The article deals with the solution of an urgent problem related to the process of filtration refining of ferroalloys. Based on the data obtained, it was found that combining the filtration process with casting ensures the flow of refined melt directly into the mold cavity and partial or complete elimination of secondary oxidation. To identify the underlying mechanism of filtration refining, the interaction of liquid metal with the filter in its separate pore channel was simulated. The main technological parameters of the process under consideration are calculated and practically confirmed

Issin, D. K.

Mazhitova, L. A.

Sirgetaeva, G. E.

Smagulova, Zh. B.

Zholdubayeva, Zh. D.

English

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142  METALURGIJA 62 (2023) 1, 142-144ZH.D. ZHOLDUBAYEVA, L.A. MAZHITOVA, D.K. ISSIN,  G.E. SIRGETAEVA, ZH.B. SMAGULOVASTUDIES OF CHANGES IN THE ACTIVITY OF DISSOLVED OXYGEN IN THE SIMULATION OF FERROMANGANESE FILTRATIONReceived – Primljeno: 2022-06-08Accepted – Prihvaćeno: 2022-08-15Preliminary Note – Prethodno priopćenjeThe article deals with the solution of an urgent problem related to the process of filtration refining of ferroalloys. Based on the data obtained, it was found that combining the filtration process with casting ensures the flow of re-fined melt directly into the mold cavity and partial or complete elimination of secondary oxidation. To identify the underlying mechanism of filtration refining, the interaction of liquid metal with the filter in its separate pore channel was simulated. The main technological parameters of the process under consideration are calculated and practi-cally confirmed.Key word: ferroalloys, refining, filtration, oxygen activity, substrate effect.ISSN 0543-5846METABK 62(1) 142-144 (2023)UDC – UDK 669.15:66.067:536.777-546.21=111INTRODUCTIONThe process of ferroalloy smelting is accompanied by significant contamination of the processed alloy with non-metallic inclusions. It is connected with to the higher multiplicity of slag than in steelmaking and the high concentration of elements in the alloy that have a strong affinity for gases. Metallographic studies show that ferroalloys contain the same inclusions as in steel, but in very larger quantities.In addition, it should be taken into account that due to the low oxygen concentrations achieved during these processes, the chemical potential of oxygen in the metal (μО2 or Ро2) becomes significantly lower than the chem-ical potential of oxygen in the phases in contact with the metal - the atmosphere, slag, and refractory lining. As a result, oxygen is transferred from these phases to the metal volume [1]. This process develops when the melt is kept in a container, but especially during the casting. In this case, the degree of oxidation depends on the size of the specific contact surface of the metal jet with the air atmosphere and the duration of this contact.Thus, during the production and casting of ball-bearing and other steel grades, the content of oxygen and nitrogen in ingots is increased 1.5 to 2 times, which contributes to significant contamination of steel with inclusions. As noted by V. P. Luzgin [2], a characteristic feature of the secondary oxidation process is contami-nation of the refined metal with non-metallic inclusions, Zh.D. Zholdubayeva, L.A. Mazhitova, D.K. Issin, Zh.B. Smagulova, Karaganda Technical University, Karaganda, Kazakhstan. G.E. Sir-getaeva, Kazakh Agrotechnical University named after S.Seifullin, Nur-Sultan, Kazakhstan. Corresponding author: Zh.Zholdubayeva (zhuma-77@mail.ru)unstable carbon monoxide of deoxidizers, and deterio-ration of mechanical and other service properties of the processed metal.As a result of secondary oxidation, not only the oxy-gen content increases, but also its activity [3]. This indi-cates that a significant part of the oxygen entering the metal does not interact immediately with the deoxidiz-ing element, that is, the melt is supersaturated with these elements, but the degree of saturation does not reach the value that ensures the formation of inclusions. The necessary saturation is created when the steel tem-perature decreases and is accompanied by the formation of secondary inclusions.Combining the filtration process with casting en-sures that the refined melt flows directly into the mold cavity and partial or complete elimination of secondary oxidation.The mechanism of filtration refining of metal melts is not limited to the stated ideas that the refining effect of filtration is only the result of cleaning from the formed non-metallic particles that are in the metal in a suspended state. There are all objective prerequisites, including thermodynamic and kinetic ones, for effec-tive purification of the melt from dissolved impurities by the mechanism called the substrate mechanism in this paper [3].The thermodynamic process of homogeneous nu-cleation can occur at any α > 1, but at its low values it is realized with low intensity. Therefore, the interaction of the deoxidizer and oxygen dissolved in the metal is de-layed in time and, under the condition of continuous cooling of the melt in the ladle and during casting, it does not have time to reach an equilibrium state.143ZH.D. ZHOLDUBAYEVA et al.: STUDIES OF CHANGES IN THE ACTIVITY OF DISSOLVED OXYGEN…METALURGIJA 62 (2023) 1, 142-144At the same time, heterogeneous nucleation of the oxide phase in liquid steel occurs much easier and 1013-1022 times faster. The activation energy of this pro-cess is significantly less than for homogeneous embryo formation.With regard to the process of separation of the oxide phase from the melt under consideration, it can be writ-ten that the activation energy of its heterogeneous crys-tallization is equal to: ΔGhet = К [(λ RmOn – λhs) / λ RmOn]2, (1)ΔGhet –  activation energy of heterogeneous formation of a new phase / J/mol;К – proportionality coefficient / J/mol;λRmOn and λhs –  geometric parameters of the crystal lat-tices of the new phase and the finished heterogeneous substrate.The closer the deoxidation product and the heteroge-neous embryo are in nature, the smaller the difference between λRmOn и λhs, which means that the activation en-ergy of the heterogeneous formation of a new phase is less. With the identity of the nature of the deoxidation product and the finished substrate, the latter essentially becomes a homogeneous embryo and ΔGhet = 0. In this case, the supersaturation of the melt will be determined only by kinetic factors – the presence of a sufficient number of ready-made germ particles and the delivery to them of the melt components involved in its deoxida-tion reaction. In real melts, there are always particles of non-metallic inclusions suspended in it, acting as ready-made substrates. The deoxidation reaction can also be realized on the surface of the refractory lining of buck-ets. However, completely supersaturation of the melt is not eliminated even under these conditions, obviously due to the insufficient development of the surface of the finished substrates and the lining of the buckets, as well as the delay in the process of delivering deoxidizer and oxygen atoms to them.The nonequilibrium of the melt with respect to the deoxidation reaction creates thermodynamic prerequi-sites for its filtration purification from dissolved oxy-gen. These prerequisites are based on the fact that the filter surface can act as a finished substrate: in this case, the deoxidation reaction is realized directly on the filter surface [4]. In this case, the filter material can partici-pate in the deoxidation reaction, reducing the activity of the RmOn deoxidation product.METHOD OF CONDUCTING THE EXPERIMENTTo identify the underlying mechanism of filtration refining, the interaction of liquid metal with the filter in its separate pore channel was simulated. For this pur-pose, a granular filter with sinuous channels of variable cross-section was presented as a filter with straight cy-lindrical channels. In this regard, cylindrical tubes made of refractory material were considered as the unit cell of the filter.Dissolved oxygen was considered as the removable impurity. This choice was due to the fact that a reliable method was developed to determine its activity based on the use of oxidation sensors – solid-electrolyte oxygen-concentration elements [5]. Similar probes are devel-oped to determine the activity of sulfur dissolved in the melt, but their reliability is still significantly lower than oxygen probes. The processes of oxide and sulfide for-mation proceed in a similar way, which means that if the efficiency of processing the melt with solid-phase adsor-bents is proved by the example of purification from dis-solved oxygen, it will also be adequate for refining the alloy from super equilibrium dissolved sulfur [6].The determination of oxygen by the electromotive force method is based on the occurrence of an oxygen-concentration element immersed directly in a liquid metal by an electromotive force proportional to the log-arithm of the activity (partial pressure) of oxygen in it.The electromotive force of this oxygen-concentra-tion element is determined by the Nernst equation:  (2а)Or , (2b)Where T is the temperature / K;E –  electromotive force of the oxygen con-centration of the elements / mV;F –  gaz constant, 8,313 J / K · mole;R –  electrochemical constant, 96 580 Cou-lomb / g-EQ; –  respectively, the oxygen activity in the reference electrode and in the analyzed liquid metal / % by weight; –  respectively, the partial pressure of oxy-gen in the reference electrode and in the analyzed liquid metal / PA.The main component of the oxygen-concentration element is a solid electrolyte made of stabilized calcium oxide, magnesium or yttrium zirconium dioxide. The specified material was selected based on almost 100 % of its ionic (oxygen) conductivity.The reference electrode is a substance with a known partial pressure of oxygen. Mixtures of Mo + 10 % (by weight) MoO2 and Cr + 2-5 % (by weight) Cr2O3 is usu-ally used as reference electrodes.The partial pressure of oxygen in these reference electrodes for the Cr-Cr2O3 mixture is calculated by the equation: ; (3)and for a mixture of Mo-MoO2 – by the equation: ; (4)Recalculation of values of  calculated from equa-tions 3 and 4 to the corresponding values  and substi-144  METALURGIJA 62 (2023) 1, 142-144ZH.D. ZHOLDUBAYEVA et al.: STUDIES OF CHANGES IN THE ACTIVITY OF DISSOLVED OXYGEN…tuting them in equation 1a allows you to get the calcula-tion formula for the reference electrode from a mixture of Cr-Cr2O3: ; (5)and for the reference electrode from a mixture of Mo-MoO2: . (6)Molybdenum current collectors are used for remov-ing the electric potential of the reference electrode and the analyzed melt.Digital voltmeters, potentiometers, and other volt-age meters with an upper measurement limit of at least 750 mV can be used as a secondary device for recording the electromotive force of oxygen-concentration ele-ments.It is advisable to use potentiometers with continuous recording of readings, such as KSP-4 with measure-ment ranges of 0-10 mV, 0-20 mV, etc. To extend the range of measured voltage, a voltage divider with a total resistance of 8 – 12 Om must be installed at the input of these devices.When setting up the experiment, we assumed that at a constant temperature and unchanged composition of the metal, a change in the concentration of oxygen dis-solved in it is equivalent to a change in its activity.Thus, to determine the efficiency of deposition of harmful ferromanganese impurities on a solid-phase surface, when processing its melt with adsorbents, the activity of dissolved oxygen was measured by the elec-tromotive force method.RESULTSFerromanganese of the FеMn78 brand used as the ob-ject of research. The filter material was quartz and alund tubes, the melt temperature maintained at 1 300 °С.Initially, the initial oxygen activity was determined, after which a package of refractory tubes of the same chemical composition with a sensor inside was injected into the melt at the same depth, followed by removal of the potentiogram. After the potentiometer readings sta-bilized (on the potentiogram – a straight line at one EMF value), the KSP-4 was turned off, and the sensor was lifted out of the melt.According to the method of measuring the activity of dissolved oxygen in a metal melt [7], the following potentiograms of oxygen activity (E / mV) obtained in the initial ferromanganese and after contact of the melt with filters made of MgO, SiO2 and Al2O3, shown in Figure 1 and Table 1.CONCLUSIONSThe Table 1 above shows that the concentration of solute in oxygen steel in contact with filters made of MgO, SiO2 and Al2O3 decreases by 28,1, 32,1 and 18,4%, respectively.Thus, the existence of a substrate mechanism for re-fining from a dissolved impurity is proved experimen-tally on the model of a filter unit cell.LITERATURE[1]  Yavoyskiy V.I., Vishkarev A.F. Zarozhdeniye i formirova. Shipilov, E.B. Tniye nemetallicheskikh vklyucheniy v me-tallicheskom rasplave, Izvestiya vuzov. Chernaya metal-lurgiya, (1980) 11, 13-16.[2]  Luzgin V.P., Yavoyskiy V.I. Gazy v stali i kachestvo metal-la. –M.: Metallurgiya, 1983. -232p.[3]  Ten E.B. Osnovy fil’tratsionnogo rafinirovaniya zhidkikh metallov // Liteynoye proizvostvo, (2013) 1, 18-22.[4] Zholdubayeva Zh.D., Isin D.K., Smagulova Zh.B., Mazhi-tova L.A. Matematicheskaya model’ podlozhechnogo ef-fekta rafinirovaniya metallicheskih rasplavov ot rastvoren-nogo kisloroda// Trudy universiteta. – Karaganda: KarGTU, (2019) 3, 366-370.[5]  Kimanov B.M. Issledovaniye mekhanizma fil’tratsionno-go rafinirovaniya metallov ot rastvora kisloroda // Trudy universiteta. –Karaganda: KarGTU, 2010, 27 - 29.[6]  Fil’trovaniye metalla v liteynoy forme. Sb. mater.po fil’tram, effektivnost’ primeneniya, opytu ispol’zovaniya v otechestvennoy i zarubezhnoy praktike) .- M .: ITTSMMe-tallurg, 2005.- 221 s.[7]  S.Een, Zh.D. Zholdubayeva, D.K. Isin, A.D. Mekhtiyev, S.S. Shipilova, E.V. Yurchenko. Refining of metal melts by filtration method. Мetalurgija, 58 (2019) 3, 303-306.Note: The translation of the N.M. Drag, Karaganda, KazakhstanFigure 1  Influence of the filter material оn changes in oxygen activity in ferromanganeseTable 1  Oxygen activity in ferromanganese before  and after contactMeltdown options Filter materials Е/ mV ΔЕ/ mV ΔЕ/ %Initial - 515 - -FilteredAl2O3 610 +95 18,4MgO 660 +145 28,1SiO2 680 +165 32,1

Studies of changes in the activity of dissolved oxygen in the simulation of ferromanganese filtration

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