Optimization of operating parameters for coarse sphalerite flotation in the HydroFloat fluidised-bed separator

Increasing the upper size limit of coarse particle flotation has been a long-standing challenge in the minerals processing industry. The HydroFloat separator, an air-assisted fluidised-bed separator, has been used in this study to float 250–1180 μm sphalerite particles in batch flotation tests and compared to results achieved utilizing a laboratory-scale conventional Denver cell. The quiescent environment within the HydroFloat cell significantly reduces the turbulent energy dissipation within the collection zone, hence decreasing the detachment of particles from bubbles during flotation. Three operating parameters including bed-level, superficial water and gas rates have been studied, and their effect on the flotation of coarse sphalerite particles is reported. It is shown that coarse sphalerite recovery increases with increasing bed-level, superficial water and gas flow rates. However, there are thresholds for each operating parameter above which recovery starts to decrease. A comparison of recovery with a conventional Denver flotation cell indicates that the HydroFloat separator vastly outperforms the conventional flotation machine for the very coarse particles (+425 μm), and this is mainly attributable to the absence of turbulence and the minimization of a froth zone, both of which are detrimental to coarse particle flotation.     

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.     

New directions in flotation machine design

The theoretical background for flotation kinetics of ultrafine and coarse particles are explored. Recent advances in flotation technology for these difficult areas are reviewed, with a focus on improving the flotation rate of ultrafines, and extending the upper limit for coarse particle flotation.For ultra-fine particles, the theory suggests that the rate of flotation can be improved by increasing the rate of shear in the suspension of particles and bubbles. A new cell has been developed, the Concorde Cell, in which the pre-aerated feed is raised to supersonic velocities before passing into a high-shear zone in the flotation cell. The local dissipation rate is of the order of 100 kW/m³, one to two orders of magnitude higher than is available in conventional mechanical cells. The Concorde Cell has been trialed on a finely ground PGM feed in South Africa, with excellent results. By recycling the tailings, and using the mass pull or solids recovery as the control variable, the Cell is capable of producing a high-grade concentrate at high recoveries, over a wide range of particle sizes.Theory for the upper limit of coarse particle flotation suggests that a quiescent flow field is necessary to prevent the particles from becoming detached from the bubbles. A liquid-fluidized bed provides a suitable environment. The flotation feed is introduced into the fluidized bed, and air bubbles are dispersed in the fluidizing water. Coarse particles attach to the bubbles rising through the bed and are lifted into the froth layer that is maintained on top of the cell in the usual way. Particles of galena up to 1 mm in diameter have been recovered in such a bed, while for particles of lower density such as quartz and coal, the upper limit for flotation has been extended to at least 2 mm and 5.6 mm respectively. The fluidized bed technology provides major advantages beyond the ability to recover coarse particles currently lost in existing operations. Thus, if the upper flotation limit can be extended, the top size for grinding can be raised, with significant reductions in energy costs. Liberation of the values is the key limitation. Also, a fluidized bed flotation cell can accept a feed with much higher percent solids, leading to significant reductions in water requirements.     

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.     

Improving the recovery of coarse coal particles in a Jameson cell

It has been observed in several Jameson cell installation where the source for flotation feed is deslime screens, that the recovery of coal particles greater than 0.5 mm is not as great as that of finer material. Consequently, a research project was undertaken at a CHPP in the Bowen Basin Queensland to assess the possibility of increasing the recovery of coarser particles (+0.5 mm) within the downcomer of the Jameson cell. The effect of decreasing turbulence and agitation in a commercial-scale downcomer was investigated to assess the effect on the recovery of both coarse and fine coal particles.This paper details the findings of the test work, summarising the results relating to differences in the operating parameters within the downcomer.     

Flotation of copper sulphides assisted by high intensity conditioning (HIC) and concentrate recirculation

This paper describes the effect of the partial concentrate (rougher floated product) recirculation to rougher flotation feed, here named concentrate recirculation flotation – CRF, at laboratory scale. The main parameters used to evaluate this alternative approach were flotation rate and recovery of fine (“F” 40–13 μm) and ultrafine (“UF” <13 μm) copper sulphide particles. Also, the comparative effect of high intensity conditioning (HIC), as a pre-flotation stage for the rougher flotation, was studied alone or combined with CRF. Results were evaluated through separation parameters, grade-recovery and flotation rates, especially in the fine and ultrafine fractions, a very old problem of processing by flotation. Results showed that the floated concentrate recirculation enhanced the metallurgical recovery, grade and rate flotation of copper sulphides. The best results were obtained with concentrate recirculation flotation combined with high intensity conditioning (CRF–HIC). The kinetics rate values doubled, the Cu recovery increased 17%, the Cu grade increased 3.6% and the flotation rates were 2.4 times faster. These were accompanied by improving 32% the “true” flotation values equivalent to 2.4 times lower the amount of entrained copper particles. These results were explained and proved to proceed by particle aggregation (among others) occurring after HIC, assisted by the recycled floatable particles. This “artificial” increase in valuable mineral grade (by the CR) resulted in higher collision probability between hydrophobic particles acting as “seeds” or “carrier”.     

Flotation of coarse composite particles in mechanical cell vs. the fluidised-bed separator (The HydroFloat™)

In principle, carrying out flotation of coarse and composite particles in a quiescent flow field is decisive to prevent particles detaching from bubbles. To overcome or limit detachment of coarse composite particles from bubbles in flotation, a fluidised bed separator, the HydroFloat™, which provides a quiescent environment has been used, and the results compared to the performance of a mechanical (Denver) cell. Model synthetic composites of quartz (value mineral) in lead borate (gangue) matrix with simple and complex locking texture were used for the study. The flotation behaviour of particles with different locking textures was studied at a coarse size distribution of 250–600 μm in both the HydroFloat separator and the Denver cell. The recovery of composite particles with the different locking textures was analysed on an un-sized and size-by-size basis. Recovery was improved in the HydroFloat separator, with both simple and complex locking composite particles having almost the same recovery. Again, comparison of recovery with the HydroFloat to the Denver cell indicates that the separator greatly out-performs mechanically agitated cells for the upper particle size of about 500 μm, with a significant effect on complex locking texture composites. This is attributed to the minimum or absence of turbulence and minimal froth zone which causes detachment of coarse particles in most conventional cells.     

Factors affecting the floto-elutriation process efficiency of a copper sulfide mineral

Coarse mineral particles exhibit poor conventional flotation efficiency because of many factors, including the low carrying capacity of bubbles, bubble/particle adhesion problems due to cell turbulence, and low degrees of liberation (low hydrophobicity). Many attempts to improve the recovery of coarse fractions have been explored, such as floto-elutriation operating at a high solid content while dispersed in a fluidized (or expanded) bed formed with a continuous injection of compressed air and an uprising water flow. This work analyzed the comparative performances of floto-elutriation (FE) and conventional flotation (CF) on a classified copper sulfide mineral feed as an example of a difficult-to-liberate low-grade ore. Contrary to expectations, CF and FE (Hydrofloat) displayed similar particle recovery rates with feed size distributions for P80s of 130, 240 and 280 μm. However, metallurgical recoveries from classified fractions of −297+210 μm were 25% higher in FE than in CF and as expected, coarse (+297 μm) particles were not recovered in the CF, but in the FE. The recovery of fine fractions in the FE process was due to high hydraulic entrainment and surprisingly the recovery of intermediate and liberated fractions (+74−149 μm) was very low, due to its low air hold-up. However, the enhancement of the holdup in FE increased the recovery of these mid-sized fractions. Because of the hydraulic carryover caused by the bubbles and water elutriation, the metallurgical grades obtained in all cases were very low compared to conventional bench flotation. It is believed that this FE equipment works better with coarse, narrowly classified particles and high-grade feeds and that performance decreases for low-grade ores requiring high liberation. Certain features of these findings are visualized.     

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.     

Separation of amine-insoluble species by flotation with nano and microbubbles

Amines (alkylamines–ether amines) are employed on a large scale to separate iron ores by reverse flotation of the gangue particles (mostly quartz and silicates). Quartz gangue particles coated with amine collector are dumped in tailings dams as concentrated pulps. Then, the fraction of the amines that detach from the surfaces and the portion that is soluble in water, contaminate surface and ground-water supplies. This work presents a novel flotation technique to remove decyl-trimethyl-ether-amine (collector employed in Brazilian iron mines) from water. This amine forms precipitates at pH > 10.5 which are removed by flotation with microbubbles (MBs: 30–100 μm) and nanobubbles (NBs: 150–800 nm). Bubbles were generated simultaneously by depressurization of air-saturated water (P_sat of 66.1 psi during 25 min) forced through a flow constrictor (needle valve). The flotation by these bubbles is known as DAF-dissolved air flotation, one of the most efficient separation technologies in water and wastewater treatment. Herein, best results (80% amine removal) were obtained only after selective separation of the MBs from the NBs exploring the fact that while the NBs remain dispersed in water, the MBs rise leaving the system. The MBs, because of their buoyancy, rise too rapidly and do not collide and adhere appropriately at the amine colloids/water interface, even causing some precipitates breakage. It was found that the “isolated” NBs attach onto the amine precipitates; aggregate (flocculate) them and entrain inside the flocs before rising by flotation. Because of the low residual amine concentration in water (6 mg L⁻¹), it is believed that this flotation technique have potential in this particular treatment of residual amine-bearing effluents.     

A mixed collector system for phosphate flotation

Flotation has been used in industry for more than a half century as the primary technique for upgrading phosphate. While the flotation of phosphate was inefficient when oleic acid was used alone as a collector, therefore a mixed collector of oleic acid (HOl), linoleic acid (LA) and linolenic acid (LNA) was employed to improve the recovery of phosphate flotation. The batch flotation results showed that the optimal composition of the mixed collector was 54 wt.% HOl, 36 wt.% LA and 10 wt.% LNA. Additionally, the effect of pH on the mixed collector application was studied while considering the surface tension, contact angle and micro-flotation. The results showed that the mixed collector should be used at a pH of 9.5. Above a pH of 9.5, the adsorption of fatty acids dimers on the apatite surface hindered phosphate flotation. The influence of the mixed collector assembly on apatite flotation was also investigated. It was demonstrated that due to its low critical micelle concentration, a sufficiently hydrophobic apatite surface could be generated at a collector concentration of 60 mg/L. In addition, zeta potential experiments suggested that collector adsorption was governed by chemisorption. FTIR and XPS spectra studies further indicated that the chemical reaction involved the carboxyl groups of fatty acids and Ca species at the apatite surface for each fatty acid in the mixed collector.     

Interaction mechanism of miscible DDA–Kerosene and fine quartz and its effect on the reverse flotation of magnetic separation concentrate

In this study, a modification of oil assisted flotation processes of quartz particles has been proposed, which is based on introduction of miscible Dodecylamine (DDA)-Kerosene as collector with DDA cationic surfactant coated on kerosene to the hydrophilic quartz particles in the pulp. The property of miscible DDA–Kerosene emulsion was investigated. Due to the adsorption of DDA at kerosene/water interface, a smaller and uniform kerosene emulsion formed. Addition of cationic surfactant to the kerosene emulsion changed the zeta potential value from negative to positive, which resulted in enhancing the adhesion of the oil droplets to negatively charged quartz. The results showed that agglomeration and flotation process can be realized simultaneously with DDA–Kerosene. The agglomeration of fine quartz minerals in the presence of miscible DDA–Kerosene led to the formation of very large compact agglomerates resulting in increasing hydrophobicity of the particles and inducing a higher probability of collision and adhesion to air bubble. Experimental data indicated that miscible DDA–Kerosene had better selectivity and stronger collectability to quartz than DDA–HCl, which can be used as an efficient collector in the reverse flotation of magnetic separation concentrate of TISCO. At the same DDA dosage (60 g/t), separation efficiency got to 18.53% when using DDA–HCl as collector; while a better result was obtained with DDA–Kerosene, the efficiency of separation reached 59.07% which was identical with 120 g/t DDA–HCl.     

CFD modelling of bubble–particle attachments in flotation cells

In recent years, computational fluid dynamic (CFD) modelling of mechanically stirred flotation cells has been used to study the complexity of the flow within the cells. In CFD modelling, the flotation cell is discretized into individual finite volumes where local values of flow properties are calculated. The flotation effect is studied as three sub-processes including collision, attachment and detachment. In the present work, these sub-processes are modelled in a laboratory flotation cell. The flotation kinetics involving a population balance for particles in a semi-batch process has been developed.From turbulent collision models, the local rates of bubble–particle encounters have been estimated from the local turbulent velocities. The probabilities of collision, adhesion and stabilization have been calculated at each location in the flotation cell. The net rate of attachment, after accounting for detachments, has been used in the kinetic model involving transient CFD simulations with removal of bubble–particle aggregates to the froth layer.Comparison of the predicted fraction of particles remaining in the cell and the fraction of free particles to the total number of particles remaining in the cell indicates that the particle recovery rate to the pulp–froth interface is much slower than the net attachment rates. For the case studied, the results indicate that the bubbles are loaded with particles quite quickly, and that the bubble surface area flux is the limiting factor in the recovery rate at the froth interface. This explains why the relationship between flotation rate and bubble surface area flux is generally used as a criterion for designing flotation cells. The predicted flotation rate constants also indicate that fine and large particles do not float as well as intermediate sized particles of 120–240 μm range. This is consistent with the flotation recovery generally observed in flotation practice. The magnitude of the flotation rate constants obtained by CFD modelling indicates that transport rates of the bubble–particle aggregates to the froth layer contribute quite significantly to the overall flotation rate and this is likely to be the case especially in plant-scale equipment.     

Fundamental studies of Rhodococcus opacus as a biocollector of calcite and magnesite

Rhodococcus opacus, a micro-organism was evaluated as a biocollector for flotation of calcite and magnesite. The electrophoretic behavior of the minerals, before and after R. opacus interaction, was evaluated and showed that the cells adhesion shifted both the minerals zeta-potential curves and the reversal charges in comparison to their original isoelectric points. Adhesion tests suggested a higher affinity of the bacteria for magnesite than calcite. The experiments of the adsorption rate of the R. opacus on calcite and magnesite showed fast kinetics, achieving a maximum of cell adsorption after 5 min. Adsorption isotherm curves for magnesite and calcite could be categorized as Langmurian (L) type II. The best bioflotability results for magnesite and calcite achieved, respectively, values around 93% for a R. opacus concentration of 100 ppm in the pH around 5.0 and 55% for a R. opacus concentration of 220 ppm in the pH around 7.0. The bioflotation experiments were discussed considering the obtained adhesion tests. The fundamental flotation studies showed the paramount potential that R. opacus presents as a biocollector and its possible application in the mineral flotation industry.     

Fluidized bed desliming in fine particle flotation – Part III flotation of difficult to clean coal

A novel flotation system was used to process fine coal feeds supplied from coal preparation plants. The system consisted of an inverted fluidized bed arranged above a system of inclined channels. High fluidization (wash water) fluxes were imposed through a distributor enclosing the free-surface, producing strong positive bias of up to 2.4 cm/s, ideal for desliming. High gas fluxes of up to 2.6 cm/s, in excess of the flooding condition, were also imposed. The presence of the inclined channels prevented the entrainment of gas bubbles into the tailings stream. This paper, which is the third in a series, examines, for the first time, the hydrodynamic performance of this system on two actual plant feeds, each known to be difficult to wash. The first feed was a poorly liberated coal with particle size <260 μm and 69% feed ash. The second was a well liberated coal with nominal size <125 μm and 83% less than 38 μm. The product ash was shown to decrease significantly with an increasing fluidization flux to gas flux ratio. The single stage flotation system demonstrated a performance capable of matching the Tree Flotation Curve with some cases in fact surpassing this result.     

New collectors for the flotation of unactivated marmatite

Three thiophenol collectors were tested for the flotation of marmatite without activation by copper sulfate. Flotation tests showed that marmatite flotation increased according to the following order: 2-fluoro thiophenol < 2-hydroxy thiophenol < 2-amino thiophenol. All these thiophenols exhibited a better flotation property for marmatite than butyl xanthate. Infrared spectra indicated that 2-amino thiophenol and 2-hydroxy thiophenol chemically adsorbed onto the surface of marmatite. 2-Fluoro thiophenol adsorbed physically onto the surface of marmatite. Quantum chemical calculations indicated that the energy of the highest occupied molecular orbital (HOMO) for 2-amino thiophenol was the highest among the tested collectors and we thus determined that its collecting ability should be the strongest. This agreed with the experimentally determined flotation results.     

Detachment of coarse composite sphalerite particles from bubbles in flotation: Influence of xanthate collector type and concentration

The force required to detach sphalerite ore particles from air bubbles has been measured in flotation concentrates, for particles in the size range of 150–300 μm and 300–600 μm with different degrees of liberation. An electro-acoustic vibrating apparatus, that produces typical force conditions experienced in a flotation cell, was used to measure particle–bubble detachment as a function of the vibrational acceleration. Sodium isopropyl xanthate (SIPX) and potassium amyl xanthate (PAX) collectors were used in flotation, at different concentrations. At a fixed frequency of 50 Hz, the maximum vibrational amplitude at which a particle detaches from bubble was used to calculate the particle detachment force. It was shown that changes in surface hydrophobicity (contact angle), due to variations in reagent conditions have significant impact on particles detaching from bubbles. On average, detachment of particles from oscillating bubble correlated well with xanthate concentration and hydrocarbon chain length of xanthate ions. Particles (300–600 μm) with high contact angle obviously required higher force to detach from bubbles than similar particles with lower contact angle. This correlated well with the flotation response at the different reagent conditions. SEM analysis of particles after detachment showed that fully liberated particles attached to bubbles more readily and also gave higher detachment force than composite particles. Moreover larger detachment forces were observed, on average, for particles with irregular shape compared to particles with rounded shape of the same size range.     

Amine–oleate interactions in feldspar flotation

This study concerns the interaction between residual amine (tallow amine acetate) and sodium oleate for floating mica and metal-oxides respectively from a feldspar ore of Cine-Milas region of Turkey. Zeta-potential measurements show the co-adsorption of oleate onto feldspar surfaces only if amine is present. Zeta-potential measurements are in good compliance with laboratory scale flotation tests. Flotation tests show that the amine concentration ought to be kept lower than 1.47 × 10⁻⁵ mol/l (≈5 ppm) to prevent feldspar loss during metal-oxides flotation stage and hence dewatering/washing operation after mica flotation seems to be a crucial step. Feldspar recovery in the concentrate, being 86.67% without dewatering/washing increases to 94.58% after three stage dewatering/washing between mica and metal-oxides flotation stages. In order to get rid of the residual amine, as an alternative to dewatering, 300 g/t bentonite as the residual amine adsorber is used right after mica flotation and it helps to remove 97.7% of the residual amine in the cell. Bentonite addition provided almost the same feldspar recoveries compared with that of dewatering/washing.     

A novel scale-up approach for mechanical flotation cells

Planning industrial flotation operation and earlier flotation equipment sizing are commonly based on batch flotation testing, where ideal operating conditions can be provided. Each plant has its own batch flotation standards and typically uses a time scale-up factor in order to compare laboratory and plant flotation performance. However, flotation scale-up is more complex, and it is not yet completely understood.In this work, a novel scale-up approach was developed, where the effects of the hydrodynamic regime (mixing), solid segregation (effective residence time) and froth recovery on the plant flotation rate were identified and evaluated. Each effect was then described by means of correction factors applied on the batch flotation rate, which was considered the optimal condition. These factors can be determined from laboratory and plant experimental data. This methodology was successfully applied at the rougher copper flotation plant of Codelco Norte Division, Codelco-Chile, for cells of 160 and 300 m³.