Optimum Treatment Time for Solid-State Extraction of Nickel from Nickel Sulfide Concentrates at 1073 K FANMAO WANG, SAM MARCUSON, LEILI TAFAGHODI KHAJAVI, and MANSOOR BARATI Solid-state thermal treatment of Ni sulfide concentrates in an inert or reducing atmosphere, and the presence of metallic Fe is proposed as a feasible route to produce ferronickel (FeNi) alloy while retaining S in iron sulfides. The present work investigated the effects of temperature and amount of Fe addition via a thermodynamic analysis, giving a suitable temperature of 973 K to 1173 K and metallic Fe to Ni concentrates mass ratio of 0.5 to 2. The minimum time required for Ni extraction at 1073 K was investigated via thermal treatment experiments of various durations, and it was determined to be 30 minutes. Under the tested experimental conditions, average Ni concentration in the resulting sulfides and the generated FeNi was found to be 0.5 ± 0.2 mass pct and 16 to 18 mass pct, respectively, and in good agreement with the thermodynamic predictions. The maximum Ni recovery to FeNi was approximately 97 pct and the characteristic particle sizes d 10 and d 80 of FeNi were 14 and 45 lm, respectively. During 360 minutes of the thermal treatment, only 0.7 mass pct of S in the concentrates was released to the off gas as SO 2 . https://doi.org/10.1007/s11663-020-01960-3 Ó The Minerals, Metals & Materials Society and ASM International 2020 I. INTRODUCTION NICKEL is a critical alloying element in steels, [1,2] other corrosion-resistant alloys, [3] and batteries. [4] Approximately, 60 pct of the annual Ni production worldwide is consumed in the stainless steel industry. [5,6] The conventional route to extract Ni from Ni sulfide concentrates involves smelting and refining to remove the Fe and S associated with Ni. During smelting, sulfur in the concentrate is oxidized to SO 2 and reports to the off gas, while Fe is oxidized to FeO and is slagged off. When SO 2 concentration in the off gas is 10 to 12 vol pct, it can be effectively captured for sulfuric acid production, [7] while more dilute gases are often vented to the atmosphere. Sulfur dioxide mitigation in Ni smelters constitutes significant operating and capital costs, sometimes greater than the smelting cost itself. Further, there are considerable environmental hazards and penalties associated with any SO 2 emitted to the atmosphere. As a result, handling SO 2 emissions from the smelter remains a challenge or a cost driver for the Ni industry. Through laboratory experiments, the concepts of selective oxidation-sulfation or oxidation-chlorination roasting followed by water leaching has been proposed as an alternative route for the conventional smelting process. [8À12] Yu et al. reported that oxidation-sulfation of nickel sulfide concentrate at 973 K for 150 minutes, with the addition of 10 mass pct Na 2 SO 4 results in recovering 79, 91, and 95 pct of Ni, Cu, and Co, respectively. [11,12] Mu et al. found that under the optimum conditions of temperature (448 K), FeCl 3 ÆH 2 O addition (50 mass pct), and time (120 minutes), the maximum recoveries of Ni and Cu were 92 and 89 pct after water leaching. [9] These alternate processes have to separate Ni and Cu from the leach solution, adding an electro-winning step to the process flowsheet. Further- more, SO 2 and SO 3 gases are still generated during the roasting step, and effluent treatment is a must. The presence of oxygen either in the conventional smelting or the aforementioned selective roasting inevitably leads to the generation of SO 2 . The concept of an oxygen-free thermal treatment process for the recovery of Ni from pyrrhotite tailings was put forward and tested by Sridhar et al. [13] In this method, the Fe/S ratio of the sulfide is increased by adding Fe or removing S, leading to precipitation of ferronickel (FeNi). Recently, this method was further developed by Liu et al. [14] and Yu et al. [15] ; they confirmed that in FANMAO WANG, SAM MARCUSON, and MANSOOR BARATI are with the Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario M5S 3E4, Canada. LEILI TAFAGHODI KHAJAVI is with the Department of Materials Engineering, University of British Columbia, 309-6350 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada. Contact e-mail: fanmao.wang@mail.utoronto.ca Manuscript submitted on May 28, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS B