Modelling of the refining space working under reduced pressure

The refining channel working under reduced pressure and at medium temperatures was modelled experimentally and mathematically. The bubble growth rate was measured in the temperature interval 1200 to 1450 °C and pressures 15 to 50 kPa for the lead glass melt with and without refining agent. The bubble nucleation temperature was determined and the glass foam on the level was observed using high-temperature observation, video recording and image analysis. The temperature-pressure areas were defined in which the bubble nucleation and glass foaming did not interfere with efficient refining. The simplified mathematical model of bubble growth or dissolution under nonisothermal conditions was formulated and applied to the mathematical modelling of glass melt and bubble behaviour in the refining channel. Having applied the critical bubble pathways, the pull rates of the modelled channel have been calculated in the experimentally examined area of admissible temperatures and pressures. Besides temperature, the reduced pressure proved to be an efficient refining factor; the temperature increase by 35 Κ corresponded to the pressure decrease by about 10 kPa under given conditions. The results were discussed with respect to simplified relations between refining efficiency and temperature or pressure. The combination of experimental and mathematical modelling showed to be a sufficient and reliable tool for acquiring the fundamental parameters of the process under real melting conditions

Verification of refining models and their application

The study compares two models of single bubble behavior in glass melts

Redox equilibria of sulphur in glassmelts

The combination of experimental and computational methods used for the study of redox equilibria of sulphur in glassmelts makes possible to reveal mechanism and time sequence of redox phenomena taking place during melting process. Both methods were applied for the optimization of sc. reducing sulphate refining realized by the addition of sulphate and reducing agent to the glass batch. The results of experiments supported by the model calculation showed that proper combination of both components at the same time improves homogenization and refining of glass melt and suppresses unwanted bubble nucleation at maximal melting temperatures

REDOX EQUILIBRIA OF SULPHUR IN GLASS MELTS

INTRODUCTION Chemical refining performed by several types of refining agents is at present the most frequent way to remove bubbles from glass melts. Among refining agents, sulphates of alkali or alkali-earth metals play an important role being used namely for float and container glass. The sulphate refining however requires high melting temperatures, often above 1500°C. In addition, secondary bubbles formation (reboil) may appear at the melt-solid, i.e. at inhomogeneities, glass electrodes or refractory materials. These disadvantages may be suppressed by sc. reducing sulphate refining facilitated by the addition of reducing agent to the batch. Carbon in the form of graphite or coke is usually used. The use of blast-furnace slag containing up to 1 wt.% of sulphides is also possible. However, theoretical description of the interaction of reducing agent with sulphate during glass melting process is complicated. Depending on the redox state, several sulphur species including sulphates, sulphites, sulphides, sulphur dioxide and elementar sulphur can exist in the melt. The aim of this work is to interpret the role of combination of sulphate and reducing agents in glass melting and refining. To study this problem, the experimental methods of evolved gas analysis and direct high temperature observation of bubble behaviour were used to define appropriate chemical reactions and to find the time sequence of corresponding processes. Experimental findings than allow formulation of theoretical model based on the reactions between components, namely reducing agent, sulphate and sulphide and calculation of simultaneous chemical equilibrium of mentioned sulphur components. EXPERIMENTAL The study was carried out on a soda-lime-silica glass of the composition (wt.%): 72.7 SiO 2 , 1.5 Al 2 O 3 , 9.7 CaO, 2.6 MgO, 13.0 Na 2 O, 0.3 K 2 O. Granulated coke of an average particle size 0.5 mm was used as reducing agent. The addition of coke varied between 0 and 0.1 wt.%. The sodium sulphate was dosed to the batch to give the concentration in glass corresponding to 0.20 and 0.25 wt.% SO 3 . Evolved gas analysis of reaction mixtures was carried out by the gas chromatography. The reaction mixture in the test tube made of silica glass was placed in a laboratory furnace heated at a rate of 5°C/min. Gases evolved from the sample were flushed out to the sampling loops by the stream of air. The loops were repeatedly connected to the stream of carrier gas of the chromatograph. The result of analysis is presented in the form of temperature dependence of evolved amounts of CO 2 and SO 2 . The residual sulphate content in melted glass samples was determined by the helium extraction followed by gas chromatographic analysis of extracted gases. The melting of glass batches and the behaviour of bubbles in the melt was observed by a high temperature method. The glass batch was placed in a silica glass observation cell, which was inserted in the observation furnace tempered at 500°C. The sample was then heated to the final temperature 1500°C at the heating rate of 10°C/min. The course of batch melting and refining was evaluated by the measurement of the average bubble growth rate at final temperature by using the image analysis. Heterogeneous nucleation of bubbles was examined by the same observation method. The formation of bubbles on a PtRh wire was observed at the tem

THE SEMIEMPIRICAL MODEL OF THE MULTICOMPONENT BUBBLE BEHAVIOUR IN GLASS MELTS

A semi-empirical model of the bubble growth and dissolution in glasses with a fining agent has been derived. This model applies the experimental data from bubble observation at melting and fining temperatures. The experimental data needed for the model involved the temperature dependences of the average growth rate of the bubble radius and the average concentration of the fining gas in the bubbles. Both sets of values were measured in the laboratory in the glass of the float type and applied in the model. The measurements of the solubilities and diffusion coefficients of the gases present in the glass – needed for the analytical model of multicomponent bubbles – were thus avoided. The course of the partial bubble absorption with the temperature decreasing was simulated by means of two factors modifying the experimental values of the bubble growth rates at constant temperature. The temperature dependence of the resulting bubble growth rate qualitatively corresponded to the experimental observations in the soda-lime-silica glass, but a more detailed experimental and comparative study has yet to be performed. Such a study is being prepared

Degradation Processes of Medieval and Renaissance Glazed Ceramics

Corrosion effects in deposit environments (soil, waste pit, etc.), together with the glaze adherence and fit, could cause severe deterioration accompanied by different types of defects or growth of corrosion products. The aim of this work was to identify the source of surface degradation of the lead-glazed ceramics sets from the Prague area from the Romanesque to the Renaissance period. A combination of X-ray fluorescence (XRF), X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), and simultaneous thermal analysis (STA) techniques along with stress state calculations was used to study the defects. Based on the interpretation of the possible sources of the observed defects, four types of degradation effects were schematically expressed for the archaeological samples. It was shown that the glazes were already appropriately chosen during the production of the Romanesque tiles and that their degradation occurred only due to long-term exposure to unsuitable environmental conditions

Thermal properties of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids with linear, branched and cyclic alkyl substituents

International audienceThe present study is focused on four isomeric and quasi-isomeric ionic liquids, namely 1-alkyl-3-butylimidazolium bis{(trifluoromethyl)sulfonyl}imides, where alkyl stands for pentyl, isopentyl, 3-pentyl, or cyclopentyl. This series of ionic liquids has formerly been studied as to their densities and transport properties and has shown interesting behaviour with respect to their chemical structure [1]. This work extends our knowledge of the thermal properties of these ionic liquids. The isobaric heat capacity and the thermal stability were measured and put into relationships with the different chemical structures studied. Because of recycling of ionic liquids used, density was measured as a benchmark of the quality of material. Newly, the density data from this work were corrected for their viscosity using experimental viscosity data. Together with measurements of velocity of sound, these data provided basis for calculation of coefficients of isobaric thermal expansivity and isentropic compressibility, quantities that are related to heat capacity. Similarly to previous work, the isomerization was found to have a pronounced effect on studied properties and the present fundamental physico-chemical data are therefore useful both in the design of new ionic liquids as well in theoretical studies and molecular modeling. The data analysis and correlation were carried out using robust non-statistical methods based on mathematical gnostics

Metodika konzervování-restaurování glazované keramiky

Metodika je zaměřena na konzervování-restaurování glazované keramiky s ohledem na stabilitu a pórovitost keramického střepu. Podává ucelený přehled o vhodných postupech pro zpracování materiálu tohoto typu. Obsahuje přehled vhodných metod pro hodnocení stupně poškození (jak střepu, tak glazury), postupy odběru vzorku a metody pro identifikaci chemického a mineralogického složení keramiky i glazury a stability jejich celkového systému. Dále popisuje možnosti čištění, postupy konsolidace, lepení a doplňování chybějících fragmentů a případné retuše střepu i glazury. Obsahuje rovněž zásady pro vhodné uložení těchto předmětů a pro pochopení jednotlivých výsledků jsou uvedeny komentované příklady

Transient melt formation and its effect on conversion phenomena during nuclear waste vitrification – HT‐ESEM analysis

International audienceAbstract Although the vitrification of nuclear waste has a decades‐long history, numerous opportunities still exist to improve its efficiency and to increase the waste loading in glass. This is especially true for the vitrification of low‐activity waste (LAW), which has been historically treated by other immobilization technologies and is less mature than high‐level waste (HLW) vitrification. In this work, we address one of the least understood phenomena during the conversion of nuclear waste feeds to glass—the formation of molten salt and transient glass‐forming melt. Using high‐temperature environmental scanning electron microscopy (HT‐ESEM) in combination with X‐ray diffraction, thermogravimetry, and evolved gas analysis, we have analyzed the complex chemical reactions and phase transitions as they occur during melting of representative HLW and LAW melter feeds. We evaluated the compositions of amorphous phases and the fractions of salt components, and estimated the fractions of molten salt phases present in the feeds as a function of temperature. We show that the maximum fraction of molten salts is ∼4 % and ∼28 % during HLW and LAW feed melting, respectively, and discuss the possibility of molten salt migration in LAW feeds. We also argue that the presence of significant fractions of molten salt phase can hinder the retention of rhenium (and, hence, radioactive technetium), and discuss how the properties of molten salt phase and transient glass‐forming melt are related to primary foam formation and behavior. Finally, we summarize key unanswered questions requiring further research to increase the understanding of the conversion process and enhance the nuclear waste vitrification efficiency

Experimental and numerical investigations of an oxygen single‐bubble shrinkage in a borosilicate glass‐forming liquid doped with cerium oxide

International audienceThe shrinkage of an oxygen single-bubble is investigated in a cerium-doped borosil-icate glass melt at 1150°C. Nine glass samples are synthesized and investigated, utilizing three different amounts of Ce2O3 and three different redox ratios (Ce-(III)/Ce total). Employing in-situ observation, the single-bubble behavior is recorded with a camera. For each glass melt, five experiments are performed with different initial bubble radii. The shrinkage rate (da/dt) depends strongly on the cerium content as well as the redox ratio. Numerical calculations are also conducted to support the understanding of the bubble shrinkage mechanism in the given cases. The model adequately estimates the experimental data for several cases, and an explanation is proposed for the cases, in which it does not. Moreover, we demonstrate, physically and mathematically, the influence of the initial radius of the bubble on the mass transfer between the rising bubble and the melt. We confirm the utilization of the "modified Péclet number", which is a dimensionless number that takes into consideration the influence of multivalent elements on mass transfer. Finally, we master the bubble shrinkage behavior by normalizing the experimental data employing a characteristic time for the mass transfer (&tau)