Gas Dispersion Properties Research Articles

Comparing the effect of salts and frother (MIBC) on gas dispersion and froth properties

The Raglan concentrator (Xstrata Nickel) does not employ frother. It was considered this might be the result of the high salt content in the process water (ca. 30 000 ppm). Two-phase (solution–air) and three-phase (slurry–air) tests were undertaken in a laboratory column to quantify the effect of inorganic ions present in the water (a range of polyvalent ions). The measurements focused on gas dispersion (bubble size and gas holdup) and froth overflow rate. The results were compared to a typical frother (MIBC) system. The two-phase tests revealed reduced bubble size, increased gas holdup and limited froth formation in salt solutions. The gas holdup correlated with ionic strength. At an ionic strength ca. 0.4 (=0.4 M NaCl) the increase in gas holdup was comparable to ca.10 ppm MIBC. In three-phase tests on a sulphide ore, bubble size and froth overflow rate were again comparable between 0.4 M NaCl and 10 ppm MIBC. The observations help explain why the Raglan plant can operate without frother addition.

Frother-related research at McGill University

Over the past ten years the Mineral Processing group at McGill University has developed techniques to determine gas dispersion properties (gas superficial velocity, gas holdup, bubble size and bubble surface area flux) in flotation machines. This work is finding application in metallurgical diagnostics and cell characterization. The picture, however, will remain incomplete until the impact of chemistry on bubble production, and hence on gas dispersion, is understood. This has prompted investigations into frothers. There are two areas addressed in this communication: frother analysis and frother characterization. Coincident with the centenary, for 100 years there was no convenient frother analysis procedure. A colorimetric technique originally developed for alcohols had been applied to MIBC (Parkhomovski, V.L., Petrunyak, D.G., Paas, L., 1976. Determination of methylisobutylcarbinol in waste waters of concentration plants. Obogashchenie Rud 21 (2), 44–45). Using this as a starting point, the technique was successfully extended to a wide range of commercial frothers and shown to be robust against most common ‘contaminants’. The technique is readily used on-site and some observations from plant surveys are described. Characterization of frothers has taken two routes, determining water carrying rate and investigating properties of thin bubble films. Second only to transporting particles the recovery of water by bubbles has the most influence on metallurgy. The question posed was whether this ‘water carrying’ property could be related to frother type. In a specially designed column the volume rate of water to the overflow per unit cross-sectional area (‘carrying rate’, J wo) and gas holdup ( ε g) at controlled froth depths were measured. The J wo– ε g relationship proved approximately linear and dependent on frother type, with four frother ‘families’ being identified. Bubble thin films have been studied for soaps and the techniques were adapted for frothers. From infrared analysis it became apparent that the frother molecule, while itself not seen, had an impact on organizing water molecules, apparently forming a film of bound water on the bubble surface. Exploiting the interference pattern generated in UV/Vis the film thickness ( d) was determined; for MIBC d was less than 160 nm while for DF250 d was ∼600 nm. Taking a representative frother from the four families identified above, the water carrying rate at a given gas holdup increased with film thickness. Possible implications of the findings on the role of frother in bubble production are explored.

Some gas dispersion characteristics of mechanical flotation machines

The gas dispersion properties that are the subjects of this communication include three measured parameters, gas superficial velocity (gas rate), gas holdup and bubble size, plus the derived parameter, bubble surface area flux. Over the past 10 years sensors to measure these parameters have been introduced and those originating from McGill University are briefly described. The sensors have seen extensive use in plant campaigns and experiences on four cell types at three sites are discussed. The impact of frother in controlling bubble size by retarding coalescence is illustrated, indicating the need to understand chemistry/machine interactions in interpreting gas dispersion results. An increase in Sauter mean bubble size ( D 32) with superficial gas velocity (gas rate, J g) was found, which was fitted empirically by D 32 = D 0 + C, J g n where D 0 ∼ 0.5–0.6 mm and C and n are fitted parameters dependent on the system (machine and chemistry). Following this, a consistent trend in bubble surface area flux ( S b) vs. J g was also observed. The relationship between Sauter mean and number mean ( D 10) is suggested to interpret the nature of the distribution. Analysing gas holdup vs. gas rate shows both a dependence on location in the cell as well as cell position in a bank, factors which must be considered when comparing data from different plants and circuits. The instrumentation and methodologies being developed are presenting new opportunities for incorporating cell hydrodynamics into metallurgical diagnostics. A case study at one plant serves as illustration.

Rate constant modelling for batch flotation, as a function of gas dispersion properties

Gas dispersion is one of the key factors in the mineral flotation process. It can be expressed by many variables such as bubble size, gas holdup, gas superficial velocity and bubble surface area flux ( S b). In particular, S b has been found to have a strong correlation with the flotation rate constant ( K). Bubble size is important because it is related with the transport capacity and with the flotation probability. It also affects the gas holdup and the bubble surface area flux. In this work bubble sizes are measured using image analysis. An empirical rate constant model is developed to relate K with S b and other dispersion variables. This model is found to be able to predict K for two different batch flotation cells.

Gas dispersion measurements in a flotation cell

Three new techniques for measuring gas dispersion properties: local gas holdup, local gas rate (superficial gas velocity) and bubble size distributions have been developed. Gas dispersion properties were measured in a 50 L lab scale flotation cell, using these techniques. The cell was operated under batch conditions, two-phase system, (gas–liquid). The experimental conditions were modified altering the operating conditions in the cell: impeller speed and gas flow rate. The experiments were conducted at a frother concentration exceeding the critical coalescence concentration. It was found that bubble surface area fluxes measured, were limited to a narrow interval, ranging from 27 to 36 m 2/m 2 s. The results suggest that bubble surface area fluxes in mechanical cells might be limited to a narrow range of values with a rather low upper limit. This paper includes a detailed description of the new methods, describing their advantages and limitations, it also discusses the outcome of the experiments.

Gas dispersion properties: bubble surface area flux and gas holdup

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 ( ε g) are related by S b ∼ 5.5 ε g. This was the case for flotation columns and mechanical cells, both laboratory and plant scale, over the approximate range S b < 130s −1 and ε g < 25%. A relationship is expected given their similar dependence on gas rate and bubble size (both increase as gas rate increases and bubble size decreases). Reasons that a relationship may be obscured, especially in full size units because of the difficulty in defining the overall S b and ε g, are discussed. The possibility to replace S b by ε g as the machine variable has the advantage that gas holdup is easier to measure.