| In recent years, the electric furnace steelmaking production has increased greatly but is restricted by the scrap shortage and deteriorating of scrap quality. DRI can be used as high-quality scrap substitutes. In addition, with the increasing scarcity of high grade iron ore resources, the use of low-grade, refractory and multi-element ore become the main trend of iron production development. In this situation, the research of direct reduction process is more and more prosperity, in particular, development of ITmk3process become a huge change for the direct reduction process.On the basis of ITmk3and Kurrup-renn process, a new process is proposed to produce iron nuggets by reduction and low temperature melting separation of ore-coal composite pellets with low carbon rate.The characteristics of reduction and low-temperature melting separation of the pellets, behavior and mechanism of melting separation, the effect of solvent addition, carbon rate and reaction time on the melting separation, and the best condition of melting separation were systematically studied in this paper. On this basis, low-temperature melting separation of limonite and low grade nickel laterite were experimentally studied to provide reference and guidance for the development of iron or ferro-nickel nuggets production process by low-temperature melting separation of ore-coal composite pellets.The main contents and achievements of the paper include:(1)In reduction process of pellets with low carbon rate, the reduction degree of pellets and FeO amount after reduction can be controlled by adjustment of carbon rate, gangue of pellets combine with FeO and generate fayalite with low melting point, slag can melt at1300℃, ensure the low-temperature melting separation of the slag and iron.(2) After reduction of pellet, the slag phase begins to melt and appear in the surface of the pellets (melting separation starting). At1300℃and carbon rate of0.75,0.8, melting separtion start time is respectively44min,46min. The theoretical analysis shows that the theoretical composition of slag phase after reduction is in liquid region lower1300℃in the SiO2-Al2O3-FeO ternary phase diagram, is more conducive to low temperature melting separtion.(3) When the slag phase melted and contacted with graphite padding, FeO is reduced by C, metallic iron is generated, w(FeO) in slag and slag amount are rapidly decreased, the mass fraction of metallic iron in slag is increased. Since separation, iron nuggets contact with graphite padding and is rapidly carburized, w(C) in nuggets go up to more than3.0%in2min, iron nuggets begin to melt, while, metallic iron generated in slag grew up by cohesion. If collisions with iron nuggets, forming a larger nuggets, the mass of iron nugget and the iron recovery are increases.(4) At the conditions of carbon rate in0.75and0.8,2%of CaO addition, the melting separation is fastest, iron recovery is also highest. As the increase of carbon rate in pellets, melting separation start time is prolonged, but the iron recovery is increased. Under the conditions of0.8of carbon rate,2.0%of CaO addition,1300℃and40min of reaction time, w(TFe) of nuggets is94.0%, metallization ratio up to93%, w(c) is about4.4%, iron nuggets can be used as high-quality raw materials for electric furnace steel-making. Iron recovery is about85%, if some of iron particles in slag can be recovered, can improve the recovery of iron.(5) In the reduction and low-temperature melting separation process, the reaction between slag and iron has a little influence on desulfurization. Sulfurate action of sulfur in padding is comparatively large, have to select padding with low sulfur content, by mixing of sulfur-fixing agent in padding, can lower sulfur content of nuggets. So it is necessary to minimize the sulfur content of pulverized coal and iron ore.(6) The melting separation of the limonite pellets was studied based on the experiment. Considering of the cracking temperature and cracking characteristics in rapid heating, the loading temperature of limonite pellets of about850℃is suitable in this process. By the investigation of effect of process parameters on the melting separation, it can be determined that the suitable condition for this process is0.7of carbon rate,2%of CaO addition, melting separation start time of the pellets is19min, w(Fe) of nuggets is94.08%, w(C) is4.35%, iron recovery is about88.3%.(7) The melting separation of nickel laterite ore pellets was also studied in this paper. High-Fe nickel laterite ore is suitable to production of ferro-nickel granules proposed. By the considering of the ferro-nickel granules formation, w(Ni) in ferro-nickel granules and the recovery of nickel, the suitable condition for melting separation is0.7of carbon rate,8%of CaO addition and40min of reaction time, w(Ni) in obtained ferro-nickel is4.75%, w(Fe) is87.51%, recovery of nickel is85%, nickel and iron is in alloy state, no significant gangue components.Summarizing the above results, iron nuggets production process proposed by this study is feasible, and has some unique advantages. The results show that, this process can produce high-quality iron nuggets at the lower temperature to meet the requirements for raw materials of electric furnace steel-making, also, can rationally use limonite or low-grade nickel laterite ore. The results of this study are important to develop direct reduction process using low quality, refractory and multi-element ore. |
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