Role of sodium hexametaphosphate in ultra-fine grinding of alumina-doped titanium dioxide

In wet stirred milling of Al-doped TiO2, sodium hexametaphosphate (SHMP) is used as a small molecule dispersant to improve milling performance. In both acidic and basic conditions, SHMP interacts strongly with Al-doped TiO2 and remains stable during milling. Considering the apparent and true particle breakage performance, at pH 10.3, the optimal dosing concentration was 0.16 wt% (0.33θ, sub-monolayer coverage), which is consistent for both dilute (5 vol%) and concentrated (≥ 19.1 vol%) suspensions. At higher SHMP concentration (0.77θ), the increased solution conductivity compromised milling performance by weakly destabilizing the Al-doped TiO2 particles. At the optimal dose concentration of 0.33θ SHMP, particle dispersion is maintained through electrostatic and steric forces which contribute to moderating changes in suspension viscosity (~10% increase) as the particle size is reduced.