1,721,030 research outputs found
Water treatment technologies for removal of acid, sulphate and metals
Get PDFThesis (Ph.D. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2005.A great deal of research effort has been undertaken to find an effective solution to the problem of acid mine drainage. Indeed, South African legislation requires mining companies to respect environmental regulations by minimising water intake from local municipalities and providing a rehabilitation plan. In order for the South African mining industry to remain competitive, the proposed solutions have to be not only efficient but also economic. This is the reason for the use of a waste material being attractive for water treatment and an integrated treatment technology being developed to treat water to different quality levels. The main objectives of this study were to develop more cost-effective treatment processes specific to the needs of the mining industry in southern Africa and to investigate the technical and environmental feasibility of utilising an alkaline waste product from the local paper industry as stabilising agent for acid mine residues. All the research and development work was carried out on laboratory and pilot scale plants. Five papers, with the present author as principal contributor, will form the basis of this thesis, of which one has been published and four are being peer reviewed, presented at international conferences (locally and overseas) and published in the proceedings of the various conferences. Another five papers, with the author as co-author, have also been presented at international conferences and are being published in the proceedings of these conferences, and are included in this thesis. The results of the laboratory and pilot scale studies have been incorporated into the design and implementation of the following full-scale plants: • A limestone handling and dosing system to supply slurried limestone of constant density to the neutralisation plant was constructed and commissioned during 2001 at Navigation Section of Landau Colliery, Witbank • A limestone handling and dosing system, including a fluidised-bed limestone neutralisation plant, was constructed and commissioned during 2001, at Ticor, Empangeni • An iron(II)-oxidation and fluidised-bed limestone neutralisation plant was constructed and commissioned during 2002 at BCL, Selebi-Phikwe, Botswana • A limestone handling and dosing system, to supply slurried limestone of constant density to the neutralisation plant, was constructed and commissioned during 2003, at Kromdraai Colliery, Witbank • An iron(II)-oxidation and fluidised-bed limestone neutralisation plant was constructed and commissioned during 2004, at the Navigation Section of Landau Colliery, Witbank. These plants consist of specific units (stages) of the completely integrated process, developed by the CS1R:Environmentek over the past four years. These stages are: • Heating unit: Production of CaO (quick lime) and C02-gas from burned coal and precipitated CaC03 (limestone) • Limestone neutralisation and partial sulphate removal to a level of 1 900 mg/l • Ca(OH)2 (hydrated lime) stage: CaO contacted with the acid water to produce Ca(OH)2 • Lime treatment stage: Partial sulphate removal as CaS04 (gypsum) to below 1 200 mg/l, and full removal of magnesium and other metals • pH adjustment stage: C02 from the heating unit applied to reduce the pH to 8.6 while CaC03 precipitates • Barium sulphide treatment or biological sulphate removal treatment: Removal of sulphate to below 200 mg/l • Production (regeneration) of barium sulphide: Heating barium sulphate from the above stage • Stripping of H2S either from the barium sulphide or the biological sulphate removal processes. H2S is contacted with Fe(lll)-rich water for elemental sulphur production.Doctora
Manipulation of gasification coal feed in order to increase the ash fusion temperature of the coal to operate the gasifiers at higher temperatures
No full textThesis (M. Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2006Coal is a crucial feedstock for South Africa's unique synfuels and petrochemicals industry and used by Sasol as a feedstock to produce synthesis gas via the Sasol-Lurgi Fixed Bed Dry Bottom (FBDB) gasification process. The ash fusion temperature (AFT) gives detail information on the suitability of a coal source for gasification purposes, and specifically to the extent ash agglomeration or clinkering is likely to occur within the gasifier. Ash clinkering inside the gasifier can cause channel burning and unstable operation. Sasol-Lurgi FBDB gasifiers are currently operated with the philosophy of adding an excess of steam to the process to control the H2/CO ratio of the syngas produced, but indirectly also to control the maximum gasifier temperature below the AFT of the coal. An opportunity exists to increase the AFT of the coal fed to the gasifiers by adding AFT increasing minerals to the coal blend before it is fed into the gasification process. For the aim of this study a typical Highveld Nr. 4 coal seam was investigated, as being used by the gasification operations in Secunda. In the drying and devolatilization zone no slag formation in the coal was observed. Based on HT-XRD analysis the predominant phases in the untreated coal sample were quartz, muscovite, calcite, dolomite. hematite. anhydrite, rutile and kaolinite. Kaolinite started to decompose to metakaolinite at ±450°C with the formation of amongst others mullite at a temperature of 850°C to 1000°C. Mullite formation can also take place if free Al2O3 is present in the coal that can react with free SiO2. However, free Al2O3 is normally not present in coal and the presence of the aluminasilicate (Al2SiO5) is formed as an intermediate phase due to the decomposition of kaolinite. From 500°C to 900°C, the carbonates, calcite and dolomite, started to decompose with the formation of lime and periclase. The feldspar (CaAl2Si2O8) observed, formed as a reaction product between the SiO2, Al2O3 and Ca-containing species present in the coal. In the gasification zone slag-liquid formed at a temperature from 1000°C. The formation of anhydrite (CaSO4) took place after the formation of calcite. At 1000°C anorthite, initially present as feldspar (CaAl2Si2O8) and gehlenite (Ca2Al2SiO7) became stable, due to partial melting of the low AFT mineral phases. Anorthite and gehlenite were formed as products from anhydrite, alumina and silica at temperatures around 900°C to 1100°C. Mullite decomposed at temperatures >1100°C, while quartz and anorthite were observed up to 1350°C. Above 1350°C the whole mineral phase assemblage in the coal sample was molten. When comparing the base case sample with the Al2O3-manipulated sample, it was clear that the mullite is one mineral that showed a significant difference in formation and mechanistic behaviours. In the combustion zone the decrease in the slag-liquid content confirmed the cooling and actual mineral formation and crystallization within the gasifier combustion zone. The representative coal ash, as it was produced after gasification, showed evidence of crystallization from the melt phase and formed due to the interaction of specific mineral species to produce a molten phase that had the correct chemistry to crystallize again. Mullite formation can also take place when free Al2O3 in the coal is available that can react with free SiO2, also present in the coal. With the addition of y-Al2O3 the free SiO2 in the coal can react with the y-Al2O3 to form mullite (Al6O5(SiO4)2) directly. The Al2O3 in the reactive form acts as a network former where SiO2 can be reacted on, to form mullite. The main conclusion of the addition of y-Al2O3 to the blend is that the slag-liquid content decreased with addition, only when the temperature was greater than 1000oC, which is of importance in Me operating region where the proposed higher gasifier temperature of more than 1250°C, is aimed for. Another observation from the AFT results was that the AFT was definitely non-additive (not a linear weighted calculated average) and not the weighted average AFT as was expected for the other coal properties such as the ash content, for example. The ash slagging behaviour is a non-additive property of individual coal sources in the blend and therefore difficult to predict. Viscosity modelling can be another tool for predicting slag mineral behaviour and used as a predicting tool, as has been done in this study. A higher viscosity for all relative density fractions were observed for all temperature ranges in comparison with the results obtained from the AFT analysis. In general it can be concluded that the unique opportunity that exists to increase the AFT, was tested, proven and mechanistically outlined in this study on the coal source fed to the Sasol-Lurgi FBDB gasifiers. The AFT can be increased to > 1350°C by adding AFT increasing minerals or species, for example Al2O3 or other mineral species, to the coal blend before it is fed into the gasification process. By increasing the AFT, the direct effect will be that steam consumption can be decreased, which in tum will improve carbon utilization.Master
Treatment of industrial effluents for neutralization and sulphate removal
Get PDFThesis (Ph.D. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2006.1.1 Background: Acid mine water containing sulphate and high concentrations of dissolved heavy metals,
including iron(II), can have pH values as low as 2.5. Environmental pollution caused by such effluents are major contributors to the salinisation of receiving water, and may prove toxic to
both fauna and flora. Acid, sulphate-rich solutions are produced bacteriologically from pyrite present in waste dumps from mining and metallurgical operations and from spent sulphuric acid used in chemical or metallurgical plants. The following large mine water treatment projects are currently receiving attention in South Africa on a national level:
1. Amanzi Water Project. The Amanzi project deals with the treatment of mine water (potentially 240 Ml/d) for the recovery of potable water and by-products (e.g. gypsum). Participating mines in the project are Randfontein Estates, First Wesgold, Durban
Roodepoort Deep, Rand Leases, ERPM and Grootvlei. The pH of these waters varies
fiom 2.8 to 6.0 and the sulphate concentrations h m 600 to 3 000 mg/l (SWaMP Steering Committee, 1998).
2.Olifants Forum. Polluted mine water, estimated at a volume of 130 Ml/d, is currently discharged to water courses on the Highveld. The mine water has a pH level between 2 and 4 and contains high sulphate concentrations (> 700 mg/l) (Van Zyl, et al., 2000). Unless neutralized, such water may not be discharged into water courses. Lime is generally used for neutralization. Neutralization costs could be reduced significantly should lime be replaced with limestone. The cost of limestone is currently R130/t compared to R700/t for lime. Furthermore, increasing pressure is being exerted by the Department of Water Affairs and
Forestry to enforce sulphate removal from effluent. Extensive studies have already been carried out by the mining industry to evaluate possible sulphate removal technologies. The high cost of these technologies are considered a major obstacle. Therefore, efforts to develop a cost-effective treatment process for the recovery of re-usable water from sulphate-rich effluents, is of national
importance. 1.2 Objectives: The objectives of this investigation were to develop processes whereby acid and/or sulphate-rich water can be treated. The specific aims of the investigation were to:
1.Develop the integrated iron(II)-oxidation and limestone neutralization process where powdered limestone is used for the neutralization of iron(II)-rich acid water in a completely-mixed reactor (Chapters 3 and 4 and Patents 1 - 3).
2.Develop the biological sulphate removal process for treatment of sulphate-rich effluents (Chapters 5 and 6). 3.Develop the barium sulphide process for treatment of sulphate-rich effluents (Chapter 7). 4.Develop a water flow and chemical mass balance model to identify the most cost-effective treatment option for a water network (Chapter 8). 1.3 Findings: The following innovative processes/models were developed for neutralization and sulphate removal from industrial effluents: 1. A limestone handling and dosing system. 2. A limestone neutralization and iron(II)-oxidation process for the removal of free acid, iron and aluminium. 3. A biological sulphate removal stage which includes biological sulphate reduction, H2S-stripping and aerobic treatment for the removal of residual organic material, and calcium carbonate precipitation. The barium process, which is similar to the biological sulphate removal process, can also be used for sulphate removal.
4. Modelling of a typical water network of a mining operation. 1.3.1 Limestone neutralization: In order to develop the limestone neutralization technology to the stage of full-scale implementation it was necessary to understand its limitations, study its kinetics, develop design criteria for full-scale plants and to protect the intellectual property through patents.
1.3.1.1 The limestone neutralization process. Limestone was not used previously on a large scale for neutralization of iron(II)-rich acid water.
The reasons were: 1. The pH of iron(II)-rich water could not be raised sufficiently with limestone to rapidly allow iron(II) to be oxidized to iron(II). Rapid oxidation of iron(II) occurs only at pH 7 and higher. This can however be achieved with lime, while limestone only raises the pH
of iron(II)-rich water to pH 6. 2. The reactivity of limestone is too low to neutralize acid water completely within an acceptably short residence time when stoichiometric dosages are applied.
3. Iron(II) passivates limestone particles due to Fe(OH)3 preferentially precipitating on the surface of the limestone particles, where the pH is the highest. 1.3.1.2 Kinetics of limestone neutralization.
Stumm and Lee (1961) investigated the rate equation for biological iron(II)-oxidation and determined that it is a function of the pH, iron(II) and oxygen concentrations. This rate equation
was investigated for the case where limestone was used as the neutralization agent. Special attention was given to the effect of suspended solids concentration on the rate of iron(II)- oxidation. 1.3.1.3 Full-scale implementation of limestone neutralization For the present investigation a demonstration plant was constructed and evaluated for iron(II)-
Oxidation/limestone neutralization (Maree, et al., 2004). A plant with a capacity of 1 Ml/d was constructed at BCL, a nickel and copper mine in Botswana. Ore tailings leachate, with an acid
concentration of 10 g/l (as CaC03), was treated. Limestone, available at a cost of R150/t, was used for neutralization of the acid water. Previously, leachate with a high acid concentration was combined with less acidic streams before it was neutralized with lime. The result of this approach was that a large volume of product water was slightly over-saturated with respect to gypsum, resulting in scaling of pipelines and other equipment. The leachate was neutralized separately
from the less acidic streams. The over-saturated fraction was first allowed to crystallize from solution in the fluidized-bed reactor before being combined with the other streams. The following patents were registered, following the investigation: 1. A patent on the integrated limestone and iron(II)-oxidation process. 2. A patent for a limestone handling and dosing system was registered where powdered precipitated CaC03 was dumped onto a concrete slab, slurried to constant density with an
automatic control, and used for neutralization of the acid water.
3. A patent on an integrated limestone and lime process for the treatment of acid and sulphate-rich effluents. This allows the following: - Stage 1 : The bulk of the acid is neutralized with limestone while C02 is produced and stripped off by aeration. - Stage 2: Lime is added to allow precipitation of magnesium and other metals as well as sulphate associated with these metals. - Stage 3: The C02 that is produced in Stage 1 is used to adjust the high pH of the water from Stage 2 to 8.3. This allows CaC03 precipitation.
1.3.2 Biological sulphate removal
A biological process was developed whereby sulphate reduction to sulphide and sulphide oxidation to elemental sulphur occur in the same reactor. The following aspects were investigated: the reaction rate of biological sulphate reduction, the effect of various parameters on the reaction rate such as temperature, sulphide and sulphate concentrations and the identification
of intermediate products formed.
Pilot scale evaluation of the following stages of the biological sulphate removal process were evaluated: 1. Heating stage. Feed water to the anaerobic stage was first contacted directly with hot coal gas to raise the temperature of the water to 30 °C. 2. Anaerobic stage. A pilot plant with a capacity of 8 m³/h was operated, using ethanol or sugar as energy source.
H2S-stripping and processing stage. A laboratory unit was operated to evaluate the suitability of the following reactor types for H2S-stripping and processing: Venturi device and a packed-bed reactor.
1.3.3 Integrated Bas process for sulphate removal Laboratory studies were carried out to demonstrate that the integrated Bas-process is technically and economically viable for sulphate removal. The Bas process consists of the following stages: 1.Thermal stage where barium sulphate is reduced to barium sulphide at 1 050°C, using coal as the reductant.
2.Sulphate removal stage 3.Sulphide stripping and processing stage 4.Softening stage where limestone is precipitated. 1.3.4 Modeling The water network of a coal mine was audited and simulated by an interactive, steady state model to determine the optimum effluent treatment process configuration. The findings from this investigation were used to optimize the mine's water management strategy. Simulation of the interactions in the water network was used to show the following: (i) Powdered CaC03 can be
used as an alternative to lime for the neutralization of acid water at a cost saving. (ii) The amount of gypsum crystallization that occurred in the primary neutralization and coal processing plants. This information was needed to plan for sludge disposal. (iii) The benefits associated with separate treatment of the most polluted stream versus combined treatment of all streams
during mine water treatment. By treating the higher polluted streams separate from the lesser polluted streams, higher salt removal efficiencies are achieved. (iv) The OSI (gypsum over saturation index) value can be controlled effectively at 1 by treating the feed water to the coal
processing, for sulphate removal. The capacity of the sulphate removal plant required was determined as well as the associated capital and running costs.
1.4 Benefits The treatment approach outlined offers the following benefits: (i) The cheapest alkali, a by-product
from the paper industry, can be used for neutralization of the acid and for the removal of the bulk of the sulphate concentration through gypsum crystallization. The more advanced
biological process is then used only for removal of the remaining sulphate, to low concentrations. (ii) A robust biological process is used for sulphate removal to produce process water which is non-scaling and suitable for discharge into public streams. (iii) This is an integrated process as CO2 produced during limestone-neutralization is used for H2S-stripping in the biological stage.
The stripped H2S-gas is utilized in the limestone-neutralization stage for precipitation of iron as iron sulphide. Iron is also removed as inert Fe(OH)3 together with gypsum in the limestone neutralization stage, after oxidation.Doctora
The leaching behaviour of a Ni–Cu–Co sulphide ore in an oxidative pressure–acid medium
Get PDFThesis (MIng (Chemical Engineering))--Potchefstroom University for Christian Higher Education, 2001Hydrometallurgical processing of sulphide concentrates is an attractive method for the
selective extraction of valuable metals. The dissolution of minerals in a leaching process
involves several electrochemical parameters that need to be investigated• to ensure the
development and growth of the base metal industry in South Africa. A study has been carried out to elucidate the leaching mechanism of a nickel-coppercobalt
sulphide concentrate in an oxidative pressure-acid medium. The sulphide
concentrate studied in this research, comprises mainly of the minerals pyrrhotite,
(Fe1_xS) with x = 0 to 0.2, pentlandite, (Ni,Fe)9S8 and chalcopyrite, (CuFeS2). The
leaching behaviour of these minerals was successfully studied by means of Atomic
Absorption (AA) measurements, Scanning '•Electron Microscopy (SEM) and Moss bauer
spectroscopy, after leaching took place in an oxidative pressure-acid medium. The dissolution of the valuable metals was achieved effectively with recoveries of well
over 90% for nickel, copper and cobalt under the specific conditions studied. Mechanical activation by means of ultra fine milling improved metal extraction with an average of approximately 40%, after a leaching period of 150 minutes. The most suitable conditions for the oxidative pressure-acid leaching of the mechanically treated nickel-copper-cobalt sulphide concentrate in a dilute sulphuric acid medium were found to be: particle size 80% - 10J.Lm; temperature l10°C; oxygen partial pressure 10 bar; sulphuric acid concentration 30 kg/ton; solids content 15% by mass and an impeller
agitation rate of 800 r/min. The values of the apparent activation energies of nickel, copper and cobalt, extracted from the sulphide concentrate, were found to be 20.6 (± 4.4) kJ/mol K, 33.6 (± 4.2) kJ/mol K and 17.4 (± 3.5) kJ/mol K respectively.Master
Remediation of AMD through bentonitic clay adsorption
Get PDFMIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2016The discharge of acid mine drainage (AMD) from several abandoned mines in South-Africa has become a serious concern and environmental hazard. Since 2002 AMD has been decanting from the Number 8 shaft of the abandoned Harmony gold mine near Krugersdorp at 15-20 million litres per day. This acidic water, which contains high concentrations of metal ions in solution, has since contaminated a number of water sources in the surrounding area, including the Tweelopiespruit and several dams in the Krugersdorp game reserve. The general method for treating AMD is by neutralising its acidity by the addition of a base compound such as lime or limestone. This method has proven to be rather ineffective, however, as iron ions present in the water can cause it to re-acidify once it has been released back into the environment. The use of lime and limestone to increase the pH of AMD also has an inherent drawback as it can lead to the formation of gypsum scaling which causes blockages of pipes and equipment, incurring additional costs for AMD processes. Bentonite clay has received a lot of attention lately for its ability to adsorb a wide range of ions from solution, a trait which can be used to remove contaminants from AMD, and several AMD treatment methods and patents have been created based on this characteristic. An added advantage is that bentonite is readily available for large scale AMD treatment processes based on the number and output of bentonite mines in South Africa. A study was undertaken to determine what effect the pH level of AMD has on the ability of bentonite to remove contaminants from it. Experiments were done where AMD samples collected from the Harmony AMD treatment site were treated with bentonite clay after the pH thereof had been altered by the addition of burnt dolomite powder, calcium hydroxide or magnesium hydroxide. The theory that burnt dolomite may be able to prevent the formation of gypsum was also investigated. It was found that increased pH levels improved the removal efficiency of contaminants such as Fe, S and Al, though treatment at pH 7 is not advised as the shift from acidic to basic causes adsorption and precipitation to become erratic. Burnt dolomite powder was also found to be incapable of prompting gypsum precipitation as the magnesium content thereof precipitated too rapidly. The addition of pure magnesium hydroxide to the AMD did however decrease both Ca and SO42- concentrations, indicating that it was successful in precipitating an amount of gyspum. It is recommended that narrower pH ranges should be tested in future to further study the effects of pH on AMD treatment. A method to increase bentonite’s adsorption capacity per weight unit by the removal of the clay’s silica content should also be investigated. Polymers created by mixing bentonite clay with compounds such as Ca(OH)2 and CaCO3, as discussed by Ntwampe et al. (2015) also deserves further study as it shows promise for AMD treatment.Master
Design of a primary off-gas scrubber for a ferro-manganese electric arc furnace process
Get PDFMSc (Chemical Engineering), North-West University, Potchefstroom Campus, 2015An electric arc furnace makes use of electrical energy in the form of an arc to heat charged material. In the
ferromanganese smelting process the ferric oxide (Fe2O3) and manganese oxide (MnO2) are reduced with coke. The basic reaction that takes place is described in the following equation Fe203 + 2Mn02+ 7C = 2FeMn + 7C0 The high quantity of carbon monoxide (CO) produced in the process, which has a significantly high calorific value, can be used to generate energy to supplement certain areas of the process. Due to the moisture in the charged material, electrolysis takes place within the furnace, generating hydrogen (H2) and oxygen (O2). The high temperature allows a percentage of the carbon monoxide to combust instantly with the available oxygen, which forms carbon dioxide (CO2). The balance of gas in the process is primarily nitrogen (N2), which comes from the air drawn into the furnace, as it is impossible to seal the furnace off perfectly. The oxygen from the air also combusts with the carbon monoxide, however there is always a small percentage of oxygen that does not combust. The following table indicates the percentiles of the different gas compositions within the furnace Gas Composition Percentage (Typical) Percentage (Range) Carbon Monoxide 51.0 % 50.0 - 65.0% Carbon Dioxide 13.0% 10.0 - 20.0% Nitrogen 25.0% 20.0 - 28.0% Hydrogen 8.30% 7.50 - 12.0% Oxygen 2.00 % 0.50 - 3.50 % Methane 0.70% 0.40 - 0.80 % Table 1: Typical gas composition percentages. The power input into the furnace process to induce the reduction of the ferric- and manganese oxide, determines the rate at which the reaction takes place. The power input is most commonly measured in MVA and then multiplied by the furnace power factor, which is a function of the electrode characteristics as an inductor, to convert to MW. The rate at which off-gas is generated does not change significantly with the change in power input, however the dust load in the off-gas stream changes exponentially. Larger particulate is generated with the increase in power, as well as the total mass of dust per cubic meter of gas. The dust loading of the off-gas plays a critical role in the design of an off-gas scrubbing system. The following table indicates the increase in dust load with the increase of power input into the furnace. Power Input [MVA] Dust Emission Rate [μg/s] Dust Emission Rate [μg/s 30 1082.877 40 2793.574 50 7206.776 60 18591.82 70 47962.61 Table 2: Dust emission rate as a function of furnace power Another critical factor of the scrubbing system design is particle size distribution (PSD). The maximum emission of a plant is dictated by environmental legislation, and needs to be adhered to. The greater the dust load in the gas
stream, the more efficient the scrubbing system needs to be, because small particulate, which are particles with a sub-micron aerodynamic diameter, is more difficult to remove from a gas stream. The greater the dust load per cubic meter, the greater the quantity of the sub-micron particulate, which significantly influences the design of the scrubber. The required increase in efficiency exponentially increases the power consumption of the scrubbing system, which greatly increases supply costs and service requirements of the plant. The following table indicates the particle size distributions Particle Size [μm] Percentage [Typical] < 1.00 20.0% 1.00 - 5.00 40.0% 5.00 - 10.0 20.0% 10.0 - 100.0 15.0% 50.0 - 100.0 4.00% 100.0 - 500.0 1.00 % Table 3: Particle size distribution at 40MW furnace load These parameters are paramount when conducting the front-end engineering of a scrubbing system for this application. Not only are there financial and commercial implications when failing to adhere to acceptable emissions, but the impact on the surrounding environment can detrimental. Diligent and accurate engineering benefits the customer, supplier and the environment, and satisfies environmental legislative requirements.Master
Effluent treatment and its re-use for the Kriel Power Station
Get PDFMSc (Engineering Sciences in Chemical Engineering), North-West University, Potchefstroom Campus, 2016Kriel Power Station generates effluent of about 5ml per day from different effluent streams of the pre-treatment plant. Currently the water is recovered into the effluent sump and pumped to the high level ash water sumps where a percentage is used for the ashing system while most water remains within the sump. There is a continuous intake of raw water to the cooling water (CW) system due to water losses through evaporation, effluent, leaks etc. Due to water scarcity in South Africa, Eskom embarked on a drive to save and protect water resources. A proposal is made through this research to recover most effluent into the CW system in order to save water and to reduce the disposal to the environment . CW system is chosen as the best opt ion for effluent recovery because there is more control over it on chemistry in terms of operation. Different technologies are discussed as options on how to recover effluent water; treat it up to acceptable Eskom Cooling Water Chemistry Standards
and re-use it into the station’s cooling water (CW) system. The study revealed savings of about R14.6 M per year if effluent could be reused in the CW instead of fresh raw water intake for make-up; which can be invested into initiating effluent recovery project.Master
The characterisation of the corrosion and degradation of atomised ferrosilicon in a dense medium circuit
Get PDFThesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2004.The Kumba Resources Sishen Iron Ore Mine is one of the biggest iron ore producers in South Afica, where beneficiation of the iron ore is done using a dense medium separation process. Ferrosilicon is used as the dense medium separation material.
About 7 years ago, Kumba Resources decided to produce its own ferrosilicon, whereas the previous supplier was Samancor. The new ferrosilicon produced by Kumba Resources, however, had different characteristics and thus caused different problems in the dense medium separation process, as was the case with the previously used ferrosilicon. A study was undertaken to identify the changes that occur in the new ferrosilicon and to understand the reasons why certain losses occur. Tests were done in the laboratory to identify the effect of attrition on the ferrosilicon used as a dense medium separation material, and the results obtained via the use of SEM analyses and Mossbauer spectroscopy, were compared
with the results obtained from samples taken in the Sishen plant itself. No definite change could be observed in the magnetic properties of the ferrosilicon during use, but rather a difference in magnetic recoverability of individual particles was found. The difference in the magnetic recoverability was assigned to the fact that the chemical compositions of individual size fractions may be different from the bulk chemical composition. Furthermore, due to a difference in magnetic recoverability, it was found that one of the main pathways of loss for the ferrosilicon was the inefficient magnetic recovery of the ferrosilicon. The abrasion process that takes place during normal use of the ferrosilicon, was also identified as a large contributing factor to the loss of ferrosilicon. It is recommended that manufacturers should ensure that the bulk chemical composition of the ferrosilicon produced be an accurate reflection of the chemical composition of individual ferrosilicon particles. This would ensure more efficient reclaiming of the used ferrosilicon.Master
Fuel bed evaluations and coal properties transformation in a Sasol–Lurgi fixed bed dry bottom gasifier operating on North Dakota lignite
Get PDFThesis (PhD (MIng (Chemical Engineering))--North-West University, Potchefstroom Campus, 2009The growth in coal consumption worldwide as well as the high oil prices in the recent past has led to the current increased interest in the application of coal gasification technologies. The Sasoi-Lurgi Fixed Bed Dry Bottom (S-L FBDB) gasification technology is one such technology that has the biggest market share in the world and maintains its competitive edge particularly with regard
to gasification of low grade and low rank coal. To ensure sustained competitive advantage through technology development, it is important to understand the fundamentals of the process as well as the behaviour of coals of different rank in the reactor. The main objective of this study was to investigate the fuel bed behaviour as
well as coal properties transformational behaviour in a S-L FBDB gasifier that gasifies North Dakota (NO) lignite. It was hypothesised in this study that using the FBDB gasifier sampling methodologies available in the literature, with some modifications to suit the context of this study, can help to explain the fuel bed behaviour as well as the coal properties transformational behaviour during gasification of lignite in the S-L FBDB process. To test the hypothesis and to achieve the objectives of the study, two MK IV S-L FBDB gasifiers (i.e. "Albert" and "Bernice") operating at the Great Plains Synfuels Plant of the Dakota Gasification Company (DGC) in the United States of America (USA) were sampled using the Turn-Out method developed by Bunt (2006) and modified in this study to suit lignite. The samples were characterised for their chemical, physical, petrographic and mineralogical properties which were then interpreted in terms of their transformation in the various reaction zones of the gasifiers. The different reaction zones In the "Bernice" and "Albert" gasifiers were successfully identified using chemical analyses (i.e. proximate and ultimate analyses as well as Fischer tar yields). Identification of reaction zones in the S-L FBDB gasifiers operating on lignite is a first in the history of the process. In comparing Secunda GG41 gasifier operating on bituminous coal with the DGC "Bernice" and "Albert" gasifiers operating on lignite, the reaction zones were found to be very different due to, amongst other things, the different operating philosophy, stability and coal rank. About two thirds of the reactor volume, in the case of DGC "Bernice" and "Albert" gasifiers, was found to be drying and devolatilizing the coal, leaving only about a third of the reactor volume for gasification and combustion. Nonetheless, due to the high reactivity of the lignite, more than 98% of the char/fixed carbon was consumed within a third of the remaining gasifier volume and this is a significant new finding. The fact that the entire reactor volume was utilized for drying, devolatilization, gasification and combustion with carbon conversion of >98%, makes the S-L FBDB gasifier very suitable for lignite gasification. In line with the Secunda GG41 gasifier, clear overlaps between the reaction zones were observed in the "Bernice" and "Albert" gasifiers. This therefore confirms the gradual transition from one reaction zone to another as reported in the literature. The volatile matter in the ash from both the "Bernice" and "Albert" gasifiers was about 10% (dry basis). This volatile matter is most probably inorganic in nature given the presence, in the samples obtained from the ash bed, of calcite (CaCO3), gypsum (CaSO4.2H2O) and melanterite (FeSO4.7H2O) which are expected to decompose during volatile matter determination at 900 °C. Using only •the volatile matter, as determined by proximate analyses, to determine the pyrolysis zone position in the reactor will in the case of DGC gasifiers therefore be delusive. As expected, most of the H, N and S were released in the pyrolysis zone of both the "Albert" and "Bernice" gasifiers. A significant increase in the reactivity of the chars from both the "Bernice" and "Albert" gasifiers was observed in the gasification zones. It is due to this increased reactivity that the char/carbon in these gasifiers were consumed ,within only a third of the gasifier volume. The increased reactivity is most probably due to the catalytic reactions effected by the organically bound alkali and alkaline earth metals, particularly calcium as the coal was found to be rich in this element. Thermal fragmentation was found to be severe with the NO lignite tested. The feed coal was found to decrease in size from 3% in the feed to 90% of <6.3 mm fine particles in the drying and pyrolysis zones of both the "Albert" and "Bernice" gasifiers. This is also a new significant finding in the history of the SL FBDB gasification process which is traditionally known to operate on coarse coal. Mineral matter in the feed coal to the "Bernice" gasifier was mainly dominated by the organically bound calcium. The crystalline phases in the gasification and combustion zones were dominated by gehlenite and bredigite which may have formed from the transformation, at higher temperatures, of the organically bound Ca and Mg to GaO and MgO and subsequent interaction with the reactive silica and transformation products of the clays. In the "Bernice" gasifier, a significant amount of calcite was found to be forming in the beginning of the gasification zone, towards the end of pyrolysis, and decomposing slightly in the hotter combustion zone. It is suggested that the calcite was formed from the reaction of GaO (formed from the transformation of the organically bound Ca) with the CO2 from the raw gas in the gasifier. As expected, the glass phase was found to be the major part of the ash minerals in the gasification and combustion zones of the "Bernice" gasifier. This phase was composed mainly of the Ca, Mg, Na aluminosilicates with some Fe. This composition is common to the slag formed from the Fort Union
lignite. There was therefore a significant amount of melting in the hotter reaction zones (i.e. gasification and combustion zones) of these gasifiers. The organically bound Ca, Mg and Na seemed to have played a significant role in the formation of this glass phase in the gasifiers. Oxygen scavenging by the ash minerals in the combustion and gasification zones of both "Albert" and "Bernice" gasifiers was observed. In the "Bernice" gasifier, it was estimated at about 16% of the oxygen fed as agent to the gasifier. From an economic viewpoint this is significant given the high cost of producing the 99% pure oxygen for gasification. The AFT in the feed coal to both "Albert" and "Bernice" gasifiers was found to be higher as compared to the ash samples from the ash bed. This may have
implications on the design and operating philosophy since the gasifiers are normally designed to operate between the initial deformation and flow temperatures of the ASTM ash, which is not the same as the ash formed in the gasifier. This is also another new significant finding. The high concentration of the fluxing elements (i.e. Ca, Mg, Na and Fe) in the dominating glass phase determined in the gasification and combustion zones of these gasifiers was most probably the reason behind this phenomenon. The char particles formed in both "Bernice" and "Albert" gasifiers were, as determined petrographically, mainly dominated by the dense chars which were highly reactive. An induced "coalification" process was observed in both "Bernice" and "Albert" gasifiers with the macerals/char particles being transformed from lignite to bituminous and anthracitic coal particles. In the "Bernice" gasifier, the average temperature of solids in the combustion zone was found to be about 700 °C, peaking at 11 00 °C in the combustion zone. The average temperature is in line with the predicted figures (i.e. 741 °C for the predicted temperature at which the water gas shift reaction was forced into equilibrium). Overall, there was an excellent match in the trends of the chemical, physical, petrographic and mineralogical properties of the samples obtained at different levels of the "Albert" and "Bernice" gasifiers. This may therefore confirm plug flow during the Turn-Out sampling methodology and hence supports the hypothesis of this study.
It is hoped that the results obtained in this study will not only benefit Sasol or Sasoi-Lurgi Technology Company with regard. to the understanding of the reactors and improvement in modelling and design, but will also assist DGC in further optimising their lignite gasification process.Doctora
Integrated barium carbonate process for sulphate removal from mine wastewater
No full textThesis (Ph.D. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2009The mining industry in South Africa is one of the primary sources of water pollution. Closed down mines leave a legacy of chemically polluted water after closure, and current operating mines are continuously discharging polluted water into the environment, consequently polluting water resources. The chemical pollutants found in this mine waste water include high concentrations of sulphate (up 5 000 mg/L), dissolved heavy metals and iron (II). This water can have pH levels as low as 2.5 and it is therefore named Acid Mine Drainage (AMD). AMD is formed as a result of the oxidation of pyritical material, which becomes exposed to oxygen and water during mining activities. While neutralization of the AMD and metal removal has been achieved by using alkaline compounds, sulphate removal to environmentally acceptable levels is still a source of controversy. Environmental, health and water governing bodies are exerting pressure on mines to treat water for sulphate content. A number of processes aimed at sulphate removal, to acceptable levels, are currently in use, and the scope of this thesis concentrates on their development and optimization. Previous research has shown that while limestone (CaC03) and lime (CaO) were conventionally used for neutralization and metal removal from AMD, these two chemicals also have a potential for partial sulphate removal from sulphate rich water, via gypsum (CaSCU) crystallization. However, a number of drawbacks such the inability to remove sulphate to low levels without addition of excess chemicals has led to the exploration of other chemicals for potential sulphate removal from AMD. Barium salts (Ba(OH)2, BaS and BaCOa) can also remove sulphate from sulphate rich water, stoichiometrically, via BaSCU precipitation. The use of these chemicals offers an added advantage of recyclability, via thermal reduction of the precipitated BaSCU to BaS, in the presence of a reducing reagent. In this thesis, the integrated barium carbonate process for sulphate and metal removal from AMD is presented and consists of the pre-treatment with lime, removal of sulphate as barium sulphate by dosing barium carbonate, the thermal reduction of BaSC>4 to BaS for sulphur production and possible BaC03 recycling, and finally H2S stripping from the concentrated solution of the recovered BaS is done, leading to sulphur production. From the beaker studies it became evident that sulphate can be removed by dosing the stoichiometrical amount of BaC03 into the sulphate rich water. The rate of sulphate removal is dependent on the BaC03 concentration and the sulphate removal is not directly inhibited by the presence of magnesium in the treated water, as was previously assumed to be the case. The sulphate removal rate is only retarded by an alkalinity > 200 mg/L (as CaC03.) An online particle size measuring experimental set-up was developed to study the precipitation process of BaSCU from the reaction of Ba+2(aq) and S04"2(aq), with the aim of enhancing particle growth over nudeation, in order to produce BaSC>4 crystals with improved settling properties. The studies have demonstrated that by changing the reactant concentration, number of feeding points into the precipitator and the stirrer speed, one can affect the extent of the feeding zone and the level of supersatu ration in this zone and control the size of the precipitated particles. The Crystal Size Distribution (CSD) analysis of the precipitation process has shown that growth takes place in all the crystal size ranges, but faster in larger crystals, suggesting a size dependent type of particle growth rate. By lowering the concentrations of the fed Ba+2 the local supersatu ration was reduced thus lowering the nudeation rate and as a result increasing particle growth rate. Increasing the number of feed points into the precipitator tube enhanced particle growth. Improved mixing due to increased stirrer speed led to increased particle growth rate, in all particle size ranges and reduced nudeation rate. However, an excessive high stirrer speed led to attrition type nudeation, the result of which is reduced/stunted growth of particles. The results from the studies, carried out using activated carbon as a reducing agent, in a furnace, have shown that the optimum temperature for the reduction of BaSCU to BaS is 950 - 1050°C, within 15 minutes for a complete reduction in a tube furnace. More than 1 hour was required for more than 60% yield to be obtained in a muffle furnace. The presence of CaCC>3 in the reaction mixture does not have a significant effect on the BaS% yield and the BaS% yield in the tube furnace is higher compared to the muffle furnace.
The TGA isothermal studies have revealed that the reduction rate of BaSCU using CO is dependent on the partial pressure of CO in the system and is also dependent on the temperature. A first order reaction rate with the average activation energy of 149 (±10) kJ/mol and constant value (k) of 0.59 were found to best describe the reaction. An effective H2S stripping depends on the balance between the C02 concentration and the sulphide concentration in the BaS solution. The molar proportionality between C02 fed and the sulphide stripped was almost equal to 1 only when the pH of the BaS solution was > 12. The results from these studies will be used in the pilot scale implementation of the integrated barium carbonate process which is currently underway at Harmony mine in Randfontein.Doctora
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