of this ancient, valued procedure is described here because of its historical
interest, and with the thought that young people coming into the field
should be aware of the basic procedures of the trade, even though they
may not pursue them. In the case of precious metal refining, it is generally
more efficient to send scrap to a reputable refiner where sophisticated
equipment and years of experience will insure accuracy. In addition, the
procedures described below use dangerous acids that mandate industrial
quality ventilation and safety precautions, which makes the decision to
send out refining chores even more compelling.
The principle of "quarter refining" The method described below is appropriate for refining batches of clean
scrap metal such as cut off pieces, sprues and filings. Polishing residue
and floor sweeps require a radically different approach because such scraps
are likely to contain a wide assortment of miscellaneous materials.
Preparing the metal
- Large pieces of foreign materials are picked out and iron or steel
particles are removed with a magnet.
- Any combustible materials such as wood, paper, pitch, wax, grease,
etc. are burned away through a gentle annealing.
- Any non-precious metal impurities such as lead or tin (from soft solders),
zinc, iron, aluminum, or other metals are dissolved in hot concentrated
hydrochloric acid. The scraps are set into a safe vessel and covered
with a generous amount of acid and remain there until bubbles no longer
rise from the scraps. This step is especially important if tin is present,
because even a minute amount will contaminate precious metal.
- The solution is poured off and the metal is washed.
- The scraps are then melted together. This process relies on the ability
of acids to attack base metals and in the process isolate gold. Because
metals become resistant to acid when combined with gold, it is vital
that the scrap contain no more than 25% gold (Au 250). If the scrap
is of a higher gold content it is first necessary to add base metals
so as to "dilute" the alloy down to a point where it will be susceptible
to acids. This is known as "quartering" the scrap, and explains the
name of the process.
- The scrap is melted together, stirred well with a quartz or carbon
rod and poured into water so as to create small spherical pieces called
shot. In the case of smaller amounts of scrap, the material is melted
to a blob on a charcoal block, quenched and rolled out to become as
thin as possible. This thin sheet is then cut into small pieces. In
both cases the intention is to create finely divided material with a
maximum surface area for attack by the purifying acid of the next step.
Dissolution in nitric acid
- Use extreme safety precautions
- The prepared metal is placed in an appropriate container and covered
with slightly diluted nitric acid. In this bath, non-precious metals
and silver are converted to nitrates, while gold falls out as a sediment.
- The nitrate-containing solution is poured off and the residue is treated
again with nitric acid, this time slightly heated to accelerate its
corrosive action. This repeats the action of the first step. When no
further dissolving appears to be taking place, this solution can be
poured off and added to the previous acid for subsequent reclaiming
of the silver present.
- The residue is washed with distilled water and caught in a paper filter.
Dissolution in Aqua Regia
- The residue is next dissolved in Aqua Regia, which dissolves impurities
and of course the gold itself. It is important that the previous step
has removed any silver present in the scrap. If more than 5% silver
were remaining at this step, the metal would become cloaked with silver
chloride, which is impervious to acid.
- Excess Aqua Regia is removed from the solution through evaporation
in the draft of a fume hood until it is as thick as syrup.
of the initial Materials
off (Removing flammable materials)
Magnet Use (Remove
Boiling in hydrocloric
acid (Sn, Pb, Zn, Fe, Al are dissolved to become chlorides)
Alloy down to
Au 250 at the highest, roll it out thinly.
in Nitric Acid
(Ag, Cu, Zn are dissolved to form nitrates)
the solution into Silver
reduce to Ag
with Zn and dilute H2SO4
Melt the silver.
Reduction of the gold
- Metallic gold is precipitated from the solution of salts that was
formed by the Aqua Regia by adding iron (II) sulfate solution. When
the addition of further iron (II) sulfate solution produces no more
precipitation the reduction is complete.
- The solution is allowed to sit for a minimum of two hours, after which
time it should have settled. The liquid is decanted to leave behind
a residue that contains fine gold. This is thoroughly rinsed, then dried.
- The fine gold is melted into an ingot, which can be tested for purity
as described earlier.
Reduction of the silver
- To reclaim silver from the solution that was set aside after the first
step in the gold reclamation process, table salt is added to provoke
the formation of silver chloride. This is added and stirred until all
precipitation of silver chloride has stopped.
- After allowing the curd-like precipitate to settle out, the solution
is stirred, which will cause the precipitate to ball up. The remaining
solution can be discarded (safely!) because the recovery of the copper
it contains does not pay for itself.
- The residue is rinsed until the runoff washing water no longer registers
acidic with litmus paper.
- The sediment is next mixed with dilute sulfuric acid and zinc is added
to a point of saturation. This will precipitate silver according to
the following equation.
- The liquid is poured off and the residue is melted to create an ingot.
It may be tested for purity as described earlier
Treatment at the Refinery The higher the proportion of precious metals in a container of material
sent for refining, the simpler the treatment at the refinery. This affects
the length of treatment time and therefore the cost of refining. For this
reason it is best to remove from scraps any foreign materials such as
pitch, plaster, wood, etc. The pile of scraps is stirred with a magnet
to remove iron and steel particles, then it is burned to remove organic
materials such as paper, wax and sawdust. It is not necessary to melt
the remaining sweeps together.
Because they are handled differently by a refiner, it is best to package
large clean pieces of scrap separate from filings, which in turn are kept
separate from sweepings. The more clearly these categories can be defined,
the easier it will be for a refiner to process. The job is made easier
still if you can be clear about what metals a given batch of scrap is
likely to contain; this is especially true where platinum metals are involved.
Scrap This category includes sheet and wire scrap that result from fabrication,
trimmings from castings such as sprues, and unwanted objects such as might
be received from a customer or bought for weight value. As far as possible
the scraps should be sorted according to fineness or purity of each alloy.
The scrap should be weighed and placed in a rigid container such as a
cardboard box or steel can. Plastic bags are not recommended because they
can be torn by the sharp edges of scrap pieces. The container should be
labeled with the total weight and, if possible, with the name or alloy
of the scrap, such as "46 ounces, Ag 925 (sterling)" or "320
grams scrap, primarily Au 333."
Filings The term filings includes not only the powder that is the result of using
a file, but the chips, curls and particles that result from drilling,
turning and grinding. Residue from abrasive papers (retrieved by burning)
are also included in this category.
Filings are weighed and collected into a plastic bag that is then packed
into a rigid box that can be well sealed with tape. It is labeled, for
instance, as "216 grams of Au 585 "lings (free from platinum
metals)" or "326 grams Ag 925 filings."
Sweeps or lemel All dust and residue swept from the work bench or retrieved from the sweeps
drawer or bench skin are called sweeps or lemel. These are kept separate
from floor sweepings or residue collected from scratchbrushing and polishing,
which have considerably less valuable. Bench sweeps are cleaned with a
magnet then packed and labeled in the same manner as filings.
Scrap containing platinum The recovery of platinum and platinum group metals is exceptionally difficult
and expensive, particularly if the individual platinum group metals have
to be separated from each other. Because of this, scrap that contains
platinum should always be carefully separated from other scraps and labeled
accordingly, such as "11.8 grams scrap with about 90% platinum content."
Even in the case of sweeps the approximate platinum content should be
stated, for instance, "19.4g mixed filings (Au, Ag and about 20%
Pt and Pd)."Electrolytic residues When the electrolytic baths used in plating become unusable, the metal
contents are precipitated out chemically and sent to a refiner for recovery.
Rhodium plating baths are concentrated by evaporation and the concentrated
solution is sent to a refiner to recover the metal.