[fractalrocks]

Greetings to all,

Is it true that zirconium and hafnium are systematically found in the same minerals all the time?

And if so, is there any explanations to this ?

regards,
jonathan
Montreal, Quebec

[AWK]

See Wikipedia: zirconium
and
hafnium

[Enthalpy]

The chemical properties are said to be very close. Metallurgy has difficulties separating both metals. For normal uses, Hf is left in Zr alloys, but at nuclear reactors where Zr is chosen because it doesn't absorb neutrons, Hf must be separated, which needs efforts that translate to cost.

Do that activity if you like worries. It was the job of the cyclist shot dead at Chevaline.

[Enthalpy]

Since I've had worries after thinking at the following, I make it public.

Three uneasy processes are known to separate Hf from Zr according to Wiki, for instance the fractional distillation of HfCl₄ and ZrCl₄. In addition, I propose to separate gaseous HfCl₄ from ZrCl₄ by centrifugation.

ZrCl₄ sublimates at 331°C=604K where HfCl₄ has 1.7atm vapour pressure. RT=5022J/mol there. A lower pressure improves a bit.

A tube of 2000MPa Maraging steel can rotate at 416m/s with 20% margin. Ti-Al6V4 and AA7075 would be good enough too, and graphite fibres much better.

Neglecting the isotopes, HfCl₄ weighs 87g per mole more than ZrCl₄: that's 29* easier than uranium enrichment. The kinetic energy differs by 7528J/mol or 1.50*RT, so after some 10 stages, the metals are pure.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Distillation at a lower pressure is often more selective. The distillator by sublimation I suggest there
http://www.chemicalforums.com/index.php?topic=86631.0
may benefit to HfCl₄ and ZrCl₄.

[Enthalpy]

Nb and Ta too are reportedly difficult to separate, so a centrifuge might help too.

A mole of Nb weighs 93g, Ta 181g, so TaCl₅ is 88g heavier than NbCl₅, 30× easier than uranium enrichment. Isotopes let that fluctuate by ±5g. After the centrifuge is built, it can optionally spend some time separating ³⁵Cl from ³⁷Cl to save time separating Nb from Ta. NbCl₅ melts at +205°C and boils at +248°C under 1atm, TaCl₅ at +216°C and +239°C (decomp).

A tube of 2000MPa Maraging steel rotating at 416m/s makes 7.7kJ/mol difference in the kinetic energy. At arbitrary +127°C=400K, that's 2.3RT, so the metals are pure in half a dozen steps, or 1 or 2 steps with tubes of graphite fibres.

The oxides mix is reduced, Cl₂ or HCl give the mix of pentachlorides, centrifuges separate the pentachlorides before reduction. No fluorine needed. I can't tell whether the process is globally advantageous.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Centrifuges have an additional trick that I had not grasped. The contents drifts slowly downwards at the bigger radius and upwards at the centre. This second movement establishes a vertical composition gradient in addition to the radial gradient, letting the centrifugal force act several times on the composition within a single tube. Very astute. They don't tell it there
https://en.wikipedia.org/wiki/Zippe-type_centrifuge

So that's why the shape is a long cylinder, despite not being the strongest against centrifugal force.

Here Hf would separate from Zr in a single tube, Ta from Nb too. But with graphite composite, the vertical drift may not even be necessary, and this allows shapes that rotate even faster.

----------

Other processes achieved isotopic separation: gaseous diffusion, vortex, nozzles...
https://en.wikipedia.org/wiki/Isotope_separation#Practical_methods_of_separation
They look less interesting than centrifuges, but would separate the elements too.

[Enthalpy]

Rare earth ore provides them all, but it's reportedly difficult to separate Sc and Y from the lanthanides. Centrifuges could do it thanks to the well spread molar masses: 45g, 89g and for La (138g) 139g with at least 44g difference, 15* easier than for uranium.

Volatile compounds are rare. ScCl₃, YCl₃ and RCl₃ need some 1800K to boil under 1atm, so hopefully 1000K make a usable vapour pressure. Monoisotopic Cl brings a bit.

Aluminum, maraging and graphite-epoxy can't operate that hot, superalloys rotate slowly. But carbon-carbon could make the rotors, maybe with a thin hermetic metal liner. 2D tubes resist even at heat around 200MPa, varying a lot among the suppliers, so near-azimuthal winding shall exceed that. 2000kg/m³ let a tube rotate at 316m/s, so 1/2*Delta(m)*V²=0.26*RT at 1000K, needing around 27 steps.

More volatile compounds would reduce RT and enable faster maraging or graphite-epoxy. Salts of organic acids seem excellent, if finding data or measuring.

Marc Schaefer, aka Enthalpy