Fundamentals on Evaporation of Alkali Compounds

Abstract

In order to obtain fundamental data for alkali metal vaporization, the evaporation rate of alkali chlorides and alkali sulfates (NaCl, KCl, Na₂SO₄, K₂SO₄), which are major alkali compounds present in coal, were measured under inert, oxidizing and reducing atmosphere by using a thermobalance. A diffusion model was applied to obtain alkali metal evaporation data and identify an evaporating path and a rate controlling phenomena.
Alkali chlorides evaporated as monomers and dimers. The evaporation rate is explained by a diffusion model. In the case of alkali sulfates, a dissociation reaction occurred simultaneously with evaporation as monomer. In inert and oxidizing atmospheres, the dissociation reaction was represented as
M₂SO₄⇒2M+SO₂+O₂
Evaporation rates can be calculated by a diffusion model in oxidizing atmosphere. In an inert atmosphere, however, when dissociation becames remarkable evaporation rate is controlled by the dissociation rate. Under a reducing atmosphere the expected reaction was represented as
M₂SO₄+2CO⇒2M+SO₂+2CO₂
This reaction does not reach equilibrium, so that the following reactions controll evaporation rate.
M₂SO₄⇒2M+COS+5CO₂
COS+2CO₂⇒SO₂+3CO