Mg-Sr alaşımları üretimi amacıyla MgO, SrO sisteminin  redüksiyon koşullarının incelenmesi
Investigation of the reduction conditions of MgO, SrO system for the production of Mg-Sr alloys
Yazar: MEHMET BUĞDAYCI
Danışman: PROF. DR. ONURALP YÜCEL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Metalurji ve Malzeme Mühendisliği Bilim Dalı
Derece: Doktora, 2017
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
Mg sıkı paket hekzagonal kristal yapısına ve iki valans elektronuna sahip olan gümüşi gri bir metaldir. Magnezyumun erime noktası ve kaynama sıcaklığı sırasıyla 650 ± 2 ºC  ve 1107 ±10 ºC'dir.
Magnezyum metal üretiminde dolomit, manyezit, karnalit ya da deniz suyu gibi hammaddeler kullanılmaktadır. Üretim elektrolitik veya metalotermik yöntemlerle yapılmaktadır. Elektrolitik yöntemle kitle halinde üretim yapılabilmektedir ve halen Dow, Magnola, Dead Sea, AM, IG Farben üretim proseslerinde hammadde olarak MgO, deniz suyu, karnalit gibi hammaddeler klorlanarak elektroliz hücrelerinde magnezyum üretilmektedir. Bu yöntemlerin en büyük dezavantajları yüksek elektrik enerjisi tüketimi ve yüksek miktarda klor gazı açığa çıkmasıdır. Kalsine dolomitin hammadde, ferrosilisyumun redükleyici olarak kullanıldığı metalotermik yöntemler hem sabit retortlarda (Pidgeon Prosesi), hem de elektrik ark fırını destekli olarak (Magnetherm Prosesi) gerçekleştirilebilmektedir.
Magnezyum yapısal metaller arasında en hafif olanıdır, bu nedenle ağırlığın kritik bir tasarım unsuru olduğu mühendislik uygulamaları için mükemmel bir seçimdir. Magnezyum, iyi bir ısıl dağılıma, darbe sönümlemesine ve kolaylıkla temin edilebilirliğe sahiptir. Magnezyumun özellikleri; kaynak, dövme, dökme veya makine yapmayı kolaylaştırır. Saf magnezyumun kullanımı, yüksek sıcaklıktaki mekanik özelliklerinde oluşan dalgalanmalar nedeniyle nadirdir ve nemli ortamlarda aşırı koroziftir. Bu nedenle, havacılık ve otomotiv parçaları tasarlarken magnezyum alaşımlarının kullanılması kritik öneme sahiptir 
Bu çalışmada vakum metalotermik yöntemle magnezyum üretimi yapılmıştır. Çalışma esnasında ilk olarak farklı indirgeyici ilavelerinin etkileri araştırılmış, kalsine dolomiti redüklemek için stokiyometrik FeSi, Si, Al ve CaC2 ilaveleri yapılmıştır. 1200°C'de gerçekleştirilen deney sonuçları incelendiğinde en yüksek verim 300 dk. Sonunda, % 96 ile ferrosilisyum ilaveli deneylerde belirlenmiştir. Artan proses süresinin ve Al'un, Si yerine indirgeyici olarak kullanılmasının, Mg verimi üzerinde pozitif bir etkisinin olduğunu deneyler sonucunda açıkca görülmüştür. En yüksek Mg verimi, Al kullanımı ile gerçekleştirilen deneyde % 88 ile gerçekleşmiştir. Proses süresi incelendiğinde en yüksek verimlerin 300 dakikalık deneylerde oluştuğu belirlenmiştir.
Stronsiyum allotropik dönüşüm gösteren bir elementtir ve üç değişik kristal yapı gösterir; 215°C'nin altındaki sıcaklıklarda yüzey merkezli kübik (YMK) yapı gösteren stronsiyum, 215-605 °C aralığında sıkı paket hegzagonal (SPH) yapısına dönüşüm gösterir, 615°C'nin üzerindeki sıcaklıkta kafes yapısı kübik hacim merkezlidir (KHM). Stronsiyumun erime noktası ve kaynama sıcaklığı sırasıyla 768 ± 2 ºC  ve 1377 ±10 ºC'dir
Kalsine dolomitteki magnezyumoksit redüksiyon şartlarının farklı redükleyiciler için araştırılmasından sonra SrO redüksiyon şartları incelenmiştir. Bu deneylerde SrO aluminyum ile indirgenmiş, verim değerlerini arttırabilmek için karışımlara (BaO, CaO, CaC2) gibi bazı fonksiyonel ilaveler yapılmıştır. Deneysel çalışmaların ilk kısmında değişen zamanla, stokiyometrik SrO-Al karışımına % 100, % 200, % 300 stokiyometrik BaO ilaveleri yapılmış, % 300 stokiyometrik karışım 1250ºC'de,  4 saat sonunda % 96,89'luk bir Sr kazanım oranıyla en yüksek değeri vermiştir. CaO ilaveli deneylerde % 200 ve %300 stokiyometrik karışımlar hazırlanmış sıcaklık değişiminin etkileri incelenmiştir. Artan sıcaklık ile, Sr verim değerleri  yükseliş  göstermiştir. En yüksek Sr kazanım değeri % 76,43 ile % 300 stokiyometrik deneyde tespit edilmiştir. CaC2 ilaveli deneylerde sarja % 0,25 molden, 2 mole kadar % 0,25'lik artan değerlerle  karpit ilavesinin etkisi incelenmiştir. Deneylerde sıcaklığın etkisi incelendiğinde Sr kazanım miktarının % 50 stokiyometrik deneyler için, 1100ºC'de elde edilen % 44'den, 1250°C'de % 74'e yükseldiği görülmüştür.
Daha sonra pilot ölçekli deneylere geçilmiş, kalsine dolomitten Mg üretiminde ağırlık değişiminin etkisi araştırılmıştır. Çalışmada 50 g.'dan, 5000g'a kadar değişen stokiyometrik karışımlar hazırlanmış, kalıntı ve elde edilen Taç Mg üzerinden geri kazanım değerleri belirlenmiştir. Kalıntıdan hesaplanan en yüksek Mg geri kazanım miktarı, 50 g'lık şarj için % 98 olarak tespit edilmiştir. Diğer taraftan, taçdan hesaplanan en yüksek Mg geri kazanım değeri 3000 g şarj miktarı ile yapılan deneyde elde edilmiş ve % 90 olarak belirlenmiştir.
Deneylerin son aşamasında MgO ve SrO'in birlikte redüksiyon koşulları incelenmiştir. Deneylerin ilk aşamasında MgO, SrO karışımı FeSi, Al karışımı ile redüklenmiştir. Deney düzenekleri belirlenirken sarja ağırlıkça % 2,5, % 5, % 7,5 ve %10 stokiyometrik Sr ilaveleri yapılmıştır. İkinci aşamada her iki reaktan Al ile redüklenmiş, SrO sarj ilavesi ilk settekiyle aynı miktarda yapılmıştır. Kalsine dolomiti redüklemek için FeSi'in kullanıldığı deneylerde en yüksek Mg verimi, 1250 °C'de, % 5'lik Sr ilavesinin olduğu deneyde, % 79,3'lük değerle belirlenirken, en yüksek Sr verimi % 2,5 ilavenin yapıldığı 1250 ºC'lik deneyde, % 63,5 olarak belirlenmiştir. Kalsine dolomiti redüklemek için Al'un kullanıldığı deneylerde en yüksek Mg verimi 1250 °C'de, % 2,5'lik Sr ilavesinin olduğu deneyde, % 89,8'lik değerle belirlenirken, en yüksek Sr verimi % 7,5 ilavenin yapıldığı 1250 ºC'lik deneyde, % 78,6 olarak belirlenmiştir.

Abstract:
Mg is a silver grey metal which has a dense hexagonal crystal structure and it has two valance electrons. Melting point and boiling point of magnesium are 650±2ºC (923K = 650°C) and 1107 ±10ºC respectively.
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. It was reported by the USGS in 2015 that the World's total magnesium production was approximately 878,000 t in 2013 and 907,000 t in 2014, thus the annual supply change is about 3%. On the other hand, the World's demand for magnesium increases nearly 10% per year.
Dolomite ore is the most important source to produce metallic magnesium. It is a compound of magnesium carbonate and calcium carbonate with a chemical formula as CaCO3.MgCO3. It theoretically consists of 45.3% MgCO3 and 54.7% CaCO3 by weight. Additionally dolomite is used in various industries such as metallurgy, glass, chemical and paper.
The major part of magnesium production is conducted via the Pidgeon Process which is a metallothermic (silicothermic) method. Metallothermic reduction reactions are normally highly exothermic. Thus, the propagation of reactions and the yield of reaction products continue in a self-sustaining mode without requiring any additional heat. On the other hand, the Pidgeon Process is highly endothermic (~ΔH298 is about 209 kJ mole of Mg), and heat supply is a critical consideration in industrial retorts (reactors). Magnesium is in gaseous phase at the temperature which is reduced, and it is collected in the form of crown at the cooling part of the retort. The Pidgeon Process is conducted through the reduction of magnesium from calcined dolomite via silicothermic reduction under vacuum atmosphere. Powdered calcined dolomite, ferrosilicon (as reductant) and a slight amount of fluorspar (CaF2 as catalyst) are mixed prior to the reduction process as raw materials. 
Although Turkey has 19 billion tones (detected and probable) of World's dolomite reserves, Turkey had not produced magnesium metal until 2015. The new Turkish magnesium plant using the silicothermic process has been started with 15000 t / year production capacity. 
In silicothermic Mg production processes, silicon or silicon based materials are used as the reductant of MgO based raw materials such as calcined magnesite and dolomite. Reduction of Mg is generally carried out via Pidgeon process. Mg is in the gas phase at reduction temperatures. This method 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.
If ferrosilicon (FeSi) contains higher than 65 mass % Si, activity of Si in ferrosilicon is close to the activity of pure silicon (0.97) at 1200 °C. Because of the lower price of ferrosilicon, the use of FeSi is more preferable than silicon in commercial applications. Al can be also used as a reductant in the Pidgeon Process, because it has thermodynamically several advantages. In the aluminothermic reduction process, reduction occurs at lower temperatures than the condition which is FeSi used as a reductant. Aluminothermic reduction is not preferable for industrial application due to economic reasons. On the other hand some solutions, such as the use of FeAl, can be suggested to make the use of aluminum feasible.
Although the use of FeSi is more economical than Si, in the common silicothermic process, it is still needed to reduce the cost of reductant. This work aims to evaluate the possibility of reducing the cost of magnesium production by using CaC2 with ferrosilicon as much as possible. According to Suchy and Seliger, MgO reduction with CaC2 is possible, but the residue of this process is highly entrained with Mg vapor. Mg vapor has the adverse effect on the reduction when it conjugate with residue, so Mg and lime must be separated from each other in order to obtain high recovery ratios. In addition, there is another disadvantage, the residue caused agglomeration of charge and the reaction mixture substrates are adapted to this agglomeration during reduction. To avoid the agglomeration silica and alumina preferably are added to mixture. With the addition of SiO2 or Al2O3 at the reaction temperature, lime converts into calcium silicate or calcium aluminate. Thus, residue of mixture does not include Mg. In the experiments, increasing proportion of CaC2 was used with ferrosilicon. Process duration and temperature were carried out as variables in order to obtain high Mg recovery ratios.
In the present study, experimental sets were developed to understand the effects of reductants type on the Pidgeon 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 ºC under 1 mbar. In all experiments process durations were 6 hours. 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 minutes. In the third experimental set, 100% stoichiometric 50 g mixtures were prepared. Stoichiometric amount of the reducing agent was calculated through sum of the reducible oxides (MgO + FeO + SiO2) contents in the calcined dolomite.  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 °C and 1250 °C under 1 mbar vacuum atmosphere for 6 hours. In the last experimental set, the experiments were conducted with increasing CaC2 addition and in different volumes of retorts as 1 l and 10 l.
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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. 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 recovery negatively it reduced from 98% to 90% which calculated from residue. Mg recovery increased from 90% to 92%. According to the results, reduction duration must be extended with increasing amount of charge.
In the second experimental set, the effects of Si, FeSi and Al addition and process duration were investigated. In these experiments, the effects of stoichiometric Si, FeSi and Al addition on Mg recovery were examined, and the results were compared to each other. All experiments were conducted under vacuum atmosphere at 1200 °C and in 1 l retort. From the results, the highest recovery was detected at silicon experiments 96%. It is clear to see that increasing process duration and the use of Al as a reductant instead of FeSi have a positive effect on the recovery of Mg. The highest Mg recovery ratio was determined as 88% at the experiment conducted with the use of Al and for the process duration of 300 minutes.
In the third experimental set, the effect of CaC2 addition and the change of reaction temperature were investigated. In these experiments, CaC2 was added to charge mixtures in specific proportions. Stoichiometric FeSi/CaC2 ratio changed from 100% FeSi - 0% CaC2 to 50% FeSi - 50% CaC2. All experiments were conducted under vacuum atmosphere at two different reaction temperatures as 1200°C and 1250°C. The experiments show that increasing of CaC2 addition in the charge decreases the Mg recovery to 82.0% for CaC2 addition of 50% at 1250°C. The decrease in terms of Mg recovery with the use of CaC2 reductant was previously explained.
In the last stage of the experiments, effects of CaC2 addition on different retort volumes were investigated. The highest Mg recovery was determined for 1 l retort 100% FeSi added experiment as 98.2%. On the other hand, the experiments conducted in the 10 l retort, the highest recovery was calculated at 10% CaC2 added experiment as 95.2% from residue and 94.7% from crown. The highest MgO amount in residue was obtained in the experiment conducted with 50% FeSi - 50% CaC2 addition ratio as 6.81%. According to chemical analysis of residues, CaC2 addition affected amount of MgO in residue, minimum amount of MgO detected in stochiometric 90% FeSi-10% CaC2 experiment with 1.73%.
Strontium belongs to the group 2 of the periodic table and, it is one of the alkali earth metals. Strontium, with atomic number 38, lies between calcium and barium in that group. It was founded by Crawford in 1790 and, metallic strontium was isolated by Davy in 1808 from a strontium carbonate compound obtained from Strontian Deposit. It is allotropic and displays three crystal structures; at temperature below 215°C (488K) it is face centered cubic, between 215-605°C hexagonal close packing, above 605°C base centered cubic. Strontium is more reactive than magnesium and calcium and less reactive than barium, so it is hard to keep stable in metallic form. Strontium reacts with H2O, O2, N2, F2 and S to produce compounds which correspond to its valence of two.
According to USGS 2016 data, mine production of strontium was 10,000 tonne in 2015 all around the world . It has many technological application areas. For instance, barium strontium titanate, strontium bismuth titanate and strontium bismuth tantalate thin films are promising materials in ferroelectric and Schottky-based microelectronics technologies especially for memory applications. Physical vapor deposition (PVD) techniques as magnetron sputtering, thermal evaporation and molecular beam epitaxy (MBE) are the most widely used methods for growth of these thin films. Strontium is principally used in the form of compounds. Other important uses of strontium compounds and strontium metal are in metallurgical industry. Strontium metal and alloys are used as getters to remove traces of gases from electronic tubes and as a scavenger in metallurgy to purify other metals. It has been also used in very small quantities to improve the hardness and durability of lead and copper. The use of strontium-silicon as inoculant has been developed in the production of high quality iron casting. Strontium metal is also used as a modifier. Strontium modifies the eutectic silicon in hypo and hyper eutectic aluminum/silicon casting alloys from coarse platelet to fine fiber form. It is known that the grain refinement is an important method of elevating properties and improving formability for magnesium alloys. 
Strontium oxide with Al, BaO, CaO and CaC2 subjected to reduction process under an average process pressure of 2 mbar, at 1050 °C, 1100°C, 1150°C and 1200 °C temperatures for 60, 120, 180 and 240 minutes (in 1 liter retort). Addition BaO is essential for high efficiency of strontinum reduction. Temperature must be above of 1150 °C, 1250 °C is enough. The maximum recoveries was observerved 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.
Recent results indicated that Sr 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. Since adding pure Sr to magnesium alloys would produce serious burning loss thus the amount of Sr is difficult to control, Sr is frequently used in master alloy form, such as Al-Sr and Mg-Sr master alloys. Recent investigations indicated that, although adding Al-Sr or Mg-Sr master alloys to magnesium alloys such as AZ31 could effectively refine the grains of magnesium alloys, the refining efficiency of the latter is higher than that of the former.
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 process alloying Mg with Sr in order to avoid burning loss and they produced Mg-Sr alloys. Magnesium main production method is Pidgeon process. In this process reduction is highly exothermic and it needs vacuum atmosphere.  Also Strontium reduction is possible by this method from Strontium-oxide (SrO). In the present study, combine metallothermic reduction of calcined dolomite and SrO production conditions alter to melt-leaching-reduction 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&Sr 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%.