The Precipitation of Alkaline-Earth Metal Carbonate Powders from Aqueous Solution. Crystal Numbers and Final Sizes

The precipitations of magnesium carbonate trihydrate, basic magnesium carbonate and calcium, strontium and barium carbonates was studied from equivalent solutions of concentrations from 0.0005 M to 1M, at pHs from 10 to 7, by optical microscopy and other methods. Crystal growth started after induction periods: the precipitations of the more sparingly-soluble metal carbonates — mainly studied at medium to high supersaturation — were homogeneously nucleated while the magnesium carbonate trihydrate precipitations — studied at low supersaturations at pH ≦ 7.6 — were heterogeneously nucleated. The crystal forms and numbers of the final precipitates depended on the type and numbers of nuclei (and crystallites) formed during the induction periods. Crystal numbers generally increased with increasing initial mean metal carbonate concentration according to the relation N = N₁C_MCO3^β; β was 3 for the metal carbonate precipitations and β was 4 for the basic magnesium carbonate precipitations. N₁ values increased in the order basic MgCO₃ (at pH ≧ 9), or MgCO₃ · 3H₂O (at pH ≦ 7.6) < CaCO₃ < SrCO₃ < BaCO₃. The final crystal lengths then generally decreased, from maximum values, with increasing initial concentration according to the relation l_fin = l₁/C^γ, where γ was 0.7 and 1.0. For precipitation at any concentration and pH, smaller crystal sizes were generally obtained in precipitates from solutions of the metal carbonate of lower solubility.