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| Instructions for Large Scale Gold Refining By the Aqua Regia Acid Method* Page 4 |
* Posted with Permission from Shor International
More aqua-regia is then added as before and the cycle repeated until the addition of fresh aqua-regia produces no reaction, i.e.
brown fumes and bubbling.
Enough aqua-regia must be added to dissolve all of the gold, however the excess aqua-regia that is required to accomplish this
will later have to be removed so large excesses should be avoided. Toward the last the reaction is much slower and it is desirable
to warm the solution and to agitate it regularly, but the aqua-regia should not be heated to boiling. If heated too much it will
produce brown fumes merely because it is too hot, this wastes acid and obscures the end of the solvent action.
The reaction also slows down near the end because of the amount of fine, sludge present which tends to restrict the contact
between aqua-regia and undissolved gold, so frequent agitation is helpful. When pieces of jewelry or larger pieces of metal are
being dissolved it often seems that the jewelry is not being attacked because it is still there in its original shape, however such
pieces usually crumble if crushed with a stirring rod. Most jewelry alloys contain silver and the aqua-regia dissolves the gold and
other alloying metals leaving insoluble silver chloride as a residue in the original size and form. It is good to break these as there
may be a yet undissolved core that will dissolve more quickly if exposed. When it is apparent that the reaction is complete the
solution should be cooled to room temperature.
brown fumes and bubbling.
Enough aqua-regia must be added to dissolve all of the gold, however the excess aqua-regia that is required to accomplish this
will later have to be removed so large excesses should be avoided. Toward the last the reaction is much slower and it is desirable
to warm the solution and to agitate it regularly, but the aqua-regia should not be heated to boiling. If heated too much it will
produce brown fumes merely because it is too hot, this wastes acid and obscures the end of the solvent action.
The reaction also slows down near the end because of the amount of fine, sludge present which tends to restrict the contact
between aqua-regia and undissolved gold, so frequent agitation is helpful. When pieces of jewelry or larger pieces of metal are
being dissolved it often seems that the jewelry is not being attacked because it is still there in its original shape, however such
pieces usually crumble if crushed with a stirring rod. Most jewelry alloys contain silver and the aqua-regia dissolves the gold and
other alloying metals leaving insoluble silver chloride as a residue in the original size and form. It is good to break these as there
may be a yet undissolved core that will dissolve more quickly if exposed. When it is apparent that the reaction is complete the
solution should be cooled to room temperature.
Filtering
The aqua-regia now contains various metal chlorides (and perhaps nitrates) in solution and insoluble silver chloride as well as a lot
of unwanted material in the sludge, and this mixture (when cooled) must be filtered. My reason for tooling is that silver chloride,
though quite insoluble in water, is slightly soluble in strong acids and this volubility is lower in cold acids. Silver is probably the major
non-gold constituent of gold refined by this procedure; though few assays have been made, these have consistently shown
996/1000 gold.
The aqua-regia solutions are filtered with a Buchner filtering funnel and a 4 liter vacuum filtering flask. Two sizes of funnel have
been employed, a small one about 125 mm diameter, and a larger one about 250 mm diameter.
The aqua-regia now contains various metal chlorides (and perhaps nitrates) in solution and insoluble silver chloride as well as a lot
of unwanted material in the sludge, and this mixture (when cooled) must be filtered. My reason for tooling is that silver chloride,
though quite insoluble in water, is slightly soluble in strong acids and this volubility is lower in cold acids. Silver is probably the major
non-gold constituent of gold refined by this procedure; though few assays have been made, these have consistently shown
996/1000 gold.
The aqua-regia solutions are filtered with a Buchner filtering funnel and a 4 liter vacuum filtering flask. Two sizes of funnel have
been employed, a small one about 125 mm diameter, and a larger one about 250 mm diameter.
Experience has shown that the paper discs usually used in these filters by chemists tend to float away when the filter is filled with
liquid, but coffee urn filters obtainable from hotel and restaurant supply shops have proved very satisfactory. These should be
large enough to line the bottom and sides of the filter funnel, inserted dry, wet thoroughly with water and firmly seated and pressed
into the corners to avoid wrinkles and vacuum leaks. Two plies of filter paper are used to help filtration and avoid breakthrough.
liquid, but coffee urn filters obtainable from hotel and restaurant supply shops have proved very satisfactory. These should be
large enough to line the bottom and sides of the filter funnel, inserted dry, wet thoroughly with water and firmly seated and pressed
into the corners to avoid wrinkles and vacuum leaks. Two plies of filter paper are used to help filtration and avoid breakthrough.
suppliers. Plastic, not metal pumps should be used, as the acid fumes from aqua-regia filtering
rapidly reduce the pumping ability of a metal pump. For the same reason mechanical vacuum
pumps are not recommended unless provided with efficient acid vapor traps. The filtrate is
usually clean and clear. If, however, some solids come through at first the filter should be stopped
after a little while and the liquid poured back for refiltering. Usually the liquid will then be clear and
clean.
rapidly reduce the pumping ability of a metal pump. For the same reason mechanical vacuum
pumps are not recommended unless provided with efficient acid vapor traps. The filtrate is
usually clean and clear. If, however, some solids come through at first the filter should be stopped
after a little while and the liquid poured back for refiltering. Usually the liquid will then be clear and
clean.
Filtering proceeds more rapidly if the clearest part of the aqua-regia is decanted onto the filter first. When the sludge and solids
get into the filter the process usually slows up. All of the solids should be washed into the filter with a small stream of water, a
wash bottle is useful for this operation.
When the filtering is complete the paper and the sludge should be washer with repeated small streams of water. This is to get all
gold chloride solution out of the filter and sludge into the liquid. The filter paper now contains the unwanted material and also the
silver chloride. It is recommended that this should be dried and saved until at least a 30 gallon drum full is accumulated. The
silver and any residual gold can be recovered separately or the material sent to a refiner. T
he filtered liquid is usually a rather handsome clear green color, due to nickel and copper etc. If only gold chloride were present it
would be yellowish. The filtered solution is poured into a plastic container (plastic buckets or 5 gallon pickle pails are suitable) for
the next steps of eliminating excess nitric acid and precipitating the gold. CONTINUED>>>
get into the filter the process usually slows up. All of the solids should be washed into the filter with a small stream of water, a
wash bottle is useful for this operation.
When the filtering is complete the paper and the sludge should be washer with repeated small streams of water. This is to get all
gold chloride solution out of the filter and sludge into the liquid. The filter paper now contains the unwanted material and also the
silver chloride. It is recommended that this should be dried and saved until at least a 30 gallon drum full is accumulated. The
silver and any residual gold can be recovered separately or the material sent to a refiner. T
he filtered liquid is usually a rather handsome clear green color, due to nickel and copper etc. If only gold chloride were present it
would be yellowish. The filtered solution is poured into a plastic container (plastic buckets or 5 gallon pickle pails are suitable) for
the next steps of eliminating excess nitric acid and precipitating the gold. CONTINUED>>>
| Updated 9/9/09 |
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