INVESTIGATIONS has been carried out to find the optimum conditions for the production of highly  reduced  iron ore briquettes suitable as a blast furnace  feed  from  Indian ore fines and low grade  coke/wool charcoal  under reduced pressure 1 x 101 mm. (Hg.) The effects of time (1/4 hr. to 6 hr.) temperature  (950°C to 1150°C  and  Fe20 1,C  ratio (1/3 to 1/5) on the rate of reduction of iron ore  brique-ttes have been studied. From the kinetic results, a mecha-nism for reduction of iron oxide in iron ore briquettes is proposed and optimum conditions for  production  of highly reduced iron ore briquettes are summarised

Koria, S C

Saxena, M N

Tripathy, A K

English

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PREPARATION OF PRE-REDUCED BRIQUETTES AND STUDIES ON THE KINETICS OF THE PROCESS UNDER REDUCED PRESSURE A. K. TRIPATHY S. C. KORIA ** & M. N. SAXENA *** Abstract INVESTIGATIONS has been carried out to find the optimum conditions for the production of highly  reduced iron ore briquettes suitable as a blast furnace feed from Indian ore fines and low grade coke/wool charcoal under reduced pressure 1 x 101 mm. (Hg.) The effects of time (1/4 hr. to 6 hr.) temperature (950°C to 1150°C and Fe20 1,C ratio (1/3 to 1/5) on the rate of reduction of iron ore briquettes have been studied. From the kinetic results, a mechanism for reduction of iron oxide in iron ore briquettes is proposed and optimum conditions for production of highly reduced iron ore briquettes are summarised. Introduction In recent years, a large number of signi-ficant developments have taken place in the technology of iron making including oxygen en-richment, humidification of the blast, higher blast temperatures, fuel injection through tuyeres and employment of high top pressure. It has been realised that by using partially reduced pellets or briquettes, the coke rate can be decreased and capacity of the furnace can be considerably increased. Republic Steel Com-pany of U.S.A. has reported (1) 9 per cent in-crease in production and 11.3 per cent decrease in coke rate by charging metallised briquettes (71 per cent Fe and 84 per cent metallisation) comprising 14 per cent of the total charge. In Japan, at Hegachida Plant at Yawata(2), it has been possible to increase the production and decrease the coke rate each by about 18 per cent by charging pre-reduced pellets (69 per cent Fe, 58 per cent metallisation) amounting to 40 per cent of the total charge. In the experimental blast furnace at Bruceton, U.S.A.(3), 24 per cent increase in production and 19 per cent decrease in 'coke rate have been reported by charging 397 kg. metallised iron powder (98.7 per cent Fe and 98 per cent metallisation) per ton of hot metal. In addition to the above mentioned ad-Vantages, the use of pre-reduced pellets or bri- quettes is further justified in view of the scant reserves of good quality coking coal and utilisa-tion of large quantity of ore fines resulting from mechanised mining. India has vast reserves of iron ore (27,270 million tons-)(4) with average iron content around 56 per cent but due to their soft nature, the ore fines are as high as 40 per cent of the total ore mined. In contrast, metallurgical grade coking coal reserves (5) of good quality are only 1600 million tons. After washing and blending, about 3000 million tons can be used for iron smelting. With the anticipated (5) demand of 23 million tons of coking coal in the steel plants in 1973-74, the reserves will last for about 70-80 years. The total reserves of non coking coal in-cluding lignite coke are estimated and reported (6) to be around 103,000 million tons. These low grade solid fuels can not be used in the blast furnaces because of their soft nature, poor strength and quality. It is therefore, high time to develop economic methods for the utilisation of ore fines and low grade solid fuels. Moreover, the existing blast furnaces are able to cope with the requirements of the country for iron and steel. Therefore, it is essential to increase the capacity of the existing blast furnaces by using pre-reduced briquettes with the regular charge. * Research Scholar, 	Bombay. ** Lecturer, Deptt. of Metallurgical Engineering, University of Roorkee, U, P. *** Professor & Head, Deptt. of Metallurgical Engineering, University of Roorkee, Roorkee, U. P. 128 'Work so far reported (7-11) in literature is mainly concerned with studies on kinetics of re-duction of pure iron oxide using activated char-coal. Not much work has been reported (12,13) regarding the use of iron ore and low grade solid fuels like charcoal, low grade coke, lignite coke etc. The present investigation has been under-taken with a view to arrive at optimum condi-tions for the preparation of highly reduced iron ore briquettes with good mechanical strength by using Indian ore fines and nonmetallurgical solid fuels like low grade coke and wood char-coal. The influence of reduction time, tempera-ture and Fe203 / .0 ratio under reduced pressure on the reduction rate of iron ore briquettes have been investigated. From the kinetic results ob-tained in these studies, a mechanism for the re-duction of iron oxide in the iron ore briquete by a solid reduction has been proposed. EXPERIMENTAL WORK Materials used The iron ore fines used in this investigation were obtained from Gua mines and contained 85 per cent Fe.203 and 15 per cent gangue. The loss on ignition was 7.8 per cent. The particle size of the fines used was in the range — 100 to + 150 mesh. The solid reductants employed in this study were low grade coke and wood charcoal, both procured from local markets of Roorkee. Wood charcoal was crushed and heated in an inert atmosphere at 900°C for 1 hr. to remove volatile matter. The size fractions of reductant used for reduction experiments were in between-100 to + 150 mesh. The proximate analysis in wt. per cent of these two reductants is given in table-1. Table-1 Proximate analysis of reductants Type of reductant Low grade coke (As received) Wood Charcoal (Treated) The ore fines and powdered reductant of the same mesh size were mixed thoroughly in a rota-ting cylinder for 8 hours. The proportion of iron oxide in iron ore to carbon was varied from mix-ture to mixture. The mixture Fe203 + 3C inves-tigated thoroughly was prepared by mixing 3 g. atoms of carbon to 1 g. mole of Fe203 Other mix- tures such as Fe,at + 3.5C, Fe203 + 4C, Fe 203 + 5C were also prepared to study the effect of Fe203 /C ratio on the kinetics of reduction. For the reduction experiments, samples weighing 6 gms were taken from the prepared mixtures. This mixture was compacted under a pressure of 470 Kg/sq cm in a universal testing machine using a dia of 1.5 cm. The length of the briquettes varied between 2 to 2.54 cms. The bri-quettes thus prepared were first air dried and then kept in an oven at 120°C for about 24 hrs to remove moisture completely. Reduction experiments were carried out in one end closed 2.54 cm. dia by 56 cm. long alu-mina tube which was kept horizontally in a kanthal wound 5 cm. dia resistant furnace. The open end of the reaction tube was closed with a solid rubber cork and connected to vacuum pump. The evacuation ackieVed was of the order of 1 x 10-1 mm. of mercury. Using a calibrated Pt-90 Pt-10 Rh thermocouple, the temperature profile along the length of the reaction tube was determined; it was found to be uniform (plus or minus 2°C) over a 4" long zone from the closed end. Two dried and weighed briquettes of the same mixture were kept at a time on the fireclay boat and then this boat was transferred to the alumina reaction tube. The tube was then evacu-ated to 10 x 10-1 mm. of Hg. It was inserted in the furnace which was maintained at the desired temperature. The furnace temperature was kept 5°C higher than required as this was found to be the temperature difference between the in-side and outside of the reaction tube. Usually 2 to 3 minutes elapsed before the samples attained the temperature of the surroundings. After the reduction was carried out under isothermal con-ditions for a pre-determined length of time, the reaction tube was taken out and cooled to room %Moisture %ash %Volatile matter %Fixed Carbon temperature under vacuum. 	 Then the vacuum seal was broken and the briquettes were taken out, weighed and analysed for residual carbon by 0.92 29.47 4.86 64.93 the usual Strohlein apparatus. 1.2 14.8 4 80 The percentage reduction of iron oxide in the briquette was calculated on oxygen basis. Let W be the weight loss after reduction, x be weight of carbon removed, y be weight of oxy-gen removed, z be the weight of volatile matter removed and w be the weight loss on ignition of iron ore then W= Y = W (x + z w) 129 Fig. I. Effect of temperature on reduction reaction Fe,0 3+ 3C (using wood charcoal). .360 60 120 f80 Z40 300 O 350.0 3 1000.0 • 1050°C O 1100°C • ff 50 '4" PERCENT  REDUCTION 7"/ML(All/V.) 100 80 O 950.0 O 1000°C • 1050.0 O f O 1150°C 01 1.6 4 108 r/ME Fig. 3 Percent reduction vs log time plots (using wood charcoal). PERCENT  REDUCTION 2.6 CO 4 20 2.2 	 2E Fig. 2 Effect of temperature on reduction reaction Fe2 03 + 3C (using low grade coke). 120 180 rimE(mliv.) O 1150 .0 O 1100.0 • 4050c • 1000°l PERCE NT REDVCrioN  404 360 240 300 PERCENT  REDUCTION  4 	 f.6 	 f 8 	 2-0 	 2.2 	 2-4 	 2-6 L. 08 771.16 Fig. 4 Percent reduction vs log time plots (using low grade coke) if YT be the oxygen removable, then y percent reduction   x 100 YT In this way, the influence of time (upto 300 mts) temperature (950 to 1150°C) and Fe203 C ratio on the rate of reduction by both low grade coke and treated wood charcoal was studied. RESULTS AND DISCUSSION A. Studies on the kinerics of reduction The percentage reduction of iron oxide in the briquettes consisting of iron ore and law grade coke or wood charcoal at constant Fe203 / C = 1/3 ratio after different intervals of time un-der reduced pressure (1 x 10-1 mm. of Hg) for  various temperatures 950 degrees Centrigade, 1050 degrees Centrigrade, 1100 degrees Centri grade and 1150 degrees Centigrade are plotted in fig. 1 and 2. From these plots, the marked influ-ence of temperature on reduction of iron oxide particularly in the initial stages (upto 30 mts.) is quite conspicuous. Further, percent reduction (R) is not directly proportional to the time (t) of re-duction. For example, percent reduction of iron oxide in iron ore-low grade coke briquettes for a period of 90 minutes increases from 40 per cent at 1000°C to N per cent at 1150 deg. C. Similar observation can also be made from fig. 2. The plots obtained in fig. 3 and 4 in both the cases confirm the following relationship R = c log t x Where c and x are constants, R indicates 130 PERCeNT REDUCT/ON 0 120 (80 240 300 Fig. 6 Log lit vs I/Tx104 (°K) for different per-centages of reduction (using low grade coke). 7.5 8.0 -1.370 icoo*c 	 s50•C al 50X R£DUCTION • 60 % REDUCTION 1 70:4 REDUCTION o so 2 RE DUCT1ON x 10 4 *K too° 00 50 100 80 7-41.1PERATURE 1050"C re, 031C Cii IP • 1/3.5 c 9/4 o f/5 1 	 1  20 	 1 1 	 80 7'719E 6411N.) Fig. 8 Effect of Fe2 O,/C ratio on percent reduction ( using !ow grade coke ) 240 	 300 	 360 ti t.) 60 4.1 ot cwi 40 OC o, 20 percentage reduction and t, the time of reduction. In this presentation, for purposes of analysing the kinetic results by semi log plot one or two initial values of reduction percent have not been considered as they have been found to be highly time sensitive. For example, using wood char- coal as reluctant, precent reduction at 1050°C was found to be .45 per cent in 15 minutes. The values of c vary from 53 to 55 or low grade coke and 50-58 for wood charcoal. in this range, the activation energy of the reduction process has been calculated in a way similar to the one T x 104 *x 	rtmE(141N.) Fig. 5 Log I/t vs I/Tx104 (°K) for different per- 	 Fig. 7 Effect of Fe,OJC ratio on percent reduco centages of reduction (using wood charcoal) 	 tion (using wood charcoal) 131 Time for Half complete reduction (Min)  	950°C 	 1000°C I 1050°C I 1100°C I 	240 	 120 	 48 	 25 210 	 90 	 33 	20 180 	 75 	 21 	20 60 	 30 	 12 	5 1/3.5 1/4 1/5 Mixture Fe203 + C Fe203 + 3 5 C Fe203 + 4C Fe203 + 5C 13 10 Fe2O3C 1/3 1150°C  15 atures, the reciprocal of time for attaining a de-cording to Arrhenius relationship at all temper-entiating R with respect to t in equation (1), it adopted by Jena and coworkers (14). By differ-is observed that the rate dR/dt is proportional to 1/t, reciprocal of time of reduction. Hence ac-finite reduction should be proportional to e-E/RT where E, R and T are activation energy, gas con-stant and absolute temperature respectively. The log lit vs 104 T'IC graphs for low grade coal and wood charcoal reductants at different reduction percentages are plotted in fig. 5 and 6 respectively. From their slopes, the activation energy values have been calculated and found to be equal to 53.9 and 57.2 K cal/g. mole for iron ore reduction with low grade coke and wood charcoal respectively. From the results reported above, it is seen that during initial stages of reduction (upto 30%), the reaction rate is relatively very high. This may be due to the surface reaction between re-actants. After 30 per cent reduction, the results obey the linear relationship (1) which suggests that the process is controlled by diffusion. The activation energy values found in case of low  grade coke (53.9 K cal/g. mole) and wood char-Loal (57.2 Kcal/g. mole) are in close agreement with the reported. value (15) (55 Kcal/mole) for the diffusion of iron through Fe204. Hence, it is suggested that diffusion of iron through Fe304 is the most probable rate determining step of 90 per cent reduction. Beyond 90 per cent reduction, the reaction becE me extremely sluggish owing to poor availa-bility of reactants. B. Effect of Fe203 ratio Experiments were conducted using the same mesh size of iron ore and low grade coke or wood charcoal as the reducing agents. The Fe203 /C ratio was varied between 1/3, 1/3.5, 1/4 and 1/5 It is discernible from the results shown in fig. 7 and 8 that as the ratio is decreased, the rate of reduction increases considerably. In other words reduction proceeds much more rapidly in mixtures with proportionately higher carbon content. The time-span data for half complete reduction for the reductants low grade coke and wood charcoal are given in table 2 & 3. TABLE II Effect of Fe2 0/C ratio on lime-span (T/2) Value for low grade coke as reductant Mixture Fe203/C ratio Time for half complete reduction (Min) 50°C 	 I 1000°C 	 I 	 1050°C 	 I 	 1100°C Fe,Os + 3C 	 1/3 480 150 	 66  30 Fe203 + 3.5C 	 1/3.5 430 120 	 54 25 Fe203 .+ 4C 	 1/4 360 90 	 42 20 Fe203 + 5C 	 1/5 300 60 	 27 10 I 1150°C 15 12 10 TABLE III Effect of Fe,03/C ratio on time-span (T1/2) values for wood charcoal as reduction 132 Conclusions 1. The effect of temperature on the degree of reduction of iron ore fines was found to be pro-nounced throughout the range of temperatures studied. It is possible to produce metallic iron from iron ore only above 1000°C. 2. In the sequence of reduction of iron ore to metallic iron, two stages were observed. Initial stage of reduction was found to be controlled by surface reaction while during the later stage diffusion of iron through Fe 30, appears to con-trol the reaction. 3. Reduction rate was found to be higher in mixtures with proportionately higher carbon content. References 1. Dastur et. al, Trans. MI, Vol. 21, June 1968, P. 92. 2. Takeda K., Blast furnace and Steel plant. Vol. 56, 1968, p. 297. 3. Woolf, Jr. of Metals, Vol.8, Feb. 1966, p. 243. 4. Balsundaram as.,, Eastern Metals Review, Annual No. April, 1972, p. 39. 5. Altekar V. A. et. al, Paper presented in 25th Annual Technical Meeting of IIM, Feb. 1972 held in Delhi. 6. Chatterjee A.B , NML Tech. Jr., Vol. 11, No. 4, 1969, p. 4. 7. Rao Y. K., Met. Transaction, Vol. 2, May 1971, p. 1439. 8. Yun T. S., Transaction of the ASM, Vol. 54, 1961, p. 129. 9. Bankloh W. and Durer R., Arch Eisenhuttenw, Vol. 5, 1931, D. 455. 10. Benkloh W. and Zinmermann G., Stahl and Eisen, Vol. 53, 1933, P. 172. 11. Rao Y. K., Paper presented at annual meet-ing of AIME, at New York, Feb. 25 1968. 12. Baldwin B. JISI,. Vol. 179, 1955. p. 179. 13. Kapoor A. N. et. al., Paper presented at IIM Symposium, Dec. 1970. 14. Jena et. al. Jr. of less Common Metals, Vol. 10, 1966, p. 185. 15. Kubaschewrki and Hopkins, Oxidation of Metals and Alloys 2nd edition, p. 33. 133 

Preparation of Pre-Reduced Briquettes and Studies on the Kinetics of the Process Under Reduced Pressure

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Preparation of Pre-Reduced Briquettes and Studies on the Kinetics of the Process Under Reduced Pressure

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