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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.3 MICROFILTRATON
4.3.1 Overview
Microfiltration is a relatively new bath maintenance technology that is applied to aqueous and semi-aqueous degreasing and cleaning baths for the removal of oil and grease. Only one respondent to the Users Survey (or 0.3%) reported the utilization of microfiltration for this application (end-of-pipe applications of microfiltration and ultrafiltration are discussed in Section 6). Respondents to the Vendors Survey indicate that hundreds of degreasing and cleaning bath applications of microfiltration have been installed. The discrepancy between the Users Survey and the Vendors Survey with regard to the use of this technology is most likely caused by the fact that most respondents to the Users Survey are electroplating job shops. According to the vendors, most microfiltration/degreaser applications are found in captive electroplating shops and non-plating facilities such as metal-working and painting shops. Also, this technology is finding use in the printed circuit board industry for recovering semi-aqueous cleaning agents from rinse water (e.g., as part of the defluxing process) (ref. 515).
In part, electroplating job shops use solvent cleaning less frequently than captive shops because many of them receive parts for plating that have been pre-cleaned and in these cases do not require as much heavy duty cleaning or degreasing. Also, the majority of the job shop respondents that used solvents at one time eliminated their use by substituting aqueous cleaners before the time when the microfiltration technology (as applied to aqueous degreasing baths) was commercially available to them (circa 1988). Most of these shops have implemented a scheme that is technically and economically satisfactory, a condition that does not promote change. Generally, the alternative approach to maintenance is to operate pre-cleaning and cleaning baths to exhaustion and either treat them on-site or haul the spent baths to off-site treatment/disposal. Also, some shops use simple oil skimming and particulate filtration to extend bath life. Conversely, a large percentage of captive shops, painting and metal-working facilities, in response to regulations, have only recently moved to eliminate chlorinated solvent use. The pre-cleaning requirements for many of these plants are more demanding and the substitution of aqueous cleaners has therefore been slower and has coincided with the advancement of microfiltration technology. Additionally, the captive facilities generally have more capital available for the purchase of equipment.
Aqueous degreasing solutions are essentially a mixture of surfactants, alkali salts, caustic soda, phosphates, silicates and complexing agents. Aqueous degreasing and cleaning baths buildup concentrations of oil, grease and soils during use. Free oils can be removed by simple skimming and most solids can be removed by settling and/or cartridge filtration. However, emulsified oils and colloidal solids are not affected by these devices. At some point, the cleaning efficiency of the bath is impaired and the solution is discarded, despite the fact that most of the bathís constituents are still usable. In many cases, heavy duty cleaners must be replaced once per week. The microfiltration technology separates the emulsified oils from the aqueous cleaning solution, thereby extending the life of the bath. This technology is also applicable to the recovery of cleaning solution drag-out from rinse waters. However, it is used much less frequently for this purpose.
In the semi-aqueous process, parts are cleaned in an organic solvent (e.g., Dupontís Axarel®, or various terpene cleaners) and subsequently rinsed in an emulsion rinse (rinse water and solvent), followed by a water rinse(s). The bulk of the cleaning solvent can be separated from the emulsion rinse by decantation and returned to the solvent cleaning tank. However, some of the solvent is carried into the water rinse. The semi-aqueous process can be operated in a near closed-loop fashion by separating the solvent from the rinse water using microfiltration, returning the solvent to the process tank and recirculating the rinse water. Although this application of microfiltration is better categorized as recovery/recycle than bath maintenance, it is discussed in this section because the equipment used for this application is similar to that used for aqueous cleaner bath maintenance.
Most commercial microfiltration systems used for this application employ ceramic filter membranes in a crossflow filtration configuration. These membranes are a new development that permits application of microfiltration to solutions and emulsions that are both heated and corrosive. Earlier efforts using polymeric membranes were unsuccessful with this application (ref. 311). The polymeric membranes deteriorate due to the high temperatures encountered and the corrosive nature of the cleaning solutions. Also, the polymeric membranes cannot be cleaned on-line with an air back-pulse, like the ceramic membranes can. The ceramic membranes are produced in a range of pore sizes that selectively permit a large percentage of the surfactants to pass through the membrane (a typical pore size is 0.8 µ and most filters have pore sizes greater than 0.2 µ). Crossflow filtration, as opposed to barrier or ìdead-endî filtration, permits the application of this technology to high solids-feed streams. As shown in Exhibit 4-2, in dead-end filtration, all of the feed solution is forced through the membrane by an applied pressure. With a high solids-feed stream, the pores of a dead-end filtration device plug. With crossflow filtration, the fluid to be filtered is pumped across the membrane, parallel to its surface. By maintaining a high velocity across the membrane, the retained material is swept off the membrane surface. This mode of operation typically requires multiple passes and consumes a greater amount of energy than with dead-end filtration. However, for high solids applications, crossflow is the only practical method (ref. 380).