High purity silicon used for photovoltaic application, namely solar grade silicon (SOG-Si) , has been commercially supplied mainly from the off-grade high purity silicon manufactured by the Siemens process. However, recent and rapid growth in solar cell production has induced a serious shortage of SOG-Si. With an aim of resolving low productivity of the Siemens process, various types of SOG-Si production/purification processes have been invented as a post-Siemens process. Some processes are coming under development aiming at establishment of new commercial process. These processes can be classified into the following three technologies: (1) decomposition and/or hydrogen reduction of silane gases by improving the current commercial Siemens-based processes, (2) metallothermic reduction of silicon halide compounds by zinc or aluminum, and (3) upgrading metallurgical-grade silicon by employing metallurgical purification methods. This paper reviews various types of SOG-Si production processes, particularly the processes based on the hydrogen reduction and/or thermal decomposition of silicon halides and silane gases. These reduction processes are classified from the viewpoint of the silicon compounds and reaction types, and the features of the processes are analyzed. The future prospect for the development of new high purity silicon production process is presented.
A considerable amount of research has been carried out in the past few decades trying to understand the mechanics of hydraulic fracturing. Conventional modeling methods using a fracture mechanics suggest that a tensile crack is generated in hydraulic fracturing, and elongates in the major-principal-stress direction. Whereas, according to the acoustic emission (AE) events recorded during the laboratory and field hydraulic fracturing experiment, most AE classified as the shear type mechanisms. Thus, the hydraulic fracturing mechanism has not been sufficiently clarified. In this study, the flow-coupled DEM (distinct element method) simulations are performed to better understand the hydraulic fracturing mechanism and the influence of fluid viscosity. The DEM can directly represent grain-scale microstructural features of rock without complicated constitutive laws. This suggests that the DEM model may be more appropriate for the analysis of rock fracturing than other numerical analysis techniques. The simulation results were in good agreement with the actual experimental results. As the results, the followings were found. When the low viscous fluid is used, the fluid is infiltrated into the fracture instantaneously, and the energy emitted from the tensile crack is small compared with that from the shear crack. Such a small AE is easily buried in a noise and hard to be measured in an experiment, the shear type AE with large energy is dominantly observed in AE measurement experiment. On the other hand, when the highly viscous fluid is used, the fluid is infiltrated slowly into the crack after the fracture elongates first. At this time, fluid pressure becomes very high. Highly pressurized fluid causes large fracture opening, and generates tensile cracks which emit large energy. Due to this reason, both tensile and shear types of AE are observed during the injection of high viscous fluid in an AE monitoring.
Quite recently, a basaltic aquifer receives particular attention as a suitable candidate for CO2 aquifer storage because basaltic rocks contain high concentrations of Ca, Mg, and Fe that can enhance CO2 geochemical trapping via acid neutralization and carbonate mineral formation. Despite the increasing interest in basaltic aquifer CO2 storage, there are few experimental researches on CO2-water-basalt interaction. In addition, the influence of rock alteration on the geochemical trapping processes has not been considered. In this study, therefore, we conducted CO2-water-basalt interaction experiments using three types of fresh to altered basaltic rocks. The fresh basalt (non-altered basalt) is composed of olivine, plagioclase, and basaltic glass. On the other hand, the basic schist (high-T altered basalt) completely consists of secondary greenschist minerals (albite, epidote, chlorite, actinolite, quartz) , and the seamount basalt (low-T altered basalt) is mainly composed of secondary clay minerals (celadonite, smectite, Fe-oxyhydroxide) . In our experiments, the cations eluted from fresh basalt and basic schist are mostly Mg and Na, while the fluids reacting with seamount basalt contain large amount of Na and K with noticeably higher pH. This suggests that Na and K eluted from clay minerals play an important role in acid neutralization. Our experiments further demonstrate that, regardless of alteration types, a Na-dominant condition emerges at an early stage of the CO2 storage in a basaltic aquifer, leading to dawsonite (NaAlCO3 (OH) 2) precipitation. The present findings lead us to propose a new perspective on carbonate formation sequence of dawsonite → siderite → calcite/dolomite in basaltic aquifers.
Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. Some extraction methods of natural gas from the MH reservoir have been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of MH that is transformed into methane gas and water. However, there are some technical and economical problems for operation of these methods. Therefore, we have proposed a new enhanced gas recovery method by nitrogen injection. Nitrogen has the effect as an inhibitor as well as methanol and salts to shift an equilibrium condition of hydrate to high temperature and low pressure. In this study, we constructed a numerical model for simulating MH dissociation process in porous media by nitrogen injection on the basis of experimental observations. The gas phase was treated as a two-component system calculation of methane and nitrogen, and equilibrium calculation of methane-nitrogen system was introduced into the numerical model. Through the history-matching of temperature change and gas production behavior in laboratory-scale experiments, we confirmed the validity of the constructed numerical model. Then, using the numerical model for nitrogen injection process, we carried out field-scale simulations. From calculation results, it was found that 1) MH dissociation zone extended depending on pressure gradient to production well, 2) At early-stages of production, water was produced depending on relative permeability characteristics, and 3) Later of the production gas front containing nitrogen and dissociated methane reached to production well and gas production was initiated. Furthermore, we discussed gas production behavior for the gas recovery method that nitrogen injection and depressurization were combined. As a result, we obtained the important knowledge that this combination method had a large advantage for dissociated gas production when well distance was longer.
Coal bed CO2 sequestration, which separates and collects the CO2 generated by a large-scale point source and then injects it into a deep underground coal bed, can be regarded as a viable form of carbon capture and storage (CCS). However, many uncertainties are still involved, due to the fact that coal reacts with CO2 in a variety of ways. In verification tests at Yubari, CO2 was injected from an injection well into the coal bed at depth of 900 m, and methane was produced from an production well. Since the injection rate of CO2 was one tenth of magnitude that estimated by the preliminary analysis, N2 was injected in an attempt to improve its performance. While the injection rate of CO2 increased temporarily, it later decreased again in a short time. To try to clarify the phenomena observed in the Yubari trials, we conducted two types of laboratory tests under stress constraint conditions. In the test I, we injected liquid CO2 into a water-saturated core specimen and heated it to make supercritical CO2. This is based on the assumption that during the initial CO2 injection at Yubari pores of the coal bed were saturated with water. In the test II, we injected supercritical CO2 into the specimen saturated with N2, and then repeatedly injected N2 and CO2. This test corresponded to the N2 injection and CO2 re-injection at Yubari. During the test I, we observed a swelling strain of 0.25 to 0.5% after injecting CO2; during test II the swelling strain was 0.5 to 0.8% after injection of supercritical CO2. Following the further injection of N2 in the test II, slow shrinkage was observed. At effective confining pressure of 2 MPa, permeability of the water-saturated specimen was 2 × 10-6 darcy. In contrast, the permeability of the N2-saturated test specimen was originally ranged during 5 × 10-4 to 9 × 10-4 darcy, and after injection of supercritical CO2 it decreased to 2 × 10-4 darcy. Further injections of N2 and supercritical CO2 caused little subsequent change in permeability. It seems that when liquid CO2 is injected into the water-saturated specimen, it does not completely replace water in the coal matrix, based on residual amounts of CO2. To explain this behavior, we developed a model in which the CO2 permeating the coal is distributed in clefts shaped like flat cracks and in fine pores in the matrix interior.
The reduction stripping methods to generate Ag and Cu fine particles from Ag(I) and Cu(II) loaded VA 10 using aqueous ascorbic acid as a reductant were studied. Principal results obtained are as followed: (1) Since the pH for 50% extraction for Ag(I) and Cu(II) were 5.1 and 3.7, respectively, there is a possibility of selective extraction of those metal ions with VA 10. (2) In the stripping of Ag(I) or Cu(II) loaded VA 10, 100% reduction precipitation of fine Ag or Cu particulates can be obtained by choosing suitable stripping conditions, using an aqueous ascorbic acid as a reductant. (3) Complete reduction precipitation of Ag was achieved using an aqueous solution with a pH of 1.8-6.2 containing an amount of ascorbic acid equivalent to that of Ag(I). (4) To obtain fine Cu particles with 100% yield, we need to use 15 times ascorbic acid in a molar ratio at 333K for 24hrs. The average Cu particle size obtained under these conditions was 1.6 μm.