This Review provides an overview of valuable metals, the supply of which has been classified as critical for Europe. Starting with a description of the current state of the art, novel approaches for their recovery from primary resources are presented as well as recycling processes. The focus lies on developments since 2005. Chemistry strategies which are used in metal recovery are summarized on the basis of the individual types of deposit and mineral. In addition, the economic importance as well as utilization of the metals is outlined.
In the field of industrial biocatalysis, formate dehydrogenase (FDH) is well established, in particular for its broad application in cofactor regeneration. Further applications have been limited by the enzyme's narrow range of substrates. These restrictions have been overcome now by the finding, that the enzyme is capable of selectively cleaving formic acid esters to the respective alcohol. Five homologous alkyl formates and phenyl formate as an aromatic ester were converted quantitatively by FDH from Candida boidinii in a batch reaction within 3 to 5 h. The substrates were turned irreversibly into carbon dioxide and the respective alcohol through hydride abstraction from the formyl group with full conversion. The mechanism shows parallels to hydrolysis reactions of the A(AC)1-type. K(M)-values and reactions rates of the tested formic acid esters display a tendency to higher conversion rates with increasing chain length. FDH emerged to be a superior deformylation catalyst compared to hydrolases as well as classical catalysts, as was shown by the selective deformylation of 1-acetoxy-4-formoxy butane (92%) and 1,3-bis(3-formoxypropyl)tetramethyldisiloxane. In particular its capability to distinguish between formic acid esters and non-formic acid esters renders the method particularly suitable for protective group chemistry. Furthermore the completeness of deformylation allows for converting substrates highly incompatible with aqueous media like siloxanes within a few hours.
Durch die Entsorgung quecksilberhaltiger Leuchtstoffe werden dem Wertstoffkreislauf jedes Jahr große Mengen an Seltenen Erden entzogen. Zur Rückgewinnung dieser Wertstoffe wurde die Feststoffchlorierung mit NH 4 Cl untersucht. Dies ist eine Aufschlussmethode abseits nasschemischer oder pyrometallurgischer Verfahren. Die betrachteten, teils Hg-haltigen Leuchtstoffmischungen enthielten neben La, Ce, Gd und Tb mit geringeren Elementgehalten vor allem Y und Eu. Durch NH 4 Cl-Zugabe und Erhitzen bis zur Zersetzungstemperatur des NH 4 Cl wurden Y und Eu in einer Sublimationsapparatur mit hohen Ausbeuten selektiv aufgeschlossen. Die Abhängigkeit des Aufschlussgrades und der Selektivität vom NH 4 Cl/ Leuchtstoffmischung-Verhältnis und der Temperatur wurden systematisch untersucht.Solid state chlorination with NH 4 Cl comprises a method for rare earth recycling apart from pyro-or hydrometallurgical strategies. The examined partially Hg-containing fluorescent lamp scraps are rich in rare earths like La, Ce, Tb and Gd, but especially in Y and Eu. By mixing with NH 4 Cl and heating up to NH 4 Cl decomposition temperature in a sublimation reactor, Y and Eu could be transferred selectively into their respective metal chlorides with high yields. The yield and selectivity depend on temperature and the ratio of NH 4 Cl to fluorescent lamp scraps, which were varied systematically.
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