How Are Gold Bars Made
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Gold Mining and Extraction
The first step in the process of making gold bars (if gold is being processed form gold ore) is that the ore must be extracted from the ground. After a gold-bearing deposit has been located, the gold is extracted by mining. In old times hydraulic mining was often performed - using high pressure water jets to mine, move and separate gold from deposits. However in modern times, explosives are used. Much of the ore from which gold is mined in modern times is not the "nugget-laden" ore one imagines from the old days of the gold rush, but is low grade ore. A ton of low-grade gold ore may only yield on average 6.5 grams of gold [1] - however the high value of the gold still makes the operation worthwhile.
After the rock containing the gold has been mined, it is crushed to grains the size of sand. Next, gold is extracted from the crushed rock using a multi-stage process. The preliminary processing varies slightly depending on the quality and composition of the ore, but eventually the gold is dissolved from the ore using cyanide, in a process called leaching: A water and cyanide solution is added to the gold, which is then ground to a mud. The mud is agitated and air is bubbled through it, which causes the gold to react with the cyanide and dissolve. Zinc powder is added to the dissolved gold solution and the resulting solids collected.
Next, a flux is added, consisting of fluorite, manganese dioxide, silica, borax and sodium nitrate. At this point the mixture is put in a smelter and heated to 1600°C. After two and a half hours, the gold (being heavier) has settled to the bottom and the "slag" is removed. The molten gold is then poured into moulds and allowed to cool. [1] The resulting ingots, known as gold doré are around 80% to 90% pure gold.
Making Poured Gold Bars
The next stage in the process is that the gold doré is refined to high purity (typically 99.9%, also known as "999 gold", which is an international standard) by a gold refiner. This process typically employed for this purification by major bullion manufacturers is either electrolysis or pyrometallurgical chlorination.
Gold scrap - for example from old jewellery, jewellery manufacturers scrap, high-grade electronic scrap, dental gold scrap and old coins - is also re-refined and recycled using either electrolysis or pyrometallurgical chlorination.
With pyrometallurgical chlorination, (known as the "Miller Process") chlorine gas is bubbled through the molten bullion. Base metals and silver react with the chlorine to form chlorides, which either form a slag on the surface of the melt or are volatilized. When the gold reaches around 99.5 to 99.7 percent purity, purple fumes of gold chloride wil begin to form and the process is halted. [2]
After this, the gold is either treated with electrolysis of a wet chemical process. With electrolysis, (known as the Wohlwill Process) the gold is first cast and the resulting bars used as anodes in an electrolyte consisting of gold chloride / hydrochloric acid. When the electric current is passed, 99.99% pure gold is gradually deposited at the cathode and this is washed, dried and then melted so as to be poured into bars.
With the wet chemical process, which is performed when the gold produced by the Miller process includes Platinum Group Metals, the gold is dissolved using aqua regia (this is a mixture of hydrochloric and nitric acids). Any silver present forms silver chloride, which is insoluble and therefore precipitates. [3] Most platinum group metals, such as titanium, iridium, ruthenium, tantalum, osmium and rhodium are insoluble in aqua regia - however platinum is not. It can however be separated from gold by virtue of the fact that gold has a lower melting point. Gold can also be precipitated by use of iron (II) chloride. Gold ultimately produced by electrolysis.
At the final stage, the gold is melted and poured into molds. The resulting bars are quenched in water and cleaned. A small sample may be taken from the bar for assay purposes.
Making Pressed Gold Bars
The first Pressed ("Minted") gold bars were made in 1952 and the method is now a popular one for the manufacture of the smaller sizes of gold bar. Pressed gold bars are made from the high purity poured gold bars already described, however several further stages are required for their manufacture. Gold is poured into a continuous casting machine, from which long thin cast bars are made. These bars are then passed through a rolling mill and pressed multiple times to approximately the right thickness. During this process, annealing (softening) of the strips may be required.
After this, the strips are passed through a gauging mill, which is a precision machine that will bring the strips to the correct thickness (to within accepted tolerances). Next, blanks are punched from the strips. The blanks are again heated in a furnace in order to soften them and their weight is tested. Any underweight or overweight blanks are recycled, however blanks that are just slightly overweight can be rubbed down.
The blanks are then polished and cleaned, and are then struck in a similar manner to coins - with a die above and below. A collar is placed around the blank and the blank is pressed using very high pressure. The design (which has been created mirror image on the die) is transferred to the blank and the pressed gold bar is finished. The bars are inspected and packaged for shipping. [4]
Gold Purification - Other Methods
Many other methods of purification of gold exist, and may be useful to an individual interested in gold recovery, purification or metallurgy. Here are a few further resources:
http://shorinternational.com/subzeroinstr.php - "Refining Gold From Circuit Boards and Electronic Scrap with Muriatic and SubZero (nitric acid Substitute)"
http://www.goldnscrap.com/index.php?option=com_content&view=category&layout=blog&id=25&Itemid=20
http://www.ganoksin.com/borisat/nenam/ajm-roads-2-recovery.htm
References:
[1] http://hight3ch.com/how-are-gold-bars-made
[2] http://www.ganoksin.com/borisat/nenam/ajm-roads-2-recovery.htm
[3] http://www.responsiblegold.org/gold_production.asp
[4] http://www.goldbarsworldwide.com/PDF/RT_2_HowMinted.pdf
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