This article presents a description of the carbothermic process concerning production of iron-silicon-aluminum alloys with 55 to 75 wt. % of silicon and 4 to 20 wt. % of aluminum in industrial conditions. For the process, mining waste resulted from mechanical processing of coal being the source of silicon and aluminum compounds as well as high-ash fine coal as a reducer was used. A modern technological line for FeSiAl smelting was described, consisting of a SAF (fitted with two 7,75 MVA three-phase transformers and six self-baking electrodes) to ensure optimum power distribution in the furnace. In addition, technical and technological parameters of the process were presented with a particular emphasis placed on Al and Si yields

S. Gil

S. Kozłowski

W. Bialik

English

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295METALURGIJA 60 (2021) 3-4, 295-298W. BIALIK, S. GIL, S. KOZŁOWSKITECHNICAL AND TECHNOLOGICAL SOLUTIONS IN DEVELOPMENT OF FeSiAl ALLOYS PRODUCTION FROM INDUSTRIAL WASTES IN SUBMERGED ARC FURNACE (SAF)Received – Primljeno: 2021-02-06Accepted – Prihvaćeno: 2021-04-20Preliminary Note – Prethodno priopćenjeW. Bialik, S. Gil, S. Kozłowski, Faculty of Materials Engineering, Sile-sian University of Technology, Katowice, Poland, e-mail: wojciech.bi-alik@polsl.pl, stanislaw.gil@polsl.pl, slawomir.kozlowski@realloys.plThis article presents a description of the carbothermic process concerning production of iron-silicon-aluminum al-loys with 55 to 75 wt. % of silicon and 4 to 20 wt. % of aluminum in industrial conditions. For the process, mining waste resulted from mechanical processing of coal being the source of silicon and aluminum compounds as well as high-ash fine coal as a reducer was used. A modern technological line for FeSiAl smelting was described, consisting of a SAF (fitted with two 7,75 MVA three-phase transformers and six self-baking electrodes) to ensure optimum power distribution in the furnace. In addition, technical and technological parameters of the process were pre-sented with a particular emphasis placed on Al and Si yields.Keywords: FeSiAl alloys, ferroalloys production, SAF, wasteINTRODUCTIONUntil middle 1950s, the scale of FeSiAl alloy pro-duction used to be minor due to a small market demand. The main reason was granulometric instability of the stocked alloy and reluctance of technologists to use a new steel deoxidizer. Ferroalloy-related literature from the first half of the 20th century also neglected prob-lems with FeSiAl production or it was very scarce [1]. Methodical research, carried out in the Soviet Union in 1950s and further in Russia and the former republics, resulted in a description of the ferrosilicon-aluminum production technology using various raw materials, in-cluding industrial waste [2-6]. In Poland, the interest in application of complex FeSiAl ferroalloys dates back to 1980s [7]. A potential for industrial waste utilization in the process of their production was also considered [8]. Further research was continued in late 2010s and result-ed in development of the FeSiAl production technology using industrial waste [9]. Ferrosilicon-aluminum man-ufacturing in a submerged arc furnace is based on Al203 and SiO2 reduction with carbon [10].Utilization of industrial waste in the process of fer-roalloy production demonstrates multidimensional eco-logical effects. This type of waste serves as a source of chemical elements that are desired for the process: sili-con and aluminum. Coal gangue provides the material containing necessary components of the alloy (silicon and aluminum) and a part of the required carbon reduc-er to the furnace reaction zone. The use of waste raw ISSN 0543-5846METABK 60(3-4) 295-298 (2021)UDC – UDK 669.17.782.71:669.168:338.455:669.187:658.567=111materials with low-carbon gangue improves economic efficiency of the enterprise and restricts the amount of waste that must be stored. Waste processing to yield a full-value product requires an advanced technology and compliance with more exact technological restrictions despite apparent simplicity of the process. An addition-al problem is coexistence of chemical, physical and electrical processes in the furnace reaction zone that are related to the way of energy delivery for reactions. The materials mentioned above must be applied in lumped forms of selected sizes.The aim of the article is to provide results of experi-ments performed during tests of the innovative ferrosil-icon-aluminum production technology based on the in-dustrial waste, particularly from coal mines.DESCRIPTION OF THE NEW  TECHNOLOGICAL LINE A six-electrode submerged arc furnace was selected for production of the FeSiAl alloys. Critical compo-nents of the technological line, responsible for the phys-icochemical processes of simultaneous reduction of silicon and aluminum oxides, are the parameters of the submerged arc furnace: geometry of the furnace bath, types of refractory materials used for the refractory lin-ing purposes, the furnace hood with the off-gas removal system. Another, equally important issue is location of the electrodes and their geometry with the fitted elec-trode columns as well as the devices that control the electrode loads through the change of the autotrans-former taps and location of the electrode tips inside the charge material. The process is controlled by means of the control and measuring equipment that ensures cur-296W. BIALIK et al.: TECHNICAL AND TECHNOLOGICAL SOLUTIONS IN DEVELOPMENT OF FeSiAl ALLOYS... METALURGIJA 60 (2021) 3-4, 295-298rent control of the physicochemical processes inside the furnace operating zone and maintenance of the technol-ogy-related parameters.The furnace is fitted with an elliptic bath:  8 500 x 820  mm, 4 370 mm (height). The entire bath is located on a steel T-bar grid seated on a reinforced concrete foundation. The grid enables air flow and natural cool-ing of the bath bottom and it compensates for free movement due to thermal expansion. The bath has two tap holes placed on the opposite side walls intended for liquid smelting product tapping directly to the tap baths.The furnace is equipped with an open hood intended to collect and deliver off-gases to the extraction system through vertical and horizontal channels and to reduce thermal emission to the environment. These gases fea-ture a large amount of enthalpy in the physical meaning that can be used analogically to other processes of fer-rosilicon production [11,12]. There are 6 holes in the hood for andalusite-sealed electrodes and a gas outlet with a flange for fixing the middle flue channel. The hood is fitted with a water cooling system. The lower edge of the hood has a ring with segmented screens hanging on it to reduce thermal emission from the upper charge material layer and burning off-gases to the out-side of the furnace.Six self-baking electrodes with diameters of 900 mm each (three electrodes for each side of the furnace, A and B) ensured optimum temperature distribution in the reaction zone and proper work of the furnace unit. In combination with two individual 7,75 MVA transform-ers and separation of the bath, this solution enables op-eration of only one side of the furnace without the need to shut down the whole furnace system.For an appropriate assessment of the furnace opera-tion, it is very important to measure electric current paths properly: single-phase active powers individually for each furnace electrode, phase and line-to-line volt-ages for each electrode, and the furnace phase currents.TECHNOLOGICAL TESTING  FOR FeSiAl PRODUCTIONThis Based on results of laboratory analyses of the selected raw material samples as well as on the model calculations for the technological process, a decision was made to use specific materials for FeSiAl produc-tion. Their technical analyses, carbon contents Cfix and chemical compositions are presented in Table 1.The parameters of the above raw materials (Table 1) were then used to calculate the final compositions of particular FeSiAl types. Results of the calculations for the final chemical compositions are presented in Table 2.Technological experiments were performed in five cycles, gradually increasing the Al content in the alloy in each of them through the increase in the percentage content of the waste aggregate in the charge material. Durations of the particular experiment cycles varied from six weeks for the first cycle to two weeks for the fourth and fifth cycles.Table 1 Technical parameters of the selected raw materials for FeSiAl productionTechnical analysis Cfix Component contents in the ash / wt. %Aa Va Wa/ wt. % SiO2 Al2O3 Fe2O3 CaO MgO TiO2 P2O5 SO3 K2O Na2O FeOQuartzite - - 1,70 - 98,51 0,44 0,28 0,01 0,04 0,085 0,005 - 0,088 - -Coal 4,41 30,50 1,68 63,41 34,15 23,94 13,91 8,92 4,89 0,92 0,58 8,99 1,9 1,58 -Fine coal 22,50 39,75 6,50 31,25 49,01 24,48 9,66 3,52 2,81 1,04 0,13 4,2 2,14 2,25 -Aggregate 87,99 9,17 0,61 2,23 64,22 22,25 5,01 1,43 1,90 1,04 0,08 0,66 3,1 - -Electrode mass 3,3 12,6 0 84,10 30,61 9 33,03 1,52 1,27 0,16 0,28 2,25 - - -Wood chips 0,2 37,2 53,6 9,00 8,7 1,76 2,7 59,75 7,17 0,01 4,35 0,61 4,3 5,9 -Iron oxides - - 4,40 - 0,31 - - - - - 0,002 - - - 99,45where: Aa - ash in analytical state, Va - volatile matter in analytical state, Wa - moisture in the analytical stateTable 2  Final chemical compositions of smelted FeSiAl typesType Chemical element contents / wt. %Si Al P S FeFeSi65Al5 65 4-6 < 0,03 < 0,01 the restFeSi65Al7 65 6-8 < 0,03 < 0,01 the restFeSi75Al7 75 6-8 < 0,03 < 0,01 the restFeSi55Al12 55 10-15 < 0,03 < 0,01 the restFeSi55Al16 55 15-20 < 0,03 < 0,01 the restTable 3 Mean raw material consumption indicators for FeSiAl productionExperiment cycleMean raw material consumption indicatorsAggregate Quartzite Hard coal Fine coal Wood chips Iron oxides Electrode weightkg / Mg kg / Mg kg / Mg kg / Mg kg / Mg kg / Mg kg / MgCycle I 425,43 1 134,25 629,01 349,77 289,16 315,22 35,63Cycle II 438,39 1 194,12 624,61 352,55 297,33 315,44 46,69Cycle III 602,35 1 681,98 912,67 442,55 503,83 219,26 50,50Cycle IV 1 501,04 532,60 259,17 1 041,89 683,89 326,72 43,81Cycle V 1 540,17 382,64 136,29 1 270,08 755,11 260,89 43,25297W. BIALIK et al.: TECHNICAL AND TECHNOLOGICAL SOLUTIONS IN DEVELOPMENT OF FeSiAl ALLOYS...METALURGIJA 60 (2021) 3-4, 295-298Table 3 presents mean raw material consumption in-dicators for all experiment cycles. In Table 4, weighted average chemical compositions of the alloys resulted from the particular experiment cycles are listed.The studies show that mining waste, as aggregate, is an effective aluminium-bearing material. Increasing the content of aggregate in the charge material instead of quartzite results in proportional increase in the Al con-tent in the finished alloy. On the other hand, the aggre-gate can provide the alloy with chemical elements which, depending on the FeSiAl purpose, can be unde-sirable components (Ca or Ti) or impurities (e.g. P) like in other ferrosilicon alloys [13]. For example, phospho-rus in FeSiAl leads to formation of compounds with calcium, aluminum and manganese that diffuse to grain boundaries where, due to the contact with moisture, they degrade and PH3 is created. One of important parameters enabling assessment of the ferroalloy production process is the yield of the main chemical element components of the alloy. In lit-erature, a yield is defined as a ratio of an element mass content in the alloy to its mass content in raw materials within an analysed time unit. It is usually expressed as percentage. Yields of the basic chemical elements of the alloy, i.e. Si and Al, for the consecutive experiment cy-cles are shown in Figure 1. The silicon yield is above 85 % in further cycles except cycle III where its value slightly exceeds 77 %. The silicon yield near 85 % is typical of a classical proper process of FeSi alloy pro-duction. The yield values for Al vary. In the first three cycles, they were close to 60 %, while significant differ-ences could be observed for the cycles IV and V: mark-edly lower and very high, respectively.The analysis of experimental data showed that a change in the quartzite percentage content in the sub-strates during the experiment cycles only slightly af-fected the silicon yield, which is illustrated in Figure 2. Even a three-fold increase in the quartzite mass content in the charge material causes extremely small fluctua-tions of the silicon yield.The influence of the percentage content of the ag-gregate waste in the substrates on the Al yield is not strictly defined. It is shown in Figure 3. Determination of such a relationship requires collection of further data from next complex FeSiAl alloy production campaigns. For their processing, advanced mathematical algorithms are necessary.The technological process resulted in trace amounts of slag relative to amounts of the alloy, which demon-strates a nearly complete reduction reactions of SiO2, Al2O3 and FeO. The resulting liquid elements formed the FeSiAl alloy.Mean electrical parameters of the furnace only slightly changed during the first four cycles of the ex-periment. The mean active power (cycles I to IV) for the A side of the furnace was 5,567 MW with the max de-viation + 1,6 % for the cycle I and – 1,5 % for the cycle IV. Regarding the B side of the furnace, fluctuations of the mean parameters were slightly higher: the mean ac-tive power (cycles I to IV) was 5,472 MW with the max deviation + 3,5 % for the cycle I and – 6,5 % for the cycle IV. The mean cosine φ value during cycles I÷IV of the experiment for the sides A and B of the furnace was 0,771 ± 0,005. For the cycle V, due to a change in the power supply parameters, the values of cos φ were 0,734 and 0,744 for the sides A and B, respectively. In Figure 1  Si and Al yields in the consecutive cycles of the experimentFigure 2  Silicon yield versus percentage content of quartzite in the substratesFigure 3 Al yield versus the percentage content of aggregate waste in the substrates298W. BIALIK et al.: TECHNICAL AND TECHNOLOGICAL SOLUTIONS IN DEVELOPMENT OF FeSiAl ALLOYS... METALURGIJA 60 (2021) 3-4, 295-298the cycle V, compared to the previous cycles, the active power values decreased to 4,676 MW and 4,688 MW for the sides A and B of the furnace, respectively. The reason was a reduction of the transformer taps from the phase voltage of 136 V to 121 V in order to increase the amperage regarding the current flow from the elec-trodes to the operating zone of the furnace bath. The effect was an increase in the chemical element yields. The silicon yield was slightly higher while for alu-minum, it was just above 80 %.The data for cycle V (the last experiment cycle) show a potential for high yields of the main alloy ele-ments. However, the cycle V duration was short and the amounts of the resulting alloy were over two-fold smaller than for the cycles II and III. To obtain more precise result interpretation and technological instruc-tions, data in future complex FeSiAl production cam-paigns should be monitored concerning the alloy of a composition com-parable to the cycle V alloy compo-sition.CONCLUSIONSThe aggregate in the form of industrial waste can be used as a cheap and effective source of aluminum and silicon oxides during production of FeSiAl with the Al content up to 20 %. A higher content of the aggregate in the furnace operating zone results in a proportionally higher Al content in FeSiAl.The phosphorus (undesirable element) content in the final product was smaller than that determined in the thermochemical calculations and remained at a low, ac-ceptable level.The electric energy consumption indicators (units per Mg of the alloy) were lower than those theoretically determined during the industrial research.The innovative technology of FeSiAl smelting from cheap aluminum-bearing mate-rials such as mining ag-gregate and high-ash fine coal, while maintaining the proper technological procedures, has a potential for ap-plication in continuous industrial production of a series of alloys with the Si content of 55 to 75 % and the Al content of up to 20 % in six-electrode submerged arc furnaces.AcknowledgementsThis article was partially supported by Silesian Uni-versity of Technology as a part of Statutory Research 11/020/BK_21/0080.REFERENCES[1] R. Durrer, G. 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Technical and technologicAl solutions in development of FeSiAl alloys production from industrial wastes in submerged ARC furnace (SAF)

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