Kinetic analysis of NaF vaporization in SiF 4 -Ar mixture
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(Na2O) ⫹ (CaF2) ⫽ (CaO) ⫹ 2NaF(g)
I. INTRODUCTION
FLUORIDE vaporization from slags containing CaF2 is a serious problem in metallurgical industries in the continuous casting of steels, particularly where, in the secondary cooling chamber of a continuous caster, enhanced corrosion of the machine structure is found. The chemistry and physiochemical properties of slags can also vary as a function of time during processing due to vaporization of fluorides leading to process inconsistency and environmental problems. There are many types of gaseous fluorides that are stable at steel casting temperatures. NaF, SiF4, AlF3, and KF are stable under the conditions found in the mold of a continuous caster and can evolve from a mold slag into the surrounding atmosphere. In humid conditions, all of these fluorides would spontaneously form HF in contact with water and/or water vapor, as shown in Eqs. [1] through [3]. 2NaF ⫹ H2O ⫽ Na2O ⫹ 2HF
[1]
SiF4 ⫹ 2H2O ⫽ SiO2 ⫹ 4HF
[2]
CaF2 ⫹ H2O ⫽ CaO ⫹ 2HF
[3]
In the case of slag containing Al2O3 and K2O, more complicated formation reactions of fluorides will also occur.[1] SiF4 can also vaporize from a slag containing SiO2 and CaF2, and this reaction is expressed in Eq. [4]. The vaporization kinetics of SiF4 from several types of slags were recently studied,[1–4] as SiF4 is a stable gas from ambient to high temperatures. However, when a slag contains Na2O, SiO2, and CaF2, the vaporization species consist of not only SiF4 but also NaF. Furthermore, as NaF deposits at moderate temperatures (⬍1273 K), precipitation of NaF on cooled surfaces can be a problem. The reactions for SiF4 and NaF vaporization are as follows: (SiO2) ⫹ 2(CaF2) ⫽ 2(CaO) ⫹ SiF4 (g)
[4]
YOSHIAKI KASHIWAYA, formerly Visiting Professor, Department of Materials Science and Engineering, Carnegie Mellon University, is Associate Professor, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Hokkaido, Japan. ALAN W. CRAMB, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted September 22, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
[5]
When SiF4 and NaF vaporize simultaneously, the reaction between SiF4 and NaF can produce sodium hexafluorosilicate (Na2SiF6), as shown in Eq. [6]. It is important to understand the behavior of Na2SiF6 during the vaporization of SiF4 and NaF and to determine its effect on the vaporization kinetics of both species. SiF4(g) ⫹ 2NaF(l;g) ⫽ Na2SiF6 (s)
[6]
Figure 1 shows the pseudobinary phase diagram in a NaFSiF4 system.[5,6,7] The phase diagram was obtained by differential thermal analysis using a sealed tantalum capsule, as the dissociation pressure of Na2SiF6 exceeds 70 atm at 1173 K. The diagram is not valid at 1 atmosphere total pressure as SiF4 is gaseous, however, Na2SiF6 can exist at 1 atm when SiF4 exists continuously in the atmosphere. The free energy of Reaction [6] in the solid state of Na2SiF6 was measured by Chiotti[7] and the free energy of a homogeneous solutio
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