RECYCLING PRACTICES OF SPENT MgO-C REFRACTORIES

The recycling options of spent MgO-C refractories from an electrical arc furnace (EAF) have been evaluated. The economic, quality of spent refractories and products made from it, the ease of implementation of a recycling practice and the interest of steel melt shops were considered. It was decided that the best option of most EAF shops would be to recycle spent MgO-C refractory as a foaming slag conditioner because of their MgO content. Crushed MgO-C spent refractories can be reused directly back into an EAF without complex and costly beneficiation. Even though this practice is simple, it is critical to know the optimum amount of MgO in the slag to achieve the best foaming quality. A computer model was designed to find the optimum MgO amount. This modeling also helps the melt shop extend refractory service life, increase the energy efficiency, increase productivity, and decrease the amount of slag. Issues related to the refractory recycling will be discussed. 70 Kyei-Sing Kwong and James P. Bennett Vol. 1, No. 2 Introduction Refractories are made from abundant industrial minerals such as SiO2, Al2O3, MgO, dolomite and chromite for usage as insulation or molten metal/slag containers. These refractories have been engineered to meet different challenging specifications such as high temperature resistance, long service life, creep or spalling resistance, and economic savings. Most refractories are consumable materials which may last from few weeks to several years depending on service conditions and material performance. In general, spent refractories do not have much value after service, creating disposal issues. In some locations in the USA, companies may pay high fees to landfill spent refractories. Over half of refractory products are used by steel producers. These steel producers are both integrated mills (BOF) and mini mills (EAF). There are about 80 EAF companies with 193 EAF furnaces in the United States producing iron steel (1). Their total annual steel production is about equal to the integrated mills (BOF). EAF companies have a wide variety of interests recycling their spent refractories. Some EAF companies are located in developing urban areas and do not have space to landfill spent refractories or they may face high disposal costs. A few companies see this as part of their quest to be a “green” company or to gain a competitive advantage of business. The processes to recycle spent refractories may be simple, however, engineering the spent refractory applications may require a strong technical background, a team approach and company commitments. This paper will emphasize the importance of a technical background and a team approach to recycling. MgO+C and MgO are widely used refractory materials in the EAF. The low value of the spent refractories makes them difficult to recycle. Only simple beneficiation of spent refractories is justified so they may compete with abundant natural materials. Two applications were selected at the Albany Research Center (ARC) to reuse these spent refractory materials. The first one is to reuse spent refractories as repair materials. However, the quality, availability, consistency, and the transportation cost are some of the major concerns for the application of spent refractories as repair materials. The second application is to reuse spent refractories as an EAF foamy slag additive. Foamy slags are widely practiced in electric arc furnaces and provide for electrical energy savings, longer refractory service life, productivity improvements, lower noise level, and lower nitrogen pick up in the steel. Foamy slags shield the electrical arcs, preventing radiation energy loss, eliminating arc flares, saving overall energy and extending refractory service life. It has been reported (2) that a foamy slag practice saves 3-10% in energy and decreases 25-63% of refractory consumption. The reuse of spent refractories as a slag additive will not change any slag quantity and will save recycling associated costs as well as operating costs. Foamy slags are obtained by the reaction of FeO with C to generate gas bubbles. “Optimum” slag chemistry is required to sustain these gas bubbles. If the slag viscosity is too Vol. 1, No. 2 RECYCLING PRACTICES OF SPENT MgO-C REFRACTORIES 71 thin, the gas bubbles can not be sustained. If the slag viscosity is too thick, it is difficult to form a foam. The “optimum” slag is a molten saturated in MgO to the point of having suspended second phase particles (MgO C FeO magnesium wustite (MW)) at operating temperature (3). The goal of this program is to reuse much of the basic MgO containing refractories as a slag additive and satisfy the MgO requirement. To melt one ton steel takes a total energy input of 560 to 680 KWh for most modern EAF operations (4), making EAF steelmaking an energy consumption industry. The Office of Industrial Technology (OIT) , USDOE funded this program along with the Steel Manufacturer Association (SMA). Its goals are to save energy and eliminate landfill disposal by reusing spent refractories as slag conditioners for foamy slag. In order to implement the recycling of spent refractories as a slag additive, the spent refractories have been characterized and a model to predict MgO saturated EAF slag chemistry has been developed. Based on the model, the amount of MgO additive can be predicted. Several issues in implementing reuse of spent refractories as slag additives will be also discussed in this paper. The characterization of MgO+C spent refractories MgO refractories are generally used for EAF linings because MgO has a high melting point, slag compatibility, and good service life. Carbon was added to the refractories to enhance their non-wetting properties. Some antioxidants such as aluminum (Al) and silicon (Si) were also added to the refractories to prevent carbon oxidation. Table I shows the chemical composition of new and spent MgO+C refractories from a EAF. The chemistry indicates that this spent refractory is fairly clean. Table I New and spent MgO+C refractory chemical composition MgO+C Al2O3 CaO FeO MgO SiO2 C N2 New 4.40 0.99 0.37 84.74 4.47 9.49 0.20 Spent 6.07 1.99 0.56 86.06 0.93 6.61 0.41 MgO+C spent refractories have been analyzed by SEM (Scanning Electron Microscope). SEM analysis indicated that carbon was present in the form of graphite and Al/Si metallic antioxidant throughout the samples. During service, elements of Al and Si can be nitridized to form AlN and Si3N4. The nitride powders react with water, releasing N2 gas, possibly causing cracks limiting their use as repair materials. A model to predict MgO saturated EAF slag chemistry EAF slags consist of five major oxides, CaO, MgO, SiO2, FeO and Al2O3. EAF slags are always characterized by an index of basicity in the steel industry. There are many different ways to calculate basicity. Based on the analysis of the SiO2-Al2O3-CaO-MgO phase diagram, the basicity of (B3 = (CaO/(SiO2+Al2O3))) is selected in this paper. Usually, the steel companies adjust the basicity of their EAF slags between 1.5 and 2.5. 72 Kyei-Sing Kwong and James P. Bennett Vol. 1, No. 2 In general, lime and dolomite are used to provide the source of CaO and MgO for adjusting the slag basicity and to saturate a slag with MgO. SiO2 and Al2O3 may be formed from the oxidation of Si and Al in the scrap, dust and coal slags. FeO may originate from the oxidation of iron and from coal slags. EAF steel making involves an oxidation process to oxidize extra carbon in the molten steel. During this operation, the FeO concentration in the slag is generally increased and dynamic changed by the lancing amount of carbon and oxygen. In the meantime, the increased FeO in the slag also affects CaO, MgO, SiO2 and Al2O3 concentration. Based on the linear relationship finding in the SiO2-CaO-FeO-MgO phase digrams, an ARC slag model has been proposed to predict MgO saturated EAF slag chemistry. Hence, the amount of slag conditioner (MgO+C spent refractory) can be predicted to saturate the slags. More detailed discussions on these relationships and models can be found in an earlier paper (5). As previously mentioned, the “optimum” slag at EAF operating temperature is a molten MgO saturated slag with the presence of suspended second phase particles (MgO C FeO magnesium wustite (MW))(3). Consequently, the basicity and FeO content affect the amount of slag conditioners needed to saturate a slag. Figure 1 demonstrates the ideal foamy slag chemistry region and the ideal foamy slag chemistry path during EAF operation with MgO injection. The crossed area is an ideal foamy slag chemistry region. Line 1, 2, and 3 are examples of slag chemistry paths. In general, the EAF process is an oxidation process to produce low carbon steel. Therefore, FeO content in slags will be increased during operation. Slag chemistry path 1 will generate a viscous slag containing too much precipitated solid particles, causing little foam. Slag chemistry path 3 will cause thin molten slag which cannot sustain gas bubbles. Slag chemistry path 2 provides an ideal situation for generating an “optimum” foamy slag. Detailed information about slag foamy practice can be found elsewhere (3).