VISCOSITY AND STRUCTURAL STATE OF IRON IN COAL ASH SLAGS UNDER GASIFICATION CONDITIONS

The effect of Fe3+/Fe2+ redox equilibrium in ferrosilicate melts and a sh slags on their viscosities is discussed. Viscosity experiments were carried out on two ash slags with low (Rochelle) and high (Illinois N o. 6) iron content in CO2/CO (40/60) and air atmospheres as a function of temperature and as a function of time at a constant temperature. T he lower values of viscosities measured in both ash slags under reduci ng conditions are assigned to the transformation of network-forming fe rric iron to the network-modifying ferrous iron. Anomalies in viscosit y are apparent in the reduced form of Illinois No. 6 ash slag during i ts oxidation in air at 1370 degrees C (slightly above the temperature of critical viscosity of the oxidized form of slag). First the viscosi ty significantly increases and then decreases to that of the oxidized form. The Fe3+/Sigma Fe ratio increases from 0.12 to 0.84. The conclus ion is that changes of local configuration in the melt (coordination n umber and charge of iron ions and stability and size of crystalline ph ases such as hematite and quartz) determine the viscosity in Illinois No. 6 ash slag. Redox ratios of iron and structural positions of Fe3and Fe2+ ions in quenched slags were determined with Mossbauer spectro scopy.