Secondary Powder Materials Raw Material Alternative for Steel Making in EAF MIRELA SOHACIU, CRISTIAN PREDESCU, AVRAM NICOLAE, DANIELA NAGY Faculty of Materials Science and Engineering POLITEHNICA University of Bucharest Splaiul Independentei 313, sector 6, Bucuresti, cod 060042 ROMÂNIA msohaciu@yahoo.com http://www.ecomet.pub.ro Abstract: - This paper aims to propose to present a theoretical and experimental analysis of the optimizing procedures of the electric arc furnaces in the case of valorization by recirculation of the secondary powder materials, resulted from steel industry in primary technological fluxes. Based on this analysis we proposed a new technology for efficient valorization of the secondary materials resources by realizing cold auto reducing briquettes chargeable in the electric arc furnaces. Based on the tests that we used for these briquettes it was established that these powder materials can fulfill the function of replacing scrap iron as well as the ecological economic function if the ration between the new and normal technology is under unit. Key Words: - ecological economic function, performances, recirculation, secondary powder materials 1 Introduction The modern requirement in the material science field imposed transformation as found as well as shape and thus the preparation has to be done in the same direction developed at the worldwide level. The specialists have to identify and develop new metallic materials, ceramics and carbonic destined for the top industry: aeronautics, robotics automatics etc. The development of new materials and their understanding is at the root of recent progress in many areas of materials science. The tendency, manifested in the world is of increasing the production capacity [4] on different procedures, is restricted by the requirements and regulations of the European environment directives. Thus the have to develop new technologies for producing materials, to valorize and develop to the highest level the raw materials but also the existent resources and simultaneously to develop recycling technologies and clean technologies to use as raw materials primary scraps, thus these technologies to contribute to the re equilibrium of the ecosystems and to prevent destroying ecosystems. In this context, today, the specialists in steelmaking industry contribute to improve the existent technologies to and replace them with other technologies with minimum pollution. These technologies are more expensive but very efficient in time, from the environmental point of view and also from the population health estate [3]. A certain policy has economical impact as well as allowing a new approach by concetrating towards the necesity of the real increase of the competitivety and sustainable development in steelmaking industry. Thus the necessity of using different sources of alternative iron in electric arc furnace charge. Due to physical and chemical characteristics, wastes from iron and steel industry represent may be used in varios industrial sectors. Now, these types of wastes are partially recovered and they represent 70% from the total waste amount. As a consequence, the specialist would like to increase the performances with electric arc furnace based on the valorization of the secondary raw materials by recirculation of the primary fluxes [1]. The necesity of recicling the primary technological fluxes of these secondary powder materials are explained by tow major causes: - crisis in the field of superior iron ore suppling; - the decrease of the quality of the iron supply due to impurities principally with nonferrous elements (Cu, Sn, Cr, Ni, Mo, etc), which affect the steel quality in the condition which the beneficiaries exigences are more and more and not lately the extends of the casting determined by the decrease the qaunties of internal scrap iron. Thus has been identified and quantified the principal resources the secondary materials, to charge the electric arc furnace (EAF dust, scale and slurry) [2] to an analyze methodology for input output materials. ISSN: 1792-6211 / ISSN: 1792-6238 45 ISBN: 978-960-474-248-6
Studies and research showed the fact that AEF dust must be appreciated function of usage degree, expressed by three values: - values of thermodynamic use that take into consideration the supplementary cost in a steelmaking company for processing in AEF the nonmetallic companions of iron by energetic consumption regarding the slag formation, the imposed alkalinity; - the values of chemical use taking into account the necessary research to compensate the negative effect of the residual metals on the quality of the steel obtained in AEF. - value of the physical usage taking into consideration the influence of the dimension and density on the costs of EAF steel making. On the analyzed effected on EAF steel regarding reparation, chemical composition and sizing distribution, we established that this can be returned in the process only as briquettes and added in classical charge. The dust resulted from the continuous casting process and heating the semi fabricates for lamination is a bulk product and non oily and can be introduce in the electric arc furnace taking into account that a very useful component (Fe>70%) that should be valorized. The slurry as a complex mixture, fine ferrous dust water oil needs special technology to allow reintroducing in EAF circuit. 2 Experimental researches In order to establish the necessary technologies in order to work three ferrous byproducts (furnace dust, scale, slurry) we used a technology to evaluate the characteristics determined for these (the Zn content, recovery of Zn, oil content and granulation) and the objective of the targeted technological objective (enriching in Zn, recovery of Zn, granulation, drying, unoiling). The EAF dust were analysed followed especialy: - the influence of the proportion of EAF dust in the furnace charge and the Zn content of the dust on the obtained steel (figure 1); - the influence of the proportion of EAF in the furnace charge and the Zn content of the the dust on the generated dust (figure 2). Fig.1 Influence of the proportion of EAF dust in the furnace charge and Zn content of the dust on the obtained steel Fig.2 Influence of the proportion of EAF dust and Zn content on content Zn on the dust generated ISSN: 1792-6211 / ISSN: 1792-6238 46 ISBN: 978-960-474-248-6
a) In the thermodynamic and kinetics processing phenomena of the powder materials we define the ratio for Zn assimilation: Characterizing the scale followed the influence of two parameters: proportion of scale in the charge network and the (Fe) tot iron content in dust [9] being analyzed the following aspects: - influence on the scale proportion in the furnace charge and the (Fe) tot on the steel quantity obtained (figure 3); - the influence of the scale proportion in the charge furnace and the content of (Fe) tot in dust in the deviation from the obtained steel quantity (figure 4). (1) - influence of the slurry proportion in the furnace charge and the (Fe) tot on the steel quantity obtained (figure 3); - influence on the slurry in furnace charge and the (Fe) tot iron content in the slurry on the deviation from the steel quantity obtained (figure 4). Subsequent to documentary and assessment analyze it was established that the valorization of these secondary resources was efficient to fabricate of cold auto reducing briquettes (EAF dust, scale and slurry) prepared after the specific technological norms and chargeable in EAF (figure 5). Fig.3 Influence of the scale proportion in the charge furnace and the content of (Fe) tot on the steel quantity obtained Fig.5 Auto reducing cold briquettes Fig.4 Influence of the scale proportion in the charge furnace and the content of (Fe) tot in dust in the deviation from the obtained steel quantity The characterization of the slurry made following the influence of two parameters: slurry proportion in the charge network and iron content (Fe) tot in slurry, being analyzed the following aspects: Establishing of these norms means the prescription of the properties and characteristics in order to establish the briquettes for using as the reducing character, auto fondant character, but also the transport resistance and charging in the furnace. Thus, the briquettes were realized based on wellestablished prescription, included, together with these secondary materials, metallurgical coke, as reducing agent, but also as a binding material, chalky stone and lime as auto fondant agents and bentonite as binding for insurance the mechanical resistance necessary for manipulation and transportation. ISSN: 1792-6211 / ISSN: 1792-6238 47 ISBN: 978-960-474-248-6
Briquettes fabricated this way were subjected to tests in order to characterize the secondary powder materials: a) Compression test realized on a universal hydraulic machine for traction compressions axial fatigue trials UHP 20 / 17358 with trials in static regime till 200 kn to establish the influence of the briquettes components on their resistance, necessary for the manipulation - transportation operations charged in the melting furnace. Experimental modeling, materiliased in regretion equation, for rupture resistance (R f ), breaking resistance (R s ) and the interval rupture breaking (r fs ) is presented in the following graphs: Fig.6 The briquettes behaviour to operation and transport, function of EAF dust weight Fig.7 The weight scale influence on strength of compression Fig.8. The variation of strength of compression, function of slurry weight b) The thermo compressions tests were realized with the aid of some furnace and the compression machine. These were realized to determine the influence of the temperature on the cracking resistance. c) Shatter tests to describe as well as possible the physical processes on degradation and determination distribution of the granulation of the cracked briquettes in similar conditions in electric furnace (falling height and temperature), (figure 10). ISSN: 1792-6211 / ISSN: 1792-6238 48 ISBN: 978-960-474-248-6
Fig.9 Behavior of briquettes to thermo compressions d) Reducing tests were effected with the aid of some installation made by an electric furnace with graphite crucible and ceramic support. These tests were realized to determine the reducing rate of the briquette material from EAF based on the gas determination during heating the briquettes (%CO) due to this can constitute a parameter of influencing the furnace productivity. e) Test regarding the electro energetical characteristics of the briquettes to observe how the electro energetical function of EAF is influenced, including the specific energetic consumption. 3 Result interpretation and conclusions The establishing of the experiments intervals (the percentage calculation of the component in fabrication prescriptions) it was realized function of some preliminary tests and taking into account the zones in the fabrication and using the briquettes to present the technological and economical signification. Based on these considerations it was admitted, for example, the significant experiment should be place on the 70-80% dust interval. After the compression and thermo compression tests it was observed that the shape of the variation curve of the breaking resistance R s present a decreasing tendency in the first zone fact that can be linked with: Increase of the dust content or slurry decrease the quantity of the ultra powder adding (bentonite, coke and lime) that negatively influence the binding, the filling of the intergranular spaces and blocking the rupture propagation; Fig.10. Shatter coefficient spatial variation in the content dust and temperature The second sector of the curve shows the increase of the resistance values that means a positive influence of the particles in the base materials and the phenomena called auto compression. We can see that the increase of the degradation interval on the fact that in the process decrease more abruptly than braking resistance. This empirical observation allow us to affirm that in the process of rupture breaking, an important role is taken by the possibility of rupture propagation n by blocking these ones on the powder materials and the binding capacity given by the lime and bentonite. After these test we underline the following conclusions: a) Briquettes are damaged during the transportation and manipulation; the compression resistance is influenced of the EAF dust content and binding content (bentonite) as well as lime content; b) During the damaging process we can observe two stages: I - the stage of starting the fracture, and II grinding stage (destroying the briquettes framework). For the second stage the grinding resistance is essential, for which an optimized content of EAF dust of 74 75% is dependent on the lime and bentonite content according of the equation in figure 11; c) After the charging in the electric furnace in a briquettes, the process of physical desintegration of their structure continuous accelerated till the temperature of 200 0 C. ISSN: 1792-6211 / ISSN: 1792-6238 49 ISBN: 978-960-474-248-6
The Shatter test shows that the particle with a diameter equivalent to 10 mm and smaller, are the most representative to the reducing and melting processes that will be developed in the furnace. Date resulted from the reducibility tests results to the following conclusions: a) Reactions of iron oxides in EAF dust briquettes begins to be significant on the temperature of 400 0 C; b) The temperature increasing have a favorable influence on the iron oxide, the reaction having a higher rate; c) Granulation of the materials brought in the furnace for reducing is determined for advance reducing of the iron oxides (the best results are obtained for 5 mm granulation and the lowest for the uncracked briquette); the mixture mix have the reductibility characteristics close to the 5 mm granulation; for higher granulation (>5mm) from the mix mixture to observe the grinding process, to the of the experiment of the mix mixture it was a 90% granulation < 5mm especially to 800 0 C. Subsequent to the tests regarding the electro energetically characteristics of the briquettes it was established that the heat transmission process and respectively the energy specific consumption are not influenced from the blacking coefficient point of view,ε. Fig.12. Variation of electrical resistivity with temperature for the three categories of briquettes Fig.11 Spatial variation of the resistance of the grinding function on the bentonite and lime content In specialty literature we can see that for scrap iron the blacking coefficient ε is 0.4 0.6 [3], and for the briquettes realized mainly from iron oxides ε is 0.3... 0.65. Admitting that a charge with 10% briquettes results: ε conventional charge = 0.5 ε briquettes charge = 0.9 x 0.5 + 0.1 x 0.6 = 0.51 The general conclusions is that using these cold auto reducing briquettes for EAF steel making can fulfill the task of scrap iron, if technologies designed will foresee using of a mixture of EAF dust briquettes and scale briquettes, the ecological function, due to the fact that introduce in the charge 1% briquette realized from EAF dust will increase the scrap iron consumption to 11,9 kg/t steel thus the EAF dust will valorize mainly fir avoiding the keeping and an ecological-economical function if the ration between the researches between the expenses with the ecological technology (when using briquettes based on EAF dust combined with scale) and these with normal technology is under unit. References: [1] Bădulescu, Aura et al., Reciclarea Deşeurilor Pulverulente din Siderurgie. Produsul Carbofer, Metalurgia, No.8, 1998, pp. 28; [2] Bredeloft, R., Recycling-und Abfallwirtschaft an der Thyssen-Krupp-Stahl, Stahl und Eisen, No.7, 2002, pp.71; [3] Dina, Vasilica, Tranfer de căldură şi instalańii termice metalurgice, Ed.Didactică şi Pedagogică, 1996; [4] Velicu Şt., et al., Research regarding the superficial hardening by phase zone modifications, Proceedings of the 2 nd WSEAS International Conference on MATERIALS SCIENCE, Baltimore, USA, 2009, pp.58-63. ISSN: 1792-6211 / ISSN: 1792-6238 50 ISBN: 978-960-474-248-6