Investigation of hydrodynamics behavior in a gas-agitated disc and doughnut column for solvent extraction with application to impurity removal from mixed nickel‑cobalt hydroxide precipitate

Abstract

Controlling the dispersed phase droplet size within a reasonable range in the extraction column is crucial for enhancing mass transfer efficiency. Traditional high energy-inputs may result in excessive droplet fragmentation in low interfacial tension systems, posing significant challenges in intensifying the extraction process. This work proposes a novel gas-agitated technology to prevent excessive droplet fragmentation. Three extraction systems, with different physical properties, were selected for the experiment: 4, 30, and 50 % (v/v) TBP/kerosene-Water. Within the range where the dispersed phase and continuous velocities vary from 1.47 × 10⁻³ m/s to 2.51 × 10⁻³ m/s, and the gas velocity ranges from 2.36 × 10⁻³ m/s to 1.18 × 10⁻³ m/s (note the order of these velocities should be clearly distinguished if they represent different phases). The droplet size (d₃₂), dispersed-phase holdup (x_d), and gas holdup (x_g) were investigated in a disc-and-doughnut column (DDC) equipped with a gas-input unit. The aim of this investigation was to understand the influence of gas energy-input on hydrodynamic parameters. The results show that gas-input can expand flooding limitation, and it has the ability to break-up droplets, reducing droplets diameter from 3.6 mm to 1.2 mm approximately, while effectively preventing excessive fragmentation. The value of x_d initially decreases and then increases with the increase in gas velocity. Within the operating gas velocity range, the value of x_g increases from 9.7 % to 22.3 %, but without the effect of two-phase velocity. The modified correlations for droplet diameter, dispersed phase holdup, and gas holdup in the gas-agitated extraction column were proposed with deviations of 11.6 %, 15.9 %, and 4.3 %, respectively. These findings are useful to strengthen the low interfacial tension extraction processes in the future. A preliminary study was conducted to investigate the mass transfer enhancement in a gas-agitated column with a typical low interfacial tension extraction system (P204/kerosene-acidic solution of nickel cobalt complex hydroxide). Under similar conditions, extraction with P204/kerosene shows the improvement of metal extraction efficiency with an increase in gas velocity for agitation. The extraction efficiencies of Mg²⁺ and Mn²⁺ were significantly higher when gas-agitated was employed compared to pulsation. This finding indicates that gas-agitated column with moderate energy input is more suitable for low interfacial tension systems.