Thermodynamics of gas phase chromium species: the chromium chlorides, oxychlorides, fluorides, oxyfluorides, hydroxides, oxyhydroxides, mixed oxyfluorochlorohydroxides, and volatility calculations in waste incineration processes

Abstract

The thermodynamics of 12 gaseous chromium chlorides and oxychlorides and 12 gaseous chromium fluorides and oxyfluorides are assessed for application to chromium volatility calculations in thermal oxidation processes such as waste incineration. A method is also proposed for estimating the thermodynamics for mixed chromium oxyfluorochlorohydroxide species which contain varying proportions of oxygen, fluorine, chlorine, and hydroxide. The thermodynamic functions, C p 0( T) and −( G 0( T) − H 0(298))/ T, and the thermodynamic constants, S 0(298) and H 0(298) − H 0(0), for the gaseous chromium chlorides, oxychlorides, fluorides, and oxyfluorides are evaluated by the molecular constant method using molecular parameters taken from the literature or estimated from known constants for related vapor species. The enthalpies of formation of the gaseous chromium chlorides, oxychlorides, fluorides, and oxyfluorides are assessed based on available data in the literature. In some cases, only minimum values of the standard formation enthalpies can be established. The thermodynamic constants, S 0(298), ΔH f 0(298), and H 0(298) − H 0(0), are tabulated for all 12 chloride and oxychloride and all 12 fluoride and oxyfluoride gaseous species. Likewise, the thermodynamic functions, −( G 0( T) − H 0(298))/ T and C p 0( T), are tabulated in polynomial form as a function of temperature. In addition, the ΔH f 0(298) values for many of the chromium hydroxides and oxyhydroxides have been recalculated based on the revised ΔH f 0(298) values for the chromium chlorides, oxychlorides, fluorides, and oxyfluorides given herein. A general assessment of the volatility of chromium in waste incineration processes at temperatures ranging from 800 to 1600 K with p(HC1) = 0.010 atm and p(HF) = 0.0032 atm yields CrO 2Cl 2(g) as the dominant oxychloride and CrO 2F 2(g) as the dominant oxyfluoride species except at higher temperatures where CrO 2Cl(g) and CrOCl 2(g) are important and where CrO 2F(g) and CrOF(g) dominate. In addition to these oxychloride and oxyfluoride vapor species, other species such as CrO 2(OH) 2(g), CrO 2ClOH(g), and CrO 2FOH(g) are expected to be important. At higher temperatures, CrO 3(g), CrOClOH(g), CrO 2OH(g), CrO(OH) 2(g), and CrOOH(g) also contribute to the overall chromium volatility.