On the mechanism of flame inhibition by alkali metal salts

Abstract

Alkali metal salts, in particular sodium and potassium bicarbonate, are increasingly used as fire-extinguishing powders. Studies have been made in the past of the relative efficiency of alkali-metal salts as flame inhibitors, and the oxalates have been shown to be particularly effective. In the present work it has been found that the high efficiency of the alkali oxalates is due to their ready decomposition in a flame, with the generation of submicron particles of alkali carbonate. Other salts such as alkali ferrocyanides have been found to behave in a similar way. With such salts the surface area required for the extinction of a diffusion flame varies with particle size, there being an optimum size below which decomposition occurs before the flame front is reached and above which little decomposition occurs within the residence time of the particle in the flame. Inhibition is considered to occur via a homogeneous mechanism, involving the volatilization and/or reaction of the initially formed submicron particles to provide gaseous hydroxide as the inhibiting species. In order that the generation of alkali oxide or hydroxide may be facilitated, the anion associated with the alkali metal should be of low acid strength.