Na2o – B2o3 İkili Curuf Sisteminin Kükürt Kapasitesinin İncelenmesi

Abstract

Metalurjik süreçlerde üretilmek istenen metalden empüritelerin giderilmesi metalin istenilen özelliklere sahip olması bakımından önemli bir proses ayağıdır. Üretilmek istenen metale göre kükürt, fosfor, arsen, hidrojen, bakır, demir vs. gibi empüriteler belirli alt limitlerle sınırlı kalmak zorundadır. Örneğin çelik üretiminde kükürt; tokluk ve süneklikte azalma, kaynaklanabilirlikte düşme ve sıcak yırtılma gibi olumsuzluklara sebep olduğu için üretim esnasında oluşan curufun kükürt kapasitesinin (Cs) ,diğer bütün metallerde olduğu gibi, mümkün olduğu kadar yüksek olması gerekmektedir. Farklı metaller için uygulanan farklı üretim süreçlerinin var olmasından ötürü üretim esnasında elde edilen curuflar da birbirinden farklılık gösterir ve üretimin farklı kademelerinde farklı bileşimlere sahip curuflar elde edilebilir. Bu yüzden gerek CaO - SiO2, FeO - MnO, Na2O - SiO2 vs. gibi iki bileşenli curuf sistemlerinin yanı sıra CaO - SiO2 - MnO, MnO - SiO2 - FeO vs. gibi üç bileşenli veya dört bileşenli curuf sistemlerinin farklı sıcaklık, konsantrasyon, kısmi oksijen ve kükürt basınçlarında kükürt kapasiteleri yıllardan beri birçok araştırmacı tarafından incelenmiştir. Bu çalışmada Na2O-B2O3 ikili sisteminin kükürt kapasitesi incelenmiştir. Yapılan çalışma sodyum boraks sisteminin kükürt kapasitesinin incelenmesi adına bir ilk teşkil etmektedir. Kükürt kapasitesinin değişen konsantrasyon ile değişimi irdelenmiştir. Söz konusu deneyler yatay tip fırında gerçekleştirilmiştir. Pota içi atmosfer koşullarının istenilen P_(O_2 ) ve P_(s_2 )değerlerinde olması için kükürt kaynağı olarak kükürtdioksit (SO2), oksijen kaynağı olarak ise CO – CO2 gazları ve bu gazların kısmi basınçlarının kontrol edilmesi amacıyla Ar gazı kullanılmıştır. Bu gazlar debimetrelerden geçip fırına girmeden önce ihtiva ettikleri nem ve CO2’ten arındırılmaları için çeşitli kimyasallar ile gaz yıkama şişelerinde yıkanmıştır. Gaz karışımının fırına girişi, tasarladığımız flanşlar vasıtasıyla olmuştur. Ayrıca flanşların gövdesinde bulunan kanallar sayesinde su soğutması yapılmıştır. Bu sayede numunelerin hızlı bir şekilde katılaşması sağlanarak termodinamik dengenin bozulmaması sağlanmıştır. Numuneler, fırın içinde alumina kayıkçıklar ile hareket ettirilmiştir. Deneylerin tamamında platin (Pt) potalar kullanılmıştır. Deney sonunda numunelere ağ. % S analizi yapılmış ve FactSage programı ile hesaplanan kısmi gaz basıncı kükürt kapasitesi formülünde yerine koyularak 1100 0C’deki kapasite değerleri tayin edilmiştir. Proje kapsamında temin edilecek hammadde ve çalışmalar sonunda elde edilmiş numunelerin kimyasal analizleri İTÜ Metalurji ve Malzeme Mühendisliği Bölümü bünyesinde mevcut bulunan cihazlar kullanılarak gerçekleştirilmiştir.

slags are complex compounds that consist of various forms of oxides, silicates, aluminates or borates. The main duty of slags is absorbing metallic or non-metallic inclusions which are dissolved in liquid metal. Beside, slags forms an interface between liquid metal and outer environment for preventing heat loss of metal. It is important to reaching demaned properties for production of metals is that process of refining. The compenents and composition of the slag decides the final properties of the metal. Since, there are a lot of different metals have been producing by several metallurgical processes on the world, there are also many kind of slags which diversed by composition from each other. However, there are some common characteristics of slags such as; lower density from liquid metal/matt, low volume and heat loss. There are mainliy two theories that express structures of slags. “Ionic Theory” claims that the basic oxides dissocates completely into ions and the acidic compounds combine with those ions to form complex compounds. On the other hand, “Molecular Theory” supports that the structures of slags consist of either simple molecule forms or complex molecul forms that are formed by combining simple molecules. In consequence of, different kind of metals has been producing on the world and there are several types of metallurgical processes, compositions of the slags show a very wide range of diversity. That’s why the many researchers have been studying the different parameters such as temperature, composition, partial oxygen and sulphur pressures of the slag systems which formed by binary systems as CaO – SiO2, FeO – MnO, Na2O – SiO2 , ternary systems as CaO – SiO2 – MnO, MnO – SiO2- FeO, quater or multi components systems. In this study, sulphur capacity of Na2O – B2O3 system examined and it has been the first research about sulphur capacity of this systems has done yet. The method that been followed is by changing the composition of the samples. On the purpose of reaching desired chemical composition of samples (% Na2O; 22.4, 27.5, 32.3, 37.1, 42, 46.9, 52, 57.1, 62.1, 67.3) chamber-type furnace used. As Na2O source Na2CO3 were used in the mixture.First, the mixed powders of Na2CO3 and B2O3 were synthesised in that furnace during 1 hour and at the temperature of 1000 0C to produce sodium borax. After that, by adding certain amount of Na2CO3 or B2O3, needful compositions were achieved. Before started to sulphide capacity experiments of sodium borax system, the experiment time had decided. The time of the experiment is crucial point, because samples had to be thermodynamically stabilised and having the information that how much time stabilising take long is definitely very important. For determining the experiment time, first 3, 4, 5, 8 and 10 hours of experiments were executed. The results were obtained from those experiments showed that choosing 10 hours for experiment time was suited for sulphide capacity experiments of sodium borax systems. The measured amount of samples are placed in tube furnace. Meanwhile the tube furnace heating up to 1100 0C from room temperature by 5 0C/min., which it takes approximately 110 min., at the temperature of 600 0C argon gas is flowed into the system with the intend of cleaning the atmosphere. The experiments practiced in a horizantal furnace. As S2 source SO2 gas, as O2 source mixture of CO – CO2 gases and balancing the mixture of those gases Ar gas used. For the purpose of decontamination of those gases from their moisture and CO2, before we passed through them from flow meters, each gas had washed in the washing bottles by several chemicals like silica gel, ascarite, magnesium per chlorate and for the Ar gas only copper chips at 600 0C. The entrance of the gas mixture to the furnace achived by flanges that had designed. Besides, water cooling accomplished by the canals sited inside the flange itself. Hereby, the thermodynamic balance had kept still during the solidification of the samples. It was estimated that duration of the experiments would be in a period of 5 – 10 hours. The motion of the samples inside the furnace provided by an alumina boat which was designed as a carrier for platinum crucibles where the smelting and thermodynamically stabilising occured in it. The important thing during the experiment was to be sure of the desired mixtured gas composition reached and constantly flowing into the system without any leak or decreasing amount of any four of them. After the experiment came to an end in certain time (3, 4, 5, 8 or 10 hours), the thing that worth of notice was taking out samples from the furnace. When the furnace started to cooling down, sampled have to pushed towards to edge’s of the furnace. As sited above, flanges were keeping cold (~ 40 0C) the edge’s of the tube. It was better to pushing samples to edge from the opposite direction of gas flow because after the mixed gases were passed through the hot zone of the furnace, they started to precipitate as yellow dust. Therefore, those precipitates were covering the surface of the tube, they caused reduction of inner tube diameter. This situation resulted in friction between the boat and the precipitates. As a result, it was possible that precipitates may dropped in the boat and it lead to wrong Cs results. Another important thing about pushing out the samples was avoiding thermal shock of the tube. Its because of edges of the tube was considerably colder than middle, during quenching samples had to be move slowly. Any fracture or breakaway cause serious problems when considering poisoneous gasa such as SO2 and CO. Subsequently, samples were cooled down and measured, they analysed of sulphide percentage. All the samples were kept in desiccator before or after the experiments and analyses. At the end of the experiments, by applying the results, which were generated from the analyzed samples and the calculation of partial pressures of oxygen and sulphur gases that was done by FactSage programme , to the formula of sulphur capacity, designation values of capacities done at the desired temperature of 1100 0C. The raw materials and chemical analyses of the samples were provided by department of Metallurgical and Materials engineering of Istanbul Technical University.