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İnce boyutlu kromit konsantrelerinden ön redüklenmiş pelet üretimi

Başlık çevirisi mevcut değil.

  1. Tez No: 75374
  2. Yazar: DURSUN ŞİRVANCI
  3. Danışmanlar: DOÇ. DR. ALİ GÜNEY
  4. Tez Türü: Yüksek Lisans
  5. Konular: Maden Mühendisliği ve Madencilik, Mining Engineering and Mining
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1998
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Maden Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Cevher ve Kömür Hazırlama Bilim Dalı
  13. Sayfa Sayısı: 144

Özet

ÖZET Bu çalışmada, %51.33 Cr203 içerikli Üçköprü (Fethiye), %54.89 Cr203 içerikli Pınarbaşı (Kayseri) ve %41.25 Cr203 içerikli Orhaneli (Bursa) bölgelerine ait ince boyutlu kromit konsantreleri ile yapılan kromit-karbon peletlerinin ön redüksiyon davranışları incelenmiş olup, optimum redüksiyon koşulları saptanmıştır. % Cr203 içerikleri farklı her üç numune ile yapılan deneylerde sırasıyla malzeme boyutu, süre, sıcaklık, redükleyici (kok tozu) miktarı, bağlayıcı (bentonit) miktarı, pelet boyutu ve CaF2 ilavesinin ön redüksiyona etkileri araştırılmıştır. Tamamı 0.5 mm, 0.106 mm, 0.075 mm ve 0.038 mm boyutlarının altına geçecek şekilde öğütülen konsantreler, bağlayıcı bentonit ve redükleyici kok tozu ile karıştırılarak -15+10 mm boyutlu peletler oluşturulmuştur. Yaş peletler 90°C'de 3 saat kurutulduktan sonra yüksek sıcaklığa dayanıklı potalara yerleştirilerek fırına şarj edilmiştir. İlk olarak fırın sıcaklığı 1200°C'de sabit tutularak 0, 30, 45, 60 ve 90 dak'lık sürelere ve malzeme boyutuna bağlı metalleşme oranları incelenmiştir. Daha sonra peletlerin fırında kalma süresi 60 ve 120 dakikada sabit tutularak 1 100, 1300, 1400°C sıcaklıklarda, sıcaklığa ve malzeme boyutuna bağlı metalleşme oranları tespit edilmiştir. Sıcaklık ve süreye bağlı redüksiyon deneylerinde elde edilen optimum süre, sıcaklık ve malzeme boyutunda, peletlere ilave edilen kok tozu miktarı stokiometrik olarak gerekli karbon miktarının katları olacak şekilde değiştirilerek, optimum kok tozu ilavesi belirlenmiştir. Belirlenen optimum çalışma koşullarında bentonit miktarının redüksiyona etki edip etmediği araştırılmıştır. Buna göre, -0.106 mm tane boyutlu 2 No'lu numuneye ait konsantreye, % 2 bentonit ve stokiometrik olarak gerekli karbon miktarının 2.24 katı olacak şekilde kok tozu ilavesi yapılarak oluşturulan peletlerin, 1300°C sıcaklıkta 120 dak süre redüklenmesi durumunda %59.25 Cr ve %79.25 Fe metalizasyonu elde edilmiştir. Bu koşullarda, pelet boyutunun redüksiyona herhangi bir etkide bulunmadığı ancak, %4 ve %8 oranlarında peletlere ilave edilen CaF2'ün metalizasyonu arttırdığı tespit edilmiştir. Optimum koşullarda, %8 CaF2 ilave edilerek oluşturulan peletler ile grafit dirençli tamman tipi fırında yapılan redüksiyon deneylerinde krom için % 73.2 oranında ve demir için % 89.65 oranında metalleşme saptanmıştır. xı

Özet (Çeviri)

SUMMARY Chromite deposites in Turkey are formed in Alpin (podiform) type and generally extended in east-west direction. They were also subjected to a intensive tectonic activity. Banded, noduled (leopard skin), and masive ores are found in the deposits. The main secondary minerals are dunite, harzburgide, olivine, serpentinite and talc. Turkey, which is an important chromite producer, has 4% of the world reserve with a standard loadable character. According to the chromite inventory studies conducted in 1995, the proven+probable+possible chromite reserve of Turkey is determined to be 308.4 Mmt. 202.2 Mmt of these reserve are proven and probable reserve. The chromite production of Turkey changes depending on the wold chromite market. Since 1924, the production of chromite in Turkey is estimated to be 36.8 Mmt and the average production for last 25 years is about 800.000 ton per year. Ferrochromium production in Turkey is made at two different plants as low and high carbon contents. Antalya ferrochromium plant, which started the production with a 8.760 ton per year, in 1963, produces low carbon content ferrochromium and silica ferrochromium. The high carbon content ferrochromium plant in Elazığ, that has an annual capacity of 150.000 tones, produces 100.000 tones of ferrochromium. Domestic ferrochromium consumption of Turkey is about 5-6 thousands tones. Therefore, most of the production is exported. The export is about 106.000 tones in 1994. Since most of the chromite reserves in Turkey are dissemine and fine size, use of these reserves as a result of the enrich of the ore must be investigated and their market possibilities must be increased XllFerrochromium, as an important alloying component in steel industry, is produced by smelting of chromite ores. Smelting process is carried out generally by reacting the mixture of ore, reducing agent and fluxing addition in a furnace. Additional to the progresses achieved in application of the smelting process in arc furnace, the so-called direct prereduction of chromite ore with carbon has recently become an important industrial process in which pelletized mixtures of chromite ore and carbon are reacted in a fuel-fired rotary furnace to produce an alloy in the solid state. The latest development in this field is to use the plasma technology in production process in spite of the difficulties in its application. The advantages of the usage of the pre reduction pelets in ferrochromium industries are; - Obrarning of cheaper and higher chromite content in the composition of the raw material. - Saving 50 % power by the usage of the pre reduction chromite-carbon pelet, instead of lump ore. - Increasing of arc furnace productivity twice. - Using cheaper coke powder instead of lump coke. In the rotary furnace ovens, at the 1300°C-1400°C, pellets which were 50-60 % pre reductions in the solid form, are feed to achieve more economic ferrochromium productions. This metod is known as SRC process and is used in a lot of ferrochromium industries. In this study, pre reduction pellets production and pre reduction conditions, which will supply the use of fine chromite in ferrochromium industries in large amounts, are researched. In the presence of carbon, carbon monoxide can reduce chromite to iron and chromium when at temperatures are higher than 1160°C. Chromite that is reduced by carbon monoxide in the presence of carbon, tough not in contact xmwith it, exhibits a reduction sequence and microstructure similar to that of chromite in chromite and carbon mixtures. Reduction of a chromite particle is a topochemical process at which the reaction interface is moved parallel to the original solid surface. This model corresponds three reaction steps as follows. The first step doesn't exist metallic structure but (Fe+2, Mg+2) (Fe+3, Cr+3, AI+3)203 composition have been reduced to (Fe+2, Mg+2) (Cr*3, Ar3)204 structure. Reduction of Fe+3 to Fe*2 starts at the contant points beween the solid chromite and the solid carbon. At the sametime, its possible to say that CO shows a reducing effect in addition to assisting to carbon reducibility. At the second step, metallic nuclei start to occur and metallic nuclei almost surround the particle surface depending on time. Metallic nuclei growth occur with increasing time. The surface surrounding metallic structure points out that the reduction might be associated by gaseous CO more than solid carbon. The third step is known as the start of shell formation of growing metallic nuclei around the particle. This metallic shell continues to enlarge at this step. Until the end of the reduction, this shell cannot remain in a compact shape and starts to break down. Since the particle was surrounded with the metallic shell reduction is controlled by diffusion at this step. Carbon dissolued in metallic structure tends facilitate the reduction phenomena. Analyses made at the unreduced oxide regions of the particle show that 3+ value chrome ions have a considerable concentration. For this reason, its possible the oxygen to pass through the oxide layer towards the metal oxide phase boundary in order to save the valancy balance. At the metal-oxide phase boundary, the oxide with -2 value reacts with the carbon dissolved in metaloxide phase boundary and causes to the formation of CO and metallic Cr. At the-reduction of chromite spinel, depending on the-reduction degrees reaction steps can be described as follows. XIV(Fe+2, Mg+2) (Fe+3, Cr+3, AI+3)204 Chromite Spinel. (Fe+2, Mg+2).(Cr+3, AI+3)204 0-4 % Reduction (Fe), Mg+2(Cr+3, AI+3)204 4-50 % Reduction *+2 (FeCr)7C3 Mg+2AI203 50-1 00 % Reduction Although the reaction rates are considerably fast in the spinel phases with Fe+2 and Fe+3, reduction of FeCr204 was found to be slow and time consuming. It was observed that the most important factor affecting the reduction rate is the spinel structure preventing the diffusion of gases towards the inside. The findings on the reduction of chromite spinel are given as. a. Reduction rates of iron and chromium are determined by solid state diffusions of Fe+2 and Cr+3 respectively, towards the lattice less in stochiometry. b. However the diffusion of Fe+3 is respectively high, the reduction at the outer surface of the particle is faster than that of the former. c. Cr+3 might leave the lattice but, Mg+3 tetrahedral places and/or Al+3 and Cr+3 oktahedral places kinetically might remain in their original places but change their shapes. For this reason reduction of chromite in Mg and Cr oxide is respectively slower than in Fe and Al oxides. In this study, the behaviour of prereduction of chromite-carbon pellet of fine chromite concentrates, taken from Üçköprü-Fethiye, Pınarbaşı-Kayseri and Orhaneli-Bursa with 51.33 %, 54.89 and 41.25 % Cr203 content respectiely, are researched and optimum reduction conditions are determined. The~effect of partical size, lime, temperature, amount of reductant (coke powder), the amount of binder (bentonite), pellet size and CaF2 addition are researched with three samples in different Cr203 contents. XVThe concentrates, wholy ground under the size of 0.5 mm, 0.106 mm, 0.075 mm and 0.038 mm are mixed with bentonite and reductant coke and pelletized in the size range of -15+10 mm. The wet pellets are charged in to the furnace after they are dried for 3 hours at 90°C. Initially, the temperature of furnace is kept as a constant at 1200°C and metallization ratio is researched due to the partical size and reduction time (0, 30, 45, 60 and 90 minutes). The reduction time of the pellets are kept constant at 60 and 120 minutes and metallization ratio are determinated due to the temperature (1100°C, 1300°C, 1400°C) and partical size. The effect of amount of reduction (coke powder), the amount of binder (bentonite), pellet size and CaF2 addition are resarched in the optimum reduction conditions. It is realized that the partical size of the chromite concentrates doesn't effect the metallization of Fe and Cr depending on time in the experiments under 1200°C The Fe metallization is more effective than Cr between 1 100°C-1200°C in pre reduction experiments. Cr metallization increases while the Fe metallization decreases at 1400°C. The reduction starts at the contact point of the chromite and carbon and spreads outwards from these points by a mechanism that involves carbon transfer from the carbon particle to the chromite across the metallized region. It seems more likely that reduction in the latter case is also by carbon monoxide. XVI

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