Preparation of α-Fe2O3 (hematite) by oxidation precipitation of aqueous solutions of iron(II) sulphate

Abstract

The oxidation precipitation of aqueous solutions of iron(U) sulphate by oxygen gas was studied; the effect of pH, temperature, and presence of x-FeOOH and 't-Fe203 crystal nuclei was examined and the conditions for the preparation of phase-pure 't-Fe203 (hematite) were sought. The hematite prepared contained 3-5% structurally bonded water. The effect of the bonded water on the properties of the product is described and the way of the bonding in the structure is discussed.Oxidation precipitation of aqueous solutions of iron(lI) salts is a complex process involving nucleation of the reaction products and crystal growth of the solid phase. In dependence on the reaction conditions, a number of iron oxides and oxide-hydroxides with variable properties are formed, e.g., 1X,,B,y,i5-FeOOH, the so-called iron gel Fe203.n H 20, or IX-Fe203' Various pathways have been suggested to account for the relation between the reaction conditions and the phase composition of the products 1 • 2 . At temperatures above 80"C, some oxide-hydroxides transform into IX-FeOOH or IX-Fe203 (refs 3 . 4 ). Based on the available thermodynamic data for the 2iX-FeOOH -+ IX-Fe203 + H 2 0 transformation, Langmuir 5 has been able to show that for spherical particles larger than 111m in diameter, goethite (IX-FeOOH) in aqueous suspension is the more stable up to 80°C, the transformation point shifting to lower temperatures as the particle size is lowered, e.g., 40 Q C for 0·1 11m or even -27 e e for 0·04 11m. In this manner it is possible to estimate the temperature region where hematite should be the product of the oxidation precipitation of iron(II) solutions if the process were controlled by thermodynamic effects solely.The stability of the solid phase is also affected by the pH and the redox potentia:of the system (E). A plot of E vs pH based on the thermodynamic data was published by Pourbaix 6 ; later this problem was studied by Detournay and coworkers 7 • The authors proceeded from the tabulated thermodynamic data at 20 0 e and considered goethite as the stable oxide phase of iron(I1I); at lower E and higher pH values, the region of its stability is bounded by the stability region of magnetite, at lower pH in the presence of sulphate, by the stability region of basic sulphates of jarosite type (e.g., NH4 Fe3(S04MOH)6)'

Unusual thermomagnetic behaviour of haematites: neoformation of a highly magnetic spinel phase on heating in air

“…It is known from the literature that most haematites produced in aqueous systems or by thermal dehydration of either hydrated nitrate salts or goethite often retain some incorporated hydroxyl groups in the structure (e.g. Gallagher & Gyorgy 1969; Wolska 1981; Šubrt et al . 1984 ; Wolska & Schwertmann 1989; Schwertmann & Cornell 1991; Waychunas 1991; Stanjek & Schwertmann 1992).…”

Reactivity of finely dispersed iron (III) oxides and oxide hydroxides in solid-state reactions

“…expressed by the formula Fe203.0.26H20 (hydrohematite [6][7][8]). 3)Hematite prepared by the annealing of goethite (~-FeOOH) at 350 ~ for 2 h. This kind of hematite also contained structurally bound water [8].…”