Imperial College London

Dr Diego Mesa Peña

Faculty of EngineeringDepartment of Earth Science & Engineering

Research Associate



+44 (0)20 7594 7144d.mesa16 Website CV




2.58DRoyal School of MinesSouth Kensington Campus





Publication Type

14 results found

Mesa D, van Heerden M, Cole K, Neethling SJ, Brito-Parada PRet al., 2022, Hydrodynamics in a three-phase flotation system-Fluid following with a new hydrogel tracer for Positron Emission Particle Tracking (PEPT), CHEMICAL ENGINEERING SCIENCE, Vol: 260, ISSN: 0009-2509

Journal article

Cole K, Brito-Parada PR, Hadler K, Mesa D, Neethling SJ, Norori-McCormac AM, Cilliers JJet al., 2022, Characterisation of solid hydrodynamics in a three-phase stirred tank reactor with positron emission particle tracking (PEPT), Chemical Engineering Journal, Vol: 433, Pages: 1-13, ISSN: 1385-8947

It is challenging to measure the hydrodynamics of stirred tank reactors when they contain multiphase flows comprising liquid, gas bubbles and particles. Radioactive particle tracking techniques such as positron emission particle tracking (PEPT) are the only established techniques to determine internal flow behaviour due to the inherent opacity and density of fluid and the vessel walls. The profiles of solids flow are an important tool for robust reactor design and optimisation and offer insight into underlying transport processes and particle–fluid–bubble interactions for applications such as froth flotation. In this work, measurements with PEPT were performed with two tracer particles differing in surface hydrophobicity to characterise the solids hydrodynamics in a baffled vessel agitated with a Rushton turbine. The location data from PEPT were averaged with time to estimate the probability density function (PDF) of particle velocity in individual voxels. The peaks of these voxel distributions were used to produce profiles of solids flow in different azimuthal and horizontal slices. Bimodal vertical velocity distributions were observed in the impeller radial jet which suggest the particles experienced trajectory crossing effects due to inertia. Statistical tests were performed to compare the velocity distributions of the hydrophilic and hydrophobic tracer particles, which indicated similar average flow behaviour in the liquid or pulp phase of the vessel and differences near the air inlet, in the impeller discharge stream and pulp–froth interface. With tracers designed to represent gangue and valuable mineral species, the differences in velocity reveal interactions such as bubble–particle attachment and entrainment.

Journal article

Mesa D, Quintanilla P, Reyes F, 2022, Bubble Analyser — an open-source software for bubble size measurement using image analysis, Minerals Engineering, Vol: 180, Pages: 107497-107497, ISSN: 0892-6875

Bubble size distribution (BSD) is a factor that is well known for influencing the performance of many industrial processes, such as froth flotation. The most commonly used method for measuring bubble size consists of processing photographs of the bubbles. However, the source code of the algorithms for performing the image processing has been seldom published. This article addresses the above by presenting a comprehensive open-source software for processing images of bubbles, allowing researchers to quantify BSD. This software - Bubble Analyser - includes a standard image processing algorithm that was tested against manually segmented images, showing errors under 5% in the calculation of the Sauter mean diameter, the most common descriptor of BSD. Additionally, Bubble Analyser has been designed to easily incorporate new segmentation algorithms developed by other researchers, in order to expand the software capabilities, allow for algorithm comparisons, and foster collaboration in research.

Journal article

Cole K, Barker DJ, Brito-Parada PR, Buffler A, Hadler K, Mackay I, Mesa D, Morrison AJ, Neethling S, Norori-McCormac A, Shean B, Cilliers Jet al., 2022, Standard method for performing positron emission particle tracking (PEPT) measurements of froth flotation at PEPT Cape Town, MethodsX, Vol: 9, Pages: 101680-101680, ISSN: 2215-0161

Positron emission particle tracking (PEPT) is a technique for measuring the motion of tracer particles in systems of flow such as mineral froth flotation. An advantage of PEPT is that tracer particles with different physical properties can be tracked in the same experimental system, which allows detailed studies of the relative behaviour of different particle classes in flotation. This work describes the standard operating protocol developed for PEPT experiments in a flotation vessel at PEPT Cape Town in South Africa. A continuously overflowing vessel with constant air recovery enables several hours of data acquisition at steady state flow and consistent flotation conditions. Tracer particles are fabricated with different coatings to mimic mineral surface hydrophobicity and size, and a data treatment derived from a rotating disk study is utilized to produce high frequency (1 kHz) location data relative to the tracer activity. Time averaging methods are used to represent the Eulerian flow field and occupancy of the tracer behaviour based on voxel schemes in different co-ordinate systems. The average velocity of the flow in each voxel is calculated as the peak of the probability density function to represent the peak of asymmetrical or multimodal distributions.•A continuously overflowing flotation vessel was developed for extended data acquisition at steady state flow.•The data treatment enabled the direct comparison of different particle classes in the flotation vessel.•The solids flow fields was described by the probability density function of tracer particle velocity measured in different voxel schemes.

Journal article

Mesa D, Cole K, van Heerden MR, Brito-Parada PRet al., 2021, Hydrodynamic characterisation of flotation impeller designs using Positron Emission Particle Tracking (PEPT), Separation and Purification Technology, Vol: 276, Pages: 1-19, ISSN: 0950-4214

Impellers play a key role in flotation cells, as the turbulence generated through agitation aids particle suspension, air dispersion and particle–bubble collision. Therefore, it is important to understand the effect that different impeller designs have on flotation hydrodynamics, as small variations could enhance flotation performance. The study of flotation hydrodynamics is, however, a complex task due to the nature of flotation systems, which are opaque, multiphase, and polydisperse. In this paper, the impact of impeller design modifications on the hydrodynamics of a flotation cell was quantified for the first time in a three-phase system. Two different impeller designs, with and without a stator, were assessed using positron emission particle tracking (PEPT), a technique that allows the position and velocity of radioactive particle tracers within an opaque vessel to be determined. A novel PEPT data analysis strategy, as well as a statistical analysis on the basis of the Jensen–Shannon distance, were used. This statistical analysis, applied for the first time to PEPT data, facilitated the comparison of the different designs, by generating a robust quantification of their hydrodynamic differences. The experimental results showed that the stator significantly modified the hydrodynamics within the flotation cell, distorting the lower mixing loop that is characteristic of radial impellers. The use of a stator also resulted in the reduction of particle velocity and swirling outside of the impeller–stator region, both at the level of the impeller and, notably, at the pulp–froth interface. These findings have important implications for impeller–stator design, evidencing that the impeller has a direct effect on the hydrodynamics of the pulp and froth.

Journal article

Quintanilla P, Neethling SJ, Mesa D, Navia D, Brito-Parada PRet al., 2021, A dynamic flotation model for predictive control incorporating froth physics. Part II: Model calibration and validation, Minerals Engineering, Vol: 173, Pages: 1-15, ISSN: 0892-6875

Modelling for flotation control purposes is the key stage of the implementation of model-based predicted controllers. In Part I of this paper, we introduced a dynamic model of the flotation process, suitable for control purposes, along with sensitivity analysis of the fitting parameters and simulations of important control variables. Our proposed model is the first of its kind as it includes key froth physics aspects. The importance of including froth physics is that it improves the estimation of the amount of material (valuables and entrained gangue) in the concentrate, which can be used in control strategies as a proxy to estimate grade and recovery.In Part II of this series, experimental data were used to estimate the fitting parameters and validate the model. The model calibration was performed to estimate a set of model parameters that provide a good description of the process behaviour. The model calibration was conducted by comparing model predictions with actual measurements of variables of interest. Model validation was then performed to ensure that the calibrated model properly evaluates all the variables and conditions that can affect model results. The validation also allowed further assessing the model’s predictive capabilities.For model calibration and validation purposes, experiments were carried out in an 87-litre laboratory scale flotation tank. The experiments were designed as a randomised full factorial design, manipulating the superficial gas velocity and tailings valve position. All experiments were conducted in a 3-phase system (solid-liquid–gas) to ensure that the results obtained, as well as the behaviour of the flotation operation, are as similar as possible to those found in industrial flotation cells.In total, six fitting parameters from the model were calibrated: two terms from the equation for overflowing bubble size; three parameters from the bursting rate equation; and the number of pulp bubble size classes. After the mode

Journal article

Mesa Pena D, Morrison AJ, Brito Parada PR, 2020, The effect of impeller-stator design on bubble size: implications for froth stability and flotation performance, Minerals Engineering, Vol: 157, ISSN: 0892-6875

The impeller in a mechanical flotation tank plays a key role in keeping particles in suspension, dispersing and breaking-up air bubbles, and promoting particle-bubble collision. However, the turbulent regime generated by the impeller can also affect the pulp-froth interface, destabilising the lower regions of the froth and affecting the overall flotation performance. The effects that pulp zone design modifications have on the froth are, however, poorly understood and have not been well-studied.In this work, we study the impact of impeller-stator design on the performance of a large laboratory-scale flotation tank. Two different impeller designs, with and without a stator, were assessed under a range of air flow rates to determine changes in pulp bubble size, froth stability, and metallurgical recovery. The results allow us to quantify, for the first time, the reduction in bubble size in a three-phase flotation system and the improvement in froth stability due to the use of a stator, and thus the enhancement in flotation performance. An inverse relationship is found between the pulp bubble size and froth stability. It is shown that the impeller designs that exhibited smaller bubble sizes resulted in higher froth stability values and also higher flotation recoveries. These findings provide insights into the links between pulp and froth zone phenomena, paving the way for improvements in flotation tank design that lead towards flotation optimisation.

Journal article

Mesa Pena D, Brito Parada P, 2020, Bubble size distribution in aerated stirred tanks: Quantifying the effect of impeller-stator design, Chemical Engineering Research and Design, Vol: 160, Pages: 356-369, ISSN: 0263-8762

Bubble size is an important variable in aerated stirred tanks as it determines the surface area available for reactions. In the bubble break-up process, impellers play a key role. Despite this importance, research into the effect of impeller design on bubble size is scarce. In this work we study the effect of two impellers, with and without a stator, as well as the effect of airflow rate, impeller speed and surfactant concentration on bubble size.Results show that there is a critical impeller speed above which bubble size is not further decreased, regardless of the airflow. Operating at this critical speed results in the smallest bubble size possible without additional turbulence. The reduction in bubble size caused by a stator was quantified for the first time and, interestingly, it was found that the stator also reduced the critical coalescence concentration. The implications of these findings for the design, evaluation and optimisation of impellers are discussed.

Journal article

Mesa Pena DA, Brito-Parada P, 2019, Scale-up in froth flotation: a state-of-the-art review, Separation and Purification Technology, Vol: 210, Pages: 950-962, ISSN: 1383-5866

Froth flotation has been one of the most important and widely used methods to concentrate minerals since its introduction over a hundred years ago. Over the last few decades, in order to process more mineral while reducing capital costs, flotation equipment has become exponentially larger. The increase in tank volume, however, has brought new challenges in the operation and design of industrial flotation tanks. This review analyses the literature on flotation tank scale-up for the first time, contrasting several techniques and approaches used in both historical and state-of-the-art research. The study of flotation scale-up is crucial for the optimisation of industrial plant performance and the maximisation of laboratory-scale research impact. While important advances in our understanding of flotation have been achieved, large flotation tank design and scale-up has, to a large extent, remained in-house know-how of manufacturing companies. This review of the literature relevant to flotation tank scale-up has resulted in a new classification, dividing the scale-up literature into two main areas of study, namely “Kinetic scale-up” and “Machine design scale-up”. This review indicates that current scale-up rules governing the design of flotation tanks focus mainly on pulp zone kinetic parameters and neglect the effects on the froth zone, despite the importance of froth stability and mobility in determining flotation performance. Froth stability and mobility are closely linked to the distance the froth needs to travel, which increases with tank diameter. Although including internal elements, such as launders and crowders, has been the industrial solution for enhancing froth transport and recovery in larger tanks, the design and scale-up of these elements have not been thoroughly studied. Gaps in our knowledge of flotation are discussed in the context of addressing the scale-up problem, considering froth transport and froth stability. Addressing thes

Journal article

Mesa Pena DA, Morrison A, Brito Parada P, 2018, Effect of impeller design on bubble size and froth stability, International Minerals Processing Congress 2018

Conference paper

Mesa D, Kracht W, Díaz G, 2016, Textural image classification of foams based on variographic analysis, Minerals Engineering, Vol: 98, Pages: 52-59, ISSN: 0892-6875

Froths can be characterised according to several features, such as colour, bubble size distribution, velocity, mobility or texture. In the case of texture, there are some alternatives that can be used to analyse and classify them, like the texture spectrum analysis, the grey-level co-occurrence matrix, or the wavelet texture analysis. In this work, a variogram-based technique is introduced. Variograms are a widely used geostatistical technique to describe the degree of spatial dependence between sample values as separation between them increases, and have been used before to analyse textures in applications that range from microscopy to satellite images. The purpose of the current work is to introduce the variogram-based technique to compare and classify foams (water-air froths) according to their texture, and studying the effect of frother type on the texture of foams generated in a quasi-2D cell and in a laboratory column. In the case of the quasi-2D foams, the variogram-based textural classification algorithm was able to classify foam images according to the frother used, with an accuracy of 88.9%. In the case of the foam images generated in the laboratory column, the results suggest that foam texture is mainly defined by froth type, with some effect of foam height. The column foam images did not show similar characteristics when grouped by foam gas holdup, which was confirmed with the variogram-based textural analysis.

Journal article

Mesa D, Kracht W, Diaz G, 2016, Effect of frother type on foam gas holdup and texture

Conference paper

Mesa D, Kracht W, Diaz G, 2016, Effect of frother type on foam structure

Conference paper

Mesa D, 2015, Estudio del efecto del tipo y concentracion de espumante sobre espumas generadas en presencia de arcillas

Thesis dissertation

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