استخراج سریوم با کاهش متالوترمیک با استفاده از تزریق پودر اکسید سریوم Cerium extraction by metallothermic reduction using cerium oxide powder injection

Al-Si alloys are mainly used in the automotive and aerospace industries[1]. The demands on their mechanical properties require a strict control on the additions of refinements and modifying agents in the molten bath. The wide variety of alloys that are produced has led to the production of master alloys that are of great importance in the aluminum smelting (Al-Sr, Al-Ce) industry. The use of rare earth metals such as cerium has been the focus on changing the morphology of the silicon eutectic in Al-Si alloys, resulting in better physical and mechanical properties[2]. Cerium is the most abundant element in the family of rare earth metals or lanthanide elements[3,4]. The objective of this study was to make a master Al-Mg-Ce alloy employing the submerged powder injection technique (reactive; CeO2) due to its very attractive advantages from both a technological and economic point of view. Also, the results concerning the feasibility to incorporate metallic cerium into an Al-Mg liquid alloy from a metallothermic reduction of cerium oxide on a laboratory scale were presented. ۱ Experimental The experimental trials were carried out in a high-frequency induction furnace with a 15 kg molten aluminum capacity. The powder injection equipment allowed for continuous and controlled feeding of solid material through an inert carrier gas. This equipment is frequently used in the process of Mg and Sb removal from molten aluminum alloy[5,6] and in the fabrication of master alloy Al-Mg-Sr[7,8]. The reactant elements used in the experiments were Al and Mg (98% purity) and CeO2 (–۱۴۰+۲۰۰ mesh[7]). The selected experimental variables and their levels were temperature of treatment (800 and 850 °C) and magnesium content (0.5 wt.%, 3.0 wt.% and 4.0 wt.%). The following parameters were kept constant: amount of liquid alloy (8+/–۰٫۱ kg) and powder flow rate to carrier gas flow rate ratio (8 g CeO2/min:6 L Ar/min). The experimental trials were conducted as follows. The Al-Mg alloy was melted in the induction furnace to the required temperature. The injection equipment loaded with CeO2 particles was positioned right above the top of the furnace. During experiments, a graphite lance was submerged into the melt to an 85% depth from the surface of the batch[9]. No fluxing or degassing of the melt was performed after melting in the final alloy, so the alloy was only treated for cerium increase. Finally, to obtain kinetic and metallographic information, the samples obtained were taken at 10 min intervals and analyzed by X-ray fluorescence, inductively coupled plasma spectrometry and scanning electron microscopy. In each experiment, dross samples taken at the end were analyzed by means of X-ray diffraction for the qualitative identification of the compounds. ۲ Results and discussion ۲٫۱ Chemical analysis From the chemical analysis results shown in Fig. 1 (a), it is observed that cerium oxide is being reduced by a metallothermic mechanism because the cerium content in the liquid bath increases with the injection time. The cerium content in the Al-Mg alloy increased to 4.3 wt.% according to the following conditions: temperature, 850 °C; magnesium content, 4 wt.%; CeO2 powder particle size, −۱۴۰+۲۰۰ mesh; treatment time, 60 min. Both the temperature and initial magnesium concentration affected the cerium incorporation rate as shown in Fig. 1 (b). As can be observed, for all initial magnesium contents and both temperatures, 800 and 850 °C, the reaction rate between the CeO2 particles and the melt alloy increased as the initial magnesium contents or temperatures increased, obtaining higher final cerium levels in the treated alloy. This behavior is associated with the chemical activity of both Mg and Al present in the bath during the treatment process. According to the literature[10,11], this phenomenon is due to the surfactant nature of magnesium because the surface tension values of this element are lower compared with those of pure aluminum (γAl=0.914 N/m, γMg=0.559 N/m). Therefore, we have concluded that the addition of magnesium reduces the surface tension of the liquid aluminum bath, which leads to an improvement in the wettability between the solid reactant and the liquid metal and, thereby, increases the kinetics of the reactions taking place at the solid-liquid interface. In this way, Mg enhances the reactions that take place at the solid-liquid interface and increases the amount of dissolved cerium in the molten bath. During testing, the initial Mg content decreased from 4 wt.% to 3.2 wt.%; at the same time, the cerium incorporation increased from 0 up to 4.3 wt.%. Similar tests[7] have been carried out in maintaining an Al-Mg liquid at a temperature of 850 ºC for up to 2 h. The reported results indicated that the loss of magnesium by oxidation or evaporation was not significant. Therefore, it can be assumed that the loss of magnesium is due to reduction reactions that occur during the stirring process.