Chromium Volatilization Research Articles

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)2O3 initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.

Occurrence and volatilization behavior of Pb, Cd, Cr in Yima coal during fluidized-bed pyrolysis ☆

The volatilization behavior of Pb, Cd, Cr and the influence of coexisting mineral matters were investigated during pyrolysis of Yima coal in a fluidized-bed reactor at temperatures ranging from 500 to 900 °C. The modes of occurrence of Pb, Cd, Cr in Yima raw coal and two char samples were determined using density fractionation, demineralization and sequential chemical extraction methods. Lead in Yima coal is mostly associated with mineral matter in various forms, mainly pyrite, sulfates and monosulfides. Large part of cadmium is associated with pyrite. Chromium in Yima coal is mainly associated with organic matter. Different trends are observed for various forms of trace metals during pyrolysis. Lead associated with pyrite, sulfates and carbonates, lead in water soluble and ion exchangeable forms, and cadmium associated with pyrite are all unstable under pyrolysis conditions. During Yima coal pyrolysis, the volatilities of lead and cadmium vary greatly with pyrolysis temperature, while chromium volatility in Yima raw coal only has slight changes over the temperature range (500–900 °C) studied. The volatility of Pb, Cd, Cr in demineralized Yima coal (YimaD) is much higher than that of Yima coal during pyrolysis. New thermally stable forms of Pb, Cd and Cr are formed during pyrolysis of Yima, whereas the sources of them are different. The interactions between chromium and its coexisting mineral matters in Yima coal retard its vaporization during pyrolysis.

The Western Maryland coal combustion byproducts/acid mine drainage initiative: the Winding Ridge demonstration project

Small-scale combustion experiments were conducted to study the vaporization of trace elements during the combustion of pulverized coal. The combustion process was sampled at high temperatures (1423 K) corresponding to in-flame conditions. Rapid quenching and dilution of the combustion products were followed by a cascade impactor to collect size-segregated ash samples. Based on the comparison of concentrations of refractory elements in size-segregated ash with their bulk concentrations, we conclude that the ash having diameters less than approximately 0.4 micrometers represents the material which vaporizes during combustion in our combustion furnace. Good mass balance closure (90% to 140%) was obtained overall for ash in the sampling system. The volatile elements As, Sb, Se, and Zn showed mass balance closures significantly less than 100%. Thermochemical equilibrium predictions reported in the literature indicate that these elements should be completely volatile in the flame, although As is predicted to form condensed calcium arsenate at the sampling temperature in our system. Little chromium was found in the vaporization mode in our experiments. The uniformly low volatility of chromium suggests that chromium may react with oxides in the bulk ash. An upper bound on the amount of the more volatile elements which vaporized was computed. This upper bound accounted for the amount of the element in the finest ash and the material “losses” indicated by the mass balance closure. The results of these experiments suggest almost complete vaporization of certain trace elements (Se, Zn) from coal combustion in the flame zone, in accordance with theoretical equilibrium predictions. Other elements (As, Sb) appeared considerably less volatile and may react with constituents in the bulk ash at combustion temperatures.

Laboratory study of trace element vaporization from combustion of pulverized coal

Small-scale combustion experiments were conducted to study the vaporization of trace elements during the combustion of pulverized coal. The combustion process was sampled at high temperatures (1423 K) corresponding to in-flame conditions. Rapid quenching and dilution of the combustion products were followed by a cascade impactor to collect size-segregated ash samples. Based on the comparison of concentrations of refractory elements in size-segregated ash with their bulk concentrations, we conclude that the ash having diameters less than approximately 0.4 micrometers represents the material which vaporizes during combustion in our combustion furnace. Good mass balance closure (90% to 140%) was obtained overall for ash in the sampling system. The volatile elements As, Sb, Se, and Zn showed mass balance closures significantly less than 100%. Thermochemical equilibrium predictions reported in the literature indicate that these elements should be completely volatile in the flame, although As is predicted to form condensed calcium arsenate at the sampling temperature in our system. Little chromium was found in the vaporization mode in our experiments. The uniformly low volatility of chromium suggests that chromium may react with oxides in the bulk ash. An upper bound on the amount of the more volatile elements which vaporized was computed. This upper bound accounted for the amount of the element in the finest ash and the material “losses” indicated by the mass balance closure. The results of these experiments suggest almost complete vaporization of certain trace elements (Se, Zn) from coal combustion in the flame zone, in accordance with theoretical equilibrium predictions. Other elements (As, Sb) appeared considerably less volatile and may react with constituents in the bulk ash at combustion temperatures.

Laboratory Scale Thermal Plasma Arc Vitrification Studies of Heavy Metal-Laden Waste.

Plasma processing has been identified as a useful tool for immobilizing heavy metal-contaminated wastes into safe, leach-resistant slag. Although much effort has gone into developing this technology on a pilot scale, not much information has been published on basic research topics. A laboratory-scale plasma arc furnace located at the University of Illinois was operated in cooperation with the U.S. Army Construction Engineering Research Laboratories in an effort to establish an understanding of the chemical and physical processes (such as metal volatilization and resultant gas evolution) that occur during thermal plasma treatment of metal-spiked samples. Experiments were conducted on nickel and chromium using a highly instrumented furnace equipped with a 75 kW transferred arc plasma torch. The volatility of nickel and chromium was examined as a function of varying oxygen partial pressures. Oxidizing conditions reduced the total dust gathered for both the nickel and chromium samples, although each dust sample was found to be metal-enriched. Plasma treating increased the leach-resistance of the slags by at least one order of magnitude when compared to unprocessed specimens. The leach- resistance of the nickel-containing slags increased in the presence of oxygen, whereas chromium samples remained relatively constant.

95/04263 Thermodynamics of gas phase chromium species: The chromium chlorides, oxychlorides, fluorides, oxyfluorides, hydroxides, oxyhydroxides, mixed oxyfluorochlorohydroxides, and volatility calculations in waste incinceration

95/04263 Thermodynamics of gas phase chromium species: The chromium chlorides, oxychlorides, fluorides, oxyfluorides, hydroxides, oxyhydroxides, mixed oxyfluorochlorohydroxides, and volatility calculations in waste incinceration

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

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.

Thermodynamics of gas phase chromium species: The chromium oxides, the chromium oxyhydroxides, and volatility calculations in waste incineration processes

The thermodynamics of three gaseous chromium oxides and twelve gaseous chromium oxyhydroxides are assessed for application to chromium volatility calculations in waste incineration processes. For the oxyhydroxides, the thermodynamic properties, S 0(298), H 0(298) − H 0(0), −(G 0(T) − H 0(298)) T , and C p 0(T), are calculated by the molecular constant method using estimated molecular parameters. The enthalpies of formation, ΔH f 0(298), of the chromium oxides and the chromium oxyhydroxides are determined either from vapor pressure data when available or from bond energy correlations which use available data on the gaseous chromium oxyhalides. The thermodynamic constants, S 0(298), ΔH f 0(298), and H 0(298) − H 0(0), are tabulated for all fifteen gaseous species. Likewise, the thermodynamic functions, −(G 0(T) − H 0(298)) T and C p 0(T), are given in polynomial form as a function of temperature for ranges from 298 to 1000 K and 1000 to 3000 K. Equilibrium vapor compositions over Cr 2O 3(s) at fixed partial pressures of oxygen (0.10 atm) and water (0.10 atm) and temperatures in the range of 800 to 1600 K show that CrO 2(OH) 2(g) is the dominant vapor species with small contributions from CrO(OH) 3(g), CrO 3(g), CrO 2OH(g), and CrO(OH) 2(g).

Effects of organic and inorganic binding on the volatilization of trace elements during coal pyrolysis

The volatility of bromine, chlorine, vanadium, managanese, selenium, arsenic, chromium, and cobalt during coal pyrolysis was determined. Volatility was determined for both hydrogenated and nonhydrogenated coal, and for coal spiked and unspiked with inorganic and organic forms of the trace elements. Volatilization of selenium and arsenic was higher when those elements were present as inorganic sulfide salts. The importance of chemical forms of elements to rates of volatilization during pyrolysis is established. (1 diagram, 9 references, 4 tables)

Determination of Chromium in Bowen's Kale, NBS Bovine Liver, and Brewers Yeast, Before and After Dry Ashing

Abstract In view of the persistent controversy about the volatility of chromium in biologic materials, we determined the element by neutron activation analysis in three reference materials before and after dry ashing up to 450°C. We obtained the following mean values: 0. 364/μg/g and 0. 362 μg/g (Bowen's kale), 0. 0806 μg/g and 0. 0789 μg/g (NBS bovine liver), 2. 043 μg/g and 2. 094 μg/g (NBS brewers yeast). We conclude that chromium does not volatilize significantly in our experimental conditions of drying and ashing.

The volatility of chromium from brewers yeast during assay

The volatility of chromium from brewers yeast during assay

Determination of lead in stainless steel by atomic-absorption spectroscopy

A method has been developed for the determination of lead in stainless steels by atomic-absorption spectrometry after removal of iron by solvent extraction and volatilization of chromium as chromyl chloride. The method proposed is suitable for the determination of lead in the range from 5 to 100 ppm.

The oxidation of chromium diffusion coatings on iron: Chromium volatility and experimental techniques

The volatility of Cr 2O 3 has been studied during the oxidation of chromium diffusion coatings on iron in dry and wet oxygen at 1000°C. The results obtained in dry oxygen as a function of gas flow rate indicate that volatilization is controlled by diffusion in the gas phase. The free energy for the reaction Cr 2O 3 (s) + 3/2 O 2 (g)2CrO 3 (g) has been calculated to be 19.6 kcal/mole CrO 3 at 1000°C. Changes in the rate of Cr 2O 3 volatilization with time at 1000°C have been shown to be due to the gradual disappearance of Cr 2O 3 from the oxide scales and the formation of spinels and iron-rich oxides. It has been demonstrated that a controlled etch in dilute HF for 15 s is the most suitable type of pre-oxidation surface treatment. Annealing in argon, electropolishing and abrasion have been shown to be unsuitable. It has been established that the best method of mounting oxidized specimens for metallographic and electron probe microanalysis examinations is one involving vacuum impregnation followed, in the case of electron probe microanlysis, by deposition of an ∼ 100 Å thick carbon film. This is superior to using a thermosetting silver cement.

The oxidation of chromium diffusion coatings on iron: Chromium volatility and experimental techniques

The oxidation of chromium diffusion coatings on iron: Chromium volatility and experimental techniques