Masters Degrees (Chemical Engineering)

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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.
Degradation of plastics in the marine environment with reference to temperature and environmental factors
(Stellenbosch : Stellenbosch University, 2020-12) Nel, Delene; Akdogan, G.; Dorfling, Christie; Chimphango, Annie F. A.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Plastic waste is an increasing problem, especially in the marine environment, where it has detrimental effects on the ecosystem . For a better comprehension of the environmental implications and eventual fate of plastic waste, research on the degradation is required. This project aimed to fill some of the knowledge gaps by completing a laboratory investigation on temperature associated plastic history, which refers to the degradation induced via temperature on plastics waste during their journey to the ocean, and the effect this has on the degradation behaviour of the plastic in the marine environment. Influences of various environments and plastic properties were also considered. Tests at various temperatures were conducted to investigate the effect of temperature associated with plastic history. The tests were conducted at the following temperatures: 25 ̊C, 100 ̊C and a weekly cycle of 25 ̊C – 60 ̊C. In these tests, three plastics, namely, black polypropylene, clear polypropylene, and clear PET, were investigated in various sizes and shapes. For example, there were large and small, circle and rectangles shapes. The temperature, size, type of plastic and colour additive was found to have important effects on the degradation rate. In the second set of tests to investigate what transpires under marine environmental conditions, samples collected after completion of the constant 25 ̊C and 100 ̊C initial tests were subjected to the following respective treatments: constant temperatures of 25 ̊C or 60 ̊C or 12-hour cycles of 65 W/m2 or 130 W/m2UV radiance – submerged in either seawater or demineralised water, respectively. The UV radiance appeared to be predominantly responsible for greater and/or accelerated degradation compared to naturally expected temperatures, especially for clear polypropylene that exhibited physically visible embrittlement under 130 W/m2 UV radiance. Nonetheless, prolonged exposure is recommended for investigating the 65 W/m2 and 25 ̊C. For both the initial and secondary tests the colour additive is suspected of hindering mechanical property degradation. The 100 ̊C initial treatment is, however, suspected of passivating the colour additive; since after the 100 ̊C treatment, the colour additive did not hinder degradation effectively. The investigation into the effect of environment indicated that under controlled conditions with identical temperatures the addition of water resulted in degradation rate increases. This suggests that the real-life phenomenon of lower degradation in the marine environment than on land could be due to water regulating the temperature. It was furthermore observed that salinity has an accelerating effect on the degradation of polypropylene. PET tended to react similarly to the salinity but the data were not conclusive enough to affirm this theory.
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.
The development of a DRI process for small scale EAF-based steel mills
(Stellenbosch : Stellenbosch University, 2010-03) Delport, Hendrikus Mattheus Wessels; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering
ENGLISH ABSTRACT: This thesis deals with the development of a new process for the production of Direct Reduced Iron (DRI), intended for use specifically by small scale Electric Arc Furnace (EAF) based steel mills, who require small volumes of DRI. The term development as used here is taken to include such aspects as conceptual design, theoretical verification and initial practical testing. The rise of EAF steelmaking brought about the metamorphosis of steel scrap from a waste product into a valuable raw material. Scrap prices rose steeply during the period 1995 to 2009 compelling EAF steelmakers, wishing to have more control over the cost of their input material, to seek for scrap supplements or alternatives. DRI has become an accepted and sought after supplement, or even complete alternative, to steel scrap. Adding DRI to an EAF charge has a range of advantages, including the dilution of tramp elements and possible cost benefits, but it does have negative effects. These include the lowering of the scrap to liquid metal yield and an increase in power consumed. The effect of charging DRI to a small EAF is quantified. The maximum DRI that may be added to the burden whilst still maintaining the present steelmaking volume, is shown to be as high as 50% if charged continuously, and the maximum price payable for DRI, is shown to be approximately 80% of base grade scrap price. Finally other requirements unique to small scale EAF operators are considered in order to prepare a schedule of requirements for a DRI plant specifically for small scale EAF steel mills. A review of published information on existing DRI production technology, processes and plants is undertaken is establish the fit of existing processes to the requirements set. Initially the thermodynamics and kinetics of iron ore reduction and coal gasification, specifically downdraft gasification are reviewed. Thereafter existing processes are reviewed. Shaft based processes and rotary kiln based processes are identified as possible suitors to the requirements. Limitations of these processes, specifically heat transfer in rotary kilns and the pressure drop over a reduction shafts are investigated. Finally a typical process in each of the main process classes is adjudicated against the set requirements. None is found to match the set requirements. A new process is proposed that is claimed to better suit to small scale operation. The uniqueness of the process is embodied in the combination of existing technologies of downdraft gasification and iron ore reduction in a shaft, in a single reactor. The process consists of two shafts, one placed above the other. Iron ore is charged into the top shaft, called the pre-heat shaft, where it is pre-heated and lightly reduced to wustite with gas from the bottom shaft, called the reduction shaft. The pre-heated ore is then charged together with coal into the reduction shaft. Gasification air is drawn into the top of the reduction shaft where the coal is gasified in a downdraft gasifier, generating reduction gas which reduces the ore as the gas moves concurrently with the iron ore. The exit gas is cleaned and pumped to the pre-heat shaft where it combusted with air to pre-heat the iron ore in the pre-heat shaft. The concept is analysed thermodynamically using amongst others, FactSage, and is shown to be thermodynamically viable. To test the concept process concept practically, an extremely small pilot plant with a production rate of 2kg DRI/h, consisting of only a gasifier/reduction shaft, was designed and constructed using reduction rate data obtained from literature supplemented with data obtained from thermogravimetric analysis of CO reduction of lump Sishen hematite. Pilot Plant trials were performed using various reductant sources. The degree of metallizaion was analysed using visual inspection of cut and polished samples compared to calibrated standards. Analysis of the results indicate that coal rate and production rate influence the degree of reduction positively and negatively. The conclusions arrived at include the fact that the process is thermodynamically viable, that it was possible to reduce iron ore in a simplified pilot plant, and that the process was found to be stable and controllable. It is recommended that a larger scale pilot plant, embodying the full proposed flow sheet be erected to test the process more completely.
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 lime additions and bulk chromium content on chromium deportment in smelter matte-slag systems
(Stellenbosch : Stellenbosch University, 2010-03) Du Preez, Rudolph C.; Akdogan, G.; Eksteen, J. J.; Georgalli, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering
ENGLISH ABSTRACT: More mining houses are turning their attention to the processing of ore mined from the UG2 reef. This is mainly due to the depletion of the Merensky reef or the low availability of ore from the Platreef. With the higher UG2 ore concentration in the process feed, companies experience more problems with the processing of the ore due to its high chrome content. Although there are various possible solutions to the problems experienced in the processing of UG2 ore, very little of thise solutions are actually implemented in the industry. When smelting chrome-bearing ore, the chrome can go to any of three phases: matte phase, spinel phase or glass phase. If it reports to the furnace matte phase it can lead to problems in the down stream processing. When chrome forms part of the spinel phase it forms a solid, refractory-like material which, to an extent, is an unwanted material inside the furnace. Lastly the chrome can report to the glass phase (liquid phase) which is the more desireable phase to which chrome should report to since it will have little or no impact on downstream processing. There were four main objectives for this research project namely to conduct a literature review to understand the problem of chrome in the smelting process, to do an experimental simulation of a matte and slag phase in one crucible, to interpret the experimental results and compare the experimental results to thermodynamic predictions obtained using FactSageTM. From literature the following aspects were found to be important: Silica structures Research has shown that silica incorporates different metal ions into its structures. When the metal ions are incorporated into the silica structures they are seen as part of the glass phase. By incorporating cations such as chromium into the silica structure the structure is able to maintain its neutral electrical charge. Basicity of slag Oxides once melted will either donate or accept oxide ions. The group of oxides that are in excess will determine whether that specific slag is acidic or non acidic. This is important to our study since the basicity can determine the stability of the spinel phase. Partial pressures The partial pressure of the system is important since it forms part of the equilibrium constant calculations - meaning that partial pressures determine the stability of certain species. Currently different arguments exist as to how the sulphur pressure inside a molten bath is maintained. What is important, however, is that for the system in this research project a log oxygen partial pressure of -8 and a log sulphur partial pressure of -3.5 was chosen. These values were based on previous research done on PGM and copper smelting processes. Chrome deportment Previous research on chrome deportment shows a relationship between temperature and the amount of chromium dissolving into the glass phase as well as an increased chromium(II)oxide solubility (when compared to chromium(III)oxide) in a silicate melt. Regarding slag chemistry and chrome deportment very little work has been published for the system found in the PGM industry but it is mentioned that by adjusting certain slag additives (alumina, lime and silica) the chrome deportment can be manipulated. The scope of this project was to investigate the effect of chromium, lime and silica on chrome deportment. More specifically, the effect of lime. The reason being that lime was originally added as 10%wt of the feed stream in the processing of Merensky ore to act as a fluxing agent. With the new furnace design and higher power densities the slag are maintained at 200oC to 350oC higher than when Merensky ore was smelted. With these higher slag temperatures fluxing agents will play a smaller role meaning that lime additions become less important. Literature studies also showed that additional lime in a slag system can stabilize the spinel structure (which is an unwanted phase). Removing lime would be advantageous out of a chrome presepective as well as an economic point of view. The research was conducted in three sections namely the determination of the time required for this particular system to reach equilibrium, the investigation of different additives on chrome deportment using a controlled atmosphere and synthetic slags and, lastly, a comparison of the experimental results obtained to thermodynamic predictions. For the equilibration studies reaction time periods of 4,7,11 and 16 hours were used. From the results it was found that the alumina crucible dissolved into the glass phase continually. This indicated that equilibrium was not reached. However, a reaction time, rather than an equilibrium time, was chosen where the species and phases had enough time to react. This was based on literature, on observations of species diffusing between the matte and slag phase as well as on two-point analysis (diffusion gradients). A reaction time of nine hours was chosen With the reaction time fixed, the effect of different slag additives on chrome deportment was investigated. For the addition of chrome it was seen that an 1.5%wt increase in the starting material increased the chrome content of the slag phase by 0.025%wt. For the same increase in chromium in the starting materials the chrome content of the spinel phase increased by 2.1% indicating that chromium has a tendency to report to the spinel phase. The increase in chromium had a minor effect on the chrome content of the matte phase, however, since the chrome content only increased from 0.025%wt to 0.028%wt. Plotting the results showed that increasing the lime in the starting material decreases the chromium content in the glass phase. For the 39%wt silica system the chromium content in the glass phase decreased from 0.75%wt to 0.46%wt for an increase in the lime content from 1.7%wt to 7.3%wt. The same trend was seen for the 33%wt silica system. For the spinel phase an increase of 1.27%wt was seen when the lime content of the starting materials was increased from 1.7%wt to 10.1%wt for a 33%wt silica system. The same increase in lime increased the chrome content of the matte phase from 0.03%wt to 0.06%wt for a 33%wt silica system. Silica also proved to affect chrome deportment. Increasing the silica content of the starting materials from 25%wt to 39%wt increased the amount of chromium in the glass phase from 0%wt to 0.46%wt for a 10%wt lime system. The same effect is seen for a lower lime content except that more chromium were incorporated into the silica structure. A silica increase from 32.4%wt to 39%wt resulted in a chromium decrease from 5.2%wt to 0% in the spinel phase for a 10%wt lime system. The same trend was seen for the 1.5%wt lime system. An increase in the silica levels lead to an increase in the chrome level of the matte phase. When silica is increased from 32%wt to 39%wt the chrome content of the matte phase increased from 0.06%wt to 0.07%wt. The last part of the research project entailed the comparison of the trends observed with the experimental results to trends obtained from thermodynamic predictions. FactSageTM is a program that uses model equations to predict the Gibbs free energies for different phases. The program is therefore also able to predict the amount of different phases present at equilibrium. This is called thermodynamic ”optimization”. In section 6 trends that were observed from FactSageTM results are compared to the trends found in the experimental results. It is important to note that it is only trends that are evaluated and not actual values since FactSageTM calculations are for a system that is at equilibrium and (as explained above) this system is not at complete equilibrium. The comparison however was good. Several trends found in the experimental results were confirmed by the results from FactSageTM. These included the relationships of chromium fed versus chromium spinel, lime fed versus chromium spinel, lime fed versus chromium in glass, silica fed versus chromium in glass, silica fed versus chromium in spinel and silica fed versus chromium in matte. To conclude, slag additions can be used to manipulate chrome deportment to an extent. Secondly, FactSageTM can be used for thermodynamic predictions but a proper understanding as well as some form of validation of the specific system investigated is still needed. Due to time constraints and the difficulty of experimentally simulating this multi-phase system in the lab, only a few parameters were investigated. In order to obtain a more complete understanding of the system the effect of partial pressures and temperature should also be investigated.
Evaluating the economics of metal recycling from end-of-life lithium ion batteries in South Africa
(Stellenbosch : Stellenbosch University, 2020-12) Smit, Mari-Alet; Dorfling, Christie; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Lithium-ion batteries (LIBs) are used in various electronic equipment as well as electric vehicles. With the rapid growth and development in technology usage, it is not surprising that the generation and safe disposal of end-of-life LIBs have become a global problem. Sustainably recycling spent LIBs will address this problem.The study aimed to investigate and compare the techno-economic feasibility of mineral acid based and organic acid based hydrometallurgical processes for metal recovery from end-of-life LIBs within a South African context. This was achieved by developing various hydrometallurgical flow sheets, completing associated mass and energy balances, calculating capital and operating costs, evaluating the profitability and performing a sensitivity analysis to investigate the influence of changing market and operating conditions on the profitability criteria. A LIB feed capacity of 868 ton per year was selected as basis for mass and energy balances. Six flow sheet alternatives using either hydrochloric or citric acid as leaching reagents were evaluated and compared.ALIB recycling facility using citric acid as leaching reagent and four selective precipitation steps for the recovery of manganese oxide, nickelhydroxide, cobaltoxalate and lithiumphosphate will be the techno-economically most favorable option returning a Net Present Value (NPV)of $ 16.4 million after 20 years.The proposed process has an estimated Capital Expenditure (CAPEX)of $ 22.8million, Operating Expenditure (OPEX)of $ 17.0millionper year and revenue of $25.5millionper year. The Present Value Ratio (PVR)of 1.8 and Discounted Cashflow Rate of Return (DCFROR)of 28.2% confirmed that profitable operation will be possible.However, if the aim of the facility is to produce only two metal products (i.e. a combined metal product that could be used in cathode material regeneration and a lithium product), the use of hydrochloric acid as leaching reagent with two subsequent precipitation steps will be most profitable and result in an NPV of $ 5.7 million.A similar flowsheet using citric acid as lixiviant may also be profitable depending on the chosen precipitant. The sensitivity analysis indicated that the profitability of the proposed facility is most sensitive to fluctuations in the feed capacity, metal selling prices and the fixed capital investment when all other parameters are kept at base values.Monte Carlo simulations evaluated the sensitivity of the profitability criteria to the random interaction between 17 variables. Depending on the simulation input specifications the probability of profitable operation ranged between 58.45% and 99.52%. It was concluded that citric acid would be a suitable alternative lixiviant for mineral acids in the LIB recycling process.Further research and experimental work should focus on in-depth process development as the current level of process integration and development is only at concept phase.Pilot-plant studies will be the best way to reduce uncertainty in mass and energy balances and to understand the technical challenges that will be faced with large-scale operation. A detailed market analysis to evaluate the current status of LIB recycling in South Africa and correspondence with key stakeholders is recommended.
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.
Feasibility of fluxless smelting of titaniferous magnetite ore in a pilot-plant open-arc DC furnace
(Stellenbosch : Stellenbosch University., 2020-03) Geldenhuys, Isabella Johanna; Akdogan, G.; Reynolds, Quinn; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Titaniferous magnetite (titanomagnetite) is nominally defined as magnetite deposits containing more than one mass percent of titanium dioxide (TiO2). Titanomagnetite deposits are known for being difficult to process; this is primarily due to the complex nature of the mineral compounds that make up the ore. The complex mineralisation means chemical upgrading processes are needed to derive economic value from the ore. While titanomagnetite deposits are widely processed for the recovery of vanadium, or vanadium and iron, titanium is not commercially recovered. The current study reviews the feasibility to extract iron and vanadium while producing a high-titanium slag via fluxless smelting in an open-arc direct current (DC) furnace. The study evaluated the technical feasibility via a pilot-scale study and furthermore considered the viability of the titania-rich furnace slag produced during the study, as a potential feedstock for the production of pigment and metal. Fluxless smelting could offer an opportunity to unlock all three valuable commodities from South Africa’s Bushveld Complex, the largest known deposit of its kind in the world. The demise of Highveld Steel and Vanadium has created an opportunity to implement the best available technology for this complex ore, leveraging the know-how and experiences of the ilmenite smelting industry in South Africa. A pilot-scale DC furnace was operated continuously for 17 days at power levels of up to 1.3 MW. During the smelting test, 108 tons of titaniferous magnetite concentrate was processed. Smelting of the concentrate involved a simple recipe which comprised of concentrate and low-ash anthracite fed to the furnace in the desired proportion to achieve the targeted metallurgical result, namely an undiluted titania-rich slag and a vanadium-bearing iron product. The test demonstrated that slag composition by virtue of the degree of reduction could be optimised for the desired specification, namely slag containing at least 85% TiO2. Due to the nature of open-arc smelting, the furnace can be operated to optimise metallurgy and is not constrained with respect to the operating power. Slag containing 89% TiO2 was consistently produced demonstrating the production of high-grade titania slag in the absence of fluxes. Phase chemically the slags were found to be dominated by the presence of an M3O5 phase (where M = Fe, Ti, Mg, Al and Mn), which is also typical of high-titania slags produced from the smelting of ilmenite. The compositional invariance observed for industrial ilmenite slags was established to also apply to the slags produced during the current study. Slags produced from titaniferous magnetite deposits will compete with higher grade titania feedstocks. The fact that the slags produced during the study resembled industrial slags is encouraging. A flowsheet was developed based on the outcomes from the pilot test, namely a mass and energy balance for fluxless smelting of titaniferous magnetite in a DC furnace. A detailed benchmarking study would be required to assess the economic viability in a highly competitive market. The results from the study supported the premise that the comprehensive processing of vanadium-bearing titaniferous magnetite deposits is technically feasible. The outcomes provide valuable insights into the behaviour of a fluxless titaniferous magnetite smelting process, presents a foundation for future work, and improves the general understanding of the requirements, challenges and benefits of fluxless smelting of titanomagnetite ores.
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.
Flow behavior, mixing and mass transfer in a Peirce-Smith converter using physical model and computational fluid dynamics
(Stellenbosch : University of Stellenbosch, 2011-03) Chibwe, Deside Kudzai; Akdogan, G.; Aldrich, C.; University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering.
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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.
Investigating the physicochemical property changes of plastic packaging material exposed to UV radiation
(Stellenbosch : Stellenbosch University, 2020-12) Conradie, Willem Johannes; Akdogan, G.; Dorfling, Christie; Chimphango, Annie F. A.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Global plastic production is increasing, and as a consequence more waste is generated and released into the environment. Oceanic weathering f actors such as ultraviolet (UV) radiation, temperature, and salinity result in the degradation of these plastics and subsequent f ormation of microplastics (MPs). These MPs in-turn pose a specific threat to ecosystems and their respective inhabitants.This study aimed to evaluate UV induced degradation of conventional packaging material made of polypropylene (PP) homopolymer and amorphous poly(ethylene terephthalate). Plastic sheets were prepared into four different shapes: small circles (6 mm dia.), large circles (12 mm d ia .), small rectangles (8x4 mm), and large rectangles (40x10 mm). Sequential degradation was considered with samples initially degraded solely by UV radiation in air. The experiments were conducted in a UV chamber that offered two levels of irradiance exposure: 65 W/m2 and 130 W/m2. After the initial degradation in air, samples were further exposed to either constant temperatures (25°C or 60°C) or cyclic UV conditions (65 W/m2 or 130 W/m2) while immersed in different aqueous solutions (demineralised water or seawater). Each experimental run commenced for six weeks, and samples were drawn and analysed fortnightly. The physicochemical properties monitored over time were mass, crystallinity, microhardness, and chemical functional groups (carbonyl and hydroxyl). These properties were measured via standard analytical techniques such as precision balance, differential scanning calorimetry (DSC), Vickers microhardness tester, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy.Results from the initial experiments indicated that UV irradiance proportionally instigated changes in plastic properties. Increased mass loss accompanied by considerable increases in carbonyl index was observed for the PPs. Shape did not significantly affect mass loss or functional group developments. Clear polypropylene (CPP) reflected the most severe degradation, resulting in the most considerable mass loss, increase in crystallinity, an d highest carbonyl content. Overall PPs degraded more than PET; differences were mainly attributed to alternative compositions, with PP having high frequencies of tertiary carbon atoms whilst PET contained stabilising aromatic rings increasing its stability towards photo-oxidative degradation. The peak wavelength sensitivity of PP also almost exactly corresponded to the peak wavelength intensity of the UV lamps used in this investigation. Furthermore, it was suspected that black polypropylene (BPP) contained a UV absorbing additive (carbon black) responsible for shielding its interior from radiation by terminating free radical reactions and converting energy to heat. Results f rom experiments performed with plastic samples immersed in aqueous solutions w ere more irregular. It was concluded that degradation occurred substantially f aster in air than in seawater. The most significant property changes in crystallinity, microhardness, and chemical functionalities were observed for material without any previous degradation history. Samples with previous histories showed more resistance to crystallinity changes. This was attributed to prior exposure weakening the material, presenting crosslinking and structural defects which inhibited polymer chains from realigning into crystalline structures. Carbonyl groups reduced f or material with previous degradation histories. This was due to the following occurrences: (i) changes in surface energy with polymer chains rearranging leaving carbonyl products concealed below the observed surface and (ii) the degraded surface layer eroding, or hydrophilic products dissolvinginto the surrounding solution medium leaving a fresh unexposed layer of plastic being analysed. Solution medium did not have a significant effect on the property changes of untreated material.
Investigating the use of printed circuit board leach residue as reductant in pyrometallurgical operations
(Stellenbosch : Stellenbosch University, 2019-04) Attah-Kyei, Desmond; Akdogan, G.; Dorfling, C.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: In recent years, there has been an increase in the generation of waste electrical and electronic equipment (WEEE) due to the advancement of technology. In addition to the environmental benefits of recycling electronic waste (e-waste), it also provides certain economic value. Printed circuit board (PCB) is the main focus of electronic waste because of the inherently high value of contained metals such as gold, silver and copper. Hydrometallurgical processes, consisting of several leaching stages, are often the most preferred option for the recovery of metals from PCB waste. However, hydrometallurgy does not address the issue of non-metallic PCB fractions that may end up being dumped at landfill sites or incinerated. It is important to reduce the environmental impact and gain value from both the metallic and non-metallic fractions of PCB waste. Several options for treatment of the non-metallic fraction including material recycling, where the residue may be used as inclusions in concrete or asphalt materials with minimal processing or chemical recycling, where chemicals and fuels are produced from the residue using techniques such as pyrolysis exist. Due to the complex composition of PCB leach residue, recovery by thermal treatment is likely to be the most feasible process route from technical and economical perspectives. In this study, the utilisation of the non-metallic PCB waste fraction as reductant in primary metal smelting operations and solid state reduction was investigated. The organic component as well as the ash composition of the PCB were characterised using Fourier transform infrared spectroscopy (FTIR) and X-ray fluorescence spectroscopy (XRF) respectively. Proximate analysis on the PCB revealed the ash and volatile matter contents being 40.1% and 44.8%, which is higher than coal used in reductive smelting operations. The elemental analysis showed carbon and oxygen content of 30.43% and 20.72% respectively. Thermodynamic modelling of chromite and iron smelting were performed using various blends of PCB and coal. The models showed that PCB residue might be used to partially replace the conventional reductants. The study revealed that in chromite smelting the optimal blend contains around 20 wt% PCB residue, with energy savings of 200 kWh/t of ore to achieve the same metal recovery. Laboratory-scale experiments simulating solid state reduction of hematite (Fe2O3) was also performed using various blends of PCB and graphitic carbon. The tests were carried out in a Differential Scanning Calorimeter (DSC) from ambient temperature to 1200℃ as well as in Single particle reactor (SPR) at 900℃ and 1000℃. The product of each test was analysed using scanning electron microscope (SEM) and X-ray powder diffraction (XRD). The degree of reduction calculated from the mass lost during the test showed that PCB acts as better reductant at lower temperatures. However, at higher temperatures the advantage shift towards carbon.
Leaching of Ni-Cu-Fe-S Peirce Smith converter matte : effects of the Fe-endpoint and leaching conditions on kinetics and mineralogy.
(Stellenbosch : Stellenbosch University, 2011-12) Van Schalkwyk, R. F.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: In a first stage atmospheric leach at the Lonmin Marikana base metals refinery, nickel-copper-iron-sulphur Peirce Smith converter matte is leached in recycled electrolyte from the electrowinning section. The electrolyte contains sulphuric acid, copper and nickel sulphates, and a small amount of iron sulphate. The converter matte contains mostly nickel, copper and sulphur (typically 48 %, 28 % and 23 %, respectively), but also minor amounts (<5 %) iron and cobalt. The matte also contains platinum group elements (PGEs) and other precious metals totalling 0.2 – 0.7 % (platinum, palladium, iridium, rhodium, ruthenium, osmium and some gold). The predominant mineral phases are heazlewoodite, chalcocite and a nickel-copper alloy phase, as well as some entrained slag and spinel minerals. The purpose of the first stage leach is to extract nickel, while simultaneously precipitating copper and PGEs contained in the recycled electrolyte. Nickel, cobalt and iron are leached by acid and oxygen. Copper is precipitated by a redox reaction in which copper ions oxidise nickel from the matte. The purpose of this study was to determine the effects of key variables on the performance of the first stage leach (specifically on the removal of PGEs and copper from solution and the overall extraction of nickel) and to improve fundamental understanding of these effects. Batch leaching tests were carried out to investigate the effects of the following factors: availability of oxygen, initial acid concentration, initial copper concentration, iron endpoint (iron content of the matte), solids/liquid ratio and stirring rate. Liquid samples were analysed with Atomic Absorption Spectroscopy (AA) to determine leaching kinetics. Characterisation of solid samples from leach tests by quantitative X-Ray diffraction (XRD) and scanning electron microscopy with an energy dispersive system (SEM-EDS) helped to improve understanding of the leaching mechanism. The oxidative leaching mechanism entails an initial period in which the alloy phase is leached by acid and oxygen, while copper reacts with the nickel-copper-alloy and heazlewoodite phases (which react galvanically with each other) to form a chalcocite precipitate. In a second reaction period, heazlewoodite was transformed to millerite by acid leaching and the particle structure became more porous. The rate of copper precipitation and nickel extraction were faster during the second reaction period than the first reaction period. Some copper leaching occurred once the leachable nickel (60 – 70 %) had been dissolved, provided that the solution was strongly acidic (pH < 2). The non-oxidative leaching mechanism entails a galvanic interaction, between the nickel-copper-alloy and heazlewoodite phases, in which nickel is leached from both phases and copper is precipitated as chalcocite. Leaching by acid was negligible in most non-oxidative tests. An initial fast period of copper precipitation was followed by a second slower period. The decrease in reaction rate can probably be linked to the decreasing availability of the nickel-copper-alloy phase. During non-oxidative leaching, the particle structure remained mostly intact. Copper precipitation kinetics under non-oxidative conditions was found to be slower than under oxidative conditions. The faster copper precipitation kinetics under oxidative conditions is most likely caused by an increase in porosity and reaction area as nickel is leached from the matte by acid and oxygen. The initial acid concentration, solids/liquid ratio and Fe-endpoint were the most important factors determining reaction kinetics under oxidative conditions. Low initial acid concentrations (37 g/L) and a high solids/liquid ratio improved the extent of copper precipitation. Nickel extraction was enhanced by low solids/liquid ratios and high initial acid concentrations (74 g/L). Nickel extraction was significantly less (56 % less in one instance) when leaching high iron mattes (5.7 % Fe) rather than low iron mattes (< 1 % Fe). Copper precipitation was initially faster when leaching a high iron matte, but slower nickel leaching from high iron mattes led to an excess of available acid, which resulted in copper being leached. The results suggest that high iron mattes will lead to poor copper and PGE precipitation in the first stage leach and also to lower nickel extractions. Consequently, Peirce Smith converting at the plant must be carefully controlled to avoid high iron mattes. Under non-oxidative conditions, the solids/liquid ratio and Fe-endpoint were the most important factors. The rate of copper precipitation was faster when a high iron matte was leached, so that a higher percentage copper was precipitated and more nickel was extracted from the matte.
Measurement of surface tension in base metal sulphide mattes by an improved sessile drop method
(Stellenbosch : Stellenbosch University, 2012-12) Hamuyuni, Joseph; Akdogan, G.; Bradshaw, S. M.; Taskinen, P.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: See item for full text
Numerical and physical modelling of tundish slag entrainment in the steelmaking process
(Stellenbosch : Stellenbosch University, 2015-12) Mabentsela, Arthur; Akdogan, G.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Extensive work has been published in the open literature concerning the flow patterns in bare tundishes and tundishes furnished with flow control devices. However, very little work has been published on the entrainment of tundish slag. The aim of this research was to study (physically and numerically) tundish slag behaviour in a bare tundish and a tundish furnished with a flow control device (FCD). Furthermore, to identify, if any, the mechanisms for tundish slag entrainment in a bare tundish and a tundish fitted with a flow control device and to make recommendations based on the findings of this study. The physical modelling was done in a 1:2 reduced scale model of an industrial tundish such as was studied by Kumar et al. (2008). In the physical model, water was used to model the steel phase while paraffin was used to model tundish slag. The numerical modelling was done in ANSYS Fluent 14.5. The Volume of Fluid (VOF) model was used to model the interface between the two phases (paraffin and water). Turbulence was modelling by making use of the Realizable k- ɛ model. Observations made whilst the model was running showed that the steel-slag interface remains immobile even in the FCD case. Furthermore entrained paraffin formed small droplets, approximately 1 mm in diameter. These droplets could be seen in small numbers throughout the tundish. Results from the physical model showed that in both the bare tundish and tundish with a FCD, areas of high concentrations of entrained slag exist near the inlet region. The entrained slag concentration decreases towards the tundish end walls. A numerical model that can be used to model tundish slag behaviour was developed using the VOF model with a mesh size of 1.4 mm at a time step of 0.125 s for the bare tundish and a tundish fitted with a FCD. The model predicted similar entrained slag patterns as those of the physical model. Analysis of flow patterns and velocity magnitudes tangential to the steel-slag interface showed that slag entrainment in both the bare tundish and tundish with a flow control device possibly takes place via two mechanisms. First the slag moves across the steel-slag interface via mass transfer, secondly small velocities tangential to the interface, 3 x 10-³ m/s for the bare tundish and 1.5 x 10-² m/s for the FCD case, at depth of greater than 10 mm below the interface carry the already “entrained” slag into the bulk steel phase. These tangential flow patterns are dominant in the inlet shroud and are to be found in both the bare tundish and in the tundish with a FCD, hence the high concentration of entrained slag in this region.
Recovery of base metals from a sulphate-based bioleach solution using commercially available chelating ion exchange resins and adsorbents
(Stellenbosch : Stellenbosch University, 2012-12) Liebenberg, Cornelius Johannes; Dorfling, C.; Akdogan, G.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Lonmin Plc. is currently investigating a hydrometallurgical process route for the recovery of base metals (BMs) and platinum group metals (PGMs) from a low grade PGM bearing ore originating from the Platreef deposit in the northern limb of the Bushveld Complex. The front-end of the flow sheet entails recovering the BM values from the ore in a heap bioleach carried out at a temperature of 65 C after which the PGMs are recovered from the solid residue of the bioleach in a second stage heap cyanide leach (Mwase et al., 2012; Mwase, 2009). Commercially available chelating ion exchange resins and chelating adsorbents, Dow M4195 (bispicolylamine functionality), Dow XUS43605 (hydroxypropylpicolylamine functionality), Amberlite IRC748 and Purolite S930 (iminodiacetic acid functionality), and Purolite S991 (mixed amine and carboxylic functionality), were investigated in this thesis for the recovery of copper, nickel and cobalt (metals of interest, or MOI) from the bioleach solution. Screening tests indicated that Dow M4195 and Dow XUS43605 were able to selectively adsorb copper to the preference of all other metals in the solution at pH 3 and 4, while the other resins only succeeded in this purpose at pH 4 in the presence of little ferric iron. Only Dow M4195 proved to be able to selectively recover nickel over other metals in the solution at pH 4. Dow M4195, Dow XUS43605 and Amberlite IRC748 were selected for further investigation. Batch kinetic and equilibrium studies were performed on these resins and they were compared on the basis of their metals uptake rate and equilibrium concentrations of the MOI. The rate of metal uptake equilibrium attainment was found to be the fastest for Dow XUS43605, followed by Amberlite IRC748 and Dow M4195. Langmuir and Freundlich isotherm models were tted to equilibrium data for copper adsorption with Dow XUS43605 and nickel adsorption with Dow M4195, and copper and nickel capacities of these two resins at pH 4 were found to be 26 g/L and g/L 30.86 g/L, respectively. Column adsorption experiments revealed that flow rate and temperature were the parameters that had the most significant effects on the copper loading achieved on Dow XUS43605 at copper breakthrough. A 36% increase in copper loading on Dow XUS43605 at copper breakthrough was observed when the temperature increased from 25 to 60 ºC, and the co-loaded nickel decreased proportionally. This increase was ascribed to the faster kinetics of copper adsorption at 60º C than at 25º C. Regarding nickel and cobalt recovery, the same trends were observed for increasing the flow rate and temperature. In addition to flow rate and temperature, an increase in initial solution pH also significantly increased metal adsorption, as would be expected. Elution studies revealed that a split elution could be performed to remove the majority of the nickel from the resin with 2 bed volumes (BV) of 20 g/L sulfuric acid to remove the majority of the co-loaded nickel, followed by 2-3 BV of 100 or 200 g/L sulfuric acid to elute the copper, thus a purer copper-rich eluate fraction could be obtained. The same was true for nickel and cobalt elution from Dow M4195. The effect of flow rate in the range of 2 to 10 BV/h did not signi cantly influence metal elution from either Dow XUS43605 or Dow M4195, whereas temperature was found to increase the rate of metal elution. Finally, two flow sheets were proposed for the recovery of the MOI. The overall recoveries of copper, nickel, cobalt and zinc for both flow sheets were 100%, but 14% nickel was lost to the copper eluate for both flow sheets, while the nickel lost to the cobalt rich effluent of the lag column was reduced from 8.3% for ow sheet option 1 to 5.6% for ow sheet option 2. By reducing the flow rate at which the process is carried out, these losses could be reduced. Also, by modifying flow sheet 2 and carrying out the copper recovery with Dow XUS43605 at a lower pH (pH 2 or 3), nickel losses to the copper eluate could be minimized as the resin's selectivity towards nickel is lower at lower solution pH values. It was further concluded that additional processing of the cobalt-rich eluate fraction of the lag column (in the lead-lag con guration of Dow M4195) is necessary to recover cobalt in a pure form.
Recovery of platinum, palladium and gold from a pregnant cyanide heap leach solution, with the use of ion exchange resins
(Stellenbosch : Stellenbosch University, 2012-12) Schoeman, Ebon; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: See item for full text

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