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This could do the mixing in easier way. You need a printer with XY moving printbed and Z-axis capable for high load.
At ebay there is iron powder and Fe2O3 powder with fine quality. I feel, mixing in this extruder will be possible but not be a good result. You have to look for metal-filled pellets or masterbatches. This exists for mass-production but it may be hard to buy 1kg. Ask for samples. Making this running would become a research work.
Filaments with metal-filling are available.

Pellets Granulate Extruder for 3d Print
by faggahz