Absorption–evaporation kinetics of water vapour on highly hygroscopic powder: Case of ammonium nitrate

Abstract

The conditioning of industrial powders or granulates is strongly dependent on their behaviour with respect to the atmospheric humidity. Adsorption and capillary condensation of water vapour at the points of contact between particles change the rheology of the powder and may result in the formation of aggregates with high mechanical resistance. In some cases water vapour causes partial dissolution of powder, as in the case of the hygroscopic fertilizer Ammonium Nitrate. Additives used as caking inhibitors are expected to act in one or both of following directions: – slackening the adsorption of water vapour and/or the dissolution of the particles in conditions of high humidity; – inhibition of the nucleation and growth during the recrystallisation of the powder in a dry atmosphere. An experimental method was developed which allowed following of the kinetics of the cycle, absorption of water vapour and formation of solution – evaporation of water – nucleation and growth of the recrystallized solid. The method is based on the variation of mass of powder sample placed in a flow of water vapour with known partial pressure at constant temperature. A model is proposed, based on the kinetic theory of gases, able to assess quantitatively the sorption/dissolution kinetics of powders exposed to high atmospheric humidity, just as the evaporation of the absorbed water and, eventually, the recrystallisation of the dissolved solid when the humidity is low. The effect of addition of Fuchsine acid (known anti-caking agent for Ammonium Nitrate) on that kinetics is found to be nil as regards the dissolution and the nucleation but slakes the subsequent recrystallisation during powder drying.