Recycling a nonionic aqueous‐based metal‐cleaning solution with a ceramic membrane: Pilot‐scale evaluation

Abstract

I N T R O D U C T I O NAqueous alkaline cleaning solutions have been commonly used for metal cleaning since the phase-out of o rganic solvent vapor degreasers [1][2][3][4][5][6][7][8]. The cleaning solutions usually are composed of alkaline builders, surfactants, co-solvents, and additives that pro m o t e / enhance detergency and longevity of the solutions. Over time, the cleaning solutions can become contaminated with oil and grease (O&G) and suspended solids, and must be recycled or disposed of as hazardous wastes. Various recycling technologies exist, but few p roduce usable recycled products [7,[9][10][11].We recently reported [12] that a 0.05 µm ZrO2 u l t r afilter (7 mm i.d. x 750 mm-long tubular filter manufact u red by U.S. Filter) effectively removed O&G, including one synthetic ester oil, two polyalphaolefin (PAO)-based oils and greases, and two petroleum hydro c a rbon-based oils and greases, from two alkaline cleaning solutions (i.e., Tu rc o ® 4 2 1 5 -N C LT and Daraclean ® 282) containing nonionic surfactants. The O&G concentrations were reduced from >10,000 mg/L to near or below analytical limits of quantification. The ZrO2 membrane was effectively cleaned after repeated use during more than 13 test runs for a total run time of over 50 hours, as indicated by restoration of its clean water permeability (CWP) (i.e., permeability to 0.2 µm m e m b r a n e -p re f i l t e red deionized water) to gre a t e r than 75% of the initial value. Furtherm o re, the membrane allowed nearly complete passage of all alkaline salts; a co-solvent, diethylene glycol monobutyl ether (DGME); a corrosion inhibitor, 2-merc a p t o b e n z o t h i azole (MBT); and an unknown organic constituent. Note: Mention of trade names or commercial pro ducts does not constitute endorsement or re c o m m e n d ation for use.The membrane, however, also removed almost all nonionic surfactants (i.e., alkylpolyoxyalkylene glycol ether [AGE] and nonylphenolpolyethylene glycol ether [NPE]) [12]. Removal of these surfactants re q u i re s proper replenishment to the recycled solutions, which adds complexity and cost to the full-scale operation of this recycling process. Also, information pertinent to the system integration (e.g., batch vs. on-line) and p e rf o rmance of re c y c l e d / replenished baths for metal