Effects of Basicity and FeO Content on the Softening and Melting Temperatures of the CaO-SiO 2 -MgO-Al 2 O 3 Slag System

Materials Transactions, Vol. 50, No. 6 (2009) pp. 1448 to 1456 #2009 The Japan Institute of Metals Effects of Basicity and FeO Content on the Softening and Melting Temperatures of the CaO-SiO 2 -MgO-Al 2 O 3 Slag System Hsin-Chien Chuang 1; * 1, Weng-Sing Hwang 1; * 2 and Shih-Hsien Liu 2 1 Department of Materials Science and Engineering, National Cheng Kung University, No. 1 Ta-Hsueh Road, Tainan 70101, Taiwan, R.O.China 2 Iron Making Process Development Section, Steel & Aluminum Research & Development Dept., China Steel Corporation, No. 1 Chung Kang Road Hsiao Kang, Kaohsiung 81233, Taiwan, R.O.China The effects of basicity (the ratio between CaO and SiO 2 ) and FeO content on softening and melting temperatures of direct reduced iron (DRI) residual, otherwise known as slag, were investigated. Sample slag pellets were prepared for two target compositions, CaO-SiO 2-10%MgO-5%Al 2 O 3 and CaO-SiO 2-5%MgO-10%Al 2 O 3. Two sets of experiments were conducted on the pellets: one varied basicity values between 1.83 and 0.55, and the other varied the FeO contents between 10% and 50% at constant basicity. The softening and melting process under elevated temperature was recorded using an optical softening-melting temperature measuring device and the temperature points were recorded at the four distinct shape changes of the sample pellets: initial deformation, sphere and hemisphere formation, and complete melting. The lowest softening and melting temperatures of the CaO-SiO 2-5%MgO-10%Al 2 O 3 samples occurred at a basicity of 0.55 while for the CaO-SiO2-10%MgO-5%Al 2 O 3 samples it occurred at 0.70. This corresponds to the liquidus temperatures on the CaO-SiO 2 -MgO-Al 2 O 3 quaternary phase diagram. At constant basicity, the deformation temperature of CaO-SiO 2-10%MgO-5%Al 2 O 3 samples was found to be higher than that of CaO-SiO 2-5%MgO-10%Al 2 O 3 samples. Lastly, the addition of FeO below 20% to the CaO-SiO 2 -MgO-Al 2 O 3 system significantly decreased the softening and melting temperatures of the slag samples. However, further addition of FeO beyond 20% produced inconclusive results. [doi:10.2320/matertrans.mra2008372] (Received October 14, 2008; Accepted March 17, 2009; Published May 13, 2009) Keywords: CaO-SiO 2 -MgO-Al 2 O 3, softening and melting temperatures, basicity, direct reduced iron 1. Introduction In integrated steel plants, dusts and sludges, commonly called residual materials, are inevitably generated during steel production. The residual materials are composed primarily of iron oxide and carbon, in addition to metallurgical slags, alkali oxides, and impurities such as chlorine, phosphorus, and sulfur. The high iron oxide and carbon content allows the residual materials to be converted into direct reduced iron (DRI) by carbothermic reduction. The reduction of iron oxide by carbon is performed with the material agglomerated in the form of pellets. The advantage of the agglomerate is a high reaction rate, due to a large contact area between the reactants. In normal operation, complete reduction is achieved in 10 to 20 min at temperatures between 1150 and 1250 C. DRI can be charged into the blast furnace to produce hot metal and to decrease fuel consumption. Mechanical strength of DRI is essential to enable handling during storage, transportation, and charging without crushing the material. 1) Meyer 2) identified that the strength of a DRI pellet depends upon bonds between metallic and slag phases. Gupta 3) found that adding slag-forming constituents such as bentonite can offer more strength to DRI. Takano 4) stated that higher content of a binder, like Portland cement, could be used to maintain the compressive strength of pellets after heating. The composition of slag affects the softening and melting temperatures. When the deformation temperature of slag is * 1 Graduate Student, National Cheng Kung University * 2 Corresponding author, E-mail: wshwang@mail.ncku.edu.tw low, the compressive strength of DRI is increased due to slag bonding. Due to its complex composition, DRI does not exhibit a discrete melting point, but rather softens and melts gradually over a wide temperature range. Phase diagrams only provide the liquidus temperature for specific slag compositions. Dashed lines on phase diagrams are merely predictive values. 5) Moreover, relatively little research has been conducted to investigate the softening and melting temperatures of slag in the carbothermic process. DRI pellets produced by the carbothermic process are composed of four main oxides: CaO, SiO 2, MgO, and Al 2 O 3. In industry, MgO and Al 2 O 3 typically account for approximately 15% of the material by mass. This will be reproduced in the experiment by using two combinations; one with 10 mass% of MgO and 5 mass% of Al 2 O 3, the other with 5 mass% of MgO and 10 mass% of Al 2 O 3. Moreover, since FeO is one of the main components in the residual materials the effects of varying its composition will be examined. This study investigates the effects of basicity and FeO content on the softening and melting temperatures of CaO- SiO 2 -MgO-Al 2 O 3 slag. This is the primary slag system used for the production of DRI from the residual material of steel plants. Two variations on this slag system are analyzed, using the compositions of MgO and Al 2 O 3 described above. The softening and melting temperatures of interest are the deformation, spherical, semi-spherical, and flow temperatures. 2. Experimental Method Slag pellets with basicity ( = CaO /SiO 2 ) ranging from 0.55 to 1.83 were prepared using a

Effects of Basicity and FeO Content on the Softening and Melting Temperatures of the CaO-SiO 2 -MgO-Al 2 O 3 Slag System 1449 Table 1 Compositions of the quaternary slag CaO-SiO 2 -MgO-Al 2 O 3 system. No. CaO SiO 2 MgO Al 2 O 3 1 55 30 10 5 1.83 2 50 35 10 5 1.43 3 45 40 10 5 1.13 4 40 45 10 5 0.89 5 35 50 10 5 0.70 6 30 55 10 5 0.55 7 55 30 5 10 1.83 8 50 35 5 10 1.43 9 45 40 5 10 1.13 10 40 45 5 10 0.89 11 35 50 5 10 0.70 12 30 55 5 10 0.55 mixture of high purity reagent grade CaO, SiO 2, MgO, and Al 2 O 3 powder to achieve the compositions shown in Table 1. For each of the CaO-SiO 2-10%MgO-5%Al 2 O 3 and CaO-SiO 2-5%MgO-10%Al 2 O 3 slag systems, six compositions were investigated. Each of these twelve powder mixtures was placed in a graphite crucible and pre-melted at C for one hour. After cooling to room temperature, the synthetic slag was ground and sized for subsequent investigation. FeO was added to each slag mixture in amounts ranging between 10 and 50 mass%, while the four basic oxides were maintained in the ratios described above. This produces sixty different compositions for investigation, as shown in Table 2. FeO was obtained from reagent grade Fe 2 O 3 powder with a purity of 95% or higher and poured into a low carbon steel crucible. It was heated gradually and held at C for 24 h under a 1:1CO and CO 2 atmosphere. The crucible and FeO sample were then quenched in a water bath under high argon atmosphere to prevent oxidation of the FeO sample due to contact with air. The FeO was then pulverized in an agate mortar filled with alcohol. Both the quaternary (CaO-SiO 2 -MgO-Al 2 O 3 ) and the quinary (CaO-SiO 2 -MgO-Al 2 O 3 -FeO) slag were mixed uniformly with light starch water and then pressed into cylindrical pellets (5 mm diameter 5 mm height). These pellets were made using a steel die, punched with a constant impact, and then left to dry at room temperature. An optical softening and melting temperature measuring device, as shown in Fig. 1, was employed in this study. The apparatus consists of three principal units mounted on an optical bench: a light source to illuminate the specimen, an electric furnace with an alumina tube for heating the specimen and introducing various gases, and a video camera unit for recording the shape change of the specimen. The specimen was placed on a ceramic plate and placed in the middle of the alumina tube. The tip of a B-type thermocouple was placed close to the specimen to measure the in-situ temperature. The specimen temperature was controlled by a programmable temperature control unit. The heating rate was set at 20 C/min. for heating from room temperature to 1000 C, and subsequently decreased to 5 C/min. to continue heating until 1600 C was Table 2 Compositions of the quinary slag CaO-SiO 2 -MgO-Al 2 O 3 -FeO system. No. CaO SiO 2 MgO Al 2 O 3 FeO 1-1 49.5 27.0 9.0 4.5 10 1.83 2-1 45.0 31.5 9.0 4.5 10 1.43 3-1 40.5 36.0 9.0 4.5 10 1.13 4-1 36.0 40.5 9.0 4.5 10 0.89 5-1 31.5 45.0 9.0 4.5 10 0.70 6-1 27.0 49.5 9.0 4.5 10 0.55 7-1 49.5 27.0 4.5 9.0 10 1.83 8-1 45.0 31.5 4.5 9.0 10 1.43 9-1 40.5 36.0 4.5 9.0 10 1.13 10-1 36.0 40.5 4.5 9.0 10 0.89 11-1 31.5 45.0 4.5 9.0 10 0.70 12-1 27.0 49.5 4.5 9.0 10 0.55 1-2 44.0 24.0 8.0 4.0 20 1.83 2-2 40.0 28.0 8.0 4.0 20 1.43 3-2 36.0 32.0 8.0 4.0 20 1.13 4-2 32.0 36.0 8.0 4.0 20 0.89 5-2 28.0 40.0 8.0 4.0 20 0.70 6-2 24.0 44.0 8.0 4.0 20 0.55 7-2 44.0 24.0 4.0 8.0 20 1.83 8-2 40.0 28.0 4.0 8.0 20 1.43 9-2 36.0 32.0 4.0 8.0 20 1.13 10-2 32.0 36.0 4.0 8.0 20 0.89 11-2 28.0 40.0 4.0 8.0 20 0.70 12-2 24.0 44.0 4.0 8.0 20 0.55 1-3 38.5 21.0 7.0 3.5 30 1.83 2-3 35.0 24.5 7.0 3.5 30 1.43 3-3 31.5 28.0 7.0 3.5 30 1.13 4-3 28.0 31.5 7.0 3.5 30 0.89 5-3 24.5 35.0 7.0 3.5 30 0.70 6-3 21.0 38.5 7.0 3.5 30 0.55 7-3 38.5 21.0 3.5 7.0 30 1.83 8-3 35.0 24.5 3.5 7.0 30 1.43 9-3 31.5 28.0 3.5 7.0 30 1.13 10-3 28.0 31.5 3.5 7.0 30 0.89 11-3 24.5 35.0 3.5 7.0 30 0.70 12-3 21.0 38.5 3.5 7.0 30 0.55 1-4 33.0 18.0 6.0 3.0 40 1.83 2-4 30.0 21.0 6.0 3.0 40 1.43 3-4 27.0 24.0 6.0 3.0 40 1.13 4-4 24.0 27.0 6.0 3.0 40 0.89 5-4 21.0 30.0 6.0 3.0 40 0.70 6-4 18.0 33.0 6.0 3.0 40 0.55 7-4 33.0 18.0 3.0 6.0 40 1.83 8-4 30.0 21.0 3.0 6.0 40 1.43 9-4 27.0 24.0 3.0 6.0 40 1.13 10-4 24.0 27.0 3.0 6.0 40 0.89 11-4 21.0 30.0 3.0 6.0 40 0.70 12-4 18.0 33.0 3.0 6.0 40 0.55 1-5 27.5 15.0 5.0 2.5 50 1.83 2-5 25.0 17.5 5.0 2.5 50 1.43 3-5 22.5 20.0 5.0 2.5 50 1.13 4-5 20.0 22.5 5.0 2.5 50 0.89 5-5 17.5 25.0 5.0 2.5 50 0.70 6-5 15.0 27.5 5.0 2.5 50 0.55 7-5 27.5 15.0 2.5 5.0 50 1.83 8-5 25.0 17.5 2.5 5.0 50 1.43 9-5 22.5 20.0 2.5 5.0 50 1.13 10-5 20.0 22.5 2.5 5.0 50 0.89 11-5 17.5 25.0 2.5 5.0 50 0.70 12-5 15.0 27.5 2.5 5.0 50 0.55

H.-C. Chuang, W.-S. Hwang and S.-H. Liu video system gas inlet stainless steel flange with water cooling jacket high purity alumina tube electric furnace sample and ceramic plate gas outlet power Temperature controller thermocouple light source 1600 1550 Fig. 1 A schematic diagram of the optical softening-melting temperature measuring device employed in this study. 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Fig. 3 Relationship between and softening and melting temperatures for the CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system. 3. Results and Discussion (a) (c) Fig. 2 A typical series of photographs showing the evolution of a slag pellet during heating, which corresponds to (a) deformation temperature, (b) spherical temperature, (c) semi-spherical temperature, and (d) flow temperature. achieved. The atmosphere around the specimen was controlled by a flow of argon of one normal liter per minute. In addition, each slag system is repeatedly tested to ensure accuracy. In this investigation, the shape change of the slag pellet as it is heated is used as an indicator for assessing the physical changes occurring in the material (softening and melting). The different stages of the process were recorded photographically or by means of a video camera. Figure 2 shows a typical series of photographs that illustrate the stages of shape change, defined in accordance with German standard DIN 51730. 6) These stages correspond to the deformation, spherical, semi-spherical, and flow temperatures of the sample. At the deformation temperature the sample shows the first signs of softening; the rounding of the edges is complete and the sample starts to fill out the gas volume between the particles. The spherical and semi-spherical temperatures correspond to the stages where the cylinder acquires approximate spherical and semi-spherical forms respectively. At the flow temperature, the solid sample has been melted to the liquid state. (b) (d) 3.1 Effects of basicity on the softening and melting temperatures of the quaternary slag CaO-SiO 2 - MgO-Al 2 O 3 system Figure 3 shows the relationship between and softening and melting temperatures in the quaternary slag CaO-SiO 2-10%MgO-5%Al 2 O 3 system. It shows that in the CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system the trends exhibited by the deformation, spherical, semi-spherical, and flow temperatures with changing are similar. The deformation, spherical, semi-spherical, and flow temperatures decrease substantially as of the quaternary slag is reduced from 1.83 to 0.70. Minimum deformation, spherical, semi-spherical, and flow temperatures are recorded when is 0.70. However, when keeps decreasing to 0.55, the softening and melting temperatures inversely rise. Figure 4 is the phase diagram of the CaO-SiO 2 -MgO with 5% Al 2 O 3 slag system. 7) When MgO content is fixed at 10%, the liquidus temperature of the slag gets lower as is reduced from 1.83 to 0.70. This trend corresponded closely with the softening and melting temperatures in Fig. 3. In addition, the liquidus temperatures when values are 0.70 and 0.55 cannot be clearly differentiated in the phase diagram. The relationship of and softening and melting temperatures in the quaternary slag CaO-SiO 2-5%MgO-10%Al 2 O 3 system is shown in Fig. 5. It shows that in the CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system the trends exhibited by the deformation, spherical, semi-spherical, and flow temperatures with changing are also similar. The deformation, spherical, semi-spherical, and flow temperatures decrease substantially as of the quaternary slag is reduced from 1.83 to 0.55. Minimum deformation, spherical, semi-spherical, and flow temperatures are recorded when is 0.55. Figure 6 is the phase diagram of the CaO-SiO 2 -MgO with 10% Al 2 O 3 slag system. 7) When MgO content is fixed at 5%, the liquidus temperatures of the slag gets lower as is reduced from 1.83 to 0.55. When is 0.55, the liquidus temperature has the lowest value. This trend corresponds very well with the softening and melting temperatures in Fig. 5. Figure 7 shows the relationship between and deformation temperature for the CaO-SiO 2-10%MgO-5%Al 2 O 3 and

Effects of Basicity and FeO Content on the Softening and Melting Temperatures of the CaO-SiO 2 -MgO-Al 2 O 3 Slag System 1451 = 0.55 = 0.70 = 0.89 = 1.13 = 1.43 = 1.83 Fig. 4 Phase diagram of CaO-SiO 2 -MgO with 5% Al 2 O 3. 7) 1250 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Fig. 5 Relationship between and softening and melting temperatures for the CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system. CaO-SiO 2-5%MgO-10%Al 2 O 3 slag systems. The deformation temperature of the CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system can be seen to be consistently higher than that of the CaO-SiO 2-5%MgO-10%Al 2 O 3 system. For the quaternary slag CaO-SiO 2-10%MgO-5%Al 2 O 3 system the deformation temperature substantially decreases as decreases, reaching a minimum of 1303 C when is 0.70. Likewise, the deformation temperature of the quaternary slag CaO-SiO 2-5%MgO-10%Al 2 O 3 system substantially decreases as decreases, reaching a minimum of 1255 C when is 0.55. 3.2 Effects of FeO on the softening and melting temperatures of the quaternary slag CaO-SiO 2 -MgO-Al 2 O 3 system Figure 8 shows the relationship between FeO content and softening and melting temperatures in CaO-SiO 2-10%MgO- 5%Al 2 O 3 slag system when is equal to 1.83. From this it can be seen that increasing the FeO content of the quaternary slag CaO-SiO 2-10%MgO-5%Al 2 O 3 system causes a decrease in the deformation, spherical, semi-spherical, and flow temperatures of the slag. The initial addition of 10 mass% FeO is found to lower the softening and melting temperatures of the slag most drastically. The softening and melting temperatures of the slag continue to decrease as the addition of FeO is increased to 50 mass%. Figures 9 through 13 show the relationship between FeO content and softening and melting temperatures in the CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system for equal to 1.43, 1.13, 0.89, 0.70, and 0.55, respectively. Again, the addition of the first 10 mass% of FeO most drastically lowers the softening and melting temperatures of the slag. The softening and melting temperatures continue to decrease as the FeO content is increased to 50 mass%, with the exception in some cases of a slight increase in these values at high FeO. The effect of FeO content on the deformation temperature of the CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system for different values of is shown in Fig. 14. From the figure, it can be seen that the deformation temperature has no clear relations with for any given FeO content ranging from 0 to 50%. However, the deformation temperature is seen to decrease as FeO content increases for all the values ranging from 0.55 to 1.83 except for the value of 0.89. It can be clearly seen that the addition of 20 mass% of FeO to CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system with a value of 0.89 yields the minimal deformation temperature, of approximately 1175 C. This peculiar phenomenon for the value of 0.89 deserves some attention and a possible explanation is proposed as the following. As described in Section 2, the samples of the CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system were pre-melted at C for one hour, then cooled and ground into powder. The free oxides in the samples have in fact formed minerals after the pre-melting treatment. Generally, the softening and melting behaviors of the minerals are very complicated. According to Fig. 4, the formed minerals are shown in Table 3 and their

1452 H.-C. Chuang, W.-S. Hwang and S.-H. Liu = 0.55 = 0.70 = 0.89 = 1.13 = 1.43 = 1.83 Fig. 6 Phase diagram of CaO-SiO 2 -MgO with 10% Al 2 O 3. 7) Deformation CaO-SiO 2-10 % MgO-5 % Al 2 O 3 CaO-SiO 2-5 % MgO-10 % Al 2 O 3 1480 1460 1440 1420 1250 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 = 1.43 FeO content Fig. 7 Relationship between and deformation temperature for the CaO- SiO 2-10%MgO-5%Al 2 O 3 and CaO-SiO 2-5%MgO-10%Al 2 O 3 slag systems. Fig. 9 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system as equals to 1.43. 1600 1550 1460 1440 1420 = 1.83 FeO content = 1.13 FeO content Fig. 8 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system as equals to 1.83. Fig. 10 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system as equals to 1.13.

Effects of Basicity and FeO Content on the Softening and Melting Temperatures of the CaO-SiO 2 -MgO-Al 2 O 3 Slag System 1453 Deformation 1250 =1.83 =1.43 =1.13 =0.89 =0.70 =0.55 = 0.89 FeO content 1150 FeO content Fig. 11 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system as equals to 0.89. Fig. 14 Effects of FeO content on the deformation temperature for CaO- SiO 2-10%MgO-5%Al 2 O 3 slag system. = 0.70 FeO content Fig. 12 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system as equals to 0.70. = 0.55 FeO content Fig. 13 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system as equals to 0.55. formulas are listed in Table 4. The information for the phase diagram of CaO-SiO 2 -MgO-Al 2 O 3 -FeO is absent so the limited reference is utilized to explain the phenomenon. Firstly eliminating FeO effect, the primary mineral phases such as pyroxene (CaOMgO2SiO 2 ), wollastonite (- CaOSiO 2 ), and melilite have liquidus temperatures lower Table 3 study. Primary phase of CaO-SiO 2 -MgO-Al 2 O 3 after pre-melting in this CaO-SiO 2-10%MgO-5%Al 2 O 3 CaO-SiO 2-5%MgO-10%Al 2 O 3 Primary phase Primary phase 0.55 pyroxene 0.55 wollastonite 0.70 wollastonite 0.70 pseudowollastonite 0.89 melilite 0.89 pseudowollastonite 1.13 merwinite 1.13 melilite 1.43 2CaOSiO 2 1.43 2CaOSiO 2 1.83 2CaOSiO 2 1.83 2CaOSiO 2 pyroxene Mineral Table 4 Formula of the various minerals. Formula MgOSiO 2 (clinoenstatite) CaOMgO2SiO 2 (diopside) wollastonite -CaOMgOSiO 2 melilite 2CaOMgO2SiO 2 (akermanite) 2CaOAl 2 O 3 SiO 2 (gehlenite) merwinite 3CaOMgO2SiO 2 pseudowollastonite -CaOSiO 2 than C as can be seen in Fig. 4. Melilite 8,9) is a solid solution of akermanite (2CaOMgO2SiO 2 ) and gehlenite (2CaOAl 2 O 3 SiO 2 ). In this case, the content percentage of akermanite is higher than that of gehlenite because MgO content is higher than Al 2 O 3. Subsequently, the effect of FeO is taken into consideration. Figure 15 shows the phase diagram of CaO-SiO 2 -FeO system. 10) It shows that the addition of FeO lowers the liquidus temperature for all the values ranging from 1.83 to 0.55. Adding 10 mass% FeO significantly reduces the liquidus temperature of CaO-SiO 2 -FeO. Further increasing the FeO content to 40 mass%, the liquidus temperature is reduced even further. Because pyroxene (CaOMgO2SiO 2 ) and melilite (2CaOMgO2SiO 2 ) both consist of MgO. Also, the mass percentage of MgO is higher than that of Al 2 O 3 in FeO- CaO-SiO 2-10%MgO-5%Al 2 O 3 system. Thus, the quaternary system CaO-SiO 2 -MgO-FeO is considered as can be seen in

1454 H.-C. Chuang, W.-S. Hwang and S.-H. Liu = 0.55 = 0.70 = 0.89 = 1.13 = 1.43 = 1.83 Fig. 15 Phase Diagram of CaO-SiO 2 -FeO n with various FeO contents and values depicted on the diagram. 10) Fig. 16 Diagram representing the CaO SiO 2 FeO MgO system. W = wollastonite, pseudowollastonite (CaOSiO 2 ); Mo = monticellite (CaOMgOSiO 2 ); Fe-Mo = iron-monticellite (CaOFeOSiO 2 ); Ak = akermanite (2CaOMgO2SiO 2 ); Fe-Ak = iron-akermanite (2CaOFeO 2SiO 2 ). 11) Fig. 16. 11) Figure 17 shows an enlargement of the portion CaOSiO 2 CaOMgOSiO 2 FeO within the CaO-SiO 2 - MgO-FeO system. 11) The liquidus temperature of mineral phase declines with the increase of FeO between 0 and 50%. As for the melilite, its minimum liquidus temperature is 1255 C as FeO content is around 24%. Furthermore, the phase area of pseudowollastonite (-CaOSiO 2 ) and olivine trend to low FeO side due to MgO addition. Reconsidering the phase diagram of CaO-SiO 2 -FeO in Fig. 15, it is suspected that the primary phase area of Wollastonite also Fig. 17 Phase diagram of CaOSiO 2 CaOMgOSiO 2 FeO. 11) tends to low FeO side and that of Rankinite tends to high SiO 2 side. Due to the above resultant effects, the minimum temperature of slag, which is the eutectic point at the intersection of Wollastonite and Rankinite, appears at a composition around 20% FeO content. Figure 17 shows the relationship between FeO content and softening and melting temperatures in CaO-SiO 2-5%MgO- 10%Al 2 O 3 slag system when is equal to 1.83. The initial addition of 10 mass% FeO is found to drastically lower the softening and melting temperatures of the slag. The softening and melting temperatures of the slag continue to decrease as the addition of FeO is increased to 50 mass%. Figures 18

Effects of Basicity and FeO Content on the Softening and Melting Temperatures of the CaO-SiO 2 -MgO-Al 2 O 3 Slag System 1455 1520 1480 1460 1440 1420 = 1.83 FeO content = 0.89 FeO content Fig. 18 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system as equals to 1.83. Fig. 21 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system as equals to 0.89. 1420 = 1.43 FeO content 1140 1120 = 0.70 FeO content Fig. 19 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system as equals to 1.43. Fig. 22 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system as equals to 0.70. melting temperatures of the slag. The softening and melting temperatures continue to decrease as the FeO content is increased to 50 mass%, with the exception in some cases of a slight increase in these values at high FeO. The trend exhibited in the softening and melting temperatures of the slag is not consistent for lower than 0.89, as shown in Figs. 20 22. The effect of FeO content on the deformation temperature of the CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system for different values of is shown in Fig. 23. It shows that as decreases, the deformation temperature of the CaO-SiO 2-5%MgO-10%Al 2 O 3 -FeO slag system decreases. = 1.13 FeO content 4. Conclusions Fig. 20 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system as equals to 1.13. through 22 show the relationship between FeO content and softening and melting temperatures in the CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system for equal to 1.43, 1.13, 0.89, 0.70, and 0.55, respectively. The addition of the first 10 mass% of FeO most drastically lowers the softening and The effect of and FeO content on softening and melting temperatures of CaO-SiO 2-10%MgO-5%Al 2 O 3 and CaO- SiO 2-5%MgO-10%Al 2 O 3 slag systems were investigated in this study. The following conclusions can be drawn: (1) The softening and melting temperatures of CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system were found to reach a minimum when was 0.55. However, for the CaO- SiO 2-10%MgO-5%Al 2 O 3 slag system the minimum occurred when was 0.70.

1456 H.-C. Chuang, W.-S. Hwang and S.-H. Liu Deformation 1140 1120 1100 1080 1250 1150 1100 = 0.55 FeO content FeO content Fig. 23 Relationship between FeO content and softening and melting temperatures for CaO-SiO 2-5%MgO-10%Al 2 O 3 slag system as equals to 0.55. =1.83 =1.43 =1.13 =0.89 =0.70 =0.55 Fig. 24 Effects of FeO content on the deformation temperature for CaO- SiO 2-5%MgO-10%Al 2 O 3 slag system. (2) The deformation temperature of the CaO-SiO 2-10%MgO-5%Al 2 O 3 system was found to be higher than that of the CaO-SiO 2-5%MgO-10%Al 2 O 3 system for a given value of. (3) The trends in the softening and melting temperatures of the slag systems closely correspond to the changes in liquidus temperature predicted by the phase diagrams of the quaternary slag CaO-SiO 2 -MgO-5%Al 2 O 3 and CaO-SiO 2 -MgO-10%Al 2 O 3 systems. (4) The addition of FeO up to 20 mass% to the CaO-SiO 2 - MgO-Al 2 O 3 system significantly decreases the softening and melting temperatures of the slag system. However, further addition of FeO does not produce consistent results. (5) The CaO-SiO 2-10%MgO-5%Al 2 O 3 slag system with a value of 0.89 and the addition of 20 mass% of FeO produced the minimum deformation temperature of approximately 1175 C. Acknowledgements The authors are grateful to the supports of China Steel Corporation for this study. The assistance from Mr. Ching- Ho Chen is also greatly appreciated. REFERENCES 1) B. Anameric and S. K. Kawatra: Miner. Process. Extr. Metall. Rev. 28 (2007) 59 116. 2) K. Meyer: Pelletizing of Iron Ores, (Springer-Verlag, Berlin, 1980) p. 292. 3) R. C. Gupta and J. P. Gautam: ISIJ Int. 43 (2003) 1913 1918. 4) C. Takano and M. B. Mourao: ISIJ Int. 41 (2001) S22 S26. 5) F. Dahl, J. Brandberg and D. Sichen: ISIJ Int. 46 (2006) 614 616. 6) A. R. Boccaccini and B. Hamann: J. Mater. Sci. 34 (1999) 5419 5436. 7) V. D. Eisenhüttenleute: Slag Atlas, (Verlag Stahleisen GmbH, Düsseldorf, 1981) p. 81. 8) E. F. Osborn, R. C. DeVries, K. H. Gee and H. M. Kraner: J. Met. 6 (1954) 33 45. 9) A. R. Lee: Blastfurnace and Steel Slag, (John Wiley & Sons, New York, 1974) p. 26. 10) V. D. Eisenhüttenleute: Slag Atlas, (Verlag Stahleisen GmbH, Düsseldorf, 1981) p. 68. 11) J. F. Schairer and E. F. Osborn: J. Am. Ceram. Soc. 33 (1950) 160 167.