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Innovation technological processes of electric furnace ferronickel refining by progressive industrial methods: information 2. thermodynamic investigations of processes and technology of ladle desulfuration of electric furnace ferronickel by sodium carbonate.

Composition of commodity ferronickel produced at PEIW meets requirements of TU U27.3-31076956009:2005.

For the purpose of improving quality and increasing competitiveness of ferronickel on international market, systemic developments directed at improvement of technology and electro-thermal equipment have been lately carried out at the integrated works, which will allow improving quality of ferronickel up to the level envisaged by the draft branch standard being developed (Table 1).

Ferronickel, produced by the method of ore-thermal reduction electric melting of import New Caledonian ore (2.2--2.5 % Ni), has the following chemical composition, wt.%: 15--17 Ni; 0.3--0.4 Co; 0.5--5.0 Si; 0.5--2.0 Cr; 1.8--2.5 C; 0.013--0.020 Cu; 0.2--0.4 S; 0.01--0.02 P, the rest--iron. For reducing content of impurity elements, electric furnace (crude) ferronickel is subjected to three-stage refining: desulfuration by soda in ladle; oxidation refining in converter with acid (silica brick) lining for the purpose of reducing content of chromium and silicon; oxidation refining in converter with basic (periclase-carbon or periclase-chromite) lining for dephosphorization, decarburization, and final oxidation of silicon and chromium under basic slag.

In this work materials of analysis of thermodynamic premises and main principles of the technology of out-of-furnace (ladle) desulfuration of electric furnace ferronickel by sodium carbonate (soda ash) are presented.

Thermodynamic premises of electric furnace ferronickel desulfuration. Electric furnace ferronickel represents an iron-carbon melt, containing 15-17 % Ni and increased content of impurity elements [1]. Impurities may be divided according to their influence on sulfur activity coefficient [f.sub.S] into three groups (Figure 1): those increasing [f.sub.S] (carbon, silicon, aluminum, phosphorus); those reducing [f.sub.S] (copper, manganese, oxygen, sulfur); and those not effecting [f.sub.S].

Influence of base (nickel and iron) and impurity elements in ferronickel on activity coefficient and, as a result, on sulfur activity may be estimated by parameters of interaction of first order (the Wagner parameter), which represent ratio of partial derivative of logarithm of activity coefficient of an element (in this case sulfur) to weight share (%) of the impurity

element [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] Positive value of [A.sup.j.sub.S] means that

this element i increases activity of sulfur; negative value means that it reduces the activity, zero value means that it does not exert any effect. Below are given values [a.sup.j.sub.S], where j are the elements, which enter into composition of ferronickel ([a.sup.j.sub.S]-100 in iron at 1600 [degrees]C):



It follows from presented data that carbon, silicon, aluminum and phosphorus increase activity coefficient of sulfur in ferronickel, while copper, chromium, manganese, sulfur and oxygen reduce it. Nickel, dissolved in iron, practically does not affect activity of sulfur in liquid iron. Activity and activity coefficient of sulfur reduce as temperature increases. Values [a.sup.j.sub.S] in different literature sources significantly vary.

Many impurity elements in ferronickel are characterized by ability to reduce activity coefficient of sulfur. But more important property of desulfurizerelements is their ability to enter into reaction with formation of sulfides, which have high thermodynamic stability and low solubility in ferronickel.

Knowing parameters of interaction, let us determine activity coefficient of sulfur in crude ferronickel [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]:


So, in crude ferronickel before its refining by soda lg [f.sup.[sigma].sub.S] = +0.6348. At the final stage of refining in basic converter content of oxygen may constitute up to 0.15 wt.%, if mass share of (each) impurity element in ferronickel is 0.01--0.02 %. At this chemical composition of ferronickel lg [f.sup.[sigma].sub.S] = 0.0137 and ratio lg [f.sup.[sigma].sub.S]: lg [f.sup.K.sub.S]) = 0.6348 : 0.0137 = 46.34, i.e. lg fP is, ap proximately, 46 times lower than in lg [f.sup.[sigma].sub.S] of crude ferronickel.


Characteristic of reagents for desulfuration of ferronickel. Different substances-reagents, including metal magnesium, calcium carbide Ca[C.sub.2], lime and sodium carbonate [Na.sub.2]C[O.sub.3] are used for commercial out-of-furnace desulfuration of cast iron. For binding 1 kg of sulfur dissolved in ferronickel it is necessary to introduce into sulfides the following amount of reagent [Q.sub.r]:


Temperature dependences of the equilibrium constant and change of the Gibbs reaction of formation of sulfides MgS, CaS and [Na.sub.2]S with application of different reagents are given in Table 2. Thermodynamic data of the reaction of [Na.sub.2]S formation with application of metal sodium were calculated by us, while the rest data were taken from works [4,5]. Conditions [DELTA][G.sup.0.sub.T] = 0 at [P.sub.Me] = 101.3 kPa for reaction with participation of magnesium, calcium and sodium are observed at the temperatures 2365, 7355, and 3937 K, respectively.

According to data presented in [4], [Na.sub.2]O is by three-four orders stronger desulfurizer than CaO. However, this conclusion does not match data of enthalpies of formation of sulfides CaS and [Na.sub.2]S. Thermodynamic preferability of [Na.sub.2]S formation in comparison with MgS in case of using respective reagents is confirmed by enthalpy of formation of sulfides:


Thermodynamic stability of compounds in the process of ferronickel desulfuration. Chemistry of the process of electric furnace ferronickel desulfuration by soda may be presented by the summary reaction:


One of the most important discussion problems of processing iron-carbon melt by sodium carbonate at high temperatures (1300--1500 [degrees]C) is mechanism of the process with estimation of sequence of separate stages of [Na.sub.2]C[O.sub.3] thermal dissociation of formation of intermediate substances and, in long run, of sulfide [Na.sub.2]S. Below results of analysis of state-of-the-art thermodynamic data on thermal dissociation of [Na.sub.2]C[O.sub.3] and probable scheme of interaction sequence of dissociation products with the sulfur of ferronickel are given.

Sodium carbonate has to be considered as product of interaction of components in the [Na.sub.2]O--C[O.sub.2] system.

Melt point of [Na.sub.2]C[O.sub.3] equals 850 [degrees]C, boil point--2597 [degrees]C. Several publications are devoted to investigation of the mechanism (chemistry) of thermal dissociation of [Na.sub.2]C[O.sub.3]. In [6] composition of vapor phase above specimens and [Na.sub.2]C[O.sub.3] was studied using effusive method and it was established that thermal dissociation occurs according to the following reaction:


Generalization and critical analysis of literature data of [Na.sub.2]C[O.sub.3] thermal dissociation in case of evapo ration of double oxides are presented in the monography [7]. It is established by isotherms of [Na.sub.2]C[O.sub.3] evaporation that at 959 [degrees]C partial pressure of sodium vapors equals P(Na) = 5.26, and at 969 [degrees]C P(Na) = = 7.43 Pa.

In [8] mechanisms of [Na.sub.2]C[O.sub.3] evaporation are studied using Knudsen method within temperature range 850--983 [degrees]C. It is established that vapor pressure of molecules [Na.sub.2]C[O.sub.3(g)] above solid carbonate at temperature about 850 [degrees]C equals 11.83 Pa, and above the melt (925 [degrees]C) [D.sub.([Na.sub.2]C[O.sub.3])] -[10.sup.3] = 243.3 Pa.

Temperature dependences of partial pressure values of molecules [Na.sub.2]C[O.sub.3] in sodium carbonate sublimation (solid state) and evaporation (liquid state) have the following form, Pa:


According to data of [7], temperature dependence of pressure of thermal dissociation products has the following form, Pa:


and heat of thermal dissociation reaction [DELTA][H.sup.0.sub.T] = = 192.28 kJ/mol.

The main product of [Na.sub.2]C[O.sub.3] interaction with sulfur of ferronickel is sodium sulfide. In analysis of the cast iron and ferronickel desulfuration process this compound is assumed to be [Na.sub.2]S. At the same time in the system Na--S exists a number of compounds, although diagram of equilibrium state of mentioned system, as it follows from reference data, is not yet, unfortunately, built. It is not mentioned in [9] about existence of any sodium sulfides. Review of 16 publications (1898--1953) is made in [10] and sulfide [Na.sub.2]S3 is noted, which melts at 230 [degrees]C and disintegrates at heating above 550 [degrees]C into [Na.sub.2][S.sub.1.9].

F. Schank [11] added data of R.P. Elliot, devoted to determination of structure of -[alpha] and [beta]-[Na.sub.2][S.sub.2], by analysis of [10]. Low-temperature modification of [alpha][Na.sub.2][S.sub.2] exists at temperatures below 100 [degrees]C. Phase [beta]-[Na.sub.2][S.sub.2] forms in heating of a-[Na.sub.2]S2 up to 250 [degrees]C. Need in new investigations for checking literature data and further study of compounds in the system Na--S is noted [12].

In [13] data are presented on formation heat, standard entropies, and structure of a number of sulfides of the system Na--S according to data of the work of K.S. Mills (1974). Values of formation heat and entropy of sodium sulfides are as follows:


Sulfide [Na.sub.2]S has crystal lattice of Ca[F.sub.2] type. Temperature dependence of the Gibbs energy change of [Na.sub.2]S formation from the elements has the form


Condition [DELTA][G.sup.0.sub.298] = 0 at pressure 101.3 kPa is fulfilled at 3346 K. These data confirm that sulfide [Na.sub.2]S is thermodynamically rather stable compound, although sulfide [Na.sub.2][S.sub.2] is also characterized by increased stability ([DELTA][H.sup.0.sub.298] = -4 32.2 kJ/mol).

Data on thermodynamic stability of [Na.sub.2]C[O.sub.3] are of interest. According to [13], standard heat of [Na.sub.2]C[O.sub.3] formation is [DELTA][H.sup.0.sub.298] = 1129.85 kJ/mol, entropy [S.sup.0.sub.298] = 138.65 J/(K-mol).

As far as thermal dissociation of [Na.sub.2]C[O.sub.3] is accompanied by formation of Nag, C[O.sub.2] and 1/2[O.sub.2], one may assume that probability of oxidation of dissolved in ferronickel silicon by soda exists with formation of sodium silicate according to the reaction


So, oxidation of dissolved in ferronickel silicon by soda according to presented endothermic reaction is possible in respect to thermodynamics. Temperature increase should enable shifting of the reaction in direction of formation of the products (2[Na.sub.2]O-Si[O.sub.2]) and 2CO.

In ladle desulfuration of ferronickel parameters of the process are optimized first of all for the purpose of achievement of as high as possible degree of desulfuration at lower specific consumption of soda.

Process of ladle desulfuration of ferronickel by soda is performed at temperature values 1450--1500 [degrees]C with formation in addition to sulfide [Na.sub.2]S of silicate [Na.sub.2]Si[O.sub.3]. Result of analysis of thermodynamic data on processes of evaporation of melts of double oxides of the system [Na.sub.2]O--Si[O.sub.2] is of interest.

In system [Na.sub.2]O--Si[O.sub.2] form a number of silicates. Melt points of the most investigated compounds are as follows:


Evaporation of the [Na.sub.2]O--Si[O.sub.2] system melts within temperature range 870--1100 [degrees]C was studied by method of mass-spectrometry (Table 3). In massspectrum of steam-gaseous phase of the [Na.sub.2]O--Si[O.sub.2] system, containing from 6 to 50 mol% [Na.sub.2]O, within temperature range 870--1100 [degrees]C only ions [Na.sup.+] and [O.sup.+.sub.2] were detected.

In [14] thermodynamic properties of the [Na.sub.2]O-Si[O.sub.2] system were studied using method of the Knudsen mass-spectrometry. In mass-spectra of saturated vapor above [Na.sub.2]O--Si[O.sub.2] ions [Na.sup.+], [Na.sub.2][O.sup.+], Na[O.sup.+] and [O.sup.+.sub.2] were detected, which formed as a result of ionization of gaseous molecules Na, [Na.sub.2]O, NaO, and Na[O.sub.2], [O.sub.2]. On the basis of these experimental data activity of Si[O.sub.2] and [Na.sub.2]O in the [Na.sub.2]O--Si[O.sub.2] system was determined.

Data of [15], in which qualitative analysis of vapor phase is performed and thermodynamic parameters of the [Na.sub.2]O--Si[O.sub.2] system (excessive molar free Gibbs energy and heat of formation) are estimated, are of interest.

Under conditions of treatment by soda of carbonaceous ferronickel with high content of silicon and carbon, probability of sodium carbonate interaction with dissolved in metal chromium with formation of sodium chromates, in which chromium has different degrees of oxidation, is insignificant. Sodium chromates are characterized by relatively low melt points: [Na.sub.2]Cr[O.sub.4]--790 [degrees]C and [Na.sub.2][Cr.sub.2][O.sub.7]--350 [degrees]C.

It was established in investigation of volatility of sodium (potassium) chromates within temperature range 714--768 [degrees]C that evaporation of [Na.sub.2]Cr[O.sub.4] occurs congruently [16]. Temperature dependence of pressure within temperature range 714--768 [degrees]C is described by the equation


Heat of solid chromate [Na.sub.2]Cr[O.sub.4] sublimation [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] equals 96.98 kJ/mol.

Process of [Na.sub.2]Cr[O.sub.4] evaporation within temperature range 577--1177 [degrees]C was studied using method of mass-spectrometry, and results of the experiments are presented graphically [17]. In the course of processing of these data sublimation heat [DELTA][H.sup.0.sub.subl] of chromate [Na.sub.2]Cr[O.sub.4] and its atomization energy [DELTA][] were calculated, [J/mol]:


So, chromate [Na.sub.2]Cr[O.sub.4] is characterized by high thermodynamic stability and in case of ferronickel desulfuration by soda (1.5--2.0 % Cr) may, evidently, form and dissolve in soda slag. Its existence in this slag depends upon content of silicon in ferronickel, whereby as content of silicon increases probability of chromate formation reduces. Presence in soda slag of small amounts of silica should enable formation of silicates of bivalent chromium 2CrO-Si[O.sub.2].

Technological operations in case of ladle desulfuration of ferronickel by soda. Through technology of ferronickel refining consists of the stage of desulfuration by soda in ladle with subsequent oxidationreduction refining processes in vertical converters with acid and basic fire-brick lining. Sodium carbonate in the form of powder (soda) or fusion cake (secondary material) is used as a desulfurizer. Ferronickel is discharged from electric furnace into a ladle with a certain amount of soda. In the process of ferronickel discharge its mixing with soda takes place, which at 850 [degrees]C transits into liquid state. For achieving efficient mixing it is necessary to determine critical height of liquid ferronickel fall.


Let us designate constant of reaction rate between ferronickel and soda melt by symbol [K.sub.r], and in case of their mixing--by symbol [K.sub.m]. We accept ratio [K.sub.r]/[K.sub.m] as a convenient criterion for quantitative change of the action of mixing components of the model.

It was established in experiments connected with modeling of influence of mixing in the system amalgam (mercury with 0.2 % Na)--H2S[O.sub.4] that pouring of amalgam into the acid ensures higher degree of mixing in comparison with blowing of the acid by gas or use of a mixer [18]. It should be noted that after exceeding critical value of the jet fall height efficiency of mixing increases insignificantly.

Products of desulfuration are thermodynamically strong sulfide [Na.sub.2]S and sodium silicate, which dissolve in liquid sodium carbonate and form <<soda>> slag. For the purpose of increasing desulfuration degree double pouring of ferronickel from a ladle into a ladle is used, whereby into the ladle designed for receiving ferronickel from notch of the furnace 70 % of the required amount of soda (fusion cake) is placed. Then ferronickel with soda slag is poured into another ladle, into which the rest amount of soda (30 %) is placed.

In case of ferronickel treatment by soda ash, degree of desulfuration constitutes 70--85 %. Activity of sulfur in electric furnace ferronickel is effected by dissolved in the latter silicon and carbon; together with increase of content of these elements activity of sulfur increases, and thermodynamic premises of more efficient desulfuration of ferronickel enhance. For establishing analytical connection of dependence of sulfur content in ferronickel and desulfuration degree upon concentration of silicon in the latter, graphic dependences (Figure 2) were obtained by means of mathematical processing of a big array of experimental data on commercial melting designed for ferronickel desulfuration by sodium carbonate, which are described by the following equations:

[[% S]] = -0.8961 lg [% Si] + 0.1211; hS = 21.9274 lg [% Si] + 65.826.

Technological operations of ferronickel ladle desulfuration finish by pumping off <<soda>> slag. At the next stage of refining ferronickel (intermediate product) is poured into converter with acid (silica brick) lining and subjected to oxygen blowing for the purpose of oxidizing silicon, chromium, and partially carbon. At the final stage ferronickel is discharged from acid converter into a ladle with its subsequent pouring into the basic converter with periclase-carbon (periclase-chromite) lining, in which carbon and phosphorus are oxidized and content of sulfur is reduced. In melting ferronickel using rich nickel ore (from New Caledonia) content of phosphorus in electric furnace ferronickel exceeds 0.015--0.017 %, that's why refining process in the basic converter is performed for the purpose of oxidizing carbon and further reduction of sulfur amount.

So, on the basis of data of thermodynamic investigations under conditions of PFIW Ltd. innovation-industrial technology was developed for out-of-furnace desulfuration of electric-furnace high-percent ferronickel by sodium carbonate.

[1.] Reznik, I.D., Ermakov, G.P., Shneerson, Ya.M. (2003) Nickel. Vol. 2. Moscow: Nauka i Tekhnologiya.

[2.] Grigoryan, V.A., Stomakhin, A.Ya., Ponomarenko, A.G. et al. (1989) Physico-chemical calculations of electric melting processes: Manual for institutes of higher education. Moscow: Metallurgiya.

[3.] (1965) Steel production in electric furnaces: Collect. Ed. by V.A. Grigoryan. Moscow.

[4.] Vegman, E.F., Zherebin, B.N., Pokhvisnev, A.N. et al. (1978) Cast iron metallurgy. Moscow: Metallurgiya.

[5.] Efimenko, G.G., Gimmelfarb, A.A., Levchenko, V.E. (1981) Cast iron metallurgy. Kiev: Vyshcha Shkola.

[6.] Semyonov, G.A., Volkov, A.D., Frantseva, K.E. (1973) Mass spectrometric examinations of carbonate sodium evaporation. Trudy Leningrad. Tekh. In-ta Tsellul.-Bumazhn. Promyshl., 30, 153 -160.

[7.] Kazenas, E.K. (2004) Thermodynamics of double oxide evaporation. Moscow: Nauka.

[8.] Kroger, C., Stratman, J. (1961) Vaporization and Silicat-glaser and decomposition pressures of alkali compounds of glass melt. Glas Tech. Ber., 34(6), 311-314.

[9.] Hansen, M., Anderko, K. (1962) Structures of binary alloys. Moscow: Metallurgizdat.

[10.] Elliot, R.P. (1970) Structures of binary alloys. Vol. 2. Moscow: Metallurgiya.

[11.] Schank, F. (1973) Structures of binary alloys. Moscow: Metallurgiya.

[12.] (1997) Constitution diagrams of binary metallic systems: Refer. Book. Ed. by N.P. Lyakishev. Vol. 3. Moscow: Mashinostroenie.

[13.] Kubashevsky, O.I., Olkokk, S.B. (1982) Metallurgical thermochemistry. Moscow: Metallurgiya.

[14.] Zajtsev, A.I., Shchelkova, N.V., Mogutnov, B.M. (2000) Thermodynamical properties of alloys of system [Na.sub.2]O--Si[O.sub.2]. Izvestiya RAN. Neorgan. Materialy, 36(6), 647 -662.

[15.] Begman, C., Bennour, F., Chastel R. et al. (1998) Etude thermodynamique des systemes d'oxydes par effusion de Knudsen couplee a la spectrometrie de masse. Rev. Met., 95(9), 1101 -1108.

[16.] Afonsky, I.S. (1962) Study of volatility of sodium and potassium chromate. Zhurnal Neorgan. Khimii, 7(11), 2640--2641.

[17.] Kuligina, L.A., Semyonov, G.A. (1979) Mass spectrometry analysis of dissociation energy of gaseous chromates of alkali metals and thallium. In: Abstr. of 8th All-Union Conf. on Calorimetry and Chemical Thermodynamics (Ivanovo, Sept. 1979).

[18.] Shanakhan, K.E. (1963) Study of effectiveness of slag and metal stir on models. In: Physical chemistry of steelmaking. Moscow: Metallurgizdat.


<<Pobuzhsky Ferronickel Integrated Works, Ltd.,>> NMetAU, Dnepropetrovsk, Ukraine
Table 1. Requirements to chemical composition of ferronickel produced
by method of ore-reduction melting in electric furnaces
(draft branch standard for ferronickel)

Ferronickel Share of elements, wt.%
 Ni Co Nu

FN-5M 4.6-50.0 Ratio of Co Not more
 to Ni is not than 0.3
 more than 1:30
FN-5K 14.0-20.0 1.5-8.0 1.0-3.0
FN-6 Not more 0.1-0.4 Not less than
 than 5.0 0.01

Ferronickel Share of elements, wt.%
 Si Cr C S P

 Not more than

FN-5M 2.0 1.5 1.5 0.08 0.04
FN-5K 0.05 0.08 0.03 0.3 0.05
FN-6 6.0 3.0 3.0 0.1 0.15

Note. The rest is iron.

Table 2. Temperature dependences of equilibrium constants and
[DELTA][G.sup.0.sub.T] (O) of desulfuration reactions of
iron-carbon melts using different

Desulfurizer- Reaction lg K(1/T)
element [DELTA][G.sup.0.sub.T],
 J/ mol

Mg [Mg.sub.g] + [S] = lg [K.sub.Mg] = lg
 Mg[S.sub.s] [[a.sub.MgS]/
 = 22750/T -9.63
 = -435138 + 183.92T

Na [Na.sub.g] + [S] = lg [K.sub.Ca] = lg [a.sub.CaS]
 Ca[S.sub.s] ([P.sub.Ca][[S].sup.f.sub.S])]
 = 29806/T -8.94
 = -570200 + 171.0T

Na 2[Na.sub.g] + [S] = lg [K.sub.Na] = lg [[MATHEMATICAL
 ASCII.]/([[P.sub.Na] [[S].sup.f
 .sub.S]] = 15190/T -4.12

CaO Ca[O.sub.s] + [S] lg [K.sub.CaO] = lg [([a.sub.CaS]
 + [] = [P.sub.CO]/([a.sub.CaO][a.sub.c]
 Ca[S.sub.s] + CO [[S].sup.f.sub.S])] = - 5540/T
 + 5.157

[Na.sub.2]O [Na.sub.2][O.sub.1] lg [MATHEMATICAL EXPRESSION
 [Na.sub.2][S.sub.1] lg. ([MATHEMATICAL EXPRESSION
 ASCII.] [a.sub.C] [[S].sup.f.
 sub.S]) = 4400/T -5.74
 [DELTA][G.sup.0.sub.T] =
 -83600 + 109.06 T

Na[N.sub.2] Na[N.sub.2s] + [S] lg [MATHEMATICAL EXPRESSION
 2 [] = lg [([a.sub.CaS][a.sup.2.sub.C]
 [[S].sup.f.sub.S]] =
 19000/T - 6.28
 [DELTA][G.sup.0.sub.T] =
 = -363240 + 120.05T

Note. In lower indices letters g, s, gr and 1 mean gaseous, solid,
graphitized and liquid.

Table 3. Value of partial pressure of vapor [Na.sub.g] above melts of
the [Na.sub.2]O--Si[O.sub.2] system [7]

[Na.sub.2]O D([Na.sub.g]) * [10.sup.3] Pa, at O, K
mole share x 1293 1423

0.06 3.99 39.9
0.22 10.64 230.1
0.36 106.4 518.7
0.40 199.5 970.9
0.50 212.8 2660.0
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Author:Novikov, N.V.; Kapran, I.I.; Gasik, M.I.; Ovcharuk, A.N.
Publication:Advances in Electrometallurgy
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Date:Apr 1, 2006
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