Gold And Silver
Recovery
Mercury Amalgamation
Amalgamation is a concentrating process in which
metallic gold or silver, or an alloy of the two, is mixed with mercury, either
in a amalgamation drum, or on an amalgamation table, where the precious metal
bonds with the mercury to form the metal laden mercury AMALGAM and the waste
(barren) ore pulp are caused to travel different paths to effect separation.
The ore containing the precious metals should be
ground fine enough to allow the maximum exposure of the gold or silver surface
to the mercury (usually between 100 mesh and 325 mesh).
Often, water is used as the ore is mixed with
water to help disperse the ore and to promote a better precious metal to
mercury interface.
Sulfide precious metal ores are difficult to
amalgamate using mercury, due to the complex iron-sulfur-gold and other metals
present, which do not allow the gold to come into contact with the mercury.
It is not uncommon for only 20% to 30% recovery
of gold and silver by mercury amalgamation from sulfide ores.
Precious metals are recovered from the mercury by retorting the mercury.
Retorting consists in distilling off the mercury
from the amalgam and is done in a cast iron retort or steel retort, a vessel
having a cover which can be fastened on so tightly that no fumes of mercury
escape, except by the condenser, which leads from the cover to a vessel
containing water, where the fumes of mercury are condensed to a metallic state.
The condenser has a water jacket surrounding it,
through which a small quantity of cold water is continuously passed during the
operation.
The open end of the vapor pipe must be lower than
where it emerges from the discharge of the retort, and its open end is
submerged in water during the distilling operation.
The retort should not be filled much over half
full, of amalgam, as room is required for the vaporized mercury.
It is good practice to make the amalgam into
several small balls, thus allowing the mercury to vaporize more rapidly.
The distillation is performed at a very low
temperature and heat applied very gradually, until 1,500 deg F is reached.
It usually takes 2 hours to retort the amalgam,
once the 1500 deg F temperature is reached.
DANGER! It should be noted that mercury is very
poisonous to humans and many forms of life.
The warning below is from the material Safety
Data Sheet, which comes with every shipment of mercury.
MAY BE FATAL IF INHALED OR SWALLOWED.
CAUSES RESPIRATORY TRACT, EYE AND SKIN IRRITATION.
MAY CAUSE EYE AND SKIN BURNS.
HARMFUL IF ABSORBED THROUGH SKIN.
CONTAINS MATERIAL WHICH CAUSES DAMAGE TO THE FOLLOWING
ORGANS: KIDNEYS, GASTROINTESTINAL TRACT, RESPIRATORY TRACT, SKIN, CENTRAL
NERVOUS SYSTEM, EYE, LENS OR CORNEA.MAY BE HARMFUL TO ENVIRONMENT IF RELEASED
IN LARGE AMOUNTS. VERY TOXIC TO AQUATIC ORGANISMS.
WARNING: This product contains a chemical(s) known to the
State of California to cause birth defects or other reproductive harm.
The essential operating requirements of the
process are maintenance of an interface that is essentially gold, mercury,
uncontaminated by either liquid or solid films; and presentation to this
interface of precious-metal particles with clean metallic surfaces.
Contamination of the interface occurs in a
variety of ways. The most usual cause of contamination is an oil that is or
contains a fatty acid or a GREASE.
Most machine and cylinder oils and greases used
around mines and mills fall into this category.
Clean, free gold from primary unoxidized ores is
bright and clean and, if brought into contact with clean mercury, amalgamates
readily and quickly.
Gold from oxidized ores, on the other hand, is
almost invariably TARNISHED (lightly filmed with a non-gold-bearing film), and
frequently COATED (rusty) with a heavy film in which oxides of iron are an
important part.
Tarnished gold does not amalgamate readily;
coated gold will not amalgamate at all.
Preparation, in such cases, must be such as will
remove the tarnish and coating; mechanical means (grinding) is almost
invariably the method employed.
Amalgams are defined as alloys of mercury and
other metals.
Considerable evidence is cited (Rose) for the
existence of a series of compounds of gold and mercury ranging from AuHg2 to
Au8Hg.
Mercury filtered from amalgams contains about
0.14% Au at normal temperatures and 0.65% at 1000 C. these figures are taken to
indicate solubility.
Information
is from Taggart, and Charles Kubach, Mining and Mineral Process Engineer
http://mine-engineer.com/mining/minproc/MercAmal.htm
http://mine-engineer.com/mining/minproc/MercAmal.htm
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