Shunsuke Kashiwakura

Removal of arsenic in coal fly ash by acid washing process using dilute H2SO4 solvent.

Coal fly ash emitted from coal thermal power plants generally contains tens ppm of arsenic, one of the hazardous elements in coal, during combustion and their elution to soil or water has become a public concern. In this study, the acid washing process developed by the authors was applied to the removal of arsenic from coal fly ash. Laboratory- and bench-scale investigations on the dissolution behavior of arsenic from various coal fly ash samples into dilute H(2)SO(4) were conducted. Arsenic in the coal fly ash samples were dissolved into H(2)SO(4) solutions rapidly. However, its concentrations decreased with an increase in the pH of H(2)SO(4) solution in some cases. The species of arsenic in the dilute H(2)SO(4) was estimated as H(3)AsO(4), and its anionic species was considered to adsorb with the elevation of pH under the presence of ash particle. Such adsorption behavior was enhanced under the presence of Fe ion in the solution. The sufficient removal of arsenic was achieved by controlling pH and avoiding the adsorption of arsenic on the surface of coal fly ash particles, and the elution of arsenic from coal fly ash sample was successfully below the regulation limit.

Analysis of Atomic Scale Chemical Environments of Boron in Coal by 11B Solid State NMR

Atomic scale chemical environments of boron in coal has been studied by solid state NMR spectroscopy including magic angle spinning (MAS), satellite transition magic angle spinning (STMAS), and cross-polarization magic angle spinning (CPMAS). The (11)B NMR spectra can be briefly classified according to the degree of coalification. On the (11)B NMR spectra of lignite, bituminous, and sub-bituminous coals (carbon content of 70-90mass%), three sites assigned to four-coordinate boron ([4])B with small quadrupolar coupling constants (≤0.9 MHz) are observed. Two of the ([4])B sites in downfield are considered organoboron complexes with aromatic ligands, while the other in the most upper field is considered inorganic tetragonal boron (BO(4)). By contrast, on the (11)B NMR spectra of blind coal (carbon content >90mass%), the ([4])B which substitutes tetrahedral silicon of Illite is observed as a representative species. It has been considered that the organoboron is decomposed and released from the parent phase with the advance of coal maturation, and then the released boron reacts with the inorganic phase to substitute an element of inorganic minerals. Otherwise boron contained originally in inorganic minerals might remain preserved even under the high temperature condition that is generated during coalification.

Chemical state of boron in coal fly ash investigated by focused-ion-beam time-of-flight secondary ion mass spectrometry (FIB-TOF-SIMS) and satellite-transition magic angle spinning nuclear magnetic resonance (STMAS NMR).

The chemical states of boron in coal fly ash, which may control its leaching into the environment, were investigated by focused-ion-beam time-of-flight secondary ion mass spectrometry (FIB-TOF-SIMS) and satellite-transition magic angle spinning nuclear magnetic resonance (STMAS NMR) spectroscopy. The distribution of boron on the surface and in the interior of micron-sized fly ash particles was directly observed by FIB-TOF-SIMS. Coordination numbers of boron and its bonding with different atoms from particles of bulk samples were investigated by STMAS NMR. Boron in coal fly ash with relatively poor leaching characteristics appears as trigonal BO(3) and coexists with Ca and Fe at the outer layer of every particle and inside CaO-MgO particles. In contrast, boron in coal fly ash with better leaching characteristics appears as CaO- or MgO-trigonal BO(3) and tetragonal BO(4), and it is distributed only on the outer surface of each ash particle without showing any correlation with a particular element.

Dissolution behavior of selenium from coal fly ash particles for the development of an acid-washing process

Coal fly ash emitted from coal-fired electric power stations generally contains environmentally regulated trace elements. In particular, boron, arsenic, and selenium have been recognized as troublesome trace elements because elutions from the fly ash contain them. In order to design an effective removal process for these trace elements, we have developed and investigated an acid-washing process. The dissolution behavior of selenium from coal fly ash particles was focused on for the improvement of the process, and was found to greatly depend on the pH of the acid solutions. The species of selenium in solutions with a pH of around 0-1 was determined to be H2SeO3. The dissolved H2SeO3 transformed into HSeO3- and adsorbed onto the surface of the ash particles in solution upon elevation of the pH. The re-elution of the absorbed HSeO3- as SeO3(2-) at a pH of 10 was also confirmed, and will cause the elution of the excess selenium from the acid-washed ash.

Comparative Studies on the Excitation Mechanism of Fe II Lines in Low-Pressure Laser-Induced Plasma of Argon and Neon

ABSTRACT Emission characteristics of Fe II lines excited from low-pressure laser-induced plasma were investigated when Ar or Ne was employed as a plasma gas. Emission intensity measured in time-resolved mode was strongly dependent both on the kind of plasma gas and on the excitation energy of Fe ionic lines. In the initial plasma, just after the breakdown, fast electrons and gas species were major energy donors for excitations of ablated Fe atoms. With an expansion of the plasma plume, particularly intense emission lines of Fe ion appeared in Ar and Ne plasmas. Especially, different emission lines of Fe ion were selectively excited when Ar or Ne were employed as the plasma gas: for example, the Fe II 242.415-nm and the Fe II 248.424-nm lines in Ar plasma, and the Fe II 250.388-nm and the Fe II 257.686-nm lines in Ne plasma. The excitation mechanism for these ionic lines are different in both plasmas: a charge-transfer collision with Ar ion principally caused their excitations in Ar plasma, while a Penning-type collision with the metastable state of Ne atom could populate the corresponding excited levels of Fe ion in Ne plasma.

Fundamental Study on Ablation Sampling of Fe-based Binary Alloys in Laser-induced Breakdown Optical Emission Spectrometry

This paper describes the fundamental process of laser ablation occurring in a laser-induced plasma. The sampling process in laser-induced breakdown plasma spectrometry is very complicated and thus has not been fully understood. Our study focused on a relationship between the composition of ablation amounts and the bulk composition, when Fe-based binary alloys were employed as test samples. For this purpose, the ablation amounts of constituent elements in the alloys were determined by ICP-OES, through a method in which ablated deposits by laser irradiation were collected on a glass plate and then dissolved in an acid solution. In Fe-Ni binary alloys, the Ni content in the ablated deposits was almost the same as the bulk composition, which implied that Ni and Fe atoms evaporated along with the chemical composition of the samples; however, in Fe-Cr binary alloys, the Cr content in the ablated deposits was half of the bulk composition, probably because Cr atoms were difficult to be released from the sample surface. X-ray photoelectron spectra of ablated Fe-Cr alloy samples indicated that the resultant surface layer after laser irradiation comprised a thin oxide layer, consisting of Cr2O3 and FeO, and a relatively thick oxide layer beneath the outermost surface oxide, of which the composition was a complex of Cr2O3, Fe and FeO. The reason for this is that the dissociation energy of Cr2O3 is obviously higher than that of FeO, and thus Cr2O3 decomposed with more difficulty and thus left preferentially in a surface oxide layer of the Fe-Cr alloys. As a result, the Cr2O3 layer could suppress the ablation of Cr.