A DIFFERENT CONSIDERATION FOR Na₂SiF₆ FORMATION/DISSOCIATION AND ITS RELATION WITH SILICON FLUORIDE VAPORIZATION IN THE STEELMAKING PROCESS

Abstract

Due to contamination and corrosion problems caused by silicon fluoride vaporization in steelmaking plants, the formation/dissociation of Na₂SiF₆ has become the subject of many studies. In all the previous works, it has been assumed that silicon tetrafluoride (SiF₄) is the only gas species formed during processing by the use of slags and fluxes in the steel industry. In the present work, it is proposed that during the thermal composition of Na₂SiF₆, SiF₄ is not the only gas species formed. Based on a study on the kinetics of decomposition of Na₂SiF₆ in nitrogen and on thermodynamic predictions, it is proposed that Na₂SiF₆ decomposes endothermically into various gaseous species (SiF₄, SiF₃, SiF₂, SiF, and Si, denoted by SiF_x) through a series of complex reactions of zero-order with respect to the gaseous products, with activation energy of 156 kJ mol⁻¹ and a rate-determiningstep given by the chemical reaction itself. The gaseous species are formed through a set of simultaneous reactions represented by the general equation

where x varies from 0 to 4 and n, given by (2 − 0.5 x), takes the values 0, 1/2, 1, 3/2, and 2 and NaF is sodium fluoride.

Keywords:

• reaction kinetics
• sodium silicofluoride
• steelmaking process
• thermal decomposition

ACKNOWLEDGEMENTS

Dr. A. L. Leal-Cruz gratefully acknowledges Conacyt (National Council of Science and Technology in Mexico) for providing a Ph.D. scholarship and for financial support (grant #CB-2005-1/24322). The authors would also like to thank Mr. Felipe Márquez Torres for technical assistance during the analysis by SEM and Mr. Francisco Botello Rionda for technical assistance during the analysis by DTA/TG.