optimum process conditions for the production of pig iron by corex process

8/12/2019 Optimum Process Conditions for the Production of Pig Iron by Corex Process

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SCANMET IV 4th International Conference on Process Developmentin Iron and Steelmaking, 10-13 June 2012, Lule , Sweden

OPTIMUM PROCESS CONDITIONS FOR THE PRODUCTION OF

PIG IRON BY COREX PROCESS

Ahmad Wafiq 1, Ahmed Soliman 1, Tarek M. Moustafa 1, and A.F. Nassar 1

1 Chemical Engineering Department, Faculty of Engineering, Cairo University

Abstract

COREX process for pig iron production is a commercially proven process, and is currentlyconsidered as the main competent to blast furnace for pig iron production. COREX processconsists of two reactors; the reduction shaft, and the melter-gasifier. The process involvesmulti-phase, multi-component, having about five raw materials, and three main products.Varying one process input can lead to operation instability and/or result in off-spec

products. Macroscopic modeling is always an attractive approach for processes with suchcomplex features. In the present paper a mathematical macroscopic process model has beendeveloped and the Indian Jindal plant was taken as case study. The macroscopic modelwas efficient tool to determine the window of possible operation scenarios while keeping all

process constraints fulfilled. The main raw material constraints are coke amount, and percentage of iron ore fines. In addition to the pig iron quality, the main productsconstraints are amount and compositions of reduction gases, export gas composition, and theslags quality.

A parametric study has been performed focusing on the freeboard zone in the melter-

gasifier. The effect of iron ore fines on the free boards temperature and export gascomposition has been analyzed. The most flexible operation scenarios have been determinedand summarized in an operation chart. This chart can be used for process control as a guideas it combines the controlling process variables. According to the market status, four mainoperation modes for COREX process have been defined; maximization of iron ore fines,minimization of fuel amount, minimization of Iron bearing materials amount, andminimization of oxygen feed amount. Including the current raw material prices, and toachieve minimum production cost, the minimum possible oxygen feed rate is economicallythe optimum mode of operation.


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Introduction

Together with the problem of the high capital cost needed, the blast furnace has other drawbacksincluding the dependence on scarce coking coal as a reductant, and the low flexibility towardsutilizing iron wastes as a substitute to the fresh iron ores [1]. Moreover, the increasedenvironmental pressures nowadays cause a lot of problems to any investor thinking to use the

blast furnace route for steel production [2]. Among the newly developed smelting reduction processes, COREX process is nearly the only competent to the blast furnace in the field of pigiron production. In COREX process, all the reactions take place in two reactors; namely, thereduction shaft (RS) and melter-gasifier (MG). Currently, 4 plants are utilizing COREX processfor pig iron production. These plants are Saldanha Steel Works in South Africa (0.8 mtpa), JindalSouth West Steel in India (2 * 0.8 mtpa), Posco in Korea (0.8 mtpa), and Baosteel in China (1.2mtpa).

COREX produces a high quality pig iron using non-coking coal and pure oxygen in anenvironmental-friendly process. COREX process eliminates the need to coking and sintering

plants conventionally used in the blast furnace route. The process is viable at lower productioncapacities, and this copes with the gradual world shift from the integrated steel plants to smallermini-mills. Moreover, COREX is more flexible to variations in compositions of raw materials,and it is insensitive to the alkali contents of the raw materials [1].

Because of having a multi-component multi-phase system, previous researches focus on performing macroscopic analysis to reach better understanding of COREX process, andassess the effect of different process parameters. Kumar et al. [3] have studied the effect ofthe cold crushing strength (CCS) of the burden on the performance of the reduction shaft. Inaddition, they studied the effect of coal size on the performance of the melter gasifier, andthey developed a regression analysis using multiple variables to get an equation for the fuelrate. The addition of coke to the melter gasifier was also a subject of many researches [4-6].In another important research [7] Kumar et al. have built a macroscopic model to predictthe changes taking place on altering any of the input variables using complex mass andenergy balance equations.

In the present paper a mathematical macroscopic process model has been developed and theIndian Jindal plant has been taken as a case study. The macroscopic model was built todetermine the window of possible operation scenarios while keeping all process constraintsfulfilled. The most flexible operation scenarios have been determined and summarized in anoperation chart. This chart can be used for process control as a guide as it combines thecontrolling process variables.

COREX Process Description

RS Process Description

As shown in Figure 1, iron ore, pellets and fluxing materials (limestone and dolomite) arecharged into RS from the top of the shaft. The reduction gas is injected from the bottom ofthe shaft at about 850 oC. The gas moves in the shaft upwards counter currently to the solid


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phase and exits from the shaft at around 250 oC. The solid product is termed as directreduced iron (DRI) and it is transported from the RS into the MG [5], [7], [8].

MG Process Description

In addition to the hot DRI from the RS, non-coking coal, iron ore fines, flux fines and somecoke are continuously charged into the MG from their charging bins. Oxygen plays a vitalrole in COREX process for generation of heat and reduction gases. It is injected through thetuyeres, where it gasifies the coal char generating CO. The hot gases ascend upward throughthe char bed [5], [7], [8]. The sensible heat of the gases is transferred to the char bed, whichis utilized for melting iron and slag and other metallurgical reactions.

The temperature of the freeboard zone (shown in figure 2) is maintained between 1000 oC to1100 oC, and this assures cracking of all the volatiles released from coal. The gas generatedinside the MG contains fine dust particles, which are separated in hot gas cyclones. The dustcollected in the cyclones is recycled back to the MG through the dust burners, where the

dust is combusted with secondary oxygen. The gas from the MG is cooled to the reductionas temperature (About 850 oC) by adding cooling gas as shown in figure 1. Most of thecombined gas is fed to the RS, while the excess gas is used to control the plant pressure [5],[7], [8].

Macroscopic Model Development

COREX Process is a complex system having a lot of raw materials and 3 main productswhich are the hot metal, slag and export gas. Changing one input variable can cause process

Figure 2 Zones in MGFigure 1 Process Flow Diagram of COREX Process


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instability, and can also result in off-spec products. Each of the 2 reactors involved (RS andMG) has one of its feeds emerging from the other reactor. Consequently, a macroscopicmodel based upon simultaneous solution of the material and energy balances on the 2reactors has been developed. A degree of freedom analysis has been applied in order todetermine the feasibility of solution and the required information. The present work will use

the data of Jindal Vijayanagar Steel Limited (JVSL) in India as a case study. Consequently,all the input data will be originated from the published information about this plant.

Macroscopic Balance on MG

In the MG there are 6 raw materials (DRI, coal and coke, iron ore fines, flux fines, primaryand secondary oxygen) and 3 products ( hot metal HM , slag, and gases). The flux finescomprise limestone, dolomite, and LD slag. From the operating experiences in Jindal plant,Kumar et al. [9] previously published the proximate analysis and ash compositions of thecoal and coke, the compositions and amounts of flux fines, composition of iron ore, andcomposition of the hot metal product. Solomon et al. [10] published a famous research

determining the compositions of the different functional groups for various coal types. Thelatter paper has been used in this model to determine the compositions of different volatilesin coal and coke. In their microscopic model for the MG, Pal and Lahiri [11] used a ratio of0.175 between the primary and secondary oxygen, and this ratio has been used in this work.

The data required to perform energy balance on the MG are simply the heats of reactions,temperatures and latent heats. The temperatures of all the fresh inputs are about 25 oC [7],and the tapping temperature of HM and slag is about 1470 oC [11]. For the complex

pyrolysis phenomenon, Strezov et al. [12] experimentally determined the heat of pyrolysisat its different stages for more than one coal type. The research finding has been used in thismacroscopic model. In addition, the final melting temperature and latent heat of melting of

slag has been got from the work of Matousek [13]. The main reactions taking place in the MG are [11], [14]:

C + 0.5 O 2 CO (1)C + CO 2 2 CO (2)C + H 2O CO + H 2 (3)CO + Fe 2O3 CO 2 + 2 FeO (4)H2 + Fe 2O3 H 2O + 2 FeO (5)CO + FeO CO 2 + Fe (6)H2 + FeO H 2O + Fe (7)

CaCO 3 CaO + CO 2 (8)CaCO 3.MgCO 3 CaO + MgO + 2 CO 2 (9)2 CO C + CO 2 (10) (MnO) + [C] [Mn] + CO (11)(SiO

2) + 2 [C] [Si] + 2 CO (12)

(P 2O5) + 5 [C] 2[P] + 5 CO (13)2 (MnO) + [Si] (SiO

2) + 2[Mn] (14)

(CaO) + [S] + [C] (CaS) + CO (15)


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CH4 C (dust) + 2 H

2 (16)

Macroscopic Balance on RS

In the RS, there are 3 raw materials (Iron ore, flux and reduction gases) and 2 products

which are DRI and gases. The same technique applied in the MG is repeated here. The mainreactions taking place in the RS are reactions 4, 5, 6, 7, 8 and 9 shown above.

Process Constraints

In addition to the well-defined givens stated above, Figure 3 and Figure 4 show another type ofinput data which are the raw materials constraints and products constraints respectively.

Figure 3 Raw material constraints for the process

Figure 4 Products constraints for the process


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530

535

540

545

550

555

0 1 2 3 4 5 O x y g e n A m o u n t i n N m 3 / T o n O r e

% Ore Fines

T = 1100 C

T= 1075 C

T= 1050 C

T=1025 C

T=1000 C

Because of the system complexity and the difficulty of assuring constant compositions of the rawmaterials and products, it is normal to have some data in the form of a range of feasible values.These data could be regarded as constraints that should be fulfilled to assure true solution. Froma Degree of Freedom perspective , there will be infinite number of solutions for the combinedMG and RS balance; however only one solution will be the optimum.

Parametric Study Using the Developed Macroscopic Model

The effect of the main operational parameters is very important for better understanding ofthe process. This is achieved by conducting a parametric study. After building themacroscopic model as shown in the previous steps, it becomes easy to perform the

parametric study. Several simulation runs have been conducted, and the results can beshown as curves. In each run, all the process constraints have been fulfilled.

In this work, the parametric study has been performed on the freeboard zone in the MG as itis considered the real process innovation. The parameter that has been changed in thedifferent runs was chosen to be the percentage of iron ore fines added to the MG. This

parameter was specifically chosen so as to assess its effect as an addi tive to the freeboardzone (referred to late r herein as dome) in the MG.

From Figure 5, it is apparent that at constant fuel amount and flux amount, as percentage orefines increase at constant oxygen amount, the dome's exit temperature decreases. This can

be attributed to the increased heat load needed to preheat the ore fines, beside theendothermic nature of the iron ore reduction (note that the heat supply is constant becauseof the constant amount of oxygen). Conversely, it is apparent that as percentage ore finesincrease at constant exit dome temperature, the amount of needed oxygen increases. This isattributed to the increased heat load needed to preheat and reduce the ore fines, and at thesame time maintain the dome's temperature within the needed range.

As shown in Figure 6, as percentage of ore fines increases, the composition of CO 2 in theexport gas increases, at nearly constant degree of metallization in RS. This is attributed tothe increased amount of CO converted to CO 2 as a result of the iron fines reduction in theMG's free board. It is to be noted that high composition of CO 2 in the export gas lowers its

Figure 5 Effect of iron ore f ines on the domes temperature and oxygen consumption


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0.345

0.35

0.355

0.36

0.365

0.37

0.375

1460

1470

1480

1490

1500

1510

1520

1530

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

% Ore Fines

% C

O 2 o u

t o f R S

I r o n B e a r i n g M a t e r i a

l s i n

k g / T H M

Iron BearingMaterials in

kg/THM% CO2 out ofRS

calorific value, and consequently affects the performance of the dependent facilities.Moreover, and because of having lower iron percent than the iron ore pellets, as the amountof iron ore fines increase, the total amount of iron bearing materials increase accordingly.

Thus, this simplified parametric study has clarified that the ore fines can be used as a

temperature control parameter for the free board where excessive temperatures can be prevented. In addition, the addition of ore fines should take into consideration thecomposition of export gas produced, and its effect on the subsequent applications.Moreover, the addition of ore fines should take into consideration the dome's temperature,and its effect of the cracking of volatiles, and consequently the environmental impacts.

Determination of the Most Flexible Operation Scenarios

During operation, and especially for this complex system, the process engineers should havea degree of insight to the process. This can be achieved by having a diagram like that shownin Figure 7. This diagram has been created by performing several runs on the developedmodel. This diagram combines the most important process variables which are the oxygenand fuel amount, percentage ore fines, and the dome's exit temperature. For certain amountof charged fuel (coal + coke), the amount of ore fines to be added can be determined. Then,its effect on the feed amount of oxygen, and on the energy and environmental efficiency ofthe process, represented in the dome's exit temperature, can be concluded.

From the diagram, it is easy to know that at certain fuel amount, increasing percentage ore

fines beyond a certain value will not achieve all the process constraints. For example, thecurve corresponding to fuel amount of 935 kg/THM has no operation points at 4% ore fines.The latter means that on using 4% ore fines, one or more of the process constraints will beviolated. It is to be also noted that the chosen scenarios in Figure 7 are the most flexible. Atlower amounts of fuel, it is difficult to utilize iron ore fines, and it is also difficult to reachhigh exit dome temperature. At higher amount of fuel, large amount of coal volatiles are

produced causing temperature drop in the dome, and excessive amount of reduction gases.In this case, purging of some gases is essential which is of course bad energy utilization.

Figure 6 Effect of iron ore fines on the composition of export gas and total amount of iron bearing materials


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Figure 7 Most flexible operation scenarios developed by the macroscopic model


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Determination of the Best Operation Modes

The iron and steel market is subjected to a lot of changes in terms of its raw material prices.Thus, for every market status, the best operation mode for COREX process should bedefined. The following four operation modes have been chosen, and the developedmacroscopic model has been used to determine the values of the corresponding main processvariables. Table 1 summarizes the results.

a) Mode 1: Maximization of iron ore fines utilization b) Mode 2: Minimization of fuel amountc) Mode 3: Minimization of iron bearing materials amountd) Mode 4: Minimization of oxygen feed amount

Table 1 Main process variables corresponding to the main modes of operation

Main Variables M ode 1 M ode 2 M ode 3 M ode 41) % Ore Fi nes 4.9 0 0 02) Ir on Bearing M ateri als in kg/TH M 1528 1454 1452 14543) Fuel Amount in kg/THM 957 907 932 907.54) O 2 Amount in Nm

3 /TH M 544.5 532 531 513.55) Dome Exit T emperature in o C 1000 1100 1038 1000

Determination of the Optimum Mode of Operation

Regarding the current raw material prices, and with the objective to achieve the minimum production cost, the optimum mode of operation was found to be the minimization of oxygen

feed amount. The high consumption of pure oxygen is a characteristic in COREX process.However, it should be noted that this is the optimized mode with regards to the productioncost minimization. Sometimes, the environmental pressures may be stronger. For example,the dome's exit temperature corresponding to oxygen feed minimization is only 1000 oC. Thistemperature may be not enough for the cracking of all the evolved coal volatiles, and this ofcourse will affect the environmental performance of the facility. Consequently, this mode ofoperation may not be applied in case of stringent environmental pressures.

Conclusions

COREX process is a multi-phase, multi-component system where varying one processinput can lead to operation instability and/or result in off-spec products. To deal withsuch complex features, macroscopic modeling approach has been applied using Jindal

plant data.The developed macroscopic model has proved that there is a wide range of operationreflecting the process flexibility. The most flexible operation scenarios can bedetermined and summarized in a chart to be used in the process control as a guide. Thedirect addition of ore fines inside the melter-gasifier can be used as a temperature control

parameter for the free board so as to prevent excessive temperature increase; however, itshould take into consideration the composition of export gas produced, and its effect onthe subsequent applications, and in addition it should take into consideration the dome'stemperature, and its effect on the cracking of volatiles, and achieving the environmental

constraints. According to the market status, there are four main operation modes forCOREX process, where the minimization of oxygen feed amount was found to be the


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optimum mode of operation to achieve minimum production cost according to thecurrent raw material prices.

References

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3. Raw Materials for COREX and their Influence on Furnace Performance. P. P. Kumar,S.C. Barman; B.M. Reddy; V.R. Sekhar : Ironmaking & Steelmaking, 2009, Vol. 36.

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9. Operating Experiences with COREX and Blast Furnace at JSW Steel Ltd. P. PrachethanKumar, P.K. Gupta, and M. Ranjan : Ironmaking & Steelmaking, 2008, Vol. 35.

10. Coal Pyrolysis: Experiments, kinetic rates and mechanisms. P. R. SOLOMON, M.A.SERIO and E.M. SUUBERG. Great Britain : Prog. Energy Combusy., 1992, Vol. 18.

11. Mathematical Model of COREX Melter Gasifier: Part I. Steady-State Model. S.Pal andA.K.Lahiri : METALLURGICAL AND MATERIALS TRANSACTIONS B, 2003, Vol. 34.

12. Experimental and modelling of the thermal regions of activity during pyrolysis ofbituminous coals. V. Strezov, J.A. Lucas, L. Strezov : Journal of Analytical and AppliedPyrolysis, 2004, Vol. 71.

13. The thermodynamic properties of slag. Matousek, J. W : JOM Journal of the Minerals,Metals, and Materials Society, 2008, Vol. 60.

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optimum process conditions for the production of pig iron by corex process

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