Remove More Nox

Q: Do you have any ideas to reduce the yellow/brown color of air emissions from our nitric acid-based aluminum bright dip process? We are currently using two wet scrubbers in series with a caustic recirculating liquor. Even though our emissions are within our permit limits, we would like to reduce this color as much as possible due to periodic complaints by neighbors.

A: In various metal finishing operations where nitric acid is utilized, the emitted plume of nitrous oxides (nitrogen oxide ,NO, and nitrogen dioxide, NO₂ , or NO_x ) from baths and dips is observed as a characteristic yellow-brown vapor. This regulated pollutant has typically been, and continues to be, controlled by contacting the gas stream with aqueous caustic (NaOH) in a single or series scrubber system. This approach has been simple and cost effective, yet presents some problems.
One problem is the varying solubility NO and NO₂ . NO₂ is very soluble and reactive in alkali solutions; however NO is not very soluble or reactive in the same conditions, often resulting in visible emissions from the scrubber. Therefore, the likely source of your remaining color is NO. Further complicating this is the formation of nitrates (NO₃) and nitrites (NO₂) in the alkali solution by the following chemical reactions:

NO + NO₂ —› N₂ O₃

N₂O3 + 2 NaOH —›2 NaNO₂ + H2O

2 NO₂ + 2 NaOH —› NaNO₂ + NaNO₃ + H₂O

As you can see from these chemical reactions, lots of sodium nitrite (NaNO2) is generated, which salts up components and results in frequent scrubber cleanings.

The main method to reduce NO_x emissions beyond that of the caustic systems has been to replace or augment the caustic in the scrubber liquor with other compounds, such as sodium hypochlorite, sodium hydrosulfide, and hydrogen peroxide.

Sodium hypochlorite is relatively cheap, easy to handle and oxidizes the NO to NO₂, leaving sodium chloride in solution. It is typically used in the first stage in conjunction with a second stage caustic scrubber. Since this also produces a salt (NaCl), frequent blow-down and bleed-off is required. It adds little to the water effluent steam over the caustic system. This is represented by the following reaction:

First stage: NO + NaClO —› NaCl + NO₂

Second stage: 2 NO₂ + 2 NaOH ? NaNO₂ + NaNO3 + H₂O

Sodium hydrosulfide is also cheap and efficient in aqueous scrubbers. The drawback to this method is the hazardous nature of NaHS, resulting in increased regulatory needs of reporting, recordkeeping, and the costs of neutralizing the spent liquor before disposal due to its high alkalinity. This is represented by the following reaction:

NaHS + 4 NO —› NaHSO4 + 2 N₂

NaHS + 2 NO₂ —› NaHSO4 + N₂

Also, the neutralization reaction produces hydrogen sulfide and mercaptans, causing a “rotten egg” odor problem.
Hydrogen peroxide is gaining favor, having not been considered at first due to its perception of high cost. However, when maintenance, salt removal, and disposal costs of the other additives are added in, this compound is very competitive. Hydrogen peroxide is as effective as caustic, and in some cases better, since its efficiency is higher at increased concentrations of NO₂. Hydrogren peroxide’s reaction with NOx is represented here by:

2 NO + 3 H₂O₂ —› 2 HNO₃ + 2 H₂O

2 NO₂ + H₂O₂ —› 2 HNO₃

Hydrogen peroxide has a few other advantages: One is the formation of nitric acid in the scrubber, which can be recycled back to the process in some applications. Two, hydrogen peroxide produces no salts, eliminating much of the maintenance, downtime and disposal costs associated with caustic. Three, H2O2 is environmentally compatible, and presents minimal handling requirements and wastewater issues at the commonly used 3% concentration. Additionally, H₂O₂ systems can be retrofitted at minimal expense.

Column design for these systems generally uses single-stage, low-column-velocity (<100 fpm), deep-bed depths (15 to 20 ft), and high water rates (30 to 40 gpm/ft² ). Materials of construction will not change for the use of hydrogen peroxide over caustic because the driving factor in corrosion is the nitric acid (HNO₃). A two-column design (if the first column has enough residence time) could be used to produce from the first column, using H₂O₂, a concentrated (30–40%) HNO₃ effluent to be recycled back into process or off-site, and the second column could be used to further reduce any nitric acid vapors or mists using your existing caustic liquor.

Any of these additions to or replacements of caustic will result in removal of NO which, even at permitted limits, appears to be causing the visible plume.