| The iron and steel industry is a high consumption, high pollution industry, and the metallurgical dust generated during the process of iron and steel making is one of the major sources of pollution. This metallurgical dust contains valuable elements, such as iron, carbon, zinc, which should be recycled. At present, using metallurgical dust as a raw material for sinter to be used in the blast furnace is one of the main ways to recycle iron and carbon in the dust. However, if the dust that contains an excessive amount of zinc which is volatile is fed into the blast furnace, the surplus zinc builds up within the furnace, thereby impeding the operation of the blast furnace and shortening its lifespan. Therefore, only dust containing low level of zinc can be treated by this method. A considerable amount of the surplus dust has been simply discarded, which not only takes a lot of land, but also brings a series of environmental problems. With the increasingly strict policies for environmental protection, looking for a technology suitable for treating dust generated in iron and steel industry is becoming more and more urgent. From several dust-treating processes now available, rotary hearth furnace (RHF) direct reduction process is considered as one of the most effective technologies for treating metallurgical dust. The RHF direct process has many advantages in the treatment of metallurgical dust. It can not only recycle valuable elements contained in the dust effectively, but be reliable and flexible, save investment, be easy to operate and maintain, and produce little secondary pollution, which has got extensive attention from iron and steel industry at home and abroad. The working process of the rotary hearth furnace is very complex, involving direct reduction of carbon-containing pellets on the hearth of RHF, gas flow, heat and mass transfer as well as fuel combustion. Now, the study about RHF direct reduction process is not extensive, further research work is of great significance to the sustainable development of iron and steel industry. In this paper, mathematical models were established to study RHF direct reduction process.First of all, a one-dimensional unsteady mathematical model was established to describe direct reduction process of a carbon-containing pellet made of metallurgical dust. The model considered heat transfer, mass transfer, iron oxide reductions, zinc oxide reduction, carbon gasification, as well as the changes of porosity and physical property parameters. The control equations of the model were discretized into algebraic equations using control volume integration method and were numerically solved by tridiagonal matrix method (TDMA). In order to verify the model, an experiment was conducted, in which chemical analysis method was used to obtain the change of chemical compositions of carbon-containing pellet during direct reduction process. Results calculated by the model were in good agreement with experimental data, which indicates the model established in this paper is reliable. The calculation results show that pellet surface and center temperatures are different during direct reduction process. At the early stage of direct reduction process of carbon-containing pellet, both metallization degree and dezincification degree increase with the decrease of pellet diameter. However, at later stage of direct reduction process, metallization degree decreases with the decrease of pellet diameter and the effect of pellet diameter on final dezincification degree is less than on final metallization degree. The metallization degree of the pellet clearly increases with the increase of C/O ranging from0.6to1.0. When C/O rangs from1.0to1.2, growth rate of metallization degree decreases with the increase of C/O. And C/O has little effect on pellet dezincification degree. Under the calculation conditions of this paper, the influence to the metallization degree decreases in the order furnace temperature, pellet diameter, reduction time, C/O, and the influence to the dezincification degree decreases in the order furnace temperature, reduction time, pellet diameter, C/O.Secondly, a mathematical model of direct reduction of carbon-containing pellet layer was established based on the mathematical model of direct reduction of a single carbon-containing pellet, after detailed analysis of heat transfer mechanism in a RHF. The model not noly considered heat transfer, mass transfer, chemical reactions, and the changes of porosity and physical property parameters, but the effect of radiation shielding of the upper layer to lower layer and the heating effect of the hearth were considered. The control equations of the model were discretized into algebraic equations using control volume integration method and were numerically solved by alternating direction implicit (ADI) method. The results indicate that staggered arrangement of carbon-containing pellets can improve average temperature of pellet layer, thus improve metallization degree and dezincification degree of the pellet layer. With the increase of furnace temperature, metallization degree of pellet layer increases rapidly. And the influence of furnace temperature on dezincification degree of pellet layer is less than on metallization degree. Reduction rates of pellets in different layers are different. With the increase of heating time, the differences of metallization degree and dezincification degree among each layer decrease. The difference of de-zinc reaction rates among each layer of pellets is lower than that of iron oxide reduction reaction rates. In practical industrial application, if only high dezincification degree of metallurgical dust is required, the reduction time can be reduced and the thickness of the pellet layer can be increased to decrease energy consumption, increase RHF production capacity and avoid re-oxidation of upper layer of pellets.Lastly, a combined model that incorporates two sub-models was established based on a pilot-scale rotary hearth furnace owned by a company. One of the sub-models was established to describe the direct reduction process of composite pellets on the hearth of RHF. The other sub-model was used to simulate gas flow and combustion process in the chamber of RHF. The layer of composite pellets on the hearth of RHF was assumed to be a porous media layer with CO source and energy sink calculated by pellet layer direct reduction model. A User-Defined Function (UDF) program in C language was developed and linked to FLUENT to consider such CO source and energy sink. A comparison was made between the predictions of the combined model and the data from a test of the pilot-scale RHF, and a reasonable agreement was found. The results calculated by the model indicate that velocity of the gas in the upper middle part of the furnace is higher than velocity of gas near the layer of composite pellets under the hearth and gas spurting from the burner inclines upward. These can help to maintain reducing atmosphere above the layer. Recirculation zones are predicted in the furnace which is good for CO post-combustion. Temperature near the layer of composite pellets under the furnace is relatively low mainly because endothermic reactions take place in the layer of the pellets. CO gas is generally concentrated at the bottom of the furnace, especially in the first and second reduction zones. Both the volume of carbon monoxide escaping from the pellet and the amount of heat absorbed by chemical reactions increase with the increase of C/O mole ratio ranging from0.6to1.0. When C/O mole ratio reaches1.0, further increase of C/O has little influence on the amount of carbon monoxide and heat absorbed by chemical reactions. Without changing air and fuel supplies, funace temperature is lower under the condition of C/O mole ratio of0.6than C/O mole ratio ranging from0.8to1.2, furnace temperature with C/O mole ratio of0.8is slightly smaller than with C/O mole ratio of1.0, and furnace tempretures are almost the same for the cases of C/O ranging from1.0to1.2.The work done in this paper lays a theoretical foundation for the design and optimization of the RHF, and can make a contribution to the development of RHF direct reduction process in our country. |
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