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| Instructions for Large Scale Gold Refining By the Aqua Regia Acid Method* Page 2 |
| Refining Procedure: |
Type of Scrap Considered and Preliminary Treatment
The gold scrap that is considered in this report is old jewelry and the material from jewelry bench work, filings, clippings, scrap
jewelrypieces, grinding wheel dust, casting spills, sprues, strip pot sludge, etc. Such material has been found to contain from 20%
gold in fairly dirty bench scrap to more than 70% gold in pure strip pot sludge. Experience indicates that most shops produce a
bench scrap (lemel) that contains 30% to 40% gold.
The dust from floor sweepings or from polishing wheel vacuum collectors and similar low-grade scrap requires extensive preliminary
treatment which is not described here.
So-called "green gold" and some low carat white golds contain considerable silver and are very difficult or impossible to dissolve in
aqua-regia as an insoluble silver chloride film is formed which prevents further action by the aqua-regia. Such golds or any high
silver alloy must be melted with several times their weight of copper or brass and shotted to permit dissolution. (See later section on
Gold Shot.)
If the scrap contains shellac, rubber wheel particles, rouge or similar material it is best simmer it in lye (sodium hydroxide and water
(a saturated solution) in a ratio of 10 volumes of lye/water to 1 volume of bench sweeps.
Massive pieces of metal take a very long time to digest in aqua-regia. Any such large pieces should be shotted as described later.
Strip pot sludge should be well washed with water to remove cyanide residues before acid is added to it. *Sediments, flour and fine
gold-and black sands receive the same treatments as follows;
The gold scrap that is considered in this report is old jewelry and the material from jewelry bench work, filings, clippings, scrap
jewelrypieces, grinding wheel dust, casting spills, sprues, strip pot sludge, etc. Such material has been found to contain from 20%
gold in fairly dirty bench scrap to more than 70% gold in pure strip pot sludge. Experience indicates that most shops produce a
bench scrap (lemel) that contains 30% to 40% gold.
The dust from floor sweepings or from polishing wheel vacuum collectors and similar low-grade scrap requires extensive preliminary
treatment which is not described here.
So-called "green gold" and some low carat white golds contain considerable silver and are very difficult or impossible to dissolve in
aqua-regia as an insoluble silver chloride film is formed which prevents further action by the aqua-regia. Such golds or any high
silver alloy must be melted with several times their weight of copper or brass and shotted to permit dissolution. (See later section on
Gold Shot.)
If the scrap contains shellac, rubber wheel particles, rouge or similar material it is best simmer it in lye (sodium hydroxide and water
(a saturated solution) in a ratio of 10 volumes of lye/water to 1 volume of bench sweeps.
Massive pieces of metal take a very long time to digest in aqua-regia. Any such large pieces should be shotted as described later.
Strip pot sludge should be well washed with water to remove cyanide residues before acid is added to it. *Sediments, flour and fine
gold-and black sands receive the same treatments as follows;
* Posted with Permission from Shor International
Mixing Aqua Regia
This and many of the operations described here should be carried out under an efficient fume extraction hood.
Aqua regia is a combination of nitric acid and hydrochloric acid (the industrial grade of hydrochloric is sometimes called muriatic
acid), it is made by mixing 1 volume of concentrated nitric acid with 4 volumes of concentrated hydrochloric acid.
If muriatic acid is used (it is usually less costly) the proportions are calculated to be: 1 nitric acid to 4.5 muriatic acid by volume.
There are reasons to err on the side of using more hydrochloric acid than theoretically necessary rather than too much nitric acid.
The precautions for mixing the acids are simple. Avoid splashes, protect eyes and work in the open or under a fume hood. These
acids mix quietly.
Both acids and especially hydrochloric emit acrid fumes. No heat is evolved when mixing but the aqua-regia at once starts to emit
chlorine gas, which evolves slowly for several days. DO NOT STOPPER aqua-regia bottles. A closed aqua-regia vessel can
develop enough chlorine pressure to burst. Store in the open or in a fume hood. The aqua-regia can be used immediately, days or
weeks and probably months after preparation. CONTINUED>>>
This and many of the operations described here should be carried out under an efficient fume extraction hood.
Aqua regia is a combination of nitric acid and hydrochloric acid (the industrial grade of hydrochloric is sometimes called muriatic
acid), it is made by mixing 1 volume of concentrated nitric acid with 4 volumes of concentrated hydrochloric acid.
If muriatic acid is used (it is usually less costly) the proportions are calculated to be: 1 nitric acid to 4.5 muriatic acid by volume.
There are reasons to err on the side of using more hydrochloric acid than theoretically necessary rather than too much nitric acid.
The precautions for mixing the acids are simple. Avoid splashes, protect eyes and work in the open or under a fume hood. These
acids mix quietly.
Both acids and especially hydrochloric emit acrid fumes. No heat is evolved when mixing but the aqua-regia at once starts to emit
chlorine gas, which evolves slowly for several days. DO NOT STOPPER aqua-regia bottles. A closed aqua-regia vessel can
develop enough chlorine pressure to burst. Store in the open or in a fume hood. The aqua-regia can be used immediately, days or
weeks and probably months after preparation. CONTINUED>>>
| Updated 9/9/09 |
| Aqua Regia Information: |
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