Metallic iron whisker formation and growth during iron oxide reduction: basicity effect K. S. Abdel Halim*, M. Bahgat, H. A. El-Kelesh and M. I. Nasr The effect of basicity on the metallic iron whisker growth during wu ¨ stite reduction was studied in the present investigation. Compacts of pure and CaO/SiO 2 doped wu ¨ stite were synthesised. The annealed compacts were isothermally reduced in thermogravimetric apparatus with CO gas at 800–1100uC. The course of reduction was followed by measuring the weight loss as a function of time. X-ray diffraction (XRD), scanning electron microscope (SEM), optical microscope and porosity measurements were used to characterise the annealed and reduced samples. The influence of temperature and basicity (CaO/SiO 2 ) on the reduction behaviour and the morphology of the annealed samples were investigated. The reduction rate increased with temperature but decreased by increasing basicity value. Metallic iron whisker shape structure was detected in the pure wu ¨ stite samples after reduction at high temperatures while in basic wu ¨ stite samples, whiskers were formed at the surface of the compacts. From the activation energy values, the reduction of pure wu ¨ stite is most likely controlled by a combined effect of gaseous diffusion and interfacial chemical reaction mechanisms. The reduction of basic wu ¨ stite compacts with 0?2 and 0?5 basicity ratios are most likely controlled by chemical reaction mechanism while for 0?8 basicity ratio, the reduction rate is most likely controlled by solid state reaction mechanism. Keywords: Swelling, Wu ¨ stite, CO gas, Metallic iron, Whisker, reduction kinetics, blast furnace Introduction In the course of iron oxide reduction during ironmaking the swelling of the reduced pellets or compacts is a very common phenomenon. The apparent volume variation due to changes in the crystal structure of the oxides during the reduction of hematite pellets to iron is around 20% which is considered normal swelling. Abnormal or catastrophic swelling occurs when the increase in the apparent volume is much greater, for instance, the volume expansion exceeds 400% under some condi- tions. 1–3 The phenomenon of catastrophic swelling leads to a subsequent degradation or disintegration of the hematite pellets and generation of dust that causes lowering of the gas permeability through packed beds and hence irregularities in the operation of blast furnace. In addition, a most serious consequence is the pollution effect of the produced dust in the waste gases. Moreover, the lost amount of the ore dust and its content of iron oxide have a considerable economic effect. Catastrophic swelling during reduction of iron oxide in CO and H 2 atmospheres was previously investi- gated. 4,5 It was found that the abnormal swelling takes place during the reduction from FeO to Fe in CO atmosphere due to the formation of metallic filaments. Nicolle and Rist 6 studied the metallic iron formation mechanism during reduction of wu ¨ stite. They stated that the overall reduction process may be divided into three mechanisms. Under pure diffusion control, nucleation takes place at many points independently in a short time until they merge to form a dense layer of iron. Under pure chemical control, iron is fed to the nucleus down a very steep gradient growth then takes place without any significant removal of oxygen around the nucleus to grow outwards. Under mixed control, the nuclei grow radially and longitudinally and assume conical shapes. Sharma et al. 7 handled the effect of firing conditions on the swelling behaviour of iron ore pellets. They concluded that the maximum swelling index of the pellets decreased with increasing in firing temperature and time because porosity of the pellets decreased with the increase of both parameters. The swelling behaviour of iron ore pellets under non-isothermal conditions in the temperature range 800–1000uC was investigated. 3 It was found that the index increased with the increase in heating rate, partial pressure, flowrate and porosity of fired pellets, but it decreased as the gangue content increased. In general, a lower swelling index is observed under non-isothermal conditions because swelling is attributed to the growth of iron whiskers which is dependent on the rate of iron production and diffusion. The effect of gangue content on the swelling behaviour of iron ore pellets was previously studied. 7,8 It was concluded that the additives like CaO, MgO and SiO 2 play an important role in decreasing the maximum Central Metallurgical Research and Development Institute (CMRDI), PO Box 87-Helwan, Cairo 11421, Egypt *Corresponding author, email khaledsaad@cmrdi.sci.eg ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 22 April 2009; accepted 1 June 2009 DOI 10.1179/174328109X463020 Ironmaking and Steelmaking 2009 VOL 36 NO 8 631