Ionic Liquid Based Separation of Precious Metals from Spent Automotive Catalytic Converters.
The paper I'll discuss in this post is this one: Application of a Novel Phosphonium-Based Ionic Liquid to the Separation of Platinum Group Metals from Automobile Catalyst Leach Liquor (Goto et al, Ind. Eng. Chem. Res., 2019, 58 (9), pp 3845–3852).
Certain elements are considered to be in critical supply. How dire this state of affairs for any particular element is not entirely clear, but it is very clear that most of the world's best ores for some metals have already been mined.
Many periodic tables showing the elements of highest concern are available; my personal favorite is the featuring the logo the scientific organization for which I am a long time proud member, the American Chemical Society. Here it is:
There are some variances in this data, and some means of address for some of the shortages. Some can be recovered from extreme low grade ores. Uranium, for example, can, at a higher price, but not one that precludes its use, recovered from the extremely low grade ore seawater.
I note that the period at most risk, in terms of the percentage of elements at risk is the 5th period. Fourteen of the 18 elements face risks to their supply. One of the elements, technetium, does not occur naturally; all of its isotopes are radioactive. I note that because of the lanthanide contraction, a quantum mechanical physical chemical effect, technetium is a suitable replacement for rhenium.
Rhenium does not appear in this particular version of the periodic table as "endangered" but probably should; it is an extremely rare element, so rare that it was the last non-radioactive natural element to be discovered, in 1923, by Ida Noddack and her husband. The chief use for rhenium is a component of "superalloys," (generally) nickel based alloys that exhibit extreme strength at high temperatures, essential to the operation of jet aircraft and high temperature combined cycle power plants.
All of the elements in period 5 are fission products found in used nuclear fuels, where they are found at various concentrations. Some of these contain significant quantities of radioactive isotopes; the degree to which this is true has to do with the timing of their isolation. It is possible to obtain non-radioactive palladium from used nuclear fuel if one isolates ruthenium promptly after irradiation. In this case the radioactive isotope Ru-106 will decay to non-radioactive (isotopically pure) palladium-106, with a half life of slightly over one year. After 20 years, the ruthenium remaining, a mixture of isotopes 100, 101, 102, 104, and 105 will be essentially non-radioactive and available for use.
Because of the high energy density of nuclear fuels, it is, however, except in special cases, unlikely that fission products can ever approximate major supplies for all of these elements, depending on their demand.
It has been argued that the supplies of the element rhodium in used nuclear fuel will be greater than the supply of the metal in ores within a few decades. (Srinivasan et al, Electrochimica Acta 53 (2008) 2794–2801)
That paper was about a class of solvents that are capturing huge attention, ionic liquids, which are salts that are liquid at temperatures approaching room temperature. In general, with some exceptions, they have organic cations and anions.
So is the paper cited at the outset of this post, although the paper just cited was about nuclear fuels and the one cited at the outset was about recovering metals from automotive catalysts, that is recycling.
In the particular case here, the ionic liquids are tetraalkyl phosphonium based salts.
From the introductory paragraphs:
The recycling and recovery of spent automobile catalysts has become a growing secondary source of PGMs, and various methods have been developed for this purpose, such as hydrometallurgical or pyrometallurgical processes.4 However, the physical and chemical properties of these metals are very similar and thus they are difficult to separate. Traditional PGM recovery methods involve physical treatments, including acid dissolution, chemical separation and refining,6,7 but have several disadvantages, such as poor selectivity, a high degree of complexity, and numerous recycling streams and refining steps. In contrast, hydrometallurgical leaching followed by solvent extraction offers greater selectivity, a scrubbing step that increases the product purity, and complete removal of metals via multistage extraction steps.8
Ionic liquids (ILs) are salts composed of organic cations and inorganic anions that are liquid at room temperature, and these substances have potential applications to metal extraction due to their unique properties. ILs also tend to have negligible vapor pressure relative to more common organic solvents and their properties can be tuned by varying the cation and anion.10 Hydrophobic ILs are also able to solvate species with a net charge, such as metal complexes,11 and thus can be suitable alternatives to organic solvents in extractions...
For this work, automotive catalysts were crushed, milled with alumina balls, and dissolved in refluxing aqueous hydrochloric acid.
The ionic liquid explored here has the following structure:
The caption:
It was used to extract the "leach liquor," the hydrochloric acid solution. Essentially all of the elements in the next graphic are present in solution at this point.
The caption:
This shows the effectiveness of the extraction as a function of the concentration of hydrochloric acid used for dissolution.
The next graphic shows the effect of contact time. The speed of contact time can be shortened by multiple extractions over shorter periods, but as this is research, and not production, it is useful to understand this effect purely in terms of time:
The caption:
What is notable here is that palladium has been effectively separated from all the elements except iron and zirconium.
The removal of iron and zirconium from the ionic liquid solution is accomplished by "scrubbing" the solvent with aqueous solutions of salts:
The caption:
The spectra show the separation.
Figure 5. UV−visible spectra of (A) iron(III)−chlorocomplexes in P8,8,8,12Cl before and after scrubbing with Na2SO3. Conditions: [HCl]aq = 5 mol L^(−1), Vaq/VIL = 2. Spectra were acquired in ethanol. (B) Iron(II)−phenanthroline complexes from scrubbing solution. Conditions: 1,10- phenanthroline = 0.01 mol L^(−1).
The palladium is stripped from the ionic liquid by extraction with an aqueous solution of thiourea:
The caption:
By adjusting the HCl concentration, utilizing the effect shown in figure two, where rhodium is more soluble in the ionic liquid if the acid concentration is 1M, it is possible to recover the rhodium:
The caption:
Overall, the flow chart for the process is here:
Figure 8. Flowchart summarizing the separation and recovery of Pd(II) and Rh(III) from an automobile catalyst leach liquor.
The caption:
All of the above was based on a simulated acid leachate.
The data below shows the results using real automotive catalysts:
The caption:
Hydrophobic ionic liquids are extremely interesting me, particularly as I am interested in the pyroprocessing of used nuclear fuels followed by electrorefining, an area in which ionic liquids hold great potential.
I note that with the exception of magnesium, aluminum and iron, all of the elements found in these automotive catalysts are also found extensively in used nuclear fuels. Aluminum is a constituent of some legacy "nuclear wastes" such as those found at the Hanford reservation, and my own preference for future nuclear fuels might well include plutonium/iron eutectic liquid fuels, which, in use, will accumulate these fission products. Magnesium is found in historic "Magnox" fuels utilized in British nuclear reactors.
I personally would prefer a world in which, to the extent that self propelled vehicles are acceptable - they are not acceptable tools of mass distribution in my view inasmuch as no bandaids can make them sustainable - they would not actually need much in the way of catalysts, something that is possible with certain kinds of fuels, in particular simple liquifiable gases like dimethyl ether.
However, it would be useful for our greatly screwed children, grandchildren and great^(n) grandchildren when they are forced to pick through our garbage, as we have left them very little, to recover valuable elements from our dead automotive catalysts.
It's an interesting paper.
Have a nice day tomorrow.
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