Phillip D. Fawell

Synthesis and characterisation of ferrihydrite/silica co-precipitates

The effect of the presence of soluble silicates on ferrihydrite precipitation and some properties of the products formed in co-precipitation of ferrihydrite and silica have been investigated. The co-precipitates were formed using a continuous crystallisation process in which a combined iron/silicon feed solution was reacted with sodium hydroxide at a constant rate, while maintaining pH at 2.65 and temperature at 85 degrees C. The products of co-precipitation and the supernatant solutions were characterised using a variety of analytical techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and surface charge measurements. The addition of silicates was shown to have a significant impact on the crystallinity and surface charge of the precipitates formed. For products collected after five residence times in the continuous crystalliser, co-precipitates formed from ferric sulfate solution were found to contain considerably less silica than those formed from ferric nitrate. We conclude that adsorption of silicate species on ferrihydrite surfaces speeds up the polymerisation process, and that sulfate ion competes with silicate for surface adsorption sites. Thus, the precipitation of silica proceeds much more rapidly in ferric nitrate media, than in ferric sulfate.

Sodalite solids formation at the surface of iron oxide and its impact on flocculation

Sodalite represents the main desilication product (DSP) phase formed from reactive silica during alkaline digestion of bauxite in the Bayer Process. Previous studies into DSP effects on bauxite residue flocculation have focused on flocculant selection or digestion optimisation, not answering a fundamental question: does DSP coat the residue and thereby change surface properties? This study sought to answer that question by contrasting the physicochemical properties of hematite slurries (as a model phase for residue) containing DSP where it was either made in-situ or added as a physical mixture. On the basis of differences found in dewatering behaviour, zeta potential, desilication rates and microscopy of the solids, it is proposed that DSP nuclei initially associate with the hematite surface and subsequently affect flocculant adsorption chemistry, resulting in different extents of flocculant adsorption and smaller aggregates. The practical implications for flocculation processes are discussed.