Direct Reduction of Ferrous Oxides to form an Iron-Rich Alternative Charge Material

Materials

The charge materials, used in production of sponge iron in a rotating furnace, are mill scale, coal and slag formers. Mill scale is a steelmaking by-product from the rolling mill in the steel hot rolling process. Mill scale contains both iron in elemental form and three types of iron oxides: wustite (FeO), hematite (α-Fe₂O₃) and magnetite (Fe₃O₄). The iron content is normally around 70%, with traces of non-ferrous metals and alkaline compounds. Mill scale is formed by flaky particles of a size of generally less than 5.0 mm. The size distribution depends on the stage in the process where the mill scale is generated [9]. In this study, the particle size of the mill scale is between 400 and 500 μm. Table 1 shows the contents of raw materials for 1 ton. The slag formers having a particle size of ~200 μm include bentonite, dolomite, limestone and are added to charge as 20 kg. Table 2 shows the contents (%) of slag formers used in the study. The content of coal used in mill scale processing is 67.25% and its particle size is ~150 μm.

Hematite ores in pellet form was supplied from ERDEMİR A.Ş. and its reduction by solid carbon having a purity of 99% was investigated as a function of reduction time and the ratio of C_fix/Fe_total. C_fix/Fe_total ratio can be described as the amount of carbon necessary to reduce completely the iron oxide that exists in the system [10]. Table 3 shows x-ray fluorescence (XRF, Rigaku) analysis of both mill scale and ore materials.

Reduction procedure

Industrially iron is produced from iron ores, principally hematite (Fe₂O₃), magnetite (Fe₃O₄) by a carbothermic reaction that is reduction with carbon, in a blast furnace at temperatures about 800–1,600°C. In the blast furnace, iron ore, carbon in the form of coke, and a flux such as limestone are fed into the top of the furnace, while blast of heated air is forced into the furnace at the bottom. Reduction of iron oxides occurs either by carbon or by carbon monoxide, formed by the gasification of carbon. The reduction process carried out by the carbon is called as direct reduction and the reaction can be defined in eq. (1).

On the other hand, the reduction process conducted with CO is called as indirect reduction and its reaction is given in eqs (2) and (3). The reduction of ferrous materials can be indirectly achieved by hydrogen and eqs (4)–(6) indicate the stages of reactions. The reduction of iron ores by hydrogen is a gas–solid reaction which occurs in two or three stages. For temperatures higher than 570°C, hematite (Fe₂O₃) is first transformed into magnetite (Fe₃O₄), then into wustite (Fe_1-yO), and finally into metallic iron whereas at temperatures below 570°C, magnetite is directly transformed into iron since wustite is not thermodynamically stable.

The iron also can be produced from its ore by the direct reduction of iron ore by a reducing agent which is coal based or may be a gaseous reducing agent, which is called direct reduced iron (DRI) or sponge iron. DRI is a good substitute of scrap for making steel in electric arc furnace, basic oxygen furnace, etc. and there has been a rapid worldwide growth in its production. DRI is a solid state product of direct reduction processes which is produced either in the form of lump or pellet. Availability of huge amounts of non-coking coal, scarcity of coking coal deposits and industrial significance of DRI led to many efforts for the development of many direct reduction processes [6, 11].

In this study, two different techniques were applied for the reduction of ferrous oxide materials. In the first stage, the reduction of mill scale was carried out in a rotating furnace using solid and gas reductants to produce sponge iron. The coal is used as solid reductant and its features have been given in “Materials” section. The source of hydrogen used as gas reductant is liquefied natural gas (LNG). The mixture of gas formed after the decomposition by combustion of LNG has 95% H₂. The flow of reduction in the rotating furnace is summarized in Table 4. The reduction in the rotating furnace was carried out in two media: (i) solid coal and (ii) solid coal and hydrogen gas. After reduction, produced sponge iron was smelted and the decomposition of metal-slag was done. In the second stage, the reduction of hematite ore formed as pellets was carried out using solid carbon in a furnace. The furnace is heated up to 1,100°C step by step with 10°C/min. The pellets having different C_fix/Fe_total were reduced at 1,100°C for 60 and 120 minutes. Table 5 shows the reduction conditions for ore in pellet form. In order to reduce all iron oxides, carbon ratio was selected as 1.5 and 2 times of the theoretical amount. The amount of binder in pellet was neglected.

Table 4

The flow of reduction in the rotating furnace used

Process
Charging of materials (mill scale + coal + slag formers) to furnace
1st reduction (entrance to furnace) : ~850–900°C
2nd reduction (middle of furnace) : ~950–1,000°C
3rd reduction (close to nozzle) : ~1,200°C

Calculation of ore reduction

The stoichiometric amount of carbon was determined using the reaction given in eq. (7). All reduced products were cooled from selected reaction temperature to room temperature in the furnace and scaled to determine %-reduction (R). This value was calculated using eq. (8).

Microscopic examinations

Solidified sponge iron and its slag were prepared by metallographic methods. Grinding was carried out with 320, 600 and 1,000 mesh size SiC abrasives, respectively and then ground surfaces were polished with 3 μm diamond solution. Etching was done with nital (% 3 HNO₃) to characterize the microstructure. Jeol JSM 6060 and JSM 840A scanning electron microscopes (SEM) were used for metallographic examinations. Energy dispersive x-ray spectrometer (EDS) was used for elemental analysis of the phases observed in the electron microscope.

Microstructural characterization of reduced mill scale

In iron-steel making industry, it is desired that sponge iron as an alternative charge material must have high degree of iron. The effect of reductant in the reduction conditions is very important in obtaining the final product. Figure 1(a) and (b) shows SEM micrographs of solidified sponge iron reduced by only solid carbon (coal). The solid metal form includes coal particle in dark contrast within the matrix and it indicates that the reduction of ferrous oxides by coal is unsufficient. An EDS (IXRF model) analysis is given in Figure 1(c) and it shows the degree of metallization in the selected image area. The amount of carbon is very high and the amount of iron is lower than that of traditional sponge iron (55–65 Fe%). Iron ore can also be reduced by hydrogen. Wagner et al. reported that (i) most of the reaction features are very similar to that of the reduction by carbon monoxide and many mechanisms are common to both of them, (ii) the reduction with hydrogen is endothermic, whereas it is exothermic with carbon monoxide and conversely, thermodynamics are more favorable with hydrogen than with carbon monoxide above 800°C, (iii) with hydrogen, the hot gas fed has to bring enough calories to heat and maintain the solid at a temperature sufficiently high for the reaction to occur, (iv) kinetics are also faster with hydrogen [11]. Figure 2(a) and (b) shows SEM micrographs of sponge iron reduced by solid + gas reductants. Hydrogen accompanies coal for the reduction mechanism and causes a high degree of metallization. EDS analysis given in Figure 2(c) indicates that the content of iron is higher and the amount of carbon is lower than the sponge iron reduced by coal only.

Figure 1

(a) and (b) SEM micrographs of solidified sponge iron after reduction by solid reductant (coal), (c) EDS analysis of matrix illustrated in (b)

Figure 2

(a) and (b) SEM micrographs of solidified sponge iron after reduction by solid + gas reductants (coal + H₂), (c) EDS analysis of matrix illustrated in (b)

Effect of reduction time and ratio of C_fix/Fe_total

Reaction kinetics in iron ore reduction deal with the rate at which iron oxides are converted to metallic iron by the removal of oxygen. The rate of a chemical reaction increases with increase in temperature. For this reason, the reaction kinetics are not generally a matter of great importance in the blast furnace because of the high temperatures at which the furnace is operated. On the other hand, in DR processes where the iron is reduced in the solid state, the maximum temperature is below the melting temperature and the reaction rates are slower. For direct reduction of iron ore, the mechanisms are complex because the oxide must go through a series of step wise changes before the conversion is complete. The slowest step in the process determines the overall reaction rate and is referred to as the rate controlling step [12]. Aguilar et al. reported the reduction process for a given spherical pellet material and they illustrated a schema explaining the reduction from outside to core. The reduction of hematite to metallic iron is carried out by the following reactions (eqs (9)–(11)) [13].

Figure 3

The reduction (%) as a function of the reduction time and the ratio of C_fix/Fe_total