Nickel recovery from serpentinitic waste materials: the potential of awaruite

The element nickel is included on the critical raw materials (CRM) list as strategic raw materials, in line with the EU Critical Raw Materials Act. It is used for making stainless steel (about two-thirds of production), and in high-performance superalloys for demanding environments like jet engines. It's also critical for electric vehicle (EV) batteries, improving energy density, and for electroplating, magnets, coins, and chemical catalysts. Nickel is mainly extracted from sulphides ores (e.g. pyrrhotite and pentlandite) or from the silicate garnierite (lateritic ore), but in recent times, the mineral awaruite, a native nickel–iron alloy Ni_2-3Fe, has gained considerable attention. Awaruite has unique characteristics compared to other nickel minerals: it does not contain sulfur, it has higher magnetic susceptibility than magnetite, and magnetic and gravity separation are feasible methods of awaruite concentration. The presence of this Ni-Fe alloy has been reported in ultramafic rocks (e.g. serpentinites) that underwent specific P-T-X conditions in prograde metamorphism, in strongly reducing conditions. For example, awaruite is an accessory mineral of the Valmalenco serpentinite (central Alps, northern Italy), a stone material used as dimension stone and for roof slabs, with different textures (massive and schistose) and color shades. The extraction and processing of serpentinite generate huge volumes of waste in the form of shapeless blocks, fragments and cutting sludge, with percentages ranging from 35 to 50 per cent of the initial volume. The waste materials were characterized from a chemical, mineralogical and petrographic point of view using ICP-OES, ICP-MS, XRPD, SEM-EDS and WDS, to assess their characteristics and potential reuse in various sectors. The whole-rock Ni content is high, ranging from 1400 to 2400 ppm, and Ni is present in trace amounts in silicates (e.g. olivine, serpentine), in the form of sulphides (pyrrhotite, pentlandite) and awaruite, in grains of approximately 10-30 µm. This Fe-Ni alloy is widespread in all commercial varieties of serpentine and represents the main Ni phase in these rocks. Among the various waste materials, the most promising is processing sludge, which has a fine grain size (less than 50 µm). In this type of waste material, the awaruite grains are already completely freed from the silicate matrix and can therefore be easily separated using magnetic and gravimetric techniques, without further grinding. The actual potential for recovering Ni from mining waste will be assessed through separation tests, combining this with the recovery of other minerals of industrial interest (e.g. olivine, serpentine) to optimize the recovery process.