Dissolver off-gases are processed for recovery of oxides of nitrogen, known collectively as NO*, in step 15. This is necessary to prevent corrosion of downstream equipment and acid contamination of the environment, and it reduces HN03 makeup. The preferred procedure is to pass the hot dissolver off-gases, which contain water vapor, nitric acid vapor, N2 04, N02, NO, N20, and added air or oxygen, successively through a downdraft condenser and a water scrubber.

In a downdraft condenser gases and condensed dilute nitric acid flow concurrently down the condenser wall. In this way the leanest gas is contacted with the maximum volume of coolest condensate, thus improving absorption. The following reactions take place:

2N02(?)-N204C?)

2N02 (or N204) (0 + H2O(0 — HNO3(0 + HN02(1)

3HNO2(0 — H2 0(/) + HN03(/) + 2NO(?) and 2NO(?) + 02(?) -> 2N02 (?)

The first three equilibrium reactions are rapid, but the fourth reaction, which eventually proceeds to completion, is slow even with excess oxygen and determines the extent of absorption.

Gases leaving the downdraft condenser are passed through a bubble-plate or packed water scrubber, where additional absorption of NO* takes place. Laboratory studies [М2] indicate that the nitrogen oxide content can be reduced to from 0.1 to 0.5 percent with a residence time of 2 min in the condenser and water scrubber. The design of scrubbers for recovery of nitrogen oxides is described in standard texts, e. g., [Р5].

The nitrogen oxide content of dissolver off-gases can be further reduced to 10 ppm by adding NH3 to the gases leaving the absorber and passing the mixture over a hydrogen mordenite catalyst [P4], which reduces NO* to N2 and H2 O.