Theses and Dissertations (Metallurgical Engineering)

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Determination and improvement of wear and abrasion properties of Glass Reinforced Plastic (GRP) piping for slurry applications
(Vaal University of Technology, 2014-09) Van der Schyff, A.; Mendonidis, P., Prof.
Glass Reinforced Plastics (GRP) are composite materials comprising a resin matrix reinforced with glass fibers, and are well known for their excellent corrosion resistance and high strength-to-weight ratios. However, their use in slurry transporting pipes is uncertain because the wear resistance of GRP pipe materials is not well documented due to the difficulty encountered in standardizing mechanical properties of GRP pipes. The aim of this project was to identify the main variables that influence the wear and abrasion properties of GRP pipes and the differences of abrasion properties between the continuous and discontinuous filament wound pipes that are manufactured by Amitech SA. In order to achieve the aim of the project, the mechanisms of wear in ductile and brittle matrix GRP composites were investigated. The Pin-on-disk and Jet-impact tests were identified as the best abrasion tests suited for testing GRP composites. Data collected from these tests indicated that the resin type, curing, additive content and- type, fiber content and manufacturing methods are factors affecting wear resistance. It was further found that hard surfaces provided better resistance to abrasive wear, whereas ductile matrices performed better under erosive wear.
Development of a beneficiation route to upgrade Sishen iron ore jig slimes for iron making
(Vaal University of Technology, 2017-11) Molebiemang, G. M.; Thubakgale, K. C.; Otunniyi, I. O., Prof.
Kumba Iron Ore’s Sishen mine is located in South Africa’s Northern Cape Province and produces about 40 million tons (Mt) of iron ore per annum. The ore body consists mainly of laminated and massive type hematite ore of which 30 Mt is being beneficiated by Dense Medium Separation (DMS), while the remaining 10 Mt is processed by the jig plant that was added as an expansion to the DMS processing route. The jigs at the jig plant are employed to treat coarse, medium and fine Run of Mine Ore (ROM). Due to the presence of finer particles on the screened ROM, hydrocyclones are employed on the secondary screen that feed the finer jig to produce fines on the overflow that is fed to the thickeners for dewatering by sedimentation, and the formation of clear liquid that can be recycled to the Jig plant. However, it was suspected that a substantial amount of the iron gets lost to the hydrocylone overflow with slimes. These slimes consist of very fine particles (about 90%-45μm) which make treatment with gravity concentration methods difficult. Sishen mine attempts to beneficiate the slimes using Pulsating High Gradient Magnetic Separators (PHGMS). However, PHGMS produce a concentrate consisting of 64.0% iron and 5.0% silica which render the slimes not suitable for pelletizing. Normally the slimes are considered as waste and are rejected to the slimes ponds. Rejecting slimes is not only a loss of resources, but it also poses severe long-term environmental problems. Recovery of iron minerals from the tailings produced in a large iron ore operation is of importance as it will result in environmental and economic benefits which indicate the importance of utilising those accumulated fines.
Evaluation of blended collectors for improved recovery of PGEs from western bushveld UG2 deposit.
(2018) Moja, Malebogo Gloria; Mendonidis, P. Prof; Otunniyi, I.O. Prof
Lonmin mining company located in the Bushveld Complex of South Africa is one of the main platinum group elements (PGEs) producers in the world. Its core operations are made up of 11 shafts and inclines. There are resources of 181 million troy ounces of 3PGE + Au, and there are reserves of 32 million ounces of 3PGE + Au. One of the ore type produced at Lonmin is UG2 ore which is dominated by the high presence of chromite. The UG2 ore is also associated with PGE assemblages divided into sulphides and non-sulphides, and it is beneficiated through the froth flotation technique. Froth flotation is a physico-chemical process that is used for separation of desired valuable minerals from the gangue minerals by utilising the difference in surface properties. The process has been achieving lower recoveries with P4 (shaft name) UG2 ore compared to Eastern Platinum Limited (EPL) UG2 ore when using similar reagents suite, this leads to loss of valuable minerals to the tailings, both ores were from Lonmin. The first step was to conduct the mineralogical analysis conducted using Scanning electron microscopy- energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and optical microscopy to study the mineral composition of the two ores, and to identify any differences between them (two ores) considering that EPL UG2 ore is a blend of P1, P2, P3 (shaft names) and P4 UG2 ores while P4 UG2 ore is not blended with any other ores. The mineralogical results showed the presence of chromite, plagioclase, enstatite and sulphide minerals. The PGEs could not be detectable by any of the techniques used due to their small size and rarity. However, X-ray diffraction detected differences in concentrations of minor gangue constituents such as talc, muscovite, chlorite and actinolite and these results suggest that reagent consuming gangue mineralogy may have contributed to the differences in PGE recoveries by flotation. Batch flotation tests were also conducted. The existing reagent suite consisted of CuSO4 as an activator, Sodium n-propyl xanthate (SPNX) as a collector, carboxymethyl cellulose CMC as a depressant, and Senfroth 200 as a frother, and this was a single collector system. Therefore it was imperative to conduct flotation n investigation on alternative collector blends in order to improve the recovery of P4 UG2 ore. SNPX was used as the primary collector and it was blended with the following co-collectors: alkyl dithiocarbamates (DTC), two formulations of S-alkyl-N-butyl thionocarbamates (ABTC C1 & ABTC C2), and two formulations O-isopropyl-N-ethyl thionocarbamate (IPETC 30 & IPETC 31), one co-collector at a time. The first test incorporated the SNPX at dosage of 150 g/t without a blend and this dosage was selected based on the current optimum practice used at Lonmin and to use as a benchmark for the project. Trying to maintain the same dosage of 150 g/t of collectors, SNPX + co-collector were blended at two different dosages of 100 g/t + 50 g/t, and was also due to the fact that the co-collectors were highly concentrated and small dosages were expected to perform very well with SNPX. Lastly, the SNPX + co-collector at dosages 100 g/t + 125 g/t, here the dosage of co-collector was very high compared to 50 g/t and this was to check the effect of high dosages of highly concentrated collectors on the performance of the ore. The flotation results showed that the use of 50 g/t of co-collectors yielded optimum PGEs + Au recoveries and grades, while the dosage of 125 g/t decreased recoveries and grades. The high dosage quantities of collectors do not necessarily mean they will yield improved recoveries and grades. Different chain structures can be used to alter the behaviour of a collector, and these may increase or decrease their capabilities to cause higher recoveries. By using a collector with a longer hydrocarbon chain the flotation limit may be extended without loss of selectivity, consequently bringing about greater water repulsion, instead of increasing the concentration of a shorter chain collector. At 100 g/t of SNPX and 50 g/t of co-collector i.e. SNPX + IPETC 30 yielded improved 3PGE + Au recovery of 85.7 % at 3PGE + Au grades of 60.14 g/t, compared to the unblended SNPX (150 g/t) which yielded 3PGE + Au recovery of 81.1 % but insignificantly higher grade of 60.53 g/t. On the other hand, SNPX + IPETC C1 blend yielded low 3PGE + Au recoveries compared to SNPX + IPETC 30 and SNPX + IPETC 31 blends, but it achieved the highest grade of 76.1 g/t. Evidently, this proves that the relationship between recovery and grade is a trade-off. The results have also shown the synergic effects, especially for SNPX blended with IPETC 30, and SNPX blended with IPETC 31 at dosage of 100 g/t (SNPX) and 50 g/t (IPETCs). It can be concluded that the different interaction obtainable from the thionocarbamate (ROCSNHR), effectively complement that from the xanthate ion (ROCS2–) to achieve more collector interaction at surface sites otherwise interactable for xanthate only. Therefore the collector blends rendered the mineral of interest hydrophobic and as a result the minerals were recovered to the concentrate. On the other hand, too much of collectors may not be beneficial. At the dosage of 100 g/t of SNPX and 125 g/t of collectors, SNPX + DTC attained lower recoveries compared to SNPX, SNPX + IPETC 30, SNPX + IPETC 31, however the grade was higher than achieved SNPX + IPETC 30, SNPX + IPETC 31 and SNPX + ABTC C1. Nevertheless, comparing these results to the dosage of 50 g/t of the co-collectors, the 125 g/t did not perform well at all. The dosage of 125 g/t of co-collectors lead to loss of collecting power and selectivity, especially for SNPX + IPETC 30, SNPX + IPETC 31, and SNPX + ABTC C1 blends. It is therefore wise to conduct an optimisation test to determine the correct dosing rate. In addition, the chromite entrainment was below the smelter limit and is very beneficial since chromite is detrimental to the furnace. Therefore, it is concluded that the blends of SNPX with IPETC 30 and IPETC 31 at a dosage 50 g/t have shown satisfying recoveries and the FeCr2O4 recovery is less than 1 % meaning there will not be any smelter penalties for FeCr2O4 content. Therefore, these are the recommended collector blends. It is recommended that further mineralogical study of the ores be conducted so that it may provide deeper insight into the causes of low recoveries under SNPX only. The system of blended collectors and its optimisation would be beneficial and can be practiced. The Chemisorption studies between the minerals and co-collectors used will provide more specific insight and details into the actual interaction synergy that gave the improved recoveries.
Biocompatibility evaluation of sintered biomedical Ti-24Nb-4Zr-8Sn (Ti2448) alloy produced using spark plasma sintering (SPS)
(Vaal University of Technology, 2018) Madonsela, Jerman S.; Yamamoto, Akiko, Dr.; Machaka, Ronald, Dr.; Shongwe, Brendon, Dr.; Matizamhuka, Wallace R., Dr.
Solid titanium (Ti), Ti-6Al-4V (wt.%), and Ti-24Nb-4Zr-8Sn (wt.%) materials were fabricated from powders using spark plasma sintering (SPS). The starting materials comprised of elemental powders of ASTM Grade 4 titanium (Ti), aluminium (Al), vanadium (V), niobium (Nb), zirconium (Zr), and tin (Sn). The powders were initially characterised and milled prior to sintering. The micronpowders were milled in an attempt to produce materials with nanostructured grains and as a result improved hardness and wear resistance. The produced solid Ti-24Nb-4Zr-8Sn alloy was compared to solid titanium (Ti) and Ti-6Al-4V (Ti64) on the basis of density, microstructure, hardness, corrosion, and biocompatibility. Relative densities above 99.0% were achieved for all three systems. CP-Ti and Ti64 had both 100% relative density, and Ti2448 showed a slightly lower density of 99.8%. Corrosion results showed that all three materials exhibited good corrosion resistance due to the formation of a protective passive film. In 0.9% NaCl Ti2448 had the highest current density (9.05 nA/cm2), implying that its corrosion resistance is relatively poor in comparison to Ti (6.41 nA/cm2) and Ti64 (5.43 nA/cm2), respectively. The same behavior was observed in Hank's solution. In cell culture medium, Ti2448 showed better corrosion resistance with the lowest current density of 2.96 nA/cm2 compared to 4.86 nA/cm2 and 5.62 nA/cm2 of Ti and Ti64 respectively. However, the current densities observed are quite low and insignificant that they lie within acceptable ranges for Ti2448 to be qualified as a biomaterial. Cell proliferation test was performed using murine osteoblastic cells, MC3T3-E1 at two cell densities; 400 and 4000 cells/mL for 7 days incubation. Pure titanium showed better cell attachment and proliferation under both conditions suggesting that the presence of other oxide layers influence cell proliferation. No significant difference in cell proliferation was observed between Ti64 and Ti2448.
Evaluation of the corrosion behaviour and biocompatibility of Ti-34Nb-25Zr alloy for biomedical applications
(Vaal University of Technology, 2018-11) Mahundla, Mithavini R.; Shongwe, Mxolisi B., Dr.; Machaka, Ronald, Dr.; Matizamhuka, Wallace R., Dr.
Pure Ti, Nb, Zr, Al and V powders were used as starting materials. Ti, Ti-6Al-4V and Ti-34Nb-25Zr materials produced by SPS were compared on the basis of density, microstructure, biocompatibility, tensile strength and corrosion resistance. In this study, powder metallurgy (PM) processing route was used to fabricate the alloys. The processing route was mechanical alloying (MA) and spark plasma sintering (SPS). Commercially pure metallic powders (Ti, Nb, Zr, V and Al) of different morphological features and different formulations were prepared. Powder mixing for ternary alloys with Ti as the matrix were conducted in a turbula mixer at a speed of 49 rpm. Followed by mechanical alloying of Ti, Ti-6Al-4V and Ti-34Nb-25Zr in a high energy ball mill for 5h at 500rpm, with a ball to powder ratio of 10:1. Spark plasma sintering of Ti, Ti-6Al-4V and Ti-34Nb-25Zr biomedical alloys was conducted using a hybrid spark plasma sintering furnace at a sintering temperature, heating rate, holding time and pressure of 1200°C, 100°C/min, 10min and 50MPa, respectively. Ti-34Nb-25Zr was fabricated in two ways, fully densified and porous samples. The fully densified sample was fabricated at a sintering temperature, heating rate and holding time and pressure of 1200°C, 100°C/min, 10min and 50MPa, respectively. Whereas, porous Ti-34Nb-25Zr was fabricated using NaCl space holder at a sintering temperature, heating rate, holding time and pressure of 750°C, 50°C/min, 5min and 50MPa, respectively. This was done to compare the solid and porous alloy biocompatibility behaviour. Microstructures, elemental compositions. Phase constitution of the sintered specimens were examined using a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive x-ray spectrometer (EDS) and an x-ray diffractometer (XRD). The microstructure of Ti-34Nb-25Zr had pores and precipitates of niobium. Relative density, micro-hardness, biocompatibility and corrosion test was also conducted on the metallographically polished cross sections of sintered specimens. Ti, Ti-6Al-4V and Ti-34Nb-25Zr alloys made from the irregularly shaped Ti powders and sintered on the hybrid sintering machine yielded higher densifications reaching up to 100 % relative densities. Hardness values ranging from 300-600Hv at a load of 0.5kg. The corrosion resistance of the alloys was higher in the range of 2-4 nA/cm2 exhibiting a passive behaviour in simulated body fluids, such as Hank’s, 0.9wt.% NaCl and eagles minimum essential + 10% fetal bovine serum (E-MEM+ 10% FBS). Biocompatibility tests were conducted (cytotoxicity by WST-1 with SaOS-2 human osteosarcoma cells, protein adsorption and surface wettability). Fibronectin adsorption was less for solid Ti and Ti-34Nb-25Zr (<2ng/mm) compared to Ti-34Nb-25Zr porous and Ti-6Al-4V (4 ng/mm). Albumin adsorption was the highest on Ti substrate (3 ng/mm) than on the fully densified and porous Ti-34Nb-25Zr surfaces followed by less adsorption on Ti-6Al-4V. Surface wettability of Ti and Ti-6Al-4V showed a high contact angle of between 93-98° compared to 86° for the Ti-34Nb-25Zr solid alloy, indicating that Ti-34Nb-Zr alloys exhibited hydrophilic behaviour. The surface wettability results correlated well to less fibronectin adsorption on Ti-34Nb-25Zr solid alloy and excellent adsorption for Ti-6Al-4V. Solid and porous Ti-34Nb-25Zr showed less cell proliferation (0.06 and 0.02% cell viability) which was possibly linked to fibronectin adsorption results. Biocompatibility behaviour of Ti-34Nb-25Zr solid and porous alloys was poorer than Ti (0.20% cell viability) and Ti-6Al-4V (0.23% cell viability). There was poor protein adsorption and cell proliferation on all the alloy substrates.
Development of a potentially low young's modulus (Ti-34Nb-25Zr-XFe) base alloy for orthopaedic device application
(Vaal University of Technology, 2019-03) Nemavhola, Mavis Khathutshelo; Machaka, Ronald, Dr.; Shongwe, Mxolisi Brendon, Dr.; Matizamhuka, Wallace R., Dr.
Elemental titanium (Ti), niobium (Nb), zirconium (Zr), and iron (Fe) powders were used to fabricate four near-β alloys with non-toxic of composition Ti-34Nb-25Zr, Ti-34Nb-25Zr-0.4Fe, Ti-34Nb-25Zr-1.2Fe, and Ti-34Nb-25Zr-2Fe (wt. %) (TNZ and TNZF) using spark plasma sintering (SPS) of nano-crystalline powders attained by high energy ball milling. The fabricated alloys were compared to Ti-34Nb-25Zr (used as a benchmark alloy in this study) and comparison was made with the commercially used Ti base alloys produced either by conventional methods or powder metallurgy. The powder mixtures were milled for 5 hours using a Simoloyer high energy ball mill with a ball to powder ratio of 10:1 and a rotational speed of 1000 rpm. This was followed by sintering the mechanically alloyed powders at 1100 ºC for 10 minutes with a pressure of 50 MPa and a heating rate of 100 ºC/min using an H-HP D25 spark plasma sintering furnace (FCT System, Germany). The powders were characterised for particle size and crystal structure using SEM and XRD. The consolidated components were characterised with regards to density, microstructure, mechanical properties. The electrochemical behaviour of the alloys was investigated using a Digi Ivy DY2300 series potentiostat. Three corrosion medium, Sodium chloride (NaCl), phosphate buffered saline solution (PBS) and Dulbecco’s modified eagle’s medium that mimic the conditions in the human body were used. Mouse myoblast cell line (C2C12) was used to investigate the biocompatibility of the sintered alloys in 1010x5 mm specimens using standard colorimetric assay MTT. Both electrochemical and biocompatibility test were conducted in triplicates and the results compared with that of the benchmark. Results of mechanical alloying of powder mixtures demonstrated an inhomogeneous structure. Milling for 5 hours resulted in agglomeration of small Fe and Zr particles. Milling for 3 hours resulted in a better distribution of elements compared to longer milling times. Therefore, sintering powders milled for 3 hours would have yielded better results. The densification results were acceptable and ranged between 97-99% of theoretical densities. Although some porosity was observed, especially on the un-etched microstructure. An insignificant decrease in density was observed when 1.2 (wt. %) Fe was added. The sintered samples had microstructures which were not homogenous. However, the addition of Fe yielded a more homogeneous microstructure compared to the one with less Fe. Therefore, TNZF with 2 (wt. %) Fe had a more homogenous microstructure. Sintering at 1100 ºC resulted in undissolved niobium and titanium which were observed in the microstructure as dark and white areas. The hardness of the TNZF alloys were comparable and lied between 373 and 432 Hv. These hardness values are higher than other similar titanium-based alloys fabricated using conventional methods. The addition of Fe to TNZ showed an insignificant decrease in hardness. The addition of Fe was found to decrease the Young’s Modulus of TNZ from 119.1 to 80 GPa with an addition of 2 wt.% Fe. However, an unacceptable reduction (230.91 to 158.2 MPa) in strength was also noticed. Pseudo passivation was observed when the alloys were immersed in 0.9 % Sodium Chloride (NaCl) which could be attributed to the inhomogeneity in the microstructure. The possibility of pitting corrosion was also observed. The alloy containing 2 Fe (wt.%) was found to be more corrosion resistant than the other alloys. The TNZF alloys exhibited better corrosion resistance in 0. 9% NaCl compared to phosphate buffered solution (PBS) and DMEM. The corrosion behaviour in PBS and DMEM cannot clearly be explained from the graphs. The morphology of the corroded samples was almost the same for all the alloys in different corrosion media. The microstructures showed pits which could have been from the pores that acted as initiation sites for pitting. In cell culture for 1 and 7 days, the cell viability for TNZF alloys was greater than that of the control group (TNZ). A significant decrease in cell viability for TNZF was observed in cell culture for 4 days. The addition of Fe on TNZ do not cause toxic effects and show good cell adhesion, indicating in-vitro cytocompatibility. The greatest cell viability of 102±3.0 % for Ti-34Nb-25Zr-2Fe. The analysis of cell morphology indicated good cell-substrate interaction. The TNZF alloys developed in this study can be suitable candidates for orthopaedic implant application due to their low Young’s modulus, corrosion resistance and superior biocompatibility. However, the strength needs significant improvement. The advantage of this biomaterial, when compared to commercial alloys, is the absence of cytotoxicity elements such as Al and V.
An investigation into the effect of physico-chemical factors affecting the extraction of calcium from iron and steel slag
(Vaal University of Technology, 2020-02) Kohitlhetse, Itumeleng; Thubakgale, C. K.; Manono, M. S., Dr.; Mendonidis, P. M., Prof.
During steel production, a large quantity of waste residue commonly known as slag is produced with a reasonable amount of metals such as calcium and magnesium. Iron and steel slags have various economically viable applications such as fertiliser production, cement production, road construction, land fill and soil reclamation. However, other applications respond to the issue of controlling and degrading high levels of anthropogenic carbon dioxide (CO2) gas emissions by carbonating calcium and magnesium leach solutions from iron and steel slags. One such technological method is the extraction of calcium by means of leaching from iron and steel slag whereby CO2 sequestration is a long-term goal. In contrast, calcium and magnesium leach solutions can produce precipitated carbonates for various uses when carbonated. In addition, it is a crucial step to study the leaching parameters and reliable solvents for the dissolution of calcium and magnesium owing to their contents in iron and steel slags. The aim of this research project was to determine the physico-chemical factors affecting leaching of calcium from blast furnace slag and thereby improve the extent and kinetics of calcium extraction. The chemical and mineralogical study by X-ray fluorescence (XRF) and X-ray diffraction (XRD) respectively, has shown that the blast furnace slag sample under study contained calcium in the form of aluminoakermanite ((Ca,Na)2(Al,Mg,Fe2+)). The minerals identified through XRD are classified as silicates with the host mineral being calcium in recoverable quantities. The initial XRF analysis showed that the slag contained 23.47% Ca, 5.21% Mg and 1.24% Fe with the rest being minor elements. The slag was received at a particle size of 20 mm. Size reduction was conducted in a dry rod mill process to produce a size of 80% -75μm. Particle size distribution was analysed by a simple laboratory sieving method. The leaching tests were conducted in an incubated orbital shaker by applying the one-factor-at-a-time method for a two-hour leaching test. Reaction temperature was controlled at 30, 50, 70 and 100˚C. Ionic strength varied at 0.5, 1.0, 1.5 and 2.0 molar (M). In addition, the solid/liquid ratio was observed in the range 200, 300, 400 and 600 gram/Litre (g/L). Acetic acid (CH3COOH), ammonium sulphate ((NH4)2SO4) and ammonium chloride (NH4Cl) were tested individually as lixiviants under varying temperatures, ionic strengths and solid/liquid ratio. The results showed that extraction of calcium from ironmaking slag by leaching is feasible. 100% Ca was extracted using CH3COOH at 100˚C, 0.5 M, 200 g/L and 120 minutes. As the reaction temperature was increased from 50˚C up to 100˚C, silica gelling was observed, which hindered filtration of a leach liquor from a residue. Silica gelling did not compromise calcium recovery. The increase in lixiviant ionic strength and solid-liquid ratio did not influence calcium recovery. The role of anions during leaching by using ammonium sulphate, ammonium chloride and acetic acid as lixiviants was identified. Sulphate ions were found to have the least influence in the increase in calcium recovery. Acetate ions have a significant role on dissolution of calcium as well as chloride ions. Consequently, the findings showed that temperature is the critical operating parameter that yielded exceptionally high calcium extraction percentages. CH3COOH yielded 80%-90% of calcium extracted under high reaction temperatures. The kinetic data for the effect of reaction temperature leaching data fitted one shrinking core model equation, which described diffusion-controlled leaching reactions. The diffusion controlled by a product layer model was fitted and the activation energy were calculated as 19.4930 kJ/mol for NH4Cl solvent. (NH4)2SO4 and CH3COOH did not fit into any model because of undefined kinetic data and formation of gelatinous silica layer during leaching process. The Arrhenius equation, confirmed that the leaching reaction by NH4Cl solvent fitted diffusion-controlled mechanism.
Selective recovery of base and precious metals from printed circuit board physical processing dust
(Vaal University of Technology, 2020-02) Oluokun, Oluwayimika O.; Ndlovu, S., Prof.; Otunniyi, I. O., Prof.
Dust generated during comminution of end of life printed circuit boards (PCB), typically having d80 of 212 μm, contains copper and gold up to 6.32 % and 635 g/ton, respectively. The dust particles being highly diverse in material makeup, an hydrometallurgical processing scheme able to selectively recover target values was studied. Use of mineral acids will result in multiple metal dissolution which will complicate subsequent solution treatments. Detailed characterization of the dust was first carried out, and leaching scheme were thereafter investigated to selectively recover gold and copper from the dust, in three leaching stages. Different conditions of ammonia and thiourea leaching were investigated to optimize agitation speed, reagents concentration, temperature and leaching time. The leaching kinetics of these elements from the dust under different prevailing leaching conditions were studied. Elemental composition of the dust size fractions indicates metal contents generally increase with decreasing dust particle size, down to – 53 μm size, which contains up to 635 g/ton Au, 25.43 % Fe, and 1.40 % Cu, compared to 51 g/ton Au, 3.07 % Fe and 6.32 % Cu in the 150–212 μm fraction. Thermodynamically, under oxidative ammonia leaching, zinc and copper ammine complex is feasible, yet zinc recovery is low. For 75 – 106 μm dust size, 2 M NH4OH, 17.5 M H2O2, 1 atm. pressure and 400 rpm in Parr reactor, Cu and Zn recoveries were 92 % and 50 %, while the activation energies evaluated within 283 – 313 K gave 47.39 kJ/mol and 33.12 kJ/mol. The kinetic analysis for copper leaching gave best correlation coefficient (R2) of 0.9804 when fitted into the chemical control model, and the rate constant was 4.4 x 10-3 at 313 K. The presence of base metals frustrates direct gold recovery from the dust using thiourea with sulphuric acid and hydrogen peroxide. Therefore, the residue obtained from the first stage copper leach was acid washed to remove iron and other residual base metal contents with 5 M H2SO4, at 333 K, 400 rpm for 2 hours. Recovery analysis shows that about 75-98 % Fe, 54-65 % Zn and 96-98 % Ni were recovered under this condition while Cu was less than 7 % at all PSDs; copper having been selectively removed at the first stage. Using 75 – 106 μm dust fraction, gold recovery was optimum when the acid wash residue was leached with 0.5 M thiourea (SC(NH2)2), 0.5 M sulphuric acid (H2SO4), 0.1 M hydrogen peroxide (H2O2) under 1 atm. pressure, 298 K and 400 rpm for 4 hours. The recovery was 98 % Au. Using this optimum for other size fractions, over 98 % gold was recovered from 150–212 μm, 106 – 150 μm and 75 – 106 μm dust while 71 % and 68 % Au were recovered from 53 – 75 μm and – 53 μm respectively. The lower recovery at the finest sizes can be due to the quantity of the gold contents deported in this particle size, which will require higher reagent dosage. The kinetic analysis gave best correlation coefficient (R2) of 0.99 when fitted to the chemical control leaching model. From this data, a process flowsheet was proposed to give separate streams rich in copper and gold values from the processed dust, with detailed processing parameters. This is considered a readily scalable process solution for retrieving gold and copper from PCB dust.
The evaluation of Ta contents on strengthening effect and oxidation resistance of gamma titanium aluminide (𝑦-TiAl-%xTa) alloys.
(Vaal University of Technology, 2020-04) Cobbinah, Prince Valentine; Machaka, Ronald, Dr.; Shongwe, Brendon, Dr.; Matizamhuka, Wallace R., Dr.
Intermetallics are described as an ordered alloy phase formed between two metallic elements. An alloy is said to be ordered provided two or more sublattices are needed to describe its atomic structure. An example is titanium aluminides (TiAl). Over the years, TiAls have been some of the most interesting and highly researched high-temperature structural materials. The benefits TiAls offer include low density, high strength-to-weight ratio, excellent corrosion and oxidation resistance, and are ductile at their operating temperatures while also maintaining good structural stability. The substitution of nickel-based superalloys (NBSAs) with the lightweight TiAls for certain stress and temperature applications stand to improve the efficiency of systems and reduce emissions of CO2 and NOx. However, at ambient temperatures, the low workability of TiAls makes their fabrication challenging. In this study, 𝛾-TiAl based alloys of composition Ti-46.5Al-%xTa (x = 0.8, 4 and 8 at.%) were fabricated through the powder metallurgy (PM) route which included mechanical alloy and consolidation by spark plasma sintering. For comparisons, a reference alloy following the renowned and commercially available Ti-48Al-2Cr-2Nb (at.%) composition was included. The main objective of the study was to evaluate the influence tantalum (Ta) contents had on strengthening effect and oxidation resistance of the 𝛾-TiAl based alloys. Particle size distribution, X-ray diffractometry (XRD) at room and high temperatures and scanning electron microscopy (SEM) analyses were used to characterise the as-received elemental powders and sintered alloys. The predominant phases observed from the XRD and SEM analyses of the sintered alloys were the 𝛼2-Ti3Al, 𝛾-TiAl and 𝛼-Al2O3. Electron backscatter diffraction (EBSD) analysis showed an additional phase of TaAl present in the alloys. Furthermore, mechanical properties were measured by compression tests carried out at room temperature and elevated temperatures up to 1000 °C. The results show that the addition of 0.8 at.% Ta increased the compressive strength of the 𝛾-TiAl alloy from 980 N.mm-2 at room temperature to 1280 N.mm-2 at 850 °C. At 1000 °C, the strength of the alloys increased with increasing Ta additions. Steady flow stresses were observed for the reference alloy and 𝛾-TiAl alloy containing 0.8 at.% Ta at their ultimate stresses. Lastly, isothermal oxidation tests in air conducted were at 850 °C for 360 hours and at 1000 °C for 100 hours. The 𝛾-TiAl alloys with 4 and 8 at.% Ta obeyed the parabolic growth law with nearly constant oxidation rates at both tested temperatures. The increasing Ta presence by doping effect suppressed pronouncedly TiO2 growth by decreasing oxygen vacancy concentrations and favoured the formation and growth of a more adherent, denser and protective Al2O3 oxide.
Development and characterization of Ti-Sn-SiC and Ti-Nb-SiC composites by powder metallurgical processing
(Vaal University of Technology, 2020-08) Mathebula, Christina; Bolokang, S., Dr.; Matizamhuka, W. R., Dr.
This work is an investigation in the development and characterisation of porous Ti-Sn-SiC and Ti-Nb-SiC composites. Pure Titanium (Ti), Tin (Sn), Niobium (Nb) and Silicon carbide (SiC) powders were used as starting materials. The Ti-Sn-SiC and Ti-Nb-SiC composites were produced by powder metallurgy (PM) press-and-sinter route. The Sn is an α-phase stabilizer while Nb is a β-phase stabilizer in Ti alloys. A systematic study of binary Ti-Sn and Ti-Nb alloys was conducted with the addition of SiC particles. The addition of Sn influences the microstructure of the titanium alloy. With increasing the percentage of Sn content, the density of the samples decreases on the Ti-Sn alloys. An increase in the Sn content from 10 to 25 wt. % content resulted in decreased hardness. The Ti-Sn binary revealed stability of the HCP phase with increasing composition of the Sn content. The porous structures of the Ti-Sn-SiC composites were evenly distributed throughout the materials. The sintered densities increase from 94.69% to 96.38%. XRD analysis detected the HCP crystal lattice structure for the Ti5.4Sn3.8SiC and Ti5.6-Sn3.8-SiC composites. XRD pattern of the Ti5.8-Sn3.8-SiC reveals both the HCP and FCC crystal structures. The HCP phase has lattice parameters a= 2.920 Å; c=4.620 Å with smaller c/a ratio of 1.589. Additionally, FCC lattice parameter a=5.620 Å Fm-3m # 225 was obtained both for Ti5.8Sn3.8SiC and Ti6.0Sn3.8SiC XRD patterns. On the other hand, Optical microscopy analysis of the Ti-Nb alloys revealed the equiaxed grains composed of the light β-phase segregating on the grain boundaries. The Ti9Nb1 has low Vickers hardness of all alloys while Ti8Nb2 and Ti7.5Nb2.5 alloys are harder due to high amount of Nb content. Generally, the densities of the Ti–Nb alloys increased with increasing Nb content. HCP and BCC phases have the lattice parameters a = 2.951 Å, c = 4.683 Å and 3.268 Å, respectively. An HCP (α′) phase was detected in the Ti8.5Nb1.5 alloy with lattice parameters a = 5.130 Å, c = 9.840 Å while a BCC phase had a = 3.287 Å. The sintered Ti8Nb2 alloy also had the α′-phase with a = 5.141 Å, c = 9.533 Å and BCC phase with a = 3.280 Å lattice parameters. On the contrary, the Ti7.5Nb2.5 alloy formed the α′-phase of a = 5.141 Å, c = 9.533 Å and BCC with a = 3.280 Å lattice parameters. For the 10 and 15 wt.% Nb alloys, very porous structures were observed. The pores appear spherical and widely distributed. As the Nb content is increased to 20 wt.% (Ti7Nb2SiC) and 25 wt.% (Ti7Nb2.5SiC), porosity was minimized. The sintered densities of the Ti-Sn alloys are decreasing from 95.90% to 92.80% with increased amount of Sn in the Ti, while the sintered densities of Ti-Sn-SiC are increasing from 94.69% to 96.38%. The high porosity, which developed in Ti7Nb1SiC and Ti7Nb2.5SiC, affected the densities of these composites. The sintered densities of Ti-Nb alloys are increasing from 92.08% to 97.65% with increased amount of Nb in the Ti. In terms of hardness Ti7Nb1SiC and Ti7Nb2.5SiC resulted in the lowest while Ti7Nb1.5SiC and Ti7Nb2SiC composites were 511.74 HV and 527.678 HV. The porosity levels were increased by the addition of SiC in the Ti-Sn-SiC and Ti-Nb-SiC composites. The XRD analysis revealed phase transformation on the Ti-Nb alloys and Ti-Nb-SiC composites.
Characterisation of Ti6Al4V lattice structures produced by direct metal laser sintering for possible biomedical applications
(Vaal University of Technology, 2022) Raselabe, Kabelo Matome; Baloyi, N.; Mendonidis, P., Prof.
The use of Ti6Al4V lattice structures as a new detergent to the stress shielding phenomenon caused during implantation has been growing. The stress shielding phenomenon is caused by the inhomogeneous transfer of load between the implant and the surrounding bone. This is mainly due to the implants containing modulus (110 GPa) being higher than that of the surrounding bone (2-20 GPa). Due to high cost and tedious process involved in manufacturing lattice structures using conventional technique, as a result of the complexity in lattice structure’s geometry, a process known as additive Manufacturing is highly recommended. Additive manufacturing processes like the use of Direct metal laser sintering (DMLS) process to create three-dimensional (3D) parts, layer by layer using computer-aided design (CAD) allows to freely build parts with geometric complexities In this dissertation, the DMLS process was used to manufacture Ti6Al4V lattice structures with Young’s modulus like that of cortical bone or within the range of 2-20 GPa. These structures were manufactured at different build orientations (Horizontal and Vertical) and strut sizes (1 and 1.5 mm). The Ti6Al4V lattice structures post-manufacturing were subjected to stress-relieving (SR) post-treatment of 650 ºC under argon atmosphere for 2 hours and 950 ºC under vacuum (VHT) atmosphere for 2 hours. The purpose of stress-relieving is to remove the residual stresses caused by the thermal gradients during manufacturing. While the vacuum heat treatment is for promoting ductility. Under these conditions (stress relieving and vacuum heat-treatment) the lattice structures were then analysed for microstructure, hardness, compression, and corrosion. For stress relieved lattice structures, DMLS's Ti6Al4V SR lattice structures possessed α' martensite and prior β grains along the build orientation. When comparing the mechanical properties of vertical and horizontal lattices, it was discovered that vertical lattice has higher hardness, compressive strength (324.1 MPa), and ductility (12%), as well as a lower Young's modulus (1.5 GPa). Vertically built lattices corroded at a faster pace than horizontally built. To change the microstructure and characteristics of Ti6Al4V lattices, vacuum heat treatment at 950 °C was performed on the SR lattices. The microstructure study demonstrated a change of α' to α+β lamellar type structure. Furthermore, the VHT enhanced ductility (18-29%) and demonstrated a Young modulus value of 2.8-3.3 GPa which is within the range for the biomedical area. The corrosion rate of horizontally built lattices was lower than that of vertically built lattices. It was then concluded that, the DMLS Ti6Al4V lattice structures yielded a Young's Modulus within the range necessary for cortical bone replacement. Which in turn mean that the Ti6Al4V lattice is more likely to be accepted in the biomedical area than fully solid/dense DMLS made Ti6Al4V, which ranges from 110 – 120 GPa. Other properties of Ti6Al4V lattice structures demonstrate that VHT lattices have higher hardness than SR lattices and that VHT lattices corrode less than SR lattices. This ultimately led to both 1 and 1,5 mm strut sizes, the Horizontally formed lattice was more corrosion resistant than the Vertically created lattice.
Studies on the mechanical properties and corrosion behaviour of AI20Be20Fe10Si15Ti35 high entropy alloy produced by mechanical alloying and SPS sintering
(Vaal University of Technology, 2022-02-22) Onawale, Omoyemi Temitope; Machaka, Ronald, Prof.; Matizamhuka, Wallace, Prof.
To meet structural, biomedical, and energy-saving application requirements, weight reduction is of utmost concern especially in the transportation sector because it is the feasible means of reducing fuel consumption as well as greenhouse gas emissions. The fabrication of light weight high entropy alloys (LWHEAs) using traditional design methods such as the addition of refractory elements or casting has proven that it is almost impossible to achieve the required properties in LWHEA. The spark plasma sintering (SPS) method was employed to consolidate wet and dry mechanically alloyed (MA) Al20Be20Fe10Si15Ti35. This is owing to the possibility of achieving desired mechanical properties and produce high-performance bulk HEAs with homogeneous and finer grains during the processing route, unlike the conventional casting techniques. The EDS, SEM and XRD examination that was conducted on both wet and dry milled alloy powders reveals a decomposition condition took place at temperatures of approx.. 2000C. We suspect that this is associated with a low melting Fe2Be17 phase, which becomes unstable with increase in temperature. This is in conformity with literature that highlights that the influence of Al and Fe on the performance of Be will depend upon which phases manifest. It has been established that volatility properties of ethanol and Fe must have aided sublimation tendency of wet milled alloy mix especially. This is in addition to evaporation tendency of Be powder. A thermo-analytical process was also conducted on both dry-milled and wet-milled alloy powder. This confirmed a decomposition leading to thermal expansion in both dry and wet milled alloy powder at a rapid rate from room temperature to ~150°C. Hence, Al20Be20Fe10Si15Ti35 milled powder is thermally unstable being the possible cause of premature sublimation that occurred during the sintering process.
The development of Light-weight High Entropy Alloy (LWHEA) composites Al35Ti35Si(20-x) Be10Bx (x=1,2,3) wt.% prepared by powder metallurgy route
(Vaal University of Technology, 2022-05-10) Dlamini, Sibongile Mabel; Machaka , Ronald; Matizamhuka, Wallace, Prof.
High entropy alloys (HEAs) are novel alloys with five or more primary elements in an equiatomic or near-equiatomic proportionate ratio. The configuration entropy in HEAs tends to stabilize the development of solid solutions like body-centred-cubic (BCC), face-centred-cubic (FCC), and hexagonal-closed-pack (HCP). Compared to traditional alloys, the increased number of primary elements present in HEAs causes severe lattice distortion, resulting in higher mechanical properties. HEAs are seen as a radical transformation for the next generation of high-temperature alloys in extreme conditions like aircraft, cutting tools, and bearings. The main objective of this dissertation was to develop new types of Al35Ti35Si(20-x)Be10Bx (x=1,2,3 wt.%) lightweight high entropy alloys using mechanical alloying and Spark plasma sintering to understand better how microstructures evolve during sintering and secondary processing, as well as the mechanical properties that can be derived. The first part of the project involved subjecting the elemental powders (aluminium, titanium, silicon, beryllium and boron) chosen for this work to mechanical alloying for 45 hours. Subsequently, applying plasma sintering to produce all the three fully densified alloy composites: Al35Ti35Si19Be10B, Al35Ti35Si18Be10B2, and Al35Ti35Si17Be10B3 at 1000 ºC with densities 3.48, 3.40 and 3.51 gꞏcm-3, respectively. The sintered alloys showed the formation of BCC and FCC solid solutions as well as ordered solid solution phases such as Ti4Si8/ Ti16Si32, Al4Ti8O2, and B2N2, with a microhardness of 957, 989, and 1093 HV, respectively. The three developed alloys also showed remarkable corrosion resistance in a 3.5 wt.% NaCl solution. Tribological characteristics of the developed Al35Ti35Si(20-x)Be10Bx (x=1,2,3 wt.%) alloys were examined under dry sliding wear conditions with stainless steel as the static friction partner under a 10 N load and a sliding duration of 60 min. The results indicated that the increase of silicon in the alloy has an impact on the friction coefficient and wear rate. High-temperature oxidation test was also conducted for Al35Ti35Si19Be10B, Al35Ti35Si18Be10B2, and Al35Ti35Si17Be10B3 alloys at 700 and 900 ºC for 400 hours and 200 hours, respectively. These alloys showed good resistance to high-temperature oxidation at 900 ºC as compared to oxidation at 700 ºC. The resistance to oxidation was indicated by low weight gain and low rate constant.

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