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Pollution Prevention and Control Technologies for Plating Operations
Section 2 - General Waste Reduction Practices
2.4 DRAG-OUT REDUCTION
2.4.1 Drag-Out Reduction Principles
Electroplaters are well aware that drag-out varies considerably among the various parts plated at their shops. For example, the volume of drag-out in rack plating differs visibly from that in barrel plating. When a barrel emerges from a process tank, it usually carries with it over 10 times more solution than does a typical rack. In addition to the obvious effects of rack and barrel design and shape of parts, there are more subtle factors that affect the volume of drag-out. These parameters include viscosity and chemical concentration, surface tension, and temperature (ref. 1).
The viscosity of a plating process solution can be described as its resistance to flow or removal by another liquid (in this case, rinse water), caused by molecular attraction forces. The difference between high and low viscosity can be demonstrated with honey and water. A much thicker film will form on a knife dipped in honey than on one dipped in water. Honey, therefore, has the higher viscosity because of its cohesive and adhesive qualities. The same effect can be observed with plating solutions. If two identical surfaces are immersed in separate chromium baths with concentrations of 53 ounces per gallon (397 g/L) and 33 ounces per gallon (247 g/L), respectively, the lower concentration bath will produce 73 percent less volume of drag-out (ref. 1).
Surface tension is another physical phenomenon that has a significant effect in the plating shop. According to kinetic theory, molecules of a liquid attract each other. At the surface of a solution, such as a plating bath, the molecules are subjected to an unbalanced force because the molecules in the gaseous phase are so widely dispersed. As a result, the molecules at the surface are under tension and form a thin, skin-like layer that adjusts to create a minimum surface area. The property of surface tension causes liquid droplets to assume a spherical shape.
In the plating process, the volume of solution that clings to a workpiece surface depends partly on surface tension. The force of surface tension appears to be most significant at the bottom edge of the part as it passes through and leaves the process solution. This force and the resultant volume of drag-out appear to be greatly affected by the orientation of the part relative to the surface of the liquid (ref. 1).
The third factor that influences drag-out volume is the temperature of the process solution. Temperature is interrelated with viscosity and surface tension. As the temperature of a plating solution is increased, its viscosity, surface tension, and, therefore, drag-out volume are reduced. A possible exception is when a part is withdrawn too rapidly from a hot process solution, evaporation concentrates the film and impedes drainage. This problem, however, can be overcome by reducing withdrawal time and using a fog spray rinse on the parts as they emerge from the plating solution (ref. 306).
Estimations of drag-out have been presented in the literature (see Exhibit 2-9). However, these values should only be used for gross estimates. Actual measurements of drag-out are needed for accurate evaluations of pollution prevention options. Techniques for measuring drag-out are presented in various references (ref. 1, 20, 305). A simple and effective method for measuring drag-out is presented in Section 2.5.3.1.