**5. Conclusions**

A comparison of two computational approaches, CFD and CFD–DEM, was carried out for the matte settling phenomena in settler geometries. The results of the small-scale model of matte droplets settling through a slag layer suggest that the settling flow pattern is channeled or funneled as a central trail of coalescing droplets. For a sounder conclusion, these results still have to be validated by a physical model. Nevertheless, as a conclusion, the applicability of the coupled CFD–DEM software equal to CFD software for the liquid–liquid system used in this study can be confirmed.

One conclusion regarding the settling pattern of matte droplets in the industrial scale settler geometry, is that it is plausible that the droplets coalesce and form centralized funnels instead of settling uniformly across the whole inlet area. This phenomenon is beneficial for the efficient separation of the slag and matte phases, and thus, for the recovery of the metal. Consistently with industrial reality, the smallest droplets were very slow to settle, and consequently they require a very long time or a very thin slag layer to reach the matte layer before the slag leaves the settler through the tapping hole. One option could be to operate with continuous tapping and a thin slag layer.

The studied case clearly indicated that the current computing power is still the limiting step in computer modeling of a full-scale industrial settler with realistic droplet coalescence calculation. Although there are some similarities in the settling pattern in the small- and full-scale models, they are too different for conclusions to be made based on the small-scale model. This poses a challenge for the validation of the full-scale model results as there are no possibilities for direct observations of the flow phenomena in the molten slag layer inside the settler. So far, the only option is to compare copper losses from the CFD model with industrial data [10].

**Author Contributions:** Conceptualization, all; methodology, J.-P.J. and N.A.K.; analysis, all; investigation, J.-P.J. and N.A.K.; resources, A.J.; writing the manuscript, J.-P.J. and N.A.K.; review and editing, A.J.; supervision, A.J.; funding acquisition, A.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** The funding from the Steel and Metal Producers' Fund in Finland, and from the Aalto University School of Chemical Engineering is greatly acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
