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 miners
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.
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 employed and the
sources of power to grind, mix, and separate. The use of water wheels
and of goats in a squirrel cage to provide power to wooden machines
are just not too common any more. However, although the chemical and
physical processes are more clearly understood by more recent authors,
the actual "hands on" instructions are remarkably similar and De Re
Metallica can still be used as a reference.
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
Some of the things that tend to frustrate the millman's attempt to get
the mercury and gold together are:
- The existence of surface coatings or encasement of the gold in
the gangue. Fine grinding and abrasion will usually solve this
- 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.
- The presence of carbon as graphite also sickens the mercury. In some
instances it can be removed by flotation prior to amalgamation.
- 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.
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.
The size of plate required will vary with the character of the ore and
the size of the gold particles. If amalgamation is used exclusively
they should be in the range of 10 to 12 square feet per ton of ore per
day. If used in conjunction with cyanidation the area can be reduced to
about 1.5 to 4.5 square feet. They should be placed at a slope of 1 1/4
to 2 1/2 inches per foot. The slope should be adjustable to correct for
inconsistencies in the ore.
Once the plates are loaded, as indicated by the hardening of the amalgam,
mercury is sprinkled on the surface to soften it and it is then scraped
with a hard wood or rubber scraper. After the amalgam has been scraped
off, mercury is once more added to the surface and the plate is returned
A more practical device for the small miner, and particularly the placer
miner, is the barrel amalgamator. This is simply a horizontal rotating
cylinder in which the slurry of finely ground ore and mercury are
placed. The rotation brings the mercury into contact with the particles
of gold. All the problems associated with flouring and sickening of
the mercury must be guarded against as with plate amalgamation. This is
nearly always a batch operation. A good rule of thumb is to use about 5
times the amount of mercury as there is gold in the batch. The addition
of 3 to 4 pounds of caustic soda per ton of ore will help keep both the
gold and the mercury clean. If necessary several rocks may be added to
the batch to increase the agitation and the abrasion. A cement mixer
with the lifts removed makes an ideal vessel for a small scale operation.
For testing or for extremely small scale purposes a copper pan can be
silvered and used in the same manner as the copper amalgamation plate.
There is other equipment available for the amalgamation of gold but they
are variations of the two basic types (1) passing the gold bearing slurry
over silvered copper plates or (2) adding mercury to the gold bearing
slurry and then retrieving the resulting amalgam.
Forgive me but I must bring up my favorite subject; the one thing that
is most often ignored! Everything that goes into the process must be
weighed, sampled, and assayed. Everything that comes out of the process
must be weighed, sampled, and assayed. Otherwise the millman will be
operating in ignorance and will have no chance at all of improving his
process or technique.
After the gold has been taken up by the mercury the amalgam must
be collected. The gold on the plates is scraped off as previously
described. That still in the slurry must be brought together into larger
globules and separated from the slurry. Gravity separation is usually
employed. Sluices, jigs, tables, wheels, pans or any other gravity device
can be used.
The excess mercury can be removed by squeezing it though damp chamois
or canvas leaving a hard lump of amalgam. The remaining mercury can then
be removed by retorting or by dissolving it in dilute nitric acid.
Because mercury vaporizes at a much lower temperature than gold it can
be driven off by heat leaving the gold behind. In the retort the mercury
vapors are captured and condensed so that the mercury can be reused. When
using a retort the temperature should be increased gradually until the
mercury begins to flow into the receiver, then backed off slightly and
maintained until distillation ceases and then increased to a dull red
finish. If heated too rapidly the amalgam may splatter and clog the
outlet which could result in the explosion of the retort.
If a retort is not available, the mercury and silver can be removed by
placing the amalgam in hot dilute nitric acid. One part acid to three
parts water is the proportion that works best. The mercury and silver are
dissolved leaving the gold. This solution can then be used to “silver”
copper plates or pans or, if economics and convenience so dictate,
it can be discarded.
To summarize, I want to once more stress the importance of the proper
care in the handling and use of mercury. The wearing of gloves and the
adequate ventilation of all work areas are simple precautions that can
prevent serious health problems.
I also want to caution you. Don't get lost in the details and mechanics
of the process and forget the fundamentals. Always keep in mind the fact
that the whole process depends upon getting clean, bright mercury into
contact with clean gold. Anything that promotes this simple concept is
good and anything that restricts it is bad.
I sincerely hope that this talk has given you enough information about
amalgamation to make you want to learn enough to make it work for you.