Characterisation of single-stream Bayer plant heat exchanger scale

• Little characterisation of single-stream heat exchanger Bayer plant scales to date
• Cancrinite, perovskite or katoite have been found to be major components.
• Minor hematite, found in all scales samples, and boehmite were present.
• The significant amorphous component is mainly poorly formed sodalite.
• Well-formed crystallites appear to be from the pre-desilicated bauxite.

During the Bayer process, deleterious scale formation, consisting largely of aluminosilicate and titanate phases, results in additional consumption of caustic soda and decreased heat transfer efficiency thereby increasing energy costs. However, little characterisation has been undertaken as to the nature of these scales, particularly from within single stream Bayer refineries. Five contrasting scale samples have been studied using SEM, EDS, XRD Rietveld analysis, and for three samples synchrotron XANES analysis. Cancrinite, perovskite or katoite were found as major crystalline phases in these scales; however, a significant amorphous content was also found to be present. In conjunction with XRD and EDS analyses, Al K-edges XANES linear combination analysis indicates that the amorphous component in the scale samples consists predominantly of poorly crystallised sodalite. Relatively minor components of hematite, found in all scales samples, and boehmite, found in some scales were distributed homogenously. It appears that scale formation in single stream heat exchangers occurs via a mixed mechanism: precipitation of aluminosilicate phases, including amorphous components, and also on occasion katoite (Ca3Al2(OH)12) and perovskite (CaTiO3) from the Bayer liquor plus cementation of pre-desilicated bauxite phases, e.g. DSP, hematite and boehmite from the slurry onto heat exchanger steel surfaces.

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