Browsing by Author "Wiese, Jennifer"

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    Open Access
    An investigation into the effect of potential modifiers on the flotation of a copper sulphide ore
    (2018) Dzinza, Lucia; Corin, Kirsten; Wiese, Jennifer
    Oxidation, adsorption and reduction reactions are electrochemical in nature in the flotation of sulphide minerals which have semiconducting properties. Electrochemical mechanisms have two valuable implications in flotation, the potential across the mineral/solution interface determines flotation recovery and the anodic oxidation reaction involving the collector is an important parameter in imparting floatability. The reactions are dependent on the redox conditions in the pulp phase. Chemical control of redox potential (Eh) using potential modifiers may be exploited in flotation processes of sulphide minerals to improve their floatability, recoveries and grades; owing to the formation of a hydrophobic dithiolate or metal thiolate in the case of thiol collectors. In addition, chemical control of Eh is advantageous as it renders a more uniform electrochemical environment around the sulphide particles as compared to the external control of pulp potential. The adjustment of pulp potential using potential modifiers is being exploited as one of the main control parameters in sulphide flotation studies as it provides a diagnostic tool to develop flotation strategies and alleviate flotation challenges. Though potential modifiers have been previously investigated, no literature has addressed the correlation between their flotation performances on copper sulphides to their respective rest potentials at different concentrations. The present study explored the use of potential modifiers such as sodium hypochlorite (NaClO), potassium permanganate (KMnO₄) and potassium dichromate (K₂Cr₂O₇) on the flotation of a copper sulphide ore from Kansanshi Copper mine in Zambia. The potential modifiers were investigated at 1x10⁻⁴, 1x10⁻³ and 1x10⁻² mols which gave rise to various Eh values for each modifier. Batch flotation and froth stability tests were carried out at the ore’s natural pH whilst varying Eh. The dynamic stability factor (Σ) was used to quantify froth stability. Electrochemical techniques have been considered as an appropriate approach in the study of collector-mineral interactions. To complement results obtained from batch flotation and froth stability tests, rest potential measurements were carried out to determine the characteristic species formed on the chalcopyrite mineral surface at specific conditions. The potential modifier-collector-mineral interactions were investigated through rest potential measurements using the aforementioned potential modifiers, a thiol collector sodium iso-butyl xanthate (SIBX) and a pure chalcopyrite mineral. It was hypothesized that assuming an X⁻/X₂ equilibrium potential below 100 mV for SIBX, a redox potential range of 100-400 mV promotes good copper floatability due to the formation of dixanthogen and thus hydrophobic mineral particles which would result in a moderately stable froth. Rest potentials above 500 mV were hypothesized to reduce copper floatability due to the presence of very hydrophobic mineral particles, which would increase bubble coalescence and bubble breakage or result in highly stable froth. In this study, the equilibrium potential of SIBX at 6.24x10⁻⁴ M was measured to be 80 mV. Furthermore, equilibrium potential of SIBX was determined to be concentration dependent. Rest potential measurements for all conditions investigated were in excess of the measured equilibrium potential, therefore implying that the dixanthogen species was formed as postulated. It was found that an increase in concentration of potential modifiers increased froth stability or bubble coalescence depending on the potential modifier used. Furthermore, concentrations of potential modifiers resulting in Eh values of 137-476 mV resulted in high copper recoveries >88%, with 1x10⁻² mols of KMnO₄ at 540 mV giving a very low copper recovery of 4.8%. However, though high copper recoveries were obtained between concentrations that gave rise to an Eh range of 137-476 mV, a slight decrease in copper recoveries of approximately <4%, was observed with even larger increases in concentrations of potential modifiers. The findings of this study showed that the use of potential modifiers improved copper grades as a result of the reduction in gangue material recovery. In addition, the present study has shown that though concentration or Eh induced by potential modifiers may affect the flotation performance of sulphide ores, the most dominant factor that has shown to have a greater impact is the nature of the potential modifier. Comparing the findings of this work to literature findings for NaClO, it was determined that different sulphide minerals indeed exhibit different rates of redox reactions at given conditions. Ultimately, an inverse relationship was determined to exist between copper recoveries and rest potential measurements. This study has provided insight into the use of potential modifiers in the flotation of copper sulphides from an electrochemical perspective.
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    Open Access
    An investigation into the effects of pulp chemistry under wet and dry grinding on the flotation response of pyrite
    (2018) Tseka, Relebohile; Corin, Kirsten; Wiese, Jennifer
    Considering the depletion of high-grade ore deposits, the mining industry is faced with the challenge of processing low grade and more complex ores in order to meet the growing demand for metals and metal products. Therefore, it is of paramount importance to have a fundamental understanding of minerals processing operations in order to improve the recoveries of valuable metals on an industrial scale. It has been acknowledged that the chemical conditions during grinding as well as pulp chemistry have a significant influence on the recovery and selectivity of most sulphide minerals in the flotation process. Floatability of ores is mostly determined by surface properties and the surface properties are essentially controlled by the grinding conditions. The flotation response of sulphide minerals is influenced by factors such as: collector-mineral interactions, mineral surface oxidation, deposition of iron hydroxides/oxides from grinding media and the attachment of inorganic ions on the surfaces of minerals. These factors are on the other hand affected by dissolved oxygen (DO), pH, ionic strength of process water and other pulp chemistry factors. With the highly instrumented Magotteaux Mill® , the effects of these variables may be investigated during grinding. Several studies have shown that the grinding environment plays a vital role in the selectivity and recovery of sulphide minerals. During wet grinding, water allows the flow of electrons within the pulp (galvanic interactions between minerals themselves and minerals and grinding media). Pyrite is reactive and can easily oxidise when exposed to air or oxygen. Pyrite and most sulphide minerals are more inert than the electrochemically reactive grinding media. Therefore, during grinding, grinding media come into frequent contact with sulphide minerals and a galvanic couple is created between the grinding media and sulphides. Due to galvanic interactions, oxygen reduction occurs on the sulphide mineral surface and iron oxidation takes place on the steel media. The redox reaction results in the formation of iron oxy-hydroxides on the surface of sulphide minerals. The oxy-hydroxide species prevent the adsorption of collector onto the mineral surface, making the mineral less floatable. Dry grinding limits the galvanic interactions present during wet grinding, due to the absence of water. Studies have been conducted and it has been shown that dry grinding yielded significantly less media wear relative to wet grinding owing to the absence of corrosive abrasion in the form of electrochemical oxidation of media during grinding. Reduced grinding media wear may imply that lesser iron hydroxide precipitates build up on the surface of the mineral hence improving collector adsorption and subsequently mineral recovery. Therefore, this suggests that dry grinding could result in improved sulphide mineral recovery as compared to wet grinding. It is necessary to consider the fundamental aspects of both grinding and flotation in order to improve concentrator performance as well as sulphide mineral recovery in the presence of nonsulphide minerals. Previous studies have investigated the influence of the grinding pulp chemistry factors on the flotation response of pyrite and other pure sulphide minerals. The possible influence that the presence of a non-sulphide gangue mineral may have during grinding and flotation has been ignored. The non-sulphide gangue cleans the surface of the sulphide minerals. Studies have shown that presence of quartz influences the formation of layers of hydrophilic species on the surface of sulphide minerals. The metal hydroxides will preferably deposit on the surface of non-sulphide mineral such as quartz rather than sulphide minerals. These studies also did not investigate the combined effects of pulp chemistry factors under dry and wet grinding. It should be noted that it is not possible to control pulp chemistry during dry grinding, thus these variables are controlled in the flotation cell in order to understand their effect on mineral surface after dry milling on pyrite flotation recovery relative to how they change the minerals surface properties during grinding. Change in chemical, surface properties of sulphide minerals can take place during milling and froth flotation. Therefore, this study aims to investigate the effects of DO, pH and grinding media type (forged steel and ceramic media) during milling and flotation process on the flotation response of pyrite (sulphide mineral) in the presence of quartz (non-sulphide gangue material). Wet milling was conducted in a Magotteaux Mill® while a Sala Batch grinding mill was used to carry out dry grinding. DO concentration and pH were controlled and measured in situ during wet grinding and were manipulated inside the flotation cell after dry grinding. The effects of the DO and pH, with changing grinding media type, on water and solids recovery, pyrite recovery and grade as well as flotation kinetic constants were studied. The EDTA extraction technique was employed to quantify the percentage of extractable oxidized iron leached from the mill product. The findings of this study have shown that under both wet grinding and dry grinding, an increase in pH from 9 to 11 resulted in increased water and solids recovery due to an increase the total concentration of OH ions in the system which led to increased froth stability owing to the reduction in pulp bubble size, as well as reduced bubble coalescence. This shows that the control of pulp chemistry during milling and flotation affected flotation process in the same way. The study has further shown that the highest recovery of pyrite, 100%, was achieved with inert grinding media (ceramic) under dry grinding. This might be due to cleaner pyrite surfaces created during dry grinding, since the prevention of media corrosion may lead to improved recoveries. During wet grinding, iron hydroxide is generated and reduces the flotation response of pyrite. Dry grinding generally produces much faster pyrite flotation kinetics than wet grinding because of the generation of particles with high surface energy and that leads to highly activated particles. It was therefore concluded that the grinding environment indeed has an effect on the flotation response of pyrite in the presence of gangue. This study has shown that careful manipulation of pulp chemistry, selection of grinding media and grinding environment may be used to manage pyrite recoveries within flotation.
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    Open Access
    Investigating electrolyte-reagent-mineral interactions in response to water quality challenges in the flotation of a PGM ore
    (2019) Manono, Malibongwe Shadrach; Corin, Kirsten; Wiese, Jennifer
    Froth flotation is a physicochemical process that enables the separation of valuable minerals and unwanted gangue minerals contained in an ore. It utilises the differences in surface properties of the minerals to be separated. Among other factors affecting flotation, water is a major factor as it acts as a reagent and transport medium. Therefore, it stands to reason that the quality of the water used in the process matters. Current environmental restrictions on water usage which are aimed at addressing the global scarcity of water require that mining operations recycle and reuse water within their operations. This necessitates proactive management strategies and initiatives aimed at understanding the impact that water could have on flotation and other water intensive processes. The development of such initiatives relies on the provision of sound and fundamental scientific evidence examining and decoupling the effects of water quality on the sub-processes of flotation. This would enable the creation of alternative operating conditions at which flotation could still occur without significant effects on production and profitability. Recycling of process water has for many years been the mining industry’s solution to reducing reliance on municipal water because mining operations are often located in arid regions. It has become clear that the recirculation of water in flotation circuits results in the accumulation of dissolved solids, electrolytes, unspent reagents and biological matter possibly resulting in poor flotation performance or alternatively high costs associated with water treatment. Given that water is both a reagent and transport medium in flotation, changes in its quality can significantly affect flotation performance through electrolyte-reagent-mineral interactions. This study has investigated whether there are any dominant or synergistic electrolyte-reagent interactions occurring during flotation which may impact negatively on the flotation performance. Interactions occurring in both the pulp phase and the froth phase were investigated through established bench scale flotation techniques. On the basis of available literature, investigations were carried out to identify inorganic electrolytes which had the biggest impact on froth stability as well as those which had a dominant role on depression, and specifically CMC efficacy on gangue management. A Merensky ore, typical of the South African Bushveld Igneous Complex was selected as previous work within the Centre for Minerals Research was conducted on ores of similar mineralogy. Threephase bench scale flotation and froth column tests were performed to examine the effect of increasing ionic strength of plant water and CMC dosage on froth stability and gangue recovery. Two-phase batch flotation and froth column tests were performed at various electrolytic conditions to assess the effect of ionic strength, electrolyte type and pH on froth stability using water recovery, foam height and foam collapse time as key performance indicators of froth stability. Settling tests, adsorption studies, zeta potential measurements as well as inorganic electrolyte speciation determination were considered in order to elucidate the role of water quality on gangue depression. Talc and pyrrhotite were selected as proxies for naturally floatable gangue (NFG) and sulphides respectively in order to simulate the possible behaviour of a Merensky ore. Increasing the ionic strength resulted in increased solids and water recoveries suggesting an enhancement in froth stability. When the effect of ionic strength on CMC behaviour was investigated under changing pH, results showed that contrary to findings at pH 9 which showed increases in solids recovery with increasing ionic strength, solids recovered decreased with increasing ionic strength at pH 11. This suggested that at higher pH levels above pH 10 there are hydroxy species present which inhibit the floatability of mineral particles either by forming layers on the mineral particles which hinder the action of the collector or through depressant efficacy enhancement. The speciation diagrams indicated that beyond pH 10, species such as CaOH+ increased in concentration especially at the higher ionic strength. Furthermore the zeta potential results for talc and pyrrhotite showed that at pH 11, the potentials were less negative compared to pH 9 for all synthetic plant waters proving that at pH 11 the pulp chemistry would exhibit a more depressive nature onto mineral particles owing to increased concentrations of positively charged hydroxo species at pH 11 compared to pH 9. These hydroxy species such as CaOH+ would adsorb onto the negatively charged mineral particle, reducing the negative surface charge of the mineral particle. Water recoveries increased with increasing ionic strength at both pH 9 and pH 11. These findings were further supported by 2-phase froth column tests in which water recoveries, foam height, and foam collapse time increased with increasing ionic strengths. This increase in froth stability with increasing ionic strength at both pH conditions is attributed to an increase in the [Ca2+], [Mg2+] and [SO4 2- ] which reduces bubble coalescence. Upon the determination of NFG recovery, entrained gangue recovery and total gangue recovery, it became clear that, at increasing ionic strength, there was a decrease in the recovery of NFG and entrained gangue per g of water recovered. The decrease in the recovery of NFG and entrained gangue per unit water was attributed to the coagulative nature of gangue in the presence of highly concentrated electrolytes and CMC. The fact that total NFG recovery did not change with water quality at a fixed CMC dosage but decreased with increasing ionic CMC dosage is indicative of the strong susceptibility of NFG depression to CMC dosage to an extent that at hyper dosages such as 500 g/t, all NFG is depressed completely. However, given the relationship between solids entrained and water recovery, the total recovery of entrained gangue increased with increasing ionic strength due to increased volumes of water which reported to the concentrate at increasing ionic strength. It was also shown that there was no change in sulphide recovery with increasing ionic strength. This was indicative of preferential adsorption of CMC onto gangue at the conditions tested. Higher solids recoveries or mass pulls were largely due to increased gangue recovery, mainly entrained gangue, which increased with increasing ionic strength. It was thus postulated that at increased ionic strengths, CMC coagulated gangue particles whilst indirectly destabilising the froth and retarding the action of electrolytes on froth stability through the removal of froth stabilising NFG. In order to examine coagulation at increasing ionic strengths, settling tests were performed on a Merensky ore and on pure talc. The results showed reduced settling time with increasing ionic strength and increasing CMC dosage suggesting that in a flotation cell, highly concentrated electrolytes would assist in depression by enhancing the coagulation of gangue and thereby decreasing their floatability. This can be attributed to increased concentrations of Ca2+ and CaOH+ which adsorb onto gangue; adsorption of which is the mechanism through which the chemisorption of CMC onto gangue occurs. In considering the effect of ionic strength and CMC dosage on froth stability, three phase froth column test results showed that the froth collapse time and froth height increased with increasing ionic strength due to an increase in the concentration of inorganic electrolytes which inhibit the coalescence of bubbles. In fact, Ca2+, Cl- , Mg2+, Na+, NO3 - and SO4 2- , which are present in the tested synthetic plant waters, are all reported in literature to have the ability to retard bubble coalescence, thus additive interactive effects in the tested systems should have been present. It was further shown that the addition of CMC resulted in a froth destabilization. The coagulation findings suggested that the presence of inorganic electrolytes enhanced the adsorption of CMC onto gangue due to changes on the mineral surface charge imposed by inorganic electrolytes. Microflotation results in the presence of CMC showed a decrease in the recovery of talc with increasing ionic strength whilst the presence of CMC did not affect the flotation behaviour of pyrrhotite. The adsorption results agreed with the microflotation results and the coagulation findings in that there was less residual CMC, meaning that more CMC was adsorbed onto the mineral surface with increasing ionic strength of plant water. In line with these findings, it was shown that the zeta potential of minerals, both talc and pyrrhotite, although investigated separately, increased (i.e. became less negative) with increasing ionic strengths. Thus, this work showed that increasing the ionic strength of plant water increased the concentration of inorganic electrolytes present in process water which acted on the mineral surface, passivating the mineral surface as seen through the less negative zeta potential in high ionic strengths. This would in turn create an environment conducive for an acid-base interaction between the hydroxyl species coated mineral particles (base) and highly negatively charged CMC ligand (acid), enhancing the preferential adsorption of CMC onto gangue as shown by the increase in the absorbed CMC concentration onto talc. The increased CMC adsorption would consequently assist in the formation of CMC-gangue mineral flocs with an induced coagulative and hydrophilic nature as shown by the shorter settling time in increasing ionic strength. Further investigations were carried out with single salts of cations and anions common in process water in order to identify whether there were any ions with the greatest froth stabilising action and gangue depression; Although Sulphide recoveries did not change with specific ions, the sulphide grades were affected by ion type owing to changes in gangue recoveries. Sulphide grades were higher with divalent ions compared to monovalent ions. It was also shown that salts containing NO3 - resulted in the lowest froth stability, as indicated by water recoveries and froth collapse time, compared to those which contained SO4 2- and Clin solution. Ca2+ and SO4 2- resulted in the highest froth stability compared to Na+. This can be attributed to a better inhibition of bubble coalescence in divalent ions compared to monovalent ions. The divalent Ca2+ and Mg2+ resulted in the lowest gangue recoveries compared to the monovalent Na+.NO3 - resulted in the least gangue recoveries compared to SO4 2- and Cl- . These findings suggested an enhanced hydrophilic nature onto gangue by divalent cations than monovalent cations with an even greater impact in NO3 - containing solutions- . Similarly, coagulation measurements showed an enhanced coagulation in NO3 - compared to SO4 2- with greater coagulation achieved in Ca2+ compared to Na+. An increase in the order of Ca(NO3)2>CaSO4>NaNO3>Na2SO4 in the zeta potential of talc and pyrrhotite was seen. This supported the enhanced coagulation and depression in Ca2+ and NO3 - containing systems. Thus, the findings of this work offer an opportunity to better understand water quality effects on flotation and their implications on froth stability and gangue management. Also, it has been shown that specific ion effects on froth stability and gangue management exist. Overall this study has shown that bench scale flotation techniques such as batch flotation, froth column flotation and microflotation can be used to understand the effect that water quality can have on a specific ore or mineral and that such techniques can be complemented with established surface chemistry laboratory techniques such as adsorption, coagulation and zeta potential to understand the interactions occurring in the air-water, air-solids and solids-water interfaces responsible for a particular flotation performance.. Through lower gangue recoveries, improved coagulation, increased adsorption and zeta potential, it can be concluded that the divalent Ca2+ is most likely to improve gangue depression and even more so in the presence of CMC compared to monovalent Na+. Moreover due to its causing a reduction of bubble coalescence, Ca2+ could result in improved froth stabilities and less entrainment. The monovalent Na+ showed higher gangue recoveries but lower water recoveries due to its weaker froth stabilising action compared to the divalent Ca2+. The higher gangue recoveries could be attributed to entrainment, meaning that given the lower froth stabilising action, Na+ richer solutions are most likely to lead to higher entrainment of gangue. Through higher water recoveries and higher froth collapse time results, it has been shown that SO4 2- ions result in better froth stabilities compared to Cl- and NO3 - ions, and would thus need to be monitored carefully for the desired froth stability. Thus, this work demonstrated the role of inorganic electrolytes on CMC efficacy and gangue depression using adsorption, coagulation and zeta potential results. These results correlated well with this study’s batch flotation and microflotation results. Also, these showed evidence to the suggestions and deductions drawn out of the bench scale flotation results on the effects and mechanisms through which inorganic electrolytes affect gangue and froth stability. This study also demonstrated that the divalent Ca2+ had the greatest froth stabilising effect and the greatest depressive effect on gangue compared to the monovalent Na+. Moreover, it provided evidence suggesting that solutions containing NO3 - were depressive on gangue and less froth stabilising compared to SO4 2- and Cl- . Findings of this work showed experimental evidence of the nature of CMC-electrolyte interaction in the pulp phase and its implications on the froth phase and gangue depression. It is believed that findings of this work offer an opportunity for flotation operations to tailor or control their water quality towards a desired flotation outcome. It may be possible that in order to combat changes in water quality, should closed water cycles be implemented, an operation could adjust their reagent suite to obtain a manageable grade and recovery and alleviate the high cost associated with cleaning of on-site water.
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    Open Access
    Investigating the effect of water quality on the adsorption of a xanthate collector in the flotation of a sulphide ore
    (2018) Manenzhe, Resoketswe; Corin, Kirsten; Wiese, Jennifer; Manono, Malibongwe
    Environmental concerns necessitate the recycling of process water within mining operations. On average, recycled water contains more dissolved solids than fresh water. Since water is used as both a transportation and process medium, it is expected that changes in its quality will affect plant processes. Flotation is a process that is acutely sensitive to the immediate conditions of the system. Literature suggests that the efficiency of flotation separation is driven by the hydrophobicity that can be achieved by the mineral particles meant to be floated. The hydrophobicity is in turn driven by the adsorption of specialised reagents i.e. the collectors. Since collectors are added such that they adsorb at the liquidparticle interface, it stands to reason that changing the chemical composition of the aqueous phase will affect the collector adsorption, and hence the flotation response of target minerals. In this study, a sulphide copper ore from the Zambian Copperbelt was floated in synthetic plant waters of varying ionic strengths, and with different concentrations of the collector sodium isobutyl xanthate (SIBX). The synthetic plant waters were prepared by adding varying concentrations of inorganic salts to distilled water in order to achieve process water compositions that reflect water compositions typically found in mining plants. Additionally, a nickel-copper ore from Lapland Finland was floated in the synthetic plant waters as well actual plant waters. To account for the latter ore’s polymetallic nature, the collectors aerophine and sodium isopropyl xanthate (SIPX) were used sequentially. The objective of the study was therefore to investigate the effect of water quality on collector adsorption in the flotation of sulphide ores. The study showed that water quality has a quantifiable effect on SIBX and SIPX adsorption. The respective effects of water quality and collector adsorption on ore flotation could not be irrefutably decoupled. However, it could be concluded that of the tested waters, the copper thickener overflow was the least conducive to xanthate adsorption and valuable mineral recovery. On the other hand, collector adsorption was favoured by waters such as the raw and standard process. However, increased adsorption did not necessarily result in improved grades and recoveries. The study further showed that in the case that the dissolved ionic species are identical, increasing the ionic strength of water yields a linear decrease in xanthate adsorption, and that recycling SIPX retained in flotation waters resulted in reduced separation selectivity.
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    Open Access
    Investigation of the effect of particle size on froth stability
    (2016) Chidzanira, Tadiwanashe; Wiese, Jennifer; McFadzean, Belinda
    The flotation process has been used for more than a century to separate valuable minerals from bulk ores. The separation process is based on utilising the differences in the physico-chemical properties of liberated particles, mainly the particle hydrophobicity which allows the particles to be attached to air bubbles rising from the pulp phase into the froth phase and subsequently collected to the launder. The stability of the froth phase which is be defined as the ability of bubbles to resist coalescing and bursting (Triffet & Cilliers, 2004), has been shown to have a significant effect on the efficiency of the flotation process. An unstable froth will result in poor valuable mineral recovery as these desired hydrophobic particles are detached from air bubbles and drain with the water back into the pulp phase due to bubble coalescence. On the other hand, a very stable froth may result in poor concentrate grade as the unwanted gangue materials are unselectively entrained to the concentrate. As a result, a substantial amount of research has been performed on improving control of froth stability by the manipulation of frother type and dosage. A recent study investigated the manipulation of flotation operating parameters such as air rate, froth height and depressant dosage which resulted in minimal changes in froth stability. The present study then investigated the effect of particle size and solids concentration on the stability of the froth phase using a UG2 ore and an Itabirite ore. Froth stability was determined using Bikerman tests on a laboratory scale non-continuous stability column. A novel continuously operated agitated hybrid cell was also used to assess froth stability, with water recovery and froth recovery used as proxies for froth stability. The agitated hybrid cell was then included in the experimental design as it allowed for continuous floatation system to be evaluated which resembles more industrial operations as compared to the stability column. The hybrid also incorporated the agitation zone benefits of a lab scale batch flotation cell which allows for better attachment of coarse particles and also allowing for the formation of deeper froths enabling improved froth stability measurements. The viability of using the top froth average bubble size and the side of froth axial bubble coalescence rate as froth stability proxies was also evaluated as the columns were clear glass.