During evaporation the following reactions are taking place:
(6) H2O (l) → H2O (g)
(7) HNO3 (aq) → HNO3 (g)
(8) HF (aq) → HF (g)
These three reactions are representing the evaporation of water, nitric acid and hydrofluoric acid. This is the first reaction after the acid is sprayed into the spray roasting reactor. The temperature in the reactor is between 300 and 700 °C. The consequence of these reaction conditions is that large parts of the gaseous nitric acid will decompose to NOx according to reaction (9-10).
(9) 2 HNO3 (g) → NO2 (g) + NO(g) + H2O(g) + O2(g)
(10) 2 NO2 ⇔ 2 NO + O2
Reaction (10) takes place in both directions, so that both NO and NO2 will be present after the reactor. Nearly the entire HNO3 will decompose to NO and NO2. In the absorption process downstream of the spray roaster, a certain amount of the NOx will react back to HNO3 again – see reactions (11-12).
(11) 3 NO2 (g) + H2O (l) ⇔ 2 HNO3 (aq) + NO (g)
(12) NO + 0.5 O2 ⇔ NO2