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Nepheline syenite as a synthetic slag addition in secondary steelmaking
Minerals Engineering, Vol. 2, No. 2, pp. 207-215, 1989 Printed in Great Britain
NEPHELINE
0892-6875/89 $3.00 + 0.00 © 1989 Pergamon Press plc
SYENITE AS A SYNTHETIC
SLAG ADDITION
IN SECONDARY STEELMAKING
P.W. KINGSTON % and W.F. CALEY § %
Ontario Ministry of Northern Development and Mines, Mines & Minerals Div., Tweed, Ontario K0K 3J0, Canada § Dept. of Mining & Metallurgical Engineering, Technical University of Nova Scotia, Halifax, Nova Scotia B3J 2X4, Canada
(Received 5 January, 1989)
ABSTRACT Recent trends in secondary steelmaking have been towards the use of synthetic slags for deoxidation, desulphurization, and inclusion control. These melts are generally a calcium aluminate material to which lime and/or fluorspar may be added, depending upon the slag/metal equilibrium desired. Due to the detrimental side-effects of fluorspar, such as enhanced refractory corrosion, a study had been undertaken to investigate the possibility of using a tailing material (nepheline syenite) to partially replace fluorspar in these slags. Based on the rheological behaviour observed, results to date suggest that nepheline syenite enhances the fluidity of such melts. These findings are discussed with reference to slag chemistry; in addition, the relative chemical reactivity of the compositions investigated with respect to refractory corrosion is addressed. Ke~words Slag viscosity; industrial minerals; nepheline syenite; flux; secondary steelmaking INTRODUCTION One of the less w e l l - k n o w n areas of industrial mineral use is in the area of fluxes, slags, additives, and s p e c i a l t y c o m p o n e n t s for the s t e e l m a k i n g industry. This is a n i n d u s t r y presently undergoing restructuring, rationalization, downsizing, and m o d e r n i z a t i o n . As a r e s u l t t h e r e a r e o p p o r t u n i t i e s for d e v e l o p m e n t of new materials such as s p e c i a l c o m p o s i t i o n refractories and for i n t r o d u c t i o n of new reagents and components, such as fluxes, synthetic slags, and ceramic coatings into the system. Overall, the steelmaking industry is large, and uses relatively large tonnages of industrial minerals. Acceptance of a small percentage increase in the use of a particular mineral, or the substitution of a relatively small percentage of an imported industrial mineral throughout the steel industry, would result in a s i g n i f i c a n t i n c r e a s e in c o n s u m p t i o n of i n d u s t r i a l m i n e r a l s . This potentially high demand makes the steelmaking industry deserving of attention as far as industrial mineral promotion is concerned. This project has focused in particular on the use of nepheline syenite as a substitute for f l u o r s p a r in the formulation of synthetic slags for use in secondary steelmaking, or ladle m e t a l l u r g y . L a d l e m e t a l l u r g y , as the n a m e implies, is c a r r i e d out in a r e f r a c t o r y - l i n e d l a d l e a f t e r the i n i t i a l steelmaking process. This batch process allows steelmakers to exercise a high degree of control on the physical and chemical properties of each ladle-full of steel. The p u r p o s e namely: I. 2.
of
a
slag
in
a
ladle
metallurgical
process
is
two-fold,
To r e m o v e c e r t a i n i m p u r i t i e s f r o m the l i q u i d m e t a l , s p e c i f i c a l l y sulphur and/or phosphorus, and To isolate the refined product from the surrounding atmosphere. 207
[I]
208.
P.W. KINGSTONand W. F. C~EY
Each of these is a function of physicochemical properties of the slag, such as activity, c o n d u c t i v i t y and v i s c o s i t y . G e n e r a l l y , a s y n t h e t i c slag o f t e n c o n t a i n i n g up to 50% calcium aluminate, blended with lime and fluorspar is used for this purpose. Slag Viscosity Slag viscosity is directly related to the temperature and composition of the melt [I]. Davies and Wright [2] explain that not only will viscosity determine the rate of fall of metal droplets through a slag phase, but in addition may well influence the rate of refining reactions, such as desulphurization. This is due to the r e l a t i o n s h i p b e t w e e n viscosity and the diffusion of gaseous species, such as 02- , to the metal phase. A metallurgical slag may be classified as being a molten silicate network [3]. In fused or vitreous pure silica, each silicon atom is bonded to four oxygen atoms and in turn each oxygen atom is bonded to two silicon atoms. However, in a m o l t e n state t h e r e is a d i s t o r t i o n of s y m m e t r y a l o n g w i t h a g e n e r a l weakening of the bonding strength between atoms primarily due to the elevated temperatures. There exists a sizeable amount of literature on the dependence of viscosity of a silicate network on its basic oxide concentration, both in the presence and a b s e n c e of f l u o r s p a r . For example, ferrous oxide is an effective fluidizer [4], although its influence was reported to diminish at temperatures in excess of 1400oc. In contrast, the behaviour of fluorspar as a fluidizer depends on its concentration and environment [5]. Efimov et al. [6] reported that CaF 2 reduces slag viscosity, whereas an i n c r e a s e in the total c o n c e n t r a t i o n of AI203 and CaO has the opposite effect. Although fluorspar is an excellent fluidizer, several problems exist with its u s e in s t e e l m a k i n g . F o r e x a m p l e , CaF 2 c h e m i c a l l y c o r r o d e s m o s t l a d l e refractories, and when heated gives off toxic fluorine-containing species to the atmosphere; in addition, it is a relatively expensive industrial mineral. Thus, it is advantageous to partially replace fluorspar with another equally effective material. Nepheline Syenite Nepheline syenite is composed essentially of nepheline (NaAISiO4), albite, and microcline [7]. The ore c o n t a i n s up to 15% a l k a l i s and at least s e v e r a l p e r c e n t iron oxide. In addition, tailings from the nepheline beneficiation process contain iron oxide l e v e l s a p p r o a c h i n g 10%. Thus, in spite of the presence of a glass former (SiO2), nepheline ore, or nepheline tailings, might be expected to behave as a reasonable fluidizer. Nepheline syenite is used as a main constituent in the production of silicab a s e d g l a s s w a r e , in c e r a m i c bodies, and in g l a z e f o r m u l a t i o n , . During b e n e f i c i a t i o n of the n e p h e l i n e ore, it is c r u s h e d and g r o u n d to release individual mineral particles. A two-stage magnetic separation process is used, f i r s t l y to r e m o v e m a g n e t i t e , w h i c h is sold as a product, and secondly to remove magnetite-containing middling grains, as well as other iron-containing mineral species, of which biotite and hornblende are the commonest. Removal of iron-containing minerals from the nepheline syenite product is essential, for in most high-temperature applications, iron included in the product would both stain or colour the melt (glass, or glaze), and lower the melting temperature of the mixture. In the only known previous use nepheline syenite ore was mixed Steel in the mid 1970s [8]. Use switched to a continuous casting
of nepheline syenite in a slag or flux, the with f l u o r s p a r in b r i q u e t t e f o r m by R o u g e was apparently discontinued when the company process. EXPERIMENTAL
Viscosity Measurement The p r o c e d u r e u s e d was to first isolate the most likely nepheline syenite tailings stream from four c a n d i d a t e s s u p p l i e d by I n d u s i m Ltd. from t h e i r Nephton deposit in Ontario. Following this, the plan was to to replace part or all of the fluorspar component of a commercial synthetic slag mix with one or
Nepheline syenite as a synthetic slag addition
209
m o r e of the w a s t e m a t e r i a l s . The tailings streams originated at different locations in the plant, and were designated Primary Dust, Wash Tailings, Final Waste, and L u r g i Waste. The fundamentals of the experimental approach have been described elsewhere [9,10]. (a) Materials Preparation: (i) Nepheline syenite tailings: Approximately 400 g of each tailings stream were isostatically pressed at 15000 p.s.i, for 3 minutes to provide a compact s t a r t i n g m a t e r i a l for viscosity measurement. Usually, 50 g of material was used per experiment. (ii) Synthetic slag mixtures: A variety of synthetic steelmaking slags is c o m m e r c i a l l y a v a i l a b l e , one b e i n g Coflux, a c a l c i u m a l u m i n a t e m a t e r i a l m a n u f a c t u r e d by E l k e m M e t a l s C o m p a n y . This is a s y n t h e t i c s l a g - f o r m i n g compound, containing 50-56 wt% CaO, 44-50 wt% A1203, with minor quantities of M g O , SiO2, and T i O 2. This m a t e r i a l may be f u r t h e r m i x e d w i t h lime and fluorspar to provide a useful ladle metallurgy slag. Compositions, in wt%, of the three mixtures used to evaluate the usefulness of nepheline syenite as a fluorspar replacement are as follows: A - 45% Coflux, 45% CaO, 10% CaF2; B45% Coflux, 45% CaO, 5% CaF2, 5% nepheline syenite; C - 45% Coflux, 45% CaO, 10% nepheline syenite. Coflux, lime, and fluorspar were supplied in sizes ranging from 1-10 cm. These materials were crushed in a Denver jaw crusher and ground in a small Denver rod mill to pass 600 microns. The remaining material, the nepheline syenite tailings, was already of sufficiently fine particle size to require only fine grinding and sieving. Approximately 50 g was used in each experiment. (b) C e r a m i c C o m p o n e n t s : The w o r k i n g c r u c i b l e s w e r e p u r c h a s e d from the M c D a n i e l R e f r a c t o r y Company (A1203) , or slip cast from Norton MgO, whereas safety crucibles (to protect the furnace muffle from slag spillage) were slip cast from high purity Reynolds AI203. Spindles consisted of 0.3 cm AI203 or MgO rods, which were in turn connected to a Brookfield pneumatic viscometer by means of a second AI203 shaft, as shown in Figure I. For all nepheline syenite t a i l i n g s experiments, AI203 working crucibles and spindles were used; these were replaced by MgO for the synthetic slag mixtures. (c) Viscometer Calibration: The viscometer was modified for high temperature use, and c a l i b r a t e d u s i n g B r o o k f i e l d s t a n d a r d silicone oils. Initial c a l i b r a t i o n was with the viscometer rotating at 100 rpm, although generally the viscometer was operated between 50 and 10 rpm, with data obtained from the h i g h e s t r o t a t i o n a l speed c o r r e s p o n d i n g to the most fluid material at the highest temperatures. (d) V i s c o s i t y - T e m p e r a t u r e Profiles: For each experiment, the spindle-shaft assemblage was suspended within the upper portion of the furnace (a Lucifer Melt-Master Elevator Furnace, Model 6000, with programmable controller), but not in contact with the sample. Thus, the a s s e m b l a g e was p r e h e a t e d b e f o r e b e i n g i n s e r t e d into the melt, while at the same time avoiding unnecessary c o n t a c t w i t h the m o l t e n m a t e r i a l . The e l e c t r i c a l s i g n a l s from b o t h the viscometer, and a Pt Pt-13% Rh thermocouple cemented to the furnace muffle, were led to a 2-pen Hewlett Packard Recorder. For all experiments, the maximum temperature, (1600C), was reached 45 minutes after commencing the heating cycle, and the spindle was immersed to within 3 m m of the c r u c i b l e b o t t o m a p p r o x i m a t e l y 6 m i n u t e s a f t e r a t t a i n i n g this temperature. Following equilibrium, the temperature was permitted to drop at the rate of 15C/min; data was recorded on a continuous viscosity/temperature t r a c e u n t i l the i n d i c a t e d v i s c o s i t y exceeded the sensitivity of t h e instrument. Refractory Corrosion Testinq In o r d e r to d o c u m e n t the effects, which substitution of nepheline syenite t a i l i n g s for f l u o r s p a r m i g h t have on r e f r a c t o r y c o r r o s i o n , a series of experiments was carried out using crucibles which were cored with a diamond core drill from high a l u m i n a (80%) r e f r a c t o r y brick. The h e a t i n g / c o o l i n g p r o f i l e c h o s e n was s i m i l a r to that used for the v i s c o s i t y measurements. Following cooling to room temperature, each of the crucibles was cut into 5 mm t h i c k d i s c s w i t h a d i a m o n d saw, to r e v e a l the r e a c t i o n zone between the refractory material and the oxide melt. Finally, the discs w e r e r e d u c e d to p o l i s h e d t h i n s e c t i o n s , and s u b j e c t e d to quantitative electron microscopy using a JEOL Superprobe (SEM/electron microprobe).
210
P.W. KINGSTONand W. F. CALEY
1
VISCOMETER
PLATINUM PINS
•
THERHOCOUPLE LEADS
PLATINUM PIN MAGNESIASHAFT
!
THERHOCOUPLE JUNCTION
i
ALUMINA SHAFT
il
lm, I! 'U (
HEATING ELEMENT ALUMINA MUFFLE ALUMINA SAFETY CRUCIBLE MAGNESIA WORKI NGCRUCIBLE PEDESTAL ELEVATOR
Fig.1
Schematic representation of experimental assemblage RESULTS
Nepheline Syenite Tailings The chemical nature of the four waste stream samples provided by Indusmin Ltd. is g i v e n in T a b l e I. Of p a r t i c u l a r note is the high Fe203 content, which approaches 10.5 wt% in the Final Waste. The resulting v i s c o s i t y - t e m p e r a t u r e profiles for the four streams are given as Figures 2 and 3. Figure 2 gives all data collected for the duplicate runs on a viscosity (poise) vs temperature (K) plot, w h e r e a s F i g u r e 3 p r e s e n t s the data in the more f a m i l i a r log viscosity (poise) vs reciprocal temperature (K -I ) format. From these plots, it is evident that the Primary Dust was the most viscous in nature, whereas the Final Waste exhibited the lowest level of viscosity for a given temperature. However, none of the samples could be termed "fluid" at 1585C, the m a x i m u m temperature attained in these experiments, and in fact all exhibited viscosity values generally found for highly silicious amorphous materials [11,12]. Synthetic Slag Mixtures The nepheline syenite tailings stream used for mixtures B and C was the "Final Waste". The viscosity-temperature p r o f i l e s for each slag i n v e s t i g a t e d are g i v e n in F i g u r e 4; for each slag a computer-generated regression curve is presented, together with error bars representing the s t a n d a r d e r r o r of the mean. As can be seen, the minimum viscosity of slag A was 2200 centipoise, whereas for B this value is about 1075 centipoise, and for C, w i t h 10 wt% nepheline syenite tailings, minimum viscosity was reached at 1750 centipoise. The log viscosity vs. reciprocal temperature plot given as Figure 5 suggests a change of slope in the 1500C range as crystals begin to precipitate from the oxide melt. This is characteristic of molten oxide melts [1,2,3,10,13]. Bulk Chemical Analysis The chemical analysis results are given in Table 2. The analysis was performed by the X-ray Assay Laboratories, Limited, Toronto, Ontario.
Nepheline syenite as a synthetic slag addition
TABLE I
211
Chemical analysis of Nephton nepheline syenite taillngs samples ( w t % ) ~ Primary Dust
Fe203** AI203 Na20 K20 CaO MgO
1.68 22.80 9.94 5.37 0.40 0.06
Wash Tailings
Final Waste
Lurgi Waste
3.94 22.70 9.58 5.32 0.62 0.12
10.50 20.40 6.33 5.43 1.43 0.27
2.46 23.00 9.75 5.36 0.44 0.07
* Reproduced from [13]. ** Total iron reported as Fe203. It is expected that a portion of the Fe203 is present as FeO or as abraded iron. 250
v 200
A • Lurgi Waste v • Final Waste o • Primary Dust
_ ~
\
O 0a
o
o o •
1100
•
,
,
1200
1300
1400
-
[ .
.
.
GQ
.
.
.
.
.
.
1500
1600
Temperature (C) Fig.2
Viscosity-temperature
profiles
for four Nephton stream samples
100
J
O
~-4
10
° ~°°° . / ~
o o
• LurslWute
~
~ F i n a l Waste ~ Primary Dust o Wash Tailing
I 5.0
I 5.5
I 6.0
I 6.5
I 7.0
7.5
Temperature K* (I04/T) Fig.3
Plot of log viscosity v s reciprocal temperature four Nephton stream samples
for
212
P . W . KINGSTONand W. F. CALEY
100 o Slag A 80-
I
I\
80o
• Slag B • Slag C
t
40-
0
20-
0
z25o
I
I
135o
z45o
I
Temperature
155o
(C)
Fig.4 V i s c o s i t y - t e m p e r a t u r e profiles for synthetic slag m i x t u r e s A, B and C. Error bars show the standard error of the mean
100.
o Slag A • Slag B • Slag C
//~
O v
o
o
5 5.3
I 5.5
5.7
I
I
I
5.9
6.1
6.3
Temperature K-I(IO~/T) Fig.5
Plot of log v i s c o s i t y vs. reciprocal t e m p e r a t u r e s y n t h e t i c slag mixtures A, B and C. E r r o r bars show the standard error of the mean
T A B L E 2:
wt%
AI
AI203 SiO 2 Fe203 CaO MgO MnO TiO 2 K20 Na20 F AI, Coflux, Coflux,
17.50 3.51 0.62 72.90 3.14 0.27 0.64 0.03 0.08 4.15
A2
16.10 3.55 0.64 70.20 3.93 0.25 0.59 0.03 0.10 3.03
Bulk Chemical
SLAG MIXTURE ~ BI
19.20 6.28 1.12 66.90 3.00 0.27 0.69 0.08 0.31 2.20
for
Analysis
B2
17.70 5.58 1.06 69.20 2.92 0.28 0.62 0.09 0.30 1.66
CI
18.0 7.39 1.46 62.20 6.76 0.30 0.64 0.37 0.61 0.06
C2
19.10 7.88 1.56 65.40 2.44 0.30 0.64 0.38 0.64 0.05
A2 45% C o f l u x , 45% CaO, 10% CaF2; B1, B2 - 45% 45% CaO, 5% CaF2, 5% n e p h e l i n e syenite; C1, C2 - 45% 45% CaO, 10% n e p h e l i n e syenite.
Nepheline syenite as a synthetic slag addition
Refractory
Corrosion
213
Testing
The r e s u l t s of the m i c r o p r o b e a n a l y s i s p e r f o r m e d on the two slag c o m p o s i t i o n s tested, A and B, w i t h the high a l u m i n a r e f r a c t o r y brick, are given in Table 3. For m i x t u r e A the r e f r a c t o r y phase was examined, w h e r e a s for slag B b o t h the slag and r e f r a c t o r y areas were analysed. In each case, random point a n a l y s e s of c o m p o s i t i o n s were taken in the g l a s s y m a t r i x and c r y s t a l l i n e phases.
TABLE
3
Refractory/slag
microprobe
analysis*
SLAG A
Analysis Point No.
REFRACTORY
- GLASS
REFRACTORY
- CRYSTAL
I 2 3 4
Wt% AI203
SiO 2
CaO
TiO 2
F
60.8 42.3
-13.6
34.5 40.2
---
0.39 0.88
10.8 6.2 48.5 39.9
--11 .6 16.2
41 .6 42.9 35.5 40.3
29.4 33.6 ---
0.98 0.72 0.99 0.97
CaO
TiO 2
SLAG B
Analysis Point No.
REFRACTORY
REFRACTORY
48.0 41 .4
2.0 . .
.
.
I 2 3
63.8 63.3 50.2
--5.8
35.6 35.6 41 .4
-. . . .
. .
. .
17.1 18.0
40.8 41 .I
---
0.6 0.8
42.4 42.2
42.7 45.3
0.6 0.9
- GLASS
I 2
39.7 39.2
SLAG
- CRYSTAL
I 2
4.8 2.0
oxides
F
4.6 5.8
SLAG
* Only m a j o r
SiO 2
38.3 50.2
- GLASS
- CRYSTAL
Wt% AI203
I .7
0.1
are presented.
DISCUSSION R e f e r r i n g to the curves of F i g u r e s 2 and 3, and the data of Table I, it may be seen that as the iron c o n t e n t of the t a i l i n g s samples increases, the v i s c o s i t y at a g i v e n t e m p e r a t u r e d e c r e a s e s , with approximately 275C separating the " b r e a k p o i n t " of t h e c u r v e s of h i g h e s t and lowest viscosity. Thus, of the samples investigated, it w o u l d a p p e a r that the N e p h t o n F i n a l W a s t e h a s the most f a v o u r a b l e r h e o l o g i c a l properties. F r o m the m e t a l l u r g i c a l point of view, none of the samples could be c o n s i d e r e d to be a " f l u x " s o l e l y f r o m their v i s c o s i t i e s . Generally, s t e e l m a k i n g slags have v i s c o s i t i e s of the order of a few h u n d r e d c e n t i p o i s e at t h e i r m a x i m u m o p e r a t i n g t e m p e r a t u r e [3], c o m p a r e d to 20,000 to 300,000 c e n t i p o i s e for the N e p h t o n samples. However, in these samples the p r e s e n c e of iron has a m a r k e d e f f e c t on the r e s u l t i n g viscosity, w i t h an i n c r e a s e in Fe203 of from 1.7 to 10.5% r e s u l t i n g in a 15 fold v i s c o s i t y d e c r e a s e at 1585C. Thus, this m a t e r i a l was c h o s e n for further evaluation.
214
P.W. KINGSTONand W. F. CALEY
Referring to the synthetic slag mixtures of this study, researchers [14,15,16] h a v e i n v e s t i g a t e d the v i s c o s i t y p r o f i l e s of v a r i o u s o x i d e m e l t s w i t h compositions similar to that of the base run of series A. Tsibul'nokov [14] tested the viscosity of a slag with a CaO/AI203 ratio of about 3.5 along with a CaF 2 concentration of 2 wt% and found that at 1500C the viscosity was 1910 centipoise. Results given in Figures 4, 5 show a viscosity of 2200 centipoise; however, the lime-alumina ratio in the present work was slightly larger, at 4. In e v a l u a t i n g the i n f l u e n c e of the f l u i d i z e r , the p r o b l e m of f l u o r s p a r instability must be considered. Kilau's [15] work showed poor reproducibility for viscosity measurements in fluorspar-containing BOF slags. It was reported that the solidification temperature of the melt increased by 61C over a period of 31 hours. He then concluded that this increase in apparent solidification temperature with time is a consequence of fluidizer depletion from the melt via f l u o r i n e v o l a t i l i z a t i o n . In the p r e s e n t work, the f l u o r i n e a n a l y s i s performed indicate a decrease in F content by approximately 20 wt%, consistent with Kilau's results. Examination of the plots given in Figures 4, 5 reveals that the B series runs demonstrated the lowest viscosity. Since the relative amount of volatilization by the f l u o r i n e in s e r i e s A and B is similar, (20%), there must exist an alternate source of fluidization. The only variable which has been changed is the addition of the nepheline syenite tailings. Therefore the fluorspar, in the presence of the alkali alumino-silicate, must be c h e m i c a l l y stabilized in such a way as to preferentially form a complex low-melting compound which thereby enhances its capability as a fluidizer. The microprobe analyses, given in Table 3, show a considerable variation in c o m p o s i t i o n b e t w e e n the r e f r a c t o r y and the slag, in b o t h of the t e s t e d systems. In addition, there are also compositional differences between the two distinct phases present within the refractory and slag. Of particular note is the CaO content of both the slag and refractory phases; as the refractory did not contain lime to start with, it is evident that e x t e n s i v e r e a c t i o n has o c c u r r e d b e t w e e n the h i g h - A l 2 0 3 r e f r a c t o r y , and the two s y n t h e t i c slag mixtures presented. The purpose of the bulk chemical analysis was to evaluate any compositional changes occurring during the experimental procedure. At the same time, it was anticipated that these results would help explain the experimental viscosity profiles found. Finally, it was hoped to determine the effect of the complex c o m p o s i t i o n of the nepheline syenite tailings, Table I, on the overall slag composition. Preliminary examination of Table 2 shows generally consistent concentrations between like runs. For example, in the A series runs the lime c o n t e n t s are 72.9 wt% and 70.2 wt% for runs I and 2 respectively. Also, the CaO concentration d e c r e a s e d w h e n the f l u o r s p a r was r e p l a c e d by the n e p h e l i n e s y e n i t e t a i l i n g s . Therefore, be decreasing the CaF 2 content there is a net decrease in the amount of calcium cations charged into the oxide melt, and in turn less CaO is produced internally. Alumina, on the other hand, responded in the opposite manner, due to its presence in the replacement tailings material (over 20%). A n o t h e r i m p o r t a n t c o m p o s i t i o n a l c h a n g e occurs in the fluorine content. In series A, 10 wt% fluorspar corresponds to about 5 wt% fluorine; thus about 20% of the f l u o r i n e has b e e n lost. A s i m i l a r d e c r e a s e in fluorine content is recognized in the series B slag; here, 2.5 wt% F changed to approximately 2.0 wt% F in the final analysis. CONCLUSIONS 1.
Of the 4 Nephton samples investigated, the Final Waste exhibited the most fluid behaviour, and the Primary Dust the least for a given temperature. At 1585C (the maximum temperature investigated), viscosities ranged from 200 to 3000 poise, typical of amorphous materials. These levels might be compared to values of a few hundred centipoise for a fluid metallurgical slag.
2.
Based upon the chemical data provided (Table I), viscosity decreased with an increase in Fe203 for a given temperature; an increase in Fe203 of from 1.7 to 10.5 wt% resulted in a viscosity decrease of 15 fold at 1585C.
Nepheline syenite as a synthetic slag addition
215
3.
Viscosity-temperature profiles of three synthetic slags were generated. The slags were produced by v a r y i n g the c o m p o s i t i o n of f l u o r s p a r w i t h n e p h e l i n e syenite tailings in a typical steelmaking synthetic slag. Two tests w e r e also c o m p l e t e d w h i c h c o m p a r e d the e x t e n t of r e f r a c t o r y c o r r o s i o n c a u s e d by two of these synthetic slags. The corrosion of the refractory brick using composition B, (5 wt% N.S.T. and 5 wt% CaF2) , was similar to that of mixture A, (10 wt% CaF2).
4.
The viscosity-temperature profiles produced indicated that synthetic B exhibits superior fluidity over the temperature range investigated.
5.
The bulk chemical analysis indicated a l o s s of f l u o r i n e d u e to volatilization by approximately 20% in both the A and B synthetic slags. Therefore, in slag B, there was the possibility that the nepheline syenite stabilized the fluorspar by forming an alkali low melting point compound, thereby enhancing the capability of fluorspar as a fluidizer.
slag
ACKNOWLEDGEMENTS This project is part of the five-year Canada-Ontario 1985 Mineral Development Agreement (COMDA), a s u b s i d i a r y a g r e e m e n t to the E c o n o m i c and R e g i o n a l Development Agreement (ERDA) signed by the Governments of Canada and Ontario. The a u t h o r s w i s h to thank both SYSCO and I n d u s m i n L i m i t e d for supplying samples, technical information and encouragement for the project. In addition, the a u t h o r s are g r a t e f u l to K e v i n M. Fogarty who carried out most of the experimental work at the Technical University of Nova Scotia. REFERENCES
I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
M i l l s K.C. & Keene B.J. Physicochemical properties of molten CaF2-based slag, Intern. Metals Review, 25-26, No. I, 21-69, (1981). Davies M.W. & Wright F.A. The viscosity of calcium fluoride-based slags, Chemistry and Industry, 3, Jan.-June, 359-363, (1970). T u r k d o g e n E.T. & Bills P.M. A critical review of viscosity of CaO-MgOA I 2 0 3 - S i O 2 melts, A m e r i c a n C e r a m i c Soc. Bull., 39, No. 11, 682-687, (1960). Kukhtin T.I. et al. Physicochemical and metallurgical properties of blast furnace slags in CaO-SiO2-AI203-MgO quaternary system with FeO additions, Steel in the U.S.S.R., 15, No. 8, 364-366, (1985). Grau A.E., Caley W.F. & Masson C.R. Thermodynamics of lead silico-fluoride melts, Canadian Metallurgical Quarterly, 15, No. 4, 267-273, (1976). E f i m o v V.A. et al. Viscosity of synthetic slags used in casting steel, Steel in the U.S.S.R., I, No. 8, 616-618, (1971). Hewitt D.F. Nepheline syenite deposits of southern Ontario, Annual Report, Ont. Dept. of Mines, LXIX, Part 8, 193, (1960). Heyman L. Personal Communication, In the files of the Resident Geologist, Ministry of Northern Development and Mines, Tweed, Ontario. Kazi A & Whiteway S.G. Viscometer for molten oxidizing slags, Can. Met. Quart., 13, No. 4, 669-670, (1974). Caley W.F., Whiteway S.G., Neil A.B. & Dugdale P.J. Viscometer for lead blast furnace slags, The Metallurgical Soc. of the CIM, Annual Vol., 1-4, (1977). Pye L.D., S t e v e n s H.J. & L a C o u r s e W.C. Introduction to Glass Science, Plenum Press, New York, 722, (1972). R a w s o n N. Inorganic Glass Formin 8 Systems, Academic Press, London, 317, (1967). Caley W.F. Viscosity of nepheline syenite tailings, Final Report, Dec. 21, 1987, Technical Univ. of Nova Scotia, Halifax. Tsibul'nikov et al. Viscosity and refining capacity of lime-alumina based slags, Steel in the U.S.S.R. 3, No. 2, 116-120, (1973). Kilau H.W., Spironello V.R. & Mahan W.M. Viscosity of BOF slags fluidized with fluorspar, colemanite, and fused boric acid, United States Bureau of Mines Report of Investigations 8292, 13-21, (1978). Manyugin A.P., Sokolov G.A. & Sergeev A.G. Investigation of the physical properties of refining slags, Steel in the U.S.S.R., 5, No. 3, 140-142, (1975).
NEPHELINE
0892-6875/89 $3.00 + 0.00 © 1989 Pergamon Press plc
SYENITE AS A SYNTHETIC
SLAG ADDITION
IN SECONDARY STEELMAKING
P.W. KINGSTON % and W.F. CALEY § %
Ontario Ministry of Northern Development and Mines, Mines & Minerals Div., Tweed, Ontario K0K 3J0, Canada § Dept. of Mining & Metallurgical Engineering, Technical University of Nova Scotia, Halifax, Nova Scotia B3J 2X4, Canada
(Received 5 January, 1989)
ABSTRACT Recent trends in secondary steelmaking have been towards the use of synthetic slags for deoxidation, desulphurization, and inclusion control. These melts are generally a calcium aluminate material to which lime and/or fluorspar may be added, depending upon the slag/metal equilibrium desired. Due to the detrimental side-effects of fluorspar, such as enhanced refractory corrosion, a study had been undertaken to investigate the possibility of using a tailing material (nepheline syenite) to partially replace fluorspar in these slags. Based on the rheological behaviour observed, results to date suggest that nepheline syenite enhances the fluidity of such melts. These findings are discussed with reference to slag chemistry; in addition, the relative chemical reactivity of the compositions investigated with respect to refractory corrosion is addressed. Ke~words Slag viscosity; industrial minerals; nepheline syenite; flux; secondary steelmaking INTRODUCTION One of the less w e l l - k n o w n areas of industrial mineral use is in the area of fluxes, slags, additives, and s p e c i a l t y c o m p o n e n t s for the s t e e l m a k i n g industry. This is a n i n d u s t r y presently undergoing restructuring, rationalization, downsizing, and m o d e r n i z a t i o n . As a r e s u l t t h e r e a r e o p p o r t u n i t i e s for d e v e l o p m e n t of new materials such as s p e c i a l c o m p o s i t i o n refractories and for i n t r o d u c t i o n of new reagents and components, such as fluxes, synthetic slags, and ceramic coatings into the system. Overall, the steelmaking industry is large, and uses relatively large tonnages of industrial minerals. Acceptance of a small percentage increase in the use of a particular mineral, or the substitution of a relatively small percentage of an imported industrial mineral throughout the steel industry, would result in a s i g n i f i c a n t i n c r e a s e in c o n s u m p t i o n of i n d u s t r i a l m i n e r a l s . This potentially high demand makes the steelmaking industry deserving of attention as far as industrial mineral promotion is concerned. This project has focused in particular on the use of nepheline syenite as a substitute for f l u o r s p a r in the formulation of synthetic slags for use in secondary steelmaking, or ladle m e t a l l u r g y . L a d l e m e t a l l u r g y , as the n a m e implies, is c a r r i e d out in a r e f r a c t o r y - l i n e d l a d l e a f t e r the i n i t i a l steelmaking process. This batch process allows steelmakers to exercise a high degree of control on the physical and chemical properties of each ladle-full of steel. The p u r p o s e namely: I. 2.
of
a
slag
in
a
ladle
metallurgical
process
is
two-fold,
To r e m o v e c e r t a i n i m p u r i t i e s f r o m the l i q u i d m e t a l , s p e c i f i c a l l y sulphur and/or phosphorus, and To isolate the refined product from the surrounding atmosphere. 207
[I]
208.
P.W. KINGSTONand W. F. C~EY
Each of these is a function of physicochemical properties of the slag, such as activity, c o n d u c t i v i t y and v i s c o s i t y . G e n e r a l l y , a s y n t h e t i c slag o f t e n c o n t a i n i n g up to 50% calcium aluminate, blended with lime and fluorspar is used for this purpose. Slag Viscosity Slag viscosity is directly related to the temperature and composition of the melt [I]. Davies and Wright [2] explain that not only will viscosity determine the rate of fall of metal droplets through a slag phase, but in addition may well influence the rate of refining reactions, such as desulphurization. This is due to the r e l a t i o n s h i p b e t w e e n viscosity and the diffusion of gaseous species, such as 02- , to the metal phase. A metallurgical slag may be classified as being a molten silicate network [3]. In fused or vitreous pure silica, each silicon atom is bonded to four oxygen atoms and in turn each oxygen atom is bonded to two silicon atoms. However, in a m o l t e n state t h e r e is a d i s t o r t i o n of s y m m e t r y a l o n g w i t h a g e n e r a l weakening of the bonding strength between atoms primarily due to the elevated temperatures. There exists a sizeable amount of literature on the dependence of viscosity of a silicate network on its basic oxide concentration, both in the presence and a b s e n c e of f l u o r s p a r . For example, ferrous oxide is an effective fluidizer [4], although its influence was reported to diminish at temperatures in excess of 1400oc. In contrast, the behaviour of fluorspar as a fluidizer depends on its concentration and environment [5]. Efimov et al. [6] reported that CaF 2 reduces slag viscosity, whereas an i n c r e a s e in the total c o n c e n t r a t i o n of AI203 and CaO has the opposite effect. Although fluorspar is an excellent fluidizer, several problems exist with its u s e in s t e e l m a k i n g . F o r e x a m p l e , CaF 2 c h e m i c a l l y c o r r o d e s m o s t l a d l e refractories, and when heated gives off toxic fluorine-containing species to the atmosphere; in addition, it is a relatively expensive industrial mineral. Thus, it is advantageous to partially replace fluorspar with another equally effective material. Nepheline Syenite Nepheline syenite is composed essentially of nepheline (NaAISiO4), albite, and microcline [7]. The ore c o n t a i n s up to 15% a l k a l i s and at least s e v e r a l p e r c e n t iron oxide. In addition, tailings from the nepheline beneficiation process contain iron oxide l e v e l s a p p r o a c h i n g 10%. Thus, in spite of the presence of a glass former (SiO2), nepheline ore, or nepheline tailings, might be expected to behave as a reasonable fluidizer. Nepheline syenite is used as a main constituent in the production of silicab a s e d g l a s s w a r e , in c e r a m i c bodies, and in g l a z e f o r m u l a t i o n , . During b e n e f i c i a t i o n of the n e p h e l i n e ore, it is c r u s h e d and g r o u n d to release individual mineral particles. A two-stage magnetic separation process is used, f i r s t l y to r e m o v e m a g n e t i t e , w h i c h is sold as a product, and secondly to remove magnetite-containing middling grains, as well as other iron-containing mineral species, of which biotite and hornblende are the commonest. Removal of iron-containing minerals from the nepheline syenite product is essential, for in most high-temperature applications, iron included in the product would both stain or colour the melt (glass, or glaze), and lower the melting temperature of the mixture. In the only known previous use nepheline syenite ore was mixed Steel in the mid 1970s [8]. Use switched to a continuous casting
of nepheline syenite in a slag or flux, the with f l u o r s p a r in b r i q u e t t e f o r m by R o u g e was apparently discontinued when the company process. EXPERIMENTAL
Viscosity Measurement The p r o c e d u r e u s e d was to first isolate the most likely nepheline syenite tailings stream from four c a n d i d a t e s s u p p l i e d by I n d u s i m Ltd. from t h e i r Nephton deposit in Ontario. Following this, the plan was to to replace part or all of the fluorspar component of a commercial synthetic slag mix with one or
Nepheline syenite as a synthetic slag addition
209
m o r e of the w a s t e m a t e r i a l s . The tailings streams originated at different locations in the plant, and were designated Primary Dust, Wash Tailings, Final Waste, and L u r g i Waste. The fundamentals of the experimental approach have been described elsewhere [9,10]. (a) Materials Preparation: (i) Nepheline syenite tailings: Approximately 400 g of each tailings stream were isostatically pressed at 15000 p.s.i, for 3 minutes to provide a compact s t a r t i n g m a t e r i a l for viscosity measurement. Usually, 50 g of material was used per experiment. (ii) Synthetic slag mixtures: A variety of synthetic steelmaking slags is c o m m e r c i a l l y a v a i l a b l e , one b e i n g Coflux, a c a l c i u m a l u m i n a t e m a t e r i a l m a n u f a c t u r e d by E l k e m M e t a l s C o m p a n y . This is a s y n t h e t i c s l a g - f o r m i n g compound, containing 50-56 wt% CaO, 44-50 wt% A1203, with minor quantities of M g O , SiO2, and T i O 2. This m a t e r i a l may be f u r t h e r m i x e d w i t h lime and fluorspar to provide a useful ladle metallurgy slag. Compositions, in wt%, of the three mixtures used to evaluate the usefulness of nepheline syenite as a fluorspar replacement are as follows: A - 45% Coflux, 45% CaO, 10% CaF2; B45% Coflux, 45% CaO, 5% CaF2, 5% nepheline syenite; C - 45% Coflux, 45% CaO, 10% nepheline syenite. Coflux, lime, and fluorspar were supplied in sizes ranging from 1-10 cm. These materials were crushed in a Denver jaw crusher and ground in a small Denver rod mill to pass 600 microns. The remaining material, the nepheline syenite tailings, was already of sufficiently fine particle size to require only fine grinding and sieving. Approximately 50 g was used in each experiment. (b) C e r a m i c C o m p o n e n t s : The w o r k i n g c r u c i b l e s w e r e p u r c h a s e d from the M c D a n i e l R e f r a c t o r y Company (A1203) , or slip cast from Norton MgO, whereas safety crucibles (to protect the furnace muffle from slag spillage) were slip cast from high purity Reynolds AI203. Spindles consisted of 0.3 cm AI203 or MgO rods, which were in turn connected to a Brookfield pneumatic viscometer by means of a second AI203 shaft, as shown in Figure I. For all nepheline syenite t a i l i n g s experiments, AI203 working crucibles and spindles were used; these were replaced by MgO for the synthetic slag mixtures. (c) Viscometer Calibration: The viscometer was modified for high temperature use, and c a l i b r a t e d u s i n g B r o o k f i e l d s t a n d a r d silicone oils. Initial c a l i b r a t i o n was with the viscometer rotating at 100 rpm, although generally the viscometer was operated between 50 and 10 rpm, with data obtained from the h i g h e s t r o t a t i o n a l speed c o r r e s p o n d i n g to the most fluid material at the highest temperatures. (d) V i s c o s i t y - T e m p e r a t u r e Profiles: For each experiment, the spindle-shaft assemblage was suspended within the upper portion of the furnace (a Lucifer Melt-Master Elevator Furnace, Model 6000, with programmable controller), but not in contact with the sample. Thus, the a s s e m b l a g e was p r e h e a t e d b e f o r e b e i n g i n s e r t e d into the melt, while at the same time avoiding unnecessary c o n t a c t w i t h the m o l t e n m a t e r i a l . The e l e c t r i c a l s i g n a l s from b o t h the viscometer, and a Pt Pt-13% Rh thermocouple cemented to the furnace muffle, were led to a 2-pen Hewlett Packard Recorder. For all experiments, the maximum temperature, (1600C), was reached 45 minutes after commencing the heating cycle, and the spindle was immersed to within 3 m m of the c r u c i b l e b o t t o m a p p r o x i m a t e l y 6 m i n u t e s a f t e r a t t a i n i n g this temperature. Following equilibrium, the temperature was permitted to drop at the rate of 15C/min; data was recorded on a continuous viscosity/temperature t r a c e u n t i l the i n d i c a t e d v i s c o s i t y exceeded the sensitivity of t h e instrument. Refractory Corrosion Testinq In o r d e r to d o c u m e n t the effects, which substitution of nepheline syenite t a i l i n g s for f l u o r s p a r m i g h t have on r e f r a c t o r y c o r r o s i o n , a series of experiments was carried out using crucibles which were cored with a diamond core drill from high a l u m i n a (80%) r e f r a c t o r y brick. The h e a t i n g / c o o l i n g p r o f i l e c h o s e n was s i m i l a r to that used for the v i s c o s i t y measurements. Following cooling to room temperature, each of the crucibles was cut into 5 mm t h i c k d i s c s w i t h a d i a m o n d saw, to r e v e a l the r e a c t i o n zone between the refractory material and the oxide melt. Finally, the discs w e r e r e d u c e d to p o l i s h e d t h i n s e c t i o n s , and s u b j e c t e d to quantitative electron microscopy using a JEOL Superprobe (SEM/electron microprobe).
210
P.W. KINGSTONand W. F. CALEY
1
VISCOMETER
PLATINUM PINS
•
THERHOCOUPLE LEADS
PLATINUM PIN MAGNESIASHAFT
!
THERHOCOUPLE JUNCTION
i
ALUMINA SHAFT
il
lm, I! 'U (
HEATING ELEMENT ALUMINA MUFFLE ALUMINA SAFETY CRUCIBLE MAGNESIA WORKI NGCRUCIBLE PEDESTAL ELEVATOR
Fig.1
Schematic representation of experimental assemblage RESULTS
Nepheline Syenite Tailings The chemical nature of the four waste stream samples provided by Indusmin Ltd. is g i v e n in T a b l e I. Of p a r t i c u l a r note is the high Fe203 content, which approaches 10.5 wt% in the Final Waste. The resulting v i s c o s i t y - t e m p e r a t u r e profiles for the four streams are given as Figures 2 and 3. Figure 2 gives all data collected for the duplicate runs on a viscosity (poise) vs temperature (K) plot, w h e r e a s F i g u r e 3 p r e s e n t s the data in the more f a m i l i a r log viscosity (poise) vs reciprocal temperature (K -I ) format. From these plots, it is evident that the Primary Dust was the most viscous in nature, whereas the Final Waste exhibited the lowest level of viscosity for a given temperature. However, none of the samples could be termed "fluid" at 1585C, the m a x i m u m temperature attained in these experiments, and in fact all exhibited viscosity values generally found for highly silicious amorphous materials [11,12]. Synthetic Slag Mixtures The nepheline syenite tailings stream used for mixtures B and C was the "Final Waste". The viscosity-temperature p r o f i l e s for each slag i n v e s t i g a t e d are g i v e n in F i g u r e 4; for each slag a computer-generated regression curve is presented, together with error bars representing the s t a n d a r d e r r o r of the mean. As can be seen, the minimum viscosity of slag A was 2200 centipoise, whereas for B this value is about 1075 centipoise, and for C, w i t h 10 wt% nepheline syenite tailings, minimum viscosity was reached at 1750 centipoise. The log viscosity vs. reciprocal temperature plot given as Figure 5 suggests a change of slope in the 1500C range as crystals begin to precipitate from the oxide melt. This is characteristic of molten oxide melts [1,2,3,10,13]. Bulk Chemical Analysis The chemical analysis results are given in Table 2. The analysis was performed by the X-ray Assay Laboratories, Limited, Toronto, Ontario.
Nepheline syenite as a synthetic slag addition
TABLE I
211
Chemical analysis of Nephton nepheline syenite taillngs samples ( w t % ) ~ Primary Dust
Fe203** AI203 Na20 K20 CaO MgO
1.68 22.80 9.94 5.37 0.40 0.06
Wash Tailings
Final Waste
Lurgi Waste
3.94 22.70 9.58 5.32 0.62 0.12
10.50 20.40 6.33 5.43 1.43 0.27
2.46 23.00 9.75 5.36 0.44 0.07
* Reproduced from [13]. ** Total iron reported as Fe203. It is expected that a portion of the Fe203 is present as FeO or as abraded iron. 250
v 200
A • Lurgi Waste v • Final Waste o • Primary Dust
_ ~
\
O 0a
o
o o •
1100
•
,
,
1200
1300
1400
-
[ .
.
.
GQ
.
.
.
.
.
.
1500
1600
Temperature (C) Fig.2
Viscosity-temperature
profiles
for four Nephton stream samples
100
J
O
~-4
10
° ~°°° . / ~
o o
• LurslWute
~
~ F i n a l Waste ~ Primary Dust o Wash Tailing
I 5.0
I 5.5
I 6.0
I 6.5
I 7.0
7.5
Temperature K* (I04/T) Fig.3
Plot of log viscosity v s reciprocal temperature four Nephton stream samples
for
212
P . W . KINGSTONand W. F. CALEY
100 o Slag A 80-
I
I\
80o
• Slag B • Slag C
t
40-
0
20-
0
z25o
I
I
135o
z45o
I
Temperature
155o
(C)
Fig.4 V i s c o s i t y - t e m p e r a t u r e profiles for synthetic slag m i x t u r e s A, B and C. Error bars show the standard error of the mean
100.
o Slag A • Slag B • Slag C
//~
O v
o
o
5 5.3
I 5.5
5.7
I
I
I
5.9
6.1
6.3
Temperature K-I(IO~/T) Fig.5
Plot of log v i s c o s i t y vs. reciprocal t e m p e r a t u r e s y n t h e t i c slag mixtures A, B and C. E r r o r bars show the standard error of the mean
T A B L E 2:
wt%
AI
AI203 SiO 2 Fe203 CaO MgO MnO TiO 2 K20 Na20 F AI, Coflux, Coflux,
17.50 3.51 0.62 72.90 3.14 0.27 0.64 0.03 0.08 4.15
A2
16.10 3.55 0.64 70.20 3.93 0.25 0.59 0.03 0.10 3.03
Bulk Chemical
SLAG MIXTURE ~ BI
19.20 6.28 1.12 66.90 3.00 0.27 0.69 0.08 0.31 2.20
for
Analysis
B2
17.70 5.58 1.06 69.20 2.92 0.28 0.62 0.09 0.30 1.66
CI
18.0 7.39 1.46 62.20 6.76 0.30 0.64 0.37 0.61 0.06
C2
19.10 7.88 1.56 65.40 2.44 0.30 0.64 0.38 0.64 0.05
A2 45% C o f l u x , 45% CaO, 10% CaF2; B1, B2 - 45% 45% CaO, 5% CaF2, 5% n e p h e l i n e syenite; C1, C2 - 45% 45% CaO, 10% n e p h e l i n e syenite.
Nepheline syenite as a synthetic slag addition
Refractory
Corrosion
213
Testing
The r e s u l t s of the m i c r o p r o b e a n a l y s i s p e r f o r m e d on the two slag c o m p o s i t i o n s tested, A and B, w i t h the high a l u m i n a r e f r a c t o r y brick, are given in Table 3. For m i x t u r e A the r e f r a c t o r y phase was examined, w h e r e a s for slag B b o t h the slag and r e f r a c t o r y areas were analysed. In each case, random point a n a l y s e s of c o m p o s i t i o n s were taken in the g l a s s y m a t r i x and c r y s t a l l i n e phases.
TABLE
3
Refractory/slag
microprobe
analysis*
SLAG A
Analysis Point No.
REFRACTORY
- GLASS
REFRACTORY
- CRYSTAL
I 2 3 4
Wt% AI203
SiO 2
CaO
TiO 2
F
60.8 42.3
-13.6
34.5 40.2
---
0.39 0.88
10.8 6.2 48.5 39.9
--11 .6 16.2
41 .6 42.9 35.5 40.3
29.4 33.6 ---
0.98 0.72 0.99 0.97
CaO
TiO 2
SLAG B
Analysis Point No.
REFRACTORY
REFRACTORY
48.0 41 .4
2.0 . .
.
.
I 2 3
63.8 63.3 50.2
--5.8
35.6 35.6 41 .4
-. . . .
. .
. .
17.1 18.0
40.8 41 .I
---
0.6 0.8
42.4 42.2
42.7 45.3
0.6 0.9
- GLASS
I 2
39.7 39.2
SLAG
- CRYSTAL
I 2
4.8 2.0
oxides
F
4.6 5.8
SLAG
* Only m a j o r
SiO 2
38.3 50.2
- GLASS
- CRYSTAL
Wt% AI203
I .7
0.1
are presented.
DISCUSSION R e f e r r i n g to the curves of F i g u r e s 2 and 3, and the data of Table I, it may be seen that as the iron c o n t e n t of the t a i l i n g s samples increases, the v i s c o s i t y at a g i v e n t e m p e r a t u r e d e c r e a s e s , with approximately 275C separating the " b r e a k p o i n t " of t h e c u r v e s of h i g h e s t and lowest viscosity. Thus, of the samples investigated, it w o u l d a p p e a r that the N e p h t o n F i n a l W a s t e h a s the most f a v o u r a b l e r h e o l o g i c a l properties. F r o m the m e t a l l u r g i c a l point of view, none of the samples could be c o n s i d e r e d to be a " f l u x " s o l e l y f r o m their v i s c o s i t i e s . Generally, s t e e l m a k i n g slags have v i s c o s i t i e s of the order of a few h u n d r e d c e n t i p o i s e at t h e i r m a x i m u m o p e r a t i n g t e m p e r a t u r e [3], c o m p a r e d to 20,000 to 300,000 c e n t i p o i s e for the N e p h t o n samples. However, in these samples the p r e s e n c e of iron has a m a r k e d e f f e c t on the r e s u l t i n g viscosity, w i t h an i n c r e a s e in Fe203 of from 1.7 to 10.5% r e s u l t i n g in a 15 fold v i s c o s i t y d e c r e a s e at 1585C. Thus, this m a t e r i a l was c h o s e n for further evaluation.
214
P.W. KINGSTONand W. F. CALEY
Referring to the synthetic slag mixtures of this study, researchers [14,15,16] h a v e i n v e s t i g a t e d the v i s c o s i t y p r o f i l e s of v a r i o u s o x i d e m e l t s w i t h compositions similar to that of the base run of series A. Tsibul'nokov [14] tested the viscosity of a slag with a CaO/AI203 ratio of about 3.5 along with a CaF 2 concentration of 2 wt% and found that at 1500C the viscosity was 1910 centipoise. Results given in Figures 4, 5 show a viscosity of 2200 centipoise; however, the lime-alumina ratio in the present work was slightly larger, at 4. In e v a l u a t i n g the i n f l u e n c e of the f l u i d i z e r , the p r o b l e m of f l u o r s p a r instability must be considered. Kilau's [15] work showed poor reproducibility for viscosity measurements in fluorspar-containing BOF slags. It was reported that the solidification temperature of the melt increased by 61C over a period of 31 hours. He then concluded that this increase in apparent solidification temperature with time is a consequence of fluidizer depletion from the melt via f l u o r i n e v o l a t i l i z a t i o n . In the p r e s e n t work, the f l u o r i n e a n a l y s i s performed indicate a decrease in F content by approximately 20 wt%, consistent with Kilau's results. Examination of the plots given in Figures 4, 5 reveals that the B series runs demonstrated the lowest viscosity. Since the relative amount of volatilization by the f l u o r i n e in s e r i e s A and B is similar, (20%), there must exist an alternate source of fluidization. The only variable which has been changed is the addition of the nepheline syenite tailings. Therefore the fluorspar, in the presence of the alkali alumino-silicate, must be c h e m i c a l l y stabilized in such a way as to preferentially form a complex low-melting compound which thereby enhances its capability as a fluidizer. The microprobe analyses, given in Table 3, show a considerable variation in c o m p o s i t i o n b e t w e e n the r e f r a c t o r y and the slag, in b o t h of the t e s t e d systems. In addition, there are also compositional differences between the two distinct phases present within the refractory and slag. Of particular note is the CaO content of both the slag and refractory phases; as the refractory did not contain lime to start with, it is evident that e x t e n s i v e r e a c t i o n has o c c u r r e d b e t w e e n the h i g h - A l 2 0 3 r e f r a c t o r y , and the two s y n t h e t i c slag mixtures presented. The purpose of the bulk chemical analysis was to evaluate any compositional changes occurring during the experimental procedure. At the same time, it was anticipated that these results would help explain the experimental viscosity profiles found. Finally, it was hoped to determine the effect of the complex c o m p o s i t i o n of the nepheline syenite tailings, Table I, on the overall slag composition. Preliminary examination of Table 2 shows generally consistent concentrations between like runs. For example, in the A series runs the lime c o n t e n t s are 72.9 wt% and 70.2 wt% for runs I and 2 respectively. Also, the CaO concentration d e c r e a s e d w h e n the f l u o r s p a r was r e p l a c e d by the n e p h e l i n e s y e n i t e t a i l i n g s . Therefore, be decreasing the CaF 2 content there is a net decrease in the amount of calcium cations charged into the oxide melt, and in turn less CaO is produced internally. Alumina, on the other hand, responded in the opposite manner, due to its presence in the replacement tailings material (over 20%). A n o t h e r i m p o r t a n t c o m p o s i t i o n a l c h a n g e occurs in the fluorine content. In series A, 10 wt% fluorspar corresponds to about 5 wt% fluorine; thus about 20% of the f l u o r i n e has b e e n lost. A s i m i l a r d e c r e a s e in fluorine content is recognized in the series B slag; here, 2.5 wt% F changed to approximately 2.0 wt% F in the final analysis. CONCLUSIONS 1.
Of the 4 Nephton samples investigated, the Final Waste exhibited the most fluid behaviour, and the Primary Dust the least for a given temperature. At 1585C (the maximum temperature investigated), viscosities ranged from 200 to 3000 poise, typical of amorphous materials. These levels might be compared to values of a few hundred centipoise for a fluid metallurgical slag.
2.
Based upon the chemical data provided (Table I), viscosity decreased with an increase in Fe203 for a given temperature; an increase in Fe203 of from 1.7 to 10.5 wt% resulted in a viscosity decrease of 15 fold at 1585C.
Nepheline syenite as a synthetic slag addition
215
3.
Viscosity-temperature profiles of three synthetic slags were generated. The slags were produced by v a r y i n g the c o m p o s i t i o n of f l u o r s p a r w i t h n e p h e l i n e syenite tailings in a typical steelmaking synthetic slag. Two tests w e r e also c o m p l e t e d w h i c h c o m p a r e d the e x t e n t of r e f r a c t o r y c o r r o s i o n c a u s e d by two of these synthetic slags. The corrosion of the refractory brick using composition B, (5 wt% N.S.T. and 5 wt% CaF2) , was similar to that of mixture A, (10 wt% CaF2).
4.
The viscosity-temperature profiles produced indicated that synthetic B exhibits superior fluidity over the temperature range investigated.
5.
The bulk chemical analysis indicated a l o s s of f l u o r i n e d u e to volatilization by approximately 20% in both the A and B synthetic slags. Therefore, in slag B, there was the possibility that the nepheline syenite stabilized the fluorspar by forming an alkali low melting point compound, thereby enhancing the capability of fluorspar as a fluidizer.
slag
ACKNOWLEDGEMENTS This project is part of the five-year Canada-Ontario 1985 Mineral Development Agreement (COMDA), a s u b s i d i a r y a g r e e m e n t to the E c o n o m i c and R e g i o n a l Development Agreement (ERDA) signed by the Governments of Canada and Ontario. The a u t h o r s w i s h to thank both SYSCO and I n d u s m i n L i m i t e d for supplying samples, technical information and encouragement for the project. In addition, the a u t h o r s are g r a t e f u l to K e v i n M. Fogarty who carried out most of the experimental work at the Technical University of Nova Scotia. REFERENCES
I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
M i l l s K.C. & Keene B.J. Physicochemical properties of molten CaF2-based slag, Intern. Metals Review, 25-26, No. I, 21-69, (1981). Davies M.W. & Wright F.A. The viscosity of calcium fluoride-based slags, Chemistry and Industry, 3, Jan.-June, 359-363, (1970). T u r k d o g e n E.T. & Bills P.M. A critical review of viscosity of CaO-MgOA I 2 0 3 - S i O 2 melts, A m e r i c a n C e r a m i c Soc. Bull., 39, No. 11, 682-687, (1960). Kukhtin T.I. et al. Physicochemical and metallurgical properties of blast furnace slags in CaO-SiO2-AI203-MgO quaternary system with FeO additions, Steel in the U.S.S.R., 15, No. 8, 364-366, (1985). Grau A.E., Caley W.F. & Masson C.R. Thermodynamics of lead silico-fluoride melts, Canadian Metallurgical Quarterly, 15, No. 4, 267-273, (1976). E f i m o v V.A. et al. Viscosity of synthetic slags used in casting steel, Steel in the U.S.S.R., I, No. 8, 616-618, (1971). Hewitt D.F. Nepheline syenite deposits of southern Ontario, Annual Report, Ont. Dept. of Mines, LXIX, Part 8, 193, (1960). Heyman L. Personal Communication, In the files of the Resident Geologist, Ministry of Northern Development and Mines, Tweed, Ontario. Kazi A & Whiteway S.G. Viscometer for molten oxidizing slags, Can. Met. Quart., 13, No. 4, 669-670, (1974). Caley W.F., Whiteway S.G., Neil A.B. & Dugdale P.J. Viscometer for lead blast furnace slags, The Metallurgical Soc. of the CIM, Annual Vol., 1-4, (1977). Pye L.D., S t e v e n s H.J. & L a C o u r s e W.C. Introduction to Glass Science, Plenum Press, New York, 722, (1972). R a w s o n N. Inorganic Glass Formin 8 Systems, Academic Press, London, 317, (1967). Caley W.F. Viscosity of nepheline syenite tailings, Final Report, Dec. 21, 1987, Technical Univ. of Nova Scotia, Halifax. Tsibul'nikov et al. Viscosity and refining capacity of lime-alumina based slags, Steel in the U.S.S.R. 3, No. 2, 116-120, (1973). Kilau H.W., Spironello V.R. & Mahan W.M. Viscosity of BOF slags fluidized with fluorspar, colemanite, and fused boric acid, United States Bureau of Mines Report of Investigations 8292, 13-21, (1978). Manyugin A.P., Sokolov G.A. & Sergeev A.G. Investigation of the physical properties of refining slags, Steel in the U.S.S.R., 5, No. 3, 140-142, (1975).