handbook of flotation reagents
TRANSCRIPT
Handbook of Flotation Reagents: Chemistry, Theory and Practice
Flotation of Gold, PGM and Oxide Minerals Volume 2
By Srdjan M. Bulatovic
ELSEVIER
Copyright © 2010 Elsevier B.V.All right reserved.ISBN: 978-0-08-093209-5
Chapter One
Flotation of Gold Ores
17.1 INTRODUCTION
The recovery of gold from gold-bearing ores depends largely on the nature of the deposit, the mineralogy of the ore and the distribution of gold in the ore. The methods used for the recovery of gold consist of the following unit operations:
1. The gravity preconcentration method, which is used mainly for recovery of gold from placer deposits that contain coarse native gold. Gravity is often used in combination with flotation and/or cyanidation.
2. Hydrometallurgical methods are normally employed for recovery of gold from oxidized deposits (heap leach), low-grade sulphide ores (cyanidation, CIP, CIL) and refractory gold ores (autoclave, biological decomposition followed by cyanidation).
3. A combination of pyrometallurgical (roasting) and hydrometallurgical route is used for highly refractory gold ores (carbonaceous sulphides, arsenical gold ores) and the ores that contain impurities that result in high consumption of cyanide, which have to be removed before cyanidation.
4. The flotation method is a technique widely used for the recovery of gold from gold-containing copper ores, base metal ores, copper nickel ores, platinum group ores and many other ores where other processes are not applicable. Flotation is also used for the removal of interfering impurities before hydrometallurgical treatment (i.e. carbon prefloat), for upgrading of low-sulphide and
refractory ores for further treatment. Flotation is considered to be the most cost-effective method for concentrating gold.
Significant progress has been made over the past several decades in recovery of gold using hydrometallurgical methods, including cyanidation (CIL, resin-in-pulp), bio-oxidation, etc. All of these processes are well documented in the literature and abundantly described. However, very little is known about the flotation properties of gold contained in various ores and the sulphides that carry gold. The sparse distribution of discrete gold minerals, as well as their exceedingly low concentrations in the ore, is one of the principal reasons for the lack of fundamental work on the flotation of gold-bearing ores.
In spite of the lack of basic research on flotation of gold-bearing ores, the flotation technique is used not only for upgrading of low-grade gold ore for further treatment, but also for beneficiation and separation of difficult-to-treat (refractory) gold ores. Flotation is also the best method for recovery of gold from base metal ores and gold-containing PGM ores. Excluding gravity preconcentration, flotation remains the most cost-effective beneficiation method.
Gold itself is a rare metal and the average grades for low-grade deposits vary between 3 and 6 ppm. Gold occurs predominantly in native form in silicate veins, alluvial and placer deposits or encapsulated in sulphides. Other common occurrences of gold are alloys with copper, tellurium, antimony, selenium, platinum group metals and silver. In massive sulphide ores, gold may occur in several of the above forms, which affects flotation recovery.
During flotation of gold-bearing massive sulphide ores, the emphasis is generally placed on the production of base metal concentrates and gold recovery becomes a secondary consideration. In some cases, where significant quantities of gold are contained in base metal ores, the gold is floated from the base metal tailings.
The flotation of gold-bearing ores is classified according to ore type (i.e. gold ore, gold copper ore, gold antimony ores, etc.), because the flotation methods used for the recovery of gold from different ores is vastly different.
17.2 GEOLOGY AND GENERAL MINERALOGY OF GOLD-BEARING ORES
The geology of the deposit and the mineralogy of the ore play a decisive role in the selection of the best treatment method for a particular gold ore. Geology of the gold deposits varies considerably not only from deposit to deposit, but also within the deposit. Table 17.1 shows major genetic types of gold ores and their mineral composition. More than 50% of the total world gold production comes from clastic sedimentary deposits.
In many geological ore types, several sub-types can be found including primary ores, secondary ores and oxide ores. Some of the secondary ores belong to a group of highly refractory ores, such as those from Nevada (USA) and Chile (El Indio). The number of old minerals and their associations are relatively small and can be divided into the following three groups: (a) native gold and its alloys, (b) tellurides and (c) gold associated with platinum group metals. Table 17.2 lists the major gold minerals and their associations.
17.3 FLOTATION PROPERTIES OF GOLD MINERALS AND FACTORS AFFECTING FLOATABILITY
Native gold and its alloys, which are free from surface contaminants, are readily floatable with xanthate collectors. Very often however, gold surfaces are contaminated or covered with varieties of impurities. The impurities present on gold surfaces may be argentite, iron oxides, galena, arsenopyrite or copper oxides. The thickness of the layer may be of the order of 1-5 μm. Because of this, the flotation properties of native gold and its alloys vary widely. Gold covered with iron oxides or oxide copper is very difficult to float and requires special treatment to remove the contaminants.
Tellurides, on the other hand, are readily floatable in the presence of small quantities of collector, and it is believed that tellurides are naturally hydrophobic. Tellurides from Minnesota (USA) were floated using dithiophosphate collectors, with over 9% gold recovery.
Flotation behaviour of gold associated in the platinum group metals is apparently the same as that for the platinum group minerals (PGMs) or other minerals associated with the PGMs (i.e. nickel, pyrrhotite, copper and pyrite). Therefore, the reagent scheme developed for PGMs also recovers gold. Normally, for the flotation of PGMs and associated gold, a combination of xanthate and dithiophosphate is used, along with gangue depressants guar gum, dextrin or modified cellulose. In the South African PGM operations, gold recovery into the PGM concentrate ranges from 75% to 80%.
Perhaps the most difficult problem in flotation of native gold and its alloys is the tendency of gold to plate, vein, flake and assume many shapes during grinding. Particles with sharp edges tend to detach from the air bubbles, resulting in gold losses. This shape factor also affects gold recovery using a gravity method.
In flotation of gold-containing base metal ores, a number of modifiers normally used for selective flotation of copper lead, lead zinc and copper lead zinc have a negative effect on the floatability of gold. Such modifiers include ZnSO4 · 7H2O, SO2, Na2S2O5 and cyanide when added in excessive amounts.
The adsorption of collector on gold and its floatability is considerably improved by the presence of oxygen. Figure 17.1 shows the relationship between collector adsorption, oxygen concentration in the pulp and conditioning time. The type of modifier and the pH are also important parameters in flotation of gold.
17.4 FLOTATION OF LOW-SULPHIDE-CONTAINING GOLD ORES
The beneficiation of this ore type usually involves a combination of gravity concentration, cyanidation and flotation. For an ore with coarse gold, gold is often recovered by gravity and flotation, followed by cyanidation of the reground flotation concentrate. In some cases, flotation is also conducted on the cyanidation tailing. The reagent combination used in flotation depends on the nature of gangue present in the ore. The usual collectors are xanthates, dithiophosphates and mercaptans. In the scavenging section of the flotation circuit, two types of collector are used
as secondary collectors. In the case of a partially oxidized ore, auxiliary collectors, such as hydrocarbon oils with sulphidizer, often yield improved results. The preferred pH regulator is soda ash, which acts as a dispersant and also as a complexing reagent for some heavy metal cations that have a negative effect on gold flotation. Use of lime often results in the depression of native gold and gold-bearing sulphides. The optimum flotation pH ranges between 8.5 and 10.0. The type of frother also plays an important role in the flotation of native gold and gold-bearing sulphides. Glycol esters and cyclic alcohols (pine oil) can improve gold recovery significantly.
Amongst the modifying reagents (depressant), sodium silicate starch dextrins and low-molecular-weight polyacrylamides are often selected as gangue depressants. Fluorosilicic acid and its salts can also have a positive effect on the floatability of gold. The presence of soluble iron in a pulp is highly detrimental for gold flotation. The use of small quantities of iron-complexing agents, such as polyphosphates and organic acids, can eliminate the harmful effect of iron.
17.5 FLOTATION OF GOLD-CONTAINING MERCURY/ANTIMONY ORES
In general, these ores belong to a group of difficult-to-treat ores, where cyanidation usually produces poor extraction. Mercury is partially soluble in cyanide, which increases consumption and reduces extraction. A successful flotation method has been developed using the flowsheet shown in Figure 17.2, where the best metallurgical results were obtained using a three-stage grinding and flotation approach. The metallurgical results obtained with different grinding configurations are shown in Table 17.3.
Flotation was carried out at an alkaline pH, controlled by lime. A xanthate collector with cyclic alcohol frother (pine oil, cresylic acid) was shown to be the most effective. The use of small quantities of a dithiophosphate-type collector, together with xanthate was beneficial.
17.6 FLOTATION OF CARBONACEOUS CLAY-CONTAINING GOLD ORES
These ores belong to a group of refractory gold ores, where flotation techniques can be used to (a) remove interfering impurities before the hydrometallurgical treatment process of the ore for gold recovery, and (b) to preconcentrate the ore for further pyrometallurgical or hydrometallurgical treatment. There are several flotation methods used for beneficiation of this ore type. Some of the most important methods are described below.
17.6.1 Preflotation of carbonaceous gangue and carbon
In this technique, only carbonaceous gangue and carbon are recovered by flotation, in preparation for further hydrometallurgical treatment of the float tails for gold recovery. Carbonaceous gangue and carbon are naturally floatable using only a frother, or a combination of a frother and a light hydrocarbon oil (fuel oil, kerosene, etc.). When the ore contains clay, regulators for clay dispersion are used. Some of the more effective regulating reagents include sodium silicates and oxidized starch.
17.6.2 Two-stage flotation method
In this technique, carbonaceous gangue is prefloated using the above-described method, followed by flotation of gold-containing sulphides using activator-collector combinations. In extensive studies conducted on carbonaceous gold-containing ores, it was established that primary amine-treated copper sulphate improved gold recovery considerably. Ammonium salts and sodium sulphide (Na2S · 9H2O) also have a positive effect on gold-bearing sulphide flotation, at a pH between 7.5 and 9.0. The metallurgical results obtained with and without modified copper sulphate are shown in Table 17.4.
17.6.3 Nitrogen atmosphere flotation method
This technique uses a nitrogen atmosphere in grinding and flotation to retard oxidation of reactive sulphides, and has been successfully applied on carbonaceous ores from Nevada (USA). The effectiveness of the method depends on (a) the amount of carbonaceous gangue present in the ore, and (b) the amount and type of clay. Ores that are high in carbon or contain high clay content (or both) are not amenable for nitrogen atmosphere flotation.
17.7 FLOTATION OF GOLD-CONTAINING COPPER ORES
The floatability of gold from gold-containing copper gold ores depends on the nature and occurrence of gold in these ores, and its association with iron sulphides.
Gold in the porphyry copper ore may appear as native gold, electrum, cuproaurid and sulphosalts associated with silver. During the flotation of porphyry copper-gold ores, emphasis is usually placed on the production of a marketable copper-gold concentrate and optimization of gold recovery is usually constrained by the marketability of its concentrate.
The minerals that influence gold recovery in these ores are iron sulphides (i.e. pyrite, marcasite, etc.), in which gold is usually associated as minute inclusions. Thus, the iron sulphide content of the ore determines gold recovery in the final concentrate. Figure 17.3 shows the relationship between pyrite content of the ore and gold recovery in the copper concentrate for two different ore types. Most of the gold losses occur in the pyrite.
The reagent schemes used in commercial operations treating porphyry copper-gold ores vary considerably. Some operations, where pyrite rejection is a problem, use a dithiophosphate collector at an alkaline pH between 9.0 and 11.8 (e.g. OK Tedi/PNG Grasberg/ Indonesia). When the pyrite content in the ore is low, xanthate and dithiophosphates are used in a lime or soda ash environment.
In more recent years, in the development of commercial processes for the recovery of gold from porphyry copper-gold ores, bulk flotation of all the sulphides has been emphasized, followed by regrinding of the bulk concentrate and sequential flotation of copper-gold from pyrite. Such a flowsheet (Figure 17.4) can also incorporate high-intensity conditioning in the cleaner-scavenger stage. Comparison of metallurgical results using the standard sequential flotation flowsheet and the bulk flotation flowsheet are shown in Table 17.5. A considerable improvement in gold recovery was achieved using the bulk flotation flowsheet.
During beneficiation of clay-containing copper-gold ores, the use of small quantities of Na2S (at natural pH) improves both copper and gold metallurgy considerably.
In the presence of soluble cations (e.g. Fe, Cu), additions of small quantities of organic acid (e.g. oxalic, tartaric) improve gold recovery in the copper concentrate.
Some porphyry copper ores contain naturally floatable gangue minerals, such as chlorites and aluminosilicates, as well as preactivated quartz. Sodium silicate, carboxy methyl-cellulose and dextrins are common depressants used to control gangue flotation.
Gold recovery from massive sulphide copper-gold ores is usually much lower than that of porphyry copper-gold ores, because very often a large portion of the gold is associated with pyrite. Normally, gold recovery from these ores does not exceed 60%. During the treatment of copper-gold ores containing pyrrhotite and marcasite, the use of Na2H2PO4 at alkaline pH values depresses pyrrhotite and marcasite, and also improves copper and gold metallurgy.
17.8 FLOTATION OF OXIDE COPPER-GOLD ORES
Oxide copper-gold ores are usually accompanied by iron hydroxide slimes and various clay minerals. There are several deposits of this ore type around the world, some of which are located in Australia (Red Dome), Brazil (Igarape Bahia) and the Soviet Union (Kalima). Treatment of these ores is difficult, and even more complicated in the presence of clay minerals.
Recently, a new class of collectors, based on ester-modified xanthates, have been successfully used to treat gold-containing oxide copper ores, using a sulphidization method. Table 17.6 compares the metallurgical results obtained on the Igarape Bahia ore using xanthate and a new collector (PM230, supplied by Senmin in South Africa).
The modifier used in the flotation of these ores included a mixture of sodium silicate and Calgon. Good selectivity was also achieved using boiled starch.
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GOLD MINING & WATER TREATMENT PROCESS EQUIPMENT
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Gold Mining Process Development
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:
1. Gravity separation
2. Flotation
3. Cyanidation
4. Refractory ore processing
5. Alternative lixiviants
6. Amalgamation
Cyanidation processes may include the following operations:
1. Agitated tank leaching
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2. Heap leaching
3. Carbon adsorption recovery
4. Zinc precipitation recovery
Carbon adsorption recovery may include the following alternatives:
1. Carbon-In-Pulp (CIP)
2. Carbon-In-Leach (CIL)
3. 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:
1. Bioleaching
2. Autoclaving (pressure oxidation)
3. Roasting
4. Clorination
5. Pre-oxidation
6. 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:
1. Bromides (Acid and Alkaline)
2. Chlorides
3. Thiourrea
4. 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:
1. Sluice boxes
2. Rocker boxes
3. Jigs
4. Spirals
5. Shaking tables
6. Centrifugal concentrators
7. 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:
1. Bio-oxidation: Bio-oxidation uses sulfur consuming bacteria in a water solution
to remove sulfur.
2. 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.
3. 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.
4. Chemical oxidation using nitric acid at ambient pressure and temperature
has also been used on a limited basis.
Other ore types considered refractory include:
1. 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.
2. Copper/gold ores that require uneconomically high quantities of cyanide to
process due to the solubility of copper in cyanide.
3. 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:
1. 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.
2. Solids liquid separation steps are not required for heap leaching.
3. 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:
1. 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.
2. In areas of high rainfall, solution balance problems can arise, resulting
in the need to treat and discharge process water.
3. 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.
4. 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.
CONCLUSION
Denver Mineral Engineers has had extensive experience with all of the
commercially viable gold and silver recovery mining processes. We can suggest
the optimal process and equipment for virtually any ore. Although we are
not a testing laboratory, we can design and coordinate your testing program.
If we don’t have the answers, our network of industry experts can be utilized.
We are not research metallurgists and we will not try to make a career out
of investigating your ore. The emphasis of our company is to build process
systems that produce profits for our clients.
Fast & Associates, LLC
Denver Mineral Engineers, Inc.
10641 Flatiron Rd.
Littleton, CO 80124 USA
Phone: (303) 932-6280
Fax: (303) 790-4701
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ChemistryInorganic ChemistryTransition Metal CompoundsMetallurgy
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Metallurgical ProcessesThe process of extracting a metal from its ore and refining it, is called metallurgical process or simply as metallurgy.
The actual process of extraction of a metal from its ore depends upon the nature of the ore and the metal. There is no universally operational method for the extraction of metals. Certain common steps however, are involved in all metallurgical processes.
Steps Involved in a Metallurgical ProcessBack to Top
The extraction of a metal from its ore involves the following steps:
Mining of OreBack to Top
Most ores generally occur deep inside the Earth. Some may occur only a few metres under the earth's surface. 'Mining' is the process of taking out the ores from the mines. When an ore occurs near the surface of the Earth, it can be directly dug out. Such mining is termed as open-pit mining. When an ore is taken out from greater depths, then the mining is termed deep-mining.
Crushing of the OreBack to Top
Extracted ore often occurs in big lumps. It is essential to break it into smaller pieces The lumps are crushed to smaller pieces by hammering in a hammer mill or by help of a jaw-crusher.
Grinding and Pulverization of the Crushed OreBack to Top
The crushed ore is then finally pulverized to fine powder state in a stamp mill or a pulveriser.
Concentration of the Ore (ore DressingBack to Top
)
The removal of the undesired foreign impurities i.e., gangue, from the ore is called concentration (or beneficiation) of the ore. Either of the following methods is used for concentrating the ores:
Hand PickingBack to Top
If the impurities present are quite distinct from the ore, and are of large size, these may be removed by hand picking. This method is slow and is generally adopted in the initial stages of concentration.
Gravity or Levigation MethodBack to Top
When the ore particles are heavier than the gangue particles, the ore is fed into a running stream of water and impurities are washed away. This separation is by way of gravity or levigation method and is commonly used for oxide ores such as hematite and native ore of Au, Ag, etc. In order to concentrate the ore in bulk, a slanting vibrating wooden table with wooden strips called riffles is introduced in the process. Such tables are termed Wilfley tables. The ore is continuously
washed with a fine spray of water and the rocking motion sieves the heavier portions, while allowing the impurities to filter away.
Fig: 10.2 - Wilfley table for washing of the ore
Sometimes in the gravity method, a hydraulic classifier based on the gravity method is used. Ore is agitated by a powerful current of water pushing upwards through the bottom of a conical reservoir. The heavier ore particles settle down and are continuously removed from another opening near the bottom, while the lighter particles are washed away by water.
Fig: 10.3 - Hydraulic classifier
Magnetic SeparationBack to Top
Magnetic separation is done especially in the case of haematite ore, whereby the powdered ore is dropped on to leather or brass conveyer belt, which moves over two rollers one of these rollers, is magnetic. When the ore passes over the magnetic roller, it sticks to the belt due to the force of attraction and falls nearer due to the force of attraction of the magnetized roller. The gangue falls over readily, further away. The ore and the magnetic impurity are collected as two separate heaps.
Fig: 10.4 - Magnetic separation
Froth Flotation ProcessBack to Top
This process is used for concentrating sulphide ores, as such ores are preferentially wetted by oil while the gangue particles are wetted by water. Powdered ore is mixed with water and a little pine oil and the mixture is vigorously stirred by passing compressed air. The froth, which is produced rises to the surface and carries the ore particles along with it. The gangue is left behind
Fig: 10.5 - The froth flotation process
Leaching ProcessBack to Top
In this method, the ore is treated chemically with a suitable reagent that preferentially dissolves the active component of the ore. The concentrated ore form is then recovered from the solution by a suitable chemical method.
A typical example of ore concentration by leaching process is the purification of bauxite using NaOH solution as a leachant. The Bauxite is digested with concentrated solution of caustic soda at 150°C in an autoclave. The Aluminium oxide dissolves in NaOH leaving behind the insoluble impurities, which are removed by filtration.
The solution of NaAlO2 (sodium meta-alumiinate) is then treated with freshly prepared Al(OH)3 when the entire aluminium in the solution gets precipitated as Al(OH)3
The precipitate of Al(OH)3 is removed, washed and dried to get Al2O3.
Leaching of Silver OreBack to Top
Leaching process is also employed in the recovery of some precious metals. Silver is extracted from its ores (argentite, Ag2S; horn silver, AgCl) by cyanide process. The finely powdered concentrated ore is treated with a dilute aqueous solution of NaCN (sodium cyanide) and a current of air is passed through the solution. Silver present in the ore gets dissolved due to the formation of soluble sodium argento-cyanide complex, Na[Ag(CN)2] viz.,
Na2S so formed gets oxidized (by air) to Na2SO3, Na2SO4 and thus allow the reaction to go in the forward direction. The solution of Na[Ag(CN)2) is then treated with zinc scrap to recover silver.
With horn silver (AgCl), the reaction with NaCN can be written as,
Leaching of Gold OreBack to Top
Gold-containing ore gets dissolved in KCN solution in the presence of air to give a solution containing K[Au(CN)2]. Gold can then be recovered from this solution by either precipitation or electrolytic method.
Electrostatic concentration and liquation are other methods of concentrating of ores. The usage of these methods depend on the nature of the ores and the type of impurities present.
CalcinationBack to Top
The concentrated ore is converted into oxide by calcination i.e., heating it strongly in the absence of air or roasting (heating it strongly in presence of air). This helps in removing volatile impurities like CO2, SO2, organic matter, and moisture from the ore. For example,
It removes moisture from bauxite.
It removes CO2 from carbonate ores e.g.,
Fig:10.6 - A reverberatory furnace
Calcination is done on the hearth of a reverberatory furnace.
RoastingBack to Top
In this process the ore (usually sulphide) is heated strongly, in the presence of excess of air but below its melting temperature. The result is
It removes moisture, CO2, SO2 and organic matter.
The sulphide ore is converted partly into its oxide or sulphate i.e.,
Similarly,
Roasting is done in a reverberatory furnace or in a blast furnace.
ProblemsBack to Top
2. How is the mixture of lead sulphide and Zinc sulphide ores concentrated?
Solution Back to Top
The mixture of lead sulphide and Zinc sulphide ores are concentrated by the method of electrostatic concentration. The powdered ore is fed upon a roller in a thin layer and subjected to the influence of a electrostatic field. Lead sulphide being a good conductor gets charged immediately and is thrown away from the roller. Zinc sulphide being a poor conductor, falls vertically from the roller.
3. What is liquation and when is it used? → Read More
Solution Back to Top
Liquation is a method for concentrating ores, which have a lower melting point than the impurities. The ores of antimony are concentrated by this method. The powdered ore is heated upon a sloping floor of the furnace. The temperature is raised above the melting point of the ore: this causes the ore to melt and flow down the floor and the impurities are left behind.
4. How is calcination used in limonite? → Read More
Solution Back to Top
Calcination is used in limonite to remove the water of hydration.
5. How are the impurities of sulphur, arsenic and phosphorous removed?
→ Read More
Solution Back to Top
The impurities of sulphur, arsenic and phosphorous are removed by roasting, where they are removed as their volatile oxides.
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