information.


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PostPosted: May 30, 2010 12:08 pm 
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Owen T wrote:
Cyril R wrote:
KCl - MgCl2 is a good one for you Kirk. Non-toxic, absurdly cheap, and a much lower melting point than NaCl-KCl. Easy to prepare because of the 2:1 eutectic (2 parts KCl for each part MgCl2). We looked at it for thermal storage (PCM or direct liquid tank storage) and it looks promising.

http://nuclear.inl.gov/deliverables/doc ... l_salt.pdf


Watch out! MgCl2(hydrate, naturally) emits unpleasant fumes when heated. I still remember that nasty whiff of HCl when I tried this eutectic. Looks good on paper, but learn to respect even the safest-looking chemicals and read their MSDS carefully. Inside all that boilerplate and derriere-covering there is also some useful information.


Why weren't you using just the anhydrous salt? Isn't it always dangerous, to heat halides in the presence of significant quantities of water (eg in hydrate form), no matter what the salt type?

MgCl2 does have a higher vapour pressure than KCl or NaCl. Which is one of the reasons the KCl MgCl2 eutectic is superiour, having less MgCl2 than the NaCl binary...


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PostPosted: May 30, 2010 2:13 pm 
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Kirk Sorensen wrote:
Guys, what do you think would be good for a nice cheap salt? Something that I could actually buy and get started working with...without spending much money (since it would be my own).

How about NaF-KF?

Or NaCl-KCl?

Anyone have any ideas for a salt that would have a sufficiently low enough melting point that you could potentially melt it with electrical-heating-wire wrapped around a pipe?

I want to "get my hands dirty" with some of these salts and am not sure which one might be best. I do think it should be a halide salt though--no nitrates or carbonates.


Try http://ras.material.tohoku.ac.jp/~molte ... query1.php . Note that it expects you to put in components in alphabetical order. If you put KCl-MgCl2-NaCl, it will list for you the 46 mol % MgCl2, 33 mol % NaCl, and 21 mol % KCl eutectic that melts at 658 K ... but I see it's saying 669 K. If you, like me, tend first to put them in in order of atomic number of the metal, NaCl-MgCl2-KCl, it finds nothing.

Also, US Patent 5215631, when downloaded as a PDF from Google Patents, lists on p. 19 many fusible salt compositions in diminishing order of temperature.

(How fire can be domesticated)


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PostPosted: May 31, 2010 2:24 am 
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Interesting, that chloride ternary melts under 400 Celcius. This could be great for a liquid salt thermal storage concept. Though Kirk may prefer the easier to prepare binary @ 435 Celcius melting point. Just put the anhydrous mix in a nickel pipe and use resistive/induction heating. I suppose melting does occur at a bit higher temperature initially, due to the heterogeneous salt mix? Oxidising salts like nitrates and carbonates can be shipped in solution and then the water can be boiled off (water is an oxide too, so no problem there). As Owen suggests, this is not a good idea for the halides, so one would always work with very dry anhydrous halide salts.


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PostPosted: May 31, 2010 7:03 am 
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Cyril R wrote:
Oxidising salts like nitrates and carbonates can be shipped in solution and then the water can be boiled off (water is an oxide too, so no problem there). As Owen suggests, this is not a good idea for the halides, so one would always work with very dry anhydrous halide salts.


It's not a problem for halides in general. Li, Na and K are all very stable and their water of hydration is all driven off before they melt. It's Mg (and perhaps some other second column or heavier elements) that have less stable halides and experience some decomposition even at lower temperatures.


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PostPosted: May 31, 2010 8:39 am 
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Owen T wrote:
Cyril R wrote:
Oxidising salts like nitrates and carbonates can be shipped in solution and then the water can be boiled off (water is an oxide too, so no problem there). As Owen suggests, this is not a good idea for the halides, so one would always work with very dry anhydrous halide salts.


It's not a problem for halides in general. Li, Na and K are all very stable and their water of hydration is all driven off before they melt. It's Mg (and perhaps some other second column or heavier elements) that have less stable halides and experience some decomposition even at lower temperatures.


So, is that also the case for the fluorides? Like FLiNaK, the high performance heat transfer salt, would that be easy to prepare?


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PostPosted: Jun 01, 2010 2:55 am 
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Cyril R wrote:
Owen T wrote:
Cyril R wrote:
Oxidising salts like nitrates and carbonates can be shipped in solution and then the water can be boiled off (water is an oxide too, so no problem there). As Owen suggests, this is not a good idea for the halides, so one would always work with very dry anhydrous halide salts.


It's not a problem for halides in general. Li, Na and K are all very stable and their water of hydration is all driven off before they melt. It's Mg (and perhaps some other second column or heavier elements) that have less stable halides and experience some decomposition even at lower temperatures.


So, is that also the case for the fluorides? Like FLiNaK, the high performance heat transfer salt, would that be easy to prepare?


The fluoride must be way more stable. My comment was from personal experience trying to find a reasonably low-melting salt eutectic with cheap and non-toxic chlorides.


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PostPosted: Jun 01, 2010 3:27 am 
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Oddly enough, Mg2Cl has the lowest Gibbs free energy of formation (at 1000K) of any chloride salt. It beats NaCl, LiCl and KCl by a mile. It should be the least corrosive salt to use.

I'd heard that the chlorides have problems with purification - getting the stuff pure enough to be non-corrosive as a heat transfer fluid. But is this really true for the common chlorides? NaCl, obviously, is very common in high purity food grade (unless you want some iodine in it).


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PostPosted: Jun 01, 2010 11:48 am 
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Cyril R wrote:
Oddly enough, Mg2Cl has the lowest Gibbs free energy of formation (at 1000K) of any chloride salt. It beats NaCl, LiCl and KCl by a mile. It should be the least corrosive salt to use.

I'd heard that the chlorides have problems with purification - getting the stuff pure enough to be non-corrosive as a heat transfer fluid. But is this really true for the common chlorides? NaCl, obviously, is very common in high purity food grade (unless you want some iodine in it).


MgCl2 stabler than CaCl2, BaCl2, SrCl2? What's your source for that?

This abstract ...

Quote:
Chlorination of UO2, PuO2 and rare earth oxides using ZrCl4 in LiCl–KCl eutectic melt

Y. SAKAMURA, T. INOUE, T. IWAI and H. MORIYAMA
J Nucl Mater, 340, 1, 39-51 (2005).

A new chlorination method using ZrCl4 in a molten salt bath has been investigated for the pyrometallurgical reprocessing of nuclear fuels. ZrCl4 has a high reactivity with oxygen but is not corrosive to refractory metals such as steel. Rare earth oxides (La2O3, CeO2, Nd2O3 and Y2O3) and actinide oxides (UO2 and PuO2) were allowed to react with ZrCl4 in a LiCl–KCl eutectic salt at 773 K to give a metal chloride solution and a precipitate of ZrO2. An addition of zirconium metal as a reductant was effective in chlorinating the dioxides. When the oxides were in powder form, the reaction was observed to progress rapidly. Cyclic voltammetry provided a convenient way of establishing when the reaction was completed. It was demonstrated that the ZrCl4 chlorination method, free from corrosive gas, was very simple and useful.


... suggests ZrCl4 could be good for pulling oxygen-in-general to the bottom and making it filterable.

(<em><a href="http://www.eagle.ca/~gcowan/">How fire can be domesticated</a></em>)


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PostPosted: Jun 01, 2010 12:17 pm 
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Sometimes I use this table:

http://en.wikipedia.org/wiki/Standard_e ... a_table%29

MgCl2 beats NaCl and KCl, though you're right that CaCl2 (and also BaCl2) are even more stable.

There are also some Gibbs figures in the coolant reference I gave in the previous post (though they are in kilocalories). Based on that MgCl2 looks as good as MgF2.


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PostPosted: Jun 01, 2010 1:41 pm 
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Cyril R wrote:
Sometimes I use this table:

http://en.wikipedia.org/wiki/Standard_e ... a_table%29

MgCl2 beats NaCl and KCl, though you're right that CaCl2 (and also BaCl2) are even more stable.

There are also some Gibbs figures in the coolant reference I gave in the previous post (though they are in kilocalories). Based on that MgCl2 looks as good as MgF2.


Table 11 of http://nuclear.inl.gov/deliverables/doc ... l_salt.pdf says, meaninglessly, "(kcal/mol–halide-element)". It and the given numbers would make sense if it said kcal per mole of halide compound.

Standard free energies, kJ/mol :

Code:
2 NaCl  +   Mg  --->    2 Na    +   MgCl2
-768.2                              -591.8


At 1000 K the increase in going from left to right should be more, because positive charges that were on one atom each are crowding onto a half-atom each (so there is an entropy reduction). Or you could say, if they were molecular substances, that chlorines that were one per metal are, on the right, crowded two per metal.

(How fire can be domesticated)


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PostPosted: Jun 01, 2010 2:26 pm 
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GRLCowan wrote:
Cyril R wrote:
Sometimes I use this table:

http://en.wikipedia.org/wiki/Standard_e ... a_table%29

MgCl2 beats NaCl and KCl, though you're right that CaCl2 (and also BaCl2) are even more stable.

There are also some Gibbs figures in the coolant reference I gave in the previous post (though they are in kilocalories). Based on that MgCl2 looks as good as MgF2.


Table 11 of http://nuclear.inl.gov/deliverables/doc ... l_salt.pdf says, meaninglessly, "(kcal/mol–halide-element)". It and the given numbers would make sense if it said kcal per mole of halide compound.

Standard free energies, kJ/mol :

Code:
2 NaCl  +   Mg  --->    2 Na    +   MgCl2
-768.2                              -591.8


At 1000 K the increase in going from left to right should be more, because positive charges that were on one atom each are crowding onto a half-atom each (so there is an entropy reduction). Or you could say, if they were molecular substances, that chlorines that were one per metal are, on the right, crowded two per metal.

(How fire can be domesticated)


Are you sure? If the MgCl2 figure is per chlorine atom then it's 2x 600 kJ isn't it?

(right now, I'm looking at my magnesium pencil sharpener, and thinking about the table salt in the kitchen, just below the furnace, and... :twisted: )


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PostPosted: Jun 01, 2010 2:42 pm 
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GRLCowan wrote:
......This abstract ...... suggests ZrCl4 could be good for pulling oxygen-in-general to the bottom and making it filterable.


One of the ORNL reports - sorry, haven't got the ref to hand - mentions that they found adding ZrF4 to be helpful in avoiding UO2 precipitation. Traces of oxide always get in, and Zr is a great oxide scavenger since the exchange reaction

ZrF4 + (2/y)MxOy = ZrO2 + (2/y)MxF2y

Strongly favours ZrO2 for most metals M. Even for thorium, it's about 40-50 kJ/mol in favour of ZrO2 and ThF4 vs ThO2 and ZrF4.

I've been wondering about exploiting this for reprocessing schemes, since Zr is a major fission product and ZrF4 is a pain to split out. It forms association complexes with LiF, and follows the FLiBe around. Adding oxide to make ZrO2 would remove it as still bottoms, and zirconia is the perfect waste form for the long half-life zirconium isotopes. Individual zirconia crystals are the oldest identifiable Earthly objects, washed from mountains to redeposit in sandstones that get made into mountains that get re-eroded..... for four billion years. Nothing touches them. Unfortunately, it doesn't work for 1- or 1.5-fluid systems, as the stability against thoria isn't enough when the system has tonnes of thorium and only a few kg of zirconium - you probably hit the thoria solubility limit first, although it would be worth testing to be sure.

Update - found the ORNL refs - further discussion in the high-temp-reprocessing thread.


Last edited by Luke on Jun 03, 2010 6:11 am, edited 2 times in total.

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PostPosted: Jun 01, 2010 4:21 pm 
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GRLCowan wrote:
....Table 11 of http://nuclear.inl.gov/deliverables/doc ... l_salt.pdf says, meaninglessly, "(kcal/mol–halide-element)". It and the given numbers would make sense if it said kcal per mole of halide compound.

Standard free energies, kJ/mol :

Code:
2 NaCl  +   Mg  --->    2 Na    +   MgCl2
-768.2                              -591.8

.......
I looked this up in the CRC book, and the above figures seem to be per mole of compound, so no fireworks from pencil sharpeners and table salt. It's much closer for the fluorides - MgF2 has about twice the formation free energy of NaF - and the other way round for the oxides, i.e Mg will displace Na from Na2O, but Na will displace Mg from MgCl2. Failing to check up on this lot before posting a comment on Atomic Insights about hazards of sodium reactivity in fast breeders made me look very foolish in public :oops: 'It's not what you don't know that gets you, it's what you think you know that just ain't so'

Try NIST - they don't have everything tabulated nicely, but they do check back to primary data and so avoid propagating errors.


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