Sorption reactions between ionic species and magnetite in aqueous solution

University dissertation from Luleå : Luleå tekniska universitet

Abstract: The surface chemistry of inorganic materials is of great significance in a number of industrially important processes such as separation of ore by flotation, catalysis, water purification, leaching, as well as in the formulation of some pharmaceutical preparations. This thesis deals with magnetite and its sorption properties. Especially it was focused on the sorption of ions present in the process water and possibly affecting the balling properties of the magnetite concentrate in the pelletizing process. It is well-known that these properties become deteriorated if the magnetite surface becomes less hydrophilic, which motivated the use of an amphiphilic adsorbate (sodium oleate) in this study.The magnetite nano-particles were synthesized and subsequently characterized by X-ray, electron microscopy, and infrared spectroscopy. The mechanisms of magnetite formation from co-precipitation of Fe (II) and Fe (III) as well as oxidation of ferrous hydroxide were evaluated using iron isotope fractionation measurements (Paper I).Since magnetite pellets are heated during the sintering process and also may contain small amounts of the hydrophobic collector used in the flotation process, it was interesting to follow what happened with a model collector such as sodium oleate upon heating the magnetite/oleate system. This was studied using a combination of thermal analysis and FTIR spectroscopy. It was found that the oleate molecules were bonded to iron atoms by predominantly a bidentate mononuclear complex and formed essentially a single layer with a distance between the oleate molecules of ~36 Å2. Thermogravimetric analysis showed indicated double bond cleavage that yielded products enriched in oxygen and also capable of forming hydrogen bonds (Paper II).To study how the magnetite surface might be modified caused by process water, the magnetite nano-particles were evenly distributed over an internal reflection element and this combination was used to study the adsorption of ions present in the process water in-situ. The ionic system included the model collector (oleate) in stead of the collector used in practise (Atrac) to separate apatite from magnetite. Among ions in the process water, the adsorption properties of sulphate, silicate, and carbonate were studied as well as the effect of calcium ions on the adsorption properties and the competition between silicate and oleate for the magnetite surface. Paper III focused on the effect of Ca (II) on the adsorption of sulphate and it could be concluded that this effect was of minor importance. On the other hand, calcium ions in solution had a large effect on the adsorption of carbonate ions onto magnetite (Paper VII). During the flotation process, silicate is added to the pulp in order to disperse the magnetite particles and make the reverse flotation of apatite from magnetite more efficient. Accordingly, the adsorption of silicate onto magnetite as well as maghemite was investigated as a function of pH (Papers IV and V). Finally, the kinetics of oleate adsorption onto magnetite and competition between sodium oleate and sodium silicate for the magnetite surface was studied. Of particular interest was to which extent oleate could possibly be substituted for silicate and vice versa. These studies are elaborated in Paper VI.

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