  The increasing importance of recycling makes the recovery of valuable
elements from slags interesting, e.g., by the concept of engineered artificial
minerals (EnAMs). In this concept, it is aimed for the formation of EnAMs,
meaning phase(s) with a high content of the to-be-recovered element(s) from
slags of pyrometallurgical recycling processes. For this, understanding the
phase constitution of the slag systems is of high importance. The system
Mg-Mn-Al-O is a metal oxide slag subsystem from Li-ion battery recycling, that
is critical for the formation of spinel phases, which are competing phases to
the possible Li-containing EnAM phase LiAlO2. Here, the phase constitution was
investigated using a thin film materials library that covers the composition
space (Mg14-69Mn11-38Al14-74)Ox. By means of high-throughput energy-dispersive
X-ray spectroscopy and X-ray diffraction, the formation of the spinel solid
solution phase was confirmed for a wide composition space. Increasing
preferential orientation of the spinel solid solution along (400) with
increasing Mg content was identified. X-ray photoelectron spectroscopy was used
to measure the near-surface composition of selected areas of the materials
library, and detailed peak fitting of the Mn 2p3/2 region revealed the Mn
oxidation state to be a mixture of Mn2+ and Mn3+. For one measurement area of
the materials library containing equal atomic amounts of Mg, Mn and Al,
transmission electron microscopy showed that the approximately 420 nm-thick
film consists of columnar spinel grains with Mg, Mn and Al being evenly
distributed. Based on these results, we suggest that the shown high likelihood
of spinel formation in slags might be influenced by controlling the Mn
oxidation state to enable the formation of desirable EnAM phases.
