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39MILL SCALE WASTE REPROCESSING BY CENTRIFUGAL METALLOTHERMIC SHS FOR PRODUCTION OF CAST FERROALLOYS Fe-(Si; Si-Al; B; B-Al)
D. M. Ikornikov*", V. N. Sanin", D. E. Andreev", and V. I. Yukhvid"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: denis-ikornikov@yandex.ru
DOI: 10.24411/9999-0014A-2019-10051
The management of wastes generated by hot metal and steel has become an important issue due to ever-tightening environment regulations. Furthermore, the depletion of iron ores necessitates extensive research work to reuse the secondary raw materials produced as a by-product in steel companies and considered as waste materials. The metallurgical industry is one of the most massive material-forming industries. In several iron and steel making processes, about 500 kg/ton of solid wastes of different nature are generated; one of these wastes is the mill scale which represents about 2% of steel produced [1].
During hot rolling of steel, iron oxides form on the surface of the metal as scales. The scale is accumulated as waste material in all steel companies. Mill scale, often called as scale, is the flaky surface consisting of the iron oxides iron (II) oxide (FeO), iron (III) oxide (Fe2O3), and iron (II,III) oxide (Fe3O4, magnetite). It can be considered as a valuable metallurgical raw material for iron and steel making industry [2]. In an integrated steel plant, portion of mill scale, the large size one, was recycled in sintering plants. But a study on recycling mill scale of steel in the sintering process showed that the sinter productivity decreased with the increase in mill scale addition due to a decrease in sinter bed permeability [2].
In the past years, steelmakers used this mill scale as oxidizer in conventional electric arc furnace steelmaking process. However, the modern electric arc furnaces are equipped with oxygen lancing system to enhance melting and oxidation processes with higher efficiency than mill scale practice [2]. A small portion of mill scale has been used by cement plants. However, the mill scale does not uniformly blend with the other feed stock materials due to its higher density than any of the blend components and thus causes a greater variation in the blend of the kiln feed. At the same time, the amount of mill scale used by cement plants, as a raw material in the manufacturing of clinker, is still rather little.
A study on laboratory scale was made to use mill scale waste to prepare iron powder. The authors used CO followed by H2 as a reducing gas. When the reduction was carried out by carbon monoxide the maximum iron content (98.40 wt % Fe) in the iron powder was obtained at 1050°C for 180min. A reduction annealing under hydrogen makes it possible to decrease carbon and oxygen contents of the reduced iron powder up to acceptable values, 0.23 and 0.28%, respectively. A recent study was made of the reduction of mill scale to sponge iron using coke at different temperatures and times. Sponge iron was successfully produced for reuse in electric furnaces as part of the metallic charge or as a raw material in the production of iron-based powder metallurgy parts.
Unfortunately, no technology has been implemented, in mass, to recover and use such materials. In some steel manufacturing companies, the bulk of mill scale waste was dumped in landfills and resulted in leaching of some percentages of heavy metals into soil and groundwater, thus threatening the environment. The continuous demand for more landfills and bad effect on the environment highlight the need for more effective methods of productive utilization of mill scale. Accordingly, most used route to recycle the iron content wastes,
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briquettes are charged into blast furnace due to its reducing atmosphere or into the electric arc furnace (EAF) during the melting of stainless steel scrap [2]. These processes allow the processing of mill scale in the metal product, but the economic efficiency remains at a sufficiently low level.
In this study, we investigated the feasibility of centrifugal metallothermic SHS to produce valuable products: cast ferroalloys Fe-(Si; Si-Al; B; B-Al) using iron oxide mill scale (FexOy) and recycled aluminum as the components of aluminothermic mixture for obtaining the cast ferroalloys as master alloys for following use in steel making industry. A promissory route to production of cast different composition ferroalloys is using technique, which was called the SHS-technology of high temperature melts or the SHS-metallurgy [3]. This is an energy-saving technique due to the use of internal energy released in high-caloric combustion reactions. Early, the possibility of preparation on cast ceramic coatings inside metallic pipes firstly was demonstrated by reprocessing of mill scale [4].
This study involved three stages:
(i) the preliminary investigation of the starting reagents (industrial mill scale) and the selection of optimal modes of their heat treatment for purification;
(ii) the study of mutual relationship the initial mixture compositions and final composition of the ferroalloys (ferrosilicon, ferrosilicoaluminum and ferroboron)
(iii) the phase and chemical composition investigations of synthesized ferroalloys and the study of experimental condition effect on formation of final compositions
The overall scheme of reaction taking place in graphite crucible can be written in the form:
viFexOy + V2AI + V3AD -> V4Fex-Si(B; Al)y + V5 AhO3
where AD stands for some alloying additive such as Si, SiO2, B, and B2O3.
Mill scales generated in the hot rolling step of steel, together with reducing agent (recycled aluminum powder) and some alloying additive (AD) were used as raw materials. Sieve analysis, chemical composition, and XRD examination of mill scale were carried out. The mill scale is not sorted by the requirements of the standard for chemical products. So preliminary analysis of the mill scale revealed that the main pollution source of the initial SHS mixture is the presence of the organic component (this is due to the specifics of the rolling production and surface treatment during the hot rolling of steel billets). In order to remove the organic wests, we calcined the scales in shallow trays and found out that the optimal conditions of such a thermal treatment were 300-350°C for 2-3 h. After calcining, the scales were ground down to a particle size of below 400 p,m.
For each experimental run, the amount of the reducing agent, AD and mill scale was calculated according to the material balance. Combustion was carried out in graphite molds 80 mm in diameter. The inner surface of the graphite molds was laminated with Al oxide (AhO3) to ensure minimal reactive interaction between the form material and metal melt. Because the attained temperatures (up to 3000°C) are well above the m.p. of reaction products, the melt represents a mixture of mutually insoluble metallic (alloy) and oxide (AhO3) phases. Due to strongly different specific weights, these phases undergo gravity-assisted phase separation and subsequent crystallization. As the result the lower layer is target material (ferroalloys) while the upper one, Al2O3.
Our previous studies have demonstrated that the SHS process carried out under high gravity conditions affords the best separation of the target product (ingot) from the slag (Al2O3) and convective mixing of all alloy components, which becomes especially important with an increase in the number and concentration for alloys with an increased number of components and their concentration of components in the alloy. Therefore, the synthesis of the as cast alloy under study was carried out in a centrifugal SHS setup [4].
The investigation is a universal axial centrifugal installation with the possibility of forming the necessary synthesis conditions in a broad range of specified overloads. Its main feature of the SHS installation included systems for initiating the reaction mixture, controlling the rotation velocity of the samples, and measuring the sample temperature after synthesis (at the cooling stage). To calculate the ratio of the starting components, we should know the mutual concentration of iron and oxygen in the scale (values x and y) exactly. The chemical and the phase analyses of two studied scale compositions (Table 1), which were supplied from different plants, revealed the difference in the mutual relationship of iron oxides.
Table 1. Main characteristics of the scale types._
Sample no. FeO : Fe2O3, wt %_Particle size, mm_Impurities, wt %
1 46 : 48 1-5 6
2 68 : 28 0.5-10 4
The first party contained a larger amount of Fe2O3 (48 wt %), while the second one, on the contrary, preferentially consisted of FeO. The size of scale particles was from 0.5 to 10 mm. Both compositions had impurities, which included inclusions of iron, silicon oxide, alumina, and organic compounds (oil, paraffin, etc.). The presence of the latter can substantially affect the flow character of the SHS process due to intense gas liberation (gasifying the impurities) and lead to the complete or the partial emission of reaction products in the course of combustion. Therefore, it became necessary to organize preliminary scale calcination. The use of various thermal treatment modes and the subsequent analysis of the composition of the processes scale allowed us to reveal the optimal temperature range for calcination, which was T = 300-350°C for 2-3 h (in the plane bottom plates). Calcination at T < 300°C led to an essential increase in the calcination time (up to 10 h), and at T < 200°C it led only to the partial gasifying of organic impurities. The perfor mance of annealing at T > 400°C is undesirable because the self-ignition of the evaporating gases occurring inbsome cases. After calcination and cooling, the scale was subjected to mechanical milling. It was found experimentally that the optimal particle size should be smaller than 400 p,m. It should be noted that the most expensive component of the starting mixture is the aluminum powder. According to our analysis, it was established that the substitution of chemically pure aluminum (brand PA-4) by a secondary one (the worked scrap) allowed us to decrease the cost of the starting mixture by more than 25%. However, the processed aluminum mostly contains impurities, the basic ones being iron (up to 5 wt %) and silicon (up to 1 wt %). It should be noted that these impurities cannot be attributed to "harmful" from the viewpoint of implementation of the SHS process. It even seems likely that their presence is desirable, since makes it possible to somewhat reduce the synthesis temperature, while the presence of silicon promotes the formation of target product (ferrosilicon and ferrosilicoaluminum). It well known that complete melting of system components is a prerequisite for good sensitivity of the process to gravity forces [5, 6]. The investigation on III stage (the effect of experimental condition on formation of final compositions) reviled that the burning velocity (U) going up with increasing a/g (overload). The effect is most pronounced within the range a = 10-200 g. Strong influence of mass forces on burning velocity was associated with intensification of convection and deformation within the reaction zone, which improves the completeness of combustion reaction. An increase in a/g (where a is the artificial gravity, g is the earth gravity) is also accompanied by a decrease in material loss (%put) and an increase in the yield of target product (^ngot). Above 60(±5) g, the ^ingot values get close to theoretical ones. The results reviled that 3 synthesized ferrosilicoaluminum alloys can be obtained within a wide range of parameters a Si (the mass fraction of Si and Al in final ingot). Thus, prepared ferrosilicon samples exhibited no residual porosity, that is, had a character of cast materials, with a small contraction cavity at the center. An increase in aSi was found to have slight influence on U it decreased from 4 to 3 cm/s.
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The XRD data in Fig. 1 suggest that cast ferrosilicon is formed by three (Fe2Si, FeSi FeSi2). The incising of aSi in initial mixture leads to formation of high concentrated ferrosilicon phases (FeSi2). Figures 2 and 3 illustrate composition and the microstructure of SHS produced as-cast ferro alloys by reprocessing of mill scale wastes.
Fig. 1. Typical diffraction pattern of Fe-40Si cast ferrosilicon synthesized by SHS.
Al Si Mn Fe 4.3 23.2 0.6 71.9
Fig. 2. The microstructure (fissure) and chemical composition of synthesized cast ferroalloy Fe-Si-Al.
Al Si Cr Mn Fe
4.0 20.0 3.4 4.8 67.8
Fig. 3. The microstructure (fissure) and chemical composition of synthesized cast ferro alloys Fe-Si-Al(Cr, Mn).
The analysis of the obtained data allows drawing a conclusion about the prospects of the materials under investigation and the method of their production for the formation of volumetric materials for steel making industry. The production of complex alloyed ferroalloys can be realized in combustion mode (SHS) for powder mixtures based on scale. The process of obtaining high-alloy ferroalloys is completely energy independent, which makes it attractive for practical realization.
The work was supported by the Program of Fundamental Research of the Presidium RAS no. 15.
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