Research Articles (Materials Science and Metallurgical Engineering)

Permanent URI for this collectionhttp://hdl.handle.net/2263/1699

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Improving performance at Rustenburg Base Metals Refiners copper tankhouse : operational review and embracing fundamentals
(Southern African Institute of Mining and Metallurgy, 2025-06) Naidoo, Kalin; Pelser, M.; Hagemann, J.; Sole, Kathryn C.
The copper tankhouse at Rustenburg Base Metals Refinery is an essential component of the Anglo American Platinum value chain. The tankhouse uses starter-sheet technology that was adopted in the 1980s, so dependence on manually intensive labour and overall operational excellence has a significant impact on process performance. An efficient, stable, and sustainable electrowinning process contributes to minimising energy consumption and reducing operational costs, and enables improved product quality and increased throughput. Historic performance of this tankhouse has often been suboptimum over prolonged periods. An in-depth operational review of performance from 2018-2024 was carried out to understand challenges, risks, and high-impact factors that contribute to poor performance. Implementation of adherence to fundamental operational basics identified from global best practice, such as the frequency and quality of cell cleaning and maintenance, preventing backlogged harvesting or maintenance, preserving cell-top furniture conditions and integrity of electrodes, preventing short circuits and poor current distribution, and maintaining a high level of operational discipline, has since contributed to an era of exceptional performance. Root-cause analysis fault trees pertaining to poor current efficiency and high scrap rate are summarised. Current efficiency has significantly improved since 2021, now consistently exceeding 88%. The copper scrap rate, previously believed to be limited to a minimum of 4%, achieved a historic monthly low record of 2.06% in 2023. Chemical and physical quality has considerably improved, and cathodes exhibit minimal morphological defects.
Comparison of Denver and Leeds laboratory flotation cells : effect of particle size and hydrodynamics
(Southern African Institute of Mining and Metallurgy, 2025-05) Tshazi, Mfesane; Leal, L.S.; Naude, Natasia; mfesane.tshazi@up.ac.za
In this paper, the authors performed experiments in the Denver and Leeds laboratory flotation devices1 at the University of Pretoria at various particle sizes to evaluate the performance of the devices. Quartz was used in a single mineral system at discrete sizes fractions, -25 µm, +2545 µm, +45-75 µm, and +75-106 µm. Hydrodynamic analysis, based on dimensionless power and Reynolds numbers, indicated that the Leeds cell required higher power input to achieve comparable flow conditions. Specifically, the Leeds cell exhibited an average power number of 1.03, whereas the Denver cell averaged 0.77 within the same impeller speed range of 1000 rpm-1500 rpm. For comparative flotation performance, impeller speeds were calibrated, resulting in operating speeds of 1200 rpm for the Denver cell and 1400 rpm for the Leeds cell. The cells performed similarly at two coarser-sized fractions. However, some deviations were observed in the finer particle size range. The -25 (fine) fraction initially achieved a recovery of only 15%. An additional reagent dosage was required to enhance the recovery of this fraction significantly. These findings highlighted the influence of particle size on flotation recovery. For all experiments, the Denver laboratory flotation cell outperformed (defined by higher recovery) the Leeds device, while still operating at a lower impeller speed. This advantage can be attributed to its impeller-stator design and air dispersion features, effectively overcoming slurry resistance and resulting in superior flotation performance compared to the Leeds cell.
Laser metal deposition of TiB/Ti6Al4V composites with first-scale network microstructure
(Sage, 2024-09) Lekoadi, Paul Masekuru; Tlotleng, Monnamme; Siyasiya, Charles Witness; Masina, Bathusile Nelisiwe
This study investigated the effect of in-situ alloying of Ti6Al4V and TiB2 during laser metal deposition for microstructure, hardness and tensile properties enhancement. It was found that the addition of TiB2 at powder feed rates of range 0.1–0.3 rpm resulted in the formation of TiB whiskers with columnar and equiaxed network morphologies, in a lamella α + β matrix. By increasing TiB2 at 0.4–0.5 rpm, a complete dissolution of the equiaxed and columnar network morphologies were achieved. The dissolution led to a random distribution of formed TiB whiskers, resulting in enhanced hardness and ultimate tensile strength (UTS), with the 0.5 rpm giving the highest hardness and UTS values of 511 ± 13 HV and 1745 MPa, respectively.
Investigation of the mechanisms affecting corrosion susceptibility of wrought aeronautical aluminium alloys Al-Cu-Li (AA2198) and Al-Cu-Mg (AA2024) for different pre-stretching levels
(Elsevier, 2025-03) Charalampidou, Christina Margarita; Pretorius, Christiaan C.E.; Salojee, Muhammed; Karousos, Dionysios; Khodja, Malika; Mostert, Roelf Johannes; Alexopoulos, Nikolaos D.; roelf.mostert@up.ac.za
Aluminium (Al) alloy sheets are usually stretched to manufacture aircraft structures with complex geometries. The corrosion susceptibility of AA2198 Al alloy is examined as a function of the extent of pre-stretching, using a wide range of advanced microscopy, electrochemical techniques, and tensile mechanical testing. Intergranular corrosion attack manifested in sub-surface secondary cracking for the 1.5 % pre-stretching level. A mechanism of deformation-induced chemical heterogeneity is believed to be responsible for the resulting intergranular corrosion, in which δ′-phase nucleation following pre-stretching allows for the removal of matrix Li within highly deformed grains, creating a micro-galvanic coupling between neighbouring grains. At pre-stretching levels of 4.0 % and higher, a transition to transgranular corrosion was observed, whilst the electron backscattered diffraction results indicated that recovery occurs at these pre-stretching levels. At 7.5. % pre-stretching level, the charge transfer resistance values were essentially increased due to the segregation of Li to sub-grain boundaries, allowing for more matrix Li removal and, hence, to the more rapid transgranular attack. On the contrary, the level of corrosion degradation of the tensile properties of AA2024-T3, although being more severe than that of the AA2198 alloy, is not essentially influenced by varying the extent of pre-stretching.
Thermomechanical control of microstructure and precipitation in vanadium microalloyed steel: influence of finish rolling and coiling temperatures
(Wiley, 2024-04) Wu, Zhangqin; Yang, Wen; Tang, Zhenghua; Siyasiya, Charles Witness; Zhang, Jianhua
Six hot compression tests are conducted using the Gleeble3500 thermomechanical simulator to investigate the microstructural evolution and precipitation behavior in low-C–Mn V-microalloyed steel. The specimens are subjected to hot isothermal compression deformation of 87%. The optical microscopy and transmission electron microscopy using carbon extraction replica method are used to characterize the microstructures and precipitation after the simulated thermomechanical controlled process and coiling. The results indicate that increasing the finish rolling temperature benefits the refinement of ferrite grains but has little influence on the refinement of the precipitates. It is also observed that lower coiling temperatures (CTs) promote the formation of fine precipitates. When the CT is 500 °C, the average precipitate size is found to be 86 nm. Furthermore, it is found that the CT significantly influences the nucleation sites of the precipitates inter alia, the matrix, interphase, grain boundaries, and dislocations. As expected, at higher CTs, nucleation is predominantly on the defects rather than the matrix.
Empirical modelling of 2205 DSS flow curves using strain-compensated Arrhenius rate-type constitutive model
(Nature Research, 2024-09-28) Gonya, Elvis M.; Siyasiya, Charles Witness; Makhatha, Mamookho E.
This work predicts, hot flow curves of 2205 DSS using strain-compensated Arrhenius rate-type constitutive model. Twenty-five (25) × Ø10 diameter × 15 mm height cylindrical samples were hot compressed at a temperature between 850 and 1050 °C at an interval of 50 °C and strain rates between 0.001 and 5 s−1, using Gleeble 1500D. After the tests, corrected flow curves were plotted followed by computation of deformations constants at various deformation conditions using steady state stress. The values of the constants were (α = 0.009708, Q = 445 kJ/mol and n = 3.7) and seemed comparable to the previous studies of DSS. Steady state predictive model was then constructed using the calculated constants and showed a reasonably good accuracy with low value of MARE = 7.78%. Furthermore, calculated strain compensated Arrhenius rate type model was used to predict flow curves at various deformation. The model had a good estimation of flow curves of flow curves at 900–1050 °C across all strain rates as reflected by MARE = 5.47%. A notable discrepancy between predicted and experimental flow stress was observed at 850 °C and across all the strain rates. A model refinement using generalised reduced gradient improved the accuracy of the model by 34.7% despite deformation conditions at 850 °C and low strain rates (0.01/ 0.1) s−1 showing minimum improvement. Further modification of Z-parameter by compensating for the strain rate improved the accuracy of the model at 850 °C/0.01 s−1/0.1 s−1. Lastly, a comparison of the current model with the other non-linear model showed that the latter was more accurate in estimation of flow curves since it relied on characteristics flow stress points controlled by underlying active deformation mechanisms.
Gas-phase reactions in nano-strand formation from Al-Fe-Ni powder reacted with CaF2-SiO2-Al2O3-MgO-MnO-TiO2 Flux at 1350 °C : SEM study and diffusion calculations
(MDPI, 2025-03) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
Please read abstract in the article.
'New mining scheme, same old problems?' ; risk factors and health challenges in underground community mining in Ghana
(IOP Publishing, 2024-12) Aram, Simon Appah; Saalidong, Benjamin M.; Yankyera, Akwasi; Akoto, Akwasi Bamfo; Amoah, Samuel Kofi
Ghana's small-scale mining sector has long been a cornerstone of its economy and a vital source of livelihood for many communities. However, the sector is plagued by significant challenges, including environmental degradation, health and safety risks, and illegal practices. In response, the Ghanaian government introduced the Community Mining Scheme (CMS), a new mining model designed to promote local community participation in artisanal small-scale gold mining (ASGM). This study examines the workplace risk factors and associated health challenges faced by ASGM miners under the CMS, drawing on interviews with fifty underground CMS miners in Ghana. Our findings reveal that miners continue to face numerous health and safety challenges, including inadequate lighting, poor ventilation, exposure to noise and odors, unsupported or poorly supported loose materials, non-use of personal protective equipment, high dust levels, and unsafe blasting practices. Additionally, health concerns such as skin rashes, musculoskeletal injuries, and the risk of mercury poisoning persist. These challenges mirror those reported in the literature on unregulated and illegal ASGM in Ghana, indicating that existing issues remain unresolved under the CMS. To prevent the undermining of this initiative, it is crucial to adopt progressive practices, such as localized coping strategies and the recruitment of trained professionals to lead ASGM operations within the CMS. Enhancing miners' risk perception through targeted outreach and awareness campaigns is also essential. Further recommendations for mitigating safety risks and health challenges are discussed in the article.
Delta ferrite in modified 9Cr–1Mo steel weld metal
(Springer, 2025) Mahlalela, Sibusiso Samuel; Pistorius, Pieter Georg Hendrik; sibusiso.mahlalela@up.ac.za
Please read abstract in the article.
Element transfer behaviors of agglomerated CaF2-ZrO2 fluxes in EH36-shipbuilding steel subject to high-heat input submerged arc welding
(Springer, 2024-10) Chen, Angran; Zhang, Yanyun; Coetsee, Theresa; Kaldre, Imants; Wang, Cong
EH36-shipbuilding steel has been welded by CaF2-ZrO2 fluxes with designed ZrO2 additions. Possible chemical and electrochemical reactions have been postulated to analyze alloying element transfer behaviors. The decomposition of ZrO2 during SAW has been validated by applying the gas–slag–metal equilibrium model and the O supply capacity of ZrO2 has been quantified. For the entire compositional range, O content has been controlled within a well-maintained range from 220 to 400 ppm, and the transferred quantity of Zr content reaches to the maximum value of 120 ppm. It is further demonstrated that ZrO2 addition incurs appreciable Si loss within the weld metal.
Green binder system for taphole clays : a potential for glycerine and phenolic resin
(Elsevier, 2025-06) Cameron, Izak Jian-Pierre; Garbers-Craig, Andrie Mariana
Recent advancements in taphole clay binder development have focused on replacing toxic coal tar/pitch, which contains polycyclic aromatic hydrocarbons (PAH), with greener alternatives. In this study, three different taphole clays were prepared in the laboratory using different binder systems which include phenolic resin: 1) coal tar, 2) glycerine, and 3) petroleum waxy oil. The clays were evaluated and compared to the conventional coal tar and phenolic resin-containing clay used in platinum smelters. The evaluation methods employed included workability and extrusion pressure ageing, hardenability, strength development, and high-temperature properties, which comprised cold crushing strength after ageing at 200 °C, carbon yield, volatile organic compound concentration and apparent porosity. The results indicated that the preferred binder to replace coal tar was glycerine, as this clay retained both its plasticity and hardenability during ageing, while also attaining comparative strength development and high-temperature properties to those of the reference clay containing coal tar.
Plasma soup for the pyrometallurgist's soul
(South African Institute of Mining and Metallurgy, 2025-03) Reynolds, Q.G.; Bowman, B.; Erwee, M.W.; Geldenhuys, I.J.; Sandrock, C.; Venter, G.A.; Xakalashe, B.S.; Zietsman, Johannes Hendrik
Understanding the behaviour of plasma arcs is an important part of the design and operation of direct current electric arc furnace smelting processes, which are used in the industrial production of many metallurgical commodities. In recent years numerical and computational modelling techniques have begun to illuminate the complexity of arc behaviour and how it is deeply connected to the thermodynamic and physical properties of the arc plasma. Plasma properties may be estimated from first principles using statistical mechanics methods, but this requires as input some knowledge of the composition of the gas phase that occurs over the process during smelting. In this paper the authors describe a workflow using thermochemistry software to predict the expected gas phase composition through a given process, followed by plasma property calculations using an open-source software tool that has been in development for several years. This tool, 'minplascalc' has been in development for several years and is also described in detail here. The workflow is then applied to the calculation of plasma properties of interest for a variety of historical and current pyrometallurgical processes, and the results are compared. The plasma property data are published in an open-access database for general use.
Engineering γ-TiAl alloys : the effects of Sn, Si and Mn on densification, microstructure, and mechanical properties
(Elsevier, 2025-06) Ellard, John J.M.; Mathabathe, M.N.; Siyasiya, Charles Witness; Bolokang, A.S.; Vilane, V.N.; Rikhotso-Mbungela, R.; McDuling, C.; Masete, S.
Please read abstract in the article.
Removal of iron from vanadium redox flow battery electrolyte using ion exchange
(South African Institute of Mining and Metallurgy, 2024-12) Makonese, Mufaro; Sole, K.C. (Kathryn); kathy.sole@up.ac.za
Please read abstract in article.
Tracking the degradation of carbon steel mechanical properties due to high-temperature hydrogen attack through strain gauge monitoring
(South African Institute of Mining and Metallurgy, 2024-11) Van Zyl, A.; Pretorius, Christiaan C.E.; Mathoho, Vhutshilo M.; Mostert, R.J. (Roelf); roelf.mostert@up.ac.za
The in-service degradation of the mechanical properties of steel components through the damage mechanism of high temperature hydrogen attack (HTHA), is a topic of concern in the refining and green hydrogen industry. This damage mechanism occurs in susceptible steels operating in environments at high temperatures and hydrogen pressures. The current investigation deals with the indirect monitoring of mechanical degradation via tracking of the swelling strain in affected structures. An autoclave with an AISI 316 shell was utilized to simulate accelerated HTHA damage at 550 °C and 46 bar for exposure times ranging from zero to 700 hours. The progress of the HTHA damage was tracked using encapsulated high-temperature strain gauges. The correlation between the swelling strain and mechanical property degradation was studied to develop a methodology for the continuous monitoring of embrittlement. The tensile sample orientation of the carbon steel plate was included as a variable, i.e., samples parallel, transverse, or perpendicular to the plate rolling direction were included, since it has been shown that the sample orientation influences the HTHA damage features. For the through-thickness orientation, and upon exceeding a threshold value of exposure time, all tensile properties were severely degraded, with values in the order of a 90 per cent reduction being observed. It was found that the degradation of carbon steel mechanical properties can be correlated to the swelling strain measured during exposure. The critical point for mechanical property degradation in the plate through-thickness orientation, whereafter a severe decrease in the ductility of the material occurs, was found to be in the order of 1% of the total swelling strain measured during exposure, equivalent to 65 microstrain. This threshold was found to be significantly lower than that expected from the literature, where limits of 400 to 1000 microstrain were postulated.
Thermochemical analysis of the behaviour of Cu in Ti nano-strand formation from low-temperature reaction of Al-Fe-Cu powder with CaF2-SiO2-Al2O3-MgO-MnO-TiO2 flux
(Elsevier, 2025-03) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
Please read abstract in the article.
Decomposition of sulfide phases and subsequent matte collection in the black top of a platinum group metal smelter
(Taylor and Francis, 2025) Rivera Li Kao, Oscar; Garbers-Craig, Andrie Mariana; andrie.garbers-craig@up.ac.za
This study explored, on a laboratory scale, how matte separates from the gangue in the black top of a PGM smelter. Two PGM concentrates, one with high sulfide content (17.3 mass%, Platreef concentrate) and the other with low sulfide content (1.3 mass%, UG-2 concentrate), were studied in the temperature range of 800°C to 1480°C, which is the temperature gradient across the black top. The results showed that effective matte separation occurs at temperatures when the matte is completely molten, and a substantial amount of liquid silicate phase has formed. The main matte separation mechanism is the coalescence of sulfide prills and their gravitational settling through the continuous path created by the liquid silicate phase. Complete matte separation occurs at lower temperatures in Platreef concentrate (1400°C in Platreef concentrate vs. 1480°C in UG-2 concentrate), due to the higher concentration of liquid silicate phase of lower viscosity, as well as the higher amount of sulfide minerals in this concentrate. PGMs already dissolve and collect in a nickel-iron-based alloy associated with the matte at temperatures as low as 900°C.
The role of niobium during heat treatment of high-vanadium high-speed steel rolls for improved mechanical properties
(Springer, 2024-12) Chauke, Rivoningo E.; Siyasiya, Charles Witness; Nyabadza, Davisn
High-speed steel (HSS) provides superior wear resistance and high surface quality of rolled-steel products. In this study, the influence of niobium addition on the solidification behavior and microstructure of high-speed steel rolls and their impact on the roll’s hardness and wear resistance were investigated by a combination of thermodynamic simulations of solidification using ThermoCalc, optical and scanning microscopy and hardness testing. The study was conducted on two centrifugal-cast commercial high-vanadium high-speed steel sleeves whose compositions differed in niobium content. Thermodynamic simulations showed that niobium promotes the precipitation of proeutectic Nb-rich MC carbide during solidification. Microscopic observations showed that the proeutectic carbide promotes carbide dispersion in the microstructure. This was found to improve the microhardness of the martensitic matrix and counteract softening during repeated tempering treatment. The addition of less than 2 wt.% Nb in the HSS roll yielded better hot hardness as a result of the roll’s better microstructural integrity and response to HT. The conclusion was that Nb-containing HSS rolls can be produced for the hot-rolling stands that operate at higher rolling force and temperature if the cost-benefit analysis is favorable.
First-principle study on tailoring the martensitic transformation of B2 Nb50−xTixRu50 shape-memory alloy for structural applications
(MDPI, 2024-09) Nkomo, Duduzile; Shen, Yu-Nien; Mostert, R.J. (Roelf); Yamabe-Mitarai, Yoko; Phasha, Maje
NbRu has a potential as a high-temperature shape-memory alloy (HTSMA) because it has a martensitic transformation temperature above 1000 ◦C. However, its shape-memory properties could be improved for consideration in the aerospace and automotive industry. The unsatisfactory shape-memory properties could be associated with the presence of a brittle tetragonal L10 martensitic phase. Therefore, in an attempt to modify the transformation path from B2→L10 in preference of either B2→orthorhombic or B2→monoclinic (MCL), an addition of B2 phase stabiliser, titanium (Ti), has been considered in this study to partially substitute niobium (Nb) atoms. The ab initio calculations have been conducted to investigate the effect of Ti addition on the thermodynamic, elastic, and electronic properties of the Nb50−xTixRu50 in B2 and L10 phases. The results showed that the B2 and L10 phases had comparable stability with increasing Ti content. The simulated data presented here was sufficient for the selection of suitable compositions that would allow the L10 phase to be engineered out. The said composition was identified within 15–30 at.% Ti. These compositions have a potential to be considered when designing alloys for structural application at high temperatures above 200 ◦C.
Effect of coiling temperature on microstructures and precipitates in high-strength low-alloy pipeline steel after heavy reduction during a six-pass rolling thermo-mechanical controlled process
(MDPI, 2024-02-18) Lei, Yicong; Yang, Wen; Siyasiya, Charles Witness; Tang, Zhenghua; charles.siyasiya@up.ac.za
Nb-Ti high-strength low-alloy pipeline steel was subjected to a six-pass rolling process followed by the coiling process at different temperatures between 600 and 650 ◦C using the thermomechanical testing system Gleeble 3500 (Gleeble, New York, NY, USA). This experimental steel was subjected to 72% heavy reduction through a thermos-mechanical controlled process. Thereafter, the microstructures were observed using optical microscopy, scanning electron microscopy, electron backscatter scanning diffraction, and transmission electron microscopy coupled with energy dispersive spectrometry and selected area electron diffraction. For the selected three coiling temperatures of 600, 625, and 650 ◦C, acicular ferrite, polygonal ferrite, and pearlite were observed, and morphology and statistical analysis were adopted for the study of precipitates. Based on the estimation by the Ashby–Orowan formula, the incremental strength through precipitation strengthening decreases with coiling temperatures and reaches 26.67 Mpa at a coiling temperature of 600 ◦C. Precipitationtime- temperature curves were obtained to explain the transformation of precipitates. The (Nb, Ti)(C, N) particles tended to precipitate in the acicular ferrite with [011](Nb, Ti)(C, N)//[011]α-Fe orientation. The lower coiling temperature provided enough driving force for the nucleation of precipitates while inhibiting their growth.

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