Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 597 of 707

Fig. 1. Higher clarification is achieved by, increasing the number of tank turnovers per hour. tank through the filter. The many filters and pumps on the market today make it possible to select and justify a cost-effective filter system for each and every solu- tion, regardless of volume. When engineering a filter system for a plating installation, it is necessary to first establish the main objectives, such as: high quality finish—maximum smoothness and brightness; optimum physical properties—grain size, corrosion, and wear resis- tance; or maximum process efficiency and control—covering power, plating rate, purification, and clarification. Then the following factors must be considered before selecting the size and materials needed for the filter media, chamber, pump, and motor: 1. Dirt load—suspended solids, size, kind, and amount; also soluble organ- ic and inorganic impurities. 2. Flow rate—turnovers per hour for a given volume of solution neces- sary to maintain clarity. 3.Frequency of filtration and purification—batch, intermittent, or con- tinuous required to remove dirt and contamination and filter servicing interval desired. When agitating solutions with air, a low-pressure blower is usually employed. This makes it virtually impossible to achieve good filtration of the air while keeping the solution clean, because the plating solution then acts like a fume scrubber. If effluent regulations make it necessary to remove or reduce total suspended solids (TSS) from wastewater, the amount discharged per hour or shift can be readily determined. For instance, a 100 gal/min (gpm) effluent containing 100 ppm TSS (100 mg/L) will generate 5 lb of solids per hour, as calculated below: 100 gpm 3.79 L/gal 100 mg/L 60 min/hr (1000 mg/g 454 g/lb) = 5 lb/hr (2.3 kg/hr) Therefore, the filter must have sufficient capacity to hold approximately 40 lb of solids/8 hr of operation. A horizontal gravity filter would be the most cost effi- 596

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