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Liquidus temperature and electrical conductivities of synthetic ferromanganese slags
Minerals Engineering, Vol. 4, No. 12, pp. 1315-1332, 1991
0892-6875/91 $3.00+0.00 © 1991 Pergamon Press plc
Printed in Great Britain
LIQUIDUS TEMPERATURE AND ELECTRICAL CONDUCTIVITIES OF SYNTHETIC FERROMANGANESE SLAGS R.H. ERIC, A.A. HEJJA and W. STANGE
Department of Metallurgy and Materials Engineering, University of the Witwatersrand, P.O. Wits, Johannesburg 2050, South Africa (Received 10 December 1990; accepted 1 January 1991)
ABSTRACT
Liquidus temperature and electrical conductivity measurements were carried out on synthetic slags prepared from pure oxides to represent a wide range of compositions likely to be encountered in the operation of ferromanganese electric furnaces. The slag constituents were in the following ranges : MnO; 5-30%, Ca0;20-35%, MgO; 5-15%, S i 0 2 ; 27-58%, A1203; 5%. The basicity ratios varied from 0.55 to 1.4. Liquidus temperatures were measured by the hot-stage microscope, the electrical conductivities with the aid of a modified Wheatstone bridge. Liquidus temperatures were found to vary from 1300 ° to 1380°C in the slags investigated, and increased with increasing basicity ratio. The electrical resistivity of slags decreased with the increase of basicity ratio from 0.55 to 1.1. Above 1.1 basicity ratio the resistivity tended to increase depending on the MnO content. Regression models were developed to predict both, ferromanganese slag liquidus temperatures and electrical conductivities as a function of slag composition. The model equations are quadratic in form, with the model coefficients calculated using the S A S package. Reasonable fits were obtained for the liquidus temperature data. The fits for the conductivity data are not as good but still useful, depending on the accuracy required from the model. INTRODUCTION The general trend in the production of ferroalloys is manifested in increasing quality demands in these alloys together with an improvement in the economy of the ore-smelting operation. Thus, the optimisation of the smelting technique is gaining greater prominence. The concept involves both furnace technology, that is size, geometry, precise electrical outline and optimum material characteristics which includes correct selection, proportioning and siting of the charge together with the quantity and physicochemical characteristics of the slag produced. An important facet of the submerged arc furnace is that the resistance of the charge and that of the slag together determines largely the electrical requirements for the economic manufacture of a given alloy. However, the exact distribution of the resistance between the two is extremely difficult to delineate even if the resistivities of the various charge components and of the slags had been determined with great accuracy. It is generally accepted that in the production of ferro=manganese the resistivity of the slag plays a considerably more important role than in the manufacture of ferrochromium. In any case the electrical conductivity (inverse of resistivity) of the charge up to 1000°C is very low as is the thermal conductivity. Logically t h e n , at least at the beginning, the smelting operation is localized in a relatively small intense region close to the electrode tip. Nevertheless, the formation of a preliminary incipient fusion zone containing ore plus 1315
1316
R . H . ERIC et al.
reductant inclusions besides gangue materials, as in the smelting of chrome ores, may not be excluded from the treatment of lower-melting manganese ores. This would probably represent the only continuous path through which current can be transferred. On account of the considerably greater influence of the molten slags as a conveyer of electricity in the smelting of manganese ores, a detailed investigation was carried out on the liquidus temperatures and electrical conductivities of slags of selected compositions likely to be produced in ferromanganese furnaces. The data then were empirically modelled with regression analysis. EXPERIMENTAL P R O C E D U R E
Preparation of Slags The slag composition ranges were selected with the help of S A M A N C O R Ltd. technical management so as to represent a wide spectrum of slag compositions in working electric furnaces. The main variables in the outline of slag composition were the MnO content, basicity ratio and the C a O / M g O ratio. The percentage (by mass) of slag constituents varied in the following ranges : MnO; 5-30%, CaO; 20-35%, MgO; 5-15%, SIO2; 27-58%, AI~O~; 5%, FeO negligible (- 0.5%). However, due to the sintered aluminium crucibles usea t-o keep the slags in electrical conductivity tests, the alumina content of the slags may have increased slightly. The highest alumina content of slags recorded in occasional chemical analyses was 6.1%. Therefore, this small variation was neglected in reporting compositions. All the other constituents remained constant within analytical error limits. Altogether six slag groups were made up based on their MnO contents which were set at 5, 10, 15, 20, 25 and 30%. In each group the basicity ratios, expressed as B = (CaO% + MgO%)/SiO 2 were then varied in four steps, viz. 0.55, 0.80, 1.10 and 1.40. In preparation of the-slags, analytical grade oxides were used. All the components except MnO were first calcined in a muffle furnace at 1200°C, then after cooling these were mixed in desired proportions to obtain the aimed slag compositions. Next the mixtures were pelletized and calcined at 1200oC for 12 hours to promote useful presintering and homogenization of the samples. U p o n cooling, the pellets were crushed and mixed with the required quantities of reagent grade MnO. The mixing operation was done in an agate mortar under acetone in order to secure the best possible mixing. After drying the mixtures were pelletized, the pellets were heated for several hours at 105°C to drive away the last traces of acetone, then cooled and stored in evacuated desiccators.
Liquidus Temperature Measurement Apparatus Slag liquidus temperatures were measured in a custom designed and built hot-stage microscope [1]. The instrument employs the principle of a thermocouple used both for temperature measuring and heating during alternate half cycles of the current [2]. The accuracy of the measured temperatures in the range of 1300 ° - 1400°C is estimated to be about -+ 3°C. The thermocouples were made of 0.35/0.50mm P t - 6 % R h / P t - 30% Rh thermocouple wire.
Electrical Conductivity Measurement Apparatus The parallel dipping electrode technique of Riebling [3] was used. In the present arrangement two platinum electrodes (2mm in diameter) were fixed to two recrystallized alumina single bore rods so that the ground and pointed end of the rod to which a 0.3mm platinum wire was welded could be placed into the bore hole in order to secure increased stability and correct positioning. This was further enhanced by connecting the electrodes with castable alumina cement to a third alumina rod, three of which were cemented together, the central rod serving as a spacer in between the two electrode holders. In this way the electrodes were positioned at a distance of 8.5mm centre to centre apart. The whole assembly was fixed to a microscope lifting device provided with a vernier scale to facilitate the correct positioning of the electrodes in the melt. The dept of immersion of the
Liquidus temperature of synthetic slags
1317
electrodes (5mm of the tips below the liquid surface) could be accurately set within 0.01mm. The electrodes were connected via platinum and copper leads to a wheatstone bridge in which the balance point or zero current flow is indicated by a null instrument. An oscilloscope, type 561 Tektronix served for a null instrument while the alternating current (to exclude the possibility of electrolysis) was obtained from the transistor R.C. oscillator type T.G.150DM, equipped with voltage (0.5 to 2.5¥) and frequency (15 to 150 kc) regulator. In all the tests the bridge was supplied with 2V and 150kc. The high frequency proved beneficial for the measurements in addition to the measuring sensitivity of the oscilloscope. Slag temperatures were recorded in a Hewlett-Packard 3467 A logging multimeter. A variable series of capacitances connected in parallel with the measuring cell were incorporated into the circuit to counteract any capacitance effect exhibited by the slags and thus to enhance further the sensitivity of the measuring system. However, as the measurements indicated, even the slightest addition of capacitance to the circuit tended to decrease the sensitivity independently from the type and nature of the slag. This fact also proved the complete absence of capacitance effects in the slags investigated. To avoid excessive heat capacities and thereby enhance the speed of operation, the size of the furnace was kept small. It was heated electrically using molybdenum wire which was protected from oxidation by ammonia gas. The alumina working tube (i.d.= 38mm, length = 460mm) was sealed ,both from bottom and top by means of water cooled brass muffles. The top brass muffle allowed the movement of electrode assembly through high quality rubber stoppers, and the lower one supported a moveable alumina rod to which a pedestal was cemented. This was also sealed by a rubber stopper. The pedestal rod, through its hole carried the measuring thermocouple and supported the alumina crucibles holding the samples by means of the pedestal. The furnace work tube was continuously flushed with argon gas during experiments. The gas was conventionally purified and was introduced to the furnace through a sealed hole at the bottom brass muffle. Measurement of Liquidus Temperatures In these measurements fineness and thorough mixing of component oxides is necessary, since the weight of the sample held by the measuring thermocouple (acting also as the sample holder) is not more than a couple of milligrams. Two disadvantages of the hotstage microscope operation are the colour of the substance and the so-called boiling effect. As regards colour, no difficulties emerged since all the samples were made of pure and prereacted (homogenised) oxides and only slightly affected by the green colour at higher contents of MnO. All the measurements were carried out under an inert argon atmosphere to retain the divalent stage of Mn in the samples. The boiling of the slag samples, however, was pronounced and in many cases measurements had to be repeated up to six times to obtain reliable data. Boiling is a phenomenon that sets in immediately at the melting of the material evidenced by extensive bubble formation. While the sudden appearance of the bubbles give rise to a reliable estimate of the onset of the bulk melting stage, in hot-stage microscopic determinations it renders the disappearance of some phases invisible making thus the exact recording of the actual melting point rather difficult. Interestingly, the vehemence of boiling appeared to be entirely independent from the MnO content of the slag. It is thus estimated that the measured liquidus temperatures are correct within _+ 3°C. Measurement of Electrical Conductivities Contrary to the liquidus temperature measurements, no boiling whatsoever was experienced in electrical conductivity tests, most probably due to very slow rate of heating of the samples. The samples, weighing 20g placed in 50mm high and 40mm o.d alumina crucibles, filled approximately half of the crucible when molten. All the slag samples were heated uniformly and slowly to 1425°C -1430°C (i.e. minimum 50oc above their melting point) and equilibrated for about four hours at these temperatures under argon atmosphere. Then the first resistance reading was taken using the previously described bridge circuit. The method of constant temperature decrease coupled with resistance readings at constant intervals was
1318
R.H.
E R I C et al.
adopted and proved successful also in checking the liquidus temperatures. Figure 1 is a typical graph depicting the behaviour of two selected slags, showing also an approximate indication of the liquidus temperature by a visible change in the slope. Temperature (" C) 1345.0
1420.0 1.50
l
I
1.20
0.90
1270.0
L, I
/
/
....o
t
t 0
J
O
J
fig
0.60
H
0.30
0.00
I
5.90
i
6.40
I
I
a
6.30
I
6.50
I
6.70
1/T x104 (K) Temperature
Fig. I Plot of resistivity vs reciprocal of temperature (schematic) Measurements obtained with the described set-up give the resistance of the slag in ohms referred to the electrode geometry, i.e. distance of the electrodes, their diameter and dept of immersion into the melt. In order to convert the data to resistivity; ohm.cm, or conductivity ohm'l.cm "1, the cell constant has to be determined. It appears that the frequency of 10kc is the most sensitive range for Wheatstone bridge technique of this kind. In the present work a series of tests were carried out using 15kc and 150kc frequencies with aqueous solutions of various concentrations. No difference whatsoever in the sensitivity of the system was noticed. The calibration of the cell containing the electrodes was performed by standard procedure using 0.01, 0.1, 0.5 and 1.0N KCI solutions with the analytical grade reagent dissolved in distilled and de-ionized water. An alumina crucible identical to those used for slags was employed. The depth of immersion of the electrodes was 5mm with the height of the liquid in the crucible being 20mm. The cell resistance varies greatly with the dept of immersion. The smaller the dept, the greater is the resistance. The resistivity of the slag is inversely proportional to the conductivity according to: R = (l/k)G
(1)
where R is the resistivity of the slag in the cell, k is the conductivity of the slag and G is the cell constant. Expressing Eq (1) in logarithmical form yields : logR=-logk+logG
(2)
Thus a plot of log R vs log k should be a straight line. From such a plot using the known conductivity values of KCI solutions and experimentally measured R values from the bridge circuit, the experimental cell constant was evaluated as
Liquidus temperature of synthetic slags
Gex p = 1.2703 ohm cm "1 = 127.03 Sm "1
1319
(3)
Equation (1) is modified to take into account external resistance of the circuit to yield conductivity : k = G/(Rm-re)
(4)
Where R Ill is the measured resistance and r e is the external resistance. It must be noted that • . G is the cell constant for a given depth of melt and depth of immersion of electrodes. Then the specific resistance in ohm.cm is given by
(5)
p = 100 (Rm-re)/127.03
Atwood [4] gave a theoretical expression for the cell constant incorporating the cell geometry, i.e. the electrode spacing; S, radius; r, and depth of immersion; ~: Gtheo r = ln[(S/2R)+{(S/2R) 2-1 )'/'1 / (Tr)0
(6)
For the present cell geometry S = 8.5ram, r = lmm, and ~ = 5mm,• G L~ ~ or. is obtained as 0.1353 ohm.mm -1 = 135.3 Sm "1. Considering a large number of interfering factors the agreement between the theoretical and experimental values is good. The determination of the external resistance, r e was carried out by shortcircuiting the electrodes in mercury using the same alumina crucible as in the calibration tests at the same depth of immersion of the electrodes. R E S U L T S AND DISCUSSION
Liquidus Temperatures The results of liquidus temperature measurements are shown graphically through Figures 2 to 5. Figure 2 summarizes the effect of basicity ratio upon the liquidus temperature of slag with the MnO content as an additional parameter. In this mode of presentation the various melting temperature ranges are shown with the stepwise increase of the MnO contents, i.e. 5, I0, 15% etc. for slags involved in each MnO content range. Within a given M n O content range for slags, for a constant basicity ratio the CaO content of the slag increases as the MgO content decreases (or vice versa), keeping the silica content constant. In this way for a given MnO content of the slags at constant basicity ratios the only variables that can affect the slag melting point are the CaO and MgO contents or the C a O / M g O ratio (% A1203 is always constant at 5%). These aspects are illustrated in Figures 3 and 4. As far as basicity ratio [B = (% CaO + % MgO) / %SIO2] is considered, the general trend is that with the increase in basicity ratio, the liquidus temperature of the slag increases• The temperature increase becomes more pronounced above a B ratio of 1.1. In general up to 25% M n O contents the change in basicity ratio from 0.55 to 1.1 has only a moderate increasing effect upon the liquidus temperature of slags. In the case of 30% MnO containing slags, the considerable increase in liquidus temperatures start from about 0.8 basicity ratio. The variation of the liquidus temperature with the C a O / M g O ratio depends also to a considerable extent on the basicity ratio and on the MnO contents. This is illustrated in Figures 3 and 4. At 0.8 basicity ratio there is a very moderate increase in the slag melting point with the increase in the C a O / M g O ratio at all MnO contents except 15 and 20% MnO. The effect of C a O / M g O ratio on liquidus temperatures is much more pronounced at high MnO contents and at 1.4 basicity ratio, where the liquidus temperature rapidly decreases with increase in C a O / M g O ratio as shown in Figure 4.
R . H . ERIC et al.
1320
% MnO
1390
•
5
o
10
•
15 20
1380 u o
1370
•
25
o
30
/ 0
/,/////.,o
5% M n O 0
/ J
J
'7.'"
/
ii /
. ~_.0 jjJ I
1330
o
10% M n O
• 15%Mn0
°
1320 v.___.._..
1310
1300
0.5
1
I
I
I
I
0.6
0.7
0.8
0.9
1.0
I 1.1
I 1.2
I t .3
I 1.4
Basicity Ratio % ( C a O + M g O ) % SlO 2
Fig.2 Slag liquids temperature as a function of the basicity ratio at various MnO contents The true effect of MnO content on liquidus temperature is illustrated in Figure 5, where averaged liquidus temperatures for each basicity ratio is plotted with respect to MnO%. Apparently, at basicity ratios of 0.55, 0.80 and 1.1 there is more or less a smooth decrease in liquidus temperature with increasing MnO content of the slags. At B = 1.1 an abrupt sharp increase in the melting point is evidenced above 25% MnO contents. At B = 1.4, the pattern of temperature variation with the rise of the MnO content from 5 to 20% was found to be erratic despite the fact that the slags were subjected to repeated careful measurements. From 20% MnO onwards, a sharp increase in liquidus temperatures is evident. This exercise of averaging out the liquidus temperatures is more justified for slags having basicity ratios between 0.55 to 1.1 where the temperature variation within each basicity range is hardly more than 10°C - 12°C for the various slag compositions. On the other hand, at B = 1.4, the temperature variation is more, reaching in some cases more than 20°C, and thus this could lead to more errors as regards the average temperatures. As far as a generalized slag-composition-liquidus temperature relationship is considered, it is quite obvious that with 10 to 25% MnO contents of slags and at basicity ratios from 0.55
Liquidus temperature of synthetic slags
1321
to 1.1 the average range of the liquidus temperatures can be expected to be between 1315°C and 1340°C with the continuous decrease of the melting point as the MnO content increases. The increase of the basicity ratio to 1.4 brings about a considerable increase in the melting points of the slags. Consequently, low melting slags will be obtained as long as their basicity ratio remains below 1.1 and the MnO content in the range of 10 to 25%, but preferably 15 to 25%. A relatively high liquidus region exist at low MnO and low SiP 2 contents. 1370
\
P
e-.-----e
0,8 BasJc~
o------o
t,1 Baslclly
/
O/
/
O~ ~
10
1340
~"~
t0
1330
/ 7 - ~ 0
1320
1310 o,-
1300
1290
•
I
,
.
,
,
,
,
2
3
4
5
6
7
. ,
....
8
,
•
,
9
I0
11
1
% C ,aq. % MgO
Fig.3 CaO/MgO ratio-liquidus temperature relationships at various MnO contents of the slag As it is seen in Figure 2, the liquidus temperature of all the slags investigated is in the relatively low range of 1310°C to 1380°C. This is understandable when the quasi-ternary phase diagram [5] CaO-MgO-SiO~ with 5% AI~Ox is taken into consideration. The slag compositions without the addition of MnO, ~tffdied in this work, falls into the low temperature valley of the ternary diagram, mostly in the pyroxene and malelite range below 1400°C melting point and a few in the monticellite and merwinite region between 1400°C and 1445°C. The addition of MnO depresses the quasi-ternary liquidus temperatures to the range that is evidenced by the present investigation and could be regarded as being representative of the melting point of slags obtained in the operation of ferromanganese furnaces. The earlier liquidus temperature data provided by Warren [6] is not directly comparable as his slags contained around 12-13% MnO only and 5-17% A120 ~ and temperature ranged from 1230°C to 1570°C in a wider range of compositions but ~vit-h less data points.
Modelling of Liquidus Temperatures Regression models, based on the data of this work and on those derived from Warren [6] were developed to predict ferromanganese slag liquidus temperatures as a function of slag composition. No theoretical basis for model development is available. This made it necessary to develop empirical models. Due to the satisfactory results obtained for a similar exercise using ferrochromium slags [7], it was decided that the models should be quadratic in form. For example, if two components are present in the slag,and their concentrations are X 1 and X 2 then the liquidus temperature Y (or conductivity) could be correlated using a model of the form: Y = a o + alX a + a2X z + a3X12 + a4X1X 2 + asXz2
(7)
1322
R . H . EPic et al.
1390 % MnO
P
i
30
1370
=
25 1360 20 1350
1340
1330
1320
1310
I
I
I
I
I
I
I
2
3
4
5
6
J
7 tC~. MgO
Fig.4 Liquidus temperature-CaO/MgO ratio relationship at 20, 25 and 30% MnO contents in the slag
B
13701-
• 0.55 o 0.80 a 1.10
\
1360
1350
1340
1330
1320
1310
i
I
I
i
I
I
5
I0
15
20
25
30 %MnO
Fig.5 Liquidus temperature as a function of the MnO content at various basicity ratios (Average temperature values)
1323
Liquidus temperature of synthetic slags
Two models were derived for predicting slag liquidus temperatures. The first was based on the data gathered in this work containing 55 data points and it was referred to as "data set 1". The second set consisted of both the data of this work and the one reported by Warren [6]. This set contained 97 points, spanning liquidus temperatures of 1230°C to 1570°(2 and it was referred to as "data set 2". The independent variables were the mass percentages of AI203, MnO, CaP, MgO and SiP z in the slag as well as the basicity ratio, B and the CaP to MgO ratio (denoted by CaOMgO), a total of seven variables. The regressions were carried out using routines from the SAS (Statistical Analysis Software) system. The first step was to use the SAS procedure RSREG which fits a complete quadratic response surface to the data. For a seven variable model, including quadratic and cross product terms, the analysis resulted in a model having 27 parameters. This was regarded as being too many. The SAS procedure RSQUARE was then used. This procedure selects the optimum combination of independent variables to produce the best fit for a given number of independent variables in the model. Using this approach it was found that essentially similar fits could be obtained using a model with far fewer parameters and this was referred to as the "Reduced model". The results are summarized in Table 1. TABLE 1 Results for Liquidus Model Data Set
Correlation Coefficient Number of parameters model
Data
1
Set 2
Complete Quadratic Model
Reduced Model
Complete Quadratic Model
Reduced Model
0.897
0.800
0.875
0.825
27
13
36
16
R2 in
Thus the equation used to predict the slag liquidus temperature using the complete quadratic model for data set 1 would be written as: T(°C) ffi ao+ a~ (% MnO) + a 3 (% CaP) + .... + alo (% MnO) 2 + ... + a24 (B x % MnO) + ... + a35 (CaO-MgO) ~ Although it may appear that the complete quadratic model provides a better fit than the reduced model, it was found that the errors in the points calculated using the reduced model were not significantly different to those calculated using the complete quadratic model. The reduced model contains half the number of parameters than the complete model does, thus making the reduced model a better choice. It was also found that the models for data set 1 provide more accurate fits for the range of values found in data set 1 than the models based on data set 2. Thus, if the slag composition falls within the range used to construct the model for data set 1, this model should be used to predict the liquidas temperature. If the slag composition falls outside this range, then the model based on the wider range of compositions in data set 2 should be used. Due to the nature of the models, it is obviously not advisable to use the models if the slag composition falls outside the range of both data sets. The model parameters are shown in Table 2. Electrical Conductivltles Each resistivity measurement on a given slag gave a set of points in terms of the log resistivities versus reciprocal temperatures from a deliberately chosen high temperature
1324
R . H . EPIC et al.
(preferably more than 30oC above the liquidus of the highest melting slag) down to about 20 ° to 30°C below the solidification point. In order to correlate the obtained resistivity data for all the slags, certain reference temperature had to be chosen for comparison. From the known melting characteristics, 1400°C and 1360°C were chosen, although at 1360°C three of the slags were at the solidification limit and were not included in the assessment. The results are sown in Figures 6 and 7 for 1400°C and 1360°C measurements respectively. The CaO/MgO ratios were not found to play a significant effect in the process of electrical conduction. This was not unexpected since in the slag composition studied these oxides replaced each other in keeping the basicity ratio constant. In Figures 6 and 7, solid lines represent limiting values, that is in the case of say 15% MnO content and at basicity ratio of 1.1 with three different CaO/MgO ratios, the two extreme values are plotted. TABLE 2 Model parameters for liquidus temperatures Coefficients for Liquidus Models
Data
Coefficient
Complete Quadratic
Set
1
Data
Reduced Model
Model 175892.0
Complete Quadratic
Set
2
Reduced Model
Model -298.0
-46339.14
-3266.7
1337.317
73.77809
Intercept
ao
AI203
a~
MnO
az
-1813.238
18.33121
975.5197
CaO
a~
2723.609
55.67750
339.6253
M~O
a4 a~
3033.391
26.79445
1568.448
99.4250
SiO~
-7433.364
876.8867
-40.6551
B
al
-141295.9
474.4984
5878.784
-
a7 al
-14.10726
-33.6818
1264.164
-
:CaO-M~O (AlzO~) z MnO
x AlTO ~
-
a9
a]g
(MnO) 2
-
-
-
-
-7.864925
-
-
-13.50180
-
-4.818810
-
-5.965896
-2.95949
-3.326358
-2.11537
1.806778
-2.20489
-0.4292552
-
CaO
x AI20 ~
all
CaO
x
all
-48.88876
-0.675465
al~
-48.72412
-0.698558
MnO
(CaO) 2 MgO
x AI~0@
MgO
x Mn0
MgO
x
all al~
CaO
-
-16.46353
-100.7227
-1.07502
-12.81905
aÂ7 a|a
SiO?
x MnO
al~
59.00428
SiO~
x
a2~
12.15613
SiO~
x M@O
(Si0~) ~
8.927958
a~
58.75614
x AI~O~
a2~
B
x MnO
B
x
CaO
az4 az~
B
x
M@O
B
x
SiO?
B2 CaO-M@O
x AI~O~
CaO-M@O
x MnO
CaO-M~O
x
CaO-M~O
x M@O
CaO-M@O
x
SiO 2
CaO-M@O
x
B
(CaO-MgO)
CaO
2
.
-
a~
B
-19.19953
-0.565592
SiO 2 x AI203
CaO
-
-52.52072
-52.52912
(MgO) 2
300.2325
-11.66325
-
-13.02977
0.223846 0.140220
-
-3.0438 0.435324
-8.932038
1.288704
-2.263144
-1.99974
-13.66380 -4.113224
0.997500
-
-83.33907
-
1520.916
-
-65.41105
-
1552.295
-
-58.26822
-
1566.986
-
12.61716
-
a??
1427.666
-
-89.54007
a?8 a2~ a~
~i012.782
-
-87.02252
a~l a~z a~ a]4 a35
-
-
0.523864
-
-33.6709
-
-16.80766
-
0.01843465
-
-14.48523
-
1.214988
-
-19.31936
-
-7.240202
-
-14.84434 -0.7695548
.997174
-
-11.40459
-
127.44202
2.197859
6.570390
-0.41043
Liquidus temperature of synthetic slags
1325
% MnO .5
o
7
o 10
l
0
0.5
016
I 0.7
I 0,8
I 0.9
11.0
11.1
112
I 1.3
1~4
Basicny Ratio
Fig.6 Slag resistivity as a function of the basicity ratio at various MnO contents, temperature 1400°C
Eq
% MnO • 5 o 40 • 15 a 20
o
v25 x30
\ Ul
0.5
I
0.6
I
0.7
I
0.8
I
0.9
i
I 0
I
I .I
I
1.2
I
1.3
1
Baslclty Ratio
Fig.7 Slag resistivity as a function of the basicity ratio at various MnO contents, temperature 1360°C
1326
R . H . ERIC et al.
It is apparent that the increase in basicity ratio from 0.55 to 1.1 decreases the resistivity of slags substantially. The effect is identical for both 1400°C and 1360°C temperatures but understandably for the latter one, the total graph is more expanded on account of the increasing resistivities with the lowering of temperature. At basicity ratios higher than I.I, in general the resistivities tend to increase depending on the MnO content. Since the effect of MnO is not clear from Figures 6 and 7, as in the case of liquidus temperatures two new plots have been made as shown in Figures 8 and 9 for 1400°C and 1360°C respectively. These show the upper and lower resistivity limits (as mentioned above) in slag composition with regard to the C a O / M g O ratios as a function of MnO content. The general pattern is similar at both temperatures, namely a high peak in resistivities is reached at 10% MnO content for basicity ratios of 0.55, 0.8 and 1.1, then with the increase of the MnO content to 25% a sharp drop in resistivities is evidenced which will be specially pronounced at low; 0.55 and 0.8 basicity ratios. Above 25% MnO content the resistivities tend to increase again. With the basicity ratio of 1.4 the rise in MnO content caused the resistivity to increase to a relatively high value at 15 to 20% MnO contents, thereafter fluctuating at lower values. Thus the decreasing effect of the MnO content upon the resistivity seems to be generally valid at basicities up to 1.1 and MnO contents between 10 and 25%. An anomalous sharp increase in slag resistivity took place at the low basicity ratio of 0.55 when the MnO content was raised from 5 to 10% for which no explanation could be found.
B • 0.55 o 0.80 1.10 x t.40
E q 6 E tO O
Top of resistivity range
X
~
0
Oz~
X
,
~.
~
~~~
I~, '~J
X
~
X
Bottom of resistivity range 0
i
5
I
I
I
I
I
I0
15
20
25
30
% MnO
Fig.8 Slag resistivity as a function of the MnO content at various basicity ratios. Temperature 1400°C In Figure 10, instead of resistivity, the conductivity data obtained as k = G / R were plotted in Sm'l units. The scatter of the data is due to the calculation method when a constant G value is divided by everchanging R values, and also to the mode of presentation of G / R with large units on the ordinate.
Liquidus temperature of synthetic slags
/ /
E q
/
% %
/
d
o
O
B 0.55 0.80 1.40 4.40
• o a x
% %
#
\
1327
% %
k
I 0
•
Top of resistivityrange
~ f %
o \
z=
0
\ \ \
x
o
\
o
~
N
'~
I
o
0 " -
-
.
~ .
.
.
.
_
_
Bottom of resistivity range 0
I
5
4i0
't15
2~3
I
25
30
%MnO
Fig.9 Slag resistivity as a function of the MnO content at various basicity ratios. Temperature 1360°C Obviously, when slag systems are subjected to high frequency effects for measuring their electrical resistivities, they may exhibit different behaviours when oscilloscope is used as a null point detector in the measuring bridge circuit. Often the introduction of an elaborate filter circuit becomes necessary to bring about the usual single narrow line in the scope screen when the point of zero current is reached in the bridge. In the lack of this arrangement a good deal of additional testing of the most suitable wave form becomes inevitable. This was the case with the present slags but with the establishment of the optimum wave form a sensitivity of 0.005 ohm could be attained with the use of the rather simple measuring system. However, it is also likely that this phenomenon results in some scatter of the data. On the whole, the data gives a hint to the practical implications of slag resistivities for the operation of the electrical furnaces when it is used carefully. Since the industrial furnaces operate at low frequencies (50 - 60c), this fact may leave some doubt as the real value of resistivities measured in the laboratory when they will have to be interpreted for largescale operating furnaces. However, a reasonable sound assumption that can be made for the present purpose is that the relative resistance of different slags say at 15kc corresponds to the relative resistance of slags at 50c. The measurement of resistivity at these low frequencies would, in any case, involve considerable difficulties say by the use of direct (i.e. voltage/current) measurements lending to impedance, rather than resistance values. Modelling of Electrical Conductivities Only the data gathered in this investigation was used to construct models for conductivity as no other data was available on similar slags. Two models were constructed, one for ME
4:12-I
1328
R.H.
Eplc et al.
1360°C and one for 1400°C. An analogous procedure to that outlined for liquidus temperatures was followed. The complete quadratic models resulted in an expression containing 27 parameters. The reduced model for 1400°C contained 14 and the reduced model for 1360°C contained only 9 parameters. The fits obtained, however, were not as good as those obtained for liquidus temperatures. The correlation coefficients (R e) for 1400°C model were 0.796 for the complete quadratic form and 0.701 for the reduced model. For the 1360°C, the correlation coefficients for the complete quadratic and reduced models were 0.793 and 0.696 respectively. Obviously the main reason for poorer fits is the more scatter in conductivity data compared with that of liquidus temperature data. However, the models for conductivity are still very useful in the prediction of ferromanganese slag electrical conductivities within the slag composition ranges investigated in this work. The model parameters are shown in Table 3. t40
Upper limit
120
tO
.-2
1013 I
=
-=-" -
o
~
i o
II /I
x
x
x
ii x
ii II
60
o
i
A
//
s
x
X
x
o
40
,"
o x
o 0
20
~ Lower limit
/s •
rS /
J
o
o
g
I
I
I
I
I
I
5
10
15
20
25
30
% MnO
Fig.10 Electrical conductivity as a function of the MnO content at various slag basicities (Notation for basicities as in previous figures) Thus the equation used to predict the slag conductivity using the complete quadratic model would be written as: k ( o h m ' l c m "1) -- a~ + at (%MnO) + a 3 (%CaO) + .... + al0 (%MnO) 2 + .... + a24 (B x % MnO) + .... + a35 (CaO -VMg0)Z
PRACTICAL IMPLICATIONS AND C O N C L U S I O N S The value of the investigations performed on slags will depend how far the results could be translated into usable form for practical furnace operation. For this purpose empirical modelling of the results were made. The prime importance of both the liquidus temperature and electrical conductivity characteristics is apparent in this instance, so what actually should be known is the degree to which the slag itself contributes to the complex operation of a ferromanganese producing furnace. Most unfortunately this point is far from being sufficiently clarified to warrant an exact quantitative treatment of the data obtained, say for optimization purposes. Liquidus temperature data are helpful in that they indicate the
1329
Liquidus temperature of synthetic slags
extent of slag overheating from which, with the knowledge of the specific heat and the mass of the charge and of the slag at any time present in the furnace, the heat utilization or wasting, may be estimated to a reasonable degree. Also, the liquidus temperatures may, to some extent, serve as indicators towards slag viscosities, these being strongly temperature dependent slag characteristics. Thus with the knowledge on slag composition, melting point and degree of superheating some measure of optimisation for thermal conditions and a greater accuracy for the prediction of the viability of slag tapping will be possible. The substantial drop in liquidus temperatures with the increase of the MnO content above 10% in the slag would suggest a reduction in power input and vice versa. TABLE 3 Model parameters for electrical conductivities Coefficients for conductivity models 1400°C
Coefficient
Complete Quadratic
1360°C
Reduced Model
Model ao
Intercept
3344.425
-96.1582
4866.2024
6.463066
-80.096863
Reduced Model
-2.5865
al
AI~O 3 MnO
a~
-75.649343
CaO
a~
-21.610706
MgO
-38.306389
SiO z
a4 a~
B
a~
-473.17585
a7 aQ
0.3177461
CaO-MgO (AIz03) 2 MnO
Complete Quadratic Model
x AlzO 3
(MnO) 2 CaO
x AlzO 3
CaO
x MnO
-103.12899
-
a9 alQ all a12
(CaO) ~
22.155033 1.680546
0.4261274 -
14.33459 -178.0534
-11.1125
-0.05557 -1.21924
-2324.8901
4.017052
0.0977123
0.210062
0.3012871 -
0.2663299
0.025700
-0.4891872
-
0.081739
-0.8101123
-
x AIzO ~
a~4
MgO
x MnO
a15
0.4617066
-0.056091
-0.3871913
MgO
x
a16
-0.09733365
0.081052
-1.4699948
a17
0.01405341
(MgO) 2 SiO2
x AI?O 3
SiO~
x MnO
SiO z x
CaO
SiO~
MgO
x
(SiO~) 2
aid al,
-
-0.1694077
MgO
CaO
0.004484
1.13661057
1.559474 -0.131759 -
-0.722310 1.6405583
a20
0.55144415
a~l
0.72259679
-0.081284
0.5691971 0.6488153
a22
0.71695819
-0.069895
1.3336187
0.001833 -0.00098 0.000395
B
x AI~O;
a~}
B
x
MnO
-
24.8555297
-
x
CaO
az4 a,~
4.94706097
B
5.29423304
-
25.9234570
-
B
x
MgO
a2&
4.40343255
-
25.0433762
-
B
x
SiO~
a??
6.64341205
-
24.6491982
a28
-3.25822761
-
-18.4073403
B2
-1.83089
CaO-MgO
x AlzO }
a~
CaO-MgO
x MnO
a}Q
0.03287501
-
0.03988242
CaO-MgO
x
CaO
a}l
0.10689761
-
0.12161739
-
CaO-MgO
x MgO
a}z
-0.33606162
-
-0.27112965
-
CaO-M~O
x
a~
CaO-M@O
x
aa4
-1.21059598
-
-1.59051458
a35
-0.20757838
(CaO-MgO)
SiOz
2
-0.20912643
-
-0.010669
1330
R . H . Epac et al.
Pertinent to the effect of electrical conductivities or slag resistivities, the question is complicated by the fact that the total resistance stipulating the power input is made up by the resistivity of the charge and that of the slag. However,the ratio between their contribution is not more than an acceptable guess say between 50% and 70% as attributable to the slag, but in general this depends to a very great extent on the thickness of the coke layer and the size of the reductant. Since it is the coke layer that provides a separation between the unmolten raw material and the liquid slag, the particle size is rather important as a balancing factor between charge resistance and slag resistance. In this instance as for an idealised case, a sudden rise in resistivity would probably indicate an abrupt decrease in particle size and the resultant thinning of the coke layer with an increased participation of the slag resistivity in the smelting operation. The heat generated preferentially in the molten charge contributes to the melting of the raw materials higher up in the furnace with rather moderate chemical reduction of the charge. The final reduction deeper in the furnace is accomplished by the heat generated near the electrode t i p s a n d should therefore result in low MnO containing slags. Quite obviously, if too much heat is consumed for the melting of the charge, a heat deficiency near the electrode tip may cause incomplete reduction. This sate of affairs brings about conditions which makes the assessment of the true effect of slag resistivity on a generalized basis difficult and rather approximate. Nevertheless, certain important points as to some of the effects of material characteristics upon the operation of electric furnaces used in ferromanganese production can be mentioned. As far as the slag is considered, the most important factors are the MnO content and the basicity ratio as it has been clearly demonstrated in this work. A rather large range in variation of furnace power may be expected with the change of the basicity ratio at MnO contents between 5 and 10%. Under these circumstances, at constant current, the power and therewith the heating rate is on the high side. As the MnO content of the slag increases, in the 20-25% MnO range, the variation of the basicity ratio will have a modest effect on the slag resistivity which can be expected to be in the low range. Therefore the aimed power input could be achieved only with a substantial increase in the furnace current to maintain an economical smelting rate for the furnace. As the slag melting points will also be the lowest at these MnO contents, extra power input will also be necessary if the superheating of the slag were to be maintained at a set level from the point of view of the metal produced. A special attention has to be drawn here to the fact that the extreme variations in slag resistivities indicated by laboratory measurements would under no circumstances bring about concurrent effects that could be expected in actual furnace operation. This is due first of all to the very limited contact (if any) between the electrodes and the slag. Secondly, the slag compositions are subject to variation both in their MnO contents and basicity ratio during the smelting of the charge. This will inevitably lead to the averaging out or to the diminishing of the extreme variations that would otherwise ensue in electrode movement, current draw etc. Despite the mentioned negative aspects, the data obtained in laboratory investigations on electrical conductivities may serve as a useful indication towards changes that may set in during furnace operation with the change in the characteristics of the produced slag. Furthermore, from the knowledge of opposite behaviour of viscosity and electrical conductivity some reasonably valid conclusions can be drawn as to the behaviour of the slags investigated in this work. For example the increase in the electrical conductivity upon the increase in the MnO content of the slag will invariably be associated with a by and large equivalent decrease in the slag viscosity. The optimum slag composition (C,,,,t) representing also optimum operating conditions for the furnace as to slag characteristiS~-is primarily a function of three parameters:
Liquidus temperature of synthetic slags
1331
Cop t = f (%MeO,k,17)
where k is conductivity and rl is viscosity. This was proved to be the case with slags used in copper matte smelting furnaces in submerged electrode resistance heated furnaces, the operation of which is of course quite different from that of submerged arc furnaces. However, the basic behaviour of the slag proper remains the same irrespective in whatever furnace it is processed in and it is only the mode of operation of the furnace together with the nature of the charge composition that brings about the differences. Following this line of thought and without knowledge of the actual slag viscosities an approximate and rather qualitative suggestion may be put forward here using the conductivity values shown in Figure I0. The conductivity curve shown in Figure 11 is roughly a mid-value between the high and low extremes. According to this illustration the furnace operating range would correspond to slag compositions having 15 to 25% MnO with an optimum around 15%. Obviously, the plot is a gross oversimplification and actual known viscosity values would probably shift the suggested range towards lower values. As it stands, the suggested range would fit very well into that found for lowest liquidus temperatures.
K Sm -t
120
\ \ \ 80
/',,
I
/ ,'\
/
40
/
I.
X ",..! i / I operating J range
/
,
I0
I t
Opt.
~ ~..J._/" I I
,
20
1
30 % MnO
Fig.l 1 Possible mode of slag optimisation from electrical conductivity and viscosity measurements as applied to the present investigations. Basis: MnO content of the slags ACKNOWLEDGEMENTS The financial assistance provided for this investigation by Ferro Alloy Producers Association and Samancor is accorded with due thanks. This paper is published by the permission granted by Samancor.
1332
R. H. ERIc et al.
REFERENCES l°
Miller, M.P. & Sommer, G. Journal of Science Instrum., vol. 43, pp 293-297 (1966)
2.
Sommer, G. and Jochens, P.R., Mineral Sci.Engr., Vol. 3, No 1, pp 3-16 (1971)
3.
Riebling, E.F. and Vogel, P.C., Rev. Sci, Instrum., Vol. 36, pp. 425-428 (1965)
4.
Atwood, S.S. Electric and Magnetic Fields, J.Wiley, New York, pp 83-160 (1949)
5.
Lewin, E.M., Robbins, C.R. and McMurdie, H.F. Phase diagrams for Ceramists 1969 Supplement, The American Ceramic Society, Columbus, Ohio, (1969)
.
Warren, G.F. Measurement of the activity of manganese (II) oxide in slags associated with the production of ferromanganese, MSc Thesis, University of the Witwatersrand (1972).
.
Pesta, M., Marjoribanks, F.M., Oosthuizen, G.A. and Salgado, G. Report no 2120, National Institute of Metallurgy, Randburg, South Africa (198 I)
0892-6875/91 $3.00+0.00 © 1991 Pergamon Press plc
Printed in Great Britain
LIQUIDUS TEMPERATURE AND ELECTRICAL CONDUCTIVITIES OF SYNTHETIC FERROMANGANESE SLAGS R.H. ERIC, A.A. HEJJA and W. STANGE
Department of Metallurgy and Materials Engineering, University of the Witwatersrand, P.O. Wits, Johannesburg 2050, South Africa (Received 10 December 1990; accepted 1 January 1991)
ABSTRACT
Liquidus temperature and electrical conductivity measurements were carried out on synthetic slags prepared from pure oxides to represent a wide range of compositions likely to be encountered in the operation of ferromanganese electric furnaces. The slag constituents were in the following ranges : MnO; 5-30%, Ca0;20-35%, MgO; 5-15%, S i 0 2 ; 27-58%, A1203; 5%. The basicity ratios varied from 0.55 to 1.4. Liquidus temperatures were measured by the hot-stage microscope, the electrical conductivities with the aid of a modified Wheatstone bridge. Liquidus temperatures were found to vary from 1300 ° to 1380°C in the slags investigated, and increased with increasing basicity ratio. The electrical resistivity of slags decreased with the increase of basicity ratio from 0.55 to 1.1. Above 1.1 basicity ratio the resistivity tended to increase depending on the MnO content. Regression models were developed to predict both, ferromanganese slag liquidus temperatures and electrical conductivities as a function of slag composition. The model equations are quadratic in form, with the model coefficients calculated using the S A S package. Reasonable fits were obtained for the liquidus temperature data. The fits for the conductivity data are not as good but still useful, depending on the accuracy required from the model. INTRODUCTION The general trend in the production of ferroalloys is manifested in increasing quality demands in these alloys together with an improvement in the economy of the ore-smelting operation. Thus, the optimisation of the smelting technique is gaining greater prominence. The concept involves both furnace technology, that is size, geometry, precise electrical outline and optimum material characteristics which includes correct selection, proportioning and siting of the charge together with the quantity and physicochemical characteristics of the slag produced. An important facet of the submerged arc furnace is that the resistance of the charge and that of the slag together determines largely the electrical requirements for the economic manufacture of a given alloy. However, the exact distribution of the resistance between the two is extremely difficult to delineate even if the resistivities of the various charge components and of the slags had been determined with great accuracy. It is generally accepted that in the production of ferro=manganese the resistivity of the slag plays a considerably more important role than in the manufacture of ferrochromium. In any case the electrical conductivity (inverse of resistivity) of the charge up to 1000°C is very low as is the thermal conductivity. Logically t h e n , at least at the beginning, the smelting operation is localized in a relatively small intense region close to the electrode tip. Nevertheless, the formation of a preliminary incipient fusion zone containing ore plus 1315
1316
R . H . ERIC et al.
reductant inclusions besides gangue materials, as in the smelting of chrome ores, may not be excluded from the treatment of lower-melting manganese ores. This would probably represent the only continuous path through which current can be transferred. On account of the considerably greater influence of the molten slags as a conveyer of electricity in the smelting of manganese ores, a detailed investigation was carried out on the liquidus temperatures and electrical conductivities of slags of selected compositions likely to be produced in ferromanganese furnaces. The data then were empirically modelled with regression analysis. EXPERIMENTAL P R O C E D U R E
Preparation of Slags The slag composition ranges were selected with the help of S A M A N C O R Ltd. technical management so as to represent a wide spectrum of slag compositions in working electric furnaces. The main variables in the outline of slag composition were the MnO content, basicity ratio and the C a O / M g O ratio. The percentage (by mass) of slag constituents varied in the following ranges : MnO; 5-30%, CaO; 20-35%, MgO; 5-15%, SIO2; 27-58%, AI~O~; 5%, FeO negligible (- 0.5%). However, due to the sintered aluminium crucibles usea t-o keep the slags in electrical conductivity tests, the alumina content of the slags may have increased slightly. The highest alumina content of slags recorded in occasional chemical analyses was 6.1%. Therefore, this small variation was neglected in reporting compositions. All the other constituents remained constant within analytical error limits. Altogether six slag groups were made up based on their MnO contents which were set at 5, 10, 15, 20, 25 and 30%. In each group the basicity ratios, expressed as B = (CaO% + MgO%)/SiO 2 were then varied in four steps, viz. 0.55, 0.80, 1.10 and 1.40. In preparation of the-slags, analytical grade oxides were used. All the components except MnO were first calcined in a muffle furnace at 1200°C, then after cooling these were mixed in desired proportions to obtain the aimed slag compositions. Next the mixtures were pelletized and calcined at 1200oC for 12 hours to promote useful presintering and homogenization of the samples. U p o n cooling, the pellets were crushed and mixed with the required quantities of reagent grade MnO. The mixing operation was done in an agate mortar under acetone in order to secure the best possible mixing. After drying the mixtures were pelletized, the pellets were heated for several hours at 105°C to drive away the last traces of acetone, then cooled and stored in evacuated desiccators.
Liquidus Temperature Measurement Apparatus Slag liquidus temperatures were measured in a custom designed and built hot-stage microscope [1]. The instrument employs the principle of a thermocouple used both for temperature measuring and heating during alternate half cycles of the current [2]. The accuracy of the measured temperatures in the range of 1300 ° - 1400°C is estimated to be about -+ 3°C. The thermocouples were made of 0.35/0.50mm P t - 6 % R h / P t - 30% Rh thermocouple wire.
Electrical Conductivity Measurement Apparatus The parallel dipping electrode technique of Riebling [3] was used. In the present arrangement two platinum electrodes (2mm in diameter) were fixed to two recrystallized alumina single bore rods so that the ground and pointed end of the rod to which a 0.3mm platinum wire was welded could be placed into the bore hole in order to secure increased stability and correct positioning. This was further enhanced by connecting the electrodes with castable alumina cement to a third alumina rod, three of which were cemented together, the central rod serving as a spacer in between the two electrode holders. In this way the electrodes were positioned at a distance of 8.5mm centre to centre apart. The whole assembly was fixed to a microscope lifting device provided with a vernier scale to facilitate the correct positioning of the electrodes in the melt. The dept of immersion of the
Liquidus temperature of synthetic slags
1317
electrodes (5mm of the tips below the liquid surface) could be accurately set within 0.01mm. The electrodes were connected via platinum and copper leads to a wheatstone bridge in which the balance point or zero current flow is indicated by a null instrument. An oscilloscope, type 561 Tektronix served for a null instrument while the alternating current (to exclude the possibility of electrolysis) was obtained from the transistor R.C. oscillator type T.G.150DM, equipped with voltage (0.5 to 2.5¥) and frequency (15 to 150 kc) regulator. In all the tests the bridge was supplied with 2V and 150kc. The high frequency proved beneficial for the measurements in addition to the measuring sensitivity of the oscilloscope. Slag temperatures were recorded in a Hewlett-Packard 3467 A logging multimeter. A variable series of capacitances connected in parallel with the measuring cell were incorporated into the circuit to counteract any capacitance effect exhibited by the slags and thus to enhance further the sensitivity of the measuring system. However, as the measurements indicated, even the slightest addition of capacitance to the circuit tended to decrease the sensitivity independently from the type and nature of the slag. This fact also proved the complete absence of capacitance effects in the slags investigated. To avoid excessive heat capacities and thereby enhance the speed of operation, the size of the furnace was kept small. It was heated electrically using molybdenum wire which was protected from oxidation by ammonia gas. The alumina working tube (i.d.= 38mm, length = 460mm) was sealed ,both from bottom and top by means of water cooled brass muffles. The top brass muffle allowed the movement of electrode assembly through high quality rubber stoppers, and the lower one supported a moveable alumina rod to which a pedestal was cemented. This was also sealed by a rubber stopper. The pedestal rod, through its hole carried the measuring thermocouple and supported the alumina crucibles holding the samples by means of the pedestal. The furnace work tube was continuously flushed with argon gas during experiments. The gas was conventionally purified and was introduced to the furnace through a sealed hole at the bottom brass muffle. Measurement of Liquidus Temperatures In these measurements fineness and thorough mixing of component oxides is necessary, since the weight of the sample held by the measuring thermocouple (acting also as the sample holder) is not more than a couple of milligrams. Two disadvantages of the hotstage microscope operation are the colour of the substance and the so-called boiling effect. As regards colour, no difficulties emerged since all the samples were made of pure and prereacted (homogenised) oxides and only slightly affected by the green colour at higher contents of MnO. All the measurements were carried out under an inert argon atmosphere to retain the divalent stage of Mn in the samples. The boiling of the slag samples, however, was pronounced and in many cases measurements had to be repeated up to six times to obtain reliable data. Boiling is a phenomenon that sets in immediately at the melting of the material evidenced by extensive bubble formation. While the sudden appearance of the bubbles give rise to a reliable estimate of the onset of the bulk melting stage, in hot-stage microscopic determinations it renders the disappearance of some phases invisible making thus the exact recording of the actual melting point rather difficult. Interestingly, the vehemence of boiling appeared to be entirely independent from the MnO content of the slag. It is thus estimated that the measured liquidus temperatures are correct within _+ 3°C. Measurement of Electrical Conductivities Contrary to the liquidus temperature measurements, no boiling whatsoever was experienced in electrical conductivity tests, most probably due to very slow rate of heating of the samples. The samples, weighing 20g placed in 50mm high and 40mm o.d alumina crucibles, filled approximately half of the crucible when molten. All the slag samples were heated uniformly and slowly to 1425°C -1430°C (i.e. minimum 50oc above their melting point) and equilibrated for about four hours at these temperatures under argon atmosphere. Then the first resistance reading was taken using the previously described bridge circuit. The method of constant temperature decrease coupled with resistance readings at constant intervals was
1318
R.H.
E R I C et al.
adopted and proved successful also in checking the liquidus temperatures. Figure 1 is a typical graph depicting the behaviour of two selected slags, showing also an approximate indication of the liquidus temperature by a visible change in the slope. Temperature (" C) 1345.0
1420.0 1.50
l
I
1.20
0.90
1270.0
L, I
/
/
....o
t
t 0
J
O
J
fig
0.60
H
0.30
0.00
I
5.90
i
6.40
I
I
a
6.30
I
6.50
I
6.70
1/T x104 (K) Temperature
Fig. I Plot of resistivity vs reciprocal of temperature (schematic) Measurements obtained with the described set-up give the resistance of the slag in ohms referred to the electrode geometry, i.e. distance of the electrodes, their diameter and dept of immersion into the melt. In order to convert the data to resistivity; ohm.cm, or conductivity ohm'l.cm "1, the cell constant has to be determined. It appears that the frequency of 10kc is the most sensitive range for Wheatstone bridge technique of this kind. In the present work a series of tests were carried out using 15kc and 150kc frequencies with aqueous solutions of various concentrations. No difference whatsoever in the sensitivity of the system was noticed. The calibration of the cell containing the electrodes was performed by standard procedure using 0.01, 0.1, 0.5 and 1.0N KCI solutions with the analytical grade reagent dissolved in distilled and de-ionized water. An alumina crucible identical to those used for slags was employed. The depth of immersion of the electrodes was 5mm with the height of the liquid in the crucible being 20mm. The cell resistance varies greatly with the dept of immersion. The smaller the dept, the greater is the resistance. The resistivity of the slag is inversely proportional to the conductivity according to: R = (l/k)G
(1)
where R is the resistivity of the slag in the cell, k is the conductivity of the slag and G is the cell constant. Expressing Eq (1) in logarithmical form yields : logR=-logk+logG
(2)
Thus a plot of log R vs log k should be a straight line. From such a plot using the known conductivity values of KCI solutions and experimentally measured R values from the bridge circuit, the experimental cell constant was evaluated as
Liquidus temperature of synthetic slags
Gex p = 1.2703 ohm cm "1 = 127.03 Sm "1
1319
(3)
Equation (1) is modified to take into account external resistance of the circuit to yield conductivity : k = G/(Rm-re)
(4)
Where R Ill is the measured resistance and r e is the external resistance. It must be noted that • . G is the cell constant for a given depth of melt and depth of immersion of electrodes. Then the specific resistance in ohm.cm is given by
(5)
p = 100 (Rm-re)/127.03
Atwood [4] gave a theoretical expression for the cell constant incorporating the cell geometry, i.e. the electrode spacing; S, radius; r, and depth of immersion; ~: Gtheo r = ln[(S/2R)+{(S/2R) 2-1 )'/'1 / (Tr)0
(6)
For the present cell geometry S = 8.5ram, r = lmm, and ~ = 5mm,• G L~ ~ or. is obtained as 0.1353 ohm.mm -1 = 135.3 Sm "1. Considering a large number of interfering factors the agreement between the theoretical and experimental values is good. The determination of the external resistance, r e was carried out by shortcircuiting the electrodes in mercury using the same alumina crucible as in the calibration tests at the same depth of immersion of the electrodes. R E S U L T S AND DISCUSSION
Liquidus Temperatures The results of liquidus temperature measurements are shown graphically through Figures 2 to 5. Figure 2 summarizes the effect of basicity ratio upon the liquidus temperature of slag with the MnO content as an additional parameter. In this mode of presentation the various melting temperature ranges are shown with the stepwise increase of the MnO contents, i.e. 5, I0, 15% etc. for slags involved in each MnO content range. Within a given M n O content range for slags, for a constant basicity ratio the CaO content of the slag increases as the MgO content decreases (or vice versa), keeping the silica content constant. In this way for a given MnO content of the slags at constant basicity ratios the only variables that can affect the slag melting point are the CaO and MgO contents or the C a O / M g O ratio (% A1203 is always constant at 5%). These aspects are illustrated in Figures 3 and 4. As far as basicity ratio [B = (% CaO + % MgO) / %SIO2] is considered, the general trend is that with the increase in basicity ratio, the liquidus temperature of the slag increases• The temperature increase becomes more pronounced above a B ratio of 1.1. In general up to 25% M n O contents the change in basicity ratio from 0.55 to 1.1 has only a moderate increasing effect upon the liquidus temperature of slags. In the case of 30% MnO containing slags, the considerable increase in liquidus temperatures start from about 0.8 basicity ratio. The variation of the liquidus temperature with the C a O / M g O ratio depends also to a considerable extent on the basicity ratio and on the MnO contents. This is illustrated in Figures 3 and 4. At 0.8 basicity ratio there is a very moderate increase in the slag melting point with the increase in the C a O / M g O ratio at all MnO contents except 15 and 20% MnO. The effect of C a O / M g O ratio on liquidus temperatures is much more pronounced at high MnO contents and at 1.4 basicity ratio, where the liquidus temperature rapidly decreases with increase in C a O / M g O ratio as shown in Figure 4.
R . H . ERIC et al.
1320
% MnO
1390
•
5
o
10
•
15 20
1380 u o
1370
•
25
o
30
/ 0
/,/////.,o
5% M n O 0
/ J
J
'7.'"
/
ii /
. ~_.0 jjJ I
1330
o
10% M n O
• 15%Mn0
°
1320 v.___.._..
1310
1300
0.5
1
I
I
I
I
0.6
0.7
0.8
0.9
1.0
I 1.1
I 1.2
I t .3
I 1.4
Basicity Ratio % ( C a O + M g O ) % SlO 2
Fig.2 Slag liquids temperature as a function of the basicity ratio at various MnO contents The true effect of MnO content on liquidus temperature is illustrated in Figure 5, where averaged liquidus temperatures for each basicity ratio is plotted with respect to MnO%. Apparently, at basicity ratios of 0.55, 0.80 and 1.1 there is more or less a smooth decrease in liquidus temperature with increasing MnO content of the slags. At B = 1.1 an abrupt sharp increase in the melting point is evidenced above 25% MnO contents. At B = 1.4, the pattern of temperature variation with the rise of the MnO content from 5 to 20% was found to be erratic despite the fact that the slags were subjected to repeated careful measurements. From 20% MnO onwards, a sharp increase in liquidus temperatures is evident. This exercise of averaging out the liquidus temperatures is more justified for slags having basicity ratios between 0.55 to 1.1 where the temperature variation within each basicity range is hardly more than 10°C - 12°C for the various slag compositions. On the other hand, at B = 1.4, the temperature variation is more, reaching in some cases more than 20°C, and thus this could lead to more errors as regards the average temperatures. As far as a generalized slag-composition-liquidus temperature relationship is considered, it is quite obvious that with 10 to 25% MnO contents of slags and at basicity ratios from 0.55
Liquidus temperature of synthetic slags
1321
to 1.1 the average range of the liquidus temperatures can be expected to be between 1315°C and 1340°C with the continuous decrease of the melting point as the MnO content increases. The increase of the basicity ratio to 1.4 brings about a considerable increase in the melting points of the slags. Consequently, low melting slags will be obtained as long as their basicity ratio remains below 1.1 and the MnO content in the range of 10 to 25%, but preferably 15 to 25%. A relatively high liquidus region exist at low MnO and low SiP 2 contents. 1370
\
P
e-.-----e
0,8 BasJc~
o------o
t,1 Baslclly
/
O/
/
O~ ~
10
1340
~"~
t0
1330
/ 7 - ~ 0
1320
1310 o,-
1300
1290
•
I
,
.
,
,
,
,
2
3
4
5
6
7
. ,
....
8
,
•
,
9
I0
11
1
% C ,aq. % MgO
Fig.3 CaO/MgO ratio-liquidus temperature relationships at various MnO contents of the slag As it is seen in Figure 2, the liquidus temperature of all the slags investigated is in the relatively low range of 1310°C to 1380°C. This is understandable when the quasi-ternary phase diagram [5] CaO-MgO-SiO~ with 5% AI~Ox is taken into consideration. The slag compositions without the addition of MnO, ~tffdied in this work, falls into the low temperature valley of the ternary diagram, mostly in the pyroxene and malelite range below 1400°C melting point and a few in the monticellite and merwinite region between 1400°C and 1445°C. The addition of MnO depresses the quasi-ternary liquidus temperatures to the range that is evidenced by the present investigation and could be regarded as being representative of the melting point of slags obtained in the operation of ferromanganese furnaces. The earlier liquidus temperature data provided by Warren [6] is not directly comparable as his slags contained around 12-13% MnO only and 5-17% A120 ~ and temperature ranged from 1230°C to 1570°C in a wider range of compositions but ~vit-h less data points.
Modelling of Liquidus Temperatures Regression models, based on the data of this work and on those derived from Warren [6] were developed to predict ferromanganese slag liquidus temperatures as a function of slag composition. No theoretical basis for model development is available. This made it necessary to develop empirical models. Due to the satisfactory results obtained for a similar exercise using ferrochromium slags [7], it was decided that the models should be quadratic in form. For example, if two components are present in the slag,and their concentrations are X 1 and X 2 then the liquidus temperature Y (or conductivity) could be correlated using a model of the form: Y = a o + alX a + a2X z + a3X12 + a4X1X 2 + asXz2
(7)
1322
R . H . EPic et al.
1390 % MnO
P
i
30
1370
=
25 1360 20 1350
1340
1330
1320
1310
I
I
I
I
I
I
I
2
3
4
5
6
J
7 tC~. MgO
Fig.4 Liquidus temperature-CaO/MgO ratio relationship at 20, 25 and 30% MnO contents in the slag
B
13701-
• 0.55 o 0.80 a 1.10
\
1360
1350
1340
1330
1320
1310
i
I
I
i
I
I
5
I0
15
20
25
30 %MnO
Fig.5 Liquidus temperature as a function of the MnO content at various basicity ratios (Average temperature values)
1323
Liquidus temperature of synthetic slags
Two models were derived for predicting slag liquidus temperatures. The first was based on the data gathered in this work containing 55 data points and it was referred to as "data set 1". The second set consisted of both the data of this work and the one reported by Warren [6]. This set contained 97 points, spanning liquidus temperatures of 1230°C to 1570°(2 and it was referred to as "data set 2". The independent variables were the mass percentages of AI203, MnO, CaP, MgO and SiP z in the slag as well as the basicity ratio, B and the CaP to MgO ratio (denoted by CaOMgO), a total of seven variables. The regressions were carried out using routines from the SAS (Statistical Analysis Software) system. The first step was to use the SAS procedure RSREG which fits a complete quadratic response surface to the data. For a seven variable model, including quadratic and cross product terms, the analysis resulted in a model having 27 parameters. This was regarded as being too many. The SAS procedure RSQUARE was then used. This procedure selects the optimum combination of independent variables to produce the best fit for a given number of independent variables in the model. Using this approach it was found that essentially similar fits could be obtained using a model with far fewer parameters and this was referred to as the "Reduced model". The results are summarized in Table 1. TABLE 1 Results for Liquidus Model Data Set
Correlation Coefficient Number of parameters model
Data
1
Set 2
Complete Quadratic Model
Reduced Model
Complete Quadratic Model
Reduced Model
0.897
0.800
0.875
0.825
27
13
36
16
R2 in
Thus the equation used to predict the slag liquidus temperature using the complete quadratic model for data set 1 would be written as: T(°C) ffi ao+ a~ (% MnO) + a 3 (% CaP) + .... + alo (% MnO) 2 + ... + a24 (B x % MnO) + ... + a35 (CaO-MgO) ~ Although it may appear that the complete quadratic model provides a better fit than the reduced model, it was found that the errors in the points calculated using the reduced model were not significantly different to those calculated using the complete quadratic model. The reduced model contains half the number of parameters than the complete model does, thus making the reduced model a better choice. It was also found that the models for data set 1 provide more accurate fits for the range of values found in data set 1 than the models based on data set 2. Thus, if the slag composition falls within the range used to construct the model for data set 1, this model should be used to predict the liquidas temperature. If the slag composition falls outside this range, then the model based on the wider range of compositions in data set 2 should be used. Due to the nature of the models, it is obviously not advisable to use the models if the slag composition falls outside the range of both data sets. The model parameters are shown in Table 2. Electrical Conductivltles Each resistivity measurement on a given slag gave a set of points in terms of the log resistivities versus reciprocal temperatures from a deliberately chosen high temperature
1324
R . H . EPIC et al.
(preferably more than 30oC above the liquidus of the highest melting slag) down to about 20 ° to 30°C below the solidification point. In order to correlate the obtained resistivity data for all the slags, certain reference temperature had to be chosen for comparison. From the known melting characteristics, 1400°C and 1360°C were chosen, although at 1360°C three of the slags were at the solidification limit and were not included in the assessment. The results are sown in Figures 6 and 7 for 1400°C and 1360°C measurements respectively. The CaO/MgO ratios were not found to play a significant effect in the process of electrical conduction. This was not unexpected since in the slag composition studied these oxides replaced each other in keeping the basicity ratio constant. In Figures 6 and 7, solid lines represent limiting values, that is in the case of say 15% MnO content and at basicity ratio of 1.1 with three different CaO/MgO ratios, the two extreme values are plotted. TABLE 2 Model parameters for liquidus temperatures Coefficients for Liquidus Models
Data
Coefficient
Complete Quadratic
Set
1
Data
Reduced Model
Model 175892.0
Complete Quadratic
Set
2
Reduced Model
Model -298.0
-46339.14
-3266.7
1337.317
73.77809
Intercept
ao
AI203
a~
MnO
az
-1813.238
18.33121
975.5197
CaO
a~
2723.609
55.67750
339.6253
M~O
a4 a~
3033.391
26.79445
1568.448
99.4250
SiO~
-7433.364
876.8867
-40.6551
B
al
-141295.9
474.4984
5878.784
-
a7 al
-14.10726
-33.6818
1264.164
-
:CaO-M~O (AlzO~) z MnO
x AlTO ~
-
a9
a]g
(MnO) 2
-
-
-
-
-7.864925
-
-
-13.50180
-
-4.818810
-
-5.965896
-2.95949
-3.326358
-2.11537
1.806778
-2.20489
-0.4292552
-
CaO
x AI20 ~
all
CaO
x
all
-48.88876
-0.675465
al~
-48.72412
-0.698558
MnO
(CaO) 2 MgO
x AI~0@
MgO
x Mn0
MgO
x
all al~
CaO
-
-16.46353
-100.7227
-1.07502
-12.81905
aÂ7 a|a
SiO?
x MnO
al~
59.00428
SiO~
x
a2~
12.15613
SiO~
x M@O
(Si0~) ~
8.927958
a~
58.75614
x AI~O~
a2~
B
x MnO
B
x
CaO
az4 az~
B
x
M@O
B
x
SiO?
B2 CaO-M@O
x AI~O~
CaO-M@O
x MnO
CaO-M~O
x
CaO-M~O
x M@O
CaO-M@O
x
SiO 2
CaO-M@O
x
B
(CaO-MgO)
CaO
2
.
-
a~
B
-19.19953
-0.565592
SiO 2 x AI203
CaO
-
-52.52072
-52.52912
(MgO) 2
300.2325
-11.66325
-
-13.02977
0.223846 0.140220
-
-3.0438 0.435324
-8.932038
1.288704
-2.263144
-1.99974
-13.66380 -4.113224
0.997500
-
-83.33907
-
1520.916
-
-65.41105
-
1552.295
-
-58.26822
-
1566.986
-
12.61716
-
a??
1427.666
-
-89.54007
a?8 a2~ a~
~i012.782
-
-87.02252
a~l a~z a~ a]4 a35
-
-
0.523864
-
-33.6709
-
-16.80766
-
0.01843465
-
-14.48523
-
1.214988
-
-19.31936
-
-7.240202
-
-14.84434 -0.7695548
.997174
-
-11.40459
-
127.44202
2.197859
6.570390
-0.41043
Liquidus temperature of synthetic slags
1325
% MnO .5
o
7
o 10
l
0
0.5
016
I 0.7
I 0,8
I 0.9
11.0
11.1
112
I 1.3
1~4
Basicny Ratio
Fig.6 Slag resistivity as a function of the basicity ratio at various MnO contents, temperature 1400°C
Eq
% MnO • 5 o 40 • 15 a 20
o
v25 x30
\ Ul
0.5
I
0.6
I
0.7
I
0.8
I
0.9
i
I 0
I
I .I
I
1.2
I
1.3
1
Baslclty Ratio
Fig.7 Slag resistivity as a function of the basicity ratio at various MnO contents, temperature 1360°C
1326
R . H . ERIC et al.
It is apparent that the increase in basicity ratio from 0.55 to 1.1 decreases the resistivity of slags substantially. The effect is identical for both 1400°C and 1360°C temperatures but understandably for the latter one, the total graph is more expanded on account of the increasing resistivities with the lowering of temperature. At basicity ratios higher than I.I, in general the resistivities tend to increase depending on the MnO content. Since the effect of MnO is not clear from Figures 6 and 7, as in the case of liquidus temperatures two new plots have been made as shown in Figures 8 and 9 for 1400°C and 1360°C respectively. These show the upper and lower resistivity limits (as mentioned above) in slag composition with regard to the C a O / M g O ratios as a function of MnO content. The general pattern is similar at both temperatures, namely a high peak in resistivities is reached at 10% MnO content for basicity ratios of 0.55, 0.8 and 1.1, then with the increase of the MnO content to 25% a sharp drop in resistivities is evidenced which will be specially pronounced at low; 0.55 and 0.8 basicity ratios. Above 25% MnO content the resistivities tend to increase again. With the basicity ratio of 1.4 the rise in MnO content caused the resistivity to increase to a relatively high value at 15 to 20% MnO contents, thereafter fluctuating at lower values. Thus the decreasing effect of the MnO content upon the resistivity seems to be generally valid at basicities up to 1.1 and MnO contents between 10 and 25%. An anomalous sharp increase in slag resistivity took place at the low basicity ratio of 0.55 when the MnO content was raised from 5 to 10% for which no explanation could be found.
B • 0.55 o 0.80 1.10 x t.40
E q 6 E tO O
Top of resistivity range
X
~
0
Oz~
X
,
~.
~
~~~
I~, '~J
X
~
X
Bottom of resistivity range 0
i
5
I
I
I
I
I
I0
15
20
25
30
% MnO
Fig.8 Slag resistivity as a function of the MnO content at various basicity ratios. Temperature 1400°C In Figure 10, instead of resistivity, the conductivity data obtained as k = G / R were plotted in Sm'l units. The scatter of the data is due to the calculation method when a constant G value is divided by everchanging R values, and also to the mode of presentation of G / R with large units on the ordinate.
Liquidus temperature of synthetic slags
/ /
E q
/
% %
/
d
o
O
B 0.55 0.80 1.40 4.40
• o a x
% %
#
\
1327
% %
k
I 0
•
Top of resistivityrange
~ f %
o \
z=
0
\ \ \
x
o
\
o
~
N
'~
I
o
0 " -
-
.
~ .
.
.
.
_
_
Bottom of resistivity range 0
I
5
4i0
't15
2~3
I
25
30
%MnO
Fig.9 Slag resistivity as a function of the MnO content at various basicity ratios. Temperature 1360°C Obviously, when slag systems are subjected to high frequency effects for measuring their electrical resistivities, they may exhibit different behaviours when oscilloscope is used as a null point detector in the measuring bridge circuit. Often the introduction of an elaborate filter circuit becomes necessary to bring about the usual single narrow line in the scope screen when the point of zero current is reached in the bridge. In the lack of this arrangement a good deal of additional testing of the most suitable wave form becomes inevitable. This was the case with the present slags but with the establishment of the optimum wave form a sensitivity of 0.005 ohm could be attained with the use of the rather simple measuring system. However, it is also likely that this phenomenon results in some scatter of the data. On the whole, the data gives a hint to the practical implications of slag resistivities for the operation of the electrical furnaces when it is used carefully. Since the industrial furnaces operate at low frequencies (50 - 60c), this fact may leave some doubt as the real value of resistivities measured in the laboratory when they will have to be interpreted for largescale operating furnaces. However, a reasonable sound assumption that can be made for the present purpose is that the relative resistance of different slags say at 15kc corresponds to the relative resistance of slags at 50c. The measurement of resistivity at these low frequencies would, in any case, involve considerable difficulties say by the use of direct (i.e. voltage/current) measurements lending to impedance, rather than resistance values. Modelling of Electrical Conductivities Only the data gathered in this investigation was used to construct models for conductivity as no other data was available on similar slags. Two models were constructed, one for ME
4:12-I
1328
R.H.
Eplc et al.
1360°C and one for 1400°C. An analogous procedure to that outlined for liquidus temperatures was followed. The complete quadratic models resulted in an expression containing 27 parameters. The reduced model for 1400°C contained 14 and the reduced model for 1360°C contained only 9 parameters. The fits obtained, however, were not as good as those obtained for liquidus temperatures. The correlation coefficients (R e) for 1400°C model were 0.796 for the complete quadratic form and 0.701 for the reduced model. For the 1360°C, the correlation coefficients for the complete quadratic and reduced models were 0.793 and 0.696 respectively. Obviously the main reason for poorer fits is the more scatter in conductivity data compared with that of liquidus temperature data. However, the models for conductivity are still very useful in the prediction of ferromanganese slag electrical conductivities within the slag composition ranges investigated in this work. The model parameters are shown in Table 3. t40
Upper limit
120
tO
.-2
1013 I
=
-=-" -
o
~
i o
II /I
x
x
x
ii x
ii II
60
o
i
A
//
s
x
X
x
o
40
,"
o x
o 0
20
~ Lower limit
/s •
rS /
J
o
o
g
I
I
I
I
I
I
5
10
15
20
25
30
% MnO
Fig.10 Electrical conductivity as a function of the MnO content at various slag basicities (Notation for basicities as in previous figures) Thus the equation used to predict the slag conductivity using the complete quadratic model would be written as: k ( o h m ' l c m "1) -- a~ + at (%MnO) + a 3 (%CaO) + .... + al0 (%MnO) 2 + .... + a24 (B x % MnO) + .... + a35 (CaO -VMg0)Z
PRACTICAL IMPLICATIONS AND C O N C L U S I O N S The value of the investigations performed on slags will depend how far the results could be translated into usable form for practical furnace operation. For this purpose empirical modelling of the results were made. The prime importance of both the liquidus temperature and electrical conductivity characteristics is apparent in this instance, so what actually should be known is the degree to which the slag itself contributes to the complex operation of a ferromanganese producing furnace. Most unfortunately this point is far from being sufficiently clarified to warrant an exact quantitative treatment of the data obtained, say for optimization purposes. Liquidus temperature data are helpful in that they indicate the
1329
Liquidus temperature of synthetic slags
extent of slag overheating from which, with the knowledge of the specific heat and the mass of the charge and of the slag at any time present in the furnace, the heat utilization or wasting, may be estimated to a reasonable degree. Also, the liquidus temperatures may, to some extent, serve as indicators towards slag viscosities, these being strongly temperature dependent slag characteristics. Thus with the knowledge on slag composition, melting point and degree of superheating some measure of optimisation for thermal conditions and a greater accuracy for the prediction of the viability of slag tapping will be possible. The substantial drop in liquidus temperatures with the increase of the MnO content above 10% in the slag would suggest a reduction in power input and vice versa. TABLE 3 Model parameters for electrical conductivities Coefficients for conductivity models 1400°C
Coefficient
Complete Quadratic
1360°C
Reduced Model
Model ao
Intercept
3344.425
-96.1582
4866.2024
6.463066
-80.096863
Reduced Model
-2.5865
al
AI~O 3 MnO
a~
-75.649343
CaO
a~
-21.610706
MgO
-38.306389
SiO z
a4 a~
B
a~
-473.17585
a7 aQ
0.3177461
CaO-MgO (AIz03) 2 MnO
Complete Quadratic Model
x AlzO 3
(MnO) 2 CaO
x AlzO 3
CaO
x MnO
-103.12899
-
a9 alQ all a12
(CaO) ~
22.155033 1.680546
0.4261274 -
14.33459 -178.0534
-11.1125
-0.05557 -1.21924
-2324.8901
4.017052
0.0977123
0.210062
0.3012871 -
0.2663299
0.025700
-0.4891872
-
0.081739
-0.8101123
-
x AIzO ~
a~4
MgO
x MnO
a15
0.4617066
-0.056091
-0.3871913
MgO
x
a16
-0.09733365
0.081052
-1.4699948
a17
0.01405341
(MgO) 2 SiO2
x AI?O 3
SiO~
x MnO
SiO z x
CaO
SiO~
MgO
x
(SiO~) 2
aid al,
-
-0.1694077
MgO
CaO
0.004484
1.13661057
1.559474 -0.131759 -
-0.722310 1.6405583
a20
0.55144415
a~l
0.72259679
-0.081284
0.5691971 0.6488153
a22
0.71695819
-0.069895
1.3336187
0.001833 -0.00098 0.000395
B
x AI~O;
a~}
B
x
MnO
-
24.8555297
-
x
CaO
az4 a,~
4.94706097
B
5.29423304
-
25.9234570
-
B
x
MgO
a2&
4.40343255
-
25.0433762
-
B
x
SiO~
a??
6.64341205
-
24.6491982
a28
-3.25822761
-
-18.4073403
B2
-1.83089
CaO-MgO
x AlzO }
a~
CaO-MgO
x MnO
a}Q
0.03287501
-
0.03988242
CaO-MgO
x
CaO
a}l
0.10689761
-
0.12161739
-
CaO-MgO
x MgO
a}z
-0.33606162
-
-0.27112965
-
CaO-M~O
x
a~
CaO-M@O
x
aa4
-1.21059598
-
-1.59051458
a35
-0.20757838
(CaO-MgO)
SiOz
2
-0.20912643
-
-0.010669
1330
R . H . Epac et al.
Pertinent to the effect of electrical conductivities or slag resistivities, the question is complicated by the fact that the total resistance stipulating the power input is made up by the resistivity of the charge and that of the slag. However,the ratio between their contribution is not more than an acceptable guess say between 50% and 70% as attributable to the slag, but in general this depends to a very great extent on the thickness of the coke layer and the size of the reductant. Since it is the coke layer that provides a separation between the unmolten raw material and the liquid slag, the particle size is rather important as a balancing factor between charge resistance and slag resistance. In this instance as for an idealised case, a sudden rise in resistivity would probably indicate an abrupt decrease in particle size and the resultant thinning of the coke layer with an increased participation of the slag resistivity in the smelting operation. The heat generated preferentially in the molten charge contributes to the melting of the raw materials higher up in the furnace with rather moderate chemical reduction of the charge. The final reduction deeper in the furnace is accomplished by the heat generated near the electrode t i p s a n d should therefore result in low MnO containing slags. Quite obviously, if too much heat is consumed for the melting of the charge, a heat deficiency near the electrode tip may cause incomplete reduction. This sate of affairs brings about conditions which makes the assessment of the true effect of slag resistivity on a generalized basis difficult and rather approximate. Nevertheless, certain important points as to some of the effects of material characteristics upon the operation of electric furnaces used in ferromanganese production can be mentioned. As far as the slag is considered, the most important factors are the MnO content and the basicity ratio as it has been clearly demonstrated in this work. A rather large range in variation of furnace power may be expected with the change of the basicity ratio at MnO contents between 5 and 10%. Under these circumstances, at constant current, the power and therewith the heating rate is on the high side. As the MnO content of the slag increases, in the 20-25% MnO range, the variation of the basicity ratio will have a modest effect on the slag resistivity which can be expected to be in the low range. Therefore the aimed power input could be achieved only with a substantial increase in the furnace current to maintain an economical smelting rate for the furnace. As the slag melting points will also be the lowest at these MnO contents, extra power input will also be necessary if the superheating of the slag were to be maintained at a set level from the point of view of the metal produced. A special attention has to be drawn here to the fact that the extreme variations in slag resistivities indicated by laboratory measurements would under no circumstances bring about concurrent effects that could be expected in actual furnace operation. This is due first of all to the very limited contact (if any) between the electrodes and the slag. Secondly, the slag compositions are subject to variation both in their MnO contents and basicity ratio during the smelting of the charge. This will inevitably lead to the averaging out or to the diminishing of the extreme variations that would otherwise ensue in electrode movement, current draw etc. Despite the mentioned negative aspects, the data obtained in laboratory investigations on electrical conductivities may serve as a useful indication towards changes that may set in during furnace operation with the change in the characteristics of the produced slag. Furthermore, from the knowledge of opposite behaviour of viscosity and electrical conductivity some reasonably valid conclusions can be drawn as to the behaviour of the slags investigated in this work. For example the increase in the electrical conductivity upon the increase in the MnO content of the slag will invariably be associated with a by and large equivalent decrease in the slag viscosity. The optimum slag composition (C,,,,t) representing also optimum operating conditions for the furnace as to slag characteristiS~-is primarily a function of three parameters:
Liquidus temperature of synthetic slags
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Cop t = f (%MeO,k,17)
where k is conductivity and rl is viscosity. This was proved to be the case with slags used in copper matte smelting furnaces in submerged electrode resistance heated furnaces, the operation of which is of course quite different from that of submerged arc furnaces. However, the basic behaviour of the slag proper remains the same irrespective in whatever furnace it is processed in and it is only the mode of operation of the furnace together with the nature of the charge composition that brings about the differences. Following this line of thought and without knowledge of the actual slag viscosities an approximate and rather qualitative suggestion may be put forward here using the conductivity values shown in Figure I0. The conductivity curve shown in Figure 11 is roughly a mid-value between the high and low extremes. According to this illustration the furnace operating range would correspond to slag compositions having 15 to 25% MnO with an optimum around 15%. Obviously, the plot is a gross oversimplification and actual known viscosity values would probably shift the suggested range towards lower values. As it stands, the suggested range would fit very well into that found for lowest liquidus temperatures.
K Sm -t
120
\ \ \ 80
/',,
I
/ ,'\
/
40
/
I.
X ",..! i / I operating J range
/
,
I0
I t
Opt.
~ ~..J._/" I I
,
20
1
30 % MnO
Fig.l 1 Possible mode of slag optimisation from electrical conductivity and viscosity measurements as applied to the present investigations. Basis: MnO content of the slags ACKNOWLEDGEMENTS The financial assistance provided for this investigation by Ferro Alloy Producers Association and Samancor is accorded with due thanks. This paper is published by the permission granted by Samancor.
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R. H. ERIc et al.
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Miller, M.P. & Sommer, G. Journal of Science Instrum., vol. 43, pp 293-297 (1966)
2.
Sommer, G. and Jochens, P.R., Mineral Sci.Engr., Vol. 3, No 1, pp 3-16 (1971)
3.
Riebling, E.F. and Vogel, P.C., Rev. Sci, Instrum., Vol. 36, pp. 425-428 (1965)
4.
Atwood, S.S. Electric and Magnetic Fields, J.Wiley, New York, pp 83-160 (1949)
5.
Lewin, E.M., Robbins, C.R. and McMurdie, H.F. Phase diagrams for Ceramists 1969 Supplement, The American Ceramic Society, Columbus, Ohio, (1969)
.
Warren, G.F. Measurement of the activity of manganese (II) oxide in slags associated with the production of ferromanganese, MSc Thesis, University of the Witwatersrand (1972).
.
Pesta, M., Marjoribanks, F.M., Oosthuizen, G.A. and Salgado, G. Report no 2120, National Institute of Metallurgy, Randburg, South Africa (198 I)