Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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THE AQUEOUS PROCESS Fig. 5. Five-tank vertical agitation and rotation system with material handling and closed-loop wastewater treatment system. System utilizes a wash,ultrasonic wash, three counterflow rinses, and rotation of baskets in a recirculation hot-air dryer. Most aqueous processes require, as a minimum, a wash, a rinse, and a dry. There may be multiple wash and rinse stages, depending upon the desired cleanliness level. Typically, wash requires a mild alka- line cleaner that is heated. Care should be taken in the selection of the cleaner to ensure that it is compatible with alloys being cleaned and has the ability to emulsify or split oils and remove oth- er contaminants. The purpose of rinses is to remove residual chemistry remaining on parts. If only one rinse is employed, a flow of clean water into the rinse tank on a con- tinual basis to an overflow dam is necessary to maintain rinse clarity. A system employing two rinses should consume half the water that a single-rinse system will require. Likewise, three rinses will utilize half the water that a two-rinse system will use to maintain final rinse tank clarity. Depending upon cleanliness specification and whether spot-free drying is necessary, RO or DI waters may be necessary for final rinses. If RO or DI water is employed consideration should be made to prevent flash rusting on ferrous components. It may be necessary to add rust inhibitors or wet- ting agents to rinse tanks to prevent flash rusting while rinsing ferrous components. WATER QUALITY If incoming municipal waters contain a high level of chlorine, sulfur, salts, or other contaminants it may be necessary to treat this water prior to use. Water hardness and poor quality of water can affect the ability of the aqueous chemistries to work. Likewise, poor water quality can affect drying and cause water spotting. SYSTEM CONSTRUCTION All aqueous cleaning systems should be constructed with stainless steel for tanks and wetted surfaces. Whereas mild steel con- struction was acceptable in the past the greatest cost factor in manufacturing an aqueous system is labor. Stainless steel, although three times the cost of mild steel, should have a cost impact of less than 20% on the overall system. Mild steel construc- tion will always oxidize in time and cause degradation of baths, compromise the abil- ity to clean, and cause rinsewater to always contain ferrous oxides. Stainless steel con- struction overcomes these shortcomings and should give an aqueous system a min- imal 20-year life. 108 Fig. 6. Vertical agitation and rotation system with access doors open. System agitates baskets 24 in. X 24 in. X 12 in. through a prewash, wash, and two counterflow rinses prior to dry.

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