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