In situ characterisation of a concentrated colloidal titanium dioxide settling suspension and associated bed development: Application of an acoustic backscatter system

• An ABS is utilised as an in situ concentration monitor in TiO2 suspensions.
• Calibration reveals the relationship between acoustic attenuation and concentration.
• Phenomenological analysis characterises concentration in a settling suspension.
• Concentration segregation in a size polydisperse dispersion is observed.
• Settling rates and settling behaviour are determined.

The ability to accurately characterise the settling of particulate suspensions is imperative in numerous industries for improved processing, optimisation and control. Characterisation is typically a non-trivial process, and in situ measurements that remove the restriction of requiring particular sample sizes are preferred. Here, for the first time, we explore the use of an acoustic backscatter system (ABS) to characterise key settling dynamics within a common colloidal mineral suspension, where backscatter attenuation is heightened due to the associated low scattering cross sections of the particles. Settling titanium dioxide dispersions with concentrations ranging from 0.05 to 3.00 vol% were successfully profiled using ABS transducers of 1, 2 and 4 MHz frequencies. This approach enabled the simultaneous visualisation of both the settling interface and sediment bed formation, generating sedimentation curves and elucidating settling velocities. Furthermore, backscatter attenuation was empirically correlated with the attenuation–concentration relationship established for homogenous dispersions, to obtain concentration profiles of the settling suspensions. The data depicted concentration changes as a function of time in a hindered settling suspension, and allowed observation of the segregation of a size polydisperse suspension. Data were compared with sedimentation data obtained via two common ex situ bench scale techniques. Critically, the acoustic backscatter method was validated as a powerful in situ characterisation tool for opaque concentrated heterogeneous dispersions, with the ability to provide concentration density information in conjunction with settling kinetics that are not easily attainable via other methods.

Illustration of the information retrieved from the acoustic backscatter measurement of a settling titanium dioxide suspension. The image on the left depicts the backscatter profiles of the settling suspension at selected times. The image on the right portrays the corresponding sedimentation curve obtained alongside the concentration changes determined at the selected times.Figure optionsDownload as PowerPoint slide