Monthly Archives: October 2014

Safe nuclear and rare earth minerals

Recently I read a article about ‘why nuclear will rely on rare earth minerals’1 which briefly introduced the important relationship between safe nuclear and rare earth minerals. Actually, as for safe nuclear, it means to make nuclear power safer.  In short, it is better to replace uranium fuel with a different element.

Fortunately, Scientists found thorium is one of the best succedaneums for nuclear power, which is also introduced in our previous blog by Mark Foreman 2. Because thorium produces little dangerous, weapons-grade waste. Especially thorium‘s waste survives for only a few hundred years, not the 10s of thousands or even millions of years for uranium. So it will reduces the weapons-proliferation threat associated with nuclear power in the future.

Thorium is a basic element of nature, like Iron and Uranium. Like Uranium, its properties allow it to be used to fuel a nuclear chain reaction that can run a power plant and make electricity (among other things). Thorium itself will not split and release energy. Rather, when it is exposed to neutrons, it will undergo a series of nuclear reactions until it eventually emerges as an isotope of uranium called U-233, which will readily split and release energy next time it absorbs a neutron. Thorium is therefore called fertile, whereas U-233 is called fissile.3

1

Photo of Monazite-(Ce) : (Ce, La, Nd,Th)(PO4), Taken from: http://www.mindat.org/photo-48025.html

Nowadays, scientists have been  working on using thorium as nuclear fuel as opposed to uranium because of some of the possible benefits, one of which is that estimated to be about three to four times more abundant than uranium, easy to mine, and refined from monazite sands as a by-product of extracting rare earth metals.4  Monazite is a phosphate mineral which is a lanthanide phosphate (LnPO4), it often contains plenty of thorium. 2

1 why nuclear will rely on rare earth minerals.  http://www.smartplanet.com/blog/intelligent-energy/why-safe-nuclear-will-rely-on-rare-earth-minerals/

2 Thorium radioactivity. I http://fp7-erean.eu/wordpress/thorium-radioactivity-i/

3 Thorium as nuclear fuel. http://www.whatisnuclear.com/articles/thorium.html

4 Thorium. http://en.wikipedia.org/wiki/Thorium

The stability of Nd-Fe-B permanent magnets

Dear reader,

         If in the last post I had spoken about magnetic losses now I will speak about the “stability”. In the following, the term “stability” used for magnets refers only to the decrease of irreversible losses, i.e. those caused by the temperature, or those produced by intense demagnetized magnetic fields, or produced by the change of the phase composition of the magnet. So if the magnet can be characterized as stable, then it can be called permanent magnet.

         The stability of the permanent magnets against the temperature is relative, because it depends not only on the material properties of the magnet and working conditions, but also depends on the form in which it presents.

         If we assume that there is no magnetic irreversible loss, when at the temperature T, the demagnetization curve is still straight, then we can define the maximum operating temperature:

µ0 HcJ(Tmax) = Br(Tmax)

because HcJ(T) = HcJ(T0) (1 + αH(T – T0)) and Br(T) = Br(T0) (1 + αB(T – T0)), where αH and αB are coefficients of reversible variation with the temperature of the coercive field and of the magnetization.

From the above relationship results Tmax

\Delta T_m_a_x = \frac{\frac{\mu_0 \cdot H_c_J \cdot (T_0)}{B_r \cdot (T_0)} -1}{\alpha_B - \alpha_H\cdot \frac {\mu_0 \cdot H_c_J \cdot (T_0)}{B_r \cdot (T_0)}}

         It result that the maximum operating temperature is even greater if µ0 HcJ(T0) / Br(T0) is higher. This means that an isotropic Nd-Fe-B magnet, which has the remanent induction approximately equal to JS / 2, will have a maximum operating temperature greater than the anisotropic magnet of the same material, which has the remanence approximately equal to JS.

         The increased of the thermal stability of Nd-Fe-B magnets is in fact the increasing of the values of intrinsic coercive field, in order to obtain an increase of the maximum operating temperature: Tmax = T0 + ΔTmax even with the presence of some demagnetized fields.

         So we can conclude that the substitution of Fe or Nd with Co or Dy can stabilize the magnet. An example is the substitution of the Nd with Dy that improves the thermal stability. For more information about the substitution, you can read another post from this site called Additives in Neodymium Iron Boron magnets

Thorium radioactivity I

Dear Reader,

Monazite is a phosphate mineral which is a lanthanide phosphate (LnPO4), it often contains plenty of thorium

When processing monazite two options exist, one of the things which the mineral processing needs to do is to manage the radioactivity in the ore and not let it contaminate the products.

Either the ore can be leached with acid or base, if the ore is leached with acid then it will be treated with sulfuric acid to form a mixture of sulfates. These can be processed by solvent extraction to give three seporate products, thorium, lanthanides and a trace of uranium. The alternative is to leach with a strong sodium carbonate solution.

Now before we get going it is important to note the half lives of the key nuclides in the thorium-232 decay chain. This will be needed later.

Nuclide Half life Decay mode Decay energy
232Th 1.405 x 1010 years α 4.083 MeV
228Ra 5.75 years β 0.046 MeV
228Ac 6.15 hours β 2.127 MeV
228Th 1.9116 years α 5.520 MeV
224Ra 3.66 days α 5.789 MeV

From looking at the literature[i] it is clear that treatment of a solution obtained by extracting monazite with sulfuric acid with a primary amine (Primene JM-T) extracts the thorium while treatment with a tertiary amine (Alamine 336) extracts the uranium. If we were to extract the uranium and thorium from the waste then the amount of radioactivity in the sulfuric acid layer would be greatly reduced in the long run. But it is possible that the actinium and the radium will follow the lanthanides in the process.

A lot will depend on the barium and strontium concentrations in the ore, if the ore is barium rich then during the sulfuric acid digestion then the release of radium from the ore could be reduced. This would reduce the contamination of the solvent extraction plant with radium-228, but if the radium never dissolves then it will stay in the insolubles which never dissolved in the sulfuric acid leach. As a result the trailings from the ore processing plant will be radioactive with radium for about 50 years. The reason why barium will alter radium chemistry is that barium and radium sulfates can form solid solutions with each other.

It is important to note that a carrier free radium could remain in solution even when sulfate is present, but as soon as barium or strontium is present I would predict that the radium will become locked away in a solid. If you want to read about this in more detail the please look at the following two papers.[ii] [iii]

If we assume that the radium is able to dissolve (low barium concentration) then things will be a little different.  When the lanthanides are precipitated from the mixture of sulfuric and phosphoric acids by the addition of sodium or potassium sulfate to form solid then it is possible that the radium will be precipitated in the form of radium sulfate if the sulfuric acid concentration is too low to keep it in solution as a bis sulfate complex such as [Ra(SO4)2]2-.

As the actinium has a short half life as soon as the lanthanides and it are separated from the radium and the thorium it will decay away quickly, so it will vanish quickly as long as the extraction is able to reject both the tetravalent thorium and the divalent radium.

The 228Ra present in a precipitated lanthanide sulfate concentrate would cause the radioactivity of the lanthanide extract to remain elevated for years. My advice to anyone trying to lower the radioactivity level of a monazite derived lanthanide stream is to extract the lanthanides with DEHPA or some other similar reagent which extracts the lanthanides but not radium, then strip the lanthanides with acid to form a solution of lanthanides in acid. If you are clever you can do a selective strip of the lanthanides with acid and set some separation between the light and the heavy lanthanides. This would allow you to make at least two lanthanide concentrates with different metals in them.

On the other hand if the thorium is extracted it will have a very high distribution ratio and it is likely that it can be separated from the lanthanides by selective stripping.

References

[i] Janúbia C.B.S. Amaral and Carlos A. Morais, Minerals Engineering, 2010, 23, 498-503.

[ii] Tieyuan Y. Zhang, Kelvin Gregory, Richard W. Hammack and Radisav D. Vidic, Environmental Science and Technology, 2014, 48, 4596-4603.

[iii] Hanna Hedstrom, HenriK Rameback and C.H: Ekberg, Journal of Radioanayltical and Nuclear Chemistry, 2013, 298, 847-852.

Rare earths in energy storage and conversion

With dwindling fossil fuel resources and oil consumption projected to reach its peak,  humanity is faced with an uphill task of exploring alternative devices for energy storage and conversion which are cost effective and environmentally benign [1]. Rare earth elements, either directly or indirectly have a key role to play in many of these devices. I plan to write a series of blog posts pertaining to the deployment of REEs in energy storage and conversion devices.  And dear reader,  this blogpost will talk about Nickel metal hydride batteries. Even if you are not familiar about the Ni-MH batteries, this blogpost can be interesting because I will break it down in similar fashion of how Spike Lee introduces his villain through a monologue in the movie “Inside man”

The What: 

Ni-MH batteries are alkaline rechargeable batteries. They have 2 electrodes, positive and negative with a porous separator in between and an alkaine electrolyte, KOH.  How good are they? Well if you click the below figure, you can see that their capacities/specific energy (60-100 Wh/Kg) lie in between lead acid and Li-ion batteries.  They have their own advantages such as being considered as a safe system, relatively inexpensive than Li based batteries and having a long cycle life. [2,3]

RAGONE

The Where: 

Where exactly are REEs used in Ni-MH batteries?  The positive electrode of Ni-MH batteries in charged state is nickel hydroxide and the negative electrode is a metal hydride alloy. Negative electrodes are essentially hydrogen storage materials and AB5 alloy is one such hydrogen storing intermetallic compound. The component A is usually Lanthanum or naturally occurring misch metal alloy or a mixture of Ce, Nd, Pr, G and Y. Component B has Co and Ni as major components with Al, Zr,Si & Ti as minor components to improve corrosion resistance.  [4,5]

The How :

What happens internally when we plug in to charge the Ni-MH battery? The positive electrode material, Ni(OH)2 reacts with hydroxyl ions and gets oxidized to nickel oxyhydroxide and  generates water and electrons. Ionic transport happens through the separator which is impregnated with alkaline electrolyte but the electronic transport happens outside the circuit. [6,7]

Ni (OH)2 + OH-  –> NiOOH + H2O + e-

Hold on, this is just the positive electrode, there is the other one, the negative electrode which has our precious REEs, what exactly happens there when we plug in to charge? The water is reduced by electrons  to hydrogen atoms at the metal (or AB5 alloy) electrode which subsequently forms a metal hydride compound.

M + H2O + e- —> MH + OH-

And what happens when we use our battery to do things we do, i,e during discharge? The reverse. In the negative electrode, the protons say goodbye to the metal hydride and combines with hydroxyl ions to form water. Just the hydroxyl ions shuttling back and forth, no fuzz of electrolytes getting consumed like in the heavy lead acid batteries.  I have attached a figure which may help understand this chemistry better. [8]

Ni MH image

 

The Why: 

The discovery of AB5′s  (esp LaNi5) ability to readily absorb and desorb hydrogen due to their crystal structure was apparently serendipitous [9]. But there are stringent and extensive requirements for hydrogen storage alloys in Ni-MH batteries, such as high storage capacity, good reversibility etc [10]. Commercially a mix of 6-8 elements are used for obtaining various required properties of the alloy.

On a concluding note, Ni-MH batteries have been preferred by some manufactures to make hybrid electric vehicles. The increased consumption of the batteries means they are also one of the potentially interesting secondary sources for recycling. From a material and application point of view, Ni-MH batteries are quite interesting devices for rare-earthers

P.S : The author regrets the fact that he was not able to use the quote “And therein as the bard would say,lies the rub” from the movie “Inside man”

 

References

1. Nature: Cleaner Fuels for the Future, Chemical education today http://pubs.acs.org/doi/pdf/10.1021/ed077p810

2. Batteries for vehicular applications, Venkat Srinivasan, LBNL, http://bestar.lbl.gov/venkat/files/batteries-for-vehicles.pdf

3. Hand book of batteries, 3rd edition, David.B.Linden and Thomas Reddy

4. Nickel/metal hydride technology for consumer and electric vehicle  batteries – a review and up-date,  Journal of Power Sources 65 ( 1997) l-7

5.  Inside the Nickel Metal Hydride Battery, Cobasys http://www.cobasys.com/pdf/tutorial/inside_nimh_battery_technology.pdf

6. Ni-MH batteries from concept to characteristics, P.H.L.Notten et al http://www.ache.org.rs/HI/2000/No03/notten.pdf

7. The Current Status of Hydrogen Storage Alloy Development for  Electrochemical Applications Materials 2013, 6, 4574-4608;

8. http://en.globtek.com/nimh-battery-safety-notes.php#

9. Rapid Solidification of AB5 Hydrogen Storage Alloys  by Sverre Gulbrandsen-Dahl, PhD thesis

10. A Nickel Metal Hydride Battery for Electric Vehicles, Science, VOL. 260,9 APRIL 1993