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| Updated 9/9/09 |
Nitric Acid Elimination
The excess aqua-regia that was added to insure complete solution of all gold is, of course, still in the solution at this stage and must
be eliminated to allow the gold to be precipitated. The classic procedure for nitric acid elimination is repeated boiling to near
dryness with the addition of hydrochloric acid with some sulphuric acid near the end. This is a lengthy and patience-trying process.
The best way is with urea. Add urea to the solution until there is no more fizzing.
The excess aqua-regia that was added to insure complete solution of all gold is, of course, still in the solution at this stage and must
be eliminated to allow the gold to be precipitated. The classic procedure for nitric acid elimination is repeated boiling to near
dryness with the addition of hydrochloric acid with some sulphuric acid near the end. This is a lengthy and patience-trying process.
The best way is with urea. Add urea to the solution until there is no more fizzing.
Precipitating The Gold
The classic method of reducing gold chloride in solution to solid gold particles is to add "copperas" to the solution. "Copperas" is an
ancient name for ferrous sulphate, a rather cheap chemical. A number of other chemicals will also 'reduce' the gold chloride but
Storm Precipitant (available from Shor) is better. In hot water, dissolve a weight of Storm Precipitant equal to the weight of dissolved
metal.
The precipitation of gold can be seen as a 'cloud' of particles in the solution. The end point of the precipitation is difficult to see,
some clues may be noted in the density of the 'cloud' of gold particles. The solution will be clearer and noticeably less yellowish
especially if a drop is viewed one white chinaware surface. This is because the yellow gold chloride is gone and the green of the
other chlorides remains. Deliberate care during this gold precipitation work is advised. Observe the signs and test the solution
frequently to avoid large excesses of Storm Precipitant. Careful addition of Storm Precipitant and a slow approach to the end point
can avoid this. The sulphur dioxide odor, however, can be used as one of the signs that gold precipitation is complete.
A problem that occurs when too much Storm Precipitant is added is that copper chloride, which is very soluble in the cupric chloride
(CuC12) form, is reduced by the excess Storm Precipitant to the cuprous chloride (CuCl) state, which forms a white precipitate.
Limited experience with this contaminant has shown that it will reduce gold quality a little and it will affect the color of the gold.
The classic method of reducing gold chloride in solution to solid gold particles is to add "copperas" to the solution. "Copperas" is an
ancient name for ferrous sulphate, a rather cheap chemical. A number of other chemicals will also 'reduce' the gold chloride but
Storm Precipitant (available from Shor) is better. In hot water, dissolve a weight of Storm Precipitant equal to the weight of dissolved
metal.
The precipitation of gold can be seen as a 'cloud' of particles in the solution. The end point of the precipitation is difficult to see,
some clues may be noted in the density of the 'cloud' of gold particles. The solution will be clearer and noticeably less yellowish
especially if a drop is viewed one white chinaware surface. This is because the yellow gold chloride is gone and the green of the
other chlorides remains. Deliberate care during this gold precipitation work is advised. Observe the signs and test the solution
frequently to avoid large excesses of Storm Precipitant. Careful addition of Storm Precipitant and a slow approach to the end point
can avoid this. The sulphur dioxide odor, however, can be used as one of the signs that gold precipitation is complete.
A problem that occurs when too much Storm Precipitant is added is that copper chloride, which is very soluble in the cupric chloride
(CuC12) form, is reduced by the excess Storm Precipitant to the cuprous chloride (CuCl) state, which forms a white precipitate.
Limited experience with this contaminant has shown that it will reduce gold quality a little and it will affect the color of the gold.
Filtration
Then the solution has been cleared of gold it should be allowed to stand for several hours. Although gold is heavy and most
settles quickly, some particles are very small and require time to settle to the bottom. Standing for a period, if possible overnight,
facilitates the subsequent filtering operation.
In the interest of reducing the time and aggravation of filtering work the clear upper portion of the barren solution can be decanted
and only the bottom few inches near the gold filtered. A simple siphon will remove the upper portion of the liquid quietly without
stirring up gold particles. The bottom few inches of liquid are then poured through the filter keeping the gold largely in the pail.
The same Buchner filters and the same kind of filter and paper are used for gold as previously used to filter aqua-regia. My gold
is washed with repeated small amounts of water until the water coming through the filter is quite clear and colorless. The gold in
the pail is then just covered with concentrated hydrochloric acid and thoroughly stirred. The acid is added to the filter and the
treatment repeated several times, followed by repeated washing with water. This treatment will remove small amounts of
contaminants including cuprous chloride.
Then the solution has been cleared of gold it should be allowed to stand for several hours. Although gold is heavy and most
settles quickly, some particles are very small and require time to settle to the bottom. Standing for a period, if possible overnight,
facilitates the subsequent filtering operation.
In the interest of reducing the time and aggravation of filtering work the clear upper portion of the barren solution can be decanted
and only the bottom few inches near the gold filtered. A simple siphon will remove the upper portion of the liquid quietly without
stirring up gold particles. The bottom few inches of liquid are then poured through the filter keeping the gold largely in the pail.
The same Buchner filters and the same kind of filter and paper are used for gold as previously used to filter aqua-regia. My gold
is washed with repeated small amounts of water until the water coming through the filter is quite clear and colorless. The gold in
the pail is then just covered with concentrated hydrochloric acid and thoroughly stirred. The acid is added to the filter and the
treatment repeated several times, followed by repeated washing with water. This treatment will remove small amounts of
contaminants including cuprous chloride.
| Instructions for Large Scale Gold Refining By the Aqua Regia Acid Method* Page 5 |
* Posted with Permission from Shor International
If cuprous chloride is present it will make the melting of the gold a memorable experience. Dense
clouds of choking white fumes will clear all persons out of the furnace room quite quickly.
Some excess of Storm Precipitant is required to cause this undesirable side reaction, and it is felt that
the advantages of no boiling and little or no emission of brown fumes make it worthwhile to use Storm
Precipitant even though larger volumes of liquids are handled and some care at the end point is
needed. If, through error, some cuprous chloride crystals are formed, they can be removed as
described later.
clouds of choking white fumes will clear all persons out of the furnace room quite quickly.
Some excess of Storm Precipitant is required to cause this undesirable side reaction, and it is felt that
the advantages of no boiling and little or no emission of brown fumes make it worthwhile to use Storm
Precipitant even though larger volumes of liquids are handled and some care at the end point is
needed. If, through error, some cuprous chloride crystals are formed, they can be removed as
described later.
When the gold has been treated with hydrochloric acid and thoroughly washed it is then ready to
transfer into the filter. A soft kitchen scraper helps move the gold into the filter and a small water
jet is very useful in clearing the last particles into the filter. Often there is gold adhering to the
walls of the pail, this can be scrubbed down with a stiff brush and washed into the filter with the
water jet. When the gold is all in the filter the vacuum should be run for a while to get the gold as
dry as possible.
transfer into the filter. A soft kitchen scraper helps move the gold into the filter and a small water
jet is very useful in clearing the last particles into the filter. Often there is gold adhering to the
walls of the pail, this can be scrubbed down with a stiff brush and washed into the filter with the
water jet. When the gold is all in the filter the vacuum should be run for a while to get the gold as
dry as possible.
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