Precipitation of Iron Oxides by Hydrolysis Reactions

Principal solid products obtained by the hydrolysis of Fe(N03)3, FeCl3, Fe(CI04)3, or Fe2(S04 3 aqueous solutions. 

Fe(N03)3 solutions. Precipitates formed from 6.25 x I0 and 6.25 x lO M Fe(N03)3 solutions at 24 and 55 C showed the characteristic pattern of a-FeOOH, whereas in 6.25 x lO M, Fe(N03)3 solution at 24°C an amorphous precipitate was formed. a-Fe203 precipitated at 90 °C. The same authors proposed (Fig. 23.2) the mechanism of hydrolysis-precipitation from Fe(N03)3 solutions, consisting of the hydrolysis to monomers (A) and dimers (B), reversible, rapid growth of small polymers (C), the formation of slowly reacting large polymers (D), and polymerization and oxolation with the formation of a solid phase (E). XRD, FT-IR, and Mbssbauer spectroscopies were used to characterize hydrolytical solid products obtained by the hydrolysis of O.l M Fe(N03)3 aqueous solutions at 90°C in the presence of hexamethylenetetramine (HMTA) [24,25]. HMTA generates OH ions at an elevated temperature in accordance with the chemical reactions 

Three solid phases were detected amorphous (low-crystalline ferrihydrite), a-FeOOH, and a-Fe203. The ratio of these fractions depended on the starting concentration of HMTA and the time of hydrolysis. 

Proposed scheme of hydrolysis/ precipitation from an aqueous Fe(N03)3 solution. 

23 MOSSBAUERSPECTROSCOPY IN THE INVESTIGATION OFTHE PRECIPITATION OF IRON OXIDES 

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The most important synthetic routes to iron oxide pigments involve either thermal decomposition or aqueous precipitation processes. A method of major importance for the manufacture of a-Fe203, for example, involves the thermal decomposition in air of FeS04-7H20 (copperas) at temperatures between 500 °C and 750 °C. The principal method of manufacture of the yellow a-FeO(OH) involves the oxidative hydrolysis of Fe(n) solutions, for example in the process represented by reaction (1). 

The type and quality of the pigment are determined not only by the nature and concentration of the additives, but also by the reaction rate. The rate depends on the grades of iron used, their particle size, the rates of addition of the iron and nitrobenzene (or another nitro compound), and the pH value. No bases are required to precipitate the iron compounds. Only ca. 3 % of the theoretical amount of acid is required to dissolve all of the iron. The aromatic nitro compound oxidizes the Fe2 + to Fe3 + ions, acid is liberated during hydrolysis and pigment formation, and more metallic iron is dissolved by the liberated acid to form iron(II) salts consequently, no additional acid is necessary. 

The exposure of sulfide minerals contained in mine wastes to atmospheric oxygen results in the oxidation of these minerals. The oxidation reactions are accelerated by the catalytic effects of iron hydrolysis and sulfide-oxidizing bacteria. The oxidation of sulfide minerals results in the depletion of minerals in the mine waste, and the release of H, SO4, Fe(II), and other metals to the water flowing through the wastes. The most abundant solid-phase products of the reactions are typically ferric oxyhydroxide or hydroxysulfate minerals. Other secondary metal sulfate, hydroxide, hydroxy sulfate, carbonate, arsenate, and phosphate precipitates also form. These secondary phases limit the concentrations of dissolved metals released from mine wastes. 

Fe(OH)2 is prepared from Fe° solutions by precipitation with alkali. When freshly precipitated under an inert atmosphere (in a Schlenck apparatus for example) Fe(OH)2 is white (Bernal et al., 1959). It is, however, readily oxidized by air or even water upon which it darkens. Fe(OH)2 has the CdL type structure with hep anions and half of the octahedral interstices being filled with Fe ions. The crystals form hexagonal platelets. In solution Fe(OH)2 transforms by a combination of oxida-tion/de-hydration/hydrolysis reactions to other iron oxides and hydroxides. The end product depends both upon the order in which these processes occur and upon their rates. 

We now consider Fe hydrolysis. The hexaaquaflFerric cation[Fe(H20)e] is more acid than hexaaquaferrous cation [Fe(H20)g]. The equilibrium constant of hydrolysis is approximately one order lower than that in phosphoric acid, whereas the equilibrium constant of the hydrolysis of Fe " is approximately one order higher than that in boric add. During the hydrolysis the following essentially mononuclear complexes are produced [FeOH] ", [Fe(OH)2]" , [Fe(OH)3(aq)]° and [Fe(OH)4]. By other reactions a series of polynuclear complexes is formed, for example, [Fe2(OH)2], [Fe3(OH)4] , [Fe4(OH)g] , etc. (for simplicity, the coordinated water molecules are omitted). First, colloid hydroxo complexes are formed and finally there is a precipitate of hydrated ferric oxide which is in fact a mixture of different polynuclear complexes. The distribution of polynudear complexes depends not only on pH, but also on the initial concentration of iron. In diluted solutions of ferric salts a precipitate of hydrated Fe203 is separated only at a higher pH. The equilibrium between particular polynuclear complexes is established only very slowly. 

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