Electroplating is a well-known process used, for example, to nickel plate workpieces such as automotive parts that are required to have a "bright" finish. In a typical example of this particular application of the process, the workpiece passes successively through one or more nickel plating baths and associated rinse baths. In each case, the bath contains a nickel salt solution and a nickel anode. The workpiece acts as the cathode and is electroplated by connecting a source of direct electric current between the cathode and anode. In one particular plating system, the workpiece receives a thin nickel coating in a first bath (often called the "nickel strike"). Additional nickel is deposited on the workpiece in the second and third baths which are referred to respectively as the "semi-bright" and "bright" baths. The nickel salt solutions in these baths contain organic "brightners".
In a commercial nickel plating operation, plating is carried out on a "batch" basis. For example, a batch of automotive parts such as bumpers may be carried by a single rack by which they are transported from bath to bath. The bumpers are electrically connected together to effectively form a single cathode and are all electroplated at the same time. As the workpieces leave the plating baths a thin film of plating solution remains on the surface. This is referred to as "dragout". Some dragout occurs no matter what the shape of the workpiece, but the amount can be substantial with workpieces of unusual shapes. Liquid transfer into a bath in this way is referred to as "dragin".
Dragout from metal plating baths represents a significant cost in terms of the value of the lost metal as well as the cost of treatment of the water used to rinse the workpieces after they have been plated. Probably even more significant is the cost and difficulty of disposal of hazardous metal hydroxide waste sludge that conventional waste treatment systems produce.
Despite the depletion effect of dragout from the plating baths, in some cases, it is unnecessary to add metal salt to the plating bath because the metal concentration in the bath tends to increase naturally due to poor cathode efficiency. In extreme cases, it may even be necessary to periodically decant some of the plating solution and replace it with water to prevent a build-up in the metal concentration in the plating bath.