 When
questions arise about manufacturing quality gold jewelry, manufacturers
are eager to talk shop with their peers and industry experts. They want
to know if they are using the right alloy for a specific application,
casting at the appropriate times and temperatures, and annealing properly
when work hardening a piece. Rarely, though, does the conversation turn
to refining-an area of jewelry manufacturing that poses more questions
than answers in many manufacturers' minds. Refining is a practice that
must be done precisely and methodically to ensure the full recovery of
gold, as well as an end product that is free of impurities, which can
lead to quality problems when the metal is reused in production. (See Cracking Up.)
But refining doesn't have to be a mystery to manufacturers. There are
several methods commonly used to recover metal. Some operations are suitable
for use by manufacturers and jewelers who wish to refine in-house, while
others are designed for commercial refiners who handle large lots. The
following are the most common methods used in the jewelry and gold refining
industries.
Cupellation
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.
Inquartation and Parting
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.
Miller Chlorination Process
A pyrometallurgical chlorination process, the Miller process is one of
the oldest and most widely used processes in large scale gold refining.
It involves bubbling chlorine gas through molten bullion. The base metals
and silver are removed as chlorides, which either volatilize or form a
molten slag on the surface of the melt. The process is complete when purple
fumes of gold chloride start to form, usually when the gold content reaches
a purity of 99.6 to 99.7 percent. Any PGMs present are not removed, and
further refining is necessary if pure gold is required.
The typical gold purity achieved by this process is 99.5 percent, with
silver as the main impurity. The process has the advantage of being quick
and is widely used for primary refining of gold doré from the mines.
Considerable technical skills are required for this process, and there
are a number of health and safety implications in the use of chlorine
gas. Expensive fume extraction and treatment facilities are essential.
Consequently, this process is not suited for small to medium scale refining
by jewelers.
Wohlwill Electrolytic Process
An old and well-established process, the Wohlwill method is widely used
in major gold refineries, often in conjunction with the Miller process.
(For typical jeweler's scraps and wastes, a preliminary refining step,
such as the Miller or inquartation process, is required.) An electrolytic
refining technique, it entails the electrolytic dissolution of an impure
gold anode in a hydrochloric acid-based electrolyte. The process results
in a deposition of 99.99 percent pure gold at the cathode. The silver
and insoluble PGMs (along with a little gold) fall out as anode slimes,
with the silver precipitated out as silver chloride, and all are recovered
later. Any base metals, platinum, and palladium remain in solution, and
can be treated later to recover the PGMs.
Gold of a purity of at least 98.5 percent is normally required for the
anode, as too much silver will result in silver chloride building up on
the anode surface and preventing dissolution of the gold. Typically, the
input material for the anode is the gold from the Miller process, described
previously.
Because it is time consuming-typically 24 hours or more-and suffers from
the lock-up of gold inventory in the electrodes and electrolyte, the Wohlwill
process is not suitable for small-scale refining.
Fizzer Cell Process
A variant of the Wohlwill electrolytic process, the Fizzer cell process
is suitable for jewelers' small-scale refining operations. In the electrolytic
cell, the cathode is contained within a porous ceramic pot, which acts
as a semi-permeable membrane; it prevents gold dissolved in the electrolyte
on the anode side of the wall from passing through and depositing on the
cathode. Thus, gold and other soluble metal chlorides build up, and insoluble
chlorides, such as those of silver and the insoluble PGMs, drop to the
bottom of the cell.
Periodically, the cell is drained and filtered, and the gold in the electrolyte
is precipitated with a selective reducing agent, as in the Aqua Regia
process described later. In this way, the dissolved PGMs are separated
from the gold, which can reach a purity of 99.99 percent.
Unlike the Wohlwill process, the Fizzer cell can treat anodes containing
up to 10 percent silver, and up to 20 percent silver if an imposed alternating
current is added. The surface of the anode may need to be scraped free
of silver chloride at regular intervals.
Aqua Regia Process
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.
In-house Versus Commercial Refining
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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