Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 80 of 707

Polypropylene or lined tanks are recommended for alkaline electrocleaners fit- ted with steel, stainless steel, or Teflon heaters. Recirculating pumps are recom- mended to prevent stratification and ensure overall homogeneity. It should be not- ed that solution inlet and outlet must be located at two opposite diagonal top and bottom corners of the tank for efficient solution movement. Cleaner filtration is gaining in popularity with the aim of prolonging the bath life between discarding and bath replacement intervals. Several filtration tech- niques have been proposed, ranging from simple bag filtration to complete systems of oil skimmers, coalescers, and ultrafiltration. Since the cost of such systems varies appreciably, a feasibility study must be undertaken before adopting a par- ticular system. In general, however, it has been reported that any type of filtration does increase the bath life at least by 20% and up to five times or more. LIQUID CLEANERS The use of liquid cleaners to replace powder versions has gained momentum and wide acceptance in the industry. These new cleaners are formulated to economically provide all the performance criteria of the powders. The advantages of liquid cleaners include the capability of automatic feeding tied to conductivity con- trollers. The automated system continually monitors the solution strength and makes additions on demand. Consequently, better bath control is achieved, elim- inating wide swings in concentrations. Automatic recording capabilities of con- centration and temperature can be achieved for statistical process control. Tank additions of liquid concentrate eliminate the hazards associated with addi- tions of alkaline powders to hot cleaner solutions. As a result of better controls, these liquid systems have substantially increased bath life in many installations. Another advantage confirmed by users of liquid cleaners is sludge reduction on waste treatment by 70–80%, which adds to the economical advantage of these systems. ELECTROCLEANING OF STRIP AND WIRE COILS Strip and wire are continuously fed through the line. There is no direct contact with anodes or cathodes in the electrocleaner tank. A bipolar electrical effect is used to provide the polarity as follows. Two separate steel grids are positioned several feet apart in the direction of work flow. The first grid is anodically connected, the second, cathodically. The strip travels between two closely spaced jaws of each grid without touching them. As a result of the bipolar effect, the strip assumes the oppo- site charge of the grid. It becomes cathodic through the first grid, then anodic through the second, achieving the desired electrocleaning effect. 79

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