Effect of mineralogy on reducibility of calcined nickel saprolite ore by hydrogen

The aim of the work reported here was to examine the effect of mineralogy on reducibility of three different nickel saprolite ores originating from Brazil (Mirabela deposit) and from Colombia (Cerro Matoso S.A. deposit) as well as find the optimal reduction conditions for the ores. The reduction experiments were carried out in a thermobalance furnace at constant temperatures 600, 750 and 900°C in H₂/N₂ (72%/28%) gas mixture, followed with scanning electron microscopy (SEM–EDS) and chemical analyses. The results clearly indicate that the reducibility of the ores depends on the initial mineralogy of the ore and on the phases formed during heating, before the reduction stage. The presence of antigorite in Colombia-1 ore results in a simultaneous dehydroxylation and transformation of magnesium silicate to olivine and pyroxene during heating. Due to the recrystallisation, the degree of removable oxygen R_t is decreased when increasing the temperature from 750 to 900°C. The low reducibility at 900°C was also confirmed by chemical analysis. Colombia-2 and Mirabela do not, however, contain antigorite and heating the samples resulted in formation of an easily reducible intermediate phase. Thus, the recrystallisation is not initiated at 750°C like in Colombia-1 and their R_t values were also enhanced at 900°C. According to SEM–EDS results, a complex series of phase and microstructure changes has been observed along with the formation of Fe–Ni alloy particles.