Gold plated pins
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Gold plated pins are found in electronics. The gold is used to avoid oxidation and create a good electrical contact between two contact elements. Most gold plated pins are only plated thick enough to give a good surface and that is achieved with 0.7 micro meters thickness. Modern contacts are often only plated on the contact surface to minimize the amount of gold used. Cheaper stuff can have even thinner plating. Thicker plating is often used on military or medical grade pins.
The plating is so thin that ordinary gold testing procedures are more or less worthless. XRF is measuring down through the plating and sees the base metals below as well as the gold surface and the touch stone shaves straight through the plating and collects base metals too. The plating is actually between 98.5% and 99.5% pure gold with some cobalt added to give a hard surface resistant to wear.
To stop migration of gold into copper, most plated pins have a layer of nickel plate between the base metal and the gold plating.
Most pins contains only a few grams per kilo of pins. Partially plated even less.
Calculating the gold content
If the thickness of the plating is known the volume and the mass of the plating can be calculated easily from the dimensions of the pin.
As there is so much base metals compared to gold in plated pins it is only economically to dissolve the base metals if a refiner have access to cheap acids. Most economically is to dissolve the gold plating without affecting the base metal.
Methods to dissolve gold plating off the base metal.
- organic solvents with gold ligands (i.e. TBP and I2, SOCl2/pyridine, NMP/HNO3/quat ammonium halide)
- Reverse plating cell
Of these only the reverse plating cell is in common usage among home refiners and among larger refineries cyanide stripping is common.
The alternative to stripping is to dissolve the base metals to create a concentrate of gold foils for further processing. Here the base metal come in play.
- Nitric acid, good for copper based pins but creates metastannic acid from alloys with tin
- Sulfuric acid, works on iron based pins as kovar legs on IC:s. (Needs verification!)
- Hydrochloric acid, works on iron based pins as kovar legs on IC:s.
- Copper chloride process, works on both copper based and iron based pins as long as there isn't too much iron.
The third alternative often used by medium sized refineries is to melt it all into a refiners bar, sample and send out to a copper refinery to be paid for both copper and precious metal content.