Şark kromları kromit konsantresinin katı redüksiyon davranışı

Abstract

Bu çalışmada Etibank Sark Kromları Kromit konsant resinin hidrojen ile ve peletinde bulunan C ile redüksi- yonu incelenmiştir. Kromit konsantresinden ve kromit konsantresine CaFp ilave edilerek hazırlanan peletler 14DD°C,de hid rojenle redüklenmiştir. Petrol koku ilave edilerek ha zırlanan peletler de 13DD-15D0 C sıcaklık aralıklarında pelette bulunan C ile redüklenmiş. Kromit konsantresi peletlerinin H" ile 1 400DC'de ^ 2 saat redüksiyonunda Cr için % 7.5 ve Fe için % 40.2 X oranlarında metalleşme elde edilirken, % 4 CaF_ ilaveli peletin 1400 C'de 2 saat redüksiyonu ile Cr içîn % 16.1 ve Fe için % 51 oranlarında metalleşme elde edilmiştir. CaF" ilavesinin redüksiyonu artırmasının yanında H" ga zı redüksiyonunda istenilen metalizasyon oranlarına ulaşılmamıştır. CaF" miktarının arttırılması da redük siyonu artırmamıştır. Petrol koku ilaveli peletlerin, karbon ile redük siyonunda sıcaklık ve süreye bağlı olarak değişim gös teren yüksek oranlarda metalizasyonlar elde edilmiştir. % 13 Petrol koku içeren peletlerde 2 saat redüksiyonla veya 15DD C'de 1 saat redüksiyonla Fe ve Cr için % 90' in üzerindeki oranlarda metalleşmeler elde edilmiştir. Redüksiyon kromit spineli parçacığının dış yüzeyde başlayıp iç bölgelere doğru ilerlemektedir. Redüksi- _ yonun ilerlemesi ile redüklenen Cr ve Fe'in birlikte oluşturdukları çözeltinin C ile yaptığı metalik faz, redüksiyonun tamamlanması ile yumrular halinde belirli bölgelerde topaklanmaktadır. Yüksek oranda redüklenen peletlerin kırılıp öğütülmüş tozlarından manyetik ayı rıcı ile bu metalik fazlar ayrılabilmektedir. Redüksiyonla oluşan ve manyetik ayırıcıda ayrılabilen metalik faz (FeCr)7C, formundaki ferrokrom karbürdür ve çelik endüstrisinde ferroalaşım ilavesi amacıyla kullanılabilir özelliktedir.

In this study, the reduction behaviour of chromite concentrates obtained from Etibank Elazığ Sark Kromları plants have been investigated. Reduction of a chromite particle is a thopochemi- cal process at which the reaction interface moved parallel to the original solid surface. This model corresponds three reaction steps as follows. The"first2step dgesn' t, exist, metallic structure but (Fe, Mg ) (Fe", Cr, Al, ^Decomposition have been reduced to iFe, Mg ") (Cr, fil )?û\ structure. Reduction of Fe to Fe starts at the contant points between the solid chromite and the solid carbon. At the sametime, its possible to say that CD shown a reducing effect in addition to its assist 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. This surface surrounding metallic structure points out that the reduction might be associated by gaseous CD more than solid carbon. The thind 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 Df the reduction, this shell cannot remain in a compact shape and starts to break down. VI 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. Fon 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 cause to the formation, of CD and metallic Cr. At the reduction of chromite spinel, depending on the reduction degrees reaction steps can be described as follows. (Fe+2, Mg+2) (Fe+3,-Cr+3, Al+3)204 Chromite Spinel, (Fe+2. Mg+2) (Cr+3, Al+3) ^ 0-4 % Reduction (Fe) Mg+2(Ci J * (Fe) Mg+2(Cr+3.Al+3)2D4 4-50% Reduction (FeCr)?C3 Mg+2A1203 50-100% Reduction Although the reaction rates şge considerably fast in the spinel phases with Fe and Fe, reduc tion of FeCr?0, 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 is given as. Vll a)- Reduction rates of inon and chromium are, determined by solid state diffusions of Fe and Cr+ respectively, towards the lattice less in stochiometry. b)- However the diffusion of Fe is respectively high, the reduction at the outer surface of the particle is faster then that of the former. c)- Cr might leave the, lattice but, Mg tetrahedral places and/or Al and Cr 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 an Al oxides. Before the experiments, pellets were produced from teh chromite concentrates. Pellets were produced in a laboratory scale pelletizing disc. Different types of blends were used. These are; i)- Chromite Concentrate ii)- Chromite Concentrate + if % CaF 2 iii)- Chromite Concentrate + B % CaF" iv)- Chromite Concentrate + 13 % petroleum coke. Pellets produced from the first three graups of blends were reduced with H" in a tubular furnace. Pellets were put into the furnace in alumina cruicibles. After the reduction experiments at different temperatures and retention times pellets were subjected to chemical analysis in order tD calculate the metallization degrees CaF" additions to pellet mixes were made to case the reduction of the chromite spinels and increase the metallization degrees. Chromite pellets with 1 3%C were reduced in tamman furnace. Pellets were charged to the furnace at desired temperature in a graphite cruicible and kept in the furnace at given retention times. Pellets in graphite cruicible were covered with petroleum coke thus preventing the oxidation of pellets with bulk oxygen. vııı üJhen composed the reduction with H" to C. it yas found that metallization degrees of pellets reduced uıith C are higher than that of H". Reduction products having comparatively high metallization degrees were subjected to magnetic seperation after they were ground. Ferro-chrome- carbide compounds was found to be (FeCr)7C_ carbide after x-rays dif fraction. observations. From the microstructural studies, structural films of reduced pellets were obtained. Hicrostructures and the growth of reduction of the chromite pellets were examined from the photographs. Metallic phases which were brought about by Cr and Fe making solution between eachother and combining with C, at;, the last steps of the reduction, become more dense in about center point of chromite particle. Results of the experiments can be given as follows * although H" is a strong reductant no more reducing effect couldn't find in the reduction of chromite spinel. After the reduction carried about 1400°C and at a retention time of 2 hours with H"0 which makes the reduction difficult, metallization degrees of Cr and Fe were found to be 7.5 % and 40.2 %, respectively., * In order to increase the reducibility with H_ CaF_ was added but, no good results were observed. Pellets with h% CaF_ additive were reduced at 14DD°C and about two hours and metallization degrees were found to be 51 % for Fe and 16.1 % for Cr. Because, the amount of slag increases with increasing CaF_. After the reduction of pellets added oil coke high metallization values were obtained depending on the temperature and retention time. From the experiments carried out at 1450DC and 2 hours or at 1500 C and 1 hour higher metallization values about 90% for Fe and Cr were determined. IX * Reduction of chromite spinel starts from the outer surface of the particle and proceeds in wands. At the last steps of the reduction occuring Cr and Fe combine with C and make a metallic phase saturation of this phase might occur in the preferred regions. This metallic phase can be seperated from the gangue content by magnetic seperation. * Obtained from reduction experiments and seperated from the gangue content the metallic phase is in the (FeCr)"C_ form and called ferro-chrome- carbide. This product can be used in steel industry as a ferro alloy.