Ring Formation in the Smelting of Saprolite Ni-ore in a Rotary Kiln for Production of Ferro-nickel Alloy: Mechanism

The clarification of mechanism of SR (Slag-Ring) formation in the smelting of saprolite Ni-ore by the rotary kiln has been successfully attempted. The walls in the kiln have a higher temperature than the raw materials, in particular, the point where the raw materials enter the bed have the highest temperature. In that point, the released low-MgO and high-SiO2·FeO silicate accompanied by the recrystallization of non-stoichiometric serpentine are transformed to the primary melt, the fine parts of ores with much FeO and Al2O3 and limestone being melted by Ostwald ripening, which allows the formation of the secondary melt. Subsequently, its entrance into the bed leads to the cooling and precipitation on the wall and accretions, which allows the increase in the sticking force. The slight transportation of raw materials from the SR sticking zone toward the discharge end leads to the rapid decrease in the sticking. This is explained from the fact that the large growth of the particles by Ostwald ripening causes the reduction of melting amount into the melt, allowing the small precipitation on the wall and accretions. The resulting SR is locally formed. The fine parts of low-MgO and high-FeO type ore have higher concentration of point defects than that of the high-MgO and low-FeO type ore. This provides the lower melting point and higher diffusion rate to the former than the latter, which allows the former to have higher possibility of SR formation than the latter.