5,851,499 Method for pressure oxidizing gold-bearing refractory sulfide ores having organic carbon

Patent Number/Link:

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

PRESS. QXID.

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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

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.

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;

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

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.

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

5,851,499

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

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

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

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.

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-

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

~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;

(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

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:

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.

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.

* * * * *