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| Aqua Regia Recovery - Continued |
| Updated 10/04/2007 |
Experience has shown that the paper discs usually used in these filters by chemists
tend to float away when the filter is filled with liquid, but coffee urn filters
obtainable from hotel and restaurant supply shops have proved very satisfactory.
These should be large enough to line the bottom and sides of the filter funnel,
inserted dry, wet thoroughly with water and firmly seated and pressed into the
corners to avoid wrinkles and vacuum leaks. Two plies of filter paper are used to
help filtration and avoid breakthrough.
tend to float away when the filter is filled with liquid, but coffee urn filters
obtainable from hotel and restaurant supply shops have proved very satisfactory.
These should be large enough to line the bottom and sides of the filter funnel,
inserted dry, wet thoroughly with water and firmly seated and pressed into the
corners to avoid wrinkles and vacuum leaks. Two plies of filter paper are used to
help filtration and avoid breakthrough.
Vacuum is produced by means of a water pump (aspirator) available from chemical laboratory suppliers.
Plastic, not metal pumps should be used, as the acid fumes from aqua-regia filtering rapidly reduce the
pumping ability of a metal pump. For the same reason mechanical vacuum pumps are not recommended
unless provided with efficient acid vapor traps.
The filtrate is usually clean and clear. If, however, some solids come through at first the filter should
be stopped after a little while and the liquid poured back for refiltering. Usually the liquid will then be
clear and clean.
Filtering proceeds more rapidly if the clearest part of the aqua-regia is decanted onto the filter first.
When the sludge and solids get into the filter the process usually slows up. All of the solids should be
washed into the filter with a small stream of water, a wash bottle is useful for this operation.
When the filtering is complete the paper and the sludge should be washer with repeated small streams
of water. This is to get all gold chloride solution out of the filter and sludge into the liquid.
The filter paper now contains the unwanted material and also the silver chloride. It is recommended
that this should be dried and saved until at least a 30 gallon drum full is accumulated. The silver and
any residual gold can be recovered separately or the material sent to a refiner.
The filtered liquid is usually a rather handsome clear green color, due to nickel and copper etc. If only
gold chloride were present it would be yellowish.
The filtered solution is poured into a plastic container (plastic buckets or 5 gallon pickle pails are
suitable) for the next steps of eliminating excess nitric acid and precipitating the gold.
Plastic, not metal pumps should be used, as the acid fumes from aqua-regia filtering rapidly reduce the
pumping ability of a metal pump. For the same reason mechanical vacuum pumps are not recommended
unless provided with efficient acid vapor traps.
The filtrate is usually clean and clear. If, however, some solids come through at first the filter should
be stopped after a little while and the liquid poured back for refiltering. Usually the liquid will then be
clear and clean.
Filtering proceeds more rapidly if the clearest part of the aqua-regia is decanted onto the filter first.
When the sludge and solids get into the filter the process usually slows up. All of the solids should be
washed into the filter with a small stream of water, a wash bottle is useful for this operation.
When the filtering is complete the paper and the sludge should be washer with repeated small streams
of water. This is to get all gold chloride solution out of the filter and sludge into the liquid.
The filter paper now contains the unwanted material and also the silver chloride. It is recommended
that this should be dried and saved until at least a 30 gallon drum full is accumulated. The silver and
any residual gold can be recovered separately or the material sent to a refiner.
The filtered liquid is usually a rather handsome clear green color, due to nickel and copper etc. If only
gold chloride were present it would be yellowish.
The filtered solution is poured into a plastic container (plastic buckets or 5 gallon pickle pails are
suitable) for the next steps of eliminating excess nitric acid and precipitating the gold.
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.
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.
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.
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.
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.
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.
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.