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.     

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.     

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.     

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).     

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).     

Effect of entrainment in bubble load measurement on froth recovery estimation at industrial scale

Froth recovery was calculated in a 130 m³ mechanical cell of a rougher flotation circuit. This was done by bubble load determinations along with mass balance surveys. Valuable grade in the bubble load decreased in the −38 μm due to fine particles entrained to the chamber of the device. The effect of fine particle entrainment on froth recovery was evaluated. A comparison between results from the raw bubble load data (assuming all particles were transported by true flotation) with those from corrected bubble load information (subtracting fine particle entrainment) was carried out. Entrainment occurred due to hydraulic transport in the bubble rear, which corresponds to the worst case scenario for froth recovery estimation. Results showed that the relative error was less than 0.3%, which allowed validation of the bubble load measurement as an effective methodology for froth recovery estimation at industrial scale.     

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.     

Review of rate constants for thiosulphate leaching of gold from ores,concentrates and flat surfaces: Effect of host minerals and pH

A review of literature data for different types of sulphide concentrates and gold ores has been carried out to examine the impact of host minerals and pH upon gold leaching. Analysis of initial rate data over the first 30–60 min of gold leaching from sulphide concentrates or silicate ores over a range of ammonia, thiosulphate, and copper(II) concentrations, pH (9–10.5) and temperatures up to 70 °C shows the applicability of a shrinking sphere kinetic model with an apparent rate constant of the order k_ss = 10⁻⁶–10⁻³ s⁻¹. The dependence of apparent rate constant on pH and initial concentrations of copper(II) and thiosulphate is used to determine a rate constant k_Au(ρr)⁻¹ of the order 1.0 × 10⁻⁴–7.4 × 10⁻⁴ s⁻¹ for the leaching of gold over the temperature range 25–50 °C (ρ = molar density of gold, r = particle radius). These values are in reasonable agreement with rate constants based on electrochemical and chemical dissolution of flat gold surfaces: k_Au = 1.7 × 10⁻⁴–4.2 × 10⁻⁴ mol m⁻² s⁻¹ over the temperature range 25–30 °C. The discrepancies reflect differences in surface roughness, particle size and the effect of host minerals.     

Prediction of the metallurgical performances of a batch flotation system by image analysis and neural networks

It is now generally accepted that froth appearance is a good indicative of the flotation performance. In this paper, the relationship between the process conditions and the froth features as well as the process performance in the batch flotation of a copper sulfide ore is discussed and modeled. Flotation experiments were conducted at a wide range of operating conditions (i.e. gas flow rate, slurry solids%, frother/collector dosage and pH) and the froth features (i.e. bubble size, froth velocity, froth color and froth stability) along with the metallurgical performances (i.e. copper/mass/water recoveries and concentrate grade) were determined for each run. The relationships between the froth characteristics and performance parameters were successfully modeled using the neural networks. The performance of the developed models was evaluated by the correlation coefficient (R) and the root mean square error (RMSE). The results indicated that the copper recovery (RMSE = 2.9; R = 0.9), concentrate grade (RMSE = 1.07; R = 0.92), mass recovery (RMSE = 1.94; R = 0.94) and water recovery (RMSE = 3.07; R = 0.95) can be accurately predicted from the extracted surface froth features, which is of central importance for control purposes.     

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%.     

The use of diffuse reflectance spectroscopy for the characterization of iron ores

The aim of this work was to develop a diffuse reflectance methodology for quantifying minerals in powdered iron ores, which is a key quality control requirement for these materials. Selected samples ranging widely in their concentrations of hematite (as specularite and martite), goethite, magnetite, and quartz were collected in mines from the Iron Quadrangle, Minas Gerais State, and also in the Carajás region, Pará State, Brazil. A chemometric analysis based on the concentrations of the different minerals as determined with a combination of conventional methods (chemical analysis, X-ray diffraction, Mössbauer spectroscopy, light-reflected microscopy, and magnetic susceptibility) and the principal components derived from the diffuse reflectance spectra in the visible range was performed. Principal component regression analysis provided successful calibration for the concentrations of goethite (r² = 0.94; standard error of validation (SEv) = 4.2%) and hematite (r² = 0.89; SEv = 7.4%), in addition to good estimates for quartz (r² = 0.83; SEv = 7.4%), specularite (r² = 0.80; SEv = 11.6%), and martite (r² = 0.78; SEv = 10.6%). Our results suggest that diffuse reflectance spectroscopy is a promising tool for the simultaneous determination of minerals in iron ores within a few minutes only.     

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.     

Single and binary component sorption of the fission products Sr2+, Cs+ and Co2+ from aqueous solutions onto sulphate reducing bacteria

This study investigates the removal of the fission products Sr²⁺, Cs⁺ and Co²⁺ in single and binary metal solutions by a sulphate reducing bacteria (SRB) biomass. The effect of initial concentration and pH on the sorption kinetics of each metal was evaluated in single metal solutions. Binary component equilibrium sorption studies were performed to investigate the competitive binding behaviour of each metal in the presence of a secondary metal ion. Results obtained from single metal equilibrium sorption studies indicated that SRB have a higher binding capacity for Sr²⁺ (q_max = 416.7 mg g^?1), followed by Cs⁺ (q_max = 238.1 mg g^?1), and lastly Co²⁺ (q_max = 204.1 mg g^?1). Among the binary systems investigated, Co²⁺ uptake was the most sensitive, resulting in a 76% reduction of the sorption capacity (q_max) in the presence of Cs⁺. These findings are significant for future development of effective biological processes for radioactive waste management under realistic conditions.     

A simple spectrophotometric method for determination of gold (III) in aqueous samples

A simple method for rapid determination of trace Au in natural water was presented by using UV–vis spectrophotometry after reaction of gold (III) with 3,3′, 5,5′-tetramethylbenzidine hydrochloride (TMBH) in acidic solution. Under the optimum conditions, in a concentration range of 100–2000 μg L^?1 of Au (III) a good linear calibration graph was obtained (r = 0.9969, n = 7). The percent relative standard deviation (RSD) for determination of 1000 μg L^?1 Au was 10% (n = 3) and limit of detection based on a signal-to-noise ratio (S/N) of 3 (3S_bl) was 50 μg L^?1. The proposed method has been successfully applied to the determination of gold spiked and real aqueous samples.     

Heavy metals removal from solution by palygorskite clay

The possible use of palygorskite clay, mined in the Dwaalboom area of the Northern Province of South Africa, as an adsorbent for the removal of metal ions such as lead, nickel, chromium and copper from aqueous solution, was investigated. In this work, adsorption of these metals onto palygorskite has been studied by using a batch method at room temperature. The results of adsorption were fitted to both the Langmuir and Freundlich models. Satisfactory agreement between experimental data and the model-predicted values was expressed by the correlation coefficient (R²). The Langmuir model represented the sorption process better than the Freundlich one, with correlation coefficient (R²) values ranging from 0.953 to 0.994. The adsorption capacity (Q₀) calculated from the Langmuir isotherm was 62.1 mg Pb(II) g⁻¹, 33.4 mg Ni(II) g⁻¹, 58.5 mg Cr(VI) g⁻¹ and 30.7 mg Cu(II) g⁻¹ at a pH of 7.0 at 25 ± 1 °C for a clay particle size of 125 μm. Kinetic investigations were performed to investigate the rate of adsorption of metal ions. The Lagergren’s first-order rate constants were calculated for different initial concentrations of metal ions. In batch mode adsorption studies, removal increased with an increase of contact time, adsorbent amount and solution pH. Adsorption of metals from the single-metal solutions was in the order: Pb > Cr > Ni > Cu. Data from this study proved that metal cations from aqueous solution can be adsorbed successfully in significant amounts by palygorskite. This opens up new possibilities and potential commercial uses in the palygorskite market.     

Biosorption of arsenic(V) with Lessonia nigrescens

Conventional treatment methods for arsenic removal from copper smelting wastewaters create sludge that is difficult to handle. Biosorption of arsenic using algae as sorbent is an interesting alternative to the conventional methods.This work shows results from biosorption of arsenic(V) by Lessonia nigrescens at pH = 2.5, 4.5 and 6.5. The adsorption of arsenic could be explained satisfactorily both by the Freundlich and the Langmuir isotherms. Maximum adsorption capacities were estimated to 45.2 mg/g (pH = 2.5), 33.3 mg/g (pH = 4.5), and 28.2 mg/g (pH = 6.5) indicating better adsorption at the lower pH. These values are high in comparison with other arsenic adsorbents reported.The sorption kinetics of arsenic by L. nigrescens could be modelled well by Lagergren’s first-order rate equation. The kinetics were observed to be independent of pH during the first 120 min of adsorption with the Lagergren first-order rate constant of around 1.07 × 10⁻³ min⁻¹.     

Interaction forces between chemically modified hydrophobic surfaces evaluated by AFM—The role of nanoscopic bubbles in the interactions

The interaction forces between a hydrophobic silicon plate and a silica particle in an aqueous solution were investigated with an atomic force microscope (AFM). The surfaces were hydrophobized chemically by a silane coupling agent, and the hydrophobicity (contact angle θ) of the surfaces was varied. The interactions were long-ranged at θ > 90° with a discontinuous step appearing in the approaching and separating force curves respectively. The range and magnitude of the interaction were decreased with decreasing θ. On the other hand, the interactions at θ = 80° was unstable and no long-range attraction was observed. When the gas phase on the surfaces was removed by flushing organic solvents between the surfaces, the interactions became short-ranged at θ > 90°, and the interaction was described DLVO theory at large distances at θ = 80°. A large number of nano-size domain structures were found on the surfaces by tapping-mode AFM. These results imply that the bridging of nanobubbles is the main origin of the long-range force between chemically hydrophobized surfaces and that the size of the bubble has critical effect on the range and magnitude of the attractive force. The short-range interactions without bubbles were found to consist of an electrostatic repulsive force at larger distances and an attractive force, which was sufficiently longer-ranged than the van der Waals force, at smaller distance.     

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.     

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.     

Oxidation of cyanide in effluents by Caro’s Acid

Caro’s Acid (peroxymonosulphuric acid: H₂SO₅) is a powerful liquid oxidant made from hydrogen peroxide that has been adopted for the detoxification of effluents containing cyanides in gold extraction plants in recent years.The present work reports the findings of a study on the kinetics of aqueous cyanide oxidation with Caro’s Acid. Experiments were conducted in batch mode using synthetic solutions of free cyanide. The experimental methodology employed involved a sequence of two 2³ factorial designs using three factors: initial [CN⁻]: 100–400 mg/L; H₂SO₅:CN⁻ molar ratio: 1–1.5–3–4.5; pH: 9–11; each one conducted at one level of Caro’s Acid strength which is obtained with the H₂SO₄:H₂O₂ molar ratio used in Caro’s Acid preparation of 3:1 and 1:1. The objective was the evaluation of the effect of those factors on the reaction kinetics at room temperature. Statistical analysis showed that the three investigated variables were found to be significant, with the variables which affected the most being the initial [CN⁻] and the H₂SO₅:CN⁻ molar ratio. The highest reaction rates were obtained for the following conditions: H₂SO₅:CN⁻ molar ratio = 4.5:1; pH = 9; and Caro’s Acid strength produced from the mixture of 3 mol of H₂SO₄ with 1 mol of H₂O₂. These conditions led to a reduction of [CN⁻] from an initial value of 400 mg/L to [CN⁻] = 1.0 mg/L after 10 min of batch reaction time at room temperature. An empirical kinetic model incorporating the weight of the contributions and the interrelation of the relevant process variables has been derived as: −d[CN⁻]/dt = k [CN⁻]^1.8 [H₂SO₅]^1.1 [H⁺]^0.06, with k = 3.8 (±2.7) × 10⁻⁶ L/mg min, at 25 °C.