Models and Experiments for Sodium Evaporation From Sodium‐Containing Silicate Melts

Abstract

Universal simulation models based on (a) validated mass transfer relations and (b) thermodynamic modeling procedures for glass melts are developed to predict the evaporation rates of volatile species from a large range of glass melt compositions. Depending on the glass composition, temperature of the surface of the melt, local composition of the atmosphere, exposure time of a melt layer to the combustion atmosphere, and local gas velocities above the glass melt surface, the evaporation rates of volatile species can be estimated. Laboratory‐scale transpiration evaporation experiments have been used to study evaporation kinetics, to derive mass transfer relations, as well as to validate the sodium evaporation modeling results for sodium‐silicate melts as well as for soda‐lime‐silicate melts. In these investigations, the molten sodium‐silicate and soda‐lime‐silicate melts are exposed to atmospheres of flowing gases with controlled composition and gas flow rates. In a humid atmosphere for example, sodium mainly evaporates as NaOH. From the measured NaOH evaporation rates and the mass transfer relations, the NaOH vapor pressures in equilibrium with the molten glass at prevalent temperature and furnace atmosphere composition were derived. The NaOH vapor pressures are assumed to be in equilibrium with the glass melt composition at the surface of the melt. During the evaporation test, the Na2O surface composition will change due to depletion. This leads to equilibrium vapor pressures decreasing with time, reflecting the changing chemical activity at the glass melt surface. The results of evaporation tests for sodium‐disilicate and soda‐lime‐silicate glass melts are shown. Chemical activities derived from these measurements are compared with the results of thermodynamic modeling, using a method based on a glass melt from ideal mixtures of associate (stoichiometric) species of structural compounds with known thermodynamic properties.

Na2O solubility in CaO–MgO–SiO2 melts

“…6 for comparison (EMF measurements in galvanic cells: Neudorf and Elliott, 1980;Yamaguchi et al, 1983;Pak et al, 1989;Kim et al, 2004;Abdelouhab et al, 2008; Knudsen effusion cells mass spectrometry: Zaitsev et al, 1999;gravimetric method: Kawahara, 1984; Ion-Molecule equilibria method: Rudnyi et al, 1988; gas phase equilibration technique: Rego et al, 1985;van Limpt et al, 2006; and chemical equilibration method: Tsukihashi and Sano, 1985;Ivanov et al, 2004).…”

“…Other methods consist to control the alkali partial pressure by equilibrating molten silicates at high temperature with alkali metal vapor pressure established by a reference system composed of simple alkali-silica binary melts or salts. For instance, Lewis et al (1993) used NaCl as a carrier to produce Na-bearing vapors, Georges et al (2000) fixed constant PK (gas) by using a mixture of K 2 CO 3 and graphite, and van Limpt et al (2006) used the evaporation of NaOH from molten sodium disilicate glasses. The use of highly volatile carriers greatly limits the region of composition space that can be explored and yields correlative high alkali metal partial pressures in their devices (Lewis et al, 1993;Georges et al, 2000;van Limpt et al, 2006).…”

“…For instance, Lewis et al (1993) used NaCl as a carrier to produce Na-bearing vapors, Georges et al (2000) fixed constant PK (gas) by using a mixture of K 2 CO 3 and graphite, and van Limpt et al (2006) used the evaporation of NaOH from molten sodium disilicate glasses. The use of highly volatile carriers greatly limits the region of composition space that can be explored and yields correlative high alkali metal partial pressures in their devices (Lewis et al, 1993;Georges et al, 2000;van Limpt et al, 2006). In contrast, Karsrud (1984), Rego et al (1985Rego et al ( , 1988, O'Neill (2005), Borisov (2008Borisov ( , 2009, Borisov et al (2006Borisov et al ( , 2008, and Wood (2008, 2010) used Na 2 O-xSiO 2 , K 2 O-xSiO 2 , Li 2 O-xSiO 2 or Rb 2 O-yNa 2 OzK 2 O-xSiO 2 melt to impose a specific Na, K, Li or Rb metal vapor pressure over their samples.…”

Control of alkali-metal oxide activity in molten silicates

“…We notice for instance that, due to such changes in composition from the nominal stoichiometry of the melt source, O'Neill [19] determined only relative values for cNaO 1=2 activity coefficients in their silicate melts and, Van Limpt et al [18] and Borisov et al [20] were obliged to deal with very short duration experiments, e.g. merely few hours.…”

“…Other methods consist in equilibrating at high temperature molten silicates with alkali metal vapor pressure established by a reference system composed of simple alkali-silica binary melts or salts. For instance, Lewis et al [16] used NaCl as a carrier to produce Na vapours, George et al [17] fixed constant K vapor pressure by using a mixture of K 2 CO 3 and graphite, Van Limpt et al [18] used the evaporation of NaOH of the molten sodium-disilicate glasses, while O'Neill [19], Borisov et al [20] and Rego et al [21,22] : ð3Þ…”