Ultra-Fine Grinding of Discretely Heterogeneous Particles: The Mode of Action of Sodium Hexametaphosphate

This work focused upon the stirred wet media milling of pigment grade titanium dioxide which is more formally called alumina doped titanium dioxide (Al-doped TiO2), with a target particle size < 300 nm required for desired product performance. This project considered the inter-dependency between stirred media milling performance, suspension/ material properties and particle-particle interactions, aiming to better accounting for relevant surface chemistry and surface forces within the mill – a colloidal science-engineering approach. Due to Al-doped titanium dioxides innate heterogeneity, the surface and materials chemistry was first considered with milling, with changes to such directly influencing both particle-particle and particle-dispersant interactions. More specifically, the process-structure relationship was deduced, with alteration of the stirrer speed leading to different breakage mechanisms and different particle surface properties. The work then shifted to the determination of the role of sodium hexametaphosphate (SHMP) within stirred media milling of Al-doped TiO2, a model dispersant of great popularity within mineral oxide processing. A two-pronged approach was used here, with SHMPs nano-architecture directly measured via colloidal probe atomic force microscopy (AFM), and SHMPs performance within the mill under a myriad of solution conditions also studied. In doing so, the full picture of SHMPs role within stirred media milling has been evaluated considering the micro-scale (dispersant conformation, particle-particle interactions), the meso-scale (suspension/ material properties) and the macro-scale (milling performance).