Reactions at the water-mineral interface of olivine and silicate modified maghemite

Abstract: The main objectives for this licentiate thesis have been to study and model the reactions at the water-mineral interface of two subsystems: maghemite-H+-silicate and olivine-H+. An increased knowledge of these systems will make it possible to further extend the complexity of the aqueous phase in future experiments. This will make it possible to further approach a composition that resembles the flotation and agglomeration process water in the production of iron ore pellets. To be able to characterise and predict the influence of different species from the process water on the different mineral particle surfaces, is an important step towards optimisation of the pellet production process and increased recirculation of the process water.
Both synthetic and natural minerals were used in the experiments, in which the protolytic surface exchange reactions of the subsystems were analysed by high precision potentiometric titrations. The minerals and their surfaces were characterised using XRD, XPS, SEM-EDS and BET, both before and after the experiments. The zeta potential of olivine and silicate modified maghemite particles were also determined as a function of pH. Surface complexation models were derived to describe the reactions in the systems. The Constant Capacitance Model (CCM) was used to model the experimental results.
The evaluation of the potentiometric data from the studies of the maghemite-H+-silicate system indicates that soluble silicates will mainly adsorb to maghemite as monodentate surface species at the defined experimental conditions. Models including polymerisation of adsorbed silicates and/or bidentate silicate ligands were tested, but could not well be fitted to titration data. The adsorption maximum of silicates was found to be within the range pH 9.0 to 9.5. At the experimental conditions used, approximately 82 % of the added silicate was adsorbed. At higher pH, the silicate started to desorb from the maghemite surface and at pH 11.1 only 60 % was still adsorbed.
Olivine ((Mg,Fe)2SiO4), one of the major rock forming silicate minerals in nature, is added in the agglomeration process to improve the performance of the iron ore pellets in the blast furnace. The results from the studies of the olivine - H+ system shows that the dissolution of olivine is essentially incongruent, with an excess of magnesium ions released in to the aqueous suspension. Studies of olivine samples equilibrated in electrolytes with magnesium ions added using XPS, SEM and zeta potential measurements have also shown that the magnesium ion release and adsorption at the surface layers is reversible, no support for surface precipitation of new phases as Mg(OH)2 could be found. Experimental potentiometric data of olivine-H+ at alkaline conditions was successfully fitted to a two pKa electrostatically corrected model.
The importance of steady state conditions when acquiring titration data was illustrated by comparing the amount of active surface sites detected by steady state titrations and considerably faster titrations.