The Coprecipitation of Magnesium Iron (III) Hydroxide Powders from Aqueous Solution: Precipitate Compositions and Coprecipitation and Ageing Mechanisms

Series of magnesium iron(111) hydroxides were coprecipitated at ambient temperature from different mixed metal cation solutions, at C M tot = 0.1 M and Mg/Fe 2 ratios from 4 to 1/4, with sodium hydroxide solution. The relevant single precipitations and the coprecipitations were monitored by potentiometric ( p H) titration and the final coprecipitate compositions were examined by chemical analysis, infra‐red spectrophotometry and thermal analysis. The coprecipitates onto aged α‐FeOOH were simple mixtures of α‐FeOOH and Mg(OH) 2 . The main coprecipitates were either «molecular inclusion mixtures» of microcrystalline α‐FeOOH with excess Mg(OH) 2 or similar mixtures of Mg(OH) 2 with excess α‐FeOOH and some ten‐twenty percent (relative to the Mg content) of magnesium hydroxo‐ferrate(III) hydrate Mg[Fe(OH) 4 ] 2 · h H 2 O. The coprecipitates aged in alkaline magnesium hydroxide suspension at 20 °C and 40 °C were mixtures of Mg(OH) 2 , α‐FeOOH and forty to ninety percent (relative to the Mg content) of Mg[Fe(OH) 4 ] 2 · h H 2 O. The related α‐FeOOHNaOH and α‐FeOOHMg(OH) 2 equilibria and the different coprecipitation mechanisms are discussed.