Karbotermik ferrobor üretim parametrelerinin optimizasyonu
The optimization of parameters for the carbothermic production of ferroboron
Yazar: ONURALP YÜCEL
Danışman: PROF. DR. OKAN ADDEMİR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Doktora, 1992
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Ferrobor = Ferroboron ; Üretim = Production

Özet:
ÖZET Ferrobor elektrik/elektronik sanayinde yumuşak (trafo saçı) ve sert magnetik malzemeler üretiminde ve çeliğin al aş i ml and iri İmasında kul! anılmaktadır. Bu çalışmada borik asit, demir oksit, odun kömürü ve tahta ta laşının farklı oranlarda karıştırılması ile elde edilen şarj harma nı, 40 kg şarj alabilen 100 kVA'lık, tek fazlı, doğru akım elektrik ark fırınına beslenerek farklı konsantrasyonlarda ferrobor üretil miştir. Deneylerde bulunan elektriksel değerler, bilgisayar programı ve data toplama sistemi kullanılarak okunmuş ve üretim parametrele rinin optimizasyonunda kullanılmıştır. Küçük çapta yapılan deneyler konsantrasyon ve verimin şaft ti pi bir fırında artırılabileceğini göstermiştir. Şarj içinde H3BO3/ Fe2Û3 oranının ve sabit karbon miktarının arttırılması ferroborda bor konsantrasyonunu arttırırken, karbon konsantrasyonunu ve enerji tüketimini azaltmıştır. H3B03/Fe203 oranının 1.12, sabit karbon mik tarının % 12.25 olduğu şarj bileşimi ile belirlenen optimum şart larda, % 18 B, % 0.2 C içeren alaşım 35.1 kWh/kgB enerji tüketim de ğeriyle üretilirken, bor kaybı sadece % 1.7 olarak saptanmıştır. Şarjdaki redükleyici madde miktarının aşırı arttırılması (% 14.88 sabit karbon), fırında metal yerine ergimemiş birikintinin oluşması na sebep olmuştur. % 16-18 B içeren ferrobor üretiminde grafit elek- trod ortalama 120.5 g/kg metal miktarında tüketilmiştir. Fırın içinde kalan sinter fazının X-ışınları ve kimyasal ana lizleri ferrobor oluşumunun demir borat bileşikleri içeren ve elek- trodun ark yaptığı bölgede toplanan sığ bir cüruf fazı üzerinden ol duğunu göstermiştir. Seçilen demir oksid ve odun kömürü cinsi ferrobor kalitesini ve üretim rejimini doğrudan etkilemiştir. Yaklaşık 40 kW güç kullanılarak, fırına 570 kW. m~2 güç yoğun luğunun uygulandığı deneylerde, voltaj 20-35 V, akım 1400-2200 A arasında salınmış, kısa devre 5-8 V arasında oluşmuştur. Demir bo- ratlı cürufun ortalama direnci 0.02 ohm olarak ölçülürken spesifik iletkenliği yaklaşık 0.8 ohm"1. cm"1 olarak saptanmıştır. - vii -

Abstract:
Ferroboron is a master alloy used in the manufacture of metal lic glass and magnetic materials, and in the alloying of steel. For decades, ferroboron has been utilized to increase the mechanical properties of steels. Recently, owing to improvements in the magne tic properties of transformers sheets, ferroboron has become impor tant role the electrical sector. The replacement of conventional silicon steel by ferroboron has resulted in reduction of one-third in the core losses of transformers, making a big impact on energy conservation. Since elemental boron is an expensive chemical, boron is usu ally introduced into ferrobor in the form of cheaper compounds such as H3BO3, B2O3, etc. There are three main process for production of ferroboron alu- minothermic, silicothermic and carbothermic metods. In conventional aluminothermic processing during the reduction of a mixture of boron oxide and iron oxide with aluminium, 1 to 6 per cent aluminium is left in the alloy. The presence of aluminium prevents the use of this product in the production of metallic glass. Silicothermic processing also couldn't find a large using. The carbothermic method of ferroboron production yields a bet ter product, which satisfies the impurity limits set by the metal - licglass industry. There are four different carbothermic processes listed in literature. These are; i) Smelting in a shoft furnace with a pre! imin arily reducing, ii) Smelting in a shoft furnace with a direct reduction, iii) Smelting in an are furnace with preliminorily reducing, - viii -iv) Smelting in an are furnace with direct reduction. Hamada and his co-workers carried out their ferroboron studies in a vertical -shaft furnace Boron oxide and boric acid were reduced in the presence of carbon, the aim being the direct utilization of the alloy in production of metallic gloss. The product obtained contoined 3 % boron, 3 % silicon, and 3 % carbon. To avoid decarburization, an alloy containing 10 % boron, 11 to 13 % silicon, and 0,3 % carbon was produced. This alloy was introduced into liquid steel for the production of metallic glass. Allied Corporation's process includes preheating and prereduction in an are furnace. The reagents used are boric acid, iron (or iron oxide), coal, and sugar, all of which are introduced at set ratios in a continuous operation. It is reported that the product contains 13,6 to 15,4 % boron and 0,3 % carbon, with a boron yield in the alloy of 68 to 86 %. In a study conducted by Mitsui Mining & Smelting Co., an alloy containing 10,5 % boron was produced, with a boron yield of 76 to 82 % and an energy consumption of 44 kWh per kilogram of boron. Seki and Hiromoto introduced 90 % of the stoichiometric carbon and produced an alloy containing 16,5 to 21,4 % boron. However, there was difficulty in the production of ferroboron in that an unsmelted deposit formed under the electrode. Hahn and his co-workers developed another method of carbothermic reduction in an electric submerged-are furnace. In that study, a mixture of wood chips, charcoal, boric acid, and iron oxide was reduced to produce an alloy containing 18 to 20 % boron and 0,02 % carbon. The process had a boron conversion rate of 95 % and an energy consumption of 35,5 kWh per kilogram of boron. Thermodynamic consideration of the reduction of Fe2Û3 and B2O3 in the presence of carbon shows that the reduction sequence with solid carbon includes Fe203, Fe3Û4, FeO, B2O3 (g), and B2O3 (-|). Iron is reduced prior to boron, and the reduction of gaseous boron starts at 1650 K. The liquid-phase reduction starts at 1900 K. Under these conditions, vapour pressure of B2O3 is 10 mm Hg. For this reason, it can be concluded that the reduction of boric oxide with solid carbon take place preferentially in the gaseous phase. The - ix -affinity of boron for iron is also higher than its affinity for carbon. Consequently, at a high iron concentration, the possibility of the formation of ferroboron is higher than the possible formation of boron carbide. In that study ferroboron alloy was produced from boric acid, hematite, charcoal and wood chips. The mixture was smelted in a 100 kVA monophase DC arc furnace. The process was optimized, with the charge composition, applied voltage and current, resistance, energy and electrode consumption, and physical conditions of furnace as parameters. The boric acid used was 99.5 % pure and contained Fe, Si, Mg, Ca and Na as impurities. The hematites were pigment grade and ore, which contained Si, Cu, Mn, Ni and Co as impurities. The particle sizes of the charcoal and wood chips were 1 to 3 mm and 5 to 15 mm respectively. The laboratory-type monophase resistant arc furnace has a 40 kg charce capacity. The smelting process was conducted with a d.c. power supply of 100 kVA. The temperature profile was monitored by the use of four thermocouples located in the furnace lining. The inner surface of the furnace was lined with graphite and, to provide the necessary heat insulation, refractory bricks were used as a backing for the graphite. The voltage and current readings were fed to a computer-control system for the acquisition of data that were used in the control of the position of the upper electrode. The charcoal and wood chips in the selected ratios of H3BO3 and Fe2C»3 were mixed in a rotary mixer for 2 hours, and 100 kg of the mixture was fed to the open heated arc. After 1 to 4 hours from start of the experimental run, the liquid metal was removed from the tap hole at the bottom and the arc was stopped. The furnace was left to cool for inspection. In the experimental evaluation, the metal and boron that remained in the furnace were taken into considera tion. The energy consumption was also evaluated accordingly. The changes in the electrodes such as consumption, erosion, shape and dimension were carefully inspected and quantified. Two H3BO3 - to- Fe2Û3 ratios were used 0.44 and 1.12. These ratios changed the content of carbon in the furnace. Generally, the addition of the increased amount of fix carbon ratio in the charge increased the concentration of boron and silicon in the alloy. The silicon arising from the charcoal and hematit is not detrimental if the production of Metglas 2605 is required. The con- - x -cent of coloured metals present in the alloy decreased with using the clean iron ore. The silicon and aluminium content of the alloy decreased with using charcoal containing more less ash. The carbon content of the alloy decreased with increasing boron concentration and, for an alloy containing 18 % boron, the carbon content decreased to 0.2 %. The energy consumption per kilogram of boron in the alloy decreased with increasing boron concentration. The production of ferroboron alloys containing 10-20 % boron consumed an extra 14.5 kWh/kg B. This means that, for every unit of boron, nearly 1.45 kWh of energy per kilogram of boron was consumed. On the other hand, for alloys containing less than 10 % boron the energy consumption was 31 kWh/kg B for every unit of boron. It is obvious that the production of high-grade boron alloys is more economical from an energy point of view. In all the experiment, curves of voltage-time, currenttime, powertime and resistance-time were obtained. During the experiments, the voltage fluctuated between 20-35 V, the amper fluctuated between 1400-1800 A. This system was short-circuited between 5-8 V. The choice of initial voltage was critical. If the voltage was less than 20 V. The arc was lost owing to a short-circuit. For this reason, the electrode spacing was adjusted to ensure the arc and to keep the voltage above 20 V. Excessive increase of the voltage caused the power consumption to increase and burnt the charge. The experiments indicated that 20 to 30 V was a satisfactory experimental condition. The movement of the electrodes was controlled during the experiments by the use of a photocell. It was observed that elec trodes were under the influence of two process: electrod erosion and the accumulation of metals or unmelted deposit at the bottom. When the metal has formed, the electrode movement was always towards the bottom. On the contrary when the unmelted deposit has formed, the movement was observed to be upwards. The erosion of the electrode was not from the sides as expec ted, but from the bottom. For this reason the charged reagents were observed to be relatively unreactive during contact with the sides of the electrode and most of the reaction took place at the arc tip of the electrode. - XTUnder optimum conditions, the electrode erosion rate is 6 cm/h and the consumption 120.5 g/kg Metal. An unmelted deposit was observed to form when an excessive amount of carbon was present during the production of high-boron ferroboron. The carbon content of the unmelted deposit was found 14 %. It was observed that the deposit contained ferroboron. The iron-borates were formed by drying of H3BO3, and by the reduction of Fe2Û3 in the furnace and they flow continued downword until coming to arc zone. This shollow ironborate slag is reduced under the electrode, to form ferroboron which accumulated at the bottom of the furnace. Unreacted B2O3 vapour evaporates upper cool parts of the furnace and condences. Only 1.7 % of boron is lost in these type of system. The overall electric resistance, in a slag resistance electric furnace, is an important factor in furnace design and performance and can be expressed as a function of slag, electrical conductivity and furnace geometric factor. Geometric factor is function of elec trical diameter, depth of electrode immersion, electrode spacing, and overall depth of the slag layer. In this study, they have been found that geometric factors were between 0.012-0.022 1/cm, slag resistances were between 15-25 mohm. As a result, the electrical conductivity of the ironborate slag has been determined as app. 0.8 ohm"l.cm~l. - xn