Zur Kenntnis der künstlichen Alterung aluminiumhaltiger Eisen(III)-hydroxide

Study of the ageing process of amorphous ferric hydroxide has shown that the presence of traces of Al³⁺ can drastically change the course of the reaction under certain conditions. Al³⁺ traces delay crystallization, an effect which is dependant on the pH of the solution in which the originally amorphous gel is boiled. With increasing pH they favour the formation of the α-F₂O₃-phase, whereby the α-FeOOH modification, the formation of which should otherwise be favoured by the dehydrative conditions employed, is not formed at all. Inhibition of the formation of the α-FeOOH phase is already crystallographically detectable at a molar ratio of Al₂O₃/Fe₂O₃=0.001. The complete disappearance of this phase at pH=10 of the ageing medium is found at the molar ratio Al₂O₃/Fe₂O₃=0.03. At higher Al³⁺ concentrations the amorphous gel is converted into α-Fe₂O₃. In the range 10–20 mol% Al₂O₃, crystallization is strongly inhibited. Further increase in the Al³⁺ content favours the formation of Bayerite-phase, which then at 50 mol% Al predominantes, alongside poorly crystalline α-Fe₂O₃. For Al₂O₃/Fe₂O₃=0.55, three phases are present, hydrohematite, bayerite and boehmite. With still greater amounts of Al³⁺ only the boehmite modification is found. By stepwise crystallographic study of the progressive dehydration the duration of reaction has been established for ferric hydroxide both in the pure state and when doped with Al³⁺.