The Behavior of Various Inorganic Ions in the Separation of Calcium Carbonate from a Bicarbonate Solution
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The author has shown the effect of various inorganic ions on the polymorphic crystallization of calcium carbonate from a bicarbonate solution. No definite conclusion regarding the mechanism to give calcite, aragonite or vaterite can yet be made, but these data help to give a pattern of the influence of inorganic ionic species and the concentration of the mother solution on the polymorphic crystallization of calcium carbonate.<BR>(1) Alkali chlorides in the mother solution inhibited aragonite formation and favored calcite formation. The extent of the inhibition of the aragonite formation decreases in the sequence: ammonium chloride>lithium chloride, sodium chloride>potassium chloride >rubidium chloride, >cesium chloride (sodium nitrate).<BR>(2) (a) Sodium chloride and sodium nitrate in the mother solution inhibited aragonite formation, sodium chloride more strongly than sodium nitrate, (b) With a moderate increase in the amount of sodium bicarbonate (sodium carbonate) added to the mother solution, the percentage of aragonite formation increased to a maximum. With a further increase in the amount, however, the percentage began to decrease and reached zero, (c) When sodium hydroxide was added to the mother solution, only calcite was formed. Aragonite was never formed from the solution which had made alkaline with sodium hydroxide, (d) The presence of sulfate ions favored aragonite formation.<BR>(3) (a) Magnesium ions in the mother solution favored aragonite formation very strongly. The presence of magnesium chloride in the mother solution was more favorable to aragonite formation than that of magnesium bicarbonate or magnesium sulfate. Magnesium was not co-precipitated with calcium carbonate, (b) The presence of calcium chloride in the mother solution had no effect on the crystallization. (c) With an increase in the strontium concentration in the original solution, the percentage of aragonite formation increased to a maximum (optimum strontium content: approximately 2 mg./l). The percentage began to decrease with a further increase in strontium concentration. The strontium was always entrained in calcium carbonate precipitates, even when the strontium concentration was very low. (d) The presence of a small number of barium ions in the mother solution favored calcite formation. The presence of a large number of barium ions favored vaterite formation. Even at a low concentration in the mother solution, barium was always entrapped in the precipitates.
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