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Thesis (M. Sc. (Physics)) -- University of Limpopo, 2021 / Titanium is the most abundant element in the earth’s crust and can be produced as both a metal and in powder form. It finds applications in various industries such as in medical and aerospace, where the fabrication of components with excellent corrosion and high temperature performance are significant. This metal also plays a significant role in the titanium production process due to its desirable physical and chemical properties. However, this process occurs in the presence of alkali metal and alkali earth metal salt mediums. In this study, a combination of computational modelling techniques was employed to investigate the LiCl, TiCl, TiCl2 and TiCl4 systems and their interaction with titanium cluster (Ti7) at various temperatures. The density functional theory-based codes were used to study the structures and stability, while the classical force-fields codes were employed to study the temperature effect on these systems. Firstly, the LiCl model was validated using Buckingham interatomic potentials from the Catlow-library, employing the GULP code. The selected potential parameters were able to reproduce the LiCl structure to within 1% in agreement with experimental data. Furthermore, the Ti-Cl and Ti-Li interatomic potential parameters from accurate first principle calculations describe the interaction of LiCl and Ti7 cluster. The new interatomic potential parameters were deduced as Ti-Cl: 𝐷𝑒= 0.400, 𝑎0= 1.279, 𝑟0=2.680 and Ti-Li: 𝐷𝑒 =0.730, 𝑎0=1.717, 𝑟0=2.000. vi Secondly, DL_POLY code was used to characterise both the bulk LiCl and Ti7/LiCl structures employing rigid ion and shell models. It was found that the diffusion coefficient of LiCl was 6.26 nm2 /s, which corresponds to the melting temperature range of 700 K – 800 K for the rigid ion model. This agrees well with the experimental melting temperature range of 877 K – 887 K. The shell model predicts a lower melting temperature range of 600 K – 700 K at a diffusion coefficient of 3.74 nm2 /s, compared to rigid ion model. This behaviour was confirmed by the broadness of peaks on the RDF graphs at this temperature. The RDF graphs for the Ti7/LiCl structure in both rigid ion model and shell model depict a change in the morphology of the system for all interactions as the temperature is increased. It was found that the shell model is preferential for the LiCl structure. Thirdly, the elastic and mechanical properties of the TiCl, TiCl2 and TiCl4 structures were evaluated. It was found that the TiCl2 and TiCl4 structures are elastically unstable. However, the mechanical properties indicated that TiCl2 and TiCl4 are mechanically stable. The TiCln structures, namely TiCl and TiCl2, were evaluated for rigid ion model, to check the transferability of potentials. It was found that the diffusion coefficient of TiCl was 32.02 nm2 /s, which corresponds to a melting temperature of 700 K. The diffusion coefficient for TiCl2 was 115.00 nm2 /s at a melting temperature of 800 K. Lastly, molecular dynamics calculations carried out on the Ti7/TiCln structure showed that an increase in temperature results in the broadening of peaks and a decrease in the peak heights. The entropy and Gibbs formation free energy for LiCl (rigid ion and shell models), vii TiCl and TiCl2 (rigid ion model) structures were estimated to determine the influence of temperature on the structures. It was found that the LiCl (shell model) structure is stable at all temperatures and that the TiCl and TiCl2 structures are favoured at lower temperatures (< 500 K). These results provided new insight into understanding the reactions and interactions of titanium clusters with salt mediums in titanium production processes. Moreover, the findings may contribute towards developing alternative ways of titanium production in continuous and less expensive processes. / Royal Society Advanced Fellowship Newton Grant (NA140447); National Research Foundation (NRF) and Titanium Centre of Competence (TiCoC)

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A stable transferred arc was produced with plasmagas containing up to 20 percent molar TiCl$ sb4$ in argon, helium and argon/hydrogen mixtures. This was achieved by replacing the commonly-used thoriated tungsten cathode tip with a tantalum carbide tip. Thus, corrosive reactions at the cathode surface, which were shown to be the cause of the observed instability, were prevented. This allowed the characteristics of stable titanium tetrachloride plasmas in a transferred arc reactor to be investigated. / Furthermore, an investigation was conducted into the feasibility of collecting titanium metal from the dissociated TiCl$ sb4$ molecule in the plasmagas. The titanium metal was collected in a molten bath, which also served as the anode in the transferred arc system. Three anode bath compositions were used in this study. Two of them, namely titanium and zirconium, were not able to reduce recombined titanium subchlorides in the bath. The third aluminum, was a reducing bath. When aluminum was used, about 60 percent of all titanium fed into the reactor was collected. / Finally, phenomena occurring on the surface of a thoriated tungsten cathode were studied in a transferred-arc reactor using argon or helium as the plasmagas. The effect of cathode geometry on the rate and mechanisms of cathode erosion were investigated. It was shown that the surface temperature of flat-tip cathodes operating in argon is near the melting point of tungsten. On the other hand, the surface temperature of flat-tip cathodes operating in helium and pointed-tip cathodes operating in either helium or argon are near the boiling point of tungsten. Some of the material vapourized from the cathode was redeposited on the cathode surface, forming crystals whose morphology and composition depended on their distance from the arc root and the plasmagas composition.

<p dir="ltr">Borane-ammonia (BH₃-NH₃) has played an essential role in shaping and promoting the field of organic chemistry. However, we believe that the potential applications of BA in organic reductions have yet to be investigated. Our studies aimed to investigate BA as a reducing agent in organic reactions and to delve into the associated reduction mechanisms. In the second chapter of our research, we discovered that a combination of borane-ammonia and titanium tetrachloride (TiCl₄) has been explored as a versatile system for reducing various carbonyl compounds. By using BA with a small amount of TiCl₄ catalyst (10 mol%) in diethyl ether (Et₂O), we reduced different aryl and alkyl ketones into secondary alcohols at room temperature in just 30 minutes. This method is much faster than traditional uncatalyzed conditions, which usually take 24 hours or more to achieve the same reduction, and it does so without impacting other functional groups. Substituted cycloalkanones are selectively reduced to the thermodynamically favored product. Our deuterium labeling experiments found that the most probable pathway involves the hydroboration mechanism involving ketones and borane-ammonia in the presence of TiCl₄.</p><p><br></p><p dir="ltr">A slight variation in this chemical system can significantly impact the deoxyhalogenation process of aryl aldehydes, ketones, carboxylic acids, and esters. This process involves using a metal halide Lewis acid as a carbonyl activator, halogen carrier, and borane-ammonia. The selectivity of this process is determined by balancing the carbocation intermediate's stability with the Lewis acid's acidity. The choice of solvent and Lewis acid depends on the substituents present, and different substitution patterns have been explored. These principles have also been applied to selectively convert alcohols into alkyl halides. Furthermore, this system is used to selectively deoxygenate carbonyls of aldehydes and ketones into methyl and methylene hydrocarbons. The substituents on the benzene ring play a significant role in the deoxygenation process of carbonyl carbons in aldehydes and ketones.</p><p><br></p><p dir="ltr">In the third chapter of the study, various applications of the titanium system are examined. The TiCl₄/BH₃-NH₃ system was used to directly reduce a range of carboxylic acids to the corresponding alcohols at room temperature with good to excellent yields. This reduction method was achieved by adjusting the stoichiometry of borane-ammonia. This process is tolerant to various potentially reactive functional groups, such as N-protected amino acids, enabling the selective reduction of acids in the presence of amides and nitriles. Further, the titanium system was used to deoxygenation aromatic and aliphatic carboxylic esters into ethers. The ratio of borane-ammonia and catalyst controls the process. This method is the first practical borane-mediated process compatible with many sensitive functional groups and can convert challenging aromatic acid esters into ethers. Using BF₃–Et₂O as the catalyst changes the result products, reducing the esters to alcohols instead.</p><p><br></p><p dir="ltr">In the fourth chapter of our exploration, we looked at various applications of this system that involved reducing aliphatic and aromatic nitriles to primary amines. This was achieved by using 2.0 equivalents of <a href="" target="_blank">BH₃-NH₃ </a>and a molar equivalent of TiCl₄. We also found that the TiCl₄/BA system in dichloroethane (DCE) under reflux temperature efficiently reduces (deoxygenates) a range of aromatic and aliphatic primary, secondary, and tertiary carboxamides. We adjusted the catalyst and reductant stoichiometry accordingly, and the resulting amines were obtained in high yields using a simple acid-base workup.</p>

The Council for Scientific and Industrial Research (CSIR) in South Africa is developing a process to produce titanium tetrachloride from a low-grade material such as ilmenite. Titanium tetrachloride can then be used as feed material for titanium metal or pigment-grade titanium dioxide production. Titanium tetrachloride is commercially produced by chlorinating synthetic rutile (<92% TiO2) or titanium dioxide slag (<85% TiO2) at ~900 ˚C. A drawback of chlorination at this temperature is that any constituents other than TiO2 will end up as hazardous waste material. A characteristic step in the CSIR’s proposed process is to nitride titanium dioxide contained in the feed material before it is sent for chlorination. The chlorination of the resulting titanium nitride is achieved at a much lower temperature (~200 ˚C) than that of the existing titanium dioxide chlorination reaction. An added advantage of the low-temperature chlorination reaction is that chlorine is selective mostly towards titanium nitride and metallic iron, which means that any other constituents present are not likely to react with the chlorine. The result is reduced chlorine consumption and less hazardous waste produced. The nitrided ilmenite must, however, be upgraded by removing all iron before it can be sent for chlorination. Commercial ilmenite upgrading processes, called synthetic rutile production, also require the removal of iron and other transition metals before chlorination. A literature review of existing ilmenite upgrading processes revealed four possible process options that could remove iron from nitrided ilmenite. Two of these process options, the Becher and Austpac ERMS SR processes, are proven process routes. The other two are novel ideas – one to passivate iron contained in the nitrided ilmenite against chlorination and the other to use ammonium chloride (as used in the Becher process) as a stoichiometric reactant to produce a ferrous chloride solution. A preliminary experimental evaluation of these process options indicated that the Austpac ERMS SR process is the most viable option for removing iron from nitrided ilmenite. The Austpac ERMS SR process was therefore selected as a template for further process development. A detailed Austpac ERMS SR process review found that two process units in the Austpac ERMS SR process could be used in a process that separates iron from nitrided ilmenite. These are the Enhanced Acid Regeneration System and the Direct Reduced Iron process units. The review also concluded that another leach unit would have to be developed. It was therefore necessary to further investigate the dissolution of nitrided ilmenite in hydrochloric acid. A detailed experimental evaluation of nitrided ilmenite dissolution in hydrochloric acid found that hydrochloric acid could be used as the lixiviant to selectively remove iron from nitrided ilmenite. The dissolution of metallic iron in 90 ˚C hydrochloric acid reached levels of at least 96% after only 60 minutes. An average “combined resistance” rate law was found that could be used to describe this dissolution reaction. The observed activation energy and Arrhenius pre-exponential factor were found to be equal to 9.45 kJ.mol-1 and 30.8 s-1 respectively. The Austpac ERMS SR process review and experimental results described above were then combined and used to propose a process that could be employed to remove iron from nitrided ilmenite. The proposed process was modelled using the Flowsheet Simulation module in HSC Chemistry 7.0 / Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2010. / Chemical Engineering / MEng (Chemical Engineering) / unrestricted