The rapid formation of crystalline double hydroxy salts and other compounds by controlled hydrolysis

Abstract

Double-cation hydroxy compounds, generally with the pyroaurite-type structure, were synthesized by a reaction involving the hydrolysis of a cation in solution by a fully hydrolysed and precipitated hydroxide of a second metallic cation. Synthesis of this type of compound is not limited to reactions between di- and trivalent cations as previously thought but can, in this process, involve divalent-divalent, divalent-tetravalent and trivalent-trivalent cations. Crystalline compounds with reasonably sharp XRD patterns formed generally within a few hours at 25°C at pH 4–8·5 depending on the cations involved. The technique presents a more rapid method of formation of these compounds than previously described syntheses, and has been called induced hydrolysis because the fully hydrolysed cation causes complete hydrolysis of the second cation at a pH below that at which this would normally occur. The minerals takovite, Ni₆Al₂(OH)₁₆CO₃·H₂O, and hydrotalcite, Mg₆Al₂(OH)₁₆CO₃·H₂O, were amongst some of the compounds formed and the reaction is considered to be more realistic in terms of conditions encountered in natural environments than previous syntheses. The reactions described could also be expected to affect the mobility of heavy elements in the environment. These double hydroxy compounds convert on heating between 300–500°C to double-metal oxides with the spinel structure.