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 H₂/N₂ gas mixtures, and leaching tests on the reduced samples have been undertaken. Phase and microstructural changes have been characterised using X-ray 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.