Metal Finishing Guide Book


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quick test of a representative sample with filter paper in a funnel will determine the nature and amount of solids present. This test will also indicate the most suitable filter medium. Bagging of soluble anodes will materially reduce the amount of sludge entering the plating bath. Airborne dirt from ceiling blowers, motor fans, hoists, or nearby polishing or buffing operations may fall into the plating tank and cause defective plating. Good housekeeping and maintenance will, of course, reduce dirt load and contamination of the plating solution. Prevention of deposit roughness is perhaps the foremost reason for filtering plating solutions. Better covering power with less chance of burning is also achieved with a clean bath. In addition to suspended solids, the plater also has to contend with organic and inorganic (metallic) impurities, which are introduced into the solution primarily by drag-in. If this contamination is allowed to build up, it will affect deposit appearance. Continuous or periodic purification of the solution with activated carbon and/or low-current-density electrolysis (dummying) will often remove these impurities before a shutdown of the plating line becomes necessary. The trend of Environmental Protection Agency (EPA) regulations is to severely restrict the amount of suspended solids and dissolved metal impurities in wastewater discharged to sewers and streams. To comply, plating plants have had to resort to some chemical treatment of their effluents to precipitate the metals as hydroxides. The filtration of these hydrated sludges is difficult and requires special separation equipment. Closed-loop systems, recycling, and recovery are being employed and require greater attention to filtration and purification. Most filtration systems consist of a filter chamber containing the filter media and a motor-driven pump to transfer or circulate the solution from the plating 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 solution, 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 resistance; 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 organic and inorganic impurities. 2. Flow rate���turnovers per hour for a given volume of solution necessary to maintain clarity. 3. Frequency of filtration and purification���batch, intermittent, or continuous 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 676

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