Impact of magnesium on the flocculation, sedimentation and consolidation of clay-rich tailings in lime-treated seawater

Abstract

The sustainability of the mining industry in times of climate change, especially in arid regions, rests on seawater. However, now more than ever, it is necessary to optimize processes regarding water consumption and increase recovery and recirculation, including water trapped in tailings dams. Seawater also offers challenges, for example, the condition in which it must be used. This study examines the effects of raw seawater (SW) and lime-treated seawater to reduce Mg content (TSW) under strong alkaline conditions (pH 11) on the flocculation, sedimentation, and consolidation of flocculated quartz-kaolin suspensions using a high molecular weight anionic polyacrylamide as a flocculant. The presence of magnesium precipitates in SW, mainly magnesium hydroxide, leads to sedimentation rates of flocculated quartz-kaolin suspensions considerably lower than in TSW because the aggregates in SW are smaller than in TSW, according to chord size analyses. The precipitates interfere in the flocculant-particle interaction, limiting the adsorption of the flocculant and its ability to form bridges of any kind, polymeric or salt, with the tailings particles. However, Mg precipitates act as short-range and limited extension binders compared to flocculants increasing the resistance of the aggregates much more in SW than in TSW in processes without and with flocculant, whether it is measured as yield stress, elastic moduli, or compressive yield stress. Thus, the new result is that Mg precipitates are responsible for most of the resistance to consolidation of quartz-kaolin suspensions and, therefore, should be avoided. Therefore, TSW should be the choice to facilitate water clarification in thickeners and water recovery from the consolidation of deformable tailings aggregates.