reaction equilibria in the production of manganese ferroalloys

Reaction Equilibria in the Production of Manganese Ferroalloys

WEIZHONG DING and SVERRE E. OLSEN

A laboratory investigation has been carried out to determine slag/metal and slag/metal/gas equilibria relevant to production of manganese ferroalloys. The metal phase was normally composed of Mn- Si-C,a, alloys, but in some experiments, the alloys contained up to 15 wt pct Fe. Different slag systems were used: MnO-SiO2, MnO-SiO2-CaO, MnO-SiO:A1203, and quaternary MnO-SiO:CaO- A1203 with fixed CaO/Al~O3 weight ratios of 1.5 and 3. The experiments were normally made in CO gas atmosphere at temperatures ranging from 1450 ~ to 1600 ~ The results give comprehensive information about equilibrium relations. Partial and complete equilibria are illustrated in equilibrium diagrams. Partial equilibrium is a situation in which equilibrium is established with respect to certain variables but not to others, in this case, between slag and metal but not with the gas phase. The effect of temperature was found to be of minor importance for the partial slag/metal equilibrium, whereas the complete slag/metal/gas equilibrium is considerably influenced by both temperature and CO pressure. As expected, increasing temperature and decreasing CO pressure will reduce the equilibrium MnO content of slags. The influence of alumina addition to the slag phase and of iron to the metal phase is also discussed.

I. INTRODUCTION

MANGANESE ferroalloys are either produced in blast furnaces or in electric furnaces. The alloys are classified as high-carbon ferromanganese, medium- and low-carbon ferromanganese, and silicomanganese. High-carbon ferromanganese and silicomanganese are produced by carbothermic reduction, whereas medium- and low-carbon ferromanganese are produced by oxygen refining of high carbon ferromanganese or by silicothermic reduction of MnO dissolved in slag.

Several investigations have been carried out over the years to study equilibrium relations associated with the production of manganese ferroalloys, tt-~~ Even though most industrial slags and alloys have been included in these stud- ies, the results have rarely been used to predict and control the production processes. The difficulty arises from a misty knowledge of the real state of equilibrium and confusion due to complex multicomponent slags.

It is well known from the literature tn-~sl that silicon addition to manganese melts gives rise to a considerably de- creased carbon solubility, for example, from 8 wt pct C for Mn-C,,, to 1.8 wt pct C for Mn-Si (18 wt pct)-C~, at 1500 ~ The solubility is somewhat affected by iron, i.e., the higher the Fe/Mn ratio, the lower the carbon content of the melt. The silicon content at the graphite/SiC coexistence point varies with the Fe/Mn ratio and the temperature, being, for example, 22.0 wt pct Si at 1550 ~ for an iron alloy (Fe-Si-C,~,) and 18.77 wt pct Si for a corresponding manganese alloy (Mn-Si-C~,). t~9,2~

WEIZHONG DING, formerly Doctorate Student, Department of Metallurgy, University of Trondheim, Norwegian Institute of Technology, is Associate Professor, Department of Metallurgy, Shanghai University of Technology, 200072 Shanghai, China. SVERRE E. OLSEN, Professor, is with the Department of Metallurgy, University of Trondheirn, Norwegian Institute of Technology, N-7034 Trondheim, Norway.

Manuscript submitted December 1, 1993.

A number of investigations have been undertaken to measure activities in manganese alloys, t2,~2,21-2sl Although the published data are not in satisfactory agreement with each other, a general tendency is obvious. Both carbon and silicon additions lower the Mn activity and give large neg- ative deviations from ideality. Activities have also been measured in the slag system. Some results have been reported for binary MnO-SiO2 slags and for different ternary and multicomponent slag systemsY 9-371

Several investigations have been carried out to determine distribution equilibria between the slag and metal phases. Tuset et aLt3] and Kol q6] studied slag/metal equilibria for low-carbon alloys, whereas carbon-saturated alloys were employed in the works of Turkdogan and Hancock,t21 Ran- kin and See, tSJ Tanaka, I7] and Gzielo and Pacula: 1~ These authors did not study the equilibrium with the gas phase, however. The equilibrium between a carbon-containing metal and an oxide slag will define a certain CO pressure. Complete equilibrium with the gas phase is only achieved when this CO pressure is equal to that of the atmosphere. Equilibrium with the atmosphere is established very slowly, however.

Both slag/metal and slag/metal/gas equilibrium relations are of considerable industrial interest. Such equilibrium relations have been studied, and the results are presented and discussed in this article.

II. CHEMICAL EQUILIBRIUM RELATIONS

Under strongly reducing conditions and in the normal temperature range of production, any iron in the raw materials goes almost completely into the metal phase. Cal- cium, aluminum, and magnesium always exist in the form of oxides. Their contents in the metal phase are very small and may be neglected. Only manganese and silicon are dis- tributed between both condensed phases. The reactions taken into consideration to evaluate the equilibrium state in production of manganese ferroalloys are the following:

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 27B, FEBRUARY 1996---5

SiO2

C~ I

Unreducible MnO oxides

Fig. 1--Illustration of partial and complete equilibria at constant temperature and pressure in a ternary slag diagram. Each set of partial equilibrium lines represents a certain alloy composition.

. . . . . Partial equilibrium [2] SiO2 Partial equilibrium [3]

equtlibrmm

Unreducible MnO oxides

Fig. 2--Effect of CO pressure on complete equilibrium at a chosen temperature (here, P~ > P2)-

(Si02) + 2C__ = Si + 2C0c~ I [I]

(MnO) + c C_ = Mn + COw) [2]

(Si02) + 2Mn = Si + 2(MnO) [3]

(Si02) + Si = 2SiO~g) [4]

Mn = Mn~) [5]

where the parentheses denote the slag phase and the un- derscored the alloy phase.

Either graphite or silicon carbide can coexist with the liquid alloys. For simplicity, the term "carbon saturated," or "C~,," will mean an alloy saturated either with graphite or with silicon carbide. For alloys with higher Si contents than those corresponding to the graphite/SiC coexistence point, solid silicon carbide in the form of cubic crystallized /3-SIC replaces graphite as the stable phase. Then, the carbon activity in the alloys will vary in accordance with the following reaction:

Si + C = SiC~,~ [6]

This equilibrium reaction defines a constant activity product of silicon and carbon.

In the temperature range of interest, say, below 1600 ~ both Psio and PM. are quite low and may be neglected; Pco

Sampling hole

Gas inlet ~ [I

/.Thermal insulation:

Graphite heater--

I ~'Water cooling tube ") _ _ ~

, e ' - - t H ' / . ~ I w~Radiati~ shield ' ' a - -

�9 ~ ~ Cooling jacket--~

l-'---'--~ Protection tub~ J " ~ ~ G a s outlet

(a) ~ T h e r m ~ o u p l e ~ (b)

Fig. 3--Furnace assemblies (a) for sampling during experiment and (b) for quenching crucible in cooling jacket.

is therefore considered to be equal to the total gas pressure. Then, the Reactions [1] through [3] will form the basis for discussion of equilibrium relations. Only two of these reactions are independent. The third will result from a com- bination of the other two.

Slag composition triangles are suitable for graphic rep- resentation of distribution equilibria. The sum of unreducible oxides, such as CaO, MgO, and AI203, is represented by the left bottom apex of the diagram, as shown in Figure 1. Slag systems with any number of unreducible oxides can be treated in the same way as a ternary slag system provided the number of restrictions equals the number of unreducible oxides minus one. Selection of mutual ratios of unreducible oxides as restrictions is practical and favorable.

Partial equilibrium is a situation where certain reaction steps may reach equilibrium whereas others are very slow. Equilibrium is established with respect to certain variables but not to others. This phenomenon was discussed particularly by Darken and Gurry.t3s] Complete equilibrium pre- vails when all phases, including the gas phase, are brought to equilibrium. Equation [3] represents the slag/metal reaction which is fast compared to the three-phase slag/metal/gas Reactions [1] and [2]. Quite often in silicothermic processes only partial slag/metal equilibrium is established.

Partial and complete equilibria for three different alloy compositions are illustrated in Figure 1 as a function of slag composition at constant temperature. The dash-dot lines correspond to the assumption that only equilibrium [2] is established for a c = I and Pco = I atm. Accordingly, aM,o = aMn/K2, and each line corresponds to a constant MnO activity. Correspondingly, lines could be drawn on the assumption that only equilibrium [1] is established. These lines would correspond to constant SiO2 activities in the slag. In practice, neither Reaction [1] nor [2] may come to partial equilibrium, because reaction [3] is the fastest. The solid lines in Figure 1 give the slag composition for the three alloys if only equilibrium [3] is established. These lines correspond to constant values for the activity ratio a~,o/as~o,. The curves [3] and [2] intersect at a point where all reactions are in equilibrium, corresponding to complete equilibrium with ac = 1 and Pco = 1 atm being established. The complete equilibrium points for the three alloy compositions are connected by the dotted line.

The influence of the CO pressure is evaluated in Figure

6---VOLUME 27B, FEBRUARY 1996 METALLURGICAL AND MATERIALS TRANSACTIONS B

slag

/ as

~ii~iiiiiiiiiiiiiill .:.:.:.:.:.:.:.:.:.:.:

ii!i!i!iiii!i!iiii!iii :.:.:.:.:.:.:.:.:.:.:.

:':':':'::':':':':':' ....................., ...................... :::::::::::::::::::::::

...................... .................-.... :::::::::::::::::::::

(a) (b)

Fig. 4--Different arrangements of slag/metal ratios in experiments (a) for partial slag/metal equilibrium and (b) for complete equilibrium.

T~(?S????

....-................... : : : : : : : : : : : : : 2 : : : : : : : :

~:~:~:i:~:~:~:~:~:i:~:~: :::::::::::::::::::::::

ii::il}!;i!g!}.!i!!-!!iil ;i}i;g}iiii!~}iii}i! ..,.................,.. . . . . . . . . . . . :::::::::::::::::::::::::::::::::: . -.-.-.................

2. The position of the partial slag/metal equilibrium line [3] is independent of the pressure, whereas the partial equilibrium line [2] for a certain alloy moves to the left with decreasing CO pressure. A new intersection point is obtained, and through this point, a new complete equilibrium line is drawn, representing the lower CO pressure. It should be noted that the complete equilibrium lines for different pres- sures are not exactly parallel with each other.

To describe both slag and alloy compositions in the same diagram, we need a certain quantity to characterize the equilibrated alloy. According to the phase rule, the carbon- saturated Mn-Si-C,a, alloy is monovariant at a given temperature. In order to define it, only one composition variable needs to be designated. The Si content of the alloy is usually selected as such a variable. Once its value is known, the alloy is exclusively defined.

Treatment of iron-containing alloys is analogous to that of slags with more unreducible oxides. An extra restriction has to be introduced, for example, fixing the iron content or the Mn/Fe ratio.

lII. EXPERIMENTAL

A. Apparatus

The equilibrium experiments were carried out in a ver- tical furnace with a graphite heating element. Two furnace assemblies were used. Assembly A, illustrated in Figure 3(a) allowed sampling during the run. Assembly B, described in Figure 3(b), was designed for quenching the crucible in a water-cooling jacket. A gas-tight motion structure was arranged at the bottom of the furnace. It allowed ver- tical movement of a stainless steel tube connected to a graphite supporter.

Crucibles (18- to 20-mm ID) made of AGR graphite

were employed in most of the experiments. One type com- monly used was a block with four holes, each acting as a separate container. Graphite crucibles were also used for experiments in which the silicon content exceeded the graphite/SiC coexistence point. It was demonstrated exper- imentally that in these cases, a silicon carbide layer was formed between the crucible wall and liquid alloy and slag. SiC crucibles were seldom used.

The crucible supporter was mounted in such a position that the sample was within the constant temperature zone of the furnace. The temperature was measured by means of a Pt/Pt-10 pct Rh thermocouple in close contact with the bottom of the crucible. The temperature difference between the measuring point and the melt was examined and com- pensated for. The temperature variation of the furnace was controlled within • 2 ~

CO gas was used in most of the experiments with a flow rate of 6 L/h. A gas mixture of CO and Ar was sometimes used to study complete equilibrium at a CO pressure of 0.3 atm. A few experiments were carried out in a flow of Ar gas to establish partial slag/metal equilibria related to low MnO-containing slags.

B. Procedure

Reagent grade chemicals were used to prepare master alloys and slags for the experiments. The melting was per- formed in an induction furnace using graphite crucibles. The melts were quenched onto a thick steel plate. Prepared master alloys and slags were analyzed to reconfirm their chemical compositions.

To ensure silica-saturated slag or slag saturated with a- CazSiO 4 or (Ca, Mn)O solid solution, a piece of quartz or lime, respectively, would be placed in the crucible.

For investigation of partial equilibrium [3], the initial

M E T A L L U R G I C A L A N D M A T E R I A L S T R A N S A C T I O N S B V O L U M E 2 7 B , F E B R U A R Y 1 9 9 6 - - - 7

Table I. Equilibrium between Binary MnO-SiO~ Slags and Mn-(Fe)-Si-C,,, Alloys

Number

Slag (Pet) Alloy (Pet)

Temperature (~ SiO2 MnO Si C Remarks

B-I* B-2* B-3 B-4 B-5 B-6 B-7 B-8 B-9 B-10 B-II* B-12 B-13 B-14 B-15*

1550 55.04 44.96 10.67 3.40 Fe = 0 51.05 48.95 10.39 3.33 47.18 51.88 9.37 3.61 44.20 55.80 6.10 4.60 45.47 54.53 7.52 4.15 45.56 54.44 7.27 4.21 48.79 51.20 9.90 - - 45.46 54.54 7.20 - - 41.15 58.85 4.00 - - 29.67 69.97 0.70 - - 54.35 45.65 11.57 - - Fe = 15 pet 45.92 54.08 8.15 - - 40.69 59.31 3.54 - - 32.86 67.14 0.84 - - 55.22 44.78 11.30 - -

*Experiment with an immersed quartz ring.

slag/metal weight ratio varied from 0.6 to 1 and the metal was fully covered with slag, as shown in Figure 4(a). Twelve to fifteen grams of metal were used. The necessary time to reach such equilibrium was 3 to 6 hours, dependent on the starting compositions.

Complete slag/metal/gas equilibrium was much more difficult to establish. It was proved in early experiments that slag and metal, with approximately equal volumes, were hardly brought to equilibrium with the ambient CO gas even after 24 hours. Therefore, in these experiments, only a small amount of slag (about 0.5 to 1.2 g) was added. The amount of metal was usually from 12 to 15 g. The slag formed a ring of oxide melt along the crucible wall, as sketched in Figure 4(b). This exposed both metal and slag to the surrounding atmosphere. Equilibrium concentrations were established by approaching equilibrium from two sides. The necessary time and the amount of slag in these experiments were in fact dependent on the reaction temperature. For example, when using 1 g of slag, about 9 hours were required to reach complete equilibrium at 1500 ~ Too long a time and initially too little slag would result in very small amounts of slag left in the crucible. Generally, experiments at higher temperatures need shorter time and more initial slag, and v ice versa.

The crucible, charged with appropriate amounts of slag and metal of desired compositions, was lowered into the reaction zone of the furnace. The system was sealed and evacuated by a rotary pump. Subsequently, CO or Ar gas was gradually filled into the system as working and/or pro- tecting gas. The crucible was heated from room temperature to the desired reaction temperature in about 1 hour.

For assembly A, slag samples were taken by dipping a steel rod into the slag and withdrawing it quickly, and a quartz tube was used to suck out metal samples. For assembly B, after equilibration, the crucible was rapidly with- drawn to the water-cooling jacket for quenching.

The metal samples were analyzed by traditional methods. The carbon content was determined by the LECO* method.

*LECO is a trademark of LECO Corporation, St. Joseph, MI.

The reproducibility was reported to be _ 0.01 pct. The sil-

12

Mn-Si-Csat ./" . . . . . . . . i 10 --..4--. Mn-Fe(15 wt%)-Si-Csat ~ (

Y 8

~ 6 "

4

2 0~

0 . . . . i 25 30 35 40 45 50 55

SiO2 %

Fig. 5--Partial slag/metal equilibria between Mn-(Fe)-Si-C,, alloys and MnO-SiO2 slags at 1550 ~

icon content was determined by the Velken method or by the ICP method. The duplicates agreed within _ 0.1 pet in the range 0.5 to 5 pct, +0.2 pct in the range 5 to 10 pet, and _ 0.3 pet in the range 10 to 20 pet. The bismuthate method was applied for determination of manganese, and the duplicates were within ___ 0.1 pet. The analyses of slag samples were carried out by X-ray fluorescence. In addition to chemical analyses, parts of the slag samples were also analyzed by electron probe microanalysis (EPMA). The bulk phase of most slag samples was observed to be un- crystallized glass, and the disturbance by inclusions, which may be metal droplets, SiC or SiO2 particles, could be over- come by using EPMA point analysis. I f the silica content was low, near the bottom liquidus, the samples would be fine crystalline. In this case, an area analysis covering about 100 /zm 2 was applied. The sum of compositions in glassy slags detected by EMPA usually varied between 98 and 100.5 pet. The typical value for crystalline slags was about 95 pct. The detected results were forced to sum to 100 pet which was necessary for plotting in ternary diagrams. The

8--VOLUME 27B, FEBRUARY 1996 METALLURGICAL AND MATERIALS TRANSACTIONS B

Table II. Equilibrium between Ternary MnO-SiOz-CaO Slags and Mn-Si-Csa, Alloys

Number

Stag (Pct) Alloy (Pct)

Temperature (~ SiO 2 MnO CaO Si C Remarks

TC-1 TC-2 TC-3 TC-4 TC-5 TC-6 TC-7 TC-8 TC-9 TC-10 TC-11 TC-12 TC-13 TC-14 TC-15 TC-16 TC-17 TC-18 TC-19 TC-20 TC-21 * TC-22 TC-23 TC-24 TC-25 TC-26 TC-27 TC-28 TC-29 TC-30 TC-31 TC-32 TC-33 TC-34 TC-35 TC-36 TC-37 TC-38 TC-39 TC-40 TC-41 TC-42 TC-43 TC-44 TC-45 TC-46 TC-47" TC-48 TC-49 TC-50* TC-51 * TC-52 TC-53 TC-54 TC-55 TC-56 TC-57 TC-58 TC-59 TC-60 TC-61 TC-62 TC-63 TC-64

1600

1550

60.04 12.62 27.33 21.51 1.17 54.41 7.87 37.71 19.63 1.65 51.36 7.21 4!.42 17.37 2.37 49.18 7.00 43.82 13.34 3.27 60.35 9.07 30.58 23.90 0.94 52.83 6.17 40.99 20.64 1.47 47.15 3.96 48.88 13.25 - - 41.49 4.74 53.77 7.34 - - 65.30 7.20 27.20 28.50 0.36 53.84 7.25 37.66 22.12 1.11 65.72 7.61 26.67 26.92 0.48 52.93 13.01 34.06 15.87 2.68 55.23 11,07 33.70 19.10 1.77 47.52 10.15 42.33 11.60 4.20 43.96 9.14 46.90 5.89 - - 55.70 I2.20 31.90 18.10 1.99 40.00 11.30 48.50 2.60 - - 39.59 9.60 50.82 2.13 - - 33.67 3.22 63.11 0.81 5.54 54.14 10.15 36.47 17.04 2.01 64.49 4.88 32.25 29.06 0.27 62.60 4.78 32.62 27.32 0.43 57.97 6.41 35.62 23.52 0.75 53.61 11.63 34.37 20.40 1.24 59.20 9.60 31.30 21.60 1.08 54.24 8.56 35.70 19.85 1.46 55.76 8.85 33.69 21.93 1.13 58.90 5.58 32.50 23.36 0.88 57.80 6.70 34.60 25.99 0.50 59.50 5.00 35.70 27.09 0.33 57.60 6.70 35.20 23.51 0.85 57.42 7.11 35.46 23.28 0.75 57.55 6.27 36.18 24.25 0.59 58.96 7.55 33.49 24.43 0.64 53.74 6.23 40.03 21.04 1.28 61.73 5.37 31.70 28.40 0.25 60.67 5.74 33.99 23.50 0.79 60.12 10.76 29.25 20.74 1,19 56.15 6.21 37.64 23.05 0.93 59.78 4.64 35.65 26.50 0.44 61.41 4.96 34.40 27.00 0.38 64.78 5.13 30.55 29.19 0.25 66.40 5.32 28.40 28.56 0.26 56.20 11.43 33.30 19.11 1.50 58.10 9.25 32.60 21.95 0.90 61.28 9.90 30.18 24.47 0.57 55.57 32.84 10.03 13.30 2.92 48.55 22.38 27.38 8.70 4.06 49.63 16.79 31.71 13.15 3.13 61.40 11.20 26.80 21.80 1,14 62.90 5.50 30.40 26.90 0.52 48.32 3.37 47.43 21,88 0.99 44,67 14.19 39.87 2.47 5.15 56,38 12.87 30.49 19.32 1.50 47.49 2.78 47.81 18.69 1.63 44,55 3.50 51.54 14.59 2.77 46.44 6.87 50.28 7.02 4.56 43,08 7.35 48.66 4.67 5.60 37.27 13.55 42.23 0.37 %46 36.97 24.32 36.37 0.33 6.68 42.15 34.92 21.44 3.72 6.90 44.29 22.94 30.67 4.74 5.24 48.67 30.39 20.94 8,29 4.13 53.68 22.91 23.41 16.89 2.58

complete equilibrium Pco = 1 atm

partial equilibrium

complete equilibrium Pco = 1 atm

partial equilibrium

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 27B, FEBRUARY 1996--9

Table 1I. (continued) Equil ibrium between Ternary MnO-SiO~-CaO Slags and Mn-Si-C,., Alloys

Number

Slag (Pct) Alloy (Pct)

Temperature (~ SiO: MnO CaO Si C Remarks

TC-65 TC-66 TC-67 TC-68 TC-69 TC-70 TC-71 TC-72 TC-73 TC-74 TC-75 TC-76 TC-77 TC-78 TC-79 TC-80 TC-81 TC-82 TC-83 TC-84 TC-85 TC-86 TC-87 TC-88 TC-89 TC-90 TC-9 l TC-92 TC-93 TC-94 TC-95 TC-96 TC-97

1550 52.79 32.70 14.51 14.52 53.37 23.74 22.89 17.51 59.61 17.33 23.06 18.10 49.55 14.66 35.79 11.73

1500 50.15 30.95 18.89 11.22 46.11 24.44 29.45 7.17 45.43 25.08 29.50 6.63 51.31 32.66 16.03 14.32 48.47 30.57 20.51 10.34 42.53 22.04 35.43 3.80 45.95 28.22 25.83 8.45 42.67 20.88 36.46 2.40 51.09 30.92 17.99 12.57 48.96 14.44 35.82 11.32 51.78 16.45 29.59 15.50 45.89 14.36 37.89 6.63 50.01 15.59 32.94 12.77 57.53 14.77 25.76 18.77 45.73 26.00 28.26 6.74 43.47 20.21 36.32 3.17 56.80 18.29 24.91 18.42 47.41 17.96 34.63 9.75

1450 42.00 32.69 25.40 2.63 46.44 44.33 9.23 7.84 48.39 41.25 10.36 9.70 43.67 47.56 8.77 6.24 42.40 46.22 I 1.38 4.56 39.68 38.97 21.35 1.76 48.37 44.56 4.31 10.02 40.08 35.97 23.85 1.30 40.43 31.31 28.27 0.68 39.67 44.52 15.81 2.20 45.37 30.52 24.11 6.15

2.62 partial 1 . 9 6 equilibrium 1.90 3.49 - - complete - - equilibrium - - Pco = 1 atm

3.79 partial 2.79 equilibrium 4.61 2.92 1.74

1.68

- - complete - - equilibrium - - "~ = 1 arm

- - partial 4.69 equilibrium


EMPA analysis was checked with wet chemical analysis and very good consistency was obtained.

IV. R E S U L T S

A. Equilibrium with Binary MnO-Si02 Slags

A total o f ten experiments were made with iron-free Mn- Si-C,at alloys and five experiments with Mn-Fe(l 5 wt pct)- Si-C~,, alloys, all equilibrated with binary MnO-SiO2 slags at 1550 ~ The results are shown in Table I and Figure 5. According to the phase rule, they represent partial slag/metal equilibrium (not equilibrated with the gas phase).

The effect o f iron is modest. At SiO2 saturation, the Si content o f the 15 wt pct Fe alloy is about 0.8 wt pet higher than in the corresponding iron-free alloy. An earlier investigation by Krrber and Oelsent~l at 1600 ~ with carbon- free alloys and SiO2-satumted slag resulted in a decrease o f 0.6 wt pct Si by addition o f 20 wt pct Fe.

The carbon content has a very pronounced effect on the equilibrium content o f Si. As seen from Figure 5, the Si content is about 10.5 wt pct in Mn-Si-C~, alloys in equilibrium with SiO2-saturated slag. This is compared with the results given by Tuset et al.,t3] showing that alloys with less than 0.3 wt pct C will contain about 18 wt pct Si in equilibrium with binary SiO2-saturated slag at 1530 ~

~ Partial slag~metal I equilJbflum at 1550"C

J S i O 2 . - Complete equilibrium 30 - . 70 at Pco=l arm

\oo

, 0 7 . . . . o ,o ,o , o ,o oo 7o ,o

MnO

WEIGHT %

Fig. 6---Eqmlibd~ relations for ternary N~O-SiO2-CaO slags in contact with Mn-Si-C~ alloys. The solid partial slag/metal equilibrium lines show isosilicon contents of the metal phase at 1550 ~ The dotted lines show complete slag/metal/gas equilibrium at Pco = 1 atm and temperatures as indicated.

The temperature has a limited influence on the partial slag/metal equilibrium. It is evaluated from data reported by Turkdogan and Hancock, t2~ for carbon-saturated alloys, that every 50 deg increase in temperature from 1350 ~ to 1450 ~ brings about an approximate increase o f 0.2 wt pct

10- -VOLUME 27B, FEBRUARY 1996 METALLURGICAL AND MATERIALS TRANSACTIONS B

Table 1II. Equilibrium between Ternary MnO-SiOz-Al20s Slags and Mn-Si-C,,, Alloys

Number

Temperature Slag (Pct) Alloy (Pct) (~ SiO 2 MnO A1203 Si C Remarks

TA- 1 1600 43.07 22.75 34.18 20.80 1.23 TA-2 1550 56.80 22.50 22.10 19.05 1.89 TA-3 57.36 22.57 20.07 19.08 1.58 TA-4 51.10 31.00 18.70 13.96 2.61 TA-5 40.10 34.60 25.30 7.23 4.08 TA-6 51.80 22.04 26.t5 20.21 1.54 TA-7 38.52 28.38 33.09 14.65 2.93 TA-8 28.05 54.88 17.07 0.65 6.85 TA-9 30.30 51.78 17.92 t.37 6.91 TA- I 0 33.98 47.67 18.35 2.98 6.42 TA-I 1 1500 55.44 30.77 13.79 15.35 2.55 TA- 12 48.67 37.09 14.24 12.30 3.62 TA- 13 38.45 38.15 23.40 8.79 4.69

partial equilibrium

I • -.~-Partial slag~metal equilibrium at 1550"C

o/1\

50 60 70 80 90 100

MnO

WEIGHT % Fig. 7--Slag/metal equilibrium relations for ternary MnO-SiO2-A120~ slags in contact with Mn-Si-C~,. Equilibrium lines show isosilicon contents of the metal phase.

Si in the metal for equilibrium with low-silica slags to about 0.8 wt pct Si for equilibrium with silica-saturated slags.

B. Equilibrium with MnO-SiO2-CaO Slags

A total of 97 experiments were made with Mn-Si-Csa, alloys in this system. Of these, 34 experiments gave complete equilibrium (Pco = 1 atrn), the remaining giving partial slag/metal equilibrium. Fifty-seven experiments were made at 1550 ~ 11 at 1600 ~ 18 at 1500 ~ and 11 at 1450 ~ The results are given in Table II and Figure 6. In order to prevent cluttering from too many data points, only accommodated curves are shown. As a typical example, the deviations of the partial slag/metal equilibrium lines for 10.5 wt pct Si and less are estimated to be within ___ 1 wt pct SiOz. The liquidus curve at 1550 ~ is drawn according to Glasser's phase diagram.t39! Solid lines in the triangle

represent partial slag/metal equilibria, giving the composition of slags in equilibrium with different alloys at 1550 ~ Each line corresponds to a certain alloy having a constant silicon content as shown. The same effect of temperature was observed as for binary slags, i.e., about 0.2 to 0.8 wt pct Si per 50 deg.

Dotted lines represent complete equilibrium compositions of slags at given temperatures and Pco = 1 atm. The intersection point of a partial slag/metal equilibrium line and a complete equilibrium line gives the composition of a slag which is in equilibrium with a certain alloy, described here by its silicon content.

The liquidus curves move up and down in the diagram with the temperature, the liquid area being larger with increasing temperature. A complete equilibrium line, obtained at a certain temperature, must end at a corresponding liquidus curve. Here, for easy comparison and application, the complete equilibrium lines for all temperatures have been copied into the same diagram.

C. Equilibrium with MnO-SiOz-Al203 Slags

A total of 13 experiments were made with Mn-Si-Csa, alloys in this system, all aiming at partial slag/metal equilibrium. Of these, nine experiments were at 1550 ~ three at 1500 ~ and one at 1600 ~ The results are shown in Table III and Figure 7. In addition, the binary slag results are also plotted in the figure. The liquidus curve is estimated based on the phase diagram proposed by Muan and Osborn.t40!

D. Equilibrium with MnO-SiO2-CaO-Al203 Slags

For a CaO/A1203 ratio of 1.5, a total of 63 experiments were made with Mn-Si-C~, alloys. Of these, 29 experiments gave complete equilibrium for Pco = 1 atm, six gave complete equilibrium for Pco = 0.3 atm, and the remaining gave partial slag/metal equilibrium. Most experiments were made at 1550 ~ but some were conducted at 1600 ~ 1500 ~ or 1450 ~ The results are shown in Table IV and as accommodated curves in Figure 8.

For a CaO/AI203 ratio of 3, a total of 54 experiments were made with Mn-Si-Cs,t alloys, 31 giving complete equilibrium for Pco = 1 atm and the remaining partial slag/metal equilibrium. As for the other systems, most ex-

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 27B, FEBRUARY 1996---1 !

Table IV. Equ i l ib r ium between Q u a t e r n a r y MnO-SiO2-CaO-AlzO 3 (CaO/Ai203 = 1.5) Slags and Mn-Si-C,,, Alloys

Temperature Slag (Pct) Alloy (Pct)

Number (~ SiO2 MnO CaO A1203 Si C Remarks

QA- 1 1600 57.32 7.35 21.58 13.57 22.10 1.16 complete QA-2 48.11 4.87 28.21 18.81 20.00 1.65 equilibrium QA-3 45.85 2.79 31.84 19.52 i6.10 2.94 Pco = 1 atm QA-4 41.59 2.17 34.48 21.73 14.70 3.12 QA-5 33.70 9.70 39.90 16.70 8.00 - - QA-6 29.50 5.70 40.90 23.90 1.40 - - QA-7 61.05 9.04 18.40 11.51 22.00 1.10 QA-8 36.95 4.79 36.13 22.13 10.75 - - QA-9 1550 58.74 11.29 18.08 11.89 20.60 1.72 QA-10 56.83 10.47 19.64 13.06 17.70 2.14 QA-11 49.70 10.58 24.15 15.57 17.35 - - QA-12 46.47 10.18 26.51 16.83 12.80 3.47 QA-13 41.00 9.23 30.70 19.10 8.80 - - QA-14 32.00 11.30 35.00 21.60 1.20 - - QA-15 37.08 10.04 31.78 21.09 5.15 - - QA-16 51.42 12.27 21.81 14.50 15.94 - - QA- 17* 62.96 4.47 18.59 13.98 23.97 0.65 partial QA-18* 61.88 11.68 14.41 12.03 21.06 1.15 equilibrium QA-19* 61.06 17.62 11.76 9.56 19.64 1.64 QA-20* 57.30 29.50 7.40 5.30 14.10 2.47 QA-21 52.52 5.83 22.51 19.39 17.27 2.49 QA-22 54.75 7.34 22.76 15.16 16.60 2.60 QA-23 56.18 13.50 17.64 11.67 14.01 3.20 QA-24 49.28 17.05 20.16 13.51 8.26 4.25 QA-25* 48.97 41.90 5.76 3.37 10.59 3.50 QA-26* 52.60 32.42 9.29 5.69 12.48 3.10 QA-27 47.71 24.39 17.73 10.17 9.02 4.25 QA-28 41.78 39.76 11.98 6.48 4.85 5.13 QA-29 49.10 39.30 6.94 4.64 9.60 - - QA-30 44.40 43.50 7.38 4.74 5.80 - - QA-31 36.80 53.70 5.74 3.77 4.00 - - QA-32 33.50 53.30 7.80 5.44 0.90 - - QA-33 60.52 6.98 19.54 12.96 25.40 0.45 QA-34 52.82 3.72 26.46 17.00 25.25 0.47 QA-35 63.07 5.40 19.10 12.43 29.52 0.25 QA-36 52.98 3.93 26.29 16.79 27.72 0.36 QA-37 1500 55.41 27.21 10.53 6.85 14.20 2.87 complete QA-38 51.78 27.77 12.20 8.23 11.90 3.62 equilibrium QA-39 47.48 29.6l 13.75 9.16 9.12 - - Pco -- 1 atm QA-40 42.41 22.77 20.24 14.57 6.26 - - QA-41 39.56 22.30 23.81 14.33 3.90 - - QA-42 34.70 18.46 27.58 19.26 1.91 - - QA-43 47.14 26.13 13.67 13.07 10.28 - - QA-44 36.12 19.18 26.73 17.96 2.32 - - QA-45 68.54 8.18 14.57 8.68 22.30 0.72 complete QA-46 63.05 13.45 14.14 9.36 18.42 1.79 equilibrium QA-47 49.85 10.15 24.18 15.82 15.60 - - Pco = 0.3 atm QA-48 40.24 6.64 33.70 19.43 10.63 - - QA-49 68.79 12.46 I 1.57 7.18 2 t .88 1.03 QA-50 34.04 6.07 34.95 24.94 5.82 - - QA-51 40.73 43.47 9.52 6.28 4.47 - - partial QA-52 33.98 52.97 7.61 5.44 1.37 - - equilibrium QA-53 1450 34.13 32.99 19.29 13.59 1.02 - - complete QA-54 38.30 35.44 15.73 10.52 2.65 - - equilibrium QA-55 44.78 43.27 7.20 4.75 7.72 - - / 'co = 1 atm QA-56 42.26 40.65 10.14 6.95 5.69 - - QA-57 32.19 30.00 22.67 15.18 0.33 - - QA-58 48.00 44.10 3.74 4.16 9.65 - - partial QA-59 49.65 37.92 7.38 5.06 12.73 3.13 equilibrium QA-60 53.00 31.13 9.56 6.31 13.24 3.17 QA-61 46.62 33.91 11.89 7.58 9.30 - - QA-62 41.44 41.64 10.08 6.85 3.74 3.18 QA-63 35.95 49.14 8.63 6.29 1.71 - -



~ Partial slag~metal SiCh equilibrium at 1550"C

2 0 . -,,80 . - Complete equilibrium ~ ~ A ~ X ' . _ a tPco= l at m

5o 1o "paso

;i+< ! i ~ 0 [ . . . . Y- Y- -~- ) 2 0

0 10 20 30 40 50 60 70 80

MnO

WEIGHT %

Fig. 8--Equilibrium relations for quaternary MnO-SiOz-CaO-AlzO3 slags (CaO/AIzO 3 = 1.5) in contact with Mn-Si-C~, alloys. The solid partial slag/metal equilibrium lines show isosilicon contents o f the metal phase at 1550 ~ The dotted lines show complete slag/metal/gas equilibrium at Pco = 1 atm and temperatures as indicated.

periments were made at 1550 ~ but some were conducted at 1600 ~ 1500 ~ or 1450 ~ The results are shown in Table V and Figure 9. Again, to prevent cluttering from too many data points, only accommodated curves are shown.

The cristobalite saturation lines at 1550 ~ for the two different CaO/AlzO3 ratios are estimated based on the CaO- A1203-SiOzt 41t phase diagram. Comparison of the two ternary phase diagrams, MnO-SiO2-CaO and MnO-SiO2- AlzO3, indicates that the presence of alumina expands the liquid region at both the upper and lower sides. However, estimation of the lower liquidus curve for these quaternary slags is difficult.

Figures 6 through 9 provide a comprehensive image of equilibrium relations for the different slag systems. Equil- ibrated with a certain alloy having a constant silicon content, the slag composition varies along a line which corresponds to a constant value of a2,o/as, o,. For lower silicon contents, say, less than 5 wt pct, just a moderate change in the silica concentration occurs along each line, as these lines are almost parallel to the bottom line of the triangle. For higher silicon contents (>20 wt pct), a substantial change in silica concentration takes place along each partial equilibrium line.

The final slag composition is determined by complete slag/metal/gas equilibrium. At a chosen CO pressure and fixed alloy composition, an increase in temperature will result in a remarkable decrease in the MnO concentration of the slag, particularly at temperatures below 1550 ~ which are considered to be those encountered in industrial production.

V. DISCUSSION

A. Effect of Al203 in the Slag

The influence of different slag-forming components has two main aspects: expansion of the liquid area and altera- tion of the activity relations. Replacement of CaO by Al:O3 in the system MnO-SiO2-CaO will, up to a certain extent, expand the liquid area toward the upper apex and also to-

ward the bottom liquidus boundary of the triangle. The larger liquid area increases the distance between the slag/metal equilibrium lines (constant a~,o/aslo~, ratios), thereby influencing the position and shape of the partial and complete equilibrium lines.

The other effect is to influence the activity relations. Alu- mina is an amphoteric oxide, and substitution of Al:O3 for the basic oxide CaO will decrease the MnO activity coef- ficient, YM,o, as demonstrated by a comparison between activity measurements of M n O - S i O 2 - C a O t29,3~ and MnO-SiOg-AlzO3 melts. I341

Figure 10 illustrates the change in position of partial and complete equilibrium lines with the addition of alumina. The different lines are taken from Figures 6, 8, and 9. The solid lines represent partial slag/metal equilibrium for an alloy with 1 wt pct Si at 1550 ~ Decreasing the CaO/A1203 ratio by addition of alumina shifts the partial slag/metal equilibrium lines to lower silica contents. The dotted lines represent complete equilibrium giving slag compositions at 1500 ~ and Pco = 1 atm. Increasing addition of Al~O3 to acid slags will shift the complete equilibrium lines to the left (lower MnO), whereas addition to more basic slags has the opposite effect (increased MnO). This effect on the complete equilibrium lines is most ap- parent at temperatures below 1550 ~

B. Effect of Temperature

As already discussed in section IVa, the partial slag/metal equilibrium is influenced relatively little by the temperature, and it is difficult to determine the change in partial equilibrium lines for a temperature change of 50 ~ The effect of temperature on multicomponent slags is likely to be as reported for the binary slag system MnO-SiOz,t 21 which indicates an approximate increase of 0.2 to 0.8 wt pct Si in the metal for every 50 deg increase in the temperature, as- suming a fixed slag composition. The diagrams in Figures 6 through 9 may therefore be used to evaluate the slag/metal equilibrium compositions for temperatures other than 1550 ~

In contrast to the small effect on the partial slag/metal equilibrium, the temperature is of vital importance for the location of the complete slag/metal/gas equilibrium lines. The MnO concentration of slags changes markedly with changing temperatures, as indicated in Figures 6, 8, and 9. An increase in temperature benefits the recovery of manganese to a great extent, provided the temperature is not so high that unacceptable manganese evaporation losses take place.

The term "slag basicity" is often used to describe the composition and properties of industrial slags. Slag basicity is the ratio of basic to acid oxides. As shown, the composition (and also the basicity) of a slag in complete equilibrium with a certain alloy at fixed CO pressure is uniquely determined by the temperature. Figure 11 illustrates how the experimental results shown in Figure 8 are used to describe the relationship between slag basicity and MnO concentration. The upper part of the figure gives slag compositions at varying temperatures in complete equilibrium with Mn-Si-C~, alloy containing 1 wt pet Si. In the middle figure, this result is plotted in terms of temperature against either slag basicity or MnO concentration. Finally, the familiar relation between MnO concentration and basicity is shown in the lower figure.

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 2713, FEBRUARY 1996--13

Table V. Equilibrium between Quaternary MnO-SiOz-CaO-Al203 (CaO/Al203 = 3) Slags and Mn-Si-C~.t Alloys

Temperature Slag (Pct) Alloy (Pct)

Number (~ SiO2 MnO CaO A1203 Si C Remarks

QB-1 1600 45.75 3.06 38.40 12.75 17.38 - - complete QB-2 38.03 2.82 44.67 14.49 9.03 - - equilibrium QB-3 32.40 5.00 45.70 16.90 2.52 - - Pco = 1 atm QB-4 50.30 5.37 33.40 10.90 20.52 1.51 QB-5 32.30 4.50 47.10 16.10 1.50 - - QB-6 54.25 7.97 26.12 11.65 21.26 1.27 QB-7 35.90 5.58 43.31 15.21 5.77 - - QB-8 61.72 8.77 22.19 7.31 24.12 - - QB-9 1550 58.83 12.36 21.34 7.47 19.25 1.63 QB- 10 52.05 12.49 26.17 9.29 15.82 - - QB-11 44.48 9.14 34.14 12.25 9.60 - - QB-12 37.79 7.50 40.43 14.29 4.31 4.54 QB-13 57.50 12.90 22.00 7.60 18.20 2.09 QB-14 50.18 9.98 29.70 10.14 16.00 2.59 QB- 15 44.89 11.32 31.75 12.03 10.40 4.27 QB-16 38.30 9.94 39.50 12.26 3.41 - - QB- 17 * 53.56 28.04 10.43 3.57 13.67 2.72 partial QB-18* 60.85 24.27 12.39 4.39 16.17 2.25 equilibrium QB- 19* 63.40 9.80 19.60 7.10 21.93 0.90 QB-20 53.30 19.20 20.60 7.20 13.64 2.81 QB-21 53.40 14.90 23.00 8.00 16.52 2.21 QB-22 47.70 23.70 20.70 7.30 7.20 4.13 QB-23 64.80 6.89 7.04 21.27 26.29 0.66 QB-24 51.20 32.30 12.20 4.37 10.40 - - QB-25 47.40 39.00 9.90 3.67 7.00 - - QB-26 38.30 48.80 9.40 3.40 3.00 - - QB-27 35.60 40.90 17.70 5.82 1.00 - - QB-28 50.60 17.10 23.90 8.37 10.90 - - QB-29 46.20 17.20 27.70 8.95 8.40 - - QB-30 39.50 36.80 17.90 5.87 2.40 - - QB-31 34.10 38.10 21.10 6.70 0.50 - - QB-32 68.33 5.83 18.70 7.13 30.30 0.45 QB-33 61.98 5.66 23.66 8.70 29.10 0.52 QB-34 49.09 2.31 35.41 13.18 25.87 0.55 QB-35 1500 46.30 22.50 23.00 8.15 8.10 - - complete QB-36 53.78 28.46 12.82 4.94 14.51 2.87 equilibrium QB-37 50.70 33.10 12.00 4.20 11.31 - - Pco = 1 arm QB-38 41.40 23.74 25.67 9.13 4.39 - - QB-39 35.14 11.99 38.83 14.04 0.44 7.40 QB-40 41.85 32.39 18.90 6.84 4.71 5.79 QB-41 50.80 29.58 14.74 4.87 12.37 - - QB-42 46.59 27.81 18.88 6.73 9.24 - - QB-43 40.71 17.77 30.46 11.07 2.64 - - QB-44 38.43 17.94 32.11 11.52 1.28 - - QB-45 1450 42.82 41.61 11.51 4.05 4.81 - - QB-46 3.69 28.02 26.09 9.20 0.58 - - QB-47 44.88 41.67 9.64 3.82 6.16 - - QB-48 37.38 31.28 23.16 8. t 8 0.76 - - QB-49 40.35 40.35 13.99 5.30 2.42 - - QB-50 47.80 43.20 6.68 2.33 9.30 - - partial QB-51 43.40 48.70 5.94 1.96 4.60 - - equilibrium QB-52 40.20 47.70 9.00 3.04 3.50 - - QB-53 34.90 45.90 13.90 5.30 0.80 - - QB-54 38.74 47.38 8.38 5.50 3.06 - -


C. Effect of CO Partial Pressure on Complete Equilibrium

In addition to temperature, the CO partial pressure also plays an equally important role in establishing complete equilibrium. An example of reduced CO pressure is pro-

duction o f ferromanganese in blast furnaces. According to the top gas composition for normal air blast operation, t421 the partial pressure o f CO in the furnace is estimated to be 0.37 atm and somewhat higher for O2-enriched operation.

Some experiments were carried out at Pco = 0.3 arm to


~ Partial slag~metal SiOz equilibrium at 1550"C

20 80 . - Complete equilibrium " 7 " " ~ " _ _ at Pco=l atm

\._ ~'~;" 40////.z~/~,,"" / ~ � 9 ~xea)

g - '

. 7 , . ~ 1 7 6 . 0 10 20 30 40 50 60 70 80

MnO WEIGHT %

Fig. 9--Equilibrium relations for quaternary MnO-SiO2-CaO-AI203 slags (CaO/A120 ~ = 3) in contact with Mn-Si-C,, alloys. The solid partial slag/metal equilibrium lines show isosilicon contents of the metal phase at 1550 ~ The dotted lines show complete slag/metal/gas equilibrium at Pco = 1 arm and temperatures as indicated.

f SiO2

40/x ,, ,,^ " ,~60

5o / ,~.';::'" \5o

/ 2 F X ~ m ~ 3 ~ 40 60 -

70 ~ 30 10 20 30 40 50 60 70

CaO+AI20 3 MnO WEIGHT %

Fig. 10--Effect of the CaOtAI203 ratio in quaternary slags. The solid lines represent partial slag/metal equilibrium with Mn-Si(l wt pct)-C~, alloy at 1550 ~ the dotted lines represent complete slag/metal/gas equilibrium with Mn-Si-C~ alloys at 1500 ~ and Pco = 1 atrn.

study the effect of a CO pressure close to the blast furnace practice. The result is illustrated in Figure 12. The solid lines represent complete equilibrium at 1500 ~ and 1550 ~ for Pco = 1 arm. The dotted line is the complete equilibrium line at 1500 ~ and Pco = 0.3 atm. As shown, the effect of reduced CO pressure by about two-thirds is equivalent to an increase in temperature of about 50 to 70 deg. This is in agreement with thermodynamic calculations.

D. Effect o f Iron and Carbon

The shape of the partial slag/metal equilibrium lines, rep- 2 resenting constant oxide activity ratios, aM~o/as~o,, is of

course independent of the iron and carbon content of the metal. However, these elements will, to a certain extent, influence the Si content of the equilibrated alloys. Only a few experiments were carried out with Fe-containing alloys. Iron seems to have little influence on the partial slag/metal equilibrium, as illustrated in Figure 5. When the silicon content exceeds about 8 wt pct, the iron-free Mn-Si-C~t

SiO~

oy~ 5~c.. r ~...

o " - - . . - / l~u ~ " ' - .~g: "'-,~-'o "~-._~---

g o / "-4:"-~:s ~oa \20 0 80

MnO

1.3 \ Complete equilibrium / 35 1 . 2 30

1.1 25

~'~ 1

~.) 0.9 = "-" 15 0 .~ o.s .2 10 0.7

0.6 / Mn-Si(l%)-Csat 5 / MnO-SiOz-CaO-AI203(C/A= 1.5)

0.5 . . . . . . . . . . . . . 0 1,45o 1,500 1,550 Temperature ("C)

35 \ Complete equilibrium

30 =

25

~" 20

10

5 Mn-Si(l%)-Csat MnO-SiO2-CaO-A1203(C/A--- 1.5)

% . . . . . . . . 0.6 0.7 0.8 0.9 I 1.1 1.2 1.3

Basicity (CaOISiO2)

Fig. 1 l--Evolution of relationship b e ~ e n basicity and MnO in slag. MnO-SiO:CaO-A1203 slags are in complete equilibrium with Mn-Si(1 pct)-C~ alloy at different temperatures and Pco = 1 atm. Data from the equilibrium diagram (Fig. 8).

alloys will equilibrate with somewhat higher silica contents in the slag compared to iron-containing Mn-Fe-Si-C~, alloys. Below 8 wt pct Si, the opposite occurs. This conclu- sion will probably also be valid for other slag systems. For complete equilibrium, further experimental work is required to identify the effect of iron in the metal.

No experiments have been carded out with low-carbon alloys due to the difficulty of finding suitable crucible materials. Judged by comparison with results given by Tuset et aL,t3] who investigated distribution equilibria between Mn-Si alloys and different slag systems, carbon will sub-


! SiOz

/ ,7" / \

0 10 20 30 40 50 60 70

MnO

WEIGHT %

Fig. 12--Effect of CO partial pressure on the complete equilibrium composition of MnO-SiO_,-CaO-A1203 slags (CaO/AI203 = 1.5) in contact with Mn-Si-C~, alloys.

stantially influence the activity ratio ae,,/asi . At constant Si content, this ratio witl rise with decreasing carbon content. In other words, the low-carbon alloy will have a higher Si content than the carbon-saturated alloy for the same slag composition. For complete equilibrium, the effect of reduced carbon activity is to shift the complete equilibrium line to higher MnO contents. An example is the manganese oxygen refining (MOR) process for production of low-carbon ferromanganese.

VI. CONCLUSIONS

Distribution equilibria between Mn-Si-C=, alloys and sil- icate slags containing various amounts of MnO, SIO2, CaO, and AlaO 3 have been measured under different conditions. The results are presented in the equilibrium diagrams (Fig- ures 6 through 9), from which the influence of different operating parameters can be summarized as follows.

The temperature has only a small effect on the partial slag/metal equilibrium. Equilibrated with a chosen alloy, the slag composition is shifted slightly toward higher silica content as the temperature decreases. Contrarily, there is a considerable influence of temperature on the complete slag/metal/gas equilibrium. The MnO content of the slag is, to a great extent, determined by the temperature, especially below 1550 ~ Increasing temperature has the advantage of lowering the manganese losses in slag.

The CO pressure also affects the complete equilibrium composition of the slag. A reduction of Pco from one to one-third of an atmosphere is shown to be equivalent to an increase in temperature of approximately 50 to 70 deg. Blast furnace production is an industrial example of reduced CO pressure.

The slag is characterized by the kind and content of unreducible oxides present, e.g., CaO, MgO, A1203, and their mutual ratios. These oxides differ noticeably in their effect on the liquidus boundaries of the slag and on the activity coefficients of MnO and SiO:. Addition of alumina to MnO-SiOz-CaO slags will, up to a certain level, expand the liquid region at a given temperature and will, for example, make it possible to produce alloys with relatively low silicon contents. Substitution of A1203 for CaO in low silica slags, as in the production of high-carbon ferromanganese, tends to shift the partial slag/metal equilibrium line down- ward to lower SiO2 and move the complete equilibrium line rightward to higher MnO.

The carbon content has a substantial influence on the activity ratio a~,/asi of the alloy and changes remarkably the composition of the metal in equilibrium with a given slag.

Iron in the metal results in small variations of the partial slag/metal equilibrium composition. Further investigation of the effect of iron on complete equilibrium is in progress.

ACKNOWLEDGMENTS

The authors wish to acknowledge the financial support provided by the Research Association of the Norwegian Ferroalloy Producers (FFF) and the Research Council of Norway (NFR). In addition, we would also like to thank Oye Smelteverk (Tinfos Jemverk A/S, Norway) for chemical analyses of slag and metal samples.

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reaction equilibria in the production of manganese ferroalloys

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