SIMS studies of oxidation mechanisms and polysulfide formation in reacted sulfide surfaces

The surface oxidation of metal sulfides in air and aqueous solution is of central importance in mineral separation and environmental control of acid mine drainage. Mechanisms of oxidation, dissolution and surface restructuring have been extensively studied using XPS. High binding energy components in S 2p XPS spectra have been attributed to metal-deficiency, formation of polysulfide S_n²⁻, elemental sulfur and electronic defect structures (ie Cu(I)/ZnS). The assignment of these components in S 2p XPS spectra has, however, left significant uncertainties particularly in the formation of SS bonding in polysulfide species requiring confirmation from other surface analytical techniques.The use of static ToF-SIMS has provided a new avenue for identification of these species and their development in oxidation of the sulfide surfaces. For the iron sulfides, there is a systematic change in the FeS₂/FeS fragment ratio from troilite (FeS) through pyrrhotite (Fe_1−xS) to pyrite (FeS₂) with ratios varying from 0.59, 1.2 to 32 respectively. Similarly, high ratios for FeS_n/FeS are found for pyrite compared with pyrrhotite and troilite mirrored in the S_n/S fragment ratios. Changes in surface oxidation, represented in atomic concentrations and S 2p XPS spectra, are seen in the ToF-SIMS signals for S_n/SO_n ratios in the same iron sulfide sequence. These mass markers, reflecting increased SS bonding, increase in surfaces after oxidation giving further confidence in XPS assignment to polysulfide species.Freshly cleaved galena PbS surfaces reacted in pH8 aqueous solution for increasing periods of time have also shown a systematic increase in S_n/S ratios with increasing at.% of oxidised S_n²⁻ species from XPS spectra. Statistical analysis of oxidised galena has shown that the ratios ²⁰⁶PbO⁺/²⁰⁶Pb⁺ and ²⁰⁸PbOH⁺/²⁰⁸Pb⁺ directly reflect the degree of oxidation of the surface lead species whilst the O⁻/S⁻, S⁻/total — ion yield and SO₃⁻/S⁻ are the best measures for following the oxidation of sulfur species. Results from these ratios suggest that initial air oxidation takes place predominantly on the S sites rather than Pb sites but, in solution at pH9, both sites are oxidised.The ToF-SIMS results appear to directly reflect the surface chemistry of the metal and sulfur species and are not consistent with recombination or fragmentation of secondary neutral or ionic species. The results strongly suggest increasing polymerisation of SS species with increasing oxidation in accord with the XPS assignment to polysulfide of increasing chain length.     

Enhanced hydrochloric acid leaching of cobalt

Dissolution rates of cobalt in hydrochloric acid with and without the presence of thiourea have been studied by means of weight loss and rotating disc (with solution analysis) methods. In the absence of thiourea an apparent activation energy value of 67.7 ± 3.4 kJ. mol⁻¹ was determined from weight loss measurements and 70.3 ± 7.5 kJ. mol⁻¹ using rotating disc techniques. These high values are inconsistent with a diffusion controlled process and this was supported by the lack of dependency of the dissolution rate on the rotational speed. A reaction controlled process is indicated.In 8.4 mol. dm⁻³ hydrochloric acid the thiourea concentration producing optimum dissolution rates was found to be ∼ 1 × 10⁻³ mol. dm⁻³. Above 60°C rates of leaching were enhanced by some 700–800%. Tests using radiochemical labelling of the sulphur and carbon in the thiourea molecule confirmed previous proposals that H₂S is produced as a by-product of the cathodic depolarisation reaction involving the thiourea and is adsorbed on the cobalt surface in the molecular state forming activated anodic sites which enhanced anodic dissolution.     

Review of theory and practice of measuring proton activity and pH in concentrated chloride solutions and application to oxide leaching

The leaching of oxides in high strength brines is advantageous because of favourable leaching kinetics even at atmospheric conditions due to enhanced proton activity, and the catalytic effect of chloride ions due to complexation with metal ions. A proper understanding of hydration theory, practice of measuring proton activities in concentrated acid–chloride systems, and their applications to rationalise leaching kinetics is beneficial for further development of chloride processes. In the present study, a hydration number of 1 or 2 for the chloride ion and a constant ionic activity coefficient for the hydrated chloride ion has been proposed for the theoretical evaluation of pH of concentrated hydrochloric acid solutions. A comparison of the results based on hydration theory and extended Debye–Hückel equation with the experimental results based on acidity functions, glass electrode, and hydrogen electrode shows the importance of liquid junction potential corrections for measured values based on potentiometric methods. The proton activity based on electrode potentials reveals a hydration number of 6–7 which is consistent with an ion-pair hydration model (H₃O⁺)(H₂O)_6–7Cl⁻. Chloride ions facilitate the dissolution of copper(II) oxide and goethite in acid solutions. The rates of dissolution of these oxides in hydrochloric acid solutions are first order with respect to proton activity due to the formation of adsorption complexes such as Cu(OH)Cl⁰ and Fe(OH)₂Cl⁰. Hydration, chloro-complexation and hydrolysis of metal ions play important roles in the selective leaching of nickel over iron from laterite ores by acid–chloride lixiviant system.     

Modification of ilmenite surface properties by superficial dissolution method

Acid surface dissolution as a pretreatment method converts Fe²⁺ ions on the ilmenite surface to Fe³⁺ ions. XPS analysis showed that the content of Fe³⁺ increases from 48.5% to 59.8% after surface dissolution for 15 min in a solution of sulfuric acid with a concentration of 10%. This conversion, without any phase transformation, decreases the zeta potential of ilmenite in a wide pH range, resulting in a shift in IEP (Iso-Electric Point) from a pH of 5.4 to 2.3. FTIR spectra and zeta potential measurements showed that the increase of oleate ions adsorption on the ilmenite surface, resulting from the surface dissolution process, is insignificant. After surface dissolution, the formation of more ferric iron oleate species (Ksp = 10^−29.7) being more stable than ferrous iron oleate (Ksp = 10^−15.5) compounds yields an increase of ilmenite hydrophobicity and floatability in a wide pH range. Using 3.65 × 10⁻⁴ M sodium oleate at a pH of 6.3, the maximum flotation recoveries are obtained as 73.5% and 92% for non-treated and acid pretreated ilmenite, respectively.     

Acid leaching of metals from deep-sea manganese nodules – A critical review of fundamentals and applications

The mass% metal composition of deep sea nodules ranges from 10–28% Mn, 4–16% Fe, 0.3–1.6% Ni, 0.02–0.4% Co, and 0.1–1.8% Cu in mixed oxide matrices with alumina and silica. The concentrations of base metal ions in sea water of pH ∼ 8 of the order 10⁻³–10² nmol/kg are shown to be dependent on the solubility products (K_SP) of carbonate sediments. Cations of higher softness have higher pK_SP and lower solubility. Previously reported leach results of nodules in H₂SO₄ and HCl under atmospheric pressure and temperatures in the range 30–90 °C and in the absence or presence of SO₂, Na₂SO₃, NaCl and CaCl₂ are used in the present study to compare, contrast and rationalise the leaching behaviour of metal values. Leach efficiency of metals increases with increasing acid concentration, and Cu(II) and Zn(II) follow the same trend in HCl. Potential–pH diagrams of base metal oxides show a higher stability of mixed metal oxides such as ferrites, magnetite and manganite, which causes partial dissolution of high-valent oxides in the absence of reducing agents. Application of a shrinking core kinetic model in both H₂SO₄ and HCl media predicts a proton diffusivity of ∼ 10⁻¹¹ cm² s⁻¹ for the dissolution of Ni from nodules. This value is of the same order as D_H⁺ for the high pressure acid leaching of Ni from limonitic laterite. A linear correlation between leaching efficiencies of Fe and Ni appears to be a result of co-dissolution of these metals from NiO·Fe₂O₃ or NiFeOOH. The first order dependence of initial dissolution rates of Cu(II) with respect to H⁺ concentration in H₂SO₄, and the beneficial effect of background chloride ions, suggests a dissolution mechanism: CuO → Cu(OH)Cl_ads/aq → CuSO₄. The Cu(II) ions in solution can also affect Ni(II) dissolution from oxide by cation exchange mechanism. The presence of SO₂ or Na₂SO₃ as reducing agents facilitates the acid leaching of high-valent oxides of Mn and Co and other metals incorporated in the mixed oxide matrix. Whilst Fe(II) ions formed during the reductive leaching of Fe(III)-oxides accelerate the dissolution of Mn(III)/(IV) oxides, the resultant Mn(II) ions accelerate the dissolution of high-valent Co-oxides. Leaching efficiency in HCl increases with temperature. As for the SO₂/H₂O system thermodynamic calculations predict a decrease in concentration of H⁺ and at high temperatures, which retards leaching efficiency. The SO₂/H₂O/air leach system enhances metal dissolution due to the production of H₂SO₄ via the transition metal catalysed oxidation of SO₂ to H₂SO₄.     

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.     

Leaching of chalcopyrite (CuFeS2) with an imidazolium-based ionic liquid in the presence of chloride

The effects of independent variables such as, temperature, concentration of ionic liquid (1-butyl-3-methyl-imidazolium hydrogen sulphate, [bmim][HSO₄]), chloride and sulphuric acid on copper extraction from chalcopyrite (CuFeS₂) ore were studied by surface optimization methodology. The Central Composite Face approach and a quadratic model were applied to the experimental design. The optimal copper extraction conditions given by the above methodology were 20% (v/v) of [bmim][HSO₄] in water, 100 g L⁻¹ chloride, and 90 °C. The concentration of chloride and the temperature together exert a synergistic effect in enhancing chalcopyrite dissolution. Experimental data were fitted by multiple regression analysis to a quadratic equation and analyzed statistically. A model was developed for predicting copper extraction from CuFeS₂ ore with variables such as Cl⁻, [bmim][HSO₄], H₂SO₄ concentrations and temperature in the range studied. The activation energy was calculated to be 60.4 kJ/mol (temperature range 30–90 °C), indicative of chemical control of the reaction and [bmim][HSO₄] acts as an acid in the reaction.     

A comparative study on thermophilic and mesophilic biooxidation of ferrous iron

High temperature biooxidation of ferrous iron was studied in a batch system, using the acidophilic thermophile Acidianus brierleyi. The effect of ferrous iron initial concentration on the growth and activity of the cells was investigated. A. brierleyi was able to grow on ferrous iron at concentrations below 7.5 kg m⁻³. The values of specific growth rate and yield were 0.043 h⁻¹ and 2.2×10¹⁴ cells/kg iron respectively. At ferrous iron concentrations of 7.5 kg m⁻¹³ and higher the growth of the cells was prohibited, however the non-growing cells were able to oxidise iron. The maximum biooxidation rate of ferrous iron, 0.105 kg m ⁻³ h⁻¹, was achieved in a culture initially containing 7.5 kg m⁻³ Fe²⁺. The mesophilic iron oxidiser Thiobacillus ferrooxidans was capable of growing on ferrous iron at concentrations as high as 30 kg m⁻³. Moreover the rate of mesophilic biooxidation offerrous iron was significantly higher than that observed in the presence of A. brierleyi.     

The reduction of chromite in the presence of silica flux

The mechanism and kinetics of the carbothermic reduction of a natural chromite was studied at 1300–1500 °C in the presence of silica. Thermogravimetry, X-ray diffraction (XRD) analysis, energy dispersive X-ray analysis (EDAX) and metallography were the experimental techniques used. Silica affected the reduction at and above 1400 °C. A two stage reduction mechanism was established. The first stage, up to about 40% reduction, is primarily limited to iron metallization and zoning is observed in partially reduced chromites. In this stage silica does not interfere with the reduction. The second stage is mainly confined to chromium metallization and formation of a silicate slag alters the reduction mechanism. Ion-exchange reactions between the reducible cations (Cr³⁺ and Fe²⁺) in the spinel and the dissolved cations (A1³⁺ and Mg²⁺) in the slag allow further reduction. Due to the very high driving force for the diffusion, the overall process is shifted toward a more chemical reaction controlled mechanism. A generalized rate equation was then applied to the individual metallization curves of iron and chromium from which respective rate constants and diffusion coefficients were derived. The rate constants were in the range 6.74 × 10⁻⁴–9.01 × 10⁻⁴ s⁻¹ for iron and 7.20 × 10⁻⁴–8.50 × 10⁻⁴ s⁻¹ for chromium reduction at 1500 °C in the presence of silica. At 1500 °C, the corresponding diffusion coefficients were in the range 3.14 × 10⁻⁸–4.78 × 10⁻⁸ m²/s for Fe²⁺ diffusion in the spinel and in the range 1.70 × 10⁻⁸–2.03 × 10⁻⁸ m²/s for the respective diffusion of Cr³⁺. Finally using Arrhenius plots activation energies were derived.     

Effect of fluidized-bed carrier material on biological ferric sulphate generation

High biomass hold-up and high iron oxidation rates of a biological ferric sulphate generating fluidized-bed reactor (FBR) requires a carrier material with high specific surface area, high porosity and inertness. In this work, the effect of activated carbon (AC), diatomaceous earth (Celite) and Al₂O₃ (Compalox) carrier materials on the ferric sulphate generation in FBRs were studied. Compalox dissolved during the experiments and formed an unfluidizable aggregate, and was therefore rejected. The slow dissolution of Celite resulted in a light, fine-grained, layer on top of the fluidized bed that had to be changed into fresh Celite. AC resisted well the friction caused by fluidization. The iron oxidation in the continuous-flow FBRs became limited by oxygen supply already at loading rates of 2.5 kg Fe²⁺ m⁻³ h⁻¹. Iron oxidation rates of 27.6 and 25.7 kg m⁻³ h⁻¹ were obtained in batch FBR experiments with AC and Celite, respectively.Biomass accumulation of 6.2 × 10¹⁰, 2.4 × 10¹⁰ and 8.0 × 10⁹ cells per g of carrier was detected on Celite, AC and Compalox, respectively. The bacterial community structures on the carrier materials were revealed by Polymerase Chain Reaction and Denaturating Gradient Gel Electrophoresis (PCR-DGGE) followed by partial sequencing of the 16S rRNA gene. Two bacterial strains, Leptospirillum ferriphilum and a strain similar to a strain unofficially named “Ferrimicrobium acidiphilum”, were detected. Examination of the carrier material surfaces with scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) revealed that all carrier materials were covered with jarosite precipitates and that the bacteria were mainly retained on the jarosite covered areas. In conclusion, AC was the most promising carrier material for a large-scale biological ferric sulphate generating FBR based on its availability, durability and the achieved high iron oxidation rates.     

Dissolution and passivation mechanisms of chalcopyrite during bioleaching: DFT calculation,XPS and electrochemistry analysis

In this work, density functional theory (DFT) calculation, X-ray photoelectron spectroscopy (XPS) and electrochemistry analysis were carried out to investigate the dissolution process and passivation mechanisms of chalcopyrite in the presence of sulfur and iron oxidizing microorganisms. Both DFT calculation and XPS analysis indicated that the formula of chalcopyrite should be Cu + Fe³ + (S²⁻)₂. Disulfide (S₂²⁻) and polysulfide (S_n²⁻) can be easily formed on the surface of chalcopyrite due to the surface reconstruction. The dissolution process of chalcopyrite in bioleaching was mainly dependent on redox potential. Chalcopyrite was predominantly directly oxidized to polysulfide when redox potential was lower than about 350 mV vs. Ag/AgCl and resulted in low dissolution rate. When redox potential was in the range of about 350–480 mV vs. Ag/AgCl, chalcopyrite was mainly transformed to intermediate species of Cu₂S rather than polysulfide, thus resulting in high dissolution rate. When redox potential was higher than about 480 mV vs. Ag/AgCl, chalcopyrite was principally directly oxidized to polysulfide which caused the passivation of chalcopyrite. Finally, a model of dissolution and passivation mechanisms of chalcopyrite in the presence of sulfur and iron oxidizing microorganisms was provided.     

Leaching behaviour of natural and heat-treated brannerite-containing uranium ores in sulphate solutions with iron(III)

Uranium leaching tests were conducted on two naturally occurring, highly metamict brannerite ores from the Crockers Well and Roxby Downs deposits, South Australia. The ores were leached over a range of temperatures and Fe^(III) and H₂SO₄ concentrations. As well, samples of the ores were calcined at 1200 °C in air to investigate the effect of thermally induced recrystallisation on uranium dissolution. For the unheated samples, a maximum of ∼80% U dissolution was obtained using an Fe^(III) concentration of 12 g/L, an acid concentration of 150 g/L H₂SO₄ and a temperature of 95 °C. The heat treated samples performed poorly under identical conditions, with maximum uranium dissolution of <10% recorded. High uranium dissolution from natural brannerite can be achieved providing; (i) acid strength, oxidant strength and temperatures are maintained at elevated levels (compared to those traditionally used for uraninite leaching), and, (ii) the brannerite has not undergone any significant recrystallisation (e.g. through metamorphism).     

Metal ions released from fluid inclusions of quartz associated with sulfides

The release of fluid inclusions has a strong potential for the unintentional activation of minerals during flotation. The present study aims to characterize fluid inclusions in natural quartz from a complex sulfide ore deposit. The results indicate that many fluid inclusions exist in the quartz. Under the experimental conditions of 2 g of quartz cleaned in 40 ml of pure deionized water under an inert atmosphere, the concentrations of Cu, Pb, Zn and Fe in aqueous solution reach concentrations of 1.92 × 10⁻⁷, 8.88 × 10⁻⁷, 8.31 × 10⁻⁷ and 90.33 × 10⁻⁷ mol/L, respectively. These values are significantly greater than those from the experimental non-oxidative dissolution of the quartz. In addition, the concentrations of metal ion released from fluid inclusions in the quartz sample at conditions approached “typical” industrial flotation environment are determined. The results indicate that the fluid inclusions of quartz represent the considerable sources of Cu, Pb, Zn and Fe in the aqueous solution. The present investigation provides a new understanding for the source of the unavoidable metal ions in the flotation pulp and may benefit understanding of the flotation theory.     

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.     

An impedance study of the adsorption of CuSO4 and SIBX on pyrrhotite samples of different provenances

The non-stoichiometric sulfide mineral pyrrhotite (Fe_(1?x)S), common to many nickel ore deposits, occurs in differing crystallographic forms and compositions. The processing of pyrrhotite from these ores through froth flotation is based on the surface properties of the sulfides and since pyrrhotite is a metallic conductor, it is of interest to characterise the surface properties of pyrrhotite with respect to its electrochemical state. In this study, a series of pyrrhotite samples derived from Canada, South Africa, and Botswana whose mineralogy is well characterised, were used for electrochemical impedance spectroscopy (EIS). The behaviour of the different pyrrhotite samples were compared in terms of the effect of pH (7 and 10), collector addition (SIBX) and copper activation and the results correlated with microflotation tests. The EIS results were then used to interpret and understand the differences in flotation performance of the pyrrhotite samples under the different reagent conditions and provide some answers as to why the success of copper activation on pyrrhotite is so variable.     

Enhanced hydrochloric acid leaching of nickel

Dissolution rates of nickel in hydrochloric acid with and without the presence of thiourea have been studied by means of rotating disc methods with solution analysis. In the absence of thiourea an apparent activation energy value of 61.1 ± 12.1 kJ. mol⁻¹ was determined. Such a high value is inconsistent with a diffusion controlled process and this was supported by the lack of dependency of the dissolution rate on rotational speed. A reaction controlled process is indicated. In 8.4 mol. dm⁻³ hydrochloric acid the thiourea concentration range producing optimum dissolution rates was 10⁻³ to 10⁻² mol. dm⁻³. At 70° C the rate of leaching was enhanced by some 105 %. Tests using radiochemical labelling of the sulphur and of the carbon separately in the thiourea molecule confirmed previous proposals, for cobalt dissolution, that hydrogen sulphide is produced as a by-product of the cathodic depolarisation reaction involving the thiourea and is adsorbed on the metal surface in the molecular state, forming activated anodic sites which enhance anodic dissolution. At the 10⁻¹ mol. dm⁻³ thiourea concentration level stimulation of the nickel dissolution process gave way to its partial inhibition with a maximum diminution at 30°C of 50 %.     

Removal of cadmium from a liquid effluent by ion flotation

The removal of Cd using sodium dodecylsulfate (SDS) as collector was studied by ion flotation at laboratory scale. The effect of frothers (iso-propanol and methyl-isobutyl-carbinol (MIBC) and ionic strength (NaCl and Na₂S0₄) were also studied, as well as characterization of the sublate by scanning electron microscopy (SEM) and the surface tension of the initial solutions. In the presence of SDS, the maximum recovery obtained at a stoichiometric metal to collector ratio of 1: 3 was 99.1 %, however a large volume of wet foam was produced. The best recovery (89.2%) with a dry foam was obtained at a stoichiometric ratio of 1:2. The introduction of frothers (iso propanol and MIBC) in the system produced the highest recovery of Cd at a concentration of 0.1% v/v, where the flotability was 98.8% and 97.7% for iso-propanol and MIBC respectively. An increase in magnitude of ionic strength from 4.7 × 10⁻⁴moles.dm⁻³to 4.7 × 10⁻¹moles.dm⁻³ significantly decreased Cd removal. Surface tension testwork indicates a decrease in flotability of Cd as surface tension drops. The SEM/EDS studies showed that the morphology of the suhlate and the foam depends on the physico-chemical conditions of the system.     

Effect of dissolved metal sulphates on gas-liquid oxygen transfer in agitated quartz and pyrite slurries

A previous study on mass transfer in agitated three phase systems showed that for quartz slurries the volumetric oxygen transfer coefficient k_La [s⁻¹] decreases with increasing solids fraction, whereas pyrite particles increase the k_La value. The present study was conducted in continuation of these results and attempts to explain why the addition of pyrite particles resulted in an increase in k_La. For this purpose, the effect of ionic strength, mineral density and oxygen consumption due to homogeneous reactions was examined. Neither the high density of pyrite nor the oxygen consumption could offer an explanation for the increased k_La values obtained for pyrite slurries. The variable mineral density did not affect k_La at all, while the consumption of oxygen was not large enough to cause an enhancement of k_La. However, by reducing bubble coalescence frequency and thus bubble size, the ionic strengths found for pyrite slurries could partially explain the increase in k_La. For 15 vol.% pyrite slurries, the ionic strengths proved to account for at least 60% of the enhancement. The variable(s) and/or mechanism(s) causing the remaining 40% of the enhancement could not be identified (yet). To demonstrate the effect of ionic strength as such, various electrolytes (viz. CuSO₄, FeSO₄, ZnSO₄ and Al₂(SO₄)₃) were added to a 15 vol.% quartz slurry. For all metal sulphates, increasing the ionic strength up to about 0.25 mol/l resulted in an increase of k_La by up to a factor 2.5.     

The flotation of magnetic and non-magnetic pyrrhotite from selected nickel ore deposits

The non-stoichiometric sulfide mineral pyrrhotite (Fe_(1?x)S), common to many nickel ore deposits, occurs in different crystallographic forms and compositions. A series of pyrrhotite samples derived from Canada, South Africa and Botswana whose mineralogy is well characterised, were selected here in order to develop the relationship between mineralogy and flotation performance. Using both oxygen uptake and microflotation tests, the behaviour of the different pyrrhotite types was compared in terms of the effect of pH and collector addition. Non-magnetic pyrrhotite was less reactive in terms of its oxygen uptake and showed the best collectorless flotation recovery. Magnetic pyrrhotite was more reactive and showed poor collectorless flotation performance that could be improved with the addition of xanthate collector, but only if it was not already passivated. These differences are interpreted to be a result of pyrrhotite mineralogy. This has implications that may aid the manipulation of pyrrhotite flotation performance in processing operations.     

A contribution to the relationship of energy and particle size in the comminution of brittle particulate materials

The contribution of this paper on the relationship of energy–particle size in the comminution of brittle particulate materials is based on two concepts: (a) the potential energy of a single particle and (b) the size distribution of particles in a particulate material. The potential energy Q_x of a single particle of size x is defined as the energy required to create this particle. By definition Q_x=q_xM_x, where q_x is the specific energy per unit mass and M_x the mass of the particle. The relationship, which relates the energy to the size of the material, is assumed to be an empirical one: (dq_x/dx)=−C(1/x^m), where C and m are constants. For particulate materials, the particle distribution is assumed to be the Gates, Gaudin, Schuhmann: P_x=W₀(x/y)^α, where P_x is the cumulative particle mass finer than x, W₀ is the total mass of the assembly, y is the maximum particle size (size modulus) and α is a constant (distribution modulus). The potential energy E_y of a particle assembly is defined as the total energy of its particles. It is shown that for m>1 and α−m≠−1 then E_y=(CW₀/(m−1))(α/(α−m+1))y^1−m and for α−m=−1 then E_y=(CW₀/(m−1))(lny^a/y^a). For m=1 and α≠0 then E_y=−CW₀(lny−1/a). For α=0, which is practically impossible, then E_y is not defined. The case for m<1 is not realistic because it gives negative values for the potential energy. The conditions for the application of the formulae above are presented in the text.