j_sum1 - 29-9-2023 at 15:17
Side topic from Finding small amounts of Pu or U235
That thread is likely destined for Detritus but it did raise some questions for me.
I posted this video which shows how Pu(NO3)4 is reduced to metallic plutonium. (The video presents a flow diagram of the process in the first 3
minutes then repeats the same information showing the actual equipment.)
Plutonium Metal Preparation
Two reactions interest me. One is the formation of a peroxide complex to precipitate plutonium nitrate.
The second is the use of elemental calcium and elemental iodine together to reduce PuF4 to Pu.
How did anyone figure out these were feasible routes?
Why would you expect the peroxide complex to be insoluble (and selective)? Why would you choose Ca and I2 for the reduction? I can't imagine anyone
had a significant amount of raw material to develop these processes. It must have been more than a hunch to derive the methodology.
And that second reaction seems a strange choice. I am guessing the reaction is
PuF4 + Ca + I2 --> Pu + CaF2 + 2IF
But other stoichiometries are possible.
Thoughts
(Placed in general chem because I am interested in the reactions and the mode of discovery.)
And, on second thoughts, moving to Radiochemistry.
[Edited on 29-9-2023 by j_sum1]
pantone159 - 29-9-2023 at 18:49
Part of the answer, was working with extremely small amounts of Pu in the early days, to study Pu chemistry and figure out purification that would
work. It was a triumph of nano-scale chemistry, especially with 1940's technology.
The Ca+I2 sounded strange to me also, but late in that video there was a comment that this heated up the Ca to where it worked for the reduction. I
wasn't sure what to make of that comment.
UC235 - 29-9-2023 at 20:06
Uranium and thorium form insoluble peroxides, so it seems like a good place to start.
[Edited on 30-9-2023 by UC235]
wg48temp9 - 29-9-2023 at 20:12
It seemed unlikely that any interhalogen compound would form, as it would combine with the plutonium metal.
Apparently, the plutonium is reduced by the calcium to form calcium fluoride. Additional calcium combines with the iodine to heat up the reaction, and
the formed calcium iodide reduces the melting point (fluxes) of the calcium fluoride to allow the plutonium to form a button at the bottom of the
crucible.
See Attachment: 4438226.pdf (894kB)
This file has been downloaded 118 times
Pumukli - 29-9-2023 at 21:02
j-sum, Ca as reducing agent seems fairly logical for me. Reductions by metals were widespread in those early days. The "tendency" of Ca2+ to combine
with F- and yield practically insoluble CaF2 makes this choice attractive.
Maybe, they tried these reactions on other "model compounds" previously and found them "worthwhile".
j_sum1 - 29-9-2023 at 21:41
So, I2 is there as an energy boost and to create a flux
Makes sense.
Still, there is a lot of creative thinking to devise such a process.
I really am in awe.
Even determining MP would have been a challenge. If I was to guess, I would have gone a lot higher.
unionised - 30-9-2023 at 02:53
Fluorine gets its name from fluorspar which is essentially naturally occurring calcium fluoride.
Fluorspar gets its name from its use in metallurgy as a flux- very loosely it means "flow stone".
So roughly 1500 years before anyone knew about plutonium, they knew that calcium fluoride was a useful flux.
Hardly shocking that they chose to use it.
[Edited on 30-9-23 by unionised]
Pentaborane - 18-3-2024 at 10:35
The heat of reaction also has a lot to do with the choice of the calciothermic route. If it does not have sufficient heat to melt the Pu and
flux/slag, the metal will be finely divided and stuck in the slag matrix (it can also reduce the yield significantly if there is no solvent effect
with the slag). It is desirable for it to melt into two distinct, nicely separated phases. The bottom metal phase solidifies and is broken out as a
"button". This is the same reason why large U metal batches may be reduced by Mg, but smaller batches (usually for criticality reasons) are often
reduced by Ca.
As for precipitation, the current "standard" method tends to be oxalate precipitation instead of peroxide as shown in the video. The oxalate is
calcined to PuO2. There was at least one plant in the US that used HF precipitation of Pu(III) (which would then be oxidized to a mixture of PuF4 and
PuO2 before reduction). There have been some interesting attempts to go to direct thermal denitration to PuO2, but this does nothing about impurities
in the feed, so the nitrate must be exceedingly pure for it to work. A nice thing about peroxide precipitation that makes it still relevant is that it
can very thoroughly decontaminate Pu from solutions... There are some uses for this in decontaminating process waste streams.
As an interesting aside, this route has been largely superseded by direct oxide reduction. The main reason is because of the (alpha, n) reaction (from
the alpha particles reacting with the fluorine) which produces a serious issue with controlling worker dose in those facilities. It is pretty common
to see water tanks built into the glove boxes for neutron shielding in these processes. PuO2 hydrofluorination (for PuF4) and PuF3 precipitation
aren't exactly a simple or nice processes either. The direct oxide reduction method does have some quality issues though, so the materially typically
needs to be electrorefined in a molten salt bath (if you've ever seen pictures of those nice Pu rings, that's the electrorefining product). I have
attached an openly published paper on electrorefining from Rocky Flats (the primary Pu processing facility in the US until it closed) it for the
curious. It also gives an idea on what it takes to work with this stuff from a practical perspective and gives an idea of how ridiculous the thread
referenced at the beginning of this thread was.
Attachment: 4434340.pdf (896kB)
This file has been downloaded 44 times
[Edited on 18-3-2024 by Pentaborane]
[Edited on 18-3-2024 by Pentaborane]
clearly_not_atara - 18-3-2024 at 12:03
The use of iodine may be a case of the crystal bar process.