Metal Finishing Industry
Design of a Modern Metal Finishing Facility
Rarely do technical assistance providers have the opportunity to assist a facility in designing or redesigning their shop. However, if this opportunity does arise, some key points should be incorporated in the layout and design of the facility. The following section provides information on designing the overall facility and key items to consider in tank design.
The first step in designing a modern shop is containing chemicals so that the likelihood of spills and property contamination is minimized. Achieving this involves the following general requirements:
Each of these characteristics is described in the following sections.
In the past, metal finishers generally designed their shops with the cleaning line in one area, the acid room in another, and the plating process in a third. Operators carried work dripping wet from one area to another with wastewater consequently tracked around the facility. In designing a facility, work should ideally enter the process line dry, stay on a single wet-processing island, and dry before leaving that island and entering the next process. Each processing line must be independent and self-sufficient with its own cleaning, pickling, plating, and post-treatment tanks (Mooney 1994).
Rinse tanks used in the process islands also should be properly designed. Proper rinse tank design is a basic component of a water conservation program. If the tanks cannot provide adequate rinsing, conservation techniques will be ineffective. Good rinse tank design is based upon the following concepts:
The methods for handling the transfer of liquids among tanks and making up of fresh water varies across facilities. The most primitive method is the use of buckets and hoses. Unless operators are conscientious, this method easily can result in overflows. If tanks are filled to less than adequate levels, the rinse will not be able to clean workpieces. Proper piping and valves to handle these problems are inexpensive and should be considered standard equipment (Hunt 1988).
The best method to transfer bath makeup, either from a rinse tank or other tanks, is to incorporate a small pump that is activated by a "dead-man's" switch. As long as the switch is depressed, transfer occurs. If the operator leaves, the transfer is shut off automatically. The best pumps are magnetically driven and seamless to preclude leaks. These systems are generally available in the range of $200 to $300; the switch is approximately $50. The complete system including piping and wiring should cost approximately $500 (Hunt 1988).
In countercurrent rinsing, rinsewater makeup comes from the final rinse tank. The simplest way to maintain proper water levels for these systems is to have a local float control valve in the first rinse. This valve is similar to a toilet float valve but is designed to control the water level in the rinse tanks with the discharge side of the valve piped to the last rinse. The flow to the final rinse should be no higher than actually required. A small fixed valve or discharge pipe can limit the flow. Facilities also can use an orifice in the line to the valve. Putting a hand valve on the inlet side of the water control valve for shutoff during non-operating hours is a cost- effective solution. Systems to automatically control makeup water are inexpensive to buy and easy to install. The float valve should cost $50 to $100; the entire system costs approximately $300 including connections and labor (Hunt 1988).
The optimum equipment system for transfers among rinse tanks is a weir box and baffle arrangement. In this system, the rinse level is controlled by diverting overflow water into a weir box that contains a baffle system. This box is piped to the bottom of the next rinse tank. The baffle height in each tank should decrease about 1 inch from the preceding tank. When parts are immersed in a tank and the level rises, rinsewater cannot flow into the cleaner rinse tank and flow can enter the box area only. The same result can be achieved by installing baffling between the compartments of rinse tanks. A facility also can use a single baffle overflow, but this method is not normally adequate because the liquid from the surface of one tank simply goes to the surface of the next. A more effective technique is a double baffle. Liquid overflows the first baffle, which controls tank level, then flows down a slot that is created by a second baffle to the bottom of the next tank. Baffles should be placed ½ to 1 inch apart. The second baffle must be higher than the liquid level and should stop short of the bottom of the next tank to allow liquid entrance (Hunt 1988).
An important feature of a well-designed baffle system is that liquid proceeds from the top of one tank to the bottom of the next to avoid stratification of water layers. Also, each successive tank level (going toward the process bath) must be lower by at least the maximum increase in water level that is anticipated when parts are immersed in the tank. In cases where only separate tanks are available, the solution is simple: assuming liner damage is not an issue, the plater can simply drill a hole at the top and bottom on the same side of each tank and install soft-sealed, screw-in bulkhead fittings (Viton seals are available for highly corrosive situations). The final rinse tank normally does not need a bottom fitting. The first tank does not need a top fitting unless it is to control water levels by overflow. Once holes are drilled and bulkhead fittings installed, install pipes from the top of one rinse to the bottom of the next, using in most cases 1½- or 2-inch fittings unless the counterflow is small (Hunt 1988).
Traditionally, plating shops often had trenches into which wastewater and waste solutions flow. This setup allowed toxic materials to seep into floor coatings and then leach into the ground. Most floor coatings go unmaintained for years or decades and, because the coating is on the floor with no observable dry space, operators are not able to detect leaks before damage is done. Allowing wastewater to flow into open trenches almost guarantees that the ground below will eventually become contaminated. Conveying waste in enclosed pipes is preferred. The pipes should be elevated off the floor, creating a space between the bottom of the pipe and the floor that can be checked for leaks. While few plating shops use double containment piping, design of any shop should allow enough space in the trenches for double contained piping to be installed in the future (Mooney 1994).
Valves and piping occasionally fail and tanks get overfilled. For this reason, as well as compliance with local sewer codes and OSHA safety rules, secondary containment is becoming increasingly necessary. Some platers believe that a curbed and coated floor is a good secondary containment device, however, plastic containment pans are often a better approach for two reasons. First, a leak in a containment pan elevated off the floor is visible and can be corrected before contamination of the ground occurs while a breech in a floor coating is almost undetectable. Second, employees must understand that the floor must be dry. Installing a floor that is designed to be wet appears to encourage operator sloppiness. Alarm horns also should be installed to detect liquids in floor sumps or containment pans (Mooney 1994).
The use of the right materials is key to a modern shop. Steel tanks and catwalks can be essentially unmaintainable. Extensive rust and corrosion can be costly, condemn housekeeping programs to failure, and demand impossibly high maintenance levels. Even worse than steel is aluminum and aluminized or galvanized materials because they corrode. In some environments, these types of products have a relatively impervious skin, but in a humid and acidic plating shop, aluminum and zinc accelerate corrosion. Steel bolts, nuts, and washers should not be used in a plating tank. Where a facility cannot easily use plastic, stainless steel hardware should be a minimum requirement.
More appropriate for use in a finishing facility is polyvinylchloride (PVC), polypropylene, and proper grades of fiberglass. These materials will last indefinitely in most plating environments without significant maintenance. Metal electrical conduits, pipes, pipe support clamps, and patent channels can be impossible to maintain in a plating environment. When made of plastic, these items can be installed and forgotten and, with occasional cleaning, will perform like new for many years.
Depending on the process chemistry, nickel plating the anode and cathode rods and saddles is usually advantageous and results in improved appearance, corrosion resistance, and durability over copper. The copper in bus runs can be lacquered to prevent corrosion and to enhance cleanability (Mooney 1994).
Trying to save energy at the cost of inadequate lighting is inefficient in a metal finishing facility. The energy requirements for lighting a plating shop are minimal compared to rectification and tank heating. Inadequate lighting often can result in increased reject rates. Appropriate lighting actually saves energy. In many areas, electric utilities help pay for energy-efficient lighting fixtures (Mooney 1994).
High-pressure sodium is adequate for outdoor lighting at night, but for most operations, lighting should be white, not orange. Fluorescent lighting is adequate, but metal halide lights are even better. These lights can be mounted in the rafters so that their exposure to corrosive tank fumes is lessened, reducing the need for constant cleaning and maintenance. Another option facilities should consider is natural lighting (Mooney 1994).
Bright paint in varied colors can imply a sense of organization and purpose. Acid tanks could be painted bright red while acid rinses could be painted bright pink. Electrical panels should be blue enamel, moving parts OSHA orange, and aisles yellow. Ceilings and walls probably should be brilliant white not only for a clean appearance, but also for reflectivity (Mooney 1994).
A plating shop that is spotless will have far less trouble with the regulators and the public.
While enforcing a program of regular cleaning is important, the shop also should be designed so that cleaning is easy and practical:
Hunt, Gary. 1988. Proper Rinse Tank Design. Raleigh, NC: North Carolina Office of Waste Reduction.
Mooney, Ted E. 1994. Plating Shops for the '90s and Beyond. Plating and Surface Finishing. January. pp. 35-37.