Oxidation of FeSO4 by air oxygen has been studied in aqueous solutions of constant acidity. The effect of various reaction conditions (of acidity, temperature, concentration of FeSO4, and of the air flow through the suspension) on the yield of various modifications of ferric oxide-hydroxides in the product mixture is described and the mechanism of the process is discussed. Reaction products prepared in the neutral and weakly alkaline solutions contain predominantly lepidocrocite (γ-FeOOH), amorphous Fe2O3.n H2O (ferrihydrite), at higher pH values also goethite (α-FeOOH) is formed. Their mutual ratios depend strongly on the solution acidity.

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)'

It is shown that amorphous precipitates of Fe(III) hydroxides are composed of two phases, namely weakly magnetic primary particles and ferrimagnetic fine‐grained particles. The relative proportion of the two phases, the type of phase transitions and the rate of amorphous precipitate crystallization during the ageing process are determined by the precipitation temperature. The crystallization of Fe(III) hydroxides during the ageing in mother liquors occurs by the “oriented growth” mechanism.

Die Oxidation von FeSO4 in wäßrigen Lösungen konstanter Acidität mit Luftsauerstoff wird untersucht und die Einflüsse von Reaktionsbedingungen wie Acidität, Temp., FeSO4‐ Konzentration und Luftstrom durch die Suspension auf die Phasenzusammensetzung des Produkts beschrieben und ein Reaktionsmechanismus diskutiert.

Amorphous FeIII hydroxides prepared at pH = 6 differ from analogous precipitates obtained in alkaline media in that they have an increased content of anions bound in their structures, and crystallize less readily both in dry thermal treatment and under hydrothermal conditions. The shapes of DTA curves obtained for freshly prepared gels and for products of ageing in mother liquor are discussed, and the effect of the anions present on the course of thermal decomposition is assessed.