Molten Slag Bath Reduction: Carbon-Thermal Reduction of Blast Furnace Dust in Molten Blast Furnace Slag

By adding BF dust into molten BF slag, it was conducted to explore the feasibility of carbon-thermal reduction of BF dust in molten BF slag bath. The experimental results indicate that, BF dust can achieve self-reduction in the bath. It is convenient for the reduction products that are iron-based metal particles separating from slag. Besides, the reduction of BF dust is endothermic. When the mass ratio between BF slag and BF dust is 10:1, the temperature of BF slag after reduction can meet the requirement of dry processing treatment. According to the comparison by X-ray diffraction, the main mineral phases of BF slag are relatively stable before and after reduction. Undoubtedly, it is a novel method to cope with the metallurgical solid waste in molten BF slag bath.


Introduction
In China, the disposal of metallurgical solid waste has always been the hot topic in ironmaking and steelmaking industry. With the overproduction of steel in recent years, it is inevitable that the generation of metallurgical solid waste has been growing significantly as well. In ironmaking, the major solid waste are blast furnace (BF) slag and BF dust. For BF slag, it is estimated that the average production of BF slag is up to 346kg per tonne hot metal [1]. Currently, these slags are either water granulated or air cooled. In general, water granulation is the most common treatment used to produce glassy materials that can be widely used in concrete and cement [2][3][4]. But before water granulation, the high quality sensible heat of molten slag is always wasted in vain without reasonable utilization [5]. It has been estimated that the waste sensible heat of the molten BF slag at 1773K reached 18.5kg CE (kilogram of coal equivalent) [6]. But at present, not only the efficiency of heat recovery is still at a low level, but also the quality of this kind of energy is poor [5]. Therefore, from the view of energy saving, it is important and valuable to use the sensible heat in a high-efficiency way. In addition to BF slag, BF dust is also a kind of by-product generated in the ironmaking process. It is estimated that the amount of BF dust is 15-50 kg per ton hot metal [7]. Since the various ingredients of BF dust are stemmed from the raw material added in BF, the dust contains high values of carbon and iron accompanied with harmful elements of Zn, K and Na. Based on this, there has been several successful attempts to separate and recycle valuable part of the BF dust, such as carbon and iron as well as zinc [8][9][10]. But the residual part that is regarded as the so-called useless waste is still a tough problem that has to be faced. Moreover, the disposal of BF dust on the landfill can obviously and inevitably cause environmental pollution as the leaching of some toxic metals (such as Pb and As) [11]. Thus, there needs more appropriate ways to recycle BF dust. Shaheer A. Mikhail et al. [12] reported that carbon-rich BF dust could be regarded as reductant, not only realizing self-reduction, but also making the iron oxides in basic oxygen furnace dust reduced at the same time. This research provides an idea that with the help of the high temperature platform, it is feasible to realize the total recycling of BF dust. Therefore, the aim of this paper is to explore the feasibility of both recycling BF dust and the direct use of high-quality sensible heat of molten BF slag. For the sake of achieving the goal, the experiments were carried out with the addition of BF dust into molten BF slag bath. Both the products and mineral phases of slag before and after reduction were studied by several methods, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectrometer (EDS). Meanwhile, the heat use of molten BF bath was evaluated as well.

Experimental materials
Based on practical composition from AnSteel, BF slag used in the experiment was synthesized by analytical reagents. BF dust was sampled from the spot in AnSteel. The composition of BF slag and BF dust was listed in table 1. The binary basicity of BF slag and BF dust were 1.11 and 1.04, respectively.

Sample and method
In the present work, analytical reagents were adopted to synthesize BF slag. Firstly, all the ingredients, including SiO 2 , CaO, MgO and Al 2 O 3 , were mixed homogeneously together as the designed mass ratio shown in table 1. Then the mixture was put into the graphite crucible and heated in a high temperature furnace for 30mins at 1500℃ in pure nitrogen, where the graphite crucible can be protected from oxidation. After synthesis, the BF slag was cooled in the air and crushed for further use. BF dust were made into pellets mixed with 2% bentonite binder and appropriate water by a pelletizer. Then, the pellets were put in a drying oven for 2 hours at 120℃. The diameter of cooled pellets is about 10.6mm shown in figure 1. Generally, combing with the output ratio between BF slag and BF dust as well as the volume of corundum crucible, the usage of BF slag was confirmed as 80g, while the usage of BF dust was decided to be 8g, 12g and 16g at each experiment respectively. Figure 2 shows the experimental apparatus used for thermal reduction of BF dust in molten BF slag bath. The experiments were conducted at 1500℃ with the temperature duration of 30mins, so that the solid BF slag could fully melt into liquid. Then, couples of BF dust pellets were added into the molten 3

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International slag bath to make carbon-thermal reduction. 30mins later, the corundum crucible was taken out and cooled in the air. The magnet was used to separate the products after reduction. X-ray diffraction (XRD, Cu Kα, λ=1.54178Å, 40kV and 40mA, 2θ =10°-90°, stepsize=0.013, interval=5s) was used for measuring the mineral phases of slag, and scanning electron microscope (SEM, type: JSM6480LV) coupled with energy dispersive spectrometer (EDS) was employed to make microstructure observation of the product. Actually, though there was obvious reduction weight loss at each experiment, this work is mainly emphasized on the effect of recycling BF dust and the sensible heat use of molten BF slag. The kinetics of reduction of BF dust will be discussed later in another paper specially.

Figure 2.
Experimental apparatus sketch at high temperature.

Thermodynamics of carbon-thermal reduction in BF slag bath
As shown in table 1, the mole radio of carbon and oxygen is calculated as 1:0.45, which means that the amount of carbon in BF dust is excess. figure 3 shows the standard Gibbs free energy changes of different reactions at high temperature [13]. It can be seen that the standard Gibbs free energy changes are all negative under the temperature from 1100℃ to 1500℃. And it is obvious that the carbon-thermal reduction reactions of metal oxides, especially NiO, CoO, FeO, Fe 3 O 4 , Fe 2 O 3 , are able to happen in molten slag bath. Figure 3 also shows the diagram of oxygen potential of part of oxides in BF dust [13]. Clearly, carbon is prior to reduce the oxide of Ni and Co instead of Fe. Meanwhile, iron oxides are doomed to be reduced into Fe step by step at 1500℃. Thus, from the view of thermodynamics, molten BF slag bath reduction is able to achieve the recycling of BF dust.  Figure 3. The oxygen potential of oxides and standard Gibbs free energy changes of reactions in slag bath at high temperatures.

Mechanism on carbon-thermal reduction of BF dust in slag bath
Since the BF dust pellets were added into the slag bath, solid-solid reactions firstly took place at the reaction point. On account of BF dust containing both carbon and metal oxides, the slag bath reduction of BF dust pellets is similar to the reduction of iron ore-coal composite pellets [14]. The direct reduction reactions can be seen in figure 3. Then, with CO gradually generated, the indirect reduction are simplified as equation (1): Where Me represents metals reduced by CO. In addition to the direct and indirect reduction, the gasification of carbon is supposed to be taken into account for the excess carbon existed in the pellets. Boudouard reaction (gasification of carbon) is expressed as equation (2): In common, the direct reduction and the gasification of carbon are endothermic process, which means that the enthalpy change of the total reduction reactions is positive ( 0  H ). So, when the temperature is increasing, it is beneficial for the generation of CO. As the bond of direct reduction and the gasification of carbon, CO plays a key role on the rate of the whole reduction. Due to the limited contact area of solid, the rate of reduction reactions is relatively slow at initial period. However, when the partial pressure of CO rises to some value, the interface of reactions turns from solid-solid area to gas-solid area, causing the reduction rate growing fast. The reduction process of BF dust pellet in slag bath is shown in figure 4. Thus, based on the analysis above, the carbon-thermal reduction of BF dust in slag bath belongs to solid-solid phase reaction and gas-solid phase reaction.

Metal particles separated from BF slag
When the pellets were added into the molten BF slag, the reduction reactions were violent in the crucible with weight loss significantly, and amounts of smoke generated accompanied with burst-out sound of pellets. Figure 5 shows that the metal particles gather and grow up with different sizes after reduction. And it is clear that most of the particles were attracted by magnet. It indicates that the metal particles are easy to be separated from the slag. SEM images in figure 6 shows that the main elements of metal particle contains Fe, Ni, Co and Si. It means that most metal oxides in the BF dust are reduced in BF slag bath, and the reduced metals are enriched in the particle where finally iron-based alloy is generated. Moreover, the physical interface of metal particle with the surrounding slag is clear, which indicates that it is easy to separate the metal particles from the slag after cooling. In figure 6(ii), it shows that there are dose of carbon inside the crack. It is probably because during the process of reduction, the metal particles were graphitized by the excess carbon in BF dust. This kind of graphitization is common to be found in the carbon-saturated iron [15].

Unsinkable behavior of metal particles in slag bath
In this work, it is very interesting to find that most of metal particles are floated on the slag surface after cooling, and it is difficult to find metal particles sinking into the slag or on the bottom of the crucible as shown in figure 5. Though it is seemingly convenient for separating metal products from the surface of slag, still the unsinkable metal particles are disadvantageous for the following treatment a) b) magnet metal particles corundum crucible slag (ii) (i) 6