A Fundamental Metric for Metal Recycling Applied to Coated Magnesium

Abstract

A fundamental metric for the assessment of the recyclability and, hence, the sustainability of coated magnesium scrap is presented; this metric combines kinetics and thermodynamics. The recycling process, consisting of thermal decoating and remelting, was studied by thermogravimetry and differential thermal analysis (TG/DTA) experiments and thermodynamic simulations. Decoating phenomena are interpreted using kinetic analysis, applying existing reaction models. The derived kinetic model parameters ln A and E a /(RT p ) are used to characterize the decoating process. The impact of inorganic coating components on remelting is quantified using exergy. Oxidation and entrapment losses, quality losses, and material resource depletion caused by the inorganic components are expressed in exergy units and combined into the single parameter R. Based on the results, the coating characteristics favorable for recycling are derived. The obtained metric is a three-dimensional (3-D) combination of ln A, E a /(RT p ), and R, which represent the decoating velocity, the ease of decoating, and the impact of coating materials on the remelting process, respectively. The metric, therefore, directly links coating characteristics, coating design, and product design with process technology and recyclability, enabling the ranking of coating alternatives in terms of their respective recyclability. Therefore, the key idea of this article is to use fundamental metallurgical theory to express the recyclability of postconsumer scrap in a unique combination of parameters. This should pave the way for ranking the sustainability of different materials.

Recycling light metals: Optimal thermal de-coating

“…These kinetics values, such as activation energy E a and frequency factor A, determine the characteristics of de-coating and in this way the coating properties can be linked to process design, as shown previously. 12 The kinetics are also dependent upon the atmosphere, with more oxygen initiating the scission and combustion regimes earlier, as shown in Table I. The small deviations lie within the uncertainty.…”

“…The residue of four different magnesium objects is discussed elsewhere. 12 These particles are common pigments, such as TiO 2 and Fe 2 O 3 , and fillers such as SiO 2 .…”

“…The natural logarithm of A is a measure of the speed of the reaction. 12 When coatings are heated it is obvious that a number of chemical reactions occur. However, one or a few reactions or mechanisms may be rate-limiting "bottlenecks."…”

Exergy-Based Efficiency Analysis of Pyrometallurgical Processes

“…Therefore the methodology needs some modifications before it can be used for exergy calculations on pyrometallurgical streams that typically have a complex composition. Only in the work of Reuter and co-workers on recyclability of metals [22][23][24] we find an adapted approach for exergy calculations on metallurgical streams, although this is not indicated explicitly. They focus on the separation and contamination of metals during recycling processes and do not consider any high temperature steps in detail.…”

“…analyses solely relying on compound based stream descriptions are not optimal and should be avoided, at least for most pyrometallurgical applications. When looking at the previously published exergy analyses mentioned in the introduction, only Reuter and co-workers took the formation of different solution phases explicitly into account [22][23][24] . Therefore we suspect that the results of most of the other analyses [11][12][13][14][15][16][17][18][19][20][21] can be improved.…”