Microstructural analysis of iron ore samples with bitumen as a binder

In the present investigation, the direct reduction of magnetite and hematite powder with and without 5% bitumen at various temperatures has been studied in a hydrogen atmosphere. The prepared samples were fired in the temperature range of 673–1373 K to observe the weight loss and to find the effect of the bitumen in the samples. The microstructural results show that bitumen mixed with iron ore powder exhibited a more porous structure with cracks after reduction at 773 K and 973 K respectively. At temperatures above 1073 K, the carbon present in bitumen contributed to an enhanced rate of reduction of iron oxide samples. It was observed that, the reactivity of samples was affected by the level of impurities, crystal structure, binder, and composition of the reducing agent.


Introduction
Iron accounts for 95% of all metals used by modern industries and is closely related to the socio-economic development of the nation [1]. World reserves of iron ores are about 230 billion tons and are expected to last for more than 50 years. Around 1.73 billion tons of crude steel was produced in 2017, used in buildings, shipping, transportation, packaging and infrastructure etc [2]. The manufacturing of steel through traditional blast furnace remained the commercial method to meet the rising demand of the steel. However, during the last decade, the direct reduction has found an economic method to produce the steel without metallurgical coke and use supplement scraps as well. In direct reduction, the iron ore is not recovered as a liquid metal, as in the case when treating the material in a conventional blast furnace (indirect reduction), but as a solid product, so-called sponge iron. The sponge iron is produced from the reduction of iron ore which is further reduced to iron through pelletization with a suitable binder which imparts the green strength to the iron oxide pellets [3]. In the direct reduction, the iron-bearing charge (pellets) has to support Graphical Abstract Page  an optimum combination of high strength and reducibility. Studies have shown that the agglomeration of iron ore is not possible without the usage of suitable binders [4]. Over the years, metallurgical dust from steel plants is a secondary resource for a carbon-containing pellet with an appropriate binder and has become increasingly more prevalent and economic. Bentonite is universally used as a binder for iron ore pellets to improve their strength, however, it contains unwanted silica and alumina impurities that decrease the iron production rate [5]. Ripe and Kawatra found that high carbon fly ash can be used as a binder with calcium hydroxide as an activator to produce pellets of acceptable strength [6]. Utilization of serval organic and inorganic binders, such as carboxymethylcellulose (CMC), calcined colemanite, Peridur C-10, Peridur CX3, corn starch, modified starch, paper sludge, lactose monohydrate (dairy waste), and DPEP06-0007 polymer sodium lignosulfonate, has been also reported in the literature [7].
The reduction reactions of iron oxide with hydrogen (H 2 ) take place in the following steps: Whether magnetite reduces directly to Fe (step 2), or via wustite (steps 3 and 4), depends on temperature. The reduction takes place stepwise from Fe 2 O 3 to Fe 3 O 4 and continues to Fe at the temperature below 843 K. The intermediate oxide FeO is unstable below 843 K; when the temperature rises to 843 K and above, then FeO also be considered in the reduction reaction [8]. With higher temperatures, the stability of FeO increases because of the free spaces in lattices other than iron ions. The thermodynamics of the above four reactions is well explained in the Baur-Glassner diagram, which illustrates the stability of different iron oxide phases as a function of temperature and gas oxidation degree [9]. However, Otsuka and Kunii explained that FeO reduction was marginally increased compared to Fe by using graphite reductant, as the gasification reaction of carbon was influenced by metallic Fe catalyzing [10].
The activation energy was calculated from the plots of the rate of weight loss (at 8% and 19.2%) against the reciprocal of absolute reaction temperature. The activation energy for the initial stages (8% weight loss), at a temperature lower than 873 K, was 42.6 kJ mol −1 , and at the higher temperature above 873 K, it was 33.18 kJ mol −1 . At a temperature above 873 K, a high value of activation energy was measured in the final stage of reduction, showing solid-state diffusion as the controlling step [11]. Furthermore, the activation energy results achieved by Wang et al. were 68.95 and 82.61 kJ mol −1 for hard and soft-coal ore (i.e., high and low volatile content) of Fe pellet; however, El-Geassy et al. conducted the same study in nitrogen (N 2 ) atmosphere where 18.81 to 28.42 kJ mol −1 of activation energy was achieved for non-isothermal reduction of composite pellets, and during the incubation period. The gaseous diffusion was found dominant for reduction [12]. Later, a comprehensive stage study was conducted by Dutta, on a low heating rate of 10.5 K min −1 for the determination of activation energy at different stages of the reduction reaction, where the results showed that during stage I volatile gases diffusion was observed, which resulted in very low activation energy from 6.1 to 13.2 kJ mol −1 ; in stage II, diffusion of H 2 gas from pores of solid increased the activation energy up to 26.4 to 42.5 kJ mol −1 ; and during the final stage, due to carbon gasification reaction, the activation energy reached at a maximum level of 183.1 to 268.5 kJ mol −1 [13]. This manuscript is a preliminary study of the direct reduction of the iron ores at temperature range from 673 to 1373 K and observation of microstructures during all the stages of the reduction of iron ore particles and pellets using bitumen as a binder.

Raw material
The first stage in producing a super concentrate (<2% gangue) is to make a high-purity concentrate by a beneficiation process. Subsequently, it can be further upgraded by magnetic separation, flotation, or high-intensity electrostatic separation. Fe 3 O 4 has been supplied by L.K.A.B of Sweden, while Fe 2 O 3 came from SAMITRI of Brazil. The chemical composition of iron oxides was determined through X-ray fluorescence spectroscopy (XRF) shown in Table 1.
A binding material, 5% bitumen was mixed with Fe 3 O 4 and Fe 2 O 3 ore in a sigmabladed mixer at a temperature of 453 K. This bitumen-coated material was compressed using an Avery compression testing machine to produce compact cylindrical pellets (6, 9, and 12 mm). These pellets were baked in an oven at 473 K to gain strength as reported in the previous work of author [11]. High-purity H 2 was used as the reducing agent for both types of pellets. To find the reduction potential of bitumen, Fe 3 O 4 and Fe 2 O 3 pellets were heated at different temperatures in the N 2 atmosphere.

Direct reduction and analytical measurement
Thermogravimetric analysis (TG) records the continuous measurement of weight change with time in the temperature range of 673 to 1373 K to reduce FeO pellets in the H 2 atmosphere in a Linseis thermogravimetric balance. N 2 gas was flushed through the system for about 7 min before allowing H 2 stream. The sample for the reduction was made up of a 6-mm diameter pellet (approximate 0.5 gm) placed in a mall platinum crucible, and the temperature control unit was set at the reaction temperature and heating rate usually 323 K min −1 . After placing the sample into the spoon, the socket was fitted tightly in the cone, ensuring that there was no leakage of gas on flowing through the reaction tube. The hot furnace was then moved over the reaction tube through which H 2 was flowing at the rate of 2.3 l min −1 for pellet samples. After completion of a run, the reducing gas was turned off and the reaction tube flushed with N 2 . The furnace was switched off and rolled away from the tube to obtain rapid cooling and hence retain the morphological condition of the specimen. The reaction takes place only on the top surface of the crucible; therefore, the rate should stay constant under the condition of chemical control, but for diffusion control, the rate should decline with time. At high temperatures above 873 K, a fast chemical reaction occurs with the formation of the product layer. As reduction proceeds, the thickness of this layer increases, and thus the rate declines. By assuming that all bitumen has been driven off, the approximate reduction of Fe 3 O 4 due to the bitumen can be calculated as [12]: where W1 is the initial weight of the sample, W2 is the final weight of the sample, f ore is a fraction of ore present in a sample = 0.95, ρ ore is a purity of ore, i.e., % of Fe 2 O 3 or Fe 3 O 4 in ore, f oxygen is a fraction of oxygen present in Fe 2 O 3 or Fe 3 O 4

Results and discussion
The SEM image in Fig. 1a and b shows that fine Fe 3 O 4 ore particles have been agglomerated more effectively than coarser Fe 2 O 3 particles (Fig. 1c, d) after mixing with bitumen. Both iron ore agglomerates show good binding capacity with bitumen.
The produced mix was compacted into pellets after cooling to room temperature. The solid particles approach each other closely under the acting pressure, forming a tight connection. The baked pellets, at low temperatures, attained high crushing strength due to the polymerization and hardening of molten bitumen film coating the ore particles. At the point of contact between particles, a solid bridge is formed. Considerably larger bridges are formed when thermoplastic material (such as bitumen) moves under the influence of pressure and friction.

Effect of temperature on the microstructures
In the case of Fe 3 O 4 super concentrate powder reduced at a lower temperature of 773 K (Fig. 3a), the number of small pores was generated, though the shape of the sample did not change. As the temperature rose to 973 K (Fig. 3b), the particles formed larger pores, but the particles' sharp corners and shape almost remained the same. After a further increase in temperature to 1273 K (Fig. 3c), the sharp corners of the original Fe 3 O 4 particles became rounded and smoother, with no open pores visible. This was due to sintering which increased the density of the porous product. A similar effect for reduced powder of the Fe 2 O 3 was observed at different temperatures. At 773 K (Fig. 3d), each grain appeared to be dense. After measuring the pore surface area of the particles, it was concluded that fine pores were present in those dense grains. At 973 K (Fig. 3e), the particles were not as dense as at 773 K, and small pores within some particles can be noticed in Fe 2 O 3 particles (×1000) (Fig. 4c) at 973 K. Few cracks are clearly shown in some reduced particles at temperatures 773 K and 973 K respectively in Fig. 3a and b; however, the shape and size of the particle appear to be the same. At 1273 K (Fig. 3f ), pores in Fe 2 O 3 particles became coarser, but the sharp edges were retained.

Reduced magnetite and hematite with and without 5% bitumen
In Fig. 4, the microstructures of reduced magnetite and hematite particles are compared with 5% bitumen-mixed samples. It can be seen that porosity increased in the case of bitumen-mixed powders of both samples. This was partially due to the evolution of volatiles which leaves behind a highly porous material. The reduction of magnetite powder at 973 K (Fig. 4a) was slower when bitumen was not added. Reduction became faster, and a rate minimum occurred at a higher degree of reduction as compared with magnetite alone. This is another reason why bitumen-added magnetite appears to be more porous as shown in Fig. 4b. Some oxide inclusions were always present in the center of the particles surrounded by cracks as shown in Fig. 4a. Some cracks are also visible in the hematite sample with and without bitumen as shown in Fig. 4c, d.
At high temperatures, such as 1273 K, an additional process occurred during magnetite-bitumen reduction, in which carbon monoxide (CO) was formed, and the reduction rate became faster than when no bitumen was present. This was the effect of high gas pressure build-up within the iron shell resulting in rupture of the shell and exposure of the unreduced oxide to the reducing gas. With H 2 reduction, the gas pressure is not high enough to break the dense iron shell. It is believed that the bursting mechanism would produce a more open structure, and the porosity of iron particles would be increased.
The effect of magnetite-bitumen and hematite-bitumen reduced by H 2 at 1273 K is shown in Fig. 4e and f. Hematite-bitumen specimen (Fig. 4f ) is more porous than those with no bitumen (Fig. 3f ). The effect of sintering at high magnification is seen in Fig. 4 in case of bitumen-added samples. The pore structure of the specimen shown in Fig. 4e and f with bitumen is coarser than without bitumen (Fig. 3c, f ). Turkdogan et al. also found that the pore structure obtained after CO reduction is always coarser than after H 2 reduction [14,15].
The combustion of organic binders at high temperatures could virtually emit no residual ash to the structure of pellets, leading to slag formation [16]. The softening of bitumen starts at 353 to 363 K; when heated at low (673 K) and high (1273 K) temperatures, the volatiles evolved at 75% and 79% respectively. This volatile reduction was also justified by chemical analysis where fixed carbon was found at 20.5%, and ash content was around 0.12%. Based on this analysis, the bitumen-magnetite mixture