OpenUCT :: Browsing by Author "Mcfadzean, Belinda"

Browsing by Author "Mcfadzean, Belinda"

Now showing 1 - 8 of 8

Open Access
A methodology for the heat of immersion as a measure of wettability of mineral mixtures
(2023) Magudu, Anam; Mcfadzean, Belinda; Geldenhuys Armand
The measure of the extent to which a mineral interacts with water is called wettability and this is important in flotation processes. This is because the interactions between solid particles and liquid molecules (water) are important in understanding the flotation mechanism and achieving high recoveries. Contact angles and work of adhesion can be used to determine the physical properties of a given solid-liquid system, but there are drawbacks to these techniques. The advancement of microcalorimetry instrumentation has led to the use of heat of immersion to determine the surface wettability of solid surfaces. Several calorimetric studies have proven that the heat of immersion can be used to determine the surface wettability of minerals. Previous research within the Centre for Minerals Research (CMR) has shown that the heat of immersion can provide a reliable measure of mineral surface wettability when it is measured by precision solution calorimetry. However, this was done only for single mineral systems and its application to real ores has not been investigated in depth. In this study, the heat of immersion as a measure of wettability is applied to a simple binary mineral mixture representative of a real ore. The binary mineral mixture consists of a hydrophobic sulphide mineral, galena and a hydrophilic silicate mineral, albite. The results in this study have shown that the heat of immersion measurements present challenges such as an unexpected endothermic response. This endothermic response is attributed to the dissolution of the mineral in water. This dissolution is due to the surface ions on the mineral being exposed to the wetting liquid. In order to predict flotation response through measuring wettability, the aim is to measure only the heat of wetting, which is an exothermic response. Therefore, the dissolution process needs to be suppressed. Alternative techniques such as a solution saturated with the mineral sample, using organic liquids as the wetting liquids, and pre-coating of the mineral particles with collector were explored. From the various approaches explored to suppress the dissolution, it was observed that the saturated solution approach was an effective technique for certain minerals such as albite but was ineffective at suppressing the dissolution process across a range of mineral types. It is, therefore, an ineffective technique for exploring the heat of immersion of binary mineral mixtures. Secondly, it was observed that the collector coating approach was effective for suppressing dissolution at surface coverages above 75%. The collector coating approach is not feasible for conducting the heat of immersion measurements for the binary mineral mixtures because it only successfully suppresses dissolution at excess surface coverages that are not necessarily those at which one would choose to do the experimental work. Additionally, collector coating does not allow for the natural wettability of the uncoated minerals to be measured. Thirdly, hexane was found as a good wetting liquid for suppressing dissolution but there were some experimental difficulties that led to this liquid not being used for the binary mineral mixtures. These experimental difficulties include a premature immersion of the mineral into the wetting liquid due to the beeswax used to seal the ampoule dissolving in the hexane. Finally, hexanol was found to be a good wetting liquid in suppressing dissolution, had no associated experimental difficulties and was able to distinguish relative hydrophobicities between different mineral surfaces. It can, therefore, be used as an effective wetting liquid for mineral dissolution suppression and hydrophobicity determination. Preliminary experimental work into the feasibility of using a binary mineral mixture as a simple model ore system was performed. A linear relationship was found between the heat of immersion and the fraction of pure mineral A in a binary A + B mineral mixture. The heat of immersion could be presented in various ways depending on what data is required and desired. The surface area fraction or mass composition can be used to create the linear relationship between the heat of immersion and the composition of the binary mineral mixture. It was shown that there is a linear relationship between the heat of immersion and mass composition or surface area fraction of the binary mineral mixture. From this linear relationship, the heat of immersion of the pure minerals comprising the mixture can be extrapolated. The linear relationship based on composition provides a simple and convenient way to estimate hydrophobicity of a floatable mineral in an ore where only the mass and mineral composition of the sample is known. This could be used in flotation modelling, where valuable mineral floatability is a required input parameter. To determine the relative hydrophobicity of a binary mineral mixture in hexanol where the mass composition is unknown, the heat of immersion or heat released by the binary mineral mixture is measured and this is correlated with the mixture's mineral weight composition. This linear relationship can then be extrapolated to zero and 100% respectively to obtain the heat of immersion of the pure minerals. These values can be read off a calibration curve such as that obtained by Taguta et al. (2018) to obtain a flotation rate constant.
Open Access
An investigation into the flotation response of sperrylite (PtAs2) by comparative evaluation of crystal structure and bonding atoms
(2022) Pikinini, Sebia; Mcfadzean, Belinda; O'connor Cyril
The Bushveld Complex in South Africa is the biggest platinum group elements reserve in the world. One of the major platinum group mineral (PGM) components in this ore is sperrylite (PtAs2), which has been found to be slow floating compared to the other PGMs and often reports to the tailings stream. Sperrylite comprises about ~21% of the platinum groups minerals (PGMs) in the Platreef ore hence, improving its recovery will be of great economic value to the South African PGM industry. The flotation of PGM ores aims to recover at least five minerals using the same reagent suite. These minerals have different physical and chemical properties and only a few studies so far have investigated the floatability of individual PGM minerals. The reasons for the poor floatability of sperrylite are not yet clear although some studies suggest that poor collector adsorption or poor collector choice might be the major cause. Molecular modelling computations have indicated that the platinum atoms on sperrylite working surfaces are not well exposed, posing some steric hindrances to the approaching collector ligands. In addition, adsorption studies, using molecular modelling computations, indicated that the OH⁻ ions had higher binding energy compared to SEX, suggesting that increasing pH may have a detrimental effect on collector adsorption. This study sought to establish the reasons why xanthate collectors, which are used as the main collector used in industry in the flotation of PGM ores, appear not to promote the flotation of sperrylite. Also, novel collectors were suggested based on the donor-acceptor theory with the aim of improving the floatability of sperrylite. It was hypothesized that increasing the electron density around the collector coordinating atoms will result in stronger bonds with sperrylite. The suggested novel collectors were screened using molecular modelling computations carried out by researchers at the University of Limpopo. Collectors that gave higher binding energies were then synthesized at BGRIMM Technology, in China. Tests were carried out to compare the floatability and related phenomena of sperrylite, pyrite (FeS2), arsenopyrite (FeAsS) and cooperite (PtS). These minerals were chosen due to their having either similar crystal structures and/or binding ligands (S, As) on the mineral surface and to ascertain how their floatability compares to that of sperrylite. Pyrite was selected as a cheap proxy for sperrylite since they have the same isometric crystal structure. Also, the effect of its S bonding ligand as opposed to As in sperrylite was investigated. Arsenopyrite was also selected to investigate the effect of a monoclinic crystal structure and the effect of the As and S ligands. Lastly, cooperite was also selected as a standard sulphide PGM mineral with a tetragonal crystal structure that is known to have a good flotation response. These minerals were included in this study in order to investigate how their floatability differs from that of sperrylite based on their crystal structures and bonding atoms. In addition, experiments were carried out to determine the energy and extent of collector adsorption on the minerals and electrochemical interactions between the minerals and standard collectors. XPS and TOF-SIMS analyses were also carried out to determine the surface products formed on sperrylite and arsenopyrite in the presence of synthetic plant water with and without collectors. Novel collectors were tested to establish the molar energy of interaction with sperrylite using isothermal titration calorimetry and this was compared to those determined by molecular modelling computations. The results indicated that the crystal structure and the binding ligands played a key role in the floatability of the selected minerals. Minerals with the isometric crystal structure had poor collectorless recoveries, viz. FeS2 (12.3%) < PtAs2 (14.5%) < FeAsS (85.4%) < PtS (94.7%) and different collector-induced flotation recoveries. However, the recovery of FeS2 (94%) improved to a greater extent when PNBX was used as a collector compared to PtAs2 (26%). Sperrylite and pyrite had poor natural floatabilities under both alkaline and acidic conditions, indicating that their floatability is strongly dependent on their interaction with collectors. The poor recovery of this relatively hydrophilic mineral is due to poor collector adsorption of the collectors at pH 9 which is the usual pH in operating plants. Rest potential tests revealed that sperrylite had the least extent of interaction with the standard collectors compared to the other minerals. Furthermore, sperrylite rest potentials remained unchanged with a change in pH, indicating its resistance to surface alteration. The minerals rest potentials at pH 9.5 followed the order FeS2 (282) > PtS (254) > PtAs2 (233) > FeAsS (177) mV, while there was a great increase for all the minerals except sperrylite at pH 3.5, with the order PtS (523) > FeS2 (407) > FeAsS (333) > PtAs2 (243) mV. Surface alteration is key in the flotation of different minerals as it either determines the natural floatability of the minerals or aids or hinders collector adsorption. Furthermore, XPS results indicated that the collectors were not chemisorbed onto sperrylite, since the chemisorption process requires concurrent oxidation of the collector and the mineral surface. It was further revealed that metal oxides were formed on the sperrylite surface when conditioned in plant water at pH 9. Plant water ions were also found to be detrimental to the recovery of sperrylite. The recovery of sperrylite with PNBX in deionised water (51%) was double that with synthetic plant water at pH 9 (26%), even though the adsorption tests which were carried out in deionised water indicated that only 5.7% of the dosed PNBX was adsorbed onto the mineral surface. Sperrylite might be also very sensitive to any pulp chemical changes. This was shown by the reduced recoveries after adding copper sulphate and after raising the pulp Eh using sodium hypochlorite. Also, the higher recoveries in distilled water can be a pointer that surface contamination could be one of the reasons for the poor recoveries of sperrylite. This study has also indicated that the reactivity of the arsenic bonding atom was dependent on the metal bonded to it (Pt or Fe). The reactivity of arsenopyrite when conditioned in synthetic plant water resulted in the formation of a hydrophobic surface layer that led to its high natural floatability and poor interaction with collectors. In the case of an unreactive surface of sperrylite, the arsenic bonding atom, which can also act as an electron acceptor could be the major reason for the poor collector adsorption at pH 9. This leads to an increase in the number of bonding sites that the sulphur ligands are attracted to (Pt, As) as opposed to pyrite (Fe), thereby reducing the overall collector binding energy with the mineral. Molecular modelling computations, in collaboration with the University of Limpopo, also showed that the sulphur ligand had the ability of bonding with both platinum and arsenic on the sperrylite mineral surface. Nevertheless, pH seemed to play a role in the poor collector adsorption and flotation recoveries of sperrylite at pH 9, however, the moderate recoveries at pH 4, in synthetic plant water, were a sign that this is not the major cause for the poor floatability of sperrylite. Moreover, recovery using PNBX remained unchanged at pH 4 despite the indication by the adsorption test results that 93% of the collector was adsorbed on the mineral surface under acidic conditions. In addition, the isothermal titration calorimetry tests indicated that the binding energy of the hydroxide ions in a high pH solution with sperrylite was lower than that with PNBX and SNDBDTC collectors. This indicates that collectors have the ability to displace the hydroxide ions which could be preferentially adsorbed on the mineral surface at high pH during the conditioning period. This is in contradiction with the molecular modelling computation findings. Ultimately this work has shown that sperrylite is a relatively hydrophilic mineral under both acidic and alkaline conditions and requires the action of collector reagents to effectively recover it. This study also corroborates the suggestion that poor collector adsorption is the major cause for the poor floatability of sperrylite. It is also important to note that all the adsorption tests were carried out in deionised water and that all the adsorbed collectors were only physisorbed onto the mineral. Chemisorption of collectors on minerals is generally key to effectively recover the valuable minerals and a degree of oxidation is also a requirement on the mineral to catalyse the oxidation of collectors. However, this study has shown that sperrylite is resistant to surface alteration, making it difficult to recover it. Even though the mineral has more binding sites as opposed to pyrite, the mineral has proved to be sensitive to any form of contamination in the form of ions present in plant water or any activator ions, such as copper ions, that could be found in the pulp. Hence, the hypothesis which proposed that a ligand with a greater inter-atomic distance between the two chelating atoms compared to that of typical thiol collectors will have a better crystal structure compatibility with PtAs2 as well as the hypothesis which proposed that linking atoms that are less electronegative (S, N), or electron-donating would be superior options were not sustained in the case of sperrylite. However, the hypotheses were sustained in the case of pyrite which is fairly reactive and also consists of a sulphur bonding ligand. Since the collectors were adsorbed to a greater extent under acidic conditions, it is recommended that collectors that have higher binding energies with sperrylite, which are stable under acidic conditions should be designed to improve the recoveries of sperrylite. Moreover, sperrylite was found to be relatively inert under conditions of increasing pH, it is suggested that a complete mapping of floatability as a function Eh and pH be made to determine floatability domains, as is common for base metal sulphide minerals.
Open Access
The development of frother optimisation techniques in full scale flotation plants
(2013) Venkatesan, Luke; Mcfadzean, Belinda; Harris, Martin
In 2012, Anglo American Platinum assembled a technical task team of metallurgists for their concentrator operations. Although there has been extensive research in literature regarding the flotation response and behaviour of reagents, there still exists a gap between fundamental laboratory scale research and plant scale application. This thesis will focus on the development of techniques for optimising and characterising frother on a full scale plant using the Anglo American Platinum Bubble Sizer (AAPBS) which is a commonly used tool by the plant metallurgist. The techniques developed have been based on the application of fundamental research of frothers in literature. This thesis consists of three main focus areas: 1) Developing a technique for measuring the relationship between sauter mean bubble diameter and frother concentration on a full scale plant. 2) Developing a technique for estimating frother concentrations in process streams in full scale plant 3) Establishing whether the relationship between sauter mean bubble diameter and superficial gas velocity in a flotation bank of identical cells in series in a plant operating at frother concentration above the CCC is identical, and whether this can be used to detect the decrease of frother concentration to below the CCC at any point in the bank. Furthermore, the metallurgical performance of a bank with a decrease in frother concentration below the CCC midway through the bank was determined before and after the addition of frother, which was added as such that all the cells in the bank operate with a frother concentration above the CCC. There were two techniques investigated for measuring the relationship between sauter mean bubble diameter and frother concentration. Both techniques involved using the AAPBS and the use of forced air mechanically agitated tank cells. Technique 1 involved using the first rougher cell on a flotation plant, dosing frother at different rates into the cell to target different frother concentrations and then measuring the resultant bubble size whilst operating at a fixed air rate. The coalescence mechanism here was occurring in a three phase solids/aqueous/air system occurring in a continuous stirred tank. Similarly Technique 2 involved using the first rougher cell; however, the first cell was depleted of frother by bypassing the frother dosage line into the next cell. This was done to isolate the first rougher cell and to minimise disturbance to the rest of the rougher bank. Known concentrations offrother were made up in potable water and these were added into the AAPBS. Bubbles from the pulp phase enter the bubble riser tube which is long and narrow (3m x 25mm diameter) which is representative of two phase aqueous/air “plug flow” system. These bubbles coalesce to different degrees based on the known frother concentration. The resulting bubble size distribution was then be measured by taking photographs at the viewing pane of the AAPBS. The air rate in the cell was kept constant to within 0.7-0.9 cm/s and the photographs were analysed using software provided by stone three to determine the sauter mean bubble diameter and the bubble size distribution. Technique 1 was applied to Plant A UG2 concentrator which was using a polyglycol type frother called Betafroth 206C which has an undisclosed composition and a molecular weight of approximately 200 g/mol. The first rougher cell used was an Outokumpu 70 m3 forced air mechanically agitated tank cell. The results showed no clear relationship between sauter mean bubble diameter and frother concentration. Furthermore, the sauter mean bubble diameter was already very small and it appeared that the changes in between runs were more strongly linked to the superficial gas velocity than frother concentration. The fact that the sauter mean bubble diameter obtained was already small implied that the frother concentration in the cell was already high. This could have been due to an additional source of frother due to spillage or in the process water that potentially elevated the actual frother concentrations in the cell. Technique 1 also resulted in significant disturbances to the entire rougher bank because it involved changing the frother dosages to the bank. This would affect the flotation performance of the bank for a prolonged period and affect plant performance. Hence it was decided that technique 1 would not be suitable.
Open Access
Investigating the role of dithiophosphate in the flotation of base metal sulfides
(2018) Jordaan,Thomas Ignatius; Mcfadzean, Belinda; O'connor, Cyril
The behaviour of sodium diethyl dithiophosphate (SEDTP) in flotation systems is of major interest to the Platinum Group Mineral (PGM) and Base Metal Sulphide (BMS) industry. Operationally, SEDTP has proved to be a point of contention as there are conflicting views regarding the role of collector with regard to its behaviour at the air-water and solid-water interface in the flotation process. The main objective of this thesis is to attempt to elucidate the surfactant behaviour of SEDTP and in particular its role at both the air-water and solidwater interface. To interpret its behaviour at the air-water interface, bubble pressure tensiometry was used to investigate the effect that SEDTP had on equilibrium surface tension. This was compared to the surface tension of a polypropylene glycol (PPG) frother, which was selected as a benchmark due to it being a surface active agent at the air-water interface and its general use in the industry as a frother. One of the most common collectors used in the PGM industry, sodium ethyl xanthate (SEX), was used as a benchmark collector, which is not known to affect the surface tension. Reagent concentrations were pushed high enough for the air-water surfactant, the frother, to reduce the surface tension (5-100 mM). The maximum concentrations of the three different reagents were tested at pH 7, 9 and 11. The pH was kept constant for other experiments at pH 9 and throughout the investigation the make-up water was deionized water (DIW). To investigate SEDTP’s behaviour at the air-water interface, a frothing column was used to determine its effect on foam stability. Reagent dosages used were similar to those used on plant operations, which are much lower than those used in surface tension experiments. Foam stability experiments were carried out at pH 9 using synthetic plant water (SPW) the constitution of which is shown in the thesis. Solids were subsequently introduced to investigate the effect that SEDTP had on froth stability (3-phase) and compare it to foam stability (2- phase). The solids used were samples from a PGM-containing silicate ore, milled to 60% passing 75 micron. As with the foam stability investigation, the froth stability experiments were carried out at pH 9. The pulp phase floatability of pyrite and galena with SEDTP was measured to investigate the effect that SEDTP had on particle hydrophobicity. Collector-less and pure reagent flotation recoveries were established to relate the effect that reagents had on the floatability. The microflotation of pyrite was carried out at pH 4 and pH 9 to investigate the effect of pH on the flotation of pyrite when using either SEDTP or SEX as single reagents and in the presence of a PPG frother. The effect of frother type and chain length in a mixture containing SEDTP was also investigated on pyrite at pH 4. Microflotation of galena was done at pH 4 to test the relative effect of SEDTP either as a single reagent or in conjunction with a frother compared to pyrite at the same conditions. Collector dosages for all microflotation experiments were determined so as achieve 50% of a single monolayer surface coverage on the mineral surface. This was done by determining the BET surface area of the mineral and using the known surface area footprint of a single collector molecule. Frother concentrations were similar to those used in previous studies. Furthermore, to minimize surface oxidation of the minerals, the samples were stored in nitrogen in a desiccator and acid-washed prior to the experiments. SPW was used to simulate a plant-like solution. Equilibrium surface tension results showed that the reagents used reduced the surface tension in the order: PPG frother, SEDTP, SEX. This is ascribed to the role of these reagents when adsorbing at the air-water interface. Foam stability tests were shown to be more sensitive than surface tension measurements in predicting the surface activity of SEDTP at much lower concentrations than the concentrations used for surface tension experiments. SEDTP did not have any significant effect on foam stability when used as a single reagent. However, when combined with a frother there was a significant improvement in the foam stability. SEX did not display any foam stabilizing effect with either a frother or in a collector mixture with a frother. This is consistent with the surface tension results, thus indicating that, compared to SEX, SEDTP has surface active properties, and more so when in the presence of a frother. The presence of solids in the froth stability experiments diminished the role of SEDTP at the air-water interface since no froth stabilizing effect was observed when it was combined with a frother compared to the two-phase foam system. This may be due to SEDTP partially adsorbing on the solid particles (as was shown by UV-Vis experiments) and thus not being available to affect the air-water interface. The collector mixture containing SEDTP and SEX decreased the froth stability. This may be attributed to increased particle hydrophobicity upon the addition of a collector, which could lend to the destabilization of the froth. Microflotation mineral recoveries are indicative of the bubble-particle attachment efficiency and hydrophobicity. At pH 9, no single reagents improved the recovery of pyrite. Combining SEDTP with a frother did, however, improve the recovery significantly. This was not observed for SEX as a single reagent or when combining SEX with a frother. However a 90 SEDTP: 10 SEX collector mixture containing frother exhibited further synergy by improving the total recovery and flotation rate of pyrite. At pH 4, single reagent flotation improved reagent-less flotation in all cases. The more acidic conditions would give rise to a more reducing environment which accommodates adsorption of surfactants at the solid-water interface. A 90 SEDTP: 10 SEX collector mixture showed synergy in terms of recovery, i.e. the combined effect was much greater than would have been expected from a weighted sum of each individual contribution. It has been proposed that this may be due to the heterogeneity of the surfaces, viz. the stronger collector adsorbing onto the coarser size fraction and weaker, possibly more selective collectors adsorbing onto a finer particle fraction. Once again, a mixture of SEDTP and a frother improved the flotation recovery synergistically, which is not observed when SEX is combined with a frother. Surfactant type, size and structure all contribute to the strength of the surfactant at the airwater interface. However, variable frother types (alcohols and PPG’s) at different molecular weights all displayed a similar synergistic effect with SEDTP. Furthermore, the mineral specificity of this synergistic phenomenon was tested on a second mineral, galena. The galena responded similarly to pyrite, in that an SEDTP-frother mixture significantly improved flotation rate and recovery above any single reagent. The findings in the thesis indicate that SEDTP plays a surfactant role as indicated by its ability to reduce surface tension and improve foam stability. However the presence of solids reduced this effect. In microflotation experiments, SEDTP displayed a synergistic effect when combined with a frother, therefore indicating that it also plays a collecting role by adsorbing at the solid-water interface. This synergistic effect between SEDTP and a frother can be explained by the ability of SEDTP, as well as the frothers, to adsorb at both the air-water interface and the solid-water interface. Furthermore, the presence of one of these surfactants at an interface improves the adsorption of the other in order to maintain electroneutrality. It is proposed that these observations provide supporting evidence for the classical Leja-Schulman penetration theory on the respective role of frothers and collectors in flotation. This theory proposes that during bubble-particle attachment, these interfaces come into contact with one another and condense to form a new mixed collector-frother monolayer at the bubble-particle film. The combination of surfactants that have bubble stabilizing ability and increase particle hydrophobicity at the bubble-particle interface will ultimately improve the flotation of the particles. However, it was shown in this dissertation, that this synergistic interaction is significant only in the case of SEDTP. SEX showed no evidence of this mechanism operating at all. This may be because all available xanthate molecules are adsorbed onto the solid surface and are unavailable to act at the airwater interface. In addition, it was shown that SEX is an extremely poor surfactant at the airwater interface. These findings have important ramifications for the current processing of PGMs and BMS where DTP is widely used.
Open Access
Investigating the use of sodium metasilicate to improve the flotation performance of altered PGE ores
(2021) Molifie, Andrea; Mcfadzean, Belinda; Becker Megan; Geldenhuys, Stefan
Mineral hydration is known to result in low flotation recoveries and grades within primary platinum group element ores worldwide. This is because the phyllosilicate minerals derived from hydration reactions of silicate minerals (i) form electrostatic coatings on valuable minerals that in turn hinders collector adsorption, (ii) alter the viscosity of the slurry leading to poor gas dispersion and (iii) decrease the concentrate grade due to naturally-floating gangue. Sodium metasilicate was investigated because its dispersant, rheology modifying, and depressant properties could prove promising in combating the problems associated with these ore types. Quantitative evaluation by scanning electron microscopy (QEMSCAN) analysis revealed large quantities of serpentine and talc present within the ore sample used in this study, which led to a poor flotation response, as indicated by batch flotation tests. Using sodium metasilicate improved the recoveries and grades at high dosages (>1000 g/t). A suite of techniques was chosen to decouple sodium metasilicates' effects to answer why an improved flotation performance occurred. The zeta potential experiments indicate that improved recoveries are, in part, as a result of the reversal of serpentines surface charge, creating electrostatic repulsion between serpentine and valuable minerals which prevents the coating of valuable minerals by serpentine slimes. This corresponded with improved recoveries of a PGM proxy in the presence of serpentine slimes and a high sodium metasilicate dosage. Ore dilution and rheology tests indicate that decreased viscosity at high dosages also improved recoveries. This was supported by slower particle settling rates at high sodium metasilicate dosages during particle settling measurements. Talc micro-flotation tests revealed that the depression of talc occurred at higher sodium metasilicate dosages, which improved concentrate grade. This was supported by a QEMSCAN concentrate analysis of the sodium metasilicate batch flotation concentrates, which confirmed that talc, and other associated silicate minerals, were depressed at high sodium metasilicate dosages. The processing of near-surface altered ores is becoming an increasing problem worldwide and the use of sodium metasilicate proved valuable in mitigating the problems associated with the altered ore investigated in this study.
Open Access
Investigating the use of sodium silicate to improve the flotation performance of Platreef and UG2 PGE ores
(2023) Nyaruwata, Everjoice; Mcfadzean, Belinda; Geldenhuys Armand
The Bushveld Igneous Complex (BIC) in South Africa is one of the largest platinum group minerals (PGM) depositories in the world. Due to the depletion in high grade ores, there has been a shift to processing low grade, mineralogically complex, finely disseminated and often altered ores. These ores require fine grinding to sufficiently liberate the very fine grained PGMs which subsequently liberates the very fine phyllosilicate gangue minerals; namely serpentine and talc. The presence of these minerals poses three major problems to valuable mineral recovery. Firstly, serpentine slimes may form a hydrophilic layer on the valuable mineral surface leading to a decrease in recovery. Secondly, serpentine and talc may form homo-coagulants that can increase the pulp viscosity and also result in a decrease in recovery. Lastly, the presence of naturally floating gangue such as talc can be recovered via true flotation and hence lower the concentrate grade. The widespread mitigation methods in commodities other than PGM's, involve use of dispersants to reverse slime coating, use of rheology modifiers to lower pulp viscosity and depressants to hinder the recovery of naturally floating gangue. All these methods target the problems individually. Sodium silicate has been previously observed to effectively address all three problems simultaneously in a highly altered PGE ore thereby enhancing flotation performance. Therefore, there is a need to assess sodium silicate performance on moderately altered to relatively unaltered PGE ores, where there is no published data in this regard. To assess how sodium silicate influences the flotation response of Platreef ore, batch flotation tests were performed at five different sodium silicate dosages in the range 0 g/t (baseline condition) to 2000 g/t sodium silicate on the secondary grind (80% passing 45 µm) using sodium metasilicate of modulus 1. For comparison purposes, batch flotation tests were also performed on the UG2 secondary grind (80% passing 45 µm) and Platreef primary grind (48% passing 75 µm) at 0 g/t and 2000 g/t sodium silicate. Additional batch flotation tests were conducted with variable moduli in the range 1 – 3 at a constant sodium silicate dosage to assess the effect of modulus on Platreef ore. Due to the pH modification properties of sodium silicate, batch flotation tests were conducted at an equivalent pH as 2000 g/t sodium silicate, without adding sodium silicate, to decouple the pH effects. Supplementary rheology and froth stability experiments were performed on both Platreef and UG2 secondary grinds to further assess sodium silicate activity. Characterization measurements such as quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) and qualitative X-ray diffraction (QXRD) were also performed on the Platreef and UG2 feed samples and Platreef concentrates to obtain the PGM characterization and bulk modal mineralogy data. The bulk modal mineralogy data showed that the Platreef and UG2 ores contain 16.4 weight percent (wt%) and 4.5 wt% alteration minerals, respectively. The four minerals being categorised as alteration minerals are serpentine, chlorite, amphibole and talc. Therefore, Platreef was expected to exhibit more detrimental effects on mineral recovery than UG2. Batch flotation tests revealed that high dosages (2000 g/t) of sodium silicate enhanced both Pt and Pd grades and recoveries in a Platreef ore. The high sodium silicate dosages also improved the rheological environmental by lowering the pulp viscosity and improved froth drainage. It was believed that slime coating reversal and a reduction in slurry viscosity which improved gas dispersion and bubble-particle interactions were responsible for the recovery enhancements. The grade improvements were attributed to the depression of talc and talc-composite particles, as well as improved froth drainage, leading to lower entrainment. QEMSCAN analysis revealed that sodium silicate mainly improved the recovery of PGE sulphides and PGE tellurides. These are believed to have been responsible for the recovery gains reported. However, sodium silicate appeared to negatively impact the floatability of PGE arsenides. The Platreef concentrates at 0 g/t and 2000 g/t sodium silicate were also split into three size fractions (-10 µm, +10 – 38 µm and +38 µm) and the recovery-by-size data showed a significant increase in the Pt and Pd recovery of the +38 µm size fraction. This was attributed to the slime cleaning effect of sodium silicate. These findings were supported by the PGM particle size distribution as it showed an increase in the amount of coarser particles at 2000 g/t sodium silicate. The modulus tests also revealed that the higher modulus had very little effect on the recovery but significantly lowered the concentrate grades. This was attributed to increased entrainment and the polymerization effect of sodium silicate at higher modulus which might have hindered the depression of naturally floating gangue. The pH tests revealed that high pH can improve Pt and Pd recoveries but at the expense of grade. This showed that high pH had no effect on talc depression. Also, the recovery enhancements at the higher pH were lower than that at high sodium silicate dosage. It was postulated that the recovery improvements were a result of both an increase in solids recoveries and the removal of slime coatings as hydroxide ions are capable of reversing mineral surface charge of the alteration minerals. A comparison between the flotation performance of the Platreef primary and secondary grinds showed that the primary grind experienced a higher concentrate upgrade than the secondary grind, although only the secondary grind experienced recovery improvements. This was expected, due to the more problematic nature of finer particle sizes, which the addition of sodium silicate is able to mitigate. It can therefore be concluded that addition of sodium silicate at the primary grind is not effective or required. A comparison between the Platreef and UG2 performance showed that sodium silicate also upgraded the UG2 concentrates, reduced the pulp viscosity and improved froth drainage, but slightly reduced the PGE recoveries. The grade improvements were also attributed to talc depression and reduced recovery of entrained gangue minerals. These findings show that Platreef is more amenable to sodium silicate than UG2 since it experienced both grade and recovery enhancements. It was postulated that the higher concentration of alteration minerals in Platreef pose a greater negative effect on flotation performance, which can be mitigated by sodium silicate addition. Therefore, the outcomes of this study revealed the dispersant, rheology modification and depressant properties of sodium silicate on a moderately altered ore as reflected by the grade and recovery enhancements and shows promise for usage as a single method for combating the three major problems associated with alteration minerals in Platreef ore. It further showed that its use is not required for coarser grinds or for ores with less alteration minerals present. Low modulus is better for both grade and recovery improvements and high pH is not an alternative to sodium silicate addition.
Open Access
Scale-up behavior of the froth stability measurement
(2018) Geldenhuys, Armand Stefan; Mcfadzean, Belinda
Froth flotation is a widely used physio-chemical separation method in the minerals processing industry. Two distinct phases are present, namely: the pulp and froth phase. Flotation research has heavily focussed on the pulp performance; however only recently it was found that the froth performance contributes significantly to the overall flotation performance. Numerous parameters have been investigated to accurately quantify froth performance with the most notably being froth recovery. That being said, experimentally gathering data to obtain froth recovery is challenging and prone to large experimental errors. For this reason, the stability of the froth phase has been highlighted as a possible characterisation tool. Froth stability is defined as the time of persistence of the froth and is usually measured using either a dynamic and/or static methodology. Although measurement of froth stability has become common place in numerous flotation research articles, little to no attention has been given to the scale-up behaviour of the measurement. It is easily thought of that a froth constrained within a froth column will behave significantly different to one in an industrial flotation cell. Two common scale parameters, froth column diameter and initial pulp bubble size, was chosen to illustrate the dependence of the current methodology on scale. This does not mean that there are not vastly more parameters that would affect the measurement (column material of construction and/or column shape); however, these are two of the most easily changed parameters from experimental setup to setup. Four different column diameters were used for this study. Column diameter experiments were done on an industrial scale by means on manual tracking of froth growth versus time. Pulp bubble size experiments was performed on a laboratory scale by using different pore size glass frits while maintaining a constant superficial gas velocity. Dynamic stability for the pulp bubble size experiments were done by means of video tracking of froth growth versus time. The column diameter data sets highlighted similar behaviour – an increase in measured dynamic stability is seen with increasing column diameter up until a maximum is reached. This behaviour was attributed to the fact that wall films are thought to drain much faster than interstitial Plateau borders. As the column diameter decreases, the relative ratio of column surface area to bulk area increases and therefore results in an increased drainage rate a subsequently less stable froth. An empirical relationship was proposed to correct for the column diameter effect which is based on a ratio of bubble size to column diameter. The pulp bubble size data sets highlighted similar behaviour – an exponential decrease in measured dynamic stability is seen with increasing pulp bubble size. This behaviour was attributed to two fundamental mechanisms occurring within larger bubbled froths. Firstly, an increase in drainage rate as well as change in the drainage regime is seen as a function of bubble size; where in general froths of larger bubble sizes drain significantly faster. Secondly, on average froths consisting of large bubbles will have less water per volume of froth due to the decrease in bubble surface area. The effect of water content in a froth is well known and it can be said that wet froths experience less coalescence and bursting events. Therefore, the combination of the two mechanism is put forth as an explanation of the observed behaviour of the experimental systems.
Open Access
The synergistic interaction between dithiophosphate and frothers at the air-water and mineral-water interface
(2022) Pienaar, Dandré; Mcfadzean, Belinda; O'connor Cyril
Collectors and frothers are reagents in the flotation process that are thought to have separate roles. Collectors selectively hydrophobized the mineral surface and frothers adsorb onto bubbles inhibiting bubble coalescence and stabilizing the froth. Interactions between these molecules have been previously reported in literature (Leja & Schulman, 1954; Lekki & Laskowski, 1971; Dai, Bradshaw & Harris, 2001; Jordaan, 2018). These interactions have been shown to improve sulphide mineral recovery. Dithiophosphate (DTP) is a collector in sulphide mineral flotation which has been shown to synergistically increase flotation recovery in the presence of frothers (Pienaar et al., 2018) but it has also been shown that DTP does not adsorb onto the sulphide mineral surface (Nagaraj & Brinen, 2001; McFadzean & O'Connor, 2014; Petrus et al., 2011; Grano et al., 1997; Guler et al., 2006; Taguta, O'Connor & McFadzean, 2017) as occurs with conventional collectors. If DTP does not adsorb onto the mineral surface and improve hydrophobicity it was not clear what the role of DTP is in the flotation process. The main objective of this research was to identify the role of DTP in sulphide mineral flotation. Each of the respective interfaces were investigated for the adsorption of dithiophosphate (DTP). The adsorption of DTP on galena, pyrite, chalcopyrite and chalcocite was measured by UV-Vis spectrophotometry and isothermal titration calorimetry. Adsorption tests in the presence of aeration were used to determine if DTP dynamically transferred from the air-water to the mineral-water interface. At the air-water interface a regular solution theory approach, critical coalescence experiments and foam stability tests were used to quantify the interactions between DTP and frothers and compare them to xanthate and frothers. Microflotation and induction time experiments were performed to determine the effect of the collectors, frothers and their mixtures on mineral recovery and bubble-particle attachment It was found that DTP in the presence of a non-ionic frother synergistically improved sulphide mineral recovery. This improvement was attributed to attractive molecular interactions which were found to occur between the collector and the frother at the air-water interface. The interaction facilitated the transfer of the frother between the air-water and mineral-water interface, which destabilised the film between the bubble and the particle, improving film thinning kinetics and synergistically enhanced mineral recovery. When DTP did not adsorb onto the mineral surface, the collector co-adsorbed with the frother at the air-water interface and transferred with the frother onto the mineral surface during bubble-particle collision.

Browsing by Author "Mcfadzean, Belinda"

Results Per Page

Sort Options