Exergy-Based Efficiency Analysis of Pyrometallurgical Processes

Exergy-based efficiency analysis provides a powerful tool for optimizing industrial processes. In this article, the use of this technique for pyrometallurgical applications is explored in four steps. First, the exergy concept is introduced, the outline of exergy calculations is presented, and the role of a reference state is discussed. Second, it is shown that an unambiguous exergy calculation for pyrometallurgical streams with a complex, unknown phase composition is not straightforward. Hence, a practical methodology is proposed in which a suitable phase-based stream description is estimated prior to the actual exergy calculation. For this, the equilibrium phase composition is calculated, whereas all known stream properties are incorporated as boundary conditions. Third, the proposed methodology is validated by recalculating literature results. This reveals significant deviations for exergy values of the same pyrometallurgical streams. Our results are probably more accurate because of the incorporation of additional phase-related information. And fourth, a full analysis of a zinc-recycling process is presented. In a base case scenario, the total exergetic efficiency turns out to be only 1.2 pct. Based on this result, different process modifications are suggested and evaluated quantitatively. We find that significant efficiency gains are possible.