Minimizing the dragout of gold solutions is of both economic and environmental concern. It is an economic advantage to decrease the cost of gold loss, and it is an environmental advantage to reduce the amount of processing needed to purify the waste stream before discharge.

Many factors affect dragout:

• The thickness of the gold plated

• The shape of the part to be plated

• The number of holes or other solution-trapping structures

• The speed of removing the plated part from the plating tank

• Provisions for air jets or wiper blades to return the drippings to the plating tank

In some cases the dragout is from 30 to 50% of the gold actually deposited. Typically, however, it is 10 to 20%. It is far better to limit the dragout than to expend effort in processing the cyanide and recovering the gold from the dragout.

Minimizing the dragout can be done with simple procedures such as training the operator to remove the rack slowly and to "nudge" or shake the withdrawn rack over the gold tank so droplets return to the tank. Barrels should be allowed to drip over the gold tank and should be rotated one-half turn or more before being dipped into the dragout recovery tank. Continuous plating machines should have an air knife or a synthetic sponge to remove excess gold solution.

All gold-plated work should be rinsed in a stagnant gold recovery tank that is treated frequently to recover the dragged-out gold. The gold can be recovered by passing the dragout solution through an appropriate ion exchange resin, or it may be recovered by plating out, in which the dragout is circulated and continuously electroplated on a carbon or wire-mesh cathode. The gold-plated cathode should periodically be sent to a refiner.

Platinum-Group Metals Plating

Ch.J. Raub, Forschungsinstitut für Edelmetalle und Metallchemie

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