Effect of agglomeration on nickel extraction and sulfur release from ultramafic nickel sulfide concentrates

•Solid-state thermal upgrading extracts over 90 % of nickel.•Pelletizing and briquetting show comparable ability to extract nickel.•Extraction of nickel is affected by hydrogen atmosphere.•Pellets release a greater amount of sulfur to off gas than briquettes. The authors have proposed an upgrading process for ultramafic nickel concentrates which is comprised of agglomeration of iron powders and concentrates followed by a two-stage thermal treatment at temperatures lower than the typical smelting conditions. This allowed the nickel to migrate from the sulfide phase to metallic ferro-nickel particles, making solid-to-solid contact important. Agglomeration by pelletizing and briquetting has been studied. Moisture added to the pellets caused oxidization of the metallic iron reducing the nickel extraction to ferronickel in the thermal upgrading process. Drying green pellets under vacuum slowed oxidation and resulted in nickel extractions greater than 90 % during thermal upgrading when an H2-Ar atmosphere was employed. The presence of H2 caused about 18 % of sulfur in the concentrate to be released to the off gas when treating pellets, around 2.5 times larger than that released from briquettes treated similarly.