CC BY
23PRODUCTION OF METALS AND ALLOYS THROUGH VACUUM METALLOTHERMIC PROCESS
O. Yucel*", K. C. Tasyurek", M. BugdayciA, and A. TuranA
aIstanbul Technical University, Metallurgy and Material Engineering Department, Chemical and Metallurgical Engineering Faculty, Istanbul, 34469 Turkey
bYalova University, Chemical and Process Engineering Department, Faculty of Engineering, Yalova, 77200 Turkey *e-mail: yucel@itu.edu.tr
DOI: 10.24411/9999-0014A-2019-10194
Mg is a silver grey metal and the consumption of magnesium in many fields, such as aircrafts, rockets and automobile industry, is expected to increase rapidly for the next decade, because magnesium has the lowest density as 1.738 g/cm3 in all structural metals and its strength/density ratio is very high. Sr has 2.63 g/cm3 density it has also a place around the light materials and a candidate material for grain refinement of magnesium. Ca has 1.55 g/cm3 density it has also a place around the light materials. Its physical and chemical properties are very similar to its heavier homologues: elements like strontium and barium. It is the fifth-most abundant element on earth.
The world's demand for magnesium is increasing by about 10% per year. However, the mechanical properties and processing performances of the developed-magnesium alloys such as AZ series alloys still not enough for some important parts in transportation, aeronautical, and helicopter parts. In order to improve this properties a lot of processes were investigated in the world, grain refinement is an alternative method for this studies. Recent results indicated that Sr element which has been widely used in industrial practice especially for the modification of Al-Si alloys, was potential effective additions of grain refinement for magnesium alloys. However adding pure Sr to Mg alloys causes burning loss. Thus, using Mg-Sr master alloys is effective way to alloying Mg. Xiang-guo et al. applied melt-leaching-reduction (MLR) process alloying Mg with Sr in order to avoid burning loss and they produced Mg-Sr alloys. Vacuum metalothermic procees is the main production method for magnesium in last decades. This process reduction is highly endothermic and it needs vacuum. Also Strontium and Ca reduction is possible by this method from their oxides. This method is an alternative process to MLR technique. In MLR tecnique metallic Sr used as strontium source which was produced from SrO. Mg-Sr alloy can produce via vacuum metallothermic process which is a one step process to obtain desired alloy and has advantage to MLR tecnique because SrO used as Sr source.
In silicothermic Mg production route, Pidgeon process is inefficient when the Mg source is MgO due to the reaction product which is in the form of MgO.SiO2. The formation of that compound stops the reduction. To avoid MgO.SiO2 formation, SiO2 activity must be decreased with some additives. In this case CaO is the most suitable additive which is provided by using calcined dolomite as reactant. The reduction of Mg from MgO becomes easier and the reduction efficiency of Mg increases in accordance with the formation of CaO.SiO2 structure in reaction products. Because of that reason, in industrial silicothermic process calcined dolomite is used as a raw material instead of MgO.
In the present study, experimental sets were developed to understand the effects of reductants type on the vacuum metallothermic process of calcined dolomite. In the first set, the change of Mg recovery was investigated with the increase in charge (reactant) weights in the case FeSi (100% stoichiometric) was used as reductant. Two different retorts were used in this experimental set. The experiment with 50 g charge weight was carried out in 1 liter (l) retort,
others (2000 g, 3000 g, and 5000 g charges) were executed in 10 liter retort at 1250°C under 1 mbar. In all experiments process durations were 6 hours (Fig. 1a). In the second experimental set, effects of Si, FeSi, and Al reductants were examined. The experiments were conducted in 1 liter retort. Effect of process duration on Mg recovery was investigated for 60, 120, 180, 240, and 300 min (Fig. 1b).
Fig. 1. (a) Mg recovery calculated from both residue and crown with the increase in charge weight. 50 g experiment was carried out in 1 liter retort, others in 10 liter retort (1250°C, 1 mbar and 6 h), (b) Mg amounts in residues and Mg recovery ratios for Si, FeSi, and Al reductants with increasing process duration (2.5% CaF2 addition at 1200°C, 50 g, 1 liter, 1 mbar). Reductants are Si (□, ■), Al (A, ▲ ), and FeSi (o, •), (c) The change of Mg recovery calculated from residue with increasing CaC2 addition ratios at different temperatures.
In the first group experimental set, effect of charge amount was investigated on magnesium recovery. 2000 g and 3000 g charge amounts respectively presented the highest recovery rates which were calculated from residue. The highest Mg recovery was detected from the charge amount of 50 g as 98%. In this experiment, crown Mg efficiency cannot be calculated, because crown Mg amount was not enough to carry out analyses. On the other hand, the highest Mg recovery calculated from crown was determined as 90% for the experiment which was conducted with 3000 g charge amount Fig. 1a. When the difference is evaluated between the Mg recovery rates calculated from residue and crown for the same conditions (such as reactant weight of 50 g) show that Mg was highly reduced, but the conditions were not enough to collect in the form of crown in a high recovery ratio. It was understood that increasing charge weight firstly affected Mg reduction rate negatively, but it had a positive effect on the collected Mg amount in condensation zone. Mg recovery, calculated from residue, decreased from 98% to 90% whereas Mg recovery, calculated from crown, increased from 90 to 92% when charge amount increased from 2000 to 3000 g. According to the results, reduction duration must be extended with increasing amount of charge. In the second experimental set, effects of Si, FeSi, and Al reductants were examined. The experiments were conducted in 1 liter retort. Effect of process duration on Mg recovery was investigated for 60, 120, 180, 240, and 300 min (Fig. 1b). In the third experimental set, 100% stoichiometric 50 g mixtures were prepared. Stoichiometric amount of the reducing agent was calculated through sum of the oxides in the calcined dolomite which are MgO, FeO and SiO2. The mixture stoichiometric ratios were changed from 100% FeSi-0% CaC2 to 50% FeSi- 50% CaC2 with 10% intervals. The change of Mg recovery with increasing CaC2 addition ratio in FeSi was carried out at 1200 and 1250°C under 1 mbar vacuum
atmosphere for 6 h (Fig. 1c). In the last experimental set, the experiments were conducted with increasing CaC2 addition and in different volumes of retorts as 1 and 10 l.
Strontium oxide with Al, BaO, CaO, and CaC2 subjected to reduction process under an average process pressure of 2 mbar, at 1050, 1100, 1150, and 1200°C temperatures for 60, 120, 180, and 240 min (in 1 liter retort). During SrO vacuum aluminothermic reduction, high amount of Sr-aluminate structure obtained. In order to eleminate this structure some functional additive added to green mixture such as BaO, CaO, and CaC2. Addition BaO is essential for high efficiency of strontinum reduction. Temperature must be above of 1150°C, 1250°C is enough. The maximum recoveries were observed as 96.89% with addition of 300% stoichiometric Al and BaO for 240 min and 96.87% with addition of 300% stoichiometric Al and BaO for 240 min. Sr recovery increases with increasing experiment time. Sr recovery increases with increasing stochiometric ratio of Al and BaO (Fig. 2a). In the CaO addition set, effect of time was examined in constant temperature 1250°C. Rising duration of experiment exponentially increase the efficiency of Sr up to 360 min, after this value, it is not changed. The highest Sr recovery detected at 1250°C for 480 min in experiment with 77.43%. Figure 2b presents effect of CaO addition on the Sr recovery at ranging durations.
Fig. 2. (a) Effect of BaO addition on Sr recovery (1 liter retort, 1250°C, 1 mbar), (b) Effect of CaO addition on Sr recovery (1 liter retort, 1250°C, 1 mbar), (c) Amount of Sr in residue with BaO addition (1 liter retort, 1250°C, 1 mbar), (d) Amount of Sr in residue with CaO addition (1 liter retort, 1250°C, 1 mbar).
In Mg-Sr alloying study, combine metallothermic reduction of calcined dolomite and SrO production conditions alter to MLR process were investigated. In experiments % 100 stochiometric mixtures prapared for Mg reduction from calcined dolomite, than (weight %2.5, %5, %7.5, %10 ) stochiometric SrO mixtures were added to charge. In the first group of Mg and Sr reduction experiments, FeSi used as a reductant in order to reduce calcined dolomite. In the second experimental set Al used as a main reductant to reduce calcine dolomite. For SrO reduction, Al used as reductant for all experiments. In the experimental set using the FeSi reductant to reduce magnesium, the highest Mg recovery was determined at 1250°C, with the 5% Sr addition with 79.3%, the highest Sr recovery was in the 2.5% Sr mixture addition experiment, at 1250°C, with 63.5%. In the experimental set using the Al reductant to reduce magnesium, the highest Mg recovery was determined at 1250°C, with the 2.5% Sr addition with 89.8%, the highest Sr recovery was in the 7.5% Sr mixture addition experiment, at 1250°C, with 78.6%.
In the last experimental set, metallic calcium production by the metallothermic process in a vacuum atmosphere were investigated. In this experimental set authors aimed evaluation of
magnesium production slag as a raw material for the production of metallic calcium. Conventionally, in metallic Ca production, limestone is used as Ca source. In that process there is a calcination step in order to obtain CaO. In this study, it is also aimed to decreasing of energy consumption required for calcination, decreasing of CO2 emission and decreasing disposal Mg production slag. In the experiments, Al powder is the only reductant used for metallothermic calcium production. The effects of Al stoichiometry, time variances, and temperature changes were investigated. The experiments were carried out at 1200, 1250, and 1300°C, and with 100, 125, and 150% Al stoichiometry to produce metallic calcium from the residue of metallic magnesium production. According to the experimental results, the highest recovery rate parameters for the reduction of calcium are 150% stoichiometric Al for 480 min at 1300°C, with 72% recovery (Fig. 3).
These reduction system were thermodynamically modelled by using FactSage 6.4 software. Table 1 was plotted to understand the change of the reduction conditions of Mg from pure MgO and calcine dolomite by Si, Al and CaC2 under 1 bar and 1 mbar pressures respectively. According to the thermodynamical investigations, the formation temperatures of the reactions are very high under 1 bar atmosphere pressure. In the condition which the reduction temperatures with others are higher than Al, these are 2143, 1789, and 1847°C for Si, FeSi, and CaC2 respectively for 1 bar. According to Fact Sage 6.4 software, the reaction beginning temperatures are 850°C for Al, 1155°C for FeSi, 1200°C for CaC2, and 1325°C for Si under 1 mbar pressure. Table 2 presents minimum reduction temperature of SrO production. According to calculation, the reduction of Sr with Si, Al, FeSi, and CaC2 at 1536, 996, 1618, and 1312°C is possible at atmospheric pressure of 1 mbar, so Al is a better choice for reduction. These values are 2654, 1884, 3050, and 2102°C for Si and Al respectively in vacuum-free atmospheric conditions. Due to the high temperature requirement in Sr production, it is necessary to use vacuum in this process. Table 3 gives reduction temperatures of MgO and CaO with the Al reductant from Mg production slag, and it can be clearly seen that CaO reduction starts at 1298°C.
Table 1. Minimum reduction temperatures of MgO alone and in calcined dolomite for different reductant.
Reductant Minimum Reduction Temp. (°C)
MgO Doliine
1 bar 1 inbar 1 bar 1 inbar
Si 2143 132; 2489 1318
Al 1475 850 1427 842
FeSi 178? 1155 1870 1098
CaC2 1847 1200 1828 1191
Table 2. Minimum reduction temperatures of SrO for different reductant.
Reductant Minimum Reduction Temp. (°C)
1 bar 1 mbar
Si 2654 1536
Al 1S84 996
FeSi 3050 1618
CaC2 2102 1312
Table 3. Minimum reduction temperatures of CaO and MgO in Mg production slag for Al reductant.
Fig. 3. Ca recovery ratios for different temperatures and reductant stoichiometry.
1. K.C. Taçyurek, M. Bugdayci, O. Yucel, Reduction conditions of metallic calcium from magnesium production residues,Met., 2018, vol. 8, no. 6, pp. 383.
2. M. Yang, F. Pan, L. Cheng, Effects of minor Sr on as-cast microstructure and mechanical properties of ZA84 magnesium alloy, J Mater EngPerform., 2010, vol. 19, no. 7, pp. 1043.
3. M. Yang, F. Pan, L. Cheng, A. Tang, Effects of Al-10Sr master alloys on grain refinement of AZ31 magnesium alloy, Transact. Nonferr. Met. Soc. China, 2008, vol. 18, iss. 1, pp. 52-58.
4. M. Aljarrah, M. Medraj, Thermodynamic modelling of the Mg-Ca, Mg-Sr, Ca-Sr, and Mg-Ca-Sr systems using the modified quasichemical model, 2008, vol. 32, pp. 240-251.
