Phase diagram study for the alkali metal-oxychloride system
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I. INTRODUCTION
MOST municipal waste in Japan is incinerated, and gas, bottom ash, and fly ash are generated and processed, as shown in Figure 1. Fly ash contains entrained particles and condensed vaporized metallic compounds trapped in dust chambers. Both bottom ash and fly ash are low in density so they must be reduced in volume for treatment at reasonable cost. These two types of ash are, therefore, mixed and melted to calcify them, but this process, in turn, generates secondary fly ash that contains heavy metals and chlorine. Basic elements of the secondary fly ash are alkali metals, heavy metals, silicon, aluminum, calcium, chlorine, and oxygen. The properties of these mixed oxychlorides is fundamental not only in the treatment of secondary fly ash but also in treatment of various other wastes. It is rarely found in the natural environment, only in artificial compounds, so very few studies have been made. The objective of this work was to understand the thermodynamic nature of waste-generated oxychlorides, to make these materials nonpoisonous stock. As a first step, the liquidus of the sodium-silicon-chlorine-oxygen system and the lithium-silicon-chlorine-oxygen system were studied. As a preliminary experiment, the NaCl-Na2CO3 system was examined by a hot filament technique. We measured the liquidus of the NaCl-Na2SiO3 system using a chemical equilibrium technique. A new method using the hot filament technique was developed for a sample with relatively high vapor pressure, and the liquidus for the NaCl-Na2SiO3 and LiCl-Li2SiO3 systems were obtained. Thus, we verified the validity of results obtained by the hot filament and the chemical equilibrium techniques. II. PRELIMINARY HOT FILAMENT EXPERIMENTS A schematic of the apparatus used in the hot filament technique is shown in Figure 2. A thermocouple (0.5 mm f Pt-6 pct Rh/Pt-30 pct Rh) is used to hold the sample, to heat it, and to measure the sample temperature all at the same
time.[1,2,3] We placed the sample on the filament, changed the temperature, and observed the state of the sample with an optical microscope; direct in situ observation was possible. The temperature was calibrated by measuring the known melting point of a pure substance such as Na2SO4, KBr, NaCl, or KNO3. The relationship between the temperature measured by the hot filament technique and reported melting points is shown in Figure 3, and it is linear. The hot filament technique was used to examine the NaClNa2CO3 system. The mixture was placed on the filament and slowly heated in the hot filament cell. This cell was purged with argon gas to eliminate water vapor; the gas was introduced at a flow rate of 50 mL/min. First, we observed the solid phase (Figure 4(a)) and then the solid-liquid phase (Figure 4(b)). We then observed the solid-liquid phases change to a complete single liquid phase (Figure 4(c)). Samples were quenched for chemical analysis. In a sample with a composition close to the eutectic composition, we observed the components shown in Figure 4(d) after quenching. Results obtai
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