| In the beneficiation of fine-grained oxidized iron ores, selective flocculation of iron oxides prior to the cationic flotation of silica is an essential step to reduce the iron loss. The selective flocculation process is extremely sensitive to the process water chemistry and the mineralogical composition of the ore. To apply selective flocculation process effectively, the control of the water chemistry, and a thorough understanding of the interaction between mineral constituents of the ore are necessary. Clay minerals present in oxidized iron ores have caused difficulties in selective flocculation. The effect of montmorillonite on the selective flocculation of a hematite-quartz mixture was studied. It adversely affected the process by indiscriminately dispersing the pulp. The adverse effect was found to be more pronounced in the presence of calcium ion. Among possible mechanisms, namely card house formation, dissolution of montmorillonite and heterocoagulation between hematite and montmorillonite, heterocoagulation was determined to be the main mechanism of the adverse effect. Several techniques were employed to ascertain the occurrence of heterocoagulation and to explain the manner in whch it affected selective flocculation. Also, the ways to counter the adverse effect of montmorillonite were explored. Two methods were investigated. The chemical method, using EDTA, STPP and sodium silicate and the mechanical method, which involved the ultrasonic treatment of the pulp. EDTA, STPP and the ultrasonic treatment were determined to be effective means. Their roles in mitigating the adverse effect were explained. It was determined that sodium silicate, a commonly used dispersant, above a certain concentration level also caused indiscriminate dispersion of a hematite-quartz mixture in the presence of calcium ion. To determine the role played by sodium silicate in the dispersion of hematite and to compare the apparent similarity between sodium silicate and montmorillonite, hematite-calcium-sodium silicate (no montmorillonite) system was studied. Presence of silicate ions or calcium silicate precipitates on the hematite surface prevented starch adsorption. Although the same was true for montmorillonite, starch could replace montmorillonite particles coating the hematite surface but silicate ions or calcium silicate precipitates were strongly held on the hematite surface and could not be replaced by starch. |
