Metal Finishing Guide Book


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will usually build up in the bath, with some of it splitting into a floating layer where it will be removed by the skimmer. Most of the aqueous and semiaqueous bath formulations contain an inhibitor to provide rust protection for steel parts. Surfactants displace oil from the parts to be cleaned and form a stable emulsion. The life of the bath is dependent upon how much soil is brought in with the parts and how much drag-out occurs as the parts are moved from the cleaning bath into the rinse tank. For many installations in surface-finishing operations continuous micro- and ultrafiltration systems using inorganic or organic membranes are successfully used to remove oils, grease, lubricants, soils, and solids from alkaline cleaners and can give the bath essentially indefinite life. An additional benefit is the steady-state condition of the cleaner, which will improve control over the process and the quality of the product being manufactured. The selection of the membranes is not only important regarding the operating temperature of the bath but also for the pore size or macromolecular structure. Elevated temperature can deteriorate organic-based membranes and too small a pore size can cause the rejection of valuable chemicals such as surfactants or inhibitors. Phosphating Baths Precipitates are formed continuously in phosphating operations presenting maintenance headaches and often resulting in the solution being discarded. Usually, the precipitates accumulate in the process tank, primarily on the heating coils. When the solution is removed from the tank this accumulation of sludge can be manually removed. The solution should be decanted back into the tank to minimize wastage but this consumes space and time so the solution is often discarded and replaced. It is far more efficient to install a continuous recirculation system through a clarifier with gentle agitation in the sludge blanket zone. This allows the solution to be used indefinitely, reduces the labor for manual clean-out of sludge, and allows a dewatered sludge to be easily removed from the bottom of the clarifier. Chromating Solutions Both ion exchange and electrochemical methods have been demonstrated to be effective for regeneration of spent chromates; however, in almost all cases, the metal finisher relies upon the proprietary chemical supplier to be responsible for the appropriate balance in the chromating bath. Either of these regenerating technologies makes the metal finisher responsible for the overall chemical maintenance of all constituents in the bath. It is possible that proprietary suppliers will provide a service to assist the finisher in maintaining a proper balance when one of the applicable techniques is applied. Economics are not likely to be attractive except in the case of high production operations using the more concentrated chromates, which give high salt spray resistance against "white rust." RECOVERY AND RECYCLING OF PRETREATED WASTEWATER Conventional techniques for water conservation (countercurrent rinsing, conductivity controls, etc.) are used extensively in the industry; however, the unavoidable end product of all waste treatment methodologies is a "salt" containing effluent, or brine. Effluent TDS from such a system can be sufficiently high to limit potential for recycle and reuse as process water without desalination. Clearly, achieving the minimum consumption and discharge of water necessitates segregated handling of concentrated solution dumps since they will carry 617

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