DEVELOPMENT THE STEEL INDUSTRY WASTE-FREE BY INTERNAL RECYCLING OF THE BLAST FURNACE DUSTS

Transcription

1 DEVELOPMENT THE STEEL INDUSTRY WASTE-FREE BY INTERNAL RECYCLING OF THE BLAST FURNACE DUSTS Anisoara CIOCAN a, Florentina POTECA U b, João Pedro VEIGA c a Faculty of Metallurgy and Materials Science, University of Dunărea de Jos Domnească Street 111 Galati Romania, aciocan@ugal.ro b Faculty of Metallurgy and Materials Science, University of Dunărea de Jos Domnească Street 111 Galati Romania, mihaela_potecasu@yahoo.com c CENIMAT/Materials Science Department, Nova University Lisbon, Caparica, Portugal, jpv@fct.unl.pt Abstract The development of steel industry is subject to solve major problems strictly targeted on pollution control and protection of natural resources and energy. The development strategies of steel integrated works around the world have two directions: development of the advanced technologies in respect to the substantially decreasing emissions and increasing the recycling ratio of the wastes and by-products. In this paper a solution is analyzed for integrated management of blast furnace dusts in accordance with their essential quantification (quantitative, qualitative and the generation sources) and transformation of these wastes into valuable by-products used directly by the company in an internal process. The physico-chemical properties that are relevant for recycling solution are discussed as respect to the optimum valorization. Also, the morphology and mineralogy of these dusts are presented in accordance with fundamental aspects of its generation process. 1. INTRODUCTION For the steel to become what is called "the environmentally preferred material" should be primarily to identify and to apply of more effective methods for retaining all possible iron-bearing sources into internal productionuse-recycling closed loop system by converting their solid wastes and by-products into valuable resources. This from-by-product-to-resource conversion conserves natural resources and diminishes energy requirements [1]. The management based on the optimum solutions in respect to natural iron bearing resources protection and to recovery of these that were consumed must ensure: strict management of wastes, reduction at source of the wastes quantity and their harmfulness, the advanced valorization of wastes by their internal reusing in various stages of the technological flow in accordance with the environmental respect and the natural resources protection. As result, no resources are depleted, all materials are perpetually reused, and no harmful waste is produced and landfilled [2]. This is a technologically and conceptually advanced integrated steelwork that harmonizes the requirements for pollution control with rational management of materials resources and energy. In this direction, one problem that must evaluate is the internal recycling ability of the valuable wastes with iron content that are generated from the off-gas dedusting plants. For a integrated steel plant with the sintering plant as upstream facility, the BF dust valorisation is an internal recycled solution. In more cases this is the single solution possible to be applied. In respect to optimum valorisation parameters into homogenizing raw materials for sintering process the good knowlodge of the wastes properties is necessary. The knowledge of the mineralogical characteristics of the dust from blast furnace is necessary to creating the fundamentals that can be utilized to design the dust recycling technology in respect to the sinter properties. The sinter quality is dependent on the properties of all 1

2 components that formed the sinter mix. The iron sinter, as iron ore, is introduced into BF furnace to produced molten iron. The quality of each feed materials components is the most important factor that conditioned the good BF furnace function. By other hand, the BF dust quantity and its properties is in accordance with its generation process in respect to the blast furnace processes (the temperature zones and the gas composition). Also, the caracteristics of the BF dust genered are influenced by the nature, the quantity and the properties (phisical, chemical, mechanical) of the feed materials (sinter, iron ores, fluxes, coal injected). This work presents the study about the characteristics of the BF furnace dust in accordance with the conditions necessary for the internal valorisation into sinter feeds. The dust properties were discussed in relationship with the fundamental aspects of its generation processes. 2. EXPERIMENTAL METHOD AND MATERIALS In these investigations we used samples of BF dusts. The sample was reduced to a necessary size after homogenization by stirring and reduction of mass by quartering. To determine the characteristics that are considered important for the valorization process by sintering the specifically methods and apparatus were utilized and the medium values were considered [3]. The chemical composition was determined by classical quantitative methods and X-ray fluorescence (XRF). The powdery material was included into synthetic resin and for a flattening surface the samples were polished. The reflected-light micrographs of the dust samples were analyzed with microscope OLYMPUS. 3. RESULTS AND DISCUSSION The dusts properties were analyzed in accordance with the processes that lead to their generation. All these (morphology, mineral phases, chemical composition and physical properties) were discussed in respect to the requirements for the wastes utilization as raw materials in the sinter process. The BF furnace dust is a powder mixture that is formed by different components of blast furnace feeding. But these have variable transformation degrees in accordance with complex phenomena that are passed into different stages of the iron making process. Therefore the nature, quantity and properties of the dust are in accordance with its generation process in respect to the blast furnace operation as the temperature zones, the gas composition, the feed materials. It is necessary to consider the zones where the dust particles are generated and the zones which are passed by the dust particles until their collecting in the dust catcher. The temperatures and the composition of the gas are different and specifically transformations are developed in the dust particles. The descending of the charge materials is accompanied by physical changes and chemical reactions. A lot of mechanical processes is occurs: abrasion, crushing, cracking. Also, the softening of materials, the formation of the liquid phases and, interactions between the gases, liquids and solides are occur. The reduction processes and oxidation are produced and these are specifically for each zone of the blast furnace, according to the temperature and the composition of the gases generated by the presence of the coke (the lower, middle and upperzone). The formation of the dust particle in the zone that have lower temperatures (so called upper zone or preparation zone where the ascending gas decreases in temperature to about C and the solids temperature rises from the ambient temperature to about C) is accompanied by the decomposition of carbonates other than calcium, vaporization of moisture and hydrated water of the burden, carbon deposition according and partial or complete reduction of hematite as well as magnetite into their lower oxides. The most part of the coarse particles are formed from the particles of the feed materials (sinter particles, iron ore particles, flux particles, carbon and ash from coke or powder coal), carbon black from the processes passed into upper zone of the charge column, the volatile mass of the 2

3 powder coal (injected through the tuyeres and condensed at surface of the dust particles), hygroscopic water and residual capillary [4]. The chemical composition is representative for the estimation the characteristics of the dust as those for the iron bearing valuable resources. The chemical composition of BF dust is given in Table 1. Table 1. Average composition of the blast furnace dust, [%wt] Fe Mn SiO 2 CaO MgO Al 2 O 3 Cl K Zn Pb C P S Oily substances ( Fe + 0.5Mn) ( SiO CaO) As result of the values of the iron content and the characteristic V m = the BF dust can be classify in the lean ores family (for that the value V m is lower than 50). But is proved these values are considerable to propose the valorization of the dust in the sintering feeds. Moreover as high carbon content the dust can be valorized in the sintering process as a carbon source. Also the BF dust gangue is acid like of CaO iron ores with i B = = and the presence of some free CaO (1.27%) (evaluated by conductometry SiO method) is favorable for basic flux addition. 2 In accordance with the chemistry of the BF dust the dust recycling in the sinter plant is conditioned by the presence of Na, K, Zn, Pb, Cd, S, Cl, cyanide, oily compounds. A lot of these elements are transferred via sinter-blast furnace. The presence of Na, K, S in the charge column of the blast furnace may lead to service interruptions. Following of cyclical reactions and damped the normal operations alkaline, elements accumulated in the blast furnace reduce permeability of the charge column and destroy refractory masonry. In addition, the cyanides of these elements alkaline lead to problems of environmental pollution. Also, zinc lead to the masonry destroy (by volatizing zinc is condensed in the upper regions of the blast furnace which are few cold) and through its participation in the cyclical processes may increase the coke consumption. For the dust analyzed the content of zinc and other harmful elements are small. These do not limit the use of dust in the mixture for homogenization. For safety the recycling process the zinc content should be kept under surveillance and continuously controlled to prevent the excessive aggregation with undesirable consequences in the blast furnace operation [4, 6]. The higher values of the calcinations losses for the blast furnace dust (21.58%) are primarily cause of the carbon content and volatile components presence which are in a large quantity into dust (source is represented by powder coal injected into blast furnaces). Also there are hygroscopic water and residual capillary that developed following processes in relationship with calcinations losses: residual capillary water is evaporating from C; molecular water is removed in the range C; hygroscopic water is removed in the range C; oily mass arising from heavy volatile compounds is removed from 660 to C. Also at the same last temperature range dissociated secondary hematite that is formed by transformations occurring in ores or sinter particles that composed the blast furnace dust. By point of view magnetic fraction of the dust (11.15%) this can be define like iron ores (hematite, limonite, siderite) in the weak magnetic materials class. The dust samples conductivity (3.02mS/cm) can be important if some modern drums for secondary mixing have continuous dust wetting plant (conductivity cell of moisture). For the dust (8.32 in the samples analyzed) ph is important in the context of its existence in the 3

4 feeds subjected to mixing with water and other components. Following leaching operations, the elutriation fraction of the dust pass by dissolution into basic solution in respect to its ph. The picnometer method was utilized to measure more dust densities. The values of the absolute density and apparent are following: 2.563(6) for the apparent powder density; for the apparent particle density; for the absolute powder density (after drying). Also the medium bulk density (also called bulk powder density) was determined. This was g/cm 3. These values must be discussed in respect to the real moisture and granulosity of the fines. The real moisture of the BF dust samples was 6.28%. It is a pollutant for environment and human operators. For avoided the environmental problems (generated by its handling and transport) is strongly recommended its directly envoys from the dry de-dusting unit of the blast furnace to the materials homogenizing sector of the sinter plant. In the sintering process, the water absorption is important. By point of view physico-chemical aspects this is a very complex process. There are two interactions water-fines: firstly between dry wastes and atmospheric moisture at deposition of materials for homogenizing and, second into homogenizing drum at the water addition. Especially the water absorption has influence on material tendency to form the micropellets at water addition. The water absorption for the BF dust analyzed is 7.27% (average values). This may be due to the macrostructure by presence of the micropores and microshakes. The BF dust has a relative high value for the intergrains porosity (55.47). It is determined by finesses characteristics of the particles that formed this material and also by the shape of its particles. We consider as a result of this property under normal atmospheric conditions these materials have high drying speed. This is advantaged by the capillarity of the material layer. This material is composed of a mixture of particles that varied into wide range of sizes, Fig. 1. This is explained by the fact that particles have different origins and is formed in different stages of iron making process. By other hand the interactions under atmospheric conditions and process characteristics amended the particles granulosity. This granulosity of the fine fraction does not prejudice, but rather promote granulation process, restrict the granulating range and increases the range of spherical granules by advanced homogenizing of to the sintering mixture. Is known as, in the first step of the sinter process the mixing operations by simultaneous balling and wetting of materials into mixing drum are developed. The various materials that are formed by varied particles with variable sizes and shapes are homogenized. As result of the wetting the fine particles are covered with a water coat and then by adherence under superficial tension and balling acts, the micropellets are formed. The diameter of these microballs is between 1 and 4 mm, while collected particles were smaller (less than 1 mm). Therefore, most dust particles will attach to the formation of large particles (to form the micropelletizing nucleus) at the materials processing into mixing drums. Also the particles of the ferrous materials with irregular polyhedral shapes have the better adherence capacity on the bigger particles considered nucleus for process [1, 4, 6]. 4

5 70 60 Frequency Number of divisions Fig. 1. Grains size distribution for the blast furnace dust (grains located on the transparent support, analyzed with calibrated micrometer from an optical microscope NEOPHOT). The transformation from division to mm: div OB x z OB = div OC x z OC; div OB = 0,01mm ( 1mm = 100 divisions); z OB = 100, is the division number on microscope objective; z OC = 63, is the division number on microscope eyepiece; div OC = div OB x z OC / z OB, [mm] In respect to morphology BF dust particles have different shapes and dimensions. A lot of dust particles have polyhedral shape and some of these have rounded shape (a little proportion), Fig. 2. X200 X100 Fig. 2. BF dust particles with varied shapes (polyhedral shapes and rounded) As a result following forecast is possible: the BF dust will has a good behavior into preparation process of the feeding materials; the formation speed of the micropellets is favorable influenced by the presence of the polyhedral particles. By point of view phase composition of the dust hematite, magnetite, ferrites, glass, and silicate phases were identified, Fig. 3. These are in accordance with the feed materials components of the blast furnace that transfer their mineralogy to mineralogical structure of the BF dust particles. Also these are in accordance with the complex phenomena that are passed into different stages of the iron making process. 5

6 a. b. Fig. 3. Reflected-light micrographs of BF dust structure: a. magnetite, hematite set in partially devitrified glass, ghelenit, silicoferrite of calcium and aluminium (x400); b. magnetite, hematite, ghelenit, olivine, calcium ferrite crystals with interstitial calcium orthosilicate (x400) 4. CONCLUSIONS The results of this study lead to the conclusion that the BF dust analyzed can be used in the sinter feeds without important restrictions. The characteristics of this dust must be homogenous and constant. That is in accordance with the stability of the blast furnace process. A stable process in respect to the materials balances and thermal (that develope a stable temperature profile) generates a dust with homogenous characteristics and quantity which can be predictible for short times and foreseeable for long time periods. A nonstable process that is variable for short times and the feed materials components with non homogenous characteristics that have always changeable proportions create conditions for very variable dust quantity and quality. The internal recycling ability of the valuable wastes is in accordance with technologically and conceptually advanced management of the integrated steelwork. It is necessary to harmonize the requirements for iron making process with optimum valorization solution of the iron-bearing wastes by sintering. That harmonizes the requirements for pollution control with rational management of materials resources and energy. LITERATURE [1]. HEINO, J., MAKKONEN, H. Recycling or utilization of dust, scales and dust from steel industry, [2]. SZEKELY, J. Steelmaking and industrial ecology. Is the steel a green material?, ISIJ International (1996) year 36, nr.1, page 121 [3]. *** SR EN /2002, SR EN /2002, SR EN /2004, SR EN /2003; SR EN 27888/1997, SR ISO 10523/1997 [4]. KALENGA, M. Investigation into the influence of magnesia content, alumina content, basicity and ignifugation temperature on the mineralogy and properties of iron sinter, Dissertation work, University of Pretoria, July 2007 [5]. FINLAY, P. Guidelines Best Available Techniques for Sinter Plants in the Iron Industry, 2004 [6]. PANAITESCU, S. Researches about quality of ferrous sinter by optimization of the feed materials composition and the parameters of the sintering process, PhD These, University of Galati,