Effect of Magnesite on the Properties of Pellets at Room and Low (900 °C) Temperatures*

By Takeshi S UGI YAMA, * * Shoji SHIR 0 UCHI, * * Osamu TS UCHI YA, * * Mamoru ONODA* * and Isao F Uf I TA* *

Synopsis II. Experimental Procedure Propertiesof acidpellets with Mg0/Si02 ratiosbetween 0 and 1.3 Specular hematite, magnesite, lime stone and silica and pelletswith Mg0 and Ca0/Si02 ratiosbetween 0 and 1.3 are sand (Table 1) were used as the raw materials for presented. The amountof magnesioferrite in thefired pellets with Mg0 can pellets. The reason why magnesite was chosen as beevaluated by thevalue of themagnetic susceptibility of the pellets, since the source of MgO was to the end that it was less it increaseswith the Mg0 contents.This amount varies with the Ca0/ influenced by components other than MgO, and the Si02 ratio whenCa0 is presentin thepellets. The Mg0 density pores, which were formed from the heat decomposi- showsstripe-like distribution when Mg++ diffuses into hematite at tem- tion of magnesite, were expected to improve the peraturesbelow 1 395 °C,and changes hyperbolicly when the temperature reducibility.6 is above1 395°C. The Mg0 densitychanges hyperbolicly at various Each of the raw materials for pellets was blended temperatureswhen Mg++ douses into magnetite in theN2 gasflow. together. As a result of firing, the MgO/Si02 ratios Thereducibility ofmagnesio ferrite is slightly inferior to that of hematite. of the pellets varied at five different levels of 0 to Therefore,the reducibilityof thepellets decreases with the amountof 1.3, and each of the ratios showed an amount of 4 magnesioferrite in thepellets. Thechanges in thevolume of magnesio- to 8 % Si02. The CaO/Si02 ratios varied from ferriteduring reduction are very slight. Swellingdecreases with increasing amountsof it in thepellets. 0.08 to 1.3 when the MgO/Si02 ratio was held at 0.7. I. Introduction To examine the influence of porosity on the reduc- There have been many reports'-4) on the influence tion properties, dextrin was added to the pellets of the MgO density on the properties of self-fluxed having the CaO/Si02 ratios of 0 to 1.5. pellets. These are continuations of the reports) They were mixed in a small drum mixer for 1 h. which stated that high temperature reduction prop- The raw materials and water were alternately in- erties of the self-fluxed pellets have been greatly troduced into a small pelletizer (400 mm diameter) improved by adding dolomite. to become green pellets of 13 mm diameter. They So far the effect of the MgO component has not were dried for two days at room temperature. Dry been sufficiently defined because pellets in the pres- pellets in a stainless basket were put into an electric ence of CaO were examined, and its influence was furnace and were heated at 1 300 °C with a flowing dominant. In addition, there are fewer studies on air of 21/min. The temperature was allowed to rise the properties of these pellets at room and low (900 °C) for 20 min and was maintained at 1 300 °C for 10 min, temperatures, which are equally important as the and then allowed to cool for 20 min. Under these properties at both low and high temperatures show conditions, the pellets with an MgO content of 8.4 a close relationship. wt% were found to have large cracks. This was pre- This report defines the effect of magnesio-ferrite vented by decreasing the heating rate between 530 °C on the physical properties of pellets at room tem- and 900 °C. perature and the reduction properties at low tem- The following method was used to study the diffu- perature, and it also clarifies the process of magnesio- sion of Mg++ into the iron oxides. Briquettes ferrite formation. (10 mmhx 10 mm~b)were made by pressing at 2 t/cm2 using Fe203 and Fe304 of reagent grade, and heated

Table 1. Chemical composition and braine index of pellet feeds.

(146) Research Article Transactions ISIJ, Vol. 23, 1983 (147) in the air at 1 450 °C and in an N2 gas flow at hematite particles with one another. Under the 1 350 °C for 5 h each. The contact surface of each presence of CaO, the pellets was reached its highest briquette was ground with emery paper (No. 1200). strength at a CaO/Si02 ratio of 0.45, before de- The MgO briquette was then put into contact with creasing. These results will be influenced by the an iron oxide briquette and fired at fixed tempera- strength of dry pellets. tures and times. The fired samples were transferred The porosity of the dry pellets showed a range of to the iron plate for 10 sec to be cooled. They were 25.ON26.6 % without any influence of the combi- used for surface and line analyses of Fe++ and Mg++ nation of raw materials. The greater the amount of density using the Electron Prove X-ray Micro- MgO is, the higher the porosity of the fired pellets analyzer (E.P.M.A.). becomes. When CaO was present, porosity of the The compressive strength was determined by pellets with CaO/Si02 ratios of 0.45 and 0.8, was measuring 10 fired pellets and indicated by the higher than that of pellets at other ratios. arithmetic mean. The porosity was calculated from The contraction degree by firing rose with an the apparent specific gravity and the true gravity, increase in the MgO/Si02 and CaO/Si02 ratios. which were measured by the JIS method (JIS M8716). The change in volume during firing can be deter- 2. Structural Observation mined by measuring 10 pellets in both dried and fired The pellets containing MgO (CaO/Si02~0) con- conditions by the above mentioned method. sisted of hematite and iron oxide with a high MgO The reducibility of the pellet was examined by density. No slag phase can be seen. The pellets using an electric furnace with thermal balance (Fig. with MgO of 4 wt% (see Photo. 1) had particles in 1). After it was heated to 900 °C in an N2 flow, a which hematite and iron oxide with a high MgO pellet was reduced for 3 h with CO/N2=30/70 (1.4 density existed in stripes. MgO was not visible in l/min). The degree of reduction was calculated from that hematite. The iron oxide with MgO showed the loss of weight. Its accuracy was within ±2 %. the structure of (Mg0.33~0.40,Fe0.s7~0.so)O . Fe203. The swelling was calculated from the volume before The density of. MgO did not depend on the MgO/ and after the reduction. Its accuracy was within Si02 ratio of the pellets. ±2.5 %. The pellets with CaO of MgO/Si02=0.72 showed different structures depending upon the CaO/Si02 III. Results ratio. The pellets with a CaO/Si02 ratio below 0.25 had particles of iron oxide in stripes in the parts where 1. Physical Properties the MgO density was 16.3 wt%. The pellets with As shown in Fig. 2 the compressive strength of the CaO/Si02 ratios of 0.45 and 0.81 had both iron dry pellets was not much influenced by the MgO/ oxide containing MgO of 10 wt% evenly, and the Si02 ratio or the amount of SiO2, and showed a value slag (Wollastnite) with 2.6 wt% MgO. within 1.0-'1.3 kg/pellet. The compressive strength The pellets with the CaO/Si02 ratio of 1.26 con- rose an additional 0.5 kg/pellet at the CaO/Si02 ratios of 0.5 and 0.8, which was higher than those at other ratios. This is due to the influence of the grain size distribution of the feed and balling operation of the pellets. The greater the amount of Si02 and the higher the MgO/Si02 ratio are, the more the strength of the fired pellets decreased. This decrease is the result of the effect that silica particles restrict to combine the

Fig. 2. Effects of Mg0/Si02i Ca0/Si02 on contraction by firing, porosity and strength of dry and fired pellets.

Photo. 1. Scanning images of characteristic X-ray of th e pellets Fig. 1. Schematic test apparatus. with Mg0/Si02 = 0.7, 5i02=8.33.

Research Article (148 ) Transactions ISIJ, Vol. 23, 1983 tamed euhedral iron oxide surrounded by slag (see ferrite (M.F.) is presumed to be generated from the Photo. 2). Mg++ was present in some areas regard- points of contact of iron oxide and MgO, as deep as less of the arrangement of the iron oxide and slag. about 101. The density of MgO in this iron oxide was 7.1 wt% and the slag was pseudo-wollastonite. 4. Reducibility The higher the MgO/Si02 ratio is, the higher the 3. Diffusion of Mg++ into Iron Oxide reduction degree of the pellets (CaO/Si02 r0) was As shown in Photo 3, both Fe++ and Mg++ counter- found to be. In any MgO/Si02 ratio, when the diffused at the contacted surface of iron oxide and amount of Si02 increased from 4.4 to 8.3 wt%, the magnesia briquettes. When Mg++ diffused into the degree of reduction increased by about 6.5 %. When Fe203 briquette under 1 350 °C in air, a striped CaO was present, the degree of reduction of the pellets structure with a high MgO density was observed, but with MgO varied with CaO/Si02 ratio as in Fig. 4. this structure was not observed at 1 400 °C. When it diffused into Fe304 in an N2 gas flow, a striped 5. Swelling structure was not observed at any temperature. Figure 5 shows the results of swelling after reduction Figure 3 shows the results of line scanning of MgO test. It fell in proportion to the rising MgO/Si02 in iron oxide by E.P.M.A. The curves of the parts ratio. At any MgO/Si02 ratio, it fell with any in- showing striped structures are drawn to connect to crease in the amount of SiO2. The swelling of pellets the parts of high MgO density. It is assumed from with both MgO and CaO decreased in proportion to the MgO-Fe203 diagram') that the striped structure the rise in the CaO/Si02 ratio. in the pellets appeared when Mg++ diffused into Fe203 held at temperatures below 1 395 °C in air. Under the firing conditions of this test, magnesio-

Photo. 2. Scanning images of characteristic X-ray of the pellets Fig. 3. Effect of temperatures on diffusion of Mg++ into with Ca0/Si02=1.26, Mg0=4.64. hematite in air.

Fig. 4. Effects of Mg0/Si02 and Ca0/Si02 in fired pellets on reduction degree at 900 °C.

Photo. 3. Scanning images of characteristic X-ray of Fe203 briquette in contact with Mg0 fired at 1 300 ° C (1) Fig. 5. Effects of Mg0/Si02 and Ca0/Si02 in fired pellets and 1 400 °C (2) for 30 min in air. on swelling by reduction at 900 °C for 3 h.

Research Article Transactions ISIJ, Vol. 23, 1983 (149)

Iv. Discussion 2. Generationof Magnesioferrite 1. Pore Formation of the Fired Pellets with Magnesite Magnesio-ferrite (M.F.) shows a reverse spinel structure8~ having a ferrite magnetism. The amount The porosity of the fired pellets are presumed to be of M.F. was estimated from the value of magnetic determined by the next two factors : the volume susceptibility (M.S.). It was measured by a magnet shrinkage (S) during firing and the porosity (Ex) meter made of BISON. Five pellets were put into which is formed from the thermal decomposition of a plastic vessel of 10 cc capacity to measure and carbonic salts. The porosity Ex on the base of dry calculate the apparent M.S. per 1 g sample. The pellet can be calculated by the following equation. pellets (CaO/SiO2N0) showed higher values in pro- Ex = (1-S/100)xEf+S-Ed portion to the increase of MgO, also slightly different lines could be seen with the increasing amount of where, E f : Porosity of fired pellets Si02 (see Fig. 7). As pointed out in Section III. 2, Ed : Porosity of dry pellets MgO density in M.F. was not influenced by the The porosity Ex, as can be seen in Fig. 6, shows amount of MgO in the pellets. So the amount of a good proportion to the MgO + CaO density in fired M.F. is proportional to both the amount of MgO in pellets. To verify whether the porosity Ex was formed pellets and the value of M.S. due only to the thermal decomposition of lime stone When CaO was present in the pellets, even if the and magnesite, the porosity E~ was calculated on the amount of MgO was fixed, the M.S. varied with the base of dry pellet using the volume loss when both CaO/Si02 ratio. MgCO3 and CaCO3 decomposed. The porosity The existence and the amount of M.F. was checked E~ was 1.0~ 1.5 vol% lower than Ex as shown in by X-ray diffraction. Figure 8 shows the results of Fig. 6. measurements. This difference was caused by two factors : (1) there In the pellets containing magnesite, a-Fe203, M.F. are differences in the porosity and composition be- (low temperature type) and a-quartz were observed. tween pure MgCO3 and magnesite, and CaCO3 and The highest peak for M.F. was superposed upon that lime stone, and (2) the cracks which generate during for a-Fe203. So it was assumed that the height of the decomposition are not to be taken into account. The porosity will be determined by (i) the porosity of dry pellets (test sample: about 26 %) which depends on balling conditions and particle size distribution of feed, (ii) the change in volume by com- bining the iron particles and slag during firing, and (iii) the porosity formed by the thermal decomposition of carbonic salts. The porosity of the pellet with lime stone varied with the basicity (CaO/Si02), because it showed the volume change at firing which depends on the slag structure.

Fig. 7. Relation between the susceptibility and content of MgO in fired pellets.

Fig. 6. Relation between the porosity formed by decomposition of both MgCO3 and CaCO3, and the contents of MgO+CaO in fired pellets. Fig. 8. X-ray diffraction pattern of the pellets without CaO.

Research Article (150) Transactions ISIJ, Vol. 23, 1983 the second peak (2, 2, 0) indicated the amount of In the case of the acid pellets (Si02=8 wt%), the M.F. And the relationship of that peak to the relationship between the degree of reduction (Rd) amount of M.F, was examined. The amount of and porosity (P f) was represented by the following M.F. in the pellets with the CaO/Si02 ratio of 0 equation: can be obtained by the results of X-ray diffraction as well as that of M.S. Rd = 2.72 XP f-16.74 (P f = 25-35 %) ...... (2) The second peak of M.F. in the pellets with CaO Equation (2) means that the reduction degree varied with the CaO/Si02 ratio. This is different changes 2.72 % per 1 % of porosity, and the dotted from the relationship between the M.S. and the CaO/ line with the slope of 2.72 is shown in Fig. 10. The Si02 ratio. Its height (d=2.96) is greatly affected by rate of increase in the reducibility of the pellets con- the presence of (Fe0.86, Cao.14)• Si02, because the taining magnesite is lower than that of the pellets second peak (d=2.93) was superposed upon it and its containing dextrin. presence increased at a CaO/Si02 ratio of above 0.45, The difference (dR) between the degree of reduc- which was made clear from the chemical analyses of tion of the pellets containing magnesite and that slag components. containing dextrin become smaller in proportion to Accordingly, the amount of M.F. in the pellets with the increase of porosity which is corresponded to CaO can not be obtained from the X-ray diffraction. M.S. (Sax 10-4) of the pellets with magnesite. So it was calculated from the material balance of MgO in chemical analyses and the composition anal- 4R(= Rm-Re) _ --0.825 xSa+b (b: Constant) yses of each phase by E.P.M.A. Figure 9 shows that ...... (3) the amount of both slag and MgO density in M.F. vary with the CaO/Si02 ratio. This variation of Equation (3) indicated that the degree of reduction M.F. with the CaO/Si02 ratio is similar to that of is lowered by about 0.8 % per M.S. (1 x 10-4) which M.S. It is possible to get the amount of M.F. from corresponded to the amount of M.F, in the fired the M.S., but further examination will be required pellets at 4.466.37 wt%. Next, whether or not to get an accurate value. these results would apply to the pellets containing CaO, was determined. 3. Influenceof Magnesite on Reducibility The reducibility was affected by the CaO/Si02 Both the porosity and the amount of M.F, in the ratio as well as by the porosity and the amount of fired pellets varies with the amount of magnesite. MgO. The degree of reduction should be evaluated These elements which influence the reducibility at a fixed porosity to eliminate that factor. The should be separated from each other in order to porosity of the pellets with CaO/Si02 ratios of 0.45 clarify the reducibility of M.F. and 0.81 was about 4 % higher than those of other Figure 10 gives the relationship between the degree basicity pellets (28.1 ±0.5 %). The degree of reduc- of reduction and the porosity of the pellets with tion when these pellets have the porosity of 28.1 % magnesite. It changed linearly. can be derived from the following equation which was obtained by using pellets containing dextrin. Rm= 1.72XPf+33.46 ...... (1) CaO/Si02 = 0.5 : R =1.27 xPi+22.15 where, Rm: Degree of reduction after 3 h (%) (P1=26.529.1 %) ...(4) P f : Porosity in the fired pellets (%). CaO/Si02 =1.0 : R =1.16 x P+33.0 In order to make clear the influence of the porosity only, the porosity of the fired pellets was controlled by (Pi =19.931.9 %) ...(5) adding dextrin which burn out during firing. where, R : Degree of reduction after 3 h (%) Pi : Porosity of fired pellets (%).

Fig. 10. Comparison of the relations between the reduction Fig. 9. Effect of Ca0/Si02 on content of ma gnesio-ferrite in degree at 900 °C and the porosity of both the pellets fired pellets. with magnesite (1) and dextrin (2).

Research Article Transactions ISIJ, Vol. 23, 1983 (151)

It was evaluated the degree of reduction at a fixed structure with a cubic system which is the same system M.S. (4x 10-4) based on the degree of reduction as wustite. change at 0.8 % per 1 X 10_4 of MS. The corrected Therefore, the following calculation could be made reduction degree still varies with the CaO/ Si02 ratio on the assumption that M.F, does not expand during (see Fig. 11). This change in the degree of reduction reduction. It is assumed that hematite only affected is attributable to the reducibility, the amount and the the swelling in the pellets without CaO. The swelling wetability of the slag. value per hematite of 1 wt% is 0.1505 vol% which is Comparing the corrected reduction to the pellets calculated from that of the acid pellets without with dextrin, the tendency, which the reduction magnesite. This value includes the changes in volume degree becomes high with an increase of CaOJSiO2 caused by transformation of hematite, cracks caused ratio, becomes similar when the CaO/Si02 ratio held by it, sintering among iron oxide particles, and solid above 0.4 (see Fig. 11). solution of gangue minerals into iron oxide. Consequently, the reducibility is determined by The swelling value of the pellets with magnesite the porosity, the amount of M.F, and the CaO/Si02 is also calculated from the amount of hematite ob- ratio. The reason why the reduction degree of the tained by subtracting the amount of gangue minerals pellets with dextrin is lower than that of the pellets except the MgO component and the M.F, from the with magnesite and especially the pellets with lower total weight. In Fig. 13, 45 represents the value Ca0/Si02 ratio than 0.4, is presumed to be a charac- obtained by subtracting the value for calculated teristic influence caused by the presence of dextrin.9~ swelling from the value for measured one, and was found to be within ±1.9 %. It becomes smaller with 4. Inluencial Factors on the Swelling an increase in the degree of reduction. Figure 12 suggests that the greater the amount of These results suggest that the factors affected by M.F, is, the smaller the swelling becomes. This is reducibility should be taken into consideration, for due to that the M.S. is directly proportional to the example, the decrease in volume caused by the sinter- amount of M.F. in the fired pellets. ing of iron oxide. Accordingly, the main factors for M.F. does not occur in the crystal transformation swelling are the transformation of hematite and the during reduction, which is expected to be the main change in volume depending on the degree of reduc- cause of normal swelling, because M.F. has a spinel tion. M.F, and gangue minerals which do not generate liquidous slag, do not influence the swelling. The swelling value of the pellets in the presence of CaO was calculated by the similar method. The amount of volume change in the slag phase during reduction was obtained from the results of measurements by H. Brill-Edward et al.'°~ (calcium- silicate +2.9 vol%, calcium-monoferrite -4.6 vol%). The following specific gravity is applied to calculate the volume of the slag phase. Fe203 = 5.2 MgFe204=4.6 Calcium-silicate= 3.4 CaO.2Fe203=4.8* (* from the specific gravity of CaO and Fe203)

Fig. 11. Effect of CaOJSiO2 in fired pellets on red uci bility at 900 °C.

Fig. 12. Relation between susceptibility of fired pellets and Fig. 13. Relation between difference of calculated swelling swelling by reduction at 900 °C. from calculated one and reducibility at 900 °C.

Research Article (152 ) Transactions ISIJ, Vol. 23, 1983

The swelling value is calculated by adding the The reducibility of M.F, is lower than that of he- amount of each volume change in the slag, hematite matite. M.F, shows only a slight change in its and M.F, phases. volume during reduction. Consequently, the greater As shown in Fig. 13, the measured value is smaller the amount of M.F, is, the lower the reducibility and than the calculated one at any Ca0/Si02 ratio and swelling become. AS decreases with a rise in the degree of reduction. When magnesite is used as a source of MgO, its Even for the pellets in which CaO was present, the thermal decomposition increases the porosity in the swelling value can be calculated just as well as for fired pellets and serves to improve the reducibility. the pellets containing magnesite. Figure 13 shows that the value of dS varies with the existence of CaO and the influence of the CaO/ REFERENCES Si02 ratio is still observed. So the effect of CaO on 1) 0. Tsuchiya, T. Sugiyama, M. Onoda and I. Fujita : Tetsu- the swelling needs to be studied further. to-Hagane, 66 (1980), 1830. 2) K. Narita, M. Maekawa, I. Shigaki and Y. Seki: Tetsu-to- V. Conclusion Hagane, 63 (1977), 1623. The properties of the pellets with MgO can be 3) P-A. Ilmoni and B. Bjorkvall: AIMS Ironmaking Proc., 36 (1977), 366. characterized by magnesio-ferrite (M.F.). 4) D. E. Morin, F. Cappuccitti, C. A. Pickles, M. U. Ross and The amount of M.F. in the fired pellets can readily T. R. Meadowcraft: AIMS Ironmaking Proc., 37 (1978), 69. be measured from its magnetic susceptibility (M.S.). 5) 0. Saeki, K. Taguchi, I. Nishida, I. Fujita, M. Onoda and The amount of M.F. in the pellets without CaO 0. Tsuchiya: Agglomeration 77, II, AIME, New York, changes in proportion to the MgO density in pellet. (1977), 803. The amount of M.F. in the pellets with CaO changes 6) I. Nishida, T. Uenaka and I. Mizuguchi: Blast Furnace with the CaO/Si02 ratio of the pellets. When the Conference, I, IRSID, Arles, June, 1980, Iv-1-1. CaO/Si02 ratio is held above 0.25, the amount of 7) B. Phillip, S. Somiya and A. Muan: J. Amer. Ceram. Soc., M.F. increases with an increase in the basicity. 44 (1961), 169. M.F, is generated in stripes when Mg++ diffuses 8) Chemical Handbook (Basic), ed, by The Chemical Society of Japan, Maruzen, Tokyo, (1975), 1240. into hematite held at temperatures below 1 395 °C 9) 0. Tsuchiya, K. Otsuki, T. Sugiyama, M. Onoda and in the air. When hematite was held at temperatures I. Fujita: Tetsu-to-Hagane, 66 (1980), 1057. above 1 395 °C, MgO distribution in hematite is 10) H. Brill-Edwards, H.E.N. Stone and B. L. Daniell: JISI, hyperbolic. 207 (1969), 1565.

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