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Characterization of Cold Briquetted Iron

2013-08-14 10:21:45


 Characterization of Cold Briquetted Iron

    Due to acute raw materials shortage for iron foundries in Nigeria, a need arises to source alternative material to replace the scarce pig iron and iron scraps used presently by local foundries. During the steel production in Delta Steel Company (DSC), Aladja, Nigeria, where direct reduced iron (DRI) is employed as raw material, large quantities of under-sized fines are generated which are not suitable for use in the plant. These metallic fines are made into briquettes using sodium silicate as binder and lime as flux . The briquettes so formed are referred to as cold briquetted iron (CBI) because they are formed when the fines are cold. Several tons of CBI are produced daily for which engineering or industrial application has not been found in Nigeria. 

    Apart from enhancing fines utilization, cold briquetting system has been found to be a very important development in the handling and storage of Midrex DRI [1]. Low specific surface area of these high-density briquettes, about 5 to 6 grams per cubic centimetre, increases their resistance to re-oxidation.   CBI has a degree of metallization of about 89% and carbon content of 3.50% , which are considered favourable for foundry furnace feed charge. However, the initial attempts made by some local foundries have shown that melting CBI is difficult with existing facilities and the losses incurred as a result of damages inflicted on the furnaces during the trials have discouraged further trial . This problem warranted the need for an investigation into the phase constituents of this material. This would help gain an insight into its nature and suggest modality for its subsequent application to foundry. This preliminary study was therefore conducted to characterize CBI and try to identify the possible causes of the problem encountered in melting it in available local foundry facilities. 

EXPERIMENTAL PROCEDURE

    Some samples of CBI were crushed and then ground to pass through a 30-micron aperture sieve. The ground material was then mounted in special flat sample holders and was measured in transmission at room temperature under rotation.  Thereafter, the rotating sample was subjected to convergent Co K- radiation (with a wavelength of 1.7890) in Debye-Scherer geometry focused by means of a curved germanium monochromator. With the aid of two-positioned STOE proportional sensitive detector (PSD), the x-ray diffraction (XRD) patterns were recorded using a STOE automatic powder diffractometer [4]. With the use of Rietveld technique [5] the produced patterns were refined (using STOE software) and the measured values were compared with the calculated data stored in an International Centre for Diffraction Data (ICDD) data bank .  

     For the micrographic study, a briquette of as-received CBI was sectioned and ground using 80, 120, 180, 320, 500, 800 and 1200 paper fineness on a Jean Wirzte Grinding Machine. With the use of a DAP-V polisher, the sample was polished using 6|ìm, 3|ìm and 1|ìm diamond laps wiStrurers DP suspensions. Thereafter, the etching was done using 2% Nital, dried and micrographs taken by means of optical and scanning electron microscope model Olympus GH2-UMA and SEM DSM-962, respectively.  

 DISCUSSION 

    CBI is found to contain prominently of ferrite (a-Fe), cementite (Fe C), silica (SiO ), iron carbide (FeC) and wustite (FeO) (Fig. 1)  which are confirmed by the observed and calculated d-spacings presented in Tables 1- 6 [6]. Alpha-iron is the most prominent phase in the sample and forms the predominant matrix (Figs. 2). Based on quantitative calculations on x-ray diffraction theory presented in references [7] and [8], -Fe phase (dark greyish) constitutes about 67 wt % of C, (blackish) constitutes about 17% of the refinable components of CBI (Table 6). Cementite, Fe3the phases in CBI. This orthorhombic structured phase is brittle [9,10] in nature. A trace of another form of iron carbide, FeC, is identified in the material (Fig. 1 and Table 5) and this has a unit cell parameters of a = 4.300?, b = 6.700? [8,9]. Due to the similarity in colour with Fe C, it 3has been difficult to distinguish the two in the micrographs (Fig. 2). This phase constitutes about 2.5 wt % of the refinable components and this brings the total iron carbide content in the sample to about 19.50 wt %.  

    Quartz, SiO , is found to be densely distributed along the cracks which look like grain 2boundaries (Fig. 3a). The SiO  particles seen are approximately spherical in shape and appear not 2to form any serious bond with one another or with neighbouring phases (Figs. 3b). These suggest  particles that that the observed cracks and most crevices in the sample originated from the SiO2cannot form strong bonds with adjacent particles. A trace of wustite FeO is also found in the sample (Fig. 1 and Table 4) which is calculated to be approximately 4.50 wt % of the refinable components in CBI. 

CONCLUSION

    CBI consists essentially of alpha iron, cementite, silica and wustite. The silica present appears in a mixture with other constituent phases and it is suspected to be the main initiator of cracks that characterised CBI. CBI is not a homogeneous material; the quantity of constituent phases and elements vary from one point to the other.  

    The difficulty experienced in melting CBI in existing foundry facilities is suspected to be attributed to the presence of fairly high quantity of unreduced iron (II) oxide and silica contents in the material. It is suggested that provision for the reduction of iron (II) oxide to iron and effective deslagging mechanism to take care of expected large volume of slag formed from the SiO  should be incorporated into a design of future foundry furnaces meant to melt CBI.