Influence of turbulence kinetic energy on bubble size in different scale flotation cells

Although the impact of hydrodynamic conditions in a flotation cell is often evaluated by correlating impeller tip speed with bubble size, the literature reports inconsistent results, some showing a reduction in bubble Sauter mean diameter (d₃₂) with increasing impeller speed, others showing little to no effect. A review of these results indicate that cell size may be a factor where small laboratory-scale cells, smaller than 50 L, tend to support the correlation while larger machines do not. This paper demonstrates an alternative approach using the average turbulent kinetic energy (TKE) in place of the impeller speed. Results were obtained using two cells with the same geometry but different size, 5 L and 60 L. Bubble size (d₃₂) was measured using the Anglo Platinum Bubble Sizer. Local velocity and velocity fluctuation were measured using a constant temperature anemometer to estimate the average turbulent kinetic energy (TKE). The effect of impeller tip speed on d₃₂ and TKE as a function of air rate was determined. Combining the results for the two cells showed that d₃₂ initially decreased with increasing TKE to become constant above a critical TKE. The TKE region below critical was associated largely with the 5 L cell and the region above critical more associated with the 60 L cell. The inconsistent data in the literature has been explained by introducing the concept of the critical TKE and it has been confirmed that the reported effect of increasing impeller speed may have its origin in the size of the cells tested: laboratory scale cells showing an effect on reducing bubble size as TKE is below critical while large and industrial scale cells may not as TKE is above critical.     

Effect of impeller rotational speed on the size dependent flotation rate of galena in full scale plant cells

The first three rougher cells in the lead circuit of the Elura concentrator (formerly Pasminco Australia Limited) were selected as the plant cells for investigation. Metallurgical surveys were performed and various hydrodynamic measurements taken, allowing the galena flotation rate constant and the bubble surface area flux (S_b) in these cells to be calculated over a wide range of gas flow rates, and at two impeller rotational speeds. It was determined that altering the impeller rotational speed did not significantly change the rate constant dependency on S_b when flotation was considered on an unsized basis.The analysis was further extended to examine the same cells parameters on a size-by-size basis. The results obtained have been used to identify differences in the flotation behaviour of the various particle size fractions, independently of surface hydrophobicity. It is shown that the physical conditions for effective flotation of fine (<9 μm) and coarse (>53 μm) particle size fractions differ substantially, suggesting that a specific hydrodynamic environment will favour a high flotation rate for fine galena, which may be detrimental to the recovery of coarse galena, and vice versa. These observations are in accord with metallurgical practice that suggest that it is difficult to improve fine particle flotation without also compromising coarse particle stability efficiency simply by modifying the cell hydrodynamics alone. A fundamental flotation model was applied to quantify differences in the flotation rate of the various particle size fractions with impeller rotational speed.     

大型机械搅拌式浮选机槽内固体悬浮的研究

固体悬浮是浮选技术中比较重要的部分,研究重点是如何以最小的能耗获得所需的悬浮效果。大颗粒达到完全离底悬浮状态所需的最低搅拌速度称为临界转速。本文在临界转速条件下进行了固体悬浮试验,从试验数据可以看出,当大颗粒达到完全离底悬浮状态时,槽内各个层面浓度分布、粒级分布及金属量分布非常均匀,说明浮选机的叶轮转速、直径及槽体结构的设计合理,JJF-200大型机械搅拌式浮选机达到了工业应用的要求。     

The effect of hydrodynamic parameters on probability of bubble–particle collision and attachment

In this study the dependence of the impeller speed on the particle size variation was investigated on the quartz particles using laboratory mechanical flotation cell. Maximum recovery was obtained at 1100 rpm. For either more quiescent (impeller speed <900 rpm) or more turbulent (impeller speed >1300 rpm) conditions, flotation recovery decreased steadily. Furthermore, amount of collision probabilities is calculated using various equations. According to this study, maximum collision probability was obtained around 48.35% with impeller speed of 1100 rpm, air flow rate of 15 l/h and particle size of 545 μm and minimum collision probability was obtained around 2.43% with impeller speed of 700 rpm, air flow rate of 15 l/h and particle size of 256 μm. Maximum attachment probability was obtained around 44.16% with impeller speed of 1300 rpm, air flow rate of 75 l/h and particle size of 256 μm. With using some frothers such as poly propylene glycol, MIBC and pine oil, probability of collision increased, respectively. Maximum collision probability was obtained around 65.46% with poly propylene glycol dosage of 75 g/t and particle size of 545 μm.     

A comparison of the critical impeller speed for solids suspension in a bench-scale and a pilot-scale mechanical flotation cell

This paper compares the critical impeller speed results for 6 L Denver and Wemco bench-scale flotation cells with findings from a study by Van der Westhuizen and Deglon [Van der Westhuizen, A.P., Deglon, D.A., 2007. Evaluation of solids suspension in a pilot-scale mechanical flotation cell: the critical impeller speed. Minerals Engineering 20, 233–240; Van der Westhuizen, A.P., Deglon, D.A., 2008. Solids suspension in a pilot scale mechanical flotation cell: a critical impeller speed correlation. Minerals Engineering 21, 621–629] conducted in a 125 L Batequip flotation cell. Understanding solids suspension has become increasingly important due to dramatic increases in flotation cell sizes. The critical impeller speed is commonly used to indicate the effectiveness of solids suspension. The minerals used in this study were apatite, quartz and hematite. The critical impeller speed was found to be strongly dependent on particle size, solids density and air flow rate, with solids concentration having a lesser influence. Liquid viscosity was found to have a negligible effect. The general Zwietering-type critical impeller speed correlation developed by Van der Westhuizen and Deglon [Van der Westhuizen, A.P., Deglon, D.A., 2008. Solids suspension in a pilot scale mechanical flotation cell: a critical impeller speed correlation. Minerals Engineering 21, 621–629] was found to be applicable to all three flotation machines. The exponents for particle size, solids concentration and liquid viscosity were equivalent for all three cells. The exponent for solids density was found to be less significant than that obtained by the previous authors, and to be consistent with values reported in the general literature for stirred tanks. Finally, a new dimensionless critical impeller speed correlation is proposed where the particle size is divided by the impeller diameter. This modified equation generally predicts the experimental measurements well, with most predictions within 10% of the experimental.     

Operating parameters that affect the carrying capacity of column flotation of a zinc sulfide mineral

Test work performed in a pilot-scale flotation column (4 m height × 0.057 m diameter) processing an industrial zinc concentrate (51% w/w Zn as sphalerite, 10.5% Fe, 0.77% Pb, 0.62% Cu, 7.3% NSG, d₈₀ = 110 μm), confirmed the findings of previous work conducted by the authors, that showed there exists a limit in the mass flow rate of solids that can be processed in the column without adversely affecting recovery and solids carrying-rate; this limit is related to the onset of an unusual accumulation of gas in the lower section of the cell due to overloading of gas bubbles. In the present work, the effect of slurry rate (J_t = 0.3–1.7 cm/s) and slurry density (15–35% w/w solids) onto solids recovery and solids carrying-rate were studied under the following experimental conditions: J_g = 1.45 cm/s, 15 ppm Dowfroth, pH = 9.5 and 60 g isopropyl xanthate/ton; froth depth = 0.3 m. The results showed that solids carrying-rate may be maximized by operating the column with a combination of a relatively dense slurry and a relatively small slurry rate. The above behavior is explained in terms of the solids load that air bubble transport under the different operating conditions imposed, which is reflected by the axial air-holdup profile established in the column, as a result of the accumulation of overloaded bubbles in the lower part of the collection zone. It is argued that the slurry rate plays an important role on the onset of this phenomenon since it directly affects the rising velocity of overloaded bubbles, thus being the responsible of such unusual accumulation of gas and of phenomena such as bubble coalescence and lost of bubble surface area.     

Effect of gas rate and impeller speed on bubble size in frother-electrolyte solutions

In a flotation cell, bubble size is a function of both coalescence and breakup phenomena. Two phase tests, conducted in a conventional 5.5 L Denver mechanical flotation cell, studied the effect of impeller speed, gas flow rate and frother concentration on bubble size in various electrolyte-frother solutions. The addition of frother to a synthetic sea salt did reduce the measured bubble size (at certain mechanical conditions); whereas the effect of frother addition to NaCl was too small (when compared to measurement errors) to make significant conclusions. This led to more detailed CCC curves (0–50 ppm MIBC) for NaCl, NaCl + MgCl₂, NaCl + CaSO₄, and NaCl + KCl solutions, at constant electrolyte concentrations, to be conducted. They showed an increase in bubble size with the addition of MIBC. This was attributed to the saturation of frother at the air-water interface, reducing local surface tension gradients that help produce smaller bubbles. This occurrence is typically masked in traditional CCC curves due to the dominance of coalescence effects at low frother concentrations.     

Critical contact angle for coarse sphalerite flotation in a fluidised-bed separator vs. a mechanically agitated cell

This work investigates the critical contact angle for the flotation of coarse (850–1180 μm, 425–850 μm and 250–425 μm) sphalerite particles in an aerated fluidised-bed separator (HydroFloat) in comparison to a mechanically agitated flotation cell (Denver flotation cell). In this study, the surface chemistry (contact angles) of the sphalerite particles was controlled by varying collector (sodium isopropyl xanthate) addition rate and/or purging the slurry with either nitrogen (N₂) or oxygen (O₂) before flotation. The flotation performance varied in response to the change in contact angle in both the aerated fluidised-bed separator and the mechanically agitated cell. A critical contact angle threshold, below which flotation was not possible, was determined for each particle size fraction and flotation machine. The results indicate that the critical contact angle required to float coarse sphalerite particles in a mechanically agitated cell was higher than that in the fluidised-bed separator, and increased as the particle size increased. At the same particle size and similar contact angles, the recoveries obtained by the aerated fluidised-bed separator in most cases were significantly higher than those obtained with the mechanically agitated flotation cell.     

Froth flotation of sphalerite: Collector concentration,gas dispersion and particle size effects

This experimental work on sphalerite flotation investigated the effect on flotation performance of three particle size fractions, namely, coarse (d₈₀ = 100 μm), medium (d₈₀ = 39 μm) and fine (d₈₀ = 15 μm), bubble size distribution, superficial air velocity, and collector dosage. Bubble size distributions were characterized with the image analysis technique. The two-phase (liquid–gas) centrifugal pump and frother addition (MIBC, 5–30 ppm) allowed generating bubble diameters between 150 and 1050 μm, and air holdup ranging from 0.2% and 1.3%. Main results showed that each particle-size distribution required an optimal bubble-size profile, and that sphalerite recovery proceeded from mechanisms involving true flotation (when J_g = 0.04 cm/s and 1.9 × 10⁻⁴ M SIPX). However, cluster-flotation occurs at high collector dosage (when J_g = 0.04 cm/s and d₃₂ between 285 and 1030 μm), and requiring further investigation.     

Enhanced Recovery and Concentration of Positively Buoyant Cenospheres from Negatively Buoyant Fly Ash Particles using the Inverted Reflux Classifier

The enhanced separation of valuable positively buoyant cenosphere particles from negatively buoyant fly ash particles using an Inverted Reflux Classifier (IRC) was examined. The effect of the suspension density on the recovery and concentration was examined in the IRC by operating at different feed pulp densities ranging from 10 wt% to 46 wt%. Using a sufficiently high fly ash concentration, it was hypothesised that a powerful bulk streaming phenomenon develops (Batchelor and Van Rensburg, 1986) within the inclined channels, driving the segregation between the positively and negatively buoyant species. With the feed flow rate, fluidization rate, and flow split to overflow and underflow fixed, the recovery of the cenospheres increased from 61.7% (at 10.1% solids) through to an optimum recovery of 89.9% (at 38.1% solids), before declining rapidly to a recovery of 60.2% (at 46.4% solids). The performance at the optimum of 38.1% pulp density was remarkable, with 3.1 t/(m² h) solids throughput, a single-stage cenosphere recovery of 89.9% and upgrade of 58.6, and throughput advantage over a conventional fluidized bed of 54. Detailed analysis indicated that the inclined channels produced an underlying throughput advantage of 18, with a further factor of 3 attributed to the bulk streaming phenomenon. The separations were also assessed in terms of the partitioning of the cenospheres between the overflow and underflow exit streams, with the sharpest size classification evident at the optimum feed pulp density, with the d₂₅ = 31.5 μm, d₅₀ = 36.5 μm, and d₇₅ = 50.0 μm. The separation was then investigated using different feed flow rates, providing the basis needed for ensuring optimum performance in future pilot scale investigation of this novel technology.     

Froth zone characterization of an industrial flotation column in rougher circuit

In this investigation the froth zone of an industrial column (4 m “diameter” × 12 m “height”) in rougher circuit was characterized. Experiments were carried out at Miduk copper concentrator, Iran. Miduk is a unique copper processing plant which utilizes columns in rougher circuit. Cleaning and selectivity actions in the rougher froth were illustrated using solids and grade profiles along with RTD data. The impact of froth depth (FD) on overall rate constant (k) and k–S_b relationship was evaluated. Dependency of overall flotation kinetics on froth depth and gas velocity (J_g) was modeled by k = 4.97(FD)^?0.87(J_g)^0.80. Froth recovery (R_f) was estimated and modeled in terms of froth residence time of slurry (FRT_Slurry) as R_f = R_f,maxexp(?k × FRT_Slurry). Finally, the correlation between k, S_b (indicative of the collection zone performance) and FRT_Slurry (indicative of the froth zone performance) was modeled by k = 0.02 (FRT_Slurry)^?0.62(S_b)^0.82.     

Selective separation of fine albite from feldspathic slime containing colored minerals (Fe-Min) by batch scale dissolved air flotation (DAF)

In this study, the separation of feldspar minerals (albite) from slimes containing feldspar and iron containing minerals (Fe-Min) was studied using dissolved air flotation (DAF) technique whereby bubbles less than 100 μm in size are produced. Before the flotation experiments with slimes, single flotation experiments with albite and Fe-Min were carried out using DAF in order to obtain optimum flotation conditions for the selective separation of feldspar from the slimes. Flotation experiments were performed with anionic collectors; BD-15 (commercial collector) and Na-oleat. The two methods of reagent conditioning were tested on the flotation performance; traditional conditioning and charged bubble technique. In addition, the effect of pH, flotation time, rising time, and drainage time which influence the selective separation in the DAF system were studied in detail. Overall, the flotation results indicated that the separation of albite from Fe-Min can be achieved with DAF at 5 min of rising time and 5 min of drainage time. Interestingly, these results also showed that the conditioning of the particles with the charged bubbles increased the flotation recovery of Fe-Min compared to the traditional conditioning. Furthermore, the flotation tests with the feldspathic slime sample were carried out under the optimum conditions obtained from the systematic studies using the single minerals. The charged bubble technique produced an albite concentrate assaying 0.33% Fe₂O₃ + TiO₂ and 11.07% Na₂O + K₂O from a slime feed consisting of 1.06% Fe₂O₃ + TiO₂ and 10.36% Na₂O + K₂O.     

A study of atmospheric acid leaching of a South African nickel laterite

Nickel and cobalt acid leaching from a low-grade South African saprolitic laterite using sulphuric acid was studied. Ore characterisation was performed by XRD and XRF. Batch agitation leaching tests were conducted at atmospheric pressure investigating main parameters: particle size and percent solids at 25 °C and 90 °C. Ore characterisation showed that the ore is a saprolitic laterite with nickel present in lizardite. Leaching tests showed that nickel and cobalt could be leached from the ore at atmospheric pressure. Nickel was found to be more leachable from the coarser −106 + 75 μm fraction, with 98% Ni being extracted at 90 °C after 480 min. Cobalt was not favoured by variation in particle size and increased percent solids. Increasing ore percent solids improved nickel extraction at 25 °C however at 90 °C extraction decreased due to a diffusion layer build-up as a result of amorphous colloidal silica. The co-dissolution of magnesium and iron was elucidated. Nickel leaching data at increased temperature and percent solids fit the shrinking core model equation, k_dt = 1−2/3x − (1 − x)^2/3 showing that nickel leaching reaction was diffusion controlled under the set conditions.     

Hydrodynamic and kinetic characterization of industrial columns in rougher circuit

In the present investigation the relationship between collection zone rate constant (k_c) and gas dispersion parameters, viz. bubble size (d_b), superficial gas velocity (J_g), gas hold-up (ε_g) and bubble surface area flux (S_b) was evaluated. Experiments were conducted in an industrial (4 m in diameter and 12 m high) and a pilot (0.1 m in diameter and 4 m high) flotation column in rougher circuit at Miduk copper concentrator in Iran. Gas hold-up was measured using pressure difference technique and mean bubble sizes were estimated from a drift flux analysis. It was found that the collection zone rate constant was not correlated with d_b and J_g solely but was linearly dependent on ε_g and S_b for the range of interest. Collection efficiency (E_k) and floatability factor (P) in the industrial columns were quantified (E_k = 3.1%; P = 7.7 × 10^?3). The influence of operating parameters comprising superficial gas velocity, slurry solids% and frother dosage/type on S_b and flotation kinetics was discussed. Analysis of available industrial data suggested that S_b and ε_g were related by S_b = 4.46ε_g over the range 30 < S_b < 60 s^?1 and 7% < ε_g < 14%.     

Improving flotation energy efficiency by optimizing cell hydrodynamics

Flotation is not a particularly energy intensive process. Therefore, flotation optimization has traditionally been focused on grade and recovery performance improvements. However, with the growing need for energy efficiency and the dramatic increase in flotation cell size in recent years it is worth considering how well energy is utilised within flotation cells. In conventional flotation cells a certain amount of energy is required to meet the basic requirements for flotation (air dispersion, solids suspension and particle-bubble collision). This paper investigates how that energy is dissipated in the flotation cell to determine the most efficient use of the imparted energy. The distribution of turbulence and its effect on flotation kinetics are investigated in a mechanical 3 m³ flotation cell for a range of hydrodynamic conditions. The effect of the different conditions are evaluated considering the Power Number (N_P); a dimensionless number that is a useful hydrodynamic indicator as it represents the ratio of energy added to the flotation cell dissipated as shear to that used to generate bulk flow. Results show that flotation rate in the collection zone and the fraction of the cell with higher turbulence increases as more of the power drawn by the impeller is dissipated as shear in the impeller-stator region (higher Power Number). This should promote higher collision rates and more efficient use of the energy imparted in the flotation cell.     

Fundamental aspects of hematite flotation using the bacterial strain Rhodococcus ruber as bioreagent

Previous research showed the effectiveness of bacterial strains as flotation reagents on Hematite beneficiation. The aim of this work is to study and evaluate Rhodococcus ruber as a biocollector. The sample was conditioned with the biomass suspension by stirring under specific conditions as particle size, biomass concentration, pH solution and conditioning time. The results showed a change in hematite zeta potential profile after interaction with R. ruber, and its adhesion onto the mineral surface was higher at pH 3 and at concentration of 0.60 g/L (10⁹ cells/mL). Flotation studies were carried out in a 0.23 L modified Partridge–Smith cell flotation, and the highest floatability (84%) was achieved at size fraction −53 μm +38 μm under the conditions mentioned before. Complementary floatability studies were performed using the conventional frother Flotanol D24 combined with the R. ruber biomass, finding interesting results for the bigger particle size range. Thus, this research aims to evaluate the efficiency of bioflotation of minerals, particularly hematite, and the potential use of R. ruber as biocollector, projecting its future application in mineral flotation industry.     

Effects of slurry filling and mill speed on the net power draw of a tumbling ball mill

The pool of slurry is known to lower the power drawn to the mill. An attempt to ascertain this observation by relating load orientation to mill power for a range of speeds and slurry fillings was undertaken.To this end, a Platinum ore (−850 μm) was used to prepare a slurry at 65% solids concentration by mass. The Wits pilot mill (552 × 400 mm), initially loaded with 10 mm balls at 20% volumetric filling, was run at 5 different speeds between 65% and 85% of critical. The net power draw and media charge position were measured. After this, the slurried ore was gradually added to the media charge for slurry filling U between 0 and 3. A proximity probe and a conductivity sensor mounted on the mill shell provided a means of measuring both the position of the media charge and that of slurry. The data collected for the load behaviour and net power draw was later analysed.It was found that Morrell’s model could not fully explain the effect of slurry volume on net power draw especially for an under-filled media charge (i.e., for U < 1). The size of lifters and grinding balls used could be the reason for this. That is why a piece-wise function was curve-fitted to the power data to help make sense of the inconsistencies observed.     

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 100–300 m³ operated with effective residence times from 2 to 7 min. 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 18–53 min. 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.     

Boundary conditions for gas rate and bubble size at the pulp–froth interface in flotation equipment

This paper describes the effective boundary conditions for the gas dispersion parameters of bubble size, superficial gas velocity and bubble surface area flux, in mechanical and column flotation cells. Using a number of previously derived correlations, with appropriate simplifying assumptions, and experimental data reported from plant practices, the boundary conditions were identified. Thus, it was shown that these constraints typically allow for a mean bubble diameter range of d_b = 1–1.5 mm and superficial gas rate of J_g = 1–2 cm/s, in order to maximize the bubble surface area flux, S_b = 50–100 s⁻¹. Under these conditions there is no carrying capacity limitation, while keeping a distinctive pulp–froth interface.     

Relationship between the bubble surface flux that overflows and the mass flow rate of solids in the concentrate of flotation processes

This paper presents the relationship between the bubble surface flux that overflows and the mass flow rate of solids in the concentrate. Even though this study was carried out in a flotation column, the knowledge derived from this paper may be applied to all froth flotation processes. The experimental set up was equipped with an image analysis system to estimate the froth bubble diameter and the air recovery. This study describes the difference between the bubble surface flux entering the froth zone (S_bI) and the flux that arrives to the top of the froth (S_bT) and then overflows to the concentrate (S_bO), the latter being most directly related to the mass flow rate of solids in the concentrate. It was observed that the superficial area of the overflow increased with increasing collector addition and air flow rate, but decreased with increasing froth depth and particle size distribution. Visual evidence and experimental results suggest that, it is common that the superficial area of air that overflows in the concentrate is covered by particles. Only when this condition is almost achieved does overflows occur; otherwise, a high level of coalescence and bubble bursting take place at the froth surface. This was concluded after finding compatible trends between the estimated and predicted mass flow rates of solids in the concentrate, when a tractable geometrical model was used (R² = 0.8).