No description available.

Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2006 / Better metallurgical accounting practices are achieved through implementation of robust and accurate analytical techniques, in particular the sample preparation techniques involved, as well as characterization of all possible errors associated with such techniques and those associated with the characteristics of the materials to be analyzed. As a contribution to the AMIRA P754 project which aims at developing standards and tools for metal balancing, reconciliation and reporting from mine resource to final product, this thesis presents best practices for the characterization of ferrochrome smelter slag. The characterization ofthe slag is also essential for process control, and thus its implications to the performance of the smelting process are also discussed in this thesis. Slag samples from a ferrochrome smelter were analyzed using an XRF powder pellet analytical technique in contrast to the rep technique used at the plant laboratory, to determine their composition. This was done to evaluate the possibility of using XRF as an alternative technique to improve the turnaround times at the plant laboratory. It was found that the XRF analysis of the composite slag by pressed powder pellets could be performed rapidly, but because of the grain size effects resulted from the entrained alloy particles which could not pulverize well, the results were not accurate. This then led to the introduction of an extra stage (oxidation of the entrained alloy particles) in the sample preparation procedure, which prolonged the turnaround time by about 16 hours. However, an important achievement was that the entrained alloy particles could be oxidized into their oxides to obtain homogeneous samples and therefore representative sub-samples. This is an important improvement in the practices at the plant laboratory which involved excluding some of the entrained alloy particles and thus analyzing biased, non-representative samples. A rapid estimation of the percentage of the entrained alloy in slag was proven to be possible using a simple model (which relates density and the mass fraction of the entrained alloy), in conjunction with a density measuring device (a pycnometer). This ability serves as a significant improvement in the mass balancing and metallurgical accounting practices at ferrochrome producers. This thesis also describes the manner in which heterogeneity can be quantified as well as the errors associated with heterogeneity and their implications to the analysis results. Heterogeneity in ferrochrome slag has been found to depend significantly on the amount of alloy entrainment in slag, and therefore the slag viscosity, tapping temperature and chemical composition. One can therefore conclude that the sampling constant and therefore sample mass is dependent on furnace operating conditions.

Ozonation, or advanced oxidation processes (utilising ozone decomposition products as oxidants) are widely used in industrial waste water and drinking water treatment plants. In these applications the use of ozone is based on ozone and its decomposition by-products being strong oxidants. A case study revealed that several waterworks in South Africa successfully utilise ozone as a pre-oxidant for the treatment of raw waters. South Africa holds more than three quarters of the world’s viable chromium ore (chromite) reserves. Subsequently the Cr-related industry-within is considerable in size and a major producer of large volumes of waste materials. Chromium also occurs commonly in other industrial waste materials (e.g. fly ash and clinkers originating from coal combustion) and is a natural occurring element in natural sediments, since chromium is the 21st most abundant element in the earth’s crust with an average concentration of approximately 100 ppm. Considering the abundance of natural and anthropogenic Cr-containing materials in South Africa the possibility exists that some of these materials might be suspended in raw water entering water treatment facilities. In this dissertation, the possible oxidation of non-Cr(VI) Cr-containing materials suspended in water during ozonation, is presented within the context of water treatment applications (Chapter 4). The results indicate that in situ formation of hazardous Cr(VI) is possible during aqueous ozonation. pH had a significant influence, since the decomposition products of aqueous O3, i.e. hydroxyl radicals that form at higher pH levels, were found to be predominantly responsible for Cr(VI) formation. Increased ozonation contact time, water temperature and solid loading also resulted in elevated Cr(VI) concentrations being formed. Occasionally these values exceeded the drinking water standard 50 ppb Cr(VI). The results therefore indicate the importance of removing suspended particulates from water prior to ozonation. Additionally, pH-control could be used to mitigate the possible formation of Cr(VI) during ozonation. In Chapter 5, exploratory work is presented on the possibility of utilising Cr(VI) formation via ozonation as a means of recovering chromium from Cr-containing waste materials. Such a study is of particular interest within the local context, considering the large volumes of waste produced by the Cr-related industry in South Africa. This exploratory work is based on the fact that unlike Cr(0) and Cr(III), most Cr(VI) compounds are relatively soluble in water. Cr(VI) is a carcinogen if inhaled, however the probability of negative health effects are substantially reduced if it occurs in solution. Thus a hydrometallurgical route of recovering Cr-units via Cr(VI) generation represents the safest route with regard to Cr(VI) exposure. Such a hydrometallurgical route could also addresses the limitations of the physical separation methods currently applied, which fails to recover fine Cr-containing solids. The degree of Cr2O3-liberation achieved in this exploratory work was relatively low. However, the Cr2O3-liberation achieved for the ferrochromium slag (15%) indicated some promise, considering the limitations of this exploratory work. Several steps can be considered in future studies, which would in all likelihood improve the Cr2O3-liberation further. / Thesis (MSc (Chemistry))--North-West University, Potchefstroom Campus, 2012

Ozonation, or advanced oxidation processes (utilising ozone decomposition products as oxidants) are widely used in industrial waste water and drinking water treatment plants. In these applications the use of ozone is based on ozone and its decomposition by-products being strong oxidants. A case study revealed that several waterworks in South Africa successfully utilise ozone as a pre-oxidant for the treatment of raw waters. South Africa holds more than three quarters of the world’s viable chromium ore (chromite) reserves. Subsequently the Cr-related industry-within is considerable in size and a major producer of large volumes of waste materials. Chromium also occurs commonly in other industrial waste materials (e.g. fly ash and clinkers originating from coal combustion) and is a natural occurring element in natural sediments, since chromium is the 21st most abundant element in the earth’s crust with an average concentration of approximately 100 ppm. Considering the abundance of natural and anthropogenic Cr-containing materials in South Africa the possibility exists that some of these materials might be suspended in raw water entering water treatment facilities. In this dissertation, the possible oxidation of non-Cr(VI) Cr-containing materials suspended in water during ozonation, is presented within the context of water treatment applications (Chapter 4). The results indicate that in situ formation of hazardous Cr(VI) is possible during aqueous ozonation. pH had a significant influence, since the decomposition products of aqueous O3, i.e. hydroxyl radicals that form at higher pH levels, were found to be predominantly responsible for Cr(VI) formation. Increased ozonation contact time, water temperature and solid loading also resulted in elevated Cr(VI) concentrations being formed. Occasionally these values exceeded the drinking water standard 50 ppb Cr(VI). The results therefore indicate the importance of removing suspended particulates from water prior to ozonation. Additionally, pH-control could be used to mitigate the possible formation of Cr(VI) during ozonation. In Chapter 5, exploratory work is presented on the possibility of utilising Cr(VI) formation via ozonation as a means of recovering chromium from Cr-containing waste materials. Such a study is of particular interest within the local context, considering the large volumes of waste produced by the Cr-related industry in South Africa. This exploratory work is based on the fact that unlike Cr(0) and Cr(III), most Cr(VI) compounds are relatively soluble in water. Cr(VI) is a carcinogen if inhaled, however the probability of negative health effects are substantially reduced if it occurs in solution. Thus a hydrometallurgical route of recovering Cr-units via Cr(VI) generation represents the safest route with regard to Cr(VI) exposure. Such a hydrometallurgical route could also addresses the limitations of the physical separation methods currently applied, which fails to recover fine Cr-containing solids. The degree of Cr2O3-liberation achieved in this exploratory work was relatively low. However, the Cr2O3-liberation achieved for the ferrochromium slag (15%) indicated some promise, considering the limitations of this exploratory work. Several steps can be considered in future studies, which would in all likelihood improve the Cr2O3-liberation further. / Thesis (MSc (Chemistry))--North-West University, Potchefstroom Campus, 2012

The production of ferrochrome (FeCr) from chromite ore is a reducing process, whereby the Cr(III) and Fe(II) in the ore are reduced to metallic chromium (Cr) and iron (Fe) in the final product. FeCr is mostly used for the production of stainless steel, which is a vital alloy in modern society. It is, however, impossible to exclude oxygen completely from all the high temperature steps during the production process and very small amounts of Cr(VI) are therefore formed, although not intended. The formed Cr(VI) is mostly associated with the off-gas of the high temperature processes, which are cleaned before it is released into the atmosphere by means of venturi scrubbers or bag filter systems. Certain Cr(VI) species are regarded as carcinogenic, with specifically airborne exposure to these Cr(VI) species being associated with cancer of the respiratory system. FeCr smelter facilities generate three main types of waste materials, i.e. slag, venturi sludge and bag filter dust (BFD). Most of the Cr in the waste materials consists mostly of Cr(III). However, BFD generated during the cleaning of the off-gas of open/semi-closed furnaces, could contain more significant levels of Cr(VI) than the slag and sludge. The aim of this study was to determine the solubility of different Cr(VI) species present in BFDs. This would allow that the Cr(VI) in BFD is categorised as water soluble Cr(VI), sparingly soluble and insoluble Cr(VI). These solubility categories can then be related to groups of Cr(VI) compounds, therefore taking the first step in better characterisation of Cr(VI) present in BFD. Four different BFD samples from FeCr producers in South Africa were characterised in detail. Analytical methods such as scanning electron microscope (SEM), SEM with energy-dispersive X-ray spectroscopy (SEM-EDS), particle size analysis, trace metal analysis with inductively coupled plasma with a mass spectrometer detector (ICP-MS) and Cr(VI) analysis with ion chromatography (IC) were utilised in order to characterise and categorise the samples. The results indicated that more Cr(VI) leached with an increase in pH. This was in contrast with the trend for most heavy metals. This was also an indication that not only soluble, but also sparingly- and insoluble Cr(VI) compounds occur in the BFD samples evaluated. Further analysis showed that approximately one third of the Cr(VI) species was insoluble and the remainder consisted of sparingly insoluble and soluble Cr(VI) compounds. The most significant finding was that the current leaching procedures applied by FeCr producers, prior to the chemical reduction of Cr(VI), do not effectively extract the sparingly water insoluble compounds. This results in Cr(VI) leaching from waste facilities at later stages, even if seemingly effective Cr(VI) treatment was applied. Therefore, it should be considered as an extremely important future perspective to develop economically feasible Cr(VI) extraction procedures that will ensure complete extraction of sparing water soluble Cr(VI) compounds together with the water soluble fraction, prior to chemical reduction of Cr(VI) and subsequent storage of the residue on a waste facility. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2015

The production of ferrochrome (FeCr) from chromite ore is a reducing process, whereby the Cr(III) and Fe(II) in the ore are reduced to metallic chromium (Cr) and iron (Fe) in the final product. FeCr is mostly used for the production of stainless steel, which is a vital alloy in modern society. It is, however, impossible to exclude oxygen completely from all the high temperature steps during the production process and very small amounts of Cr(VI) are therefore formed, although not intended. The formed Cr(VI) is mostly associated with the off-gas of the high temperature processes, which are cleaned before it is released into the atmosphere by means of venturi scrubbers or bag filter systems. Certain Cr(VI) species are regarded as carcinogenic, with specifically airborne exposure to these Cr(VI) species being associated with cancer of the respiratory system. FeCr smelter facilities generate three main types of waste materials, i.e. slag, venturi sludge and bag filter dust (BFD). Most of the Cr in the waste materials consists mostly of Cr(III). However, BFD generated during the cleaning of the off-gas of open/semi-closed furnaces, could contain more significant levels of Cr(VI) than the slag and sludge. The aim of this study was to determine the solubility of different Cr(VI) species present in BFDs. This would allow that the Cr(VI) in BFD is categorised as water soluble Cr(VI), sparingly soluble and insoluble Cr(VI). These solubility categories can then be related to groups of Cr(VI) compounds, therefore taking the first step in better characterisation of Cr(VI) present in BFD. Four different BFD samples from FeCr producers in South Africa were characterised in detail. Analytical methods such as scanning electron microscope (SEM), SEM with energy-dispersive X-ray spectroscopy (SEM-EDS), particle size analysis, trace metal analysis with inductively coupled plasma with a mass spectrometer detector (ICP-MS) and Cr(VI) analysis with ion chromatography (IC) were utilised in order to characterise and categorise the samples. The results indicated that more Cr(VI) leached with an increase in pH. This was in contrast with the trend for most heavy metals. This was also an indication that not only soluble, but also sparingly- and insoluble Cr(VI) compounds occur in the BFD samples evaluated. Further analysis showed that approximately one third of the Cr(VI) species was insoluble and the remainder consisted of sparingly insoluble and soluble Cr(VI) compounds. The most significant finding was that the current leaching procedures applied by FeCr producers, prior to the chemical reduction of Cr(VI), do not effectively extract the sparingly water insoluble compounds. This results in Cr(VI) leaching from waste facilities at later stages, even if seemingly effective Cr(VI) treatment was applied. Therefore, it should be considered as an extremely important future perspective to develop economically feasible Cr(VI) extraction procedures that will ensure complete extraction of sparing water soluble Cr(VI) compounds together with the water soluble fraction, prior to chemical reduction of Cr(VI) and subsequent storage of the residue on a waste facility. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2015

No description available.

No description available.

Neste trabalho estuda-se o comportamento de redução para a obtenção da liga FeCrAC a partir da pelota auto-redutora feita de minério de cromita, coque de petróleo, ferro-silicio, cal hidratada, sílica e cimento portland ARI. As principais variáveis consideradas são: teor de redutor na composição da pelota, quantidade do redutor, temperatura e tempo. Inicialmente os materiais (cromita, ferro-silício, coque de petróleo, cal hidratada, sílica e cimento Portland ARI), foram caracterizados por: análise química e análise granulométrica. Após a caracterização os materiais (cromita, ferro-silício, coque de petróleo e cimento Portland ARI) foram aglomerados na forma de pelotas juntamente com cal hidratada e sílica para ajuste da basicidade quaternária da escória. A redução das pelotas foi feita num forno de indução que pode atingir temperaturas de até 1973K (1700oC). Todos os experimentos de redução foram realizados no aparato experimental utilizando-se cadinhos de grafite nas temperaturas de 1773K (1500oC), 1823K (1550oC) e 1873K (1600oC). Após os ensaios de redução os produtos obtidos (escória e metal) foram analisados por microscopia ótica, por microscopia eletrônica de varredura (MEV) e análise por EDS. O efeito do aumento da temperatura na redução da cromita é significativo. Houve aumento na velocidade de redução de 4 a 6 vezes com o aumento de 1773K (1500oC) para 1873K (1600oC). Os resultados indicam um efeito marcante de pequenas adições de Fe-Si na velocidade de redução da cromita. Na temperatura de 1773K (1500oC) as adições até ~2% de Fe-Si são benéficas e para adições maiores praticamente não há vantagens técnicas e econômicas. Os tempos necessários para atingir a fração unitária de redução foram 12, 7,5 e 5 minutos para adições de Fe-Si de 0, ~1%, e ~2%, respectivamente; a temperatura de 1823K (1550oC). À temperatura de 1873K (1600oC) as adições de Fe-Si na pelota apresentam também efeitos significativos na velocidade de redução, porém adições de ~1%, e ~2% mostraram os mesmos resultados, indicando que o teor ótimo de adição de Fe- Si na pelota deve estar em torno de 1%. Verificou-se que a utilização de pelotas auto-redutoras contendo 26% em excesso, sobre o estequiométrico, de coque de petróleo aumentou o rendimento de recuperação de Cr de 96% para 98%. O rendimento e a eficiência do processo de auto-redução supera aos processos convencionais de produção de FeCrAC, obtendo-se altas recuperações de cromo na faixa de 96% até 98% para Cr. / The reduction behaviors, at high temperature, of the self-reducing pellets of chromites for production of high carbon ferro-chromium are studied in this work. The influences of the temperature, of the excess of reductant and the small addition of the Fe-Si were analyzed. The materials used (chromites, petroleum coke, Portland cement, hydrated lime and silica) were characterized chemically and by size distribution. The composite pellets (self-reducing) were produced aiming a quaternary basicity of 0.91. The reductant was calculated considering a stoichiometry of reduction and dissolution of 4wt%C in the final metallic phase. The reduction experiments were made in a special system, in argon atmosphere, heated by induction and at temperatures of 1773, 1823 and 1873K. The dried pellets were placed into a pre-heated graphite crucible and left there along up to no gas evolution was observed. The results of the reacted fraction with time were plotted and the obtained product (metallic and slag phases) after experiments were analyzed by optical and by electron micrograph. The chemical estimations were made by micro-analysis (EDS) The effect of increasing the temperature of reduction was sensitive, such that, the reduction rate increased 4 to 6 times with increase of temperature from 1773 to 1873. The small additions, up to 2% of Fe-Si, for substituting the equivalent fixed carbon of the petroleum coke showed to improve substantially the reduction rate, almost doubling it in comparison with pellets without any addition. The use of excess of 26%, over the stoichiometry, of the petroleum coke decreased around 50% of the chromium content in the slag, with relation to pellet without excess. The chromium recovery yield reached 98%. This result coupled with very high reduction rate of self-reducing pellets show the potential for self-reducing processes for ferro-chromium production.

Neste trabalho estuda-se o comportamento de redução para a obtenção da liga FeCrAC a partir da pelota auto-redutora feita de minério de cromita, coque de petróleo, ferro-silicio, cal hidratada, sílica e cimento portland ARI. As principais variáveis consideradas são: teor de redutor na composição da pelota, quantidade do redutor, temperatura e tempo. Inicialmente os materiais (cromita, ferro-silício, coque de petróleo, cal hidratada, sílica e cimento Portland ARI), foram caracterizados por: análise química e análise granulométrica. Após a caracterização os materiais (cromita, ferro-silício, coque de petróleo e cimento Portland ARI) foram aglomerados na forma de pelotas juntamente com cal hidratada e sílica para ajuste da basicidade quaternária da escória. A redução das pelotas foi feita num forno de indução que pode atingir temperaturas de até 1973K (1700oC). Todos os experimentos de redução foram realizados no aparato experimental utilizando-se cadinhos de grafite nas temperaturas de 1773K (1500oC), 1823K (1550oC) e 1873K (1600oC). Após os ensaios de redução os produtos obtidos (escória e metal) foram analisados por microscopia ótica, por microscopia eletrônica de varredura (MEV) e análise por EDS. O efeito do aumento da temperatura na redução da cromita é significativo. Houve aumento na velocidade de redução de 4 a 6 vezes com o aumento de 1773K (1500oC) para 1873K (1600oC). Os resultados indicam um efeito marcante de pequenas adições de Fe-Si na velocidade de redução da cromita. Na temperatura de 1773K (1500oC) as adições até ~2% de Fe-Si são benéficas e para adições maiores praticamente não há vantagens técnicas e econômicas. Os tempos necessários para atingir a fração unitária de redução foram 12, 7,5 e 5 minutos para adições de Fe-Si de 0, ~1%, e ~2%, respectivamente; a temperatura de 1823K (1550oC). À temperatura de 1873K (1600oC) as adições de Fe-Si na pelota apresentam também efeitos significativos na velocidade de redução, porém adições de ~1%, e ~2% mostraram os mesmos resultados, indicando que o teor ótimo de adição de Fe- Si na pelota deve estar em torno de 1%. Verificou-se que a utilização de pelotas auto-redutoras contendo 26% em excesso, sobre o estequiométrico, de coque de petróleo aumentou o rendimento de recuperação de Cr de 96% para 98%. O rendimento e a eficiência do processo de auto-redução supera aos processos convencionais de produção de FeCrAC, obtendo-se altas recuperações de cromo na faixa de 96% até 98% para Cr. / The reduction behaviors, at high temperature, of the self-reducing pellets of chromites for production of high carbon ferro-chromium are studied in this work. The influences of the temperature, of the excess of reductant and the small addition of the Fe-Si were analyzed. The materials used (chromites, petroleum coke, Portland cement, hydrated lime and silica) were characterized chemically and by size distribution. The composite pellets (self-reducing) were produced aiming a quaternary basicity of 0.91. The reductant was calculated considering a stoichiometry of reduction and dissolution of 4wt%C in the final metallic phase. The reduction experiments were made in a special system, in argon atmosphere, heated by induction and at temperatures of 1773, 1823 and 1873K. The dried pellets were placed into a pre-heated graphite crucible and left there along up to no gas evolution was observed. The results of the reacted fraction with time were plotted and the obtained product (metallic and slag phases) after experiments were analyzed by optical and by electron micrograph. The chemical estimations were made by micro-analysis (EDS) The effect of increasing the temperature of reduction was sensitive, such that, the reduction rate increased 4 to 6 times with increase of temperature from 1773 to 1873. The small additions, up to 2% of Fe-Si, for substituting the equivalent fixed carbon of the petroleum coke showed to improve substantially the reduction rate, almost doubling it in comparison with pellets without any addition. The use of excess of 26%, over the stoichiometry, of the petroleum coke decreased around 50% of the chromium content in the slag, with relation to pellet without excess. The chromium recovery yield reached 98%. This result coupled with very high reduction rate of self-reducing pellets show the potential for self-reducing processes for ferro-chromium production.