Browsing by Author "Petersen, Joachim"

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    An investigation into the enzymatic activity of deepsea actinobacteria in decolourising crystal violet dye
    (2019) Davids, Natasha; Petersen, Joachim; Huddy Robert
    Crystal Violet (CV) decolourising deep-sea actinobacteria could provide a great source of novel redox biocatalysts that can be used in various applications such as removal of triphenylmethane dyes from contaminated wastewater and soil, degradation of aromatic environmental pollutants, biotransformation of antimicrobial agents and degradation of xenobiotics. CV is a triphenylmethane dye that has various applications, including use in medical, research and industrial applications, but its release into the environment poses a threat to aquatic life as it has characteristics of a biocide. Only a limited number of microorganisms are able to decolourise and degrade CV, and one of these proposed mechanisms by which they do so is the catalytic effect of oxidoreductase enzymes, including peroxidases, polyphenol oxidases and laccases. Triphenylmethane reductase has also been reported to be involved in decolourising CV, but the reaction involving this enzyme has not been studied systematically. Eleven deep-sea actinobacteria were investigated and found to decolourise CV by either biodegradation or biosorption. Gordonia sp. JC 51 was selected as a candidate for further study as it could decolourise CV efficiently and could tolerate high concentrations (1mM) of CV. A combination of spectral scan studies, dye decolourisation, biodegradation assays, enzymatic assays, SDS-PAGE, Native PAGE, TLC and LC/MS/MS methods revealed the mechanism involved in the decolourisation of CV. Gordonia sp. JC 51 decolourised CV via enzymatic and non-enzymatic mechanisms. However, true decolourisation of CV was performed via biodegrading enzymes. Triphenylmethane reductase and polyphenol oxidase was confirmed to be the enzymes involved. Leucocrystal Violet was identified as the metabolite produced. CV also was sequentially N-demethylated, oxidised and cleaved into smaller compounds such as Michler’s Ketone. In conclusion, Gordonia sp. JC 51 has potential as a whole cell biocatalyst and should be investigated further.
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    An investigation into the precipitation of Fe(III) oxyhydroxides in lime neutralization processes
    (2019) Mangunda, Cledwyn Tawanda; Lewis, Alison; Petersen, Joachim
    Acid Mine Drainage (AMD) and hydrometallurgical wastewater are products of extractive activities that pose environmental risks as they contain heavy metals. The contents of these streams, which is dominated by iron, necessitates an effective treatment protocol. The tendency of iron to precipitate as a sludge has drawn attention to the mechanisms and the associated kinetics of Fe(III) oxyhydroxide precipitation and the post precipitation solidliquid separation processes within the high density sludge (HDS) treatment process. The HDS process is affordable to operate and has the potential benefit of the Fe(III) oxyhydroxide precipitates removing other pollutants through coprecipitation and adsorption, but it has its own shortcomings. These include, but are not limited to, the tendency of lime to be affected by armouring which hinders further lime dissolution, the decreased solubility of lime at high pH and elevated temperature and the formation of a gelatinous low-density sludge comprised of minuscule particles of Fe(III) oxyhydroxides and gypsum. The sludge is characterized by poor dewatering behaviour and postneutralization instability which increases the risk of the adsorbed or encapsulated dissolved metals to re-leach from the disposal sites, thus lessening the likelihood for metal value recovery, waste material reuse or recycle. These negative attributes result in considerable water losses and a large land area being required for sludge disposal to landfill. The literature reviewed indicates that researchers have not reached a consensus on the actual causes of sludge formation and transformation in the course of Fe(III) oxyhydroxide precipitation, with reported reaction kinetics and mechanisms being significantly different. In light of the above difficulties and lack of sufficient progress in body of knowledge, the objectives of the current research were - to gain an in-depth understanding of the dewatering behaviour of primary Fe(III) oxyhydroxide precipitates through investigating the effect of Fe(III) concentration; [Fe(III)] and OH/Fe molar ratios on the performance (settling and filtration rates and specific resistance to filtration - SRF) of products generated in the precipitation process so as to establish the mechanistic relationships that exist, and - to understand the mechanisms and kinetics through which ageing of secondary Fe(III) oxyhydroxide precipitates occurs at elevated temperature and prolonged ageing time and to infer the effects of ageing on the dewatering behaviour of secondary Fe(III) oxyhydroxide precipitates. The mineral phase identification, though acknowledged to be valuable in iron precipitation, was seen to have negligible value in relation to describing dewatering behaviour, hence was not focused on. In order to achieve the objectives above, experiments were conducted in a 1.5 L glass mixed-suspension-mixed-product-removal (MSMPR) reactor under ambient laboratory temperature at 21 ± 1 °C. The agitated MSMPR was equipped with a lid containing ports for pH and temperature probes and four uniformly spaced stainless steel baffles to maximize mixing of the reagents. A synthetic acid mine drainage (SAMD) solution made from aqueous Fe2(SO4)3 was treated in the MSMPR reactor with a reagent grade Ca(OH)2 aqueous stream, with the Ca(OH)2 solution being used for pH adjustment thereby effecting neutralization and precipitation. A residence time of 15 minutes, as estimated from the reactor volume and feed solution flow rate, was used. The number of residence times for steady state attainment were identified as 16 from the particle size distribution (PSD) measurements, although the reactor was allowed to run up to 19 residence times (19 τ = 575 min) before sampling commenced. The ferric concentration ([Fe(III)]) in the feed was kept constant at six different levels of 50, 100, 200, 300, 400 and 800 mg L-1 (± 5 mg L-1 ) whilst adding the corresponding OH, at a molar ratio (OH/Fe - referred to as R) of 3, from the Ca(OH)2 solution. The reaction conditions were such that no gypsum was precipitated. The experiments to investigate the effect of molar ratios were conducted at OH/Fe molar ratios ranging from 3/1 (R = 3), 2.5/1 (R = 2.5) and 2/1 (R = 2) whilst the [Fe(III)] in the feed was kept constant at 100, 300, and 800 mg L-1 (± 5 mg L-1 ). The experiments to aid the understanding of ageing effects on the product characteristics (particle size, PSD, morphology and structure) and dewatering behaviour were conducted under two regimes, the continuous and the batch system. Under continuous conditions and a residence time of 15 minutes, the feed [Fe(III)] into the primary reactor (MSMPR) was set at 100, 300, and 800 mg L-1 and the aqueous stream of reagent grade Ca(OH)2 was added at a molar ratio of OH/Fe of 2.5. The primary reactor was set up on a secure elevated platform and was operated such that the overflow was continuously fed into a 1 L jacketed glass MSMPR reactor (ageing reactor). The ageing reactor was supplied with heated deionized water, operated at 30, 40 and 50 (± 1) °C and agitated at a rate of 230 rpm by a four bladed 45° pitched blade turbine. The batch ageing experiments were conducted in a 1 L jacketed reactor which was allowed to attain the setpoint temperature of 40 ± 1 °C before being filled with fresh Fe(III) oxyhydroxides from the primary reactor precipitated at a molar ratio of OH/Fe of 2.5 (R = 2.5) and [Fe(III)] of 300 mg L-1 . The agitated (230 rpm) reactor was run for 24 hours with samples collected at set times of 1, 2, 4, 6, 7, 8, 9, 10, 16 and 24 hours. The PSD results of the samples were used to establish the mechanisms and kinetics of the system by using the method of moments. The samples taken from the conditions investigated, as outlined above, were analysed for [Fe(III)] in the feed, precipitate and supernatant using spectrophotometry and the 1–10 phenanthroline method as the basis. Spectrophotometry was also used to determine the calcium and sulphate concentrations in the feed, precipitate and supernatant. The PSD and aqueous specific surface area were measured by means of laser diffraction that used static light scattering. Surface charge inferences were made through zeta potential measurements (ζ-potential) using electrophoresis to determine the isoelectric point (IEP). The pH dependence of the ζ-potential was investigated using multi-purpose titrations (MPT-2) with the titrants being 0.01 M H2SO4 and 0.01 M and NaOH. The crystal habit was analysed using scanning electron microscopy (SEM) whilst a combination of SEM with EDX was used for composition determination. Transmission electron microscopy was used to determine the morphology and size. The slurry viscosity was measured using a rheometer operating with the vane and cup arrangement. Iron mineral phase identification was done through Raman microscopy and XRD in order to verify if the mineral phases from a slurry based analysis and a solid sample analysis were the same. The initial studies on the dewatering behaviour indicated that the [Fe(III)] in the feed influenced (to varying extents) the dewatering trend of the precipitation product, via particle size, particle size distribution (PSD), surface charge, morphology and structure. It was established that there was a corresponding change in product characteristics and performance as the [Fe(III)] was increased. A transition was observed at [Fe(III)] of 300 mg L-1 where the settling rate (2.4 ± 0.04 mm min-1 ), particle size (D4,3 of 27 µm) and supersaturation passed through a maximum whilst the filtration rate (3 ± 0.46 cm3 /cm2 /min) and zeta potential approached a minimum. These conditions resulted in enhanced agglomeration, with the improved dewatering behaviour being attributed to a dense network of interlocked and interconnected bridges in the crystal habit of the particles. The results did show that there was a complicated relationship between the underlying micro- and nano-scale properties and the product performance from which dewatering behaviour is inferred. The OH/Fe molar ratio variations were described as low (R = 2.5) and high (R = 3) due to contact nucleation prompting the exclusion of the R = 2 results from further analysis. These molar ratios had an effect on product performance which was improved at the low OH/ Fe molar ratio. A low R positively affected the product characteristics which led to the fastest settling rate of 3.3 ± 0.2 mm min-1 , an increase in the filtration rate from 3.7 ± 0.4 to 5.1 ± 0.5 cm3 cm-2 min-1 and a decrease in the specific resistance to filtration (SRF) from 1.42 x 1012 to 6.71 x 1011 m kg-1 at R = 2.5 and a [Fe(III)] = 300 mg L-1 . The improved product performance was attributed to the dominance of large (D4,3 of 48.3± 0.05 µm and D3,2 of 30.8 ± 0.05 µm), regularly shaped particles which had a unimodal distribution and a notable decrease in the peak number of fine particles from 3.5 × 1013 (1 µm size) to 1.7 × 1012 (3.90 µm size). The Fe(III) oxyhydroxides formed at [Fe(III)] of 100 and 800 mg L-1 displayed insignificant product performance changes with variation in R with the exception of the 800 mg L-1 case which produced the largest change in settling rate (threefold – from 0.8 to 2.4 ± 0.2 mm min-1 ) at R = 2.5. These results reinforced the change in settling and dewatering behaviour observed at [Fe(III)] of 300 mg L-1 . It was noted that the lower R value (R = 2.5) resulted in a lower pH in solution, leading to decreased nucleation. However, higher agglomeration rates were observed, contrary to other studies. This was attributed, in the present study at low R, to a reduction in the surface charge related electrostatic repulsion, whilst van der Waals attractive forces had been favoured by the decrease in supersaturation. The product performance of the precipitates which underwent structural maturation in a continuous system were found to improve after elevated temperature treatment. The highest settling rate was found to have been reached at 40 °C and at [Fe(III)] of 300 mg L1 . This was ascribed to the increase in agglomeration which yielded a high D4,3 (26.3 ± 0.05 µm) mean particle size at this temperature; hence bigger particles implied faster settling. The filterability of the aged precipitates, at a given [Fe(III)], passed through a maximum as the temperature increased. Another transition was observed at Fe(III) of 300 mg L-1 where there was a decrease in surface charge with an increase in temperature as evidenced by the low zeta potential values and a steady state pH which was close to the isoelectric point. This was evidence of a circum-neutral surface charge that favoured agglomeration. The change in micro and nano-structure was seen in the image analysis results. However, these imaging results show evidence of deagglomeration at 50 °C which probably explains the improved dewatering behaviour. The batch ageing system revealed that structural maturation in the ageing process, at elevated temperature, has multiple particle rate processes that occur simultaneously but that which one dominates varies with time. The ageing process results identified four broad time intervals, namely 0 to 6, 6 to 8, 8 to 10 and 10 to 24 hours, through which the different mechanisms were active. The settling trend for the samples collected at the different ageing times displayed a high initial settling rate within the free settling zone of the settling curve. The solids from interval 1 and 2 (0 to 6 and 6 to 8 h) displayed the fastest settling rates within the hindered settling zone with interval 2 solids dominating the compression settling zone. It was noted, however that no significant improvements in settleability were observed with an increase in ageing time except at 8 hours. The filterability on the other hand experienced improvements with ageing especially in the interval 3 and 4 (8 to 10 and 10 to 24 h) where the specific resistance to filtration (SRF) was at its lowest value of 8.78 × 1011 m kg-1 at 8 hours. This was attributed to a combination of an increase in particle size, a decrease in the number of fine particles and a transition in the crystal habit and morphology of the particles as evidenced in the image analysis findings. The product performance followed a trend where it decreased at the onset of interval 1 and remained fairly constant throughout the interval. The onset of interval 2 saw an increase in product performance which reached a distinct maximum at 8 hours after which it reverted back, interval 3 and 4, to levels similar to those in interval 2. The mechanisms that were inferred from the moments of the distribution alluded to the nucleation of new particles (interval 1), the agglomeration and limited growth of those particles which involved structural incorporation (at 8 hours in interval 2) and transformation (of ferrihydrite into goethite particles via a dissolution and reprecipitation process which led to the goethite being sparsely distributed on top of the ferrihydrite base) (interval 3 and 4). The graphical illustration of these processes and their impact on settling and dewatering behaviour is illustrated below. The findings of this work on primary precipitation and ageing of iron during lime neutralization have implications on how iron is treated in AMD and hydrometallurgical processes. A slight elevation in post-precipitation operating temperature under reduced lime usage offers an attractive alternative to improving dewatering behaviour of the precipitated sludge with the potential of reducing metal leachate from landfills.
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    Assessment and modelling of chromium release in minerals processing waste deposits
    (1998) Petersen, Joachim; Petrie, James G
    The minerals processing industry is by far the largest generator of mineral solid wastes, which are commonly stored in large scale landfill deposits. The potential environmental impact of these is directly linked to the time-dependent process of leachate generation within these deposits. Rainwater draining through the porous matrix of a deposit creates a slowly moving aqueous environment within the deposit. Heavy metal species that may be contained in trace amounts in the waste material can be mobilised into the aqueous phase by various chemical reactions and be transported by mechanisms of diffusion and convection to the base of the deposit and from there further into the surrounding environment. Laboratory assessment methods aim to provide indicators to the leachate generation potential of a particular waste material, often based on "worst case" assumptions, but generally fail to offer a meaningful appreciation of the time-dependent leach behaviour of the material in a full scale deposit. This is to a large part due to the lack of a thorough description - in terms of a rigorous mathematical model - of the leachate generation process itself. Such a model is developed in the present work, building on an existing model for heap leaching, which, conceptually, is very similar to the leachate generation process. The model is based on the continuity equation formulated for reaction-diffusion processes at the level of an individual porous particle and for convection-dispersion transport at the bulk level. This is combined with a number of reaction models, both kinetic rate expressions and thermodynamic equilibrium models, to describe the release process of individual species at the solid liquid interface and also within the aqueous phase. The model has been translated in the WASTESIM computer code within which waste iv Abstract material and disposal scenario are characterised by a number of parameters, such as those describing reaction modes and constants, particle size and pore diffusion effects as well as bed transport and saturation. The program was found to be a versatile tool for modelling a wide range of multi-species, multi-reaction deposit and batch leach scenarios. However, for modelling real waste materials the model parameters have to be established from a systematic laboratory investigation. An assessment methodology is proposed which aims to combine lysimeter studies with bench scale leach and physico- chemical characterisation experiments to enable determination of all model parameters entirely on the basis of laboratory experiments and validate them at this level against the results from independent lysimeter studies with the modelling tool. It is argued that, if all model parameters are validated at the laboratory scale in this way, modelling of full scale scenarios involving the same waste material can be conducted with some confidence. This approach has been put to the test with two waste materials from the ferro-alloy industry - a furnace emission control dust and a smelter slag. The contaminant species of particular interest for both these materials was chromium, especially Cr(VI), and therefore it was the release behaviour chromium on what much of the work presented herein has focused. The aqueous and environmental chemistry of chromium is extensively reviewed and, as a side aspect, the long-term atmospheric oxidation of Cr(III) to Cr(VI) has been positively identified by experimental work with a third chromium-containing waste material. The two test materials have been subjected to intensive characterisation in terms of column and batch leach experiments, adsorption studies, column tracer studies and physical characterisation experiments. The results are carefully interpreted with a view to establishing a complete set of parameters to simulate the leachate generation behaviour with respect to chromium species in a deposit scenario. It is demonstrated Abstract V that the modelling tool can in fact also be used for the interpretation of batch leach data through curve fitting exercises. For both materials the WASTESIM code, calibrated with parameters established entirely through the laboratory experimentation, has been used to simulate the leach curves of two independent lysimeter experiments, which are then compared to the measured data. In both cases the modelled and measured curves compared reasonably well and in most regards discrepancies can be explained by insufficient characterisation in the bench-scale experiments. The overall approach is therefore seen as valid in principle, but it is acknowledged that further experimental work and model development would be needed to take account of the remaining discrepancies. Two aspects were found to be particularly significant. The first relates to slow reaction mechanisms, which may go unnoticed in short-term laboratory experiments, but may become significant in full scale deposits given their long life-span. The slow atmospheric oxidation of chromium is a point in case. The second aspect relates to the hydro-dynamic characterisation of flow through unsaturated beds. Both model and laboratory assessment methods are insufficiently developed to account for effects such as dead pore diffusion and a distribution of flows. Recommendations for further development work should focus on these two aspects and on expansion of the approach to heavy metal species other than chromium. It is hoped that the modelling and assessment methodology will ultimately find welcome application in the environmental risk assessment of mineral processing waste disposal operations.
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    Evaluation of solvent swelling pre-treatment combined with ammonia leaching using waste etchant from printed circuit board manufacturing for copper recovery from waste printed circuit boards
    (2023) Kondo, Takunda; Petersen, Joachim
    Electronic waste (e-waste) has emerged as a rapidly growing waste stream, growing at a global rate of 2 Mt per annum and is expected to exceed 74 Mt by 2030. E-waste contains a high content of precious and base metals which can be recovered for economic benefit. This has made the recycling of this waste stream of interest, with particular emphasis on the recovery of metals from waste Printed Circuit Boards (PCBs) as they contain a high content of copper as well as a significant quantity of precious metals such as gold, silver, and palladium. The major economic driver for the beneficiation of PCBs is the recovery of these precious metals followed by the recovery of copper which can constitute more than 20% of the value share. In the South African context, 5-7% of e-waste generated is collected and formally recycled. There exists potential to increase the proportion of e-waste recycled through the development of accessible beneficiation techniques that can be implemented by small scale industries. This study builds on previously completed research on alkaline ammonia leaching for the recovery of copper from waste PCBs that have undergone various physical or chemical pretreatment processes for metal liberation. Some PCB manufacturing processes employ a similar alkaline ammonia oxidative leaching system as one of the key process-steps in the manufacturing of PCBs as is evident by the case study of a local PCB manufacturing company. There exists an opportunity to utilize waste etchant from the manufacturing process as lixiviant for the recovery of copper from waste PCBs. This will enhance resource efficiency in line with the United Nations sustainable development goal 12, responsible consumption and production by extracting value from both the copper loaded waste etchant and waste PCBs. The potential to recover copper from both waste PCBs and the waste etchant through a leach circuit that utilizes the excess lixiviant contained in the waste etchant is explored. To do this, the copper distribution of a custom-made 4-layer PCB is established through a breakdown of the readily accessible top and bottom surface copper, referred to as the “surface copper” as well as the tightly laminated inner layer copper inaccessible to the lixiviant. Pre-treatment utilizing chemistry from the PCB manufacturing desmear process is then explored as a viable pretreatment method for liberating the inner layer copper with a key focus on solvent swelling with N-Methyl-2-pyrrolidone (NMP) and NMP based solvent (Solvent B). The characterization of the custom-made PCB determined that 47.2% of the copper was located on the surface of the PCB and 52.8% in the inner layers. The success of the solvent swelling pre-treatment is measured by the extent to which this 52.8% is unlocked for access by the lixiviant. The degree of metal liberation by the chemical pre-treatment method employed was determined by diagnostic column leaching using primarily samples of waste etchant from a local PCB manufacturing factory as well as synthetic etchant modelled after a typical waste etchant stream. Solvent swelling yielded good recoveries above 90%, with NMP solvent swelling at 150C achieving the highest copper recovery of the two solvents at 93.4%. It was found that the PCBs that did not undergo solvent swelling yielded recoveries mostly from the surface copper with a recovery of 50% from boards that had been soaked in sodium hydroxide for removal of the solder mask coating. A combination of both physical and chemical pretreatment by combining 6 pass shredding with solvent swelling yielded a copper recovery of 68.9% using Solvent B. This was comparable to 88.4% obtained from solvent swelling with the same solvent. Solvent swelling alone was found to be an effective method for liberating copper from the inner layers and combining swelling with mechanical shredding was found to yield lower copper recoveries as a result of the preg-robbing phenomenon. Three lixiviant systems were analyzed for performance, namely ammonium sulphate, waste etchant and synthetic etchant. Of the three, ammonium sulphate with initial copper concentration of 100 ppm was significantly outperformed in leaching rate by waste etchant and synthetic etchant which required shorter leaching times to recover the readily accessible surface copper, however waste etchant was found to have limitations of copper saturation leading to precipitation due to its high copper content.
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    Hydrometallurgical Processing Of Rare Earth Elements From Ion Adsorption Clays
    (2023) Miiro, Eddy; Petersen, Joachim
    The surface mining and heap leaching of China's ion-adsorption rare-earth resources have caused severe environmental damage, there is a need to better understand Rare Earth leaching from Clay ore with the use of agglomerates that improve the permeability of the ore during heap leaching operation. The practice of in-situ leaching has also revealed serious environmental problems including underground contamination, mine collapses, and landslides. The performance of lixiviants in extracting Rare Earth has been compared in many studies however this has been carried out in agitated systems and the heap leach scenario is different from agitated systems. This limits the understanding of poor heap permeability and post-closure stability whereas un-agitated leaching of Rare Earth from Clay agglomerates gives a better understanding of diffusion, heap permeability, and material balance to mimic the heap leach scenario. A comprehensive understanding of the leaching mechanism is crucial for achieving high extraction efficiency with low cost and less environmental impact. A series of inorganic salts with different concentrations were employed to leach the Ion-adsorption rare earth agglomerates, and the relationship between the leaching efficiency of rare earth and reagent type was investigated in unagitated systems. This study showed that ammonium sulfate extracts less aluminum content than magnesium chloride. Even when magnesium sulfate (hydrate) extracts less aluminum (impurity in Pregnant Leach Solution) content than the other lixiviants used in the study, magnesium sulfate extracts less Rare Earth Elements (REE) compared to the other lixiviants in the same leaching time, this was an unexpected result when magnesium sulfate hydrate was used instead of anhydrous magnesium sulfate that was much more expensive (per mass) compared to the prices of the other leaching reageants. Therefore the lixiviant of preference in terms of REE extraction from ion adsorption clays in un-agitated systems is in the order (NH4)2SO4 > MgCl2.6H2O > NaCl > MgSO4.7H2O.
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    Phase equilibria studies and beneficiation of titaniferous slags
    (2019) Goso, Xolisa Camagu; Petersen, Joachim; Nell, Johannes
    Titaniferous magnetite (titanomagnetite) offers a unique opportunity for the production of three valuable products from one resource. It generally contains economically appreciable reserves of vanadium and iron as well as significant contents of titanium. Titanomagnetite is typically smelted in blast or electric furnaces in the presence of reductant and fluxes (dolomite and quartz) to produce a valuable vanadium bearing pig iron and a virtually valueless titaniferous slag. The titaniferous slag by-products are generally defined by the Ca-Mg-Al-Si-Ti-O system. These fluxed slags can contain as high as 20-40wt% TiO2 (titania). The lack of interest in processing titaniferous slags to produce saleable titania materials is attributed to the presence of chemically inert phases, like the spinel solid solution [Mg(Al,Ti,V)2O4] that cannot be handled by the available titania slag upgrading technologies. The understanding of phase relations in titaniferous slags is thus important in order to be able to implement a suitable fluxing strategy for the production of a treatable titaniferous slag with no inert spinel phase. The available phase equilibria data established in air for titaniferous slags is inconclusive about the possible crystallisation of the detrimental spinel. However, literature on phase compositions of plant titaniferous slags are conclusive about the crystallisation of Mg(Al,Ti,V)2O4 in high MgO and Al2O3 bearing slags. It is thus clear that the understanding of phase relations in titaniferous slags requires further development. The objective of the current project was to investigate the phase equilibria and beneficiation of titaniferous slags to produce a saleable titania product. As a development to previous work, the composition of the slag for review was based on the available work, namely; TiO2 = 37.19wt%, SiO2 = 19.69wt%, and Al2O3 = 13.12wt%, at varying proportions of CaO (30- 0wt%) and MgO (0-30wt%). The phase equilibria studies followed a systematic approach involving the review and validation of the available equilibrium phase diagram produced in air, followed by the determination of updated phase equilibria at low oxygen partial pressures (pO2) of 10-16 atm applicable to titanomagnetite smelting. The generic approach of studying phase equilibria in multicomponent systems was followed, namely; (1) literature survey of the available thermodynamic and phase equilibria data applicable to the reviewed system, (2) calculation and re-drawing of the equilibrium phase diagrams using FactSage thermochemical software, and (3) equilibration-quench-(electron probe micro analysis) (EPMA) experiments to the validate calculated equilibrium phase relations. A titaniferous slag with little crystallisation of the inert spinel phase, based from the best fluxing strategy with an MgO-free limestone, was produced by smelting in conventional (using alumina crucible) and cold copper crucible induction furnaces for subsequent beneficiation using the established Upgraded Slag (UGS) process. A conceptual flowsheet for the production of vanadium, steel and titanium products was therefore designed and subsequently subjected to economic evaluation using the discounted cash flow (DCF) modelling approach. Thermodynamic and phase equilibria literature for the Ca-Mg-Al-Si-Ti-O system demonstrated that this system and some subsystems are well researched in air, and not as much in low pO2 atmospheres applicable to smelting operations. The FactSage software used in the current study for the calculation of phase equilibria in the Ca-Mg-Al-Si-Ti-O system applicable to titaniferous slags does not have tialite (Al2TiO5) modelled as a component in the customary pseudobrookite solution database - Al2TiO5 is a component of the pseudobrookite solution reported in literature for the current system. Hence, a private pseudobrookite solution database applicable to the reviewed system, i.e. MgTi2O5-Al2TiO5-Ti3O5, was developed and incorporated into FactSage before any calculation could be conducted. Thermodynamic modelling of the MgTi2O5-Al2TiO5-Ti3O5 system was conducted through the CALculation of PHase Diagram (CALPHAD) method. The sublattice model coupled with compound energy formalism (CEF) and Redlich-Kister polynomial were adopted. The model information was incorporated into FactSage to create a private database for subsequent calculations of phase equilibria of titaniferous slags. The equilibrium phase diagram for the Ca-Mg-Al-Si-Ti-O system in the same compositional range as in the available literature was then calculated in air. The liquidus surfaces and phase relations in the equilibrium phase diagrams of available literature and FactSage calculation are fairly comparable. However, at high MgO concentrations: FactSage calculation predicts that Mg(Al,Ti)2O4 is the primary phase, followed by successive crystallisation of pseudobrookite solid solution (MgTi2O5-Al2TiO5) and forsterite (Mg2SiO4); and the available literature reports MgTi2O5-Al2TiO5 as the primary phase, followed by Mg2SiO4. The crystallisation of spinel phase in the available phase diagram produced in air is not predicted. The crystallisation of the spinel solid solution phase in titaniferous slags is extensively reported in the open literature. Equilibration-quench-EPMA experimental results produced in air generally compared well to the FactSage calculations. The inability of the available phase diagram to predict spinel phase crystallisation was attributed to the lack of sensitive analytical techniques in the late 1960s, when the available phase diagram was developed. The phase equilibria of titaniferous slags were further calculated at low pO2 atmospheres of 10-16 atm. In the reviewed compositional range of titaniferous slags, the liquidus surface and Ti3+/Ti4+ mass fraction ratio increased with decreasing the pO2. There was no significant difference in terms of the crystallisation of phases between the calculated results in air and at pO2 of 10-16 atm, except that the size of the primary phase field at higher MgO concentrations than the composition for the minimum liquidus temperature increased and the pseudobrookite solution included Ti3+ bearing phase, i.e. MgTi2O5-Al2TiO5-Ti3O5. Equilibration-quench-EPMA experimental results produced at pO2 of 10-16 atm generally compared well to the FactSage calculations. The new phase equilibria at low pO2 of 10-16 atm shows that the crystallisation of the chemically inert Mg(Al,Ti)2O4 in titaniferous slags would occur if the slag contains high Al2O3 concentration and MgO concentration of 2wt% and above. However, the crystallisation of Mg(Al,Ti)2O4 in titaniferous slag is not significantly sensitive to variation in the TiO2 concentration in, and basicity of the slag. To produce a titaniferous slag with minimum possible inert spinel content for subsequent beneficiation, the South African Main Magnetite Layer (MML) titanomagnetite concentrate was smelted in the presence of an MgO-free lime and low ash Sasol carbon (SASCARB) reductant. This smelting approach would produce a titaniferous slag with about 4wt% MgO, which would come from the titanomagnetite - based on the phase equilibria, this slag should crystallise a small amount of the inert spinel. When the smelting was conducted in an alumina crucible placed inside a conventional induction furnace, the slag was inevitably contaminated by Al2O3 from the refractory container. This slag crystallised a significant Mg(Al,Ti)2O4 with the content approximated by the MgO concentration - the significant Mg(Al,Ti)2O4 crystallisation was attributed to the Al2O3 saturation in the titaniferous slag. A titaniferous slag containing a treatable ulvospinel phase was produced in a cold copper crucible induction furnace - the crystallisation of the ulvospinel, instead of the chemically inert spinel solid solution was attributed to the saturation of the slag by iron from the incomplete reduction process due to the inevitable stoppage of the heat supply to the induction furnace as soon as the susceptor iron metal and produced pig iron settled at the bottom of the copper crucible. For the purpose of demonstrating the feasibility of producing titania products from titaniferous slags, this slag was also subjected to beneficiation using UGS process. The current study successfully demonstrated that the titaniferous slags can be beneficiated to saleable titania products using the UGS process: the TiO2 in the Mg(Al,Ti)2O4 bearing waste titaniferous slag produced by the defunct Evraz Highveld Steel and Vanadium Corporation (EHSV) was upgraded from about 33wt% to 75wt%, while the TiO2 in the titaniferous slags produced in conventional and cold crucible induction furnaces were upgrade from about 30wt% to 67wt% and 22.00wt% to 90.45wt%, respectively. The remaining impurities in the 75wt% and 67wt% TiO2 UGS products were mainly MgO and Al2O3 contained in the refractory spinel solid solution. In the case of the 67wt% TiO2 product, there was excess Al2O3 in the spinel structure - the excess Al2O3 remained in the glass phase. Though the 90.45wt% TiO2 product is attractive for use as feedstock for the production of the preferred chloride pigment, this product however contained finer PSD and higher concentrations of impurities such as SiO2, Al2O3, and CaO, than the specification for the chloride process feedstock. This product was thus not a suitable feedstock for the chloride pigment production. Further optimisation of the UGS conditions has a potential to reduce the concentrations of impurities to levels suitable for feedstock for the preferred chloride pigment production process. Further investigations are also required to study the feasibility of the chlorination of micro-pellets of the UGS product. Since the UGS product is mainly composed of rutile structure, it would not be a suitable feed for the sulfate pigment production as the sulfuric acid lixiviant is unable to dissolve the rutile structure. Only if soluble in sulfuric acid, this high TiO2 bearing UGS product produced from titaniferous slags could be used as advantageous feedstock for the sulfate pigment production in terms of the minimization of the reagent consumption and the amount of the toxic sulfate waste. Based on the work of the current study, literature data and Pyrosim simulations, a conceptual process flowsheet for the production of vanadium slag, steelmaking pig iron and titania product was proposed. The economic modelling of the conceptual flowsheet for a 20 year operational projection showed that the process is economically viable. The process economic viability is sensitive to variation in the Opex and Revenue. In addition, additives, such as the amount and type of reductant, fluxes, and reagents account for about 75% of the Opex. It is possible that the additives are overestimated in the process as the recycle streams were not included in the proposed process and economic model. At the same time, the economic model does not consider the environmental and waste management costs. Hence, the economic analysis is considered to be preliminary in nature, or indicative at best. The current study has demonstrated that (1) a titaniferous slag containing little or no inert spinel phase that is suitable for upgrading can be produced when the MgO content in the slag is below 2wt% - the best approach to producing a slag with minimum possible MgO content would be to smelt the titanomagnetite in the presence of an MgO-free limestone flux and low ash reductant, and (2) it is technically and economically feasible to produce three products, i.e. V slag, steelmaking pig iron, and titania product, from titanomagnetite.
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    Residence time investigation of artificial silver ores in heap leaching using cyanide lixiviant
    (2020) Gibson, Borbor Auzzel Kwaku Kudar; Petersen, Joachim
    Heap Leaching has gained much relevance in the processing of low-grade mineral resources - ores considered uneconomical for beneficiation through conventional concentration and tank leaching. However, it is at the same time characterized by extended leaching periods due to slow mineral conversion and low rates of recovery as a major challenge. Interactions within the heap bed are not fully understood as chemical leaching and hydrodynamics interact in a complex manner. To study these interactions, a number of investigations have focused on the hydrodynamic interaction using conventional residence time distribution (RTD) studies in laboratory columns. In these RTD studies, the flows of tracer exiting through the effluent stream provide information of its paths, where some flows might relate to fast movement, slow convoluted channels, or micro/macro stagnated regions. This information is usually interpreted through simplified reactor models representing the bed as a combination of plug flow reactors (PFRs), continuous stirred tank reactors (CSTRs) and dead zones. Using columns as reactors to approximate heap leaching on a laboratory scale, it is anticipated that the RTD flow distribution response should be similar to the distribution of a PFR with associated dead zones. While some literature sources have alluded to the response in columns being similar to a plug flow response, recent sources using a similar hydrodynamic RTD approach reported column reactor distribution resembling flow more typical of a continuous stirred tank, CSTR, system instead of plug flow. Given that packed ore beds are not agitated, this appears paradoxical. It is hypothesised that the CSTR-like response is a result of the distribution of convoluted flow channels through the ore bed, which perform overall like a bundle of PFRs of different lengths. To discern the two patterns the use of a reactive leaching on a well characterized ore material is proposed. Therefore, this work aims to study simultaneously the flow and leach performance of a laboratory column reactor, utilizing a novel reactive leaching approach with a lixiviant amendable to a well-characterized homogeneous solid material. The objective of this work is to establish flow distribution performance in packed bed columns and correlate such performance to the ultimate extraction from the packed bed. The study was performed using a nonreactive solution tracer (potassium nitrate) to characterise flow through a column packed with an artificial silver ‘ore' (silver grains embedded in concrete), followed by a reactive leaching study using sodium cyanide which would leach the silver. The artificial silver ore was developed with the aim to exhibit four ideal properties, namely homogeneous porosity, uniform grain size, homogenous dispersion of the grains throughout the ore, and even grade distribution of the different size fractions after crushing. Leaching and micro-XCT characterization studies were performed in order to determine the leaching properties of the artificial silver ore and validate the ore corresponds to these ideal properties. This validation was indeed achieved; however, the inner-particle pores were not found to be continuous at the scale of resolution of the instrument (4.8 microns). Poor extraction from the coarse particles in both the leaching characterization and reactive column leaching investigations suggested that this network was not well established and may exist only at the sub-micron scale. Leach tests were performed on individual particle sizes in both shake flask and circulating bed reactor tests. Extraction from the leaching of the coarse synthetic silver ore particles was observed to be very low relative to dissolution from pure silver metal grains. Diagnostic shrinking core and the extended mixed topology models were used to determine the controlling dissolution mechanism. Both models demonstrated that a diffusion-reaction mechanism governed the dissolution extraction from the large particles. RTD column leaching studies were performed utilizing flow rate and PSD as investigated parameters. The nonreactive tracer produced a step-change flow response that was more similar to a characteristic plug flow type distribution but showed distinct deviations towards CSTR behavior, especially for the beds containing a higher degree of fines. Reactive column leaching experiments were performed under similar conditions as the nonreactive RTD, introducing a step-change of the cyanide reagent. Rapid silver leaching occurred initially, but equally rapidly declined to very low rates. The leach curves were interpreted by translating the information obtained from the nonreactive RTDs into a distribution of parallel plug flow channels. The extent of reaction for each of these channels is derived from the surface reaction model for the individual size classes, put together for the corresponding PSD in each experiment. RTD specific PSDs tested using this approach assumed that longer residence times correspond to the prevalence of finer material. The validity of the approach was tested by comparing the extraction determined from the particle leaching kinetics studies to the reactive column data through modelling. The model is based on the weighted average leaching from a population of particles, calibrated against kinetic models formulated for individual size classes. This model is further linked to a distribution of flow channels determined from the RTD studies. The prediction of the model did not compare too well against the raw silver dissolution data of the columns. This was attributed to the model having been calibrated against kinetic data that did not fully consider the smaller size classes below -4/+2.8 mm – considered to be the key source of rapid surface reaction in the packed bed. Although the concept proposed in this project was not fully proven, further test work is recommended to expand on the approach presented here.
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    The hydrated lime dissolution kinetics in acid mine drainage neutralization
    (2021) Mgabhi, Senzo Mntukhona; Petersen, Joachim; Lewis, Alison; Rampai, Tokoloho
    Hydrated lime, Ca(OH)₂, has been rediscovered as an environmentally sustainable product, which could be of help in the remediation of acid mine drainage (AMD), especially in the AMD neutralization process. This is due to its ease of acquisition, affordable price and unique versatile properties such as reactivity and neutralization efficiency. AMD is an acidic wastewater containing high concentrations of sulphates and dissolved heavy metals mainly ferrous iron. The dissolution of Ca(OH)₂ in aqueous solution is complex, which make its kinetics in AMD neutralization difficult to understand. The aim of this study was therefore to understand the Ca(OH)₂ kinetics in simplified solutions such as de-ionized water and CH₃COOH. The neutralization process is an acid-base reaction; therefore, pH was used as a critical parameter in determining Ca(OH)₂ dissolution rate. The determination of the dissolution rate was attempted in two ways – measurement of dissolved calcium and determining change of particle size distribution. There were two methods of determining calcium assays investigated, that is EDTA-EBT titration method and OCPC spectrophotometric method. Both methods worked successfully for a Ca(OH)₂-H₂O system. The EDTA-EBT titration method worked better even at higher concentrations of calcium (up to 100 ppm) while the complexometric spectrophotometric method was consistent with Beer-Lambert Law for a narrow calcium concentration range of 1 to 2 ppm, when a small amount of magnesium was introduced. However, both methods failed in the presence of appreciable quantities of magnesium, sulphates and ferric ion. The investigation for particle characterization found that image analysis of SEM images was a better particle-size characterization option than laser diffraction measurement, which tended to cause blinding of the instrument window, but still yielded only qualitative results. There were four reactor configurations used, that is batch reactor for determining the effect of the hydrodynamics (stirring rate and powder addition) and three types of slurry CSTRs. The jacketed chemostat was found to be the optimal reactor configuration while the other two were used as base cases. The Ca(OH)₂ dissolution rate in de-ionized water decreased from 4.0×10⁻⁵ to 1.6×10⁻⁵ mol‧L⁻¹‧s⁻¹ when the temperature was increased from 26 °C to 42 °C. Correspondingly, the pH decreased with Ca(OH)₂ dissolution rate from 11.89 to 11.6. The dissolution rate expression was first order and consistent with the Nernst-Brunner Equation, with the dissolution rate constant of 2.34×10⁻³ s⁻¹ and the activation energy of 18.1 kJ mol ⁻¹ respectively. The overall Ca(OH)₂ dissolution rate in CH₃COOH solution decreased from 2.6×10⁻⁴ to 1.7×10⁻⁴ mol‧L⁻¹‧s⁻¹ when the temperature was increased from 25 °C to 44 °C. At constant ambient temperature (22°C), the Ca(OH)₂ dissolution rate increased with the decrease in pH from 12.1 to 4.38, then decreased with the decrease in pH from 4.38 to 3.5. Using pH to correlate dissolved calcium data and then to determine the rate of reaction, it was found that the dissolution rate is zeroth-order to hydrogen proton and first-order with respect to calcium concentration with the dissolution rate constant of 1.2×10⁻² s⁻¹ and the activation energy of 5.7kJ mol ⁻¹ respectively. These results confirmed that the dissolution of Ca(OH)₂ in DI water and the acetic acid solution is complex. The lower values of the activation energies (5.7 – 18.1 kJ mol ⁻¹), signifies that the kinetics of the Ca(OH)₂ dissolution are mass transfer controlled. Furthermore, these results were confirmed by the weak dependence of the dissolution rate to temperature. However, it was found that slurry CSTR is an efficient reactor system to study the effect of pH on the kinetics of hydrated lime at steady-state conditions.