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Influence of calcium ions on the crystallization of sodium bicarbonate

par Zhu, Yi ;Demilie, Paul ;Davoine, Perrine;Cartage, Thierry ;Delplancke, Marie-Paule
Référence Journal of crystal growth, 275, 1-2, page (e1333-e1339)
Publication Publié, 2005-02

Article révisé par les pairs

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Résumé :

In industrial crystallization of sodium bicarbonate (sodium hydrogenocarbonate), the presence of calcium ions in solutions is unavoidable due to the production process. The understanding of the Ca2+ role in NaHCO3 crystallization would be helpful for improving the quality of the final products. The influence of calcium ions on NaHCO3 crystallization was investigated in a 5-l mixed suspension mixed product removal crystallizer under controlled conditions. A density meter was used for continuous supersaturation monitoring. After a steady state had been reached, different CaCl2 amounts were added at a constant flow rate. It was found that limited calcium ion levels in the system reduce drastically the nucleation frequency of NaHCO3 and has a limited influence on crystal growth rate. The supersaturation measurements and other methods confirmed this phenomenon. The relationship between the Ca2+ influence on NaHCO 3 crystallization, the calcium carbonate solubility and its metastable zone in concentrated NaHCO3 solution was established. In fact, Ca2+ has a maximum effect on NaHCO3 crystallization kinetics when the saturation of calcium carbonate in NaHCO3 solution has been reached, and the effect is constant in the metastable zone. The excess of Ca2+ precipitates in NaHCO3 solution as CaCO 3, as observed by energy dispersive X-ray and X-ray diffraction. This explained why an increasing Ca2+ concentration in the solution has a limited influence on NaHCO3 crystal size distribution and habit, but decreases the crystal purity. It is also confirmed that an impurity as Ca 2+ has no influence on the equilibrium NaHCO3-Na 2CO3. © 2004 Elsevier B.V. All rights reserved.