Melt/freeze heat transfer measurements in cryolite-based electrolytes

Taylor, Marilyn; Welch, B. J.

doi:10.1007/bf02656158

Cited by 25 publications

(39 citation statements)

References 8 publications

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“…The element mapping of the whole surface was carried out under low vacuum condition. Results were correlated with earlier published observations about crystal topography obtained on the same faces, using secondary electron imaging mode (SEI) (JEOL Ltd., Tokyo, Japan) with SEM (JEOL JSM 6480 LV with EDS Oxford SiLi detector, (JEOL Ltd., Tokyo, Japan) [10]. Figure 1 shows the SEM-SEI images of the surfaces of both transient and steady-state samples in a plane perpendicular to the probe-sample contact face.…”

Section: Chemical Element Mappingsupporting

confidence: 85%

“…using secondary electron imaging mode (SEI) (JEOL Ltd., Tokyo, Japan) with SEM (JEOL JSM 6480 LV with EDS Oxford SiLi detector, (JEOL Ltd., Tokyo, Japan) [10]. Figure 1 shows the SEM-SEI images of the surfaces of both transient and steady-state samples in a plane perpendicular to the probe-sample contact face.…”

Section: )mentioning

confidence: 99%

“…Some information can be found about the structure and the chemical composition of frozen bath in sideledge [2][3][4][5][6][7][8][9][10] and in the crust [1,11] using samples taken from industrial cells. However, those samples were formed under unknown and varying conditions.…”

Section: Introductionmentioning

confidence: 99%

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Impact of the Solidification Rate on the Chemical Composition of Frozen Cryolite Bath

Poncsák

Kiss

Guérard

et al. 2017

Metals

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Solidification of cryolite (Na3AlF6)-based bath takes place at different rates along the sideledge, and around alumina rafts and new anodes. The solidification rate has a significant impact on the structure and the chemical composition that determine the thermal conductivity and thus the thickness of sideledge, or the duration of the existence of the temporary frozen bath layers in other cases. Unfortunately, samples that can be collected in industrial cells are formed under unknown, spatially and temporally varying conditions. For this reason, frozen bath samples were created under different heat flux conditions in a well-controlled laboratory environment using the so-called cold finger technique. The samples were analyzed by X-ray Diffractometer (XRD) and Scanning Electron Microscope (SEM) in Back Scattering (BS) mode in order to obtain spatial distribution of chemical composition. Results were correlated with structural analysis. XRD confirmed our earlier hypothesis of recrystallization of cryolite to chiolite under medium heat flux regime. Lower α-alumina, and higher γ-alumina content in the samples obtained with very high heating rate suggest that fast cooling reduces α-γ conversion. In accordance with the expectation, SEM-BS revealed significant variation of the Na/Al ratio in the transient sample.

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Section: Chemical Element Mappingsupporting

confidence: 85%

Section: )mentioning

confidence: 99%

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Impact of the Solidification Rate on the Chemical Composition of Frozen Cryolite Bath

Poncsák

Kiss

Guérard

et al. 2017

Metals

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“…The convective heat transfer coefficient between the bath and freeze lining in aluminium reduction cells was also investigated in laboratory experiments [6,35]. A rotating steel tube, cooled by nitrogen gas, with a heat exchanger attached to the tip of the tube was used for this purpose.…”

Section: Previous Researchmentioning

confidence: 99%

“…From equations (2-3) and (2)(3)(4)(5)(6)(7)(8), it is clear that the interface temperature is one of the key factors determining the heat flow through and thickness of the freeze lining. This so-called hotface temperature has long been assumed to equal the liquidus temperature of the slag, based on observations in molten salt systems [36,50].…”

Section: Steady Statementioning

confidence: 99%

Fundamental studies on the chemical aspects of freeze linings

Crivits¹

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The application of freeze lining technologies has been shown to be highly effective in extending the working lives of pyrometallurgical furnaces operating under aggressive process conditions and corrosive liquid melts. Freeze linings are obtained by cooling the outer furnace walls, resulting in the formation of solid protective deposits on the inner walls of the furnace linings. Of particular interest to process operators are the freeze lining thickness and the heat loss through the furnace walls, both of which are directly related to the bath-freeze lining interface temperature.In most of the predictions for thermal steady state conditions, the assumption has been made that the temperature at the freeze lining/liquid interface is equal to the liquidus temperature of the bulk slag, and that the primary phase forms a dense sealing layer at this deposit/liquid interface. As a result of recent research on these systems, there is now extensive experimental evidence to demonstrate that the interface temperature can be below the liquidus and that the primary phase is not necessarily present at the deposit/liquid interface at thermal steady state conditions. These observations clearly indicate that other factors, in addition to thermal parameters of the system, need to be taken into account in the design of freeze lining systems.In the present study, heat and mass transfer and elementary reaction steps, and the associated thermal, physical, and chemical factors that can influence freeze lining behaviour are identified.From experiments undertaken using CaCl2-H2O solutions, accurate measurements have been made of deposit growth and microstructural changes taking place in the deposit over time were directly observed. These experiments have demonstrated the importance of nucleation rate of the primary phase and the thermal history of the freeze lining.High-temperature experiments in the 'Cu2O'-'Fe2O3'-MgO-SiO2 system in equilibrium with metallic copper have clearly demonstrated that at thermal steady state, bath/freeze lining interface temperatures are lower than the liquidus temperatures, and the possibility of operating processes at subliquidus bath temperatures. These experiments also indicated that an increase in bath temperatures results in an increase of the steady state interface temperature.High-temperature experiments in the Al2O3-CaO-SiO2 system were designed to study the effect of slag viscosity. Differences in the characteristics of the deposit/liquid interface were observed, however, it was unclear whether the observed differences in freeze lining behaviour were caused by the changes in viscosity, or the differences in morphology of the primary phase. The spontaneous decrepitation of freeze linings containing the dicalcium silicate phase on cooling clearly illustrated iv the need for caution when designing freeze lining systems that may form compounds undergoing significant changes in volume due to polymorphic transformations.The research supports the view that deposit/liquid interface temperatures in ...

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Dynamic Response of Cryolitic Bath and Influence on Cell Heat and Mass Balance With Large Scale Potline Power Shifts

Liu¹,

Taylor²,

Dorreen³

2016

Light Metals 2016

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An appropriate energy balance is critical to achieve high energy efficiency in aluminium electrolysis. However, to satisfy the electricity market, and the market demand for aluminium, a wider operational energy input window is now also required. Unfortunately this would require large scale adjustment of input power and heat dissipation, and is not possible with existing smelter technologies in which heat dissipations are not able to be adjusted widely or regulated. The associated wider cell operational window would also need to be matched to flexible energy supply without causing alumina concentration problems. Side ledge control is also a challenge under such a flexible energy input scenario, and the bath chemistry will be perturbed by dynamic ledge behaviour in conventional cryolitic baths. Thus, observations of the bath behaviour and heat transfer rates in the cell system are necessary in order to understand the limits of long term power adjustment. In this paper an analogue laboratory experiment is proposed, to model the industrial cell system and allow prediction of the effect of large scale power shifts.

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Melt/freeze heat transfer measurements in cryolite-based electrolytes

Cited by 25 publications

References 8 publications

Impact of the Solidification Rate on the Chemical Composition of Frozen Cryolite Bath

Impact of the Solidification Rate on the Chemical Composition of Frozen Cryolite Bath

Fundamental studies on the chemical aspects of freeze linings

Dynamic Response of Cryolitic Bath and Influence on Cell Heat and Mass Balance With Large Scale Potline Power Shifts

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