Gold

Of all the precious metals, gold is the softest, has the lowest tensile strength, and is tied with silver for the greatest ductility. Gold is one of only three colored metals. It combines easily with other metals to create alloys and thus can vary widely in strength and color. However, in contrast to silver, gold is extremely resistant to oxidation and most acids. It can only be chemically dissolved in a cyanide solution or strong oxidizing agent such as aqua regia. Gold can be easily cast, thanks to its low melting point for a precious metal. Its excellent ductility and workability, combined with its color variations in alloys, have predestined gold to play a key role in the production of jewelry and artwork. Its high electrical conductivity—topped only by silver and copper—in combination with impressive chemical stability are ideal prerequisites for utilizing gold in high-performance applications for the electronics industry. Gold primarily occurs with a valence of +3 in compounds, but occasionally +1 as well.

The gold that was mined in prehistoric times is found mostly pure or in sulfidic ores, where it is often enclosed in quartz veins. Top producers include South Africa and China at approx. 10% each of annual global production, followed by Australia and the USA. The simplest way to separate pure gold is “panning” with gravity separation, an effective method because of its high density. Another common method is cyaniding pulverized gold ore to dissolve it to produce complex gold cyanide. The gold cyanide solution is then reduced to raw gold and refined with electrolysis down to pure gold. Gold is also recovered from anode slime as a byproduct of copper and silver electrolysis refining, and in small quantities from platinum ore refining as well. The jewelry and dental industries are other important secondary sources. At Heraeus, gold is refined on a scale of several hundred tons in Hong Kong, Hanau and at Argor-Heraeus in Switzerland.

The largest portion of available gold (approx. 60%) is fabricated into jewelry and pieces of art, where experts use it in a diverse array of alloys to give it the desired color and workability qualities. Another 35% can be found in the form of gold bars and coins in monetary institutions, or in the hands of private owners as an investment. Gold is particularly well suited to applications in the electronics industry due to its impressive electrical properties and resistance to corrosion. An exceptionally ductile material, it can be drawn into the finest bonding wires for use in high-quality electronic components for the semiconductor industry. Gold pastes for contact surfaces and solder play a key role in circuit board technology, and gold-plated switching contacts and plug connections have become the standard in the electronics industry. Its extraordinary color and variable luster has given this precious metal a broad spectrum of decorative uses in the ceramic and glass industries. As a target material, gold is used to coat glass and to manufacture heat reflectors. And finally, gold’s aesthetics, malleability and biocompatibility have also helped it gain entry into the medical product industry, particularly as dental filling or implant material.