Formation mechanism of layered double hydroxides in Mg2 +-, Al3 +-, and Fe3 +-rich aqueous media: Implications for neutralization in acid leach ore milling

Hydrotalcite-like layered double hydroxides (HT-LDHs) are important due to their extensive use in catalysis, polymer science, medicine, refractory materials, and the remediation of contaminated waters. The inclusion of several cations and anions in their structure can result in complex formation mechanisms depending on the chemical nature of the respective species. A systematic evaluation of the formation mechanism of HT-LDH from Mg2 +Al3 +Fe3 +-rich aqueous media at ambient conditions was conducted to determine formation pathways in natural settings and in conditions observed in ore milling processes. Neutralization from pH 1.7 to 12.5 (using NaOH) of individual M2 + and M3 + ions demonstrated that M3 +-hydroxides precipitate first, followed by M2 +-hydroxides in the order of Fe(OH)3, Al(OH)3, then Mg(OH)2. The rate of formation accelerates as pH increases. The addition of M2 + ions to M3 +-hydroxides produces HT-LDH while the addition of M3 + ions to M2 +-hydroxides does not. HT-LDH precipitates immediately under alkaline conditions (pH = 13.2) with available M2 + + M3 + ions, while formation progresses slowly as initially acidic conditions (pH = 1.7) are neutralized (to pH = 12.5) via the precipitation of M3 +-hydroxides and subsequently HT-LDH. This latter formation pathway is consistent with observations of the formation of MgFeAl HT-LDH during the neutralization process in an acid-leach process uranium mill: HT-LDH is first observed at pH ~ 6.4 and is dominant at pH ~ 8.0 or greater. At pH ≥ 6.4, HT-LDH is co-associated with Fe3 +-hydroxides. The early formation of M3 +-hydroxides and substitution by M2 + ions into the structure appear to govern the overall HT-LDH formation mechanism, with the resulting excess positive charge counterbalanced by CO32− ions through atmospheric CO2 dissolution.