Gaseous reduction of nickel calcines in hydrogen-2 and carbon monoxide between 400 degrees C and 850 degrees C

The behaviour of two roasted nickel sulphide concentrates when heated in argon, hydrogen and carbon monoxide atmospheres was characterized by combining thermogravimetric analysis with off-gas analysis. Mass changes in inert atmospheres were attributed to sulphur dioxide evolution from solid state reactions between pyrrhotite and iron oxides (above 300°C) and from the decomposition of sulphates (above 550°C). In reducing atmospheres, sulphate decomposition was achieved at lower temperatures (250°C in H2; 350°C in CO; reduction of the hematite phase prevented SO2 emissions from the solid state reactions. Direct reduction of metal sulphides yielding H2S and COS was detected above 500°C.; The rate of oxygen removal from two roasted nickel concentrates by H 2 and CO reductants was measured using thermogravimetric analysis. Non-isothermal and isothermal techniques were applied. It was found that hydrogen reduction obeyed a uniform internal reduction model. The temperature dependence of the rate constant obeyed an Arrhenius expression. Using isothermal analysis, different rate constants expressions were measured for the two calcines, however their activation energies were statistically the same (55 kJ/mol). The difference between the two expressions was attributed to the difficulty in measuring the degree of conversion in complex solids.; Isothermal and non-isothermal thermogravimetric analysis failed to provide consistent rate expressions for CO reduction of calcines. The reduction rate was noticeably retarded above 600°C. In addition the deposition of carbon on the calcine pores was observed. It was concluded that carbon deposition had interfered with access of the gaseous reductant to the solid oxide, and hence lowered the reaction rate.; Reoxidation of deeply reduced calcines in air was measured. Above 500°C, reoxidation caused samples to gain mass at a rate of ≈8%/min. Calcines pre-reduced in H2 reoxidized approximately 20% faster than calcine pre-reduced in CO. From the relatively low activation energy of 12 kJ/mol, it was concluded that oxidation was diffusion controlled. This was in agreement with other researchers.