Induration Of Iron Ore Pellets Research Articles

Rigorous modeling of the Traveling Grate stage in the iron ore pellet induration process

This work presents a robust phenomenological model for the Traveling Grate system in an iron ore pellet induration plant on an industrial scale. The system consists of a bed of pellets in constant movement, heated by perpendicular air currents. The aim is to integrate into a single model the details of the chemical and physical phenomena that occur in the system, including gas/pellet energy transfer, a three-stage drying mechanism, compressive strength, and kinetic models for coke combustion, magnetite oxidation, and carbonate calcination. The model demonstrated adherence to process data, with a maximum relative deviation of 2.35%. Simulations enabled the assessment of the influence of key parameters on the bed temperature, observing an inversely proportional relationship of the profile for grate velocity, pellet diameter, and bed height. Pellet quality was analyzed under different composition conditions and void fractions, highlighting the significant influence of the coke on compressive strength, limited by the vitrification phenomenon at high temperatures. This model enables critical analysis of phenomena that are difficult to detect in conventional operations and helps in defining operational ranges to optimize product quality and process energy efficiency.

Experimental and numerical investigation of iron ore pellet firing using coupled CFD-DEM method

Iron ore pellets are the main feedstock in ironmaking processes. While extensive research has addressed numerical modeling of the iron ore pellet induration process, little effort has been made to describe the intricate thermochemical processes occurring within the reactor starting from the pellet and particularly at the intra-particle scale. In this regard, discrete-continuous methods like CFD-DEM can generate more realistic, irregular particle assemblies, which leads to significantly more accurate predictions of voidage variation, wall effects, temperature distribution, and associated mass transfer phenomena. This study presents a numerical model based on computational fluid dynamics (CFD) coupled with the discrete element method (DEM) to simulate the thermal induration process of iron ore pellets. The presented model solving heat, mass, and momentum conservation equations for both continuous and discrete phases, provides detailed information on the thermochemical aspects of the process. Pilot-scale induration experiment was conducted to validate model predictions in terms of thermal history and final conversion fraction. It was found that inlet charge specifications, such as particle and pellet size, significantly impact the productivity of pelletizing plants, highlighting the potential of the presented model to optimize the process and improve plant productivity.

Comparative analysis of straight grate machines and combined facilities grate-tubetype kiln-cooler for selection technologies of pellets production

At present, mainly straight grate machines (SGM) and combined facilities grate-tube-type kiln-cooler (GKC) are used for heat-strengthening induration of iron ore pellets. Their total share in the produced iron ore pellets in the world accounts for 93%, of which SGM takes 60% and GKC – 33%, which speaks about high efficiency of both methods of induration. At the same time, when making decision on construction a pelletizing plant, a question of selection of most effective technology of iron ore pellets production by SGM and GKC often arises. Results of comparative analysis of efficiency of technologies of iron ore pellets production by SGM and GKC presented. Features of various ore types pellets induration considered as well as possibilities of ensuring the required quality of finished pellets at application for induration SGM and GKC. Data on maximum productivity of the considered induration facilities, amount of dust, fines formation, emissions of harmful substances into environment and electric energy consumption presented. Importance of a possibility of accounting of fuel type selection, of heat expenses for heat treatment, of expenses for maintenance, capital and operation costs were noted. It was shown that choice of a particular variant of technology should be done at the stage of elaboration feasibility study accounting existing experience of facilities running, availability of fuel types, cost of energy resources, climate zone and assembling solutions.

Modeling and simulation of iron ore pellet drying and induration process with accurate bed void fraction calculation

ABSTRACTAs key parameters of modeling iron ore pellet drying and induration process, the properties of a packing bed are calculated by our proposed method. Derived from realistic spheres-dropping phenomena, the dropping and rolling rules make the moving sphere finally reach its minimum gravitational potential energy. Furthermore, the plastic deformation of green pellets and mechanic vibration are also considered in the model. The model is based on the laws of mass and heat transfer, physical process, and chemical reaction. The partial differential equations of the process kinetic model involve derivatives in time and bed depth. Comparing the results with measured data proves that the model can be used for optimizing the iron ore pellet induration process and as an effective tool for other, similar thermodynamic process simulations.

Predictive models and operation guidance system for iron ore pellet induration in traveling grate–rotary kiln process

Thermal state of iron ore pellets in industrial traveling grate–rotary kiln process cannot be revealed straightforward, which is unfavorable for field operations. In this study, coupled predictive models of pellet thermal state within traveling grate and rotary kiln were established. Based on the calculated temperature profiles, predictive model of pellet compression strength was also established to assist in process optimization. All the models proposed were validated by the industrial data collected from a domestic plant, and the results show that grate model possesses a high accuracy, kiln model is considered to be accurate to within 10–15% of actual values, and strength model can identify the variation of pellet strength caused by the thermal changes. The proposed models were embodied into an operation guidance system developed for a large-scale pelletizing plant, and the system running results illustrate that the predictive models and expertise rules established can optimize the process very well.

Energy and exergy analyses of a mixed fuel-fired grate–kiln for iron ore pellet induration

Many models of iron ore pellet induration have been developed on the basis of the first law of thermodynamics. However, the exergy analysis, well grounded on the first and second law, of the process is rare. Therefore, exergy balance test was systematically carried out on a grate–kiln, and energy and exergy analyses have been conducted to investigate irreversibility of the process. A model of iron oxide pellet exergy (IOPEM) is presented according to oxidation kinetic characteristics. It is found that the maximum effect (−6.8%) of the characteristics on the pellet exergy appears in the raised end of the kiln. The exergy efficiency of the system is determined to be 10.7% whereas its energy efficiency 59.9%, indicating a great potential for energy-saving improvements. The exergy analysis reveals exergy destruction ratio of kiln and cooler is 14.1%, and 7.7% respectively, and the largest exergy destruction (74.2%) results from the grate. No violent effect of reference environment temperature on exergy efficiency of major components is observed, indicating the grate is the major source of irreversibility. In a word, this study provides a better understanding of the energy and exergy flows of iron ore pellet induration in the grate–kiln and helps to economize energy.

Interactions between Magnetite Oxidation and Flux Calcination during Iron Ore Pellet Induration

Many chemical reactions take place simultaneously during the induration of iron ore pellets produced from magnetite concentrates. Two of the most important are magnetite oxidation and calcination of carbonate fluxes. The first reaction consumes oxygen diffusing into the pellet, while the second reaction produces carbon dioxide that must diffuse out of the pellet. A mathematical model combining the two reactions and gaseous diffusion within the pellet has been developed to quantify the interaction between the two reactions. This combined mathematical model showed that current induration plant mathematical models for the mass and energy balance around a pellet furnace are inaccurate in treating magnetite oxidation and flux calcination as separate reactions. Assuming separate reactions can lead to an error of up to 20 pct conversion of magnetite at the end of the preheat stage. This combined mathematical model, confirmed by experiments with single pellets, also demonstrated that calcination of fluxes also tends to follow a “shrinking core” model rather than reacting simultaneously across the pellet, as existing whole plant models assume. Modifying induration plant mathematical models in accordance with the findings of this article could lead to further savings in energy costs for pellet plants.

Mathematical model for a straight grate iron ore pellet induration process of industrial scale

A mathematical model is developed for induration of the iron-ore pellets in an industrial-scale plant based on the laws of mass, heat and momentum transfer. Physicochemical processes occurring during the induration of pellets such as drying of moisture, calcination of limestone and coke-magnetite reaction are considered to evaluate the gradient of temperature within the pellets. Differential equations describing transport phenomena in a grate bed are simultaneously solved by dividing the space in both x- and z- directions and numerically solving the phenomenological equations. A kinetic model is used for estimation of the change of the strength of the pellets due to firing. The model involves gas-flow through different zones of a typical industrial plant. Different conditions are used for determination of the effect of addition of carbon to the green pellets on performance of the induration process. The optimum carbon content is determined to be 0.75% for green pellets. This results in a productivity enhancement of 20%, an N.G. fuel consumption decrease of 17% and an electrical power saving of 17%. Based on the model calculations, the volumes of the wasted greenhouse gases diminish with addition of the carbon content of the pellets.

Interactions among magnetite oxidation, calcination of carbonate minerals and melt formation in iron ore pellets

During induration of iron ore pellets produced from magnetite concentrates, there are many reactions occurring simultaneously. While the magnetite is being oxidised to haematite, carbonate fluxes are being calcined, leading to the formation of complex oxides. Calcination of fluxes and the formation of calcium and magnesium ferrites and silicates during the preheat stage can influence the magnetite oxidation process. As the calcination reaction proceeds quicker than oxidation at temperatures experienced during the preheat stage (700–1200°C), the bulk flow of CO2 emanating from within the pellet should disrupt the diffusion of O2 to the magnetite reaction as the gas fluxes are similar in magnitude. This increases the time required for the oxidation of the pellet. Using an infrared rapid heating furnace, experiments were performed on heating individual magnetite green balls in an air atmosphere. These experiments showed that time played an important role in the extent of oxidation and calcination and temperature significantly influenced the initial evolution of high temperature phases such as magnesioferrite, calcium ferrites and calcium silicates. Understanding the kinetics of all these reactions for a particular pellet feed and their interaction is important for optimising the design of the induration furnace.

Thermal Implications of Phase Transformations during Induration of Iron Ore Pellets Produced from Hematite

Although extensive development has occurred in recent years for models of induration of iron ore pellets, none of these models have taken into account the partial melting of some of the raw materials. To determine the importance of partial melting and melt phase formation to the energy balance for induration, estimates have been made of the thermodynamic properties of silicoferrite of calcium, SFC, using published techniques. SFC was used in this paper as an example of the initial melt forming minerals in the pellets. Owing to the complexity of the structure of SFC, there was some doubt as to the accuracy of these estimates as they suggested that SFC was not a thermodynamically stable phase, though it exists between 1050 and 1250°C. As no experimental data was available, however, these ‘best available’ estimates have been incorporated into a mathematical model to determine the effect of melt phase formation on the induration process. It was found that while there was little effect on the pellet temperature profiles, the overall amount of energy required to indurate the pellets increased by about 1-1.5% when melt phase formation was included. This suggests that experimental determination of the thermodynamic properties of SFC and other phases produced from the melt would be of benefit in modelling the energy requirements for induration of pellets more accurately.

Mathematical model of magnetite oxidation in a rotary kiln furnace

AbstractA novel method of accounting for the oxidation of magnetite during iron ore pellet induration in a rotary kiln is presented. The technique is based on the mathematical description of a cascading bed of pellets in a rotary kiln by Saeman (1951). For the purpose of illustration, this method was incorporated into a simple one‐dimensional model of heat and mass transfer in a natural gas fired rotary kiln furnace. The model predicts a fraction of magnetite oxidation less than 5%, which is in agreement with plant observations.

GASFLO—Airflow distribution evaluation software tool for ducting systems of pellet induration processes

Models for the evaluation of flow distribution in a gas flow networks are well established. However, such models are typically homogenous (i.e., the components are generically similar) and all sources/sinks are specified. Iron ore pellet induration systems involve the flow of gas through a complicated network of distinct components and a range of unknown leakages (i.e., sources or sinks). This paper describes the dvelopment and validation of a computational model to analyze flows in pellet induration systems. The resulting model of neccesity involves original algorithms and is embedded within a software tool, GASFLO.

Modelling of a Pilot Scale Iron Ore Pellet Induration Furnace

AbstractIn this work, a mathematical model of the pilot scale iron ore pellet induration furnace is presented. The validity of the model has been verified by performing separate experiments of subprocesses: drying, firing and cooling. The experiments were performed in such a way that the phenomena of heat transfer, drying and coke combustion could be studied separately. To do this, the green pellets were first dried, then heated to elevated temperatures at which the coke combustion commences and the pellet strength begins to increase. Finally, the pellets were cooled by passing ambient air across the bed.The pellet induration model is based on the equations describing the phenomena of heat and mass transfer, gas flow, drying and combustion of coke. The experiments and the pellet induration cycle were simulated by solving the differential equations. The pellet bed was divided into differential height elements and the phenomenological equations were solved by numerical methods. This type of model is very us...

Modelling of a Pilot Scale Iron Ore Pellet Induration Furnace

Modelling of a Pilot Scale Iron Ore Pellet Induration Furnace

Energy cost minimization in grate/kiln induration

A mathematical model of the grate/kiln process for the induration of iron ore pellets, verified for an operating plant, has been used to investigate the effects of process variables on fuel and power consumption, and on certain key temperatures and flowrates. The variables investigated include temperatures reached by pellets at the bottom of the preheated bed and in the kiln, feed diameter and temperature, height and void fraction of the grate furnace bed, kiln speed and flame dimensions, recuperation air temperature, and relative air rates for the two stages of cooling. Simulations in which variables were investigated individually provide an improved understanding of the complex interactions within the process, and show how energy consumption might be reduced. From these simulations, cases involving simultaneous changes to a number of variables were developed. One of these has been optimized by stepwise variation of the first stage cooling air rate and the temperature attained by pellets at the bottom of the grate furnace bed. Calculated fuel savings of 8 to 9 pct demonstrate the value of process models for energy minimization.

Development and validation of a mathematical model for the moving grate induration of iron ore pellets

A mathematical model is described for the induration of iron ore pellets on a moving grate. The validity of the model has been demonstrated, both for the performance of an operating plant and for the firing of pellets in a laboratory pot grate furnace. The model permits calculation of pellet bed temperature profiles and the resulting product quality, together with overall heat and mass balances for the induration machine. Indications are also given of the occurrence and extent of the spalling that can occur during removal of both free and chemically combined water in the drying stages. The work has demonstrated the significant effect of gas composition on pellet temperatures and indicated that difficulties can arise when comparing plant data with that from laboratory pot grate simulators.

Solid state sintering in the induration of iron ore pellets

Data is developed to show the relationship between pellet strength and shrinkage which accompanies induration. Systematic measurements of shrinkage kinetics of a pure hematite reagent powder and of a commercial magnetite concentrate are presented, and rate constants and activation energies evaluated. The discussion links the results of pellet induration research with sintering theory, developed in support of powder metallurgy and ceramics. Transport mechanisms active during strength development of pellets are dealt with.

Mechanisms and kinetics underlying the oxidation of magnetite in the induration of iron ore pellets

The chemical and transport phenomena that may participate in controlling the oxidation rate of magnetite pellets were examined. Reaction rates were measured under a varying range of conditions including temperature, oxygen partial pressure, and porosity of the pellet. Models based on the initial as well as developing reaction rates were used to explain the experimental findings. In contrast with many experimentally-made agglomerates, all pellet samples were made with a uniform internal structure. Such a condition was a prerequisite to obtaining a meaningful correlation of the experimental data with the theoretical base. A study of the initial reaction rates revealed that two mechanisms were operative in the first stages of pellet oxidation. Up to about 420°C a surface type of chemical reaction was the controlling step, while above 420°C mass transfer through the gaseous boundary layer was the controlling step and dominated the reaction rate. As the reaction developed within the pellet, mass transfer through the gaseous boundary layer as well as that through the product layer became the controlling steps. These mechanisms were studied in the temperature range of from 1000° to 1200°C and the role played by each was found to be a function of the amount of oxidation which had taken place.