Apparently there is a process to recover REE (Rare Earth Metals / Rare Eath Elements) from Magnesium Dross as a possible added benefit.  Perhaps this could be something worthwhile to investigate in your efforts to recover magnesium from the dross residue (thinking something akin to the exisiting relationship / JV with the Aluminum Dross high value chemicals recovery)?

Recycling of black dross containing rare earths originating from melting and recycling of magnesium alloys - ScienceDirect