•High extraction efficiency for unusual high molar concentrations of Co/Ni.•Novel process windows: dimensionless numbers determined by space minerals processing.•Real meteorite material leached and microfluidically extracted.•First step towards substantial water savings in space mining. This research proposes a pathway for the last step of the asteroid mining process: the purification of the adjacent metals, cobalt and nickel, in the frame of in-situ resource utilization (ISRU) in space. Major technological and economic challenges will need to be overcome, and one main issue to be tackled is the reduction of water usage in this process. Therefore, the leached metal solutions are expected to contain ultra-high metal concentrations, up to 10 mol/l. These solutions will have challenging thermodynamic properties (increased density, viscosity and interfacial tension). As a result, an analysis of dimensionless numbers for fluidics and mass transport was made, showing that some of these are favourable under the constraints of accessible microfluidic operations. Experiments were performed with advanced microfluidic reactors (a coiled-flow inverter (CFI) and an industrial re-entrance flow reactor from Corning®) at high metal concentrations and high nickel to cobalt ratios (3:0.3 mol/l Ni:Co). Using Cyanex 272 as a selective extractant for cobalt, extraction efficiencies of 60% with high separation factors (>1000) were reached in just one extraction stage. The CFI showed high extraction efficiency for low fluid velocities and a residence time of 60 s. For the Corning® reactor, high fluid velocities or the use of many modules (>3) are needed to obtain an emulsion, resulting in high extraction efficiencies at a very short residence time of 13 s. The comparison between the CFI and the Corning® reactor shows that they share the best operation point (at 120 ml/h), but the Corning® reactor performs better at higher flow rates and thus can leverage higher productivity. However, the CFI is easier to operate and has a much lower pressure drop, resulting in low energy input. Finally, an iron meteorite sample was leached and efficiently extracted in both microfluidic reactors.