 |
 |
| Instructions for Large Scale Gold Refining By the Aqua Regia Acid Method |
| Updated 10/4/2007 |
These are probably very similar to the units used by many large professional refiners. These
electrolytic methods eliminate some of the problems of the acid method described here, but they too
have their own set of disadvantages and are not as suitable for the small lots and needs of individual
jewelry manufacturers.
The amounts which have been refined range from small lots of about 45 g (1½ oz Troy) of fine gold
recovered from about l00 g (3 on Troy) of scrap, to more than 3 Kg (100 oz Troy) from about 6 Kg
(200 on Troy) of scrap. The latter is about the maximum that is reasonably handled in the equipment
herein described.
The usual refining lot in practice is 300-1000 g (10 to 30 oz Troy) of fine gold recovered from
600-3000 g (20 to 100 oz Troy) of scrap.
A knowledge of chemistry is not required to do this work. However, for those so inclined some
chemical equations and discussion are included in Appendix No. 1.
OUTLINE OF THE PROCESS
The gold refining technique described here is the rather ancient wet chemical method whereby the
gold-bearing scrap is dissolved in aqua-regia. m is gold solution is then filtered and the jewelers
bench dirt, sandpaper grit, grinding wheel grains and similar material remains on the filter as a solid
sludge, together with any silver present which will be in the form of silver chloride.
The filter and sludge are washed with repeated small amounts of water to wash all gold chloride
solution down through the filter. Other metals that were in the alloy or in the scrap (nickel, zinc,
copper, iron, etc.) are also in this solution which is usually green in color. However, if no nickel or
copper is present it will most likely be the characteristic yellow of gold chloride.
The nitric acid from the excess of aqua-regia used in the digestion is removed either by boiling or
chemical reaction.
To recover the gold as metal a reducing chemical is added to selectively change the gold chloride into
solid gold particles and leave the other metal chlorides unchanged and in solution. When tests show
this to be complete the solution is filtered and the gold in the filter thoroughly washed. The clean
gold is then melted and poured into molds or made into shot.
The acids used in the process are very corrosive and highly toxic fumes are produced. Appendix No.
2 discusses safety precautions in the operation of the process and should be carefully studied before
any part of this work is undertaken.
electrolytic methods eliminate some of the problems of the acid method described here, but they too
have their own set of disadvantages and are not as suitable for the small lots and needs of individual
jewelry manufacturers.
The amounts which have been refined range from small lots of about 45 g (1½ oz Troy) of fine gold
recovered from about l00 g (3 on Troy) of scrap, to more than 3 Kg (100 oz Troy) from about 6 Kg
(200 on Troy) of scrap. The latter is about the maximum that is reasonably handled in the equipment
herein described.
The usual refining lot in practice is 300-1000 g (10 to 30 oz Troy) of fine gold recovered from
600-3000 g (20 to 100 oz Troy) of scrap.
A knowledge of chemistry is not required to do this work. However, for those so inclined some
chemical equations and discussion are included in Appendix No. 1.
OUTLINE OF THE PROCESS
The gold refining technique described here is the rather ancient wet chemical method whereby the
gold-bearing scrap is dissolved in aqua-regia. m is gold solution is then filtered and the jewelers
bench dirt, sandpaper grit, grinding wheel grains and similar material remains on the filter as a solid
sludge, together with any silver present which will be in the form of silver chloride.
The filter and sludge are washed with repeated small amounts of water to wash all gold chloride
solution down through the filter. Other metals that were in the alloy or in the scrap (nickel, zinc,
copper, iron, etc.) are also in this solution which is usually green in color. However, if no nickel or
copper is present it will most likely be the characteristic yellow of gold chloride.
The nitric acid from the excess of aqua-regia used in the digestion is removed either by boiling or
chemical reaction.
To recover the gold as metal a reducing chemical is added to selectively change the gold chloride into
solid gold particles and leave the other metal chlorides unchanged and in solution. When tests show
this to be complete the solution is filtered and the gold in the filter thoroughly washed. The clean
gold is then melted and poured into molds or made into shot.
The acids used in the process are very corrosive and highly toxic fumes are produced. Appendix No.
2 discusses safety precautions in the operation of the process and should be carefully studied before
any part of this work is undertaken.
The gold scrap that is considered in this report is old jewelry and the material from jewelry bench
work, filings, clippings, scrap jewelry pieces, 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;
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.
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.
| Mixing Aqua Regia |
It is known that the aqua-regia technique herein described is not the only method.
It may not be the best, and for the sake of experience electrolytic cells for
refining precious metals have been examined
It may not be the best, and for the sake of experience electrolytic cells for
refining precious metals have been examined
| Refining Procedure |