5,851,499
Dec. 22, 1998
[11]
[45]
111111111111111111111111111111111111111111111111111111111111111111111111111
US005851499A
Patent Number:
Date of Patent:
United States Patent [19]
Gathj e et al.
[54] METHOD FOR PRESSURE OXIDIZING
GOLD-BEARING REFRACTORY SULFIDE
ORES HAVING ORGANIC CARBON
[75] Inventors: John C. Gathje, Longmont, Colo.;
Gary L. Simmons, Albuquerque, N.
Mex.
OTHER PUBLICATIONS
Ketcham, Yl. et aI., "The Lihir Gold Project; Process Plant
Design", Minerals Enginerring, vol. 6, Nos. 8-10, pp.
1037-1065, 1993.
[73] Assignee: Newmont Gold Company, Denver,
Colo.
Primary Examiner-Wayne Langel
Attorney, Agent, or Firm-Holme Roberts & Owen
FOREIGN PATENT DOCUMENTS
References Cited
Appl. No.: 712,252
Filed: Sep. 11, 1996
Int. C1.6 COlG 7/00; C22B 11/08
U.S. Cl. 423/23; 423/29; 423/30
Field of Search 423/23, 30, 29
Provided is a method for treating refractory gold-bearing
ores that have both sulfide material with which the gold is
associated and from which the gold is difficult to separate
and having organic carbonaceous material having an affinity
for at least one of gold and a gold complex. The mineral
material is pressure oxidized in the presence of a halogencontaining
material in a manner to reduce the susceptibility
of the organic carbonaceous material to capture and hold
gold during the pressure oxidation. Also provided is a
gold-bearing effluent product from the pressure oxidation
process. The effluent product comprises gold which can be
effectively recovered by carbon-in-Ieach cyanidation of the
effluent product. In many cases, gold recovery according to
the present invention is increased substantially over standard
pressure oxidation techniques.
[57] ABSTRACT
11/1985 Mason et al. 423/30
9/1986 Weir et al. 423/30
5/1990 Ramadorai et al. 423/30
3/1992 Anderson et al. 75/734
7/1994 Han et al. 423/32
10/1995 Simmons 423/30
8/1997 Gathje et al. 423/30
U.S. PATENT DOCUMENTS
4,552,589
4,610,724
4,923,510
5,096,486
5,328,669
5,458,866
5,653,945
[21]
[22]
[51]
[52]
[58]
[56]
WO 89/12699 12/1989 WIPO. 31 Claims, 8 Drawing Sheets
2
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5,851,499
2
Eqn 2: Precipitation of gold by carbon of organic carbonaceous
material
For the reactants of Eqn. 1, the O2 is primarily supplied
from oxygen gas introduced into the pressure oxidation
process to oxidize the sulfide sulfur and the H+ is primarily
supplied by sulfuric acid added to the feed slurry or generated
during the pressure oxidation. The C in Eqn. 2 is
65 provided primarily by the organic carbonaceous material.
In one aspect of the present invention, a method is
provided for pressure oxidation of a gold-bearing mineral
als that have both sulfide material and organic carbonaceous
material is detrimentally affected when the pressure oxidation
is conducted in the presence of one or more halogencontaining
materials. Halogen-containing materials may be
5 introduced into the pressure oxidation process in a variety of
ways. For example, the mineral material may contain
naturally-occurring halogen-containing material. Or,
halogen-containing material may be introduced into the
pressure oxidation in fresh or recycled process water, such
10 as in the form of dissolved halide salts. The source of
halogen-containing materials in recycle water could come
from the build-up of material released from the mineral
material or could come from various reagents, such as
caustic used to neutralize the oxidized slurry or cyanide used
15 to leach gold following neutralization. In fresh process
water, the halogen-containing material may be naturally
occurring or may be introduced into the water through water
pretreatment operations, such as water softening.
The low gold recoveries following pressure oxidation in
20 the presence of halogen-containing material of mineral
materials having both sulfide material and organic carbonaceous
material is particularly surprising because the low
gold recoveries occur even when tests of residues from
pressure oxidation show that the preg-robbing ability of the
25 organic carbonaceous material has been successfully eliminated
and essentially all of the sulfide material has been
decomposed to release the gold. Furthermore, gold recoveries
have been found to decrease in the presence of
halogen-containing materials with increased severity of
30 pressure oxidation operating conditions. For example, gold
recoveries have been observed to decrease with increasing
temperature during pressure oxidation. These observations
are contrary to conventional thought that increased temperature
results in increased gold recovery due to more complete
35 decomposition of the sulfide material and, accordingly, more
complete freeing of the gold.
It has been discovered, however, in developing the present
invention, that when in the presence of a halogen during
pressure oxidation, elemental gold that is freed from asso-
40 ciation with the sulfide material is susceptible to being
captured and held by the organic carbonaceous material.
This is a considerably different effect than that of pregrobbing,
which involves the adsorption of a gold-cyanide
complex. Rather, it is believed that capture of the elemental
45 gold by the organic carbonaceous material occurs through an
intermediary of a gold-halide complex. Not to be bound by
theory, but to aid in the understanding of the present
invention, the mechanism for elemental gold capture by the
organic carbonaceous material is believed to involve the
50 following chemical reactions, which are shown representatively
for chlorine as the halogen:
Eqn 1: Dissolution of gold as halide complex
BACKGROUND OF THE INVENTION
1
METHOD FOR PRESSURE OXIDIZING
GOLD-BEARING REFRACTORY SULFIDE
ORES HAVING ORGANIC CARBON
FIELD OF THE INVENTION
SUMMARY OF THE INVENTION
It has been found that gold recoveries from typical
pressure oxidation processing of refractory mineral materi-
The present invention involves a method for pressure
oxidizing gold-bearing refractory sulfide mineral materials
having organic carbonaceous material, and particularly
relates to pressure oxidation of such mineral materials in the
presence of a halogen-containing material.
Gold is difficult to separate from several gold-bearing ores
by standard recovery techniques, such as by standard cyanidation
processing. These difficult ores are often referred to
as refractory. One reason that an ore may be refractory is that
the gold in the ore is associated with sulfide minerals from
which the gold is difficult to separate. One method for
treating these refractory sulfide ores is to pressure oxidize
the ore in a acidic environment in the presence of oxygen gas
to decompose the sulfide minerals, thereby releasing the
gold for subsequent recovery.
Another reason that an ore may be refractory is that the
ore contains a significant amount of organic carbonaceous
material that is capable of capturing and holding the gold, to
the detriment of gold recovery operations. For example,
when gold is recovered by cyanidation, a soluble goldcyanide
complex is formed during a cyanide leach. The
complex is then adsorbed onto activated carbon granules,
from which the gold may subsequently be separated to
obtain the gold. When organic carbonaceous material is
present in an ore, however, the organic carbonaceous material
can adsorb the gold-containing cyanide complex,
thereby significantly reducing the amount of the goldcyanide
complex that is available for adsorption on the
activated carbon granules, and causing a corresponding
reduction in gold recovery. Adsorption of the gold-cyanide
complex by the organic carbonaceous material in competition
with a gold recovery operation is often referred to as
preg-robbing, because the organic carbonaceous material is
"robbing" the gold from a solution that is "pregnant" with
gold in the form of a soluble gold-cyanide complex.
Refractory ores that contain organic carbonaceous material
have not traditionally been considered to be satisfactorily
treatable by pressure oxidation because of the pregrobbing
problem. In commonly owned U.S. Pat. No. 5,536,
480, however, a method is disclosed for pressure oxidizing
such carbonaceous refractory ores. The method disclosed in
that application involves a combination of very fine sizing of
the ore feed with severe pressure oxidation processing to
oxidize and/or passivate the preg-robbing organic carbonaceous
material.
Ores that are refractory due to the presence of both sulfide
minerals and organic carbonaceous material are particularly
problematic. The method described in U.S. Pat. No. 5,536,
480 can be used for many ores containing both sulfide
minerals and organic carbonaceous material. It has been 55
found, however, that under some conditions, ores having
both sulfide minerals and organic carbonaceous material are
difficult to treat even with the process disclosed in U.S. Pat.
No. 5,536,480.
Based on the foregoing, there is a need for additional 60
methods for treating gold-bearing ores that are refractory
due to the presence of both sulfide minerals and organic
carbonaceous material.
5,851,499
3 4
DETAILED DESCRIPTION OF THE
INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow diagram of one embodiment of a method
of the present invention for producing a gold-bearing effluent
product.
FIG. 2 is a graph showing the effect of pressure oxidation
temperature and retention time on gold recoveries according
to the present invention.
FIG. 3 is a graph showing the effect of temperature and
retention time on gold recovery according to the present
invention.
FIG. 4 is a graph showing the effect of temperature on
gold recovery according to the present invention.
FIG. 5 is a graph showing the effect of the weight ratio of
sulfide sulfur to carbonate in feed to pressure oxidation on
gold recovery according to the present invention.
FIG. 6 is a graph showing the effect of free acid in liquid
discharge from pressure oxidation relative to gold recovery
according to the present invention.
FIG. 7 is a graph showing the effect of blending ores with
different carbonate contents on gold recovery according to
the present invention.
FIG. 8 is a graph showing the effect of the addition of a
chloride salt to pressure oxidation feed.
FIG. 9 is a graph showing the effect of pressure oxidation
temperature on gold recovery according to the present
invention.
The gold-bearing mineral material feed of the process of
the present invention is refractory to gold recovery due to the
presence of sulfide material with which the gold is associated
and from which the gold is difficult to separate. The
mineral material feed also comprises organic carbonaceous
9. Conducting the pressure oxidation in a batch operation
rather than in a continuous operation;
10. Conducting the pressure oxidation with a retention
time of shorter than about 45 minutes; and
11. Conducting the pressure oxidation with a mineral
material feed that comprises a blend of a first mineral
material with a second mineral material that has a
higher carbonate content than the first mineral material.
In another aspect, the present invention provides a gold-
10 containing effluent product from pressure oxidation, in the
presence of a halogen-containing material, of a gold-bearing
mineral material having sulfide material and organic carbonaceous
material. The effluent material comprises solid
effluent and aqueous liquid effluent from the pressure
15 oxidation, with the solid effluent comprising gold from the
mineral material feed and carbonaceous residue of the
organic carbonaceous material of the mineral material feed.
The effluent product is further characterized by the presence
in the solid effluent material of gold from the original
20 mineral material feed, with greater than about 75%, and
preferably even more, of the gold in the solid effluent
material being removable by carbon-in-Ieach cyanidation.
This gold-containing effluent material is a valuable product
that can be further processed to recover gold from the solid
25 effluent by carbon-in-Ieach cyanidation or other known
recovery methods. The gold in the solid effluent is substantially
not held by residue of organic carbonaceous material
in a manner that would inhibit standard recovery operations,
such as standard carbon-in-Ieach cyanidation.
30
material having sulfide material from which the gold is
difficult to separate and having organic carbonaceous material
that has an affinity, in the presence of a solubilized
halogen, for at least one of gold and a gold-halide complex.
Feed to the pressure oxidation includes a halogen-containing 5
material having halogen that, during pressure oxidation, is in
a solubilized form that is capable of complexing with gold
that becomes freed from association with the sulfide material.
The pressure oxidation of the present invention is
conducted under conditions to suppress the susceptibility of
gold to be captured and held by the organic carbonaceous
material, such that greater than about 75%, and preferably
even more, of the gold from the original mineral material is
removable, by carbon-in-Ieach cyanidation, from solid residue
of the pressure oxidation. The gold-bearing mineral
material may be an ore, ore concentrate, tailings from prior
processing, or other material having gold-bearing mineral
components.
According to the present invention, suppression of the
susceptibility of gold to be captured and held by the organic
carbonaceous material may be accomplished in a variety of
ways. For example, reaction conditions may be varied to
reduce the kinetics of gold dissolution (Eqn. 1) and/or gold
deposition on the organic carbonaceous material (Eqn. 2).
Also, reactive conditions could be varied to shift to the left
the equilibrium of one or both of the gold dissolution
reaction (Eqn. 1) and the gold deposition reaction (Eqn. 2).
Alternatively, a component could be introduced during the
pressure oxidation to bind the halogen in a form that
substantially prevents participation of the halogen in the
dissolution and subsequent capture of elemental gold by the
organic carbonaceous material.
Specific operating conditions for pressure oxidation to
suppress the susceptibility of gold to be captured and held by
organic carbonaceous material, according to the present 35
invention, include one or more, in any combination, of the
following:
1. A temperature of lower than about 2150 c.;
2. A ratio, on a weight basis, of sulfide sulfur to carbonate
in the mineral material feed of smaller than about 4:1; 40
3. Maintaining aqueous effluent liquid from pressure
oxidation at an oxidation/reduction potential (ORP) of
smaller than about 700 millivolts, relative to a standard
hydrogen electrode;
45
4. Maintaining aqueous effluent liquid from pressure
oxidation at an acid level of less than about 28 grams
of free sulfuric acid per liter of the effluent liquid;
5. Conducting the pressure oxidation in the presence of
carbon dioxide at a partial pressure larger than that 50
which would be exerted by carbon dioxide generated
from carbon released by decomposition of the mineral
material during pressure oxidation;
6. Conducting the pressure oxidation in the presence of a
component causing formation of a halogen-containing 55
reaction product that is insoluble during the pressure
oxidation, to prevent halogen in the reactive product
from forming a complex with gold;
7. Conducting the pressure oxidation in the presence of a
component capable of causing formation of a stable, 60
soluble complex with halogen present during pressure
oxidation, such that formation of a gold complex with
the halogen is inhibited;
8. Restricting oxygen gas used for the pressure oxidation
so that reaction conditions in the reactor are such as to 65
prevent substantially complete oxidation of ferrous iron
to ferric iron during the pressure oxidation;
5,851,499
5
material that is capable of capturing and holding gold during
pressure oxidation when a halogen-containing material is
part of the pressure oxidation feed. The mineral material
feed is typically sized such that at least about 80 weight
percent of the feed particles are smaller than about 75
microns (200 mesh) and more preferably smaller than about
38 microns (400 mesh). The mineral material feed is slurried
with water to prepare a feed slurry for the pressure oxidation
processing. The present invention is particularly useful for
mineral material feeds in which less than about 60% of the
gold is recoverable by direct cyanidation techniques, and
even more particularly useful for ores in which less than
40% of the gold is recoverable by direct cyanidation.
The sulfide material, with which the gold is associated,
typically comprises one or more of the following sulfide
minerals: pyrite, marcasite, pyrrhotite and arsenopyrite. The
organic carbonaceous material may be any carbonaceous
material having an affinity for at least one of gold, a gold salt
or a gold complex. This affinity, however, may vary widely
depending upon the type, origin, hydrophobicity, porosity
and other properties of the carbonaceous material.
Generally, the amount of organic carbonaceous material in a
mineral material is determined as the total amount of carbon
in the mineral material except that which is present in a
carbonate group. The present invention is particularly useful
for whole ores, and concentrates of such whole ores, having
greater than about 0.3 weight percent of the organic carbonaceous
material. The present invention is more particularly
useful for ores having greater than about 0.4 weight percent
of the organic carbonaceous material and even more particularly
beneficial for ores having greater than about 0.6
weight percent of the organic carbonaceous material.
Although the severity of the gold capture problem is generally
expected to increase with increasing amounts of the
organic carbonaceous material, the organic carbonaceous
material of some ores is particularly active and even small
amounts, such as around 0.3 weight percent or less, can
cause significant problems during pressure oxidation processing.
The halogen-containing material may be part of the
mineral material feed or may be introduced into the feed in
another manner. The present invention is particularly useful
for mineral materials having naturally-occurring halogencontaining
material that, when the mineral material is pressure
oxidized, provides one or more halogen in a solubilized
form, such as a soluble halide, that is available to complex
with gold and thereby facilitate capture of gold by organic
carbonaceous material. Naturally-occurring halogencontaining
material in ores is often in the form of small
liquid inclusions having dissolved halide salts. The halogencontaining
material need not, however, be part of the mineral
material feed. For example, when recycle water is used to
prepare the feed slurry for pressure oxidation, significant
concentration levels of dissolved halide salts can build up in
the recycle water, which can cause problems during pressure
oxidation. Also, a significant amount of dissolved halides
can be introduced into recycle water by various reagent
formulations. For example, caustic formulations used to
neutralize the effluent slurry from pressure oxidation and
sodium cyanide formulations used to extract gold from the
effluent slurry both often contain significant quantities of
halide salts. Furthermore, fresh process water can sometimes
contain significant quantities of dissolved salt, especially
when sea water or brackish water is used. Moreover, if fresh
process water is softened prior to use, many water softening
operations can introduce significant quantities of dissolved
halides into the water.
6
The detrimental effect from any given level of halogencontaining
material will depend upon the specific conditions
for any pressure oxidation process, including the activity of
the organic carbonaceous material and the composition of
5 the mineral material. The method of the present invention is
generally useful, however, for pressure oxidation in which
the dissolved halogen concentration in effluent aqueous
liquid from pressure oxidation is larger than about 5 milligrams
per liter of the effluent liquid and especially when the
10 concentration is larger than about 15 milligrams per liter. In
some instances, however, significant problems do not occur
until dissolved halogen concentrations reach 50 to 100
milligrams of halogen per liter, or more. As used herein,
halogen includes one or a combination of any of chlorine,
15 bromine and iodine, with chlorine being the most common
encountered. A halogen-containing material is a material
having the halogen.
The process of the present invention is well-suited for
pressure oxidation feeds having halogen-containing material
20 with halogen in an amount of greater than about 20 parts per
million by weight relative to the weight of mineral material.
The pressure oxidation process of the present invention is
particularly useful for feeds having greater than about 35
parts per million, and more particularly for feeds having
25 greater than about 50 parts per million, of halogen by weight
relative to the mineral material. During the pressure
oxidation, the halogen of the halogen-containing material is
in a solubilized form, typically a soluble halide.
The present invention is particularly well-suited for pres-
30 sure oxidation treatment of refractory gold-bearing sulfide
mineral materials that demonstrate poor gold recoveries due
to the presence of organic carbonaceous material when
subjected to traditional pressure oxidation conditions in the
presence of the halogen-containing material. Traditional
35 severe pressure oxidation conditions include high
temperatures, high oxygen input and high levels of acid in
the discharge. Unexpectedly, and surprisingly, however, the
mineral materials most suited for treatment by the present
invention exhibit poor gold recoveries from typical pressure
40 oxidation processing conditions when pressure oxidized
with the halogen-containing material. The solid residue of
the organic carbonaceous material, following pressure
oxidation, may hold up to 50% or more of the gold in a
manner to render that gold unrecoverable by standard
45 carbon-in-Ieach cyanidation techniques. By adjusting operating
conditions of the pressure oxidation according to the
present invention, however, the amount of gold held by the
organic carbonaceous residue following pressure oxidation
may be reduced to less than about 25%, and preferably to
50 less than about 10%, of the gold in the solid residue. With
the present invention, gold recovery may be increased from
as low as 60% or less to greater than about 75%, more
preferably greater than about 80%, and most preferably
greater than about 85% of the gold originally present in the
55 mineral material feed.
For example, typical severe, continuous pressure oxidation
processing of a gold-bearing refractory sulfide ore may,
for reference purposes, include a temperature of about 2250
c., an oxygen gas overpressure of 100 psi, a residence time
60 of about one hour, and effluent liquid having about 30 grams
per liter of sulfuric acid. These reference operating conditions
are adequate for treating most refractory sulfide mineral
materials. These reference conditions, however, have
not been found adequate for treating mineral materials
65 having both sulfide material and organic carbonaceous material
when processed with a halogen-containing material.
With the process of the present invention, however, gold
5,851,499
7 8
the detrimental effects of the halogen-containing material
can be sufficiently controlled without major changes to
typical pressure oxidation operating conditions. For some
mineral materials, however, blending alone is not adequate,
5 perhaps due to the presence of an extremely active organic
carbonaceous material. In those circumstances, it is generally
necessary to make other adjustments to the pressure
oxidation operating conditions, such as lowering the treating
temperature, reducing oxygen input or making other adjustments
as discussed herein. Often, best results will be
attained by combining a number of the different control
techniques discussed herein.
One way to reduce the susceptibility of gold to be
captured and held by the organic carbon is to operate the
pressure oxidation in a way to keep the ORP from becoming
too high in effluent liquid from pressure oxidation. As used
herein the ORP is relative to a standard hydrogen reference
electrode. According to this embodiment of the present
invention, the ORP of the effluent liquid from pressure
oxidation should be kept at a level that is at least about 25
20 millivolts below the ORP that would be exhibited if the
mineral material feed had been pressure oxidized according
to the reference conditions stated previously. Typically, with
the present invention, the ORP of the effluent liquid should
be lower than about 700 millivolts, and preferably lower
25 than about 650 millivolts, although the exact ORP will vary
depending upon the specific mineral material. The ORP may
be held at a low level by, for example, reducing the input of
oxygen into the pressure oxidation or reducing the residence
time.
Another way to reduce the susceptibility of the gold to be
captured and held by the organic carbonaceous material is to
maintain effluent liquid from pressure oxidation at a free
acid content of less than about 28 grams of free sulfuric acid
per liter of effluent liquid, preferably smaller than about 25
35 grams per liter and more preferably smaller than about 20
grams per liter. Particularly preferred with the present invention
is to maintain the effluent liquid at a free sulfuric acid
content of from about 7 grams per liter to about 25 grams per
liter.
Another way to reduce the susceptibility of the gold to be
captured and held by the organic carbonaceous material is to
maintain a partial pressure of carbon dioxide at a partial
pressure that is higher than the partial pressure of carbon
dioxide that would be generated in the autoclave from
45 decomposition of the mineral material feed. For example,
gaseous carbon dioxide could be fed into a pressure oxidation
autoclave in addition to the oxygen gas. Increasing the
partial pressure of carbon dioxide will tend to drive the
equilibrium of the reaction of Eqn. 2 to the left, to reduce the
50 capture of gold by the organic carbonaceous material.
Another way to reduce the susceptibility of the gold to be
captured and held by the organic carbonaceous material is to
restrict the amount of oxygen gas fed to the pressure
oxidation. Reducing the concentration of oxygen during
55 pressure oxidation drives the equilibrium of the reaction of
Eqn. 1 to the left, and thereby tends to reduce the concentration
of the gold-halide complex that is available for
participation in the deposition reaction of Eqn. 2. According
to this embodiment of the present invention, oxygen to the
60 reactor is kept at a level that is below a level at which
substantially all iron dissolved in effluent liquid from the
pressure oxidation process is in the ferric state. Preferably at
least about 10 mole % of soluble iron in the effluent liquid
is in the ferrous state, more preferably greater than about 20
65 mole %, and most preferably greater than about 50 mole %.
Another way to reduce the susceptibility of gold to be
captured and held by the organic carbonaceous material is to
recovery may be increased by at least 10%, preferably by at
least 25%, and most preferably by at least 50% (based on
carbon-in-Ieach cyanidation following pressure oxidation),
relative to gold recovery when the pressure oxidation is
conducted at the above-stated reference conditions.
The key to high recoveries according to the process of the
present invention is to conduct the pressure oxidation operation
in a way to reduce the susceptibility of gold to be
captured and held by the organic carbonaceous material
during pressure oxidation, thereby rendering the residue 10
from pressure oxidation more susceptible to high gold
recoveries during subsequent gold recovery processing
(typically carbon-in-Ieach cyanidation).
One way to reduce the susceptibility of gold to be
captured and held by the organic carbonaceous material is to 15
conduct the pressure oxidation at a temperature that is
smaller than about 2150 c., and more preferably smaller
than about 2050 C. Particularly preferred is a temperature of
smaller than about 1950 C.
The use of a lower pressure oxidation temperature is
believed to reduce the amount of gold captured and held by
the organic carbonaceous material by reducing the kinetics
of formation of a gold-halide complex according to Eqn. 1.
When less gold is complexed, less gold is available for
capture by the organic carbonaceous material according to
Eqn.2.
Another way to reduce the susceptibility of the gold to be
captured and held by the organic carbonaceous material
during pressure oxidation is to feed a carbonate-rich material 30
into the pressure oxidation along with the mineral material
feed. Materials such as dolomite or limestone could be used
as the carbonate-rich material. Also, the mineral material
feed could comprise a blend of two or more different
gold-bearing mineral materials, one having a lower carbonate
content and one having a higher carbonate content. For
example, a low carbonate-containing mineral material, such
as one having a weight ratio of sulfide sulfur to carbonate of
greater than 5:1, could be blended with a second mineral
material having a very high carbonate content, with the 40
resulting blend the desired ratio of sulfide sulfur to carbonate.
When blending, it is preferred that the blended mineral
material feed have a weight ratio of sulfide sulfur to carbonate
of smaller than about 4:1, more preferably smaller
than about 3: 1, and even more preferably smaller than about
2:1. Particularly preferred are blended mineral material
feeds having a weight ratio of sulfide sulfur to carbonate of
from about 0.5:1 to about 2:1. Below a ratio of about 0.5:1,
however, acid production during pressure oxidation may be
insufficient to obtain adequate oxidation of the sulfide minerals.
In computing the weight ratio of sulfide to carbonate,
weight attributable to the sulfide sulfur in the sulfide materials
is divided by the weight attributable to carbonate (C03 )
in carbonate-containing components. It is believed that
having a low ratio of sulfide sulfur to carbonate in the feed
is beneficial because the amount of carbon dioxide produced
during pressure oxidation is increased due to decomposition
of the additional carbonate. An increased concentration of
carbon dioxide drives the reaction of Eqn. 1 to the left by
reducing available acid and also drives the reaction of Eqn.
2 to the left, thereby decreasing the amount of gold that is
captured and held by the organic carbonaceous material.
Blending to attain a low ratio of sulfide sulfur to carbonate
in the mineral material feed works well with a number of
mineral materials to provide satisfactory gold recoveries
following pressure oxidation. In those circumstances when
high carbonate content materials are available for blending,
5,851,499
9 10
50 'Ounces of gold per standard short ton of ore
"Total of CI-, Br- & r- (titratable with silver nitrate)
TABLE 1
EXAMPLE 1
Gold Sulfide Sulfur Organic Carbon Halogen2 C03
Sample Oz/st' wt.% wt.% wt.% wt.%
A 0.270 5.68 0.46 0.02 0.69
B 0.196 5.91 0.39 0.01 0.51
C 0.342 4.93 0.45 0.09 0.46
0 0.248 4.72 0.41 0.04 4.18
E 0.186 3.68 0.13 7.43
F 0.209 4.13 0.24 0.03 9.25
G 0.160 5.99 0.35 <0.02 0.06
H 0.104 2.05 1.66 <0.005 4.96
r 0.232 1.65 1.60 <0.005 20.3
EXAMPLES
The following examples further demonstrate the present
invention, without limiting the scope thereof. Representative
analyses for ore samples A through I used in the following
examples are shown in Table 1. All of these ore samples are
from gold-bearing deposits in Nevada, U.S.A. Ore samples
A-F, however, are drill samples obtained using a potassium
chloride drill fluid. Those samples are, therefore, contaminated
with potassium chloride from the drilling operation.
Sample G was taken from the same zone as sample B, but
using a drill fluid not having a chloride salt, so that Sample
G is not contaminated with a chloride. Sample G has no
appreciable amount of naturally-occurring halogen. Samples
H and I are ore samples that have particularly active organic
carbonaceous material and a sufficient amount of naturallyoccurring
halogen to present a problem under standard
pressure oxidation processing conditions.
This example demonstrates the effect of pressure oxida-
55 tion temperature and residence time on gold recovery of
refractory sulfide ores having organic carbonaceous material
and that are pressure oxidized in the presence of a halogencontaining
material.
A continuous pressure oxidation pilot plant is conducted
using a four stage, stirred autoclave. Ore samples are fed one
at a time into the autoclave in a slurry with water at about
40% solids. Each ore sample is sized so that 80% of the
particles are smaller than about 20-22 microns. During
pressure oxidation, oxygen overpressure to the autoclave is
65 held at about 100 psi and retention time in the autoclave is
about 60 minutes. The autoclave is operated at 2000 C. for
some runs and 2250 C. for other runs.
aqueous liquid effluent. The aqueous liquid effluent has a pH
of smaller than about 1.5, and preferably has free sulfuric
acid, as discussed previously, in an amount of smaller than
about 28 grams per liter of the aqueous effluent liquid. The
5 solid effluent of the effluent product 8 comprises substantially
all of the gold from the feed slurry 2. Greater than
about 75%, preferably greater than about 80% and more
preferably greater than about 85% of gold in the solid
effluent is removable from the solid effluent by carbon-in-
10 leach cyanidation. The high gold recoveries from the effluent
product 8 of the present invention are attainable because
only a small percentage, typically less than about 25% or
less, of the gold in the solid effluent is held by residue of the
carbonaceous material.
Typically, the effluent product 8 of the present invention
is neutralized and subjected to carbon-in-Ieach cyanidation
to recover the gold therefrom. Other gold recovery methods
could, however, be used instead.
perform the pressure oxidation as a continuous process with
a short retention time. Preferably the retention time is shorter
than about 45 minutes, and more preferably shorter than
about 30 minutes. It is believed that a short retention time
during continuous processing provides operating conditions
embodying lower ORP and higher ratios of Fe+2 and Fe+3
,
which reduces the kinetics of goldlhalide dissolution (Eqn.
1).
Although a continuous process is preferred for the pressure
oxidation, an alternative is to operate the pressure
oxidation in a batch mode to reduce susceptibility of gold to
be captured and held by the organic carbonaceous material.
It has been found that when the pressure oxidation is
conducted in a batch mode with a sufficiently long retention,
that the organic carbonaceous material is satisfactorily
destroyed and/or passivated to the extent that it is incapable 15
of holding a substantial quantity of gold.
Yet another way to reduce the susceptibility of the gold to
be captured and held by the organic carbonaceous material
is to introduce a component into the pressure oxidation that
is capable of reacting with halogen from the halogen- 20
containing material, to thereby prevent the halogen from
complexing with gold. Accordingly, the amount of gold that
is susceptible to capture by the organic carbonaceous material
is reduced. In one embodiment, the feed comprises a
component that, during pressure oxidation, causes the for- 25
mation of an insoluble reaction product involving the
halogen, thereby removing the halogen as a threat for gold
complexation and reducing the amount of gold-halide complex
that is available to participate in the capture of gold by
organic carbonaceous material. Example components of this 30
variety include silver, mercury, lead and bismuth metals and
compounds having those metals in a form that is capable of
reacting to form the insoluble reaction product. In another
embodiment, the component may form a stable, soluble
complex with a halogen, thereby inhibiting the halogen from 35
complexing with gold. Examples of components of this
variety include copper, lead, zinc, cobalt, bismuth and tin
and compounds having those metals in a form capable of
reacting to form the stable, soluble complex. Some
components, such as lead and bismuth, will form either a 40
soluble or insoluble complex with the halogen, depending
upon the specific attributes of the mineral material and
specific pressure oxidation operating conditions. In any
event, the component may be a discrete component that is
added to the feed or may be part of a mineral material that 45
is blended with one or more other mineral materials to form
the mineral material feed.
The present invention also provides an effluent product
resulting from pressure oxidation, in the presence of a
halogen-containing material, of a gold-bearing mineral
material feed comprising sulfide material and organic carbonaceous
material. The effluent product is valuable in that
gold in the effluent material is easily recoverable by a variety
of recovery techniques, and especially by carbon-in-Ieach
cyanidation.
Referring to FIG. 1, one embodiment of the process is
shown for producing the effluent product of the present
invention. As shown in FIG. 1, a feed slurry 2 comprising the
mineral material feed slurried in an aqueous liquid, is
subjected to pressure oxidation 6, which is typically carried 60
out in one or more autoclaves. A gas 4 that is rich in oxygen
is contacted with the feed slurry to provide oxygen to
oxidize sulfide sulfur in the mineral material to a sulfate
form. Offgases 10 are collected for treatment and eventual
release.
The effluent product 8 includes solid effluent, which is the
solid residue from the pressure oxidation 6, mixed with an
11
5,851,499
12
Ore samples Band C are processed in the pilot plant.
During pressure oxidation, samples are taken from each of
the four compartments of the autoclave as well as from the
autoclave discharge. Each sample is neutralized to a pH of
about 10.5 using milk of lime. The neutralized samples are 5
then subjected to a 24-hour laboratory bottle-roll carbon-inleach
cyanidation test to determine gold extractions. Results
for autoclave compartment samples provide information
concerning the effect of pressure oxidation as a function of
time. 10
FIG. 2 shows a graph of gold recovery versus autoclave
retention time for the two temperature conditions for ore
Sample B and FIG. 3 shows the same information for ore
Sample C. As seen in FIGS. 2 and 3, for pressure oxidation
at 2250 c., recoveries drop significantly for the longer 15
retention times. This is contrary to conventional thought
concerning pressure oxidation, which is that gold recovery
increases with increased residence times. Much higher gold
recoveries are attained in the 2000 C. tests than in the 2250
C. tests. Also, for the 2000 C. tests, there is a range of 20
retention times in a maximum gold recovery region, with
gold recovery dropping off significantly for both shorter and
longer retention times. The test results are tabularly shown
in Table 2.
that gold recoveries increase with increasing pressure oxidation
temperature.
TABLE 3
Sample Temp. C. Gold Recovery
A 180 91.1
A 190 90.0
A 200 91.1
A 210 88.2
A 225 69.5
EXAMPLE 3
This example demonstrates the effect of varying the
weight ratio of sulfide sulfur to carbonate in the pressure
oxidation feed.
Using ore Sample A, a pilot plant is conducted as
described in Example 1. Various amounts of limestone,
dolomite or acid are added to ore Sample A to form the feed
to the autoclave. Autoclave discharge samples are collected,
neutralized and subjected to carbon-in-Ieach testing to determine
gold recovery as described in Example 1.
The results are shown graphically in FIGS. 5 and 6 and
tabularly in Table 4. As shown in FIGS. 5 and 6 and Table
TABLE 2
Total Halogen
Temp. Time Gold Recoveries % Cone.
Sample cc. Min Stage 1 Stage 2 Stage 3 Stage 4 Discharge mg/'
B 225 63 92.5 93 88 78.5 71.9 60
B 200 64 69 91.5 92.5 88.5 86.7 130
C 225 68 96 85 76 76.5 71.1 140
C 200 66 83 91 91.5 83 81.6 150
'Total of halogens titrated by silver nitrate (Cd-, r- & Be) in the autoclave discharge liquid
EXAMPLE 2
This example further demonstrates the effect of temperature
on gold recovery for refractory sulfide ores having
organic carbonaceous material pressure oxidized in the
presence of a halogen-containing material.
Using ore Sample A, a pilot plant is operated as described
in Example 1, but with the temperature being varied from
1800 C. to 2250 C. Samples of autoclave discharge are
subjected to cyanide-in-Ieach cyanidation as described in
4, gold recovery increases with a decreasing ratio of sulfide
40
sulfur to carbonate in the feed and with decreasing free acid
concentrations in the discharge liquid from the autoclave. It
should be noted, however, that if carbonate levels in the feed
become too high, then sufficient acid will not be generated
45 during pressure oxidation to adequately oxidize the sulfide
sulfur.
TABLE 4
Additive to
Sample Temp cc. Feed
wt. ratio
Sulfide sulfur
to carbonate
Gold
Recovery % Acid' gil
Halogen2
mg/l
A
A
A
A
225 45 lb/st acid3
225 None
225 5% dolomite
225 15% limestone
9.30
8.23
1.48
0.61
67.9
69.5
94.3
95.4
33.8
30.8
21.5
7.3
134
'Grams free H2S04 per liter of discharge liquid
2Total of halogens titrated by silver nitrate (CI-, Be, r-) in discharge liquid
3Pounds of sulfuric acid added per standard short ton of ore sample
Example 1. The results are shown graphically in FIG. 4 and
tabularly in Table 3. As seen in FIG. 4 and Table 3, gold 65
recovery declines substantially at higher temperatures.
Again, such a result is contrary to the conventional belief
EXAMPLE 4
This example demonstrates use of a feed having a blend
of two different ores, one having a greater carbonate content
5,851,499
13
than the other, to alter the weight ratio of sulfide sulfur to
carbonate in the feed.
Pressure oxidation is conducted on five feed blends, each
having a different blend of ore samples as shown in Table 5.
The blended feed samples are subjected to pressure oxidation
in a continuous pilot plant as described in Example 1,
at a temperature of 2250 C. Following pressure oxidation,
samples of the autoclave discharge are subjected to carbonin-
leach cyanidation to determine gold recoveries as
described in Example 1.
Results are shown graphically in FIG. 7 and tabularly in
Table 5. As seen in FIG. 7 and Table 5, gold recovery
generally increases for blends having a lower weight ratio of
sulfide sulfur to carbonate.
14
A composite sample is made from 50 weight percent of
ore sample Hand 50 weight percent of ore sample I. This
composite sample is subjected to batch pressure oxidation
with a retention time of at least 120 minutes and at varying
5 temperatures. Results are shown tabularly in Table 6 and
graphically in FIG. 9. Table 6 and FIG. 9 show that gold
recovery is highly sensitive to temperature, with gold recovery
at 1800 C. being at around 80 percent compared to a gold
10 recovery of only about 30% percent at 2250 C. Again, such
results are contrary to conventional thought concerning the
effect of increased pressure oxidation temperature on gold
recovery.
TABLE 5
Blend Wt. Ratio
A 0 E F Temp Sulfide Sulfur Gold Halogen'
wt% wt% wt% wt% 0c. to Carbonate Recovery % mg/l
100 0 0 0 225 8.23 69.5
85 15 0 0 225 4.56 77.2 247
75 25 0 0 225 3.49 70.0 146
66 34 0 0 225 2.85 95.2 110
56.5 0 6.0 37.5 225 1.13 95.5 229
'Total of halogens titrated by silver nitrate (CI-, Be, r-) in discharge liquid
EXAMPLE 7
'Total of halogens titrated by silver nitrate (CI-, Be, r-) in discharge liquid
This example demonstrates reduction in the susceptibility
of gold to be captured and held by organic carbonaceous
material through restriction of the amount of oxygen fed into
the autoclave during pressure oxidation, preventing substantially
complete oxidation of ferrous iron to ferric iron.
A blend of 50 weight percent ore sample Hand 50 weight
percent of ore sample I is subjected to batch pressure
oxidation processing at a treating temperature of 2250 C. and
a retention time of 90 minutes. In one test, the batch
autoclave is loaded with the slurried sample and oxygen gas
to a sufficient excess so that an unrestricted supply of oxygen
will be available during pressure oxidation. In a second test,
the amount of oxygen is limited to a stoichiometric amount
assuming complete oxidation of all sulfide sulfur to a sulfate
form and assuming that at the end of the pressure oxidation
all iron will be in the ferrous state. Discharge from the
autoclave for each test is subjected to carbon-in-Ieach cyanidation
as described in Example 1. Gold recovery is measured
at 74.3% for the test with restricted oxygen, compared
to 31.9% for the test with unrestricted oxygen.
Preferred embodiments of the pressure oxidation processing
in the present invention have been described herein. It
65 should be recognized, however, that the present invention is
not so limited. Any feature of any embodiment may be
combined with any compatible feature of any other embodi-
30
TABLE 6
Temp 0c. Gold Recovery % Halogen' mg/l
225 31.9 15
35 200 71.9 41.7
190 77.4 60.6
180 80.1 20.0
160 68.6 41.7
EXAMPLE 6
EXAMPLE 5
This example demonstrates pressure oxidation of ore
samples having naturally-occurring halogen-containing
material and sensitivity of gold recovery to the treating
temperature.
This example demonstrates the detrimental effect of the
presence of a chloride-containing material during pressure
oxidation of a gold-bearing sulfide ore that also contains
organic carbonaceous material.
Ore sample G is subjected to pressure oxidation in a batch
autoclave under semicontinuous conditions. The autoclave
is operated at 2250 C. with an oxygen overpressure of 100
psig.
A slurry of the feed sample is first treated with sulfuric
acid to a pH of 2. The feed samples is then subjected to
pressure oxidation in a batch autoclave at the desired treat- 40
ing conditions for two hours. Batches of additional feed
slurry are then pumped into the autoclave every fifteen or
twenty minutes, depending on the desired cycle time, and an
equivalent amount of material is removed from the autoclave.
Four consecutive discharge samples are combined 45
and subjected to carbon-in-Ieach cyanidation as described in
Example 1.
After the pressure oxidation has initially stabilized, new
batches introduced into the autoclave are spiked with potassium
chloride in an amount to provide 0.015 weight percent
of chloride relative to the weight of the ore sample of each 50
new batch. Twelve cycles later the chloride level in the feed
was increased to 0.030 percent chloride. Twenty cycles later,
potassium chloride in the feed batch is discontinued. The test
is run for an additional ten cycles and discontinued. During
the last ten cycles, two-batch composites are subjected to 55
carbon-in-Ieach cyanidation rather than the normal fourbatch
composites previously described.
Results are shown graphically in FIG. 8. From an initial
base gold recovery of over 90%, gold recoveries dropped to
below 70% after the addition of the potassium chloride. 60
After cessation of chloride addition, gold recoveries climbed
back to over 90%, indicating that the system was recovering.
5,851,499
15
~ent. Furthermore, the foregoing description of the inventIon
has been presented for purposes of illustration and
description. The description is not intended to limit the
variations and modifications commensurate with the above
teachings, and the skill or knowledge in the relevant art are 5
within the scope of the present invention. The preferred
embodiment described above is further intended to explain
the best mode known of practicing the invention and to
enable others skilled in the art to utilize the invention in
various embodiments and with the various modifications
required by their particular applications or uses of the 10
invention. It is intended that the appended claims be construed
to include alternative embodiments to the extent
permitted by the prior art.
What is claimed is:
1. A method of oxidatively treating a gold-bearing mineral 15
material feed to facilitate recovery of gold, the mineral
~aterial feed having sulfide material from which gold is
dIfficult to separate and having organic carbonaceous material
t?a~ has an affinity for at least one member of the group
conslstmg of gold and a gold complex, the mineral material 20
feed being pressure oxidized in the presence of a solubilized
form of a halogen which, when in the presence of the organic
carbonaceous material, is capable of interfering with gold
recovery, the method comprising the steps of:
pressure oxidizing, in an oxidizing environment at 25
elevated temperature and elevated pressure a feed
having said mineral material feed slurried with aqueou~
feed liquid, to free gold from said association with said
sulfide material to facilitate recovery of said gold;
said feed comprising halogen-containing material having 30
halogen, wherein during said step of pressure oxidizing
said halogen is in a solubilized form that is capable of
complexing with gold that is freed from said association
with said sulfide material; 35
said organic carbonaceous material, during said step of
pressure oxidizing, being capable of capturing and
holding gold when said organic carbonaceous material
is in the presence of said solubilized form of said
halogen; 40
effluent from said step of pressure oxidizing comprising
solid residue and aqueous effluent liquid;
wherein, said step of pressure oxidizing is conducted
under conditions to suppress the susceptibility of said
gold to be captured and held by said organic carbon- 45
aceous material, such that following said step of pressure
oxidation, greater than about 75 percent of said
gold from said mineral material feed is removable from
said solid residue by carbon-in-Ieach cyanidation;
said conditions including at least one condition selected 50
from the group consisting of:
(i) a temperature of lower than about 2150 C .
(ii) a ratio, on a weight basis, of sulfide ~~lfur to
carbonate in said mineral material feed of smaller
than about 4 to 1; 55
(iii) said aqueous effluent liquid has an oxidation/
reduction potential of smaller than about 700
. mill~volts, relative to a standard hydrogen electrode;
(IV) saId aqueous effluent liquid comprises sulfuric acid
in an amount of less than about 28 grams of free 60
sulfuric acid per liter;
(v) said step of pressure oxidizing is conducted in the
presence of carbon dioxide at a partial pressure that
is larger than a partial pressure that would be exerted
by carbon dioxide generated during said step of 65
pressure oxidizing from carbon released by decomposition
of said mineral material feed;
16
(vi) said step of pressure oxidizing is conducted in the
presence of a component that causes formation of a
halogen-containing reaction product that is insoluble
in aqueous liquid present during said step of pressure
oxidizing;
(vii) said step of pressure oxidizing is conducted in the
presence of a component capable of forming a
complex, that does not comprise gold, with said
solubilized form of said halogen to bind at least a
portion of said halogen and thereby inhibit formation
of a gold complex with said solubilized form of said
halogen, said complex being soluble during said step
of pressure oxidizing;
(viii) said step of pressure oxidizing is conducted in a
reactor with the addition of oxygen gas to the reactor
in an amount that is small enough, under reaction
conditions present in said reactor, to prevent substantially
complete oxidation of ferrous iron to ferric
iron during said step of pressure oxidizing;
(ix) said step of pressure oxidizing is conducted in a
batch operation;
(x) said step of pressure oxidizing is for a time of
shorter than about 45 min; and
(xi) said mineral material feed comprises a blend of a
first mineral material and a second mineral material
wherein said second mineral material has a highe;
carbonate content than said first mineral material.
2. The method of claim 1, wherein:
said solid residue comprises carbonaceous solid residue of
said organic carbonaceous material and said step of
pressure oxidizing is performed under conditions such
that, in said solid residue, less than about 25 percent of
go.ld ori~inally in said mineral material feed is held by
saId solId carbonaceous residue.
3. The method of claim 1, wherein:
said halogen, of said halogen-containing material, is
present in said feed in an amount that is larger than
about 20 parts per million by weight of halogen relative
to the weight of said mineral material.
4. The method of claim 1, wherein said halogen, of said
halogen-containing material, is present in said feed in an
am?unt that. is larger than about 35 parts per million by
weIght of saId halogen relative to the weight of said mineral
material.
5. The method of claim 1, wherein:
said halogen is selected from the group consisting of
chlorine, bromine, iodine and combinations thereof.
6. The method of claim 1, wherein:
said solubilized form of said halogen comprises a halide.
7. The method of claim 1, wherein:
said halogen comprises chlorine.
8. The method of claim 1, wherein:
said mineral material feed is such that less than about 60%
of said gold in said mineral material feed is removable
from said mineral material feed by direct cyanide
leaching of said mineral material feed.
9. The method of claim 1, wherein:
said organic carbonaceous material comprises greater
than about 0.3 weight percent of said mineral material
feed.
10. The method of claim 1, wherein:
said step of pressure oxidizing is conducted at a temperature
of lower than about 2050 C.
11. The method of claim 1, wherein:
said step of pressure oxidizing is conducted at a temperature
of lower than about 1950 C.
5,851,499
17
12. The method of claim 1, wherein:
said mineral material feed comprises a ratio, on a weight
basis, of sulfide sulfur to carbonate of from about 0.5 to
1 to about 2 to 1.
13. The method of claim 1, wherein: 5
said aqueous effluent liquid comprises sulfuric acid in an
amount of less than about 20 grams of free sulfuric acid
per liter.
14. The method of claim 1, wherein:
10
said aqueous effluent liquid comprises sulfuric acid in an
amount of from about 7 grams of free sulfuric acid per
liter to about 25 grams of free sulfuric acid per liter.
15. The method of claim 1, wherein:
said aqueous effluent liquid has an oxidation/reduction 15
potential of smaller than about 650 millivolts, relative
to a standard hydrogen electrode.
16. The method of claim 1, wherein:
said aqueous effluent liquid has an oxidation/reduction
potential that is at least about 25 millivolts, relative to 20
a standard hydrogen electrode, smaller than an
oxidation/reduction potential that would exist if said
feed were pressure oxidized in a continuous operation
under the following conditions: a temperature of about
2250 c., an oxygen overpressure of about 100 psia, a 25
residence time of about one hour, and free sulfuric acid
in said aqueous effluent liquid of about 30 grams per
liter.
17. The method of claim 1, wherein:
said step of pressure oxidizing is for a time of shorter than 30
about 30 minutes.
18. The method of claim 1, wherein:
said step of pressure oxidizing is conducted in the presence
of a component that causes formation, during said
step of pressure oxidizing, of a halogen-containing 35
complex that is insoluble in aqueous liquid present
during said step of pressure oxidizing.
19. The method of claim 18, wherein:
said component comprises at least one member selected 40
from the group consisting of silver, mercury, lead and
bismuth.
20. The method of claim 1, wherein:
said step of pressure oxidizing is conducted in the presence
of a component capable of forming a soluble 45
complex that does not comprise gold and that is soluble
during said step of pressure oxidizing, said complex
comprising said solubilized form of said halogen to
bind at least a portion of said halogen to inhibit
formation of a gold complex with said solubilized form 50
of said halogen.
21. The method of claim 20, wherein:
said component comprises at least one member selected
from the group consisting of copper, lead, zinc, cobalt,
bismuth, zirconium and tin.
18
22. The method of claim 1, wherein:
said mineral material feed comprises a blend of a first
mineral material and a second mineral material,
wherein said second mineral material is richer in carbonate
than said first mineral material; and
said mineral material feed comprises a ratio of sulfide
sulfur to carbonate, on a weight basis, that is smaller
than about 3 to 1.
23. The method of claim 22, wherein:
said sulfide sulfur to carbonate ratio is from about 0.5 to
1 to about 2 to 1.
24. The method of claim 1, wherein:
said halogen-containing material is initially dissolved in a
recycle aqueous liquid that makes up, at least in part,
said aqueous feed liquid.
25. The method of claim 1, wherein:
said halogen, in said feed, is substantially entirely a part
of said mineral material feed.
26. The method of claim 1, wherein:
aqueous effluent liquid from said step of pressure oxidizing
has a pH of smaller than about pH 1.5.
27. The method of claim 1, wherein:
said solid residue from said step of pressure oxidizing is
subjected to cyanide leaching; and
during said step of cyanide leaching, greater than about
75% of gold originally in said mineral material feed is
removed from said solid residue.
28. The method of claim 27, wherein:
said cyanide leaching comprises a carbon-in-Ieach cyanidation
of said solid residue.
29. The method of claim 27, wherein:
during said step of cyanide leaching, greater than about
80% of gold originally in said mineral material feed is
removed from said solid residue.
30. The method of claim 27, wherein:
during said step of cyanide leaching, greater than about
85% of gold originally in said mineral material feed is
removed from said solid residue.
31. The method of claim 1, wherein:
said step of pressure oxidizing is conducted under continuous
operating conditions other than comparison
continuous operating conditions consisting essentially
of the following: temperature of about 2250 c., oxygen
overpressure of about 100 psia, residence time of about
one hour, effluent liquid having sulfuric acid in an
amount of about 30 grams of free sulfuric acid per liter;
and
greater than about 25% more gold is removable from said
solid residue by carbon-in-Ieach cyanidation than if
said step of pressure oxidizing is conducted under said
comparison operating conditions.
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