<p>The depletion of massive sulfide reserves has shifted attention to low-grade ultramafic ores, which are abundant, underutilized, and capable of meeting rising global Ni demand. However, their high MgO content hinders flotation through slime coating, increases acid consumption during leaching, and elevates slag liquidus temperatures during smelting. To address these challenges, this study investigates a thermal upgrading approach in which Fe is added to an ultramafic concentrate and heated to ~ 920 °C. During treatment, Fe captures sulfur from (Fe, Ni)₉S₈, forming nonmagnetic FeS, while excess Fe alloys with Ni, forming a magnetic FeNi. Operating under relatively mild conditions (&lt; 950 °C), the process mitigates SO₂(g) and enables magnetic separation. Because magnetic separation efficiency depends on particle size, FeNi growth was examined under varying thermal treatment conditions. FeNi particle size increased modestly from 10.9 to 21.0 <i>µ</i>m between 500 °C and 850 °C. In contrast, heating to 920 °C resulted in a nearly tenfold increase to ~ 118.9 <i>µ</i>m, indicating a transition from slow, solid-state, diffusion-controlled growth to a liquid-film-assisted transport regime following liquid-phase formation at ~ 900 °C. The liquid FeS formed a continuous network, enabling FeNi particle migration and coalescence. Other parameters, including holding time, hydrogen concentration, and briquetting pressure, had secondary effects.</p> Graphical Abstract <p></p>