Masters Degrees (Chemical Engineering)

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Aerosol synthesis of ceramic particles by seed growth : analysis of process constraints
(Stellenbosch : Stellenbosch University, 2002-04) Human, Chris; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Aerosol synthesis involves the formation of condensable product species by gas-phase reaction, and the simultaneous growth of particles by coagulation. For the production of ceramic particles, reaction temperatures higher than 700 K are commonly used, and a maximum fusible particle size is observed. Coagulation-controlled growth yields spherical particles up to the maximum fusible size (approximately < 50 nm). Such particles coalesce rapidly and completely upon collision with other particles, whereas larger particles reach a meta-stable equilibrium for solid-state coalescence. Agglomerates with weak Van der Waal's bonds between particles inevitably form in the cooling/collection process. Coagulation of particles larger than the maximum fusible particle size yields agglomerates with significant neck growth between the primary particles. Spherical ceramic particles in the order of 1 J-Lm are favourable precursors for bulk electronic applications that require high purity. Such large spherical particles may possibly be produced in conditions of seed growth, which involves the deposition of small newly formed clusters onto larger existing particles. The central focus of the present work is to evaluate whether spherical ceramic particles significantly larger than the maximum fusible size may be produced by seed growth. The evaluation is done by modelling of process constraints and interpretation of published results. The modelling of constraints is based on a mathematical framework for comparison of different values of reactor design parameters. This framework comprises a simplified model system, a typology of quantities, and isolation of a set of independent design parameters. Comparison is done on the basis of fixed initial (seed) and final (product) particle sizes. The reactor design framework is used to evaluate the hypothesis on spherical seed growth, by assessing whether a reactor can be designed that satisfies all the process constraints. Future extension of the framework may allow optimisation for seed growth in general. The model system assumes laminar flow and isothermal conditions, and neglects the effect of reactor diameter on wall-deposition. The constraints are graphically represented in terms of the design parameters of initial reactant concentration and seed concentration. The effects of different temperatures and pressures on the constraints are also investigated. In a separate analysis, the suitability of turbulent flow for seed growth is assessed by calculating Brownian and turbulent collision coefficients for different colliding species. As turbulent intensity is increased, the seed coagulation rate is the first coagulation rate to be significantly enhanced by turbulence, resulting in a lowering of the maximum seed concentration allowed by the constraint for negligible seed coagulation. This tightening of a constraint by turbulence is the justification for considering only laminar flow for evaluating the hypothesis on spherical seed growth. Quantitative application of the model of constraints, as well as experimental and modelling results from the literature, did not demonstrate that significant spherical seed growth is possible without seed coagulation (agglomeration). As part of the conceptual effort in becoming familiar with aerosol reactor engineering, a simple two-mode plug-flow aerosol reactor model was developed, and verified with published results. This model has some novel value in that it translates the equations for aerosol dynamics into the terminology of reactor engineering.
Atmospheric acid leaching of oxidised and mixed copper cobalt ores mined in the Democratic Republic of the Congo
(Stellenbosch : Stellenbosch University, 2019-04) Pretorius, Petrus Johannes; Akdogan, G.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Kamoto-Oliveira-Virgule (KOV) located in the Democratic Republic of the Congo, operates its mining operation on the north-western boundary of the approximately 350km long Congolese copper belt. This deposit accounts for nearly half of all the Cu-Co resources within the DRC. The concentrator and refinery being studied was launched as a brownfields project, designed to receive concentrated copper oxides that feed the leaching circuit directly. Copper sulphide concentrate was received by the fluidized bed roasters after sulphide and oxide flotation. Current open pit operation has predominantly oxidised copper ore and mining the mineral resources from the open pit is considerably more economical than running the underground operation that produces predominantly copper sulphides. Consequently, the decision to build a Flotation Tailings Acid Leach (FTAL) plant was made which allows for the copper oxide flotation process to be eliminated completely by increasing the leach circuit capacity to process all the ore from the open pit operation, the ore will only undergo a pre-flotation process to recover the copper sulphides fraction. The justification for the FTAL plant lies with the mineral recovery gained by eliminating the copper oxide flotation circuit. The purpose of this study is to characterize the ideal leaching conditions of flotation tails for ore mined from KOV and its respective orebodies. Therefore, determining if the mining, concentration and refining operation can run a metal recovery process from mine to metal efficiently by evaluating the leach performance and characteristics of all the available resources including the ores containing dolomite and calcite. Core samples were received from FNSR, Oliveira, Virgule and Variant ore bodies in the KOV pit. The samples were crushed and milled to a desired P80 particle size followed by a 4-minute flotation step to remove the copper sulphide minerals present in the sample. The concentrate from this step was submitted to the laboratory for analysis. The flotation tailings were dried and sent for chemical analysis and labelled as the leach feed. The flotation tails were then re-pulped and vigorously agitated and leached at a controlled temperature using diluted sulphuric acid, at the desired pH for a period of 4 hours. After the leach process, the acidic slurry was filtered, and washed. The initial experimental results revealed that particle with a P80 of 75μm, 150μm and 212μm had leach recoveries of 91%, 89%, 89% respectively and average acid consumption values of 141, 132 and 128 kg/t respectively, but the sulphides fraction recovery dropped by 20% from 212μm to 150μm. The fresh acid consumption (FAC) decreased from 142kg/MT to 86kg/MT for leach tests performed at pH values of 1.0 and 2.0 respectively and leach recovery decreased from 95.4%CuOx to 93%CuOx respectively. The effect on cobalt however was much more pronounced as the total cobalt recovery dropped from 82% to 60% for pH values of 1.0 and 2.0 respectively. A change in the percentage solids in the leach slurry showed that the optimal leach conditions was at 30% solids with a copper leach recovery of 98.2%. Leaching at 30°C, 45°C and 60°C resulted in copper leach recoveries of 98.2%, 98.2% and 98.5% respectively. The leaching of cobalt was much more affected by the change in temperature, the total cobalt leach efficiency varied from 78.6% to 88.0% for tests conducted at 30°C and 60°C respectively. A very strong correlation between the contained calcium in the feed and the gangue acid consumption value was found, which would make it uncomplicated to create an advanced blending strategy if the operation would invest in online analysers placed on the conveyors that feed the stockpiles. Furthermore, the mineralogy revealed that the percentage cobalt in the ore had a profound precipitation effect on the already leached copper in solution due to the electron negativity of the Co3+ found in heterogenite. The optimised leaching conditions were used to leach a large quantity core samples to verify the results from the initial core samples.
The characterization and carbothermic reduction of furnace dust from the TKZN heavy mineral sands operation
(Stellenbosch : Stellenbosch University, 2016-12) Khesa, Makhale Elia; Akdogan, G.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Titaniferous ores serve as a major feedstock for the production of a white titanium dioxide pigment, a titanium metal and several other titanium-based products. Such ores exist mainly as heavy mineral sands. Ilmenite, which is a subset of those ores, is often upgraded to intermediate products namely synthetic rutile and titania slag through the synthetic rutile route and the reductive smelting route respectively. Like in most mineral sands operations, Tronox KwaZulu Natal Sands (Pty) Ltd recovers and disposes of a furnace dust, which is produced from the reductive smelting route, as a waste material. However, previous investigations have shown that such furnace dust can contain significant amount of titanium-bearing minerals. This study was therefore initiated to better understand the characteristics of this furnace dust and subsequently to investigate the potential of transforming such a metallurgical waste stream into a valuable stream through the production of a titania slag within the context of ilmenite smelting. The specific objectives of the study were to study the chemical and mineralogical characteristics of the furnace dust that was obtained from Tronox KwaZulu Natal Sands, to simulate the carbothermic reduction process of the furnace dust and to experimentally investigate the potential of producing a titania slag and a metallic iron from the furnace dust. The characteristics of the furnace dust were examined by several analytical techniques including x-ray fluorescence, x-ray diffraction, microscopy and laser-diffraction size analyses. The observations from the characterization work showed that the furnace dust was rich in the oxides of titanium and iron (49.39wt% 𝑇𝑖𝑂2 and 29.51wt% 𝐹𝑒2𝑂3). However, these oxides were associated with significant amount of impurities, with silica being the largest of such impurities. In order to understand the thermodynamic feasibility of producing a titania slag and a metallic iron from the furnace dust, a series of thermodynamic simulations were performed using FactSage between 1500˚C and 1700˚C and at different carbon additions. The FactSage simulations showed that more than 85wt% of the equivalent titanium dioxide content within the slag could be obtained under relatively high carbon additions and at temperatures between 1650˚C and 1700˚C. However, such strongly reducing conditions favoured a significant reduction of the oxides of titanium to metallic titanium without significantly reducing the impurity oxides from the slag phase. On the other hand, the degree of iron metallization was found to be above 90wt% at carbon additions that favoured less titanium loss from the slag phase. The simulation work also showed that the extent of reduction of 𝐹𝑒𝑂 from the slag phase was mainly influenced by the carbon addition, but less influenced by temperature. An experimental production of a titania slag and a metallic iron was investigated on a laboratory scale, PVT 18/75/350 Carbolite® vertical tube furnace at 1500˚C, 1600˚C and 1650˚C with the reduction times up to 10 minutes. From these experimental reduction tests, the slag samples with the equivalent titanium dioxide content ranging within 70.87-76.87wt% at 1600˚C and 1650˚C were obtained. Such equivalent titanium dioxide content was slightly lower than the 79wt% that was calculated in thermodynamic simulations. The 𝐹𝑒 content of the metallic iron samples that were produced ranged within 93.03-97.13wt%. However, a titania slag sample that was produced at 1500˚C for 10 minutes exhibited the equivalent titanium dioxide content of only 65.87wt%. That relatively lower content of the equivalent titanium dioxide was attributed to an insignificant removal of the impurity oxides and further insignificant removal of iron oxide from the slag as the slag was partially molten. The metallic iron sample that was produced at 1500˚C showed the 𝐹𝑒 content of 98.47wt%. The overall experimental reduction tests were in good agreement with the simulation results and both test works evidenced the potential of producing a titania slag and a metallic iron from the furnace dust. The production of both titania slag and metallic iron from this furnace dust demonstrated the potential of such material to supplement the natural titaniferous ores in order to meet the ever-increasing market demands in the white titanium dioxide pigment industry and to substantiate the feedstock into the foundry market.
Computational fluid dynamic modelling of an electric smelting furnace in the platinum recovery process
(Stellenbosch : Stellenbosch University, 2008-12) Bezuidenhout, Johan Jacobus; Eksteen, J. J.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
The electric smelting furnace is found at the heart of the platinum recovery process where the power input from the electrodes produces a complex interplay between heat transfer and fluid flow. A fundamental knowledge of the dynamic system hosted by the electric furnace is valuable for maintaining stable and optimum operation. However, describing the character of the system hosted by the electric furnace poses great difficulty due to its aggressive environment. A full-scale threedimensional Computational Fluid Dynamics (CFD) model was therefore developed for the circular, three-electrode Lonmin smelting furnace. The model was solved as time dependent to incorporate the effect of the three-phase AC current, which was supplied by means of volume sources representing the electrodes. The slag and matte layers were both modelled as fluid continuums in contact with each other through a dynamic interface made possible by the Volume of Fluid (VOF) multi-phase model. CO-gas bubbles forming at electrode surfaces and interacting with the surrounding fluid slag were modelled through the Discrete Phase Model (DPM). To account for the effect of concentrate melting, distinctive smelting zones were identified within the concentrate as assigned a portion of the melting heat based on the assumption of a radially decreasing smelting rate from the centre of the furnace. The tapping of slag and matte was neglected in the current modelling approach but compensation was made for the heating-up of descending material by means of an energy sink based on enthalpy differences. Model cases with and without CO-gas bubbles were investigated as well as the incorporation of a third phase between the slag and matte for representing the ‘mushy’ chromite/highly viscous slag commonly found in this region. These models were allowed to iterate until steady state conditions has been achieved, which for most of the cases involved several weeks of simulation time. The results that were obtained provided good insight into the electrical, heat and flow behaviour present within the molten bath. The current density profiles showed a large portion of the current to flow via the matte layer between the electrodes. Distributions for the electric potential and Joule heat within the melt was also developed and showed the highest power to be generated within the immediate vicinity of the electrodes and 98% of the resistive heat to be generated within the slag. Heat was found to be uniformly distributed due the slag layer being well mixed. The CO-gas bubbles was shown to be an important contributor to flow within the slag, resulting in a order of magnitude difference in average flow magnitude compared to the case where only natural buoyancy is at play. The highest flow activity was observed halfway between electrodes where the flow streams from the electrodes meet. Consequently, the highest temperatures are also observed in these regions. The temperature distribution within the matte and concentrate layers can be characterized as stratified. Low flow regions were identified within the matte and bottom slag layer which is where chromite and magnitite deposits are prone to accumulate. The model results were partially validated through good agreement to published results where actual measurements were done while also falling within the typical operating range for the actual furnace. The modelling of the electric furnace has been valuably furthered, however for complete confidence in the model results, further validation is strongly recommended.
Control performance assessment for a high pressure leaching process by means of fault database creation and simulation
(Stellenbosch : Stellenbosch University, 2016-03) Miskin, Jason John; Auret, Lidia; Dorfling, C.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Platinum group metal (PGM) producing companies typically extract PGMs from a nickel-copper ore through a combination of processes including comminution, flotation, smelting, converter treatment, and leaching (Dorfling, 2012; Lamya, 2007; Liddell et al., 1986). The latter processing step is a hydrometallurgical process which aims to dissolve base metals (i.e. copper and nickel sulphides) out of a converter matte and into the liquid phase by means of oxidative reaction, while limiting the dissolution of PGMs. Dorfling (2012) developed an open-loop dynamic process model within MATLAB™ comprising the second and third stage pressure leaching system and surrounding process units of Western Platinum base metal refinery (BMR). The dynamic process model was subsequently reprogrammed into Simulink™ by Haasbroek and Lindner (2015). The developed dynamic process model is a powerful tool which can be used to investigate and possibly improve several aspects of the Western Platinum BMR operation. This project aims to improve the dynamic process model to mimic the Western Platinum BMR operation, and to ultimately use the model to analyse the process performance during the occurrence of faults (i.e. abnormal events that potentially lead to failure or malfunction of equipment which causes significant process performance degradation). The updated dynamic process model will allow the possibility of developing and testing fault detection and diagnostic algorithms for Western Platinum BMR. The Simulink™ dynamic process model was firstly validated using an approach developed by Sargent (2005). This approach validates the entire model on four different levels namely conceptual model validation, computerised model verification, operational validation and data validation. A total of 34 dynamic process model issues divided into the four validation categories of Sargent (2005) were identified. It was concluded that the reaction kinetics used within the baseline dynamic process model might cause inaccurate leaching predictions. This is attributed to issues existing in both the rate expressions and the experimental data used to fit the kinetics. Most of the other issues which effect the model predictability were addressed. The dynamic process model is therefore valid for predicting general process behaviour, but invalid for exact leaching predictions. The affect which a variety of variable step-changes has on the direction of leaching behaviour is however as expected. Several control layers which exist at Western Platinum BMR were implemented on the Simulink™ open-loop dynamic process model. This includes regulatory control, supervisory control, alarm systems and safety interlock systems. The addition of control layers ensures that the dynamic process model mimics and acts in a similar manner than the actual process. A total of 35 sensors; 21 actuators; 30 regulatory controllers; 33 alarms systems; 37 safety interlocks; and 4 supervisory controllers was implemented into the open-loop dynamic process model. These control layers correspond to that which is used at Western Platinum BMR. The developed closed-loop dynamic process model is a useful tool which can be used to train operators and therefore assist in developing operator decision making. A fault database was developed which contains entries of faults which commonly occur at Western Platinum BMR. Valuable fault characteristics (Himmelblau, 1978; Isermann, 2005; Patton et al., 2013) such as transition rate, frequency of occurrence, fault type and symptoms were included for each fault present in the fault database. Faults were organised based on their point of origin (Venkatasubramanian et al., 2003). Several faults were modelled which ultimately served as a tool to perturb the process so as to assess the process performance during fault occurrences. A total of 17 faults with the necessary fault characteristics were gathered during a site visit (McCulloch et al., 2014) and composed into a fault database. This includes common faults such as valve wear, valve stiction, pump impeller wear, and controller misuse. A total of 12 faults were subsequently modelled. The fault database can serve as a means of information transfer between several Western Platinum BMR operators and personnel. The control performance was expressed in terms of control and operational key performance indicators which were calculated at several locations within the dynamic process model. The control and operational key performance indicators (Gerry, 2005; Marlin, 1995; McCulloch et al., 2014; Zevenbergen et al., 2006) include integral absolute error, maximum deviation, time not at set-point, valve reversals, valve saturation; and throughput, extent of base metal leaching, extent of PGM leaching, spillage; respectively. The process performance during the occurrence of faults was compared to a faultless baseline run. The control performance during the occurrence of 8 independent fault cases was investigated. The extent in which process performance degraded varied significantly between faults. Two faults namely pump impeller wear and solid build-up in cooling coils proved to be the faults which caused the largest process upset. This is attributed to significant autoclave pressure and temperature variations, and the activation of safety interlocks. These two faults also proved to have the least localised symptoms. This is attributed to the major effect they have early in the process which results in a propagation of symptoms. Two faults namely valve wear and level sensor blockage on the other hand caused minimal deviation in process performance while also propagating through only a few of the measured key performance indicators. These faults occur in the latter part of the process which explains their localised symptoms. The extent to which the process performance was degraded by the level sensor blockage corresponds with expert knowledge (McCulloch et al., 2014); while the model underpredicts the process performance degradation caused by valve wear. The updated closed-loop dynamic process model together with the modelled faults can be used to develop and test fault detection and diagnostic algorithms for Western Platinum BMR. Moreover, fault signatures produced in this this project could possibly be used as a baseline at Western platinum BMR in an attempt to detect and identify fault occurrences though expert interpretation.
Determination of drainage rates of heavy media for different aperture sizes on a vibrating screen
(Stellenbosch : Stellenbosch University, 2018-03) Kabondo, Leonard; Snyders, Neil; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH SUMMARY: Media losses are a significant contributor to the operational cost in a dense media separation (DMS) circuit. Of these losses, up to 80 wt% can occur on the drain and rinse vibrating screens. Although these screens are an integral part of any DMS circuit, surprisingly little work can be found in open literature regarding the effect of different screening panels on medium recovery, and particularly on ferrosilicon. Hence, considering the cost implication of heavy medium to the DMS circuit, the project focused on the recovery of medium particles. In this case, the effect of slurry density, volumetric flowrate and slot size variation were investigated. To execute the thesis, experimental works were conducted on a 0.6 x 1.2 m vibrating screen with polyurethane, rubber, and poly-wedge slot apertures at slurry density between 1.6 – 2.7 kg/L and volumetric flowrate of 18 – 26 m3/h for both magnetite and ferrosilicon. Medium drainage rates were established with and without ore material for the entire underflow stream. Samples from the feed, underflow and overflow streams were collected for particle size distribution analysis, percent moisture, medium carryover and mass balance calculations. Results obtained showed that increasing volumetric flowrate from 19.9 – 23.7 m3/h led to an increase in ferrosilicon drainage rate, percent moisture and medium carryover. However, once a critical volumetric flowrate was exceeded, a further increase in volumetric flowrate led to a decrease in drainage rate with a sharp increase in moisture and medium bypass to the oversize stream. A shift in the critical volumetric flowrate from 23.7 m3/h for fresh ferrosilicon to 24.5 m3/h for degraded material was observed. Comparable results obtained on magnetite showed different critical volumetric flowrates for different screen panels with 1x13 mm and 0.8x8.8 mm at 20.8 m3/h, 1x12 mm rubber panels and 0.63 mm poly-wedge at 21.3 m3/h, and 0.63x12 mm and 0.63x8.8 mm at 20.4 m3/h. The increase in ferrosilicon slurry density from 1.9 to 2.45 kg/L led to a gradual decrease in medium drainage rate with increase in moisture and medium bypass to the oversize stream. However, a sharp drop in the drainage rate coupled with a significant increase in moisture and medium carryover was observed at slurry density between 2.45 – 2.7 kg/L. Conversely, increase in magnetite slurry density from 1.64 to 1.84 kg/L led to a gradual decrease in medium drainage rate across all the panels tested. On the other hand, aperture size increase from 0.63x8.8 mm to 0.8x8.8 mm to 1.0x13 mm resulted in an increase in ferrosilicon drainage rate of about 1.4 - 1.9 m3/m2/h with a reduction in moisture and medium carryover of about 1.0 – 1.2 w/w% and 14.1 – 20.2 kg/t/m respectively depending on the volumetric flowrate. The increase in slot width from 0.63 to 0.8 mm led to an increase in magnetite drainage rate of about 1.0 - 1.4 m3/m2/h at volumetric flowrate between 20.4 – 22.8 m3/h. Howbeit, the increase in slot length from 8.8 to 12 mm led to an increase in magnetite drainage rate of about 0.1 - 0.5 m3/m2/h at volumetric flowrate between 19.94 – 22.8 m3/h. Increase in slurry density from 1.9 to 2.45 kg/L led to a steady decrease in the sharpness of separation with the cut sizes becoming finer postulating a rise in moisture and medium carryover to the oversize stream. Beyond 2.45 kg/L, a sharp decrease in the efficiency of separation and cut sizes with an increase in water split was observed. Increase in volumetric flowrate from 21.8 to 24.5 m3/h led to a drop in the sharpness of separation and cut size values with an upsurge in water carryover to the oversize stream. However, higher volumetric flowrate and slurry density led to a sharp decrease in the sharpness of separation and cut sizes with a marked increase in moisture bypass.
Development of a cellulose acetate hollow-fine-fibre membrane
(Stellenbosch : University of Stellenbosch, 2010-03) Tawari, Akram; Bradshaw, S. M.; Jacobs, E. P.; University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: The goal of this study is to produce cellulose acetate (CA) hollow-fine-fibre membranes with good water flux performance in the 95 – 96% salt retention range for brackish water desalination from first principles. First, the acceptable range of fibre dimensions was determined by means of a collapse pressure calculation using the elastic buckling pressure equation (thin shell assumption). Second, the pressure drop across the fibre wall in the hollow-fine fibre was determined by using the Hagen-Poiseuille equation, in order to determine how this would affect the chosen fibre dimensions. It was determined that the acceptable range of fibre dimensions was 222 – 247 m, and the wall thickness was 50 m. Fibres with these dimensions exhibited a high resistance to brackish water operating pressure of 20 – 25 bar, without collapse. The pressure drop calculations of these dimensions showed a sufficiently low pressure drop across the fibres. A dry-wet spinning technique was used for the preparation of the hollow-fine-fibre membranes. Hollow-fine fibres were spun using CA dissolved in a suitable solvent and non-solvent mixture comprising acetone and formamide. The effects of the dope composition and spinning parameters such as solvent to non-solvent ratio, bore fluid ratio, take-up speed, dope extrusion rate and heat treatment on the membrane morphology and performance were investigated. The spun fibres showed a good morphological structure, with no macrovoids (sponge-like structure), which is favourable for reverse osmosis (RO) applications. The hollow-fine-fibre membranes showed a good brackish water desalination performance within brackish water operating conditions. Statistical analysis was used to generate a fabrication formulation for producing cellulose acetate hollow-fine-fibre membrane for brackish water desalination with improved salt retention and flux. A three-level three-factor factorial was used to the study of the effect of spinning parameters (solvent to non-solvent ratio, bore fluid ratio and air gap distance). A regression equation was successfully established and was used to predictably produce membranes with good performance within the limits of the factors studied. RO performance of these hollow-fine-fibre membranes was good: The salt retention ranged from 96 to 98% and the permeate flux ranged from 60 to 46 L/m2.d (2 000 ppm, NaCl, 20 bar, 24 oC).
Development of an environmentally friendly lithium-ion battery recycling process
(Stellenbosch : Stellenbosch University, 2019-04) Musariri, Bruce; Akdogan, G.; Dorfling, C.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: The main aim of this work was to evaluate the technical feasibility of using organic acids as lixiviants for Co, Li and Ni recovery from lithium-ion batteries (LIBs) and to recover the metals from the resulting pregnant leach solution (PLS). Batch leaching tests to investigate the effects of H2O2 addition, temperature and acid concentration on metal dissolution were performed in a glass jacketed reactor with 300 ml working volume, using citric acid and DL-malic acid as lixiviants. Initial tests to investigate the effects of H2O2 addition indicated that it speeds up the leaching kinetics, hence it was included in successive leaching tests. Leaching tests were performed to investigate the effect of temperature and acid concentration on metal dissolution. Temperature levels of 30℃, 60℃ and 95℃ were used and acid concentration levels of 1 M, 1.25M and 1.5 M were used, with the H2O2 concentration and pulp density being kept constant at 2 % v/v and 20g/L, respectively. Results revealed that the performances of both acids were almost similar with over 95% metal dissolution within 30 minutes, using 1.5M citric acid and 1M DL-malic acid in the presence of 2% v/v H2O2 at 95℃ and 20g/L pulp density. After considering the cost of each acid, citric acid was selected as the more suitable lixiviant and was used in successive tests. Batch solvent extraction tests were performed, with the aim of separating Mn and Al from Co, Li and Ni in the PLS, using D2EHPA as extractant in kerosene diluent. The following variables at the given levels were investigated: D2EHPA concentration (10% v/v and 20% v/v), pH (2.5, 3.0 and 3.5) and organic/aqueous phase ratio (O/A) (1, 2, 3, 4, and 5). The best separation results were obtained using 10% v/v D2EHPA at pH 2.5 and organic phase/aqueous phase O/A ratio 5, where 94% Mn was extracted within 15 minutes, with 47% Al, 7% Co, 9% Li and 3% Ni co-extraction, in one stage. The McCabe-Thiele method was employed under the optimum conditions and it predicted that over 99% Mn can be extracted in two stages. This was verified experimentally and 99% Mn and 89% Al were extracted in two stages, with 13% Co, 17% Li and 6% Ni co-extraction. Metal precipitation tests were carried out at 50℃, 60℃, 70℃ and 80℃ using NaH2PO4 as precipitating agent. The results revealed that the solubility of Li3PO4 decreases with temperature increase, while the solubilities of Co3(PO4)2, Mn3(PO4)2 and Ni3(PO4)2 were not affected, in the investigated temperature range. Five scenarios for the recovery of metals from solution were considered and the proposed separation order in each scenario was experimentally investigated. For each scenario a flowsheet was constructed and mass balances were performed. Comparisons were made based on the mass balances, and the flowsheet in scenario four was selected as the most efficient one. It involves Mn and Al extraction from PLS using D2EHPA, followed by phosphate precipitation at 50℃ (targeting Co and Ni) and subsequent phosphate precipitation at 80℃ (targeting Li). This yields three products: a 93% pure Mn product, a Co-Ni product with 42 wt. % Co and 57 wt. % Ni and a Li product with 89 wt. % Li.
Effect of temperature, contact time and agitation speed during pre-treatment on elution of gold
(Stellenbosch : Stellenbosch University, 2015-12) Oladele, Temitope Philip; Bradshaw, S. M.; Snyders, Neil; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Please see full text for abstract
Evaluation of precipitation processes for the removal of iron from chloride-based copper and nickel leach solutions
(Stellenbosch : Stellenbosch University, 2015-12) Masambi, Saviour; Dorfling, Christie; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: A process route is being developed to recover nickel and copper from chloride leach solutions contaminated with iron. The concentrations of nickel and copper are approximately 3 g/L each, while that of iron is about 45 g/L. Iron contamination causes problems in processes typically used for the recovery of nickel and copper from leach solutions, such as solvent extraction or sulphide precipitation. This study focused on the removal of iron from the chloride-based leach solution. Iron is commonly removed from hydrometallurgical solutions by the process of precipitation. In the leach solution under investigation, iron mainly exists as iron(II) chloride. Iron(II) generally precipitates at pH above 7 while iron(III) can be precipitated at pH above 0. In this study, it was desired to oxidize iron(II) to iron(III) using oxygen gas at a temperature below 100oC and to subsequently precipitate iron(III). It was sought to produce an environmentally friendly precipitate, with minimal nickel and copper co-precipitation and easily separate the solids from the liquid. The effect of hydrochloric acid and copper concentration on the rate of iron(II) oxidation were experimentally determined. Several concentrations of hydrochloric acid, ranging from about 0.7 M to 6.4 M, were investigated while the copper concentrations investigated were 0.3 g/L and 3 g/L. The effects of temperature (40oC, 60oC, 80oC and 90oC), pH (0, 1, 2 and 3) and 30 g/L seeding on the quality and extent of iron removal were experimentally determined. The conventional hematite precipitation process was compared to the iron phosphate process in terms of iron removal, nickel and copper co-precipitation, and solid-liquid separation. The experiments were conducted in a 1.6 L glass reactor using synthetic as well as plant solutions. Synthetic solutions contained about 45 g/L iron, 3 g/L nickel and copper. Plant solutions contained significantly higher iron and nickel, with traces of copper. The concentrations of iron and nickel in plant solutions were approximately 120 g/L and 12 g/L respectively. The rate and extent of oxidation of iron(II) , using synthetic solutions, increased with both acid and copper concentrations. Experimental data and equilibrium calculations were used to prove that the mole ratio of associated acid to iron needed to be greater than or equal to 1 for rapid oxidation of iron(II) to occur. It was experimentally shown that oxidation in the presence of 3 g/L copper concentration yielded higher iron(III) compared to oxidation in the presence of 0.3 g/L copper concentration under uniform conditions. Iron precipitation from synthetic solutions was complete at all pH points investigated (0, 1, 2, 3) in both iron phosphate and hematite precipitation processes. The co-precipitation of nickel and copper increased with pH for both precipitation processes. The co-precipitation of nickel and copper in the iron phosphate process increased with an increase in temperature from 40oC while the co-precipitation of nickel and copper increased with reduction in temperature from 80oC in the hematite precipitation process. Seeded iron phosphate precipitation at pH 1 and 40oC resulted in over 99% iron removal with averages of 5% and 11% nickel and copper co-precipitation respectively. Increasing the pH to 3 resulted in complete iron removal at the expense of over 50% losses in nickel and copper. Seeded hematite precipitation at pH 1 and 80oC yielded over 99% iron removal with averages of 6.5% nickel and 7% copper losses. Upon increasing the pH to 3, nickel and copper losses were above 35%. The iron phosphate precipitation was complete within 30 – 60 minutes while hematite precipitation was complete after 60 – 120 minutes. All seeded precipitation experiments produced easily filterable precipitates. Attempts to precipitate unseeded hematite at 80oC and pH 1 resulted in higher nickel and copper losses (about 16% and 27% respectively), with the precipitates practically impossible to filter. The unseeded iron phosphate precipitates produced at 40oC and pH 1 were filterable however relatively higher losses of nickel and copper were observed (about 11% and 22% respectively). Settling experiments showed that iron phosphate precipitates completely settled within 26 minutes. Hematite precipitates did not settle after 8 h. Plant solutions were tested to validate the direct applicability of the results obtained using synthetic solutions. It was observed that complete oxidation was achieved after 180 minutes. Iron phosphate precipitation at pH 1 and 40oC achieved complete iron removal after 60 minutes and nickel losses of approximately 7.8% after 120 minutes. Hematite precipitation at pH 1 and 80oC resulted in complete iron removal after 60 min and nickel co-precipitation of 12% after 120 minutes.
The extraction of precious metals from an alkaline cyanided medium by granular activated carbon
(Stellenbosch : Stellenbosch University, 2012-12) Ngoie Mpinga, Cleophace; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: A 2 stage heap leach process to extract base and precious metals from the Platreef ore is currently being investigated industrially. A first stage bioleach is used to extract the base metals. In the 2nd stage, cyanide is used as the lixiviant at high pH to extract the platinum group metals and gold. By analogy with current gold recovery practices, the present study investigates the preferential and quantitative adsorption of precious metals (Pt, Pd, Rh and Au) over base metals (Cu, Ni and Fe) from an alkaline cyanide medium, by means of granular activated carbon. Experiments were designed statistically to optimise the process parameters using synthetic alkaline cyanide solutions close in composition to those expected from plant leach solutions. The statistical approach allowed the development of a reliable quantitative approach to express adsorption as a response variable on the basis of a number of experiments. A 2IV(7-2) fractional factorial design approach was carried out in a batch adsorption study to identify significant experimental variables along with their combined effects for the simultaneous adsorption of Pt(II), Pd(II), Rh(III) and Au(I). The adsorbent was characterized using SEM-EDX, and XRF. Precious metals adsorption efficiency was studied in terms of process recovery as a function of different adsorption parameters such as solution pH, copper, nickel, free cyanide ion, thiocyanate, initial precious metal (Pt, Pd, Rh and Au) ion and activated carbon concentrations. It was shown that adsorption rates within the first 60 minutes were very high (giving more than 90% extraction of precious metals) and thereafter the adsorption proceeds at a slower rate until pseudo-equilibrium was reached. Among the different adsorption parameters, at 95% confidence interval, nickel concentration had the most influential effect on the adsorption process followed by the adsorbent concentration. Adsorption of Ni was found to proceed at approximately the same rate and with the same recovery as the precious metals, showing a recovery of approximately 90% in two hours. The kinetics of Cu adsorption were slower, with less than 30% being recovered at the 120 minute period. This suggests that the co-adsorption of Cu can be minimised by shortening the residence time. Adsorption of Fe was found to be less than 5%, while the recovery of Rh was negligibly small. The effect of thiocyanate ion concentration was not as important as the effect of free cyanide ion concentration but still had some influence. The correlation among different adsorption parameters was studied using multivariate analysis. The optimum experimental conditions resulted in a solution with pH of 9.5, [Cu(I)] of 10 ppm, [Ni(II)] of 10 ppm, [CN ] of 132.44 ppm, [SCN ] of 98.95 ppm, [PMs] of 2.03 ppm and [AC] of 10 g/L. Under these conditions, predicted adsorption percentages of Pt, Pd and Au were approximately 98, 92 and 100%, at the level of 95% probability within two hours as an effective loading time. The negative values of ΔG° for all ions under optimum conditions indicate the feasibility and spontaneous nature of the adsorption process. Chemisorption was found to be the predominant mechanism in the adsorption process of Pt(II), Pd(II) and Au(I). Based on their distribution coefficients, the affinity of activated carbon for metal ions follows the selectivity sequence expressed below. Au(CN) > Pt(CN) > Pd(CN) > Ni(CN) > Cu(CN) Finally, it is important that additional research and development activities in the future should prove the economic viability of the process. Future work is also needed to investigate the adsorption of precious metals (PMs) by comparing the efficiencies and kinetics of adsorption when using sodium hydroxide (in this study) or lime, respectively, in order to control the pH.
Fault pattern recognition in simulated furnace data
(Stellenbosch : Stellenbosch University, 2021-03) Theunissen, Carl Daniel; Louw, Tobias M.; Bradshaw, S. M.; Auret, Lidia; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Modern submerged arc furnaces are plagued by blowbacks; hazardous occurrences where hot, toxic furnace freeboard gases are blown into the environment. While common occurrences, their causes are currently unknown, hence they cannot be predicted with mechanistic models. Data-driven models use data recorded from modern processes, like submerged arc furnaces, to recognize specific process conditions. This project aimed to identify and compare fault pattern recognition models that could be used for detecting and recognizing blowback-preceding conditions. A simple submerged arc furnace model that emulates blowbacks was developed with which to generate large volumes of data for model comparison. This submerged arc furnace model was developed from mass- and energy balances over distinct furnace zones, and yielded a large dataset with dynamic- and nonlinear characteristics. This dataset contained observations from multiple distinct operating modes, and was deemed suitable for fault pattern recognition model evaluation. A semi-supervised learning approach was selected as most suitable for recognizing blowback preceding conditions. Semi-supervised fault pattern recognition models are trained on a set of only blowback-preceding observations; this fits the typical constraints imposed by industrial datasets, where data is poorly defined and only a few observation of the target fault are labelled as such. Principal component analysis (PCA), kernel PCA and input-reconstructing neural networks called auto-encoders are established semi-supervised pattern recognition methods. One-dimensional convolutional auto-encoders are neural network architectures that effectively compress multivariate time series, but their application to on-line fault pattern recognition is relatively novel. This work applied these methods to on-line fault pattern recognition for blowback prediction, and presented algorithms for applying these methods for semi-supervised fault pattern recognition tasks. Feature engineering has the largest impact on fault pattern recognition performance, therefore feature engineering techniques were applied as part of an overall approach to data-driven fault pattern recognition. The investigation into the above fault pattern recognition models showed that kernel PCA’s superior performance over standard PCA is limited to smaller datasets, and that large datasets must be compressed significantly before kernel PCA can be applied. Consequently this investigation found linear PCA to be superior to nonlinear kernel PCA for modelling large datasets. Both auto-encoders and the developed convolutional auto-encoders outperformed linear PCA modelling, highlighting the improved fault pattern recognition capabilities of nonlinear models. This investigation found that one-dimensional convolutional auto-encoders were far more effective than the other presented models when applied to raw multivariate time series data, confirming that one-dimensional convolutional auto-encoders are effective at processing time series. However, the best performance was observed for auto-encoders models when applied to feature engineered data. This highlighted the guiding role that feature engineering should have in developing and implementing fault pattern recognition models.
Flow analysis of a four-strand steelmaking tundish using physical and numerical modelling
(Stellenbosch : Stellenbosch University, 2014-04) Cloete, Jan Hendrik; Akdogan, G.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: In modern steelmaking a tundish serves as an important metallurgical reactor to remove inclusions and maintain thermal and chemical homogeneity in the product. In this study the flow behaviour in a four strand tundish was investigated by means of a 1/2 scale water model, based on Froude number similarity, as well as by using numerical modelling. Both the numerical study and physical model were used to characterise residence time distribution (RTD) in the vessel and to calculate properties pertaining to the tundish flow regime. The three different tundish configurations investigated in this study are: a bare tundish with no flow control devices, a tundish with a turbulence inhibitor and a tundish using a turbulence inhibitor with holes in combination with dams. Preliminary investigations focussed on the framework for obtaining an accurate numerical solution within reasonable computational times. The effect of assuming symmetry and dynamically steady flow in the numerical model was shown to be small relative to the effect of grid size and justifiable by the savings in computational time. The grid independence study indicated the importance of using a finer mesh in areas of high velocity gradients to obtain realistic results and also to limit the number of computational cells. A procedure using gradient adaptation was used to refine the mesh automatically in the required regions for different tundish geometries. Results also showed that the inlet boundary of the numerical model should be selected at the ladle outlet, since assuming a flat velocity profile at the nozzle port resulted in significant changes in the RTD response. Comparison of the results obtained using the numerical model with those from physical experiments yielded an average error of less than 10%. This was assumed to be a good prediction, considering the assumptions employed in the numerical model. Both the physical and numerical models showed that a tundish without flow control devices was prone to significant short circuiting. The addition of a turbulence inhibitor was shown to be successful in preventing short circuiting and provided surface directed flow, which is thought to aid inclusion removal in the slag. Additionally, the minimum, peak and mean residence times and plug flow volume fraction were increased significantly, while the dead volume fraction decreased. However, using a turbulence inhibitor with holes in combination with dams showed that this configuration may cause increased refractory wear together with increased risk of slag entrainment due to flow patterns with increased surface turbulence. It also showed that the short-circuiting might not be eliminated completely. This indicates that certain design changes to tundish flow control systems can introduce problems that outweigh the benefits of the altered flow patterns. Furthermore, the numerical method, which was based on the water model, was modified to simulate the high temperature steel process. A very good match was obtained between the results using the two different numerical models. This serves as additional evidence that tundish water modelling based on Froude number similarity provides a good representation of the actual industrial process. Using the numerical model based on the high temperature steel process the effect of turbulence inhibitor shape was studied for four different turbulence inhibitor designs. Results showed the best performance, based on flow characteristic properties and surface turbulence values, was achieved for the design using a rectangular box-like shape with flanges at the top. However, the comparison emphasized the effect of the turbulence inhibitor shape on the flow behaviour, as each design yielded completely different flow patterns. It was also observed that a good turbulence inhibitor provided an optimum amount of turbulent suppression. Insufficient suppression would cause fast flows, which will result in insufficient residence time for inclusion flotation and high surface turbulence values, which may cause slag entrainment. On the other hand, too much suppression may increase the variation between strands.
Flowsheet development and comparison for the recovery of precious metals from cyanide leach solutions
(Stellenbosch : Stellenbosch University, 2014-04) Van Wyk, Andries Pieter; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: The Platreef ore deposit, situated in the Bushveld Igneous Complex, is one of the world’s largest platinum group metal (PGM) resources. The mineralogy of this resource is, however, unique as it consists of complex PGM mineralization with mainly copper and nickel, at very low PGM grades. The PGMs are mainly present in the ore as slow floating refractory minerals resulting in marginal process economics when processing via traditional mill-float-smelt processes. A new process is currently being investigated to extract PGMs from low grade Platreef ore and concentrate using a sequential heap leach process entailing heap bioleaching and high temperature cyanide leaching. The heap bioleach extracts the base metals in an acidic sulphate medium using a mixed culture of mesophiles and thermophiles. After heap bioleaching, the heap will be reclaimed, rinsed and restacked for high temperature cyanide leaching where the cyanide liquor is directly heated via solar energy in panels. Platinum, palladium and gold are extracted during the cyanide leaching stage and then recovered from the pregnant liquor either by adsorption onto activated carbon or ion exchange resins. Final metal recovery will proceed by techniques such as electrowinning and precipitation. In this thesis, process options for the recovery of platinum group metals from cyanide solutions were identified with different flowsheet alternatives developed utilizing these options. Simulations were made for the different processing alternatives with the objective of finding the alternative flowsheet to maximise net present value. The various processing options were simulated, combining data from concurrent experimental studies and data reported in literature with kinetic adsorption models. This was combined with economic models to arrive at an optimum design for each flowsheet alternative. Seven different processing alternatives for the recovery of platinum group metals from cyanide solutions were developed and investigated. These included two different activated carbon flowsheets as well as five different ion exchange resin flowsheets. The flowsheets differ in the elution procedures as well as the use of single or multiple resins. The well-known Merrill Crowe precipitation process was investigated but was found to yield unsatisfactory results. In each alternative, the cyanide solution is sent to a SART (sulphidization, acidification, recycling and thickening) plant to remove copper, nickel and zinc from solution prior to upgrading by means of adsorption onto activated carbon or ion exchange resins and subsequent elution. The platinum group metals are recovered from the eluate by precipitation using an autoclave, producing a solid product consisting of base and precious metals, while gold is recovered by electrowinning. It was found that the overall performance of the resin-in-solution (RIS) flowsheets were superior to that of the carbon-in-solution (CIS) flowsheets, from an overall PGM recovery perspective and product grades. The superior adsorption kinetics and high selectivity of the resins for the PGMs resulted in excellent overall plant performances, with PGM extractions in excess of 97%. Gold extraction efficiencies with resins were found to be lower than those achieved in the CIS flowsheets, mainly due to the higher selectivity of the resins for the divalent platinum and palladium cyanide complexes and poor gold elution efficiencies. The gold concentrations in the feed streams to these processes were, however, very low, at only 8.5% of the total precious metal content. The overall precious metal recoveries of the RIS flowsheets were thus higher than the CIS flowsheets due to the superior PGM extractions. From the cost analyses performed it was found that the RIS flowsheets requires lower initial capital costs, almost 28% lower than that require for the CIS flowsheets, while the operating cost requirements were found to be ±10% lower. This, combined with the high overall precious metal extractions, resulted in the RIS flowsheets to achieve higher net present values than those of the CIS flowsheets over an assumed project life of 15 years. The optimum flowsheet proposed for the recovery of precious metals from cyanide leach solutions was a RIS flowsheet option that employed the Amberlite PWA 5 resin, capable of extracting platinum, palladium and gold from solution, with elution being performed with a zinc cyanide solution. This process option had the lowest capital and operating cost requirements while achieving similar overall precious metal recoveries as the other flowsheets. Economic analysis of this process yielded the highest net present value, with a 31% increase in the overall return on investment compared to the optimal CIS flowsheet. Based on this, it was concluded that resin technology would be the best process option for recovering precious metals from cyanide leach solutions, however, additional research is required as the current level of process development is only at a concept phase.
Flux enhancement using flow destabilization in capillary membrane ultrafiltration
(Stellenbosch : Stellenbosch University, 2000-12) Botes, Jacobus Petrus; Bradshaw, S. M.; Jacobs, E. P.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: The aim of the thesis was to investigate the use of flow destabilization methods, combined with permeate backflushing (BIF) or on their own, on flux recovery and maintenance in capillary UF membrane systems under cross-flow (XF) and dead-end (DE) operating conditions. Various hydraulic and mechanical methods have been used to remove the accumulated cake layer and improve steady state process flux. Permeate backflushing (B/F) is the most widely used but the drawbacks are loss of product and extensive down-time. In a pilot plant study for ultrafiltration of surface waters containing high NOM, turbidity and cation loads, the use of flow destabilization, or feed flow reversal (FFR) combined with cross-flow B/F was able to improve the normalised flux by 10.7 ± 3.4 %, compared with 3.2 ± 1.6 % improvement for BIF without FFR. When a second B/F included FFR, the flux improvement was 7.0 ± 2.0 % compared with 4.3 ± 2.5 % for a B/F without FFR. The hypothesis was proposed that the flow destabilization caused slight lifting of the oriented cake layer, while the cross-flow B/F was able to sweep the lifted cake out of the lumen. If the flow destabilization may be effected by a simple but effective and low-cost method, and if this flow destabilization may be combined with reverse flow for short durations, the "lift-and-sweep" approach will be the ideal method of maintaining process flux and increasing membrane life. Such a flow destabilization method, now named "reversepressure pulsing" (RIP), was developed. The method involves circulation of feed water in a recycle loop for 2 s to gain momentum, followed by closure of a fast-action valve upstream of the modules. The momentum of the water in the concentrate loop carries it into an air-filled feed accumulator, while concentrate and reverse-flow permeate (which also lifts the fouling layer) are discharged to the atmosphere using the recycle pump for 15 s. When the valve opens again, the air in the accumulator forces the water under pressure through the membrane lumens, causing a pressure pulse and flow perturbations that lift, shift and break up the fouling layer. During 3 such "lift-and-sweep" events, the cake is lifted and the debris is swept out of the lumen. Experimental results for uninterrupted dead-end filtration at a UF pilot plant using RIP only on a severely fouled membrane, indicated that the RIP increased the flux by 18.4 % and decreased the dP by 8.2 % over a 7.2 h period. The method is effective in removing the cake layer intermittently and no long-term flux decline occurred for a period of 555 h since the previous chemical cleaning.
The hydrodynamic characterisation of an axial-flow membrane module
(Stellenbosch : Stellenbosch University, 2001-12) Marais, Pierre Charl; Bradshaw, S. M.; Jacobs, E. P.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: The hydrodynamics of a hollow fibre membrane module for the ultrafiltration of potable water were investigated. The purpose was to use a hydrodynamic model to predict the permeate flux for modules of various dimensions. Various models were considered, but most of them could not account for important effects such as macroscopic radial gradients and wet fibre expansion, found in hollow-fibre membrane modules. The Porous Medium Model was found to be a suitable model and it was used together with a finite element software package, Fastflo, to solve for the pressure distributions inside the membrane modules and predict permeate flux. The permeability of the membranes was obtained using a combination of numerical and experimental procedures and was found to be 2.3 x 10-13m. A cost analysis was performed to find the most economical module dimensions (outer diameter and length) for any required product flow rate. It was assumed that the cost of the fibres and module housing comprised the capital cost, while the operating cost consisted of the pumping energy. A capital recovery factor of 0.3 was used to convert capital costs to a yearly cost. It was found that the optimum module dimensions are an outer diameter of between 90mm and 160mm and a length of 0.6m. Finally the pressure distributions on the lumen and shell sides during both cross-flow filtration and backwash were examined. Shade plots proved useful for identifying possible areas of stagnant flow, as well as indicating where backwash is the most effective.
Improving the control structure of a high pressure leaching process
(Stellenbosch : Stellenbosch University, 2015-03) Knoblauch, Pieter Daniel; Bradshaw, S. M.; Dorfling, Christie; Auret, Lidia; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: The main purpose of the base metal refinery (BMR) as operated by Lonmin at their Western Platinum Ltd BMR, is to remove base metals – such as copper and nickel – from a platinum group metal (PGM) containing matte. The leaching processes in which this is done pose several challenges to the control of the process. The most significant of these is the slow dynamics of the process, due to large process units, as well as the continuously changing composition of the first stage leach residue, which is not measured on-line. This is aggravated by the fact that the exact leaching kinetics (and therefore the effect of the disturbances) are not understood well fundamentally. The slow process dynamics mean that controllers cannot be tuned aggressively, resulting in slow control action. The large residence times and off-line composition analyses of major controlled variables also mean that the effects of operator set point changes are visible only the following day, often by a different shift of operators. Dorfling (2012) recently developed a fundamental dynamic model of the pressure leach process at Lonmin‟s BMR. This dynamic model incorporates 21 chemical reactions, as well as mass and energy balances, into a system of 217 differential equations. The model provides a simulation framework within which improved control strategies can be investigated. The primary aims of this study are twofold. The first is to validate the model for the purpose of the investigation and development of control structure improvements. This is done by comparing the model to plant data, and adapting it if necessary. The second aim to reconsider the current control philosophy to the extent that is allowed by the model‟s determined validity. The current plant control philosophy aims to maintain a PGM grade of 65%, while the copper in the solids products of the second and third leaching stages should be below 25% and 3.5% by mass, respectively. Two areas of particular concern in this process that have been raised by Lonmin are the control of the temperature of the first compartment and the addition of pure sulphuric acid to control the acid concentration in the second stage leach. Dynamic plant data were used to calibrate the model, which was migrated from its received MATLAB platform to Simulink, to assist with control development. Flow rates were imported from the data, with some data values adapted for this purpose, due to mass balance inconsistencies. The outputs from the calibrated model were compared with corresponding data values. The model was found to be suitable for the investigation and development of the control structures of pressure, temperatures and inventories (termed basic regulatory control) and the acid concentration and solids fraction in the preparation tanks (termed compositional regulatory control). It was, however, found to be inadequate for the investigation and development of supervisory control, since it does not provide accurate compositional results. The leaching of copper is especially under-predicted, with the predicted copper concentration in the second stage product being approximately 46% lower than data values. The basic and compositional regulatory control structures were investigated. For each of these a base case was developed which aimed to represent the relevant current control structure, assuming optimal tuning. The variable pairings for the basic regulatory control were reconsidered using a method proposed by Luyben and Luyben (1997), since this part of the process does not permit the generation of a relative gain array (RGA) for variable pairing. The resulting pairing corresponds with Lonmin‟s current practice. Considering the temperature control of compartment 1, it was found that the addition of feed-forward control to the feedback control of the level of the flash tank improves the temperature control. More specifically, during an evaluation where the temperature‟s set point is varied up to 1%, the IAE of the temperature of compartment 1 was decreased with 7.5% from the base case, without disturbing the flash tank. The addition of feed-forward control allows for more rapid control and more aggressive tuning of this temperature, removing the current limit on ratio between the flash recycle stream and the autoclave feed. The compositional control was investigated for the second stage leach only, due to insufficient flow rate and compositional information around the third stage preparation tank. Variable pairing showed that three additive streams are available for the preparation tanks of the second and third stage leach to control the acid concentration and solids fraction in those tanks. Focussing on the second stage, the aim was to determine whether the acid concentration in the flash tank can be successfully controlled without the addition of pure acid to the tank. With four streams available around the second stage preparation tank to control its mass/level, the acid concentration and solids fraction, three manipulated variables were derived from these streams. The resulting pairings were affirmed by an RGA. Control loops for the control of acid concentration and solids fraction in the flash tank were added as cascade controllers, using the preparation tank‟s control as secondary loops. The added compositional control was evaluated in two tests. The first of these entailed the adding of typical disturbances, being the flash recycle rate, the solids and water in the feed to the second stage preparation tank and the acid concentration in copper spent electrolyte. In the second test the control system was tested for tracking an acid concentration set point. It was found that the cascade structure controls the acid concentration in the flash tank less tightly than the base case (with an IAE that is 124% and 80.6% higher for the two tests), but that it decreases the variation of solids fraction (lowering the IAE with 40.8% with the first test) in the same tank and of the temperature in the first compartment (lowering the IAE with 73.6% in the second test). It is recommended that the relative effects of these three variables on leaching behaviour should be investigated with an improved model that is proven to accurately predict leaching reactions in the autoclave.
Improving the interpretability of causality maps for fault identification
(Stellenbosch : Stellenbosch University, 2020-12) Van Zijl, Natali; Louw, Tobias M.; Bradshaw, S. M.; Auret, Lidia; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Worldwide competition forces modern mineral processing plants to operate at high productivity. This high productivity is achieved by implementing process monitoring to maintain the desired operating conditions. However, a fault originating in one section of a plant can propagate throughout the plant and so obscure its root cause. Causality analysis is a method that identifies the cause-effect relationships between process variables and presents these in a causality map which can be used to track the propagation path of a fault back to its root cause. A major obstacle to the wide acceptance of causality analysis as a tool for fault diagnosis in industry is the poor interpretability of causality maps. This study identified, proposed and assessed ways to improve the interpretability of causality maps for fault identification. All approaches were tested on a simulated case study and the resulting maps compared to a standard causality map or its transitive reduction. The ideal causality map was defined and all comparisons were performed based on its characteristics. Causality maps were produced using conditional Granger causality (GC), with a novel heuristic approach for selecting sampling period and time window. Conditional GC was found to be ill-suited to plant-wide causality analysis, due to large data requirements, poor model order selection using AIC, and inaccuracy in the presence of multiple different residence times and time delays. Methods to incorporate process knowledge to constrain connections and potential root causes were investigated and found to remove all spurious connections and decrease the pool of potential root cause variables respectively. Tools such as visually displaying node rankings on the causality map and incorporating sliders to manipulate connections and variables were also investigated. Furthermore, a novel hierarchical approach for plant-wide causality analysis was proposed, where causality maps were constructed in two subsequent stages. In the first stage, a less-detailed plant-wide map was constructed using representatives for groups of variables, and used to localise the fault to one of those groups of variables. Variables were grouped according to plant sections or modules identified in the data, and the first principal component (PC1) was used to represent each group (PS-PC1 and Mod-PC1 respectively). PS-PC1 was found to be the most promising approach, as its plant-wide map clearly identified the true root cause location, and the stage-wise application of conditional GC significantly reduced the required number of samples from 13 562 to 602. Lastly, a usability study in the form of a survey was performed to investigate the potential for industrial application of the tools and approaches presented in this study. Twenty responses were obtained, with participants consisting of Stellenbosch University final-year/postgraduate students, employees of an industrial IoT firm, and Anglo American Platinum employees. Main findings include that process knowledge is vital; grouping variables improves interpretability by decreasing the number of nodes; accuracy must be maintained during causality map simplification; and sliders add confusion by causing significant changes in the causality map. In addition, survey results found PS-PC1 to be the most user-friendly approach, further emphasizing its potential for application in industry.
Investigation of geometric properties of media particles for floating media filter
(Stellenbosch : University of Stellenbosch, 2010-12) Brika, Bashir; Bradshaw, S. M.; Jacobs, E. P.; University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: In a floating medium filter, polymeric beads with a density less than that of water form a floating bed which removes suspended material. Polyolefinic beads (polypropylene and polyethylene) are commonly used as filter media in this application. The geometric properties of the beads, and to a lesser extent the surface properties, strongly influence the performance of the filter. In the case of water treatment, the primary performance requirement is the production of a filtrate with turbidity ≤ 1.0 NTU. The influence of geometric properties on the performance of existing upflow filtration systems has not been extensively researched. The aim of this thesis was therefore to investigate the effects of floating medium granule size and shape on the performance of the floating medium filter (FMF). Towards this goal a pilot plant consisting of a dosing and flocculation unit and a clear PVC column with an inner diameter of 0.3 m and height of 2.8 m was designed and constructed, allowing the effect of media type, bed depth and filtration conditions to be investigated. Artificial feed water for use during the experimental work was made up by dissolving 250 mg/L of bentonite in tap water (≈ 60 NTU). Four median grain sizes (d50 = 2.28, 3.03, 3.30, and 4.07 mm) of polypropylene plastic granules were used. Two media shapes (cubic and disc) were evaluated. The effect of filtration rising velocity, medium depth, and coagulant chemical dosage were investigated using a complete 23 full factorial experimental design. Filter performance was evaluated in terms of filtrate turbidity and headloss development. The direction of filtration was upward in all the experiments. It was found that optimal conditions for turbidity removal were low filtration rate (36.8 L/m2· min), longer media depth (0.6 m) and optimum coagulant dose (23 mg/L). At these conditions the best medium was the one with d50 = 2.28 mm, for which a minimum turbidity of 0.4 NTU was achieved, and which was able to provide 624 L of filtrate of ˂ 1.0 NTU using a bed of 0.014 m3. For this medium headloss was 109 mm H2O at breakthrough, while the other three media showed a headloss of 42 mm H2O at breakthrough. Visual observation indicated that removal of solids took place primarily in the first 0.3 m of the floating bed in the case of the smallest medium, but that solids removal took place over the full depth of the bed for the other three media. It was found that bed depth had the strongest influence on performance for a given medium type. Experimental observation showed that coagulant dosage played an important role in floc size. A higher coagulant dosage (23 mg/L) resulted in a larger floc size which gave better performance. A lower velocity gradient was favourable for the formation of larger flocs. Some effect of media shape was noted, although it appeared that media size was dominant. It is concluded that FMF show promise for application in the water treatment. FMF, however, can be applied successfully as pre-filtration unit for treatment of high turbid water. Proper medium selection in conjunction with operating conditions can enhance performance of the filter. Smaller medium would give better turbidity removal but high headloss development and more frequent backwashing becomes necessary than with larger medium.
A mathematical modelling study of fluid flow and mixing in gas stirred ladles
(Stellenbosch : Stellenbosch University, 2008-12) Cloete, Schalk Willem Petrus; Eksteen, J. J.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
A full scale, three dimensional, transient, mathematical model was developed to simulate fluid flow and mixing in gas stirred ladles. The volume of fluid (VOF) and discrete phase (DPM) models were used in combination to account for multiphase aspects, and a slightly modified version of the standard - model was employed for turbulence modelling. The model was validated to compare well against published physical modelling results. Model results were interpreted from the fundamental grounds of kinetic energy transport within the ladle. This approach led to the specification of three key measures of mixing efficiency: The rate and efficiency of kinetic energy transfer from the buoyant gas to the bulk steel as well as the total kinetic energy holding capacity of the ladle. These measures describe the quantity of mixing in any specific ladle setup, whereas the traditional measure of mixing time reflects mixing quality, i.e. the degree of kinetic energy distribution through the entire ladle. The model was implemented in designed experiments to assess various operating and design variables pertaining to mixing quantity and quality. Considerable time was invested in finding the correct balance between numerical accuracy and computational time so that the model could be used to generate the required data within the given time frame. Experiments on the operating variables drew an important distinction between factors influencing the shape and the strength of gas induced flow patterns. Flow pattern strengthening variables, such as gas purge rate, significantly affected the quantity of mixing, but had a limited effect on mixing quality. Variables such as mass loading that influence the shape of the flow patterns had much larger potential to influence both the quantity and quality of mixing. Minimization of turbulence losses in the region of the plume eye was identified as the primary outcome of ladle design. It was shown that a taller vessel allowed more distance over which the plume could disperse, thereby reducing velocity gradients and subsequent turbulence generation at the free surface. Multiple tuyere systems yielded similar improvements by dividing the gas flow into several weakened plumes. Surface wave formation was investigated as an added mixing mechanism and demonstrated to be impractical for application in full scale gas stirred ladles. The conditions for resonance between the surface wave and the bubble plume were met only in vessels with a very low aspect ratio. Performance improvements offered by swirl in these ladles could easily be replicated in more practical ways. This study demonstrated the potential of mathematical modelling as a tool for in-depth investigation into fluid flow and mixing in the hostile environment of a full scale gas stirred ladle. Scaled-down cold models are the only alternative and can offer no more than a reasonably reliable predictive framework. The ease of flow data extraction from the numerical model also proved invaluable in facilitating a fundamental understanding of the effects of various important independent variables on ladle hydrodynamics. At this stage of development, however, the model is recommended for use on a comparative basis only. Two important developments are required for complete quantitative agreement: The inclusion of turbulence modulation by the bubbles and the increased turbulence kinetic energy dissipation rate in the vicinity of the free surface. A general strategy was developed to account for these effects and it compared favourably with published cold model results. Further research is required to generalize this approach for application in full scale gas stirred ladles.

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Browsing Masters Degrees (Chemical Engineering) by browse.metadata.advisor "Bradshaw, S. M."

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