Molten Metal-Slag-Refractory Reactions During Converting Process

Transcription

1 International Journal of Engineering & Technology IJET-IJENS Vol: 10 No: Molten Metal-Slag-Refractory Reactions During Converting Process Hady Efendy 1, Mochamad Safarudin 1, Haeryip Sihombing. 2 1 Fakulti Kejuruteraan Mekanikal, Universiti Teknikal Malaysia (UTeM) Malaka, Malaysia 2 Fakulti Kejuruteraan Pembuatan, Universiti Teknikal Malaysia (UTeM) Malaka, Malaysia Correspondance : hady@utem.edu.my Abstract.- Magnesia carbon (MgO-C) refractories are widely used in converter because of their favorable properties such as low wetting by corrosive steelmaking slags chemical compatibility with basic slags and better thermal properties. The molten slag is in contact with the refractory during the converting process, where temperatures >1450 C are common. Local convection currents develop near the slag refractory nickel mette air intersection that leads to small-scale circulating flows that increase dissolution. In this report the effects of dissolution of MgO-C refractory samples into nickel matte and Fe 2 O 3 -SiO 2 -MgO slag were observation by optical microscope and SEM/EDS. The experimental results show that the rate of dissolution of MgO-C refractory materials increased with the temperature and immersion time. This supports the assumption that the diffusion of magnesium through the slag boundary layer formed around the refractory samples would be the rate-determining step. The formation of a thin oxide layer at the interface is due the reaction between magnesium vapor and the CO generated by the reaction MgO and C in the refractory walls. The oxide inclusions formed in the matte have been shown mainly to consist of MgO, Fe 2 O 3 and a mixture of them. The rate of corrosion increased with temperature and immersion time and decreased when the slag was nearly saturated with MgO. The experimental results confirm the assumption that the diffusion of magnesium oxide through the slag phase boundary layer controls the corrosion process. The corrosion mechanism seems to be the dissolution of elements in the refractory materials into the slag, followed by penetration into the pores and grain boundaries. Finally, grains are loosened from the refractory into the slag. Keywords: re-oxidation, converter, inclusions, refractory, molten slag, corrosion rate, corrosion mechanisms. I. INTRODUCTION The converter removes the remaining silica, iron and iron oxide, which are referred to collectively as slag from the nickel matte product. This is achieved by heating the molten matte and selectively oxidizing the iron by blowing air through the molten liquid. The oxidation of the iron is an exothermic reaction and release heat into the converter. Silica flux is added which melts and together with the iron oxide forms converter slag. The addition of scrap used to help control the temperature of the converter content. The converters are batch process. Furnace nickel matte is put into the converter followed by a quantity of flux material and scrap material from which nickel is to be recovered. The converter are then blown (air is blow into the molten slag-matte batch through the tuyere system). At the completion of each blow the slag is poured out of the converters into ladles for dumping. Then more furnace matte, scrap and flux material is added, and the blowing process is repeated. When the proportion of nickel in the converter has risen to the required level, the final high nickel converter slag is poured off and finally the converter matte is poured off and sent to the granulation system for finally processing and packaging.

2 International Journal of Engineering & Technology IJET-IJENS Vol: 10 No: The main chemical reactions in the converters are: 2FeS + 2O 2 2FeO + SO 2 (1) FeO + xsio 2 FeO (SiO 2 ) x (2) During this procedure a coating is formed on the refractory material. This coating is made of slag coming from pellet s dust and impurities. As the thickness of the reaction layer increase during time, the weight of the layer involve a chipping off of big peaces of slag causing damage to the refractory (peaces of brick crack and stay fixed to the slag blocks). Due to this phenomenon the converter has to be stopped every year for a maintenance period. The converter is then cleaned and the damaged bricks are replaced. The replacement of the bricks is an expensive operation and it involves the complete stop of matte production that induces a big overall cost. The composition of converters matte is given in the following table 1: TABLE 1. THE COMPOSITION OF FURNACE NICKEL MATTE Ni Co Fe SiO2 MgO S Materials involved This part will present the materials that have been used in this study and give their main properties: Bricks, Slag, SiO 2 and Matte (The composition was given in table 1). Bricks The Bricks used in the converters are made of MgO-C refractory materials. Their composition is given in table 2. The main proprieties needed for refractory materials are their high heat resistance, low thermal conductivity, mechanical resistance, and thermal stresses resistance, resistance to corrosion, resistance to erosion, liquid and gas permeability [1]. TABLE 2. THE COMPOSITION OF MGO-C BRICK MgO Al 2 O 3 CaO SiO 2 Cr 2 O 3 Fe 2 O 3 C Slag The slag, which forms on the walls of the converter, is mainly constituted of disintegrated furnace matte, silica and impurities. During the converting process this FeS is transformed into FeO. FeO is one of the stable oxide forms of iron that is finding in nature (the other one is magnetite). It got its name from a Greek word meaning blood-like, because of its red color. The slag is then constituted of FeO, impurities coming from the brick, and SiO 2 introduced by the blowing. Its composition is given in the following table 3: TABLE 3. THE COMPOSITION OF THE CONVERTER SLAG Ni Co Fe SiO2 MgO We can see in this chemical analysis that the main impurities in the Slag after FeO are SiO 2 and MgO. SiO 2 is forming a glassy phase that bounds the dust together and permits the slag to enter into the pores of the refractory materials and be fixed. II. SAMPLE SOURCE AND PREPARATION Magnesia-graphite Converter slag bricks were recovered from brick piles after lining tear-out. The compositions of these bricks were MgO-C and slag coated samples were specifically chosen to increase the odds of retaining slag line reaction products. A typical post-mortem MgO- C brick specimen from a converter slag line was about 20 cm long and included a 10 to 25 mm slag coating. Magnesia grains were generally translucent and clear white at the slag-brick interfaces and at the bottom (cold face) of the bricks and blackened in the interior of the bricks. The cold zones of the bricks were also loosely held and disintegrated easily. The bricks were sectioned perpendicular to the hot face, vacuum impregnated with a low viscosity resin and cured at 70 o C. Both polished and polished thin sections were made from the impregnated specimens. The final polishing was completed with a vibrating polisher with 1 to 0.5 micron diamond paste and lapping oil. The papers explained the mechanism responsible for the penetration of slag into the

3 International Journal of Engineering & Technology IJET-IJENS Vol: 10 No: brick and the different parameters that may influence the degradation of the refractory. Using SEM and optical microscopy the influence of slag penetration on surface brick has been studied and the resistance of the bricks after using in the converting process. Than concluded that a combination between diffusion and infiltration of the slag was responsible of the slag attack. III. OBSERVATION AND RESULTS Infiltration of iron slag into refractory bricks Because pores exist in a refractory, liquids penetrate into refractory through the open pore in contact with liquid. The mechanism of the penetration differs a little according to the lining orientation of the refractory or the pressure applying on it, but the main driving force of penetration is the suction of liquid like molten slag due to capillarity. Figure 1 showing a depth penetration of slag into refractory brick. Wet ability between a refractory and a liquid like molten slag is an important factor influencing penetration. Also, surface energies of refractory materials, surface tension of molten slag, as well as interfacial energy between the solid and liquid are factors related to penetration. Basic refractories such as MgO which are easily penetrated may be impregnated with tar or pitch to fill the pores, so that residual carbon in the pores prevents wetting by molten slag. Figure 1. Depth Penetration Slag into Refractory Brick Slag coating In contact with slag (Figure 2), a dense spinell layer is formed in a first stage. This layer becomes enriched very quickly in MgO on the refractory side. It is completely transformed into magnesia after several minutes. Then the thickness does not increase in time and remains about 150 mm. The decarburized zone has a much smaller thickness than for the nondeoxidized grade and it does not form a continuous layer. Metal infiltrations in the refractory have been observed, and they increase with time. After application in steel making process, a composition gradient of the inclusionary cluster is observed with magnesia on the refractory side. Fig. 2. Photograpf of The Slag- MgO-C Refractory Interface At the working surface, the refractory is eroded by molten FeO, while the back of the refractory is decarburized by air. When the working surface reached the carburized part, the corrosion rate abruptly increases due to spalling. The slag-refractory interface is characterized by presence of metallic Fe beads and crystallization spinel (MgAl 2 O 4 ) crystal [2]. Metallic iron beads at the interface are always associated with graphite in the sample and often from oxidized magnetite and hematite rims. Such association indicates reduction of the FeO component of the slag by graphite to from metallic Fe and CO gas at the hot face of refractory. The following reaction describes the observed behavior [3], FeO(l) + C(s) => Fe(s) + CO(g) (3) Representative microstructures of the refractoryslag interface for a post-mortem MgO-C are shown in Figure 2. Confirmation of dense layer formation on the surface of the refractories was

4 International Journal of Engineering & Technology IJET-IJENS Vol: 10 No: achieved by observation of the cross section of the specimen after converter process (Figure 3 and Figure 4). Slag Interface Refractory MgO-C Figure 3. Cross Section View of The MgO-C Refractory Interface Blocky and euhedral spinel crystals at the interface exhibit and form an irregular and often discontinuous chain-like structure. IV. DISCUSSION The wear mechanism of refractory materials by slag is complex phenomenon. The experimental results indicate that apart from chemical attack of the slag an the MgO-C refractory brick, penetration of the slag cause serious direct loss of the MgO-C refractory brick. The dissolution rate of MgO-C refractory brick depends upon the some factor, such as temperature converter process and viscosity slag. The investigation of the sample after converter process show, that there has been a formation of inclusion on the molten metal. The inclusions found in the molten metal were examined using SEM/EDX. The result showed that the inclusions contained MgO. The formation of spinel is practically a very significant aspect of the reaction between MgO-C refractory and slag. As the magnesium gas diffuses into the slag the following reaction is taking place: MgO.Al 2 O 3 (s) => Mg (s) + 2Al (s) + 4O (5) Fig. 4. Cross Section View Detail of The MgO- C Refractory Interface Interface The slag-brick interface is characterized by the presence of metallic Fe beads and crystallization of euhedral spinel (MgO.Al 2 O 3 ) crystals. Metallic iron beads at the interface are always associated with graphite in the brick and often form oxidized magnetite and hematite rims. Such association indicates reduction of the FeO component of the slag by graphite to form metallic Fe and CO gas at the hot face of the brick. The following reaction describes the observed behavior [3], FeO(l) (in slag) + C(s) => Fe(s) + CO(g) (4) As the initial alloys did not contain any magnesium, the presence of MgO in the inclusions should indicate a result from the contamination by the refractory/slag reaction. The reaction MgO (s) + C (s) ==> Mg (g) + CO (g) proceeds to the right at higher temperatures and Mg(g) diffuses toward the free surface of the sample where it encounter a higher PO 2 [4]. Thereafter, magnesium is oxidized to MgO, were it condenses and forms a MgO layer. At the same time the CO (g) formed during MgO (s) reduction by carbon will diffuses to the interface where it will react with the molten slag forming MgO according to the following reaction [5]: Mg (s) + CO (g) ===> MgO (s) + C (s) (6) The reaction occurs immediately after the reactive CO gas come into contact with the surface of the slag. As a result, a thin oxide film at MgO is formed at the interface. The formation of a surface layer will inhibit any

5 International Journal of Engineering & Technology IJET-IJENS Vol: 10 No: further oxidation by CO, by retarding the diffusion of carbon and oxygen a cross the layer. The dissolution process in the refractory material is supported by optical microscope and SEM investigations of the samples. The slag penetrated the refractory material in pores and crack. It is possible to observe that the slag phase has a concentration gradient at the boundary layer between slag/refractory. The corrosion of oxide often occurs not by dissolution or evaporation of the oxide, but by the penetration of the solid by some all the elements from the fluid slag [6]. The liquid phase may be pulled into the open porosity of the solid by capillary forces, and species from the fluid will diffuse both down the grain boundaries and into the bulk of the solid. The higher wetting angle makes it more difficult for the slags penetrate pores and crack in the refractory. This is not the only think that affects the infiltrating depth. The infiltrating depth is also affected by the temperature gradient in the brick. The temperature gradients will cause the viscosity to increase and then the infiltration depth will decrease. V. CONCLUSIONS During the production of nickel matte the degree of oxide inclusions partly depends on the reaction of the melt with the converter lining and the pouring system. The refractory material may be eroded by the molten steel and slag as well as corroded through chemical reactions with the slag and molten steel and the deoxidation products. In this report of dissolution of MgO-C refractory into CaO-Al 2 O 3 -SiO 2 -MgO slag were examined after converting process. The results show that the infiltration of slag into MgO-C refractory and dissolution of MgO-C refractory on the molten slag. This supports the assumption that the diffusion of magnesium through the slag boundary layer formed around the refractory samples would be the rate-determining step. The formation of a thin oxide layer at the interface is due the reaction between magnesium vapor and the CO generated by the reaction MgO and C in the refractory walls. The oxide inclusions formed in the steel have been shown mainly to consist of MgO, Al 2 O 3 and a mixture of them. REFERENCES [1] N. P. Cheremisinoff, Handbook of Ceramics and Composites, CRC Press, 1990, ISBN [2] Chen Y., Brooks G., Nightingale S., Slag Line Dissolution of MgO Refractory, Canadian Metalurgical, Vol.44, pp , 2005 [3] Camelli S., Labadie M. Analysisi of Wear Mechanis of MgO-C Slag Line Bricks For Steel Ladle, International Feuerfst- Kolloqium, Instituto Argentono de Siderurgia, San Nicolas, Argentina, 2006 [4] Watanabe A., Takahashi H., and Nakatami F., Mechanism of Dense Magnesia Layer Formation near Surface of Magnesia- Carbon Brick, J.Am.Ceram.Soc.69, pp , [5] Poirier J., Thillou B., Guiban M.A., and G. Provost, Mechanism and Countermeasures of Alumina Clogging in Submerged Nozzles, 78 th Steelmaking Conf. Proc., Nasville, USA, Vol.78, pp , 1995 [6] Cooper A.R., Kinetic of Refractory Corrosion, Ceram.Eng. and Sci. Proc., No.2, pp , 1982