Seawater flocculation of clay-based mining tailings: Impact of calcium and magnesium precipitation

Abstract

In areas where access to water of good quality is limited, the use of seawater in mineral processing can be an option. However, the dewatering of waste tailings is adversely affected, particularly when this follows processing at higher pH. The flocculation response of a synthetic clay-based tailings in seawater to different pH conditions was examined in detail, focusing on conditions promoting precipitation of the divalent cations present. Flocculated aggregate growth and breakage during turbine mixing were monitored in-situ in terms of chord length distributions (by focused beam reflectance measurement) and aggregate image capture, with the resultant hindered settling rates also determined. It was found that flocculation in seawater was substantially impaired at pH values greater than 10.3 due to the precipitation of magnesium species. The addition of the precipitated phase to the synthetic tailings increases both the surface area exposed to flocculant and the number of particles or micro-aggregates then needing further aggregation to attain acceptable settling rates. The direct use of seawater in processing operations without resorting to full desalination does not necessarily have detrimental impacts on tailings flocculation and thickening, provided the operation is conducted at a pH that prevents the precipitation of magnesium present within the process liquors.