Note: Descriptions are shown in the official language in which they were submitted.
A 7544
2Qfi5$~7
PATENT
' 362100-2024
HACRGROUND OF THE INtIENTION
Technical Field of the Invention.
This invention relates to recovering precious metal
and/or metal values from ores including refractory.ores, ore
concentrates, or ore tailing which include arsenic-, carbon-
and/or sulfur-containing components and ores which are refractory
to the recovery of precious metal values.
Background Art
Precious metals, such as gold, occur naturally in ores
in different forms. Unfortunately, precious metal ores also
frequently contain other materials which interfere with the
recovery of these precious metal values, rendering these ores
refractory to precious metal recovery. Furthermore, the precious
metal content may be at a relatively low level. This low level
content compounds the effect of the refractory nature of these .
ores.
The following patents are illustrative of attempts to
deal with refractory components in precious metals and other
metals recovery a well as efforts in distinctly different fields
addressed to solving the arsenic contamination problems
encountered when roasting precious metal and other metal ores
havl.ng arsenic as an unwanted component present in the ore.
U.S. Patent No. 360,904 to Elizabeth B. Parnell relates
to roasting gold or silver bearing ores using a double roasting
schedule with the first roasting at 1100 to 1300 degrees
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Fahrenheit and the second roasting to 1200°F to 1600°F (the
time
occupied in the second roasting can be reduced by supplying
oxygen along with the air).
U.S. Patent No. 921,645 to J.E. Greenwalt discloses the
roasting of ore by heating the ore on a porous granular bed
through which air is forced from below.
U.S. Patent No. 1,075,011 to N.C. Christensen, Jr.
discloses a process for treating ore by means of a roasting oven
which, by regulation of the fuel supply, may be either oxidizing,
reducing, or neutral.
U.S. Patent No. 2,056,564 to Bernart M. Carter
discloses suspension roasting of finely divided sulfide ores.
Roasting is in air or oxygen in which the temperature of the
mixture entering the roasting chamber is controlled and to a
corresponding degree the temperatures within the roasting chamber
are thus controlled in an effort to prevent the formation of
accretions on the walls of the apparatus.
U.S. Patent No. 2,209,331 to Ture Robert Haglund
discloses a process for the production of sulfur from the
roasting of sulfide material in oxygen or air enriched with
oxygen so that as soon as the free oxygen has been consumed in
the formation of 502, the iron sulfide reacts with the sulfur
diox~.de forming free sulfur and iron oxides.
U.B. Patent No. 2,536,952 to Kenneth D. McCean relates
to roasting mineral sulfides in gaseous suspension.
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U.S. Patent No. 2,596,580 to James B. McKay et al. and
U.S. Patent No. 2,650,169 to Donald T. Tarr, Jr. et al., relates
to roasting gold-bearing ores which contain commercially
significant amounts of gold in association with the mineral
arsenopyrite. The patent describes the importance of closely
regulating the availability of oxygen in order to provide enough
oxygen so that volatile compounds of arsenic are formed while the
formation of nonvolatile arsenic compounds is minimized.
U.S. Patent No. 2,867,529 to Frank A. Forward relates
to treatment of refractory ores and concentrates which contain at
least one precious metal, sulfur and at least one arsenic,
antimony or lead compound by roasting in a non-oxidizing
atmosphere at a temperature above 900 degrees Fahrenheit, but
less than the fusion temperature of the material being roasted.
U.S. Patent No. 2,927,017 to Orrin F. Marvin relates to
a method for refining metals, including precious metals, from
complex ores which contain two or more metal values in chemical
union or in such physical union as to prevent normal mechanical
separation of the values. The method uses multiple roasting '
steps.
U.S. Patent No. 2,993,778 to Adolf Johannsen et al.
relates to roasting a sulfur mineral with its objects being-the
production of sulfur dioxide, increasing the completeness of
roasting and the production of metal oxides.
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U.S. Patent No. 3,172,755 to Angel Vian-Ortuno et al.
relates to a process for treating pyrite ores bearing arsenic by
subjecting the arsenic-containing pyrite ore to partial oxidation
so as to oxidize only the labile sulfur of the arsenic-containing
pyrite and subsequently heating the pyrite ore in a non-oxidizing
gas to separate the arsenic from the ore and to form a residual
ore free of arsenic.
U.S. Patent No. 4,731,114 Gopalan Ramadorai et al.
relates to a process for the recovery of precious metals from
low-grade carbonaceous sulfide ores using partial roasting of the
ores following by aqueous oxidation in an autoclave.
U.S. Patent No. 4,919,715 relates to the use of pure
oxygen in roasting of refractory gold-bearing ores at
temperatures between about 1000°F (537.8°C) and about
1200°F
(648.9°C). This patent fails to address the problem of arsenic
volatilization, is silent on the arsenic content in the ore, and
does not address in that context the optimizing of gold recovery
from refractory sulfidic, carbonaceous ores or separation of
cyanide consuming components before recovery of gold from the
ore. The disclosed method requires two fluid beds and stage-wise
roasting in these beds and the use of substantially pure oxygen
(substantially pure oxygen being defined as at least about 80% by
weight.)
European Patent Specification 0 128 887 discloses
roasting sulfide concentrates having an average particle size
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362100-2024
below 1 mm and containing copper and noble metals as valuable
metals as well as arsenic as an impurity. Volatization of
arsenic: is in a circulating fluidized bed under an oxygen partial
pressure of 10-14 to 10-l6bars and at low temperatures, i.e.
temperatures which exceed the breakdown and decomposition
temperatures of arsenic compounds. A major part of the solids is
removed under the same conditions in a hot cyclone from the
suspension discharged from the fluidized bed reactor and is
recycled to the ~fluidized bed reactor. Additional solids are
removed from the gas in a second cyclone. After an optional fine
purification in an electrostatic precipitator the exhaust gas is
discharged through a chimney. The calcine from the circulating
fluidized bed and eventually solids collected in the second
cyclone are fed to a classical fluidized bed, in which the sulfur
containing materials which are present are roasted at an
increased oxygen potential. In the event the temperature falls
below the sublimation temperature of the arsenic oxides contained
in the exhaust gas from the circulating fluidized bed, arsenic
oxides may be removed together with the residual solids. That
exhaust gas may also contain volatilized sulfur.
German Patent Specification 15 83 184 discloses the
removal of arsenic from iron ores and calcined pyrites in a
process in which the ores are mixed with calcium oxide or calcium
carbonate in an amount of 0.5% to 5% as Ca relative to the weight
of the ore and axe heated in an oxidizing atmosphere to 800°C to
NEH2024sAppln 5
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1000°C so that the arsenic is concentrated in a fine-grained
fraction. This fraction is separated from the coarser fraction
and is leached with acids to remove arsenic. In this patent, in
the description of the state of the art in the roasting of
pyrites, an addition is described of oxides, hydroxides and
various salts of alkali metals and alkaline earth metals. From
these additives, corresponding water-soluble arsenates may be
formed from the arsenic contained in the ore. The effect of
these additives in the roasting stage is constrained by the
formation of the corresponding sulfates. The sulfates are almost
entirely inactive in a reaction for partitioning arsenic. When
the above substances are added to calcined pyrites in an
oxidizing atmosphere at 500°C to 900°C, arsenates will be
formed,
which may be leached with salt solutions or acid solution. These
arsenates should not be dumped in open air dumps. Moreover, the
leaching results in an arsenic-containing solution, which is
nearly impossible to dispose environmentally in an acceptable
manner.
For sulfide ores, any arsenic which is present is an undesired
accompanying element and must be removed from the calcine and
from the roaster gas. This is typically accomplished by a so-
called dearsenication roasting. The arsenic content of the
material is volatilized in a roasting zone having a low oxygen
content and enters the gaseous effluent as arsenic vapor or
arsenic oxide vapor and arsenic sulfide vapor. The above
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mentioned U.S. Patent art deals with such roasting. In the ,
gaseous effluent, arsenic and arsenic sulfides are oxidized to
form arsenic oxide vapors under a relatively high oxygen partial
pressure.
However, a number of problems are encountered. The
dustlike solids contained in the roaster gas are removed at a
temperature exceeding the sublimation temperature of the arsenic
oxides, which are subsequently separated at lower gas
temperatures, or~the solids and the arsenic oxides are jointly
removed at lower gas temperatures. In the first case,
contaminated arsenic oxides will be formed. In the second case,
the arsenic which has been removed will be recycled in the
process scheme. Recycling is together with the other solids
which have been separated, particularly if the solids~contain
valuable metals and for that reason alone must be recirculated,
or the removed solids may be dumped only after taking special
precautionary measures because of the arsenic content. In the
second case there is also a risk that part of the arsenic oxide
may undesirably and unpredictably react with metal oxides to from
metal arsenates, e.g., with Fe203 to form FeAS04. The metal
arsenates deposit on e.g. the ore particle surfaces and clog the
pores of the particle. ,
Particularly in the roasting of gold ores, the
formation of FeAs04 on the particle surfaces will involve a
NEN2024:Appln 7
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362100-2024
higher cyanide consumption in the leaching and a lower yield of
gold.
German Patent Specification 1,132,942 disclosed a
process of roasting iron-containing sulfide ores, particularly
pyrites in which the ores are roasted in a single stage fluidized
bed roaster with oxygen-containing gases at 800°C to 90o°C under
an oxygen partial pressure not in excess of 2.9 x10"8 atm so that
the iron content is reacted to form Fe304, some sulfur is
-- sublimated and arsenic, arsenic sulfides and arsenic oxides are
vaporized. Solids entrained by the roaster exhaust gas are
subsequently removed at temperatures exceeding the condensation
temperatures of sulfur and arsenic and the roaster gas is after-
burned with a supply of air or oxygen so that the oxygen partial
pressure is sufficiently increased to ensure a complete
combustion of the sulfur in the purified roaster gas. The
arsenic oxides produced by the after burning and removed from the
gas stream, will be contaminated by residual dust.
German Patent Specification 1,458,744 discloses the
roasting of iron sulfides by a process in which the ores are
roasted in a single stage fluidized bed roaster with oxygen-
containing gases at 700°C to 1100°C and under an oxygen partial
pressure of about 10-2 to 10"15 atm, whereby Fe2o3 is partly
formed, the arsenic which is present is substantially volatilized
as As203 and the sulfur is volatilized as elementary sulfur. ,
After the solids have been removed from the roaster gas, the
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362100-2024
oxygen partial pressure in the roaster gas is increased by a
supply of air and the elementary sulfur and the arsenic compounds
are oxidized. In that process too the volatile arsenic oxides
are contaminated by residual dust as they are removed from the
gas stream.
From German Patent Specification 30 33 635 it is known
that arsenic-containing material, particularly non-ferrous metal
ores, may be treated and the arsenic may be volatilized in a
.- first stage at temperatures of 627°C to 927°C and under
oxygen
partial pressures of about 10-16 bars. The solids are roasted
under oxidizing conditions in a second stage. The gas from the
second stage is fed in part to a gas purifier and in part to the
first stage. Sulfur and oxygen are added to the exhaust gas from
the second stage and the arsenic contained therein is completely
reacted to form arsenic sulfides, which are partly present as
fine dust and partly as vapor. In a scrubber the vaporous
arsenic sulfides are condensed and removed together with the
solid arsenic sulfides. The arsenic sulfides which have been
removed from the scrubbing water are dumped. The presence of SOZ
involves a risk of a formation of arsenic oxides, which must not
be dumped because of their solubility. Besides, a high
consumption of elementary sulfur is involved.
None of these patents teaches or suggests roasting ores
or refractory ores, ore concentrates or ore tailings of the type
described herein for recovery of metals such as precious metals
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CA 02065837 2003-11-27
in an oxygen-enriched gaseous environment under conditions as
described herein in order to minimize and/or eliminate arsenic
volatilization, facilitate arsenic conversion to an insoluble,
environmentally acceptable form immobilized in a waste product
while reducing the effects of carbon- and sulfur-containing
components on metal recovery such as precious metal recovery.
Moreover, none of the references deals with the conversion of
arsenic to arsenates to environmentally very stable compounds
during e.g. a ~~ingle stage circulating fluid bed roasting of
ores. In fact,, the opposite is true. The present invention
achieves excelT~ent results in a simpler more efficient manner
with outstanding metal, e.g. gold recovery with facile arsenic
elimination as an environmental problem, while minimizing
leaching cyanide consumption and conserving heat given-off
in the roasting process.
More particularly, the present invention proposes
a process for treating ores in the form of ore particles,
having recoverable precious metal values and metal values
and including arsenic-, carbon- and sulfur-containing
components which comprises:
roasting said ore particles in presence of or
with a sufficient addition of at least one substance
selected from the group consisting of a free oxide,
carbonate, sulfate, hydroxide or chloride of calcium,
magnesium, iron and barium, a pyrite or iron in an oxygen-
enriched gaseous atmosphere having a total initial oxygen
content of less than 65% by volume while maintaining a
reaction temperature from 475°C to 900°C during said
roasting, without formation of a molten phase on the
surface of said ore particles so as to form stable
arsenates; and
CA 02065837 2002-08-27
recovering a thus-roasted ore a.s calcine whereby
said calcine is amenable to recovery of precious metal
values in said calcine; wherein said sufficient addition of
said substance in a hyperstoichiometric amount on mole
basis, to react with arsenic: in said ore, wherein:
said roasting is in presence of water vapor up to
10% by weight of ore.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Figure 1 is a flow diagram of the process of the
present invention;
Figure 2 is a side elevation in vertical section
of the roasting apparatus in accordance with the present
invention showing a circulating fluidized bed;
Figure 3 is a side elevations in vertical section
of the roasting apparatus in accordance with the present
invention showing an ebiallating flua_dized bed;
10a
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PATENT
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Figure 4 is a graph of the percent of gold extraction
versus the reaction temperature of the oxygen-enriched gaseous
atmosphere during roasting based on both leaching with a carbon-
in-leach/sodium cyanide leaching and a carbon-in-leach/sodium
cyanide leaching with a sodium hypochlorite pretreatment of the
roasted ore;
Figure 5 is a graph of the percent gold extraction
versus the percent oxygen by volume in the feed gas to the
._ oxygen-enriched,gaseous roasting atmosphere;
Figure 6 is a graph of the percent of gold extraction
versus the reaction temperature of the air atmosphere during
roasting based on leaching with a carbon-in-leach/sodium cyanide
leaching of the roasted ore;
Figure 7 is a schematic drawing of an industrial
embodiment of the present invention;
Figure 8 is a flow chart illustrating the process in
accordance with the invention wherein various oxygen amounts are
introduced in different sections of a circulating fluid bed;
Figure 9 illustrates the range in which stable
arsenates are formed as a function temperature and oxygen partial
pressure and in which the process in accordance with the
invention is carried out. Some of the arsenates formed in the
range in which normal arsenates are formed are water-soluble,
however, increased oxygen content in the roasting gas reduces
NE~f2024:Appln 11
362100-2024
arsenic solubility especially in presence of iron additives, e.g.
pyrites, iron oxides or iron sulfates;
Figure 10 shows the range in which arsenic is
volatilized in the Fe203 range as a function of temperature and
oxygen partial pressure.
Figure 11 is another flow scheme illustrating the
process in accordance with the invention.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the present invention precious metal
and metal values may be recoverable from ore, ore concentrates or
tailings which have arsenic- carbon- and sulfur-containing
components by
1) comminuting the material to a desired particle w
size;
2) roasting the comminuted material under the
conditions set forth herein which oxidizes, or burns off, the
carbon and sulfur values and provides a calcined product amenable
to efficient gold recovery; while .
3) sequestering in and/or converting arsenic to an
insoluble form during roasting of the comminuted material, and
4) leaching with increased efficiency the precious
metal values from the roasted materials.
Hence, it is a desideratum to roast refractory gold
ores in such a manner that cyanide leaching will result in a high
yield of gold, will involve a low consumption of cyanide, and
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will assure economic environmentally acceptable disposal of
arsenic-containing solids.
In accordance with the present invention, the above
objective is accomplished by a process of roasting ores
containing metal values or refractory gold ores or gold ore
concentrates or tailings whereby the roasting is carried out:
a) at temperatures which are between 450°C to 900°C
and below the temperature at which a molten phase of a roasted
ore material is formed;
b) in an oxygen-containing atmosphere of at least 1%
oxygen, on basis of volume, and referenced to a basis amount of
oxygen in air;
e) in the presence of or with an addition of at least
one or more substances of the group consisting of the free
oxides, carbonates, sulfates, hydroxides, and chlorides of
calcium, magnesium, iron and barium, or of pyrites, in an amount
which is in excess or the amount which is stoichiometrically
required to form a stable arsenate; and
d) in the presence of water vapor.
An S02-containing exhaust gas obtained in such reaction
is thereafter purified, and may be sent to an acid plant
producing sulfuric acid wherein surplus oxygen employed in such
acid plant to obtain sulfuric acid is recirculated to an
appropriate place in the process, e.g. circulating fluid bed or
NEHZ024:Appln 1 3
362100-2024
calcine coolers or ore heaters to utilize more efficiently in
such combination oxygen employed in this process.
According to a preferred feature the oxygen content of
the gas defined in b) amounts to 20% to 50% by volume amounts as
high as 65% by volume may be employed.
Other advantages of the present process will be further
explained such as improved heat recovery, fast reaction rates,
lowered emission of gases such as fluorine, etc.
Still further, this invention relates to a process of
removing arsenic vapor and arsenic-compound vapor from dust-
containing hot gases such as during ore roasting, wherein solids
are separated from the gas at a temperature above the
condensation temperature of the arsenic and arsenic compounds.
These arsenic components are subsequently oxidized with a supply
of oxygen-containing gases and immobilized for disposal in an
environmentally acceptable manner meeting with ample margins of
safety the accptable environment disposal requirements.
An another aspect of this invention and as a result of
the novel manner at looking to solve the arsenic problem plaguing
the industry, this invention is to provide an economic process by
which the metallic arsenic and the arsenic compounds found with
mineral values upon roasting and contained in the gases are
converted to a Eorm that these values may be dumped in an
environmentally acceptable manner.
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362100-2024
The above is accomplished, in accordance with the
invention thusly:
i) solids are removed from the gas;
ii) one or more substances are added to the gas, these
substances comprise the group consisting of the oxides,
hydroxides, carbonates, and sulfates or iron, calcium, magnesium
and barium or pyrites; moreover, these substances have a particle
size below 3 mm;
iii) the gas and the added substances are treated in
the presence of water vapor and at temperatures of about 300°C to
about 800°C under oxidizing conditions in such a manner that the
exhaust gas contains at least 1% oxygen and the arsenic content
is reacted to form stable arsenates; and
iv) these stable arsenates are removed from the gas
stream and carried away.
The arsenic compound vapors contained in the gas to be
treated may consist of arsenic oxides and arsenic sulfides. The
percentages are in percent in volume with reference to gases.
Depending on the source of the gas, it may be free of
802 or may contain 502. As discussed above, S02-containing gases
are produced, e.g., by the roasting of sulfur-containing
materials, such as sulfidic non-ferrous metal ores. SOZ-free
gases are produced, e.g., by the thermal processing of arsenic-
containing intermediate products and waste materials, such as
sludges, dusts and solutions as it is known in the metallurgical
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362100-2024
industry. The solids are suitably removed from the gas in
cyclones and/or ceramic filters, such as candle filters and/or
hot electrostatic precipitators.
The above recited additives in ii) may consist of waste
products, such as red mud formed by processes employed in the '
alumina production industry, filter salts and waste gypsum.
Particularly suitable additives are sulfates, e.g. iron sulfates.
The particle size of the additives should be as small as possible
because small particles will reduce the reaction time and the
amount of reactant which is required. The term "stable
arsenates" designates those arsenates which have only a low
solubility in rainwater. The additives are added in amounts
which are sufficient for the formation of the arsenates.
Mixtures of additive used. Water required for the water vapor
content in the gas phase may be introduced into the gas to be
treated by a corresponding supply of steam, as moisture or even
as water of crystallization in the ore or additives. The
arsenates are preferably formed at a temperature of 500°C to ,
600°C. The maximum oxygen content of the exhaust gas is not
critical and may be, e.g., 50% of volume. If the exhaust gas
contains 502, it may be processed in a suitable plant for the
production of sulfuric acid. The treatment may be effected in a
circulating fluidized bed, an ebullating fluidized bed, a
classical fluidized bed, a rotary kiln or a multiple-hearth
furnace; a circulating fluidized bed is preferred.
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The solubility of the stable arsenates is so low that
these may be dumped without special precautionary measures.
According to a preferred feature at least 80% of the
additives employed have a particle size of about 10 to about 200
Vim. With that particle size a substantially complete and fast
formation of arsenates will be effected.
According to a preferred featur~~the water vapor
content of the exhaust gas is adjusted to 0.5% to 10%. This
content will result in a formation of stable arsenates having a
particularly low solubility, e.g. such as scorodites or scorodite
like compounds.
According to a preferred feature, gases in which the
dust has no content or only a low content of metal are treated to
remove only that amount of solids which exceeds the amount of
solids required to form arsenates. A typical example for such
aspect of the invention is in the roasting of pyrites or calcined
pyrites or in the processing of gases in which the dust content
consists of iron compounds. It is possible to utilize at least a
part of the additives for the reaction with a containing arsenic
values and thus these additives need not be separately obtained
and added.
According to another preferred feature, the solids
suspended in the gas are substantially removed therefrom if the
dust in the gas has a valuable metal, e.g. gold. In that case
the valuable metal will substantially be introduced into the
NEII2024:Appln 17
362100-2024
calcine and can be recovered therefrom. It will then be
necessary to add the required additives in the necessary amount
to immobilize the arsenic.
Refractory ores which include carbon-and sulfur-
containing components, such as organic and inorganic carbonaceous
materials and sulfidic minerals, respectively, pose an especially
severe problem in the economical, commercial recovery of precious
metals,.such as gold, because the efficiency and completion of
recovery is dependent on the content of those carbon- and sulfur-
containing components. The recovery yield of precious metal
values in refractory ores can be significantly increased by
oxidizing carbon- and sulfur-containing components. The
efficient oxidation of carbon is especially important because
residual carbon in the roasted ore, or calcine, reduces precious
mtetal recovery during leaching by "preg robbing" because it takes
up or "robs" leachant solubilized gold.
However, refractory ores which further include arsenic-
containing components pose an even more complex problem. This
arsenic content, while amenable to oxidation as discussed above,
poses a problem in that the arsenic component or an intermediate
product of roasting may volatilize at roasting temperatures,
thereby requiring supplemental precautionary processing measures
or the oxidized end product in the calcine solubilizes to a
presently unacceptable level during leaching and/or after the
NEY2024sAppln 1 8
362100-2024
exhausted calcine, i.e. tailings have been discarded and stored
in a heap.
The improved process specifically for precious metal
recovery from these refractory ores or their concentrates or
tailings may be practiced with improved yields. Thus, not only
can improved yields be achieved in an economically efficient
manner, but also the problem of arsenic volatilization~can be
controlled. Consequently, preferrably arsenic is immobilized in
the calcine upon roasting but further roaster gas treatment such
as in the fluidized beds) be practiced to immobilize arsenic in
the event a gas phase treatment of the volatilized arsenic
compounds is desired. As a side benefit, fluorine (while present
in very small amounts in the form of HF) is also converted to an
unknown insoluble form in the calcine such that only a small
percentage must further be treated thereby reducing fluorine
levels. On an elemental basis, the reduced HF and arsenic
immobiliztion levels achieved by the present process are far
below the present day required limits.
Furthermore, the lower temperatures and lower oxygen
concentrations make the process more economically efficient.
The process for the recovery of precious metals from refractory
ores or their concentrates or tailings (here referred to
generically for the sake of simplicity simply as "ore" or "ore
material" or "ore particles") which include arsenic-, carbon- and
sulfur-containing components according to the present invention
Nt1YL024;Appln 19
CA 02065837 2002-08-27
includes roasting that ore in an oxygen-enriched gaseous
atmosphere such as oxygen augmented air having an initial oxygen
content of less than about 65 percent by volume and recovering
the thus-roasted ore, whereby the ore is amenable to recovery of
the precious metal values in it. In the event a reduced content
oxygen atmosphere .is used for a vaporized arsenic compount
treatment in a gas phase, the specific steps will be discussed
proceeding from the above base case as first disclosed in the
continuation-in-part application, Serial. No. 07/684,649, filed
April 12, 1991 and now U.S. Patent No. 5,123,956.
The term "free oxides" in item c) above indicates that
said substances are not present as compounds with arsenic or
sulfur but in a form free of these. If calcium and magnesium as
carbonates are available in a free form i.n the ore in a
sufficient amount, it will be unnecessary to add said substances.
If iron compounds are present, even in a large excess,
an addition will always be required, i.e. i.f below a ratio of 3.5
to 4.0 moles iron to a mole of arsenic, because a major part of
the iron will always be included in compounds with arsenic or
sulfur. Hence, iron must be present of at least 3:5 moles of
iron for each mole of arsenic. The additives may consist of
waste products, such as red mud from the alumina industry, filter
salts and waste gypsum. Sulfates are particularly suitable. As
seen from the data herein, iron compounds are preferred. The use
of an additive is preferable because the additive, in particle
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362100-2024
form will then be present close to the ore particles and will be
able to combine immediately with arsenic which may have been
vaporized from the ore particles at the higher temperatures
discussed herein.
The term "stable arsenates" designates those arsenates
which have only a low solubility in rainwater when stored in a
waste dump of an exhausted calcine. Proper roasting is also
related to the iron content in the ore, e.g., as pyrites in the
ore, the partition of arsenic between oxidation and reaction with
an iron, or other compound in the ore, or an added additive and
the role of iron in added form (if addition is necessary to the
orej the conversion of arsenic to scorodite or scorodite
compounds during roasting and like effects.
The process of the present invention is preferably
suitable for use on candidate precious metal ores having
arsenic-, sulfur- and carbon-containing components. Typically,
iron is in the form of the sulfides in such ores, i.e. pyrites.
Water required for the water vapor may be fed to the
reactor by a suitable addition of steam, as moisture or water in
the ore, of crystallization in the additives or as a water of
crystallization in a component in the ore. Depending on the sot
content, the exhaust gas may be processed for a production of
sulfuric acid or may be scrubbed to remove the S02 or the SOZ
content may be liquefied.
N8S12024;Appln 21
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Preferably, the ore is roasted in the form of fluidized
solids, and more preferably, the ore circulates as fluidized
solids in a circulating fluidized bed or in an ebullating
fluidized bed (which has a circulation feature to it). The ;
precious metal content can be recovered from the thus-roasted ore
or ore concentrate or tailings by separation of cyanide consuming ,
components by solubilization of these and then leaching through
cyanidation, carbon-in-leach cyanidation or carbon-in-pulp
cyanidation.
The advantage afforded by the process in accordance
with this invention resides in that the calcine which is produced
has a very good leachability, with e.g. cyanide, resulting in a
high yield of gold and in a low consumption of cyanide.
Moreover, the arsenic is bound in the form of stable arsenates,
which do not disturb the leaching and which have an extremely low
solubility in rainwater such that these calcines may be dumped
without a need for special precautionary measures or further
treatment(s).
The ores or concentrates may contain up to about 1
arsenic and even up to 2% and more. In addition to the roasting
being effected in a circulating fluidized bed, a stationary
Pluidized bed having a defined upper surface mayy also be used.
Further, an ebullating fluid bed, a rotary kiln or a multiple-
hearth furnace, may be employed, provided the proper reactions
may be obtained. The temperature at which an undesirable molten
NBHt024:Apptn 2 2
2os~s~v
PATENT
362100-2024
phase is formed depends on the composition of the ore in molten
phase in or on the ore particle even a partial molten phase, e.g.
partial sintering is undesirable as metal recovery by leaching is
undesirably affected. The percentages for the gases are stated
in percent by volume.
In the event of a low arsenic content in an ore, the
gas which is fed is adjusted to have a higher oxygen content.
The reaction temperature is achieved by a feeding of hot gases
and/or by an addition of fuel. If fuel is added, oxygen in the
amount required for the combustion of fuel must be added. If a
reaction temperature is low, the required heat is introduced by
feeding of suitable hot gases and/or by a sufficient preheating
of the charged materials.
Roasting, with two stage oxygen injection may be
carried out particularly conveniently. The roasting in the lower
portion of the circulating fluid bed reactor is carried out as
the first stage. A fluidizing gas contains an oxygen-containing
atmosphere having an oxygen content below about 1%. The second
oxygen injection during this roasting stage is carried out in the
upper portion of the reactor with a supply of secondary gas and
optionally even with a supply of tertiary gas having yet more
oxygen injected in that phase at a corresponding higher oxygen
content.
NFH2024:AppM 2 3
PATENT
362100-2024
The candidate ores may have the following levels of
arsenic, carbon and sulfur components on a percent by weight
basis:
Arsenic up to 1.0% or higher
Carbon 2.5% Maximum
Sulfur 5.0% Maximum
w (All percentages are on a weight-to-weight basis unless otherwise
stated.)
The ore is primarily pyritic-carbonaceous-siliceous.
Candidate ores may be found in the region around Carlin, Nevada.
Other types of ores which may be used have been identified as
siliceous-argillaceous-carbonate-pyritic, pyritic-siliceous, and
carbonaceous-siliceous. Small amounts of dolomite, calcite and
other carbonate materials may be present in the ore,
A typical mineralogical analysis of these ores snows:
Quartz 60-85 Percent
Pyrite 1-10 Percent
Carbonate 0-30 Percent
Kaolinite 0-l0 Percent
Fe O 0-5 Percent
Illite 0-5 Percent
Alunite 0-4 Percent
Barite o-4 Percent
NES12024 sAppln 2 4
'~06~83?
PATENT
362100-2024
A typical chemical analysis of the ore shows an average
composition as follows:
Arsenic 0.2 Percent
Sulfur (Total) 4.0 Percent
Carbon (Total) 1.0 Percent
Iron 3.5 Percent
Zinc 0.08 Percent
Strontium 0.03 Percent
.. Gold 0.15 Ounces per ton
This ore, if so treated, typically shows gold recovery
of less than 10 percent by simple cyanidation and less than 20
percent by simple carbon-in-leach cyanidation.
On the other hand, gold recovery by using the process
of the present invention yields from about 75 percent to about 90,
percent (and even higher) gold recovery.
While the primary application of the present invention
relates to ores (as opposed to ore concentrates or tailings), it
appears that ore concentrates may be used or that ore tailings
may be used from the recovery of precious metal, or other values.
The term "ore" as it is used throughout the remainder of this
description encompasses and contemplates not only ores but also
ore concentrates and ore tailings.
According to another feature of this invention, the
roasting treatment according to items a) to d) described above is
NFlIt024:Appln , 25
20683?
PATENT
362100-2024
preceded by a first roasting stage, in which the roasting is
effected at temperatures which are between 450° and 900°C,
preferably below 575°C, and below the temperature at which a
molten phase is formed of an ore material and in an oxygen-
containing atmosphere having an oxygen content below 1%. Such
roasting assures vaporization and an immediate reaction of the
arsenic with the additive. At the second oxygen injection point.
A roasting with two stage oxygen injection may be necessary if
the ores contain, more than about 1% arsenic but may also be
adopted if the ores have a lower arsenic content and are
particularly refractory. The additives according to c).and the
water vapor according to d) need not be present in the first
roasting stage but are preferably added already in the first
roasting stage.
According to a preferred feature the water vapor
content of the gas defined in d) ranges from about 0.5% to 10% by
weight. Arsenates having a particularly low solubility such as
Escorodites will be formed if the water vapor content is in that
range.
The advantages set forth herein-before will be achieved
even with ores which contain about 1% to 2% arsenic if the
roasting is effected by two stage oxygen injection. Roasting in
two stages will produce particularly good results with ores which
contain less than about 1% arsenic although equivalent results
NEH2024:Appln 2 6
206~83'~
PATENT
362100-2024
will also be obtained by proper use of arsenic immobilizing
additives and oxygen content in the roasting gas.
According to a desired feature, provided that no molten
phase forms on or within the ore particle, the roasting is
effected at temperatures of 550°C to 750°C. In case the
formation of a molten phase may reliably be avoided, and the heat
consumption may be low, the arsenic will effectively be bound and
immobilized and the calcine will have a good leachability.
According to a preferred feature the substances defined
in c) are present in at least about 1.5 to about 3 to 4 times the
stoichiometric quantity depending on the particular compound and
ore used. This will result in an effective binding of the
arsenic in conjunction with a relatively small amount of solids.
The amount of the substance added is, of course, determined by
the solubility of arsenic in the exhausted calcine.
According to a preferred feature, the substances
defined in c) are added in a particle size below 1 mm, That
particle size will result in an effective contact and binding of
arsenic present in the ore material.
According to a preferred feature 80% of the substances
defined in c) are added in a particle size of 10 to 50 Vim.
Arsenic will be bound very effectively using that particle size.
The ore is comminuted, or ground, before roasting to a
range of particle sizes, i.e., from about 50% to about 90%
passing through about 200 mesh (-200M) sieve (U. S. or Tyler
IIEW2024sApptn , 27
PATENT
362100-2024
size), and of a set moisture content, i.e., from about 0% to
about 5% (and preferably less than about 1% if clays having water
of crystallization are present).
Next, the ground ore is roasted in an oxygen-enriched
gaseous. atmosphere wherein the carbon and sulfur content is
substantially completely oxidized from an initial roaster feed to
a final calcine content as follows:
COMPONENT ROASTER FINAL
FEED CALCINE
CONTENT
From To About From To
About About About
Arsenic 0.1% 1.0% 0.1% 1.0%
Carbon 0.5% 2.5% 0.02% 0.1%
(total)
Sulfur 0.5% 5.0% 0.05% 0.1%
(total)
Ninety-eight percent or greater of the sulfur content and 90
percent or greater of the carbon content are respectively
oxidized during roasting. For extraction of gold from these
refractory ores, an important consideration is the completeness
of the oxidation of the carbon and sulfur values. Final carbon
values at 0.05% to 0.1% provide good results. The same applies
to sulfide sulfur levels, with final sulfide sulfur values at
0.05% to 0.1% providing good results. However, the final carbon
level is important since it can negatively affect gold recovery
by "preg robbing" during the leaching operation.
NEW2024:App4n 2 8
PATENT
362100-2024
While there is no seemingly apparent reduction in
arsenic content, this is highly desirable sine it is indicative
of the lack of volatilization and/or immobilization of the
arsenic content and ability of iron and other additives to
sequester and/or react with the arsenic in the ore and keep it in
a form without causing any interference with gold recovery and
subsequent long term arsenic solubilization. In other words, the
arsenic content is beneficially retained in the solid phase
ore/calcine rather than being volatilized (with a consequent need
for supplemental~precautionary measures.)
Typically, greater than about 95% of the arsenic is
fixed in the calcine by the presence of a e.g: proper amount of
iron. If desired, additional iron may be added to facilitate
this conversion to an insoluble form. By having greater than a
ratio from about 3.5:1 and e.g. 4:1 of iron to arsenic (molar
ratio), ferricarsenate compounds'fonaed during roasting render
the arsenic in a fixed form in the calcine. Further, the
ferricarsenate compound is insoluble in the subsequent leaching
and from the tailings in dump storage after the gold values are
extracted. Consequently, not only are the arsenic values not
volatilized by the process of the present invention by retaining
them in the calcine fn a nonvolatile form, but also these arsenic
values can be retained in a form which is insoluble to the '
leaching and insoluble over long period while in a dump. A
NEt12024 sApptn 2 9
~O~e'~H~~ PATENT
362100-2024
triple benefit results - reduced arsenic volatilization, long-
term arsenic immobilization, and no impairment of gold recovery.
For the present invention the reaction temperature of
the oxygen-enriched gaseous atmosphere during roasting is
controlled preferably such that it is from about 475°C to about
600°C.
In another aspect of the invention and especially when
volatilized arsenic compounds are formed at higher temperatures
and thereafter converted to resoluble compounds, higher
temperatures are used. However, for the arsenic sequesteration
without arsenic volatilization and/or solubilization, sintering
is to be avoided, i.e. molten phase formation should also be
prevented since molten phase silicates formed, upon even partial
sintering, make the precious metal content of the ore less
amenable to recovery. Further, the reaction temperatures in the
reactor apparatus must be sufficiently high to optimize the
oxidation reaction, particularly the oxidation of carbon- and
sulfur-containing components and formation of e.g. ferricarsenate
compounds. It has been found that a reaction temperature in the
reaction apparatus for the oxygen-enriched gaseous atmosphere of
from about 475°C to about 600°C is desirable, while a preferred
temperature range is from about 500°C to about 575°C.
While the objective of the oxidatian of the carbon and
sulfur content is the formation of oxides wherein carbon and
sulfur are as completely oxidized as possible, the situation with
NEH2024:Appln 3 0
e~ ~ ~ ~ 3621.00-2024
respect to arsenic has more subtle ramifications since certain of
its intermediate oxides, such as arsenic trioxide (As203)
(boiling point 465°C), volatilize at elevated temperatures as do
certain of its sulfides, such as As2S2 (boiling point 565°C), and
As2S5 (sublimates at 500°C). The focus, therefore, is on the
formation of insoluble compounds with the substances recited
above, such as ferricarsenate compounds, e.g. scorodite, to avoid
the volatization problem and to keep arsenic values out of the
process off-gas and keep these in a highly insoluble state. This
control is one of the desirable results that the present
invention achieves by a combination of steps including the
reaction conditions, oxygen content, roasting residence time,
iron content, step wise oxygen injection, etc. However, the
present invention also addresses, as will be further discussed
herein and shown by examples, the volatilized arsenic treatment
in the off-gas by the proper formation of insoluble arsenic
compounds.
The gaseous atmosphere in which, e.g. the gold ore is
roasted is an oxygen-enriched gaseous atmosphere, such as oxygen-
enriched air, having a total initial oxygen content, after
enrichment, of less than about 65 percent (by volume), and
desirably from about 25 percent (by volume) to about 6o percent
(by volume); industrially a range of oxygen of 35% to 55% by
volume is indicated fox the process.
Weuzou:npptn 31
362100-2024
The ground ore is roasted as fluidized solids in the
oxygen-enriched gaseous environment. In effect, the fluidized
ore in the gaseous roasting atmosphere forms a two phase
suspension in which ore is a discontinuous phase composed of
discrete solid particles and the gaseous atmosphere is the
continuous phase. In most instances, the ore concentrates will
have sufficient oxidizable content that there will be an
autothermal oxidation reaction during roasting. In those
instances where there is not sufficient oxidizable content, such
as for ore which does not support an autothermal reaction,
additional oxidizable content is provided by adding a comburant
so that there will be a thermal reaction during roasting.
Typically a low ignition point fuel is added, e.g. coal or
butane/propane. Hence, desirably the ignition point should be
that of propane or below.
Fluidizing the ore facilitates the transfer of
reactants and heat produced by the oxidation reaction, i.e., from
the ore to the gaseous atmosphere and vice versa. It also
increases both reaction velocity and reaction uniformity.
Further, as a result of these factors and the law of mass
reaction, reaction of e.g. the iron and arsenic values to
ferricarsenate compounds and, therefore, arsenic volatilization
can be controlled. The reaction pathway for iron and arsenic
values appears to be the oxidation of iron arid arsenic values to
Eorm ferricarsenates. Because of the great complexity of
NH1f2024;Appl n 3 2
s e7 g 3 ~ PATENT
362100-2024
reactions in any ore during roasting such pathway as arsenic to
ferricarsenate is merely surmised but the important point is e.g.
the scorodite formation. For the other substances disclosed
herein, similar end results are obtained. However, the
ferricarsenates are the desirable end products such as in the
scorodite form.
While t:~e oxidation reaction of the carbon- and sulfur-
containing components is generally exothermic, it may be '
necessary to raise initially the temperature of the ore and the
temperature of the gaseous reaction atmosphere in order to
initiate the oxidation. This may be accomplished by initially
adding a comburant, such as a carbonaceous comburant like coal,
or butane typically coal; or other low combustion, i.e. flash
point fuel. Moreover, if the stoichiometry of the ore is such
that supplemental heat input is needed, the below-described fluid
beds lend themselves well to such supplementation without any
disadvantages.
As another embodiment, an ebullating bed may be used
-- with the overflow from the ebullating bed being constantly
circulated. The reaction velocity may be lower in an ebullating
fluid bed. Efficiency and control over the oxidation and
reaction conditions are improved by circulating the ore as
fluidized solids. An advantage of a circulating fluid bed or an
ebullating fluid bed is the precise control of the bed
temperature; and although an employed temperature is ore specific
NEIf2024 sApptn 3 3
pATENT
362100-2024
within the above ranges, the control is maintained within ~15°C
in a broader aspect; with ~10°C being more typical and ~5°C
being
preferred. Such temperature range permits even greater control
over oxidation of the arsenic-, carbon- and sulfur-containing
components and over reaction of the iron- and arsenic-containing
components with each other while minimizing arsenic
volatilization.
According to a preferred feature the roasting is
performed in a circulating fluidized bed. The fluidized bed
system consists of a fluidized bed reactor, a recycling cyclone
and a recycling line. That fluidized bed differs from a
classical fluidized bed, in which a dense phase is separated by a
distinct density step from the overlying gas space and exhibits
states of distribution having no defined boundary layer. There
is no density step between the dense phase and an overlying dust
space but the solids concentration in the reactor decreases
Continuously from bottom to top. A gas-solid suspension is
discharged from the top of the reactor. In a definition of the
operating conditions by the Froude and Archimedes numbers the
following ranges are obtained:
0.1 5 3/4 x Fr2 x f a S 10
J k - ! g
and 0.01 S Ar 5 100
wherein
NB112024:Appln 3 4
PATENT
362100-2024
Ar - aka~k ~
Jg x vx v
Fr2 = u2
g X dk
and
a the relative gas velocity in m/sec
Ar the Archimedes number
Fr the Froude number
f the density of the gas in kg/m3
9
fk the density of the solid particle in kg/m3
dk the diameter of the spherical particle in m
V the kinematic viscosity in m2/sec
g the constant of gravitation in m/sec2
The suspension discharged from the fluidized bed
reactor is fed to the recycling cyclones) of the circulating
fluidized bed and substantially all solids are removed from the
suspension in said cyclone(s). The solids which have been
removed are returned to the fluidized bed reactor in such a
manner that the solids circulated in the circulating fluidized
bed systems amount to at least four times the weight of solids
contained in the fluidized bed reactor.
Circulating fluidized bed technology is further
discussed in e.g. G. Folland et al., "Lurgi's Circulating Fluid
Bed Applied to Gold Roasting", E & MJ, 28-30 (October 1989) and
Paul Broedermann, "Calcining of Fine-Grained Materials in the
NEU2024:Appln 3,5
CA 02065837 2002-08-27
Circulating Fluid Bed", Lurgi Express Information bulletin - C
1384/3.81,
The residence time of the ore in the oxygen-enriched
gaseous atmosphere should be from about 8 to 10 minutes
preferably from about 10 minutes to about 12 oz- more, but
constrained by practical design considerations such as vessel
size; pump size etc. It should be understood that residence time
is a function of ore mineralogy. Control of residence time at
temperature also controls silicate melting which is to be avoided
since the porosity created by sulfidic sulfur oxidation is then
vitiated. High porosity and low sintering is desirable for the
subsequent leaching of gold.
Following roasting, the precious metal values are
recovered from the thus-roasted ore, or calcine, by leaching,
such as by cyanidation, carbon-in-leach cyanidation or carbon-in-
pulp cyanidation. Such leaching techniques are known in the art
and are described in general in U.S. Patient Nos. 4,902,345 and
4,923,510.
As a bench mark comparison of the roasting efficiency
and completion of the present invention, conventional fluid bed
roasting for equivalent length of time at the same conditions
provides a measure by which the present invention may be
evaluated. Another measure of efficiency and completion are the
36
~O~~S~~ PATENT
362100-2024
amount of cyanide used to extract an equivalent amount of gold,
or residual amounts of gold in ore after standard extraction
procedures. According to the above measures, evaluation of ore
of the same mineralogy will give the outstanding advantages of
the present invention.
The thus-roasted gold ore may be subjected to an oxygen
or chlorine treatment after roasting and prior to leaching. This
treatment may be in the form of bubbling gaseous oxygen or
chlorine through,a suspension or a slurry of the thus-roasted ore
either in a bath at ambient pressure or in a closed vessel at
ambient or elevated pressure prior to leaching the ore.
The precious metal recovery provided by the present
invention from refractory ores which include arsenic-, carbon-
and sulfur-containing components is much improved,. reaching
levels of 75-90% and in some cases higher, such as 92%. It must
be understood that the mineralogy of the ore will influence the
results. Conventionally pyritic sulfides, sulfides and carbon
affect recovery arid higher or lower arsenic content makes it more
or less expensive to treat the ore to meet today's environmental
demands.
DESCRIPTION OF THE ILLUSTRATIONS BROWN IN THE DRAWING
In Figure 1 a self-explanatory flow diagram has been
provided. This generic flow diagram should be considered in
combination with a schematic industrial embodiment shown in
NEN2024sAppln 3 7
362100-2024
Figure 7 for gold recovery from gold ores and also amplified
further herein by the dada shown in Table 7.
As one of the advantageous aspects of this invention,
heat recovery (i.e. as a cost advantage) in this process may be
readily practiced. For example heat may be recovered not only
from the off-gases from the one stage roasting such as derived
from a circulating fluid bed or an ebullating fluid bed, but also
by pooling a calcine with air or air enriched with oxygen e.g. of
up to 65% oxygen, by volume. Such air cooling is taught in U.S.
Patent 4,919,715 to supposedly reduce the recovery of gold,
apparently by as much as 2%, but we have found it not to be
detrimental, if anything, such heat recuperation seems to have
improved the yields.
Another aspect of the invention which has not been
mentioned or apparent from the immediately above-mentioned patent
is that subsequent liquid quenching allows reduction of cyanide
consuming materials. These materials are rendered soluble by the
low temperature oxygen roasting and low temperature oxygen post-
finishing of the calcine during cooling. Such post-finishing
provides excellent sulfation at acidic conditions, e.g. making of
Fez(S04)3 and like compounds of metals such as copper, nickel,
antimony, zinc, lead etc. The removal of these compounds during
liquid quench reduces cyanide consumption during leaching from 2
to 10 pounds more typically from 5 to 10 pounds of cyanide per
NEN2024:Apptn 3 8
362100-2024
ton of calcine to less than one pound e.g. typically 0.3 pound of
cyanide per ton of calcine.
In Figure 2 a schematic representation of appropriately
labeled circulating fluidized bed (CFB) has been shown. The air
input at the bottom of the bed with the recirculating material
from the hot cyclone (or a plurality of cyclones in parallel,
e.g. two) keep the bed in a high degree of turbulence assuring
excellent i.e. almost instantaneous temperature uniformity and
reaction conditions. Typically the complete residence time in
such bed may be based on a number of passes of the bed contents
through the bed, but it is best to express it as overall nominal
residence time for the bed contents. It should be understood
that a residence time is a summation time of the circulating
particles in such bed. It is believed that the post-finishing of
the calcine during cooling has the above-mentioned advantageous
effect for any particle which may have escaped the necessary
residence time in the circulating fluid bed, yet at no overall
reduction of residence efficiency and gold recovery.
Figure 3 shows an ebullating fluid bed which is an
embodiment of a fluid bed suitable as another approach in the
disclosed process. The appropriately labeled illustration
provides for another circulation approach when roasting an ore
material.
Figures 4 to 6 will be further explained in conjunction
with the Examples. Figures 4 and 6 illustrate the "knee-in-the-
NEW2024:Apptn 3 9
i
PATENT
362100-2024
curve" found for the roasting conditions existing as a function
of roasting temperature, oxygen content in roasting gas i.e. air,
and as a function of gold extraction.
In Figure 7 an embodiment showing a schematic
industrial application of the process is illustrated in greater
detail and amplifies the flow chart of Figure d. Other Figures,
i.e. 8 to 11 will be discussed in conjunction with the Examples 8
and 9 herein.
A circulating fluid bed (CFB) reactor 100 is fed from
an ore preheat stage identified with stream 200 corresponding to
the same stream number in Table 7 further disclosed herein. A
start-up gas stream such as butane/propane has been shown
entering the CFB reactor 100 at the bottom thereof.
Additionally, a combined stream of oxygen unexhausted off-gas and
fresh oxygen via preheater 102 is introduced into the CFB reactor
100. The combined stream is identified as 201. Further, a
preheated, oxygen supplemented air stream 208 is introduced in
the CFB reactor 100 and is coming from the post-finishing calcine
treatment which will be discussed below. A single cyclone 103
has been shown in Figure 7, but more than one may be operated in
parallel or in series to assure greater particulate removal from
the off-gas. Cyclone 103 bottoms i.e. underflow collections are
partially reintroduced into the CFB reactor 100 via seal pot 104.
A slip stream 105 of calcined product is also taken from seal pot
104 and introduced into a four stage pre-heaters (recuperators)
weuxou~~~w 4 0
362100-2024
107 to 110 which are in a heat recovery unit 106. Air augmented
with oxygen is brought up to about 450°C in heat recovery unit
106. The unit 106 consists of four pre-heaters in the form of
fluidized beds 107, 108, 109 and 110, respectively: Because the
conditions in each of the pre-heater beds are different, these
pre-heaters 107, 108, 109 and 110 have been identified by
separate numbers. Typically, the CFB reactor 100 is operated at
550°C. The resulting calcine (of retention time of 10 minutes in
reactor 100) is introduced in the first pre-heater 107. The
calcine is at a temperature of about 525°C and has a residence
time of about 15 minutes in preheater 107; in the second pre-
heater 108; the calcine temperature is about 475°C and residence
time is about 10 minutes; in the third pre-heater 109 the calcine
temperature is at about 420°C and the residence time is about 8
minutes; in the fourth pre-heater 110 the calcine temperature is
about 350°C and the residence time is about eight minutes. Air
and oxygen enter these preheaters in parallel, fluidize in each
the calcine, and is mixed and cleaned in cyclone 112. After
separation of particulates in cyclone 112, air and oxygen is
introduced as stream 208 into the CFB reactor 100. A second pre-
heater unit (not shown) of the same type may be operated in
parallel to the first pre-heater unit 106. The seal pot 104 or a
second seal pot (not shown) may feed the second pre-heater unit.
In the data shown in Table 7, these are referred to two parallel
NEHZ024:Apptn 4 1
362100-2024
identical pre-heater units such as 106, two parallel cyclones '
such as 112, and two parallel seal pots such as 104.
Heated air and oxygen from all four pre-heaters is used
and is at about 450°C as shown in Table 7. However, in addition
ambient air is introduced via pump 113 into heating coils 114
immersed in the fluidized calcine in pre-heaters 109 and 110.
This air is used to pre-heat in a CFB type vessel (not shown) the
ore introduced as stream 200 in the CFB reactor 100. Hot air
exits heating coils 114 at 200°C. As contemplated, but subject
to change in the mineralogy of the ore, the balance of.the energy
requirement for roasting is made up by the addition of butane or
pulverized coal to the CFB reactor 100. Calcine in stream 209 is
quenched in water in tank 115 to a 15% solids content and further
worked-up as previously described for removal of a cyanicide
materials, neutralization and subsequent leaching.
Off-gases, i.e. cyclone 103 overflows are introduced
into a waste heat boiler 116 where the off-gas temperature is
reduced to about 375°C, dust from the waste heat boiler 116 is
introduced into the pre-heater unit in an appropriate place, e.g.
pre-heater 108 and combined with calcine. From waste heat boiler
116, the off gases are introduced into an electrostatic
precipitator 117, e.g. a five field, hot electrostatic
precipitator, to remove substantially all residual dust in the
off-gas. A number of precipitators 117 may be used. The exit
temperature of the off-gas from the electrostatic precipitator
NBS12024 sAppln 4 2
20G583'~
PATENT
362100-2024
117 is at about 350°C and the off-gas comprises about 36% by
volume of oxygen. About half of the exit gases are recycled via
pump 118 to the CFB reactor 100. This recycle is a significant
benefit because the off-gas cleaning system becomes about half
the size if the off-gas is recycled. Precipitates from the
electrostatic precipitator are also introduced into the calcine
pre-heat units) 106. The 502 laden exit gases may be sent
directly to an acid plant and further amounts of oxygen
introduced (as needed, for conversion of S02 to an acid as it is
well known in the art). However, the excess oxygen rich gas from
such plant may be recycled to the roasting side of the process
and introduced such as in the CFB reactor 100 or used for calcine
post-finishing, e.g. in fluidized beds 107, 108, 109 and 110 to
aid in sulfating i.e. solubilizing the otherwise cyanide
consumers.
In accordance with the present invention, a series of
experimental runs were conducted which established the
significant process parameters which show the previously
unachieved results of which the present invention is capable.
The following examples illustrate the process of the
present invention in the context of the recovery of gold.
Example 1.
The ore used in these runs came from a random sampling
of arsenic-, sulfidic-, organic carbon-containing, gold-bearing
ores~from the region around Carlin, Nevada. This ore, for the
series of runs showed an average gold content of about 0.16
NEN2024:Appln 4 3
206e~~~~ PATENT
362100-2024
ounces of gold per ton of ore and up to 0.20 oz. of gold per ton,
an average content of 0.08 percent arsenic, 2.49 percent sulfide
sulfur (2.81 percent total sulfur) and 0.79 percent organic
carbon (0.84 percent total carbon.) The ore was classified as
pyritic-carbonaceous-siliceous ore and had the following
.mineralogical and chemical analyses:
~tineralocical Analysis
A typical analysis of this ore shows:
Quartz ~ 68 Percent
Kaolinite 10 Percent
Sericite or Illites 8 Percent
Pyrite 5 Percent
Jarosite 4 Percent
Alunite 3 Percent
Fe O 1 Percent
Barite 1 Percent
Carbonates 0 Percent
IIEIf1024sAppln 4 4
PATENT
362100-2024
Chemical Analysis:
A chemical analysis of the ore shows an average
composition as follows:
Arsenic 824 parts per million
_
Carbon (Total) 0.84 Percent
Sulfur (Total) 2.81 Percent
Gold 0.164 ounces per ton
Iron 4.0 Percent
Zinc 400 parts per million
Strontium 0.02 Percent
The ore was ground in a small ball mill to 100
percent -65 mesh (except as otherwise noted), i.e., 100 percent
passed through a 65 mesh sieve, and it had a bulk density of
about 57 pounds per cubic foot and a moisture content of about 1
percent.
The ground ore was placed in a simple rotating tube
reactor and roasted in a batch operation to evaluate various
reaction conditions using a residence time of two hours for the
sake of consistency.
The roasted ore, or calcine, was treated by a carbon in
leach cyanidation leach using a dosage of 6 pounds of sodium
cyanide per ton of roasted ore and 30 grams per liter of
activated carbon (available Prom North American Carbon.)
NENZ024:Apptn 4 5
PATENT
362100-2024
The leaching was conducted in a continuously rolling
bottle under the following conditions:
1. 200 grams of calcine per leach test
2. 40% solids and '
3. 24 hours leaching time.
A first series of runs was made roasting the ore with
40% oxygen (by volume) initially in the feed gas, or gaseous
atmosphere, at the following temperatures and with the following
results:
Roasting Temperaturefold
(Degree C) Extraction
(Percent)
450 84
475 92
500 86.5
525 82
550 80
600 76.8
(The symbol * in the graph in Figure 4 also shows these results.)
When the roasted ore is treated with sodium hypochlorite at a
rate of 25 pounds per ton of ore and using the same leaching
technique, the results were as follows:
NE1f2024:Appln 4 6
265837
PATENT
362100-2024
Roasting Temperature Gold Arsenic in
(Degree C) Extraction Taiiinge
(Percent)
ppm %
450 86 939 0.094
475 92.5 913 0.091
500 87.3 934 0.093
525 82.5 918 0.092
550 80.3 950 0.095
600 78 898 0.090
(The symbol o on the graph in Figure 4 also shows these results.)
A second run was undertaken in which the roasting
temperature was held at 475 degrees Centigrade and the retention
time at 2 hours, but the percent oxygen (by volume) in the feed
gas, i.e., the total initial oxygen content of the gaseous
atmosphere, was varied as follows and the following percentages
of gold extraction were observed:
Total Oxygen (by Volume)Gold Extraction
in Feed Gas (air + added(Percent)
oxygen) (Percent)
80 '
85.5
87.5
92
(These results are also shown in the graph of Figure 5.)
Further, the following additional results were observed
in the roasted are and are set forth in Table 1.
NEII2024:Appln 4 7
2065837
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2065837
PATENT
362100-2024
ERample 2
A series of air roasting tests was run in a six-inch
rotating tube furnace with off gas oxygen content. (This resulted
in approximately 4% to 6% oxygen by volume in the off-gas.)
These tests used specimens of the same composition as the sample
used in Example 1. The ore for this series of test runs showed
an average gold context of about 0.164 ounces of gold per ton,
2.49 percent sulfide sulfur and 0.79 percent organic carbon. The
ore was classified as sulfidic-carbonaceous ore. Sample
preparation and test procedures used were the same as in Example
1. Table 2 arid Figure 5 present the comparative results. These
tests demonstrate that low gold recoveries are achieved when
roasting is conducted with air as the oxidizing atmosphere.
These tests also demonstrate that the process of the present
invention using oxygen-enriched air (such as 40% oxygen by
volume) allows better process control - at lower temperatures -
for maximum gold recoveries.
NCN2024sAppln 4 9
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206~8~'~
PATENT
362100-2024
EXAMPLE 3
The ore used in these runs came from a random sampling
of arsenic-, sulfidic-containing, gold bearing ores from the
region around Carlin, Nevada. The ore for this series of runs
showed an average gold content of about 0.14 ounces of gold per
ton of cre, an average content of 0.15 percent arsenic, 2.15
percent sulfide sulfur (2.50 percent total sulfur) and 0.35
percent organic carbon (0.39 percent total carbon.) The ore was
classified as pyritic-siliceous ore and had the following
mineralogical analysis:
I~~,neraloQiaal Analysis:
A typical analysis of this ore shows: '
Quartz 80 Percent
Sericite 6 Percent
Pyrites 4 Percent
Jarosite 4 Percent
Kaolinite 3 Percent
Alunite 2 Percent
Barite 1 Percent
Fe O 0 Percent
NEN2024:Apptn 5 1
206837
PATENT
362100-2024
Chemical Anal3rsis
An elemental analysis of the ore shows an average
composition as follows:
Arsenic 0.15 Percent
Carbon (Organic) 0.35 Percent
Sulfur (Sulfide) 2.15 Percent
Gold 0.14 Percent
Iron 2.0 Percent
Zinc 0.06 Percent
Strontium 0.05 Percent
The ore was ground in a small ball mill to 100
percent -100 mesh, i.e., 100 percent passed through a I00 mesh
sieve (except as otherwise noted) and it had a bulk density of
approximately 62 pounds per cubic foot and a moisture content of
approximately 1 percent.
The ground ore was placed in a simple rotating tube
reactor and roasted in a batch operation to evaluate various
reaction conditions using a residence time of two hours for the
sake of consistency. The ore feed to roast was 800 grams at -100
mesh.
The roasted ore, or calcine, was treated by a carbon-
in-leach cyanidation leach using 5 pounds of sodium cyanide per
ton of roasted ore and 30 grams per liter of, activated carbon
(available from North American Carbon.)
NEN202G:Appln 5 2
20658~~
PATENT
362100-2024
The leaching was conducted in a continuously rotating
bottle under the following conditions:
1. 200 grams of calcine per leach test
2. 40% solids and
3. 24 hours leaching time.
The series of runs was made roasting the ore with 40%
total oxygen (by volume) initially in the feed gas, or gaseous
atmosphere, at the following temperatures and with the following
results:
Roasting Temperature Gold Extraction
(Degree C)
(Percent)
450 72.2
475 84.9
500 82.5
525 76.8
550 ' 77.7
600 75.5
Table 3 also shows these and additional results.
NEN2024:Apptn 5 3
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2os~~~~
PATENT
362100-2024
ERamDle 4
A series of roast tests was run in a six-inch rotating
tube furnace with air as the input stream. (This resulted in
approximately 4% to 6% oxygen by volume in the off-gas.)
Specimens from the same sample as in Example 3 were used for
these tests. Sample preparation and test procedures were the
same as in Example 1. Table 4 presents the test results. These
tests also demonstrate that when comparing to Table 3 results,
the former show that gold recovery is maximized when oxygen-
enriched air, e.g., 40% total oxygen in the feed gas, is used as
the oxidizing medium.
NE1t2024: Appl n 5 5
206~8~7
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PATENT
362100-2024
Example 5
A series of tests was conducted in a six-inch rotating
tube furnace on a sample with high carbonate content to
demonstrate that the high gold recoveries are achieved with the
process of the present invention. For comparison, three air
roasts are presented along with the example that illustrates the
present invention. Sample preparation and test procedures used
were the same as in Example 1. Table 5 shows the test results.
The analysis of the sample was:
Chemical Analysis:
Chemical Analysis
Gold 0.66 Ounces per ton
Carbon (total) 3.5 Percent
Carbon (organic) 0.0 Percent
Sulfur (total) 2.6 Percent
Sulfur (sulfide) 2.2 Percent
Iron 2.8 Percent
Arsenic 0.43 Percent
Mercury 56 Parts per million
NEH2024:Appln 5 7
pATENT
362100-2024
Mineraloaiaal Analyses:
X-RAY Diffraction X-RAY Fluorescence
Analysis Analysis
Quartz 29 PercentZirconium .03 Percent
Sericite 4 PercentTitanium .04 Percent
Kaolinite 18 PercentBarium .85 Percent
Alunite 26 PercentNickel .02 Percent
Jarosite 9 PercentVanadium .02 Percent
Pyrite 3 PercentStrontium .04 Percent
Barite 1 PercentZinc .03 Percent
Fexoy 2
Percent
Diopside 7 Percent
. ,
NEH2024:Appln 5 g
PATENT
362100-2024
Table 5.
TEST RESULTS
FOR THE
HIGH CARBONATE
SAMPLE
ROAST LEACH GOLD -200 COMMENTS
TEMP. RESIDUE EXTRACTION MESHl
DEG. C Au % %
oz/ton
525 .077 88 80 Oxygen-
Enriched
Roast2
550 .105 84 80 Air
Roast3
600 .132 80 89 Air Roast3
650 .138 79 86 Air Roast3
i ~ i - ~ ~
Passed through a 20o mesh sieve
Feed gas was air enriched to 40% total oxygen content
(by volume.)
Feed gas was air and the off-gas composition was
maintained at 6% to 8% oxygen by volume.
NEH2024:Appln 5 9
r
4
pATENT
362100-2024
Example 6
A series of pilot plant tests was conducted in a six-
inch fluidized bed reactor and an eight-inch fluidized bed
reactor on a sulfidic carbonaceous sample with the following
chemical and min 1 1 't'
Chemical An
era ogica
compose
ion:
alyais:
Chemical
Analysis
Gold 0.13 Ounces per
ton
Carbon (total) .82 Percent
Carbon (organic) .78 Percent
Sulfur (total) 3.1 Percent
Sulfur (sulfide) 2.6 Percent
Iron 2.7 Percent
Arsenic 0.09 Percent
Mercury 4.7 Parts per
million
NEN2024;Appln 6 0
PATENT
362100-2024
Mineralogical Analvses:
X-RAY Diffraction X-RAY Fluorescence
Analysis Analysis
Quartz 71 Percent Zirconium .01 Percent
Sericite 5 Percent Titanium .12 Percent
Kaolinite 11 Percent Barium .85 Percent
Alunite 3 Percent Nickel .03 Percent
Jarosite 5 Percent Vanadium .05 Percent
Pyrite 4 Percent Strontium .05 Percent
Barite 1 Percent Zinc .10 Percent
Fe O 0 Percent Lead .01 Percent
The sample preparation procedure for this series of
tests included crushing, wet grinding in a ball mill to 100%
passing through a 65 mesh sieve, solid/liquid separation, and
drying prior to roasting. The dry sample was fed to the roaster
via a screw feeder with the combustion gas consisting of either
air alone or air enriched to 40% total initial oxygen content by
volume. Solids exiting the roaster were carbon-in-leach cyanide
leached at the same conditions as in Example 1.
Table 6 presents the test results. From the results it
is seen that maximum gold recoveries are achieved by using the
process of the present invention. By way of comparison, several
afr roasts conducted in a circulating fluidized bed roaster and a
stationary fluid bed roaster are presented along with three
examples that illustrate the present invention.
NE112024:Appln 61
e7 ~ ~ ~ 362100-2024
Residual sulfide sulfur content and organic carbon
content of the solids exiting from the pilot plant roaster were
less than 0.05 percent by weight in all the tests from this
series. Table 6.
Test esults m Pilot lant Fluidized Bed Roasters
R Fro P in
ROAST OXYGEN LEACH CALC GOLD COMMENTS
TEMP. IN OFF- RESIDUE HEAD EXTRN
DEG.C GAS oz/ton oz/ton %
%
525 37 .019 .131 85 Oxygen Roasts
550 38 .020 .137 85 Oxygen Roasts
550 38 .016 .131 88 Oxygen Roastz
625 6 .046 .131 65 Air Roast3
675 6 .044 .137 68 Air Roast3
725 6 .044 .133 67 Air Roast4
600 6 .034 .134 75 Air Roasts
600 6 .028 .133 79 Air Roasts
Test conducted in a six-inch circulating fluidized bed
roaster with a combustion gas of air enriched to 40% oxygen by
volume.
Same as in footnote 1 but the test was conducted in an
eight-inch circulating fluid bed roaster.
Test conducted in a six-inch circulating fluid bed
roaster with air as the combustion gas and the composition of the
off-gas was maintained at 6% oxygen by volume.
Same as in footnote 3 but the test was conducted in an
eight-inch circulating fluid bed roaster.
Test conducted in a six-inch stationary fluid bed
roaster with air as the combustion gas and the composition of the
off-gas was maintained at 6% oxygen by volume.
NE112024 :Apps n 6 2
362100-2024
The foregoing examples demonstrate that the process of
the present invention produces significantly desirable results
from refractory ores with arsenic-, carbon- and sulfur-containing
components while reducing the cost of oxygen-based roasting and
minimizing arsenic volatilization.
It is noteworthy, particularly by comparing air
roasting, such as those in Example 2, with oxygen-enriched air
roasting, such as those in Example 1, that the present invention
y effectively lowers the temperature at which optimum gold recovery
occurs. This is graphically demonstrated by comparing Figure 6,
which is for air roasting, with Figure 4 which is for 40% oxygen-
enriched air roasting. In Figure 6 (air roast) the maximum gold
recovery is at 600 degrees Celsius while in Figure 4 (oxygen-
enriched air roast) the maximum gold recovery is at 475 degrees
Celsius. The importance of this is that the process of the
present invention is more energy-economical. Figure 5 shows that
the percent gold extraction generally increases as the total
oxygen content in the feed gas increases, with a practical,
economical upper range based on other considerations such as
operating costs, oxygen gas costs, equipment costs, etc.
Example 7
In a schematic industrial illustration shown in Figure
7 and described above, the following process data illustrate the
application of the present invention.
Neuzazu s~~t n 6 3
~O~c7~~P~ PATENT
362100-2024
The base case roaster feed analysis is as follows:
Carbon Organic 0.8%
Sulfide Sulfur 2.5%
Weight Loss on 6.0%
Ignition - L.O.I.
As 1200 ppm
C1 100 ppm
F 1000 ppm
Pb 25 ppm
Hg 5 ppm '
Sb 80 ppm
Zn 1000 ppm
Si0 80 %
A1 O 7 %
The following x-ray diffraction analysis was used to
Further characterize the above ore mixture:
Sericite 5%
Kaolinite 11%
Alunite 3%
Jarosite 5%
The ore feed had a specific gravity 2.52; and a bulk
density (loose) of 1.0 m.t./m3 and bulk density (packed) of 1.25
m.t./m3. Roaster feed (D50) was: 50% passed at 19~, size and 80%
passed at 70~ (estimate). The design roast temperature was 550°C
and the 02 concentration in off-gas was 36 vol.% wet basis.
NEN2024:Appln 6 4
PATENT
362100-2024
Organic carbon burn-off was assumed to be 0.7% (for energy
calculations).
As illustrated by the above x-ray diffraction analysis
it shows the ores to contain a variety of clay compounds
predominantly kaolinite but also alunite, jarosite and sericite.
These compounds all have varying decomposition energies (all
assumed to be endothermic). At a roasting temperature of 525-
550°C all of the clays would be decomposed and hence all of the
waters of crystallization would end up in the vapor phase.
Volatilization in roaster was taken for each elements
as follows: Mercury 100%; Arsenic 1%; Fluorine 15% and Chlorine
100%.
Based on the above data, an illustration of an
industrial operation as described in conjunction with Figure 7 is
shown in Table 7; this table must be read in conjunction with the
description of the process in Figure 7.
NEH2024:Appln 6 5
206~$~7
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ATENT
62100-2024
For the above illustration, a carbon content in the ore
was provided for at 0.8% level, but should also be provided for a
range from about 0.4% to about 1.15%. However, at still lower
amounts of carbon in ore, more coal or fuel needs to be added,
while at higher amounts of carbon in ore less or no coal is
required (autothermal conditions). Hence, about 330 kg/hr of
coal calculated as carbon is added for the above ore in Table 7.
Besides the heat recovered in heat recovery unit 106, the waste
heat boiler 116 produces at the specified conditions about 6 tons
per hour of 55 bar steam.
In the above illustration, it is noted that a total "at
temperature" time for the calcine (before quenching) is about 30
minutes. Such "at temperature" time is a combined time in the
CFB reactor 100 and during post-finishing in heat recovery unit
106. This "at temperature'' time may range from about 25 minutes
to 50 minutes and does not adversely affect the gold recovery
even for the longer period; therefore, this process has an
advantage because it is also free from the heat sensitivity, i.e.
"at teraperature" time limits such as cautioned against in some of
the prior art processes and disclosures thereof.
While the above process has been illustrated as capable
of treating ores of various particulate sizes, the advantageous
size is determined for each ore and is typically from about -14
mesh to about -100 and less. At finer particulate sizes e.g. -
100 mesh there is no need to wet grind the calcine after
quenching in tank 105 but before leaching.
IIEN1024:Appln
PATENT
g 3 ~ 362100-2024
Example 8
Figure 8 illustrates a roasting with two stage oxygen
injection carried out in a circulating fluidized bed. The
circulating fluidized bed system consists of a fluidized bed
reactor 301, a recycling cyclone 302, and a recycling line 303.
The fluidized bed reactor 301 was 0.16 m in diameter and had a
height of 4 m. By means of a metering screw (not shown) a
mixture of refractory gold ore and additives at a rate of l0 kg/h
was charged through line 304 into the reactor 301. The gold ore
contained 0.8% arsenic, 1.4% sulfide sulfur and 13 g gold per
1000 kg. It had a particle size below 0.1 mm with a median value
(D50) of 20 Vim. The types and quantities of the additives are
apparent from the following Table 8. 80% of the additives had a
particle size below 20 to 50 ~.m. A gas which contained 0.9%
oxygen was fed at a rate of 10 sm3/h through line 305 into the
gas heater 306 and was heated therein to 550°C and then fed
through line 307 into the reactor 301 as a fluidizing gas. The
reactor 301 was indirectly heated and a temperature between 550°
and 570°C was adjusted in the reactor. The reactor 301 was fed
through line 308 with secondary oxygen containing gas and through
line 309 with tertiary oxygen containing gas. The secondary and
tertiary gases consisted of preheated air and oxygen,
respectively, and were used to adjust in the upper roasting stage
the oxygen content indicated in the table. The calcine was
withdrawn through line 310. A gas-solid suspension was fed from
the reactor 301 through line 311 to the recycling cyclone 302 and
the solids separated therein were recycled through the recycling
NEN2024:Appln 6 8
PATENT
O ~ ~ ~ ~ ~ 362100-2024
line 303 into the reactor 301. The exhaust gas discharged
through line 312 contained 0.1% to 0.5% S02 by volume.
In the following Table 8 the yield of gold and the
solubility of arsenic in the cyanide leaching are indicated for
various additives and oxygen contents. Whereas the addition of
sodium compounds gives good results as regards the yield of gold,
the solubility of arsenic will be excessively high in that case.
HE1l2024: Appl n 6 9
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206837
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PATENT
.3 ~ ~ ~ 362100-2024
Based on the experiments described above a representative,
schematic presentation of arsenic immobilization
is evident from the oxygen content versus temperature curves from
soluble and substantially insoluble arsenate formation. While it
is evident from the composite curves shown above that as oxygen
and temperature increases arsenic immobilization occurs, it is
also evident that for efficient leaching such temperatures must
be kept below ore component fusion temperatures which prevent ,
good cyanide leaching. At an oxygen partial pressure of log.p02
of -3.0, the arsenate (in case of ferricarsenate -- as shown in
Figure 10) must be also analyzed as only one component which
needs to be considered. Carbon and sulfur must also be
eliminated and efficient elimination calls for balancing of
temperature and oxygen content. Additional substances such as
Ca804. 2H20 also favorably immobilize arsenic. Moreover, pyrites
in the ore being in intimate contact with arsenic compounds in
ore, as shown above, react favorably to immobilize arsenic
especially at higher oxygen content in the reactant gas.
Example 9
According to Figure 11 the first circulating fluidized
bed system consists of the fluidized bed reactor 401, the
recycling cyclone 402, and the recycling line 403. The fluidized
bed reactor 401 was 0.2 m in diameter and had a height of 6 m.
By a metering screw feeder, gold ore concentrate at a rate of 15
kg/h was charged through line 404 into the reactor. The
concentrate contained 2.1% arsenic, 15% sulfide sulfur and 45 g
gold per 1000 kg. The particle size was below 0.2 mm with a
NEN2024:Appln 7 1
s
PATENT
362100-2024
_...
me,~ian size (D50) of 70 ~Cm. Air at a rate of fi sm3/h (sm3 =
standard cubic meter) was fed through line 405 into the heat
exchanger 406 and was preheated therein to 600°C and then fed
through line 407 into the reactor 401 as a fluidizing gas. The
reactor 401 was fed through line 408 with secondary air at a rate
of 9 sm3/h and through line 409 at a rate of 3 sm3/h with
tertiary air, which served to combust the residual sulfur in the
reactor 401. By the distribution of the air supply, the oxygen
potential was adjusted to be in the range in which arsenic is
volatilized in the Fe203 range (Figure 10), below the range in
which iron arsenate is formed.
The temperature in the reactor was between 700°C and
750°C. The calcine withdrawn through line 410 contained 0.02%
arsenic and 0.1% sulfur. The leaching of the calcine resulted in
a recovery of gold with a yield of 96%. The solubility of
arsenic during the leaching of gold was very low and amounted
only to less than 2 mg/1.
A gas-solid suspension was fed from the reactor 401
through line 411 into the recycling cyclone 402. The solids
collected there were recycled through the recycling line 403 into
the reactor 401. The exhaust gas conducted in line 412 was
dedusted in two cyclones (not shown) and in a candle filter 413
at about 600°C. The collected dusts were returned to the reactor
401 through line 414. The dust-free exhaust gas contained S02
and As203 and was fed through line 415 to the fluidized bed
reactor 416 of a sE;cond circulating fluidized bed system.
The reactor 416 was 0.16 m in diameter and had a height
of 4 m. It was heated by indirect electric heating. Hematinic
NE1J2024: Appl n 7 2
i
2 0 6 ~ ~ ~ ~ 362100-2024
iLon ore having a particle size below 0.5 mm, with a medium size
of 30 Vim, was charged through line 417 at a rate of 0.3 kg/h.
Fluidizing air at a rate of 15 sm3/h was fed into the reactor
416.
The suspension leaving through line 419 was adjusted to
contain 6% oxygen and 4% water vapor so that the conditions for a
formation of stable arsenates (Figure 9) were established. To
adjust a water vapor content of 4%, the moisture content of the
iron ore charged through 417 was controlled in dependence on the
water vapor content of the gas entering through line 415 and of
the fluidizing air entering through line 418.
The solids collected in the recycling cyclone 420 were
returned through the recycling line 421 into the reactor 416.
The arsenic-free roaster gas contained 9.1% S02 and was fed
through line 22 to a gas purifier and subsequently to a plant for
producing sulfuric acid. The solid material which was discharged
through line 423 from the reactor 416 contained 17.3% arsenic.
Leaching tests with water (corresponding to a DEV-S4 leaching
test) showed that the solubility of arsenic was less than 1"mg/1.
According to a preferred feature of the embodiment
shown in Example 9, the dust-containing gases which contain
arsenic vapor and arsenic compound vapors) are produced by
roasting e.g. of sulfide materials which contain iron and
arsenic. Such materials are roasted in the Fe203 range at
temperatures of 500°C to 1100°C in a first stage, which is
supplied with oxygen-containing gases. In these materials,
arsenic is volatilized mainly as arsenic oxides and part of the
sulfur content is volatilized as elementary sulfur. Solids are
NE4t2024 : Appl n 7 3
2 0 fi e,~ ~ ~ ~ 362100-2024
r owed from the exhaust gas at temperatures above the
condensation temperature of the volatilized components, and the
solids are discharged as calcine.
The sulfide materials may consist of arsenic-containing
ores or ore concentrates, such as gold ores, copper ores, silver
ores, nickel ores, cobalt ores, antimony ores, lead ores and iron
ores as well as of arsenic-containing sulfide residues and
intermediate products. By the roasting, a small part of the
arsenic content is reacted to form arsenic sulfides. In the
processing of gold ores or gold ore concentrates, environmentally
acceptable dumps of residues are obtained. Further, a product
from which gold can be leached with cyanides in a high
yield.
Although the above illustrations concerning metal
recovery has been with reference to gold, other precious metal
and metal recovery of arsenic containing ores may be practiced as
described herein -- thereby realizing the advantages of the
present process, i.e. low temperature (e. g. less than 700°C),
oxygen enriched air roasting in presence of substances such as ''
iron or calcium to immobilize arsenic as e.g. ferricarsenate in
the form of scorodite or scorodite like compounds. Scorodite
like compounds are intended to mean compounds of ferricarsenate
with water of crystallization of varying mole amounts. For
scorodite two moles of water of crystallization is typically
shown but the amounts of water crystallization may vary. As
shown above, the presence of water of crystallization in the
added substance the roasting atmosphere or in the ore components,
e.g. aids in the immobilization of arsenic. However, the measure
NEN202G:Apptn 7 4
362100-2024
f~~ immobilization, i.e. insolubility, is scorodite and
represents the level of insolubility which is desired. A
"scorodite like" compound is intended to have insolubility of
about the same order of magnitude as scorodite.
Moreover, while the process for gold recovery has been
found best conducted with the indicated oxygen levels for other
metal recovery from ores which contain arsenic, such process may
be practiced with even higher oxygen levels (and also temperature
levels) as shown above because the improvement concerning arsenic
recovery as such may even be practiced with pure oxygen used as
the oxidizing medium. When using higher temperatures, i.e. as
shown in Example 9, the combination of first stage and second
stage treatment provides a double measure of safety that any
arsenic which may have been volatilized may be separately
immobilized to assure an environmentally double safe treatment of
any off gas. Such combination also provides for employment
choice of a lower oxygen content in first stage and higher in the
second stage. In part such effect may also be achieved by the
multiple oxygen injection as shown for the gold ores treated in
the combination shown in Example 8.
Because of these advantages including those derived
from e.g. circulating fluidized beds, the present invention
provides improvements over those shown by the prior art as
previously described and pointed out with reference to that art.
While the exact reasons that cause the process of the
present invention to produce the herein-observed results are
unknown and could not be predicted, the results themselves
bespeak the achievements that have been obtained - based merely
NEN202b:Appln 7 5
PATENT
'~ ~ 362100-2024
on the percent of gold extraction and arsenic immobilization -
from these refractory ores at great savings of oxygen usage and
using a less complicated approach than the best prior art
technology can show. It is especially noted for conditions such
as apply when using a circulating fluidized bed which provides
for significant heat recovery and reutilization.
It is also evident from the above that various
combinations and permutations may well be practiced and advanced,
but these are not to be understood as limiting the invention
which has been defined in the claims which follow.
NE41Z024:Appln
