Thermal decomposition mechanism of ammonium nitrate and potassium chloride mixtures

Abstract

The thermal decomposition of ammonium nitrate (AN) and potassium chloride (KCl) mixtures was investigated. The thermal properties were studied using differential scanning calorimetry (DSC), and the evolved gas was analyzed using thermogravimetry with mass spectrometry and pressurized DSC coupled with mass spectrometry (TG–DTA–MS and PDSC–MS). DSC measurements of AN/KCl mixtures in sealed and sealed-separated sample pans showed a sharp exothermic decomposition and lower onset temperature than pure AN, whereas AN/KCl in an open pan exhibited an endothermic reaction. In the sealed-separated pan, a separator with a pinhole divided the KCl and AN physically and the evolved gas could interact with both AN and KCl. TG–DTA–MS results revealed that HCl gas was evolved from AN/KCl, which indicated that the reaction of KCl with HNO₃ dissociated from AN formed HCl, and subsequent destabilization of AN. However, the TG–DTA–MS results did not indicate the violent exothermic reaction due to using an open pan and ordinary pressure conditions. PDSC–MS was used to observe two exothermic reactions of AN/KCl and analyze the evolved gases from the reactions. A violent first exothermic reaction was accompanied by a large amount of N₂ and N₂O gases without H₂O, and a second exothermic reaction accompanied by H₂O, N₂O, and other gases occurred subsequently. The reactions are ${\text{HCl }} + {\text{HNO}}_{ 3} \to {\text{NO}}_{ 2} {\text{Cl }} + {\text{H}}_{ 2} {\text{O }} \to {\text{Cl }} + {\text{NO}}_{ 2} + {\text{H}}_{ 2} {\text{O}}$, which have a lower energy barrier by 103 kJ mol⁻¹ than the energy barrier that is needed for HNO₃ homolysis cleavage, which is triggered by pure AN decomposition, ${\text{HNO}}_{ 3} \to {\text{OH}} + {\text{NO}}_{ 2}$. We therefore concluded that AN reacts with KCl to produce Cl radicals via HCl and NO₂Cl, and the Cl radical triggers a radical chain reaction of AN.