Vakumda metalotermik yöntem ile kalsiyum redüksiyonuna etki eden parametrelerin incelenmesi
Investigation of the parameters that affect the vacuum metalothermic method parameters and the calcium reduction
Yazar: KEREM CAN TAŞYÜREK
Danışman: PROF. DR. ONURALP YÜCEL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Makine Mühendisliği Ana Bilim Dalı / Metalurji ve Malzeme Mühendisliği Bilim Dalı
Derece: Doktora, 2018
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Kalsiyum toprak alkalileri grubundan olan metalik bir elementtir ve yeryüzünde en çok bulunan elementler arasında 5. sırada yer almaktadır. İsmi Latincede "kireç" anlamına gelmekte olan "calx" sözcüğünden türetilmiştir. İlk olarak 1808'de Lumphru Davy tarafından kalsiyum hidroksitten elektroliz yoluyla elde edilmiştir.
Spesifik ağırlığı 1,55 g/cm³ ve ergime sıcaklığı 851°C'dir. Kaynama sıcaklığı ise 1440°C'dir. İnsan vücudunda makro yapı olarak bulunmaktadır ve elektrik iletkenliği  açısından diğer metallere göre daha iyi özellik göstermektedir. Sertlik skalasında sodyum ve alüminyum arasında bulunur.
Kalsiyum metali üretimi için birden çok yöntem uygulanmıştır. Metalik kalsiyum ilk defa eritilmiş kalsiyum klorürün elektrolizi yöntemi ile elde edilmiştir. Bu prosesde elektrolit kabı olarak porselen veya demir kaplar kullanılmaz bunun nedeni yüksek sıcaklıklarda yapılan bu proseste erimiş kalsiyum klorür bu tür kaplara tesir etmesidir. Bu nedenle genellikle grafitten yapılmış kaplar kullanılmaktadır. Diğer bir yöntemde ise eritilmiş kalsiyum iyodürü sodyum ile reaksiyona sokulmakta ve CaI2 + 2Na → Ca + 2NaI göre ayrılmış olan kalsiyum, sodyumun fazlasıyla alaşım yapar. Sonrasında kristallerden saf alkol ile sodyum uzaklaştırılarak kalsiyum metali elde edilir.
Kalsiyum metali genellikle yüksek sıcaklıkta CaO'nun alüminyum metali ile indirgenmesi yoluyla elde edilir. Bu işlemi gerçekleştirmek için düşük basınç ve yüksek sıcaklık gerekmektedir. Bu şartlar sağlandıktan sonra kalsiyum metali vakum ortamında redüksiyon sonrasında diğer maddelerden ayrışıp saflaştırılır. Endüstriyel üretimde kalsiyum metali bikarbonat, eritilmiş kalsiyum klorür ve cevherin hidroklarit asit ile proses edilmesiyle elde edilir. Yapılan bu işleme elektroliz adı verilir. Elektroliz işleminde grafit kap anot olarak kullanılırken, erimiş kalsiyum klorüre batırılmış demir çubuk da katot görevini yapmaktadır. İşlem sonrasında ise metalik kalsiyum katot etrafında toplanır.
Günümüzde metalik kalsiyum üretimi uygun kireç taşının hammadde olarak kullanıldığı prosesler ile üretilmektedir. Kireç taşı (CaCO3) kalsinasyon işlemi sonucunda CaO'e dönüştürülmekte ve bu kalsine vakumda alüminyum ile redüksiyona sokularak kalsiyum elde edilmektedir. Bu prosesde kalsinasyon işleminde açığa çıkan CO2 prosesin çevre ve enerji açısından sorgulanmasına yol açmaktadır.
Yapılan bu tez çalışmasında kalsiyum üretiminde çevreye olumsuz etkisi olan CO2 salınımına ve yüksek enerji tüketimine sebep olan kalsinasyon kademesini ortadan kaldırmak amacıyla pirometalurjik magnezyum redüksiyon atıkları hammadde olarak seçilmiştir. Bu atıklar %57,65 CaO içermekte olup vakumda metalotermik prosesle 1 ve 10 lt'lik retortlarda farklı ilave, süre ve sıcaklıklarda reaksiyona sokulmuş, bu değişkenlerin kalsiyum redüksiyon verimi ve nihai kalıntının bileşimi üzerindeki etkileri incelenmiştir. Redükleyici madde olarak seçilen alüminyumun redüksiyon davranışına etkisi önce termodinamik hesaplamalar daha sonra da deneysel çalışmalar ile ortaya koyulmuştur.
Deneylerin ilk aşamasında Mg redüksiyon curufu, Al redüktan kullanılarak vakum ortamında 1 litrelik retort içerisinde redüklenmiştir. Deneyler 60 dk, 120 dk, 180 dk ve 240 dk'da 1200℃, 1250℃ ve 1300℃ sıcaklıklarda yapılmıştır. Mg redüksiyon curufundan metalik kalsiyum üretimi için en yüksek verim olan %72,2 için proses parametreleri olarak 1300℃'de %150 Al stokiyometri ilavesi ve 480 dk olarak belirlenen deney şartlarında elde edilmiştir.
Yarı pilot çapta ise 10 litrelik retortlarda yapılan kalsiyum redüksiyon çalışmalarında metalik kalsiyumun taç olarak elde edilmesi amaçlanmıştır. Yapılan deney sonucunda 1300℃, %150 Al stokiyometri ve 480 dk proses parametrelerinde metalik kalsiyum üretim verimi %61,3 olarak elde edilmiştir.

Abstract:
Calcium is a chemical element which is represented as Ca. An alkaline earth metal, calcium is a reactive dull-yellow metal which forms a dark oxide-nitride layer when exposed to air. 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.
Calcium is included in the IIA group within the periodic table. Its atomic number is 20 and its atomic weight is 40.08. Calcium is present in the earth alkali metal element series; it has a silver-white appearance and is found in a solid state.
From past to present, humans have been using calcium compounds to make cement. Limestone (calcium carbonate) was called "Calx" by the Romans, who heated it up to drive off carbon dioxide and obtain calcium oxide. To make cement, all one has to do is mix calcium oxide with water. The Romans built vast amphitheaters and aqueducts using cement made of calcium oxide to bond stones together. Calcium compounds have a long history, but the element itself was not discovered until electricity became available for use in experiments.
Calcium is the most abundant of all the metallic elements found in the human body. The average adult body contains about 1 kg of calcium, 99% of which is present in bones and teeth. Indeed, oxygen, carbon, hydrogen, and nitrogen are more abundant in our bodies as compared to calcium.
Calcium reacts with almost any form of metal oxide at high temperatures and acts as a good reducing agent. It is used for lead refining (specifically, the separation of bismuth), steel purification (desulfurization and deoxidation), and as an alloying agent for silicon and lead. Calcium oxide, which is present in refractory metals (e.g. chromium, rare earths, and thorium), is used in the recovery and reduction of uranium dioxide. CaO is used together with Al2O3-MgO as refractory material for the industrial applications.
There are several calcium resources, among which the most important ones are calcite, dolomite, gypsum and anhydride. The major industrial calcium manufacturers are Russia, China, America and France. Russian calcium production facilities are producing 4000 tons of metallic calcium annually. China as a main producer has increased the quantity of calcium production in last 5-6 years, and the common reason for this increase is related to  two companies producing the calcium with the electrolytic way in 1960's. However, later production by aluminothermic process has begun and lots of companies have started the metallic calcium production. China has been producing 30000-35000 tons of calcium with aluminothermic production method, while still produces 4000-6000 tons of calcium using the electrolytic process. USA produces approximately 1000-2000 tons of calcium using the aluminothermic method. Also, France has some facilities for the production of calcium metal but they aren't producing at present. Calsium reserves are almost unlimited due to the presence of living organisms.
There are several methods for the production of calcium metal. Metallic calcium is obtained by electrolysis of molten calcium chloride. In this process, porcelain or iron containers are not used as the electrolyte housing. This process made at high temperatures, molten calcium chloride, such containers affect and for this reason, containers made out of graphite are usually used. Furthermore, calcium can also be obtained by chemical means. The other is to treat the molten calcium iodide with sodium: Calcium separated by the equation CaI2 + 2Na → Ca + 2NaI, the sodium excessively alloys with hot, calcium metal is obtained by removing sodium from the crystals in the cold.
Calcium is generally obtained as a metal by reduction with caustic aluminum metal by high-temperature processes. Low pressure and high temperature are required to perform this operation. Once these conditions are met, the calcium metal is separated and purified from the other substances in the vacuum environment after reduction. In industrial production, calcium metal is brought to the market by processing a carbonate, melted calcium chloride and ore with hydrochloric acid. This process is called electrolysis. While the graphite container is used as an anode in the electrolysis process, the melted calcium chlorite-immersed iron rod also serves as a cathode. After the process, metallic calcium is collected around the cathode.
The first calcium production was conducted by the electrolysis of the anhydrous, molten calcium chloride, in 1808. Then, the aluminothermic process was developed by Hans Goldschmidt in 1898 with the reduction of metal oxides by a metal exchange reaction. This process enhances the significance of the extraction technology of refractory metals. After that, electrolytic calcium production was replaced by the metallothermic reduction process. In China, the consumption quantities per ton of magnesium in the reactors are as follows: 0.9 MWh of electricity, 10.5 tons of coal, 11 tons of dolomite, 82% magnesium efficiency, and 6.2 tons of residues. Starting from high-purity dolomite raw materials (99.5% CaCO3·MgCO3) makes it possible to achieve 99.95% or more magnesium production in commercial quantities.
In the past, Turkey didn't produce magnesium metal until 2015. However, Turkey's dolomite reserves amounted to 16 billion tones (identified and probable). New Turkish plant has started the production capacity of 15.000 T/Year with uses the silicothermic process. Yucel et al. Since 2002, using with the Turkish minerals, they were investigated the parameters for the silicothermic reduction of calcined dolomite production. It means that, Turkey has 8.454 tones magnesium production residues and it's the raw material of the production of metallic calcium. Nowadays pure calcium metal is obtained by the industrial scale metallothermic reduction of calcium oxide with the aluminum. The mixture of calcium oxide and aluminum powder is heated up 1200°C and the reduction begins producing calcium metal. Thermite reaction is described the exothermic reductions of the metallic oxides. This kind of process release huge heat and its sufficient way to heat the product phases above their melting points. Recently thermite reaction described as a broader class of reactions. It defined as a metal reacting with a metallic or a non-metallic oxide to produce more stable or corresponding metal.
Pidgeon et.al. was made the first calcium production is conducted via the Pidgeon Process which they were used aluminum as a reducing agent. The reduction is done at 1170°C in a steel retort under vacuum atmosphere. The stoichiometry of aluminum had been taken 100%, 110% and 120% respectively. The reaction was realized in 100 pounds' retort, recoveries are obtained 85.8%, 92.0% and 96.0% of calcium have been observed, respectively. Heating cycles of the experiments are 12, 17 and 24 hours. Pidgeon et.al. used CaO as a raw material during their experiments. On the other hand, in this study magnesium production residues were used as a raw material instead of CaO. In both studies Al used as a main reductant. In order to reduce process cost and evaluate magnesium residues, effects of time, temperature and reductant stoichiometry on the results were investigated in the experiments. Experimental studies were made under vacuum atmosphere with the steel retort into the horizontal furnace, the effects of variable process parameters on Ca recovery efficiency were discussed and kinetic model studies were performed with the obtained data.
This study had three fundamental objectives. First, to recycle the Mg production residue to use it as a raw material for Ca production. Starting from the residue, experiments were reduced CO2 emission and eliminated energy loss in the calcination process, which is needed for the decomposition of CaCO3 to CaO and for eliminating energy consumption for the calcination process.
The present study began with the theoretical requirement of 100% Al stoichiometric reductant, and most importantly, its time effect was investigated for calcium reduction at 1200°C. The raw materials, such as metallic calcium production residue and 100% Al stoichiometric powder mixtures, were prepared, and experiments were conducted with the same amount of mixtures, 9 g. The recovery rate of calcium was calculated from the residue of metallic calcium production. It was clearly seen that the 100% Al reductant and the extended time did not affect the recovery rate of calcium. The recovery rate results were found to be 3.2% to 19.5% in the first experiments. After that, Al stoichiometry was increased to 125%, and the recovery rate results were 5.4% to 21.3% in these experiments. Then, 150% Al stoichiometry was examined, and the recovery rate results were 5.3% to 19.6%. These data showed us that the quantity of the reductant was not enough for Ca reduction with the high recovery rates.
In the second set of experiments, the reaction temperature was fixed at 1250°C, and the highest recovery was detected for the charge amount of 9 g, 150% Al stoichiometry, 1250°C, and 240 minutes: 22% Ca in the residue. At 1250°C, the differences between the Ca recovery rates increased with time and Al stoichiometry. The recovery results were 7.4% to 19.9% at 100% Al stoichiometry. After that, Al stoichiometry was increased to 125%, and the recovery results were 6.4% to 20.1%. Then, the set of experiments was repeated at 150% Al stoichiometry, for which the recovery rate results were 11.2% to 22%. This showed us that amount of the reductant was not enough to get Ca reduction with high recovery rates.
In the third set of experiments, the reduction temperature was set at 1300°C, the highest CaO amount in residue was obtained in the experiment conducted with 150% Al and 480 minutes with 72.2% recovery rate. The recovery results were 25.4% to 61.2% at 100% Al stoichiometry. After that Al stoichiometry was changed to 125%, the recovery results were 36.1% to 62.4%. At the end, Al stoichiometry was fixed 150% and the recoveries were 36.6% to 72.2%. It was showed us the amount of the reductant was enough for getting Ca reduction with the high recovery rates. Frankly, increasing time and the reductant stoichiometry is increasing the recovery of the Ca.
In this study, parameters affecting calcium production by vacuum metallothermic method were investigated. During the experiments, different stoichiometries of Al reductant, which will be used for reducing the raw material with residual temperature, time and magnesium production residue, have been tested under the previously prepared reduction conditions. When the characterization results of the experimental studies are examined, the highest yields were obtained in experiments where 150% stoichiometry Al was reductively added. Experimental studies have clearly shown increased process times and positive effects of Al on Ca yield. Experiments with the highest Ca yield and Al reduction were obtained at 72.2% Al. When the process time and temperature of the experiment were examined, the highest yield was obtained at 480 min and 1300℃.
At the first stage of the experiment, Mg production residues were reduced in the 1-liter retort vacuum environment using Al reduction. Experimental studies were carried out at 1200°C for 100 min; the 125% and 150% stoichiometric Al additions were carried out for 60 min, 120 min, 180 min, or 240 min for the first set after the test conditions were regulated. In the second stage, the same process conditions were set at 1250°C or 1300°C for the same set. The highest value for metallic Ca production from Mg residues was 72.2% for the process parameters of 1300°C, 150% Al stoichiometry, and 480 min.
In the industrial-scale application, Ca reduction studies carried out in 10-liter retorts in the literature were examined and we aimed to obtain metallic Ca as a crown in the reduction experiment. As a result of the experiment, 61.3% efficient metallic Ca production was obtained at 1300 ℃, 150% Al stoichiometry, and 480 min.