Reaction Characteristics of Carbothermic Reduction with Tall Pellets Bed

Reaction Characteristics of Carbothermic Reduction With Tall Pellets Bed

X. Jiang*1, S.H. Liu2, W.K. Lu3, T.Y. Huang2, G.Q. Zhang1, H. Guo1, G.H. Shiau2, H.Y. Zheng1, F.M. Shen1

1 Northeastern University, No. 11, Lane 3, Wenhua Road, Heping District, Shenyang, Liaoning, China 110004 Phone: 86-18904015206 * Email: [email protected]

2 China Steel, No. 1, Chung Kang Rd., Hsiao Kang, Kaohsiung 81233, Taiwan 3 McMaster University, Hamilton, Ontario, Canada L8S4L7

ABSTRACT Recently, more and more attention has been paid on alternative ironmaking processes due to the sustainable development. Aimed for the development of a new direct reduction technology, PSH process, the reaction characteristics of the carbothermic reduction with high pellets bed are investigated at lab-scale in the present work. The experimental results show that when the height of pellets bed is 80mm (pellet diameter:16~18mm, 5 layers), the optimum parameters investigated in this paper are: C/O=0.95, 50 minutes of heating time at 150ıɗ, and hot charging operation. Under these optimum conditions, the metallization degree of DRI can reach 85%, and the productivity can reach 85 kg-DRI/m2.hr or 59 kg-M.Fe/m2.hr. The operational characteristics of the carbothermic reduction with high pellets bed can be summarized as “double high”: high temperature and high pellets bed. And the production characteristics of the carbothermic reduction with high pellets bed can be summarized as “four high”: high metallization degree, high productivity, high energy efficiency, high strength and density of DRI. Keywords: Reaction Characteristics, Carbothermic Reduction, Metallization Degree, Productivity, Direct Reduced Iron

1 BACKGROUND For the coming decades of years, Blast Furnace (BF) will be the dominant ironmaking reactor in the world. But there have been continuing efforts all over the world in searching for alternative ironmaking process because of high capital investment, coke requirement, and environmental concerns associated with preparatory steps of raw materials for BF, e.g. coke making and sintering. Many alternative ironmaking processes have been developed in the past decades of years, including shaft furnace, rotary kiln, tunnel kiln, rotary hearth furnace (RHF), and some smelting processes, e.g. COREX, FINEX, and HiSmelt etc [1-12]. Each process may have its advantage and applicability, for example treating some waste oxides and special ores, but it also has disadvantage and inapplicability. So none of these alternative ironmaking processes has the overwhelming advantage and could defeat others. Recently, a new direct reduction process (Paired Straight Hearth Furnace, Abbreviated as PSH) was proposed and developed in North America and China. PSH process was considered to produce Direct Reduced Iron (DRI)

with high DRI quality, low carbon rate, and consequentially low CO2 emission. In this reearch work, the reaction characteristics of the carbothermic reduction with high pellets bed are experimentally investigated, including amount of carbon addition (denoted as C/O), reducing time, metallization degree (MD), and productivity of DRI.

AISTech 2015 Proceedings © 2015 by AIST 1342 2 INTRODUCTION ON PSH PROCESS 2.1 Origination of PSH Process The chemically self-sufficient green balls (ore-coal composite pellets), which contain carbonaceous reductants, need heat only to convert green balls to DRI. Currently, a hearth-type furnace, Rotary Hearth Furnace (RHF), is a proper choice for these green balls to commercially produce DRI due to their weakness in mechanical strength.. Generally, the low metallization degree of DRI is an inevitable disadvantage of RHF process due to the contradictory requirements of fuel in oxidation compartment and in reduction compartment (see Fig. 1). In oxidation compartment, in order to increase the energy efficiency of fuel, the ratio of CO/CO2 is lower, which is easy to re-oxidize the newly formed DRI and results in a low metallization degree. In reduction compartment, in order to avoid DRI from re-oxidation, the high ratio of 4 CO/CO2 is necessary and the flame temperature is low too. Then the radiation heat transfer, which is proportional to T of heat source, will be lower and results in a low energy efficiency and metallization degree too. By trials and errors, the following compromise for the contradictory requirement of fuel was established in the current

commercial RHF process. (1) The CO/CO2 ratio in gas is kept at or larger than 2.0 to moderate its oxidizing potential with respect to metallization of DRI. (2) The flame temperature aimed for is 1300-1350ɗ. But under these two compromised conditions, the re-oxidation of DRI is still inevitable and the metallization of DRI in commercial RHF process is lower than 70%, which cannot be directly used in steelmaking process without obvious refractory corrosion. Basically, the productivity of RHF process is lower too, due to its shallow pellets bed (20~25mm).

Fig. 1 Schematic diagram of the carbothermic reduction with shallow bed

Based on the reduction of ore-coal composite pellets on furnace hearth, and aimed to solve the problem on the contradictory requirement of fuel in oxidation compartment and in reduction compartment, Professor Wei-kao Lu of McMaster University in Canada proposed a new direct reduction process, which is named as PSH (Paired Straight Hearth) process [13]. In PSH process, the metallization degree of DRI and energy efficiency can be simultaneously increased, and the productivity should be increased too. Same as RHF process, PSH process may be divided into oxidation compartment and reduction compartment (see Fig. 2). In comparison with commercial RHF practice, PSH process has two operational characteristics, high temperature (1500~1550ć) in oxidation compartment and tall pellets bed (80~120mm) in reduction compartment. In oxidation compartment, CO is fully

combusted to CO2 to obtain high temperature (1500~1550ć), so the prevention of newly formed DRI from re-oxidation is very important issure during the process. In reduction compartment, the height of pellets bed is increased to 80~100 mm by 5~7 layers of pellets.

Fig. 2 Schematic diagram of the carbothermic reduction with high pellets bed

It should be pointed out that there are two core technologies, (1)fast heat transfer from the top to bottom of pellets bed, (2) prevention of newly formed DRI from re-oxidation by fully combusted gas, for application and achievement of PSH process,.

2.2 Theoretical Fundamentals (High Temperature and Tall Pellets Bed) 2.2.1 High temperature

In oxidation compartment, high temperature is obtained by the full combustion of CO to CO2, the gas composition is nearly 0% of CO. Generally, high temperature operation has the following advantages. (1) Shrinkage of DRI. From the view point of the process control, the radiation heat transfer from heating source to the bottom of bed is the critical step of the process. In the typical RHF process, the flame temperature is low (about 1300-1350ɗ), and heat transfer from top to bottom of the pellets bed by radiation is slow if the layer of pellets is more than 2 or 3. That’s why a shallow bed is adopted in RHF process. But in PSH furnace, the newly formed DRI in top layer will shrink under the high temperature (1500ɗ) and the pellet shrinkage result in a larger space for the passage of radiative heat flux. Then the 2nd layer of DRI shrink and generate the larger passge, then the 3rd layer…… (see Fig. 3). Therefore, high temperature can promote the shrinkage of DRI and is benefit for heat transfer by radiation.

Fig. 3 Large space created inside pellets bed due to the shrinkage of DRI

(2) Radiation heat transfer. Heat energy derived from heat source is consumed to support the endothermic reactions and sensible heat of substances. The temperature of heat source is the most important variable for enhancing the rate of heat transfer in the whole system because the intensity of thermal radiation is proportional to T4 of heat source. Therefore, high temperature can drastically improve heat transfer in the process. (3) Metallization degree. It is well known that higher reducing temperature will result in higher metallization degree of DRI if the newly formed DRI can be effectively protected from re-oxidation by oxidizing gas. Furthermore, higher temperature is benefit for the coalescence of metallic iron grains and result in big size of metallic phase. The big size of metallic phase is very important for treatment waste oxides and some special ores. (4) Heat conduction. Heat conduction within a pellet and between pellets across the contact points is important for pellets of lower layers, especially for the bottom layer. Higher metallization degree of pellets will be benefit to increase the effective heat conductivity between and inside pellets. (5) Energy efficiency.

If DRI can be effectively protected from re-oxidation, then the complete combustion of CO to CO2 can increases the energy

efficiency of fuel. Consequently, the CO2 emission is reduced.

2.2.2 Tall pellets bed In reduction compartment, the pellets bed is increased to 80~100mm by 5~7 layers of pellets. Basically, the tall pellets bed operation has the following advantages. (1) Protect DRI from re-oxidation The newly formed DRI at the top of the bed is protected from re-oxidation by the upward gas flow (basically the gas is CO rich) generated during the reduction of pellets in the tall bed, and enhanced by the high volatile coal in green ball over a longer period of the reducing time. (2) Energy efficiency Using upward gas to prevent DRI at the top of pellets bed from re-oxidation would allow the pellets bed to be heated to higher temperature. So all combustibles can be fully combusted to gain high temperature and high energy efficiency in

oxidation compartment, which is the key to decrease the coal rate and CO2 emission in the DRI production process.

AISTech 2015 Proceedings © 2015 by AIST 1345 (3) More layers of pellets are simultaneously heated and reduction reaction occurs in different layers at the same time. So the productivity of DRI can be effectively increased in PSH process compared with RHF process.

2.2.3 Simultaneous high temperature and high pellets bed A very important phenomenon in PSH process is recognized that the flame temperature and the pellets bed should be increased simultaneously. One is not effective without another one, because, (1) For the case of tall pellets bed only without high temperature, DRI of top layer cannot effectively shrink at lower temperature. If the space created by pellets shrinkage in the pellets bed is small, the radiation heat transfer through the bed will be limited. Therefore, the pellets of bottom layer cannot be effectively reduced. (2) For the case of high temperature only without tall pellets bed, the newly formed DRI will be re-oxidized due to the lack of the protection of the upward gas. The consequence is likely to obtain the DRI with low degree of metallization or liquid and corrosive slag. Therefore, it’s the basis that high temperature and tall pellets bed should be adopted simultaneously. More efficient process and drastic improvement of productivity and DRI quality (metallization degree and density) are the result of “high flame temperature and tall pellets bed". These “two high” are summarized as the operational characteristics of PHS process.

3 EXPERIMENTAL PROCEDURE FOR ELECTRIC MUFFLE FURNACE 3.1 Experimental Set-Up PSH experiments can be carried out in an electric muffle furnace and a natural gas-fired furnace. In the muffle furnace, the advantage is smaller size of furnace chamber, i.e. even temperature distribution, and little raw materials required for the experiment. The disadvantage is that only temperature profile of PSH furnace can be simulated, but the atmosphere profile cannot be simulated in the muffle furnace due to heating by electric heating elements. In the natural gas-fired furnace, the advantage is that both the temperature profile and the atmosphere condition of PSH furnace can be simulated. In this research work, the extensive investigation on the optimum condition with respect to variables such as amount of carbon addition (C/O), reducing time, charging method, etc. was performed in the electric muffle furnace. In the future work, the optimum conditions decided by the experimental results of the muffle furnace will be used in natural gas-fired furnace. The normal experimental procedure (not including the hot charging operation) in electric muffle furnace consists of the following steps. (1) Making pellets (pelletization). The diameter of ore/coal composite pellet is about 16~18mm. (2) Making special crucible for holding composite pellets in muffle furnace. The crucible consists of two parts, a) the mullite ring (the inner diameter is 80mm) to retain the upward gas in vertical direction in pellets bed, and b) the insulating materials surrounding the mullite ring to obstruct the heat transfer from the horizontal direction of pellets bed (Fig. 4). (3) Placing the dried composite pellets into a special crucible. The height of pellets bed is about 80mm. (4) Putting the special crucible into muffle furnace, which has been controlled to stay at 1200ɗ in air atmosphere. (5) Staying the furnace temperature at 1200ɗ for 5 minutes and at 1500ɗ for the residual time. The total reducing time for pellets in furnace is 50 minutes or 60minute in the present work. The entire temperature profile control is illustrated in Fig. 5. (6) When pre-determined end point of the reducing time is reached, taking the crucible out of the furnace and taking the surrounding insulating materials away from the crucible assembly immediately to stop the reduction reaction. And, flowing nitrogen into the hot DRI for quenching.

Fig. 4 Special crucible for the carbothermic reduction with high pellets bed in muffle furnace

1600

Ļ 1500 ķ

'HFUHDVL QJ 'L VFKDUJL QJ DQG 1400 7H PSHUDW XUH FRRO HG XQGHU 1 ĸ ĺ &KDUJL QJ 1300

1200 Temperature, C Temperature,

1100 Ĺ

1000 0 20406080100 7L PH PL QXW HV

Fig. 5 Schematic diagram of temperature profile control of electric muffle furnace

3.2 Raw Materials The chemical composition of iron ore concentrate used in this work is shown in Table 1. This iron ore consists mainly of magnetite. The iron ore concentrate is ground by shatter-box down to -200mesh for pelletization. Table 1 Chemical composition of iron ore concentrate, mass%

TFe FeO SiO2 CaO MgO Al2O3 LOI H2O

63.26 26.99 6.68 0.15 0.12 0.20 -1.45 7.22

The proximate analysis and ash composition of the pulverized coal is shown in Table 2. The volatile matter (VM) in the coal is about 26%. For the preparation of coal /ore composite pellets, coal is ground down to -60mesh for pelletization.

Proximate analysis Ash analysis

Fixed C Total C Ash VM SiO2 Al2O3 Fe2O3 MgO CaO

61.31 77.5 9.38 26.00 49.14 29.98 10.22 0.70 5.37

4 EFFECT OF AMOUNT OF CARBON ADDITION (C/O) ON CARBOTHERMIC REDUCTION 4.1 Analysis on Metallization degree Definition of amount of carbon addition is the gram-atomic ratio of the fixed carbon in the coal added to the combined oxygen in iron oxides, denoted as C/O (g-atomic / g-atomic). In this work, C/O = 0.80, 0.95, 1.10. If “C” is calculated based on total carbon, then the CO is 1.0, 1.2, and 1.4, respectively. Other experimental parameters are 16~18mm in pellet diameter, 5 layers of pellets (about 80mm of bed height), 1500ć of reducing temperature, ,50 minutes of reducing time. In this work, the metallization degree (MD) of DRI specimens with different C/O is shown in Fig. 6. It can be seen, (1) In case of C/O=0.80, when the reducing time is 50 minutes, MD of 1st layer, 2nd layer, 3rd layer (order from top to bottom of the pellets bed) is lower (74%, 72%, and 60% respectively), because the reductant is not sufficient (reducing time is enough for the upper layers). MD of total DRI bed is only 50.56%. (2) In case of C/O=0.95 and C/O=1.10, MD of 1st layer, 2nd layer, 3rd layer are similar (more than 80%%, which is obviously higher than that of C/O=0.85), which indicate reductant in these pellets is sufficient. (3) In case of C/O=0.95, the MD of 4th layer and 5th layer is lower compared with C/O=1.10.

100 C/O=0.80 C/O=0.95 80 C/O=1.10

20 Metallization Degree / %

0 12345 Layer No. Fig. 6 Effect of C/O on metallization degree of DRI

4.2 Metallographic Analysis ŕũŦġtypical metallographic picture and energy spectrum of DRI specimen are shown in Fig. 7. It can be concluded that there are mainly three phases, (1) white phase, pointed by A, is metallic iron phase. (2) Light grey phase, pointed by B, is melting

slag, consisting compound minerals of silicon oxide, iron oxides, and aluminum oxide (mainly, fayalite, 2FeO·SiO2). (3) Deep grey phase, pointed by C, is quartz. In addition, the black phase in Fig. 7, pointed by D. is the area of pores inside the pellets.

400 FeKa Point A

300 FeLa

200 Counts

100

0 0 5 10 15 Energy, KeV 500 SiKa SiKa 400 Point B Point C 400

300 300

200 Counts OKa FeLa Counts 200 AlKa FeKa OKa 100 100

CaKa 0 0 0 5 10 15 0 5 10 15 Energy, KeV Energy, KeV Fig. 7 Typical metallographic picture and energy spectrum of DRI (C/O=0.95, 50 minutes, 1st Layer)

Other than the typical metallographic picture shown in Fig. 7, in the metallographic picture of 5th layer DRI of C/O=1.1, see Fig 8, there is some black and loose powder. By using the element analysis on the powder area, it can be seen that there is some carbon (fixed C in coal) and silica (ash in coal). Therefore, it can be proven that the coal addition (C/O) is excessive, and some residual carbon remains in DRI, which is not benefit for DRI production. Hence, residual carbon of 5th layer DRI (bottom layer) with different C/O was analyzed and shown in Fig. 9.

500

SiKa 400

300 CKa

FeKa

Counts 200 OKa

100

0 0 5 10 15 Energy, KeV

Fig. 8 Metallographic and energy spectrum analysis of DRI in 5th layer (C/O=1.4)

16 C/O: 0.80 C/O: 0.95 12 C/O: 1.10

4 Residual Carbon /% Residual Carbon

0 50 55 60 Reducing Time /minutes

Fig. 9 Residual carbon of DRI in 5th layer (bottom layer)

Generally, C/O should be is adjusted to meet the minimum requirement of the complete reduction in ore/coal composite pellet, more or less is not suitable for the pellets to produce good quality of DRI. If more excessive residual carbon remains in DRI, there are the following disadvantages: (1) Grade of total iron is decreased, which is not good for the next step of the steelmaking process. (2) Carbonaceous resource is wasted. (3) Ash content increases with increasing coal addition. Then, the density and strength of DRI is weakened, which is not good for storage and long-distance transportation. The metallization degree of total DRI bed are similar in cases of C/O=0.95 and C/O=1.10, as seen in Fig. 6 and Fig. 9, It is indicated that the residual carbon in the case of C/O=1.10 is much more than that in the case of C/O=0.95. Therefore, C/O=0.95 is regarded as optimum carbon addition in the next stage of experiments.

5 EFFECT OF REDUCING TIME ON CARBOTHERMIC REDUCTION 5.1 Normal Operation of Experiments Based on the preliminary experiments, for 5 layers of pellets, the reducing time of 50 minutes at least. So in the present work, the reducing time are 50 minutes and 60 minutes. Other experimental parameters are C/O=0.95, 16~18mm in the pellet diameter, 5 layers of pellets (about 80mm of bed height), reducing temperature 1500ć. The metallization degree (MD) of DRI with different reducing time are shown in Fig. 10. From the figure, it can be seen that the MD of 1st layer, 2nd layer, 3rd layer is relatively higher, about 80% in the case of 50 minutes. ,The MD of 4th layer and 5th layer are lower,35% and 13%, respectively. The MD of total DRI bed is about 57%. In the case of 60 minutes, the MD of 1st layer, 2nd layer, 3rd layer is still higher, about 85~90%. And, the MD of 4th layer and 5th layer are increased to 75% and 63%. respectively. The MD of total DRI bed is about 80%.

100 C/O=0.95 50mins 60mins 80

20 Metallization Degree / %

0 12345

Layer No. Fig. 10 Metallization degree of DRI with different reducing time (50 minutes and 60 minutes)

In the metallographic picture and energy spectrum of 1st layer DRI specimen for of 60 minutes reducing time, shown in Fig. 11. the white phase is metallic iron phase. Deep grey phase, pointed by A, is quartz, which is still a zone of unmelted solid grains due to its high melting point. Light grey phase (pointed by B) and middle grey phase (pointed by C) are melted slag

(mainly fayalite, 2FeO·SiO2). It can be interpreted that in the later stage of reduction, the up-ward protective gas evolved less and less, so a little bit of newly formed metallic iron is re-oxidized to FeO , and then FeO reacts with SiO2 to form 2FeO·SiO2, which is liquid and corrosive slag. Light grey phase is precipitated from middle grey phase during cooling due to their different melting points, and then forms the banding structure. Therefore, the longer the reducing time, the more melting slag formed, which is not good for the operation of PSH process.

600 Point A SiKa

400

Counts 200

OKa

0 0 5 10 15 Energy, KeV

500 500 Point B Point C SiKa SiKa 400 400

300 300

OKa AlKa Counts 200 Counts 200 OKa FeKa

100 100 FeKa

0 0 0 5 10 15 051015 Energy, KeV Energy, KeV

Fig. 11 Metallographic picture and energy spectrum of 1st layer DRI reduced by 60 minutes

5.2 Hot Charging Operation In this studźĭġ ŵhe hot charging operation of experiments is that the ore-coal composite pellets are directly charged into a crucible which has been pre-heated to 1200ć in the furnace.. The metallization degree of DRI with hot charging operation is shown in Fig. 12. It can be seen that the metallization degree of each layer DRI with hot charging operation is relatively higher than that with cold charging operation, especially for the lower layers (4th layer and 5th layer). Compared with cold charging the MD of 4th layer increases from 35% to 77%, and the MD of 5th layer increases from 13% to 79%. Then the MD of total DRI bed increases from 57% to 85%. The reason for the MD difference between hot charging and cold charging is that the pellets bed can be heated from the bottom of crucible by the heat pre-stored in the refractory..In fact, the hot charging can not only increase metallization degree of DRI, but also increase productivity. The productivity can be increased up to 85kg-DRI/m2.h or 59kg-MFe/m2.h with the hot charging operation in this study.

Cold Charge (Normal) 100 Hot Charge

20 Metallization Degree / % / Degree Metallization

0 12345 Layer No. Fig. 12 Effect of hot charge on metallization degree of DRI

6 CONCLUSIONS In the present work, the reaction characteristics of the carbothermic reduction with high pellets bed are investigated. The main findings could be summarized as follows: (1) When the height of pellets bed is 80mm (16~18mm, 5 layers), the optimum parameters for the carbothermic reduction are C/O=0.95, 50 minutes of reducing time, and hot charging operation. Under the optimum condition, the metallization degree of DRI can reach 85%, and the productivity can reach 85kg-DRI/m2.h or 59kg-MFe/m2.h. (2) The operational characteristics of the carbothermic reduction with the tall pellets bed can be summarized as “double high”: high temperature and high pellets bed. (3) The production characteristics of the carbothermic reduction with the tall pellets bed can be summarized as “four high”: high metallization degree, high productivity, high energy efficiency, high strength and density of DRI.

ACKNOWLEDGMENT The authors wish to acknowledge the contributions of associates and colleagues in Northeastern University of China and China Steel Coorperation of Taiwan. Also, the financial support of National Science Foundation of China (NSFC 51404059 and NSFC 51374061) and the Fundamental Research Funds for the Central Universities (N140204009) are very appreciated.

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