Interactive effects of the type of milling media and CuSO4 addition on the flotation performance of sulphide minerals from Merensky ore Part II: Froth stability

The limitations of pulp chemistry measurements in the flotation of a platinum group mineral (PGM) bearing Merensky ore were demonstrated in Part 1 of this article. In this paper the importance of the contribution of the froth structure due to changing froth stability is analysed using the batch flotation data. The effects of mild steel (MS) and stainless steel (SS) milling media and the addition of copper sulphate on the flotation performance of the sulphide minerals in Merensky ore have been evaluated in relation to the changes in stability of the froth phase. The effects of pulp chemistry and froth stability on the flotation of sulphide minerals were distinguished by using two different rate constants (k_t and k_w). The rate constant (k_w) calculated as a function of cumulative water recovery was used to describe characteristics of froth phase and k_t was calculated as a function of flotation time. The results revealed that the type of grinding media and copper sulphate addition had an interactive effect on the froth stability. While mild steel (MS) milling increased the froth stability due to the presence of hydrophilic iron hydroxides and colloidal metallic iron, the addition of copper sulphate reduced the stability, especially with stainless steel (SS) milling. Copper sulphate addition had a dual role in the flotation of Merensky ore in that it caused destabilisation of the froth zone as well as activation of selected sulphide minerals. The dominant effect was found to depend on the type of milling media and floatability of the mineral in question and this work has demonstrated the importance of using a combination of measurements to evaluate flotation performance holistically.     

Continuous air rate measurement in flotation cells: Some fundamental considerations

A robust and reliable sensor to measure gas (air) superficial velocity (J_g) continuously in flotation systems is introduced. It is based on the sampling of bubbles by buoyancy with the accumulating air allowed to exit through an orifice. At steady state, pressure drop is measured and related to the J_g by prior calibration. The continuous device is readily automated and extended to a multi-unit set-up. The sensor and data collections are described. The governing gas flow equation and models of the dynamic response to air flow rate set point change and fluctuations in froth depth are derived. Model predictions are confirmed against plant data. By taking a moving average, the J_g remains valid in the face of typical plant disturbances.     

The effect of air flow rate on the kinetics of flotation. Part 2: The transfer of material from the froth over the cell lip

The flotation rate of galena was investigated as a function of air flow rate (AFR) and froth thickness (from 0 to 6 cm) in a batch flotation cell designed to produce a quiescent froth-liquid interface. This cell design limits mechanical breakage of the froth and prevents the hydraulic entrainment of fine particles into the froth phase. The overall transfer rate was characterized by a first-order rate constant, the overall rate constant (ORC). The ORC was found to increase with increasing AFR and with decreasing froth thickness. The transfer rate of material from the slurry to the froth was also determined as a function of AFR, and characterizes by its first-order rate constant, the flotation rate constant (FRC).The froth transport constant (FTC), the first-order constant which characterizes the transport of galena from the froth over the cell lip, was then determined from the ORC and the FRC. The FTC was found to increase with increasing AFR and decreasing froth thickness. For a froth thickness of 5 cm and low-to-intermediate AFR, the FTC was found to be significantly lower than the FRC.The FTC of galena particles of 0–12, 12–32 and 22–40 μm was also determined. At a froth thickness of 5 cm, the FTC decreased with increasing particle size over the full experimental range of AFRs.     

A modified laboratory cell for the flotation of coal

The need to obtain accurate, operator-independent data from small-scale laboratory flotation tests has become particularly apparent in the treatment of fine coal. The work described suggests a method for reducing operator dependency by removing the flotation froth uniformly. This is achieved using a perspex insert to change the geometry of the cell, by maintaining a constant pulp level height and by removing froth from the full width of the cell and at a fixed depth.A comparison is made between the modified cell and a conventional Denver cell by obtaining equilibrium and kinetic data for each cell from two independent operators.The comparison shows a reduction in the variability of the results obtained from the modified cell.     

Determination of the contributions of true flotation and entrainment in batch flotation tests

Laboratory batch flotation tests were carried out on a deslimed (+6 μm) sulfiderich cassiterite ore, an ultrafine fraction (?6 μm) of a cassiterite ore and a bituminous coal. Chemical conditions were kept constant but the water recovery was varied by changing the height of the froth column and the rate and depth of froth removed. The recovery of the floatable mineral in each system was then found to be linearly related to the weight of water recovered. The intercept of the regression line on the mineral recovery axis, where the water recovery is zero, was interpreted as the recovery due to true flotation. The entrainment contribution was proportional to the slope of the line. In this way the contributions of entrainment and true flotation to overall recovery were separated.     

A compartment model for the mass transfer inside a conventional flotation cell

A model is developed by taking into account the simultaneous mechanisms of true flotation and entrainment in a conventional flotation cell. The total volume of the cell is divided into three compartments: pulp collection zone, pulp quiescent zone and froth region, with the mechanisms being modeled as occurring at the same time but originating at different places: true flotation from the collection zone and entrainment from the quiescent one. A particle is referred to as suspended in water or attached to an air bubble, depending upon its original state before crossing the pulp–froth interface (whether or not it remains in that state all the way to the concentrate launder). The model is obtained by solving a set of equations describing the mass conservation of solids and water between adjacent compartments. The principal mass transfer factors are identified as: the flotation rate constant, the mean residence time in the collection zone, the froth recovery of attached particles, the degree of entrainment through the froth and the water recovery from the feed to the concentrate. The development presented here allows the intricate nature of the mass transfer in a flotation cell to be reduced to one single equation, overcoming the need of numerical methods for simulation purposes. Moreover, it is shown that reliable prediction of grade and recovery can be obtained without detailed information on the pulp hydrodynamics or on any froth sub-process either than drainage, bubble bursting and bubble coalescence.     

Roughing and cleaning flotation behaviour and the realistic simulation of complete plant performance

Data are presented which illustrate a range of flotation behaviour observed for roughing and cleaning operations in industrial plants. Differences are observed in the size by size recoveries in cleaner flotation from those measured in the roughers.The implications of these differences to flotation modelling are discussed, and the importance of conducting confirmatory laboratory batch flotation tests is emphasized.Batch flotation tests have indicated that for certain minerals the flotation characteristics in the cleaner tests are substantially different from those in rougher tests conducted under the same conditions of pulp level, impeller speed, aeration rate and froth removal. The differences are most pronounced for the coarse particles.Computer simulations of a rougher-cleaner circuit have been done to illustrate the effects of these differences on predicted circuit performance. The differences are significant and arise mainly from differences in the behaviour of coarse particles.     

Strain analysis of deformed granitic rocks (Helikian), Muskoka District,Ontario

Three-dimensional analysis of irrotational, longitudinal, finite strain was carried out on samples from a crescentic sheet which intruded and was deformed with a host gneiss unit of probable Helikian age. Analytical methods were compared using deformed feldspar grains representing four ideal degrees of strain intensity observed in the porphyritic sheet. The polar plot and R_f/φ, R_s methods proved most reliable and sensitive.Data derived from fabric and strain analysis at 38 sites in the units suggest a two-stage deformational sequence. The first stage produced recumbent, isoclinal, similar (class 2) folds with northwest-trending hinge surface traces. This fold form was modified during the second stage to produce an overall type 2 fold interference pattern. The second stage produced upright, open buckle folds as well as the resultant strain fabric currently observed. Strain analysis confirms the general fold geometries of the model, and also documents competency contrasts between the matrix and feldspar grains with increased strain intensity and magnitude. Deformation of feldspar grains in the sheet involved modification of a fabric of low strain magnitude (?_s = 0.3) and a k value near unity to magnitudes of ?_s = 2.6 and k = 0.6. Matrix strain intensities and magnitudes are consistently higher than those of the feldspar markers in the sheet. This variation is related to competency differences between the matrix and the feldspar grains. Fabric anisotropy accounts for the strain gradient observed between the sheet and gneiss.     

Connections between surface complexation and geometric models of mineral dissolution investigated for rhodochrosite

Mineral dissolution rates have been rationalized in the literature by surface complexation models (SCM) and morphological and geometric models (GM), and reconciliation of these conceptually different yet separately highly successful models is an important goal. In the current work, morphological alterations of the surface are observed in real time at the microscopic level by atomic force microscopy (AFM) while dissolution rates are simultaneously measured at the macroscopic level by utilizing the AFM fluid cell as a classic flow-through reactor. Rhodochrosite dissolution is studied from pH = 2 to 11 at 298 K, and quantitative agreement is found between the dissolution rates determined from microscopic and macroscopic observations. Application of a SCM model for the interpretation of the kinetic data indicates that the surface concentration of >CO₃H regulates dissolution for pH < 7 while the surface concentration of >MnOH₂⁺ regulates dissolution for pH > 7. A GM model explains well the microscopic observations, from which it is apparent that dissolution occurs at steps associated with anisotropic pit expansion. On the basis of the observations, we combine the SCM and GM models to propose a step-site surface complexation model (SSCM), in which the dissolution rates are quantitatively related to the surface chemical speciation of steps. The governing SSCM equation is as follows: R = χ_1/2(k_co + k_ca)[>CO₃H] + χ_1/2(k_mo + k_ma)[>MnOH₂⁺ ], where R is the dissolution rate (mol m⁻² s⁻¹), 2χ_1/2 is the fraction of surface sites located at steps, [>CO₃H] and [>MnOH₂⁺ ] are surface concentrations (mol m⁻²), and k_co, k_ca, k_mo, and k_ma are the respective dissolution rate coefficients (s⁻¹) for the >CO₃H and the >MnOH₂⁺ surface species on obtuse and acute steps. We find k_co = 2.7 s⁻¹, k_ca = 2.1 × 10⁻¹ s⁻¹, k_mo = 4.1 × 10⁻² s⁻¹, k_ma = 3.7 × 10⁻² s⁻¹, and χ_1/2 = 0.015 ± 0.005. The rate coefficients quantify the net result of complex surface step processes, including double-kink initiation and single-kink propagation. We propose that the SSCM model may have general applicability for dissolution far from equilibrium of flat mineral surfaces of ionic crystals, at least those that dissolve by step retreat.     

Relationship between hydrogeological parameters for data-scarce regions: the case of the Araripe sedimentary basin, Brazil

This paper applies and validates a method for generating spatially distributed hydraulic conductivity (k) based on the specific capacity (Q _s) for data-scarce regions. This method has been applied to the Araripe sedimentary basin, Brazil, and consists of four steps: (1) selection of (32) wells for which both k and Q _s data are available; (2) estimation of k as a function of Q _s for the (128) wells for which only specific capacity data are available; (3) spatial distribution of k using the kriging geostatistical tool; (4) validation of the method, using (17) representative wells with k measured data. The equation relating k and Q _s showed a statistically significant linear relationship (R = 0.93), from which a database has been generated using kriging with the spherical model. The results showed a calibration coefficient of Nash and Sutcliffe (NS) of 0.54 and moderate spatial dependence ratio of 69 %. The validation process provided only a moderate efficiency (NS = 0.22), possibly due to the geological complexity of the focus system. Despite its limitations, the method indicates the possibility of application of ordinary kriging to generate reliable data from auxiliary variables, especially for the water management of data-scarce areas.     

Mixing and gangue dispersion in flotation machine pulps

Tracer experiments using quartz in sized ranges to simulate gangue minerals provide information on how impeller speed, air rate, and particle size determine the concentration of gangue in the uppermost pulp layers. Tests with and without a froth will contribute towards an understanding of the selective action of the froth column.Results indicate that the laboratory machine-operating conditions required for flotation coincide with a hydrodynamic region which is responsible for fine-gangue transfer from the pulp into the froth column by elutriation and entrainment process acting in the pulp, thus compromising the selective action of the flotation process itself.Topics are suggested requiring further investigation and mention is made of machine design features to which attention should be paid in attempting to reduce gangue transfer from pulp to froth.     

Time-Variant Reliability Analysis of Unsaturated Soil Slopes Under Rainfall

Time-variant reliability analysis for a typical unsaturated soil slope is performed. Eight rainfall conditions are considered, and three slope models are set up for studying the influence of shear strength parameters, hydraulic conductivity parameters, rainfall intensity and duration on the reliability of the soil slope. Sensitivity analysis shows that when the saturated hydraulic conductivity (k _s) is very small, the variation of hydraulic conductivity has little effect on the reliability index (β). For saving the computation effort, only the shear strength parameters are needed in performing the reliability analysis in this condition. With the increase of k _s, the importance of hydraulic conductivity becomes large. The reliability index of the soil slope is changing with time (t), and the shape of β–t curves for different slope model is quite different for they depend on the value of k _s. When k _s is very small, β keeps decreasing for all the 18 simulation days. With the increase of k _s, β decreases to its minimum value at about the cessation day of rainfall events, and it then increases gradually due to the redistribution of suction in the soil slope.     

Studies on the flotation of sulphides. I. The effect of Cu(II) ions on the flotation of zinc sulphide

The effect of Cu(II) ions on the froth flotation of ZnS in the absence of any collector has been studied in a Fuerstenau microflotation cell. The flotation of pure synthetic ZnS, in both cubic and hexagonal modifications, can be greatly enhanced by the adsorption of traces of Cu(II) ions. In the case of cubic ZnS, the “activating” effect of Cu(II): (a) involves the stoichiometric replacement of one Zn(II) ion in the lattice by one Cu(II) ion from solution at a rate which is independent of pH (1–5); (b) is noticeable at coverages corresponding to as little as 0.1% of a monolayer, reaches a maximum at coverages of 2–100% of a monolayer, and decreases again at higher coverages; (c) is unaffected by the presence or absence of O₂, by variation of the pH of flotation (6–10), by heating in vacuum at 250°C for two hours or by storing in the dry state for over one year; and (d) does not appear to involve any significant change in surface area or in the state of aggregation of the particles.     

A study of a plug-flow model for flotation froth behaviour

The importance of the cleaning action of the froth has been recognised. In order to observe the effect of various mechanisms on the gradients of mineral concentration in the froth, experiments were performed in a flotation cell using a deep froth phase where transverse motion of the froth was reduced to a minimum. The variables chosen for study include gas rate, baffling of the froth and product removal rate. The mineral studied comprised a mixed Cu, Zn and Fe sulphide with a siliceous gangue. A model based on counter-current plug-flow assumptions is advanced to describe some of the effects observed. A number of these effects are simulated using this model, leading to a better understanding of some of the processes occurring in the froth.     

Formation and recognition of alkyl xanthyl thiosulphates in sulphide ore flotation liquors

Alkyl xanthyl thiosulphates (R.OCSS.S₂O₃^?) (RXT^?) are formed in solution by mild oxidation (e.g. by I₂) of solutions containing both xanthate and thiosulphate. They can also be formed by reaction of Cu²⁺ with xanthate and thiosulphate, reaction of dixanthogen with thiosulphate, and by reaction of xanthate with tetrathionate; these last three reactions can occur in flotation pulps in slightly acid or alkaline solutions (pH 5–10).Alkyl xanthyl thiosulphates are stable in acid and neutral solution; the solutions have a UV absorption maximum at 289 nm. In strongly alkaline solution (pH 12) RXT^? decomposes within a few minutes to yield a xanthate (mostly) plus a little perxanthate. At pH 10 this decomposition to xanthate takes about 48 h. At pH 7–9 RXT^? is relatively stable. RXT^? is not extracted from aqueous solution with common solvents (chloroform, iso-octane, cyclohexane, or ether). It forms a water-insoluble adduct with cetyltrimethyl-ammonium bromide (CTAB); this adduct can be extracted into chloroform, and the extract has a UV absorption maximum at 296 nm.RXT^? was found in solutions from the gangue-sulphide flotation section at Renison Ltd, the zinc flotation circuit and the copper flotation circuit at Mount Isa Mines Ltd, and the lead flotation section of The Zinc Corporation Ltd. The presence of RXT^? in operating flotation plants has various practical and theoretical implications.     

On the hydrodynamics of flotation machines

Flotation is realized under highly turbulent flow conditions, so that turbulence may be process-determining, too. Because the principles of turbulent multi-phase flow have still not been fully worked out, one needs integral quantities to characterize the state of turbulence indirectly.To show the hydrodynamic conditions diagrams are introduced in which the power intensity $\text{P}\text{V}{}_{\mathrm{<mtext>t</mtext>}}$ is a function of air flow number c_L; impeller rotational speed n and specific air rate q_L are additional parameters. Diagrams of this kind are also useful for the characterization of the optimum operation conditions. For the flotation of different particle sizes the realization of closely defined power inputs is an important condition to obtain an optimum recovery.     

Modelling transport phenomena in a flotation de-inking column: Focus on gas flow,pulp and froth retention time

A laboratory flotation column using Venturi aerators and a vacuum system to remove froth was used to investigate the contribution of gas flow, pulp flow, cell volume and froth retention time on the ink removal efficiency and on cellulose fibres and mineral fillers loss. The increase in the gas flow from 4 to 8 L/min gave a general rise of particle transport from the pulp slurry to the froth with an ensuing strong increase in ink removal, from 75% to 85%, and water and total loss, from 10% to 40% and 15% to 30%, respectively. Whereas, the increase of the cell volume from 14 to 24 L improved ink removal from 72% to 80% without considerably affecting flotation loss. The rise of the froth retention time in the flotation cell from 5 to 20 s before removal gave a general decrease in the flotation loss from 20% to 11% without a corresponding decrease in ink removal. This trend was interpreted as reflecting poor ink drainage through the froth. The increase of both pulp and froth retention time in the flotation cell appeared as the most favourable way to improve ink flotation selectivity. A mathematical model, describing particle removal during flotation in terms of true flotation, entrainment and drainage, was proposed and used to fit experimental data.     

Studies on the flotation of sulphides. II. The effect of urea on the flotation of zinc sulphide

The effect of three hydrophilic non-electrolytes on the flotation of ZnS has been studied using a Fuerstenau microflotation cell. The rate of flotation increases with increasing concentrations of urea(OC(NH₂)₂), but is unaffected by thiourea and slightly decreased by sucrose. This effect of urea was observed with all samples of ZnS (synthetic and natural) studied, is unaffected by extensive purification of the urea and by the presence or absence of O₂, and is reversed by dilution. Urea also increases the rate of flotation (in a Partridge and Smith cell) in the absence of frother. Surface tension measurements showed the absence of detectable concentrations of surfactants in the urea. The presence of urea has no effect on the rate of sedimentation (i.e. on the degree of aggregation) of ZnS or on the volume of water carried over during flotation. It is suggested that the effect of urea is (at least in part) to increase the rate of bubble-particle attachment by modifying the structure of water.