The effect of chlorine on the viscosity of Na2O-Fe2O3-Al2O3-SiO2 melts

Abstract

The shear viscosities of Cl-bearing melts in the system Na 2 O-Fe 2 O 3 -Al 2 O 3 -SiO 2 were determined at temperatures of 550-950 °C in the range of 10 8.5 to 10 12.0 Pa s using the micropenetration technique. The compositions are based on addition of Fe 2 O 3 or FeCl 3 to aluminosilicate glasses with a fixed amount of SiO 2 (67 mol%). Although there was loss of Cl - during the glass syntheses, no loss occurred during the viscometry experiments. It is to be expected that Cl - takes the structural position of O 2- , and thus reduces the polymerization of the melt structure, and therefore the viscosity of the melt; as F - does. Our measurements show that the presence of Cl - increases or decreases the viscosity of the melts as a function of melt composition. In the present melts, at least 10% of the Fe exists as network-modifying or charge-balancing Fe 2+ ; whereas the rest exists as network-forming Fe 3+ . It is proposed here that the different effects of Cl on viscosity are due to the preferred Cl - -Fe 2+ NBO bonding together with the different structure of peralkaline and peraluminous melts. In peralkaline aluminosilicate melts, the addition of Cl 2 O -1 will destroy 2 NBOs and create one BO if Cl - bonds primarily to the Fe 2+ creating non-bridging O atoms. This would result in an increase in viscosity. In peraluminous melts, the addition of Cl 2 O -1 may result in Cl - bonds to the charge-balancing Fe 2+ , creating 2 new tri-clusters [assuming (Al 3+ ,Fe 3+ )Si 2 O 5 tri-clusters exist]. The preference of Cl - to form bonds to the NBO-forming Fe 2+ is indicated by the small amount of Cl - soluble in the peraluminous melt structure in comparison to that soluble in the peralkaline structure.