C.O. Gomez

Gas dispersion properties: bubble surface area flux and gas holdup

Abstract Bubble surface area flux has been related to flotation performance and advocated as a key “machine variable”. Analysis of available data suggests that bubble surface area flux (S b ) and gas holdup ( e g ) are related by S b ∼ 5.5 e g . This was the case for flotation columns and mechanical cells, both laboratory and plant scale, over the approximate range S b −1 and e g b and e g , are discussed. The possibility to replace S b by e g as the machine variable has the advantage that gas holdup is easier to measure.

Technical note bubble size measurement in flotation machines

A direct bubble size measurement technique which employs a device to collect and expose bubbles has been revived. Rising bubbles are collected into a vertical tube filled with water and directed to a flat viewing chamber at the top for imaging. The dimensions of the device were selected to make the unit suitable for use in industrial mechanical flotation cells and columns. The bubble images, collected using a video camera, are manually processed off-line in the version described here. Operational details as well as practical limitations of the technique are illustrated though measurements completed in two banks of four pilot-scale Denver cells processing a sulphide ore.

Industrial testing of a gas holdup sensor for flotation systems

Abstract The role of gas holdup in flotation has long been discussed but never demonstrated, arguably because a reliable measurement technique has not been available. Work was initiated to develop a gas holdup sensor for industrial operations based on the use of two so-called flow cells for measuring the conductivity of the pulp with and without air. These are the measurements required to estimate gas holdup using Maxwell’s equation that relates conductivity to concentration of a dispersed non-conducting phase (i.e., bubbles) in a continuous liquid phase (pulp in this case). After a series of prototypes a unit robust enough for industrial use that continuously measures and delivers signals easily integrated into a plant PLC system has been developed. This communication describes the working principle along with some construction details. The experience of plant tests, ranging from paper to mineral pulps, and mechanical cells to columns, is reviewed.

Gas dispersion and de-inking in a flotation column

The role of four gas dispersion parameters in ink particle collection was investigated in 4″ and 20″ flotation columns. Gas holdup (eg) and superficial gas velocity (Jg) were measured on-line and bubble size (db) was estimated using drift flux analysis that enabled bubble surface area flux (Sb) to be calculated. Operating with approximately zero froth depth ink recovery as a function of retention time (controlled by underflow rate) was determined. Using a mixing model, the collection zone flotation rate constant (kc) was estimated from the recovery––time data. The rate constant was not related to Jg or db but was linearly dependent on eg and Sb, similar to findings in mineral flotation studies.

AIR ASSISTED SOLVENT EXTRACTION

Abstract Solvent extraction using a solvent coated bubble (air assisted solvent extraction) is introduced. Coating of a bubble with an organic solvent consisting of 3 v/v% (di-(2-ethylhexyl)-phosphoric acid) DEHPA in kerosene is shown to be thermodynamically favourable and is demonstrated experimentally. Aset up to generate a single stream of solvent coated bubbles is described. Trials to extract copper (Cu++) from acidified copper sulphate solution illustrate the concept. On introduit l’extraction par solvant utilisant une bulle revêtue de solvant (“extraction par solvant assistée à l’air”). On montre que le revêtement d’une bulle avec un solvant organique consistant de 3% v/v de DEHPA dans du kérosène est favorable thermodynamiquement et on le démontre expérimentalement. On décrit un montage produisant un courant unique de bulles revêtues de solvant. Des essais d’extraction de cuivre (Cu++) d’une solution acidifiée de sulfate de cuivre illustre le concept.

Characterizing Frothers using Gas Hold-Up

Abstract A way to characterize (classify) frothers using gas hold-up as a surrogate for bubble size was explored. Nine surfactants with a range in chemical structures were selected and tested in a bubble column instrumented to measure gas hold-up and superficial gas velocity. A correlation between frother type and gas hold-up was observed: for alcohols, gas hold-up increases with hydrocarbon chain length and the effect is the same whether the chain is branched or straight; for polyglycols, gas hold-up increased with the number of propoxy groups. The ranking by gas hold-up gave the same result as other, more complex frother characterization techniques.

An automated data acquisition technique for settling tests

An automated technique to measure the settling velocity of particle suspensions has been developed. The technique relies on the measurement of conductance as solids settle through a conductivity cell. The cell consists of two electrodes mounted flush with the wall of a cylinder and separated by a set distance. The technique is illustrated by determining settling velocities for single and mixed sizes of silica, and as means of detecting particle agglomeration.

REVIEWING THE EXPERIMENTAL PROCEDURE TO DETERMINE THE CARRYING CAPACITY IN FLOTATION COLUMNS

Abstract Carrying capacity is a parameter required in the sizing of flotation columns that depends on the characteristics of the generated bubble population and on the mineral density and particle size distribution. Two alternatives were originally proposed by Espinosa-Gomez et al. in the mid-80s for its evaluation: a semi-empirical model based on then available data and an experimental procedure for direct estimation. The model has been improved since, but its calibration still depends on values obtained using the original experimental procedure. To re-evaluate the experimental procedure, tests for different combinations of collection and froth zone heights were performed at three different gas velocities in a laboratory column processing a mixture of hematite/silica particles. The column was instrumented to allow for the continuous calculation of bubble surface area flux which was used to account for changes in the characteristics of the bubble population generated in the column and to explain concentrate particle size variations by the estimation of particle coverage of the bubble surface. The experimental results demonstrate that the collection and froth zone heights as well as the gas rate affect the determination of the carrying capacity. Particle coverage was between 10 and 18% and showed almost no variation in spite of the large increase in the concentrate solids rate. This increase was compensated by the reduction in concentrate particle size. These results lead to the conclusion that both the concept of carrying capacity and its experimental determination method require a deep revision. La capacité de transport, un des paramètres requis pour le dimensionnement des colonnes de flottation, dépend des caractéristiques de la population de bulles générées et de la distribution granulométriques des particules minérales à flotter. Deux alternatives pour estimer sa valeur furent proposées par Espinosa et al. lors de l’introduction de ce paramètre dans les années 80: un model semi-empirique basé sur les données alors disponibles et une procédure expérimentale pour son estimation directe. Le modèle a été amélioré par la suite, mais pour son calibrage, des valeurs encore obtenues par la méthode expérimentale originale sont requises. À objet de re-évaluer cette procédure, des essais réalisés avec différentes combinaisons de hauteur de collecte et de nettoyage à trois taux d’aération différents furent complétés dans une colonne de laboratoire traitant un mélange de particules de silice et d’hématite. La colonne était instrumentée afin de permettre l’estimation en continu du taux surfacique de bulles Sb, variable utilisée pour représenter les caractéristiques de la population de bulles générées dans le diffuseur, ainsi que pour expliquer les variations rencontrées dans la granulométrie du concentré produit, par estimation de la couverture minérale de la surface de bulles. Les résultats expérimentaux obtenus démontrent que la hauteur des zones de collecte et de nettoyage ainsi que le débit de gaz utilisés affectent la détermination de la capacité de transport. Des valeurs de 10 à 18% de couverture furent obtenues, et ce malgré l’augmentation importante du taux de production de concentré observée. Une explication possible serait le fait que cette augmentation ait été compensée par la réduction de la taille des particules récupérées au concentré. Ces résultats indiqueraient que le concept même de capacité de transport ainsi que la procédure actuellement employée pour sa détermination expérimentale requièrent d’une sérieuse révision.

Flotation column amenability and scale-up parameter estimation tests

Abstract A preliminary step in the decision to install flotation columns in an existing separation circuit is the so-called amenability testing. This testing consists of comparing metallurgical results from laboratory columns with, for example the performance of the existing circuit or with laboratory mechanical cells. The following step is to select the size and number of columns required for the duty. One way this is achieved is by using a computer simulator based on a scale-up model. This model requires, among other parameters, the flotation rate constants and the solids removal froth capacity, which have to be experimentally determined. This paper describes the apparatus and methodology used to conduct flotation column amenability and scale-up tests and discusses problems encountered during experience at a number of different concentrators. Amenability tests and scale-up procedure are illustrated using examples from two particular case studies: Mount Isa Mines and Falcombridge Ltd.

Estimation of gas holdup in froths by electrical conductivity: Application of the standard addition method

Abstract Gas holdup has been estimated in air-water froths from electrical conductivity measurements using the “standard addition method”. The technique was verified by estimating the liquid conductivity as well as gas holdup and comparing with independent estimates. The technique was used to measure gas holdup as a function of position in a froth. The resulting “gas holdup profile” suggests a greater variation than in previous work.