Investigation of the reduction roasting of saprolite ores in the Caron process: microstructure evolution and phase transformations

Abstract

Fundamental studies have been undertaken to determine the microstructural and phase transformations occurring during the reduction roasting of saprolite ores. Laboratory studies have been undertaken to simulate the conditions occurring during the reduction roast step of the Caron Process. Selected serpentine samples have been treated at temperatures between 500°C and 800°C in H 2 /N 2 gas mixtures, and leaching tests on the reduced samples have been undertaken. Phase and microstructural changes have been characterised using Xray Powder Diffraction (XRD), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) techniques.A series of complex physical, microstructure and phase changes has been shown to occur during reduction roasting involving: (i) the dehydration of serpentine, (ii) the formation of a high-silica amorphous phase, (iii) the formation of Ni-Fe nanoparticles, and (iv) the nucleation, growth and recrystallisation of the forsterite (olivine) phase. The principal mechanism of removal of nickel from the reduced ore has been shown to occur through the selective leaching of the Ni-Fe alloy nano-particles from the surfaces in the product oxide.

Investigation of the reduction roasting of saprolite ores in the Caron process: effect of sulphur addition

“…It has been observed previously (Chen et al 2019a) that during the reduction roasting of saprolite, the ore undergoes a series of complex transformations including dehydroxylation, reduction of NiO to Ni-Fe alloy, sintering of dehydroxylated material, and the formation and recrystallisation of olivine. Each of these processes has been found to influence the final Ni recovery from the reduced ore.…”

“…It can be clearly seen that these particles are enriched in sulfur and nickel, indicating the formation of a nickel sulfide phase. Unlike the Ni-Fe alloy observed when reduction was carried out without sulphur (Chen et al 2019a), these sulphide phases consist of predominantly nickel and sulphur with relatively little iron. Whilst no alloy particles are observed following reduction at temperatures up to 700°C, it has been found that alloy is formed at high reduction temperatures, e.g.…”

“…It has been shown in (Chen et al 2019a(Chen et al , 2019b) that the low nickel recovery obtained at high reduction temperatures is principally due to the encapsulation of reduced Ni-Fe alloy in the sintered olivine phase; this alloy then becomes inaccessible to the leach liquor. In present study, the nickel recovery from reduced ore with 1 wt% S addition was also found to decrease at high reduction temperature; recovery is, however, found to be significantly higher than in samples without S addition.…”

“…Despite these tests, there remains a lack of fundamental understanding of the effect of S on the saprolite phase during reduction. The microstructural changes and principal elemental processes taking place during reduction roasting of saprolite have been clearly identified in recent studies by Chen et al (2019aChen et al ( , 2019b. The effects of thermal history, temperature and time on the rates and extents of these processes, and on nickel recoveries have also been investigated.…”

Investigation of the reduction roasting of saprolite ores in the Caron process: reaction mechanisms and reduction kinetics

“…At this point it is recalled that in (Chen et al 2018) it was shown, using transmission electron microscopy (TEM), that after direct reduction of the ore at 800°C…”