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On transportation of nitrogen through molten slag.

Nitrogen in steels is considered not just as a harmful impurity, but also as an independent alloying element. So, in melting or secondary remelting it is necessary in some cases to remove nitrogen from the metal, while in other cases retain it in the metal or alloy metal by it. It is considered that it is impossible in presence of a molten slag, which participates in many metallurgical processes, to alloy metal by nitrogen directly from gaseous phase. That's why alloying of a metal by nitrogen should be performed by introduction into molten metal of nitrated ferroalloys [1, 2] or nitrides of alloying elements [3]. Removal of nitrogen from the metal occurs only as a result of assimilation by slag of nitride inclusions, but not due to transportation of nitrogen to the metal though slag into the gaseous phase [4].

Investigation of nitrogen behavior in melting of steel in electric furnaces and electroslag remelting showed that denitration and nitration are possible, depending upon created conditions of melting [5, 6]. So, in order to answer the question about feasibility of nitration of the metal, covered by a layer of molten slag, directly from the gaseous phase, it is necessary to consider metal melting process for the purpose of determining conditions of nitrogen transportation through slag.

Transportation of nitrogen though slag is called gas permeability, which is determined as product of ultimate nitrogen solubility in slag [C.sub.N] and nitrogen diffusion coefficient in slag [D.sub.N] [7-9]:

[P.sub.N] = [C.sub.N][D.sub.N].

Two kinds of diffusion (molecular and convective ones) are present in the slag thickness, but it is not found out yet, which of them limits nitrogen transportation process. Taking into account methodology of estimation using Peclet number [10, 11], one may state that in the process of nitrogen mass transfer in slag the share of molecular diffusion, in comparison with the convective one, is negligently small.

Solubility of nitrogen in slag was determined in investigation of the double system gas-slag, while nitrogen permeability assumes transfer of the gas phase nitrogen through slag to the metal in the triple system gas-slag-metal. It was established in investigation of the latter that transportation properties of slag are determined by thermodynamic conditions, created not just on the interface gas-slag, but also on the interface slag-metal [12-14]. That's why it's not so much solubility of nitrogen in slag, as driving force of the whole process of nitrogen transfer from gas to the metal, which consists in difference of the nitrogen chemical potential value in different areas of the gas-slag-metal system, has to be taken into account. So, nitrogen permeability, as well as diffusion, is an energy-dependent process and occurs more intensively under action of additional energy pulses supplied from outside, for example, in case of arc action on the gas-slag-metal system [6].

For the purpose of checking mentioned assumptions we carried out comparative experiments--two series of melting in the Tamman furnace in nitrogen with application of graphite crucibles. In the first series slag, and in the second series metal (steel Kh18N10) under layer of slag was melted. Slag of oxide and fluoride-oxide systems was used. Content of nitrogen in the slag and the metal was determined using Kjeldahl method.

Results of the investigations showed that weight share of nitrogen in slag is higher in case of interaction of the gas phase nitrogen with slag without participation of metal than in case of interaction of nitrogen with slag in presence of metal. So, for example, in case of the gas phase contact with slag without metal solubility of nitrogen in slag CaO-[Al.sub.2][O.sub.3]-15% Ti[O.sub.2] constitutes 0.136%, in flux AN-295 (16% Ca[F.sub.2]-51% [Al.sub.2][O.sub.3]-31% CaO)--0.119%, while in presence of metal solubility in them equals 0.026 and 0.052%, respectively. Content of nitrogen in the metal equals 0.035% (slag CaO-[Al.sub.2][O.sub.3]-15% Ti[O.sub.2]) and 0.026% (slag AN-295), which is lower than equilibrium concentration 0.17%, calculated for conditions of interaction of molten metal with the nitrogen atmosphere. So, slag under described conditions transfers nitrogen from gas phase to the metal and does not keep it in its volume.


For explanation of noted ambiguities of gas permeability processes of slag melts it is necessary to present on the basis of literature data mechanism of nitrogen transport through molten slag, and consider for this purpose structure of the latter.

There are several models of slag structure, which originated not simultaneously, but by means of the liquid structure theory development. According to state-of-the-art assumptions, a slag represents solution of ions [15] with available in the solution free electrons [16]. Ions may be in the form of inactive multi-atom complexes and in the form of freely moving multi-atom and free complexes. Presence of free ions in oxide systems is improbable, but it is probable in fluoride and fluoride-oxide systems, in which exchange reactions may proceed with formation of volatile fluoride compounds and free ions [17-19]. Constructively the slag melt is a set of closely packed spheres (single- or several atom ions [20, 21]), between which, according to the Frenkel's liquid <<hole>> model, cavities (<<holes>>) are present. In them (depending upon their size) may be located dissolving in slag single or complex ion. These cavities, in contrast to vacancies in crystal bodies, may have different sizes and changeable parameters [22].

Number of <<holes>> determines value of such structurally-sensitive properties of a molten slag as density and surface tension. According to the <<hole>> model, probable volume of a <<hole>> V and surface tension s are connected by the following dependence:

V = 0.68 [(kT/[sigma]).sup.3/2], (1)

where k is the Boltzmann's constant; Tis the absolute temperature.

Having determined radius of the <<hole>>, one can estimate size of the particles, which are able to be placed in them.

Viscosity of a liquid depends upon both value of the viscous flow activation energy and free volume level. This dependence is expressed by formula of A.I. Bachinsky [9]:

where A is the constant; [V.sub.S] is the free volume of the liquid equal to the difference of its own and specific volumes.

So, free volume of the liquid equals total of the volumes of all <<holes>> [V.sub.S] = nV, and its viscosity is connected with surface tension by the expression, obtained as a result of integration of the dependences (1) and (2):

[eta] = A/0.68n [([sigma]/kT).sup.3/2]

The higher is number of <<holes>>, the lower is viscosity, and the higher is surface tension, the higher is viscosity. It is stated in descriptions of some investigations [23, 24] that when the slag melts are saturated by nitrogen, their viscosity and surface tension increase. So, during dissolution nitrogen and its compounds fill the <<holes>> and reduce free volume, thus causing growth of the value of structurally sensitive properties.

Mentioned changes of the structure are proved by the following. If, for example, to construct dependence of viscosity of the slag melts upon temperature in coordinates lg [eta]-1/T (in the Figure the dependence is shown for slag 50% Ca[F.sub.2], 25% CaO, 25% [Al.sub.2][O.sub.3]), registered breaks on the diagrams will correspond to the temperature of structural change of the melts that causes variation of the number (or size) of the <<holes>> [24, 25]. Similar regularities were detected in processing of the results of electric conductivity investigations of various kinds of slag [26].

Solubility of nitrogen in slag, in opinion of many researchers, follows Sieverts law and depends upon degree of oxidation of the gas phase above molten slag [16, 27]. Driving force of nitrogen movement is difference of its concentrations according to the degree of oxidation of different molten slag layers. And at the same time it should be noted that nitrogen is present in molten slag not as an independent ion, but in the form of compounds with a cation, characterized by high affinity to nitrogen. From the whole set of frequently occurring slag components, one may single out such cations with high affinity to nitrogen as titanium, aluminum, calcium and silicon, and carbon.

Silicon is present in slag in the form of thermodynamically strong dioxide formed strong oxide complexes. That's why silicon, evidently, can not be bound with nitrogen. Titanium, as well as silicon, is present in slag in the form of thermodynamically strong oxides and, probably, can not form nitride compounds. Calcium (under certain conditions and, possibly, aluminum) can precipitate, as a result of exchange reactions, in free state and form nitride compounds in the slag melt [28, 29].

Metal calcium, added into slag in remelting, not just deoxidizes molten slag, but, when evaporating (due to high pressure of its vapor) reduces oxidation potential of the gas phase and enables improved transportation of nitrogen from gas into the metal [30, 31].

In opinion of many researchers, carbon can form in slag melts radicals of the type of cyanides [CN.sup.-] or cyanamides [CN.sup.2-.sub.2] [32, 33]. But these radicals can not, in all evidence, exist by themselves and should have connection with cations of metals.

That's why our further investigations will be directed at study of transport properties of fluoride-oxide slag systems.

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[21.] Sokolsky, V.E., Kazimirov, V.P., Kuzmenko, V.G. (2000) Model of structure of oxide-fluoride melts on the base of close packing of oxygen atoms. Problemy Spets. Elektrometallurgii, 4, 63 -73.

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[24.] Zhmojdin, G.I., Moldavsky, O.D. (1971) Toughness of fluorine-containing melts. Izvestiya AN SSSR. Metally, 1, 70-73.

[25.] Zhmojdin, G.I. (1969) Fusibility of fluorine-containing slags. Ibid., 6, 9-16.

[26.] Zhmojdin, G.I. (1970) Electroconductivity of fluorine-containing melts. Ibid., 3, 69-74.

[27.] Utochkin, Yu.I., Pavlov, A.V., Froyde, T. et al. (1993) Effect of oxidation potential on solubility of nitrogen in slag melts. Izvestiya Vuzov. Chyorn. Metallurgiya, 3, 10-15.

[28.] Povolotsky, D.Ya., Krichevets, M.I., Kozheurov, V.A. (1966) Distribution of calcium between steel and slag containing two anions of different valency. Izvestiya AN SSSR. Metally, 2, 5-8.

[29.] Filippov, S.I., Dedushev, L.A., Klyuev, M.M. (1972) Analysis of process of calcium reduction with aluminium from fluoride slags. Izvestiya Vuzov. Chyorn. Metallurgiya, 3, 19-24.

[30.] Ryabtsev, A.D., Troyansky, A.A., Korzun, E.L. et al. (2002) Metal alloying with nitrogen from gas phase in ESR process. Advances in Electrometallurgy, 4, 2-6.

[31.] Grigorenko, G.M., Lakomsky, V.V., Ryabtsev, A.D. et al. (2003) About feasibility of nitrogen transport through fluoride slags, containing metal calcium, in ESR. Ibid., 4, 11-13.

[32.] Kamyshov, V.M., Esin, O.A., Chuchmarev, S.K. (1964) Nitrogen solubility in iron-free slags. Izvestiya Vuzov. Chyorn. Metallurgiya, 7, 24 -28.

[33.] Ershov, G.S., Orlov, Yu.G. (1965) Nitrogen behavior in slag and metal phases during melting of alloy steels. Izves tiya AN SSSR. Metally, 6, 28-37.


E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine
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Author:Lakomsky, V.V.; Pomarin, Yu.M.; Grigorenko, G.M.; Orlovsky, V.Yu.
Publication:Advances in Electrometallurgy
Article Type:Report
Date:Jul 1, 2006
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