Solid State Reactions in the Na2CO3, CaF2 and CaSiO3 System.
The Case of a Bifurcation Point
Marco Alloni, Riccardo Carli
(Prosimet S.p.A., Filago (BG), Italy)
Abstract: Different studies have been conducted, beginning in 1979, on mineralogical phases which evolve in mold
powders during their use in continuous casting of steel.
Literature reports many studies which focus on melting, crystallization or devitrification processes and on equilibrium of
mineralogical phases formed by fluxes.
Most of these studies point out that, depending on different composition factors such as the basicity index, CaO/SiO2 wt
%/wt%, alkali or fluorine content, these systems evolve to different silicate and fluoro-silicate phases. In many
publications the most common identified phase is cuspidine, Ca4Si2O7F2, which is also an important phase in controlling
thermal exchange between steel strand and mold wall.
In the present work, while data available in open literature on these systems have been qualitatively confirmed, the
conditions which lead, during melting process, to formation of the different crystalline fluoro-silicate phases such as
cuspidine, Ca4Si2O7F2 and dicalcium sodium fluoro-silicate, Ca2SiO4.NaF, have been deeply investigated using hot stage
microscope, x-ray difractometry, DSC-TGA and SEM.
This study led to identification of a well-defined bifurcation point which has been found to be correlated with existence
of an eutectic point in the pseudo binary system CaF2 and Na2CO3.
Solid state reaction paths have been considered to explain the observed phenomena pointing out the crucial role of the
liquid phase formed by this eutectic in melting process of mold fluxes and crystalline phases formation.
Key words: mold flux, melting behavior, cuspidine formation, Na2CO3-CaF2 pseudo-binary system, eutectic reaction
an eutectic reaction in the pseudo – binary system
Na2CO3-CaF2[10].
The present study aims to further investigate
chemical conditions for formation of fluorosilicate
phases in mold fluxes during melting and their
correlation to existence of the eutectic reaction.
1. Introduction
Modern continuous casting technology takes
advantage of slag formed by melting of mold
fluxes to provide lubrication in the mold/solidying
strand shell system. Mold fluxes may be considered
as high temperature lubricants which rheological
properties are controlled through chemical
formulation[1].
Melting of mold flux producing this
lubricating liquid slag requires heat and time.
Development of a method to control this period of
time, i.e. reduction of the melting rate, has been
one of most important advance in this
technology[2].
More recently the appeal of higher casting
speeds led to requirement for faster melting fluxes.
This has been achieved through use of large
amounts of fluxing components and through use of
low temperature melting glasses which allow to
inject heat into the system as chemical energy.
Studies on melting of mold fluxes and on phase
transformation induced by heating when mold
fluxes are molten down, have been carried out by
many research groups[3-7]. In our previous works
[8,9,10], in situ HTC-XRD analyses showed that
melting process of mold fluxes goes through
formation of cuspidine before melting. Cuspidine,
Ca4Si2O7F2,
is a calcium fluorosilicate phase
which is responsible for the heat transfer properties
of recrystallized mold fluxes into mold-strand
gap[11]. While many studies have been devoted on
formation of cuspidine during recrystallization,
only few researches can be found on the processes
that leads to its formation in mold fluxes before
melting.
It has been shown that cuspidine starts
developing at temperatures as low as 853K (580°C)
seemingly correlated to the melting temperature of
2. Experimental
Samples used in this work to investigate
chemical conditions of formation of fluorosilicates
were formulated using reagent grade CaSiO3,
Na2CO3 and CaF2 (Table 1) milled and sieved
through ASTM 170 mesh sieve, dried 24 hours at
393 K (120°C) and mixed in a proper lab mixer for
two hours.
The samples were then inserted in a platinum
crucible and heated in an oven at 1173 K for 24
hours with six intergrindings. When last collected
the mixtures were cooled on a copper plate and
grinded for the subsequent analyses.
Chemical analysis was conducted on each
calcinated sample, a PW 2400 XRF Sequential
Spectrometer with VCR-2540 sample changer
equipment by PANalytical was used to determinate
Ca, Si, Na. An electrochemical method was used to
determinate fluorine content.
The phases formed after heating treatment have
been studied with the X-Ray Diffractometer
described in previous works[8,9].
DSC-TGA analyses were also conducted by
means of a STA F3 Jupiter instrument by Netzsch on
some of the samples before heat treatment to
identify the reaction temperatures involved in
formation of the different phases in the ternary
system and also on mixtures of CaF2-Na2CO3 to
identify the melting point at different ratios and the
carbonate decomposition temperature.
Table 1 Compositions of the samples before calcination and main fluorinated phase at 1173 K
Sample
Na2CO3
CaF2
CaSiO3
Main fluorinated
phase
1
55,1
39,9
5,1
liquid
2
52,2
37,8
10,0
liquid
3
49,3
35,7
14,9
liquid
4
14,1
10,4
75,5
dicalcium
sodium
fluorosilicate
5
12,1
12,6
75,3
cuspidine
6
16,3
7,8
75,9
cuspidine
7
9,9
13,2
76,9
cuspidine
8
17,6
12,7
69,7
dicalcium
sodium
fluorosilicate
9
45,3
12,7
42,0
dicalcium
sodium
fluorosilicate
10
13,0
33,3
53,7
cuspidine
11
21,7
9,3
69,0
dicalcium
sodium
fluorosilicate
12
25,8
15,7
58,5
dicalcium
sodium
fluorosilicate
13
10,5
36,8
52,6
cuspidine
14
34,6
26,9
38,5
dicalcium
sodium
fluorosilicate
15
14,4
1,0
83,6
NaF
16
45,0
5,0
50,0
NaF
17
60,0
6,7
33,3
NaF
18
4,3
10,3
85,5
cuspidine
19
9,1
22,7
68,2
cuspidine
64,3
dicalcium
sodium
fluorosilicate
50,0
dicalcium
sodium
fluorosilicate
20
28,6
7,1
21
40,0
10,0
3. Results and Discussion
Table 1 reports composition of samples and
main fluorinated phases formed by the CaSiO3 Na2CO3 - CaF2 mixtures after heating at 1173 K
which are also plotted in Fig. 1.
X-ray diffraction measurement pointed out
presence of three main fluorinated phases formed
during heating of the prepared mixtures. Two of
these phases were fluorosilicates (cuspidine and
dicalcium sodium fluorosilicate, Ca2SiO4.NaF)
while the third one was villiaumite (NaF).
!
Fig. 1 Main fluorinated phase at different compositions
A liquid slag region (dark blue dots) still
existed for a content of CaSiO3 up to 15% in weight.
DSC-TGA analyses have been conducted on binary
mixtures of sodium carbonate and fluorspar to
determinate the extension of this liquid phase on the
CaF2 – Na2CO3 plane.
a
b
Endothermic peak temperature and carbonate
decomposition temperature in Fig. 2b agree with
endothermic peak temperature in Fig. 2a and with
reportedand
in calcium
previousfluoride
works[8,9,10]
for
Fig. 2a - 2b – 2c DSC-TGA analyses of a 60%-40% mixture temperatures
of sodium carbonate
and DSCcuspidine
formation
proving
that
the
reaction
is
TGA of sample 8 and sample 16
c
associated with formation of liquid phase.
It has been possible to prove that liquid slag
On the contrary in Fig. 2c the endothermic peak
was forming at a temperature of 850 K for CaF2/
had an higher temperature, 1125 K, and carbonate
Na2CO3 ratios (wt./wt.) going from 1:5 to 1:1 and
decomposition started at 1032 K. These
that carbonate decomposition occurred at an higher
temperatures are consistent with data on
temperature, 1012 K.
decomposition of sodium carbonate when mixed
If content of wollastonite was above 15% in
with silicon dioxide[12]. In this case no interaction
weight, the system was solid and cuspidine (red
between calcium fluoride and sodium carbonate to
dots) and dicalcium sodium fluorosilicate (light blue
produce liquid slag at 850 K has been observed,
dots) were main phases containing fluorine formed.
combined with a lack of fluorosilicate phases in the
However in the region where the CaF2/Na2CO3 ratio
sample.
was lower than 1:5 the main fluorinated phase found
Based on this results it may be said that eutectic
by XRD was NaF (black dots) and no fluorosilicate
reaction between Na 2 CO 3 and CaF 2 is the
phases were found.
bifurcation point in the reaction paths responsible
DSC-TGA measurements carried out on a 60%
for formation of intermediate phase, such as
- 40% Na2CO3 - CaF2 mixture and CaF2 - Na2CO3 cuspidine, that allows the system to melt down.
CaSiO3 mixes with same proportions as sample #8
and sample #16 are reported in Fig. 2a, 2b and 2c.
These last two compositions were chosen
4. Conclusions
because the CaF2/Na2CO3 ratios were respectively
in and out of the range found for the eutectic
An investigation on fluorinated phases formed
reaction.
in the ternary system Na2CO3 – CaF2 – CaSiO3
was carried out emphasizing the correlation existing
between a liquid phase in the Na2CO3 – CaF2
forming at a relatively low temperature and the
generation of these phases. Furthermore a region of
compositions at an higher sodium carbonate –
calcium fluoride ratio compared to the low
temperature liquid phase has been observed in which
major fluorinated phase found was villiaumite.
The phenomenon has been studied in depth
through use of DSC-TGA analyses which confirmed
that an endothermic reaction, which involves
carbonate decomposition and which generates
fluorosilicate phases, occur in the ternary mixture at
the same temperature as the eutectic reaction in the
pseudo-binary system. Samples, which did not show
presence of fluorosilicates phases during XRD
analyses, were same samples that during DSC-TGA
analyses did not display an endothermic peak at 878
K.
The data obtained in this research suggest that
eutectic reaction between Na2CO3 and CaF2 is the
bifurcation point in the reaction paths responsible
for formation of intermediate phase that allows the
system to melt down.
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