Thesis (Ph.D.(Physics)) -- University of Limpopo, 2020 / The effects of O2, H2O and OH− and collectors are the major factors that determine the flotability behaviour of minerals. In particular, the influence of the chain length variation on xanthate collectors gives rise to increased recovery rates, and are still the most versatile collector for most minerals. This study explores the bonding behaviour, adsorption energies and electronic properties directly related to the reactivity of O2, H2O and OH−, ethyl xanthates (EX), normal propyl-xanthate (nPX), normal-butyl-xanthate (nBX–) and amyl-xanthate (AX) with the platinum antimony mineral surfaces: (100), (110) and (111) surfaces. We employed the ab-initio quantum mechanical density functional theory to investigate their adsorption and their electronic properties. In order to attain precise calculations, the cut-off energy of 500 eV was used for the bulk PtSb2, which was also transferred to the surfaces. To obtain accurate results the k point used for both the bulk and surfaces were 6x6x6 and 4x4x1, respectively. The bulk relaxation was found to give final lattice parameter of 6.531 Å. The DOS (Density Of States) indicated that both bulk and surfaces of PtSb2 had a metallic character, thereby indicating semiconducting behaviour. In cleaving the surfaces, all possible terminations were considered and the slab thickness was varied to obtain the desired stable surfaces. Their relaxed surface energies were 0.807 J.m-2, 1.077 J.m-2 and 1.074 J.m-2 for the (100), (111) and (110), respectively. These indicated that the (100) surface was the most stable and dominant plane for the platinum antimony. This fact is also observed in other minerals in general that low-index surfaces with lower surface energies indicates structural stability. The DOS showed stability with the EF (Fermi level/ Fermi energy) falling deep into the pseudo gap for all surface. The valence electrons on the surface were 5d96s1 for Pt and 5s25p3 for Sb as depicted from the Mulliken population charges and these electrons were actively involved in the hybridisation. The oxidation showed that the oxygen molecules preferred interacting with the Sb atoms than the Pt atoms for all surfaces. For the (100) surface we found that the Pt-O2peroxide adsorption site gave the strongest adsorption, while for the (110) surface we noted that the Sb2-O-O-Sb3 bridging gave the most exothermic adsorption. The case of the (111) surface showed the Sb2-O-O-Sb2 bridging to give the strongest exothermic adsorption, which dissociated and resulted in atomic bonding. Their atomic charges indicated that the oxygen molecules gain charges from the Pt and Sb atoms. In all cases, PtSb2 Bulk PtSb2 (100), (110) and (111) surfaces O2, H2O, OH-and Xanthates adsorptions the O2 interacting with Sb gained more charges, thus showing preferential adsorption to the Sb atoms. In addition, the Sb/Pt-bonded oxygens were more negative than the terminal or end-bonded oxygen atom for superoxide modes. These suggested that the 2p-orbital spin-down unoccupied orbital (LUMO) of O2 is fully occupied. The case of H2O molecules adsorptions on the three PtSb2 mineral surfaces indicated that the H2O adsorbed through van der Waals forces, in particular for multi adsorptions by physisorption process for the (100) and the (110) surfaces. However, on the (111) surface we observed chemisorption adsorption. For the (100) surface we found that the H2O-Pt was exothermic, while the H2O-Sb was endothermic and only showed exothermic from 5/8-8/8 H2O/Sb. The case of the (110) surface showed stronger adsorption of H2O on Pt than on Sb atoms, with a weaker adsorption on Sb2 atoms, while the adsorption on the (111) surface was stronger on Sb3 and weaker on Sb2 atoms. The full-coverage for the (110) surface gave –35.00 kJ/mol per H2O molecule, which is similar to the full coverage on the (100) surface (–38.19 kJ/mol per H2O molecule). Furthermore, the full monolayer adsorption on Sb2 and Sb3 for the (111) surface gave much stronger adsorption (–55.54 kJ/mol per H2O). In addition, the full-coverage on the (111) surface (i.e. on Pt1 and all Sb atoms) gave adsorption energy of –54.95 kJ/mol per H2O molecule. The adsorption of hydroxide on the surfaces showed stronger affinity than the water molecules. This suggested that they will bind preferentially over the water molecules. We also found that the OH–preferred the Sb atoms on the (100) surface, with a greater adsorption energy of –576.65 kJ/mol per OH– molecule for full-surface coverage. The (110) surface adsorption energy on full-surface coverage was –541.98 kJ/mol per OH molecule. The (111) surface full-coverage yielded adsorption energy of –579.53 kJ/mol per OH– molecule. The atomic charges related to both hydration and hydroxide adsorption showed charge depletion on both Pt/Sb and O atoms of the H2O and OH–. This suggested that there is a charge transfer into other regions within the orbitals. The adsorption of collectors on the PtSb2 surfaces to investigate their affinity with surfaces were performed considering different adsorption sites in order to find the most stable exothermic preferred site. In respect of the (100) surface, we noted that the bridging on Pt and Sb atoms by the collector involved the S atoms for all xanthates. Their adsorption energies showed that EX had strong affinity with the surface and the order was as: EX ≈ AX > nBX > nPX. In the case of the (110) surface the bridging on Pt atoms were PtSb2 Bulk PtSb2 (100), (110) and (111) surfaces O2, H2O, OH- and Xanthates adsorptions the most preferred sites for EX, nPX, nBX and AX. The order of adsorption energies was: nBX > nPX ≈ AX > EX. The (111) surface was observed to have the bridging on Sb2 and Sb3 atoms most exothermic for EX, nBX and AX, while the nPX showed the bridging on Pt1 and Sb3 atoms. The adsorption energies were found to have the nPX more stronger on the surface, with EX weaker and the order decreased as: nPX > nBX > AX > EX. This gave insights in the recovery of the minerals during flotation, that the use of EX or AX may float the platinum antimonide better based on the adsorption trends on the (100) surface, which is the most stable surface plane cleavage for platinum antimonide. The analysis of the electronic structures of the collector on the surface from density of states showed stability bonding of the collector on the surface, due to the EF falling deep into the pseudo gap for collector S atoms and surface Pt and Sb PDOS. The atomic charges computed indicated that the collectors behave as electron donors and acceptors to the Pt and Sb on the surface, respectively for the (100) surface. Interestingly for the (110) surface we observed that both surface Pt and Sb atoms lost charges, with a loss of charges on the collector S atoms. These observations suggested that the collectors S atoms offer their HOMO electrons to Pt and Sb atoms to form bond and simultaneously the Pt and Sb atoms donate their d-orbital and p-orbitals electrons to the LUMO of the collectors to form a back donation covalent bond, respectively. The (111) surface clearly showed that the surface Pt and Sb atoms lose charges to the collector S atoms, suggested a back donation covalent bonds. / National Research Foundation (NRF) and CSIR (Council for Scientific and Industrial Research) through Centre for High Performance Computing (CHPC)

No description available.

This work begins with an introduction to catalysis focusing on heterogeneous systems and surface science phenomena. A study on the partial oxidation reaction of n-butane to maleic anhydride (MA) is presented in the first part. MA supplies are barely adequate for market requirements due to continued strong demand. Only slight improvement in catalytic performance would be welcome in the industrial community. The vanadium phosphorus oxide (VPO) catalyst was used in this work. The reaction is highly exothermic and the need to properly support the catalyst, not only for good dispersion but adequate heat dissipation is of crucial importance. For this, alpha-SiC commercial powders were used in synthesizing the catalyst due to its high thermal conductivity. Up to 25% MA yields were obtained and the reaction temperature was lowered by up to 28% using SiC/VPO mixed catalysts. The second part of this work is focused on the Fischer-Tropsch synthesis (FTS) process using cobalt silica supported catalysts. The main objective is the production of synthetic ultra high purity jet fuel (JP5). This is a very timely topic given the energy issues our world is facing. Almost all aspects of the FTS process have been extensively studied, however the effects of calcination temperature and silica support structure on the catalyst performance are lacking in literature. The catalysts were prepared using various silica supports. The catalysts had different drying and calcination temperatures. It was found that lower support surface area and calcination temperature catalysts exhibited higher activity due to lower support cobalt phase interaction. Co/silica catalysts calcined at 573K showed the highest CO conversion and the lowest CH4 selectivity. Catalysts prepared with 300m²/g support surface area exhibited 79.5% C5+ selectivity due to higher reducibility and less metal support interaction. The properties and performance of various prepared catalysts in both VPO and Co/silica systems are characterized by FTIR, XRD, BET, GC and XPS techniques. Theoretical FTS deactivation by sintering calculations and SiC/VPO particle temperature gradient calculations are presented as well. Finally, conclusions and future work on improving the yield and selectivity and scaling up the bench top setups are also presented.

This master's thesis deals with the influence of the exothermic riser sleeves on the solidification time of aluminum alloys. The thesis is divided into five parts. The first chapter describes solidification of cast alloys. The second chapter deals with the designing of risers. The third chapter is focused on the types of risers. The fourth chapter describes ways of intensification of risers and chemical composition of exothermic and insulating mixtures for production of riser sleeves. The main fifth chapter focuses on experimental measurements of temperatures and solidification time of aluminum alloy in castings, risers and exothermic and insulating riser sleeves.

During exothermic reactions a significant amount of heat is released. This heat can be further utilized for heating up chemical substances, chemical reaction initiation or welding. The first part of this thesis contains survey of thermodynamics and thermodynamics and thermochemistry of exothermic reactions, overview of commonly used exothermic processes as thermites and NanoFoil, summary of intermetallic systems and possibilites of powder metallurgy as a fabrication process of exothermic powder blends. Based on the survey part is designed experimental powder blend obtained by powder metallurgy. 18 powder samples were analysed; chemical composition was obtained by XRD and EDS analysis, on scaning electron microscope the morphology of powder particles was evaluated and by differential scanning calorimetry (DSC) the temperature of exothermic intermetallic reaction was determined. Furthermore a bulk intermetallic sample was sintered by spark plasma sintering process (SPS).

There is a considerable scope for improving the temperature control of exothermic processes. In this thesis, a sub-optimal control strategy is developed through utilizing the dynamic, simulation tool. This scheme is built around easily obtained knowledge of the system and still retains flexibility. It can be applied to both exothermic batch and fed-batch processes. It consists of servo and regulatory modes, where a Generalized Predictive Controller (GPC) was used to provide self-tuning facilities. The methods outlined allow for limited thermal runaway whilst keeping some spare cooling capacity to ensure that operation at constraints are not violated. A special feature of the method proposed is that switching temperatures and temperature profiles can be readily found from plant trials whilst the addition rate profile Is capable of fairly straightforward computation. The work shows that It is unnecessary to demand stability for the whole of the exothermic reaction cycle, permitting a small runaway has resulted in a fast temperature response within the given safety margin. The Idea was employed for an exothermic single Irreversible reaction and also to a set of complex reactions. Both are carried out in a vessel with a heating/cooling coil. Two constraints are Imposed; (1) limited heat transfer area, and (11) a maximum allowable reaction temperature Tmax. The non-minimum phase problem can be considered as one of the difficulties in managing exothermic fed-batch process when cold reactant Is added to vessel at the maximum operating temperature. The control system coped with this within limits, a not unexpected result. In all cases, the new strategy out-performed the conventional controller and produced smoother variations in the manipulated variable. The simulation results showed that batch to batch variations and disturbances In cooling were successfully handled. GPC worked well but can be susceptible to measurement noise.

No / Since batch reactors are generally applied to produce a wide variety of specialty products, there is a great deal of interest to enhance batch operation to achieve high quality and purity product while minimizing the conversion of undesired by-product. The use of process optimization in the control of batch reactors presents a useful tool for operating batch reactors efficiently and optimally. In this work, we develop an approach, based on an on-line dynamic optimization strategy, to modify optimal temperature set point profile for batch reactors. Two different optimization problems concerning batch operation: maximization of product concentration and minimization of batch time, are formulated and solved using a sequential optimization approach. An Extended Kalman Filter (EKF) is incorporated into the proposed approach in order to update current states from their delayed measurement and to estimate unmeasurable state variables. A nonlinear model-based controller: generic model control algorithm (GMC) is applied to drive the temperature of the batch reactor to follow the desired profile. A batch reactor with complex exothermic reaction scheme is used to demonstrate the effectiveness of the proposed approach. The simulation results indicate that with the proposed strategy, large improvement in batch reactor performance, in term of the amount of a desired product and batch operation time, can be achieved compared to the method where the optimal temperature set point is pre-determined.

There is a considerable scope for improving the temperature control of exothermic processes. In this thesis, a sub-optimal control strategy is developed through utilizing the dynamic, simulation tool. This scheme is built around easily obtained knowledge of the system and still retains flexibility. It can be applied to both exothermic batch and fed-batch processes. It consists of servo and regulatory modes, where a Generalized Predictive Controller (GPC) was used to provide self-tuning facilities. The methods outlined allow for limited thermal runaway whilst keeping some spare cooling capacity to ensure that operation at constraints are not violated. A special feature of the method proposed is that switching temperatures and temperature profiles can be readily found from plant trials whilst the addition rate profile Is capable of fairly straightforward computation. The work shows that It is unnecessary to demand stability for the whole of the exothermic reaction cycle, permitting a small runaway has resulted in a fast temperature response within the given safety margin. The Idea was employed for an exothermic single Irreversible reaction and also to a set of complex reactions. Both are carried out in a vessel with a heating/cooling coil. Two constraints are Imposed; (1) limited heat transfer area, and (11) a maximum allowable reaction temperature Tmax. The non-minimum phase problem can be considered as one of the difficulties in managing exothermic fed-batch process when cold reactant Is added to vessel at the maximum operating temperature. The control system coped with this within limits, a not unexpected result. In all cases, the new strategy out-performed the conventional controller and produced smoother variations in the manipulated variable. The simulation results showed that batch to batch variations and disturbances In cooling were successfully handled. GPC worked well but can be susceptible to measurement noise. / Higher Education Ministry and Scientific Research

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-06-13T19:56:21Z No. of bitstreams: 1 FRANCISCO PINTO FILHO – TESE (PPGEP) 2016.pdf: 2405186 bytes, checksum: d7b5b1d68600f138fc5134159b39d6fc (MD5) / Made available in DSpace on 2018-06-13T19:56:21Z (GMT). No. of bitstreams: 1 FRANCISCO PINTO FILHO – TESE (PPGEP) 2016.pdf: 2405186 bytes, checksum: d7b5b1d68600f138fc5134159b39d6fc (MD5) Previous issue date: 2016-08-29 / Capes / A busca por inovações tecnológicas nos últimos anos cada vez mais se intensifica. Neste contexto, dois importantes grupos, os lantanídeos e as hidantoínas se destacam com importantes contribuições em diversas áreas de pesquisas apresentando vasto campo de aplicações. Nesses termos, esta pesquisa teve como objetivo sintetizar e caracterizar nanocomplexos dos íons lantanídicos: Eu +3, Er+3 e Nd+3 com 5-(4-metilfenil)-3-fenil-2-tioxo-imidazolidin-4-ona (HPA) e um segundo ligante, 1,10-fenatrolina (Phen), almejando obter complexos com propriedades biológicas. Partindo da reação entre os cloretos dos respectivos lantanídeos com os ligantes orgânicos, sob refluxo de aproximadamente 8 horas e temperatura de 60°C, foram obtidos os complexos de európio, érbio e neodímio. Os nanocomplexos em pó foram caracterizados utilizando técnicas de análises espectroscópicas, térmicas e estrutural. Os complexos de íons lantanídicos apresentaram resultados da análise elementar conforme a estequiometria proposta (1:3:1). A coordenação dos ligantes com os íons lantanídicos ocorreu através dos átomos de oxi gênio e enxofre da estrutura do 5-(4-metilfenil)-3-fenil-2-tioxo-imidazolidin-4-ona (HPA) e dos átomos de nitrogênio (C=N) da 1,10’- fenantrolina, que podem ser observados nos espectros de infravermelho, através dos deslocamentos dessas bandas presentes nos ligantes livres e coordenados. Os espectros UV-Vis apresentaram discretos deslocamentos e mudanças nas intensidades das bandas de absorção dos complexos em relação ao ligante HPA. As curvas TG/DTG do ligante HPA apresentou 3 etapas de decomposição, enquanto, que os complexos apresentaram 4 etapas. O nanocomplexo de Eu+3 apresentou menor estabilidade térmica, o que possibilita o uso destes pós como fármacos. As curvas DSC dos nanocomplexos apresentaram uma série de eventos endotérmicos e exotérmicos correspondentes a processos de decomposição, desidratação, fusão e volatilização que corroboram com as curvas termogravimétricas. Todos os complexos lantanídicos obtidos apresentaram dimensões nanométricas, e o complexo de neodímio apresentou a estrutura mais cristalina. / The search for technological innovations in recent years increasingly intensifies. In this context, two major groups, the lanthanides and hydantoins stand out with important contributions in many areas of research showing wide field of applications. In these terms, this research aims to synthesize and characterize complex of lanthanide ions: Eu+3, Ho+3, Er+3 and Nd+3 with 5-(4-methyl-phenyl)-3-phenyl-2-thioxo-imidazolidin-4-one (HPA) and a second linker, 1,10-fenatrolina (Phen), aiming to obtain complexes with biological properties. From the reaction between the chlorides of lanthanides with the respective organic ligands under reflux for approximately 8 hours and 60°C, were obtained complexes of europium, holmium, erbium and neodymium. The powder complexes were characterized using spectroscopic techniques, thermal and structural analyzes. The complexes of lanthanide ions showed elemental analysis results according to the proposed stoichiometry (1:3:1). The coordination of the ligands with lanthanide ions occurred through oxygen and sulfur atoms in the structure of 5 - (4-methylphenyl) -3-phenyl-2-thioxo-imidazolidin-4-one (HPA) and nitrogen atoms (C = N) of 1,10'- phenanthroline, which can be observed in the infrared spectra, through the displacement of these bands present in free and coordinated ligands. UV-Vis spectra showed discrete displacements and changes in the intensities of the absorption bands of the complex compared to the HPA binder. The TG/ DTG curves HPA binder 3 had decomposition steps, while the complexes showed 4 and 5 steps. The complex synthesized Er+3 showed higher thermal stability. The DSC curves of the complexes showed a series of endothermic and exothermic events corresponding to decomposition processes, dehydration, fusion and volatilization, corroborating the thermogravimetric curves. All lanthanide complexes obtained showed nanometric dimensions, and neodymium complex presented the most crystalline structure.

The master’s thesis deals with the influence of the exothermic risers on the solidification of aluminum alloys. The work is divided into three parts. The first theoretical part describes the differences of properties of cast aluminum alloys compared to other based metals, especially iron alloys. It also describes the methodology of designing risers, composition of exothermic mixtures and finally gives an overview on the simulation programs on the market, their application in foundry practice and briefly explains how to define the material in the simulation program ProCAST. The second part describes the experimental measurement of the solidification time of aluminum alloy AlSi7Mg0,3 in the sand form with exothermic riser. The final section describes the simulation of experimental measurements. Further with help of simulation software it is tried to find the general principles that have an influence on feed module rate of exothermic riser.