Impact of modular sinter characteristics on the extraction of alumina when processing nepheline ores

Processing alkaline aluminosilicates (as an alternative to bauxite ores) provides Russian aluminum smelters with domestic alumina up to 41 %. The Achinsk Alumina Plant manufactures alumina by alkaline sintering of nepheline charge. To obtain very water- and alkaline-soluble sodium and potassium aluminates and low-soluble dicalcium silicate, a charge is used, consisting of nepheline, limestone, white sludge (from the aluminate solution desiliconization) and recycle water (to obtain fluid pulp). The raw materials are dosed to obtain certain CaO : SiO2 (lime module) and (Na,K)2O : Al2O3 (alkaline module) molar ratios. The charge is fed to a rotary kiln (185 m long and 5 m in diameter) and sintered when heated to 1250-1300 °C. According to the plant’s practice, periods occur when the Al2O3 extraction reduces due to an increase in the content of sulfur and iron compounds in the sinter. To define the sinter’s optimal chemical composition considering these impurities, sintering of nepheline charges with different modular characteristics has been studied: Mlime = 1.90-1.93 units, Malk = 1.06-1.08 units (first option); Mlime = 1.90-1.93 units, and Malk Fe,S (alkaline module, considering iron and sulfur impurities in the sinter) = 0.87-0.91 units (second option). The study results show that to obtain the sinter’s optimal chemical composition, the alkaline module (considering iron and sulfur impurities) and the lime module should be maintained at 0.89 and 1.92 units, respectively. Such conditions allow for achieving an 84 % alumina recovery under normal operating modes of sintering kilns.