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NNadir

(33,368 posts)
Sat May 16, 2020, 04:50 PM May 2020

The Preparation of Ultra High Purity Tungsten Free Molybdenum.

The paper I'll discuss in this post is this one: Sustainable Extraction and Complete Separation of Tungsten from Ammonium Molybdate Solution by Primary Amine N1923 (Zhang et al., ACS Sustainable Chem. Eng. 2020, 8, 18, 6914-6923).

In the periodic table the lanthanides, the elements, shown in the graphic below, sequentially add electrons to f orbitals, which in general do not contribute significantly to their bonding.



The directional nature of the f orbitals results in incomplete shielding from the nuclear charge for the outer electrons resulting in what is known as the "lanthanide contraction," a decrease in the atomic radius of the elements and their ions. This has an effect on the chemistry of the elements that emerge sequentially after the lanthanides, halfnium, tantalum, and tungsten since as a result of the contraction, the atomic radius is very close to their cogener elements, respectively, zirconium, niobium and molybdenum. The similarity in size makes the separation of high purity samples of these elements difficult. In fact, the name of tantalum, a "conflict metal" that is an important component of cell phones, derives from the greek myth of Tantalus, because the process of isolating pure tantalum was frustrating. (This difficulty would be expected for the separation of rhenium from technetium, but technetium does not occur naturally except in small traces in uranium ores which generally do not contain rhenium. Therefore this difficulty is not observed in isolating rhenium from its rare ores, although this separation might become important in a future wherein wisdom prevails, not that history suggests that periods of international wisdom are rare.)

Although in my daily ruminations I am far more interested in tungsten than I am in molybdenum, molybdenum is a very important element, and is widely used in alloys and in applications as a refractory in high temperature systems. Pure molybdenum has the sixth highest melting point of the commonly available elements; only tantalum, osmium, rhenium, its cogener tungsten, and carbon have higher melting points. In addition, molybdenum oxides, molybdenates, are important catalysts for many reactions. (The tungstates are also used in many settings.)

(An interesting fact about molybdenum is that it is the heaviest element, except for iodine, that was essential to life before the invention of the Haber process in the early 20th century. Molybdenum coordinating proteins fix nitrogen in the natural world. A few proteins containing tungsten are known is rare species, but they are not nearly as important as molybdenum containing nitrogenases.)

In any case, the authors of this paper describe some of the applications of very high purity molybdenum. From the introduction to the paper:

Molybdenum, a national strategic resource, has unique mechanical, thermal, electrical, and corrosion-resistant properties, making it widely used in electronics, metallurgy, aerospace applications, and other fields.(1,2) The increasing demand for high-purity metallic molybdenum and its intermediate has become a challenge for metallurgists. For example, for mammography applications in the medical field, ultrapure molybdenum is necessary for the sputter target.(3) The manufacture of corrosion-resistant molybdenum materials by powder metallurgy has a Mo purity requirement of 99.95–99.99% because the toughness of the molybdenum metal depends largely on its purity.(4) In recent decades, many purification procedures have been proposed and applied for the removal of alkali and alkaline-earth metals as well as transition elements from molybdate solutions.(5?8) Overall, impurities containing P, As, Si, and some other elements are relatively easy to remove, but tungsten, whose physicochemical properties (ionic radius, aqueous chemical behaviors) are similar to those of molybdenum, is not.(9) However, it is almost impossible to separate tungsten from molybdenum during pyrometallurgical refining due to their similar physical properties. Therefore, the hydrometallurgical preparation of ammonium molybdate is the only and best choice for tungsten removal since the difference between tungsten and molybdenum is magnified in solution systems by polymerization.(10,11)...


The nation for which molybdenum is a national strategic resource as discussed in this paper is China, where incidentally, molybdenum is mined; China has the world's largest reserves of this element. (The United States also has significant reserves, albeit, ironically, low in Tungsten.)

The author's review methods by which molybdenum and tungsten are separated when separation is required. Since these methods cannot be metallurgical, they require significant chemistry, and include solvent extraction, selective crystallization, adsorption and ion exchange methods.

The author's here have opted to refine an extraction method, where the extractant is a commercially available reagent known as "primary amine 1923" which is widely utilized in metal extraction chemistry. The name refers to an amino group located on a carbon that features long aliphatic carbon chains on either side. These chains range between 19 and 23 carbons in length, hence, the "1923." By a careful theoretical evaluation of the equilibria obtained in the complex molybdenate/tungstate oxo anion system, they have worked to improve the environmental impact of this separation, in particular by recovery and reuse of primary amine 1923.

The authors write:

Compared to ion exchange, solvent extraction has a higher separation coefficient and production efficiency; however, the extraction law details and the separation mechanism of W(VI) and Mo(VI), such as the effects of concentration and pH, are still unclear. These issues make the removal of trace amounts of tungsten from molybdenum solution a bottleneck problem. Therefore, to achieve complete separation of tungsten from molybdate solution with different W(VI) concentrations and ultimately obtain ultra-high-purity molybdenum products, it is necessary to study the polymerization and extraction mechanism of tungsten and molybdenum systems. Furthermore, sustainable operations are increasingly needed to alleviate the environmental impact of such processes while maintaining efficient selectivity. To better understand the theoretical basis of the separation of tungsten from molybdate solution and increase the feasibility of practical separation processes, in the present study, the laws governing the variation in tungsten and molybdenum complexes at pH 6.0–8.0 in the H+–MoO42––WO42– system were investigated by thermodynamic analysis and experimental examination. Then, the extraction process and the mechanism of the complete separation of tungsten from ammonium molybdenum solution with primary amine N1923 were systematically studied, focusing on the effects of the concentration of tungsten and the equilibrium pH. Finally, a series of systematic methods for completely separating different grades (from trace to macro) of tungsten from ammonium molybdate solution were evaluated. During the process, primary amine N1923 was recycled, and using inexpensive mineral acids and ammonia, a highly effective and selective separation of tungsten from molybdenum was achieved. In addition, the waste regeneration liquor (waste acid), after enrichment, could be reused as the regeneration agent. The proposed process is cheap and sustainable and has a broad spectrum and high efficiency.


The authors, using theoretical and experimental tools propose to govern the extraction with highly accurate and precise control of the pH of the solutions.

The chemistry of the oxoions of molybdenum and tungsten is very complex below pH 8, and highly pH dependent and even more complex when the elements are both present. This is because the oxoions can form polymeric species with themselves and with each other.

Table 2 from the paper details some species found in the solutions:



The (p, q, r) values listed in the table reflect the following equation:



To get a feel for the complexity of this system, consider the equation for the total concentration of Tungsten, [W]T:



The equation for total Molybdenum is similar. Note that the terms with large exponents in these equations appear along side terms for the hydrogen ion concentration, which is obtained from the pH. This expression, and the corresponding expression for total Molybdenum demonstrate the sensitivity of concentrations to pH.

This graphic from the paper shows the pH dependency of this system's components:



The caption:

Figure 1. Distribution of species in aqueous systems as a function of pH at 25 °C: (a) in the H+–WO42– system (solid line, [W]T = 0.5 mol/L; dashed line, [W]T = 0.01 mol/L); (b) in the H+–MoO42– system (solid line, [Mo]T = 0.5 mol/L; dashed line, [Mo]T = 0.01 mol/L); (c) in the H+–MoO42––WO42– system (solid line, [W]T = 0.5 mol/L; dashed line, [Mo]T = 0.5 mol/L).


The next figure shows the species dependence on pH at differing initial concentrations of total tungsten:



The caption:

Figure 2. Diagram of the mole fractions of the main W(VI) and Mo(VI) species in a pH range of 6.0–9.0 in the system H+–MoO42––WO42– at 25 °C: (a) the scatter solid line denotes [W]T = 0.5 mol/L, and the dashed line denotes [Mo]T = 1.0 mol/L; (b) the scatter solid line denotes [W]T = 0.01 mol/L, and the dashed line denotes [Mo]T = 1.0 mol/L; (c) the scatter solid line denotes [W]T = 0.001 mol/L, and the dashed line denotes [Mo]T = 1.0 mol/L.


Next the separations as a function of extractant volumes between the aqueous and organic (primary amine N1932) layers:



The caption:

Figure 3. Experimental results for the equilibrium pH experiments: (a) 31.4 g/L W and 26.2 g/L Mo in the feed solution, 10% (V/V) N1923 as the extractant, and a 1/1 extraction O/A ratio (V/V); (b) 1.26 g/L W and 71.8 g/L Mo in the feed solution, 5% (V/V) N1923 as the extractant, and a 1/2 extraction O/A ratio (V/V); and (c) 20.65 mg/L W and 58.85 g/L Mo in the feed solution, 5% (V/V) N1923 as the extractant, and a 1/1 extraction O/A ratio (V/V).


The following graphic gives a breakdown of W/Mo extraction ratios as a function of pH using different extraction conditions:



The caption:

Figure 4. Extraction percentage and distribution of W and Mo species as a function of equilibrium pH: (a) 31.4 g/L W and 26.2 g/L Mo in the feed solution, 10% (V/V) N1923 as the extractant, and a 1/1 extraction O/A ratio (V/V); (b) 1.26 g/L W and 71.8 g/L Mo in the feed solution, 5% (V/V) N1923 as the extractant, and a 1/2 extraction O/A ratio (V/V); and (c) 20.65 mg/L W and 58.85 g/L Mo in the feed solution, 5% (V/V) N1923 as the extractant, and a 1/1 extraction O/A ratio (V/V).


All of this leads to a flow chart for the separation process:



The caption:

Figure 5. Flowchart of the sustainable process: (a) three-stage counter current extraction, 1-stage scrubbing and 2-stage stripping tests; (b) 3-stage counter current extraction and 2-stage stripping test; and (c) 2-stage counter current extraction and 1-stage stripping test.


This table shows the conditions and purity of molybdenum under these conditions obtained in these processes:



After much research in the 1950's, in the 1960's scientists and engineers at Los Alamos National Laboratory ran a nuclear reactor, the LAMPRE reactor, using liquid eutectic of plutonium iron metal as a fuel. The metal chosen to contain the plutonium containing the eutectic was tantalum. Tantalum is a conflict metal; it's use is problematic because of conditions of essentially human slavery where its ores occur in the largest amounts. I personally do not feel comfortable encouraging new uses for tantalum. Moreover, it is a fairly rare metal, subject to depletion.

Nevertheless, more than half a century latter, it occurs to me that the idea behind the LAMPRE reactor had much to recommend it. The rising availability of plutonium that has a wide distribution of isotopes, particularly in used MOX fuels, on an industrial scale, makes the idea even more attractive. This would be particularly true if humanity decides on nuclear weapons disarmament, not a likely outcome, since the value of wisdom is declining, not rising, but still an outcome for which we can hope. The use of plutonium with a large distribution of isotopes makes instantaneous denaturation of weapons grade plutonium possible.

The threat of climate change, is of course, worse than the threat of nuclear war, since climate change is observed and nuclear war is not. Climate change is a certainty; nuclear war a possibility. We can, and should, in a time of wisdom, lower the probability of nuclear war by converting weapons grade plutonium into reactor grade plutonium.

All of this relates to tungsten, because during research at Los Almos in the 1950's, it was discovered that tungsten had low solubility in liquid plutonium. The reason that tantalum was chosen over tungsten is that while tungsten has the highest melting point of all known metals, it is very difficult to machine. The machinability of tungsten can be greatly improved by addition of rhenium, but rhenium is a very expensive and very rare element.

However, technetium, a component of used nuclear fuel that is often regarded in what I regard as abysmal ignorance, as so called "nuclear waste" is an excellent substitute for rhenium in almost all applications.

The amount of technetium available in the 1950's was tiny; it is now available on a ton scale.

There are many different ways to contain liquid plutonium I expect, including many types of materials that were unknown in the 1950's or available at only a tiny scale, nanolayered ceramics, metalloceramics such as the MAX phases. It is also conceivable to explore peritectic systems to address the corrosive nature of liquid plutonium.

However machinable alloys of tungsten are an excellent default.

In "breed and burn" nuclear systems containing tungsten as a structural component - systems designed to operate for many decades without refueling - tungsten, which is relatively rare but certainly available in industrial quantities, particularly in mass efficient systems like nuclear reactors, will be slowly converted into the far more valuable metals rhenium, osmium and iridium. This of course, would be a good thing.

Tungsten alloyed with technetium, and placed under neutron irradiation for a fair portion of a century will also contain the metal ruthenium from the transmutation of technetium. Ruthenium is another valuable metal which is also a fission product. However in recovering the rhenium some residual technetium will remain, and therefore, the separation of rhenium and technetium, not observed in nature, may require similar study to that conducted here for molybdenum and tungsten here.

All of this is esoteric I know, and has little to do with politics, but it shows some interesting light that may someday pierce the current darkness in a better world.

I trust you will have an opportunity for some small pleasures in these dire times. I wish you peace.
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NNadir

(33,368 posts)
3. There will be a world beyond him. He's an ephemeral stain on the history of this country...
Sat May 16, 2020, 05:14 PM
May 2020

...and future generations will be more interested in recovering from him than in who or what passed through his withered mind.

 

jberryhill

(62,444 posts)
4. It's hard to know if the moly you get on the street is pure
Sat May 16, 2020, 05:24 PM
May 2020

I’ll bet half the stuff at raves isn’t even moly.

Say what you want about mushrooms, but if it stains blue when cut and has dark purple spores, it’s probably psilocybe cubensis. Or were you just going to take a whirl on that dodgy piece of blotter paper that ended up in the cellophane of your cigarette pack in the parking lot of a concert somewhere?

Some day I’ll have to post my long unpublished laboratory entries on home methods for the concentration of mescaline from purée of Echinopsis pachanoi, the lovely ornamental San Pedro cactus.

Although given the time required and the initial emesis which remains a characteristic of mescaline and related cactus alkaloids, I might just look into that moly stuff in the OP.

Backseat Driver

(4,333 posts)
5. Hah - when I was a kid I visited Colorado. We drove by
Sat May 16, 2020, 07:47 PM
May 2020

green forests surrounded by wire and scary signs indicating some sort of element was being obtained just past the forest. We pronounced it MOLLY-BE-DE-NUM, and laughed and laughed about this multi-syllable mystery, never ever before hearing about molybdenum. Five decades later, this is still quite a mysterious substance to me, and your post is quite technical and still so way beyond my ken of metallurgy. So glad that you are fascinated by it too so I won't have to figure out what this article means in and to my life, though I'd venture I'd appreciate it more if I did learn anything about molybdenum. Thanks for posting, I think!

NNadir

(33,368 posts)
7. It turns out that there's a ghost town in Colorado named "Tungsten."
Sun May 17, 2020, 09:10 AM
May 2020

Many times I write these posts that touch on the availability, of certain elements. I knew that there are US supplies of molybdenum, but I wondered why most of the world's tungsten is produced in China since the geochemistry of these two elements is so similar. I usually google and look among the links produced for the USGS.

When I did that this time, I came across this link: Tungsten Colorado

Canoe52

(2,944 posts)
6. Well you just made my high school level chemistry brain explode!
Sun May 17, 2020, 12:47 AM
May 2020

But thanks for sharing, always enjoy your posts!

eppur_se_muova

(36,227 posts)
8. Man, I hate finding tungsten in my molybdenum ... throws the flavor completely off.
Sun May 17, 2020, 05:42 PM
May 2020

Seriously, though, this is why I prefer organic chemistry. I like all those nice barriers of activation between reactant and product. Can you imagine doing a 20-step drug synthesis in which every step involved equilibration and separation at just the right pH, temperature, concentration, ionic strength, and spectator ions ?

NNadir

(33,368 posts)
11. Well, I'm an old man, but I note that these kids today...
Sun May 17, 2020, 07:43 PM
May 2020

...are moving ever closer to continuous processes. This is a huge movement in chemical engineering classes taught in many academic institutions and in fact, in industrial chemistry. I went to a real nice organic process meeting at Rutgers back in the pre-covid days, where almost all the talks were on precisely these topics. It is somewhat amazing the number of steps that can be condensed with very careful reaction controls in line. The advantage is cost and time and, I think, greatly reduced waste.

Organic chemistry is not what it was when I was a kid; but frankly, when I was in the lab, it was possible to develop habits that were kind of sloppy. I worked on many reactions that had to be very tightly controlled, and we got to understand the nature of these controls without robots and without in line analysis. Things took a very long time, and there was a lot of waste.

When I was a kid I was involved in the delivery of ton scale intermediates for a highly potent anti-neoplastic. It was a 25 step process.
My company was making 3 ton quantities in France, shipping to the United States for additional steps, whereupon an intermediate a few steps from the penultimate intermediate was shipped to Ireland. In Ireland, the yield was 5 kilograms.

It was one of those deals where the anti-neoplastic was basically available from a very rare and slow growing tree in a politically problematic area and treating the patients with the naturally obtained product would have made the tree extinct.

The drug has been commercial for many years, and hopefully saved, or extended, many lives. It took, however, many years to develop the process, and with modern technology, it may have been much faster.

In that lecture series I attended, I saw some very complex syntheses done completely without the isolation of intermediates. I was beautiful and scary at the same time.

JustFiveMoreMinutes

(2,133 posts)
9. I am so sorry.. but I did not see any explanation of how the extraction method impacted...
Sun May 17, 2020, 05:58 PM
May 2020

.. either your results or completely ignored to go along with the outcomes you presented.

You can't say the extraction method was completely isolated and had no actual impact.

NNadir

(33,368 posts)
10. These are not my results; I am merely commenting on the original paper.
Sun May 17, 2020, 07:29 PM
May 2020

I'm not sure entirely what you're asking, and I cannot, of course, simply post the complete paper, but can only excerpt it and post the pictures.

That said, I think figures 3, 4, and 5 and their captions, as well as table 3, give some insight to process utilized, which is a counter current extraction with the only variable being the relative volumes between the aqueous solutions with a tightly controlled pH and volume ratios between the aqueous phase and the organic phase (primary amine N1923). In these graphics this is referred to as O/A ratios, i.e. organic/aqueous ratios.

Having access to the full paper as I do, I believe the authors were fairly clear on this point. Their work strikes me as exceedingly precise, and it was a pleasure to read.

Regrettably most academic libraries are closed for Covid-19; however, when they reopen, if you are interested in the details you may travel to one to read the full paper, or if you are in an academic setting, use electronic library capabilities for access to the full paper.

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