For a number of years membrane separation processes have been very effectively utilized as essential components of electrodeposition (ED) paint processes. As the two technologies grow and mature, applications for membranes related to ED paint continue to be developed. A case history is presented emphasizing the technological and economic considerations inherent to the practical development of a new application. Reverse osmosis was utilized to treat the UF permeate generated by an ED paint process, resolving a waste disposal problem while generating significant economic benefit through reuse of recovered materials.
Crossflow membrane systems are proven unit operations used in Electrodeposition (ED) paint processes. The systems provide effective means of separating fluid constituents and are essential components of the total process. The two primary categories of membrane systems used in the ED paint operations are Ultrafiltration (UF) and Reverse Osmosis (RO). Both categories operate in a crossflow mode to prevent blinding of the membrane surface, which would occur very rapidly if operated in a perpendicular or normal filtration mode. The basics of crossflow membrane technology, including system components and configurations, have been presented in other works (1) and this paper will only briefly touch on them.
Ultrafiltration effects separation primarily by size of the component molecules. Organic molecules of a given molecular weight or larger are retained and concentrated while water, ions, and smaller organic molecules permeate the membrane. The molecular weight of the smallest constituents, which are retained by the membrane, is specified as the molecular weight cut-off (MWCO) of the membrane.
Ultrafiltration concentrates components in the 0.002 to 0.2 micrometer range, which may be more usefully described using an MWCO range of 1000 to 300,000. These membranes are effectively used to treat the ED paint baths, retaining the paint solids while permeating dissolved ionic contaminants, water and solvents. This application of UF has been employed for many years and is essential for maintaining the paint bath integrity and minimizing paint solids loss. In fact, electrodeposition of paint would probably not be economical today without the use of UF. (2)
Reverse osmosis uses more complex mechanisms to effect separation both at the micromolecular and ionic size ranges. Compared to UF, the RO membrane has a significantly smaller nominal pore size and therefore a much lower MWCO range. In addition to retaining most dissolved ions, high rejection RO membranes can reject and concentrate solutes down to a molecular weight of 150 and lower. This ability to retain ionic contaminants facilitates the use of these membranes for processing permeate generated by ultrafiltration of the ED paint bath.
This paper investigates the recent development of a new application of RO to treat UF permeate and resolve a waste discharge concern. The case history presents the economic and environmental benefits while defining the engineering and design considerations involved.