Surface coatings are a potential method to prevent the formation of gibbsite scale on equipment surfaces in contact with green liquor in the Bayer circuit, or to at least reduce scale adhesion and thus facilitate descale. To identify the optimum materials for reducing scale adhesion, the nature of the bond between scale and the substrate surface needs to be understood: Why does one material adhere to another? What determines the strength of the bond? Does a material that reduces scale adhesion also reduce scale nucleation?
The literature indicates various theoretical considerations in regard to adhesion between two materials, including surface roughness/porosity, “Surface Free Energy”, and the nature, spacing, and orientation of surface functional groups.
In a current study, the potential for molecular modeling to predict the nature and strength of scale-to-substrate bonding is being investigated. In parallel, a range of analytical techniques have been used to observe and quantify the nature and strength of scale adhesion to various substrates, to examine the mechanisms of scale nucleation and growth/development, as well as the role played by physical and chemical degradation or changes in substrates in the Bayer liquor environment. Techniques used include optical microscopy, Scanning Electron Microscopy, Atomic Force Microscopy (topographic, amplitude and phase imaging; force spectroscopy), contact angle, Raman spectroscopy, and X-ray Diffraction.