Vol.5 No.1 2012
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Research paper : An analysis method for oxygen impurity in magnesium and its alloys (A. Tsuge et al.)−27−Synthesiology - English edition Vol.5 No.1 (2012) the oxygen analysis in Mg, because Mg has a low boiling point and high oxygen affinity.Figure 3 shows the result of monitoring the extracted amount of carbon monoxide (or the temperature-increased inert gas fusion profile) when the magnesium oxide was placed in the graphite crucible with increase in the applied power. Although the carbon monoxide extraction started at around 2,400 W, the temperature of the graphite crucible corresponding to the applied power was estimated to be about 2,000 °C, being much higher than the boiling point of Mg, 1,090 °C. It implies that before the magnesium oxide in Mg starts to react with carbon, the matrix material Mg begins to boil at lower temperature. When the sample boils, the molten sample in the crucible causes boiling out due to the force of generated Mg vapor and eventually makes the analysis impossible.To solve this issue, we adopted the “multistep temperature increase method” where the oxide included in the sample is separated and followed by the oxygen analysis. This method is inspired by the refining method of iron and Mg.As mentioned earlier, the IGF-IRA method uses the reduction reaction of the sample by carbon. This is an irreversible reaction where the gaseous reaction product, carbon dioxide or carbon monoxide, desorbs from the reaction system. It can be considered that oxygen analysis corresponds to obtaining the gas phase product (carbon monoxide), contrary to obtaining the liquid phase one (metal) in iron refining.In the Pidgeon method, which is the major thermal refining method for Mg, the raw material of magnesium oxide ore is mixed and heated with the powder of iron-silicon alloy (ferrosilicon). When the oxygen in magnesium oxide transfers to the ferrosilicon, the metalized Mg evaporates and desorbs from the reaction system. This reaction is not associated with reverse reaction, where the oxygen transfers to the ferrosilicon, and resulting in the collection of the evaporated Mg by coagulation in the low temperature area. In other words, the metal Mg is refined from the gas phase product of the irreversible reaction. Therefore, we reached a conclusion that analysis of the residual liquid phase, in which the oxygen was remaining, provides the oxygen content.When certain metal with higher boiling point than Mg is mixed with the Mg sample as the oxygen receptor, the oxygen would remain in the metal even after Mg evaporated from the sample. The oxygen that desorbed from the Mg sample could be measured as the oxygen in the receptor metal.At the beginning of the research, we believed that it was necessary to transfer the oxygen of the oxide in the Mg sample to the receptor by redox reaction, and that a metal with strong oxygen affinity like aluminum was considered as a receptor candidate. However, we eventually came across an idea that the magnesium oxide, which is the primary oxide in the sample, can be directly transferred to the receptor. Consequently, tin (Sn) was selected as the candidate of oxygen receptor metal. Since Sn has a low melting point of 232 °C but has a high boiling point of 2,602 °C, it mixes with the molten Mg in a liquid state, and the oxide was expected to be received into it easily. Also, Sn is often used as the metal bath material in the inert gas fusion process and has advantages of low oxygen content as well as ready availability. On the other hand, aluminum is not usually used as the metal bath material, and specimen of low oxygen content is not readily available.Figure 4 shows the conceptual diagram of the newly developed analysis method in which Sn is used as the oxygen receptor metal. After 0.3 g of Mg was melted with 0.5 g of Sn at about 900 °C, the temperature was increased gradually to 2,000 °C in more than 1,000 seconds. The residue found at the bottom of the crucible was slightly less than 0.5 g. As the vapor pressure of Sn at 2,000 °C was a few kPa, it is considered that Sn evaporated resulting in the weight loss from the original amount. This temperature also surpassed the boiling point of Mg, and it is inferred that Mg was almost thoroughly evaporated off. The appropriate heating condition for separating Mg from a receptor was determined through weight measurement of the eutectic material with systematic changes in temperature increasing rate and heating duration.The residue in the crucible was considered to be composed Fig.3 Temperature-increase inert gas fusion profile of magnesium oxideFig. 4 Conceptual diagram of the analysis method for oxygen in magnesium by multistep temperature increase method02000 040 2400 3600 4800 6000 80 120 160 200 240 280 320 360 400 1200 1000 3000 4000 5000 6000 Rate of carbon monoxide extraction (arbitrary unit) Measurement time (s)Applied power (W) 2000 ℃ 900 ℃400s Sn 0.5 g Mg 0.3 g 900 ℃ 2000 ℃ 1000s 2500 ℃ CO MgOMgOMgOMgO+CMg+CO MgSn SnMgTo detector

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