Exergetische duurzaamheidsanalyse van zinkrecyclageprocessen

Een exergieanalyse laat toe om de efficiëntie van een proces objectief te kwantificeren via een thermodynamische beschrijving van zijn processtromen. Dit werk maakt voor het eerst gebruik van aangepaste modellen om de exergie van complexe pyrometallurgische processtromen nauwkeurig te berekenen vanuit hun evenwichtstoestand. Met behulp van deze implementatie wordt aangetoond dat de gebruikte referentietoestand, de thermodynamische gegevensbron en de stroombeschrijving het resultaat van een exergieanalyse aanzienlijk kunnen beïnvloeden. Dit bemoeilijkt een vergelijking van verschillende analyses. Zolang een consistente werkwijze aangehouden wordt, biedt exergieanalyse echter een krachtige techniek voor het evalueren en optimaliseren van industriële processen. Een analyse van een recent ontwikkeld zinkrecyclageproces toont dit aan. Door gerichte ingrepen kan de efficiëntie van dit proces substantieel worden verhoogd. Tenslotte wordt in dit werk de exergetische efficiëntieanalyse in een algemene methodiek gekaderd die ook de milieu-impact van processtromen in rekening brengt en waarmee de volledige duurzaamheid van processen kan worden gekwantificeerd.Eindwerk ingediend ter verkrijging van de graad van Burgerlijk Materiaalkundig Ingenieur (Katholieke Universiteit Leuven, 2008) Promotoren: Prof. dr. ir. B. BLANPAIN (Katholieke Universiteit Leuven), Prof. dr. ir. P. WOLLANTS (Katholieke Universiteit Leuven) en Prof. dr. ir. J. DEWULF (Universiteit Gent)status: publishe

Gas Bubbles in Liquid Metal in a Hele-Shaw Cell; A Mesoscopic Study (Gasbellen in vloeibaar metaal in een Hele-Shaw cel; Een mesoscopische studie)

Gas injection reactors in general, and bubble column reactors in particular, are key elements of many pyrometallurgical flow charts. Nevertheless, the phenomena and interactions that govern these reactors are not yet fully understood. From a multiscale point of view, the main bottleneck is situated at a mesoscopic level on which individual bubbles are considered. While simulations and water models can be very helpful to widen this bottleneck, experimental observations of gas bubbles in real pyrometallurgical systems remain indispensable for validation and fine-tuning of mesoscopic system descriptions. Unfortunately, the opacity of these systems enforces the use of indirect imaging techniques with limited temporal or spatial resolution. In addition, accurate tracking of the gas-liquid interface requires tomography, further complicating the design of an experimental setup. In this doctoral study an alternative and innovative approach is suggested that circumvents these two main restrictions. By injecting gas in a thin sheet of liquid entrapped between two flat and closely spaced plates, bubbles in a Hele-Shaw flow regime are generated. The resulting quasi-two-dimensional multiphase flow phenomena can be fully captured from a single point of view. Moreover, when using a transparent plate material that is not wetted by the liquid, even bubbles in opaque liquids can be visualized directly. This approach is explored for inert gas injection in liquid metals. To demonstrate the feasibility of the suggested Hele-Shaw based approach, buoyancy driven nitrogen bubbles in liquid mercury are observed at room temperature in a Hele-Shaw cell of 1 mm thickness. By using a moving high speed camera to make continuous close up recordings of individual bubbles, the position and geometry of these bubbles are quantified with a high resolution along their entire path. After a thorough evaluation of the experimental accuracy, this information is used for a detailed analysis of bubble volume variations. It is clear that a hydrostatic pressure gradient accounts for the most of the observed bubble growth. Yet, a careful assessment of the variations for smaller bubbles suggests that an accurate bubble description should also account for significant dynamic pressure variations that are largely flow regime dependent. Contrary to aqueous systems, in mercury transitions between these regimes can readily be observed along the trajectory of individual, expanding bubbles. A mapping for bubbles with diameters between 4 mm and 20 mm shows the existence of two different regimes: smaller bubbles adopt a constant round-to-elliptical shape and travel along a linear path while larger bubbles exhibit a periodically distorted shape and follow a swirling trajectory. The transition between this linear and periodic regime can be attributed to a shift from capillary to inertia dominated flow with increasing bubble size. Furthermore, a quantitative mapping of the bubble velocity shows that the transition goes hand in hand with a steep acceleration, and that the linear regime is marked by a negative correlation between the Eötvös number and the Reynolds number while the opposite is true for the periodic regime. To demonstrate the applicability of the suggested Hele-Shaw based approach for industrially relevant systems, a high-temperature experimental setup is developed. With this setup, nitrogen bubbles are observed in liquid zinc at 700°C in a fused quartz cell with a thickness of 1.5 mm. At low oxygen levels, cell walls are not wetted by the liquid zinc and bubbles can be visualized directly through the transparent cell walls, using the same moving high speed camera as for mercury. In the range of diameters between 5.9 and 9.0 mm, this reveals a single periodic flow regime in which bubbles follow a sinusoidal path with a characteristic frequency of 3.31 Hz. In addition, systematic intermediate accelerations are observed of which the exact origin remains unexplained. A direct extrapolation of the observations in a Hele-Shaw cell to industrially relevant geometries is not straightforward. As a consequence, such experiments will never be able to completely replace three-dimensional observations. Nevertheless, the suggested approach seems very promising as a first step in the study of gas bubbles in liquid metals. After all, in a Hele-Shaw cell, the same effects and interactions that govern unconfined bubbles can be studied. In addition, the resolution of the observations is unseen for bubbles in liquid metals, especially at high temperatures. This makes them highly suitable for detailed validations of simulation results. Therefore it is expected that this approach can contribute to a better understanding of the mechanisms that govern gas injection in pyrometallurgy and support future developments in this field.nrpages: 144status: publishe

Reactive gas bubbling in metals in a Hele-Shaw Cell

Bubble column reactors are employed in various pyrometallurgical smelting and refining processes. A fundamental understanding of their operation requires detailed information on the hydrodynamic and thermodynamic phenomena governing the behavior of individual gas bubbles in the melt. In this work, the opportunities of a Hele-Shaw cell based experimental setup for investigating reactive gas bubbles in liquid metals are evaluated as an alternative to tomography techniques, which are often difficult to apply at high temperatures. Our setup consists of two transparent closely spaced parallel plates. The gap between the plates is filled with liquid metal and gas bubbles with a significantly larger equivalent diameter than the gap width are injected in the melt. The relatively high surface tension of the liquid metal prevents the formation of a wetting layer between the bubbles and the wall, allowing the accurate tracking of the gas-liquid interface. In this way, bubble properties and 2D flow patterns in the system can be monitored under controlled circumstances and allows the in-situ study of the interactions between thermodynamic and hydrodynamic phenomena.status: publishe

Observing nitrogen bubbles in liquid zinc in a vertical Hele-Shaw cell

Observations of gas bubbles in liquid metal are strongly hindered by the opacity of metals. To circumvent this limitation, the authors recently proposed to study such systems under quasi-2D flow conditions in a Hele-Shaw cell. The current paper presents a successful application of this approach for nitrogen bubbles in liquid zinc at 973 K (700 °C) in a fused quartz cell with a thickness of 1.5 mm. At low oxygen levels, the cell walls are not wetted by the liquid zinc, and bubbles can be observed directly through the transparent cell walls. Furthermore, using a moving high-speed camera that travels upwards with the bubbles, their properties are quantified in detail along the entire trajectory. In the range of equivalent diameters between 5.9 and 9.0 mm, this reveals a single periodic flow regime in which bubbles follow a sinusoidal path with a characteristic frequency of 3.31 Hz. In addition, systematic intermediate accelerations are observed of which the origin remains unexplained. Considering the unprecedented resolution of such observations for bubbles in liquid metals, especially at high temperatures, it is expected that this approach will contribute to a better understanding of the mechanisms that govern gas injection in pyrometallurgy.status: publishe

A study of gas bubbles in liquid mercury in a vertical Hele-Shaw cell

High quality observations of mesoscopic gas bubbles in liquid metal are vital for a further development of pyrometallurgical gas injection reactors. The opacity of metals however enforces the use of indirect imaging techniques with limited temporal or spatial resolution. In addition, accurate interface tracking requires tomography which further complicates the design of a high temperature experimental setup. In this paper an alternative approach is suggested that circumvents these two main restrictions. By injecting gas in a thin layer of liquid metal entrapped between two flat and closely spaced plates, bubbles in a Hele-Shaw flow regime are generated. The resulting quasi-2D multiphase flow phenomena can be fully captured from a single point of view and, when using a non-wetted transparent plate material, the bubbles can be observed directly. The feasibility of this approach is demonstrated by observations on buoyancy driven nitrogen bubbles in liquid mercury in a vertical Hele-Shaw cell. By using a moving high speed camera to make continuous close up recordings of individual bubbles, the position and geometry of these bubbles are quantified with a high resolution along their entire path. After a thorough evaluation of the experimental accuracy, this information is used for a detailed analysis of the bubble expansion along the path. While the observed bubble growth is mainly caused by the hydrostatic pressure gradient, a careful assessment of the volume variations for smaller bubbles shows that an accurate bubble description should account for significant dynamic pressure variations that seem to be largely regime dependent.status: publishe

Comparison of electric arc furnace dust treatment technologies using exergy efficiency

Multiple methods exist to treat electric arc furnace dust (EAFD), aiming at high metal recovery and low landfilling needs. However, a technology that is energy efficient and recovers both Zn and Fe has only recently been developed to a commercial scale with the Rotary Hearth Furnace. Various other technologies have been proposed as alternatives to the historically preferred Waelz Kiln process. This gate-togate study presents an objective method to compare the overall thermodynamic performance and efficiency of treatment technologies through the use of an exergy analysis incorporating the dominant factors without the need of examining every process detail. In addition, the influence of the EAFD zinc content on the exergy efficiency of both the Waelz Kiln and Rotary Hearth Furnace process was studied. The efficiencies of these high temperature metal recovery processes were also compared to the efficiency of the newly proposed In-Process Separation technology.status: publishe

Exergy based comparison of the Waelz Kiln process and RHF for the treatment of EAF dust

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Experimental and numerical study of buoyancy-driven single bubble dynamics in a vertical Hele-Shaw cell

Buoyancy-driven single bubble behaviour in a vertical Hele-Shaw cell with various gap Reynolds numbers Re(h/d) 2 has been studied. Two gap thicknesses, h = 0.5 mm (Re(h/d) 2 = 1.0–8.5) and 1 mm (Re(h/d) 2 = 6.0–50) were used to represent low and high gap Reynolds number flow. Periodic shape oscillation and path vibration were observed once the gap Reynolds number exceeds the critical value of 8.5. The bubble behaviour was also numerically simulated by taking a two-dimensional volume of fluid method coupled with a continuum surface force model and a wall friction model in the commercial computational fluid dynamics package Fluent. By adjusting the viscous resistance values, the bubble dynamics in the two gap thicknesses can be simulated. For the main flow properties including shape, path, terminal velocity, horizontal vibration, and shape oscillation, good agreement is obtained between experiment and simulation. The estimated terminal velocity is 10%–50% higher than the observed one when the bubble diameter d ≤ 5 mm, h = 0.5 mm and 9% higher when d ≤ 18 mm, h = 1.0 mm. The estimated oscillation frequency is 50% higher than the observed value. Three-dimensional effects and spurious vortices are most likely the reason for this inaccuracy. The simulation confirms that the thin liquid films between gas bubbles and the cell walls have a limited effect on the bubble dynamics.status: publishe

Exergy based efficiency analysis of pyrometallurgical processes

Exergy based efficiency analysis provides a powerful tool for optimizing industrial processes. In this paper, the use of this technique for pyrometallurgical applications is explored in four steps. First, the exergy concept is introduced, the outline of exergy calculations is presented and the role of a reference state is discussed. Second, it is shown that an unambiguous exergy calculation for pyrometallurgical streams with a complex, unknown phase composition is not straightforward. Hence, a practical methodology is proposed in which a suitable phase-based stream description is estimated prior to the actual exergy calculation. For this, the equilibrium phase composition is calculated while all known stream properties are incorporated as boundary conditions. Thirdly, the proposed methodology is validated by recalculating literature results. This reveals significant deviations for exergy values of the same pyrometallurgical streams. Our results are probably more accurate due to the incorporation of additional phase related information. And fourthly, a full analysis of a zinc recycling process is presented. In a base case scenario, the total exergetic efficiency turns out to be only 1.2%. Based on this result, different process modifications are suggested and quantitatively evaluated. We find that significant efficiency gains are possible.status: publishe