Theses and Dissertations (Materials Science and Metallurgical Engineering)

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The influence of ageing treatment on the microstructural evolution and the mechanical properties of the TI-6AL-4V alloy
(University of Pretoria, 2017-02) Mostert, R.J. (Roelf); Smasete@discoverymail.co.za; Masete, Mosimanegape S.
The Ti-6Al-4V alloy is used in aerospace parts and its microstructure and properties can be controlled by solution treatment or solution treatment followed by ageing. However, there are different views on whether solution treatment above the beta transus temperature followed by ageing can improve the microstructure and mechanical properties of the Ti-6Al-4V alloy. This study investigated the influence of ageing treatment conditions (ageing time, temperature and cooling rate) on the microstructural evolution and the response of the mechanical properties. Specimens of the Ti-6Al-4V alloy were solution-treated at 1050 C for 30 minutes in a furnace in an oxygen atmosphere and water-quenched (WQ), or furnace-cooled (FC) to room temperature. The specimens were subsequently aged at temperatures of 500 C to 900 C for 0.5 hours to 48 hours, followed by either FC or WQ. Microstructural analysis, hardness measurements, tensile properties, fracture toughness, and fatigue crack growth rate were determined. The ageing temperatures of 500 C to 650 C produced greater hardness, yield strength, and ultimate tensile strength with optimum ageing times of 0.5 hours for samples that were solution-treated and WQ. As ageing temperature and time increased, hardness, yield strength and ultimate tensile strength decreased due to the increase in the volume fraction of the α phase and the dissolution of the martensitic needles. The best strength-ductility balance was obtained from specimens that were solution-treated and WQ, followed by ageing at 900 C for 24 hours and FC. Ageing of samples that were solution-treated and FC (colony lamellar) showed hardness, yield strength, and ultimate tensile strength to increase as ageing time and temperature increased. The increase was attributed to the increase in the beta content with ageing and subsequent martensitic phase transformation after quenching. Solution treatment and WQ followed by ageing was found to result in improved mechanical properties of the Ti-6Al-4V alloy for use in the aerospace industry. The colony lamellar specimens gave greater fracture toughness values than the fully martensitic specimens. In addition, the fatigue crack growth resistance was superior for the colony lamellar specimens than for the fully martensitic specimens. The random orientation of the α/β-colonies in the lamellar microstructure led to crack branching and formation of secondary cracks, resulting in higher fracture toughness and fatigue crack growth resistance. The fully martensitic morphology had brittle fractures leading to smoother fracture surfaces. The aged, fully martensitic specimens gave rise to greater fracture toughness than the fully martensitic specimens due to the larger α-plates, while the fatigue crack growth resistance was found to be similar.
Performance and hydrodynamic characterisation of laboratory batch flotation cells
(University of Pretoria, 2024-03-11) Naude, Natasia; Leal Filho, Laurindo de Salles; mfesane.tshazi@up.ac.za; Tshazi, Mfesane
The purpose of this study was to compare the Denver and Leeds laboratory flotation cells by evaluating their performance in the flotation of quartz of four sizes (−25 μm, +25–45 μm, +45–75 μm, and +75–106 μm). This study was conducted at the Materials Science and Metallurgical Engineering laboratory of the University of Pretoria. The impeller diameters of the Denver and Leeds cells were measured to be 0.07 m and 0.074 m, respectively. The impeller speeds of the cells were calibrated for identical flotation performance (assessed using quartz recoveries without interfering with the cell design), with the Denver cell set at 1200 rpm and the Leeds cell at 1400 rpm. The reagent regime was kept constant, using 25 g/t Flotigam EDA ether amine as a collector, no frother, and NaOH to modify the pH to 9.5. The air flow rate was kept constant at 2 L/min in each of the 3.5 L cells. Flotation kinetics tests were conducted at the optimal rotation speeds, and the results were similar. Both cells achieved similar quartz recoveries of over 70 % for the three +25 μm fractions, but only 15 % for the −25 μm fraction. An additional collector was required to improve the recovery of the −25 μm fraction significantly. These findings demonstrated the effect of particle size on flotation recovery, and the finer particles requiring more reagent due to their larger surface areas. The performance of these cells was further evaluated using dimensionless numbers and with a chemical tracer. The use of dimensionless numbers, such as Power and Reynolds numbers, allowed for a detailed analysis of the cells' hydrodynamics. Additionally, a chemical tracer (NaOH), was used to assess the mixing efficiency of the impellers. The Denver flotation cell exhibited superior performance compared to the Leeds cell. It managed to achieve higher recovery rates while consuming less power. This can be attributed to the effective design of its impeller and stator, which enabled it to overcome the resistance posed by the slurry, allowing it to operate at optimal levels that surpassed the capabilities of the Leeds cells. The performance of the Leeds cell was found to be inferior to that of the Denver cell, and this is attributed to several factors. One of the main reasons is the slightly larger bubble size of 3.5 mm in the Leeds cell, compared to the Denver cell's average of 2.5 mm. Therefore, the surface area available for particle attachment was still low for the Leeds cell, even at higher impeller speeds. Additionally, the power numbers for the Leeds cell were higher, averaging at 1.03 between 1000 iii and 1500 rpm in the presence of solids, while the Denver cell averaged 0.77 under the same conditions. This indicates that the Leeds cell requires more power to create the necessary flow. All this leads to deterioration in particle collection efficiency and an overall reduction in performance.
Fundamental study of taphole clay behaviour with respect to the binder system and evaluation of alternative non-toxic binders
(University of Pretoria, 2024-07-04) Garbers-Craig, Andrie Mariana; Ramjee, Shatish; izakcameron@outlook.com; Cameron, Izak Jian-Pierre
The aim of this study was to determine the critical properties that a binder for use in taphole clay should have, identify alternative non-toxic binders that could potentially replace the currently used toxic high-temperature coal tar (CTht), and evaluate taphole clays in which these alternative non-toxic binders were used. The work presented in the first part of this thesis investigates the behaviour of the CTht. Analytical techniques were employed to describe the molecular composition and toxicity (Fourier-transform infrared spectroscopy, gas chromatography-mass spectroscopy; targeted and untargeted), thermal behaviour (thermogravimetric analysis, differential scanning calorimetry) and rheological properties of this binder. A set of alternative binders from various sources, including coal sources (a coal tar pitch-merisol oil blend, a low-PAH synthetic coal tar pitch), petroleum sources (a crude waxy oil, a distilled crude waxy oil, a mesophase-forming pitch), wood/plant sources (a beechwood tar, a pinewood tar, a vegetable tar), and a glycerine-resole resin mixture, were then considered and evaluated as non-toxic alternatives to CTht. Using a ranking system based on the results from the analytical techniques, the most suitable non-toxic binders to CTht were identified. The results indicated that the binders had molecular structures that were either cyclic aromatic hydrocarbons (aromatic benzene), chain hydrocarbons (aliphatic), or a combination of both. The toxicity (BE-values) ranged from 0.03-1.67 with the coal-based binders having the highest toxicity values. The thermal analysis indicated an average mass loss of the binders ranging from 0.28-1.79 g/°C with the glycerine-based binders having the highest average mass loss. Rheology results indicated that the vegetable tar and pinewood tar had limited thermal stability to be used as binders and was disqualified as potential alternative binders. The characterization and ranking process indicated that beechwood tar and a glycerine-resole resin mixture were the most suitable replacements for CTht in taphole clay, with BE-values (toxicity) of zero. Pinewood and vegetable tars were disqualified as potential binder replacements due to their limited thermal stability. In the second part of this thesis, the evaluation of the top two selected non-toxic alternative binders in a taphole clay formulation is discussed. Both non-standardized tests (workability and extrusion pressure ageing, hardenability, strength development) and standardized tests (volatile organic compounds, cold crushing strength, apparent porosity, carbon yield) were used to describe the behaviour of the taphole clay and the changes that occur due to the binder substitutions. Mixing procedures during the pilot-scale manufacturing of the taphole clay highlighted that beechwood tar was not a suitable binder for use in taphole clay. The crude waxy oil and the glycerine-resole resin mixture were selected to be evaluated in the taphole clay. The glycerine and phenolic resin binder caused the lowest decay in plasticity (7%) and smallest increase in extrusion pressure (17.5%) during aging of the THC. Thermal ageing results indicated that glycerine and phenolic resin binder had the lowest hardenability whilst the crude waxy oil had a higher hardenability. The strength development results indicated insufficient strength development for the crude waxy oil-containing clay which was manifested in the fact that the sample could not keep its shape after firing. The glycerine and phenolic resin binder clay had comparative compressive strength values (2.4 MPa) compared to the reference clay sample (3.1 MPa). The clay containing glycerine and phenolic resin binder had a lower volatile organic compound release. The taphole clay evaluations confirmed that the glycerine-resole mixture was the most suitable replacement for CTht in taphole clay due to similar process parameters (workability and extrusion pressure ageing, hardenability, strength development) as well as standardized test results showing similar performance to the clay with the CTht binder.
Back-annealing of hot dip galvanised strip steel microalloyed with vanadium
(University of Pretoria, 2024-05-17) Siyasiya, Charles Witness; Westraadt, Johan E.; Banks, Kevin M.; Mostert, R.J. (Roelf); steynianjs@gmail.com; Steyn, Johannes Sebastian
The steel market increasingly demands higher strength thinner products with reduced weight and decreased cost, in particular galvanised products for the roofing, automotive and solar industries, which are in high demand. A viable option to meet this demand is via back-annealing, also known as partial or recovery annealing, which consists of annealing cold-worked material at low temperatures. Back-annealing aims to only utilise the recovery process, i.e. to only restore a fraction of the elongation whilst retaining most of the strength obtained from cold-rolling. The targeted specification is EN 10346 S700GD, i.e. the desired properties are in the region of 800 MPa tensile strength and 10% total elongation. Therefore, the purpose of this study was to examine the viability of using V-N microalloying for high-strength recovery annealed steel. In this work, back-annealing (recovery) was applied to heavily cold-worked low C-microalloyed V-N steels in an attempt to achieve a yield strength of 700 MPa and a minimum A80 elongation of 10%. Laboratory simulations of annealing during hot dip galvanising (HDG) were employed to compare recovery and recrystallisation kinetics of V-N, Nb-Ti and plain low C steel. Based on these results, industrial slabs were subjected to conventional hot rolling and back-annealing cycles during HDG. Precipitation in both V-N and Nb-Ti steels extended the back-annealing window by about 50 °C compared to low C steel. Initial hardening due to precipitation during annealing was observed in V-N and Nb-Ti steels. A temperature-time parameter M was used to compare the results. The Mrs (recrystallisation start) in both V-N and Nb-Ti steels was 20.1 compared to 18 in plain low C-Mn. The EN 10346 S700GD specification was achieved by subjecting low C-microalloyed V-N steel to low finishing rolling temperatures and coiling temperatures followed by back-annealing of the heavily cold-worked strip at an M value to prevent full recrystallisation. The V-N steel showed improved stability during annealing when compared to Nb-Ti, due to the unstable nature of the NbC precipitation during the back-annealing process. In other words, the V-N steel was found to be friendlier in terms of microstructural control in order to achieve consistent mechanical properties.
Characterisation of wear in natural rubber and polyurethane hydrocyclones used for minerals processing applications
(University of Pretoria, 2020-04) Naude, Natasia; simeoncombrink@gmail.com; Combrink, Simeon
Slurry wear is a common problem in the minerals processing industry. Hydrocyclones are widely used in wet mineral classification applications and can be notoriously prone to wear. This study investigated wear in hydrocyclones, with specific attention to wear of spigots in polyurethane- and natural rubber-lined cyclones. Hydrocyclone wear, and more specifically spigot wear and its influence on efficiency (imperfection value), was investigated for a Multotec HC250 (250 mm barrel diameter) hydrocyclone. The HC250 hydrocyclone comprised a metal shell lined with vulcanized natural rubber, with a thermoset polyurethane spigot and vortex finder. Tests were conducted on this hydrocyclone in a pilot-scale recirculating test setup. The wear rate of the spigot was quantified by measuring its volume change with operating time using microcomputed-tomography scans. After 122 days in operation (recirculating feed materials in a closed loop), wear in the hydrocyclone led to an increase of 13.67% in the volume of the spigot. This relatively low wear rate was related to the fact that material recirculation led to particle smoothening and a reduction in particle size. This result also showed the enormous influence of particle shape and size on wear. A large single wear groove at the bottom of the natural rubber lower cone formed due to localized porosity (a compression-moulding defect), as determined by microcomputed tomography. The worn natural rubber lining also showed signs of chemical attack (devulcanization) occurring at microscopic level. In terms of efficiency, it was found that wear in the spigot resulted in a lower imperfection value of 0.48 compared with that of 0.39 for a new spigot. The relative density of the underflow decreased significantly from a value of 1.359 for the new hydrocyclone to 1.183 when worn. Although the worn spigot recovered a larger amount of water to the underflow, misplacement of fines and the fish-hook effect were greater for the new spigot. Multotec VV165 polyurethane (165 mm barrel diameter) hydrocyclones, used at the Exxaro Grootegeluk coal dense medium separation plant (South Africa) for magnetite densification, were also evaluated to determine their wear rate and enable more informed decisions on replacement intervals. It was found that after 90 days of operation in densifying magnetite medium (−45 µm), these hydrocyclones showed no wear, based on the change in volume in the spigot as measured by microcomputed tomography. These findings showed the importance of particle size on the wear rate of a hydrocyclone spigot and confirmed the requirement of a critical diameter at a set velocity to cause wear. The magnetite medium of −45 µm was below the critical particle diameter that would cause wear within this time period.
Vanadium recovery in the ferrovanadium production process
(University of Pretoria, 2020-08) Garbers-Craig, Andrie Mariana; Cromarty, Robert Douglas; marnucj@gmail.com; Van der Merwe, Marnu C.J.
Ferrovanadium can be produced in a DC arc furnace by reducing V2O3 by means of an aluminothermic reaction. Vanadium recovery is decreased by the formation of vanadium oxides within the slag, along with ferrovanadium droplet entrainment in the slag. High refractory wear rates increase the operational cost of the process, which is mainly a result of the high operating temperature (slag temperature of 2100˚C) and incompatibility between the slag and the refractory. The effect of slag composition on the extent of refractory wear was investigated by heating slags of different MgO contents to 1800˚C in the contact with magnesia refractory. The chemical interaction between the slag and refractory was investigated, along with the dimensional change of a piece of refractory which was reacted with slag to determine the extent of slag-refractory interaction. Changing the slag composition to favour an increase in vanadium recovery and implementing a two stage melting process were both investigated. The (CaO+MgO): Al2O3 ratio was varied to alter the amount of spinel (MgAl2O4) that forms in the slag in order to establish the effect of spinel formation on vanadium recovery. The two stage process included adding excess aluminium to the first stage to increase V2O3 reduction and adding Fe2O3 to the second stage to produce a ferrovanadium product with an aluminium content below 1.5 wt%. The slag-refractory test results indicated that the extent of refractory wear can be decreased by increasing the MgO content of the slag in contact with the MgO refractory. The results also indicated that refractory wear was driven by the formation of MgAl2O4, which is the product of a chemical reaction between the slag and the refractory. The amount of spinel formed with a change in (CaO+MgO): Al2O3 ratio did not influence the vanadium recovery. The change in (CaO+MgO): Al2O3 ratio of the slag did influence the vanadium recovery due to the effect it has on the activity coefficient of V2O3. The vanadium recovery decreased linearly from 75 wt% to 25 wt% as the (CaO+MgO): Al2O3 ratio increased from 0.30 to 1.43. The vanadium recovery was also strongly influenced by the calculated slag volume, with the metal recovery decreasing with an increase in slag volume. A two stage process was simulated whereby excess aluminium was added in the first stage to increase vanadium recovery and hematite was added in the second stage to decrease the aluminium content of the ferrovanadium. The two stage process tests done on a laboratory scale indicated that it is possible to increase the overall vanadium recovery by increasing the aluminium used to reduce the V2O3, followed by the addition of Fe2O3 to produce a metal product with an aluminium content below 1.5 wt%. The optimum aluminium addition for the first stage was determined to be 3.5 wt% excess, which resulted in a vanadium recovery of 73.9 wt%. The vanadium recovery of the two stage process was 72.6 wt%, which is higher than the 71.4 wt% obtained for the single stage process. The use of Fe2O3 did however decrease the vanadium content of the metal from 68.6 wt% to 63.5 wt%.
Study of the wear and rolling contact fatigue of class B wheel against the R350HT and R260 rail steels using the twin-disc wear simulator
(University of Pretoria, 2023-09-25) Siyasiya, Charles Witness; Mostert, Roelf; tshenolophinah@gmail.com; Leso, Tshenolo Phinah
Maintenance due to the replacement of damaged wheels and rails as a result of wear and rolling contact fatigue (RCF) has been found to be the major problem to rail operating companies. This problem tends to lead to poor availability of railway networks. In order to solve this problem, cost effective wear simulators are developed to predict the wear behaviour of the rails and wheels in order to improve the preventive maintenance in pursuit of operational efficiency. Therefore, more studies that simulate a combination of rolling and sliding wear, together with RCF, are required, specifically for the Southern African region, where good rail wear simulators are not readily available. The problem with wear and RCF simulators is high production costs and, therefore, part of this work was to solve this problem by developing a cost-effective wear test rig that would be able to simulate RCF, sliding and rolling wear as experienced by the wheel and rail during movement of train on rail tracks. For this work, it was decided that twin-disc concept would be used, since literature clearly demonstrated that the method was successful in simulating the three damage mechanisms mentioned. The developed twin-disc wear test rig was successful in simulating both rolling and sliding wear and parameters such as applied load and speed (slip) were easily varied so to simulate the actual contact conditions between the wheel and rail. Outputs such as coefficient of friction and wheel disc temperature were obtained. The results obtained from the developed test rig agreed with literature as they are repeatable and comparable. To validate the performance and accuracy of the rig, typical South African wheel and rail materials were used. The wear and RCF performance of AAR class B wheels against BS EN 13674 R350HT and R260 rail steels under different slip ratios and applied loads were investigated. The results showed that the severity of wear is heavily dependent on slip ratio i.e., increased with slip ratio. Severe plastic deformation was also observed at high values of slip ratio. The AAR class B wheels performed better against the softer R260 rail as compared to the harder R350HT rail. As expected, the R350HT performed better than the R260 due to it having higher hardness values and finer interlamellar spacing. Three wear regimes were identified from the plots of wear rate versus wear index (Tγ/A) namely; mild wear, severe wear and catastrophic wear. The effects of introducing water and oil on the contact surface were investigated. It was found that wear was much lower when water or oil was introduced at the wheel-rail interface compared to dry conditions, for all slip ratios. When water was used, the main cause of RCF was found to be fluid crack pressurisation. The RCF cracks were also observed under dry contact. Therefore, the rig was successful in simulating wear and RCF at the wheel/rail contact under different contact conditions as experienced by the wheel and rail during movement of train on rail tracks. For the wear rates model, a data correlation coefficient to link twin disc and field wear rates was determined and used to predict the reprofiling times of wheels for two different distances of 100 000 and 200 000 km. Therefore, the purpose of this study was achieved in providing a tool for the predictive maintenance for the local rail industry that uses AAR class B wheels against R350HT or R260 rail combinations.
Evaluation of the reduction roast – magnetic separation process to upgrade low-grade ferruginous manganese ore fines
(University of Pretoria, 2022) Cromarty, Robert; yathilmatabadal@gmail.com; Matabadal, Yathil
Manganese is an important raw material in many industries, such as the steel, non-ferrous and battery industries. Manganese ores are typically mined and are then fed to submerged arc furnaces where a manganese ferroalloy, the most common form in which manganese is used, is produced. For economic ferromanganese production, the ore used should have a manganese grade above 40% and a Mn/Fe ratio greater than 7,5. With the increase demand for manganese in recent years, resources for high grade manganese ores are gradually being depleted. In order to deal with the demand, it has now become imperative to make use of lower grade manganese ores which have historically been dumped due to selective mining of higher-grade ores. Many processes have previously been investigated in order to upgrade these manganese ores, such as physical methods, pyro- and hydrometallurgical methods. One such process that has been investigated has the ability to produce a product with a high manganese grade and Mn/Fe ratio (Elliott & Barati, 2020). This process is the reduction roast – magnetic separation process. In order to upgrade the ore, it is first subject to a carefully controlled reduction process to produce phases with different magnetic susceptibilities, such as MnO and Fe or MnO and Fe3O4. This work focused on the optimization of this process for use with ores obtained from the Nchwaning mine slimes dam. Ore was obtained from the slimes dam, reduced to smaller sample sizes and was then subject to a mineralogical study. XRF and XRD was carried out on the samples, and it indicated that the Mn grade was in the region of 44 wt% Mn, which is typical of a high-grade ore. The Mn/Fe ratio, however, was found to be 3 – much lower than the 7,5 required for economic ferromanganese production. The main manganese minerals were found to be braunite I and braunite II, with a small amount of bixbyite and hausmannite present as well. The main iron mineral was hematite. A significant amount of calcite was also found. An optimization study was carried out in the form of a central composite design, with the variables investigated being temperature and reductant ratio. The conditions were selected based on the thermodynamics of the reduction to obtain the desired phases. The reduction step for the process was carried out in a retort furnace, after which the reduced briquettes were milled and subject to a wet magnetic separation process using a Davis Tube. The resultant magnetic and non-magnetic streams were then analysed with XRF and XRD. The results indicated that the reduction roast – magnetic separation process may not be suitable for this specific ore. The best results showed a manganese grade of the product increase from 44% to 47% and the Mn/Fe ratio increase from 3,00 to 3,72. Despite the poor observed separation, the relevant review metrics; namely the Mn/Fe ratio, the percentage Fe removed and the percentage Mn lost, were modelled and response surfaces were developed. Making use of an optimization technique known as desirability functions, it was concluded that the optimized conditions were a temperature between 850K – 900K and a CO content of the reducing gas between 30 – 35 vol%. In order to determine the reasons for the poor separation, SEM and reduction progress tests were carried out. Solid solution phase formation was evident from the SEM images, with the reduction progress tests indicating that the cause of this solid solution formation was not excessive reduction times or temperatures, but due to the mineralogy of the ore itself. The ore was then subject to a kinetic analysis to understand the reaction mechanisms and rate controlling steps. Two separate kinetic studies were carried out, one with a CO-CO2 gas mixture and another with a H2-H2O gas mixture. The use of hydrogen is growing in popularity due to the formation of water vapour rather than CO2 in the reduction reaction. The different sets of conditions for both kinetic studies were the same as those used for the optimization study. In the case of the CO kinetic study, it was found that the Avrami model fit experimental data the best. This model describes nucleation and growth as the rate limiting step. In the case of the H2 kinetic study, the rate limiting step was found to be activation control. A comparison between H2 and CO reduction indicated that the former occurred at significantly higher rates and reached higher reduction extents than the latter.
Geometric acceleration of complex chemical equilibrium calculations for inclusion in multiphysics and process models
(University of Pretoria, 2023-07-27) Zietsman, Johan; willem.ab.roos@gmail.com; Roos, Willem Abraham
Incorporating multi-component, multi-phase, high-temperature, complex chemical equilibrium calculations into multiphysics and process models can provide valuable insights into industrial processes and equipment that current modelling methods and measurements cannot. The equilibrium state of a thermochemical system is determined by minimising the Gibbs energy for a given set of system component concentrations, temperature, and pressure, and becomes computationally expensive when a large number of these calculations have to be performed. This makes direct integration of chemical equilibrium calculations into models infeasible. There have been many attempts to, in one way or another, accelerate these calculations. The strengths of these existing acceleration methods, together with fundamental thermochemical theory, were used to conceptualise and develop a new accelerator algorithm. The accelerator algorithm uses a system's phase diagram and the Gibbs phase rule to map the thermochemical system to geometric space by storing calculated physical and thermochemical properties in-situ for later recall and interpolation. Linear interpolation with the lever rule in geometric space is less computationally expensive than Gibbs energy minimisation. The advantage of populating a database in-situ is that data is only generated and stored in the regions accessed by the model as it is being solved. The accelerator algorithm is based on established thermochemical theory, and the generality thereof allows the accelerator to be used in any system, regardless of the number of components. The performance of the accelerator algorithm was tested on a number of two- and three-component systems as well as on two industry-related processes; a simplified four-component ilmenite smelting system and a simplified five-component iron- and steelmaking system. As the number of system components increase, so does the computational expense of equilibrium calculations. This translated to larger acceleration factors being achieved as the number of system components increased -- from as high as 20 in two-component systems to 1000 in the four- and five-component systems. Interpolation errors made on phase compositions were in the order of 10E-2 mole\mole and less. This would translate to an interpolated phase composition being accurate to within 99% of the calculated phase composition. The majority of interpolation errors made on physical and thermochemical properties were in the order of 1% and less. The developed algorithm showed noteworthy acceleration of equilibrium calculations when tested on the two industry-related processes while maintaining acceptable levels of accuracy. There is great potential for the accelerator algorithm to make the inclusion of equilibrium calculations in models with many system components feasible. The performance of the accelerator can be improved by transferring the algorithm to a more computationally efficient compiled programming language and utilising a more performant database system.
Leaching of crude titanium powder produced by metallothermic reduction : effects of leaching conditions on final powder quality
(University of Pretoria, 2021) Sole, Kathryn; Coetsee, Theresa; Fazluddin, S.; matsie.serwale@gmail.com; Serwale, Matsie Rinny
A low-cost titanium production process, the CSIR-Ti powder process, which aims to produce titanium powder directly by metallothermic reduction of titanium tetrachloride with lithium, has been under development at the Council for Industrial and Scientific Research (CSIR). Crude titanium powder produced using the CSIR-Ti process is inevitably contaminated with by-products such as lithium chloride, lithium and titanium dichloride. These by-products tend to become sources of impurities in titanium powder, specifically oxygen and chloride impurities. The presence of oxygen and chloride impurities has marked effects on the mechanical properties of titanium finished products. Consequently, for the crude titanium powder to be rendered useful downstream, it must be purified and the by-products reduced to concentrations specified in the commercial standards. The present study was undertaken to examine whether acid leaching could be used to selectively dissolve and prevent hydrolysis of the by-products—specifically excess lithium and unreacted titanium dichloride in the crude titanium powder produced by the CSIR-Ti process. A further objective was to determine whether a purified product that meets both oxygen and total residual chloride content as specified by the standards can be achieved. The effects of key leaching variables and their interaction were also investigated to gain fundamental understanding of these effects on the by-products leaching behaviour. A literature study to select a suitable lixiviant and to establish the aqueous chemistry of the byproducts and their effect on the leaching conditions was undertaken. It showed that of the various acids suggested in the literature, hydrochloric acid was the cheapest and that it was more suited for the CSIR-Ti leaching process than nitric acid, due to the common ion chloride. This simplifies the leachate purification process downstream. The literature study established that Ti(II) has no aqueous chemistry but instead is oxidised to Ti(III) in solution. It was found that Ti(III) is easily oxidised to TiO2+ by dissolved oxygen and water. However, the oxidation rate was slow in hydrochloric acid solutions with the advantage that hydrolysis of the ions could be minimised and the precipitation of the oxides or oxychlorides prevented. It was further revealed that the lithium neutralisation reaction is highly exothermic, with the possibility of raising the leachate temperature to 60°C, resulting in the contamination of the titanium powder particles by the oxide layer and precipitated hydrolysis products. Batch leaching tests were carried out using factorial design of experiments to investigate the effect of initial hydrochloric acid concentration, which was estimated by varying the concentration between 0.032 M and 1 M; particle size, which was varied between −10 mm and +10 mm; and the initial temperature, varied between 14°C and 30°C. The resulting data were modelled and analysed using the analysis of variance statistical method. The solid residues were analysed for oxygen and total residual chloride content. The solid residue was also characterised by scanning electron microscopy (SEM) to examine the morphology of the leached particles. Leaching kinetics model fitting was also conducted. The statistical analysis showed that of the three factors investigated, temperature was the factor with the most statistical significance on both the oxygen and chloride concentration in the purified product, followed by particle size. The effect of acid concentration proved to be minimal, a phenomenon attributed to low concentrations of acid-consuming impurities, specifically excess lithium in the crude product. Thus, the two concentrations of hydrochloric acid investigated were found to be efficient to prevent hydrolysis product formation. Scanning electron micrographs revealed that crushing the crude product with a jaw crusher occluded crude titanium pores, thus locking in some by-products in addition to the pores locked by sintering during the metallothermic reduction. The observation showed that residual chloride impurities in the purified product are not just a consequence of hydrolysis products but also byproducts locked deeper in the pores of the product. Based on the parameter ranges evaluated in the study, a product that satisfied both oxygen and chloride standard specifications was achieved when the crude product was leached in both 1 M and 0.032 M initial HCl concentrations, temperature of 30°C and particle size of +10 mm. The combination of (−10 mm and 14°C) at all concentrations also yielded acceptable oxygen and chloride content levels. Overall, it was concluded from the present work that purification of crude CSIR-Ti product by leaching in dilute HCl is technically feasible.
Corrosion characterisation of solid and lattice AlSi10Mg manufactured by laser powder bed fusion
(University of Pretoria, 2021) Möller, Heinrich; Du Plessis, Anton; u11200597@tuks.co.za; Taute, Carlien
Additive manufacturing can be used to produce complex, custom geometries, consolidating different parts into one. This reduces the required number of assemblies and allows distributed manufacturing with short lead times. Defects, such as porosity and surface roughness, associated with parts manufactured by laser powder bed fusion, can severely limit industrial application. The effect these defects have on corrosion and hence long term structural integrity must also be taken into consideration. This project aimed to characterise porosity in both solid and lattice cube samples produced by laser powder bed fusion, with the differences in porosity induced by changes in the process parameters, and subsequently, characterising the effect porosity has on corrosion. The alloy used in this investigation is AlSi10Mg, which is widely used in the aerospace and automotive industries. Samples were studied before and after corrosion using X-Ray computed tomography (CT scanning), metallographic examination and scanning electron microscopy (SEM), as well as compression testing for the lattice cubes. It was found that higher laser power leads to more porosity and lower surface roughness. CT scanning was a very effective method to study corrosion using aligned CT images of before-after states. Porosity did not have an effect on the corrosion during the early corrosion stages (168 hours). The manufacturing process parameters induced differences in porosity and surface conditions, but did not strongly affect corrosion. It is probable that crack initiation sites such as internal porosity and defects are filled with corrosion product, delaying the onset of cracking and failure, and the corrosion product that fill the voids adding to the full strength of the lattice will also slightly increase the compressive strength of the samples.
The effect of ternary alloying on the shape memory properties of titanium-platinum alloys
(University of Pretoria, 2020) Siyasiya, Charles Witness; Chikosha, S.; u04413571@tuks.co.za; Daswa, Pfarelo
Among many shape memory alloys, Ti-Pt alloys have potential as a high temperature shape memory alloy. The alloy undergoes a reversible martensitic transformation at temperatures above 1000 oC. However, the alloy’s negligible shape memory effect could limit its application. Solid solution strengthening has been suggested as an alternative to improve the shape memory properties of Ti-Pt. In this study, the effect of partially substituting either platinum (Pt) or titanium (Ti) with vanadium (V) on the shape memory properties of Ti-Pt was investigated in the as-cast and solution heat treated conditions. The study focused on investigating the effect of V on the structure, phase transformation, mechanical properties and shape memory behaviour of Ti-Pt alloy using scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), X-ray diffraction analysis (XRD), differential scanning calorimeter (DSC), compression tests and a macro Vickers hardness tester. Ti-Pt50-x-Vx and Ti50-x-Vx-Pt (x = 0, 6.25, and 12.5 at.%) alloys were produced by an arc melting method using elemental powders of Ti, Pt and V. Solution heat treatment (SHT) was carried out at a temperature 1250 oC for 72 hours, followed by ice water quenching. The SEM-EDX revealed that the addition of V to Ti-Pt resulted in multiple phase microstructures in both the as-cast and solution heat treated conditions. The partial substitution of Pt with V did not occur, since the alloy composition was within in a three-phase region of the Ti-Pt-V ternary phase diagram with increasing V content. Solution heat treatment increased the amounts and size of the other phases with increasing V content. The partial substitution of Ti with V resulted in alloy compositions that were in a single TiPt phase region. The addition of 6.25 and 12.5 at.% V did not change the structure of B19 martensite phase and this phase was the major matrix phase in all compositions. The partial substitution of Pt with V decreased the transformation temperatures of the Ti(Pt,V) phase with increasing V content, while the partial substitution of Ti with V increased the transformation temperatures of (Ti,V)Pt phase with increasing V content. Both substitutions widened the thermal hysteresis compared to TiPt, with Ti(Pt,V) exhibiting the highest. For mechanical properties, the addition of V to Ti-Pt exhibited a single yielding behaviour and, the yield strength and hardness of Ti-Pt were significantly increased with the increasing V content. Ternary alloying with V degraded the ductility and plastic strain of Ti-Pt-V with increasing V content. The partial substitution of Pt with V did not improve the shape memory effect and shape recovery of Ti-Pt-V in the as-cast condition. However, the partial substitution of Ti with 6.25 at.% V improved the shape memory effect and shape recovery of Ti-V-Pt in the as-cast condition. The SHT improved the shape recovery of Ti-Pt-V and Ti-V-Pt alloys with increasing V content, but were still lower than in the as-cast condition.
Influence of primary cooling conditions and austenite conditioning on the hot ductility of simulated continuous cast peritectic steels
(University of Pretoria, 2020) Mostert, R.J. (Roelf); Siyasiya, Charles Witness; u13409035@tuks.co.za; Lekganyane, Kedibone Melita
Surface transverse cracking is still one of the main problems in the continuous casting of steel. The cooling rate at the corners of the slab and strand is usually the highest. Therefore, depending on the cooling regime, the initial temperature drop (primary cooling to the Tmin values) in the corner regions can result in temperatures that fall into the low-temperature range of the austenite region or the α+γ transformation zone. This can cause ferrite formation or promote the precipitation of non-metallic inclusion particles at the grain boundaries and in ferrite due to the lower solubility of these particles in ferrite than in austenite. The objective of this study was to simulate the effect of the initial austenite conditioning, the extent of primary cooling, the magnitude of the temperature rebound and the unbending temperature on the ductility properties of a plain carbon peritectic steel grade under conditions resembling the commercial continuous casting process. The austenite grain conditioning was studied using two methods, the 1350 °C treatment and the simulated in-situ melting conditionings. Both of these conditionings were utilised to accomplish the initial austenite grain sizes similar to the as-cast microstructure in the magnitudes of ± 500 μm to ± 1000 μm. Bähr DIL805 Dilatometer equipment was used to simulate the heat treatments which allowed the study of the initial austenite grain size distributions. The Gleeble 1500D thermomechanical simulator was used to study the hot ductility behavior of the plain carbon peritectic steel grade. During the hot ductility test, the tensile specimens are usually solution treated at high temperatures, followed by cooling to the unbending temperatures and then fractured isothermally. However, in this study, instead of cooling the specimens directly to the unbending temperatures after the austenite treatment, the specimens were subjected to simulated primary cooling, followed by temperature rebound (i.e. ΔTr) of either 200 °C or 300 °C as well as a simulated secondary slow cooling process (at a cooling rate of 0.1 °C/s) and then isothermally deformed to fracture in the temperature range of 630–1060 °C. In both cases of the austenite conditioning, the ductility was observed to be high when the hot deformation specimens were subjected to Tmin (830 °C), this temperature being the minimum temperature reached after primary cooling and was very close to the equilibrium austenite start transformation temperature, 840 °C. In both cases of Tmin values closer to the equilibrium austenite start transformation temperature, the coarse-grained (± 500 μm) specimens showed better ductility results, compared to the abnormally large grained (±1000 μm) specimens. This was attributed to the differences in the microstructure such as the initial austenite grain sizes, the segregation effects and high fraction of non-metallic inclusion particles at the austenite grain boundaries. The influence of the magnitude of the rebound steps (i.e. ΔTr) was also studied. The result showed that for the specimens subjected to the Tmin (830 °C), ductility increased as the ΔTr increased from 200 °C to 300 °C. Moreover, with the rebound step of 300 °C ductility values increased further with an increase in the unbending temperatures (TU) and this was observed for the specimens heated to 1350 °C. In contrast to this observation for the specimens treated at 1350 °C, small ΔTr (200 °C) showed better hot ductility values than large ΔTr (300 °C) for the specimens molten in-situ condition and this was observed in the unbending temperature range of 830-940 °C. However, the hot ductility values of these specimens were observed to increase with an increase in unbending temperature range of 980-1040 °C. In both cases of the austenite conditionings, the relatively good ductility results were attributed to the beneficial effect of Tmin values. These temperatures were 10 °C and 30 °C below the equilibrium austenite start transformation temperature, Ae3 for the specimens treated at 1350 °C and molten in-situ conditions, respectively. After quenching the specimens from these temperatures (Tmin), no grain boundary films of ferrite were observed. Due to the absence of ferrite, a smaller density of inclusion particles at the grain boundaries was expected. Furthermore, the effect of Tmax values (e.g. 1030 °C and 1130 °C) and high unbending temperatures (830-1060 °C and 830-960 °C) were also thought to have contributed towards good ductility results. The hot ductility values only decreased when the unbending temperatures fell below the Ar3S (~788 °C) temperature and this was observed for both austenite conditionings.
Modelling deformation in a DC smelting furnace lining
(University of Pretoria, 2020) Zietsman, Johannes Hendrik; Kok, Schalk; u10098862@tuks.co.za; Kotze, Herman Hermanus
No abstract
Investigating the Effect of Adding a Disc Liner to the Mill Shell of a Vertical Stirred Media Mill
(University of Pretoria, 2020) Naude, Natasia; u21045772@tuks.co.za; Ford, Elizma Maria
Comminution processes are used in the metallurgical industry to reduce the particle size of mined ores in order to liberate valuable minerals for downstream separation and extraction. Comminution is a very energy intensive process and an incentive therefore exists for metallurgical operations to optimise the efficiency of the comminution processes employed. Stirred media mills are more efficient for regrinding and fine grinding duties as compared to ball mills that have traditionally been used in these applications. The efficiency of stirred media mills are influenced by both the operating conditions and physical design of the mill. Using the shear based power model, Radziszewski, 2013, hypothesised that the power draw of a vertical stirred media mill operating with pin or disc type stirrers could be increased by adding stationary liners to the mill shell. If this hypothesis holds true this approach might be used to improve the design of stirred media mills for new applications or it might be used to optimise existing mills by modifying the mill internals to improve on the stirrer and mill shell designs. The aim of this investigation was to test the hypothesis and to evaluate the effect of liner addition on the productivity and energy efficiency of the mill. An experimental test work programme was developed to measure the mill power draw of four different mill geometries under comparative conditions. Two different stirrer designs were evaluated, the first stirrer consisted of a pin type agitator and the second stirrer consisted of a ring type agitator design. Testing was conducted with the two different stirrers operating in both a smooth mill vessel and in a mill vessel fitted with a stationary disc liner. The test work programme also included grinding tests to compare the productivity and energy efficiency of the four different mill configurations. Milling tests were conducted on mono-sized quartz feeds in particle size ranges of around -150 + 106 μm and -106 + 75 μm. Results of these tests were used to calculate both the time based specific rate of breakage (Si) and the energy normalised specific rate of breakage (SiE). The breakage rates were used to compare the four different mill geometries in terms of productivity (Si) and energy efficiency (SiE). Further tests were conducted on quartz feed material with a natural feed size distribution. The specific energy requirement (kWh/t) and milling times (minutes) to reach a given target product grind were used to compare the performance of the four different mill geometries. Discrete Element Modelling (DEM) was used to qualitatively study and compare the charge conditions in each of the four geometries. The experimental results supported the hypothesis and showed that the power draw of the pin and ring stirrer mills increased with the addition of a stationary disc liner to the mill shell. The additional mill power draw resulted in an increase in the productivity of the mill without any negative effect on the energy efficiency. The results of this work shows that there could be a potential to improve the milling performance of stirred media mills by adding stationary discs to the mill shell. Specifically a higher mill power draw could lead to either a smaller equipment footprint in the case of new mills or to a larger throughput capacity or finer product grind in existing mills that have been retrofitted with an improved internal design. Further work would be required to confirm the results of this study on a larger scale and in a continuous milling configuration. Some general observations were made on the shear based power model proposed by Radziszewski, 2013. When applied to this current investigation the shear based power model correctly predicted that the addition of the disc liner to the mill vessel will result in an increase in the mill power draw. However the model did not correctly predict the relative power draw of the pin versus ring stirrer designs. The shear based power model predicted that the ring stirrer will draw more power than the pin stirrer, but the experimental work showed the opposite result. The pin stirrer had a higher power draw than the ring stirrer. DEM data showed that the pin stirrer resulted in a higher average bead velocity in the mill as compared to the ring stirrer. It is therefore postulated that the reason for the higher power draw with the pin stirrer was that this design provided a better transfer of movement from the mill shaft to the mill charge.
The effect of autogenous gas tungsten arc welding parameters on the solidification structure of two ferritic stainless steels
(University of Pretoria, 2019) Pistorius, P.G.H. (Pieter); prinsheinrich@gmail.com; Prins, Heinrich Johann
Ferritic stainless steel is typically used in the automotive industry to fabricate welded tube that is plastically deformed for flanging, bending and necking. The effect of welding parameters during autogenous gastungsten arc welding (GTAW) of thin sheet on the weld metal structure and tensile properties were determined. Two grades of ferritic stainless steels, a titanium-containing Grade 441 and a titanium-free molybdenum-containing Grade 436, were used as base metal. Statistical analysis was used to determine the influence of welding parameters on the microstructure of autogenous GTAW welds. The results of Grade 441 indicated that the welding speed and peak welding current had a statistically significant influence on the amount of equiaxed grains that formed. For Grade 436, the same welding parameters (welding speed and peak welding current) had a statistically significant influence on the grain size of the weld metal grains. The ductility of a tensile test coupon machined parallel to the weld direction, for both base metal grades, was unaffected by the welding parameters or the weld metal microstructure. The elongation was determined by the amount of weld metal in the gauge area of a tensile coupon. The titanium content of the base material seems to have the most significant effect on the formation of equiaxed grains.
Microstructure and mechanical properties as a function of process parameters for Ti6Al4V produced by high power laser powder bed fusion
(University of Pretoria, 2019) Pistorius, P.G.H. (Pieter); Möller, Heinrich; u12373606@tuks.co.za; Madungandaba, Pam Mana
Laser-powder bed fusion (L-PBF) processing finds its application in various metal forming industries such as aerospace, automotive and medical industries. Ti6Al4V alloy is widely used in aerospace applications. The main interest of studies on additive manufacturing of Ti6Al4V is to investigate the material properties (strength, toughness and corrosion resistance) with regards to applications in the aerospace industry. The L-PBF process allows great flexibility with regards to process control and process design, and therefore control over microstructure and properties. The aim of the project was to study the effect of process parameters (laser power, scanning speed, hatch spacing, spot size and energy density) on Ti6Al4V microstructure and hardness of samples produced by L-PBF processing. The main objective was to analyse, and statistically predict part properties based on selected process parameters in order to enhance process understanding. The equipment that was used to manufacture the samples is a prototype powder bed fusion setup, with an Ytterbium laser system housed in a LENS (laser engineering net shaping) chamber. Experiments were carried out using a laser power of 1 to 3 kW, 2 to 4 m/s scanning speed, 0.10 to 0.24 mm hatch spacing, 250 to 450 μm spot size, and laser energy density of 33 to 200 J/mm3. Porosity analysis was conducted using the OHAUS Explore® balance precision weighing equipment. Optical microscope (OM) and EBSD analysis scanning electron microscope (SEM) was used to analyse microstructures of the samples. Porosity was found to be a function of laser power, scanning speed, hatch spacing and energy density. Linear regression relationships were developed to predict porosity of Ti6Al4V under the set of parameters used in the study. The lowest level of fraction porosity obtained from the built parts was 0.6% (2 kW laser power, 2 m/s scanning speed, 0.24 μm hatch spacing and 450 μm spot size). The amount of porosity varied with laser power. A higher laser power resulted in increased micro round porosity. A microstructure of acicular  martensite within columnar prior  grains was obtained for all energy density values used. Changes in process parameters used in the project scope were found to have a significant effect on the microstructure and not so much on the hardness range. However, through electron backscatter diffraction analysis a change in β content of (0.2 to 5.5%) was found with increasing energy densities, whilst content decreased with increasing energy densities. The hardness was between 326 and 418 HV (300 g).
Investigation into the dephosphorization of ferromanganese alloys for production of advanced high strength steels
(University of Pretoria, 2020) Pistorius, Petrus Christiaan; Steenkamp, Joalet; patriciam@mintek.co.za; Maphutha, Mamaotseng Patricia
The focus of the current study was to investigate dephosphorization of ferromanganese to produce a low P containing alloy that could effectively be used for the production of AHSS. The study involved conducting laboratory scale testwork to investigate dephosphorization of of FeMn (HCFeMn and MCFeMn) alloys by CaO-based slag systems. Addition of Na2O, BaO, and CaF2 to MnO-CaO-SiO2 slag was investigated to study the influence on dephosphorization. The effect of MnO-BaO-BaF2 slag system without CaO was also investigated in a preliminary way. The testwork was carried out in a 25 kVA induction furnace at temperatures of 1350°C, 1400°C, and 1450°C at different retention times of 5 minutes, 30 minutes and 60 minutes. The analytical results of the product samples were used to evaluate the P-partition ratio. In general, analysing the low P contents of the slags and alloys proved to be a challenge and at the lowest concentrations the uncertainty was large. The analytical results of the slags generally reported higher MnO-contents than was anticipated. This resulted in dilution of the other major slag components, i.e. SiO2 and CaO. Low P2O5 contents were found in the slags, suggesting that the removal of P from the alloys was minimal. The alloy results reported C-pickup after the tests due to the dissolution of the graphite from the crucible. Loss of Mn from the alloys was also observed. The P-content of the alloys were generally higher than in the feed alloy. The results generally showed the 〖 L〗_p remained small at <1 which is an indication that dephosphorization had not been achieved. The baseline slag comprising of 40%CaO-40%SiO2-20%MnO reported relatively higher 〖 L〗_p values. Addition of Na2O and CaF2 did not show any added benefit. Substituting half of CaO by BaO, resulted in similar L_p values to those of the baseline slag under a few conditions, namely 1350°C and 1450°C at 30 minutes. The baseline slag was however not outperformed by the BaO-bearing slag under any of the other conditions. Increasing the temperature generally resulted in lower 〖 L〗_p values. This may be attributed to the exothermic nature of the phosphorus removal reaction which should be favourable at lower temperatures. Increasing the basicity (%CaO/%SiO2 ratio) of the baseline slag showed an initial increase in 〖 L〗_p value for basicities of 0.7 to 0.9, thereafter a slight reduction in the 〖 L〗_p value was obtained. The latter results were not expected as higher basicity is anticipated to improve the P capacity of the slag. Increasing the basicity resulted in increased slag liquidus temperature of about 1500°C which negatively affected dephosphorization. In summary, based on the 〖 L〗_p obtained, the conditions investigated with the CaO-based slags appeared to have been unfavourable for dephosphorization of FeMn alloys, as most of this impurity element remained in the alloy. The BaO-BaF2-MnO slag showed potential to dephosphorise HCFeMn alloy, however the slag posed numerous challenges and the slag should be investigated further
The development of a risk-based model to predict corrosion fatigue failures in subcritical boilers
(University of Pretoria, 2020) Mostert, R.J. (Roelf); Wannenburg, Johann; u27198457@tuks.co.za; Rode, Bianca
The increased energy demand within South Africa has led to continued periods of load shedding. This has had an adverse impact on industry, quality of life and the economy as a whole. A larger requirement for production time, reduced downtime and an enlarged focus on health and safety have steered industry towards a paradigm shift in inspection and maintenance. These activities have progressed from a predominantly time-based (prescriptive) approach towards a risk-based approach. Generally accepted standards like BS EN 16991:2018 and API RP 580 give a comprehensive outline of the basic elements for developing, implementing and maintaining a risk-based inspection program. API RP 581 takes this outline one step further and contains the quantitative methods that support the minimum guidelines presented by API RP 580. Similarly, Kent W. Mühlbauer’s approach has developed a relative risk ranking model for petroleum and gas pipelines, which outlines a qualitative method for representing risk. None of these models are however directly applicable to predicting the failure of pressurised boiler equipment due to the mechanism of corrosion fatigue. API RP 580 / 581 was primarily developed for the oil and gas industry and have practical limitations when applied to pressurised equipment typically found in utilities. BS EN 16991:2018 supplies a framework for utilities, but doesn’t go into the specific detail of how to structure, formulate and apply a risk based management model. The methodology laid out by Kent W. Mühlbauer, while practical and easily implemented, was designed for oil and gas pipelines. A systematic methodology to evaluate the risk associated with specific failure mechanisms in boilers, such as corrosion fatigue, does not exist or is not readily available. A comprehensive risk-based predictive model, using aspects of the abovementioned standards and guides, was developed to demonstrate the predictability of corrosion fatigue in sub-critical boilers. Weightings were assigned to contributory causes to corrosion fatigue, which then allocated relative risk ranks to certain segments within a boiler. Operators and owners of boilers can derive benefit from this model by focusing inspection, maintenance and alteration activities on those equipment locations with the highest relative risk score.
Investigating premature ageing of blast furnace taphole clay containing a resole resin and liquid pitch binder
(University of Pretoria, 2021) Garbers-Craig, Andrie Mariana; Ramjee, Shatish; u12011020@tuks.co.za; Cameron, Izak Jian-Pierre
This report investigates the cause of the reduction in workability and increased ageing of a blast furnace taphole clay. The taphole clay contains 60 mass% alumina, with a phenol-formaldehyde resole resin (PFR) and liquid pitch as a binder system. The clay aggregates as well as powder matrix raw materials were analysed using XRF, XRD and SEM-EDS for characterisation and impurity detection such as sulphur and free lime. The presence of free lime can cause premature cross-linking of the resin in the binder while the presence of sulphur can reduce the curing time of the resin. The wettability as well as particle size distribution (PSD) of all the raw materials were investigated to confirm a uniform particle size distribution of the dry aggregate of the taphole clay and wettability compatibility between the dry raw material and both resin and liquid pitch. The resin and liquid pitch characterisation, as well as interaction between resole resin and liquid pitch, were evaluated using viscosity measurements, Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The analyses confirmed a chemical interaction between the resole resin and liquid pitch, where the chemical structure of the resole resin was broken down when the two liquids were mixed. This prevented curing of the resin to occur. After ageing of the resin and liquid pitch mixtures, premature cross-linking of the resin occurred, causing the curing process to move to lower temperatures, i.e. earlier onset of curing. This reduction in curing temperature, after ageing, was confirmed by an increase in binder viscosity at lower temperatures (starting at 60°C) than the curing temperature (121-126°C) of the virgin resole resin in the binder. This increase in viscosity of the binder mixture is the primary cause of the reduced workability, increased ageing and increasing Marshall extrusion pressure (MEP) of the taphole clay.

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