Hydrodynamic Study of a Column Flotation using Electrical Resistance Tomography (ERT) and CFD Techniques

Vadlakonda, Balraju and Mangadody, Narasimha (2017) Hydrodynamic Study of a Column Flotation using Electrical Resistance Tomography (ERT) and CFD Techniques. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Column flotation has been widely used in mineral processing and coal preparation industry. Although the basic concept of column flotation is relatively simple, the fundamental principles governing column performance are complex and not easily quantifiable. Performance of a column flotation mainly depends on the hydrodynamic parameters of the process such as gas dispersion, which includes bubble size distribution (BSD), gas hold-up, air superficial velocity (Jg) and bubble surface-area flux (Sb). Because of its high speed competence, low price measurement, robust sensors and non-intrusive nature, Electrical resistance tomography (ERT) is considered to be the most powerful tool compare with other techniques. Apart from experimental methods, there are computational models based on fluid mechanics, if developed for a specific process, would offer better understanding of multi-phase interactions especially for column flotation. The complete understanding of hydrodynamics of column flotation is essential to model and develop design criteria. However its similarity to the bubble column provides a basis for improved understanding of fluid flow in the column flotation. In this work we intend to develop a CFD model for hydrodynamics of column flotation process mainly two-phase (air-water) flow followed by extending to three phase flow (air-water-solids) system. To test the reliability of the CFD models, this work also focuses on experimental investigation of column hydrodynamics using high speed ERT, pressure transducers (PT) and high speed video camera (HSVC). Tomography experiments are carried out to analyse the gas-liquid two phase flow behaviour in fabricated bubble column and column flotation. The gas dispersion characteristics in a 100 mm laboratory column flotation have been investigated in terms of the local and mean gas hold-up, bubble rise velocity and bubble size distribution across the column. This experimental data has been used to demonstrate the validation of the two-fluid CFD model predictions in the column flotation. Using the ERT system, measurement of two phase distributions are examined for a wide range of design and operating conditions of the column including different spargers and frother dosage, where the flow changes from homogenous to transition bubbly flow. It is confirmed by ERT that the gas-holdup increases with an increase in the viii sparger porosity, air superficial velocity, liquid height and liquid feed flow rate. Dynamic gas disengagement technique (DGD) coupled with ERT is utilized to measure bubble rise velocity and sauter mean bubble diameter. For three phase flow of column flotation, the distribution characteristics of combined solid and gas hold-up is studied using ERT coupled with pressure transducers (PT) in the column flotation. The effect of superficial gas velocity, feed slurry flow rate, slurry height, sparger pore number density and frother dosage in the column on mean gas hold-up and its radial distribution has been analysed for three phase systems. Mean gas and solids hold-ups extracted from ERT have been critically assessed for the column operating in various flow regimes. The results show that gas hold-up (εg) increases in the column with an increase in the Jg, whereas solids holdup distribution is very homogeneous for high gas velocities. The presence of solids renders the bubble rise velocity thereby decreases the local gas hold-up. Further feed slurry flow rate and frother effects on column hydrodynamics have been explored and quantified. Some contradictions are observed in the literature regarding gas hold-up change in the presence of solids. Using the DGD method, the effect of solids on gas hold-up has been assessed in terms of bubble rise velocity and sauter mean bubble diameter. DGD would assume that the solids radial distribution is invariant in axial direction in the column. The bubble sauter mean diameter is increased in the presence of solids, indicating possibility of the bubble coalescence. Due to coalescence phenomena, the gas hold-up is less in presence of solids compare with without solids. Initially CFD studies were carried out in rectangular bubble column and in house 100 mm fabricated bubble column. Two-fluid model (TFM) with k-ε turbulence model is employed in these simulations. Suitable interphase forces accounting for virtual mass, lift and drag force are tested and evaluated for bubble plume evaluation and gas dispersion dynamics. Different drag models based on bubble shape and size are tested for correct flow dynamics. Population balance method is coupled with two fluid model to evaluate the evolving transitional flow behaviour by consider bubble break-up and coalescence sub-processes. The liquid-phase velocities and gas hold-up predictions are validated against the measured data in a bubble column. The simulation values are matches well matched with Buwa and Ranade (2003) experimental data for different air superficial velocities. Numerical simulations have been carried out to analyse the hydrodynamic parameters such as mean gas hold-up, axial liquid velocity, sauter mean diameter, and number density fraction for column flotation. The simulations are performed for two CF; 100 mm fabricated laboratory column flotation and literature based 500 mm column flotation. PBM has been used to consider the bubble interactions at various operational parameters such as air superficial velocity, liquid height, sparger pore density and liquid velocity. Simulations also being carried out without PBM. Implementation of PBM method leads to better agreement with experimental data especially in the evolving transitional flow regime (1.2 & 1.8 cm/s air superficial velocity). Because of homogeneous bubbly flow at low air superficial velocities (up to 0.9 cm/s), the predicted mean gas hold-up values are well matched with and without PBM simulation values. The predicted hydrodynamic parameters are validated with ERT experimental values. Three phase CFD simulations are also performed in column by considering hydrophilic silica as a solid phase similar to experiments. Euler-Euler model with three phases coupling with PBM model has been used to explore the hydrodynamics. The solid and gas radial distribution and effect of solids concentration on combined hold-up are investigated in the column. The effect of operational parameters such as air superficial velocity on gas and solid hold-up have been estimated. The predicted numerical simulations values are validated with ERT experimental data. The simulations values are well matched with ERT coupling with PT experimental data.

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