5,653,945 Method for processing gold-bearing sulfide ores involving preparation of a sulfide concentrate

Patent Number/Link:

5,653,945

[45] Date of Patent: Aug. 5, 1997

I~I.~IIIIIIIIIIIIIII

US005653945A

United States Patent [19] [11] Patent Number:

Gatbje et aI.

[54] METHOD FOR PROCESSING GOLD·

BEARING SULFIDE ORES INVOLVING

PREPARATION OF A SULFIDE

CONCENTRATE

[75] Inventors: John C. Gathje, Longmont. Colo.;

Gary L. Simmons, Albuquerque, N.

Mex.

[73] Assignee: Santa Fe Pacific Gold Corporation,

Albuquerque, N. Mex.

[21] Appl. No.: 423,839

[22] Filed: Apr. 18, 1995

[51] filt. CI,6 B03D 1/00; COlG 7/00

[52] U.S. Cl. 423/26; 423/27; 423/29;

423/30

[58] Field of Search 423/26. 27, 29,

423/30, 579, DIG. 15

Berglund et al.. "Influence of Different Gases In Flotation Of

Sulphide Minerals," Proceedings of An Engineering Foundation

Conference on Advances in Coal and Mineral Processing

Using Flotation (1989) pp. 71-76, Society for Mining,

Metallurgy and Exploration, Inc., Littleton, Colorado,

Dec., 1989.

Martin et al.. "Complex Sulphide Ore Processing With

Pyrite Flotation By Nitrogen," International Journal of Mineral

Processing, 26 (1989) pp. 95-110, Elsevier Science

Publishers B.V.. Amsterdam. no month.

Jones, "Some Recent Developments in the Measurement

and Control of Xanthate, Perxanthate, Sulphide. and Redox

Potential in Flotation," International Journal of Mineral

Processing, 33 (1991) pp. 193-205, Elsevier Science Publishers

B.V., Amsterdam. no month.

(List continued on next page.)

Primary Examiner-Steven Bos

Attorney, Agent, or Finn-Holme Roberts & Owen LLP

Burger, "Froth Flotation Development: This Industry Workhorse

From Strength to Strength," E&MJ (Sep. 1983) pp.

67-75.

Onstott et al., "By-Product Molybdenum Flotation From

Copper Suliide Concentrate With Nitrogen Gas in Enclosed

Wemco Nitrogen Flotation Machines", Preprint No. 84-65

(1984) Society of Mining Engineers of AIME, no month. 39 Claims, 9 Drawing Sheets

Provided is a method for processing a gold-bearing sulfide

ore which involves maintaining the ore in a substantially

oxygen free environment, preferably beginning with comminution

of the ore and ending when a desired final

concentrate, enriched in sulfide minerals, is obtained by

flotation. In one embodiment, nitrogen gas is used to substantially

prevent contact between the ore and air during

comminution of the ore and during flotation operations. It is

believed that oxygen gas present in air detrimentally affects

the recovery of sulfide minerals in a flotation concentrate

through surface oxidation of sulfide mineral particles. The

use of a gas such as nitrogen can significantly reduce the

potential for such surface oxidation. Additionally, gases

separated from an oxygen plant may be beneficially used,

with an oxygen gas stream being used, for example, for

pressure oxidation of sulfide mineral materials, and with a

nitrogen gas stream being used in comminution and/or

flotation operations, resulting in advantageous use of a

nitrogen gas by-product stream which has previously been

vented to the atmosphere as waste.

[56]

809,959

1,045,970

1,505,323

3,655,044

3,834,896

4,571,263

4,571,264

4,605,439

4,797,202

5,013,359

5,074,993

5,245,110

References Cited

U.S. PATENT DOCUMENTS

1/1906 Kirby 209/166

12/1912 Greenway 209/166

8/1924 Eberenz 209/166

4/1972 Delaney 209/167

9/1974 Eisele et aI 423/26

2/1986 Weir et aI 423/150.4

8/1986 Weir 423/140

1/1989 K1irnpe1 et aI 423/26

5/1991 Fair et aI 423/26

12/1991 Kerr et aI 209/167

9/1993 Van Dijk et aI. 423/579

OTHER PUBLICATIONS

[57] ABSTRACT

AIR

132

flOTATION

TAIL

118

OXIDIZEO

MATERIAL

126

5,653,945

Page 2

OTHER PUBLICATIONS

Berglund, ''Pulp Chemistry in Sulphide Mineral Flotation",

International Journal of Mineral Processing, 33 (1991) pp.

21-31. Elsevier Science Publishers B.V.• Arnbsterdam.

Klymowsky et al., 'The Role of Oxygen in Xanthate Flotation

of Galena, Pyrite and Chalcopyrite," CIM, Bulletin

for June. pp. 683-688 (1970), Jun., 1970.

Rao et al.. ''Possible Applications of Nitrogen Flotation of

Pyrite." Minerals, Materials and Industry (ed. M.T. Jones),

Institute of Mining and Metallurgy, pp. 285-293 (1990), no

month.

Rao et al., "Adsorption of Anyl Xanthate at Pyrrhotite in the

Presence of Nitrogen and Implications in Flotation." Can.

Metall. Q., vol. 30, No.1, pp. 1-6 (1990), no month.

Xu et al.. "Sphalerite Reverse Flotation Using Nitrogen,"

Proc. Electrochem Soc., vol. 92-17. Proc. Int. Syrup. Electrochem.

Miner. Met. Process. III, 3rd, pp. 170-190 (1992),

no month.

Van Deventer et al., 'The Effect of Galvanic Interaction of

the Behaviour of the Froth Phase During the Flotation of a

Complex Sulfide Ore," Minerals Engineering. vol. 6, No. 12,

pp. 1217-1229 (1993), no month.

Author unknown. title unknown, Chapter IV. Gases and

Aeration, pp. 63-70. date unknown.

Plaskin et al.. "Role of Gases in Flotation Reactions."

Acacemy of Sciences, U.S.S.R Moscow. date unknown.

u.s. Patent Aug. 5, 1997 Sheet 1 of 9 5,653,945

RATE

MINERAL

MATERIAL

FEED

102 l'

1--08 GAS COMMINUTION SOURCE

104 110

106 1

114 I

r •

FLOTATION -- FLOTATIO 112 CONCENT

116 '. FLOTATION

TAIL

118

Fig. 1

u.s. Patent Aug. 5, 1997

PARTICULATE

MINERAL

MATERIAL

110

Sheet 2 of 9

AIR

132

OXYGEN

PLANT 130

5,653,945

.Ir

FLOTATION

112

• r

FLOTATION

TAIL

118

_ 114 -

116

128

PRESSURE

OXIDATION

124 -

OXIDIZED

MATERIAL

126

Fig. 2

u.s. Patent Aug. 5, 1997 Sheet 3 of 9 5,653,945

FIRST

MINERAL MATERIAL

FEED

138

, Ir

FLOTATION 116

112

114

FLOTATION

TAIL

118

-...

OXYGEN

PLANT 130

Fig. 3

SECOND

MINERAL MATERIAL

FEED

,Ir 140

MIXING

142

,Ir 144

PRESSURE

OXIDATION

124

OXIDIZED

MATERIAL

126

u.s. Patent Aug. 5, 1997 Sheet 4 of 9 5,653,945

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u.s. Patent Aug. 5, 1997 Sheet 7 of 9 5,653,945

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u.s. Patent Aug. 5, 1997 Sheet 9 of 9 5,653,945

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5,653,945

gold-bearing sulfide ores. however, is that a significant

amount of the gold-bearing sulfide mineral often reports to

the wrong flotation fraction. representing a significant loss

of gold.

There is a significant need for an improved method for

processing many gold-bearing sulfide ores that avoids the

high costs associated with oxidatively treating whole ores

without the significant loss of gold associated with concentrating

sulfide ores by flotation.

SUMMARY OF THE INVENTION

The present invention involves a method for processing

gold-bearing sulfide ores to facilitate gold recovery without

the burden of pressure oxidizing or roasting a whole ore and

15 without the substantial loss of gold value associated with

preparation of an ore concentrate by conventional flotation.

It has been found that air, which is used as the flotation gas

in conventional flotation. detrimentally affects flotation

separation of gold-bearing sulfide minerals, and that significantly

enhanced flotation performance may be obtained by

maintaining the sulfide ore in an environment substantially

free of air until a desired final flotation concentrate is

obtained.

It is believed that oxygen gas present in air tends to

oxidize the surface of certain gold-bearing sulfide mineral

particles, with the effect that flotation of those sulfide

mineral particles is reduced, resulting in a significant

amount of sulfide mineral which fails to float during

flotation. and, therefore, remains with the gangue.

By using a flotation gas that is deficient in oxygen gas

relative to air, however. the problems associated with the use

of air can be reduced. The result is an increased recovery of

sulfide materials in the concentrate, and correspondingly, an

increase in the recovery of gold in the concentrate.

In one embodiment. the gold-bearing sulfide minerals in

a sulfide ore are maintained in an environment that is

substantially free of oxygen beginning with comminution of

the ore and ending with recovery of a desired final sulfide

mineral concentrate. An oxygen deficient gas can be intro-

40 duced prior to or during comminution to displace any air that

may be present in the ore feed and to prevent air from

entering during comminution. Oxygen in the air that would

otherwise be present during comminution is, thereby, prevented

from oxidizing newly exposed sulfide mineral surfaces

created during comminution.

In one aspect, the present invention involves the advantageous

utilization. in the processing of gold-bearing sulfide

ores, of gases which may be separated from air. In one

embodiment, a flotation operation, conducted substantially

50 in the absence of oxygen gas, is combined with oxidative

treating to decompose sulfide minerals. freeing gold for

possible subsequent dissolution using a gold lixiviant. such

as a cyanide. The preferred oxidative treating is pressure

oxidation, although another oxidative treatment such as an

55 oxidizing roast may be used instead. Such oxidative treating

often requires a source of purified oxygen gas, which is often

produced by separation from air in an oxygen plant. A

by-product gas from such an oxygen plant is deficient in

oxygen gas and rich in nitrogen gas. The by-product gas is,

60 therefore, an ideal source of gas for use during comminution

and/or flotation of a gold-bearing sulfide ore. This

by-product gas is normally vented to the atmosphere in

current gold processing operations and is, therefore, wasted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing one embodiment of the

present invention;

FIELD OF THE INVENTION

BACKGROUND OF THE INVENTION

METHOD FOR PROCESSING GOLDBEARING

SULFIDE ORES INVOLVING

PREPARATION OF A SULFIDE

CONCENTRATE

Significant amounts of gold are found in sulfide ores. in

which the gold is associated with sulfide mineralogy. The

gold is difficult to recover from such sulfide ores, because

the gold is typically bound in sulfide mineral grains in a

manner that renders the ore refractory to many traditional 25

gold recovery techniques, such as direct cyanidation of the

ore. Therefore, sulfide ores are commonly treated to chemically

alter the sulfide mineral to permit dissolution of the

gold during subsequent gold recovery operations.

One technique for treating a gold-bearing sulfide ore in 30

preparation for gold recovery is to subject the ore to an

oxidative treatment to oxidize sulfide sulfur in the sulfide

minerals, thereby rendering the gold more susceptible to

recovery. One method for oxidatively treating a sulfide ore

is pressure oxidation, in which a slurry of the ore is subjected 35

to oxygen gas in an autoclave at elevated temperature and

pressure to decompose the sulfide mineral, freeing the gold

for subsequent recovery. Other oxidative treating methods

include roasting and bio-oxidation of the ore in the presence

of air or oxygen gas.

Treating whole ores by pressure oxidation or by oxidative

roasting is expensive. Part of the expense is due to energy

consumed in heating gold-barren gangue material in the

whole ore, and especially the energy required to heat water

in which the gangue material is slurried in the case of 45

pressure oxidation. Also, process equipment for treating a

whole ore must be sized to accommodate the throughout of

gangue material, in addition to the throughput of the goldbearing

sulfide minerals, thereby significantly adding to the

cost of process equipment. Moreover, side reactions may

occur involving gangue material which can detrimentally

affect the oxidative treating or can produce hazardous materials

which require special handling.

One way to reduce the high energy and process equipment

costs associated with oxidative treating of a whole ore. as

well as the potential for problems associated with side

reactions, would be to remove gangue material from the ore

prior to the oxidative treatment. For example, one method

that has been used to remove gangue material from goldbearing

sulfide ores is flotation. In flotation, air is bubbled

through a slurry of ore particles which have been treated

with reagents and the particles of the ore which are less

hydrophilic tend to rise with the air bubbles. thereby permitting

separation of the ore into two fractions. Flotation has

been used to prepare concentrates of gold-bearing sulfide 65

minerals which are rich in the sulfide minerals and relatively

free of gangue material. One problem with flotation of many

The present invention involves a method for processing

gold-bearing sulfide ores to facilitate recovery of gold from

the sulfide ore. In particular, the present invention involves 10

flotation processing of gold-bearing sulfide ores in a manner

that reduces problems associated with conventional flotation

to produce an ore concentrate. The present invention also

involves the flotation processing in combination with oxidative

treating, such as pressure oxidation. and use of

by-product gas from an oxygen plant used to supply oxygen

gas for the oxidative treating.

5,653,945

mineral particles of a size suitable for flotation. The particulate

mineral material 106 is preferably sized such that at

least 80 weight percent ofparticles in the particulate mineral

material are smaller than about 100 mesh. more preferably

5 smaller than about 150 mesh, and still more preferably

smaller than about 200 mesh. The size at which 80 weight

percent of a material passes is often referred to as a P80 size.

Any suitable grinding and/or milling operation may be used

for the comminution 104. Wet grinding and/or milling

10 operations are generally preferred due to their relative ease

and low cost compared to dry operations.

The comminution 104 is conducted in the presence of a

blanketing gas 108 which is obtained from a gas source 110.

During, or prior to. the comminution 104, the mineral

15 material feed 102 is mixed with the blanketing gas 108,

which contains oxygen gas, if at all, at a lower volume

fraction of oxygen gas than is present in ambient air, to

reduce problems that could be caused by the presence of air

during the comminution 104. During the comminution 104,

20 it is preferable to maintain a positive pressure of the blanketing

gas 108 into any grinding and/or milling apparatus to

assist mixing of the mineral material feed 102 with the

blanketing gas 108, and to displace any air which may have

been present with the mineral material feed 102.

After the comminution 104, the particulate mineral material

106 is subjected to flotation 112 to separate sulfide

minerals, with which the gold is associated. from non-sulfide

gangue material. During flotation, a slurry of the particulate

mineral material 106 is aerated with a flotation gas 114 from

30 the gas source 110. Any suitable flotation apparatus may be

used for the flotation 112. such as a one or more of a

conventional flotation cell or a flotation column. Preferably,

however. the flotation apparatus is such that a small positive

pressure of the flotation gas 114 may be maintained in the

35 apparatus to prevent the entry of air into the apparatus. The

flotation gas 114 has oxygen gas, if at all, at a reduced

volume fraction relative to the volume fraction of oxygen

gas in ambient air, to reduce the problems associated with

using air as a flotation gas. Although not required, the

40 flotation gas 114 will normally be of substantially the same

composition as the blanketing gas 108 used in the comminution

104. Additionally, normal reagents may be added

during or prior to the flotation III to assist in flotation

separation. Such reagents may include frothing agents,

45 activators, collectors, depressants, modifiers and dispersants.

Preferably, the flotation 112 is conducted at ambient

temperature and a natural pH produced by the mineral

material. Operating conditions such as pH may, however, be

adjusted as desired to optimize flotation separation for any

50 particular mineral material.

Exiting from the flotation 112 is a flotation concentrate

116, which is recovered from the flotation froth and which

is enriched in sulfide minerals, and consequently is also

enriched in gold.. Also exiting from the flotation 112 is a

55 flotation tail 118, which is enriched in non-sulfide gangue

materials, and consequently contains low,levels of gold.. The

flotation concentrate 116 may be further processed to

recover the gold by any suitable technique, if desired..

Alternatively, the flotation concentrate 116 may be sold as a

60 valuable commodity for processing by others to recover the

gold.

As noted previously, the flotation gas 114 and the blanketing

gas 108 each comprise oxygen gas, if at all, at a

volume fraction that is less than the volume fraction of

65 oxygen gas in ambient air. Preferably, however, the amount

of oxygen gas in the flotation gas 114 and/or blanketing gas

108 is less than about 15 volume percent, and more prefer-

The present invention provides a method for processing a

gold-bearing sulfide mineral material, such as a gold-bearing

sulfide ore, to facilitate recovery of the gold from the

mineral material. The method involves preparation of a

flotation concentrate in a manner that reduces problems 25

associated with conventional flotation. It has, surprisingly,

been found that the problems associated with concentrating

a gold-bearing sulfide ore by conventional flotation may be

significantly reduced by the use of a flotation gas which

comprises a lower volume fraction of oxygen gas than is

present in ambient air. Preferably, the flotation gas should be

substantially free of oxygen gas. When air is used as a

flotation gas, the oxygen gas in the air appears to detrimentally

affect the floatability of the sulfide minerals. This may

be due to a surface oxidation of sulfide mineral particles

caused by the presence of the oxygen gas. The surface

oxidation would tend to depress the sulfide mineral particles

during flotation. Furthermore, the detrimental effects of

oxygen gas may be further reduced by maintaining the ore

in an environment that is substantially free of oxygen gas

during comminution, mixing, pumping and all other processing

steps until a final flotation concentrate has been

obtained.. For example, when multiple flotation steps are

used, it is desirable to maintain the ore in an environment

that is substantially free of oxygen gas between the flotation

steps.

By reducing the apparently detrimental effects of oxygen

gas, it is possible to recover a greater amount of the sulfide

mineral in the flotation concentrate. The present invention,

therefore, facilitates the recovery of gold from sulfide mineral

material which may have previously been discarded as

waste, either with the gangue in a flotation tail or as subgrade

ore previously believed to be uneconomical for gold recovery.

One embodiment in accordance with the present invention

is shown in FIG. 1. With reference to FIG. 1, a gold-bearing

mineral material feed 102 is provided for processing. The

mineral material feed 102 may be any gold-bearing material

comprising one or more sulfide mineral with which the gold

is predominantly associated, and from which the gold is

difficult to recover. The sulfide mineral could include one or

more mineralogy including pyrite, marcasite, arsenopyrite,

arsenous pyrite and pyrrhotite. The mineral material feed

102 is typically a whole ore, butmay be a residue from other

processing or a previously discarded tail.

The mineral material feed 102 is subjected to comminution

104 to obtain a particulate mineral material 106 having

FIG. 2 is a flow diagram showing another embodiment of

the present invention;

FIG. 3 is a flow diagram showing yet another embodiment

of the present invention;

FIG. 4 is a graph of the grade of concentrate recovered

from flotation versus grind size Examples 1--6;

FIG. 5 is a graph of the grade of tails from flotation versus

grind size Examples 1--6;

FIG. 6 is a graph of concentrate weight percent recovery

from flotation versus grind size for Examples 1--6;

FIG. 7 is a graph of gold recovered in concentrate from

flotation versus grind size for Examples 1--6;

FIG. 8 is a flow diagram for one embodiment of the

present invention relating to a pilot plant for Example 7; and

FIG. 9 is a graph of gold recovery in concentrate from

flotation versus grind size for Examples 8-15.

DErAILED DESCRIPTION OF THE

PREFERRED EMBODIMENT

5,653,945

ably less than about 5 volume percent. Most preferably, both The gas source 110 may be any source providing a

the flotation gas 114 and the blanketing gas 108 are sub- suitable flotation gas 114 and blanketing gas 108. One

stantially free of oxygen gas. preferred gas source 110 is a facility in which nitrogen gas

To aid in the understanding of the present invention, but is separated from air, with the separated nitrogen gas being

not to be bound by theory, it is believed that oxygen gas, if 5 used as the blanketing gas 108 and the flotation gas 114.

Several processes are known for separating nitrogen from

present in any appreciable quantity, tends to oxidize the air, including cryogenic separation and membrane separasurface

of particles of certain gold-bearing sulftde minerals, tion. One particularly preferred gas source 110 is an oxygen

which can have the effect of depressinE: flotation of the

~ plant, which is commonly found at existing facilities where

gold-bearing su1fi.de mineral particles during the flotation 10 gold-bearing sulftde ores are processed. An oxygen plant is

112. By reducing the amount of oxygen gas that comes into typically required, for example, when a pressure oxidation

contact with a mineral material, it is believed that any operation or an oxidative roasting operation is used in the

surface oxidation effect is reduced, resulting in enhanced processing of gold-bearing su1fi.de ores. In the oxygen plant,

flotali'on of su1fi.de lDl'neral parti'cles and a corresponding

oxygen is separated from air, such as by cryogenic separaincrease

in the amount of su1fi.de mineral, and therefore gold, 15 tion or membrane separation, and the separated oxygen gas

recovered in the flotation concentrate 116. Therefore, it is

is used in the pressure oxidation or oxidative roasting

preferred that the flotation gas 114 and the blanketing gas operation. A by-product of such an oxygen plant is an

108 consist essentially of components which could not effluent gas stream which is enriched in nitrogen gas and is

oxidize the surface of gold-bearing su1fi.de mineral particles. suitable for use as the blanketing gas 168 an<:L'or the flotation

It is preferred that the flotation gas 114 and the blanketing 20 gas 114. This by-product stream has previously been vented

gas 108 predominantly comprise one or more gases other to the atmosphere and has, therefore, been wasted. With the

than oxygen gas. Suitable gases include nitrogen, helium, present invention, however, the by-product stream may be

argon and carbon dioxide. Preferably, one or more of these beneficially used to produce the flotation concentrate 116, in

gases should comprise greater than about 95 volume percent addition to using the oxygen gas product stream for the

of the flotation gas 114 and the blanketing gas 108, and more 25 pressure oxidation or oxidative roasting operation.

preferably greater than about 98 volume percent. Still more FIG. 2 shows one embodiment of the present invention in

preferable is for the blanketing gas 108 and the flotation gas which both the oxygen gas product stream and the nitrogen

114 to consist essentially of one or more of these gases. gas by-product stream from an oxygen plant are both used

Nitrogen gas is particularly preferred because of its rela- to process gold-bearing su1fi.de mineral material. Referring

tively low cost. Carbon dioxide is less preferred because it 30 to FIG. 2, particulate mineral material 110 is subjected to the

forms an acid when dissolved in water, which could corrode flotation 112 to produce the flotation concentrate 116 and the

process equipment or produce conditions less conducive to flotation tail 118, as previously described. The flotation gas

optimum flotation. 114 is a nitrogen gas enriched by-product stream from an

The blanketing gas 108 andlor the flotation gas 114 may oxygen plant 130, in which air 132 is separated into an

be introduced into process apparatus in any appropriate 35 oxygen enriched gas stream and nitrogen enriched gas

manner. Such gases may be fed under positive pressure or stream. The flotation concentrate 116, which is enriched in

may be induced into the apparatus by creating a suction gold-bearing sulftde minerals, is subjected to pressure oxiwhich

pulls the gas in. Preferably, however, the apparatus is dation 124 to decompose su1fi.de minerals, producing an

designed to substantially prevent introduction of air into oxidized material 126 from which the gold could be recovcomminution

and flotation apparatus. 40 ered by dissolution using any suitable gold lixiviant, such as

In one embodiment, the possible detrimental effects of a cyanide. The pressure oxidation 124 involves treating a

any surface oxidation of sulfide mineral particles that may slurry of the flotation concentrate 116 in an autoclave at a

be present in a mineral material feed may be counteracted by temperature of greater than about 1500 C. and an elevated

the addition of a sulfidizing agent, to at least partially replace pressure in the presence of an overpressure of a treating gas

the oxidized coating with a su1fi.de coating. Any material 45 128, which is rich in oxygen. It should be noted that other

capable of reacting to form the desired su1fi.de coating of the oxidative treating steps could be used instead of the pressure

mineral particle could be used. Suitable su1fi.dizing agents oxidation 124. For example, an oxidative roasting or bioinclude

alkali metal su1fi.des and bisulftdes, such as Na2S, oxidation could be used to produce the oxidized material

NaHS, etc. Such su1fi.dizing agents could be added just 126 using the treating gas 128.

before or during any stage of the flotation 112. 50 A further embodiment in accordance with the present

With the present invention, greater than about 80 weight invention is shown in FIG. 3 which uses the product and

percent of su1fi.de minerals from the particulate mineral by-product gas streams from an oxygen plant to process a

material 106 may be recovered in the flotation concentrate gold-bearing su1fi.de mineral material provided in two sepa-

116, and preferably greater than about 90 weight percent of rate feed streams. Referring to FIG. 3, a particulate first

those sulftde minerals are recovered in the flotation concen- 55 mineral material feed 138 is subjected to the flotation 112 to

trate 116. produce the flotation concentrate 116 and the flotation tail

One major advantage of the process of the present inven- 118, as previously described. The flotation gas 114 is a gas

tion is that, in addition to permitting a high recovery of stream enriched in nitrogen from the oxygen plant 130. A

gold-bearing sulftde minerals in the flotation concentrate particulate second mineral material feed 140 is combined

116, it permits a high rejection of gangue material into the 60 with the flotation concentrate 116 in a mixing step 142. The

flotation tail 118. Relative to the use of air as a flotation gas, combined stream 144. in the form of a slurry, is subjected to

the present invention permits the same recovery of gold to the pressure oxidation 124 to produce the oxidized material

be obtained in a concentrate of smaller weight. This provides 126, from which gold could be recovered.

a significant economic advantage because less gangue mate- One advantage of the embodiment shown in FIG. 3 is that

rial is present in the concentrate, from which the gold must 65 it permits the processing of multiple ores having different

ultimately be separated to produce a purified gold product, characteristics. For example, the first mineral material feed

if desired. 138 may comprise a lower grade gold-bearing su1fi.de ore

5,653,945

7 8

For each example, the ore sample is ground to the desired

size. A first portion of the ore sample is subjected to flotation

in a laboratory-scale flotation cell using air as the flotation

5 gas. A second portion of the ore sample is subjected to

flotation under the same conditions, except using a flotation

gas which consists essentially of nitrogen gas. During each

flotation test, a flotation froth is collected from the top of the

flotation cell to recover a flotation concentrate which is

enriched in sulfide minerals, and which is, therefore, also

enriched in gold. The flotation tail is that material which is

not collected in the froth. For each flotation test, the flotation

conditions are substantially as follows: A natural pH and

addition of potassium amyl xanthate and mercaptobenzothiazole

as collectors, copper sulfate for activation of

sulfides and MIBC as a frother. Flotation times range from

20 to 30 minutes.

than the second mineral material feed, which may comprise

a higher grade gold-bearing sulfide ore. The higher grade ore

may be suitable for pressure oxidation in a whole ore fonn,

whereas the lower grade ore must be upgraded to a concentrate

form to be suitable for pressure oxidation.

Alternatively, the second mineral material feed may comprise

a gold-bearing sulfide ore which has a significant

amount of carbonate material which would consume acid

produced during the pressure oxidation 124, and which

could, therefore, detrimentally interfere with proper opera- 10

tion of the pressure oxidation 124. A high sulfide sulfur

content in the flotation concentrate 116, however, tends to

produce additional acid during pressure oxidation to at least

partially offset the acid consuming effect of carbonate material

in the second mineral material feed. Almost all carbonate 15

material that may have been present in the first mineral

material feed, if any, would ordinarily have been removed

during the flotation 112.

The present invention is further described by the following

examples, which are intended to be illustrative only and 20

are not intended to limit the scope of the present invention.

EXAMPLES

Examples 1-6

Examples 1-6 demonstrate the use of nitrogen gas as a

flotation gas during flotation of a gold-bearing sulfide ore to

produce a sulfide enriched concentrate which could be

further processed to recover gold, if desired.

The results for examples 1-6 are shown tabularly in Table

2 and graphically in FIGS. 4-7 and reveal a significant

25 increase in the amount of gold recovered in the concentrate

when nitrogen gas is used as the flotation gas, especially at

smaller grind sizes.

TABLE 2

LONE TREE SUBGRADE BATCH JESTS

CoIlCentrate Concentrate Gold Reporting

Grind Grade Tail Grade Recovery to Concentrate

Exam- P80 oz goldlstC') oz goldlstC2) wt. %(4) %(5)

pIe Mesh(l) air nitrogen air nitrogen air nitrogen air nitrogen

1 100 0.31 0.35 0.19 0.20 15 15 75 75

2 150 0.28 0.31 0.21 0.16 15 16 71 79

3 200 0.33 0.29 0.21 0.16 15 19 74 81

4 270 022 0.25 022 0.12 20 24 72 86

5 325 0.23 0.20 0.22 0.16 20 25 73 81

6 400 0.14 0.14 0.29 0.12 29 33 67 85

(1)g0 weight percent of material passing the indicated size

(2)01mces of gold per short ton of cOIlCenlrate

(3)onnces of gold per short ton of tail

(4lweight percent of ore sample feed reporting to concentrate

(5)% of gold in ore sample feed reporting to cOIlCenlrate

LONE TREE SUBGRADE SULFIDE ORE

REPRESENOOIVE HEAD ANALYSIS

TABLE 1

(1)onnces per short ton of ore

For each ofExamples 1-6, an ore sample is provided from 50

Santa Fe Pacific Gold Corporation's Lone Tree Mine in

Nevada. The ore samples are of a low grade sulfide ore

which would be unsuitable for economic pressure oxidation

in a whole ore form. A representative assay of an ore sample

is shown in

FIG. 4 graphically shows the grade of the flotation concentrate

(measured as ounces of gold per short ton of

concentrate material) as a function of the grind size. As seen

in FIG. 4, no identifiable effect on the grade of the concentrate

is apparent from using nitrogen gas relative to using air

in the flotation. FIG. 5, however, shows that the flotation tail,

55 at smaller grind sizes, contains a significantly lower gold

value when using nitrogen gas as a flotation gas than when

using air. Therefore, when using nitrogen gas, more of the

gold-bearing sulfide minerals are recovered in the

concentrate, apparently without any detrimental effect to the

60 grade of the concentrate recovered. FIG. 6 shows that the

---------------------- amount of material recovered in the concentrate may be

~~~r O~~~ ~~:;:; significantly higher when using nitrogen gas as a flotation

Total Sulfur 1.75 wt. % gas than when using air, especially at the smaller grind sizes.

Sulfide Sulfur 1.66 wI. % FIG. 7 shows that gold recovery in the concentrate may be

Arsenic 1440 ppm. by wI. increased by almost 15% at a P80 grind of 270 mesh, when

_____________________ 65 using nitrogen gas as a flotation gas as opposed to air, again

without detrimental effect to the grade of concentrate recovered.

5,653,945

It should be noted that at a P80 grind of 100 mesh, there

is no significant difference in flotation performance when

using nitrogen gas as opposed to air as the flotation gas. It

is, therefore, surprising and unexpected that the performance

using nitrogen gas would improve so markedly relative to air 5

at the smaller grind sizes. Typically, it is expected that

flotation performance should improve with a smaller grind

size due to a more complete liberation of sulfide minerals

from non-sulfide gangue material. As seen in FIG. 7,

however. the gold recovery in the concentrate when using air 10

as the flotation gas is flat. at best When using nitrogen gas,

however, gold recovery generally increases with decreased

grind size due to increased sulfide mineral particle

liberation, as would normally be expected.

One way to explain the unexpectedly poor flotation per- 15

formance when using air. to assist in the understanding in the

present invention but not to be bound by theory, is that some

detrimental chemical process may be occurring when air is

used as a flotation gas, with the detrimental chemical process

counteracting the normally beneficial effects of a smaller 20

grind size. It was observed that when air is used as the

flotation gas, the pH of the slurry in the flotation cell drops

rapidly for several minutes, sometimes falling by as much as

0.5-2 pH units. Therefore, it appears that oxygen in the air

may be oxidizing the surface of sulfide mineral particles,

producing sulfuric acid and lowering the slurry pH. Such

surface oxidization of the sulfide mineral particles could

render them less responsive to flotation. As the grind

becomes smaller. the surface area available for oxidation of

the sulfide minerals increases significantly and, accordingly,

any beneficial effect from more complete liberation of

sulfide mineral due to the smaller grind size is offset by

increased surface oxidation, further depressing flotation of

the sulfide mineral particles. Nitrogen gas, however. would

not oxidize the surface of sulfide minerals and, therefore, 35

permits better flotation of sulfide mineral particles, resulting

in a higher recovery of sulfide minerals at the smaller grind

sizes, as would normally be expected.

cleaner flotation step 180 is accomplished in a series of three

dual compartment flotation cells. As shown in FIG. 8,

nitrogen gas 192 is supplied from gas tank 194 and is fed to

each of the comminution step 168. the rougher flotation step

172, the scavenger flotation step 178 and the cleaner flotation

step 180. The nitrogen gas 192 is used as the flotation

gas in each of the flotation steps and is used as a blanketing

gas to prevent air from oxidizing ore particles during the

comminution 168. The nitrogen gas is also used to blanket

all other process equipment, not shown, such as pumps and

mixing tanks. Gold-bearing sulfide minerals in the ore

sample 166 are, therefore. maintained in a substantially

air-free environment through the entire pilot plant, until the

gold-bearing sulfide minerals have been recovered in a

desired concentrate product.

The results of the pilot plant are shown in Table 3, which

shows that the final concentrate 190 from the pilot plant is

of a higher quality than the concentrates shown in Examples

1-6. Addition of the scavenger flotation step 178 and the

cleaner flotation step 180 in the pilot plant significantly

improves the grade of concentrate finally recovered, without

any appreciable loss of gold recovery.

(1180 weight percent of material passing the indicated size

(2101mces of gold per short ton of respective concentrate

(310unces of gold per short ton of final tail

(4lweight percent of ore sample feed reporting to respective concentrate

(5)% of gold in concentrate relative to feed fur the respective flotation step

0.085 071~11

0.28 071~11

6.45 WI. %

6.27 WI. %

1630 ppm by wI.

TABLE 4

Twin Creeks SUBGRADE SULFIDE ORE

REPRESENTATIVE HEAD ANALYSIS

Gold

Silver

Total Sulfur

Sulfide Sulfur

Arsenic

The results of Example 8 are graphically shown in FIG.

9 which shows a plot of gold recovery in the concentrate as

a function of grind size. As seen in FIG. 9, the use of

nitrogen gas generally results in a significantly higher recovery

of gold in the concentrate compared to the use of air as

a flotation gas.

60 (110unces per short ton of ore

Example 8

Laboratory tests are performed on samples of a low grade

gold-bearing sulfide ore from Santa Fe Pacific Gold Corporation's

Twin Creeks Mine in Nevada. A representative

45 analysis of an ore sample is shown in Table 4. For each test,

a sample is ground to the appropriate size and a portion of

each sample is then subjected to flotation using air as a

flotation gas and another portion is subjected to flotation

using nitrogen as a flotation gas. Substantially the same

50 flotation conditions are used as described for Examples 1--6.

Example 7 40

This example further demonstrates the beneficial use of

nitrogen gas in the flotation of gold-bearing sulfide ores. and

the use of a rougher-scavenger-cleaner arrangement of flotation

to enhance recovery of concentrate.

Aflotation pilot plant is operated using a low grade sulfide

ore from the Lone Tree Mine, as previously described with

Examples 1-6. The pilot plant flow is shown in FIG. 8.

With reference to FIG. 8, the ore sample 166 is subjected

to comminution 168 in a ball mill to a P80 size of 270 mesh.

The ground ore, in a slurry 170, is introduced into a rougher

flotation step 172. In the rougher flotation step 172. an initial

flotation separation is made with a rougher concentrate 174

being collected with the flotation froth and a rougher tail 176

being sent to a scavenger flotation step 178. material col- 55

lected in the flotation froth of the scavenger flotation step

178 is repulped and introduced, as a slurry 179, to a cleaner

flotation step 180, where a final flotation separation is made

to produce a cleaner concentrate 182 from the froth and a

cleaner tail 184. The cleaner tail 184 is combined with a

scavenger tail 186, from the scavenger flotation step 178, to

produce the final tail 188. The rougher concentrate 174 and

the cleaner concentrate 182 are combined to form a final

concentrate 190. In this example, the rougher flotation step

172 is accomplished in a single dual compartment flotation 65

cell, the scavenger flotation step 178 is accomplished in a

series of three dual compartment flotation cells, and the

5,653,945

10. The method of claim 1, wherein:

said step of providing a particulate gold-bearing mineral

material comprises comminuting a coarse gold-bearing

mineral material in an environment which is substantially

free of oxygen gas.

11. The method of claim 10, wherein:

said sulfide mineral is maintained in an environment that

is substantially free of oxygen between and during said

comminution and said flotation.

12. The method of claim 1, wherein:

subsequent to said flotation, at least a portion of said

flotation concentrate is subjected to oxidative treating

in the presence of a treating gas, which is enriched in

oxygen gas relative to ambient air, to oxidize at least a

portion of sulfide sulfur in said sulfide mineral, to assist

in freeing at least a portion of said gold from association

with said sulfide mineral and to facilitate possible

subsequent recovery of said gold.

13. The method of claim 12, wherein:

said oxidation treating comprises biooxidation of said

sulfide material.

14. The method of claim 12, wherein:

said flotation gas comprises an oxygen deficient

by-product gas from an oxygen plant which produces

an oxygen enriched gas from air; and

in said step of oxidative treating, said treating gas comprises

at least a portion of said oxygen enriched gas

from said oxygen plant.

15. The method of claim 12, wherein:

said oxidative treating comprises pressure oxidizing a

slurry of said sulfide mineral at an elevated temperature

and an elevated pressure in the presence of said treating

gas.

16. The method of claim 12, wherein:

said oxidative treating comprises roasting of said sulfide

mineral at an elevated temperature in the presence of

said treating gas.

17. The method of claim 12, wherein:

subsequent to said step of flotation, at least a portion of

said flotation concentrate is blended with a whole ore

comprising a sulfide mineral to form a blend; and

said blend is subjected to said oxidative treating.

18. The method of claim 17, wherein:

said oxidative treating comprises pressure oxidizing a

slurry of said sulfide mineral at an elevated temperature

and an elevated pressure in the presence of said treating

gas;

said whole ore comprises carbonate material which consumes

acid during said pressure oxidizing; and

said flotation concentrate is enriched in sulfide sulfur

which, during said pressure oxidizing, contributes to

production of sulfuric acid which at least partially

offsets acid consumption by said carbonate material.

19. The method of claim 12, wherein:

following said oxidative treating, gold which has been

freed from association with said sulfide mineral during

pressure oxidation, is recovered by dissolution into a

leach solution comprising a lixiviant for gold.

20. The method of claim 1, wherein:

said flotation concentrate comprises greater than about 80

weight percent of said sulfide mineral from said mineral

material.

21. The method of claim 1, wherein:

said flotation concentrate comprises greater than about 90

weight percent of said sulfide mineral from said mineral

material.

The present invention has been described with reference

to specific embodiments of the present invention. According

to the present invention, however, any of the features shown

in any embodiment may be combined in any way with any

other feature of any other embodiment. For example, any 5

feature shown in anyone of FIGS. 1-3 and 8 can be

combined with any other feature shown in any of those

figures. Furthermore, while various embodiments of the

present invention have been described in detail, it is apparent

that modifications and adaptations to those embodiments 10

will occur to those skilled in the art. It is to be expressly

understood that such modifications and adaptations are

within the scope of the present invention, set forth in the

following claims.

What is claimed is:

1. A method for processing a gold-bearing mineral material

having a sulfide mineral with which gold is associated,

the method comprising the steps of:

(a) providing a particulate gold-bearing mineral material,

wherein said mineral material comprises gold and a 20

sulfide mineral with which said gold is associated, and

wherein said mineral material also comprises nonsulfide

material as gangue;

(b) subjecting said mineral material to flotation with a

flotation gas to separate said mineral material into at 25

least two fractions, a first fraction being a flotation

concentrate, collected from flotation froth, enriched in

said sulfide mineral and said gold and a second fraction

being a flotation tail enriched in said non-sulfide material

and depleted in said gold; 30

wherein said flotation gas comprises no greater than

about 15 volume percent of oxygen gas;

and wherein, when pyrrhotite is present in said mineral

material, said flotation concentrate is enriched in said

pyrrhotite.

2. The method of claim 1, wherein:

said flotation gas comprises a by-product gas, enriched in

nitrogen gas relative to air, from an oxygen plant in

which an oxygen enriched gas is produced from air.

3. The method of claim 1, wherein:

said flotation gas comprises less than about 5 volume

percent oxygen gas.

4. The method of claim 1, wherein:

said flotation gas is substantially free of oxygen gas.

5. The method of claim 1, wherein:

said flotation gas comprises greater than about 85 volume

percent nitrogen gas.

6. The method of claim 1, wherein:

said flotation gas comprises greater than about 95 volume 50

percent nitrogen gas.

7. The method of claim 1, wherein

said flotation gas is substantially free of components

capable of oxidizing, during said flotation, sulfide sul- 55

fur in said sulfide mineral.

8. The method of claim 1, wherein:

said flotation gas comprises greater than about 95 volume

percent of gas selected from the group consisting of

nitrogen gas, helium gas, argon gas, carbon dioxide gas 60

and combinations thereof.

9. The method of claim 1, wherein:

said step of providing a particulate gold-bearing mineral

material comprises comminuting a coarse gold-bearing

mineral material in the presence of a blanketing gas 65

comprising no greater than about 15 volume percent of

oxygen gas.

5,653,945

22. The method of claim 1. wherein:

said flotation concentrate is enriched in, and said flotation

tail is depleted in, said gold and at least one of pyrite,

marcasite, arsenopyrite. arsenous pyrite and pyrrhotite.

23. A method for processing a gold-bearing mineral 5

material having a sulfide mineral with which gold is

associated. the method comprising the steps of:

(a) providing a coarse gold-bearing mineral material.

wherein said mineral material comprises gold and a

sulfide mineral with which said gold is associated. and 10

wherein said mineral material also comprises nonsulfide

material as gangue;

(b) mixing a blanketing gas with said mineral material;

presence of said blanketing gas to form a particulate

gold-bearing mineral material;

(d) subjecting said particulate mineral material to flotation

with a flotation gas, to separate said mineral material

into at least two fractions. a first fraction. collected 20

from flotation froth, being a flotation concentrate

enriched in said sulfide mineral and said gold, and a

second fraction being a flotation tail enriched in said

non-sulfide material and depleted in said gold;

wherein, when said blanketing gas comprises oxygen gas 25

said blanketing gas comprises less than about 15 volume

percent of said oxygen gas.

24. The method of claim 23. wherein:

during said mixing, said blanketing gas displaces air from 30

the vicinity of said coarse mineral material.

25. The method of claim 23. wherein:

said blanketing gas comprises less than about 5 volume

percent oxygen gas.

26. The method of claim 23. wherein: 35

said blanketing gas comprises greater than about 95

volume percent nitrogen gas.

27. The method of claim 23. wherein:

said blanketing gas and said flotation gas have substantially

the same gas composition. 40

28.Amethod for using diverse gas streams separated from

air to assist in processing a gold-bearing mineral material

having a sulfide mineral with which gold is associated, the

method comprising the steps of:

(a) separating a quantity of air into at least two gas 45

streams, with a first gas stream being enriched in

nitrogen gas relative to said air and a second gas stream

being enriched in oxygen gas relative to said air;

(b) providing a feed of particulate mineral material com- 50

prising gold and a sulfide mineral with which said gold

is associated. and wherein said mineral material also

comprises non-sulfide material;

flotation to separate said mineral material into at least 55

two fractions, with a first fraction being a flotation

concentrate which is enriched in said sulfide mineral

and said gold relative to said mineral material and said

gold in said feed and a second fraction being a flotation 60

tail which is enriched in said non-sulfide material and

depleted in said gold relative to said mineral material in

said feed;

said flotation comprising subjecting at least a portion of

said feed to a flotation gas including at least a portion 65

of said first gas stream, which is enriched in nitrogen

gas; and

(d) oxidative treating of at least a portion of said mineral

material, said oxidative treating comprising contacting

said portion of said mineral material with at least a

portion of said second gas stream. which is enriched in

oxygen gas, to oxidize at least a portion of sulfide sulfur

in said sulfide mineral to produce an oxidized material

in which at least some of said gold is freed from

association with said sulfide mineral, facilitating possible

subsequent recovery of gold from said oxidized

material.

29. The method of claim 28, wherein:

said step of providing said feed of particulate mineral

material comprises comminuting a coarse mineral

material in the presence of at least some of said first gas

stream, which is enriched in nitrogen gas.

30. The method of claim 28, wherein:

at least a portion of said mineral material, which is

subjected to said step of oxidative treating, comprises

at least a portion of said flotation concentrate.

31. The method of claim 28, wherein:

at least a portion of said mineral material, which is

subjected to said step of oxidative treating. comprises

at least a portion of said feed blended with at least a

portion of said flotation concentrate.

32. The method of claim 28. wherein:

said oxidative treating comprises pressure oxidizing a

slurry of said sulfide mineral at an elevated temperature

and an elevated pressure in the presence of said second

gas stream. which is enriched in oxygen gas.

33. The method of claim 28. wherein:

said oxidative treating comprises oxidative roasting of

said mineral material at an elevated temperature in the

presence of said second gas stream, which is enriched

in oxygen.

34. The method of claim 28, wherein:

said first gas stream comprises greater than about 95

volume percent nitrogen gas.

35. A method for processing a gold-bearing mineral

material having a sulfide mineral with which said gold is

associated. the method comprising the steps of:

(a) providing, in at least two portions, particulate mineral

material comprising gold, with a first feed portion of

said mineral material having a first average gold concentration

and a second feed portion of said mineral

material having a second average gold concentration

that is smaller than said first average gold concentration;

each of said first feed portion and said second feed

portion comprising a sulfide mineral with which gold

is associated and from which gold is difficult to

recover, and each of said first feed portion and said

second feed portion also comprising non-sulfide

material;

(b) oxidative treating of said first feed portion, said

oxidative treating comprising contacting said first feed

portion with a treating gas comprising oxygen gas, to

oxidize at least a portion of sulfide sulfur in said sulfide

mineral to produce an oxidized material in which at

least some of said gold is freed from association with

said sulfide mineral; and

feed portion, to flotation, comprising treating a liquid

5,653,945

slurry of said second feed portion with a flotation gas

to separate said second feed portion into at least two

fractions, a first fraction being a flotation concentrate

enriched in said sulfide mineral and said gold, and a

second fraction being a flotation tail enriched in said 5

non-sulfide material and depleted in said gold;

said flotation gas comprising no greater than about 15

volume percent of oxygen gas.

36. The method of claim 35, wherein: 10

said flotation gas comprises less than about 5 volume

percent oxygen gas.

37. The method of claim 35, wherein:

said flotation gas comprises greater than about 95 volume

percent nitrogen gas.

38. The method of claim 35, wherein:

at least a portion of said flotation concentrate is blended

with said first feed portion prior to said step of oxidative

treating.

39. The method of claim 35, wherein:

said oxidative treating comprises at least one of: (i)

pressure oxidizing a slurry of said first feed portion of

said mineral material in the presence of said treating

gas at elevated temperature and at elevated pressure,

(ii) oxidative roasting of said first feed portion in the

presence of said treating gas at elevated temperature,

and (iii) biooxidation of said first feed portion in the

presence of said treating gas.

* * * * *