Mechanical Flotation Cell Research Articles

Computational fluid dynamics simulation of gas-liquid two phases flow in 320 m3 air-blowing mechanical flotation cell using different turbulence models

According to the recently developed single-trough floating machine with the world’s largest volume (inflatable mechanical agitation flotation machine with volume of 320 m3) in China, the gas-fluid two-phase flow in flotation cell was simulated using computational fluid dynamics method. It is shown that hexahedral mesh scheme is more suitable for the complex structure of the flotation cell than tetrahedral mesh scheme, and a mesh quality ranging from 0.7 to 1.0 is obtained. Comparative studies of the standard k-e, k-ω and realizable k-e turbulence models were carried out. It is indicated that the standard k-e turbulence model could give a result relatively close to the practice and the liquid phase flow field is well characterized. In addition, two obvious recirculation zones are formed in the mixing zones, and the pressure on the rotor and stator is well characterized. Furthermore, the simulation results using improved standard k-e turbulence model show that surface tension coefficient of 0.072, drag model of Grace and coefficient of 4, and lift coefficient of 0.001 can be achieved. The research results suggest that gas-fluid two-phase flow in large flotation cell can be well simulated using computational fluid dynamics method.

Modelling of residence time distribution of liquid and solid in mechanical flotation cells

This paper presents the results of modelling the residence time distribution (RTD) in mechanical flotation cells, measured in several plants, using the radioactive tracer technique. Results include mechanical cells of 100–300m3 operated with effective residence times from 2 to 7min. Data were obtained in forced air and self-aerated cells for liquid and non-floatable solids (per size classes). Different RTD model structures including perfect mixing, Large and Small Tanks in Series (LSTS) and by-pass flow plus perfect mixing were evaluated and compared for single cells. The actual mixing regimes were related to the effective residence times and cell designs based on the model fitting. By-pass flow percentages were typically lower than 10%, therefore the perfect mixing model plus a dead time was a suitable model structure in most cases.On the other hand, the arrangement of cells in series was in most cases effective in reducing the bypass flow observed in single cells. In addition, the N mixed tanks-in-series model was used to represent the mixing regime along flotation banks, which were operated with mean residence times in the range of 18–53min. Results showed that the N equivalent tanks-in-series values were closer to the actual values in most cases. In some applications, slight by-pass propagation was observed because of circuit layout, problems with the level control system, solid settling and/or high flowrates.

Effect of ionic strength on bubble coalescence in inorganic salt and seawater solutions

Bubble size is of fundamental importance in the flotation process, as it provides the surface area for particle collection. Typically, weak surfactants (frothers) are added to process water to reduce bubble coalescence. Certain inorganic electrolytes, which occur naturally in some flotation process water, have been shown to mimic the role of frothers. The concentration at which bubble coalescence is inhibited, the critical coalescence concentration, was determined in a 5.5‐L mechanical flotation cell for a series of coalescence inhibiting inorganic salts. To mimic some industrial flotation process water, a synthetic sea salt solution was also tested. It was found that when the multicomponent sea salt solution was broken down into its constituent parts, the addition of the ionic strength of each ion correlated well with the overall ionic strength curve of all the salts tested. The critical coalescence ionic strength ranged from 0.22 to 0.35, with sea salt being 0.26. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2489–2496, 2015

Effect of Dimensionless Parameters and Bubble Surface Area Flux on Flotation Rate Constant

In this study, the effect of dimensionless hydrodynamic parameters (Reynolds Number–Re, Froude Number–Fr, and Weber Number–We) and bubble surface area flux, S b, on flotation rate constant, k, of quartz particles was investigated using laboratory mechanical flotation cell. Maximum flotation rate constant was obtained at Re = 89, 800, Fr = 2.4, and We = 1558. For either more quiescent (Re < 73, 500 and Fr < 1.61) or more turbulent (Re > 98, 000 and Fr > 2.85) conditions, flotation rate constant decreased steadily. Furthermore, the effect of Reynolds number in the k–S b relationship was investigated, and model of was obtained.

Micro- and nano-scale phenomena effect on bubble size in mechanical flotation cell

The micro/nano-scale phenomena affecting the bubble size observed in a mechanical flotation cell were investigated using an extensive dynamic surface property study with commercial laboratory instruments. The results highlighted two distinct dynamic surface properties: a rapid adsorption/desorption rate leading to undetectable dilatation surface elasticity, and a slow adsorption/desorption process taking place within minutes and leading to a significant increase in the dilatation elasticity of the interface. The latter is observed in highly surface-active reagent grade Polypropylene Glycol and in commercial Dowfroth 250 frothers while the rapid adsorption/desorption rate is a characteristic of weakly surface-active reagent grade 1-Pentanol and commercial Dowfroth 200.The bubble size distribution and therefore the Sauter mean diameter results also highlighted the unambiguous similarity in the effect of frother on bubble size between the I-Pentanol and the Dowfroth 200 and indeed between the Polypropylene Glycol and the Dowfroth 250. The present paper examines the similarity between the two groups in order to gain understanding on how the dynamic surface properties affect the bubble size in two-phase systems.

Flotation Separation of Nanyang Kyanite Ore

It is a developing trend for the refractory materials industry that producing refractories using the kyanite ore with minute amounts of titanium. The tests were conducted with an ore sample by a mechanical flotation cell. The isoelectric point of the kyanite ore was found to be pH 6.6. Petroleum sulfonate was found to be an effective collector for kyanite flotation. 55.85% Al2O3is produced with 84.41% kyanite recovery by grinding, desliming, reverse flotation and kyanite flotation.

The influence of submicron particles and salt on the recovery of coarse particles

Coarse particles are more difficult to float. One of the factors that contributes to poor floatability is the stability of froth. The froth formed in industrial flotation cells is typically not strong enough to provide adequate support for coarse and dense particles. The present study investigates how the presence of hydrophobic submicron particles at low concentration increases the recovery of relatively coarse particles through improvement in the froth stability. Silica particles with d80 of approximately 230μm were floated in a laboratory mechanical flotation cell in a collector-free environment in the presence of poly(propylene glycol) 425 as a frothing agent. The hydrophobicity of the feed particles was modified through an esterification process with different alcohols ranging from 3 to 8 hydrocarbon groups to form a coating of intermediate hydrophobicity. Hydrophobised silica submicron particles of 300nm in size were added to the flotation cell at 0.01 and 0.1wt% concentration. The effect of electrolyte, sodium chloride, in the concentration range 10−5–10−1M on the recovery of coarse particles was also investigated. For the feed employed, 1-butanol was found to provide relatively good flotation properties with a possibility for improvement by stabilising the froth phase. Both additives slightly stabilised the froth phase, which resulted in an increase in the maximum recovery of up to approximately 8%. It appeared that the additives had no significant effect on the first-order flotation rate constant.

Evaluation of effect of viscosity changes on bubble size in a mechanical flotation cell

In operating flotation plants, the viscosity of the pulp can vary significantly. Consequently, the resulting impact on bubble size is of interest as many plants experience seasonal changes in water temperature, or particle size changes as ore hardness, mineralogy and throughput fluctuate. However, given its importance in flotation, there existed no mathematical relationship linking bubble size created in flotation machines to the key process variable of fluid viscosity. In this study, a program of investigation to develop such a model was utilizing a pilot-scale mechanical flotation machine, to investigate the effect of water viscosity due to temperature on bubble size distribution. The bubble sizes were determined using a specific bubble viewer and imaging technology. The temperature itself was varied as a method for introducing significant viscosity change. The viscosity-temperature effect introduced a correspondingly significant change in the water viscosity (1619 to 641 μPa·s). It is suggested that a considerably stronger relationship may exist, yielding D32 versus (μ/μ20)0.776, and hence viscosity becomes an important design consideration for plants operating where pulp temperature fluctuations, very small particles or high solid fractions are present.

Frother function–structure relationship: Dependence of CCC95 on HLB and the H-ratio

Although the number and diversity of commercial frothers has steadily increased to meet flotation industry demands, frother selection is still mainly empirical. As part of a general structure–property study, the paper presents a correlation between the critical coalescence concentration (CCC95) and H-ratio for surfactants used as flotation frothers. The CCC95 were determined in 0.8m3 mechanical flotation cell. The H-ratio was a substitution of hydrophile-lipophile balance (HLB) and determined through high resolution proton nuclear magnetic resonance (1H NMR) spectrometry. A large data set, consisting of 45 surfactants from four frother families, was used to develop the correlation. It is shown that the H-ratio can substitute for HLB. The potential of NMR both to identify the frother family and to derive the H-ratio in predicting CCC95 for commercial frothers is discussed.

Critical coalescence concentration of inorganic salt solutions

Critical coalescence concentration (CCC) was determined in a laboratory-scale mechanical flotation cell for a series of coalescence inhibiting inorganic salts (KCl, NaCl, Na2SO4, CaCl2 and MgSO4) compared to two commercial frothers (methyl isobutyl carbinol, Dowfroth 250C). The salt CCC values ranged from 0.07M (MgSO4) to 0.31M (KCl and NaCl) and correlated with ionic strength. The CCC values are compared to transition concentrations in the literature. The effect of salts on gas dispersion in flotation systems is discussed.

Validity of critical coalescence concentration in dynamic conditions

The aim of the study was to confirm that the concept of the critical coalescence concentration (Cho and Laskowski, 2002) is only applicable to static conditions.A new experimental setup was developed. The experimental idea was to sample the same batch of bubbles at their formation point and later, during the bubble rise in static and dynamic conditions. The difference in the measured bubble size in the two sampling areas gives indication on the effect of frothers on bubble coalescence and breakup that takes place after the bubble creation due to the dynamic conditions.Three commercial frothers, DF200, NF240 and DF250, were tested in the experiments.In static conditions, the size of the forming and rising bubbles became equal at the frother concentration equal to the critical coalescence concentration (CCC) observed previously in mechanical flotation cell. On the other hand, in dynamic conditions, the size of rising bubbles was significantly larger than the size of freshly formed bubbles not only below the CCC as expected based on the literature. This finding confirms the hypothesis that the CCC is only applicable in static conditions, or in conditions where hydrodynamic forces do not contribute to bubble coalescence or breakup. The results showed that the frothers have a major effect on bubble size already at the early stage of bubble formation.

Gas dispersion measurements in mechanical flotation cells: Industrial experience in Chilean concentrators

This technical note presents results of gas dispersion measurements carried out in industrial mechanical flotation cells from Chilean concentrators. Results showed that superficial gas rate, gas holdup and bubble size were within ranges found in literature. The bubble surface area fluxes (SB) were computed using the superficial gas rates and the Sauter mean diameters, obtaining approximate SB ranges from 20m2/s/m2 to 60m2/s/m2 for both self-aerated and forced-air flotation cells. Despite the different cell types, sizes and wide range of operating conditions, an exponential relationship between superficial gas rate (JG) and Sauter mean diameter (D32) was obtained: D32=D0·eγ·JG,D0=0.77±0.11,γ=0.77±0.09 This result allows predicting D32 as a function of JG. Data on gas holdup (εG) as a function of Sauter mean diameter were also presented: For JG<1.2cm/s, gas holdup increases as superficial gas rate increases, indicating a dominant effect of superficial gas rate; for JG>1.2cm/s, gas holdup decreases as D32 increases, indicating a predominant effect of bubble coalescence.

TPH removal from oily wastewater by combined coagulation pretreatment and mechanically induced air flotation

The objective of this study was to combine coagulation and mechanically induced air flotation in order to determine the optimum operational condition for the removal of total petroleum hydrocarbons (TPHs) from oily wastewater. A mixture of water, gas oil, and emulsifier was used to generate a stable emulsion as the simulated wastewater. Aluminum sulfate, ferric chloride, and polyaluminum chloride were employed as coagulants to destabilize the emulsions, and the subsequent separation of hydrocarbons was carried out in a mechanical flotation cell. Considering the multiplicity of factors in both stages of coagulation and flotation, the effects of each factor were studied using Taguchi design of experiments technique, and the optimum conditions were revealed. The results showed that at the optimal conditions, about 93% TPH removal was achieved by addition of 50 mg/l aluminum sulfate at pH 4, flotation time of 10 min, impeller speed of 1,000 rpm, and airflow rate of 4.5 l/min.

Purification of Refractories Andalusite of High Quality

It is a developing trend for the refractory materials industry that producing refractories using the andalusite ore with minute amounts of titanium. The tests were conducted with an ore sample by a high gradient magnetic separator and a mechanical flotation cell. The isoelectric point of the andalusite ore was found to be pH 5.9. Petroleum sulfonate was found to be an effective collector for andalusite flotation. 52.08% Al2O3is produced with 52.92% andalusite recovery by grinding, desliming, high gradient magnetic separation and andalusite flotation.

Study of solid and liquid behavior in large copper flotation cells (130 m2) using radioactive tracers

The behavior of the solid and liquid phases, in large flotation cells, was characterized by means of the radioactive tracer technique. The use of radioactive tracers enabled the identification of the Residence Time Distribution, of floatable and non-floatable solid, from continuous (on-line) measuring at the output streams of the flotation cells. For this study, the proper radioactive tracers were selected and applied in order to characterize the different phases; i.e. for liquid phase Br-82 as Ammonium Bromide, for floatable solid recovered in the concentrate Cu-64, and for non-floatable solid in three particle size classes (coarse: >150 μm, intermediate: <150 μm and >45 μm, and fine: <45 μm), Na-24. The experimental results confirmed the strong effect of particle size on the Residence Time Distribution, and mean residence time of solids in larger flotation cells, and consequently in flotation hydrodynamics. From a hydrodynamic point of view, the experimental data confirmed that a single mechanical flotation cells, of large size, can deviate significantly from perfect mixing. The experimental work was developed in a 130 m3 industrial flotation cell of the rougher circuit at El Teniente Division, Codelco-Chile.

Correlation of air recovery with froth stability and separation efficiency in coal flotation

Recent research progress in hard rock mineral flotation shows that froth stability can be represented by air recovery, which is defined as the fraction of air entering a flotation cell that overflows the weir in unburst bubbles, and that air recovery has strong correlation with the separation performance of mineral flotation. Yet no experimental work on air recovery has been devoted to coal flotation. This paper studies air recovery in coal flotation and examines the links between air recovery, froth stability and coal flotation performance. A series of experiments were conducted using a laboratory-scale mechanical flotation cell at various methyl isobutyl carbinol (MIBC) concentrations and aeration rates. It was found that air recovery has a strong correlation with dynamic froth stability determined by measuring the maximum froth height in a non-overflowing froth column. At a fixed aeration rate (hydrodynamic condition) and various MIBC concentrations, a strong correlation between air recovery and coal flotation performance was also observed.

Control of rougher flotation circuits aided by industrial simulator

Today, large size mechanical flotation cells of 100–300m3 are used in rougher operation in different flotation industrial plants, in Chile and worldwide. However, in spite of the advances in fundamental research and the notable growing in equipment size, with more complete instrumentation systems, there are still a lack of reliable data for industrial flotation modeling and simulation to advance in better control systems design. In this work, a procedure for modeling and simulation of rougher flotation banks, based on operating variables and parameters fitted from industrial data, is presented in condensed form. Recently, a new methodology for describing the industrial flotation, separating the collection and froth zones, has been developed. A non-linear distributed model to simulate rougher flotation circuits was developed, based on measurements of the main operating variables. The simulator was calibrated and tested using experimental data from the rougher operation at El Teniente Division, Codelco-Chile. The simulator was first used to sensitize the effect of changing the froth depth profile on target variables, and then to find out the best profile to reject common disturbances coming into the plant, considering some operating constraints. The flotation circuit simulator was then redesigned to incorporate an expert system following the proposed control algorithm. Several tests were performed by simulation to evaluate the control responses to different correlated series of input variables, such as feed grade, tonnage, particle size distribution, and solid percentage. Control alternatives based on the circuit simulator are discussed.

Flotation of a refuse tailing fine coal slurry

The flotation process performance of a refuse tailing fine coal slurry sample from a Colombian washing coal plant was studied using a mechanical flotation technique. Flotation parameters investigated included dosage of frother methyl-isobutyl-carbinol (MIBC) and collector (kerosene), as well as scrubbing time for the mechanical flotation cell. Flotation kinetics was also studied in the mechanical flotation cell. The results showed that the refuse tailing fine coal slurry was very straightforward to upgrade by flotation. Flotation yield of 45% (dry basis, db), combustible recovery of 92%, product ash of 20%, db and separation efficiency of 79% were obtained when MIBC was used as frother and kerosene as collector. Those results are in agreement with those obtained in the kinetics study, where the best operation condition was using a collector concentration of 2lb/t and a frother concentration of 30ppm of MIBC, which produced the highest flotation rate constant of 0.9457min−1.

Detachment of particles from bubbles in an agitated vessel

The flotation process is widely used to separate valuable minerals from waste in the minerals industry. It is well known that recovery decreases with increasing particle size, and a reason often given is that large particles detach from bubbles in the turbulent shear flow induced by the impeller in mechanical flotation cells. The energy produced by the impeller is transferred to the liquid in the cell and dissipates throughout the tank. In the impeller region, the turbulence is non-isotropic, but in a very short distance, it becomes isotropic, with the typical cascade of eddies from large to micro-scale length scales.The aim of this study is to observe the behavior of particle–laden bubbles in the turbulent shear flow near a rotating impeller in a flotation cell. An agitated vessel was constructed in which bubbles could be introduced beneath the impeller. Bubbles were generated in a liquid-fluidized bed in a special chamber beneath the cell, in which the fluidizing liquid contained a collector that enabled the particles in the bed to adhere to the bubbles. When the bubbles entered the impeller zone, some particles were observed to detach, while others remained attached to the bubble and rose out of the vessel as a flotation product. The fractional detachment of particles was related to the mechanical energy dissipation rate in the region of the impeller. A standard Rushton turbine was used. The results are compared with theoretical predictions of Schulze (1977, 1982).

Characterizing Frothers through Critical Coalescence Concentration (CCC)95-Hydrophile-Lipophile Balance (HLB) Relationship

Frothers are surfactants commonly used to reduce bubble size in mineral flotation. This paper describes a methodology to characterize frothers by relating impact on bubble size reduction represented by CCC (critical coalescence concentration) to frother structure represented by HLB (hydrophile-lipophile balance). Thirty-six surfactants were tested from three frother families: Aliphatic Alcohols, Polypropylene Glycol Alkyl Ethers and Polypropylene Glycols, covering a range in alkyl groups (represented by n, the number of carbon atoms) and number of Propylene Oxide groups (represented by m). The Sauter mean size (D32) was derived from bubble size distribution measured in a 0.8 m3 mechanical flotation cell. The D32 vs. concentration data were fitted to a 3-parameter model to determine CCC95, the concentration giving 95% reduction in bubble size compared to water only. It was shown that each family exhibits a unique CCC95-HLB relationship dependent on n and m. Empirical models were developed to predict CCC95 either from HLB or directly from n and m. Commercial frothers of known family were shown to fit the relationships. Use of the model to predict D32 is illustrated.