Research Articles (Materials Science and Metallurgical Engineering)

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

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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.
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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
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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; theresa.coetsee@up.ac.za
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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.
Correlation between elastic properties and phase transformation of Zr50Pd50-xRux high-temperature shape memory alloys designed by DFT
(Springer, 2024-09) Shen, Yu-Nien; Nkomo, Duduzile; Matsunaga, S.; Phasha, M.J.; Yamabe-Mitarai, Y.
The effect of Ru addition on the martensitic transformation and mechanical properties of Zr50Pd50 xRux SMAs shape memory alloys has been studied. Strain–temperature experiments, differential scanning calorimeter, and scanning electron microscope analyses were performed to validate the theoretical predictions by DFT calculation regarding the influence of the C¢ parameter on martensitic stability. Despite the absence of an observed shape memory effect under the tested conditions, the findings suggest that ZrPdRu alloys hold promise for high-temperature applications.
Incompressible versus compressible fluid flow models : a case study on furnace tap-hole lancing
(Southern African Institute of Mining and Metallurgy, 2024-03) Erwee, Markus Wouter; Reynolds, Q.G.; Zietsman, Johannes Hendrik
Pyrometallurgical furnaces, essential for metal extraction, operate at temperatures exceeding 1600°C and represent complex multiphase systems that challenge direct industrial research. Multiphysics models play a key role in shedding light on their intricate behaviours, supporting the refinement of design and operational strategies. Integral to the operation are the tap-holes, which facilitate the removal of molten products and are routinely opened by lancing, a process comparable to the use of a cutting torch, where high temperatures result from oxygen reacting with an iron lance. When the lance pierces the clay, oxygen gas enters the furnace, which could influence the behaviour of the molten material inside. In this work, a multiphase fluid flow model was used to investigate bulk flow dynamics, with a focus on the effects of the lancing process on the inside of the furnace, immediately behind the tap-hole. Incompressible and compressible multiphase fluid solvers were used and compared with respect their performance – the intention was to assess whether using a compressible solver would yield a different solution to the incompressible one. It was concluded that there are negligible disparities in bulk fluid flow behaviour between the solvers for the case studies examined, indicating that solver selection might be less consequential for certain aspects of oxygen lancing.
Extraction of rare earth elements from Phalaborwa phosphogypsum
(Southern African Institute of Mining and Metallurgy, 2024-10) Wildenboer, R.A.; Sandenbergh, Roelf
Rare earth elements (REE) are present at concentrations of approximately 0.36 % in phosphogypsum stacks located in Phalaborwa, South Africa. The REE are present in the phosphogypsum in solid solution in the gypsum and as fluorine and aluminium rich precipitates. Recovery of the REE offers opportunities for valorization and reducing the environmental impact of the phosphogypsum waste. This paper presents the results of investigations into the leaching behaviour of rare earth elements and other impurities from phosphogypsum using aqueous sulfuric acid. The effects of acid concentration, temperature, residence time, and the influence of impurities were evaluated for their impact on the solubility of rare earth elements in a sulfate medium. It was found that a significant fraction of the REE can be leached from the phosphogypsum in a range of conditions much less severe than traditional hard rock rare earth processing conditions. The REE were leached at a sulfuric acid concentration of 110 g/l, temperature of 30°C – 40°C and a residence time of approximately 8 hours.
Investigating optimum hot working window of 2205 duplex stainless steel using modified dynamic material modeling
(Springer, 2024-12) Gonya, Elvis M.; Makhatha, Mamookho E.; Siyasiya, Charles Witness; Silubane, Ndumiso M.; Kibambe, Ngeleshi M.
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Timed thermodynamic process model applied to submerged arc welding modified by aluminium-assisted metal powder alloying
(Springer, 2024-11) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
An EERZ (effective equilibrium reaction zone) model was applied to the modified SAW (submerged arc welding) process to simulate the SAW process metallurgy in the gas-slag-metal reaction system. The SAW process was modified by adding Al as a de-oxidizer with alloying metal powders of Cr, Cu, and Ti. The static gas-slag-metal equilibrium model can accurately calculate the weld metal oxygen content (ppm O) for conventional SAW but not for the modified SAW process. The static equilibrium model overpredicts the reaction of Al. EERZ model runs were made for 2000–2500°C because this is the reported temperature range in the SAW arc cavity. The weld metal composition was adequately calculated, especially the weld metal ppm O, at the following effective equilibrium temperatures: 2400°C for Al-Cr additions, 2200°C for Al-Cr-Cu additions, and 2000°C for Al-Cr-Cu-Ti additions. Model results show that Ti metal powder can serve a de-oxidizer role in the presence of Al, resulting in Ti loss to the slag. Ti is also lost to the gas phase as TiF3(g) and TiF2(g) compared to little loss of Cr to the gas phase as Cr(g) and CrO to the slag phase.
Mechanical and thermal properties of calcined clay mixed with agrowaste char for sustainable building applications
(Wiley, 2024) Agyei-Tufour, Benjamin; Sewordor, Michael E.; Bafoe, John F.; Forson, Martha; Ampah-Essel, John Ekow; Ardayfo, Beatrice; Kadiri-English, Bashiru; Marfo, Kwame K.; Adjah, John; Asante, Joshua Tuah; Nyankson, Emmanuel; Dodoo-Arhin, David
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Development of an efficient e-waste recycling and beneficiation method into separable precious metals
(EDP Sciences, 2024-03) Nkhoma, Richard; Ngonda, Tiyamike; Mwale, Vincent Dodoma; Falayi, Thabo; Siyasiya, Charles Witness
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Gas formation of cobalt and copper in the application of unconstrained Co-Cr-Al-Cu metal powders in submerged arc welding : gas phase thermodynamics and 3D slag SEM evidence
(MDPI, 2023-04-05) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
Aluminium metal is not typically added to the submerged arc welding (SAW) process because it is easily oxidised to form unwanted slag in the weld pool. The successful application of aluminium as a de-oxidiser is illustrated in this study by preventing oxidation of Cr and Co to their oxides, thereby preventing element loss to the slag. Unconstrained pure metals of Al, Cr, Co and Cu were applied to investigate the gas formation behaviour of these elements in the SAWarc cavity. Of interest is the effect of copper in the arc cavity in terms of its possible substitution for aluminium. The results confirmed that the Al-Cr-Co-Cu alloyed weld metal total oxygen content was lowered to 176 ppm O, in comparison to 499 ppm O in the weld metal formed from welding with the original flux, which excluded metal powder additions. This lower ppm O value of 176 ppm O confirms that the added aluminium powder effectively lowered the original flux-induced partial oxygen pressure in the arc cavity, and at the molten flux–weld pool interface. Carbon steel was alloyed to 5.3% Co, 5.5% Cr, 5.3% Cu and 4.5% Al at 78% Co yield, 82% Cr yield, 78% Cu yield and 66% Al yield. Thermochemical equilibrium calculations confirm the partial oxygen pressure-lowering effect of aluminium when considering the gas–slag–alloy equilibrium. BSE (backscattered electron) images of the three-dimensional (3D) post-weld slag sample show dome structures which contain features of vapour formation and re-condensation. SEM-EDX (scanning electron microscope-energy dispersive X-ray) maps show that the dome surface matrix phase consists of Al-Mg-Ca-Si-Na-K-Ti-Fe- Mn oxy-fluoride. The spherical 3D structures of 10–40 m in diameter consist of Fe-Mn-Si fluorides with some Cr, Cu and Co contained in some of the spheres. Cr and Co were observed in distinctive porous structures of approximately 10 m in size, consisting partly of Cr oxy-fluoride and partly of Co oxy-fluoride. Nano-sized oxy-fluoride strands and spheres in the dome structures confirm vaporisation and re-condensation of oxy-fluorides. Cu and Na formed a distinct condensation pattern on the surface of the Si-Cu-Na-Mn-Fe-Co oxy-fluoride sphere. The results confirm the importance of including gas phase reactions in the interpretation of SAW process metallurgy.
EERZ (effective equilibrium reaction zone) model of gas-slag-metal reactions in the application of unconstrained Al-Ni-Cr-Co-Cu metal powders in submerged arc welding : model and 3D slag SEM evidence
(MDPI, 2023-07-14) Coetsee, Theresa; De Bruin, Frederik Johannes
The scope of this work is to improve the SAW process understanding and present an improved description of the SAW process in terms of gas-slag-metal reactions with alloy powder and Al powder additions. The scope does not include the materials properties of the weld metal. The latter may easily be optimised in the future by changing the weld metal chemistry once the process reactions of different element powders in SAW are understood. Aluminium as de-oxidiser element was applied to SAW to lower the oxygen partial pressure in the process. The results show the Al-Ni-Cr-Co-Cu alloyed weld metal total oxygen content was reduced to 257 ppm O, compared to the base case weld metal at 499 ppm O, made with the same flux and no metal powder additions. Thus, the aluminium that was added as a de-oxidiser element to the SAW process effectively lowered the original flux-induced partial oxygen pressure, both in the arc cavity and at the interface of the molten flux–weld pool phases. This partial oxygen pressure lowering effect of Al also prevents oxidation of Cr, preventing loss of Cr to the slag. Carbon steel was alloyed to 3.9% Al, 4.8% Ni, 4.9% Cr, 4.8% Co, 4.7% Cu at 62% Al yield, 76% Ni yield, 77% Cr yield, 75% Co yield, 74% Cu yield. SEM (scanning electron microscope) work on the three-dimensional (3D) post-weld slag sample show dome cavities with 3D rounded structures embedded in the dome cavity walls, as well as shards and nano-strands on the dome cavity walls. The 3D structures indicate vapour formation and re-condensation of oxy-fluorides. The novel application of the EERZ (effective equilibrium reaction zone) model simulates the mass transfer effects in the SAW process. This model is novel because it is the first model used to calculate the gas-slag-metal phase chemistry changes in SAW as a function of welding time. The novel SAW process modification of adding Al de-oxidiser powder with alloying element powders of the unique combination of Co-Cr-Co-Ni-Al was successfully applied. The results confirm that the gas phase and its reactions must be included in the interpretation and modelling of SAW process metallurgy.
Chemical behaviour of copper in the application of unconstrained Cr-Ni-Al-Cu metal powders in submerged arc welding : gas phase thermodynamics and 3D slag SEM evidence
(MDPI, 2023-02) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
Unconstrained metal powders of Cu, Cr, Ni and Al were applied to submerged arc welding (SAW) to clarify the chemical behaviour of copper in this modified SAW process. Aluminium metal is avoided in SAWbecause it is easily oxidised. Excessive aluminium oxides in the form of slag or inclusions in the weld metal will lead to poor weld metal materials properties. Aluminium is an effective deoxidiser and can be used to prevent Cr and Ni loss to the slag by preventing oxidation of these metals. The results show that carbon steel was alloyed to 5.3% Cr, 5.3% Ni, 3.6% Al and 5.2% Cu at 80% Cr yield, 81% Ni yield, 54% Al yield and 79% Cu yield. BSE (backscattered electron) images of the three-dimensional (3D) post-weld slag sample show 3D structures within the slag dome. The 3D structures contain features of vapour formation and recondensation. In addition, nano-strands appear in the 3D structures and confirm the vaporisation and recondensation of fluorides. The chemical behaviour of copper metal powder added in SAWis to vaporise as metallic copper and incorporate in the Al-Si-Mg-Ca-Mn-Fe-Cu-Na-Cr-Ni fluoride. Copper, in combination with aluminium, has a stabiliser effect in SAW due to its formation of an initial alloy melt of low liquidus temperature, thus decreasing the temperature required to melt high-melting-point metals such as Cr into the weld pool. Although Al and Cu have similar vapour pressures at specific temperatures, it appears that Cu does not substitute for Al in the gas phase. Gas-slag-alloy thermochemical equilibrium calculations confirm the partial oxygen pressure lowering effect of aluminium and the vaporisation of copper as metallic copper with very little copper-fluoride species expected to form. The quantity of metallic copper vaporisation calculated in the gas-slag-alloy thermochemical equilibrium is much higher than the vaporisation quantity measured in welding. This may be due to recondensation of vaporised copper which is not accounted for in the equilibrium calculation at the set arc cavity temperature, as well as the effect of surface-active elements such as sulphur and oxygen in limiting the vaporisation reaction of copper.

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