Abstract
The maintenance of several steel-making equipment of the Companhia Siderúrgica Nacional (CSN/Brazil) builds up about 11 000 tons of refractory waste per year. Most of that refractory waste is disposed off in landfill sites without any application. That action permanently excludes an important alternative source of raw material from the productive cycle. In this context, in the particular case in CSN, all critical points of the generation cycle and recycling of refractory waste were investigated, aiming to use these residues as reprocessed raw material in the Refractory Industry – reverse logistics. The project implementation, through an integrated solution, was technically and financially feasible with an economic return of about US$ 1 million per year in terms of net present value. The reuse of this refractory waste also minimises the environmental impact generated by the production, consumption and disposal of refractory materials from steel production.
Introduction
Every industrial activity that produces and processes material generates residues a lesser or greater degree. These residues do not always have a destination for reuse or environmentally appropriate place of disposal. However, in some cases these residues can be reused as raw material in other industrial processes after a suitable treatment. 1,2 This substantial action is above all, to protect the environment. Continuous extraction of ore from natural sources results in great damage, such as pollution of rivers and soil. Reuse of industrial refractory residues is a necessary action mainly in developing countries. The exhaustive ore extraction in order to produce industrial products can result in permanent environmental damage, as have happened in Brazil, where recently a whole river and town were destroyed by an ecological fallout of mud and iron-ore residues from the ore extraction industry BHP Billiton-Vale, and nowadays these toxic materials flows down the Doce River into the Atlantic Ocean.
In a context of balance between consumption and conservation of energy and environmental resources, recycling plays an important role in the preservation of scarce deposits of raw materials, also reduces the energy consumption and the environmental damage associated with the extraction/production process. 2
Sustainable development, legal specifications, environmental compliance, continuous improvement, and others, are phrases in the day to day of the business world. However, most of the actions required to apply are classified as economically impracticable or too difficult to realise. With the increasing restrictions imposed by the environmental legislation, as well as market demands for environmentally friendly products and processes, many studies have been developed in order to reduce the generation of waste, through treatment and reuse. 3
New research segments seek the potential development of these residues as raw material for creating new composite materials made partly or entirely by waste. The rise of industrial waste as a category of alternative source of mineral resource requires the adoption of a qualified methodology, based on the environmental policies of classification and characterisation, besides the conduction of technical and economically feasible studies for applying these residues. 3,4
Problems associated with the production, consumption and disposal of refractory materials are an important environmental issue. Several types of refractory materials are used in the steel industry and the working line of various equipment of reduction metallurgy, such as MgO–C, Al2O3–MgO–C, and others. When refractory materials meet the end of their useful life, they are replaced by new refractories made from raw materials. After that, the residues are usually landfilled, wasting up valuable natural resources. 1–3
The steel industry consumes about 70% of the current refractory production. 5 In this way, it is essential that the commitment and actions of Steel Industry joins the development of new routes for the reuse of refractory materials. Furthermore, the disposal of waste in faraway landfills greatly increases the cost involved. 1
Increasing the useful life of refractory has a direct effect in reducing both the generation of waste and the extraction of raw materials from ore mines. The use of higher quality and performance materials, associated with the implementation of scheduled maintenance techniques and staff training are actions that provide an increased lifespan to the refractory. The consumption of refractory (refractory kg ton−1 steel) decreased dramatically in recent years due to the efforts in R & D areas of both Industries of Steel and Refractories.
The contamination of refractory waste generated in the steel industry processes restricts the possibility of reuse. The success or failure of the recycling refractory waste depends fundamentally on the separation of these impurities. Another hindrance cited in the literature is the high cost of recycling compared to the tangible benefits generated. 4,5
From the point of view of quality control after the refractory be used and demolished, it should be classified as materials based on MgO, MgO–C, Al2O3, silica-alumina and others. Even so, these materials can be mixed with different types of refractories, metal and slag. To remove impurities and obtain uniformed particle sizes, the used refractory material should be crushed, milled and sieved.
Refractory materials used in steel-making equipment vary greatly in quality according to the application sites, due to different requests. Hanagiri et al. 6 developed a method of recycling of processed refractory waste, called downward position. In this method, the reprocessed raw materials are recycled in order to produce a material with lower quality than their original one. 6,7
The processing of some products such as alumina refractory bricks – silicon carbide – carbon refractory bricks, for example, present no technological difficulty (low complexity) and result in a reprocessed raw material with high added value (more noble). The silica-alumina bricks also do not present technological difficulties in its processes (low complexity), but the resulting raw material has low added value, being classified as less noble. On the other hand, the products such as alumina-carbon valves involve technological challenges in its processes due to the presence of metal capsules. In this case, the raw material reprocessed has low added value (less noble), furthermore requiring a greater complexity process, which make them economically unviable.
The processing of magnesia-carbon products is costly due to the presence of metal compounds in its composition, whose volumetric stabilisation is achieved by thermal decomposition. The valves submerged with Zirconia inserts also present high technological challenges in its processing. However, a raw material of high added value is obtained.
This project had as main objective to add value and promote a proper disposal of waste refractory generated in CSN. In this context, all critical points of generation and recycling of refractory waste cycle were investigated, in order to enable its technical and financial application as a reprocessed raw material to their own industry refractories, using a reverse logistic. This project was developed by CSN through technical and economic cooperation agreement with the Engineering School of Lorena of University of São Paulo (EEL-USP), with the participation of Magnesita Refractories S.A, Brazil.
The refractory products made from the use of recycled materials have the same properties when compared with those made from raw materials, because it has a low contamination level. Furthermore, this processed raw material is also indicated to be applied in refractories that operate in processes with temperatures lower than 1500°C. This is an important requirement because all mineralogical phases of these raw materials have a melting point higher than 1500°C, which is the average temperature of steel production. In addition, the process flow sheet for manufactured products using recycled refractories is the same of those produced with new raw materials, there is no need of any further modifications.
Experimental procedure
The project was composed by the following steps: (1) preliminary assessment, (2) research and development, (3) transportation, handling, processing and (4) application, validation and deployment. The diversity of refractory products used in the steel industry demands strict control of all stages, the cycle of generation and recycling of refractory waste is composed by the followings steps: demolition, segregation, packaging, identification, transportation, receiving, processing, quality control and disposal. These steps are designed to become technologically and economically feasible the use of reprocessed raw material in refractory plants.
Stage 1 – preliminary assessment
Generation survey
The generation survey of refractory waste at CSN covered the metallurgical reduction equipment (Blast Furnaces and Torpedo Cars) and steel metallurgy (Steel Ladle, LD converters, HR and Continuous Casting Distributors) that have regular schedule of maintenance repairs throughout the year. The first part of the article shows the results for the recycling of refractory from CSN reduction metallurgy. The generation of refractory waste was calculated taking in account the type (located, partial and general) and the amount of annual repair each equipment. Based on CSN's historical production data, it is estimated a residual percentage of 20–30% from the originally applied refractories. (ii) Demolition and segregation
The demolition of the refractory lining of various equipments and subsequent segregation of refractory waste was accompanied with a focus on the following points: (a) personnel safety: the manual segregation of the material shall comply with the company's safety standards, and the demolition activities and segregation of the material may not result in delay in the return of the equipment to operation; (b) segregation: the materials contaminated with slag and metal located in the refractory contact that faces the atmosphere of the equipment is discarded. The materials without visible contamination are manually separated for processing purposes; (c) identification: designed to avoid the mixing of different families of refractory products; (d) cost: the number of man-hour (MH) and the equipment hours are recorded to assess the financial viability of the Project.
Stage 2 – research and development
Material samples collected after manual segregation were submitted to a beneficiation process on a pilot scale for the preliminary assessment of technical and economic feasibility and destination decision. The refractory materials were comminuted in the pilot-scale plant in fractions 19.00–4.75 mm, 4.75–2.36 mm and below 2.36 mm. These fractions were subjected to separation in a 5000–6000 Gauss magnetic drum. Then, different magnetic and non-magnetic particle size fractions were characterised in terms of chemical composition, combined carbon content, moisture, lost on ignition, density, porosity, mineralogical composition by XRD and microstructural analysis through Scanning Electron Microscope (SEM).
Stage 3 – transport, handling and processing
After separation, the refractory material was benefited on an industrial scale in the CSN's Civil Construction and Waste Recycling Plant. The beneficiation process involved the steps of primary magnetic separation to remove scattered metal fragments, which are recycled in the metallurgical processes of the CSN itself, crushing and then going through the secondary magnetic separation to remove the metal fraction still attached and/or infiltrated into the material. After these steps, the screening and particle size classification of the material were performed. The fraction below 1 mm was discarded because it has some slag adhered and/or infiltrated. The contaminated fraction with slag waste is friable and tends to segregate in the fine fraction. The fraction above 76 mm returned to the crusher for additional comminution. The submerged tubes have metal capsules that are removed with the assistance of compressing rollers.
Stage 4 – implementation, validation and implementation
Aiming at the market the families of raw materials were bid with two purchasing options: (i) selling raw materials of individual families or (ii) selling the whole family package of raw materials.
Results and discussion
For the purpose of application, validation and implementation, the various refractory residues were classified in six different families of reprocessed raw materials, as shown in Table 1. The CSN produces annually 11 000 tons of refractory waste and the steel metallurgy is responsible for about 85% of this generation, Fig. 1. This paper presents the results of the processing of refractories residues produced by both Blast Furnace Taphole and Torpedo Car, families 1 and 2, respectively. Annual generation of refractory waste at the UPV/CSN
Raw materials families reprocessed
Chemical composition in percentage of refractory waste of taphole before magnetic separation
Chemical composition in percent of non-magnetic fraction of the refractory waste of the blast furnaces taphole
The XRD results for fractions 19.00 < Φ < 4.75 mm, 4.75 < Φ < 2.36 mm and 2.36 mm Φ < showed that the fractions are composed of alumina, quartz, mullite, silicon carbide, guelenita, anorthite and iron titanate. The fraction of the initial fine after magnetic separation, showed the hercinita phase (FeO·Al2O3) apart from the compounds mentioned for the other fractions.
Microstructural analysis through SEM-EDS reveals the presence of refractory aggregate contaminated with slag and pig iron, Fig. 2
a, the slag is indicated by the light grey areas and the metal (Fe) by the lighter regions. The metal fragment is shown highlighted in Fig. 2
b. However, after magnetic separation (fraction 4.75 < Φ < 2.36 mm) it presents refractory aggregate-free or with low slag infiltration. In this particle size fraction infiltrations with pig iron were not observed.
a, b Micrography of the fraction 4.75 < Φ < 2.36 mm of the refractory waste of blast furnaces taphole before magnetic separation
Figure 3
a illustrates the presence of aggregates free of contamination and aggregates contaminated with slag and Fig. 3
b shows refractory fragments corroded by slag, but free of pig iron infiltration. The results of several characterisations performed have signalled the technical practicability of beneficiation of refractory wastes from taphole of blast furnaces for application as raw material reprocessed in Refractory Industry. Micrography of the fraction 4.75 < Φ < 2.36 mm of the refractory waste of blast furnaces taphole after magnetic separation
The characterisation of Torpedo car's refractory waste, based on alumina-carbide of silicon-carbon from Family 2, was performed after the beneficiation in the waste recycling plant, Fig. 4. In the case of Torpedo car refractory's, the fraction 8.00 < Φ < 4.75 mm is the particle size range of most interest to the industry of refractory production, because it has lower fraction of both metal and slag presence. Table 4 shows the results of chemical composition, density (MEA) and apparent porosity (AP) of Torpedo Car's refractory waste after beneficiation, compared to the technical specification of refractory manufacturers. According to the results, the material was approved to be used as raw material reprocessed for application in the refractories industry. Construction waste recycling plant and demolition, CSN
Characterisation results of torpedo car refractory waste after beneficiation in the RCD plant, fraction 8.00 < Φ < 4.75 mm
Conclusions
The segregation and benefaction process of refractory waste generated at the UPV–CSN for reuse in the Refractory Industry as raw material is technically feasible. The project implementation, through an integrated solution, also presents financial viability, with an economic return of US $ 1 million per year in terms of net present value (NPV), and also reduces the environmental impacts of this particular industrial activity. In addition to the economic advantages, the transformation of this refractory waste in a new alternative source of raw material has an amount of social and environmental benefits. The reuse of these materials offers two major advantages to the environment: first preserves the finite natural mineral reserves and then reduces the generation of refractory waste, which lowers the resulting environmental impact.