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.     

Evaluation of solids suspension in a pilot-scale mechanical flotation cell: The critical impeller speed

This paper investigates solids suspension in a pilot-scale mechanical flotation cell in terms of the critical impeller speed, N_js. Understanding solids suspension has become increasingly important in recent years due to dramatic increases in flotation cell sizes but appears to be relatively poorly researched. The critical impeller speed is commonly used to indicate the effectiveness of solids suspension in stirred tanks, but has seldom been investigated in flotation cells. In this study, critical impeller speeds were visually determined and concentration profiles were measured through sample withdrawal in a 125 l Batequip (Bateman) pilot flotation cell. Two solids size fractions (75–106 and 150–250 μm) were tested, in ungassed and gassed conditions (J_G = 0, 1 cm/s) and at various impeller speeds (300–900 rpm). The effectiveness of solids suspension was quantified in three ways; (i) the extent of off-bottom solids suspension, (ii) the extent of axial solids distribution (suspension height) and, (iii) the variability of axial solids distribution. Consistent trends were found when these effectiveness criteria were considered against relative impeller speed, in terms of percentage of critical impeller speed, N/N_js. As per definition, off-bottom solids suspension was found to be complete at or above 100% of N_js, with significant sedimentation occurring as the impeller speed dropped below 60% of N_js. Suspension heights consistently reached a level equivalent to 90% of the tank diameter at the critical impeller speed, and dropped off significantly as N/N_js dropped below 60%. The relative standard deviation of the vertical solids distribution consistently reached a value of around 30% at N_js. The paper concludes that the critical impeller speed, as per stirred tanks, is an appropriate measure for describing and benchmarking the effectiveness of solids suspension in a mechanical flotation cell. The authors speculate that, when solids suspension is considered as a precondition to flotation, flotation cells should not be operated at specific ‘impeller speeds’ but rather at specific ‘percentages of critical impeller speed’ analogous to the ‘critical speed’ used in the operation of grinding mills.     

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.     

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.     

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.     

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.     

Packing of particles on the surface of bubbles

In this paper, geometrical packing models were derived to determine the coverage of particles on an air bubble. Nearly spherical glass beads of two size fractions and galena particles were used in the study. The coverage of air bubbles by glass beads was carried out in the concentration range 2.74 × 10^?5–1.65 × 10^?3 mM of CTAB. The results indicated that coverage at all concentrations could be approximated with a hexagonal model with monodispersed particles using the value of d_[4,3]. This could be done with a relative deviation of the packing factor within 15%. The coverage of an air bubble by galena particles was carried out in a collectorless environment. The best models were found to be a hexagonal or square cell using the value of d_[1,0]. Experimental observations on particle packing are given and implications for the froth phase of flotation are discussed.     

Estimation of the actual bubble surface area flux in flotation

The bubble surface area flux, S_B, defined as the ration between the superficial gas rate J_G and the Sauter mean bubble diameter D₃₂, has been widely used to describe the gas phase dispersion efficiency in flotation machines, and from this predict flotation performance, notable mineral recovery to forecast plant economics.In this work, results of bubble size distribution (BSD) generated in a pilot column are analyzed. Using video and image analysis techniques, the impact of different sampling rates on the BSD was evaluated. Measurements were carried out for D₃₂ = 1–2 mm, J_G = 0.5–1.5 cm/s and two frother concentration, with a maximum sampling rate of 100 fps. In addition, the bubble rise velocity in the bubble swarm was measured, as a function of the individual bubble diameter, for different operational conditions.The identification of the BSD depends on the proper selection of the visual field and sampling rate for acquisition and processing of bubble images. Distortion in the estimation occurs because a larger holdup of small bubbles is observed, relative to the overall data set, due to their lower velocity.The actual BSD was obtained by correcting the observed population, considering the effect of bubble rise velocity. Thus, the actual bubble surface area flux, S_B, was calculated. The results were evaluated at a pilot scale (air–water system) as well as an industrial plant scale (air-pulp system).     

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.     

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%.     

Using a high pressure hydrocyclone for solids classification in the submicron range

A 10 mm hydrocyclone was operated using a barite suspension with a maximum particle size of d^max = 7 μm. The test rig was equipped with a piston diaphragm pump for pressures up to 60 bar. At 40 bar and 20 °C, cut sizes d₅₀ were obtained down to 0.7 μm; increasing the temperature to 50 °C resulted in d₅₀ values down to 0.5 μm for a throughput of 0.6 m³/h. Another experiment was conducted at 40 bar using a batch hydrocyclone technology. Only the overflow was recirculated to the feed box, whereas the underflow was discharged via a collection box. Increasing the number of recirculations increased the separation of fines in the submicron range. The results showed that after 20 min particles with d^max = 1 μm were obtained in the hydrocyclone overflow. After 120 min, the particles size distribution had a d^max = 0.5 μm and a mean size of d⁵⁰ = 0.2 μm. This procedure requires high energy consumption and is thus suitable only for fractionating small quantities of particles in the submicron range.     

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.     

Evaluation of a MicrocelTM sparger in the Red Dog column flotation cells

Column flotation cells have been installed in numerous base metal operations around the world. The majority of these cells utilize conventional air-only spargers to introduce air into the bottom of the column. The recent development of instruments to measure the bubble characteristics in these columns has provided a renewed understanding of column behaviour. These new tools provided insight into why the columns at Red Dog Mine had never performed up to expectations. After efforts to optimise the spargers failed to substantially change the bubble size and air efficiency, alternate sparger systems were investigated. The Metso Minerals CISA Microcel sparger system appeared promising and was selected for a full-scale plant trial. The Microcel sparger system was originally developed for the coal industry at the Virginia Centre for Coal and Minerals Processing.In October 2003, a Microcel was retrofitted into one of two 3.66 m diameter flotation columns in the zinc retreat circuit at Red Dog Mine. The operation of the two different sparger systems in parallel allowed a detailed comparison.This paper discusses the performance of the Microcel based on the data collected during several detailed surveys. Bubble size measurements carried out in the pulp zone using the McGill University bubble viewer showed a significant difference in bubble size. The mean Sauter diameter of the bubbles decreased from 3.4 mm for the jetting-type sparger to 1.9 mm for the Microcel sparger. The overall recovery and the recovery by size fraction for both valuable and gangue minerals were compared. Paired t-tests demonstrated that the Microcel column produced a higher concentrate grade (0.6% zinc absolute) and a higher unit recovery (2.8% zinc absolute) than the existing Canadian Process Technologies Inc (CPT) SlamJet column. These improvements provided a payback period of 1.5 months for the $109,000 investment.     

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.     

Effect of frothers and dodecylamine on bubble size and gas holdup in a downflow column

Bubble size and gas holdup were characterized in a two phase gas–water system in a laboratory downflow column. The effect of the cationic surfactant dodecyl amine (MW 185, HLB 10.7) and the frothers MIBC (MW 102, HLB 6.05) and polyglycol F507 (MW 425, HLB 8.63) on the bubble size and gas holdup were investigated. In addition, the effect of blends of MIBC-dodecyl amine (DDA) and F507-DDA on these parameters was assessed. The bubble Critical Coalescence Concentration (CCC) followed the order MIBC > DDA > F507. When blending the frothers with DDA at a concentration below its CCC, the frother CCC decreased and bubbles of finer size were obtained below and above the frother CCC. Static surface tension measurements of aqueous solutions with frothers and DDA as well as with frothers-DDA blends show coadsorption of DDA at the air/aqueous solution interface. The surface tension of aqueous solutions prepared with the blends decreased with the addition of DDA and varied linearly with the frother concentration within the concentration range studied. The gas holdup in the downflow column was determined by the bubble size and decreased with the bubble size. It is shown that frother-DDA blends gave the lowest gas holdup in the downflow column. This work is relevant for the reverse flotation of quartz from iron ores using amine collectors in cells with downflow systems.     

Bubble size measurement in electroflotation

A feature of electroflotation is the ability to create very fine bubbles, which are known to improve flotation performance of fine particles. This study was aimed at determining the hydrogen bubble size generated as a function of current density and electrode geometry. Experiments were performed in a viewing cell that allowed direct visualization of hydrogen bubbles being generated and transported away from platinum wire electrodes of 90, 120 and 190 μm in diameter. The detached bubble diameters varied between 15 and 23 μm in diameter, and for each wire diameter, were little influenced by the applied current in the range 150–350 A/m². The measurements were consistent with those predicted from a simple force balance analysis based on a H₂–Pt–0.2M Na₂SO₄ contact angle of 0.18°. Interestingly, upon detachment, the bubble size increased rapidly, recording up to an 8-fold increase in volume in the first few millimeters of rise, before approaching the steady state diameter of between 30 and 50 μm in the bulk. This increase in bubble size was found to be mostly due to the transfer of dissolve hydrogen into growing bubble while moving through the electrolyte super saturated by dissolved hydrogen gas. The equilibrium bulk diameter was found to be a function of the rate of hydrogen production, bubble nucleation rate, and dissolved gas concentration field. Consequently, electroflotation cells need to be designed to optimise the contact between the supersaturated liquid and the rising bubble plume. By doing this, the volumetric flux of bubbles will be maximised leading to improved flotation performance.     

Removal of Phormidium sp. by positively charged bubble flotation

The objectives of this study were to investigate the behavior of Phormidium sp. during flocculation and negatively or positively charged bubble flotation in order to optimize algal removal processes and identify mechanisms underlying the efficiency of flotation with positively charged bubbles. The nuisance of Phormidium sp. significantly decreases water quality in natural watershed and clogs filter bed in water treatment plant. Although dissolved air flotation has been recently adopted for algae removal, the best method has not been fully investigated. According to theories on dissolved air flotation, the operational conditions affect removal of the process and in this study, the optimum bubble generations was also investigated for better algal removal. Bubbles were generated at two levels of saturated pressure and measured at different bubble concentrations (10%, 20% and 30%), in the absence and presence of coagulants. Bubbles forming at 6 bars and 3 bars were observed at zeta potentials of −30 mV to + 27 mV. The chain-like algae were cultured in the laboratory for 20 days. At the stationary phase, Phormidium sp. sizes ranged from 2 μm to 10 μm in diameter and about 100–200 μm in length. Over a pH range of 4.0–7.0 (increments of 0.5), the negative zeta potentials were −4 mV to −12 mV. Algal removal by flocculation was determined by jar tests and by the batch dissolved air flotation (BDAF) method with bubble generation and flotation. We obtained optimal Phormidium sp. removal with positively charged bubble flotation at a 30% bubble rate at >16 mV and a bubble formed at 6 bars, with removal of up to 85% and 93% of cells and chlorophyll a, respectively. We also demonstrated the efficacy of using positively charged bubbles to remove Phormidium sp. cells and the importance of positively charged bubbles in the rarely reported interaction between bubbles and chain-like algae.     

Kinetics of high-sulphur and high-arsenic refractory gold concentrate oxidation by dilute nitric acid under mild conditions

On one hand, high-sulphur and high-arsenic refractory gold concentrate (HGC) leads to regional ecological damage. On the other hand, it contains a lot of valuable elements. So utilization of it can bring social and environmental benefits. In this paper, the kinetics of HGC oxidation by dilute nitric acid under mild conditions was investigated. The effects of particle size (50–335 μm), reaction temperature (25–85 °C), initial acid concentration (10–30 wt.%) and stirring speed (400–800 rpm) on the iron extraction rate (C_r) were determined. It is obvious that C_r increases with the rise of initial nitric acid concentration, reaction time and stirring speed, but decreases with the increase of particle size. Oxidation kinetics indicates that the rate of reaction is diffusion controlled. The activation energies were determined to be 10.70 kJ/mol in the 10% HNO₃ and 12.25 kJ/mol in the 25% HNO₃.     

Kinetics of saprolitic laterite leaching by sulphuric acid at atmospheric pressure

Mineralogical analyses of the saprolitic laterite material have been characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermal analysis, scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDAX). Results showed that the saprolitic laterite material consists mainly of nickel-substituted lizardite showing the pebble-like morphology and traces of magnetite and phlogopite. Leaching results showed that as much as 84.8% nickel could be leached under the experimental conditions of 10% (v/v) H₂SO₄, 90 °C reaction temperature, leached within 5 min, particle size d₅₀ = 25 μm, stirring at 500 rpm and liquid to solid ratio 3:1. The kinetics of nickel and magnesium leaching from the saprolitic laterite material have been investigated in a mechanically stirred reactor and the activation energies were determined to be 53.9 kJ mol^?1 for nickel and 59.4 kJ mol^?1 for magnesium respectively, which are characteristic for a chemical reaction controlled process. The similarity of the activation energies of nickel and magnesium leaching from the saprolitic laterite material by sulphuric acid means that nickel in lizardite is loosely bound within the octahedral layer and almost all of the nickel could be leached simultaneously with magnesium but without complete decomposition of the silicate structure.