Behavior of mercury release from iron ores during temperature-programmed heat treatment in air.

The behavior of Hg release from iron ores during temperature-programmed heat treatment (TPHT) in air is studied, primarily using an online monitoring method. The Hg release behavior during TPHT depends significantly on the type of ore being processed, involving the evolved forms, Hg0 and Hg2+, and those that remain thermally stable up to 950 °C. Furthermore, TPHT experiments for model Hg compounds suggest the presence of several types of Hg forms (HgCl2, Hg2Cl2, HgS, HgO, HgSO4, and associated mineral Hg) in the considered iron ores. The findings of this study provide insights for designing an efficient method for the removal of Hg from iron ore and gaseous Hg.

Production of Silicone Tetrachloride from Rice Husk by Chlorination and Performance of Mercury Adsorption from Aqueous Solution of the Chlorinated Residue.

The production of silicone tetrachloride (SiCl4) from rice husk char by chlorination was investigated, and the effect of the char preparation temperature on SiCl4 volatilization and the coexisting element species in the char was examined. The behavior of chlorine (Cl) and the change in pore properties during char chlorination were analyzed, and the reaction mechanism was discussed. The performance of Hg ion removal of the chlorination residue was also investigated. At 1000 °C chlorination, the optimum rice husk pyrolysis temperature for attaining high ash-release extent was 800 °C. Ash volatilization during char chlorination with heat treatment mainly occurred at >300 °C and reached a release extent of ∼75% by 1000 °C. Si and P volatilization started at >300 °C and reached 70-75% by 1000 °C. In contrast, Na and K the volatilization occurred at >700 °C, with a 50% volatilization extent by 1000 °C. Mg and Ca had a volatilization rate of <20% by 1000 °C. When the char was held at 1000 °C, the release extent of Si and P reached 75-80% by 10 min. Na and K volatilized almost completely by 10 min, and the release extent of Mg and Ca increased with increasing holding time and became 10-50% by 60 min. The Cl content in the residue obtained at each chlorination temperature increased from 300 to 700 °C and then decreased with increasing temperature. The majority of Cl taken up in the residue was an H2O insoluble form. The surface area and pore volume of the chlorinated residue tended to increase with increasing chlorination temperature, with the former increasing to 335 m2/g at 1000 °C and 10 min holding. The maximum mercury adsorption amount of the chlorinated residue obtained at 1000 °C, 10 min holding was 620 mg/g, indicating the mercury ion adsorption performance of the chlorinated residue.

Electronic State of Low-Rank Coals with Exchanged Sodium Cations.

Our previous experimental study showed that Na+-exchanged coal prepared from low-cost natural soda ash is an excellent catalyst for steam gasification of low-rank coals using fixed-bed quartz reactors. However, it is difficult to experimentally clarify the effect of Na ion exchange on low-rank coal. In order to investigate the influence of Na+ ions on low-rank coal, this study determined the electronic state between the Na+-exchanged coal model and raw coal model and compared them using RHF/6-311G* and B3LYP/6-31G*. The experiments revealed that Na ion exchange has a significant effect on low-rank coal gasification. The model structure of low-rank coal is considered to change significantly in terms of the electronic state before and after Na exchange even with a simple main molecular structure. Molecular models where H of COOH/OH was ion-exchanged with one, two, and three Na ions were developed, and quantum chemical calculations were performed. The results showed that when the number of Na+-exchanged sites is increased, the electron state on the coal molecule becomes more negatively charged in the case of the Na+-exchange coal model. It is presumed that this contributes to enhancing the reactivity of low-rank coal and water vapor. In addition, weak bonds in the Na+-exchanged coal molecule were examined by calculating the difference in the value of the Mulliken and Löwdin bond orders before and after Na+ exchange. The results showed that the increase in the number of exchanged Na+ in the low-rank coal molecule model increased the number of weak bonds in the molecule. It is presumed that this contributes to enhancing the decomposition of low-rank coal.

Upgrading Low-Grade Iron Ore through Gangue Removal by a Combined Alkali Roasting and Hydrothermal Treatment.

In this study, a combination of alkali roasting and hydrothermal treatment is used as a method of gangue (Si, Al, and P) removal from iron ores as a means to upgrade low-grade iron ore (limonite) into a high-grade iron ore with low gangue content, low porosity, and high Fe and Fe2O3 content to enhance the sustainable development of iron and steel industries. The effects of the combined treatments (NaOH hydrothermal treatment and H2O/NaOH hydrothermal treatment of the alkali roasted sample), the iron ore type, their physical properties, and their calcination/roasting temperatures on the removal extent of gangue are investigated. The extent of Si, Al, and P removal by subjecting iron ores to a 5 M NaOH hydrothermal treatment at 300 °C reached 10-91%, 39-70%, and 38-76%, respectively. When the iron ores are roasted with NaOH at 350 °C, α-FeOOH in limonite transfers to NaFeO2. On the other hand, for alkali roasted iron ores that inherently contain Fe2O3, Fe2O3 and Na2CO3 are also observed after the roasting treatment. Higher Al and P removal extents are observed for H2O leaching at room temperature in the prepared roasted samples (Roasting/H2O_RT) as compared to NaOH hydrothermal treatment, whereas that of Si is low for all samples, except the iron ore with the highest Fe content. After the H2O leaching process, the Fe form is found to be in the amorphous form for all samples, except for the iron ore sample of the highest Fe content. The reason for this is thought to be due to the large amount of unreacted Fe2O3 with NaOH during the roasting process. The specific surface area significantly increases after the Roasting/H2O_RT treatment in all samples due to the dehydration of goethite (α-FeOOH → Fe2O3 + H2O) during the roasting treatment and gangue removal during H2O leaching. When the roasted samples are supplied for hydrothermal treatment by H2O at 300 °C (Roasting/H2O_SC), the removal rate of Si and P increases as compared with the Roasting/H2O_RT treatment. The influence of temperatures of calcination and the roasting treatment on the extent of gangue removal in 5 M NaOH hydrothermal, Roasting/H2O_RT, and Roasting/H2O_SC treatments is small. When NaOH hydrothermal treatment is carried out on the samples that have undergone the Roasting/H2O_RT treatment, a gangue removal extent of above 70-97% was achieved, except for the iron ore with the lowest P content, which had the largest loss of ignition and the lowest Fe content. In addition, it is revealed that low-grade iron ore with a high pore properties, α-FeOOH content, and gangue content can be upgraded to a high-grade iron ore with a low pore property (low specific surface area and pore volume), high Fe2O3 content, and low gangue content using the above method. Therefore, this method is promising as a method for upgrading low-grade iron ore.

Chemical characterization of unburned carbon in coal fly ashes by use of TPD/TPO and LRS methods.

Functional forms of the unburned carbon present in six kinds of coal fly ashes have been examined mainly by the temperature-programmed desorption (TPD)/temperature-programmed oxidation (TPO) and laser Raman spectroscopy (LRS) methods. The carbon contents of the ash samples range from 0.4 to 4.1 mass%. The LRS analysis shows that the C consists of both amorphous and crystallized forms, and the proportion of the former is as large as 50-65 C%. Further, the TPD measurement exhibits that the C contains several types of surface oxygen species, such as carboxyl and lactone/acid anhydride groups, which can readily be decomposed into CO2 up to 700 °C to provide active carbon sites. The results of the TPD also indicate that the ashes have surface CaCO3, and most of this species can be converted to CaO and CO2 around 600-700 °C. Interestingly, there is a significant correlation between organic fluorine concentrations and carboxyl/lactone/acid anhydride groups or surface CaCO3 contents in the ash samples. It might thus be possible that the formation of organic F forms proceeds through gas-solid-solid interactions among HF (and/or F2) in flue gas, active carbon sites and surface Ca species produced around 600-700 °C downstream of coal-fired boilers.

Chemical characterization of dust particles recovered from bag filters of electric arc furnaces for steelmaking: some factors influencing the formation of hexachlorobenzene.

To make clear some factors controlling the formation of hexachlorobenzene (HCB) in the process of electric arc furnace (EAF) steelmaking, six dust samples recovered from different bag filters in commercial EAF steelmaking plants have been characterized with XRD, SEM-EPMA, XPS and temperature-programmed desorption (TPD) techniques. These dust samples contain 1.9-8.0 mass% of chlorine element, and the XPS and TPD measurements exhibit that the Cl is enriched at the dust surface and composed of the inorganic and organic functionalities, part of the Cl being evolved as HCl in the temperature region of flue gas treatment. All of the samples also include 2.1-6.4 mass% of carbon element, and some of the C can release CO(2) in the TPD up to 300°C to form active carbon sites. The number is related closely to HCB concentration of each dust. Further, it is suggested that the Zn present in the samples consists of ZnFe(2)O(4), ZnO and surface ZnCO(3), and the dust with a larger content of the ZnCO(3) has a higher concentration of HCB. It is possible that HCB formation occurs via gas-solid-solid interactions among gaseous Cl-containing compounds in flue gas, active carbon sites and surface Zn-species produced in exhaust ducts and bag filters.

Effect of nitrogen-containing compounds on polychlorinated dibenzo-p-dioxin/dibenzofuran formation through de novo synthesis.

An experimental study was conducted to clarify the suppression effect of nitrogen-containing compounds, that is, ammonia and urea, on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) through the de novo synthesis reaction. In the experiment, graphite and copper chloride contained in a mixture were used as sources of carbon and chlorine, respectively. The granulated sample mixture was charged as a packed-bed in the glass tube and heated at 300 degrees C in the flow of Ar-O2 gas mixture. In some cases, urea was added as aqueous solution to the sample, while ammonia was added to the gas flowed through the sample bed. The amount of PCDD/Fs formed decreases significantly by the addition of both ammonia and urea. Particularly, the addition of urea reduces the amount of PCDD/Fs discharged in the outlet gas by approximately 90%. The oxidation rate of carbon in the early stage of the experiment, that is, the heating period, is promoted bythe addition of nitrogen-containing compounds. However, soon afterthe temperature reaches 300 degrees C, the formation rate becomes lower than that of the case without the addition of nitrogen-containing compounds. On the other hand, organic compounds containing amino (-NH2) or cyanide (-CN) groups and those containing nitrogen within the carbon ring frame were detected in the outlet gas in the case of urea addition. Typically observed aromatic compounds are chlorobenzonitriles, chlorobenzeneamines, and chloropyridines. This suggests a possibility that hydrogen and/or chlorine combined with PCDD/Fs are also substituted by such nitrogen-containing groups, and this decreases the formation rate of their frame of carbon rings. This phenomenon was also consistent with the fact that a significant reduction was observed in the amount of PCDD/Fs released to the outlet gas when urea was added.