The effect of iron sources on caustic and alumina recovery from synthetic bayer DSP (sodalite)

Abstract

The formation of desilication product (DSP) in the processing of high silica bauxite is both a significant cost to production and a potential threat to the environment due to the high level of caustic soda it contains. Hydrothermal reprocessing of DSP to recover caustic is a promising solution to the problem. The basis of this technology is the formation of iron-rich hydrogarnet and calcium silicate by redigesting DSP in caustic solution with lime and an “active” iron source. This paper highlights the influence of iron sources on phase formation during re-processing of synthetic DSP (sodalite) as well as the efficiency of soda and alumina extraction and lime consumption. Sodium ferrite, ferric nitrate, ferric chloride and hematite were chosen as iron sources for this study.Different iron sources significantly affected the mineralogical composition of the reprocessed residues and the efficiency of the process. Under conditions of relatively low temperatures (200-230°C), digestion time of 10 to 40min and lime charge of molar CaO/SiO2 ratio ~1-2, ferrite and sodalite readily reacted to an iron-rich hydrogarnet. Ferric nitrate and chloride also produced hydrogarnet which has less iron incorporation because of competing reactions to form hematite. This drawback was minimized by separate injections of lime and~neutral salt solutions into digestion. Hematite did not react with lime and iron hydrogarnets did not form. Instead tobermorite formed as the dominant calcium-containing phase which is more efficient in lime consumption (molar CaO/NaOH~<1) and soda extraction, and equally good in alumina extraction.