The Basic Processes of gold recovery
INTRODUCTION
Man has held a fascination with recovering and acquiring gold almost since
the beginning of time. This paper will attempt to put the multitude of recovery
processes into a current day perspective.
An underlying theme of this paper is that the mineralogy of the ore will
determine the best recovery process and that metallurgical testing is almost
always required to optimize a recovery flowsheet.
The major categories of commercially viable recovery processes include the
following:
Gravity separation
Flotation
Cyanidation
Refractory ore processing
Alternative lixiviants
Amalgamation
Cyanidation processes may include the following operations:
Agitated tank leaching
Heap leaching
Carbon adsorption recovery
Zinc precipitation recovery
Carbon adsorption recovery may include the following alternatives:
Carbon-In-Pulp (CIP)
Carbon-In-Leach (CIL)
Carbon-In-Column (CIC)
Refractory ore processing methods almost always serve only one purpose, to
treat ores that will not liberate their values by conventional cyanide leaching.
The refractory ore treatment process is then followed by a conventional
cyanidation step. Refractory ore processing methods include:
Bioleaching
Autoclaving (pressure oxidation)
Roasting
Clorination
Pre-oxidation
Lime/caustic pretreatment
Amalgamation is one of the oldest processes available. It relies upon the
contact of ore with mercury to form a gold-mercury amalgam. The process is still used extensively by artesian mines in third world countries and at small “mom and pop” mines, due to its simplicity.
GRAVITY CONCENTRATION
Gravity concentration processes rely on the principal that gold contained
within an ore body is higher in specific gravity than the host rocks that
contain the gold. Elemental gold has a specific gravity of 19.3, and typical
ore has a specific gravity of about 2.6. All gravity concentration devices
create movement between the gold and host rock particles in a manner to separate
the heavy pieces from the lighter pieces of material.
The prospector’s gold pan is the most familiar gravity concentration
device. To function properly, the ore must be broken down to particles small
enough to provide a significant specific gravity difference among the particles.
Placer mining has generally been where gravity concentrates have been most
widely applied. In a placer deposit, there has generally been a pre-concentration
of gold made naturally by gravity concentration due to ore particles being
transported by water. Mechanical concentration is used to continue the process
until sufficient concentration is obtained.
Gravity concentration works when gold is in a free elemental state in particles
large enough to allow mechanical concentration to occur.
The number of types of gravity concentration devised that have been used
is almost limitless. Some of the more popular ones are:
Sluice boxes
Rocker boxes
Jigs
Spirals
Shaking tables
Centrifugal concentrators
Dry washers
In addition to specific gravity differences, the performance of gravity
concentration is also affected by particle shape, as can be imagined by comparing
a falling leaf to a twig falling in air.
The process flowsheet generally consists of conditioning and sizing of the
feed material followed by ore or two stages of recovery.
FLOTATION
The flotation process consists of producing a mineral concentrate through
the use of chemical conditioning agents followed by intense agitation and
air sparging of the agitated ore slurry to produce a mineral rich foam
concentrate. The process is said to have been invented by a miner who watched
the process happening while washing dirty work clothing in his home washing
machine.
Specific chemicals are added to either float (foam off) specific minerals
or to depress the flotation of other minerals. Several stages of processing
are generally involved with rough bulk flotation products being subjected
to additional flotation steps to increase product purity.
The flotation process in general does not float free gold particles but is
particularly effective when gold is associated with sulfide minerals such
as pyrites. In a typical pyrytic gold ore, the gold is encapsulated within
an iron sulfide crystal structure. Highly oxidized ores generally do not
respond well to flotation.
Advantages of the flotation process are that gold values are generally liberated
at a fairly coarse particle size (28 mesh) which means that ore grinding
costs are minimized. The reagents used for flotation are generally not toxic,
which means that tailings disposal costs are low.
Flotation will frequently be used when gold is recovered in conjunction with
other metals such as copper, lead, or zinc. Flotation concentrates are usually
sent to an off-site smelting facility for recovery of gold and base metals.
Cyanide leaching is frequently used in conjunction with flotation. Cyanidation
of flotation concentrates or flotation tailings is done depending upon the
specific mineralogy and flowsheet economics.