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| Aqua Regia - Digesting Scrap |
| Updated 10/4/2007 |
Gold Chloride forming
Nitrous Oxide fumes forming
Precipitating the gold from Leach
Draining the leach
treated in these flasks. This quantity requires considerable agitation and stirring to keep the reaction
going and is better done in a jar under a fully ventilated fume hood.
The amount of aqua-regia required for a given batch varies and depends on the proportion that is acid
soluble and the quality of the metal present. It has been found that from 3.5 to 5 liters of aqua-regia
are required per kilo of scrap, most batches fall in the range of 4 to 4.5 liters of aqua-regia per kilo of
scrap, in smaller amounts this is equivalent to 4 to 4.5 ml (cc) per-gram of scrap.
If the scrap is the usual filings and dust from jewelers benches, the aqua-regia will react very rapidly
and may boil over, so the acid must be added slowly and with care.
The scrap gold is placed in the digesting vessel. Glass may be used for small
batches. Teflon plastic is also suitable for the strong oxidizing conditions of
the aqua-regia and Teflon will tolerate heating if done with care.
For many batches 6 liter Ehrlenmeyer flasks are used, or the glass jars
illustrated are suitable if heated with much care.
1 to 2 kilos of scrap material in a 6 liter Ehrlenmeyer flask is typical though up
to as much as 35 kilos have been
batches. Teflon plastic is also suitable for the strong oxidizing conditions of
the aqua-regia and Teflon will tolerate heating if done with care.
For many batches 6 liter Ehrlenmeyer flasks are used, or the glass jars
illustrated are suitable if heated with much care.
1 to 2 kilos of scrap material in a 6 liter Ehrlenmeyer flask is typical though up
to as much as 35 kilos have been
A slow flow or drop wise addition can be made from a bottle (with a bottom outlet) set on a shelf
above the flask. If the aqua-regia is several days old and is no longer producing chlorine gas a siphon
from a high container is also a convenient way of adding aqua-regia slowly.
above the flask. If the aqua-regia is several days old and is no longer producing chlorine gas a siphon
from a high container is also a convenient way of adding aqua-regia slowly.
If the scrap is in the form of old jewelry or metal shot or other large pieces, the reaction will be
slower and a considerable amount of aqua-regia can just be poured onto the scrap. Care is advised
as the reaction is often quite slow to start and then after some minutes becomes very, very active.
The jar or flask may get quite hot which increases the reaction speed.
When there is jewelry with diamonds, rubies, sapphires and similar acid-resistant gems these can be
left in place and recovered from the filter.
The reaction of the aqua-regia with the metals in the scrap produces nitrogen oxides. Some of
these are red-brown in color, others are colorless but take up oxygen as soon as they reach air and
then turn red-brown. These fumes are acrid, choking and extremely toxic; they dissolve quite
easily in water and in caustic solutions; they are heavier than air and the aqua-regia digestion
should be done under a good fume hood.
The preferred practice is to add the aqua-regia to the batch in two or three separate additions.
Up to about half or three-quarters of the expected amount of aqua-regia is added and the mixture
is allowed to stand for some time. Occasional agitation is good, especially with finely divided
material.
When brown fumes are no longer evolved and the bubbling of the solution is quiet a little
hydrochloric acid is usually added. Sometimes a further spurt of activity is seen, the original
hydrochloric acid having been depleted leaving some unused nitric acid available, excess
hydrochloric acid is not harmful. If no more brown fumes appear the liquid is carefully poured off
into a glass or plastic container, being careful not to disturb the solid material in the reaction
vessel.
slower and a considerable amount of aqua-regia can just be poured onto the scrap. Care is advised
as the reaction is often quite slow to start and then after some minutes becomes very, very active.
The jar or flask may get quite hot which increases the reaction speed.
When there is jewelry with diamonds, rubies, sapphires and similar acid-resistant gems these can be
left in place and recovered from the filter.
The reaction of the aqua-regia with the metals in the scrap produces nitrogen oxides. Some of
these are red-brown in color, others are colorless but take up oxygen as soon as they reach air and
then turn red-brown. These fumes are acrid, choking and extremely toxic; they dissolve quite
easily in water and in caustic solutions; they are heavier than air and the aqua-regia digestion
should be done under a good fume hood.
The preferred practice is to add the aqua-regia to the batch in two or three separate additions.
Up to about half or three-quarters of the expected amount of aqua-regia is added and the mixture
is allowed to stand for some time. Occasional agitation is good, especially with finely divided
material.
When brown fumes are no longer evolved and the bubbling of the solution is quiet a little
hydrochloric acid is usually added. Sometimes a further spurt of activity is seen, the original
hydrochloric acid having been depleted leaving some unused nitric acid available, excess
hydrochloric acid is not harmful. If no more brown fumes appear the liquid is carefully poured off
into a glass or plastic container, being careful not to disturb the solid material in the reaction
vessel.
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.
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.
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.
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.