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| Updated 9/9/09 |
| Gold Chloride forming |
| Nitrous Oxide fumes forming |
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.
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.
Digesting the Scrap
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 treated in these flasks.
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 treated in these flasks.
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.
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.
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. CONTINUED>>>
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. CONTINUED>>>
| Instructions for Large Scale Gold Refining By the Aqua Regia Acid Method* Page 3 |
* Posted with Permission from Shor International
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.
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.
| Aqua Regia Information: |
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