Comparative Reduction Behavior of Various Cement Coated Iron Ore Pellets

Transcription

1 doi: /me Comparative Reduction Behavior of Various Cement Coated Iron Ore Pellets M. Bahgat* 1, S. Niaz 2, S. Lakdawala 3 and H. Hanafy 4 1 Currently Lead scientist, Metal technology section, Saudi Basic Industries Corporation, P.O. Box 11669, Jubail 31961, Saudi Arabia, Formerly, Assoc. Prof., Minerals Technology Dep., CMRDI, Cairo, Egypt 2 Chief scientist, Metal technology section, SABIC Technology Center Jubail, Saudi Basic Industries Corporation, P.O. Box 11669, Jubail 31961, Saudi Arabia 3 Senior Process Engineer, SPDR Saudi Iron and Steel Company, P.O Box 10053, Jubail 31961, Saudi Arabia 4 Sr. Engineer, Metals SBU, Technology Programming, Saudi Basic Industries Corporation, Jubail 31961, Saudi Arabia * 1 saddikmb@sabic.com; 2 AhsanSN@SABIC.com; 3 LakdawalaS@hadeed.sabic.com; 4 hanafiha@sabic.com Abstract In the moving bed shaft direct reduction processes, such as Midrex and HYL III, the avoidance of sticking is absolutely essential. In order to prevent sticking of pellets a coating material, basically inactive under the reducing conditions prevailing in the shaft furnace, should be applied to cover the outer layer of the pellets. In the present work cement coating is applied for iron ore pellets in various concentrations. 12%, 16% and 20% cement slurry concentrations were used to have a coated cement amount of 0.5, 0.75 and 1.0 Kg/ Ton iron ore. Using thermogravimeteric technique the coated samples were reduced with hydrogen/carbon monoxide mixture which is somewhat simulating the composition of the reducing gas in Midrex furnace. The iron ore pellets and reduced samples were characterized by XRD, XRF and SEM. The influences of various coating conditions on the reduction behavior and the morphology were investigated. As reduction proceeds, the rate of reduction was the highest at the early stages and decreased as reduction proceeds till the end of reduction. Comparatively the reduction rate of the coated iron ore pellets are clearly effected by the different coating conditions. Keywords Ironmaking; Direct Reduced Iron; Cement Coating; Iron Ore Pellets; Reduction Behavior Introduction In the moving bed shaft direct reduction processes, such as Midrex and HYL III, the avoidance of sticking is absolutely essential. The sticking tendency imposes an upper limit on the reduction temperature and, hence, on the productivity of the process. In the DR processes the product is freshly reduced iron in a solid state. It is, therefore, crucial for the material flow in the reducing module that the solid product does not form aggregates, blocking the material flow within and out of the reactor [1]. If pellets have little or no tendency to stick, the reduction temperature and therefore throughput will increase. It has been reported that an increase of C in the reduction temperature can bring about a significant increase in throughput [2]. High reduction temperature is also essential to minimize degradation and re oxidation of reduced product, which is an important fact placing additional emphasis on the sticking behavior of pellets. The sticking problem has been studied by many researchers [3 8]. In order to prevent sticking of pellets a coating material, basically inactive under the reducing conditions prevailing in the shaft furnace, should be applied to cover the outer layer of the pellets. The technique of coating of the direct reduction (DR) iron ore pellets is well known for a few years, and it has been widely used in the DR process to reduce the clustering of direct reduced iron, DRI, in DR shaft furnaces. Several investigators had investigated coating methods to prevent sticking. Hayashi and Igushi [5] found that increase in the gangue content of pellets improves their non sticking behavior. This, however, will increase the solid waste in downstream processing of the reduced iron. Hayashi et al. [6] tried three methods of coating, namely powder, sol, and slurry. Pellets coated with calcium carbonate slurry showed very promising results. Cano and Wending [9] claimed that the sticking problem is eliminated by increasing the MgO content of pellets. In the present investigation comparative reduction behavior of various cement coated iron 31

2 Journal of Metallurgical Engineering (ME) Volume 4, 2014 ore pellets was investigated. The reduction process was performed using hydrogen/carbon monoxide mixture with ratio of 63% 37% which is somewhat simulating the composition of the reducing gas in Midrex furnace. Experimental Work Raw Material Iron ore pellets (mainly hematite) in size ranges mm were used in the present experiments. High purity (99.99%) H2, CO and N2 were used as experimental gas. Portland cement was used as coating material. The reducing gas was a mixture of H2 and CO with gas ratio about 63/37 respectively to simulate the used Midrex gas. Characterization of Prepared Coated Samples Iron ore pellets samples were characterized with X ray diffraction analysis (XRD), X ray fluorescence (XRF) and scanning electron microscope (SEM). It can be seen that iron oxide (Fe2O3) is the major phase with presence of SiO2, CaO and Al2O3 as minors components (Fig. 1 & Table 1). For SEM the sample to be examined was fixed into an epoxy resin mould, and then examined under scanning microscope. The SEM photos for SAMARCO iron ore samples are shown in Fig 2 a and b. It is observed that grain coalescence with very low micropores and some macropores took place in a dense structure. FIG. 1: XRD ANALYSIS FOR IRON ORE PELLETS TABLE 1: CHEMICAL ANALYSIS BY XRF ANALYZERS FOR IRON ORE PELLETS Compounds Conc. % Elements Conc. % O Na2O Na MgO Mg Al2O Al SiO Si P2O P K2O K CaO Ca TiO Ti V2O V Cr2O Cr MnO Mn Fe2O3 Balance Fe Balance NiO Ni

3 FIG. 2: SEM MICROGRAPHS OF IRON ORE PELLETS Experimental Procedure Iron ore pellets were coated with various concentrations of cement suspension and then applied for comparative reducibility test. 1) Coating Process One thousand grams of iron ore pellets were used in each coating test. Pellets were placed in a disc pelletizer of 50cm diameter rotating at 20rpm. Coating was applied by spraying a suspension of coating material (cement). Various solid concentrations (0.5, 0.75 & 1 Kg cement/ton of iron ore) using different cement suspension concentration (12%, 16% & 20%) were applied. Coated pellets were left to air dry, which was followed by reducibility test. FIG. 3: VISUAL PHOTOS OF IRON ORE PELLETS BEFORE AND AFTER CEMENT COATING Spectrum In stats. C O Na Mg Al Si Ca Fe Total P1 Yes P2 Yes All results in weight% FIG. 4: SEM PHOTO AND EDX ANALYSIS FOR CEMENT COATING LAYER OF IRON ORE PELLETS 33

4 Journal of Metallurgical Engineering (ME) Volume 4, 2014 Figure 3 shows the visual photo for the ore pellets before and after cement coating. Also to investigate the coating layer, a coated pellet was examined with SEM and EDX analysis as shown in Fig. 4. It can be seen that to some extent most of the pellets surfaces were covered by the cement in different thickness. The EDX analysis confirmed the presence of cement material on the pellet s surface. 2) Reduction Process The apparatus consists of a reduction furnace supported with a system to supply and regulate the gases, a reduction tube of heat resistance steel, a weighing device to determine the oxygen loss at regular intervals and an electrically heated furnace to heat the test portion to the specified temperature. The sample weight used for each reduction test is about 500g in the size range mm. The flow rate of reducing gas during the test period is maintained at about 50 l/min while the temperature is maintained at C (10 0 C above the current in Midrex). The schematic diagram and visual photo of experimental apparatus used in this study are presented in Figs. 5 and 6 respectively. FIG. 5. SCHEMATIC DIAGRAM OF THE REDUCTION APPARATUS 1 WEIGHTING SYSTEM, 2 TUBE FURNACE, 3 SAMPLE HOLDER, 4 SAMPLE BED IN CERAMIC BALLS, 5 THERMOCOUPLE AND 6 UPPER GRID FIG. 6. VISUAL PHOTO OF THE REDUCTION APPARATUS The sample is introduced into the chamber. In order to achieve a more uniform gas flow, a two layer bed of porcelain pellets having a size range of mm are placed between the perforated plate and the test 34

5 portion. The control panel is used to create the desired program. The reduction tube is closed, inserted into the furnace and suspended centrally from the weighing device. The test samples are dried inside the furnace then Nitrogen gas is allowed to pass through the reduction tube at a flow rate of approximately 25 l/min and the heating is commenced. After the mass of the test portion become constant H2 CO gas mixture is introduced at a rate of 31.5 and 18.5 l/min respectively. The mass of the test portion is recorded continuously. The degree of reduction, R, is calculated as follow, R% = Wt of O removed from iron oxide / Total wt of removable O in iron oxide Results and Discussion Reduction Behavior of Cement Coated Pellets Using thermo gravimetric technique, iron ore pellets coated with different cement concentrations were reduced at C with H2/CO gas mixture. The reduced iron ore samples were characterized with X ray diffraction analysis (XRD), X ray fluorescence (XRF) and scanning electron microscope (SEM). It can be seen that metallic iron (Fe) is the major phase with presence of SiO2, CaO and Al2O3 as minors components (Fig. 7 & Table 2). The visual observation of coated iron ore pellets after reduction is shown in Fig. 8. It is obvious that there is no catastrophic swelling or cluster formation taking place on the reduced pellets. The influence of both solid cement concentration per Ton iron ore pellets and cement slurry concentrations on the reduction behavior and structural characteristics of the reduced products was studied in order to elucidate the optimum coating conditions. FIG. 7: XRD ANALYSIS FOR REDUCED IRON ORE PELLETS FIG. 8: VISUAL PHOTOS OF COATED IRON ORE PELLETS AFTER REDUCTION 1) Influence of Solid Concentration TABLE 2: CHEMICAL ANALYSIS BY XRF ANALYZERS FOR REDUCED IRON ORE PELLETS Elements Conc. % C Na Tracing Mg Al Si P S K Ca Ti V Cr Mn Fe Balance The effect of solid concentration per Ton iron ore pellets (0.5, 0.75 & 1.0 Kg cement/ton iron ore) at constant 35

6 Journal of Metallurgical Engineering (ME) Volume 4, 2014 cement slurry concentration was studied. The reduction curve of iron ore pellets coated with 0.5, 0.75 & 1.0 Kg cement/ton iron ore using 12% cement slurry concentration is shown in Fig. 9. For each single reduction curve, the rate of reduction was the highest at the early stages and was decreased as reduction proceeds till the end of reduction. The reduction rate value came very similar for the different samples. However, the sample coated with 1.0 Kg cement has a little bit higher reduction rate. The SEM micrographs for these reduced iron ore pellets are shown in Fig. 10 (a, b and c). At the various coating conditions the metallic iron is homogenously formed in a very porous structure with presence of large number of micro and macro pores forming channels in structure which allowed for a good access of reducing gas to the oxides grains. Also some little whiskers were observed through the porous structure. FIG. 9: REDUCTION CURVES OF REDUCED IRON ORE PELLETS COATED WITH a. 12% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS b. 12% CEMENT SLURRY CONC. & 0.75 Kg CEMENT /TON IRON ORE PELLETS c. 12% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS FIG. 10: SEM MICROGRAPHS OF REDUCED IRON ORE PELLETS COATED WITH a. 12% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS b. 12% CEMENT SLURRY CONC. & 0.75 Kg CEMENT /TON IRON ORE PELLETS c. 12% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS 36

7 The reduction curve of iron ore pellets coated with 0.5, 0.75 & 1.0 Kg cement/ton iron ore using 16% cement slurry concentration is shown in Fig. 11. It is observed that during the fast reduction stages (initial and intermediate stages) the reduction rate is very similar for the various samples while after that it increased with decreasing the coated cement amount from 1.0 Kg to 0.5 Kg gradually. This is might be due to the decreasing in pellets porosity with increasing the coated cement. Also the SEM examination of these samples in Fig. 12 (a, b and c) show the formation of relatively same porous structure with presence of some whiskers. FIG. 11: REDUCTION CURVES OF REDUCED IRON ORE PELLETS COATED WITH a. 16% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS b. 16% CEMENT SLURRY CONC. & 0.75 Kg CEMENT /TON IRON ORE PELLETS c. 16% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS FIG. 12: SEM MICROGRAPHS OF REDUCED IRON ORE PELLETS COATED WITH a. 16% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS b. 16% CEMENT SLURRY CONC. & 0.75 Kg CEMENT /TON IRON ORE PELLETS c. 16% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS The reduction curve of iron ore pellets coated with 0.5, 0.75 & 1.0 Kg cement/ton iron ore using 20% cement slurry concentration is shown in Fig. 13. Comparatively, the influence of solid concentration became so clear on using higher slurry concentration. In the present case (20%), the solid influence was observed from the initial 37

8 Journal of Metallurgical Engineering (ME) Volume 4, 2014 reduction stages whereas the reduction rate increased with decreasing the coated cement amount from 1.0 Kg to 0.5 Kg. As mentioned above this is owing to the decreasing in pellets porosity with increasing the coated cement amount. The morphological observation of these samples are shown in Fig. 14 (a, b and c). It can be seen that generally porous structure is formed with presence of some whiskers. FIG. 13: REDUCTION CURVES OF REDUCED IRON ORE PELLETS COATED WITH a. 20% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS b. 20% CEMENT SLURRY CONC. & 0.75 Kg CEMENT /TON IRON ORE PELLETS c. 20% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS 2) Influence of Slurry Concentration FIG. 14: SEM MICROGRAPHS OF REDUCED IRON ORE PELLETS COATED WITH a. 20% CEMENT SLURRY CONC. & 0.5 KG CEMENT /TON IRON ORE PELLETS b. 20% CEMENT SLURRY CONC. & 0.75 KG CEMENT /TON IRON ORE PELLETS c. 20% CEMENT SLURRY CONC. & 1.0 KG CEMENT /TON IRON ORE PELLETS The influence of cement slurry concentrations (12%, 16% & 20%) at constant solid cement concentration per Ton iron ore pellets was studied. 38

9 The reduction curve of iron ore pellets coated with 0.5 Kg cement/ton iron ore using 12%, 16% and 20% cement slurry concentration is shown in Fig. 15. Also as mentioned above, for each single reduction curve, the rate of reduction was the highest at the early stages and decreased as reduction proceeds till the end of reduction. From the initial up to the final reduction stages, the influence of cement slurry concentration is observed clearly. It was found that the reduction rate decreased with decreasing the slurry concentration from 20% to 12% gradually. This is might be caused by the comparative amount of solution that was used to get the same solid concentration (0.5 Kg cement/ton iron ore). More solution was used from 12% slurry compared to other slurries (16% & 20%) that could relatively more isolated the pellet s surface from the reducing gas. FIG. 15: REDUCTION CURVES OF REDUCED IRON ORE PELLETS COATED WITH a. 12% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS b. 16% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS c. 20% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS In case of iron ore pellets coated with 0.75 Kg cement/ton iron ore using 12%, 16% and 20% cement slurry concentration, the reduction curve is shown in Fig. 16. It can be seen that the reduction rate at the three coating conditions are very similar during all the reduction stages. However, slightly higher reaction rate can be observed for the 12% slurry at the intermediate reaction stage. Thus, with increasing the coated cement from 0.5 up to 1.0 Kg cement/ton iron ore, the effect of using excess amount of lower slurry concentration (12%) to get the same coated amount decreased gradually. This is confirmed during the reduction of SAMARCO iron ore pellets coated with 1.0 Kg cement/ton iron ore using 12%, 16% and 20% cement slurry concentration. It is found that the reduction rate increased with decreasing the slurry concentration from 20 to 12% as shown in Fig. 17. Accordingly, using higher concentration slurry (20%) to get a higher coated amount (1.0 Kg) decreasing the gas excess to the iron ore pellets. FIG. 16: REDUCTION CURVES OF REDUCED IRON ORE PELLETS COATED WITH a. 12% CEMENT SLURRY CONC. & 0.75 Kg CEMENT /TON IRON ORE PELLETS b. 16% CEMENT SLURRY CONC. & 0.75 Kg CEMENT /TON IRON ORE PELLETS c. 20% CEMENT SLURRY CONC. & 0.75 Kg CEMENT /TON IRON ORE PELLETS 39

10 Journal of Metallurgical Engineering (ME) Volume 4, 2014 FIG. 17: REDUCTION CURVES OF REDUCED IRON ORE PELLETS COATED WITH a. 12% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS b. 16% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS c. 20% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS From the above it is found that the optimum reduction behavior is obtained during the coating with 12% slurry concentration and 1.0 kg cement/ton iron ore pellets and also with 20% slurry concentration and 0.5 kg cement/ton iron ore pellets. The reduction of uncoated pellets is done and compared to the reduction of pellets coated with the optimum condition that mentioned above as shown in Fig. 18. FIG. 18: REDUCTION CURVES OF REDUCED IRON ORE PELLETS COATED WITH a. UNCOATED PELLETS b. 12% CEMENT SLURRY CONC. & 1.0 Kg CEMENT /TON IRON ORE PELLETS c. 20% CEMENT SLURRY CONC. & 0.5 Kg CEMENT /TON IRON ORE PELLETS Conclusions The influence of solid concentration (0.5, 0.75 & 1 Kg cement) is observed clearly during the using of higher cement slurry concentration. With increasing the slurry concentration, the reduction rate increased with decreasing the coated cement amount from 1.0 Kg to 0.5 Kg gradually. This is might be due to the decreasing in pellets porosity with increasing the coated cement. The reduction rate decreased with decreasing the slurry concentration from 20% to 12% gradually. More solution was used from 12% slurry compared to other slurries (16% & 20%) that could relatively more isolated the pellets surfaces from the reducing gas. This effect decreased gradually with increasing the coated cement from 0.5 up to 1.0 Kg cement/ton iron ore. So when the cement concentration is 0.75 and 1.0 kg/ton pellets, the reduction rate increased with decreasing the slurry concentration. Thus, using higher concentration slurry (20%) to get a higher coated amount (1.0 Kg) decreasing the gas excess to the iron ore pellets. 40

11 The optimum reduction behavior was obtained during the coating with 12% slurry concentration and 1.0 kg cement/ton iron ore pellets and also with 20% slurry concentration and 0.5 kg cement/ton iron ore pellets. REFERENCES [1] Direct reduced iron: Technology and Economics of Production and Use, ed. by J. Feinman and D. R. Mac Rae, ISS, Warrendale, PA, (1999). [2] Wong PLM, Kim MJ, Kim HS, Choi CH. Ironmaking Steelmaking, 1999: 26: [3] R. Nicolle and A. Rist: Metall. Trans. B, 10B (1978), 429. [4] H. W. GUDENAU, H. P. EISEN, and Y. QI: Stahl Eisen, 1991, 111, (9), [5] S. HAYASHI and Y. IGUSHI: ISIJ Int., 1992, 32, (9), [6] S. HAYASHI, S. SAWAI, and Y. IGUSHI: ISIJ Int., 1993, 33, (10), [7] Jian Hua SHAO, Zhan Cheng GUO and Hui Qing TANG ISIJ International, Vol. 51 (2011), No. 8, pp [8] Ben Zhang, Zhi Wang, and Zhancheng Guo, Powder Technology, 225, (2012) 1 6 [9] Cano JAM, Wendling F. Mining Eng 1993: 45: