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
Today, cyanide leaching is the method of choice for the recovery of most of the world’s gold production. There are however, many other chemical leaching processes that have been sporadically or historically used. In most instances, cyanide leaching will provide a more technologically effective and cost efficient method. Alternative lixiviants include:
- Bromides (Acid and Alkaline)
- Chlorides
- Thiourrea
- Thiosulfate
Amalgamation is one of the oldest processes available. It relies upon the contact of ore with mercury to form a gold-mercury amalgam. This process is strongly out of favor with the major mining companies, due to the extremely toxic nature of mercury and the processes inferior performance when compared to the available alternatives. The process is still used extensively by artesian mines in third world countries and at small "mom and pop" mines, due to its simplicity.
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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 performance of the various categories of gravity separators are as generally depicted on the following illustration.
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.
CYANIDATION
Cyanide leaching is the standard method used for recovering most of the gold throughout the world today. The process originated around 1890 and quickly replaced all competing technologies. The reason was strictly economical in nature. Where amalgamation plants could recover about 60% of the gold present, cyanide could recover about 90%. Because of the improved recovery, many of the old tailings piles from other processes have been economically reprocessed by cyanide leaching. Cyanide is as close to a "universal solvent" for gold as has been developed. Other leaching reagents will only work on very specific types of ore.
The standard cyanide leach process consists of grinding the ore to about 80% - 200 mesh, mixing the ore/water grinding slurry with about 2 pounds per ton of sodium cyanide and enough quick lime to keep the pH of the solution at about 11.0. At a slurry concentration of 50% solids, the slurry passes through a series of agitated mixing tanks with a residence time of 24 hours. The gold bearing liquid is then separated from the leached solids in thickener tanks or vacuum filters, and the tailings are washed to remove gold and cyanide prior to disposal. The separation and washing take place in a series of units by a process referred to as counter current decantation (CCD). Gold is then recovered from the pregnant solution by zinc precipitation and the solution is recycled for reuse in leaching and grinding.
REFRACTORY ORE PROCESSING
The common definition of "refractory" gold ores, are those ores that do not allow the recovery of gold by standard gravity concentration or direct cyanide leaching.
One major category of refractory ores are gold values contained within the crystalline structure of sulfide minerals such as pyrite and arsenopyrite. For cyanide to leach gold, the cyanide solution must come into direct contact with gold molecules. With many sulfide ores, the ore cannot practically be ground down fine enough to expose the gold particles. The objective of pretreatment for these ores is to remove enough of the sulfide so that at least a small portion of all gold particles are directly exposed to the elements. Processes available for treatment all involve oxidation of sulfur to form water soluble sulfates or sulfur dioxide. The main sulfur oxidation processes include:
- Bio-oxidation: Bio-oxidation uses sulfur consuming bacteria in a water solution to remove sulfur.
- Pressure oxidation: Utilizes oxygen and heat under pressure in a liquid medium, to effect oxidation of sulfur by way of a controlled chemical reaction. High pressure autoclaves are used for the reactors. Reactor operation is under alkaline or acidic conditions, depending upon the specific process.
- Roasting: Roasting uses heat and air to burn away the sulfur from dry ore. Roasting was the standard method for sulfur oxidation years ago when it was considered environmentally acceptable to emit large quantities of sulfur dioxide gas into the atmosphere. Today’s roasting plants employ elaborate gas scrubbing systems that frequently produce sulfuric acid as a byproduct.
- Chemical oxidation using nitric acid at ambient pressure and temperature has also been used on a limited basis.
Other ore types considered refractory include:
- Carbonaceous ores that allow cyanide to dissolve gold but quickly adsorb gold back onto the active carbon in the ore. Treatment processes include chlorination for carbon deactivation, roasting to burn away carbon and carbon-in-leach which introduces competing high activity carbon to preferentially adsorb gold that can be conveniently separated from the leach slurry.
- Copper/gold ores that require uneconomically high quantities of cyanide to process due to the solubility of copper in cyanide.
- A multitude of other unfavorable constituents including pyrrhotite, tellurides, antimony, and arsenic.
It should be noted that most of the refractory ore treatment processes are expensive and frequently economical only with higher grade ores and high processing rates.
HEAP LEACHING
Heap leaching was introduced in the 1970’s as a means to drastically reduce gold recovery costs. This process has literally made many mines by taking low grade geological resources and transforming them to the proven ore category. Ore grades as low as 0.01 oz Au per ton have been economically processed by heap leaching.
Heap leaching involves placing crushed or run of mine ore in a pile built upon an impervious liner. Cyanide solution is distributed across the top of the pile and the solution percolates down through the pile and leaches out the gold. The gold laden pregnant solution drains out from the bottom of the pile and is collected for gold recovery by either carbon adsorption or zinc precipitation. The barren solution is then recycled to the pile.
Heap leaching generally requires 60 to 90 days for processing ore that could be leached in 24 hours in a conventional agitated leach process. Gold recovery is typically 70% as compared with 90% in an agitated leach plant. Even with this inferior performance, the process has found wide favor, due to the vastly reduced processing costs compared with agitated leaching.
The cost advantage areas are largely as follows:
- Comminution: Where as heap leaching is typically done on –3/4 inch rock, agitated leaching requires reduction to –200 mesh. This additional step is typically done with large grinding mills that consume roughly one horsepower per ton per day of capacity.
- Solids liquid separation steps are not required for heap leaching.
- Tailings disposal costs are quite high for a modern agitated leach plant. Large expensive liquid containment dams are required. By comparison, heap leach pads can generally be left in place after reclamation.
Disadvantages, in addition to lower recovery of heap leaching compared with agitated leaching, include:
- The stacked ore must be porous enough to allow solution to trickle through it. There have been many recovery failures due to the inability to obtain solution flow. This is widely experienced when ores have a high clay content. This problem is often alleviated by agglomeration prior to heap stacking.
- In areas of high rainfall, solution balance problems can arise, resulting in the need to treat and discharge process water.
- In extremely cold areas, heap freezing can result in periods of low recovery. Operational procedure modifications such as subsurface solution application have reduced, but not eliminated, this concern.
- Ice and snow melting can result in excessive accumulation of leach solutions. This concern can often be mitigated by use of diversion structures.
Quite frequently, mines will use agitated leaching for high grade ore and heap leaching for marginal grade ores that otherwise would be considered waste rock. A common recovery plant is often employed for both operations.
MERRILL-CROWE RECOVERY
The traditional method for gold recovery from pregnant cyanide solutions is zinc precipitation. Originally, solutions were passed through boxes containing zinc metal shavings. Gold and silver would precipitate out of solution by a simple replacement reaction procedure. Around 1920, zinc shaving precipitation was replaced by the Merrill-Crowe method of zinc precipitation.
The Merrill-Crowe process starts with the filtration of pregnant solution in media filters. Filter types used include pressure leaf filters, filter presses, and vacuum leaf filters. Generally, a precoat of diatomaceous earth is used to produce a sparkling clear solution.
Clarified solution is then passed through a vacuum deaeration tower where oxygen is removed from the solution.
Zinc powder is then added to the solution with a dry chemical feeder and a zinc emulsification cone. The reaction of the special fine powder zinc with the solution is almost instantaneous.
Precipitated gold is then typically recovered in a recessed plate or plate and frame filter press.
CARBON ADSORPTION RECOVERY
Granular coconut shell activated carbon, is widely used for recovery of gold from cyanide solutions. The process can be applied to clean solutions through fluidized bed adsorption columns, or directly to leached ore slurries by the addition of carbon to agitated slurry tanks, followed by separation of the carbon from the slurry by coarse screening methods.
Gold cyanide is adsorbed into the pores of activated carbon, resulting in a process solution that is devoid of gold. The loaded carbon is heated by a strong solution of hot caustic and cyanide to reverse the adsorption process and strip the carbon of gold. Gold is then removed from the solution by electrowinning. Stripped carbon is returned to adsorption for reuse.
The major advantage of carbon-in-pulp recovery over Merrill Crowe recovery is the elimination of the leached ore solids and liquid separation unit operation. The separation step typically involves a series of expensive gravity separation thickeners or continuous filters arranged for countercurrent washing or filtration of the solids. For ores exhibiting slow settling or filtration rates, such as ores with high clay content, the countercurrent decantation (CCD) step can become cost prohibitive.
Ores with high silver content will generally suggest that Merrill-Crowe recovery be used. This is because of the very large carbon stripping and electrowinning systems required for processing large quantities of silver. The typical rule of thumb states that economic silver to gold ratios of greater than 4 to 1, will favor installation of a Merrill-Crowe system, but this decision can be altered if the ore exhibits very slow settling rates.
There are several variations to the carbon adsorption process including:
1. Carbon-In-Column (CIC): With carbon-in-column operation, solution flows through a series of fluidized bed columns in an upflow direction. Columns are most frequently open topped, but closed top pressurized columns are occasionally used.
Carbon columns are most commonly used to recover gold and silver from heap leach solutions. The major advantage of fluidized bed carbon columns is their ability to process solutions that contain as much as 2 to 3 wt% solids. Heap leach solutions are frequently high in solids due to fine particle washing from heaps. Down flow carbon columns are rarely used for gold recovery, because they act like sand filters and are subsequently subject to frequent plugging.
2. Carbon-In-Pulp (CIP): Carbon-in-pulp operation is a variation of the conventional cyanidation process. Ore is crushed, finely ground, and cyanide leached in a series of agitated tanks to solubilize the gold values. Instead of separating solids from the pregnant solution, as in the traditional cyanidation process, granular activated carbon is added to the leached slurry.
The carbon adsorbs the gold from the slurry solution and is removed from the slurry by coarse screening. In practice, this is accomplished by a series of five or six agitated tanks where carbon and ore slurry are contacted in a staged countercurrent manner.
This greatly increases the possible gold loading onto the carbon while maintaining a high recovery percentage. Carbon is retained within the individual CIP tanks by CIP tank screens. The opening size of the CIP tank screens is such that the finely ground ore particles will pass through the screens, but the coarse carbon will not. Almost every imaginable type of screen has been tried for this application, with some types being much more successful than the rest.
3. Carbon-In-Leach (CIL): The carbon-in-leach process integrates leaching and carbon-in-pulp into a single unit process operation. Leach tanks are fitted with carbon retention screens and the CIP tanks are eliminated. Carbon is added in leach so that the gold is adsorbed onto carbon almost as soon as it is dissolved by the cyanide solution. The CIL process is frequently used when native carbon is present in the gold ore. This native carbon will adsorb the leached gold and prevent its recovery. This phenomenon is referred to commonly as "preg-robbing". The carbon added in CIL is more active than native carbon, so the gold will be preferentially adsorbed by carbon that can be recovered for stripping. The CIL process will frequently be used in small cyanide mills to reduce the complexity and cost of the circuit.
There are several disadvantages to CIL compared with CIP. Carbon loading will be 20 to 30% less than with CIP, which means more carbon has to be stripped. (This disadvantage may be overcome by a hybrid circuit, incorporating a cross between CIL and CIP.) The CIL process requires a larger carbon inventory in the circuit, which results in a larger in-process tie up of gold. The larger carbon inventory can also result in higher carbon (and gold) losses through carbon attrition.
