A new dense-pack ceramic membrane cross-flow filtration technology can provide an innovative way to efficiently maintain pretreatment baths as well as electrocoat paint baths, according to developer The Hilliard Corp. (Elmira, NY).
Hilliard’s patented ceramic filter membranes remove oils, greases, and other soils, allowing continuous filtration of pretreatment baths. Hilliard says continuous filtration enables consistent bath quality; extends bath life by reducing new chemical purchases; reduces disposal, energy, and labor costs; and results in higher finish quality.
The ceramic membrane technology can operate at temperatures to 200ºF and does not remove measurable amounts of surfactants and other bath additives, according to Hilliard. It’s also said to be less susceptible to fouling than other membrane filters, and can be aggressively cleaned to recover a higher percentage of permeate after fouling.
The system’s ceramic membrane support structure consists of mullite, a mechanically, chemically, and thermally stable ceramic. The filter element contains 1,800 flow channels within its 5.66-inch diameter × 34-inch-long body. A majority of the channels perform the membrane separation, producing the filtrate (permeate) while others are subsequently used to efficiently collect the filtrate generated. This approach allows the entire filter diameter to be utilized and results in higher process flux, according to Hilliard.
The system operates in a cross-flow mode, where the influent stream flows across the membrane surface. This results in two outlet streams, permeate (filtrate) and concentrate (retentate). Differential pressure from the inlet to the outlet of the filter keeps things moving, according to Hilliard. This mode of operation is in contrast to bag or cartridge filters—what
Hilliard calls “dead-end” filtration systems—where there is only one outlet stream, the permeate.
Hilliard says the ceramic elements have excellent chemical resistance over a pH range of 2–13, can be aggressively cleaned and have a low coefficient of thermal expansion, resulting in stable membrane pore size. They can be subjected to backpulsing, are tolerant of pressure excursions, function with high solids loadings, and can be steam sterilized, the company adds.
In one test on an alkaline cleaner bath used as part of an E-coat pretreatment process, ceramic membrane filters effectively removed free and emulsified oil, attaining <100 ppm levels of oil in the permeate. Hilliard says bath life can be extended to more than four times that of a bath with a filtration system using an oil skimmer, oil coalescer and bag filters and, more recently, polymeric membrane crossflow ultrafiltration. Improved tank life leads to lower maintenance and disposal costs, less paperwork, and lower waste volumes as water and cleaning chemicals are recovered for reuse.
Filtration is also a key to controlling E-coat paint bath conductivity and soluble contaminants, and producing permeate for the subsequent rinse stage to rinse and recover drag-out paint. Hilliard says ceramic filter elements used in paint bath applications can be cleaned using aggressive solvents and temperatures to provide better, faster cleaning. The result is less cleaning downtime throughout the year, thereby reducing labor costs and perhaps cleaning chemical volume.
Ceramic filter elements are available in two sizes, the previously mentioned 5.66-inch diameter × 34-inch-long (surface area 120 ft2) or 1.05-inch × 12-inch (surface area 1.5 ft2. Users can select four membrane pore sizes—0.005, 0.01, 0.2 and 0.5 μm.