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Recent Submissions

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    Evaluation of microstructure and mechanical properties of 0.5Cr-0.5Mo-0.25V steam piping after long-term service in creep conditions
    (University of the Witwatersrand, Johannesburg, 2025-10) Mngoma, Sibusiso Siphesihle; Maledi, Nthabiseng; Chown, L.
    This work is part of the ongoing research by the South African power utility aimed at addressing the issues related to creep damage and safe extension of service life to meet the country’s increasing electricity demand. Creep damage in low-alloy steels remains a major concern for the safe and reliable operation of high-temperature components in power plants. This study evaluates the microstructural evolution and mechanical property degradation of ½Cr–½Mo–¼V (14MoV6-3) steam piping that was in service for more than 270,000 hours. Optical microscopy, scanning electron microscopy, and X-ray diffraction were used to characterize microstructural evolution of the new and service exposed materials, while Thermo-Calc software was employed to predict equilibrium carbides. Mechanical performance was assessed through hardness, tensile tests at room temperature and high temperatures (500°C –560°C), impact testing at-30℃ to 200℃, and creep testing using standard and Gleeble® machine. The results revealed significant microstructural changes in the service-exposed material, with the disappearance of ferrite–pearlite/bainite structures and the presence of ferrite and carbides, predominantly M23C6 and M6C along grain boundaries. Mechanical properties largely remained within the lower acceptance bounds but showed pronounced deterioration at elevated temperatures. Replica analysis confirmed creep damage to be most severe in the heat affected zone, consistent with Type-IV failure. Cross-weld creep testing aligned with the lower limit of the European Creep Collaborative Committee (ECCC) master curve. The deviations from ECCC curve were attributed to prior long-term exposure. Overall, the findings highlight that prolonged creep service significantly reduces creep strength, underscoring the need for additional testing to establish reliable strength reduction factors for life extension assessments.
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    Identifying the effects of the energy transition on the South African coal mining sector
    (University of the Witwatersrand, Johannesburg, 2025) Masindi, Shale lnnocentia; Tholana, Tinashe
    South Africa’s coal mining sector is undergoing significant disruption due to the global energy transition towards cleaner and renewable energy sources. As a country historically dependent on coal for both domestic energy production and export revenue, the shift presents complex economic, social, and environmental challenges. This research analysed the effects of this transition through a review of policy documents, economic data, and literature to provide a clear understanding of the sector’s trajectory. Economically, the declining global demand for coal has placed pressure on export volumes, investment, and local employment. Forecasts suggest that South Africa will lose between 25 000 and 35 000 coal mining jobs by 2035, with provinces like Mpumalanga expected to experience the most severe impact. These figures highlight the urgent need for strategic economic diversification and investment in alternative industries to reduce regional dependency on coal. Socially, coal dependent communities are at risk of economic marginalisation due to job losses and reduced municipal revenues. The study identifies the importance of proactive labour market interventions such as retraining, skills development, and infrastructure investment to support a just transition. Without these, the energy transition can worsen unemployment and deepen social inequality. Environmental considerations were also analysed, emphasising the imperative for South Africa to align its energy policies with global environmental social and governance goals. The study not only scrutinised the environmental impacts of coal mining but also advocates for a sustainable transition strategy that harmonises economic interests with ecological responsibility. This appeal for alignment acknowledges the link between economic growth and environmental responsibility, encouraging South Africa to take an active part in the worldwide fight against climate change. The research calls for an active involvement of government, industry stakeholders, and local communities to navigate this transformative period, to have a fair and sustainable transition for all stakeholders involved. This will balance economic objectives with social and environmental responsibility, to have a resilient and sustainable future.
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    An investigation into wear of media in stirred media milling
    (University of the Witwatersrand, Johannesburg, 2025) Manzini, Tinashe Arison; Bwalya, Murray Mulenga
    The beneficiation of low grade and finely disseminated ores has increased the adoption of stirred media mills because of their superior energy efficiency compared to tumbling mills. However, the highly dynamic and energy intensive milling environment accelerates grinding media wear, affecting both process efficiency and operating costs. This study investigated the wear behaviour of ceramic grinding media in stirred media mills through combined experimental and numerical (Discrete Element Method, DEM) approaches. The work aimed to identify operating conditions and media interactions that promote wear and to evaluate the capability of DEM to analyse energy transfer mechanisms associated with media wear. Direct wear quantification was not possible due to limitations such as uncharacterised media, the absence of the slurry phase, and the inability to resolve separate normal and tangential forces required to compute shear impact energies. Experimental tests were conducted using a laboratory-scale stirred mill with ceramic beads as grinding media. Specific energy input was varied at 5, 10, and 20 kWh/t, while feed sizes of -1180 + 850 µm and -650 + 425 µm were used to evaluate their influence on grinding performance and wear. Results showed that increasing specific energy input produced finer products, confirming the relationship between energy input and grinding efficiency. Coarser feed exhibited higher power draw and torque due to increased resistance and viscous losses, consuming more energy for the same milling duration. Wear tests performed on the coarse feed for 2 h and 4 h showed measurable bead degradation, with a linear wear-rate trend under the tested conditions. DEM simulations revealed predominantly low-energy collisions through the collision energy spectra, consistent with abrasion dominated milling environments. Predicted power draw values were 13% to 34% lower than experimental measurements, primarily due to the exclusion of the slurry phase, which typically accounts for up to 34% of total power losses in stirred mills. Despite these constraints, DEM reproduced realistic energy transfer trends, complementing the experimentally observed trends. The study confirms that integrating experimental data with DEM analysis offers valuable insights into the mechanisms governing energy dissipation and media wear in stirred milling environments. Further work should employ DEM software capable of resolving stress fields for integration into energy based wear models, include detailed characterisation of ceramic media properties, and incorporate breakage models to establish a more comprehensive representation of the grinding process.
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    Survival outcomes of HIV-positive adults on peritoneal dialysis at Helen Joseph renal unit
    (AOSIS Publishing, 2023) Thomas, Kagisho L.; Davies, Malcolm
    Background: HIV is a risk factor for the development of chronic kidney disease. People with chronic kidney disease in the state sector are likely to be prescribed continuous ambulatory peritoneal dialysis (CAPD). Previous studies have raised concern about the safety of CAPD in people living with HIV (PLWH) compared to HIV-negative patients. Objectives: To compare the risk of peritonitis, and modality and patient survival by HIV status in patients receiving CAPD at Helen Joseph Hospital. Method: A retrospective study of patients receiving CAPD between 01 January 2007 and 31 December 2017 was undertaken. Five-year patient and modality survival were modelled for PLWH and HIV-negative subgroups and analysed using the log-rank test; the effect of CD4 count, HIV viral load, and duration of antiretroviral therapy on these parameters in PLWH were additionally modelled using the Cox Proportional Hazards technique. Results: Eighty-four patients, comprising of 21 PLWH and 63 HIV-negative patients, were analysed. No difference was observed in the proportion of patients who had at least one episode of peritonitis between PLWH (61.2%) and HIV-negative patients (63.5%) (P = 0.547). A trend towards increased risk of peritonitis due to Gram-negative organisms in PLWH was noted (odds ratio: 3.20, 95% confidence interval: 0.86–11.9, P = 0.083). No difference was observed in 5-year patient or modality survival on CAPD between PLWH (log-rank P = 0.161) and HIV-negative patients (log-rank P = 0.240). Conclusion: People living with HIV should not be excluded from CAPD as a mode of kidney replacement therapy (KRT).
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    Knowledge and practices associated with exposure to nail products among informal nail technicians in Braamfontein, South Africa
    (University of the Witwatersrand, Johannesburg, 2025) Mmamogopodi, Frengy Mapogo
    Background The global nail care industry is an annual $13 billion industry and attracts a large workforce market, comprising both formal and informal nail technicians. In Braamfontein, one of the busiest and mixed-use zones in Johannesburg, South Africa, the informal nail sector is thriving, with numerous salons operating from often overcrowded and poorly ventilated rooms. Exposure to nail polish, gel removers, adhesives, acrylics and other nail products increases the risk of acute respiratory and chronic health issues. Assessing the knowledge and practices of informal nail technicians regarding exposure to nail products is critical to help develop interventions to promote occupational health and safety. Objectives i. To describe the knowledge and practices of informal nail technicians regarding the chemicals in the products they use. ii. To identify factors (sex, age and work experience) that are associated with informal nail technicians’ knowledge about these chemicals. iii. To identify factors (sex, age and work experience, knowledge) that are associated with nail technicians’ practice regarding the use of nail products. Methods This was a quantitative, cross-sectional study, for which we recruited 60 participants, comprising 31 informal nail technicians who participated in a previous larger study on exposure to chemicals in nail products and 29 additional nail technicians who were selected using convenience sampling. The data were collected using an English interviewer-administered questionnaire with close-ended questions. The questionnaire was divided into sections related to 1) demographic characteristics, 2) knowledge, 3) symptoms, and 4) practices. Sections 2 to 4 consisted of multiple- choice questions with responses such as “yes” or “no”, and “harmful” or “not harmful”. Knowledge and practice scores were allocated for responses and then summed, after which participants were grouped into high or low (knowledge or practice) categories. Descriptive statistics were used to summarise the demographic and work-related characteristics of the study participants, and their reported symptoms, and their knowledge and practice scores. Multivariable logistic regression analysis was used to assess the relationships between socio-demographic factors, and both practices and knowledge. Results Most of the participants were females (n=40, 66.7%). The mean age was 33 ±6.6; 50% had a tertiary education. Twenty-two (36.7%) reported experiencing symptoms that are associated with exposure to nail products. The average knowledge and practice scores were 33.3% and 78.3%, respectively. Work experience was negatively associated with knowledge: for every increase in years of work experience, the odds of having higher knowledge decreased by 19% (OR = 0.81, 95% CI = 0.67–0.97, p= 0.025). However, the association was not statistically significant in the adjusted model (OR = 0.85, 95% CI = 0.66–1.08, p=0.174). Age was positively associated with knowledge: with every year of increase in age, the odds of having higher knowledge increased by 14% (OR = 1.14, 95% CI = 1.02–1.26, p=0.021). However, this result was also not significant in the adjusted model (OR = 1.08, 95% CI = 0.95–1.24, p=0.237). There were no associations between demographic and work-related factors, and level of practice. Conclusion In this study, we found no significant associations between socio-demographic or work-related characteristics and the knowledge or practices of informal nail technicians regarding the chemical composition of nail products and associated health hazards. This is possibly due to the limited population to participate in the study. Overall, the findings suggest a disconnect between knowledge and practice. Although informal nail technicians demonstrated relatively good safety practices, their limited knowledge of chemical hazards may explain the persistence of exposure-related symptoms. These results highlight the urgent need for targeted educational programs to improve awareness and safety interventions to promote healthier practices in the informal sector.