Solid-liquid mass transfer in agitated vessels with high solids concentration
Creators: name: Bong, Eng Ying
Publisher: name: RMIT University
Year: 2024

description: Solid-liquid agitated vessels are widely used in a number of chemical and mineral process industry operations such as adsorption, ion-exchange, leaching, dissolution and crystallisation. Due to the strong demand in the mineral processing industry to process more ore, these vessels are often required to process slurries with higher solids concentration than normally used. The increase in solids concentration will have an effect on impeller power draw and solid-liquid mass transfer coefficient thereby affecting the overall mass transfer rate. It is of interest to the industry to intensify the solid-liquid mass transfer operations to process more minerals whilst minimising energy consumption. Therefore, the main objective of this work is to determine the ‘optimum solid concentration’ and the best impeller type and baffle arrangement that will ensure high impeller energy efficiency input and highest achievable mass transfer coefficient. Experiments were carried out in 0.20 and 0.30 m diameter cylindrical tanks. Four equally spaced vertical baffles were used to study the effect of baffles on mass transfer, critical impeller speed for off-bottom suspension (N<sub>js</sub>) and impeller power draw. Aqueous NaOH solution and cationic ion-exchange resins were used as the liquid and solid phases, respectively. Impellers used in this work were: Rushton disc turbine, 45<sup>o</sup> pitched blade turbine and A310 impeller. Experimental results show the presence of an optimum solids concentration (C<sub>v</sub>)<sub>op</sub>, at which the specific power value is minimum, for all impeller types and it varies between 0.20 and 0.25 (v/v) depending on the impeller type and baffle arrangement. These results indicate that the energy efficiency of solid-liquid mixing vessels can be increased by operating them at an optimum solids concentration, which is higher than those normally used. The specific power values under unbaffled condition are much lower than those under baffled condition indicating that the agitator energy efficiency at high solids concentrations can be improved for all impeller types by removing the baffles. Rushton turbine is found to be more energy efficient compared to other impellers under unbaffled condition whereas it is found to be the least energy efficient under baffled condition. Mass transfer experiments conducted at the critical impeller speed N<sub>js</sub> show that the solid-liquid mass transfer coefficient increases with an increase in solid concentration up to 0.20 (v/v) and decreases thereafter. The solids concentration at which the highest mass transfer coefficient is obtained is designated as the effective solids concentration (C<sub>v</sub>)<sub>eff</sub>. Rushton turbine is found to produce the highest mass transfer coefficient values under both baffled and unbaffled conditions as it has the highest agitation energy efficiency. Removal of baffles has no significant effect on mass transfer coefficient values regardless of the solids concentration and impeller type used. A correlation to estimate the solid-liquid mass transfer coefficient is developed using experimental results and the concept of the Kolmogoroff’s theory of isotropic turbulence to estimate the energy dissipation rate in agitated vessels.<br>