Marko Kekkonen

Thermal behaviour of hydrous nickel–magnesium silicates when heating up to 750°C

Abstract The thermal decomposition sequence of hydrous nickel–magnesium silicates from Colombia and Brazil was studied under air/Ar atmosphere from room temperature up to 750°C by differential scanning calorimetry–thermogravimetry followed by X-ray diffraction and scanning electron microscopy analyses. Differential scanning calorimetry curves of the samples obtained showed three endothermic peaks at 100, 250 and 600°C due to the release of free water, the dissociation of goethite and the release of crystalline water respectively. To determine the mineral species and microtexture, the ores were studied by scanning electron microscopy. Scanning electron microscopy–energy dispersive spectroscopy analyses showed that the ores are rich in Mg and Mg–Fe silicates, Cr spinel, Mn oxide, goethite and silica and exhibit complex alteration texture. X-ray diffraction analyses of Colombia-2 and Mirabela (Brazil) after the experiments showed that the dehydroxylation produces an amorphous intermediate phase, which is supposed to be due to the exsolution of silica. However, Colombia-1 sample, which was confirmed to contain antigorite mineral, was observed to undergo dehydration and recrystallisation simultaneously.

Effect of mineralogy and reducing agent on reduction of saprolitic nickel ores

Abstract The aim of this work was to explore the effect of mineralogy and gas reductant on the reduction of nickel laterites. Three different nickel saprolitic ores reduced by carbon monoxide were examined, and the results were compared with hydrogen reduction of the same ores. The reduction experiments were carried out in a thermobalance furnace which allows continuous measurement of mass change. The ores were reduced in CO/CO2 (72%∶28%) at 750 and 900°C for 90 min. Chemical analysis was utilised to determine the degree of metallisation of the reduced pellets. It was found that the reduction rate and degree of removable oxygen at 750 and 900°C in CO/CO2 was lower compared to reduction experiments in H2/N2 (72%∶28%). The reducibility of the samples seems to depend on mineralogy and their modes of decomposition and recrystallisation. The results clearly indicate that using various reducing gases has an impact on the reduction behaviour and generation of metallics of nickel saprolitic ores.

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

Abstract 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 Rt 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 Rt 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.

Effect of sulphur addition on roasting reduction of saprolitic nickel ores in CO/CO2 gas mixture

Abstract The objective of the work presented here was to examine the effect of sulphur on formation of high temperature recrystallised phases and further on reducibility of saprolitic nickel ores. The ores blending with FeS2 were first heated up to 1300°C to investigate the changes that might happen upon heating. As a result of sulphur addition, the exothermic peak, which is evidently associated with crystallisation of new phases, changed in size compared to ores without sulphur addition. In addition, the dehydration and recrystallisation of minerals was also observed to begin earlier, when sulphur is added in the ores. Additionally, the ores were reduced in CO/CO2 atmosphere at temperatures 750 and 900°C, in order to determine the effect of sulphur and temperature on the reducibility. Although sulphur prevented the formation of difficult reducible olivine, as well as pyroxene phases in Colombia-2 and Mirabela, the degree of removable oxygen was not enhanced when increasing the temperature from 750 to 900°C. This is due to low reduction of iron oxides to metalic iron at 900°C. The beneficial effect of sulphur on metallisation degree of the ores was observed by comparison of these results with the experiments carried out with the pellets without sulphur addition. The results obtained in this study clearly indicate that the reduction of saprolitic nickel ores appears to be strongly dependent on the characteristics of the starting materials, amount of additive used and the process conditions.

Melting Behavior of Sn–Bi Alloy Powder Compacts Observed Using Optical Dilatometry

Abstract The melting behavior of Sn – Bi alloy powder compacts was observed using optical dilatometry. The change in standard deviation of compact width with temperature obtained by dilatometry was used to evaluate the melting behavior by considering the factors in the shape change of the compact under heating. The effects on melting behavior of alloy composition, particle size and pressing load at compacting are discussed in this study. The difference in composition affected the melting behavior in accordance with the phase diagram. Smaller particle size showed a lower degree of melting. The effects of these two parameters, viz. composition and particle size, however, are highly dependent on surface oxidation of powders. The influence of surface oxidation, on the other hand, was reduced by producing the compacts with a higher pressing load.