Browsing by Author "Taskinen, Pekka, Prof., Aalto University, Finland"
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- The effect of mineralogy, sulphur, and reducing gases on the reducibility of saprolitic nickel ores
School of Chemical Technology | Doctoral dissertation (article-based)(2013) Bunjaku, AliThe economically important nickel ores can be divided into two types: sulphide and laterite ores. Because of the recent decline in sulphide nickel reserves, laterites are becoming an increasingly important source of nickel. The challenge facing the production of nickel from laterites is the energy consumption, as the process requires significantly more energy than the one involving sulphides. The high energy consumption and greenhouse gas footprints for laterite ores will be critical sustainability issues affecting the future of the nickel laterite industry. One commonly used processing method for nickel extraction via the pyrometallurgical treatment of laterite ores involves smelting to ferronickel in a rotary kiln-electric furnace process. The reduction roasting of laterite ore in a rotary kiln has a significant impact on the final ferronickel production; the higher the degree of the necessary pre-reduction of the calcine, the greater the energy saving during the smelting in the electric furnace. Better knowledge of nickel-bearing minerals in saprolite ores and their behaviour at high temperatures is important for developing the pre-reduction process in the rotary kiln. The aim of the thesis reported here was to examine: (i) the thermal behaviour of different saprolite ores in order to determine the microstructures and phases that formed as a result of dehydroxylation and to detect the phases where nickel is predominantly abundant after dehydroxylation; (ii) the reducibility of the ores, in order to explore the relationship between the mineralogy of the ore, the phases formed, and the reducibility, as well as to define the optimal reducing conditions for the ores; (iii) the effect of sulphur on the formation of high-temperature phases during heat treatment and further on the reducibility of saprolite ores. The results reveal that during heat treatment, three endothermic peaks at 100, 250, and 600°C, caused by the removal of free water, thermal dissociation of goethite, and dehydroxylation reactions were observed. Followed by an exothermic process at ~820°C, which is evidently associated with the crystallisation of new phases. The formation of new phases (talc-like, amorphous, olivine, and pyroxene) appears to depend on the type of minerals in the initial ore and the phases formed during heating were observed to affect the reducibility of the ores. The type of reducing gas also appears to have an impact on the reduction behaviour and metallisation of the components in saprolitic ores. Considering the results from reduction experiments, ~750°C appears to be the optimal reduction temperature for the ores, CO reducing gas is recommended in order to achieve high nickel metallisation, and finally, the addition of sulphur has a great effect on the reducibility of the ore and thus the formation of metallic particles. - Fundamentals of SO2 depolarized water electrolysis and challenges of materials used
School of Chemical Technology | Doctoral dissertation (monograph)(2013) Lokkiluoto, AnuSulfur dioxide depolarized water electrolysis (SDE) produces sulfuric acid and hydrogen. Due to its lower cell voltage, the process requires far less electricity than traditional water electrolysis. When SO2 is obtained from flash smelting, sulfides roasting, sulfur combustion, or any other similar operation, SDE is a part of the OutotecR open cycle process. In the present work, materials to be used in SDE were studied together with the fundamentals of the process. Data on the co-production of concentrated acid and hydrogen are scarce in the earlier literature. Theoretical modeling work covering the entire concentration range was deemed necessary in addition to experimental testing to settle this issue. Based on the literature concerning PEM Fuel Cells, it was assumed that the water transport properties of the membrane used as a separator in the electrolysis have a significant effect on the water balance. The activities of the electrolyte components were calculated for the whole concentration range of the ternary H2O-H2SO4-SO2 system relevant for the SDE process using a mathematical model elaborated during the present work. The calculated activities were combined with the model for water transport through a Nafion membrane, and an overall model for SDE was built. The reversible cell potential was calculated for the entire concentration range. The model was used to predict the overall water balance of the electrolysis process with changing electrolyte concentrations. The stability of thin gold coatings and their activity towards electrochemical oxidation of SO2 were studied first in autoclave tests and with linear sweep voltammetry, and the performance of the coatings was compared to that of gold. Next, the performance of the gold-coated electrodes was tested in bench-scale SDE. Even though the oxidation of SO2 has previously been studied on gold, such experiments have seldom been carried out in as high concentrations of sulfuric acid as in this work, and the results obtained with gold coatings are believed to be original. The modeling results were compared with the actual performance of the electrolyzer. Based on the combined results of the experimental and theoretical work, it was possible to explain and predict the remarkable changes in the SDE process that take place with changing electrolyte concentrations. A significant amount of water is transported from the anolyte to the catholyte through the membrane due to the electro-osmotic water drag effect. This phenomenon is beneficial for the production or concentration of sulfuric acid by SO2 depolarized electrolysis. Thin gold coatings on stainless steel surface can be used to catalyze the anodic oxidation of SO2, and similar electrodes can be used as cathodes. - Technology and theory of producing APT from tungsten concentrates by sulfuric acid conversion-ammonium salt leaching
School of Chemical Technology | Doctoral dissertation (article-based)(2019) Shen, LeitingTungsten is considered as a strategic and critical raw material, due to its remarkable physical and chemical properties, wide industrial applications and non-substitutability. Ammonium paratungstate is the main intermediate product extracted from tungsten ores with metallurgical processes, intrinsically linking to the tungsten industry across all supply chain stages. An efficient and cleaner technology for producing ammonium paratungstate is crucial to the sustainable development of the tungsten industry. This work presents a novel process by the sulfuric acid conversion-ammoniacal ammonium carbonate leaching route. The complete conversion of tungsten concentrates in H2SO4 solutions can be achieved by controlling the sulfuric acid concentration and adding an oxidizing agent. The formation mechanism of the H2WO4 layer on unreacted tungsten particles is explained by isopolytungsten ions diffusion-tungsten acid deposition in the diffusion layer. The more difficulty of wolframite conversion than scheelite in H2SO4 solutions is attributed to the thermodynamics, especially the accumulation of Fe2+ and/or Mn2+ in the solutions. The kinetics of scheelite conversion in moderate H2SO4 solutions agrees with the shrinking core model under chemical surface reaction control. Subsequently, an ammonium tungstate solution is directly obtained by leaching converted products in ammoniacal (NH4)2CO3 solutions at 30 °C, with WO3 leaching yield of >99%. The transformation of calcium sulfates to carbonates influences the WO3 leaching yield by secondary reactions, which can be suppressed by an excess (NH4)2CO3 in solutions through the formation of more stable vaterite and calcite. The leaching agent can be recovered to restore the leaching system. Additionally, thermodynamic modelling of the CaSO4–H2O system is carried out to better understand the thermodynamic property of CaSO4 solution and facilitate the solutions cycle and scaling prevention, as well as the upcoming CaSO4–H2SO4–H2O system. The critically revaluated solubility data were assessed with the NPL Pitzer model by MTDATA software to optimize the Pitzer parameters. The obtained model predicts very well the CaSO4–H2O system up to 300 °C, and agrees with most published solubility data.This work makes it possible to produce ammonium paratungstate cleanly and economically, featuring circulation of the leaching reagents and bypassing the conversion of Na2WO4 to (NH4)2WO4, which will hopefully be adopted in industrial practices.