FREQUENTLY ASKED QUESTIONS
Machinery
Cupellation is the technique that forms the first part of the fire assay process, in which lead is added
to the unrefined gold material. The mixture is heated in air to between 1,830°F and 2,010°F (1,000°C
and 1,100°C), at which point the gold-containing metal dissolves in the lead. All base metals,
including the lead, are oxidized to form a lead oxide slag. A gold-silver bullion, which also contains any
platinum group metals (PGMs) present, remains. If pure gold is required, additional refining steps are
necessary to separate out the gold.
While this procedure can be used on the very small scale (roughly up to 10 grams) such as in fire
assay, its use on a small to medium scale (roughly 100 grams to 10 kg) is not recommended because
it emits copious quantities of toxic lead oxide fumes. These fumes give rise to environmental pollution
unless expensive fume abatement systems, also known as gas scrubbers, are installed.
In the inquartation and parting process, the refinable material is melted with additional silver or
copper to produce an alloy containing 25 percent or less gold. The dilution ensures that all the base
metals and silver can be dissolved out in nitric acid.
Next, the molten alloy should be grained to maximize surface area. The grained alloy is attacked with
nitric acid to dissolve out all the base metals and silver, leaving behind a gold sludge. This sludge is
then washed, filtered, and dried.
Any platinum and palladium present will also be dissolved out (although the process may need to be
performed twice to ensure their complete removal), but insoluble PGMs will remain. In such cases,
further refining is necessary if pure gold is needed.
When used for refining material that doesn't contain PGMs, the inquartation and parting process is
capable of producing gold of up to 99.99 percent purity. The process is particularly suited for
treatment of low karat gold scrap, since large additions of copper or silver are unnecessary to achieve
the desired 25 percent-or-less gold content. On the contrary, this process may not be as desirable for
an operation making predominantly medium to high karat gold jewelry, as scraps from production
may need to be substantially diluted with copper or silver.
In addition, inquartation and parting can be used as a preliminary step to reduce the silver content of
silver-rich refinable materials from 40 to 50 percent to below 10 percent prior to refining by the Aqua
Regia process, which is explained below.
The Aqua Regia process can produce gold of up to 99.99 percent purity. It is based on the fact that
Aqua Regia (a mixture of hydrochloric and nitric acids in a 4.5:1 ratio) can dissolve gold into soluble
gold chloride. The process is most suited to medium- to large-scale operations. A typical batch size is
4 kg of scrap, and equipment in a range of capacities is commercially available from several suppliers.
The main limitation of the process is that the feed material should have a silver content of 10 percent
or less to avoid blocking up the dissolution of the scrap. Because of this, pretreatment by the
inquartation process to reduce the silver content may be necessary. Alternatively, the low silver
content may be achieved by a judicious blending of batches of scrap. Thus, the process is more suited
for medium to high karat gold scrap refining.
In practice, the scrap is grained to increase surface area and treated with a series of Aqua Regia acid
additions. Gentle heating speeds up dissolution.
Copious brown fumes of nitrogen oxide are emitted while the gold is being dissolved. Fume abatement
systems are required to stop emission of these toxic fumes and to comply with pollution laws. It is
also worth noting that these strong acids require suitable storage and safety procedures.
Once the gold is dissolved, the yellow-green solution must be filtered to remove the insoluble silver
chloride, the insoluble PGMs, and any non-metallics, such as abrasives and inclusions. The gold can
then be selectively precipitated using a number of reducing agents, such as ferrous sulphate (also
known as Copperas), sodium bisulphite, and sulphur dioxide gas. Other less frequently used agents
include hydrazine, formaldehyde, oxalic acid, and hydroquinone. Some emit copious quantities of gas
and some are carcinogenic.
One refining expert, Roland Loewen of Alchemy Gold Refining in Baytown, Texas, favors an aqueous
solution of sodium bisulphite over ferrous sulphate. This is added slowly until the yellow color of the
solution disappears. He notes that a smell of sulphur dioxide may be apparent at this point.
Completion of the reaction can be ascertained with the stannous chloride test. (In this test, drops of
stannous chloride are added to the solution. If gold is present, the colloidal gold formation will create a
purple coloration in the solution.)
After precipitation, the solution should stand overnight to allow the fine gold particles to settle as a
sludge on the bottom. Most of the liquid can be decanted off and the remaining portion with the gold
can be filtered. To ensure that all other metals are dissolved away, the filtrate is washed with
hydrochloric acid and then water. It is then dried and placed in a crucible for melting and graining.
Occasionally, jewelers who try this process complain that they have lost a considerable amount of the
gold. This suggests that either they are not fully dissolving all the gold in the first stage or, more
probably, not precipitating all the gold in the reducing step. To see if you are guilty of the latter,
analyze the liquid for gold content using the stannous chloride test.
As with some of the other refining methods, the dangers of handling strong acids are present in the
Aqua Regia process. Anyone using this process must be aware of the risks and ensure that they have
trained chemists and safe facilities.
In fact, all of the refining processes described in this article require technical expertise and safe
implementation. Each process entails permit licensing and regulation by the EPA. For all of these
reasons, as well as the safety requirements described in this article, it is often safer-and more cost-
effective-to leave refining to the experts.
When choosing between sending your scrap out for refining or doing it yourself in-house, consider the
following factors:
In any strategy to recover precious metals, there is no sense in spending more on processing costs
than the value of metal recovered. Compare the overall cost of in-house refining to the recovery
efficiency (the amount of gold and other precious metals) achieved by an outside refiner. You may find
that low-grade scraps and wastes are not economic to recover in-house and are best treated by a
commercial refiner. Silver and platinum group metal (PGM) recovery will also play a part in
determining the economic viability of in-house processing.
The gold purity obtained will vary depending on refining technique and operating skill. If the gold is
being used for re-alloying in-house and you have access to analytical facilities to obtain gold purity,
this may not be important.
If you are re-using the gold for new alloy production, be aware that some impurities may not be
removed in the refining process. For example, PGMs are not removed by some techniques, and they
can affect the new alloy's color or properties.
It is essential to make sure that all the gold is recovered. This requires an understanding of the
underlying technology and good process control.
Think about health, safety, and environmental pollution. Local legislation on disposing of effluents
and release of toxic fumes may restrict your choice of technique. Also, many refining techniques
require the use of strong acids; the safe storage and handling of these chemicals may restrict your
choice.
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