Sodium Aluminosilicate Solid Phase Specific Fouling Behaviour
Jonas Addai-Mensah, Jun Li, Marek Zbik and Scott Rosencrance
Process heat transfer equipment fouling due to sodium aluminosilicate precipitation is a serious problem that confronts high-level nuclear waste and alumina refining plants. The fouling of stainless steel substrate by stable and unstable sodium aluminosilicate polytypes: amorphous solid, zeolite A, sodalite and cancrinite crystals, has been studied under isothermal, batch precipitation conditions at 65°C. Fouling invariably occurred via heterogeneous nucleation, crystal growth and particulate adsorption processes. This was accompanied by solution-mediated, phase transformation and morphological changes where the scale deposit comprised a less thermodynamically stable phase (e.g., amorphous, zeolite A and sodalite). The scale layer coverage/growth characteristically increased and then decreased, in a manner whose periodicity appeared to be kinetically related to polytypic phase transformations: amorphous gel Æ zeolite Æ A Æ sodalite Æ cancrinite. For cancrinite scale, the amount deposited increased systematically with increasing crystallization time before levelling off as a result of depleted supersaturation.