Browsing by Author "Taguta, Jestos"

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    Open Access
    The relationship between enthalpy of immersion, and its derived wettability parameters, to flotation response
    (2019) Taguta, Jestos; McFadzean, Belinda; O’Connor, Cyril
    The wettability of mineral surfaces plays an important role in the flotation process. A wettable mineral is hydrophilic while a non-wettable mineral is hydrophobic. In the flotation process, sufficiently hydrophobic particles are collected by rising air bubbles and they report to the concentrate. On the other hand, hydrophilic particles do not attach to the air bubbles and they report to the tailings. Some of the conventional methods used to characterise mineral surface wettability include contact angle, inverse gas chromatography (iGC), time of flight secondary ion mass spectrometry (ToF-SIMS) and induction time measurements. The measurement of contact angles on flat, smooth and ideal surfaces is relatively simple, straight forward and well-described, but the measurement of powder contact angles is not so straight forward. The iGC technique is a gas phase technique while ToF-SIMS exposes the particles under high vacuum compared to an aqueous environment in real flotation systems. This thesis has investigated the use of the enthalpy of immersion as an indicator of the wettability of mineral surfaces. The enthalpy of immersion is the heat change arising from the replacement of the solid-gas interface with the solid-liquid interface when a solid surface is immersed in a liquid. Although immersion calorimetry has been established as a reliable means of determining the wettability of solid surfaces, it has found only limited applications in flotation research where wettability of mineral ores is a key variable. In this study, precision solution calorimetry was employed to measure the enthalpies of immersion of different minerals in water. The Washburn method and a microflotation system were used to measure the corresponding powder contact angles and the flotation responses of the same minerals respectively. Two mineral systems were investigated in this study, viz: different pure minerals in their natural form as well as collector-coated sulphide minerals. Furthermore, to assess whether the enthalpy of immersion is able to differentiate between the amounts of minerals of different wettabilities in a mineral mixture, a synthetic ore comprising of different proportions of a sulphide mineral (realgar) and a silicate gangue mineral(albite) was also investigated. The surface energetics of different minerals and the synthetic ore were also characterised by measuring the enthalpies of immersion in different probe liquids and applying the van Oss-Chaudhury-Good (VOCG) model. The VOCG model is reported to give consistent results in terms of surface energetics of surfaces. It has been found that the enthalpy of immersion technique was capable of distinguishing differences in the wettabilities of different minerals and these differences were explained in terms of the solid state properties of the minerals. The enthalpy of immersion method was also able to assess the changes in the surface chemical properties of the galena and realgar surfaces resulting from collector adsorption. The magnitude of the enthalpy of immersion was inversely related to the surface coverage of potassium amyl xanthate on both galena and realgar. The enthalpy of immersion measurements correlated well with powder contact angle measurements, but, most importantly, the enthalpy of immersion measurements were found to be more reproducible and sensitive than the contact angle measurements. It has also been shown that the enthalpy of immersion is a more widely applicable measure of the hydrophobicity of mineral particles typical of those used in the flotation process as opposed to the contact angle. It is therefore concluded that the enthalpy of immersion is a superior indicator of the extent to which minerals are hydrophobic or hydrophilic either in their natural form or after treatment with a collector. Furthermore, it was found that there was a strong inverse relationship between the enthalpy of immersion of the minerals studied and their flotation response. The strong inverse relationship has potential to be used in pulp phase flotation models, although this was not the focus of this thesis. In addition, a value, termed the critical enthalpy of immersion (CEI), was observed above which no flotation occurred. The CEI was in the region of -200 mJ/m². At values less exothermic than the CEI, the flotation response was found to be inversely related to the enthalpy of immersion. At values more exothermic, viz. more negative, than the CEI, no flotation occurs. The significance of this finding is that for any mineral whose flotation behaviour is unknown, the measurement of the enthalpy of immersion appears to be able to predict the flotation response of the mineral. The variance in the inverse relationship between enthalpy of immersion and rate of flotation was reduced when the data was normalized with respect to particle density which was the only variable in the flotation studies in terms of particle-bubble encounter efficiency. These results have shown that the enthalpy of immersion is an excellent indicator of both the natural mineral hydrophobicity and of the extent to which collectors render a mineral hydrophobic. The relative strength of the acid-base sites was shown to depend on the mineral type. The surface energetics obtained in this study were consistent with the hydrophobichydrophilic nature of these minerals. The basic, polar components as well as the total surface energy decreased in the following order: silicates > metallic sulphide minerals and talc. It was observed that the higher the total surface energy, the lower the hydrophobicity of the mineral. The acid-base characteristics of the minerals, measured by solution calorimetry, can give a detailed insight into the surface energies of different mineral types and may be useful in optimising processing strategies. Using the surface energetics, two important parameters were calculated, viz: the interfacial free energy of interaction between mineral particles and bubbles immersed in water (∆Gpwb) as well as the work of adhesion for water (Wadh). Interestingly, and not surprisingly, the trends in both of these parameters coincided with the trend in the enthalpies of immersion of the different minerals in water. Critical values of ∆Gpwb and Wadh parameters in the region of 200 mJ/m² and 320 mJ/m² respectively were observed above which no flotation occurs. At values less than the critical values, both parameters were inversely related to the flotation response. The enthalpy of immersion was able to differentiate between the amounts of minerals of different wettabilities in a mineral mixture. The enthalpy of immersion became increasingly exothermic as the percentage of albite in the realgar-albite mixtures increased. The experimentally determined enthalpies of immersion in water were in excellent agreement with the weighted enthalpies of immersion for the realgar-albite mixtures. The weighted enthalpies of immersion of the synthetic ore were calculated based on the specific surface areas of both realgar and albite. Therefore, it is possible to calculate the enthalpy of immersion of a synthetic ore (mineral mixtures) from the knowledge of the proportion and the enthalpy of immersion in water of the individual minerals comprising the synthetic ore. The surface energetics of the synthetic ore showed that there is a relationship between the mass recovery and the calculated relative surface polarity, based on the individual polar and total surface energies. As the relative surface polarity increases, there is a significant decrease in the mass recovery after a relative surface polarity of about 0.4. Thus, the enthalpy of immersion has the potential to be used to predict the wettability and the floatability potential of mineral mixtures. It is proposed that this work should be extended to other mineral mixtures, with careful measurement and calculation of the surface energetics of these mixtures. Thus, this work presents an opportunity for further study to investigate the use of the enthalpy of immersion to characterise the wettability of real ores.
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    Open Access
    The Use of Acid Mine Drainage (AMD) in the Flotation of a Platinum-Group-Minerals-Bearing Merensky Ore
    (Multidisciplinary Digital Publishing Institute, 2022-10-05) Taguta, Jestos; Peku, Zandile; Sehlotho, Nthapo; Corin, Kirsten
    Water scarcity is compelling mining houses to not only recycle process water but to also identify alternative sources of make-up water in concentrators. South Africa has significant volumes of acid mine drainage (AMD) generated from vast mining operations. This study investigated the viability of using AMD as a replacement for potable water in the flotation of a platinum-group-minerals (PGM)-bearing Merensky ore. Rougher and cleaner flotation testwork was conducted at laboratory scale to compare the performances of potable water (baseline water), AMD treated with Ca(OH), and AMD treated with the Veolia process. Water analysis showed that the three water types differed in pH, water hardness, conductivity, and total dissolved solids. The results showed the AMD treated with Ca(OH) was detrimental to PGM recovery compared to potable water at depressant dosages of 50 g/t. Specifically, AMD treated with Ca(OH) achieved a PGM rougher recovery of 67.8%, while potable water achieved a PGM rougher recovery of 88.4%. Depressant dosage optimisation and treatment of the AMD using the Veolia process were investigated as potential strategies to mitigate the detrimental effects of the AMD treated with Ca(OH) on the flotation performance of a Merensky ore. The AMD treated with the Veolia process achieved a PGM rougher recovery of 70.8%. Thus, treatment of the AMD was beneficial, though the PGM and base metal sulphides (BMS) recoveries were still lower than those achieved in potable water. Reducing the depressant dosage to 25 g/t in AMD treated with Ca(OH) resulted in the highest PGM, Cu, and Ni rougher recoveries of 91%, 60.2%, and 58%, respectively. The AMD treated with Ca(OH) at lower depressant dosage outperformed the potable water in terms of PGM and BMS recoveries and concentrate grades, indicating that AMD has the potential to replace potable water as make-up water in Merensky ore processing plants. The results showed that depressant optimisation is important to achieve superior metallurgical results when using AMD treated with Ca(OH). The use of AMD in Merensky ore processing plants not only conserves freshwater in minerals processing plants but also reduces high volumes of contaminated effluents.
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    Open Access
    The thermochemical behaviour of thiol collectors and collector mixtures with sulphide minerals
    (2015) Taguta, Jestos; McFadzean, Belinda
    Thiol collectors continue to dominate the base metal sulphide (BMS) and platinum group mineral (PGM) flotation industry. The behaviour of thiol collectors and collector mixtures with sulphide mineral surfaces has been extensively studied using different techniques such as XPS, ToF-SIMS, UV-VIS, etc. However, most of these techniques require a collector dosage above monolayer coverage, take place under equilibrium conditions and may not simulate flotation conditions. Moreover, most of the studies focus on isolated minerals yet exploitable ores exist as an association of sulphide minerals. The use of thiol collector mixtures in the base metal sulphides (BMS) and platinum group mineral (PGM) flotation industries has been reported to offer several performance benefits by many researchers. However, the mechanism whereby these collector mixtures adsorb onto a specific mineral surface is still not clearly understood. This study used isothermal titration microcalorimetry to monitor sub-monolayer reactions by continuously measuring the heats of adsorption as the reactions between thiol collector (and collector mixtures) and sulphide minerals (and mixed minerals) proceed in real time. The enthalpy of adsorption does not only characterise the intensity of adsorption between a collector and mineral surface but also gives insight into the reaction mechanism, whether physisorption (less negative than -40 kJ/mol) or chemisorption (more negative than -40 kJ/mol). Microflotation was also used to determine the hydrophobicity imparted onto the mineral particles as a result of the mineral-collector interactions. Sulphide minerals investigated were chalcopyrite, pyrite, pyrrhotite and galena. Thiol collectors investigated were xanthates of varying chain length (SEX, SIBX, PNBX and PAX) as well as dithiocarbamates (diethyl-DTC and n-butyl DTC) and diethyl-DTP. The current study seeks to gain knowledge of which collector interacts best with which mineral and an understanding of the mechanism behind the reactions. This study also seeks to investigate the performance and adsorption mechanisms when single minerals are interacted with thiol collectors singly and also in a mixture. A better understanding of how to design collector mixtures is to be gained. Furthermore this study seeks to understand the effect of mineral-mineral interactions on thiol collector adsorption and on the floatability of the pure minerals.