35    IMPROVED SCRUBBING OF NOx EMISSIONS
FROM AN ALUMINUM ANODIZING BRITE-DIP
SYSTEM USING ALKALINE PERMANGANATE
Hugh G. Allerton
TRW Vehicle Safety Systems Inc.
Washington, Michigan
Rodney Todd
TRW Vehicle Safety Systems Ltd.
Midland, Ontario
Jerome H. Jacobs and Richard Prober
Metcalf & Eddy, Inc.
Cleveland, Ohio
ABSTRACT
This chapter describes the scrubbing system for control of NOx emissions from aluminum anodizing. Changing from an alkaline sodium sulfide scrubbing liquor to alkaline potassium permanganate, with continuous bleed and replenishment of scrubbing liquor, has eliminated incidents of visible brown plumes or acid mists.
INTRODUCTION
The former Decor Products Division of TRW located in Midland, Ontario (now KLECO) produces extruded aluminum products such as railings for automatic passive track seat belts and picture frames. During the anodizing process, the parts undergo Brite-Dip treatment by sequential
immersion in tanks containing concentrated sulfuric acid, phosphoric acid, and nitric acid. The
Brite-Dip treatment generates fumes containing gaseous oxides of nitrogen (NOx), principally
NO,, as well as nitric acid vapor and entrained droplets of the acid solutions. Exhaust air containing these acidic fumes discharged directly to the atmosphere until about 1980, when a
packed-column wet scrubber was installed.
Figure 1 is a schematic flow diagram of the existing air scrubbing system for the Brite-Dip
process. An exhaust air hood collects and conveys the fumes to the scrubber, an 8-ft diameter
Fiberglas column containing 8 feet of 2-inch Tripack packing. The air flow (14,000 CFM) is
countercurrent to scrubbing liquor (240 US gpm), which is injected through nozzles above the
packing. Scrubbed air discharges into the atmosphere through a 3 foot, 8-inch diameter stack
containing a chevron-type mist eliminator. Scrubbing liquor recirculates from a 1700 US gallon
sump. Spent scrubbing liquor is neutralized in the plant wastewater pretreatment system, which
discharges into the Town of Midland sanitary sewer system. The municipal waste water treatment plant (WWTP) is located within 1 km of the Decor facility.
ORIGINAL SCRUBBING CHEMISTRY
Before 1993, the scrubbing solution was an alkaline liquor containing sodium sulfide. It was
made up in 500 US gallon batches from 50% caustic soda and technical grade sodium sulfide. As
indicated schematically in Figure 2. the principal liquid phase reactions were as follows:
52nd Purdue Industrial Waste Conference Proceedings. 1997, Ann Arbor Press. Chelsea. Michigan 48118. Printed in
U.S.A.
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