Simulation of iron oxide/silica precipitation in the paragoethite process for the removal of iron from acidic zinc leach solutions

An investigation of the simultaneous precipitation of iron oxide and silica species from acidic solutions was conducted, simulating the paragoethite process, an iron-removal stage employed in zinc hydrometallurgy. Laboratory-based continuous crystallisation experiments were carried out under the primary conditions employed in industry, with pH (at 85 °C) maintained at 2.65, the combined Fe and Si concentration at 0.1128 M (unless otherwise stated) and Si:Fe molar ratios varied from 0 to 0.43. The crystal structure of the residues produced was characterised, with the extent of iron and silicate removal quantified and properties relating to aggregate structure also measured.It is shown that the degree of silica polymerisation prior to the precipitation reaction dictates the mechanism of co-precipitation and thereby the properties of the residue. In the presence of polymerised silica, less dense aggregates of finer mean particle size are formed, which has negative implications for dewatering. Conversely, where silica is allowed to polymerise prior to the iron precipitation reaction, a greater proportion of silica is removed from the solution. As these effects are key to an effective precipitation stage in the process, the influence of silica polymerisation must figure in considerations when handling high-silicate concentrates.

Depiction of ferrihydrite/silica co-precipitation mechanisms in the case of both monomeric and polymerised silica. Each process leads to significant variations in chemical and physical properties.Figure optionsDownload as PowerPoint slideHighlights
► The paragoethite process precipitation stage was simulated at the lab scale.
► Iron and silicate tested without complex plant solution matrix.
► Monomeric and polymeric silica influence the residue properties differently.
► Due to different mechanism of co-precipitate formation and structure.