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| Mercury and Black Sands part 3 |
| Updated 11/7/2006 |
7. Mix and pour in a solution of nitric acid and allow it to bubble until there is no visible
reaction. BE CAREFUL NOT TO BREATHE THE FUMES GIVEN OFF BY THE CHEMICAL
REACTION!
8. Pour off the acid solution into another glass jar or a beaker; so that, the mercury in
solution can be recovered later (see how to do below).
9. If all of the mercury has not been dissolved from step 7, with the gold back in its natural
flake and powder form, pour fresh water into the jar and use an old screwdriver to poke it
around and break it up. Pour out the water and pour in another solution of nitric acid.
Sometimes it is necessary to poke at the gold just a bit to break it up while it's being worked
on by the acid. An old screwdriver works well for this.
10. When the reaction stops, flush with fresh water. If the gold is still not back in its natural
form, repeat the above steps. When dealing with small amounts of amalgam, usually the gold
will be thoroughly cleaned of it after step 7. Sometimes when working with larger amounts of
amalgam, it is necessary to do the steps a few times as described above, or to use a
stronger acid solution. NOTE: if you have a large amount of concentrates, you may wish to
ignore steps 3 and 4 and place the concentrates and an estimation of the correct amount of
mercury into a rock tumbler and allow it to turn for several hours. Some large scale
operations employ the use of portable cement mixers. If a new cement mixer is used, run it
first with a full load of sharp sand and gravel for 10 to 12 hours to scour out any paint that
may be present, as it will contaminate the mercury.
RECOVERING THE MERCURY
FROM THE NITRIC ACID SOLUTION
To recover the mercury in solution (see step 8), simply drop some aluminum foil into the acid
solution. A chemical reaction takes place and the acid solution will drop the mercury to attack
the aluminum. This causes the mercury to revert to its natural liquid metal form at the bottom
of the jar. Then rinse out the acid solution and you will be left with most of your original
mercury.
CLEANING MERCURY
After mercury has been used a number of times in the process of amalgamation, it becomes
dirty and tends to break down into smaller, separate balls instead of it all coming together
into a single mass. To clean dirty mercury, you simply soak it in a nitric acid solution of 30:1
part of acid. This will clean the impurities out and allow it to amalgamate properly again.
Mercury can be used over and over to amalgamate and cleaned when necessary in this way.
reaction. BE CAREFUL NOT TO BREATHE THE FUMES GIVEN OFF BY THE CHEMICAL
REACTION!
8. Pour off the acid solution into another glass jar or a beaker; so that, the mercury in
solution can be recovered later (see how to do below).
9. If all of the mercury has not been dissolved from step 7, with the gold back in its natural
flake and powder form, pour fresh water into the jar and use an old screwdriver to poke it
around and break it up. Pour out the water and pour in another solution of nitric acid.
Sometimes it is necessary to poke at the gold just a bit to break it up while it's being worked
on by the acid. An old screwdriver works well for this.
10. When the reaction stops, flush with fresh water. If the gold is still not back in its natural
form, repeat the above steps. When dealing with small amounts of amalgam, usually the gold
will be thoroughly cleaned of it after step 7. Sometimes when working with larger amounts of
amalgam, it is necessary to do the steps a few times as described above, or to use a
stronger acid solution. NOTE: if you have a large amount of concentrates, you may wish to
ignore steps 3 and 4 and place the concentrates and an estimation of the correct amount of
mercury into a rock tumbler and allow it to turn for several hours. Some large scale
operations employ the use of portable cement mixers. If a new cement mixer is used, run it
first with a full load of sharp sand and gravel for 10 to 12 hours to scour out any paint that
may be present, as it will contaminate the mercury.
RECOVERING THE MERCURY
FROM THE NITRIC ACID SOLUTION
To recover the mercury in solution (see step 8), simply drop some aluminum foil into the acid
solution. A chemical reaction takes place and the acid solution will drop the mercury to attack
the aluminum. This causes the mercury to revert to its natural liquid metal form at the bottom
of the jar. Then rinse out the acid solution and you will be left with most of your original
mercury.
CLEANING MERCURY
After mercury has been used a number of times in the process of amalgamation, it becomes
dirty and tends to break down into smaller, separate balls instead of it all coming together
into a single mass. To clean dirty mercury, you simply soak it in a nitric acid solution of 30:1
part of acid. This will clean the impurities out and allow it to amalgamate properly again.
Mercury can be used over and over to amalgamate and cleaned when necessary in this way.
TREATING GOLD ORES BY AMALGAMATION
Circular No. 27, March, 1987
by Richard R. Beard, Mining Engineer
Text of a presentation given at an Ehrenberg, Arizona miner’s seminar.
Introduction
The variations of the amalgamation process are as diverse as are the ores that are treated or the metallurgists who treat
them.
In this talk I will try to cover a little of the history, the hazards, and the theory of amalgamation. I will then try to show how
this experience and theory can be applied in a practical way to the extraction of gold from its ores by describing examples
of a couple of ancient but still viable devices. No attempt will be made to give you a cookbook type method since each
ore will demand slightly different details. What I hope to do is provide a basic starting point that can be adjusted as
necessary.
Circular No. 27, March, 1987
by Richard R. Beard, Mining Engineer
Text of a presentation given at an Ehrenberg, Arizona miner’s seminar.
Introduction
The variations of the amalgamation process are as diverse as are the ores that are treated or the metallurgists who treat
them.
In this talk I will try to cover a little of the history, the hazards, and the theory of amalgamation. I will then try to show how
this experience and theory can be applied in a practical way to the extraction of gold from its ores by describing examples
of a couple of ancient but still viable devices. No attempt will be made to give you a cookbook type method since each
ore will demand slightly different details. What I hope to do is provide a basic starting point that can be adjusted as
necessary.
Historic Background
Amalgamation is one of the oldest methods of extracting gold from its ores. According to Herbert Hoover, in a footnote
to his translation of Georgius Agricola’s De Re Metallica, the amalgamation of gold possibly dates from Roman
times. The extraction of silver by amalgamation is generally attributed to the Spanish in Latin America in the late sixteenth
century even though Biringuccio describes the fundamental principals of the “patio process� about a half a century
earlier.
The amalgamation of gold ores as described by Agricola in De Re Metallica, which was published in 1556, is essentially
the same as modern practice. The only changes are in the machinery
Amalgamation is one of the oldest methods of extracting gold from its ores. According to Herbert Hoover, in a footnote
to his translation of Georgius Agricola’s De Re Metallica, the amalgamation of gold possibly dates from Roman
times. The extraction of silver by amalgamation is generally attributed to the Spanish in Latin America in the late sixteenth
century even though Biringuccio describes the fundamental principals of the “patio process� about a half a century
earlier.
The amalgamation of gold ores as described by Agricola in De Re Metallica, which was published in 1556, is essentially
the same as modern practice. The only changes are in the machinery
Safety
Before going any further into the subject of amalgamation, I will say a few words about the hazards of working with
mercury. Like most things in our environment it is safe enough if it is treated with due respect and reasonable
precautions are observed. However, the effects of mercuric poisoning are cumulative and it can do considerable
damage if mistreated over a long period of time.
Mercury can be absorbed through the skin or inhaled in vapor form. Gloves should be worn whenever it is handled.
Be sure that you are in a well ventilated area if there is any possibility of vapors forming. Avoid breathing any of the
vapor at all costs.
Theory of Amalgamation
The physical and/or chemical characteristics that make amalgamation work are not clearly understood to this day.
However, it is known that if clean mercury is brought into contact with clean gold, the gold is wetted and “drawn
into� the mercury. This results in a solution of gold in mercury or an alloy of gold and mercury called amalgam.
After the mercury has gathered in the gold it can be removed by dissolving it in nitric acid or by driving it off as a
vapor by heat. The gold will remain behind.
The mill operator’s problem is to get the gold and the mercury into intimate contact with each other. To do this
he must: (1) liberate the gold particles from the gangue; (2) remove any coating which may be covering the gold; (3)
keep the mercury clean and bright; and (4) bring the gold and mercury into intimate contact. Then he must allow the
amalgam to coalesce, separate it from the pulp, and extract the gold.
Amalgamation generally works best on relatively coarse gold that can be liberated from the gangue and abraded
clean without too much difficulty. Since mercury will not penetrate into minute crevasses or pores, the ore must be
ground fine enough to expose the gold at the surface. If the gold is very fine, cyanidation is more effective and, in
practice, a combination of amalgamation and cyanidation is usually employed. Gravity and flotation are also
frequently used in conjunction with amalgamation.
Some of the things that tend to frustrate the millman’s attempt to get the mercury and gold together are: (1) The
existence of surface coatings or encasement of the gold in the gangue. Fine grinding and abrasion will usually solve
this problem. (2) The presence of oil, grease, clay or iron and base metal sulfides may result in sickening or flouring
of the mercury. Grinding in lime or some other alkali will usually prevent this occurrence. (3) The presence of carbon
as graphite also sickens the mercury. In some instances it can be removed by flotation prior to amalgamation. (4)
The presence of sulfides of arsenic, antimony or bismuth will cause flouring and sickening of the mercury. This
usually makes the recovery of gold by amalgamation impractical.
Before going any further into the subject of amalgamation, I will say a few words about the hazards of working with
mercury. Like most things in our environment it is safe enough if it is treated with due respect and reasonable
precautions are observed. However, the effects of mercuric poisoning are cumulative and it can do considerable
damage if mistreated over a long period of time.
Mercury can be absorbed through the skin or inhaled in vapor form. Gloves should be worn whenever it is handled.
Be sure that you are in a well ventilated area if there is any possibility of vapors forming. Avoid breathing any of the
vapor at all costs.
Theory of Amalgamation
The physical and/or chemical characteristics that make amalgamation work are not clearly understood to this day.
However, it is known that if clean mercury is brought into contact with clean gold, the gold is wetted and “drawn
into� the mercury. This results in a solution of gold in mercury or an alloy of gold and mercury called amalgam.
After the mercury has gathered in the gold it can be removed by dissolving it in nitric acid or by driving it off as a
vapor by heat. The gold will remain behind.
The mill operator’s problem is to get the gold and the mercury into intimate contact with each other. To do this
he must: (1) liberate the gold particles from the gangue; (2) remove any coating which may be covering the gold; (3)
keep the mercury clean and bright; and (4) bring the gold and mercury into intimate contact. Then he must allow the
amalgam to coalesce, separate it from the pulp, and extract the gold.
Amalgamation generally works best on relatively coarse gold that can be liberated from the gangue and abraded
clean without too much difficulty. Since mercury will not penetrate into minute crevasses or pores, the ore must be
ground fine enough to expose the gold at the surface. If the gold is very fine, cyanidation is more effective and, in
practice, a combination of amalgamation and cyanidation is usually employed. Gravity and flotation are also
frequently used in conjunction with amalgamation.
Some of the things that tend to frustrate the millman’s attempt to get the mercury and gold together are: (1) The
existence of surface coatings or encasement of the gold in the gangue. Fine grinding and abrasion will usually solve
this problem. (2) The presence of oil, grease, clay or iron and base metal sulfides may result in sickening or flouring
of the mercury. Grinding in lime or some other alkali will usually prevent this occurrence. (3) The presence of carbon
as graphite also sickens the mercury. In some instances it can be removed by flotation prior to amalgamation. (4)
The presence of sulfides of arsenic, antimony or bismuth will cause flouring and sickening of the mercury. This
usually makes the recovery of gold by amalgamation impractical.
There is some confusion about the meaning of the words flouring and sickening of the mercury. If the mercury will
not wet or take up the gold or coalesce into larger globules it is said to be sick. Sickening is most likely caused by
impurities in or on the surface of the mercury. The most common impurities are oil, grease, clay, manganese and
iron sulfates, and base metal and iron sulfides. Flouring in the strict sense is the division of the mercury into
extremely small globules. This gives it a white flour-like appearance. This is not bad in itself but the mercury seems
to be more susceptible to sickening while in the finely divided state. Therefore, it does not coalesce but stays in a
floured condition and is lost to tails. Any gold that it took up before becoming sickened is also lost to tails.
Mechanisms
The traditional use of amalgamation involved the stamp mill and amalgamation plates. A variation using agitation
tubs for amalgamation was described in some detail in De Re Metallica. Today, with the exception of a few
traditionalists, the stamp mill has been replaced by the ball mill for this purpose. The mercury is fed into the ball mill
with the ore and is then passed over prepared plates.
The plates, which should be of pure annealed copper, preferably at least 1/8 inch think to prevent buckling, must be
scoured with sand and lye to remove any coating or oxidation. The resulting bright metallic copper is then rinsed
with clean water and washed with a 2 to 3% solution of cyanide if available. Next a mixture of sal ammoniac and fine
sand in equal proportions containing mercury is scrubbed onto the plate and as much mercury as the plate will
adsorb is added. After coating the plate should be washed again with clean water and rinsed with the cyanide
solution. Keeping the mercury clean will be a problem until the plate builds up a good gold amalgam coating. To
alleviate this problem the plates can be silvered by applying silver amalgam to the prepared plate.
not wet or take up the gold or coalesce into larger globules it is said to be sick. Sickening is most likely caused by
impurities in or on the surface of the mercury. The most common impurities are oil, grease, clay, manganese and
iron sulfates, and base metal and iron sulfides. Flouring in the strict sense is the division of the mercury into
extremely small globules. This gives it a white flour-like appearance. This is not bad in itself but the mercury seems
to be more susceptible to sickening while in the finely divided state. Therefore, it does not coalesce but stays in a
floured condition and is lost to tails. Any gold that it took up before becoming sickened is also lost to tails.
Mechanisms
The traditional use of amalgamation involved the stamp mill and amalgamation plates. A variation using agitation
tubs for amalgamation was described in some detail in De Re Metallica. Today, with the exception of a few
traditionalists, the stamp mill has been replaced by the ball mill for this purpose. The mercury is fed into the ball mill
with the ore and is then passed over prepared plates.
The plates, which should be of pure annealed copper, preferably at least 1/8 inch think to prevent buckling, must be
scoured with sand and lye to remove any coating or oxidation. The resulting bright metallic copper is then rinsed
with clean water and washed with a 2 to 3% solution of cyanide if available. Next a mixture of sal ammoniac and fine
sand in equal proportions containing mercury is scrubbed onto the plate and as much mercury as the plate will
adsorb is added. After coating the plate should be washed again with clean water and rinsed with the cyanide
solution. Keeping the mercury clean will be a problem until the plate builds up a good gold amalgam coating. To
alleviate this problem the plates can be silvered by applying silver amalgam to the prepared plate.