Experimental and computational approaches to study the chlorination mechanism of pentlandite with ammonium chloride

Pentlandite (Fe_4.5Ni_4.5S₈) is the primary source for the metallurgical production of nickel worldwide, however it usually coexists with copper sulfide in nature. To develop an efficient and green process for the separation and extraction of valuable metals from the nickel sulfide concentrate, herein we conducted experimental studies and density functional theory (DFT) calculations to elucidate the chlorination mechanism of pentlandite using ammonium chloride (NH₄Cl). First, low-temperature chlorination roasting experiments with NH₄Cl were performed in which pentlandite was successfully converted into the corresponding metal chlorides (FeCl₂ and NiCl₂). Then, the chlorination product was analyzed via energy dispersive spectrometry to reveal the elemental distribution at the cross-section. Results reveal that Fe atoms in pentlandite underwent preferential chlorination to form a chloride layer, whereas Ni atoms remained at the center of the grain. Furthermore, density functional theory calculations were performed to investigate the chlorination mechanism of pentlandite by exploring two possible pathways, involving the adsorption of oxygen (O₂), ammonium chloride (NH₄Cl) and chlorine (Cl₂) on both the (001) and (010) surfaces of pentlandite. Considering that the chlorination of pentlandite was achieved in air atmosphere, we first consider the direct chlorination of pentlandite by NH₄Cl in the presence of oxygen. Dissociative oxygen adsorption was found to promote the chlorination process by providing oxygen sites for the dissociation of HCl, which is decomposed from NH₄Cl, eventually leading to the formation of H₂O and FeCl₂ species. Alternatively, the reaction between pentlandite and Cl₂ was proved to be feasible thermodynamically.