Faculty of Engineering and the Built Environment (ETDs)

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Constrained solutions to IFPPS by finite-dimensional approximation
(University of the Witwatersrand, Johannesburg, 2024) McDonald, Andr´e Martin; van Wyk, Anton
The inverse Frobenius-Perron problem (IFPP) refers to the probabilistic modelling problem that requires the design of a discrete-time, one-dimensional dynamical system (i) that is ergodic and (ii) generates a zero-input time response that possesses prescribed statistical metrics as specifications. Systems designed using solutions to the IFPP hold potential as highly efficient and versatile random signal generators as well as solutions to practical radar signal processing problems. Distinct formulations of the IFPP have appeared in the literature: IFPP-I: The system is designed such its zero-input response possesses a pre- scribed probability density function (PDF). IFPP-II: The system is designed such that its zero-input response simultaneously possesses a prescribed PDF and a prescribed power spectral density (PSD). IFPP-III: An unknown system is reconstructed from sequences of PDF estimates of its zero-input response, which are derived from measurements of the unknown system. The work presented in this thesis contributes towards IFPP-II and IFPP-III by developing novel methods that improve upon exiting solutions. These novel methods construct candidate systems that admit an exact finite-dimensional representation for the FPO, which governs the evolution of density functions of the system’s response. The finite dimensionality of the FPO yields practical analytical methods for designing dynamical systems with prescribed statistical metrics as specifications, and for the consistent reconstruction of unknown dynamical systems. However, the structure of these systems imposes restrictions on the statistical metrics that are realisable, and only permit the approximation of arbitrary metrics that may be prescribed. Examples are presented that demonstrate the realisation of power spectra with prescribable poles and multiple peaks that are adjustable, and the reconstruction of iii unknown systems with consistent branch monotonicity and power spectrum mode characteristics, using the novel methods. Although the application-specific design and performance evaluation of random signal generators constructed using the proposed methods are not considered in this work, it is anticipated that these methods will be of interest in applications such as low-complexity random signal generation for Digital Radio Frequency Memory (DRFM) systems.
Port Hamiltonian Modelling of an Integrated Mechanical Ventilator-Human Respiratory System
(University of the Witwatersrand, Johannesburg, 2024) Madahana, Milka Cynthia Ijunga; Nyandoro, Otis. T. C.
Mechanical ventilation is a life-saving treatment for critically ill patients who are struggling to breathe independently due to injury or disease. Globally, large numbers of individuals have always required mechanical ventilation per year. In this research work, a compre- hensive integrated Mechanical ventilator human respiratory system which is a complex dynamical system is modelled using the Port Hamiltonian approach. The Port Hamilto- nian framework provides a powerful tool for the analysis, modelling, and control of such systems. The objectives of this research work are separated into four main outcomes: The first objective is formulation of the human respiratory system using a Port Hamiltonian based approach. In this formulation, the first abstraction is a nominal Port Hamiltonian mechanical ventilator human respiratory system model. The second level of abstraction formulates a comprehensive integrated Port Hamiltonian model of the human respiratory system which includes the alveoli. The third abstraction is the Multi-physics and Multi- scale Port Hamiltonian model of the human respiratory system, which is integrated with the mechanical ventilator. The second objective is the formulation of a comprehensive Port- Hamiltonian model of a mixed finite/infinite dimensional electro-hydro-mechanical model of the mechanical ventilator. The third objective is the presentation of a multi-physics Port Hamiltonian generalized model of a Mechanical Ventilator-Human respiratory sys- tem with Naiver stokes for the fluid flow in the mechanical ventilator respiratory system model. The generalized formulation leads to a new class of algebraic constraints in the modelling of the mechanical ventilator respiratory system. The main result presented is a novel interconnected Dirac structure of the fluid/electrically coupled system. Finally, two novel applications of the mechanical ventilator human respiratory system model are presented. Simulations for this research work are conducted using the Matlab platform and the respiratory system model results and the mechanical ventilator results are com- pared to results in existing literature and are found to be comparable. This work can be developed further in future to include robust controllers in the Port Hamiltonian frame- work for improved mechanical ventilator patient dynamics.
A bottom-up smart city approach to solid waste management: the case of ICT-enabled waste reclaimers system in two South African cities
(University of the Witwatersrand, Johannesburg, 2023) Siwawa, Vincent
This study delves into the realm of waste management in the context of bottom-up smart cities, focusing on the implementation of an innovative ICT-enabled waste reclaimer system in Cape Town and Johannesburg, South Africa. The central inquiry pertains to the pivotal role played by the bottom-up smart city paradigm in addressing waste management challenges and fostering the inclusion of waste reclaimers within the framework of smart cities. The inadequacies inherent in prevailing top-down smart city approaches and techno-centric solutions extends to the lack of mechanisms within the conventional informal recycling system to furnish reliable, immutable, and transparent waste-related data, thereby compromising security. To address these challenges, a novel approach emerges, fusing the capabilities of the Internet of Things (IoT) and blockchain technology into the informal recycling sector. This ICT-enabled waste reclaimer system introduces a comprehensive framework encompassing training initiatives, the provisioning of protective equipment, smartphones to facilitate communication between households and waste reclaimers, measuring instruments, tricycles, and dedicated spaces for sorting and storing recyclable materials. Employing a qualitative research methodology, this study incorporates a blend of document analysis, integrative literature review, and semi-structured interviews with key stakeholders. The selection of case studies, namely BanQu, Kudoti, and Regenize, is underpinned by purposive sampling. An array of research instruments including webinars, photography, participant observations, and transect walks contribute to the rich data collection process. The study draws upon the socio-technical transition theory to sustainability and the Multi-Level Perspective (MLP) as conceptual frameworks to dissect the digital transformation of the informal waste sector through the lens of the ICT-enabled waste reclaimer system. Findings underscore the potential of this innovative system to foster symbiotic connections between waste reclaimers and stakeholders embedded within the recycling value chain. This, in turn, culminates in enhanced working conditions and augmented income for waste reclaimers. Crucially, the ICT-enabled waste reclaimer system offers mechanisms for waste monitoring and tracking, while concurrently introducing incentives and rewards. By generating precise, secure, and reliable data, this system engenders a paradigm shift from a conventional cash- based payment structure to a virtual and electronic payment mechanism. Preliminary evidence showcases a notable threefold increase in recyclable collection by waste reclaimers compared to municipal efforts. In culmination, this study delivers both theoretical and empirical contributions by shedding light on the integration of waste reclaimers and waste management within the context of a bottom-up smart city approach. The study posits a promising trajectory for future research and ushers in new avenues for the development of bottom-up smart cities within the ambit of developing nations.
Assessment of the Performance of Corn Cob Ash as a Partial Replacement for Portland Cement in Concrete
(University of the Witwatersrand, Johannesburg, 2023) Fadele, Oluwadamilola Adepeju; Otieno , Mike
The production of Portland cement is associated with the release of greenhouse gases especially carbon dioxide which is estimated to be about a ton per every ton of clinker produced contributing to climate change. Several mitigation strategies have been proposed but the most viable remains the use of supplementary cementitious materials as partial replacement for Portland cement. There have been considerable success with the use of some industrial by-products (fly ash and slag) and natural materials (calcined kaolin clay) as supplementary cementitious materials. However, the non-availability of these by-products in countries like Nigeria calls for the investigation of locally available substitutes. Supplementary cementitious materials are either pozzolanic or possess latent hydraulic properties making them choice materials as partial Portland cement replacement. The classification and choice of a material as supplementary cementitious material lies in the understanding of their characteristic properties (chemical composition and mineralogy) and subsequent performance in cementitious systems. The performance of corn cob ash calcined at 700°C and 800°C as partial replacement for Portland cement (PC) compared to Portland cement and fly ash (FA) was studied with the following objectives: to determine the influence of calcination temperature on the reactivity of corn cob ash; investigate the effects of corn cob ash content at varying w/b ratio on the i) hydration reaction of Portland cement; ii) the compressive strength of concrete iii) drying shrinkage strain of mortar iv) penetrability of concrete v) microstructure of concrete The laboratory investigation involves using corn cob ash to partially replace Portland cement at two levels of 15% and 30% by mass using two w/b ratios of 0.4 and 0.6 at a water content of 205 kg/m3. The corn cob ashes in binary combination with either Portland cement or fly ash were used to prepare concrete samples which were used for the determination of compressive strength, durability index tests (namely oxygen permeability, water sorptivity and chloride conductivity) to assess the durability of concrete, and microstructural development. The concrete was designed using the South African Cement and Concrete Institute method of mix design. Also, mortar samples made from one part of cement to three parts of sand were prepared for the investigation of drying shrinkage and estimation of strength activity index while paste samples were prepared for determining reactivity of the ashes and effect on Portland cement hydration. iv Reactivity of the ashes was measured using both strength activity index and R3 reactivity test. Strength activity index was estimated from the compressive strength of 50 mm cube mortars at the ages of 28, 56 and 90 days of curing in order to better understand the mechanism of reaction of the ash, while R3 test was performed on model paste using the bound water approach at the age of 7 days. The amorphous content of ash calcined at 700°C and 800°C is 1.9% and 2.4% respectively while the gain in strength of mortar cubes prepared with only Portland cement, Portland cement/fly ash, Portland cement/corn cob ash calcined at 700°C and 800°C between 28 and 90 days are 14%, 24%, 10% and 9% respectively. The surface area of the Portland cement, fly ash, corn cob ash calcined at 700°C and at 800°C is 2.38, 2.224, 3.122 and 2.751 m2/g respectively. The results indicate that the corn cob ashes (CCA) calcined at 700°C (C700) and 800°C (C800) are low reactive materials with limited pozzolanic reactivity while the mechanism of reaction is largely influenced by filler effect due to finer particle size than plain PC. The compressive strength of concrete containing 15% CCA calcined at 700°C and 85% Portland cement ranges between 40 to 58 MPa between 3 and 90 days of curing at w/b ratio of 0.4 compared to 56 to 83 MPa for Portland cement concrete and 48 to 82 MPa for fly ash/Portland cement concrete at the same replacement level. The porosity of concrete containing 15% C700 and C800 at w/b ratio of 0.4 is 9.66 and 6.9% respectively at 28 days of curing compared to 8.37% for PC and 6.52% for fly ash at the same age and replacement level. The presence of CCA affects the heat of hydration of plain PC by prolonging the induction phase by about 12 hours which delayed the evolution of main heat peak. The use of CCA lead to a reduction in strength compared to PC/FA system with compressive strength decreasing with increasing w/b ratio and increasing PC replacement level. CCA has a high potassium oxide content which is highly soluble with a high concentration in the pore solution of concrete. CCA influences volume change leading to a high drying shrinkage strain compared to plain PC and FA. CCA also affects the durability of concrete by increasing the penetrability of concrete which increases with increasing ash content. In terms of the studied properties of cementitious systems, there is no marked difference in the effect of C700 compared to C800 while the effects recorded becomes significant with increasing PC replacement level. In comparison to FA, the effect of CCA on the properties studied was inferior due to the largely crystalline nature resulting in limited pozzolanic activity
Modelling the comminution process in the rotary offset crusher
(University of the Witwatersrand, Johannesburg, 2024) Nghipulile, Titus; Bwalya, M.
There is always a search for size reduction solutions due to the inherent energy inefficiency associated with comminution devices. The rotary offset crusher (ROC) is a new comminution device with a promising performance potential in terms of throughput due to enhanced speed of transportation induced by the centrifugal force of the discs and high frequencies of closure and opening of crushing chamber. The lack of fundamental understanding of the micro- processes that are facilitating both comminution and material transport in this new crusher necessitated a thorough investigation of key factors that drive comminution in this equipment. A combination of experimental and numerical modelling techniques was used to study the effect of key operating variables such as speed of the discs, offset between the vertical axes of the discs, feed size distribution and feed rate. The offset provides some flexibility in the system, but it does not significantly improve the crushing efficiency or the throughput. The speeds of the discs proved to be the key determinants of the degree of breakage achieved in the crusher. Both compressive and shear energies are active, with compressive energy dominating at lower speeds and decreasing with gradual increase in speed while the shear force increases with the increasing speed. Those trends render the device operating at higher speeds to be analogous to a laboratory pulveriser. Feed rates of at least 10 tph and feeds with wide size distributions (including near-vertical exit gap particles) are recommended for future studies in the quest to optimize the throughput of the crusher. Further work is recommended to investigate the effect of crusher profiles on the product quality (size and shape), power draw and throughput. It is anticipated that modifying the crusher profile can further intensify the crushing forces imposed on the particles. Consideration should also be given to increase the opening of the comminution cavity in order to be able to choke feed the crusher and thereby promoting rock-on-rock crushing. It is also recommended that the vibratory motion for the top disc be considered in order to intensify the compressive loads experienced by the particles. Overall, the redesign of this engineering device is necessary to reduce comminution by attrition due to disc-particle contacts and intensify the compressive forces.
Hydrometallurgical extraction of metals from secondary resources using various reagents
(University of the Witwatersrand, Johannesburg, 2024) Teimouri, Samaneh; Billing, Caren; Potgieter, Herman
The advancement and widespread applications of metals in the modern world have led to a growing demand which outstrips their supply. This has resulted in a vital need for recovering precious and critical metals from waste materials, known as secondary resources. Recovering precious, critical and heavy metals not only improves the circularity of metals, but also minimises the deleterious effects of waste materials on the environment. To achieve this crucial aim, research in this thesis focuses on improving gold (Au) yield by finding different ways for pretreatment to break down pyrite, the predominant sulfidic mineral that encapsulates gold in mine tailings. In addition, the research focuses on extracting critical metals such as indium (In) and gallium (Ga) from industrial waste, in this case electric arc furnace dust (EAFD). The results achieved in this research are presented in five publications as explained in brief below:  The dissolution of pyrite – the predominant host mineral encapsulating gold – to improve gold extraction from mine tailings was studied in a nitric acid (HNO3) solution. The study showed that when the concentration of HNO3 is above 2 M, it acts as a powerful acid and oxidant to break down the pyrite structure, while simultaneously exposing the enclosed gold through oxidative dissolution. The conducted experiments confirmed that within 2 h, 3 M HNO3 effectively dissolved 95% of FeS2 to release the remaining gold in pyrite at 75 °C. The kinetics of pyrite dissolution was also examined in the temperature range of 25 to 85 °C. The results indicated that the mixed controlled model (1/3Ln (1–X)+[(1–X)–1/3–1)] = k.t, where X is the fractional conversion, k the apparent reaction rate constant, and t leaching time) describing the interfacial transfer and diffusion was governing the kinetics of pyrite dissolution in nitric acid. The activation energy required at low temperatures (25-45 °C) was 145.2 kJ/mol and it reduced at higher temperatures (55-85 °C) to 44.3 kJ/mol. Therefore, nitric acid pretreatment is an effective method for mine tailings containing pyrite with enclosed gold. Nitric acid can be recovered in an eco-friendly manner by capturing the emission of NOx gases from the nitric acid decomposition and can be economically attractive when regenerating the starting acid/oxidant (see publication: “The Kinetics of Pyrite Dissolution in Nitric Acid Solution”).  To gain insight into the dissolution of minerals encapsulating gold, such as pyrite and chalcopyrite, an electrochemical study was conducted in nitric acid media using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV scans were measured to identify the oxidation-reduction peaks for pyrite and chalcopyrite. Based on the corresponding CV scans and visual observations, anodic and cathodic reactions for oxidised and reduced species were iii deduced for each identified peak which occurs at a specific potential. An EIS study was then conducted at the particular oxidative potentials, to gain further indications on the relevant reactions, hence providing supporting evidence of the dissolution mechanism. The EIS study at low potential (i.e. 0.5 V vs Ag/AgCl (3 M KCl) reference electrode) showed that the dissolution reaction was controlled by a diffusion process due to the accumulation of certain species, i.e. Fe(OH)3 and S0, on the pyrite electrode, and Cu1-xFe1-yS2-z, CuS2, and S0 in the case of chalcopyrite. This was confirmed in the EIS curve through the appearance of the linear Warburg diffusion effect. Increasing the potential beyond 0.7 V leads to reactions at which the previously formed species covering the surface of the electrodes and causing a diffusion barrier, oxidised further converting them to soluble species. This was reinforced by the omission of Warburg-like effects in the EIS data (see publication: “A comparision of the Electrochemical Oxidative Dissolution of Pyrite and Chalcopyrite in Diluted Nitric Acid Solution”).  Due to environmental awareness, neoteric eco-friendly solvents like ionic liquids (ILs) and deep eutectic solvents (DESs), which can be used as alternatives to conventional leaching reagents for recovering metals, are gaining increasing attention among researchers. Hence, a new hydrometallurgical method using ILs to extract Zn, In, and Ga, along with Fe as a common impurity from EAFD, that was spiked with 5% of both In and Ga, was examined. EAFD is a valuable metal- containing waste generated in significant amounts during the process of steelmaking from iron scrap material in an electric arc furnace. With critical metal recovery as the main goal, two ILs: [Bmim+HSO4–] and [Bmim+Cl–], were studied in conjunction with three oxidants Fe2(SO4)3, KMnO4, and H2O2, to determine which IL and oxidant combination performs best for extracting the target metals. Following the initial tests, the influence of parameters such as the IL concentration, oxidant concentration, solid-to-liquid ratio (S/L), time, and temperature were optimised to achieve the maximum extraction of the target metals. Results from a series of experiments found the optimum condition to be; 50 ml 30% v/v [Bmim+HSO4–], 1 g of Fe2(SO4)3 oxidant (2%), S/L ratio of 1/20, at 85 °C for 240 min leaching time, resulting in extractions of 92.7% Zn, 80.2% Fe, 97.4% In, and 17.03% Ga. The dissolution kinetics of the studied metals over a temperature range of 55– 85 °C was diffusion-controlled (see publication: “A New Hydrometallurgical Process for Metal Extraction from Electric Arc Furnace Dust Using Ionic Liquids”).  Environmental and safety concerns about traditional methods for gold extraction, and the potential volume of enclosed gold in mine tailings in sulfidic minerals (i.e. pyrite), were the motivations to find effective, efficient and ecologically benign ways to break down the pyrite structure to expose the locked gold to improve its extraction. Hence, the feasibility of the dissolution of pyrite was studied in a deep eutectic solvent (DES) as a novel solvent. DESs are an iv analogue of ILs, which are gaining increasing attention as a potential solvent with eco-friendly features. Therefore, the viability of pyrite dissolution in a DES containing choline chloride – a quaternary ammonium salt [(CH3)3NCH2CH2OH]+Cl−] – and ethylene glycol [HO-CH2-CH2-OH], named ethaline, was examined both theoretically through density functional theory (DFT) calculations and experimentally. DFT calculations determined whether Cl– and/or [C2H4O2]2−, the two ligands provided by ethaline, can make the most probable and stable complex with Fe2+ and/or Fe3+. To do so, the reaction Gibbs free energies (G) for possible complexes that Cl– and [C2H4O2]2− can form with Fe were calculated. In addition, the energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gap) were considered. Among the proposed complexes, the tetrahedral complex [Fe(C2H4O2)2]−, with Fe3+ chelation taking place through the O-donors of the ligand [C2H4O2]2−, had the lowest G (– 71.4 eV) indicating the simultaneous formation of the complex, as well as the largest HOMO- LUMO gap (1.3 eV) specifying the most stable complex. For experimental evaluation, the effect of the pH of the ethaline solvent mixed with hydrogen peroxide (H2O2) oxidant, and the different S/L ratios on Fe extraction (indicating pyrite dissolution) were examined. The results show that a pH of 8 provides the desired condition at which ethylene glycol is deprotonated to [C2H4O2]2−, was the favorable ligand for Fe complexation. It was found that the S/L ratio of 1/20 was optimal and achieved 23.7% Fe extraction. The theoretical and experimental work correlated in indicating [C2H4O2]2− as the favourable ligand. However, the ethaline solvent as the leaching solution did not achieve adequate Fe extraction, as it did not succeed in properly breaking down pyrite to expose the locked gold (see publication: “The Feasibility of Pyrite Dissolution in a Deep Eutectic Solvent Ethaline: Experimental and Theoretical Study”).  DFT modelling was also applied to theoretically calculate the possibility of the extraction of In and Ga, in the IL medium. To investigate this aim, three imidazolium-based ILs, namely [Bmim+HSO4– ], [Bmim+Cl–], and [Bmim+NO3–] were selected for DFT calculations. They all have the same cationic part [Bmim+], but different anionic parts, i.e. [SO42–], [Cl–], and [NO3–], which are similar to the most commonly used mineral acids H2SO4, HCl, and HNO3, respectively. The G for different complexes were calculated to determine which of the available ligands, i.e. sulfate (SO42–), chloride (Cl–), and nitrate (NO3–), provided by each IL most likely form the most stable complex with In and Ga. The obtained values for G confirm that IL [Bmim+HSO4–], owing to the [SO42–] O-donor ligand, resulted in the dimer complexes of [In2(SO4)3] and [Ga2(SO4)3] having the lowest G and the largest HOMO-LUMO gap, indicating the most probable and stable complexes (see publication: “Indium and Gallium Extraction Using Ionic liquids: Experimental and Theoretical Study”).
Optimisation of the mineral grading wind sifter separator for coal beneficiation
(University of the Witwatersrand, Johannesburg, 2024) Alade, Jimmy Joanah; Bada, Samson
The concept of wind sifting for particle separation has been successfully implemented for various concentration purposes. Diverse configurations based on this concept have been fabricated over many decades. It has been used in recycling, agriculture, electronic waste sorting, furniture, food and beverages, and mineral processing industries to some extent. The approach’s effectiveness stems from the capability of separating lighter particles from heavier ones. This study utilised an optimised version of the wind-sifter that was designed and fabricated by the author during his master's degree. The drawback experienced while testing the first prototype wind-sifting separator led to this investigation. This study used a computer simulation technique (the Lagrangian particle tracking method). This resulted in observing the effectiveness of the separation process in the newly designed separator. The design of the new separator was made flexible in its mode of operation by fabricating detachable collecting bins to the separator assembly. This means the optimised separator can be operated with or without the coal collecting bins, unlike the prototype version, which could only run with its bins. The design of the separator was done with the aid of Autodesk Inventor, and simulation was carried out using Star-CCM+TM computer software. The simulation tests were performed for different particle sizes (−6.7+3.35 mm), (−3.35+1.0 mm) and (−1.0+0.2 mm) at different airstream velocities. The optimal airstream velocities from the previous study (at a cut point of 1.6 g/cm3) were also used in this study when the separator was run with its bins. These airstream velocities were 6.0 to 4.0 m/sec, 4.2 to 2.0 m/sec and 1.7 to 1.0 m/sec for the (−6.7+3.35 mm), (−3.35+1.0 mm) and (−1.0+0.2 mm) particle sizes, respectively. A simulation was used to determine the airstream velocity ranges of the separator without collecting bins. These were 10.5 to 9.0 m/sec for the (−6.7+3.35 mm), 7.0 to 5.0 m/sec for the (−3.35+1.0 mm) and 3.5 to 2.5 m/sec for the (−1.0+0.2 mm) particles. For the –1.0 mm size faction, three particle size distributions (–1.0+0.1 mm), (–1.0+0.15 mm) and (–1.0+0.2 mm) were simulated. The best airstream velocities of 1.7 m/sec and 3.5 m/sec were achieved, respectively, for the closed and opened bins. The results of the simulation study led to the fabrication of the optimised wind sifter used in this study. From the sink and float test conducted on two sets of feed-coal (coal A and coal B), the extent to which the separator could beneficiate coal was determined. The sink and float analysis revealed that coal A has a higher ash content than coal B. Coal A, at (−1.0+0.2 mm) size iii fraction, has an ash content of 4.29% at 1.3 relative density (RD). This is followed by the (−3.35+1.0 mm) particle size with 5.55% ash content and the (−6.7+3.35 mm) size fraction with an ash content of 5.89%. At an RD of 1.5, coal A has a specific ash content of 16.65%, and coal B has an ash content of 13.56% for the (−6.7+3.35 mm) fraction. Upon separating with the wind sifter, the clean coal products from coal A have a higher ash content compared to those from coal B. Running the separator without the bins, clean coal products with cumulative ash content ranging from 22.42% to 19.44% for the (−6.7+3.35 mm), 24.61% to 21.43% for the (–3.35+1.0 mm) and 27.54% to 22.51% for the (–1.0+0.2 mm) fractions were obtained. For the particle size of (−6.7+3.3 mm) and with the bins closed, a clean coal product of 20.21% was obtained from coal A (feed coal with 37.38% ash content). A coal product with 19.55% ash content was obtained from coal B (from feed coal of 26.65% ash). A second-stage test conducted on a first-stage coal product of 23.48% ash content yielded a coal product of 21.79% ash and 80% yield for coal A at (6.7+3.35 mm). This trend was also observed for other first-stage products at the three particle sizes used in this study. The Ep values obtained from this separator ranged between 0.035 and 0.16, with the Ep values increasing as the airstream velocity was reduced. For the (6.7+3.35 mm) fraction, Ep values (Probable Error of Separation) of 0.035, 0.095, and 0.16 were obtained at 6.0, 5.0, and 4.0 m/sec air velocities, respectively. Overall, the cleanest coal product with 16.73% ash and 26.70 MJ/kg was obtained in this study from coal A at bin 2. According to the study’s results, the separator was highly adaptable. The separator could also be used for upgrading and pre- concentrating other minerals in the mineral processing industry.
Performance evaluation of reduced graphene oxide incorporated dye-sensitized solar cells for stable power generation
(University of the Witwatersrand, Johannesburg, 2024) Ikpesu, Jasper Ejovwokoghene; Iyuke , Sunny E.
Dye-sensitized solar cells (DSSCs) have emerged as a promising alternative to traditional silicon-based solar cells due to their low cost, easy fabrication, and high efficiency in converting sunlight into electricity. However, the performance of (DSSCs) is limited by the charge transfer and recombination processes at the interfaces between the different components of the device. In the recent years, Graphene Oxide (GO) and reduced Graphene Oxide (rGO) have been proposed as potential interfacial components to improve the performance of DSSCs. This research looked at the performance of reduced graphene oxide in dye-sensitized solar cells for sustained power generation. Most dye-sensitized solar cells have benefited from the usage of reduced graphene oxide. This research focuses on the performance of reduced graphene oxide in dye-sensitized solar cells for sustained power generation. Most dye-sensitized solar cells have benefited from the usage of reduced graphene oxide. This is due to its characteristics such as high surface area, superior transparency in the visible region, light absorption, and charge transport. Aerosol Assisted Chemica1 Vapour-Deposition (AACVD) was used to create a fluorine doped tin oxide (FTO) layer. The Hummer's Method was applied to synthesize reduced graphene oxide (rGO). Titanium (IV) oxide – reduced graphene oxide (TiO2-rGO) composite was synthesized at the photoanode via screen printing and spin coating, then inserted in DSSC using roselle dye as a natural dye sensitizer to investigate the cell performance. X-ray diffraction (XRD), ultraviolet diffuse reflectance spectroscopy, scanning electron microscopy/energy dispersive X – ray (SEM/EDX), and profilometry were used to characterize the synthesized/fabricated samples. Using a solar simulator and air mass A.M. 1.5 (100 mw/cm2), current-voltage (I-V) measurements were obtained to determine the performance of the cells. Tin oxide thin films doped with fluorine (F:SnO2) were successfully deposited on a glass substrate using the AACVD process at various doping percentages and the optical properties of the FTO and substrate layers were investigated using absorbance spectra. A low absorption value of 12% F: SnO2 resulted in a higher transmittance of 90% were achieved. This shows that the optical and electrical properties of the DSSC were altered by fluorine doped tin oxide. The results of the developed spin coated TiO –rGO Nano composites revealed that the Hall Effect increases with thin-film thickness iv while mobility increases with carrier concentration. The optical absorption results of TiO2–rGO nanocomposites demonstrate that as dopant amount increases, the band gap energy falls from 3.6 eV to 1.4 eV. The findings indicate that there is a strong interaction between Titanium dioxide (TiO2) and reduced Graphene Oxide (rGO), which could result in better visible light absorption and consequently improve light harvesting efficiency when utilized in a dye sensitized solar cell. Open circuit voltage (0.53 V), short circuit photocurrent (0.12 mA/cm2), fill factor (0.02), and photoelectric conversion efficiency (11.52%) were the simulation results for the cell parameters obtained for the DSSC manufactured using reduced graphene oxide. Meanwhile, the open circuit voltage (0.56 V), short circuit photocurrent (0.63 mA/cm2), fill factor (0.03), and photoelectric conversion efficiency (4.70%) of the DSSC without reduced graphene oxide were obtained. The power conversion efficiency of the dye-sensitized solar cell with graphene oxide was 6.82 % greater than that of the cell without graphene oxide. A four-week stability test was also performed on the DSSC fabricated with reduced graphene oxide to measure the extent of electrolyte deterioration. After the first 24 days, the short circuit current value reduced by 39% from its original value of 0.1190 mA/cm2. Under white light irradiation, the efficiency value of the DSSCs was found to be stable for the first 12 days before gradually decreasing to 24% of its initial value. The improved performance of DSSCs with reduced graphene oxide may be ascribed to an increase in electron transport efficiency and visible light absorption. When reduced graphene oxide was used, the performance improved due to increased light absorption, a wider range of absorption wavelengths, faster electron transport, and suppression of charge recombination. Based on this research, comparing the absorbance and transmittance of fluorine- doped tin oxide (F: SnO2) at 4%, 8%, and 12% with regard to wavelengths (ℷ) at 230 nm and 1100 nm; It shows that at 12% (F: SnO2), the lowest absorbance yields better transmittance, and increasing the cell's efficiency by 11.52%. The power conversion efficiency (PCE) of DSSCs demonstrates that DSSCs fabricated with reduced graphene oxide improved cell performance and outdoor stability. Which confirm that the TiO2 - rGO is a good material for solar application.
Bio-tribocorrosion behavior of low- cost titanium alloys in simulated body fluids
(University of the Witwatersrand, Johannesburg, 2023) Rundora, Nicola Ruvimbo; Bodunrin, M.O.; Merwe, Van der; Klenam, D.E.P
Titanium and its alloys are considered the gold-standard material in the manufacturing of biomedical implants. This is due to their superior mechanical properties and resistance to corrosion in comparison to the other biomaterials on the market. Titanium and its alloys are however very expensive in comparison and their application and accessibility is thus limited. Issues of toxicity as in the case of the commercial alloy (Ti-6Al-4V) have also been reported. One approach to combat these issues has been the development of new low-cost and less expensive α+β type titanium alloys through the modification of the commercial alloy composition. These efforts led to the development of the following alloys: Ti-3Fe, Ti-4.5Al-1V-3Fe, and Ti-6Al-1V-3Fe. To determine whether these alloys can be utilised for the manufacturing of biomedical implants, they have to undergo a series of screening tests. Some of those tests include wear, corrosion, and tribocorrosion tests. This is the basis of this study. Dry sliding ball-on-disk wear tests were conducted on these alloys. Ti-4.5Al-1V-3Fe and Ti-6Al-1V-3Fe had superior wear resistance to Ti-3Fe, the alloy where the full substitution of V with Fe was employed. Ti-4.5Al-1V-3Fe and Ti- 6Al-1V-3Fe alloys wear resistance was closely comparable to that of the commercial alloy, Ti- 6Al-4V. The corrosion behavior of the experimental alloys was investigated in 0.9 wt.% NaCl solution, Hanks Balanced Salt Solution, prediabetic Hanks Balanced Salt Solution, and diabetic Hanks Balanced Salt Solution. It was important to study the behavior of these alloys in a simulated diabetic environment because of the increase in the number of people living with diabetes and their increased risk of developing different bone and joint disorders. The corrosion behavior of Ti-3Fe was superior to that of the commercial alloy and the other experimental alloys in 0.9 wt.% NaCl solution and Hanks Balanced Salt Solution. In the prediabetic and diabetic Hanks Balanced Salt Solution, the experimental alloys resistance to corrosion was comparable to that of the commercial alloy with Ti-4.5Al-1V-3Fe having the best resistance to corrosion in the prediabetic solution and Ti-6Al-1V-3Fe having the best resistance to corrosion in the diabetic solution. Ti-4.5Al-1V-3Fe and Ti-6Al-1V-3Fe showed tribocorrosion resistance that was comparable to the commercial alloy in both the normal HBSS and diabetic HBSS under both open circuit potential and potentiostatic conditions. These alloys showed better tribocorrosion behavior in the normal HBSS under potentiostatic conditions compared to Ti-3Fe and the commercial alloy. In the diabetic HBSS under potentiostatic conditions, Ti-4.5Al-1V-3Fe had the best resistance to tribocorrosion. The iv | P a g e glucose addition had a slightly negative effect on the tribocorrosion behavior as the wear rates and total worn volume values in such solutions were slightly higher than in the normal solutions. Ultimately, under corrosion and tribocorrosion testing conditions, the partial substitution of V with Fe as beta stabilisers in low-cost titanium alloy designs offered superior properties over the total replacement of V with Fe.
Flammability, corrosion resistance, and environmental friendliness of coal composites produced from various coal fines
(University of the Witwatersrand, Johannesburg, 2024) Vatsha , Mhlawakhe; Bada, Samson O.
The concept of recycling carbon-containing waste into secondary raw materials is highly promising for fostering a resource-efficient and circular economy, given the increasing scarcity of natural resources and growing population. The effectiveness of coal tar modified by air- blowing technique as a binder “pitch” for coal fines in the production of structural composites is highlighted in this study. In addition, the feasibility of commercial dimethylpolysiloxane as a binder for coal carbon composite production was assessed and compared to that produced using coal tar-pitch. The two coal fines (GG1 and GS) used in this study have an ash content of 84.02% and 62.27%, respectively, and can be classified as rock. Their fixed carbon content ranges between 5.44% and 16.27%, compared to coal tar (35.23%). The coal tar has a volatile matter content of 64.50%, and with the air-blowing pretreatment, the tar was converted to pitch with a low volatile matter content of 11.27%. A pitch with the highest fixed carbon content of 87.68% and total carbon content of 96.01% was produced. Various ASTM standard test methods were used in the investigation to characterise and evaluate samples, including mineral phases and functional groups in the raw and coal composites produced. The composites were fabricated using a circular mould with a diameter of 30 mm and a 40 mm square mould. In the study, it was found that composites with a low H/C atomic ratio had low water absorption. Additionally, composites with high volatile matter content had high water absorption. However, the sample with the highest water absorption (19%) was the GG1 50/50 coal tar pitch 400-composite, which falls within the range (0-25%) for building materials. The composites with an intense O-H group had high compressive and flexural strengths ranging from 106.58 to 344.71 MPa and 48.75 to 159.30 MPa, respectively. The flammability of all composites was low. The highest flammability mass ablation rate and linear ablation rate were found to be 0.008 g/sec and 0.00983 mm/sec, respectively. In terms of the corrosion rate, the GS 80%/20% dimethylpolysiloxane coal composite had the highest corrosion rate (0.081 μmpy), which is minimal compared to some commercial ceramic tiles. The composites' environmental friendliness was determined by leaching them at various pH levels. The test was conducted by comparing the concentration of heavy toxic elements in the solution to the leachable concentration threshold for waste management standards (NEM WA Act No. 59 of 2008). All composites were environmentally friendly, meeting the moderate risk leaching concentration threshold. The composites that were produced in this study from South iii African discard coal can be used in large quantities in the environment without any danger or hazards, as demonstrated. Based on this study's overall results, repurposing South African discard coal as carbon/ceramic composites for building materials could play a role in the country's Just Transition initiatives. In addition to waste reduction, this strategy could reduce operational emissions, improve circularity, and address associated environmental risks.

Faculty of Engineering and the Built Environment (ETDs)

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