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Pollution Prevention and Control Technologies for Plating Operations


Section 4 - Chemical Solution Maintenance

4.3 MICROFILTRATON

4.3.3 Applications and Restrictions

The application of microfiltration to an aqueous degreasing operation is shown in Exhibit 4-3, which is based on a case study presented by Schwering, Golisch and Kemp (ref. 311). This application was a retrofit to an existing metal cleaning line. Prior to conversion (see top diagram), the degreasing bath was discarded after 80 hours of operation when its oil content reached 8 g/l. The bath was remade using the solution in the drag-out tank as make-up. The conversion (see lower diagram) consisted of dividing the degreaser tank into two sections, changing the rinsing configuration and adding a microfiltration unit to maintain the solution in the first degreaser section. The concentration of cleaner in the first section (primary degreaser) was increased to 50 g/l and the temperature was raised to 185°F. The concentration of cleaner in the second section (secondary degreaser) was set at 20 g/l. With the installation of the microfiltration unit, a constant oil concentration of 1.0 g/l was maintained in the primary degreaser and 0.2 g/l oil in the secondary. The final rinse contained 10 mg/l oil when operated with a 3.2 gpm feed rate (67 lbs/yr oil). In addition to the rinse water discharge, 308 gpy of oil and contaminants (30% emulsion) were discarded off-site.

Microfiltration is not applicable to all aqueous degreasing and cleaning applications. Usually, shops implementing this technology must consider changing to an alternate cleaner. The most applicable type of cleaner is a simply structured non-silicate cleaner that operates in the higher temperature range (160° to 175°F or 70° to 80°C) and that can easily be replenished with additives (ref. 311). As suggested by Schwering, Golisch and Kemp, cleaners that are particularly well suited for recycling are ones that emulsify oils at the high temperature range and then have the ability to release these emulsified oils at lower temperatures and during plant shutdown (ref. 311).

Cleaning formulations with a high silicate content are generally less amenable to microfiltration recycle. These cleaners contain colloidal silicic acid, which has a tendency to plug the pores of the ceramic membrane (ref. 311). However, the only respondent to the Users Survey that employs this technology operates a bath formulated with sodium metasilicate.

Microfiltration is not applicable to aluminum cleaning solutions since the dissolved aluminum concentration will buildup over time because it will be unaffected by the filtration process.

An application of microfiltration to semi-aqueous cleaning is diagrammed in Exhibit 4-4. The semi-aqueous (SA) solvent tank contains 100% solvent. In this tank, soils are dislodged and/or dissolved in the solvent. Agitation (e.g., pumping) and heat can be applied to minimize the cleaning time. Following the cleaning step, the parts are transferred to the emulsion tank. Air knives are sometimes employed to minimize the carry-over of solvent. The emulsion tank contains a relatively high concentration of semi-aqueous cleaning agent. The concentration is maintained at a relatively constant level by employing a decanter/coalescer to separate the solvent and water. The solvent is returned to the solvent tank (except for periodic blowdown) and the water is returned to the emulsion rinse. A low solvent concentration in the emulsion rinse is desired in order to minimize the carry-over of solvent to the water rinses. However, the lowest practical emulsion concentration is the minimum concentration that will separate in the decanter (e.g., the minimum concentration for DuPontís Axarel® is about 2 percent) (ref. 515). Ultrasonics and mechanical agitation (pumping) are often used in the emulsion tank to improve the efficiency of rinsing. Subsequently, parts are transferred to water rinses (usually two rinses connected in a counterflow configuration). Again, air knives are often used to minimize the carry-over of drag-out. The water rinses remove the vast majority of the remaining solvent from the parts. Due mainly to the presence of the organics in the rinse water, firms sometimes separate the solvent from the rinse water using microfiltration, recover the solvent and recycle the water. Competing technologies for this application include carbon treatment and ion exchange.


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