Conference Dates

May 18-22, 2003

First Page



Process heat transfer equipment fouling due to sodium aluminosilicate precipitation is a serious problem that confronts high-level nuclear waste liquor and Bayer process alumina processing plants. The fouling of 316 stainless steel substrate by thermodynamically stable and unstable sodium aluminosilicate polytypes: amorphous solid, zeolite A, sodalite and cancrinite crystals, been has studied in an isothermal, batch precipitation system at 65 °C. Fouling invariably occurred via heterogeneous nucleation, crystal growth and particulate adsorption processes, accompanied by solution-mediated, phase transformation and morphological changes. For the thermodynamically stable cancrinite fouling, the amount of scale deposited increased systematically with increasing crystallization time before levelling off as a result of depleted supersaturation. Where the deposited scale involved a less stable phase (e.g., amorphous, zeolite A), transformation to a more stable phases (e.g., sodalite/cancrinite) occurred. The scale layer coverage/growth characteristically increased and then decreased in a periodic manner. The periodicity of this unusual behaviour appeared to be directly and kinetically related to polytypic phase transformation of amorphous to zeolite A, zeolite A to sodalite and sodalite to cancrinite.